wdiff rfc8490v1.txt rfc8490.txt

Internet Engineering Task Force (IETF) R. Bellis
Request for Comments: 8490 ISC
Updates: 1035, 7766 S. Cheshire
Category: Standards Track Apple Inc.
ISSN: 2070-1721 J. Dickinson
S. Dickinson
Sinodun
T. Lemon
Nibbhaya Consulting
T. Pusateri
Unaffiliated
February
March 2019

DNS Stateful Operations

Abstract

This document defines a new DNS OPCODE for DNS Stateful Operations
(DSO). DSO messages communicate operations within persistent
stateful sessions using Type Length Value (TLV) syntax. Three TLVs
are defined that manage session timeouts, termination, and encryption
padding, and a framework is defined for extensions to enable new
stateful operations. This document updates RFC 1035 by adding a new
DNS header opcode OPCODE that has both different message semantics and a new
result code. This document updates RFC 7766 by redefining a session,
providing new guidance on connection reuse, and providing a new
mechanism for handling session idle timeouts.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8490.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Session Management . . . . . . . . . . . . . . . . . 9
4.1.2. Long-Lived Subscriptions . . . . . . . . . . . . . . 9
4.2. Applicable Transports . . . . . . . . . . . . . . . . . . 10
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 11
5.1. DSO Session Establishment . . . . . . . . . . . . . . . . 12
5.1.1. DSO Session Establishment Failure . . . . . . . . . . . . 13
5.1.2. DSO Session Establishment Success . . . . . . . . . . . . 14
5.2. Operations After after DSO Session Establishment . . . . . . . . . 14
5.3. DSO Session Termination . . . . . . . . . . . . . . . . . . . 15
5.3.1. Handling Protocol Errors . . . . . . . . . . . . . . 15
5.4. Message Format . . . . . . . . . . . . . . . . . . . . . 16
5.4.1. DNS Header Fields in DSO Messages . . . . . . . . . . 17
5.4.2. DSO Data . . . . . . . . . . . . . . . . . . . . . . 19 18
5.4.3. DSO Unidirectional Messages . . . . . . . . . . . . . 20
5.4.4. TLV Syntax . . . . . . . . . . . . . . . . . . . . . 21
5.4.4.
5.4.5. Unrecognized TLVs . . . . . . . . . . . . . . . . . . 22
5.4.6. EDNS(0) and TSIG . . . . . . . . . . . . . . . . . . 24 23
5.5. Message Handling . . . . . . . . . . . . . . . . . . . . 25 24
5.5.1. Delayed Acknowledgement Management . . . . . . . . . 26 25
5.5.2. MESSAGE ID Namespaces . . . . . . . . . . . . . . . . 27 26
5.5.3. Error Responses . . . . . . . . . . . . . . . . . . . 28 27
5.6. Responder-Initiated Operation Cancellation . . . . . . . 29 28
6. DSO Session Lifecycle and Timers . . . . . . . . . . . . . . 30 29
6.1. DSO Session Initiation . . . . . . . . . . . . . . . . . 30 29
6.2. DSO Session Timeouts . . . . . . . . . . . . . . . . . . 31 30
6.3. Inactive DSO Sessions . . . . . . . . . . . . . . . . . . 32 31
6.4. The Inactivity Timeout . . . . . . . . . . . . . . . . . 33 32
6.4.1. Closing Inactive DSO Sessions . . . . . . . . . . . . 33 32
6.4.2. Values for the Inactivity Timeout . . . . . . . . . . 34 33
6.5. The Keepalive Interval . . . . . . . . . . . . . . . . . 35 34
6.5.1. Keepalive Interval Expiry . . . . . . . . . . . . . . 35 34
6.5.2. Values for the Keepalive Interval . . . . . . . . . . 35 34
6.6. Server-Initiated DSO Session Termination . . . . . . . . . . 37 36
6.6.1. Server-Initiated Retry Delay Message . . . . . . . . 38 37
6.6.2. Misbehaving Clients . . . . . . . . . . . . . . . . . 39 38
6.6.3. Client Reconnection . . . . . . . . . . . . . . . . . 39 38
7. Base TLVs for DNS Stateful Operations . . . . . . . . . . . . 41 40
7.1. Keepalive TLV . . . . . . . . . . . . . . . . . . . . . . 41 40
7.1.1. Client Handling of Received Session Timeout Values . 43 42
7.1.2. Relationship to edns-tcp-keepalive EDNS0 EDNS(0) Option . . . 44 43
7.2. Retry Delay TLV . . . . . . . . . . . . . . . . . . . . . 45 44
7.2.1. Retry Delay TLV Used as a Primary TLV . . . . . . . . 45 44
7.2.2. Retry Delay TLV Used as a Response Additional TLV . . 47 46
7.3. Encryption Padding TLV . . . . . . . . . . . . . . . . . 48 46
8. Summary Highlights . . . . . . . . . . . . . . . . . . . . . 49 47
8.1. QR Bit and MESSAGE ID . . . . . . . . . . . . . . . . . . 49 47
8.2. TLV Usage . . . . . . . . . . . . . . . . . . . . . . . . 50 48
9. Additional Considerations . . . . . . . . . . . . . . . . . . 52 50
9.1. Service Instances . . . . . . . . . . . . . . . . . . . . 52 50
9.2. Anycast Considerations . . . . . . . . . . . . . . . . . 53 51
9.3. Connection Sharing . . . . . . . . . . . . . . . . . . . 54 52
9.4. Operational Considerations for Middlebox . Middleboxes . . . . . . . 55 53
9.5. TCP Delayed Acknowledgement Considerations . . . . . . . 56 54
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 59 57
10.1. DSO OPCODE Registration . . . . . . . . . . . . . . . . 59 57
10.2. DSO RCODE Registration . . . . . . . . . . . . . . . . . 59 57
10.3. DSO Type Code Registry . . . . . . . . . . . . . . . . . 59 57
11. Security Considerations . . . . . . . . . . . . . . . . . . . 60 59
11.1. TLS 0-RTT Zero Round-Trip Considerations . . . . . . . . . . . . . . . . 61 59
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 62 60
12.1. Normative References . . . . . . . . . . . . . . . . . . 62 60
12.2. Informative References . . . . . . . . . . . . . . . . . 63 61
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 65 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65 63

1. Introduction

This document specifies a mechanism for managing stateful DNS
connections. DNS most commonly operates over a UDP transport, but it
can also operate over streaming transports; the original DNS RFC
specifies DNS over TCP DNS-over-TCP [RFC1035], and a profile for DNS over TLS DNS-over-TLS
[RFC7858] has been specified. These transports can offer persistent
long-lived sessions and therefore, when using them for transporting
DNS messages, it is of benefit to have a mechanism that can establish
parameters associated with those sessions, such as timeouts. In such
situations, it is also advantageous to support server-initiated
messages (such as DNS Push Notifications [DNS-PUSH]). [Push]).

The existing Extension Mechanism for DNS (EDNS(0)) [RFC6891] is
explicitly defined to only have "per-message" semantics. While
EDNS(0) has been used to signal at least one session-related
parameter (edns-tcp-keepalive EDNS0 EDNS(0) Option [RFC7828]), the result
is less than optimal due to the restrictions imposed by the EDNS(0)
semantics and the lack of server-initiated signaling. For example, a
server cannot arbitrarily instruct a client to close a connection
because the server can only send EDNS(0) options in responses to
queries that contained EDNS(0) options.

This document defines a new DNS OPCODE, 6, OPCODE for DNS Stateful Operations (DSO).
(DSO) with a value of 6. DSO messages are used to communicate
operations within persistent stateful sessions, expressed using Type
Length Value (TLV) syntax. This document defines an initial set of
three TLVs used to manage session timeouts, termination, and
encryption padding.

All three TLVs defined here are mandatory for all implementations of
DSO. Further TLVs may be defined in additional specifications.

DSO messages may or may not be acknowledged; this is signaled by
providing acknowledged. Whether a non-zero DSO message ID for messages that must
is to be acknowledged (DSO (a DSO request messages), and a zero message ID for
messages that are message) or is not to be
acknowledged (DSO (a DSO unidirectional
messages). This message) is also specified in the
definition of a that particular DSO message type. The MESSAGE ID is
nonzero for DSO request messages, and zero for DSO unidirectional
messages. Messages are pipelined; answers pipelined and responses may appear out of
order when more than one answer is pending. multiple requests are being processed concurrently.

The format for DSO messages (Section 5.4) differs somewhat from the
traditional DNS message format used for standard queries and
responses. The standard twelve-byte header is used, but the four
count fields (QDCOUNT, ANCOUNT, NSCOUNT, ARCOUNT) are set to zero,
and accordingly their corresponding sections are not present.

The actual data pertaining to DNS Stateful Operations (expressed in
TLV syntax) is appended to the end of the DNS message header. Just
as in traditional DNS over TCP DNS-over-TCP [RFC1035] [RFC7766], the stream
protocol carrying DSO messages (which are just another kind of DNS
message) frames them by putting a 16-bit message length at the start.
The length of the DSO message is therefore determined from that
length rather than from any of the DNS header counts.

When displayed using packet analyzer tools that have not been updated
to recognize the DSO format, this will result in the DSO data being
displayed as unknown additional extra data after the end of the DNS message.

This new format has distinct advantages over an RR-based format
because it is more explicit and more compact. Each TLV definition is
specific to its use case and, as a result, contains no redundant or
overloaded fields. Importantly, it completely avoids conflating DNS
Stateful Operations in any way with normal DNS operations or with
existing EDNS(0)-based functionality. A goal of this approach is to
avoid the operational issues that have befallen EDNS(0), particularly
relating to middlebox behavior (see, for example, Sections 3.2 (see sections discussing EDNS(0), and 4
problems caused by firewalls and load balancers, in the recent work
describing causes of [FTR]). DNS failures [Fail]).

With EDNS(0), multiple options may be packed into a single OPT
pseudo-RR, and there is no generalized mechanism for a client to be
able to tell whether a server has processed or otherwise acted upon
each individual option within the combined OPT pseudo-RR. The
specifications for each individual option need to define how each
different option is to be acknowledged, if necessary.

In contrast to EDNS(0), with DSO there is no compelling motivation to
pack multiple operations into a single message for efficiency
reasons, because DSO always operates using a connection-oriented
transport protocol. Each DSO operation is communicated in its own
separate DNS message, and the transport protocol can take care of
packing several DNS messages into a single IP packet if appropriate.
For example, TCP can pack multiple small DNS messages into a single
TCP segment. This simplification allows for clearer semantics. Each
DSO request message communicates just one primary operation, and the
RCODE in the corresponding response message indicates the success or
failure of that operation.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Terminology

DSO: DNS Stateful Operations.

connection: a bidirectional byte (or message) stream, where the
bytes (or messages) are delivered reliably and in order, such as
provided by using DNS over TCP DNS-over-TCP [RFC1035] [RFC7766] or DNS over TLS DNS-over-TLS
[RFC7858].

session: The the unqualified term "session" in the context of this
document refers to a persistent network connection between two
endpoints that allows for the exchange of DNS messages over a
connection where either end of the connection can send messages to
the other end. (The term has no relationship to the "session
layer" of the OSI "seven-layer model".)

DSO Session: a session established between two endpoints that
acknowledge persistent DNS state via the exchange of DSO messages
over the connection. This is distinct from a DNS-over-TCP session
as described in the previous specification for DNS over TCP DNS-over-TCP
[RFC7766].

close gracefully: a normal session shutdown where the client closes
the TCP connection to the server using a graceful close such that
no data is lost (e.g., using TCP FIN; see Section 5.3).

forcibly abort: a session shutdown as a result of a fatal error
where the TCP connection is unilaterally aborted without regard
for data loss (e.g., using TCP RST; see Section 5.3).

server: the software with a listening socket, awaiting incoming
connection requests, in the usual DNS sense.

client: the software that initiates a connection to the server's
listening socket, in the usual DNS sense.

initiator: the software that sends a DSO request message or a DSO
unidirectional message during a DSO session. Session. Either a client or
server can be an initiator.

responder: the software that receives a DSO request message or a DSO
unidirectional message during a DSO session. Session. Either a client or a
server can be a responder.

sender: the software that is sending a DNS message, a DSO message, a
DNS response, or a DSO response.

receiver: the software that is receiving a DNS message, a DSO
message, a DNS response, or a DSO response.

service instance: a specific instance of server software running on
a specific host (Section 9.1).

long-lived operation: an outstanding operation on a DSO session Session
where either the client or server, acting as initiator, has
requested that the responder send new information regarding the
request, as it becomes available.

Early Data: A

early data: a TLS 1.3 handshake containing early data on the first flight
that begins a DSO session Session (Section 2.3 of the TLS 1.3
specification [RFC8446]). TCP Fast Open [RFC7413] is only
permitted when using TLS.

DNS message: any DNS message, including DNS queries, responses,
updates, DSO messages, etc.

DNS request message: any DNS message where the QR bit is 0.

DNS response message: any DNS message where the QR bit is 1.

DSO message: a DSO request message, DSO unidirectional message, or
DSO response to a DSO request message. If the QR bit is 1 in a
DSO message, it is a DSO response message. If the QR bit is 0 in
a DSO message, it is a DSO request message or DSO unidirectional
message, as determined by the specification of its primary Primary TLV.

DSO response message: a response to a DSO request message.

DSO request message: a DSO message that requires a response.

