wdiff rfc8765.original rfc8765.txt

Internet Engineering Task Force (IETF) T. Pusateri
Internet-Draft
Request for Comments: 8765 Unaffiliated
Intended status:
Category: Standards Track S. Cheshire
Expires: April 15, 2020
ISSN: 2070-1721 Apple Inc.
October 13, 2019
June 2020

DNS Push Notifications
draft-ietf-dnssd-push-25

Abstract

The Domain Name System (DNS) was designed to return matching records
efficiently for queries for data that are relatively static. When
those records change frequently, DNS is still efficient at returning
the updated results when polled, as long as the polling rate is not
too high. But But, there exists no mechanism for a client to be
asynchronously notified when these changes occur. This document
defines a mechanism for a client to be notified of such changes to
DNS records, called DNS Push Notifications.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum 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 six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on April 15, 2020.
https://www.rfc-editor.org/info/rfc8765.

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
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Fatal Errors . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. State Considerations . . . . . . . . . . . . . . . . . . . . 6
5. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8
6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 13
6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13
6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16
6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 20
6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20
6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 26
6.4.1. UNSUBSCRIBE Message . . . . . . . . . . . . . . . . . 26
6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 28
6.5.1. RECONFIRM Message . . . . . . . . . . . . . . . . . . 29
6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 31
6.7. Client-Initiated Termination . . . . . . . . . . . . . . 32
6.8. Client Fallback to Polling . . . . . . . . . . . . . . . 33
7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 35
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 35
7.3. TLS Early Data . . . . . . . . . . . . . . . . . . . . . 36
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 40
Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42

1. Introduction

Domain Name System (DNS) records may be updated using DNS Update
[RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] [RFC8766] can
also generate changes to a DNS zone. This document specifies a
protocol for DNS clients to subscribe to receive asynchronous
notifications of changes to RRsets of interest. It is immediately
relevant in the case of DNS DNS-based Service Discovery [RFC6763] but is
not limited to that use case, and case; it provides a general DNS mechanism for
DNS record change notifications. Familiarity with the DNS protocol
and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895].

1.1. 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. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.

1.2. Fatal Errors

Certain invalid situations are described in this specification, like such
as a server sending a Push Notification subscription request to a
client, or a client sending a Push Notification response to a server.
These should never occur with a correctly implemented client and
server, and if they do occur occur, then they indicate a serious
implementation error. In these extreme cases cases, there is no reasonable
expectation of a graceful recovery, and the recipient detecting the
error should respond by unilaterally aborting the session without
regard for data loss. Such cases are addressed by having an engineer
investigate the cause of the failure and fixing the problem in the
software.

Where this specification says "forcibly abort", it means sending a
TCP RST to terminate the TCP connection, connection and the TLS session running
over that TCP connection. In the BSD Sockets API, this is achieved
by setting the SO_LINGER option to zero before closing the socket.

2. Motivation

As the domain name system continues to adapt to new uses and changes
in deployment, polling has the potential to burden DNS servers at
many levels throughout the network. Other network protocols have
successfully deployed a publish/subscribe model following the
Observer design pattern [obs]. XMPP [OBS]. Extensible Messaging and Presence
Protocol (XMPP) Publish-Subscribe [XEP0060] and Atom [RFC4287] are
examples. While DNS servers are generally highly tuned and capable
of a high rate of query/response traffic, adding a publish/subscribe
model for tracking changes to DNS records can deliver more timely notification
notifications of changes with reduced CPU usage and lower network
traffic.

The guiding design principle of DNS Push Notifications is that
clients that choose to use DNS Push Notifications, instead of
repeated polling with DNS queries, will receive the same results as
they could via sufficiently rapid polling, except more efficiently.
This means that the rules for which records match a given DNS Push
Notification subscription are the same as the already established
rules used to determine which records match a given DNS query
[RFC1034]. For example, name comparisons are done in a case-
insensitive manner, and a record of type CNAME in a zone matches any
DNS TYPE in a query or subscription.

Multicast DNS [RFC6762] implementations always listen on a well known well-known
link-local IP multicast group address, and changes are sent to that
multicast group address for all group members to receive. Therefore,
Multicast DNS already has asynchronous change notification
capability. When DNS DNS-based Service Discovery [RFC6763] is used
across a wide area network using Unicast DNS (possibly facilitated
via a Discovery Proxy [DisProx]) [RFC8766]), it would be beneficial to have an
equivalent capability for Unicast DNS, DNS in order to allow clients to
learn about DNS record changes in a timely manner without polling.

The DNS Long-Lived Queries (LLQ) mechanism [LLQ] [RFC8764] is an existing
deployed solution to provide asynchronous change notifications, notifications; it
was used by Apple's Back to My Mac [RFC6281] service introduced in
Mac OS X 10.5 Leopard in 2007. Back to My Mac was designed in an era
when the data center operations staff asserted that it was impossible
for a server to handle large numbers of mostly-idle TCP connections, so LLQ
was defined as a UDP-based protocol, effectively replicating much even if
those connections carried very little traffic and spent most of their
time idle. Consequently, LLQ was defined as a UDP-based protocol,
effectively replicating much of TCP's connection state management
logic in user space, space and creating its own imitation of existing TCP
features like the three-way
handshake, flow control, reliability, and reliability. the three-way handshake.

This document builds on experience gained with the LLQ protocol, with
an improved design. Instead of using UDP, this specification uses
DNS Stateful Operations (DSO) [RFC8490] running over TLS over TCP,
and therefore doesn't need to reinvent existing TCP functionality.
Using TCP also gives long-lived low-traffic connections better
longevity through NAT gateways without depending on the gateway to
support NAT Port Mapping Protocol (NAT-PMP) [RFC6886] or Port Control
Protocol (PCP) [RFC6887], or resorting to excessive keepalive
traffic.

3. Overview

A DNS Push Notification client subscribes for Push Notifications for
a particular RRset by connecting to the appropriate Push Notification
server for that RRset, RRset and sending DSO message(s) indicating the
RRset(s) of interest. When the client loses interest in receiving
further updates to these records, it unsubscribes.

The DNS Push Notification server for a DNS zone is any server capable
of generating the correct change notifications for a name. It may be
a primary, secondary, or stealth name server [RFC7719]. [RFC8499].

The "_dns-push-tls._tcp.<zone>" SRV record for a zone MAY reference
the same target host and port as that zone's
"_dns-update-tls._tcp.<zone>" SRV record. When the same target host
and port is offered for both DNS Updates and DNS Push Notifications,
a client MAY use a single DSO session to that server for both DNS
Updates and DNS Push Notification Subscriptions. subscriptions. DNS Updates and DNS
Push Notifications may be handled on different ports on the same
target host, in which case they are not considered to be the "same
server" for the purposes of this specification, and communications
with these two ports are handled independently. Supporting DNS
Updates and DNS Push Notifications on the same server is OPTIONAL. A
DNS Push Notification server is not required to support DNS Update.

Standard DNS Queries MAY be sent over a DNS Push Notification (i.e.,
DSO) session. For any zone for which the server is authoritative, it
MUST respond authoritatively for queries for names falling within
that zone (e.g., the "_dns-push-tls._tcp.<zone>" SRV record) both for
normal DNS queries and for DNS Push Notification subscriptions. For
names for which the server is acting as a recursive resolver (e.g.,
when the server is the local recursive resolver) for any query for
which it supports DNS Push Notification subscriptions, it MUST also
support standard queries.

DNS Push Notifications impose less load on the responding server than
rapid polling would, but Push Notifications do still have a cost, so cost.
Therefore, DNS Push Notification clients MUST NOT recklessly create
an excessive number of Push Notification subscriptions.
Specifically:

(a) A subscription should only be active when there is a valid
reason to need live data (for example, an on-screen display is
currently showing the results to the user) user), and the subscription
SHOULD be
cancelled canceled as soon as the need for that data ends (for
example, when the user dismisses that display). In the case of
a device like a smartphone which, that, after some period of
inactivity, goes to sleep or otherwise darkens its screen, it
should cancel its subscriptions when darkening the screen (since
the user cannot see any changes on the display anyway) and
reinstate its subscriptions when re-awakening reawakening from display sleep.

