wdiff rfc8547.original rfc8547.txt

Network Working Group
Internet Engineering Task Force (IETF) A. Bittau
Internet-Draft
Request for Comments: 8547 Google
Intended status:
Category: Experimental D. Giffin
Expires: December 31, 2018
ISSN: 2070-1721 Stanford University
M. Handley
University College London
D. Mazieres
Stanford University
E. Smith
Kestrel Institute
June 29, 2018
May 2019

TCP-ENO: Encryption Negotiation Option
draft-ietf-tcpinc-tcpeno-19

Abstract

Despite growing adoption of TLS, a significant fraction of TCP
traffic on the Internet remains unencrypted. The persistence of
unencrypted traffic can be attributed to at least two factors.
First, some legacy protocols lack a signaling mechanism (such as a
"STARTTLS"
STARTTLS command) by which to convey support for encryption, thus
making incremental deployment impossible. Second, legacy
applications themselves cannot always be upgraded, requiring upgraded and therefore
require a way to implement encryption transparently entirely within
the transport layer. The TCP Encryption Negotiation Option (TCP-ENO)
addresses both of these problems through a new TCP option-kind option kind
providing out-of-band, fully backward-compatible negotiation of
encryption.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.

Internet-Drafts are working documents
evaluation.

This document defines an Experimental Protocol for the Internet
community. 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 the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents valid approved by the IESG are candidates for a maximum any level of
Internet Standard; see 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 December 31, 2018.
https://www.rfc-editor.org/info/rfc8547.

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. Requirements language Introduction . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . 3
1.1. Design Goals . . . . . . . . . . . . . . . . . . . . 3
2.1. Design goals . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. TCP-ENO Specification . . . . . . . . . . . . . . . . . . . . 5
4.1. ENO Option . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. The Global Suboption . . . . . . . . . . . . . . . . . . 8
4.3. TCP-ENO Roles . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Specifying Suboption Data Length . . . . . . . . . . . . 10
4.5. The Negotiated TEP . . . . . . . . . . . . . . . . . . . 11
4.6. TCP-ENO Handshake . . . . . . . . . . . . . . . . . . . . 12
4.7. Data in SYN Segments . . . . . . . . . . . . . . . . . . 13
4.8. Negotiation Transcript . . . . . . . . . . . . . . . . . 15
5. Requirements for TEPs . . . . . . . . . . . . . . . . . . . . 15
5.1. Session IDs . . . . . . . . . . . . . . . . . . . . . . . 16
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Future Developments . . . . . . . . . . . . . . . . . . . . . 20
8. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Handshake Robustness . . . . . . . . . . . . . . . . . . 21
8.2. Suboption Data . . . . . . . . . . . . . . . . . . . . . 21
8.3. Passive Role Bit . . . . . . . . . . . . . . . . . . . . 21
8.4. Application-aware Application-Aware Bit . . . . . . . . . . . . . . . . . . 22
8.5. Use of ENO Option Kind by TEPs . . . . . . . . . . . . . 23
8.6. Unpredictability of Session IDs . . . . . . . . . . . . . 23
9. Experiments . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
12. Acknowledgments References . . . . . . . . . . . . . . . . . . . . . . . 27
13. Contributors . . 26
12.1. Normative References . . . . . . . . . . . . . . . . . . 26
12.2. Informative References . . . . 27
14. References . . . . . . . . . . . . . 27
Acknowledgments . . . . . . . . . . . . 27
14.1. Normative References . . . . . . . . . . . . . 28
Contributors . . . . . . . . . 27
14.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29

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.

2. Introduction

Many applications and protocols running on top of TCP today do not
encrypt traffic. This failure to encrypt lowers the bar for certain
attacks, harming both user privacy and system security.
Counteracting the problem demands a minimally intrusive, backward-
compatible mechanism for incrementally deploying encryption. The TCP
Encryption Negotiation Option (TCP-ENO) specified in this document
provides such a mechanism.

Introducing TCP options, extending operating system interfaces to
support TCP-level encryption, and extending applications to take
advantage of TCP-level encryption all require effort. To the
greatest extent possible, the effort invested in realizing TCP-level
encryption today needs to remain applicable in the future should the
need arise to change encryption strategies. To this end, it is
useful to consider two questions separately:

1. How to negotiate the use of encryption at the TCP layer, and layer

2. How to perform encryption at the TCP layer. layer

This document addresses question 1 with a new TCP option, ENO. TCP-
ENO
TCP-ENO provides a framework in which two endpoints can agree on a
TCP encryption protocol (_TEP_) (TEP) out of multiple possible TEPs. For
future compatibility, TEPs can vary widely in terms of wire format,
use of TCP option space, and integration with the TCP header and
segmentation. However, ENO abstracts these differences to ensure the
introduction of new TEPs can be transparent to applications taking
advantage of TCP-level encryption.

Question 2 is addressed by one or more companion TEP specification
documents. While current TEPs enable TCP-level traffic encryption
today, TCP-ENO ensures that the effort invested to deploy today's
TEPs will additionally benefit future ones.

2.1.

1.1. Design goals Goals

TCP-ENO was designed to achieve the following goals:

1. Enable endpoints to negotiate the use of a separately specified
TCP encryption protocol (_TEP_) (TEP) suitable for either opportunistic
security [RFC7435] of arbitrary TCP communications or stronger
security of applications willing to perform endpoint
authentication.

2. Transparently fall back to unencrypted TCP when not supported by
both endpoints.

3. Provide out-of-band signaling through which applications can
better take advantage of TCP-level encryption (for instance, by
improving authentication mechanisms in the presence of TCP-level
encryption).

4. Define a standard negotiation transcript that TEPs can use to
defend against tampering with TCP-ENO.

5. Make parsimonious use of TCP option space.

6. Define roles for the two ends of a TCP connection, so as to name
each end of a connection for encryption or authentication
purposes even following a symmetric simultaneous open.

3. Terminology

Throughout

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, we use the following terms, several 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

Throughout this document, we use the following terms, several of
which have more detailed normative descriptions in [RFC0793]:

SYN segment
A TCP segment in which the SYN flag is set

ACK segment
A TCP segment in which the ACK flag is set (which includes most
segments other than an initial SYN segment)

non-SYN

Non-SYN segment
A TCP segment in which the SYN flag is clear

SYN-only segment
A TCP segment in which the SYN flag is set but the ACK flag is
clear

SYN-ACK segment
A TCP segment in which the SYN and ACK flags are both set

Active opener
A host that initiates a connection by sending a SYN-only segment.
With the BSD socket API, an active opener calls "connect". In
client-server configurations, active openers are typically
clients.

Passive opener
A host that does not send a SYN-only segment, segment but responds to one
with a SYN-ACK segment. With the BSD socket API, passive openers
call "listen" and "accept", rather than "connect". In client-
server configurations, passive openers are typically servers.