DSO unidirectional message: a DSO message that does not require and
cannot induce a response.

Primary TLV: The the first TLV in a DSO request message or DSO response; in the
DSO message,
unidirectional message; this determines the nature of the
operation being performed.

Additional TLV: any TLVs in a DSO message response that follow the
primary Primary TLV in a DSO
request message or DSO unidirectional message.

Response Primary TLV: in a DSO response, any TLVs with the (optional) first same DSO-
TYPE as the Primary TLV from the corresponding DSO request
message. If present, any Response Primary TLV(s) MUST appear
first in a the DSO response message, before any Response Additional
TLVs.

Response Additional TLV: any TLVs in a DSO response that follow the
(optional) Response Primary TLV. TLV(s).

inactivity timer: the time since the most recent non-keepalive DNS
message was sent or received (see Section 6.4).

keepalive timer: the time since the most recent DNS message was sent
or received (see Section 6.5).

session timeouts: the inactivity timer and the keepalive timer.

inactivity timeout: the maximum value that the inactivity timer can
have before the connection is gracefully closed.

keepalive interval: the maximum value that the keepalive timer can
have before the client is required to send a keepalive (see
Section 7.1).

resetting a timer: setting the timer value to zero and restarting
the timer.

clearing a timer: setting the timer value to zero but not restarting
the timer.

4. Applicability

DNS Stateful Operations are applicable to several known use cases and
are only applicable on transports that are capable of supporting a
DSO Session.

4.1. Use Cases

Several use cases for DNS Stateful operations Operations are described below.

4.1.1. Session Management

In one use case, establishing session parameters such as server-
defined timeouts is of great use in the general management of
persistent connections. For example, using DSO sessions Sessions for stub-to-
recursive DNS-over-TLS [RFC7858] is more flexible for both the client
and the server than attempting to manage sessions using just the
edns-tcp-keepalive EDNS0 EDNS(0) Option [RFC7828]. The simple set of TLVs
defined in this document is sufficient to greatly enhance connection
management for this use case.

4.1.2. Long-Lived Subscriptions

In another use case, DNS-based Service Discovery (DNS-SD) [RFC6763]
has evolved into a naturally session-based mechanism where, for
example, long-lived subscriptions lend themselves to 'push'
mechanisms as opposed to polling. Long-lived stateful connections
and server-initiated messages align with this use case [DNS-PUSH]. [Push].

A general use case is that DNS traffic is often bursty, but session
establishment can be expensive. One challenge with long-lived
connections is to maintain sustaining sufficient traffic to maintain NAT and
firewall state. To mitigate this issue, this document introduces a
new concept for the DNS -- DSO "keepalive traffic". This traffic
carries no DNS data and is not considered 'activity' in the classic
DNS sense, but it serves to maintain state in middleboxes and to
assure the client and server that they still have connectivity to
each other.

4.2. Applicable Transports

DNS Stateful Operations are applicable in cases where it is useful to
maintain an open session between a DNS client and server, where the
transport allows such a session to be maintained, and where the
transport guarantees in-order delivery of messages on which DSO
depends. Examples of Two specific transports that can meet the requirements to
support DNS Stateful Operations are DNS-over-TCP [RFC1035] [RFC7766]
and DNS-over-TLS [RFC7858].

Note that in the case of DNS over TLS, DNS-over-TLS, there is no mechanism for
upgrading from DNS-over-TCP to DNS-over-TLS mid-connection (see
Section 7 of the DNS-over-TLS specification [RFC7858]). A connection
is either DNS-over-TCP from the start, or DNS-over-TLS from the
start.

DNS Stateful Operations are not applicable for transports that cannot
support clean session semantics or that do not guarantee in-order
delivery. While in principle such a transport could be constructed
over UDP, the current DNS specification over UDP transport of DNS-over-UDP [RFC1035] does
not provide in-order delivery or session semantics and hence cannot
be used. Similarly, DNS-over-HTTP [RFC8484] cannot be used because
HTTP has its own mechanism for managing sessions. This sessions, which is
incompatible with the mechanism specified here.

No other transports

Only DNS-over-TCP and DNS-over-TLS are currently defined for use with
DNS Stateful Operations. Such Other transports can may be added in the future
if they meet the requirements set out in the first paragraph of this
section.

5. Protocol Details

The overall flow of DNS Stateful Operations goes through a series of
phases:

Connection Establishment: A client establishes a connection to a
server (Section 4.2).

Connected but Sessionless: A connection exists, but a DSO session Session
has not been established. DNS messages can be sent from the
client to server, and DNS responses can be sent from servers the server to
clients.
the client. In this state, a client that wishes to use DSO can
attempt to establish a DSO session Session (Section 5.1). Standard DNS-
over-TCP inactivity timeout handling is in effect [RFC7766] (see
Section 7.1.2 of this document).

DSO Session Establishment in Progress: A client has sent a DSO
request within the last 30 seconds, but has not yet received a DSO response.
response for that request. In this phase, the client may send
more DSO requests and more DNS requests, but MUST NOT send DSO
unidirectional messages (Section 5.1).

DSO Session Establishment Failed: Timeout: A client has sent a DSO request,
and after 30 seconds has still received no DSO response for that
request. This means that the server is now in an indeterminate
state. The client forcibly aborts the connection. The client MAY
reconnect without using DSO, if appropriate.

DSO Session Establishment Failed: A client has sent a DSO request,
and received a corresponding DSO response with a nonzero RCODE.
This means that the attempt to establish the DSO
session Session did not
succeed. At this point, the client is permitted to continue
operating without a DSO session Session (Connected but Sessionless) but
does not send further DSO messages (Section 5.1).

DSO Session Established: A client has sent a DSO request, and
received a corresponding DSO response with RCODE set to NOERROR
(0). A DSO Session has now been successfully established. Both
client and server may send DSO messages and DNS messages; both may
send replies in response to messages they receive (Section 5.2).
The inactivity timer (Section 6.4) is active; the keepalive timer
(Section 6.5) is active. Standard DNS-over-TCP inactivity timeout
handling is no longer in effect [RFC7766] (see Section 7.1.2 of
this document).

Server Shutdown: The server has decided to gracefully terminate the
session and has sent the client a Retry Delay message
(Section 6.6.1). There may still be unprocessed messages from the
client; the server will ignore these. The server will not send
any further messages to the client (Section 6.6.1.1).

Client Shutdown: The client has decided to disconnect, either
because it no longer needs service, the connection is inactive
(Section 6.4.1), or because the server sent it a Retry Delay
message (Section 6.6.1). The client closes the connection
gracefully (Section 5.3).

Reconnect: The client disconnected as a result of a server shutdown.
The client either waits for the server-specified Retry Delay to
expire (Section 6.6.3) or else contacts a different server
instance. If the client no longer needs service, it does not
reconnect.

Forcibly Abort: The client or server detected a protocol error, and
further communication would have undefined behavior. The client
or server forcibly aborts the connection (Section 5.3).

Abort Reconnect Wait: The client has forcibly aborted the connection
but still needs service. Or, the server forcibly aborted the
connection, but the client still needs service. The client either
connects to a different service instance (Section 9.1) or waits to
reconnect (Section 6.6.3.1).

5.1. DSO Session Establishment

In order for a session to be established between a client and a
server, the client must first establish a connection to the server
using an applicable transport (see Section 4). 4.2).

In some environments, it may be known in advance by external means
that both client and server support DSO, and in these cases either
client or server may initiate DSO messages at any time. In this
case, the session is established as soon as the connection is
established; this is referred to as implicit session DSO Session
establishment.

However, in the typical case a server will not know in advance
whether a client supports DSO, so in general, unless it is known in
advance by other means that a client does support DSO, a server MUST
NOT initiate DSO request messages or DSO unidirectional messages
until a DSO Session has been mutually established by at least one
successful DSO request/response exchange initiated by the client, as
described below. This is referred to as explicit session DSO Session
establishment.

Until a DSO session Session has been implicitly or explicitly established, a
client MUST NOT initiate DSO unidirectional messages.

A DSO Session is established over a connection by the client sending
a DSO request message, such as a DSO Keepalive request message
(Section 7.1), and receiving a response with a matching MESSAGE ID,
and RCODE set to NOERROR (0), indicating that the DSO request was
successful.

Some DSO messages are permitted as early data (Section 11.1). Others
are not. Unidirectional messages are never permitted as early data,
unless an implicit session DSO Session exists.

If a server receives a DSO message in early data whose primary Primary TLV is
not permitted to appear in early data, the server MUST forcibly abort
the connection. If a client receives a DSO message in early data,
and there is no implicit DSO session, Session, the client MUST forcibly abort
the connection. This can only be enforced on TLS connections;
therefore, servers MUST NOT enable TCP Fast Open (TFO) when listening
for a connection that does not require TLS.

5.1.1. DSO Session Establishment Failure

If the response RCODE is set to NOTIMP (4), or in practice any value
other than NOERROR (0) or DSOTYPENI (defined below), then the client
MUST assume that the server does not implement DSO at all. In this
case, the client is permitted to continue sending DNS messages on
that connection but MUST NOT issue further DSO messages on that
connection.

If the RCODE in the response is set to DSOTYPENI ("DSO-TYPE Not
Implemented"; RCODE 11), this indicates that the server does support
DSO but does not implement the DSO-TYPE of the primary Primary TLV in this
DSO request message. A server implementing DSO MUST NOT return
DSOTYPENI for a DSO Keepalive request message because the Keepalive
TLV is mandatory to implement. But in the future, if a client
attempts to establish a DSO Session using a response-requiring DSO
request message using some newly-defined DSO-TYPE that the server
does not understand, that would result in a DSOTYPENI response. If
the server returns DSOTYPENI, then a DSO Session is not considered
established. The client is, however, permitted to continue sending
DNS messages on the connection, including other DSO messages such as
the DSO Keepalive, which may result in a successful NOERROR response,
yielding the establishment of a DSO Session.

Two other possibilities exist: the

When a DSO message is received by an existing DNS server might drop that doesn't
recognize the connection,
or DSO OPCODE, two other possible outcomes exist: the
server might send no response to the DSO message.

In message, or the first case, server might
drop the client SHOULD mark that service instance as
not supporting DSO, and not attempt a DSO connection for some period
of time (at least an hour) after connection.

If the failed attempt. The client MAY
reconnect but not use DSO, if appropriate (Section 6.6.3.2).

In server sends no response to the second case, DSO message, the client SHOULD
wait 30 seconds, after which time the server will be assumed not to
support DSO. If the server doesn't respond within 30 seconds, the
client MUST forcibly abort the connection to the server since the
server's behavior is out of spec does not conform to the specification, and hence
its state is undefined. The client MAY reconnect, but not use DSO,
if appropriate (Section 6.6.3.1).

If the server drops the connection the client SHOULD mark that
service instance as not supporting DSO, and not attempt a DSO
connection for some period of time (at least an hour) after the
failed attempt. The client MAY reconnect but not use DSO, if
appropriate (Section 6.6.3.2).

5.1.2. DSO Session Establishment Success

When the server receives a DSO request message from a client, and
transmits a successful NOERROR response to that request, the server
considers the DSO Session established.

When the client receives the server's NOERROR response to its DSO
request message, the client considers the DSO Session established.

Once a DSO Session has been established, either end may unilaterally
send appropriate DSO messages at any time, and therefore either
client or server may be the initiator of a message.

5.2. Operations After after DSO Session Establishment

Once a DSO Session has been established, clients and servers should
behave as described in this specification with regard to inactivity
timeouts and session termination, not as previously prescribed in the
earlier specification for DNS over TCP DNS-over-TCP [RFC7766].

Because a server that supports DNS Stateful Operations MUST return an
RCODE of "NOERROR" when it receives a Keepalive TLV DSO request
message, the Keepalive TLV is an ideal candidate for use in
establishing a DSO session. Session. Any other option that can only succeed
when sent to a server of the desired kind is also a good candidate
for use in establishing a DSO session. Session. For clients that implement
only the DSO-TYPEs defined in this base specification, sending a
Keepalive TLV is the only DSO request message they have available to
initiate a DSO Session. Even for clients that do implement other
future DSO-TYPEs, for simplicity they MAY elect to always send an
initial DSO Keepalive request message as their way of initiating a
DSO Session. A future definition of a new response-requiring DSO-
TYPE gives implementers the option of using that new DSO-TYPE if they
wish, but does not change the fact that sending a Keepalive TLV
remains a valid way of initiating a DSO Session.

5.3. DSO Session Termination

A "DSO Session" DSO Session is terminated when the underlying connection is closed.
DSO Sessions are "closed gracefully" as a result of the server
closing a session DSO Session because it is overloaded, because of the client
closing the session DSO Session because it is done, or because of the client
closing the session DSO Session because it is inactive. DSO Sessions are
"forcibly aborted" when either the client or server closes the
connection because of a protocol error.

o Where this specification says "close gracefully", it means sending
a TLS close_notify (if TLS is in use) followed by a TCP FIN, or
the equivalent for other protocols. Where this specification
requires a connection to be closed gracefully, the requirement to
initiate that graceful close is placed on the client in order to
place the burden of TCP's TIME-WAIT state on the client rather
than the server.

o Where this specification says "forcibly abort", it means sending a
TCP RST or the equivalent for other protocols. In the BSD Sockets
API, this is achieved by setting the SO_LINGER option to zero
before closing the socket.