(b) A DNS Push Notification client SHOULD NOT routinely keep a DNS
Push Notification subscription active 24 hours a day, 7 days a
week, just to keep a list in memory up to date so that if the
user does choose to bring up an on-screen display of that data,
it can be displayed really fast. DNS Push Notifications are
designed to be fast enough that there is no need to pre-load a
"warm" list in memory just in case it might be needed later.

Generally, as described in the DNS Stateful Operations specification
[RFC8490], a client must not keep a DSO session to a server open
indefinitely if it has no subscriptions (or other operations) active
on that session. A client may close should begin closing a DSO session
immediately after it becomes idle, and then then, if needed in the future,
open a new session when required. Alternatively, a client may
speculatively keep an idle DSO session open for some time, subject to
the constraint that it must not keep a session open that has been
idle for more than the session's idle timeout (15 seconds by default)
[RFC8490].

Note that a DSO session that has an active DNS Push Notification
subscription is not considered idle, even if there is no traffic
flowing for an extended period of time. In this case case, the DSO
inactivity timeout does not apply, because the session is not
inactive, but the keepalive interval does still apply, to ensure 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 is described in Section 6.2 of the DSO specification [RFC8490].

4. State Considerations

Each DNS Push Notification server is capable of handling some finite
number of Push Notification subscriptions. This number will vary
from server to server and is based on physical machine
characteristics, network bandwidth, capacity, and operating system resource
allocation. After a client establishes a session to a DNS server,
each subscription is individually accepted or rejected. Servers may
employ various techniques to limit subscriptions to a manageable
level. Correspondingly, the client is free to establish simultaneous
sessions to alternate DNS servers that support DNS Push Notifications
for the zone and distribute subscriptions at the client's discretion.
In this way, both clients and servers can react to resource
constraints.

5. Transport

Other DNS operations like DNS Update [RFC2136] MAY use either DNS
over User Datagram Protocol (UDP) [RFC0768] or DNS over Transmission
Control Protocol (TCP) [RFC0793] as the transport protocol, in keeping with provided
they follow the historical precedent that DNS queries must first be
sent using DNS over UDP and only switch to DNS over TCP if needed
[RFC1123]. This requirement to use prefer UDP has subsequently been
relaxed [RFC7766].

In keeping with the more recent precedent, DNS Push Notification is
defined only for TCP. DNS Push Notification clients MUST use DNS
Stateful Operations [RFC8490] running over TLS over TCP [RFC7858].

Connection setup over TCP ensures return reachability and alleviates
concerns of state overload at the server, which is a potential problem with
connectionless protocols, which can be more vulnerable to being
exploited by attackers using spoofed source addresses. All
subscribers are guaranteed to be reachable by the server by virtue of
the TCP three-way handshake. Flooding attacks are possible with any
protocol, and a benefit of TCP is that there are already established
industry best practices to guard against SYN flooding and similar
attacks [SYN] [RFC4953].

Use of TCP also allows DNS Push Notifications to take advantage of
current and future developments in TCP, TCP such as Multipath TCP (MPTCP)
[RFC6824],
[RFC8684], TCP Fast Open (TFO) [RFC7413], the TCP RACK fast loss
detection algorithm [I-D.ietf-tcpm-rack], [TCPRACK], and so on.

Transport Layer Security (TLS) [RFC8446] is well understood, understood and is
used by many application-layer protocols running over TCP. TLS is
designed to prevent eavesdropping, tampering, and message forgery.
TLS is REQUIRED for every connection between a client subscriber and
server in this protocol specification. Additional security measures
such as client authentication during TLS negotiation may also be
employed to increase the trust relationship between client and
server.

6. Protocol Operation

The DNS Push Notification protocol is a session-oriented protocol, protocol and
makes use of DNS Stateful Operations (DSO) [RFC8490].

For details of the DSO message format format, refer to the DNS Stateful Oper-
ations
Operations specification [RFC8490]. Those details are not repeated
here.

DNS Push Notification clients and servers MUST support DSO. A single
server can support DNS Queries, DNS Updates, and DNS Push
Notifications (using DSO) on the same TCP port.

A DNS Push Notification exchange begins with the client discovering
the appropriate server, using the procedure described in Section 6.1,
and then making a TLS/TCP connection to it.

After making the TLS/TCP connection to the server, a typical DNS Push
Notification client will then immediately issue a DSO Keepalive
operation to establish the DSO session and request a session timeout
and/or keepalive interval longer than the 15-second default values,
but this is not required. A DNS Push Notification client MAY issue
other requests on the session first, and only issue a DSO Keepalive
operation later if it determines that to be necessary. Sending
either a DSO Keepalive operation or a Push Notification subscription
request over the TLS/
TCP TLS/TCP connection to the server signals the
client's support of DSO and serves to establish a DSO session.

In accordance with the current set of active subscriptions, the
server sends relevant asynchronous Push Notifications to the client.
Note that a client MUST be prepared to receive (and silently ignore)
Push Notifications for subscriptions it has previously removed, since
there is no way to prevent the situation where a Push Notification is
in flight from server to client while the client's UNSUBSCRIBE
message cancelling canceling that subscription is simultaneously in flight from
client to server.

6.1. Discovery

The first step in establishing a DNS Push Notification subscription
is to discover an appropriate DNS server that supports DNS Push
Notifications for the desired zone.

The client begins by opening a DSO Session session to its normal configured
DNS recursive resolver and requesting a Push Notification
subscription. This connection is made to TCP port 853, the default
port for DNS-over-TLS DNS over TLS [RFC7858]. If the request for a Push
Notification subscription is successful, and the recursive resolver
doesn't already have an active subscription for that name, type, and
class, then the recursive resolver will make a corresponding Push
Notification subscription on the client's behalf. Results received
are relayed to the client. This is closely analogous to how a client
sends a normal DNS query to its configured DNS recursive resolver resolver,
which, if it doesn't already have appropriate answer(s) in its cache,
issues an upstream query to satisfy the request.

In many contexts, the recursive resolver will be able to handle Push
Notifications for all names that the client may need to follow. Use
of VPN tunnels and Private DNS [RFC8499] can create some additional
complexity in the client software here; the techniques to handle VPN
tunnels and Private DNS for DNS Push Notifications are the same as
those already used to handle this for normal DNS queries.

If the recursive resolver does not support DNS over TLS, or supports
DNS over TLS but is not listening on TCP port 853, or supports DNS
over TLS on TCP port 853 but does not support DSO on that port, then
the DSO Session session establishment will fail [RFC8490].

If the recursive resolver does support DSO on TCP port 853 but does
not support Push Notification subscriptions, then it when the client
attempts to create a subscription, the server will return the DSO
error code DSOTYPENI (11).

In some cases, the recursive resolver may support DSO and Push
Notification subscriptions, subscriptions but may not be able to subscribe for Push
Notifications for a particular name. In this case, the recursive
resolver should return SERVFAIL to the client. This includes being
unable to establish a connection to the zone's DNS Push Notification
server or establishing a connection but receiving a non success non-success
response code. In some cases, where the client has a pre-established
trust relationship with the owner of the zone (that is not handled
via the usual mechanisms for VPN software) software), the client may handle
these failures by contacting the zone's DNS Push Notification server
directly.

In any of the cases described above where the client fails to
establish a DNS Push Notification subscription via its configured
recursive resolver, the client should proceed to discover the
appropriate server for direct communication. The client MUST also
determine on which TCP port on the server is listening for connections,
which need not be (and be, and often is not) the typical not, TCP port 53 (traditionally used
for conventional DNS, DNS) or TCP port 853 (traditionally used for DNS
over TLS. TLS).

The discovery algorithm described here is an iterative algorithm,
which starts with the full name of the record to which the client
wishes to subscribe. Successive SOA queries are then issued,
trimming one label each time, until the closest enclosing
authoritative server is discovered. There is also an optimization to
enable the client to take a "short cut" directly to the SOA record of
the closest enclosing authoritative server in many cases.