Simultaneous open
The act of symmetrically establishing a TCP connection between two
active openers (both of which call "connect" with BSD sockets).
Each host of a simultaneous open sends both a SYN-only and a SYN-
ACK segment. Simultaneous open is less common than asymmetric
open with one active and one passive opener, but it can be used
for NAT traversal by peer-to-peer applications [RFC5382].

TEP
A TCP encryption protocol intended for use with TCP-ENO and
specified in a separate document. document

TEP identifier
A unique 7-bit value in the range 0x20-0x7f that IANA has assigned
to a TEP. TEP

Negotiated TEP
The single TEP governing a TCP connection, determined by use of
the TCP ENO option specified in this document. document

4. TCP-ENO Specification

TCP-ENO extends TCP connection establishment to enable encryption
opportunistically. It uses a new TCP option-kind option kind [RFC0793] to
negotiate one among multiple possible TCP encryption protocols
(TEPs). The negotiation involves hosts exchanging sets of supported
TEPs, where each TEP is represented by a _suboption_ suboption within a larger
TCP ENO option in the offering host's SYN segment.

If TCP-ENO succeeds, it yields the following information:

o A a negotiated TEP, TEP represented by a unique 7-bit TEP identifier,

o A a few extra bytes of suboption data from each host, if needed by
the TEP,
o A a negotiation transcript with which to mitigate attacks on the
negotiation itself,

o Role role assignments designating one endpoint "host A" and the other
endpoint "host B", and

o A a bit available to higher-layer protocols at each endpoint for
out-of-band negotiation of updated behavior in the presence of TCP
encryption.

If TCP-ENO fails, encryption is disabled and the connection falls
back to traditional unencrypted TCP.

The remainder of this section provides the normative description of
the TCP ENO option and handshake protocol.

4.1. ENO Option

TCP-ENO employs an option in the TCP header [RFC0793]. Figure 1
illustrates the high-level format of this option.

byte 0 1 2 N+1 (N+2 bytes total)
+-----+-----+-----+--....--+-----+
|Kind=|Len= | |
| TBD 69 | N+2 | contents (N bytes) |
+-----+-----+-----+--....--+-----+

Figure 1: The TCP-ENO option Option

The contents of an ENO option can take one of two forms. A SYN form, SYN-form
ENO option, illustrated in Figure 2, appears only in SYN segments. A non-SYN
form,
non-SYN-form ENO option, illustrated in Figure 3, appears only in
non-SYN segments. The
SYN form of SYN-form ENO option acts as a container for
zero or more suboptions, labeled "Opt_0", "Opt_1", ... in Figure 2.
The non-SYN form, non-SYN-form ENO option, by its presence, acts as a one-bit
acknowledgment, with the actual contents ignored by ENO. Particular
TEPs MAY assign additional meaning to the contents of non-SYN non-SYN-form
ENO options. When a negotiated TEP does not assign such meaning, the
contents of a non-SYN non-SYN-form ENO option MUST be zero bytes (i.e., N =
0) in sent segments and MUST be ignored in received segments.

byte 0 1 2 3 ... N+1
+-----+-----+-----+-----+--...--+-----+----...----+
|Kind=|Len= |Opt_0|Opt_1| |Opt_i| Opt_i |
| TBD 69 | N+2 | | | | | data |
+-----+-----+-----+-----+--...--+-----+----...----+

Figure 2: SYN form of SYN-Form ENO Option
byte 0 1 2 N+1
+-----+-----+-----...----+
|Kind=|Len= | ignored |
| TBD 69 | N+2 | by TCP-ENO |
+-----+-----+-----...----+

Figure 3: Non-SYN form of ENO, where Non-SYN-Form ENO option, Where N MAY be Be 0

Every suboption starts with a byte of the form illustrated in
Figure 4. The high bit "v", when set, introduces suboptions with
variable-length data. When "v v = 0", 0, the byte itself constitutes the
entirety of the suboption. The remaining 7-bit value, called "glt",
takes on various meanings, meanings as defined below:

o Global configuration data (discussed in Section 4.2), 4.2)

o Suboption data length for the next suboption (discussed in
Section 4.4), or 4.4)

o An offer to use a particular TEP defined in a separate TEP
specification document. document

bit 7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
| v | glt |
+---+---+---+---+---+---+---+---+

v - non-zero for use with variable-length suboption data
glt - Global suboption, Length, or TEP identifier

Figure 4: Format of initial suboption byte Initial Suboption Byte

Table 1 summarizes the meaning of initial suboption bytes. Values of
"glt"
glt below 0x20 are used for global suboptions and length information
(the "gl" in "glt"), while those greater than or equal to 0x20 are
TEP identifiers (the "t"). When "v v = 0", 0, since the initial suboption
byte constitutes the entirety of the suboption and suboption, all information is
expressed by the 7-bit "glt" glt value, which can be either a global
suboption or a TEP identifier. When "v v = 1", 1, it indicates a suboption
with variable-length suboption data. Only TEP identifiers have
suboption data, not global suboptions. Hence, Therefore, bytes with "v v = 1" 1
and "glt glt < 0x20" 0x20 are not global suboptions but rather length bytes
governing the length of the next suboption (which MUST be a TEP
identifier). In the absence of a length byte, a TEP identifier
suboption with "v v = 1" 1 has suboption data extending to the end of the
TCP option.

+-----------+---+-------------------------------------------+
| glt | v | Meaning |
+-----------+---+-------------------------------------------+
| 0x00-0x1f | 0 | Global suboption (Section 4.2) |
| 0x00-0x1f | 1 | Length byte (Section 4.4) |
| 0x20-0x7f | 0 | TEP identifier without suboption data |
| 0x20-0x7f | 1 | TEP identifier followed by suboption data |
+-----------+---+-------------------------------------------+

Table 1: Initial suboption byte values Suboption Byte Values

A SYN segment MUST contain at most one TCP ENO option. If a SYN
segment contains more than one ENO option, the receiver MUST behave
as though the segment contained no ENO options and disable
encryption. A TEP MAY specify the use of multiple ENO options in a
non-SYN segment. For non-SYN segments, ENO itself only distinguishes
between the presence or absence of ENO options; multiple ENO options
are interpreted the same as one.