5.3.1. Handling Protocol Errors

In protocol implementation, there are generally two kinds of errors
that software writers have to deal with. The first is situations
that arise due to factors in the environment, such as temporary loss
of connectivity. While undesirable, these situations do not indicate
a flaw in the software and are situations that software should
generally be able to recover from.

The second is situations that should never happen when communicating
with a compliant DSO implementation. If they do happen, they
indicate a serious flaw in the protocol implementation beyond what is
reasonable to expect software to recover from. This document
describes this latter form of error condition as a "fatal error" and
specifies that an implementation encountering a fatal error condition
"MUST forcibly abort the connection immediately".

5.4. Message Format

A DSO message begins with the standard twelve-byte DNS message header
[RFC1035] with the OPCODE field set to the DSO OPCODE. OPCODE (6). However,
unlike standard DNS messages, the question section, answer section,
authority records section, and additional records sections are not
present. The corresponding count fields (QDCOUNT, ANCOUNT, NSCOUNT,
ARCOUNT) MUST be set to zero on transmission.

If a DSO message is received where any of the count fields are not
zero, then a FORMERR MUST be returned.

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| MESSAGE ID |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|QR | OPCODE (6) | Z | RCODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| QDCOUNT (MUST be zero) |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| ANCOUNT (MUST be zero) |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| NSCOUNT (MUST be zero) |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| ARCOUNT (MUST be zero) |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |
/ DSO Data /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

5.4.1. DNS Header Fields in DSO Messages

In a DSO unidirectional message, the MESSAGE ID field MUST be set to
zero. In a DSO request message, the MESSAGE ID field MUST be set to
a unique nonzero value that the initiator is not currently using for
any other active operation on this connection. For the purposes
here, a MESSAGE ID is in use in this DSO Session if the initiator has
used it in a DSO request message for which it is still awaiting a
response, or if the client has used it to set up a long-lived
operation that has not yet been canceled. For example, a long-lived
operation could be a Push Notification subscription [DNS-PUSH] [Push] or a
Discovery Relay interface subscription [MDNS-DISCOVERY-RELAY]. [Relay].

Whether a message is a DSO request message or a DSO unidirectional
message is determined only by the specification for the Primary TLV.
An acknowledgment cannot be requested by including a nonzero message MESSAGE
ID in a message that is required according to its primary Primary TLV to be
unidirectional. Nor can an acknowledgment be prevented by sending a
message
MESSAGE ID of zero in a message that is required to be a DSO request
message according to its primary Primary TLV. A responder that receives
either such malformed message MUST treat it as a fatal error and
forcibly abort the connection immediately.

In a DSO request message or DSO unidirectional message, the DNS
Header Query/Response (QR) bit MUST be zero (QR=0). If the QR bit is
not zero, the message is not a DSO request or DSO unidirectional
message.

In a DSO response message, the DNS Header QR bit MUST be one (QR=1).
If the QR bit is not one, the message is not a DSO response message.

In a DSO response message (QR=1), the MESSAGE ID field MUST NOT be
zero, and MUST contain a copy of the value of the (nonzero) MESSAGE
ID field in the DSO request message being responded to. Also, the MESSAGE ID field MUST NOT be zero. If a DSO
response message (QR=1) is received where the MESSAGE ID is zero,
this is a fatal error and the recipient MUST forcibly abort the
connection immediately.

The DNS Header OPCODE field holds the DSO OPCODE value. value (6).

The Z bits are currently unused in DSO messages; in both DSO request
messages and DSO responses, the Z bits MUST be set to zero (0) on
transmission and MUST be ignored on reception.

In a DSO request message (QR=0), the RCODE is set according to the
definition of the request. For example, in a Retry Delay message
(Section 6.6.1), the RCODE indicates the reason for termination.
However, in most DSO request messages (QR=0), except where clearly
specified otherwise, the RCODE is set to zero on transmission, and
silently ignored on reception.

The RCODE value in a response message (QR=1) may be one of the
following values:

Use of the above RCODEs is likely to be common in DSO but does not
preclude the definition and use of other codes in future documents
that make use of DSO.

If a document defining a new DSO-TYPE makes use of response codes not
defined here, then that document MUST specify the specific
interpretation of those RCODE values in the context of that new DSO
TLV.

The RCODE field is followed by the four zero-valued count fields,
followed by the DSO Data.

5.4.2. DSO Data

The standard twelve-byte DNS message header with its zero-valued
count fields is followed by the DSO Data, expressed using TLV syntax,
as described in Section 5.4.3. 5.4.4.

A DSO request message or DSO unidirectional message MUST contain at
least one TLV. The first TLV in a DSO request message or DSO
unidirectional message is referred to as the "Primary TLV" and
determines the nature of the operation being performed, including
whether it is a DSO request or a DSO unidirectional operation. In
some cases, it may be appropriate to include other TLVs in a DSO
request message or DSO unidirectional message, such as the DSO
Encryption Padding TLV (Section 7.3), and these extra 7.3). Additional TLVs are referred to as follow the "Additional TLVs" and
Primary TLV. Additional TLVs are not limited to what is defined in
this document. New "Additional TLVs" Additional TLVs may be defined in the future.
Their definitions will describe when their use is appropriate.

A DSO response message may contain no TLVs or may be specified to
contain one or more TLVs appropriate to the information being
communicated. This includes "Primary TLVs" and "Additional TLVs"
defined in this document as well as in future TLV definitions. It
may be permissible for an additional

An unrecognized Primary TLV to appear results in a response to a
primary TLV even though the specification of that primary TLV does
not specify it explicitly. See Section 8.2 for more information. DSOTYPENI error response.
Unrecognized Additional TLVs are silently ignored, as described in
Sections 5.4.5 and 8.2.

A DSO response message may contain no TLVs, or may contain one or
more TLVs, appropriate to the information being communicated.

Any TLVs with the Primary
TLV DSO-TYPE the same DSO-TYPE as the Primary TLV from the
corresponding DSO request message, or it message are Response Primary TLV(s) and
MUST appear first in a DSO response message. A DSO response message
may contain zero multiple Response Primary TLVs, or more Additional TLVs only.
The a single Response
Primary TLV, or in some cases, no Response Primary TLV. A Response
Primary TLV is not required; for most DSO operations the MESSAGE ID
field in the DNS message header is sufficient to identify the DSO
request message to which this a particular response message relates.

Any other TLVs in a DSO response message may contain one or more are Response Additional
TLVs, such as the DSO Encryption Padding TLV (Section 7.3). Response
Additional TLVs with DSO-TYPEs
different from follow the Response Primary TLV from TLV(s), if present.
Response Additional TLVs are not limited to what is defined in this
document. New Response Additional TLVs may be defined in the corresponding future.
Their definitions will describe when their use is appropriate.
Unrecognized Response Additional TLVs are silently ignored, as
described in Sections 5.4.5 and 8.2.

The specification for each DSO request
message, TLV determines what TLVs are required
in which case those a response to a DSO request message using that TLV. If a DSO
response is received for an operation where the specification
requires that the response carry a particular TLV or TLVs, and the
required TLV(s) are referred to as "Response
Additional TLVs".

Response Primary TLV(s), if not present, then this is a fatal error and the
recipient of the defective response message MUST occur first in forcibly abort the
connection immediately. Similarly, if more than the specified number
of instances of a given TLV are present, this is a fatal error and
the recipient of the defective response message before any Response Additional TLVs. MUST forcibly abort
the connection immediately.

5.4.3. DSO Unidirectional Messages

It is anticipated that most DSO operations will be specified to use
DSO request messages, which generate corresponding DSO responses. In
some specialized high-traffic use cases, it may be appropriate to
specify DSO unidirectional messages. DSO unidirectional messages can
be more efficient on the network because they don't generate a stream
of corresponding reply messages. Using DSO unidirectional messages
can also simplify software in some cases by removing the need for an
initiator to maintain state while it waits to receive replies it
doesn't care about. When the specification for a particular TLV used
as a Primary TLV (i.e., first) in an outgoing DSO request message
(i.e., QR=0) states that a message is to be unidirectional, the
MESSAGE ID field MUST be set to zero and the receiver MUST NOT
generate any response message corresponding to that DSO
unidirectional message.

The previous point, that the receiver MUST NOT generate responses to
DSO unidirectional messages, applies even in the case of errors.

When a DSO message is received where both the QR bit and the MESSAGE
ID field are zero, the receiver MUST NOT generate any response. For
example, if the DSO-TYPE in the Primary TLV is unrecognized, then a
DSOTYPENI error MUST NOT be returned; instead, the receiver MUST
forcibly abort the connection immediately.

DSO unidirectional messages MUST NOT be used "speculatively" in cases
where the sender doesn't know if the receiver supports the Primary
TLV in the message because there is no way to receive any response to
indicate success or failure. DSO unidirectional messages are only
appropriate in cases where the sender already knows that the receiver
supports and wishes to receive these messages.

For example, after a client has subscribed for Push Notifications
[DNS-PUSH],
[Push], the subsequent event notifications are then sent as DSO
unidirectional messages. This is appropriate because the client
initiated the message stream by virtue of its Push Notification
subscription, thereby indicating its support of Push Notifications
and its desire to receive those notifications.

Similarly, after a Discovery Relay client has subscribed to receive
inbound multicast DNS (mDNS) [RFC6762] traffic from a Discovery
Relay, the subsequent stream of received packets is then sent using
DSO unidirectional messages. This is appropriate because the client
initiated the message stream by virtue of its Discovery Relay link
subscription, thereby indicating its support of Discovery Relay and
its desire to receive inbound mDNS packets over that DSO session
[MDNS-DISCOVERY-RELAY].

5.4.3. Session
[Relay].

5.4.4. TLV Syntax

All TLVs, whether used as "Primary", "Additional", "Response
Primary", or "Response Additional", use the same encoding syntax.

Specifications

A specification that define defines a new TLVs TLV must specify whether the DSO-TYPE DSO-
TYPE can be used as the a Primary TLV, and whether the DSO-TYPE can be
used as an Additional TLV, or in either
context, both TLV. Some DSO-TYPEs are dual-purpose and can
be used as Primary TLVs in the case of requests some messages, and responses. in other messages as
Additional TLVs. The specification for a TLV DSO-TYPE must also state
whether, when used as the Primary (i.e., first) TLV in a DSO message
(i.e., QR=0), that DSO message is unidirectional unidirectional, or is a DSO request
message that requires a response.

If the a DSO request message requires a response, the specification must
also state which TLVs, if any, are to be included in the
response. response and
how many instances of each of the TLVs are allowed. The Primary TLV
may or may not be contained in the response depending on what is
specified for that TLV. If multiple instances of the Primary TLV are
allowed the specification should clearly describe how they should be
processed.

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| DSO-TYPE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| DSO-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| |
/ DSO-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

DSO-TYPE: a A 16-bit unsigned integer, in network (big endian) byte
order, giving the DSO-TYPE of the current DSO TLV per the IANA
"DSO Type Codes" registry registry.

DSO-LENGTH: a A 16-bit unsigned integer, in network (big endian) byte
order, giving the size in bytes of the DSO-DATA

DSO-DATA: Type-code specific format. The generic DSO machinery
treats the DSO-DATA as an opaque "blob" without attempting to
interpret it. Interpretation of the meaning of the DSO-DATA for a
particular DSO-TYPE is the responsibility of the software that
implements that DSO-TYPE.

5.4.3.1. Request TLVs

The first TLV in a DSO request message or DSO unidirectional message
is the "Primary TLV", which indicates the operation to be performed.
A DSO request message or DSO unidirectional message MUST contain at
least one TLV, the Primary TLV.

Immediately following the Primary TLV, a DSO request message or DSO
unidirectional message MAY contain one or more "Additional TLVs",
which specify additional parameters relating to the operation.

5.4.3.2. Response TLVs

Depending on the operation, a DSO response message MAY contain no
TLVs, because it is simply a response to a previous DSO request
message, and the MESSAGE ID in the header is sufficient to identify
the DSO request in question. Or, it may contain a single response
TLV with the same DSO-TYPE as the Primary TLV in the request message.
Alternatively, it may contain one or more TLVs of other types, or a
combination of the above, as appropriate for the information that
needs to be communicated. The specification for each DSO TLV
determines what TLVs are required in a response to a DSO request
message using that TLV.

If a integer, in network (big endian) byte
order, giving the size in bytes of the DSO-DATA.

DSO-DATA: Type-code specific format. The generic DSO response is received for machinery
treats the DSO-DATA as an operation where opaque "blob" without attempting to
interpret it. Interpretation of the
specification requires that meaning of the response carry DSO-DATA for a
particular TLV or
TLVs, and the required TLV(s) are not present, then this DSO-TYPE is a fatal
error and the recipient responsibility of the defective response message MUST
forcibly abort the connection immediately.

5.4.3.3. software that
implements that DSO-TYPE.

5.4.5. Unrecognized TLVs

If a DSO request message is received containing an unrecognized
Primary TLV, with a nonzero MESSAGE ID (indicating that a response is
expected), then the receiver MUST send an error response with a
matching MESSAGE ID, and RCODE DSOTYPENI. The error response MUST
NOT contain a copy of the unrecognized Primary TLV.

If a DSO unidirectional message is received containing both an
unrecognized Primary TLV and a zero MESSAGE ID (indicating that no
response is expected), then this is a fatal error and the recipient
MUST forcibly abort the connection immediately.