1. The client begins the discovery by sending a DNS query to its
local resolver, with record type SOA [RFC1035] for the record
name to which it wishes to subscribe. As an example, suppose the
client wishes to subscribe to PTR records with the name
_ipp._tcp.headoffice.example.com
"_ipp._tcp.headoffice.example.com" (to discover Internet Printing
Protocol (IPP) printers [RFC8010] [RFC8011] being advertised in
the head office of Example Company.). Company). The client begins by
sending an SOA query for _ipp._tcp.headoffice.example.com "_ipp._tcp.headoffice.example.com" to
the local recursive resolver. The goal is to determine the
server that is authoritative for the name _ipp._tcp.headoffice.example.com.
"_ipp._tcp.headoffice.example.com". The closest enclosing DNS
zone containing the name
_ipp._tcp.headoffice.example.com "_ipp._tcp.headoffice.example.com" could
be example.com, "example.com", or
headoffice.example.com, "headoffice.example.com", or _tcp.headoffice.example.com,
"_tcp.headoffice.example.com", or even
_ipp._tcp.headoffice.example.com.
"_ipp._tcp.headoffice.example.com". The client does not know in
advance where the closest enclosing zone cut occurs, which is why
it uses the iterative procedure described here to discover this
information.

2. If the requested SOA record exists, it will be returned in the
Answer section Section with a NOERROR response code, and the client has
succeeded in discovering the information it needs.

(This language is not placing any new requirements on DNS
recursive resolvers. This text merely describes the existing
operation of the DNS protocol [RFC1034] [RFC1035].)

3. If the requested SOA record does not exist, the client will get
back a NOERROR/NODATA response or an NXDOMAIN/Name Error
response. In either case, the local resolver would normally
include the SOA record for the closest enclosing zone of the
requested name in the Authority Section. If the SOA record is
received in the Authority Section, then the client has succeeded
in discovering the information it needs.

(This language is not placing any new requirements on DNS
recursive resolvers. This text merely describes the existing
operation of the DNS protocol regarding negative responses
[RFC2308].)

4. If the client receives a response containing no SOA record, then
it proceeds with the iterative approach. The client strips the
leading label from the current query name, and if the resulting
name has at least two labels in it, then the client sends an SOA
query for that new name, name and processing continues at step 2 above,
repeating the iterative search until either an SOA is received, received or
the query name consists of a single label, i.e., a Top Level Top-Level
Domain (TLD). In the case of a single-label name (TLD), this is
a network configuration error, which should not happen, and the
client gives up. The client may retry the operation at a later
time,
time of the client's choosing, such as after a change in network
attachment.

5. Once the SOA is known (either by (by virtue of being seen either in the
Answer Section, Section or in the Authority Section), the client sends a
DNS query with type SRV [RFC2782] for the record name
"_dns-push-tls._tcp.<zone>", where <zone> is the owner name of
the discovered SOA record.

6. If the zone in question is set up to offer DNS Push Notifications
Notifications, then this SRV record MUST exist. (If this SRV
record does not
exist exist, then the zone is not correctly configured
for DNS Push Notifications as specified in this document.) The
SRV "target" contains the name of the server providing DNS Push
Notifications for the zone. The port number on which to contact
the server is in the SRV record "port" field. The address(es) of
the target host MAY be included in the Additional Section,
however, the address records SHOULD be authenticated before use
as described
below in Section 7.2 and in the specification for using DANE
DNS-Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records [RFC7673], if applicable.

7. More than one SRV record may be returned. In this case, the
"priority" and "weight" values in the returned SRV records are
used to determine the order in which to contact the servers for
subscription requests. As described in the SRV specification
[RFC2782], the server with the lowest "priority" is first
contacted. If more than one server has the same "priority", the
"weight" indicates the weighted probability that the client
should contact that server. Higher weights have higher
probabilities of being selected. If a server is not willing to
accept a subscription request, or is not reachable within a
reasonable time, as determined by the client, then a subsequent
server is to be contacted.

Each time a client makes a new DNS Push Notification subscription, it
SHOULD repeat the discovery process in order to determine the
preferred DNS server for that subscription at that time. If a client
already has a DSO session with that DNS server server, the client SHOULD
reuse that existing DSO session for the new subscription, subscription; otherwise,
a new DSO session is established. The client MUST respect the DNS
TTL values on records it receives while performing the discovery
process and store them in its local cache with this lifetime (as it
will generally be do anyway for all DNS queries it performs). This
means that, as long as the DNS TTL values on the authoritative
records are set to reasonable values, repeated application of the
discovery process can be completed nearly practically instantaneously by the
client, using only locally-stored locally stored cached data.

6.2. DNS Push Notification SUBSCRIBE

After connecting, and requesting a longer idle timeout and/or
keepalive interval if necessary, a DNS Push Notification client then
indicates its desire to receive DNS Push Notifications for a given
domain name by sending a SUBSCRIBE request to the server. A
SUBSCRIBE request is encoded in a DSO message [RFC8490]. This
specification defines a primary DSO Primary TLV for DNS Push Notification
SUBSCRIBE Requests (tentatively DSO (DSO Type Code 0x40). 0x0040).

DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted
in TLS early data, provided that the precautions described in
Section 7.3 are followed.

The entity that initiates a SUBSCRIBE request is by definition the
client. A server MUST NOT send a SUBSCRIBE request over an existing
session from a client. If a server does send a SUBSCRIBE request
over a DSO session initiated by a client, this is a fatal error and
the client MUST forcibly abort the connection immediately.

Each SUBSCRIBE request generates exactly one SUBSCRIBE response from
the server. The entity that initiates a SUBSCRIBE response is by
definition the server. A client MUST NOT send a SUBSCRIBE response.
If a client does send a SUBSCRIBE response, this is a fatal error and
the server MUST forcibly abort the connection immediately.

6.2.1. SUBSCRIBE Request

A SUBSCRIBE request begins with the standard DSO 12-byte header
[RFC8490], followed by the SUBSCRIBE primary Primary TLV. A SUBSCRIBE
request is illustrated in Figure 1.

The MESSAGE ID field MUST be set to a unique value, value that the client is
not using for any other active operation on this DSO session. For
the purposes here, a MESSAGE ID is in use on this session if either
the client has used it in a request for which it has not yet received
a response, or if the client has used it for a subscription which that it
has not yet cancelled canceled using UNSUBSCRIBE. In the SUBSCRIBE response response,
the server MUST echo back the MESSAGE ID value unchanged.

The other header fields MUST be set as described in the DSO spec-
ification
specification [RFC8490]. The DNS OPCODE field contains the OPCODE
value for DNS Stateful Operations (6). The four count fields must be
zero, and the corresponding four sections must be empty (i.e.,
absent).

The DSO-TYPE is SUBSCRIBE (tentatively 0x40). (0x0040).

The DSO-LENGTH is the length of the DSO-DATA that follows, which
specifies the name, type, and class of the record(s) being sought.

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) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = SUBSCRIBE (tentatively 0x40) (0x0040) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /

Figure 1: SUBSCRIBE Request

The DSO-DATA for a SUBSCRIBE request MUST contain exactly one NAME,
TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP,
multiple SUBSCRIBE DSO request messages can be concatenated in a
single TCP stream and packed efficiently into TCP segments.

If accepted, the subscription will stay in effect until the client
cancels the subscription using UNSUBSCRIBE or until the DSO session
between the client and the server is closed.

SUBSCRIBE requests on a given session MUST be unique. A client MUST
NOT send a SUBSCRIBE message that duplicates the NAME, TYPE name, type and CLASS class
of an existing active subscription on that DSO session. For the
purpose of this matching, the established DNS case-insensitivity case insensitivity for
US-ASCII letters [RFC0020] applies (e.g., "example.com" and
"Example.com" are the same). If a server receives such a duplicate
SUBSCRIBE message, this is a fatal error and the server MUST forcibly
abort the connection immediately.

DNS wildcarding is not supported. That is, a wildcard an asterisk character
("*") in a SUBSCRIBE message matches only a literal wildcard asterisk
character ("*") in
the zone, a name and nothing else.

Aliasing is not supported. That is, Similarly, a CNAME in a
SUBSCRIBE message matches only a literal CNAME record with that name in the zone,
zone and no other records with the same owner that name.

A client may SUBSCRIBE to records that are unknown to the server at
the time of the request (providing that the name falls within one of
the zone(s) the server is responsible for) for), and this is not an error.
The server MUST NOT return NXDOMAIN in this case. The server MUST
accept these requests and send Push Notifications if and when
matching records are found in the future.