4.2. The Global Suboption

Suboptions 0x00-0x1f are used for global configuration that applies
regardless of the negotiated TEP. A TCP SYN segment MUST include at
most one ENO suboption in this range. A receiver MUST ignore all but
the first suboption in this range in any given TCP segment so as to
anticipate updates to ENO that assign new meaning to bits in
subsequent global suboptions. The value of a global suboption byte
is interpreted as a bitmask, bit mask, illustrated in Figure 5.

bit 7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 |z1 |z2 |z3 | a | b |
+---+---+---+---+---+---+---+---+

b - Passive role bit
a - Application-aware bit
z* - Zero bits (reserved for future use)

Figure 5: Format of the global suboption byte Global Suboption Byte

The fields of the bitmask bit mask are interpreted as follows:

b
The passive role bit MUST be 1 for all passive openers. For
active openers, it MUST default to 0, but implementations MUST
provide an API through which an application can explicitly set "b b = 1"
1 before initiating an active open. (Manual configuration of "b"
is only necessary to enable encryption with a simultaneous
open, open
and requires prior coordination to ensure exactly one endpoint
sets "b b = 1" 1 before connecting.) See Section 8.3 for further
discussion.

a
Legacy applications can benefit from ENO-specific updates that
improve endpoint authentication or avoid double encryption. The
application-aware bit "a" is an out-of-band signal through which
higher-layer protocols can enable ENO-specific updates that would
otherwise not be backwards-compatible. backwards compatible. Implementations MUST set
this bit to 0 zero by default, and MUST provide an API through which
applications can change the value of the bit as well as examine
the value of the bit sent by the remote host. Implementations
MUST furthermore support a _mandatory_ mandatory application-aware mode in
which TCP-ENO is automatically disabled if the remote host does
not set "a a = 1". 1. See Section 8.4 for further discussion.

z1, z2, z3
The "z" bits are reserved for future updates to TCP-ENO. They
MUST be set to zero in sent segments and MUST be ignored in
received segments.

A SYN segment without an explicit global suboption has an implicit
global suboption of 0x00. Because passive openers MUST always set "b b
= 1", 1, they cannot rely on this implicit 0x00 byte and MUST include an
explicit global suboption in their SYN-ACK segments.

4.3. TCP-ENO Roles

TCP-ENO uses abstract roles called "A" and "B" to distinguish the two
ends of a TCP connection. These roles are determined by the "b" bit
in the global suboption. The host that sent an implicit or explicit
suboption with "b b = 0" 0 plays the A role. The host that sent "b b = 1" 1
plays the B role. Because a passive opener MUST set "b b = 1" 1 and an
active opener by default has "b b = 0", 0, the normal case is for the active
opener to play role A and the passive opener role B.

Applications performing a simultaneous open, if they desire TCP-level
encryption, need to arrange for exactly one endpoint to set "b b = 1" 1
(despite being an active opener) while the other endpoint keeps the
default "b b = 0". 0. Otherwise, if both sides use the default "b b = 0" 0 or if
both sides set "b b = 1", 1, then TCP-ENO will fail and fall back to
unencrypted TCP. Likewise, if an active opener explicitly configures
"b
b = 1" 1 and connects to a passive opener (which MUST always have "b b =
1"),
1), then TCP-ENO will fail and fall back to unencrypted TCP.

TEP specifications SHOULD refer to TCP-ENO's A and B roles to specify
asymmetric behavior by the two hosts. For the remainder of this
document, we will use the terms "host A" and "host B" to designate
the hosts with roles A and B, respectively, in a connection.

4.4. Specifying Suboption Data Length

A TEP MAY optionally make use of one or more bytes of suboption data.
The presence of such data is indicated by setting "v v = 1" 1 in the
initial suboption byte (see Figure 4). A suboption introduced by a
TEP identifier with "v v = 1" 1 (i.e., a suboption whose first octet has
value 0xa0 or higher) extends to the end of the TCP option. Hence,
if only one suboption requires data, the most compact way to encode
it is to place it last in the ENO option, after all other suboptions.
As an example, in
In Figure 2, for example, the last suboption, "Opt_i", Opt_i, has suboption
data and thus requires "v v = 1"; however, 1. However, the suboption data length is
inferred from the total length of the TCP option.

When a suboption with data is not last in an ENO option, the sender
MUST explicitly specify the suboption data length for the receiver to
know where the next suboption starts. The sender does so by
introducing the suboption with a length byte, depicted in Figure 6.
The length byte encodes a 5-bit value "nnnnn". nnnnn. Adding one to "nnnnn" nnnnn
yields the length of the suboption data (not including the length
byte or the TEP identifier). Hence, a length byte can designate
anywhere from 1 to 32 bytes of suboption data (inclusive).

bit 7 6 5 4 3 2 1 0
+---+---+---+-------------------+
| 1 0 0 nnnnn |
+---+---+---+-------------------+

nnnnn - 5-bit value encoding (length - 1)

Figure 6: Format of a length byte Length Byte

A suboption preceded by a length byte MUST be a TEP identifier ("glt (glt
>= 0x20") 0x20) and MUST have "v v = 1". 1. Figure 7 shows an example of such a
suboption.

length byte - specifies nnnnn
TEP identifier - MUST have v = 1 and glt >= 0x20
suboption data - length specified by nnnnn+1

Figure 7: Suboption with length byte Length Byte

A host MUST ignore an ENO option in a SYN segment and MUST disable
encryption if either: either of the following apply:

1. A length byte indicates that suboption data would extend beyond
the end of the TCP ENO option, or option.

2. A length byte is followed by an octet in the range 0x00-0x9f
(meaning the following byte has "v v = 0" 0 or "glt glt < 0x20"). 0x20).

Because the last suboption in an ENO option is special-cased to have
its length inferred from the 8-bit TCP option length, it MAY contain
more than 32 bytes of suboption data. Other suboptions are limited
to 32 bytes by the length byte format. The However, the TCP header
itself can only accommodate a maximum of 40 bytes of options, however. Hence, options.
Therefore, regardless of the length byte format, a segment would not
be able to contain more than one suboption over 32 bytes in size.
That said, TEPs MAY define the use of multiple suboptions with the
same TEP identifier in the same SYN segment, providing another way to
convey over 32 bytes of suboption data even with length bytes.

4.5. The Negotiated TEP

A TEP identifier "glt" glt (with "glt glt >= 0x20") 0x20) is _valid_ valid for a connection when:
when all of the following hold:

1. Each side has sent a suboption for "glt" glt in its SYN-form ENO
option,
option.

2. Any suboption data in these "glt" glt suboptions is valid according to
the TEP specification and satisfies any runtime constraints,
and constraints.

3. If an ENO option contains multiple suboptions with "glt", glt, then such
repetition is well-defined by the TEP specification.

A passive opener (which is always host B) sees the remote host's SYN
segment before constructing its own SYN-ACK segment. Hence, Therefore, a
passive opener SHOULD include only one TEP identifier in SYN-ACK
segments and SHOULD ensure this TEP identifier is valid. However,
simultaneous open or implementation considerations can prevent host B
from offering only one TEP.

To accommodate scenarios in which host B sends multiple TEP
identifiers in the SYN-ACK segment, the _negotiated TEP_ negotiated TEP is defined as
the last valid TEP identifier in host B's SYN-form ENO option. This
definition means host B specifies TEP suboptions in order of
increasing priority, while host A does not influence TEP priority.

4.6. TCP-ENO Handshake

A host employing TCP-ENO for a connection MUST include an ENO option
in every TCP segment sent until either encryption is disabled or the
host receives a non-SYN segment. In particular, this means an active
opener MUST include a non-SYN-form ENO option in the third segment of
a three-way handshake.