If a DSO request message or DSO unidirectional message is received
where the Primary TLV is recognized, containing one or more
unrecognized Additional TLVs, the unrecognized Additional TLVs MUST
be silently ignored, and the remainder of the message is interpreted
and handled as if the unrecognized parts were not present.

Similarly, if a DSO response message is received containing one or
more unrecognized TLVs, the unrecognized TLVs MUST be silently
ignored and the remainder of the message is interpreted and handled
as if the unrecognized parts are not present.

5.4.4.

5.4.6. EDNS(0) and TSIG

Since the ARCOUNT field MUST be zero, a DSO message cannot contain a
valid EDNS(0) option in the additional records section. If
functionality provided by current or future EDNS(0) options is
desired for DSO messages, one or more new DSO TLVs need to be defined
to carry the necessary information.

For example, the EDNS(0) Padding Option [RFC7830] used for security
purposes is not permitted in a DSO message, so if message padding is
desired for DSO messages, then the DSO Encryption Padding TLV
described in Section 7.3 MUST be used.

A DSO message can't contain a TSIG record because a TSIG record is
included in the additional section of the message, which would mean
that ARCOUNT would be greater than zero. DSO messages are required
to have an ARCOUNT of zero. Therefore, if use of signatures with DSO
messages becomes necessary in the future, a new DSO TLV would have to
be defined to perform this function.

Note, however, that while DSO *messages* cannot include EDNS(0) or
TSIG records, a DSO *session* is typically used to carry a whole
series of DNS messages of different kinds, including DSO messages and
other DNS message types like Query [RFC1034] [RFC1035] and
Update[RFC2136]. Update
[RFC2136]. These messages can carry EDNS(0) and TSIG records.

Although messages may contain other EDNS(0) options as appropriate,
this specification explicitly prohibits use of the edns-tcp-keepalive
EDNS0
EDNS(0) Option [RFC7828] in *any* messages sent on a DSO Session
(because it is obsoleted by the functionality provided by the DSO
Keepalive operation). If any message sent on a DSO Session contains
an edns-tcp-keepalive EDNS0 option, EDNS(0) Option, this is a fatal error and the
recipient of the defective message MUST forcibly abort the connection
immediately.

5.5. Message Handling

As described in Section 5.4.1, whether an outgoing DSO message with
the QR bit in the DNS header set to zero is a DSO request or a DSO
unidirectional message is determined by the specification for the
Primary TLV, which in turn determines whether the MESSAGE ID field in
that outgoing message will be zero or nonzero.

Every DSO message with the QR bit in the DNS header set to zero and a
nonzero MESSAGE ID field is a DSO request message, and MUST elicit a
corresponding response, with the QR bit in the DNS header set to one
and the MESSAGE ID field set to the value given in the corresponding
DSO request message.

Valid DSO request messages sent by the client with a nonzero MESSAGE
ID field elicit a response from the server, and valid DSO request
messages sent by the server with a nonzero MESSAGE ID field elicit a
response from the client.

Every DSO message with both the QR bit in the DNS header and the
MESSAGE ID field set to zero is a DSO unidirectional message and MUST
NOT elicit a response.

5.5.1. Delayed Acknowledgement Management

Generally, most good TCP implementations employ a delayed
acknowledgement timer to provide more efficient use of the network
and better performance.

With a bidirectional exchange over TCP, such as with a DSO request
message, the operating system TCP implementation waits for the
application-layer client software to generate the corresponding DSO
response message. It The TCP implementation can then send a single
combined packet containing the TCP acknowledgement, the TCP window
update, and the application-generated DSO response message. This is
more efficient than sending three separate packets, as would occur if
the TCP packet containing the DSO request were acknowledged
immediately.

With a DSO unidirectional message or DSO response message, there is
no corresponding application-generated DSO response message, and
consequently, no hint to the transport protocol about when it should
send its acknowledgement and window update.

Some networking APIs provide a mechanism that allows the application-
layer client software to signal to the transport protocol that no
response will be forthcoming (in effect it can be thought of as a
zero-length "empty" write). Where available in the networking API
being used, the recipient of a DSO unidirectional message or DSO
response message, having parsed and interpreted the message, SHOULD
then use this mechanism provided by the networking API to signal that
no response for this message will be forthcoming. The TCP
implementation can then go ahead and send its acknowledgement and
window update without further delay. See Section 9.5 for further
discussion of why this is important.

5.5.2. MESSAGE ID Namespaces

The namespaces of 16-bit MESSAGE IDs are independent in each
direction. This means it is *not* an error for both client and
server to send DSO request messages at the same time as each other,
using the same MESSAGE ID, in different directions. This
simplification is necessary in order for the protocol to be
implementable. It would be infeasible to require the client and
server to coordinate with each other regarding allocation of new
unique MESSAGE IDs. It is also not necessary to require the client
and server to coordinate with each other regarding allocation of new
unique MESSAGE IDs. The value of the 16-bit MESSAGE ID combined with
the identity of the initiator (client or server) is sufficient to
unambiguously identify the operation in question. This can be
thought of as a 17-bit message identifier space using message
identifiers 0x00001-0x0FFFF for client-to-server DSO request
messages, and 0x10001-0x1FFFF for server-to-client DSO request
messages. The least-significant 16 bits are stored explicitly in the
MESSAGE ID field of the DSO message, and the most-significant bit is
implicit from the direction of the message.

As described in Section 5.4.1, an initiator MUST NOT reuse a MESSAGE
ID that it already has in use for an outstanding DSO request message
(unless specified otherwise by the relevant specification for the
DSO-TYPE in question). At the very least, this means that a MESSAGE
ID can't be reused in a particular direction on a particular DSO
Session while the initiator is waiting for a response to a previous
DSO request message using that MESSAGE ID on that DSO Session (unless
specified otherwise by the relevant specification for the DSO-TYPE in
question), and for a long-lived operation, the MESSAGE ID for the
operation can't be reused while that operation remains active.

If a client or server receives a response (QR=1) where the MESSAGE ID
is zero, or is any other value that does not match the MESSAGE ID of
any of its outstanding operations, this is a fatal error and the
recipient MUST forcibly abort the connection immediately.

If a responder receives a DSO request message (QR=0) where the
MESSAGE ID is not zero, the responder tracks request MESSAGE IDs, and
the MESSAGE ID matches the MESSAGE ID of a DSO request message it
received for which a response has not yet been sent, it MUST forcibly
abort the connection immediately. This behavior is required to
prevent a hypothetical attack that takes advantage of undefined
behavior in this case. However, if the responder does not track
MESSAGE IDs in this way, no such risk exists. Therefore, tracking
MESSAGE IDs just to implement this sanity check is not required.

5.5.3. Error Responses

When a DSO unidirectional message type is received (MESSAGE ID field
is zero), the receiver should already be expecting this DSO message
type. Section 5.4.3.3 describes the handling of unknown DSO message
types. Parsing errors MUST also result in the receiver forcibly
aborting the connection. When a DSO unidirectional message of an
unexpected type is received, the receiver SHOULD forcibly abort the
connection. Whether the connection should be forcibly aborted for
other internal errors processing the DSO unidirectional message is
implementation dependent according to the severity of the error.

When a DSO request message is unsuccessful for some reason, the
responder returns an error code to the initiator.

In the case of a server returning an error code to a client in
response to an unsuccessful DSO request message, the server MAY
choose to end the DSO Session or MAY choose to allow the DSO Session
to remain open. For error conditions that only affect the single
operation in question, the server SHOULD return an error response to
the client and leave the DSO Session open for further operations.

For error conditions that are likely to make all operations
unsuccessful in the immediate future, the server SHOULD return an
error response to the client and then end the DSO Session by sending
a Retry Delay message as described in Section 6.6.1.

Upon receiving an error response from the server, a client SHOULD NOT
automatically close the DSO Session. An error relating to one
particular operation on a DSO Session does not necessarily imply that
all other operations on that DSO Session have also failed or that
future operations will fail. The client should assume that the
server will make its own decision about whether or not to end the DSO
Session based on the server's determination of whether the error
condition pertains to this particular operation or to any subsequent
operations. If the server does not end the DSO Session by sending DSO Session by sending
the client a Retry Delay message (Section 6.6.1), then the client
SHOULD continue to use that DSO Session for subsequent operations.

When a DSO unidirectional message type is received (MESSAGE ID field
is zero), the receiver should already be expecting this DSO message
type. Section 5.4.5 describes the handling of unknown DSO message
types. When a DSO unidirectional message of an unexpected type is
received, the receiver SHOULD forcibly abort the connection. Whether
the connection should be forcibly aborted for other internal errors
processing the client a Retry Delay DSO unidirectional message (Section 6.6.1), then the client
SHOULD continue is implementation dependent
according to use that DSO Session for subsequent operations. the severity of the error.

5.6. Responder-Initiated Operation Cancellation

This document, the base specification for DNS Stateful Operations,
does not itself define any long-lived operations, but it defines a
framework for supporting long-lived operations such as Push
Notification subscriptions [DNS-PUSH] [Push] and Discovery Relay interface
subscriptions [MDNS-DISCOVERY-RELAY]. [Relay].

Long-lived operations, if successful, will remain active until the
initiator terminates the operation.

However, it is possible that a long-lived operation may be valid at
the time it was initiated, but then a later change of circumstances
may render that operation invalid. For example, a long-lived client
operation may pertain to a name that the server is authoritative for,
but then the server configuration is changed such that it is no
longer authoritative for that name.

In such cases, instead of terminating the entire session, it may be
desirable for the responder to be able to cancel selectively only
those operations that have become invalid.

The responder performs this selective cancellation by sending a new
DSO response message with the MESSAGE ID field containing the MESSAGE
ID of the long-lived operation that is to be terminated (that it had
previously acknowledged with a NOERROR RCODE) and the RCODE field of
the new DSO response message giving the reason for cancellation.

After a DSO response message with nonzero RCODE has been sent, that
operation has been terminated from the responder's point of view, and
the responder sends no more messages relating to that operation.

After a DSO response message with nonzero RCODE has been received by
the initiator, that operation has been terminated from the
initiator's point of view, and the canceled operation's MESSAGE ID is
now free for reuse.

6. DSO Session Lifecycle and Timers

6.1. DSO Session Initiation

A DSO Session begins as described in Section 5.1.

The

Once a DSO Session has been created, client or server may perform initiate as
many DNS operations as it wishes they wish using the
newly created DSO Session.

When the client an initiator has multiple messages to send, it SHOULD NOT wait
for each response before sending the next message.

The server

A responder MUST act on messages in the order they are received, but and
SHOULD NOT delay sending return responses to those request messages as they become
available in order to return them available.
A responder SHOULD NOT delay sending responses for the purpose of
delivering responses in the same order that the corresponding
requests were received.

Section 6.2.1.1 of the DNS-over-TCP specification [RFC7766] specifies
this in more detail.

6.2. DSO Session Timeouts

Two timeout values are associated with a DSO Session: the inactivity
timeout and the keepalive interval. Both values are communicated in
the same TLV, the Keepalive TLV (Section 7.1).

The first timeout value, the inactivity timeout, is the maximum time
for which a client may speculatively keep an inactive DSO Session
open in the expectation that it may have future requests to send to
that server.

The second timeout value, the keepalive interval, is the maximum
permitted interval between messages if the client wishes to keep the
DSO Session alive.

The two timeout values are independent. The inactivity timeout may
be lower, shorter, the same, or higher longer than the keepalive interval, though
in most cases the inactivity timeout is expected to be shorter than
the keepalive interval.

A shorter inactivity timeout with a longer keepalive interval signals
to the client that it should not speculatively keep an inactive DSO
Session open for very long without reason, but when it does have an
active reason to keep a DSO Session open, it doesn't need to be
sending an aggressive level of DSO keepalive traffic to maintain that
session. An example of this would be a client that has subscribed to
DNS Push notifications. In this case, the client is not sending any
traffic to the server, but the session is not inactive because there
is an active request to the server to receive push notifications.

A longer inactivity timeout with a shorter keepalive interval signals
to the client that it may speculatively keep an inactive DSO Session
open for a long time, but to maintain that inactive DSO Session it
should be sending a lot of DSO keepalive traffic. This configuration
is expected to be less common.

In the usual case where the inactivity timeout is shorter than the
keepalive interval, it is only when a client has a long-lived, low-
traffic operation that the keepalive interval comes into play in
order to ensure that a sufficient residual amount of traffic is
generated to maintain NAT and firewall state, and to assure the
client and server that they still have connectivity to each other.

On a new DSO Session, if no explicit DSO Keepalive message exchange
has taken place, the default value for both timeouts is 15 seconds.

For both timeouts, lower values of the timeout result in higher
network traffic and a higher CPU load on the server.

6.3. Inactive DSO Sessions

At both servers and clients, the generation or reception of any
complete DNS message (including DNS requests, responses, updates, DSO
messages, etc.) resets both timers for that DSO Session, with the one
exception being that a DSO Keepalive message resets only the
keepalive timer, not the inactivity timeout timer.

In addition, for as long as the client has an outstanding operation
in progress, the inactivity timer remains cleared and an inactivity
timeout cannot occur.

For short-lived DNS operations like traditional queries and updates,
an operation is considered "in progress" for the time between request
and response, typically a period of a few hundred milliseconds at
most. At the client, the inactivity timer is cleared upon
transmission of a request and remains cleared until reception of the
corresponding response. At the server, the inactivity timer is
cleared upon reception of a request and remains cleared until
transmission of the corresponding response.

