TCP RST Diagnostic Payload
draft-boucadair-tcpm-rst-diagnostic-payload-15
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| Authors | Mohamed Boucadair , Tirumaleswar Reddy.K , Jason Xing | ||
| Last updated | 2026-02-27 (Latest revision 2026-02-25) | ||
| Replaced by | draft-ietf-tcpm-rst-diagnostic-payload | ||
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draft-boucadair-tcpm-rst-diagnostic-payload-15
TCP Maintenance and Minor Extensions M. Boucadair
Internet-Draft Orange
Intended status: Standards Track T. Reddy
Expires: 29 August 2026 Nokia
J. Xing
Tencent
25 February 2026
TCP RST Diagnostic Payload
draft-boucadair-tcpm-rst-diagnostic-payload-15
Abstract
This document specifies two diagnostic payload formats returned in
TCP RST segments. Such payloads are used to share with an endpoint
the reasons for which a TCP connection has been reset. Sharing this
information is meant to ease diagnostic and troubleshooting.
This specification builds on provisions that are already present in
RFC 9293 "Transmission Control Protocol (TCP)". As such, this
document does not require any change to RFC 9293.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the TCP Maintenance and
Minor Extensions mailing list (tcpm@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tcpm/.
Source for this draft and an issue tracker can be found at
https://github.com/boucadair/draft-boucadair-tcpm-rst-diagnostic-
payload.
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 of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents 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 29 August 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. RST Diagnostic Payload . . . . . . . . . . . . . . . . . . . 4
3.1. Compact Format . . . . . . . . . . . . . . . . . . . . . 4
3.2. Free-description Format . . . . . . . . . . . . . . . . . 5
3.3. Behavior . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Some Examples . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Operational Considerations . . . . . . . . . . . . . . . . . 7
5.1. Backward Compatibility . . . . . . . . . . . . . . . . . 8
5.2. Multiple RSTs . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Manageability . . . . . . . . . . . . . . . . . . . . . . 8
6. Socket API Considerations (Informative) . . . . . . . . . . . 9
6.1. Socket Options . . . . . . . . . . . . . . . . . . . . . 9
6.1.1. Enable the Sending of the Diagnostic Payload
(TCP_RST_REASON_ENABLE) . . . . . . . . . . . . . . . 9
6.1.2. Get or Set the Diagnostic Payload as Code
(TCP_RST_REASON_CODE) . . . . . . . . . . . . . . . . 10
6.1.3. Get or Set the Diagnostic Payload as Description
(TCP_RST_REASON_DESC) . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7.1. Compact Format . . . . . . . . . . . . . . . . . . . . . 11
7.2. Free-description Format . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8.1. New Registry for TCP Failure Causes . . . . . . . . . . . 12
8.2. Guidelines for the Designated Experts . . . . . . . . . . 14
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Implementation and Experimental Validation in
Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 16
A.1. Implementation . . . . . . . . . . . . . . . . . . . . . 16
A.2. Experimental Validation . . . . . . . . . . . . . . . . . 19
A.2.1. Functional Verification . . . . . . . . . . . . . . . 19
A.2.2. Compatibility Verification . . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
A TCP connection [RFC9293] can be reset by a peer for various
reasons, e.g., received data does not correspond to an active
connection. Also, a TCP connection can be reset by an on-path
service function (e.g., Carrier Grade NAT (CGN) [RFC6888], NAT64
[RFC6146], or firewall) for several reasons. Typically, a Network
Address Translator (NAT) function can generate an RST segment to
notify an endpoint upon the expiry of the lifetime of the
corresponding mapping entry or because an RST segment was received
from a peer (Section 2.2 of [RFC7857]).
A TCP connection can also be closed by a user or an application at
any time. However, the peer that receives an RST segment does not
have any hint about the reason that led to terminating the
connection. Likewise, the application that relies upon such a TCP
connection may not easily identify the reason for the connection
reset. Troubleshooting such events at the remote side of the
connection that receives the RST segment may not be trivial.
This document fills this void by specifying formats of the diagnostic
payload returned in an RST segment. This design is backward
compatible with TCP as further clarified in Section 5.1.
The generic procedure for processing an RST segment is specified in
Section 3.5.3 of [RFC9293]. Only the deviations from that procedure
to insert and validate a diagnostic payload is provided in Section 3.
This document specifies the formats and the overall approach to ease
maintaining the list of codes while allowing for adding new codes as
needed in the future and accommodating any existing vendor-specific
codes. An initial version of error codes is available in Table 2.