If neither TYPE nor CLASS are ANY (255) (255), then this is a specific
subscription to changes for the given NAME, TYPE name, type, and CLASS. class. If one
or both of TYPE or CLASS are ANY (255) (255), then this subscription
matches
any type all types and/or any class, all classes as appropriate.

NOTE: A little-known quirk of DNS is that in DNS QUERY requests,
QTYPE and QCLASS 255 mean "ANY" "ANY", not "ALL". They indicate that the
server should respond with ANY matching records of its choosing, not
necessarily ALL matching records. This can lead to some surprising
and unexpected results, where a query returns some valid answers answers, but
not all of them, and makes QTYPE = 255 (ANY) queries less useful than
people sometimes imagine.

When used in conjunction with SUBSCRIBE, TYPE 255 and CLASS 255
should be interpreted to mean "ALL", not "ANY". After accepting a
subscription where one or both of TYPE or CLASS are 255, the server
MUST send Push Notification Updates for ALL record changes that match
the subscription, not just some of them.

6.2.2. SUBSCRIBE Response

A SUBSCRIBE response begins with the standard DSO 12-byte header
[RFC8490]. The QR bit in the header is set indicating it is a
response. The header MAY be followed by one or more optional TLVs,
Additional TLVs such as a Retry Delay Additional TLV. A SUBSCRIBE
response is illustrated in Figure 2.

The MESSAGE ID field MUST echo the value given in the MESSAGE ID
field of the SUBSCRIBE request. This is how the client knows which
request is being responded to.

A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a
client receives a SUBSCRIBE response message containing a SUBSCRIBE
TLV
TLV, then the response message is processed but the SUBSCRIBE TLV
MUST be silently ignored.

Figure 2: SUBSCRIBE Response

In the SUBSCRIBE response response, the RCODE indicates whether or not the
subscription was accepted. Supported RCODEs are as follows:

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

+-----------+-------+-----------------------------------+
| Mnemonic | Value | Description |
+-----------+-------+-----------------------------------------------+
+===========+=======+===================================+
| NOERROR | 0 | SUBSCRIBE successful. |
+-----------+-------+-----------------------------------+
| FORMERR | 1 | Server failed to process request due to a |
| | | due to a malformed request. |
+-----------+-------+-----------------------------------+
| SERVFAIL | 2 | Server failed to process request due to a |
| | | due to a problem with the server. |
+-----------+-------+-----------------------------------+
| NOTIMP | 4 | Server does not implement DSO. |
+-----------+-------+-----------------------------------+
| REFUSED | 5 | Server refuses to process request for policy |
| | | for policy or security reasons. |
+-----------+-------+-----------------------------------+
| NOTAUTH | 9 | Server is not authoritative for the requested |
| | | the requested name. |
+-----------+-------+-----------------------------------+
| DSOTYPENI | 11 | SUBSCRIBE operation not |
| | | supported. |
+-----------+-------+-----------------------------------------------+
+-----------+-------+-----------------------------------+

Table 1: SUBSCRIBE Response codes Codes

This document specifies only these RCODE values for SUBSCRIBE
Responses. Servers sending SUBSCRIBE Responses SHOULD use one of
these values. Note that NXDOMAIN is not a valid RCODE in response to
a SUBSCRIBE Request. However, future circumstances may create
situations where other RCODE values are appropriate in SUBSCRIBE
Responses, so clients MUST be prepared to accept and handle SUBSCRIBE
Responses with any other nonzero RCODE value. error values.

If the server sends a nonzero RCODE in the SUBSCRIBE response, that
means:

a. the client is (at least partially) misconfigured, or
b. the server resources are exhausted, or
c. there is some other unknown failure on the server.

In any case, the client shouldn't retry the subscription to this
server right away. If multiple SRV records were returned as
described in Section 6.1, Paragraph 9, Item 7, a subsequent server
MAY be tried immediately.

If the client has other successful subscriptions to this server,
these subscriptions remain even though additional subscriptions may
be refused. Neither the client nor the server are is required to close
the connection, although, although either end may choose to do so.

If the server sends a nonzero RCODE RCODE, then it SHOULD append a Retry
Delay Additional TLV [RFC8490] to the response specifying a delay
before the client attempts this operation again. Recommended values
for the delay for different RCODE values are given below. These
recommended values apply both to the default values a server should
place in the Retry Delay TLV, Additional TLV and the default values a
client should assume if the server provides no Retry Delay Additional
TLV.

For RCODE = 1 (FORMERR) (FORMERR), the delay may be any value selected by
the implementer. A value of five minutes is RECOMMENDED, RECOMMENDED to reduce
the risk of high load from defective clients.

For RCODE = 2 (SERVFAIL) (SERVFAIL), the delay should be chosen according to
the level of server overload and the anticipated duration of that
overload. By default, a value of one minute is RECOMMENDED. If a
more serious server failure occurs, the delay may be longer in
accordance with the specific problem encountered.

For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
implement DNS Stateful Operations [RFC8490], it is unlikely that
the server will begin supporting DSO in the next few minutes, so
the retry delay SHOULD be one hour. Note that in such a case, a
server that doesn't implement DSO is unlikely to place a Retry
Delay Additional TLV in its response, so this recommended value in
particular applies to what a client should assume by default.

For RCODE = 5 (REFUSED), which occurs on a server that implements
DNS Push Notifications, Notifications but is currently configured to disallow DNS
Push Notifications, the retry delay may be any value selected by
the implementer and/or configured by the operator.

If the server being queried is listed in a
"_dns-push-tls._tcp.<zone>" SRV record for the zone, then this is
a misconfiguration, since this server is being advertised as
supporting DNS Push Notifications for this zone, but the server
itself is not currently configured to perform that task. Since it
is possible that the misconfiguration may be repaired at any time,
the retry delay should not be set too high. By default, a value
of 5 minutes is RECOMMENDED.

For RCODE = 9 (NOTAUTH), which occurs on a server that implements
DNS Push Notifications, Notifications but is not configured to be authoritative
for the requested name, the retry delay may be any value selected
by the implementer and/or configured by the operator.

For RCODE = 11 (DSOTYPENI), which occurs on a server that
implements DSO but doesn't implement DNS Push Notifications, it is
unlikely that the server will begin supporting DNS Push
Notifications in the next few minutes, so the retry delay SHOULD
be one hour.

For other RCODE values, the retry delay should be set by the
server as appropriate for that error condition. By default, a
value of 5 minutes is RECOMMENDED.

For RCODE = 9 (NOTAUTH), the time delay applies to requests for other
names falling within the same zone. Requests for names falling
within other zones are not subject to the delay. For all other
RCODEs
RCODEs, the time delay applies to all subsequent requests to this
server.

After sending an error response response, the server MAY allow the session to
remain open, or MAY send follow it with a DNS Push Notification DSO Retry Delay
Operation TLV operation (using
the Retry Delay Primary TLV) instructing the client to close the session,
session as described in the DSO specification [RFC8490]. Clients
MUST correctly handle both cases. Note that the DSO Retry Delay
operation (using the Retry Delay Primary TLV) is different to the
Retry Delay Additional TLV mentioned above.

6.3. DNS Push Notification Updates

Once a subscription has been successfully established, the server
generates PUSH messages to send to the client as appropriate. In the
case that the answer set was already non-empty at the moment the
subscription was established, an initial PUSH message will be sent
immediately following the SUBSCRIBE Response. Subsequent changes to
the answer set are then communicated to the client in subsequent PUSH
messages.

A client MUST NOT send a PUSH message. If a client does send a PUSH
message, or a PUSH message is sent with the QR bit set indicating
that it is a response, this is a fatal error and the receiver MUST
forcibly abort the connection immediately.

6.3.1. PUSH Message

A PUSH unidirectional message begins with the standard DSO 12-byte
header [RFC8490], followed by the PUSH primary Primary TLV. A PUSH message
is illustrated in Figure 3.

In accordance with the definition of DSO unidirectional messages, the
MESSAGE ID field MUST be zero. There is no client response to a PUSH
message.

The DSO-TYPE is PUSH (tentatively 0x41). (0x0041).

The DSO-LENGTH is the length of the DSO-DATA that follows, which
specifies the changes being communicated.

The DSO-DATA contains one or more change notifications. A PUSH
Message MUST contain at least one change notification. If a PUSH
Message is received that contains no change notifications, this is a
fatal error, error and the client MUST forcibly abort the connection
immediately.