For long-lived DNS Stateful operations Operations (such as a Push Notification
subscription [DNS-PUSH] [Push] or a Discovery Relay interface subscription
[MDNS-DISCOVERY-RELAY]),
[Relay]), an operation is considered "in progress" for as long as the
operation is active, i.e., until it is canceled. This means that a
DSO Session can exist with active operations, with no messages
flowing in either direction, for far longer than the inactivity
timeout. This is not an error. This is why there are two separate
timers: the inactivity timeout and the keepalive interval. Just
because a DSO Session has no traffic for an extended period of time,
it does not automatically make that DSO Session "inactive", if it has
an active operation that is awaiting events.

6.4. The Inactivity Timeout

The purpose of the inactivity timeout is for the server to balance
the trade-off between the costs of setting up new DSO Sessions and
the costs of maintaining inactive DSO Sessions. A server with
abundant DSO Session capacity can offer a high inactivity timeout to
permit clients to keep a speculative DSO Session open for a long time
and to save the cost of establishing a new DSO Session for future
communications with that server. A server with scarce memory
resources can offer a low inactivity timeout to cause clients to
promptly close DSO Sessions whenever they have no outstanding
operations with that server and then create a new DSO Session later
when needed.

6.4.1. Closing Inactive DSO Sessions

When a connection's inactivity timeout is reached, the client MUST
begin closing the idle connection, but a client is not required to
keep an idle connection open until the inactivity timeout is reached.
A client MAY close a DSO Session at any time, at the client's
discretion. If a client determines that it has no current or
reasonably anticipated future need for a currently inactive DSO
Session, then the client SHOULD gracefully close that connection.

If, at any time during the life of the DSO Session, the inactivity
timeout value (i.e., 15 seconds by default) elapses without there
being any operation active on the DSO Session, the client MUST close
the connection gracefully.

If, at any time during the life of the DSO Session, twice the
inactivity timeout value (i.e., 30 seconds by default), or five
seconds if twice the inactivity timeout value is less than five
seconds, too much time
elapses without there being any operation active on the DSO Session,
then the server MUST consider the client delinquent and MUST forcibly
abort the DSO Session. What is considered "too much time" in this
context is five seconds or twice the current inactivity timeout
value, whichever is greater. If the inactivity timeout has its
default value of 15 seconds, this means that a client will be
considered delinquent and disconnected if it has not closed its
connection after 30 seconds of inactivity.

In this context, an operation being active on a DSO Session includes
a query waiting for a response, an update waiting for a response, or
an active long-lived operation, but not a DSO Keepalive message
exchange itself. A DSO Keepalive message exchange resets only the
keepalive interval timer, not the inactivity timeout timer.

If the client wishes to keep an inactive DSO Session open for longer
than the default duration, then it uses the DSO Keepalive message to
request longer timeout values as described in Section 7.1.

6.4.2. Values for the Inactivity Timeout

For the inactivity timeout value, lower values result in more
frequent DSO Session teardowns and re-establishments. Higher values
result in lower traffic and a lower CPU load on the server, but a
higher memory burden to maintain state for inactive DSO Sessions.

A server may dictate any value it chooses for the inactivity timeout
(either in a response to a client-initiated request or in a server-
initiated message) including values under one second, or even zero.

An inactivity timeout of zero informs the client that it should not
speculatively maintain idle connections at all, and as soon as the
client has completed the operation or operations relating to this
server, the client should immediately begin closing this session.

A server will forcibly abort an idle client session after either five
seconds or twice the inactivity timeout value or five seconds, value, whichever is greater.
In the case of a zero inactivity timeout value, this means that if a
client fails to close an idle client session, then the server will
forcibly abort the idle session after five seconds.

An inactivity timeout of 0xFFFFFFFF represents "infinity" and informs
the client that it may keep an idle connection open as long as it
wishes. Note that after granting an unlimited inactivity timeout in
this way, at any point the server may revise that inactivity timeout
by sending a new DSO Keepalive message dictating new Session Timeout
values to the client.

The largest *finite* inactivity timeout supported by the current
Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
days).

6.5. The Keepalive Interval

The purpose of the keepalive interval is to manage the generation of
sufficient messages to maintain state in middleboxes (such at NAT
gateways or firewalls) and for the client and server to periodically
verify that they still have connectivity to each other. This allows
them to clean up state when connectivity is lost and to establish a
new session if appropriate.

6.5.1. Keepalive Interval Expiry

If, at any time during the life of the DSO Session, the keepalive
interval value (i.e., 15 seconds by default) elapses without any DNS
messages being sent or received on a DSO Session, the client MUST
take action to keep the DSO Session alive by sending a DSO Keepalive
message (Section 7.1). A DSO Keepalive message exchange resets only
the keepalive timer, not the inactivity timer.

If a client disconnects from the network abruptly, without cleanly
closing its DSO Session, perhaps leaving a long-lived operation
uncanceled, the server learns of this after failing to receive the
required DSO keepalive traffic from that client. If, at any time
during the life of the DSO Session, twice the keepalive interval
value (i.e., 30 seconds by default) elapses without any DNS messages
being sent or received on a DSO Session, the server SHOULD consider
the client delinquent and SHOULD forcibly abort the DSO Session.

6.5.2. Values for the Keepalive Interval

For the keepalive interval value, lower values result in a higher
volume of DSO keepalive traffic. Higher values of the keepalive
interval reduce traffic and the CPU load, but have minimal effect on
the memory burden at the server because clients keep a DSO Session
open for the same length of time (determined by the inactivity
timeout) regardless of the level of DSO keepalive traffic required.

It may be appropriate for clients and servers to select different
keepalive interval values intervals depending on the nature type of the network they are on.

A corporate DNS server that knows it is serving only clients on the
internal network, with no intervening NAT gateways or firewalls, can
impose a higher longer keepalive interval because frequent DSO keepalive
traffic is not required.

A public DNS server that is serving primarily residential consumer
clients, where it is likely there will be a NAT gateway on the path,
may impose a lower shorter keepalive interval to generate more frequent DSO
keepalive traffic.

A smart client may be adaptive to its environment. A client using a
private IPv4 address [RFC1918] to communicate with a DNS server at an
address outside that IPv4 private address block may conclude that
there is likely to be a NAT gateway on the path, and accordingly
request a lower shorter keepalive interval.

By default, it is RECOMMENDED that clients request, and servers
grant, a keepalive interval of 60 minutes. This keepalive interval
provides for reasonably timely detection if a client abruptly
disconnects without cleanly closing the session. Also, it is
sufficient to maintain state in firewalls and NAT gateways that
follow the IETF recommended Best Current Practice that the
"established connection idle-timeout" used by middleboxes be at least
2 hours and 4 minutes [RFC5382] [RFC7857].

Note that the lower shorter the keepalive interval value, the higher the
load on client and server. Moreover, for a keep-alive keepalive value that is
smaller
shorter than the time needed for the transport to retransmit, the
loss of a single packet loss would cause a server to overzealously abort
the connection. For example, a (hypothetical and unrealistic)
keepalive interval value of 100 ms would result in a continuous
stream of ten messages per second or more (if allowed by the current
congestion control window) in both directions to keep the DSO Session
alive. And, in this extreme example, a single retransmission over a
path with, as an example, 100 ms RTT would introduce a momentary
pause in the stream of messages long enough to cause the server to
abort the connection.

Because of this concern, the server MUST NOT send a DSO Keepalive
message (either a DSO response to a client-initiated DSO request or a server-
initiated
server-initiated DSO message) with a keepalive interval value less
than ten seconds. If a client receives a DSO Keepalive message
specifying a keepalive interval value less than ten seconds, this is
a fatal error and the client MUST forcibly abort the connection
immediately.

A keepalive interval value of 0xFFFFFFFF represents "infinity" and
informs the client that it should generate no DSO keepalive traffic.
Note that after signaling that the client should generate no DSO
keepalive traffic in this way, the server may at any point revise
that DSO keepalive traffic requirement by sending a new DSO Keepalive
message dictating new Session Timeout values to the client.

The largest *finite* keepalive interval supported by the current
Keepalive TLV is 0xFFFFFFFE (2^32-2 milliseconds, approximately 49.7
days).

6.6. Server-Initiated DSO Session Termination

In addition to canceling individual long-lived operations selectively
(Section 5.6), there are also occasions where a server may need to
terminate one or more entire DSO sessions. An entire DSO session may
need to be terminated if the client is defective in some way or
departs from the network without closing its DSO session. DSO
Sessions may also need to be terminated if the server becomes
overloaded or is reconfigured and lacks the ability to be selective
about which operations need to be canceled.

This section discusses various reasons a DSO session may be
terminated and the mechanisms for doing so.

In normal operation, closing a DSO Session is the client's
responsibility. The client makes the determination of when to close
a DSO Session based on an evaluation of both its own needs and the
inactivity timeout value dictated by the server. A server only
causes a DSO Session to be ended in the exceptional circumstances
outlined below. Some of the exceptional situations in which a server
may terminate a DSO Session include:

o The server application software or underlying operating system is
shutting down or restarting.

o The server application software terminates unexpectedly (perhaps
due to a bug that makes it crash, causing the underlying operating
system to send a TCP RST).

o The server is undergoing a reconfiguration or maintenance
procedure that, due to the way the server software is implemented,
requires clients to be disconnected. For example, some software
is implemented such that it reads a configuration file at startup,
and changing the server's configuration entails modifying the
configuration file and then killing and restarting the server
software, which generally entails a loss of network connections.

o The client fails to meet its obligation to generate the required
DSO keepalive traffic or to close an inactive session by the
prescribed time (either (five seconds or twice the time interval dictated
by the
server or five seconds, server, whichever is greater, as described in Section 6.2).

o The client sends a grossly invalid or malformed request that is
indicative of a seriously defective client implementation.

o The server is over capacity and needs to shed some load.

6.6.1. Server-Initiated Retry Delay Message

In the cases described above where a server elects to terminate a DSO
Session, it could do so simply by forcibly aborting the connection.
However, if it did this, the likely behavior of the client might be
simply to treat this as a network failure and reconnect immediately,
putting more burden on the server.

Therefore, to avoid this reconnection implosion, a server SHOULD
instead choose to shed client load by sending a Retry Delay message
with an appropriate RCODE value informing the client of the reason
the DSO Session needs to be terminated. The format of the DSO Retry
Delay TLV and the interpretations of the various RCODE values are
described in Section 7.2. After sending a DSO Retry Delay message,
the server MUST NOT send any further messages on that DSO Session.

The server MAY randomize retry delays in situations where many retry
delays are sent in quick succession so as to avoid all the clients
attempting to reconnect at once. In general, implementations should
avoid using the DSO Retry Delay message in a way that would result in
many clients reconnecting at the same time if every client attempts
to reconnect at the exact time specified.

Upon receipt of a DSO Retry Delay message from the server, the client
MUST make note of the reconnect delay for this server and then
immediately close the connection gracefully.

After sending a DSO Retry Delay message, the server SHOULD allow the
client five seconds to close the connection, and if the client has
not closed the connection after five seconds, then the server SHOULD
forcibly abort the connection.

A DSO Retry Delay message MUST NOT be initiated by a client. If a
server receives a DSO Retry Delay message, this is a fatal error and
the server MUST forcibly abort the connection immediately.

6.6.1.1. Outstanding Operations

At the instant a server chooses to initiate a DSO Retry Delay
message, there may be DNS requests already in flight from client to
server on this DSO Session, which will arrive at the server after its
DSO Retry Delay message has been sent. The server MUST silently
ignore such incoming requests and MUST NOT generate any response
messages for them. When the DSO Retry Delay message from the server
arrives at the client, the client will determine that any DNS
requests it previously sent on this DSO Session that have not yet
received a response will now certainly not be receiving any response.

Such requests should be considered failed and should be retried at a
later time, as appropriate.

In the case where some, but not all, of the existing operations on a
DSO Session have become invalid (perhaps because the server has been
reconfigured and is no longer authoritative for some of the names),
but the server is terminating all affected DSO Sessions en masse by
sending them all a DSO Retry Delay message, the reconnect delay MAY
be zero, indicating that the clients SHOULD immediately attempt to re-
establish
re-establish operations.

It is likely that some of the attempts will be successful and some
will not, depending on the nature of the reconfiguration.

In the case where a server is terminating a large number of DSO
Sessions at once (e.g., if the system is restarting) and the server
doesn't want to be inundated with a flood of simultaneous retries, it
SHOULD send different reconnect delay values to each client. These
adjustments MAY be selected randomly, pseudorandomly, or
deterministically (e.g., incrementing the time value by one tenth of
a second for each successive client, yielding a post-restart
reconnection rate of ten clients per second).

6.6.2. Misbehaving Clients

A server may determine that a client is not following the protocol
correctly. There may be no way for the server to recover the DSO
session, in which case the server forcibly terminates the connection.
Since the client doesn't know why the connection dropped, it may
reconnect immediately. If the server has determined that a client is
not following the protocol correctly, it may MAY terminate the DSO
session
Session as soon as it is established, specifying a long retry-delay
to prevent the client from immediately reconnecting.

6.6.3. Client Reconnection

After a DSO Session is ended by the server (either by sending the
client a DSO Retry Delay message or by forcibly aborting the
underlying transport connection), the client SHOULD try to reconnect
to that service instance or to another suitable service instance if
more than one is available. If reconnecting to the same service
instance, the client MUST respect the indicated delay, if available,
before attempting to reconnect. Clients should not SHOULD NOT attempt to
randomize the delay; the server will randomly jitter the retry delay
values it sends to each client if this behavior is desired.