A host MUST disable encryption, refrain from sending any further ENO
options, and fall back to unencrypted TCP if any of the following
occurs:

1. Any segment it receives up to and including the first received
ACK segment does not contain a an ENO option (or contains an ill-
formed SYN-form ENO option), option).

2. The SYN segment it receives does not contain a valid TEP
identifier, or
identifier.

3. It receives a SYN segment with an incompatible global suboption.
(Specifically, incompatible "incompatible" means the two hosts set the same
"b"
value value, or the connection is in mandatory application-aware
mode and the remote host set "a a = 0".) 0.)

Hosts MUST NOT alter SYN-form ENO options in retransmitted segments,
or between the SYN and SYN-ACK segments of a simultaneous open, with
two exceptions for an active opener. First, an active opener MAY
unilaterally disable ENO (and thus remove the ENO option) between
retransmissions of a SYN-only segment. (Such removal could enable
recovery from middleboxes dropping segments with ENO options.)
Second, an active opener performing simultaneous open MAY include no
TCP-ENO option in its SYN-ACK if the received SYN caused it to
disable encryption according to the above rules (for instance instance,
because role negotiation failed).

Once a host has both sent and received an ACK segment containing an
ENO option, encryption MUST be enabled. Once encryption is enabled,
hosts MUST follow the specification of the negotiated TEP and MUST
NOT present raw TCP payload data to the application. In particular,
data segments MUST NOT contain plaintext application data, but rather
ciphertext, key negotiation parameters, or other messages as
determined by the negotiated TEP.

A host MAY send a SYN-form ENO option containing zero TEP identifier
suboptions, which we term a _vacuous_ "vacuous" ENO option. If either host's
SYN segment contains a vacuous ENO option, it follows that there are
no valid TEP identifiers for the connection connection, and hence therefore the
connection MUST fall back to unencrypted TCP. Hosts MAY send vacuous
ENO options to indicate that ENO is supported but unavailable by
configuration, or to probe network paths for robustness to ENO
options. However, a passive opener MUST NOT send a vacuous ENO
option in a SYN-ACK segment unless there was an ENO option in the SYN
segment it received. Moreover, a passive opener's SYN-form ENO
option MUST still include a global suboption with "b b = 1", 1 as discussed
in Section 4.3.

4.7. Data in SYN Segments

TEPs MAY specify the use of data in SYN segments so as to reduce the
number of round trips required for connection setup. The meaning of
data in a SYN segment with an ENO option (a SYN+ENO segment) is
determined by the last TEP identifier in the ENO option, which we
term the segment's _SYN TEP_. "SYN TEP". A SYN+ENO segment MAY of course
include multiple TEP suboptions, but only the SYN TEP (i.e., the last
one) specifies how to interpret the SYN segment's data payload.

A host sending a SYN+ENO segment MUST NOT include data in the segment
unless the SYN TEP's specification defines the use of such data.
Furthermore, to avoid conflicting interpretations of SYN data, a
SYN+ENO segment MUST NOT include a non-empty TCP Fast Open (TFO)
option [RFC7413].

Because a host can send SYN data before knowing which if any TEP the
connection will negotiate, hosts implementing ENO are REQUIRED to
discard data from SYN+ENO segments when the SYN TEP does not become
the negotiated TEP. Hosts are furthermore REQUIRED to discard SYN
data in cases where another Internet standard specifies a conflicting
interpretation of SYN data (as would occur when receiving a non-empty
TFO option). This requirement applies to hosts that implement ENO
even when ENO has been disabled by configuration. However, note that
discarding SYN data is already common practice [RFC4987] and the new
requirement applies only to segments containing ENO options.

More specifically, a host that implements ENO MUST discard the data
in a received SYN+ENO segment if any of the following applies:

o ENO fails and TEP-indicated encryption is disabled for the
connection,
connection.

o The received segment's SYN TEP is not the negotiated TEP, TEP.

o The negotiated TEP does not define the use of SYN data, or data.

o The SYN segment contains a non-empty TFO option or any other TCP
option implying a conflicting definition of SYN data.

A host discarding SYN data in compliance with the above requirement
MUST NOT acknowledge the sequence number of the discarded data, but
rather MUST acknowledge the other host's initial sequence number as
if the received SYN segment contained no data. Furthermore, after
discarding SYN data, such a host MUST NOT assume the SYN data will be
identically retransmitted, and MUST process data only from non-SYN
segments.

If a host sends a SYN+ENO segment with data and receives
acknowledgment for the data, but the SYN TEP in its transmitted SYN
segment is not the negotiated TEP (either because a different TEP was
negotiated or because ENO failed to negotiate encryption), then the
host MUST abort the TCP connection. Proceeding in any other fashion
risks misinterpreted SYN data.

If a host sends a SYN-only SYN+ENO segment bearing data and
subsequently receives a SYN-ACK segment without an ENO option, that
host MUST abort the connection even if the SYN-ACK segment does not
acknowledge the SYN data. The issue is that unacknowledged data
could nonetheless have been cached by the receiver; later
retransmissions intended to supersede this unacknowledged data could
fail to do so if the receiver gives precedence to the cached original
data. Implementations MAY provide an API call for a non-default mode
in which unacknowledged SYN data does not cause a connection abort,
but applications MUST use this mode only when a higher-layer
integrity check would anyway terminate a garbled connection.

To avoid unexpected connection aborts, ENO implementations MUST
disable the use of data in SYN-only segments by default. Such data
MAY be enabled by an API command. In particular, implementations MAY
provide a per-connection mandatory encryption mode that automatically
aborts a connection if ENO fails, and they MAY enable SYN data in
this mode.

To satisfy the requirement of the previous paragraph, all TEPs SHOULD
support a normal mode of operation that avoids data in SYN-only
segments. An exception is TEPs intended to be disabled by default.

4.8. Negotiation Transcript

To defend against attacks on encryption negotiation itself, a TEP
MUST
MUST, with high probability probability, fail to establish a working connection
between two ENO-compliant hosts when SYN-form ENO options have been
altered in transit. (Of course, in the absence of endpoint
authentication, two compliant hosts can each still be connected to a
man-in-the-middle attacker.) To detect SYN-form ENO option
tampering, TEPs MUST reference a transcript of TCP-ENO's negotiation.

TCP-ENO defines its negotiation transcript as a packed data structure
consisting of two TCP-ENO options exactly as they appeared in the TCP
header (including the TCP option-kind option kind and TCP option length byte as
illustrated in Figure 1). The transcript is constructed from the
following, in order:

1. The TCP-ENO option in host A's SYN segment, including the kind
and length bytes. bytes

2. The TCP-ENO option in host B's SYN segment, including the kind
and length bytes. bytes

Note that because the ENO options in the transcript contain length
bytes as specified by TCP, the transcript unambiguously delimits A's
and B's ENO options.