The change notification records are formatted similarly to how DNS
Resource Records are conventionally expressed in DNS messages, as
illustrated in Figure 3, and are interpreted as described below.

The TTL field holds an unsigned 32-bit integer [RFC2181]. If the TTL
is in the range 0 to 2,147,483,647 seconds (0 to 2^31 2^(31) - 1, or
0x7FFFFFFF), then a new DNS Resource Record with the given name,
type, class class, and RDATA is added. Type and class MUST NOT be 255
(ANY). If either type or class are 255 (ANY) (ANY), this is a fatal error, error
and the client MUST forcibly abort the connection immediately. A TTL
of 0 means that this record should be retained for as long as the
subscription is active, active and should be discarded immediately the moment
the subscription is cancelled. canceled.

If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record
with the given name, type, class class, and RDATA is removed. Type and
class MUST NOT be 255 (ANY). If either type or class are 255 (ANY) (ANY),
this is a fatal error, error and the client MUST forcibly abort the
connection immediately.

If the TTL has the value 0xFFFFFFFE, then this is a 'collective'
remove notification. For collective remove notifications notifications, RDLEN MUST
be zero zero, and consequently consequently, the RDATA MUST be empty. If a change
notification is received where TTL = 0xFFFFFFFE and RDLEN is not
zero, this is a fatal error, error and the client MUST forcibly abort the
connection immediately.

There are three types of collective remove notification: notification. For
collective remove notifications, if notifications:

* If CLASS is not 255 (ANY) and TYPE is not 255 (ANY) (ANY), then for the
given name name, this removes all records of the specified type in the
specified class.

For collective remove notifications, if

* If CLASS is not 255 (ANY) and TYPE is 255 (ANY) (ANY), then for the
given name name, this removes all records of all types in the specified
class.

For collective remove notifications, if

* If CLASS is 255 (ANY), then for the given name name, this removes all
records of all types in all classes. In this case case, TYPE MUST be
set to zero on transmission, transmission and MUST be silently ignored on
reception.

Summary of change notification types:

* Remove all RRsets from a name, name in all classes classes:
TTL = 0xFFFFFFFE, RDLEN = 0, CLASS = 255 (ANY) (ANY).

* Remove all RRsets from a name, name in given class:
TTL = 0xFFFFFFFE, RDLEN = 0, CLASS gives class, TYPE = 255 (ANY) (ANY).

* Remove specified RRset from a name, name in given class:
TTL = 0xFFFFFFFE, RDLEN = 0 0,
CLASS and TYPE specify the RRset being removed removed.

* Remove an individual RR from a name:
TTL = 0xFFFFFFFF 0xFFFFFFFF,
CLASS, TYPE, RDLEN RDLEN, and RDATA specify the RR being removed removed.

* Add individual RR to a name name:
TTL >= 0 and TTL <= 0x7FFFFFFF 0x7FFFFFFF,
CLASS, TYPE, RDLEN, RDATA RDATA, and TTL specify the RR being added added.

Note that it is valid for the RDATA of an added or removed DNS
Resource Record to be empty (zero length). For example, an Address
Prefix List Resource Record [RFC3123] may have empty RDATA.
Therefore, a change notification with RDLEN = 0 does not
automatically indicate a remove notification. If RDLEN = 0 and TTL
is the in the range 0 - to 0x7FFFFFFF, this change notification signals the
addition of a record with the given name, type, class, and empty
RDATA. If RDLEN = 0 and TTL = 0xFFFFFFFF, this change notification
signals the removal specifically of that single record with the given
name, type, class, and empty RDATA.

If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value
in the range 0 - to 0x7FFFFFFF, then the receiver SHOULD silently
ignore this particular change notification record. The connection is
not terminated and other valid change notification records within
this PUSH message are processed as usual.

For efficiency, when generating a PUSH message,

In the case where a server SHOULD
include as many single change notifications as it has immediately available
to send, rather affects more than sending each change notification as a separate
DSO message. Once it has exhausted the list of change notifications
immediately available to send, a server SHOULD then send the one active
subscription, only one PUSH message immediately, rather than waiting to see if additional change
notifications become available. is sent. For efficiency, when generating example, a PUSH message, a server SHOULD use
standard DNS name compression, with offsets relative to the beginning
of the DNS
message [RFC1035]. When multiple change notifications in adding a single PUSH message have given record may match both a SUBSCRIBE request with
the same owner name, this name compression TYPE and a different SUBSCRIBE request with TYPE = 255
(ANY). It is not the case that two PUSH messages are sent because
the new record matches two active subscriptions.

The server SHOULD encode change notifications in the most efficient
manner possible. For example, when three AAAA records are removed
from a given name, and no other AAAA records exist for that name, the
server SHOULD send a "Remove specified RRset from a name in given
class" PUSH message, not three separate "Remove an individual RR from
a name" PUSH messages. Similarly, when both an SRV and a TXT record
are removed from a given name, and no other records of any kind exist
for that name in that class, the server SHOULD send a "Remove all
RRsets from a name in given class" PUSH message, not two separate
"Remove specified RRset from a name in given class" PUSH messages.

For efficiency, when generating a PUSH message, rather than sending
each change notification as a separate DSO message, a server SHOULD
include as many change notifications as it has immediately available
to send to that client, even if those change notifications apply to
different subscriptions from that client. Conceptually, a PUSH
message is a session-level mechanism, not a subscription-level
mechanism. Once it has exhausted the list of change notifications
immediately available to send to that client, a server SHOULD then
send the PUSH message immediately rather than waiting speculatively
to see if additional change notifications become available.

For efficiency, when generating a PUSH message a server SHOULD use
standard DNS name compression, with offsets relative to the beginning
of the DNS message [RFC1035]. When multiple change notifications in
a single PUSH message have the same owner name, this name compression
can yield significant savings. Name compression should be performed
as specified in Section 18.14 of the Multicast DNS specification
[RFC6762],
[RFC6762]; namely, owner names should always be compressed, and names
appearing within RDATA should be compressed for only the RR types
listed below:

NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC

Servers may generate PUSH messages up to a maximum DNS message length
of 16,382 bytes, counting from the start of the DSO 12-byte header.
Including the two-byte length prefix that is used to frame DNS over a
byte stream like TLS, this makes a total of 16,384 bytes. Servers
MUST NOT generate PUSH messages larger than this. Where the
immediately available change notifications are sufficient to exceed a
DNS message length of 16,382 bytes, the change notifications MUST be
communicated in separate PUSH messages of up to 16,382 bytes each.
DNS name compression becomes less effective for messages larger than
16,384 bytes, so little efficiency benefit is gained by sending
messages larger than this.

If a client receives a PUSH message with a DNS message length larger
than 16,382 bytes, this is a fatal error, error and the client MUST forcibly
abort the connection immediately.

1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| MESSAGE ID (MUST BE ZERO) | \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|QR| OPCODE(6) | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = PUSH (tentatively 0x41) (0x0041) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TTL | |
| (32-bit unsigned big-endian integer) | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| RDLEN (16-bit unsigned big-endian integer) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA (sized as necessary) \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
: NAME, TYPE, CLASS, TTL, RDLEN, RDATA : |
: Repeated As Necessary : /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /

Figure 3: PUSH Message

When processing the records received in a PUSH Message, the receiving
client MUST validate that the records being added or removed
correspond with at least one currently active subscription on that
session. Specifically, the record name MUST match the name given in
the SUBSCRIBE request, subject to the usual established DNS case-
insensitivity for US-ASCII letters. For individual additions and
removals, if the TYPE in the SUBSCRIBE request was not ANY (255) (255),
then the TYPE of the record must either be CNAME or match the TYPE
given in the SUBSCRIBE request, and if the CLASS in the SUBSCRIBE
request was not ANY (255) (255), then the CLASS of the record must match
the CLASS given in the SUBSCRIBE request. For collective removals,
at least one of the records being removed must match an active subscription. If a
matching active subscription on that session is not found, then that
particular addition/removal record is silently ignored. Processing
of other additions and removal records in this message is not
affected. The DSO session is not closed. This is to allow for the
unavoidable race condition where a client sends an outbound
UNSUBSCRIBE while inbound PUSH messages for that subscription from
the server are still in flight.