If the a particular service instance will only be out of service for a
short maintenance period, it should use indicate a retry delay value that
is a little longer than the expected maintenance window. It should
not default to a very large delay value, or clients may not attempt
to reconnect promptly after it resumes service.

If a particular service instance does not want a client to reconnect ever
(perhaps the service instance is being decommissioned), it SHOULD set
the retry delay to the maximum value 0xFFFFFFFF (2^32-1 milliseconds,
approximately 49.7 days). It is not possible to instruct a client to
stay away for longer than 49.7 days. If, after 49.7 days, the DNS or
other configuration information still indicates that this is the
valid service instance for a particular service, then clients MAY
attempt to reconnect. In reality, if a client is rebooted or
otherwise loses state, it may well attempt to reconnect before 49.7
days elapse, e.g., for as long as the DNS or other configuration
information continues to indicate that this is the service instance
the client should use.

6.6.3.1. Reconnecting After after a Forcible Abort

If a connection was forcibly aborted by the client, client due to
noncompliant behavior by the server, the client SHOULD mark that
service instance as not supporting DSO. The client MAY reconnect but
not attempt to use DSO, or it may connect to a different service
instance if applicable.

6.6.3.2. Reconnecting After after an Unexplained Connection Drop

It is also possible for a server to forcibly terminate the
connection; in this case, the client doesn't know whether the
termination was the result of a protocol error or a network outage.
When the client notices that the connection has been dropped, it can
attempt to reconnect immediately. However, if the connection is
dropped again without the client being able to successfully do
whatever it is trying to do, it should mark the server as not
supporting DSO.

6.6.3.3. Probing for Working DSO Support

Once a server has been marked by the client as not supporting DSO,
the client SHOULD NOT attempt DSO operations on that server until
some time has elapsed. A reasonable minimum would be an hour. Since
forcibly aborted connections are the result of a software failure,
it's not likely that the problem will be solved in the first hour
after it's first encountered. However, by restricting the retry
interval to an hour, the client will be able to notice when the
problem has been fixed without placing an undue burden on the server.

7. Base TLVs for DNS Stateful Operations

This section describes the three base TLVs for DNS Stateful
Operations: Keepalive, Retry Delay, and Encryption Padding.

7.1. Keepalive TLV

The Keepalive TLV (DSO-TYPE=1) performs two functions. Primarily, it
establishes the values for the Session Timeouts. Incidentally, it
also resets the keepalive timer for the DSO Session, meaning that it
can be used as a kind of "no-op" message for the purpose of keeping a
session alive. The client will request the desired session timeout Session Timeout
values and the server will acknowledge with the response values that
it requires the client to use.

DSO messages with the Keepalive TLV as the primary Primary TLV may appear in
early data.

The DSO-DATA for the Keepalive TLV is as follows:

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| INACTIVITY TIMEOUT (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| KEEPALIVE INTERVAL (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

INACTIVITY TIMEOUT: The inactivity timeout for the current DSO
Session, specified as a 32-bit unsigned integer, in network (big
endian) byte order in units of milliseconds. This is the timeout
at which the client MUST begin closing an inactive DSO Session.
The inactivity timeout can be any value of the server's choosing.
If the client does not gracefully close an inactive DSO Session,
then after either five seconds or twice this interval or five seconds, interval, whichever is
greater, the server will forcibly abort the connection.

KEEPALIVE INTERVAL: The keepalive interval for the current DSO
Session, specified as a 32-bit unsigned integer, in network (big
endian) byte order in units of milliseconds. This is the interval
at which a client MUST generate DSO keepalive traffic to maintain
connection state. The keepalive interval MUST NOT be less than
ten seconds. If the client does not generate the mandated DSO
keepalive traffic, then after twice this interval the server will
forcibly abort the connection. Since the minimum allowed
keepalive interval is ten seconds, the minimum time at which a
server will forcibly disconnect a client for failing to generate
the mandated DSO keepalive traffic is twenty seconds.

The transmission or reception of DSO Keepalive messages (i.e.,
messages where the Keepalive TLV is the first TLV) reset only the
keepalive timer, not the inactivity timer. The reason for this is
that periodic DSO Keepalive messages are sent for the sole purpose of
keeping a DSO Session alive when that DSO Session has current or
recent non-maintenance activity that warrants keeping that DSO
Session alive. Sending DSO keepalive traffic itself is not
considered a client activity; it is considered a maintenance activity
that is performed in service of other client activities. If DSO
keepalive traffic itself were to reset the inactivity timer, then
that would create a circular livelock where keepalive traffic would
be sent indefinitely to keep a DSO Session alive. In this scenario,
the only activity on that DSO Session would be the keepalive traffic
keeping the DSO Session alive so that further keepalive traffic can
be sent. For a DSO Session to be considered active, it must be
carrying something more than just keepalive traffic. This is why
merely sending or receiving a DSO Keepalive message does not reset
the inactivity timer.

When sent by a client, the DSO Keepalive request message MUST be sent
as a DSO request message with a nonzero MESSAGE ID. If a server
receives a DSO Keepalive message with a zero MESSAGE ID, then this is
a fatal error and the server MUST forcibly abort the connection
immediately. The DSO Keepalive request message resets a DSO
Session's keepalive timer and, at the same time, communicates to the
server the client's requested Session Timeout values. In a server
response to a client-initiated DSO Keepalive request message, the
Session Timeouts contain the server's chosen values from this point
forward in the DSO Session, which the client MUST respect. This is
modeled after the DHCP protocol, where the client requests a certain
lease lifetime using DHCP option 51 [RFC2132], but the server is the
ultimate authority for deciding what lease lifetime is actually
granted.

When a client is sending its second and subsequent DSO Keepalive
request messages to the server, the client SHOULD continue to request
its preferred values each time. This allows flexibility so that if
conditions change during the lifetime of a DSO Session, the server
can adapt its responses to better fit the client's needs.

Once a DSO Session is in progress (Section 5.1), a DSO Keepalive
message MAY be initiated by a server. When sent by a server, the DSO
Keepalive message MUST be sent as a DSO unidirectional message with
the MESSAGE ID set to zero. The client MUST NOT generate a response
to a server-initiated DSO Keepalive message. If a client receives a
DSO Keepalive request message with a nonzero MESSAGE ID, then this is
a fatal error and the client MUST forcibly abort the connection
immediately. The DSO Keepalive unidirectional message from the
server resets a DSO Session's keepalive timer and, at the same time,
unilaterally informs the client of the new Session Timeout values to
use from this point forward in this DSO Session. No client DSO
response to this unilateral declaration is required or allowed.

In DSO Keepalive response messages, exactly one instance of the
Keepalive TLV is REQUIRED MUST be present and is used only as a Response Primary
TLV sent as a reply to a DSO Keepalive request message from the
client. A Keepalive TLV MUST NOT be added to other responses as a
Response Additional TLV. If the server wishes to update a client's
Session Timeout values other than in response to a DSO Keepalive
request message from the client, then it does so by sending a DSO
Keepalive unidirectional message of its own, as described above.

It is not required that the Keepalive TLV be used in every DSO
Session. While many DNS Stateful DSO operations will be used in conjunction with
a long-lived session state, not all DNS Stateful DSO operations require a long-lived long-
lived session state, and in some cases the default 15-second value
for both the inactivity timeout and keepalive interval may be
perfectly appropriate. However, note that for clients that implement
only the DSO-TYPEs defined in this document, a DSO Keepalive request
message is the only way for a client to initiate a DSO Session.

7.1.1. Client Handling of Received Session Timeout Values

When a client receives a response to its client-initiated DSO
Keepalive request message, or receives a server-initiated DSO
Keepalive unidirectional message, the client has then received
Session Timeout values dictated by the server. The two timeout
values contained in the Keepalive TLV from the server may each be
higher, lower, or the same as the respective Session Timeout values
the client previously had for this DSO Session.

In the case of the keepalive timer, the handling of the received
value is straightforward. The act of receiving the message
containing the DSO Keepalive TLV itself resets the keepalive timer
and updates the keepalive interval for the DSO Session. The new
keepalive interval indicates the maximum time that may elapse before
another message must be sent or received on this DSO Session, if the
DSO Session is to remain alive.

In the case of the inactivity timeout, the handling of the received
value is a little more subtle, though the meaning of the inactivity
timeout remains as specified; it still indicates the maximum
permissible time allowed without useful activity on a DSO Session.
The act of receiving the message containing the Keepalive TLV does
not itself reset the inactivity timer. The time elapsed since the
last useful activity on this DSO Session is unaffected by exchange of
DSO Keepalive messages. The new inactivity timeout value in the
Keepalive TLV in the received message does update the timeout
associated with the running inactivity timer; that becomes the new
maximum permissible time without activity on a DSO Session.

o If the current inactivity timer value is less than the new
inactivity timeout, then the DSO Session may remain open for now.
When the inactivity timer value reaches the new inactivity
timeout, the client MUST then begin closing the DSO Session as
described above.

o If the current inactivity timer value is equal to the new
inactivity timeout, then this DSO Session has been inactive for
exactly as long as the server will permit, and now the client MUST
immediately begin closing this DSO Session.

o If the current inactivity timer value is already greater than the
new inactivity timeout, then this DSO Session has already been
inactive for longer than the server permits, and the client MUST
immediately begin closing this DSO Session.

o If the current inactivity timer value is already more than twice
the new inactivity timeout, then the client is immediately
considered delinquent (this DSO Session is immediately eligible to
be forcibly terminated by the server) and the client MUST
immediately begin closing this DSO Session. However, if a server
abruptly reduces the inactivity timeout in this way, then, to give
the client time to close the connection gracefully before the
server resorts to forcibly aborting it, the server SHOULD give the
client an additional grace period of either five seconds or one
quarter of the new inactivity timeout or five seconds, timeout, whichever is greater.

7.1.2. Relationship to edns-tcp-keepalive EDNS0 EDNS(0) Option

The inactivity timeout value in the Keepalive TLV (DSO-TYPE=1) has
similar intent to the edns-tcp-keepalive EDNS0 EDNS(0) Option [RFC7828]. A
client/server pair that supports DSO MUST NOT use the edns-tcp-
keepalive EDNS0 EDNS(0) Option within any message after a DSO Session has
been established. A client that has sent a DSO message to establish
a session MUST NOT send an edns-tcp-keepalive EDNS0 EDNS(0) Option from
this point on. Once a DSO Session has been established, if either
client or server receives a DNS message over the DSO Session that
contains an edns-tcp-keepalive EDNS0 EDNS(0) Option, this is a fatal error
and the receiver of the edns-tcp-keepalive EDNS0 EDNS(0) Option MUST
forcibly abort the connection immediately.

7.2. Retry Delay TLV

The Retry Delay TLV (DSO-TYPE=2) can be used as a Primary TLV
(unidirectional) in a server-to-client message, or as a Response
Additional TLV in either direction. DSO messages with a Relay Delay
TLV as their primary Primary TLV are not permitted in early data.

The DSO-DATA for the Retry Delay TLV is as follows:

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RETRY DELAY (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

RETRY DELAY: A time value, specified as a 32-bit unsigned integer in
network (big endian) byte order, in units of milliseconds, within
which the initiator MUST NOT retry this operation or retry
connecting to this server. Recommendations for the RETRY DELAY
value are given in Section 6.6.1.

7.2.1. Retry Delay TLV Used as a Primary TLV

When sent from server to client, the Retry Delay TLV is used as the Primary TLV in a DSO unidirectional message. message, the
Retry Delay TLV is sent from server to client. It is used by a
server to instruct a client to close the DSO Session and underlying
connection, and not to reconnect for the indicated time interval.

In this case, it applies to the DSO Session as a whole, and the
client MUST begin closing the DSO Session as described in
Section 6.6.1. The RCODE in the message header SHOULD indicate the
principal reason for the termination:

o NOERROR indicates a routine shutdown or restart.

o FORMERR indicates that a client DSO request was too badly
malformed for the session to continue.

o SERVFAIL indicates that the server is overloaded due to resource
exhaustion and needs to shed load.

o REFUSED indicates that the server has been reconfigured, and at
this time it is now unable to perform one or more of the long-
lived client operations that were previously being performed on
this DSO Session.

o NOTAUTH indicates that the server has been reconfigured and at
this time it is now unable to perform one or more of the long-
lived client operations that were previously being performed on
this DSO Session because it does not have authority over the names
in question (for example, a DNS Push Notification server could be
reconfigured such that it is no longer accepting DNS Push
Notification requests for one or more of the currently subscribed
names).

This document specifies only these RCODE values for the DSO Retry
Delay message. Servers sending DSO Retry Delay messages SHOULD use
one of these values. However, future circumstances may create
situations where other RCODE values are appropriate in DSO Retry
Delay messages, so clients MUST be prepared to accept DSO Retry Delay
messages with any RCODE value.

In some cases, when a server sends a DSO Retry Delay unidirectional
message to a client, there may be more than one reason for the server
wanting to end the session. Possibly, the configuration could have
been changed such that some long-lived client operations can no
longer be continued due to policy (REFUSED), and other long-lived
client operations can no longer be performed due to the server no
longer being authoritative for those names (NOTAUTH). In such cases,
the server MAY use any of the applicable RCODE values, or
RCODE=NOERROR (routine shutdown or restart).