5. Requirements for TEPs

TCP-ENO affords TEP specifications a large amount of design
flexibility. However, to abstract TEP differences away from
applications requires fitting them all into a coherent framework. As
such, any TEP claiming an ENO TEP identifier MUST satisfy the
following normative list of properties. properties:

o TEPs MUST protect TCP data streams with authenticated encryption.
(Note that "authenticated encryption" refers only to the form of
encryption, such as an AEAD Authenticated Encryption with Associated
Data (AEAD) algorithm meeting the requirements of [RFC5116]; it
does not imply endpoint authentication.)

o TEPs MUST define a session ID whose value identifies the TCP
connection and, with overwhelming probability, is unique over all
time if either host correctly obeys the TEP. Section 5.1
describes the requirements of the session ID in more detail.

o TEPs MUST NOT make data confidentiality dependent on encryption
algorithms with a security strength [SP800-57part1] [NIST-SP-800-57] of less than
120 bits. The number 120 was chosen to accommodate ciphers with
128-bit keys that lose a few bits of security either to
particularities of the key schedule or to highly theoretical and
unrealistic attacks.

o TEPs MUST NOT allow the negotiation of null cipher suites, even
for debugging purposes. (Implementations MAY support debugging
modes that allow applications to extract their own session keys.)

o TEPs MUST guarantee the confidentiality of TCP streams without
assuming the security of any long-lived secrets. Implementations
SHOULD provide forward secrecy soon after the close of a TCP
connection,
connection and SHOULD therefore bound the delay between closing a
connection and erasing any relevant cryptographic secrets.
(Exceptions to forward secrecy are permissible only at the
implementation level, level and only in response to hardware or
architectural constraints--e.g., constraints -- e.g., storage that cannot be securely
erased.)

o TEPs MUST protect and authenticate the end-of-file marker conveyed
by TCP's FIN flag. In particular, a receiver MUST MUST, with
overwhelming probability probability, detect a FIN flag that was set or
cleared in transit and does not match the sender's intent. A TEP
MAY discard a segment with such a corrupted FIN bit, bit or MAY abort
the connection in response to such a segment. However, any such
abort MUST raise an error condition distinct from an authentic end-of-
file
end-of-file condition.

o TEPs MUST prevent corrupted packets from causing urgent data to be
delivered when none has been sent. There are several ways to do
so. For instance, a TEP MAY cryptographically protect the URG
flag and urgent pointer alongside ordinary payload data.
Alternatively, a TEP MAY disable urgent data functionality by
clearing the URG flag on all received segments and returning
errors in response to sender-side urgent-data API calls.
Implementations SHOULD avoid negotiating TEPs that disable urgent
data by default. The exception is when applications and protocols
are known never to send urgent data.

5.1. Session IDs

Each TEP MUST define a session ID that is computable by both
endpoints and uniquely identifies each encrypted TCP connection.
Implementations MUST expose the session ID to applications via an API
extension. The API extension MUST return an error when no session ID
is available because ENO has failed to negotiate encryption or
because no connection is yet established. Applications that are
aware of TCP-ENO SHOULD, when practical, authenticate the TCP
endpoints by incorporating the values of the session ID and TCP-ENO
role (A or B) into higher-layer authentication mechanisms.

In order to avoid replay attacks and prevent authenticated session
IDs from being used out of context, session IDs MUST be unique over
all time with high probability. This uniqueness property MUST hold
even if one end of a connection maliciously manipulates the protocol
in an effort to create duplicate session IDs. In other words, it
MUST be infeasible for a host, even by violating the TEP
specification, to establish two TCP connections with the same session
ID to remote hosts properly implementing the TEP.

To prevent session IDs from being confused across TEPs, all session
IDs begin with the negotiated TEP identifier--that identifier -- that is, the last
valid TEP identifier in host B's SYN segment. Furthermore, this
initial byte has bit "v" set to the same value that accompanied the
negotiated TEP identifier in B's SYN segment. However, only this
single byte is included, not any suboption data. Figure 8 shows the
resulting format. This format is designed for TEPs to compute unique
identifiers; it is not intended for application authors to pick apart
session IDs. Applications SHOULD treat session IDs as monolithic
opaque values and SHOULD NOT discard the first byte to shorten
identifiers. (An exception is for non-security-relevant purposes,
such as gathering statistics about negotiated TEPs.)

byte 0 1 2 N-1 N
+-----+------------...------------+
| sub-| collision-resistant hash |
| opt | of connection information |
+-----+------------...------------+

Figure 8: Format of a session Session ID

Though TEP specifications retain considerable flexibility in their
definitions of the session ID, all session IDs MUST meet the
following normative list of requirements:

o The session ID MUST be at least 33 bytes (including the one-byte
suboption), though TEPs MAY choose longer session IDs.

o The session ID MUST depend depend, in a collision-resistant way way, on all
of the following (meaning it is computationally infeasible to
produce collisions of the session ID derivation function unless
all of the following quantities are identical):

* Fresh data contributed by both sides of the connection, connection
* Any public keys, public Diffie-Hellman parameters, or other
public asymmetric cryptographic parameters that are employed by
the TEP and have corresponding private data that is known by
only one side of the connection, and connection

* The negotiation transcript specified in Section 4.8. 4.8

o Unless and until applications disclose information about the
session ID, all but the first byte MUST be computationally
indistinguishable from random bytes to a network eavesdropper.

o Applications MAY choose to make session IDs public. Therefore,
TEPs MUST NOT place any confidential data in the session ID (such
as data permitting the derivation of session keys).

6. Examples

This subsection illustrates the TCP-ENO handshake with a few non-
normative examples.

(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK ENO<b=1,Y>
(3) A -> B: ACK ENO<>
[rest of connection encrypted according to TEP Y]

Figure 9: Three-way handshake Three-Way Handshake with successful Successful TCP-ENO negotiation Negotiation

Figure 9 shows a three-way handshake with a successful TCP-ENO
negotiation. Host A includes two ENO suboptions with TEP identifiers
X and Y. Host A does not include an explicit global suboption, which
means it has an implicit global suboption 0x00 conveying passive role
bit "b b = 0". 0. The two sides agree to follow the TEP identified by
suboption Y.

(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK
(3) A -> B: ACK
[rest of connection unencrypted legacy TCP]

Figure 10: Three-way handshake Three-Way Handshake with failed Failed TCP-ENO negotiation Negotiation

Figure 10 shows a failed TCP-ENO negotiation. The active opener (A)
indicates support for TEPs corresponding to suboptions X and Y.
Unfortunately, at this point point, one of several things occurs:

1. The passive opener (B) does not support TCP-ENO, TCP-ENO.

2. B supports TCP-ENO, TCP-ENO but supports neither of the TEPs X and Y, and
so it does not reply with an ENO option, option.