In the case where a single change affects more than one active
subscription, only one PUSH message is sent. For example, a PUSH
message adding a given record may match both a SUBSCRIBE request with
the same TYPE and a different SUBSCRIBE request with TYPE = 255
(ANY). It is not the case that two PUSH messages are sent because
the new record matches two active subscriptions.

The server SHOULD encode change notifications in the most efficient
manner possible. For example, when three AAAA records are removed
from a given name, and no other AAAA records exist for that name, the
server SHOULD send a "remove an RRset from a name" PUSH message, not
three separate "remove an individual RR from a name" PUSH messages.
Similarly, when both an SRV and a TXT record are removed from
subscription. If a given
name, and no matching active subscription on that session is
not found, then that particular addition/removal record is silently
ignored. The processing of other additions and removal records of any kind exist in
this message is not affected. The DSO session is not closed. This
is to allow for that name, the
server SHOULD send a "remove all RRsets from unavoidable race condition where a name" PUSH message,
not two separate "remove client sends
an RRset from a name" outbound UNSUBSCRIBE while inbound PUSH messages.

A messages for that
subscription from the server SHOULD combine multiple change notifications are still in a single
PUSH message when possible, even if those change notifications apply
to different subscriptions. Conceptually, a PUSH message is a
session-level mechanism, not a subscription-level mechanism. flight.

The TTL of an added record is stored by the client. While the
subscription is active, active the TTL is not decremented, because a change
to the TTL would produce a new update. For as long as a relevant
subscription remains active, the client SHOULD assume that when a
record goes away away, the server will notify it of that fact.
Consequently, a client does not have to poll to verify that the
record is still there. Once a subscription is cancelled canceled
(individually, or as a result of the DSO session being closed) closed),
record aging for records covered by the subscription resumes and
records are removed from the local cache when their TTL reaches zero.

6.4. DNS Push Notification UNSUBSCRIBE

To cancel an individual subscription without closing the entire DSO
session, the client sends an UNSUBSCRIBE message over the established
DSO session to the server.

The entity that initiates an UNSUBSCRIBE message is by definition the
client. A server MUST NOT send an UNSUBSCRIBE message over an
existing session from a client. If a server does send an UNSUBSCRIBE
message over a DSO session initiated by a client, or an UNSUBSCRIBE
message is sent with the QR bit set indicating that it is a response,
this is a fatal error and the receiver MUST forcibly abort the
connection immediately.

6.4.1. UNSUBSCRIBE Message

An UNSUBSCRIBE unidirectional message begins with the standard DSO
12-byte header [RFC8490], followed by the UNSUBSCRIBE primary Primary TLV.
An UNSUBSCRIBE message is illustrated in Figure 4.

In accordance with the definition of DSO unidirectional messages, the
MESSAGE ID field MUST be zero. There is no server response to an
UNSUBSCRIBE message.

The DSO-TYPE is UNSUBSCRIBE (tentatively 0x42). (0x0042).

The DSO-LENGTH field contains the value 2, the length of the 2-octet
MESSAGE ID contained in the DSO-DATA.

The DSO-DATA contains the value previously given in the MESSAGE ID
field of an active SUBSCRIBE request. This is how the server knows
which SUBSCRIBE request is being cancelled. canceled. After receipt of the
UNSUBSCRIBE message, the SUBSCRIBE request is no longer active.

It is allowable for the client to issue an UNSUBSCRIBE message for a
previous SUBSCRIBE request for which the client has not yet received
a SUBSCRIBE response. This is to allow for the case where a client
starts and stops a subscription in less than the round-trip time to
the server. The client is NOT required to wait for the SUBSCRIBE
response before issuing the UNSUBSCRIBE message.

Consequently, it is possible for a server to receive an UNSUBSCRIBE
message that does not match any currently active subscription. This
can occur when a client sends a SUBSCRIBE request, which subsequently
fails and returns an error code, but the client sent an UNSUBSCRIBE
message before it became aware that the SUBSCRIBE request had failed.
Because of this, servers MUST silently ignore UNSUBSCRIBE messages
that do not match any currently active subscription.

Figure 4: UNSUBSCRIBE Message

6.5. DNS Push Notification RECONFIRM

Sometimes, particularly when used with a Discovery Proxy [DisProx], [RFC8766], a
DNS Zone may contain stale data. When a client encounters data that
it believes may be stale (e.g., an SRV record referencing a target
host+port that is not responding to connection requests) requests), the client
can send a RECONFIRM message to ask the server to re-verify that the
data is still valid. For a Discovery Proxy, this causes it to issue
new Multicast DNS queries to ascertain whether the target device is
still present. How the Discovery Proxy causes these new Multicast
DNS queries to be issued depends on the details of the underlying
Multicast DNS implementation being used. For example, a Discovery
Proxy built on Apple's dns_sd.h API [SD-API] responds to a DNS Push
Notification RECONFIRM message by calling the underlying API's
DNSServiceReconfirmRecord() routine.

For other types of DNS server, the RECONFIRM operation is currently
undefined,
undefined and SHOULD result in a NOERROR response, but otherwise it need not
cause any other action to occur.

Frequent use of RECONFIRM operations may be a sign of network
unreliability, or some kind of misconfiguration, so RECONFIRM
operations MAY be logged or otherwise communicated to a human
administrator to assist in detecting and remedying such network
problems.

If, after receiving a valid RECONFIRM message, the server determines
that the disputed records are in fact no longer valid, then
subsequent DNS PUSH Messages will be generated to inform interested
clients. Thus, one client discovering that a previously-advertised previously advertised
device (like a network printer) is no longer present has the side
effect of informing all other interested clients that the device in
question is now gone.

The entity that initiates a RECONFIRM message is by definition the
client. A server MUST NOT send a RECONFIRM message over an existing
session from a client. If a server does send a RECONFIRM message
over a DSO session initiated by a client, or a RECONFIRM message is
sent with the QR bit set indicating that it is a response, this is a
fatal error and the receiver MUST forcibly abort the connection
immediately.

6.5.1. RECONFIRM Message

A RECONFIRM unidirectional message begins with the standard DSO
12-byte header [RFC8490], followed by the RECONFIRM primary Primary TLV. A
RECONFIRM message is illustrated in Figure 5.

In accordance with the definition of DSO unidirectional messages, the
MESSAGE ID field MUST be zero. There is no server response to a
RECONFIRM message.

The DSO-TYPE is RECONFIRM (tentatively 0x43). (0x0043).

The DSO-LENGTH is the length of the data that follows, which
specifies the name, type, class, and content of the record being
disputed.

A DNS Push Notifications RECONFIRM message contains exactly one
RECONFIRM Primary TLV. The DSO-DATA for in a RECONFIRM message Primary TLV MUST
contain exactly one record. The DSO-DATA for in a RECONFIRM message Primary TLV
has no count field to specify more than one record. Since RECONFIRM
messages are sent over TCP, multiple RECONFIRM messages can be
concatenated in a single TCP stream and packed efficiently into TCP
segments. Note that this means that DNS name compression cannot be
used between different RECONFIRM messages. However, when a client is
sending multiple RECONFIRM messages this indicates a situation with
serious network problems, and this is not expected to occur
frequently enough that optimizing efficiency in this case is
important.

TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value
ANY (255).

DNS wildcarding is not supported. That is, a wildcard an asterisk character
("*") in a RECONFIRM message matches only a literal wildcard asterisk
character ("*") in
the zone, a name and nothing else.

Aliasing is not supported. That is, Similarly, a CNAME in a
RECONFIRM message matches only a literal CNAME record with that name in the zone,
zone and no other records with the same owner that name.

Note that there is no RDLEN field, since the length of the RDATA can
be inferred from DSO-LENGTH, so an additional RDLEN field would be
redundant.

Following the same rules as for PUSH messages, DNS name compression
SHOULD be used within the RDATA of the RECONFIRM message, with
offsets relative to the beginning of the DNS message [RFC1035].

Figure 5: RECONFIRM Message

6.6. DNS Stateful Operations TLV Context Summary

This document defines four new DSO TLVs. As recommended in
Section 8.2 of the DNS Stateful Operations specification [RFC8490],
the valid contexts of these new TLV types are summarized below.