However, the authoritative source to retrieve the full list of error
codes is the IANA-maintained registry (Section 8.1).
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Section 4 provides a set of examples to illustrate the use of TCP RST
diagnostic payloads.
Section 6 provides an informative discussion of socket API
considerations. Implementation and experimental validation are
detailed in Appendix A.
2. Conventions and Definitions
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.
This document makes use of the terms defined in Section 4 of
[RFC9293].
SEG.LEN is defined in Section 3.3.1 of [RFC9293].
This document uses the following terms:
RST diagnostic payload: The payload of an RST message that conveys
diagnostic data.
RST with diagnostic payload: An RST segment that includes diagnostic
payload.
3. RST Diagnostic Payload
This section defines two message formats to convey RST diagnostic
payload:
* Compact format (Section 3.1): This format is designed to minimize
the length of the payload.
* Free-description format (Section 3.2): This format is designed to
accommodate, in particular, applications that don't maintain a
reset cause registry but need sharing reason codes not covered in
IANA-maintained registry (Section 8.1).
3.1. Compact Format
The format of the compact RST diagnostic payload is shown in
Figure 1.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x33AA | reason-code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| pen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Structure of the Compact RST Diagnostic Payload
The RST diagnostic payload comprises a magic number that is used to
unambiguously identify an RST payload that follows this
specification. It MUST be set to 0x33AA for the compact encoding
shown in Figure 1.
The descriptions of other fields shown in Figure 1 are as follows:
reason-code: This field takes a value from an available registry
(IANA or vendor-specific).
Value 0 is reserved and MUST NOT be used.
The reason code is taken from the "TCP Failure Causes" registry
(Section 8.1) if "pen" is set to 0.
If the "pen" is not set to 0, then the reason code refers to the
registry of the entity specified by the "pen" parameter.
pen: Includes a Private Enterprise Number (PEN)
[Private-Enterprise-Numbers].
The reserved PEN value "0" is used to indicate that the reason
code refers to the IANA-maintained registry (Section 8.1).
SEG.LEN MUST be 8 for an RST with compact diagnostic payload.
3.2. Free-description Format
The format of the RST diagnostic payload with a reason description is
shown in Figure 2. This format is useful to convey reset reasons
that are not yet registered or for application-specific reset
reasons.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xF317 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
: reason-description :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Structure of the RST Diagnostic Payload with Reason
Description
The RST diagnostic payload comprises a magic number that is used to
unambiguously identify an RST payload that follows this
specification. It MUST be set to 0xF317 when the free-description
format is used.
The description of the other field shown in Figure 2 is as follows:
reason-description: Includes a brief description of the reset reason
encoded as UTF-8 [RFC3629].
The length of the reason-description is "SEG.LEN - 2".
3.3. Behavior
Senders are RECOMMENDED to use the compact format. It is RECOMMENDED
that both formats are supported at the receiver side.
Malformed RST diagnostic payloads that include the magic numbers
(0x33AA or 0xF317) MUST be silently ignored by the receiver. RSTs
that carry such malformed diagnostic payloads are handled like an RST
without payload.
A peer that receives a valid diagnostic payload may pass the reset
reason information to the local application in addition to the
information (MUST-12) described in Section 3.6 of [RFC9293]. That
information may also be logged locally, unless a local policy
specifies otherwise. How the information is passed to an application
and how it is stored locally is implementation-specific.
4. Some Examples
Figure 3 depicts an example of an RST diagnostic payload that is
generated to inform the peer that the TCP connection is reset because
an ACK was received from that peer while the connection is still in
the LISTEN state (Section 3.10.7.2 of [RFC9293]).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x33AA | 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Example of an RST Diagnostic Payload with Reason Code
An RST diagnostic payload may also be sent by an on-path service
function. For example, the following diagnostic payload is returned
by a NAT function upon expiry of the mapping entry to which the TCP
connection is bound (Figure 4).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x33AA | 0x0E |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Example of an RST Diagnostic Payload to Report
Connection Timeout
Figure 5 illustrates an RST diagnostic payload that is returned by a
peer that resets a TCP connection for a reason code 1234 defined by a
vendor with the private enterprise number 32473 (0x7D9).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x33AA | 0x4DE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x7D9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example of an RST Diagnostic Payload to Report Vendor-
Specific Reason Code
Figure 5 uses the Enterprise Number 32473 defined for documentation
use [RFC5612].