3. B supports TCP-ENO, TCP-ENO but has the connection configured in mandatory
application-aware mode and thus disables ENO because A's SYN
segment contains an implicit global suboption with "a a =
0", or 0.

4. The network stripped the ENO option out of A's SYN segment, so B
did not receive it.

Whichever of the above applies, the connection transparently falls
back to unencrypted TCP.

(1) A -> B: SYN ENO<X,Y>
(2) B -> A: SYN-ACK ENO<b=1,X> [ENO stripped by middlebox]
(3) A -> B: ACK
[rest of connection unencrypted legacy TCP]

Figure 11: Failed TCP-ENO negotiation because Negotiation Because of option stripping Option Stripping

Figure 11 Shows another handshake with a failed encryption
negotiation. In this case, the passive opener B (B) receives an ENO
option from A and replies. However, the reverse network path from B
to A strips ENO options. Hence, Therefore, A does not receive an ENO option
from B, it disables ENO, and it does not include a non-SYN-form ENO
option in segment 3 when ACKing B's SYN. Had A not disabled
encryption, Section 4.6 would have required it to include a non-SYN non-SYN-
form ENO option in segment 3. The omission of this option informs B
that encryption negotiation has failed, after which the two hosts
proceed with unencrypted TCP.

(1) A -> B: SYN ENO<Y,X>
(2) B -> A: SYN ENO<b=1,X,Y,Z>
(3) A -> B: SYN-ACK ENO<Y,X>
(4) B -> A: SYN-ACK ENO<b=1,X,Y,Z>
[rest of connection encrypted according to TEP Y]

Figure 12: Simultaneous open Open with successful Successful TCP-ENO negotiation Negotiation

Figure 12 shows a successful TCP-ENO negotiation with simultaneous
open. Here Here, the first four segments contain a SYN-form ENO option,
as each side sends both a SYN-only and a SYN-ACK segment. The ENO
option in each host's SYN-ACK is identical to the ENO option in its
SYN-only segment, as otherwise otherwise, connection establishment could not
recover from the loss of a SYN segment. The last valid TEP in host
B's ENO option is Y, so Y is the negotiated TEP.

7. Future Developments

TCP-ENO is designed to capitalize on future developments that could
alter trade-offs and change the best approach to TCP-level encryption
(beyond introducing new cipher suites). By way of example, we
discuss a few such possible developments.

Various proposals exist to increase the maximum space for options in
the TCP header. These proposals are highly experimental-- experimental --
particularly those that apply to SYN segments. Hence, Therefore, future
TEPs are unlikely to benefit from extended SYN option space. In the
unlikely event that SYN option space is one day extended, however,
future TEPs could benefit by embedding key agreement messages
directly in SYN segments. Under such usage, the 32-byte limit on
length bytes could prove insufficient. This draft document intentionally
aborts TCP-ENO if a length byte is followed by an octet in the range
0x00-0x9f. If necessary, a future update to this document can define
a format for larger suboptions by assigning meaning to such currently
undefined byte sequences.

New revisions to socket interfaces [RFC3493] could involve library
calls that simultaneously have access to hostname information and an
underlying TCP connection. Such an API enables the possibility of
authenticating servers transparently to the application, particularly
in conjunction with technologies such as DANE DNS-Based Authentication of
Named Entities (DANE) [RFC6394]. An update to TCP-ENO can adopt one
of the "z" bits in the global suboption to negotiate the use of an
endpoint authentication protocol before any application use of the
TCP connection. Over time, the consequences of failed or missing
endpoint authentication can gradually be increased from issuing log
messages to aborting the connection if some as yet unspecified DNS
record indicates authentication is mandatory. Through shared library
updates, such endpoint authentication can potentially be added
transparently to legacy applications without recompilation.

TLS can currently only be added to legacy applications whose
protocols accommodate a STARTTLS command or equivalent. TCP-ENO,
because it provides out-of-band signaling, opens the possibility of
future TLS revisions being generically applicable to any TCP
application.

8. Design Rationale

This section describes some of the design rationale behind TCP-ENO.

8.1. Handshake Robustness

Incremental deployment of TCP-ENO depends critically on failure cases
devolving to unencrypted TCP rather than causing the entire TCP
connection to fail.

Because a network path might drop ENO options in one direction only,
a host needs to know not just that the peer supports encryption, but
that the peer has received an ENO option. To this end, ENO disables
encryption unless it receives an ACK segment bearing an ENO option.
To stay robust in the face of dropped segments, hosts continue to
include non-SYN form non-SYN-form ENO options in segments until such the point as that
they have received a non-SYN segment from the other side.

One particularly pernicious middlebox behavior found in the wild is
load balancers that echo unknown TCP options found in SYN segments
back to an active opener. The passive role bit "b" in global
suboptions ensures encryption will always be disabled under such
circumstances, as sending back a verbatim copy of an active opener's
SYN-form ENO option always causes role negotiation to fail.

8.2. Suboption Data

TEPs can employ suboption data for session caching, cipher suite
negotiation, or other purposes. However, TCP currently limits total
option space consumed by all options to only 40 bytes, making it
impractical to have many suboptions with data. For this reason, ENO
optimizes the case of a single suboption with data by inferring the
length of the last suboption from the TCP option length. Doing so
saves one byte.

8.3. Passive Role Bit

TCP-ENO, TEPs, and applications all have asymmetries that require an
unambiguous way to identify one of the two connection endpoints. As
an example, Section 4.8 specifies that host A's ENO option comes
before host B's in the negotiation transcript. As another example,
an application might need to authenticate one end of a TCP connection
with a digital signature. To ensure the signed message cannot not be
interpreted out of context to authenticate the other end, the signed
message would need to include both the session ID and the local role,
A or B.

A normal TCP three-way handshake involves one active and one passive
opener. This asymmetry is captured by the default configuration of
the "b" bit in the global suboption. With simultaneous open, both
hosts are active openers, so TCP-ENO requires that one host
explicitly configure "b b = 1". 1. An alternate design might automatically
break the symmetry to avoid this need for explicit configuration.
However, all such designs we considered either lacked robustness or
consumed precious bytes of SYN option space even in the absence of
simultaneous open. (One complicating factor is that TCP does not
know it is participating in a simultaneous open until after it has
sent a SYN segment. Moreover, with packet loss, one host might never
learn it has participated in a simultaneous open.)

8.4. Application-aware Application-Aware Bit

Applications developed before TCP-ENO can potentially evolve to take
advantage of TCP-level encryption. For instance, an application
designed to run only on trusted networks might leverage TCP-ENO to
run on untrusted networks, but, importantly, needs to authenticate
endpoints and session IDs to do so. In addition to user-visible
changes such as requesting credentials, this kind of authentication
functionality requires application-layer protocol changes. Some
protocols can accommodate the requisite changes--for instance changes -- for instance, by
introducing a new verb analogous to "STARTTLS"--while STARTTLS, while others cannot do
so in a backwards-compatible manner.