Note that the selection of RCODE value in a DSO Retry Delay message
is not critical since the RCODE value is generally used only for
information purposes such as writing to a log file for future human
analysis regarding the nature of the disconnection. Generally,
clients do not modify their behavior depending on the RCODE value.
The RETRY DELAY in the message tells the client how long it should
wait before attempting a new connection to this service instance.

For clients that do in some way modify their behavior depending on
the RCODE value, they should treat unknown RCODE values the same as
RCODE=NOERROR (routine shutdown or restart).

A DSO Retry Delay message (DSO message where the Primary TLV is Retry
Delay) from server to client is a DSO unidirectional message; the
MESSAGE ID MUST be set to zero in the outgoing message and the client
MUST NOT send a response.

A client MUST NOT send a DSO Retry Delay DSO message to a server. If a
server receives a DSO message where the Primary TLV is the Retry
Delay TLV, this is a fatal error and the server MUST forcibly abort
the connection immediately.

7.2.2. Retry Delay TLV Used as a Response Additional TLV

In the case of a DSO request message that results in a nonzero RCODE
value, the responder MAY append a Retry Delay TLV to the response,
indicating the time interval during which the initiator SHOULD NOT
attempt this operation again.

The indicated time interval during which the initiator SHOULD NOT
retry applies only to the failed operation, not to the DSO Session as
a whole.

Either a client or a server, whichever is acting in the role of the
responder for a particular DSO request message, MAY append a Retry
Delay TLV to an error response that it sends.

7.3. Encryption Padding TLV

The Encryption Padding TLV (DSO-TYPE=3) can only be used as an
Additional or Response Additional TLV. It is only applicable when
the DSO Transport layer uses encryption such as TLS.

The DSO-DATA for the Padding TLV is optional and is a variable length
field containing non-specified values. A DSO-LENGTH of 0 essentially
provides for 4 bytes of padding (the minimum amount).

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
/ /
/ PADDING -- VARIABLE NUMBER OF BYTES /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

As specified for the EDNS(0) Padding Option [RFC7830], the PADDING
bytes SHOULD be set to 0x00. Other values MAY be used, for example,
in cases where there is a concern that the padded message could be
subject to compression before encryption. PADDING bytes of any value
MUST be accepted in the messages received.

The Encryption Padding TLV may be included in either a DSO request
message, response, or both. As specified for the EDNS(0) Padding
Option [RFC7830], if a DSO request message is received with an
Encryption Padding TLV, then the DSO response MUST also include an
Encryption Padding TLV.

The length of padding is intentionally not specified in this document
and is a function of current best practices with respect to the type
and length of data in the preceding TLVs [RFC8467].

8. Summary Highlights

This section summarizes some noteworthy highlights about various
aspects of the DSO protocol.

8.1. QR Bit and MESSAGE ID

In DSO Request Messages, request messages, the QR bit is 0 and the MESSAGE ID is
nonzero.

In DSO Response Messages, response messages, the QR bit is 1 and the MESSAGE ID is
nonzero.

In DSO Unidirectional Messages, unidirectional messages, the QR bit is 0 and the MESSAGE ID is
zero.

The table below illustrates which combinations are legal and how they
are interpreted:

+------------------------------+------------------------+
| MESSAGE ID zero | MESSAGE ID nonzero |
+--------+------------------------------+------------------------+
| QR=0 | DSO Unidirectional Message unidirectional message | DSO Request Message request message |
+--------+------------------------------+------------------------+
| QR=1 | Invalid - Fatal Error | DSO Response Message response message |
+--------+------------------------------+------------------------+

8.2. TLV Usage

The table below indicates, for each of the three TLVs defined in this
document, whether they are valid in each of ten different contexts.

The first five contexts are DSO requests or DSO unidirectional
messages from client to server, and the corresponding responses from
server back to client:

o C-P - Primary TLV, sent in DSO Request request message, from client to
server, with nonzero MESSAGE ID indicating that this request MUST
generate response message.

o C-U - Primary TLV, sent in DSO Unidirectional unidirectional message, from client
to server, with zero MESSAGE ID indicating that this request MUST
NOT generate response message.

o C-A - Additional TLV, optionally added to a DSO request message or
DSO unidirectional message from client to server.

o CRP - Response Primary TLV, included in response message sent back
to the client (in response to a client "C-P" request with nonzero
MESSAGE ID indicating that a response is required) where the DSO-
TYPE of the Response TLV matches the DSO-TYPE of the Primary TLV
in the request.

o CRA - Response Additional TLV, included in response message sent
back to the client (in response to a client "C-P" request with
nonzero MESSAGE ID indicating that a response is required) where
the DSO-TYPE of the Response TLV does not match the DSO-TYPE of
the Primary TLV in the request.

The second five contexts are their counterparts in the opposite
direction: DSO requests or DSO unidirectional messages from server to
client, and the corresponding responses from client back to server.

o S-P - Primary TLV, sent in DSO Request request message, from server to
client, with nonzero MESSAGE ID indicating that this request MUST
generate response message.

o S-U - Primary TLV, sent in DSO Unidirectional unidirectional message, from server
to client, with zero MESSAGE ID indicating that this request MUST
NOT generate response message.

o S-A - Additional TLV, optionally added to a DSO request message or
DSO unidirectional message from server to client.

o SRP - Response Primary TLV, included in response message sent back
to the server (in response to a server "S-P" request with nonzero
MESSAGE ID indicating that a response is required) where the DSO-
TYPE of the Response TLV matches the DSO-TYPE of the Primary TLV
in the request.

o SRA - Response Additional TLV, included in response message sent
back to the server (in response to a server "S-P" request with
nonzero MESSAGE ID indicating that a response is required) where
the DSO-TYPE of the Response TLV does not match the DSO-TYPE of
the Primary TLV in the request.

+-------------------------+-------------------------+
| C-P C-U C-A CRP CRA | S-P S-U S-A SRP SRA |
+------------+-------------------------+-------------------------+
| KeepAlive | X X | X |
+------------+-------------------------+-------------------------+
| RetryDelay | X | X X |
+------------+-------------------------+-------------------------+
| Padding | X X | X X |
+------------+-------------------------+-------------------------+

Note that some of the columns in this table are currently empty. The
table provides a template for future TLV definitions to follow. It
is recommended that definitions of future TLVs include a similar
table summarizing the contexts where the new TLV is valid.

9. Additional Considerations

9.1. Service Instances

We use the term "service instance" to refer to software running on a
host that can receive connections on some sets set of { IP address and address, port }
tuples. What makes the software an instance is that regardless of
which of these tuples the client uses to connect to it, the client is
connected to the same software, running on the same logical node (but see (see
Section 9.2), and will receive the same answers and the same keying
information.

Service instances are identified from the perspective of the client.
If the client is configured with { IP addresses and address, port number } tuples, it has
no way to tell if the service offered at one tuple is the same server
that is listening on a different tuple. So in this case, the client
treats each such different tuple as if it references a separate different service
instance.

In some cases, a client is configured with a hostname and a port
number. The port number (either implicitly, where may be given explicitly, along with the
hostname. The port number is omitted may be omitted, and
assumed, or explicitly, assumed to have some
default value. The hostname and a port number may be learned from
the network, as in the case of DNS SRV records). records. In these cases, the (hostname/port)
{ hostname, port } tuple uniquely identifies the service
instance (hostname comparisons are instance,
subject to the usual case-insensitive [RFC1034]). DNS comparison of names
[RFC1034].

It is possible that two hostnames might point to some common IP
addresses; this is a configuration error anomaly that the client is not
obliged to detect. The effect of this could be that after being told
to disconnect, the client might reconnect to the same server because
it is represented as a different service instance.

Implementations SHOULD NOT resolve hostnames and then perform the
process of matching IP address(es) in order to evaluate whether two
entities should be determined to be the "same service instance".

9.2. Anycast Considerations

When an anycast service is configured on a particular IP address and
port, it must be the case that although there is more than one
physical server responding on that IP address, each such server can
be treated as equivalent. What we mean by "equivalent" here is that
both servers can provide the same service and, where appropriate, the
same authentication information, such as PKI certificates, when
establishing connections.

If a change in network topology causes packets in a particular TCP
connection to be sent to an anycast server instance that does not
know about the connection, the new server will automatically
terminate the connection with a TCP reset, since it will have no
record of the connection, and then the client can reconnect or stop
using the connection as appropriate.

If, after the connection is re-established, the client's assumption
that it is connected to the same service instance is violated in some way,
that would be considered an incorrect behavior in this context. It
is, however, out of the possible scope for this specification to make
specific recommendations in this regard; that would be up to follow-
on documents that describe specific uses of DNS stateful operations. Stateful Operations.

9.3. Connection Sharing

As previously specified for DNS over TCP DNS-over-TCP [RFC7766]:

To mitigate the risk of unintentional server overload, DNS
clients MUST take care to minimize the number of concurrent
TCP connections made to any individual server. It is RECOMMENDED
that for any given client/server interaction there SHOULD be
no more than one connection for regular queries, one for zone
transfers, and one for each protocol that is being used on top
of TCP (for example, if the resolver was using TLS). However,
it is noted that certain primary/secondary configurations
with many busy zones might need to use more than one TCP
connection for zone transfers for operational reasons (for
example, to support concurrent transfers of multiple zones).

A single server may support multiple services, including DNS Updates
[RFC2136], DNS Push Notifications [DNS-PUSH], [Push], and other services, for one
or more DNS zones. When a client discovers that the target server
for several different operations is the same service instance (see
Section 9.1), the client SHOULD use a single shared DSO Session for
all those operations.

This requirement has two benefits. First, it reduces unnecessary
connection load on the DNS server. Second, it avoids paying the TCP
slow start penalty when making subsequent connections connection
startup time that would be spent establishing each new additional
connection to the same DNS server.

However, server implementers and operators should be aware that
connection sharing may not be possible in all cases. A single host
device may be home to multiple independent client software instances
that don't coordinate with each other. Similarly, multiple
independent client devices behind the same NAT gateway will also
typically appear to the DNS server as different source ports on the
same client IP address. Because of these constraints, a DNS server
MUST be prepared to accept multiple connections from different source
ports on the same client IP address.

9.4. Operational Considerations for Middlebox Middleboxes

Where an application-layer middlebox (e.g., a DNS proxy, forwarder,
or session multiplexer) is in the path, care must be taken to avoid a
configuration in which DSO traffic is mishandled. The simplest way
to avoid such problems is to avoid using middleboxes. When this is
not possible, middleboxes should be evaluated to make sure that they
behave correctly.

Correct behavior for middleboxes consists of one of the following:

o The middlebox does not forward DSO messages and responds to DSO
messages with a response code other than NOERROR or DSOTYPENI.

o The middlebox acts as a DSO server and follows this specification
in establishing connections.

o There is a 1:1 correspondence between incoming and outgoing
connections such that when a connection is established to the
middlebox, it is guaranteed that exactly one corresponding
connection will be established from the middlebox to some DNS
resolver, and all incoming messages will be forwarded without
modification or reordering. An example of this would be a NAT
forwarder or TCP connection optimizer (e.g., for a high-latency
connection such as a geosynchronous satellite link).

Middleboxes that do not meet one of the above criteria are very
likely to fail in unexpected and difficult-to-diagnose ways. For
example, a DNS load balancer might unbundle DNS messages from the
incoming TCP stream and forward each message from the stream to a
different DNS server. If such a load balancer is in use, and the DNS
servers it points to implement DSO and are configured to enable DSO,
DSO session Session establishment will succeed, but no coherent session will
exist between the client and the server. If such a load balancer is
pointed at a DNS server that does not implement DSO or is configured
not to allow DSO, no such problem will exist, but such a
configuration risks unexpected failure if new server software is
installed that does implement DSO.

It is of course possible to implement a middlebox that properly
supports DSO. It is even possible to implement one that implements
DSO with long-lived operations. This can be done either by
maintaining a 1:1 correspondence between incoming and outgoing
connections, as mentioned above, or by terminating incoming sessions
at the middlebox but maintaining state in the middlebox about any
long-lived operations that are requested. Specifying this in detail
is beyond the scope of this document.

9.5. TCP Delayed Acknowledgement Considerations

Most modern implementations of the Transmission Control Protocol
(TCP) include a feature called "Delayed Acknowledgement" [RFC1122].

Without this feature, TCP can be very wasteful on the network. For
illustration, consider a simple example like remote login using a
very simple TCP implementation that lacks delayed acks. When the
user types a keystroke, a data packet is sent. When the data packet
arrives at the server, the simple TCP implementation sends an
immediate acknowledgement. Mere milliseconds later, the server
process reads the one byte of keystroke data, and consequently the
simple TCP implementation sends an immediate window update. Mere
milliseconds later, the server process generates the character echo
and sends this data back in reply. The simple TCP implementation
then sends this data packet immediately too. In this case, this
simple TCP implementation sends a burst of three packets almost
instantaneously (ack, window update, data).

Clearly it would be more efficient if the TCP implementation were to
combine the three separate packets into one, and this is what the
delayed ack feature enables.

With delayed ack, the TCP implementation waits after receiving a data
packet, typically for 200 ms, and then sends its ack if (a) more data
packet(s) arrive, (b) the receiving process generates some reply
data, or (c) 200 ms elapse without either of the above occurring.

With delayed ack, remote login becomes much more efficient,
generating just one packet instead of three for each character echo.