The client TLV contexts are:

C-P: Client request message, primary Primary TLV
C-U: Client unidirectional Unidirectional message, primary TLV
C-A: Client request or unidirectional message, additional Additional TLV
CRP: Response back to client, primary Primary TLV
CRA: Response back to client, additional Additional TLV

+-------------+-----+-----+-----+-----+-----+
| TLV Type | C-P | C-U | C-A | CRP | CRA |
+-------------+-----+-----+-----+-----+-----+
+=============+=====+=====+=====+=====+=====+
| SUBSCRIBE | X | | | | |
+-------------+-----+-----+-----+-----+-----+
| PUSH | | | | | |
+-------------+-----+-----+-----+-----+-----+
| UNSUBSCRIBE | | X | | | |
+-------------+-----+-----+-----+-----+-----+
| RECONFIRM | | X | | | |
+-------------+-----+-----+-----+-----+-----+

Table 2: DSO TLV Client Context Summary

The server TLV contexts are:

S-P: Server request message, primary Primary TLV
S-U: Server unidirectional Unidirectional message, primary TLV
S-A: Server request or unidirectional message, additional Additional TLV
SRP: Response back to server, primary Primary TLV
SRA: Response back to server, additional Additional TLV

+-------------+-----+-----+-----+-----+-----+
| TLV Type | S-P | S-U | S-A | SRP | SRA |
+-------------+-----+-----+-----+-----+-----+
+=============+=====+=====+=====+=====+=====+
| SUBSCRIBE | | | | | |
+-------------+-----+-----+-----+-----+-----+
| PUSH | | X | | | |
+-------------+-----+-----+-----+-----+-----+
| UNSUBSCRIBE | | | | | |
+-------------+-----+-----+-----+-----+-----+
| RECONFIRM | | | | | |
+-------------+-----+-----+-----+-----+-----+

Table 3: DSO TLV Server Context Summary

6.7. Client-Initiated Termination

An individual subscription is terminated by sending an UNSUBSCRIBE
TLV for that specific subscription, or all subscriptions can be
cancelled
canceled at once by the client closing the DSO session. When a
client terminates an individual subscription (via UNSUBSCRIBE) or all
subscriptions on that DSO session (by ending the session) session), it is
signaling to the server that it is no longer interested in receiving
those particular updates. It is informing the server that the server
may release any state information it has been keeping with regards to
these particular subscriptions.

After terminating its last subscription on a session via UNSUBSCRIBE,
a client MAY close the session immediately, immediately or it may keep it open if
it anticipates performing further operations on that session in the
future. If a client wishes to keep an idle session open, it MUST
respect the maximum idle time required by the server [RFC8490].

If a client plans to terminate one or more subscriptions on a session
and doesn't intend to keep that session open, then as an efficiency
optimization
optimization, it MAY instead choose to simply close the session,
which implicitly terminates all subscriptions on that session. This
may occur because the client computer is being shut down, is going to
sleep, the application requiring the subscriptions has terminated, or
simply because the last active subscription on that session has been
cancelled.
canceled.

When closing a session, a client should perform an orderly close of
the TLS session. Typical APIs will provide a session close method
that will send a TLS close_notify alert (see as described in Section 6.1
of the TLS 1.3 specification [RFC8446]). [RFC8446]. This instructs the recipient
that the sender will not send any more data over the session. After
sending the TLS close_notify alert alert, the client MUST gracefully close
the underlying connection using a TCP FIN, FIN so that the TLS
close_notify is reliably delivered. The mechanisms for gracefully
closing a TCP connection with a TCP FIN vary depending on the
networking API. For example, in the BSD Sockets API, sending a TCP
FIN is achieved by calling "shutdown(s,SHUT_WR)" and keeping the
socket open until all remaining data has been read from it.

If the session is forcibly closed at the TCP level by sending a RST
from either end of the connection, data may be lost.

6.8. Client Fallback to Polling

There are cases where a client may exhaust all avenues for
establishing a DNS Push Notification subscription without success.
This can happen if the client's configured recursive resolver does
not support DNS over TLS, or supports DNS over TLS but is not
listening on TCP port 853, or supports DNS over TLS on TCP port 853
but does not support DSO on that port, or for some other reason is
unable to provide a DNS Push Notification subscription. In this case
case, the client will attempt to communicate directly with an
appropriate server, and it may be that the zone apex discovery fails,
or there is no "_dns-push-tls._tcp.<zone>" SRV record, or the server
indicated in the SRV record is misconfigured, overloaded, or is
unresponsive for some other reason.

Regardless of the reason for the failure, after being unable to
establish the desired DNS Push Notification subscription, it is
likely that the client will still wish to know the answer it seeks,
even if that answer cannot be obtained with the timely change
notifications provided by DNS Push Notifications. In such cases cases, it
is likely that the client will obtain the answer it seeks via a
conventional DNS query instead, repeated at some interval to detect
when the answer RRset changes.

In the case where a client responds to its failure to establish a DNS
Push Notification subscription by falling back to polling with
conventional DNS queries instead, the polling rate should be
controlled to avoid placing excessive burden on the server. The
interval between successive DNS queries for the same name, type type, and
class SHOULD be at least the minimum of: of 900 seconds (15 minutes), minutes) or
two seconds more than the TTL of the answer RRset.

The reason that for TTLs shorter than up to 898 seconds the query should not be
reissued until two seconds *after* _after_ the answer RRset has
expired expired, is
to ensure that the answer RRset has also expired from the cache on
the client's configured recursive resolver. Otherwise (particularly
if the clocks on the client and the recursive resolver do not run at
precisely the same rate) rate), there's a risk of a race condition where
the client queries its configured recursive resolver just as the
answer RRset has one second remaining in the recursive resolver's
cache. The client would then receive a reply telling it that the answer
RRset has one second remaining, and then remaining; the client would then re-query requery the
recursive resolver again one second later
when later. If by this time the
answer RRset has actually expires, and only then would expired from the recursive resolver's
cache, the recursive resolver would then issue a new query to fetch new
fresh data from the authoritative server. Waiting until the answer
RRset has definitely expired from the the cache on the client's
configured recursive resolver avoids this race condition and any
unnecessary additional queries it causes.

Each time a client is about to reissue its query to discover changes
to the answer RRset, it should first make a new attempt to establish
a DNS Push Notification subscription, subscription using previously cached DNS
answers as appropriate. After a temporary misconfiguration has been
remedied, this allows a client that is polling to return to using DNS
Push Notifications for asynchronous notification of changes.

7. Security Considerations

The Strict Privacy Usage Profile profile for DNS over TLS is REQUIRED for DNS Push
Notifications [RFC8310]. Cleartext connections for DNS Push
Notifications are not permissible. Since this is a new protocol,
transition mechanisms from the Opportunistic Privacy profile are
unnecessary.

Also, see Section 9 of the document Usage Profiles for DNS over
(D)TLS Usage Profiles document [RFC8310] for additional recommendations for various versions
of TLS usage.

As a consequence of requiring TLS, client certificate authentication
and verification may also be enforced by the server for stronger
client-server security or end-to-end security. However,
recommendations for security in particular deployment scenarios are
outside the scope of this document.

DNSSEC is RECOMMENDED for the authentication of DNS Push Notification
servers. TLS alone does not provide complete security. TLS
certificate verification can provide reasonable assurance that the
client is really talking to the server associated with the desired
host name, but since the desired host name is learned via a DNS SRV
query, if the SRV query is subverted subverted, then the client may have a
secure connection to a rogue server. DNSSEC can provide added
confidence that the SRV query has not been subverted.

7.1. Security Services

It is the goal of using TLS to provide the following security
services:

Confidentiality: All application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.

Data integrity protection: Any changes made to the communication in
transit are detectable by the receiver.

Authentication: An end-point endpoint of the TLS communication is
authenticated as the intended entity to communicate with.

Anti-replay protection: TLS provides for the detection of and
prevention against messages sent previously over a TLS connection
(such as DNS Push Notifications). If prior messages are re-sent
at a later time as a form of a man-in-the-middle attack attack, then the
receiver will detect this and reject the replayed messages.

Deployment recommendations on the appropriate key lengths and cypher cipher
suites are beyond the scope of this document. Please refer to the
current TLS Recommendations [BCP195] for the best current practices.
Keep in mind that best practices only exist for a snapshot in time time,
and recommendations will continue to change. Updated versions or
errata may exist for these recommendations.