The application-aware bit "a" in the the global suboption provides a
means of incrementally deploying TCP-ENO-specific enhancements specific to TCP-ENO to
application-layer protocols that would otherwise lack the necessary
extensibility. Software implementing the enhancement always sets "a a = 1"
1 in its own global suboption, but only activates the new behavior
when the other end of the connection also sets "a a = 1". 1.

A related issue is that an application might leverage TCP-ENO as a
replacement for legacy application-layer encryption. In this
scenario, if both endpoints support TCP-ENO, then application-layer
encryption can be disabled in favor of simply authenticating the TCP-
ENO session ID. On the other hand, if one endpoint is not aware of
the new TCP-ENO-specific mode of operation, operation specific to TCP-ENO, there is little
benefit to performing redundant encryption at the TCP layer; data is
already encrypted once at the application layer, and authentication is
only has meaning with respect to this application-layer encryption.
The mandatory application-aware mode lets applications avoid double
encryption in this case: the mode sets "a a = 1" 1 in the local host's
global
suboption, suboption but also disables TCP-ENO entirely in the event that
the other side has not also set "a a = 1". 1.

Note that the application-aware bit is not needed by applications
that already support adequate higher-layer encryption, encryption such as those
provided by TLS [RFC5246] [RFC8446] or SSH [RFC4253]. To avoid double- double
encryption in such cases, it suffices to disable TCP-ENO by
configuration on any ports with known secure protocols.

8.5. Use of ENO Option Kind by TEPs

This draft document does not specify the use of ENO options beyond the
first few segments of a connection. Moreover, it does not specify
the content of ENO options in non-SYN segments, only their presence.
As a result, any use of option-kind TBD option kind 69 after the SYN exchange does
not conflict with this document. Because, in In addition, because ENO guarantees
at most one negotiated TEP per connection, TEPs will not conflict
with one another or ENO if they use ENO's option-kind option kind 69 for out-of-band
signaling in non-SYN segments.

8.6. Unpredictability of Session IDs

Section 5.1 specifies that all but the first (TEP identifier) byte of
a session ID MUST be computationally indistinguishable from random
bytes to a network eavesdropper. This property is easy to ensure
under standard assumptions about cryptographic hash functions. Such
unpredictability helps security in a broad range of cases. For
example, it makes it possible for applications to use a session ID
from one connection to authenticate a session ID from another,
thereby tying the two connections together. It furthermore helps
ensure that TEPs do not trivially subvert the 33-byte minimum length minimum-length
requirement for session IDs by padding shorter session IDs with
zeros.

9. Experiments

This document has experimental status because TCP-ENO's viability
depends on middlebox behavior that can only be determined _a
posteriori_. a
posteriori. Specifically, we need to determine to what extent
middleboxes will permit the use of TCP-ENO. Once TCP-ENO is
deployed, we will be in a better position to gather data on two types
of failure:

1. Middleboxes downgrading TCP-ENO connections to unencrypted TCP.
This can happen if middleboxes strip unknown TCP options or if
they terminate TCP connections and relay data back and forth.

2. Middleboxes causing TCP-ENO connections to fail completely. This
can happen if middleboxes perform deep packet inspection and
start dropping segments that unexpectedly contain ciphertext, or
if middleboxes strip ENO options from non-SYN segments after
allowing them in SYN segments.

Type-1 failures are tolerable, tolerable since TCP-ENO is designed for
incremental deployment anyway. Type-2 failures are more problematic,
and, if prevalent, will require the development of techniques to
avoid and recover from such failures. The experiment will succeed so
long as we can avoid type-2 failures and find sufficient use cases
that avoid type-1 failures (possibly along with a gradual path for
further reducing type-1 failures).

In addition to the question of basic viability, deploying TCP-ENO
will allow us to identify and address other potential corner cases or
relaxations. For example, does the slight decrease in effective TCP
segment payload pose a problem to any applications, requiring which would
require restrictions on how TEPs interpret socket buffer sizes?
Conversely, can we relax the prohibition on default TEPs that disable
urgent data?

A final important metric, related to the pace of deployment and
incidence of type-1 failures, will be the extent to which
applications adopt TCP-ENO-specific enhancements specific to TCP-ENO for endpoint
authentication.

10. Security Considerations

An obvious use case for TCP-ENO is opportunistic encryption--that is, encryption, e.g.,
encrypting some connections, but only where supported and without any
kind of endpoint authentication. Opportunistic encryption provides a
property known as _opportunistic security_ "opportunistic security" [RFC7435], which protects
against undetectable large-scale eavesdropping. However, it does not
protect against detectable large-scale eavesdropping (for instance,
if ISPs terminate TCP connections and proxy them, them or simply downgrade
connections to unencrypted). Moreover, opportunistic encryption
emphatically does not protect against targeted attacks that employ
trivial spoofing to redirect a specific high-value connection to a
man-in-the-middle attacker. Hence, the mere presence of TEP-
indicated encryption does not suffice for an application to represent
a connection as "secure" secure to the user.

Achieving stronger security with TCP-ENO requires verifying session
IDs. Any application relying on ENO for communications communication security MUST
incorporate session IDs into its endpoint authentication. By way of
example, an authentication mechanism based on keyed digests (such as
Digest Access Authentication [RFC7616]) can be extended to include
the role and session ID in the input of the keyed digest.
Authentication mechanisms with a notion of channel binding (such as
SCRAM
Salted Challenge Response Authentication Mechanism (SCRAM) [RFC5802])
can be updated to derive a channel binding from the session ID.
Higher-layer protocols MAY use the application-aware "a" bit to
negotiate the inclusion of session IDs in authentication even when
there is no in-band way to carry out such a negotiation. Because
there is only one "a" bit, however, a protocol extension that
specifies use of the "a" bit will likely require a built-in
versioning or negotiation mechanism to accommodate crypto agility and
future updates.

Because TCP-ENO enables multiple different TEPs to coexist, security
could potentially be only as strong as the weakest available TEP. In
particular, if TEPs use a weak hash function to incorporate the TCP-
ENO transcript into session IDs, then an attacker can undetectably
tamper with ENO options to force negotiation of a deprecated and
vulnerable TEP. To avoid such problems, security reviewers of new
TEPs SHOULD pay particular attention to the collision resistance of
hash functions used for session IDs (including the state of
cryptanalysis and research into possible attacks). Even if other
parts of a TEP rely on more esoteric cryptography that turns out to
be vulnerable, it ought nonetheless to be intractable for an attacker
to induce identical session IDs at both ends after tampering with ENO
contents in SYN segments.

Implementations MUST NOT send ENO options unless they have access to
an adequate source of randomness [RFC4086]. Without secret
unpredictable data at both ends of a connection, it is impossible for
TEPs to achieve confidentiality and forward secrecy. Because systems
typically have very little entropy on bootup, implementations might
need to disable TCP-ENO until after system initialization.