The logic of delayed ack is that the 200 ms delay cannot do any
significant harm. If something at the other end were waiting for
something, then the receiving process should generate the reply that
the thing at the other end is waiting for, and TCP will then
immediately send that reply (and (combined with the ack and window
update). And if the receiving process does not in fact generate any
reply for this particular message, then by definition the thing at
the other end cannot be waiting for anything. Therefore, the 200 ms
delay is harmless.

This assumption may be true unless the sender is using Nagle's
algorithm, a similar efficiency feature, created to protect the
network from poorly written client software that performs many rapid
small writes in succession. Nagle's algorithm allows these small
writes to be combined coalesced into larger, less wasteful packets.

Unfortunately, Nagle's algorithm and delayed ack, two valuable
efficiency features, can interact badly with each other when used
together [NagleDA].

DSO request messages elicit responses; DSO unidirectional messages
and DSO response messages do not.

For DSO request messages, which do elicit responses, Nagle's
algorithm and delayed ack work as intended.

For DSO messages that do not elicit responses, the delayed ack
mechanism causes the ack to be delayed by 200 ms. The 200 ms delay
on the ack can in turn cause Nagle's algorithm to prevent the sender
from sending any more data for 200 ms until the awaited ack arrives.
On an enterprise Gigabit Ethernet (GigE) backbone with sub-
millisecond round-trip times, a 200 ms delay is enormous in
comparison.

When this issues is raised, there are two solutions that are often
offered, neither of them ideal:

1. Disable delayed ack. For DSO messages that elicit no response,
removing delayed ack avoids the needless 200 ms delay and sends
back an immediate ack that tells Nagle's algorithm that it should
immediately grant the sender permission to send its next packet.
Unfortunately, for DSO messages that *do* elicit a response,
removing delayed ack removes the efficiency gains of combining
acks with data, and the responder will now send two or three
packets instead of one.

2. Disable Nagle's algorithm. When acks are delayed by the delayed
ack algorithm, removing Nagle's algorithm prevents the sender
from being blocked from sending its next small packet
immediately. Unfortunately, on a network with a higher round-
trip time, removing Nagle's algorithm removes the efficiency
gains of combining multiple small packets into fewer larger ones,
with the goal of limiting the number of small packets in flight
at any one time.

For DSO messages that elicit a response, delayed ack and Nagle's
algorithm do the right thing.

The problem here is that with DSO messages that elicit no response,
the TCP implementation is stuck waiting, unsure if a response is
about to be generated or whether the TCP implementation should go
ahead and send an ack and window update.

The solution is networking APIs that allow the receiver to inform the
TCP implementation that a received message has been read, processed,
and no response for this message will be generated. TCP can then
stop waiting for a response that will never come come, and immediately go
ahead and send an ack and window update.

For implementations of DSO, disabling delayed ack is NOT RECOMMENDED
because of the harm this can do to the network.

For implementations of DSO, disabling Nagle's algorithm is NOT
RECOMMENDED because of the harm this can do to the network.

At the time that this document is being prepared for publication, it
is known that at least one TCP implementation provides the ability
for the recipient of a TCP message to signal that it is not going to
send a response, and hence the delayed ack mechanism can stop
waiting. Implementations on operating systems where this feature is
available SHOULD make use of it.

10. IANA Considerations

10.1. DSO OPCODE Registration

The IANA has assigned the value 6 for DNS Stateful Operations (DSO)
in the "DNS OpCodes" registry.

10.2. DSO RCODE Registration

IANA has assigned the value 11 for the DSOTYPENI error code in the
"DNS RCODEs" registry. The DSOTYPENI error code ("DSO-TYPE Not
Implemented") indicates that the receiver does implement DNS Stateful
Operations, but does not implement the specific DSO-TYPE of the
primary
Primary TLV in the DSO request message.

10.3. DSO Type Code Registry

The IANA has created the 16-bit "DSO Type Codes" registry, with
initial (hexadecimal) values as shown below:

+-----------+------------------------+-------+----------+-----------+

+-----------+-----------------------+-------+-----------+-----------+
| Type | Name | Early | Status | Reference |
| | | Data | | |
+-----------+------------------------+-------+----------+-----------+
+-----------+-----------------------+-------+-----------+-----------+
| 0000 | Reserved | NO | Standard Standards | RFC 8490 |
| | | | Track | |
| | | | | |
| 0001 | KeepAlive | OK | Standard Standards | RFC 8490 |
| | | | Track | |
| | | | | |
| 0002 | RetryDelay | NO | Standard Standards | RFC 8490 |
| | | | Track | |
| | | | | |
| 0003 | EncryptionPadding | NA | Standard Standards | RFC 8490 |
| | | | Track | |
| | | | | |
| 0004-003F | Unassigned, reserved | NO | | |
| | for DSO session- | | | |
| | management TLVs | | | |
| | | | | |
| 0040-F7FF | Unassigned | NO | | |
| | | | | |
| F800-FBFF | Experimental/local use | NO | | |
| | use | | | |
| | | | | |
| FC00-FFFF | Reserved for future | NO | | |
| | expansion | | | |
+-----------+------------------------+-------+----------+-----------+
+-----------+-----------------------+-------+-----------+-----------+
The meanings of the fields are as follows:

Type: the The 16-bit DSO type code code.

Name: the The human-readable name of the TLV TLV.

Early Data: If OK, this TLV may be sent as early data in a TLS 0-RTT zero
round-trip (Section 2.3 of the TLS 1.3 specification [RFC8446])
initial handshake. If NA, the TLV may appear as a secondary an Additional TLV
in a DSO message that is sent as early data.

Status: IETF Document RFC status (or (e.g., "Standards Track") or "External" if not
documented in an
IETF document) RFC.

Reference: a A stable reference to the document in which this TLV is
defined
defined.

Note: DSO Type Code zero is reserved and is not currently intended
for allocation.

Registrations of new DSO Type Codes in the "Reserved for DSO session-
management" range 0004-003F and the "Reserved for future expansion"
range FC00-FFFF require publication of an IETF Standards Action
document [RFC8126].

Any document defining a new TLV that lists a value of "OK" in the
0-RTT column must include a threat analysis for the use of the TLV in
the case of TLS 0-RTT. See Section 11.1 for details.

Requests to register additional new DSO Type Codes in the
"Unassigned" range 0040-F7FF are to be recorded by IANA after Expert
Review [RFC8126]. The expert review should validate that the
requested type code is specified in a way that conforms to this
specification, and that the intended use for the code would not be
addressed with an experimental/local assignment.

DSO Type Codes in the "experimental/local" range F800-FBFF may be
used as Experimental Use or Private Use values [RFC8126] and may be
used freely for development purposes or for other purposes within a
single site. No attempt is made to prevent multiple sites from using
the same value in different (and incompatible) ways. There is no
need for IANA to review such assignments (since IANA does not record
them) and assignments are not generally useful for broad
interoperability. It is the responsibility of the sites making use
of "experimental/local" values to ensure that no conflicts occur
within the intended scope of use.

Any document defining a new TLV that lists a value of "OK" in the
Early Data column must include a threat analysis for the use of the
TLV in the case of TLS zero round-trip. See Section 11.1 for
details.

11. Security Considerations

If this mechanism is to be used with DNS over TLS, DNS-over-TLS, then these
messages are subject to the same constraints as any other DNS-over-
TLS messages and MUST NOT be sent in the clear before the TLS session
is established.

The data field of the "Encryption Padding" TLV could be used as a
covert channel.

When designing new DSO TLVs, the potential for data in the TLV to be
used as a tracking identifier should be taken into consideration and
should be avoided when not required.

When used without TLS or similar cryptographic protection, a
malicious entity may be able to inject a malicious unidirectional DSO
Retry Delay message into the data stream, specifying an unreasonably
large RETRY DELAY, causing a denial-of-service attack against the
client.

The establishment of DSO sessions Sessions has an impact on the number of open
TCP connections on a DNS server. Additional resources may be used on
the server as a result. However, because the server can limit the
number of DSO sessions Sessions established and can also close existing DSO
sessions
Sessions as needed, denial of service or resource exhaustion should
not be a concern.

11.1. TLS 0-RTT Zero Round-Trip Considerations

DSO permits zero round-trip operation using TCP Fast Open [RFC7413] with
TLS 1.3 [RFC8446] 0-RTT early data to reduce or eliminate round trips in
session establishment. TCP Fast Open is only permitted in
combination with TLS 0-RTT. 1.3 early data. In the rest of this section, we
refer to TLS 1.3 early data in a TLS 0-RTT zero round-trip initial
handshake message, regardless of whether or not it is included in a
TCP SYN packet with early data using the TCP Fast Open option, as
"early data."

A DSO message may or may not be permitted to be sent as early data.
The definition for each TLV that can be used as a primary Primary TLV is
required to state whether or not that TLV is permitted as early data.
Only response-requiring messages are ever permitted as early data,
and only clients are permitted to send any a DSO message as early data
unless there is an implicit DSO session (see Section 5.1).

For DSO messages that are permitted as early data, a client MAY
include one or more such messages as early data without having to
wait for a DSO response to the first DSO request message to confirm
successful establishment of a DSO session. Session.

However, unless there is an implicit DSO session, a client MUST NOT
send DSO unidirectional messages until after a DSO Session has been
mutually established.

Similarly, unless there is an implicit DSO session, a server MUST NOT
send DSO request messages until it has received a response-requiring
DSO request message from a client and transmitted a successful
NOERROR response for that request.

Caution must be taken to ensure that DSO messages sent as early data
are idempotent or are otherwise immune to any problems that could
result from the inadvertent replay that can occur with zero round-
trip operation.

It would be possible to add a TLV that requires the server to do some
significant work and send that to the server as initial data in a TCP
SYN packet. A flood of such packets could be used as a DoS attack on
the server. None of the TLVs defined here have this property.

If a new TLV is specified that does have this property, that TLV must
be specified as not permitted in 0-RTT zero round-trip messages. This
prevents work from being done until a round-trip has occurred from
the server to the client to verify that the source address of the
packet is reachable.

Documents that define new TLVs must state whether each new TLV may be
sent as early data. Such documents must include a threat analysis in
the security considerations section for each TLV defined in the
document that may be sent as early data. This threat analysis should
be done based on the advice given in Sections 2.3, 8, and
Appendix E.5 of the TLS 1.3 specification [RFC8446].

12. References

12.1. Normative References

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<https://www.rfc-editor.org/info/rfc1918>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.

[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<https://www.rfc-editor.org/info/rfc6891>.

[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.

[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/info/rfc7830>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2. Informative References

[DNS-PUSH]
Pusateri, T. and S. Chesire, "DNS Push Notifications",
Work in Progress, draft-ietf-dnssd-push-16, November 2018.

[FTR]

[Fail] Andrews, M. and R. Bellis, "A Common Operational Problem
in DNS Servers - Failure To Respond", Work in Progress,
draft-ietf-dnsop-no-response-issue-12, November 2018.

[MDNS-DISCOVERY-RELAY]
Lemon, T. and S. Chesire, "Multicast DNS Discovery Relay", Communicate", Work in
Progress, draft-ietf-dnssd-mdns-relay-01, July
2018. draft-ietf-dnsop-no-response-issue-13, February
2019.

[NagleDA] Cheshire, S., "TCP Performance problems caused by
interaction between Nagle's Algorithm and Delayed ACK",
May 2005,
<http://www.stuartcheshire.org/papers/nagledelayedack/>.

[Push] Pusateri, T. and S. Chesire, "DNS Push Notifications",
Work in Progress, draft-ietf-dnssd-push-16, November 2018.

[Relay] Lemon, T. and S. Chesire, "Multicast DNS Discovery Relay",
Work in Progress, draft-ietf-dnssd-mdns-relay-01, July
2018.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.

[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<https://www.rfc-editor.org/info/rfc5382>.

[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.

[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.

[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.

[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828,
DOI 10.17487/RFC7828, April 2016,
<https://www.rfc-editor.org/info/rfc7828>.

[RFC7857] Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
S., and K. Naito, "Updates to Network Address Translation
(NAT) Behavioral Requirements", BCP 127, RFC 7857,
DOI 10.17487/RFC7857, April 2016,
<https://www.rfc-editor.org/info/rfc7857>.

[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8467] Mayrhofer, A., "Padding Policies for Extension Mechanisms
for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
October 2018, <https://www.rfc-editor.org/info/rfc8467>.

[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.

Acknowledgements

Thanks to Stephane Bortzmeyer, Tim Chown, Ralph Droms, Paul Hoffman,
Jan Komissar, Edward Lewis, Allison Mankin, Rui Paulo, David
Schinazi, Manju Shankar Rao, Bernie Volz, and Bob Harold for their
helpful contributions to this document.

Authors' Addresses

Ray Bellis
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
United States of America

Phone: +1 (650) 423-1200
Email: ray@isc.org

Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, CA 95014
United States of America

Phone: +1 (408) 996-1010
Email: cheshire@apple.com
John Dickinson
Sinodun Internet Technologies
Magadalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom

Email: jad@sinodun.com

Sara Dickinson
Sinodun Internet Technologies
Magadalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom

Email: sara@sinodun.com

Ted Lemon
Nibbhaya Consulting
P.O. Box 958
Brattleboro, VT 05302-0958
United States of America

Email: mellon@fugue.com

Tom Pusateri
Unaffiliated
Raleigh, NC 27608
United States of America

Phone: +1 (919) 867-1330
Email: pusateri@bangj.com