7.2. TLS Name Authentication

As described in Section 6.1, the client discovers the DNS Push
Notification server using an SRV lookup for the record name
"_dns-push-tls._tcp.<zone>". The server connection endpoint SHOULD
then be authenticated using DANE TLSA records for the associated SRV
record. This associates the target's name and port number with a
trusted TLS certificate [RFC7673]. This procedure uses the TLS
Server Name Indication (SNI) extension [RFC6066] to inform the server
of the name the client has authenticated through the use of TLSA
records. Therefore, if the SRV record passes DNSSEC validation and a
TLSA record matching the target name is useable, usable, an SNI extension must
be used for the target name to ensure the client is connecting to the
server it has authenticated. If the target name does not have a
usable TLSA record, then the use of the SNI extension is optional.
See Usage Profiles for DNS over TLS and DNS over DTLS [RFC8310] for
more information on authenticating domain names.

7.3. TLS Early Data

DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted
in TLS early data. Using TLS early data can save one network round
trip,
trip and can result in the client obtaining results faster.

However, there are some factors to consider before using TLS early
data.

TLS Early Data early data is not forward secret. In cases where forward secrecy
of DNS Push Notification subscriptions is required, the client should
not use TLS Early Data. early data.

With TLS early data data, there are no guarantees of non-replay between
connections. If packets are duplicated and delayed in the network,
the later arrivals could be mistaken for new subscription requests.
Generally
Generally, this is not a major concern, concern since the amount of state
generated on the server for these spurious subscriptions is small and
short-lived,
short lived since the TCP connection will not complete the three-way
handshake. Servers MAY choose to implement rate-limiting measures
that are activated when the server detects an excessive number of
spurious subscription requests.

For further guidance on use of TLS early data, please see discussion
of zero round-trip data
(Section 2.3, Section in Sections 2.3 and 8, and Appendix E.5) in E.5, of
the TLS 1.3
specification, specification [RFC8446].

7.4. TLS Session Resumption

TLS Session Resumption session resumption [RFC8446] is permissible on DNS Push
Notification servers. However, closing the TLS connection terminates
the DSO session. When the TLS session is resumed, the DNS Push
Notification server will not have any subscription state and will
proceed as with any other new DSO session. Use of TLS Session
Resumption session
resumption may allow a TLS connection to be set up more quickly, but
the client will still have to recreate any desired subscriptions.

8. IANA Considerations

This document defines a new service name, only applicable for the TCP
protocol, to be which has been recorded in the IANA Service Type Registry
[RFC6335][SRVTYPE].

+-----------------------+------+----------------------+-------------+ "Service Name and
Transport Protocol Port Number Registry" [RFC6335] [SRVTYPE].

Table 4: IANA Service Type Assignments

This document defines four new DNS Stateful Operation TLV types to be types,
which have been recorded in the IANA DSO "DSO Type Code Registry [RFC8490][DSOTYPE].

+-------------+------------+--------+-----------------+-------------+ Codes" registry
[RFC8490] [DSOTYPE].

Table 5: IANA DSO TLV Type Code Assignments

This document defines no new DNS OPCODEs or RCODEs.

9. Acknowledgements

The authors would like to thank Kiren Sekar and Marc Krochmal for
previous work completed in this field.

This draft has been improved due to comments from Ran Atkinson, Tim
Chown, Sara Dickinson, Mark Delany, Ralph Droms, Jan Komissar, Eric
Rescorla, Michael Richardson, David Schinazi, Manju Shankar Rao,
Robert Sparks, Markus Stenberg, Andrew Sullivan, Michael Sweet, Dave
Thaler, Brian Trammell, Bernie Volz, Eric Vyncke, Christopher Wood,
Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text
that was greatly appreciated.

10. NO | Standards Track | 6.5 |
+-------------+--------+------------+-----------------+---------+

Table 5: IANA DSO TLV Type Code Assignments

This document defines no new DNS OPCODEs or RCODEs.

9. References

10.1.

9.1. Normative References

[DSOTYPE] "DSO Type Code Registry", IANA, "Domain Name System (DNS) Parameters",
<https://www.iana.org/assignments/dns-parameters/>.

[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[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>.

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.

[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>.

[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.

[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.

[RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
2015, <https://www.rfc-editor.org/info/rfc7673>.

[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>.

[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>.

[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>.

[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.

[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>.

[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.

[SRVTYPE] IANA, "Service Name and Transport Protocol Port Number
Registry", <http://www.iana.org/assignments/service-names-
port-numbers/>.

10.2. <https://www.iana.org/assignments/service-
names-port-numbers/>.

9.2. Informative References

[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015,
<http://www.rfc-editor.org/info/bcp195>.

[DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", draft-ietf-dnssd-hybrid-10 (work in
progress), March 2019.

[I-D.ietf-tcpm-rack]
Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
a time-based fast loss detection algorithm for TCP",
draft-ietf-tcpm-rack-05 (work in progress), April 2019.

[LLQ] Cheshire, S. and M. Krochmal, "DNS Long-Lived Queries",
draft-sekar-dns-llq-03 (work in progress), March 2019.

[obs]
<https://www.rfc-editor.org/info/bcp195>.

[OBS] Wikipedia, "Observer Pattern",
<https://en.wikipedia.org/wiki/Observer_pattern>. pattern", February 2020,
<https://en.wikipedia.org/w/
index.php?title=Observer_pattern&oldid=939702131>.

[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.

[RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes
(APL RR)", RFC 3123, DOI 10.17487/RFC3123, June 2001,
<https://www.rfc-editor.org/info/rfc3123>.

[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.

[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>.

[RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
"Understanding Apple's Back to My Mac (BTMM) Service",
RFC 6281, DOI 10.17487/RFC6281, June 2011,
<https://www.rfc-editor.org/info/rfc6281>.

[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>.

[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.

[RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol
(NAT-PMP)", RFC 6886, DOI 10.17487/RFC6886, April 2013,
<https://www.rfc-editor.org/info/rfc6886>.

[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.

[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>.

[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.

[RFC8010] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Encoding and Transport", STD 92, RFC 8010,
DOI 10.17487/RFC8010, January 2017,
<https://www.rfc-editor.org/info/rfc8010>.

[RFC8011] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Model and Semantics", STD 92, RFC 8011,
DOI 10.17487/RFC8011, January 2017,
<https://www.rfc-editor.org/info/rfc8011>.

[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.

[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.

[RFC8764] Cheshire, S. and M. Krochmal, "Apple's DNS Long-Lived
Queries Protocol", RFC 8764, DOI 10.17487/RFC8764, June
2020, <https://www.rfc-editor.org/info/rfc8764>.

[RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
2020, <https://www.rfc-editor.org/info/rfc8766>.

[SD-API] "dns_sd.h API", Apple Inc., "dns_sd.h",
<https://opensource.apple.com/source/mDNSResponder/
mDNSResponder-878.70.2/mDNSShared/dns_sd.h.auto.html>.

[SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The
Internet Protocol Journal, Cisco Systems, Volume 9, Number
4, December 2006. 2006,
<https://www.cisco.com/web/about/ac123/ac147/
archived_issues/ipj_9-4/ipj_9-4.pdf>.

[TCPRACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
a time-based fast loss detection algorithm for TCP", Work
in Progress, Internet-Draft, draft-ietf-tcpm-rack-08, 9
March 2020,
<https://tools.ietf.org/html/draft-ietf-tcpm-rack-08>.

[XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, July 2010. October 2019,
<https://xmpp.org/extensions/xep-0060.html>.

Acknowledgments

The authors would like to thank Kiren Sekar and Marc Krochmal for
previous work completed in this field.

This document has been improved due to comments from Ran Atkinson,
Tim Chown, Sara Dickinson, Mark Delany, Ralph Droms, Jan Komissar,
Eric Rescorla, Michael Richardson, David Schinazi, Manju Shankar Rao,
Robert Sparks, Markus Stenberg, Andrew Sullivan, Michael Sweet, Dave
Thaler, Brian Trammell, Bernie Volz, Éric Vyncke, Christopher Wood,
Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text
that was greatly appreciated.

Authors' Addresses

Tom Pusateri
Unaffiliated
Raleigh, NC 27608
USA
United States of America

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

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

Phone: +1 (408) 996-1010
Email: cheshire@apple.com