With a regular three-way handshake (meaning no simultaneous open),
the non-SYN form non-SYN-form ENO option in an active opener's first ACK segment
MAY contain N > 0 bytes of TEP-specific data, as shown in Figure 3.
Such data is not part of the TCP-ENO negotiation transcript, transcript and
hence
therefore MUST be separately authenticated by the TEP.

11. IANA Considerations

[RFC-editor: please replace TBD in this section, in Section 4.1, and
in Section 8.5 with the assigned option-kind number. Please also
replace RFC-TBD with this document's final RFC number.]

This document defines a new TCP option-kind option kind for TCP-ENO, assigned a
value of TBD 69 from the TCP option space. This value is defined as:

+------+--------+----------------------------------+-----------+
| Kind | Length | Meaning | Reference |
+------+--------+----------------------------------+-----------+
| TBD 69 | N | Encryption Negotiation (TCP-ENO) | [RFC-TBD] RFC 8547 |
+------+--------+----------------------------------+-----------+

Table 2: TCP Option Kind Numbers

Early implementations of TCP-ENO and a predecessor TCP encryption
protocol made unauthorized use of TCP option-kind option kind 69.

[RFC-editor: please glue the following text to the previous paragraph
iff TBD == 69, otherwise delete it.] These earlier
uses of option 69 are not compatible with TCP-ENO and could disable
encryption or suffer complete connection failure when interoperating
with TCP-ENO-
compliant TCP-ENO-compliant hosts. Hence, legacy use of option 69 MUST be
disabled on hosts that cannot be upgraded to TCP-ENO.

[RFC-editor: please glue this to the previous paragraph regardless of
the value of TBD.] More recent
implementations used experimental option 253 per [RFC6994] with
16-bit ExID 0x454E. Current and new implementations of TCP-ENO MUST
use option TBD, 69, while any legacy implementations MUST migrate to
option TBD. 69. Note in particular that Section 4.1 requires at most one
SYN-form ENO option per segment, which means hosts MUST NOT not include
both option TBD 69 and option 253 with ExID 0x454E in the same TCP
segment.

[IANA is also requested to update the entry for TCP-ENO in the TCP
Experimental Option Experiment Identifiers (TCP ExIDs) sub-registry
to reflect the guidance of the previous paragraph by adding a note
saying "current and new implementations MUST use option TDB." RFC-
editor: please remove this comment.]

This document defines a 7-bit "glt" glt field in the range of 0x20-0x7f,
for which 0x20-0x7f.
IANA is to create has created and will maintain a new registry entitled titled "TCP
encryption protocol identifiers" under
Encryption Protocol Identifiers" under the "Transmission Control
Protocol (TCP) Parameters" registry. The Table 3 shows the initial
contents of the TCP
encryption protocol identifier registry is shown in Table 2. this registry. This document allocates one TEP
identifier (0x20) for experimental use. In case the TEP identifier
space proves too small, identifiers in the range 0x70-0x7f are
reserved to enable a future update to this document to define
extended identifier values. Future assignments are to be made upon
satisfying either of two policies defined in [RFC8126]: "IETF Review"
or (for non-IETF stream specifications) "Expert Review with RFC Required."
Required". IANA will furthermore provide early allocation [RFC7120]
to facilitate testing before RFCs are finalized.

+-----------+------------------------------+-----------+
| Value | Meaning | Reference |
+-----------+------------------------------+-----------+
| 0x20 | Experimental Use | [RFC-TBD] RFC 8547 |
| 0x70-0x7f | Reserved for extended values | [RFC-TBD] RFC 8547 |
+-----------+------------------------------+-----------+

Table 2: 3: TCP encryption protocol identifiers Encryption Protocol Identifiers

12. Acknowledgments

We are grateful for contributions, help, discussions, and feedback
from the IETF and its TCPINC working group, including Marcelo
Bagnulo, David Black, Bob Briscoe, Benoit Claise, Spencer Dawkins,
Jake Holland, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Watson
Ladd, Kathleen Moriarty, Yoav Nir, Christoph Paasch, Eric Rescorla,
Adam Roach, Kyle Rose, Michael Scharf, Joe Touch, and Eric Vyncke.
This work was partially funded by DARPA CRASH and the Stanford Secure
Internet of Things Project.

13. Contributors

Dan Boneh was a co-author of the draft that became this document.

14. References

14.1.

12.1. Normative References

[NIST-SP-800-57]
National Institute of Standards and Technology,
"Recommendation for Key Management - Part 1: General",
NIST Special Publication, 800-57, Revision 4,
DOI 10.6028/NIST.SP.800-57pt1r4, January 2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-57pt1r4.pdf>.

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

[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>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.

[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>.

[SP800-57part1]
Barker, E., "Recommendation for Key Management, Part 1:
General", NIST Special Publication 800-57 Part 1, Revision
4, January 2016,
<http://dx.doi.org/10.6028/NIST.SP.800-57pt1r4>.

14.2.

12.2. Informative References

[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, DOI 10.17487/RFC3493, February 2003,
<https://www.rfc-editor.org/info/rfc3493>.

[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.

[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.

[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[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>.

[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
DOI 10.17487/RFC5802, July 2010,
<https://www.rfc-editor.org/info/rfc5802>.

[RFC6394] Barnes, R., "Use Cases and Requirements for DNS-Based
Authentication of Named Entities (DANE)", RFC 6394,
DOI 10.17487/RFC6394, October 2011,
<https://www.rfc-editor.org/info/rfc6394>.

[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>.

[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>.

[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.

[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
<https://www.rfc-editor.org/info/rfc7616>.

[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>.

Acknowledgments

We are grateful for contributions, help, discussions, and feedback
from the IETF and its TCPINC Working Group, including Marcelo
Bagnulo, David Black, Bob Briscoe, Benoit Claise, Spencer Dawkins,
Jake Holland, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Watson
Ladd, Kathleen Moriarty, Yoav Nir, Christoph Paasch, Eric Rescorla,
Adam Roach, Kyle Rose, Michael Scharf, Joe Touch, and Eric Vyncke.
This work was partially funded by DARPA CRASH and the Stanford Secure
Internet of Things Project.

Contributors

Dan Boneh was a coauthor of the draft that became this document.

Authors' Addresses

Andrea Bittau
Google
345 Spear Street
San Francisco, CA 94105
US
United States of America

Email: bittau@google.com

Daniel B. Giffin
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
US
United States of America

Email: dbg@scs.stanford.edu daniel@beech-grove.net

Mark Handley
University College London
Gower St.
London WC1E 6BT
UK
United Kingdom

Email: M.Handley@cs.ucl.ac.uk

David Mazieres
Stanford University
353 Serra Mall, Room 290
Stanford, CA 94305
US
United States of America

Email: dm@uun.org

Eric W. Smith
Kestrel Institute
3260 Hillview Avenue
Palo Alto, CA 94304
US
United States of America

Email: eric.smith@kestrel.edu