5. Operational Considerations
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5.1. Backward Compatibility
Returning diagnostic data in an RST segment is consistent with the
provision in Section 3.5.3 of [RFC9293] for RST segments, especially:
| "TCP implementations SHOULD allow a received RST segment to
| include data (SHLD-2)."
Also, this document does not change the conditions under which an RST
segment is generated (Section 3.5.2 of [RFC9293]).
5.2. Multiple RSTs
Per Section 3.6 of [RFC9293], one or more RST segments can be sent to
reset a connection.
Sending more RST segments to reset a connection can be used to
mitigate deployment contexts where some on-path devices may discard
RSTs with payload data.
Whether a TCP endpoint elects to send more than one RST with only a
subset of them that include the diagnostic payload is implementation-
specific.
5.3. Manageability
TCP server implementations should support the following parameters:
* A parameter to control the activation of RSTs with diagnostic
payload.
* A parameter to accept/deny RSTs with the format defined in
Section 3.2).
* A parameter to set a maximum length of acceptable reason
description (Section 3.2), when enabled.
* A parameter to control whether "empty" RSTs are also sent together
with an RST with diagnostic payload.
* A rate-limit of RSTs with diagnostic payload.
* Counters to track sent/received RSTs with diagnostic payload.
These counters should be structured per encoding format described
in Sections 3.1 and 3.2.
* Counters to track received invalid RSTs with diagnostic payload.
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6. Socket API Considerations (Informative)
This section describes how the socket API can be extended to provide
a way for an application to use the functionality described in this
document.
This section is informational only.
The API described in this section can change in a non-backwards
compatible way during the evolution of this document due to changed
functionality or gained experience during the implementation.
6.1. Socket Options
Table 1 provides an overview of the IPPROTO_TCP-level socket options
defined in this section.
+=======================+=======================+=====+=====+
| Option Name | Data Type | Set | Get |
+=======================+=======================+=====+=====+
| TCP_RST_REASON_ENABLE | uint32_t | X | |
+-----------------------+-----------------------+-----+-----+
| TCP_RST_REASON_CODE | struct tcp_rst_reason | X | X |
+-----------------------+-----------------------+-----+-----+
| TCP_RST_REASON_DESC | char[] | X | X |
+-----------------------+-----------------------+-----+-----+
Table 1: Socket Options
6.1.1. Enable the Sending of the Diagnostic Payload
(TCP_RST_REASON_ENABLE)
Using setsockopt() with the IPPROTO_TCP-level socket option with the
name TCP_RST_REASON_ENABLE enables or disabled the sending of the
diagnostic payload using a reason-code and pen. The option_value of
type uint32_t specifies the pen in host byte order to use. When 0 is
used (Section 3 of [RFC9371]), the reason-codes from the registry
specified in Section 8.1 are used. When 0xffffffff is used
(Section 3 of [RFC9371]), the sending is disabled. The default is
that the sending of a diagnostic payload is disabled. An
implementation might not support the use of PENs different from zero
and 0xffffffff.
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6.1.2. Get or Set the Diagnostic Payload as Code (TCP_RST_REASON_CODE)
Using getsockopt() with the IPPROTO_TCP-level socket option with the
name TCP_RST_REASON_CODE allows the caller to retrieve the reason-
code and the pen of the diagnostic payload in the received RST
segment, which terminated the corresponding TCP connection.
Using setsockopt() with this socket option allows the caller to
provide reason-code and pen to be sent as part of the diagnostic
payload when the application triggers the sending of a RST segment by
using close(). In addition to using close() in combination with the
SOL_SOCKET-level socket option with name SO_LINGER, the application
can just provide the TCP_RR_RST_ON_CLOSE flag in trr_flags. This way
the application can trigger the sending of a RST segment by calling
setsockopt() once followed by close().
For accepted sockets, this socket option is inherited from the
listening socket.
The following structure is used as the option_value:
struct tcp_rst_reason {
uint16_t trr_flags;
uint16_t trr_code;
uint32_t trr_pen;
};
trr_flags: This field is reported as 0 for getsockopt() calls.
For setsockopt()
calls, the following flag can be used:
TCP_RR_RST_ON_CLOSE: When this flag is set, calling
close() triggers the sending of a RST segment similar to case,
where the SOL_SOCKET-level socket option with name SO_LINGER is
used to enable lingering with the linger time of 0. When this
flag is cleared, the corresponding functionality is disabled.
trr_code: The reason-code in host byte order to be interpreted in
combination with the PEN provided in trr_pen. In case of trr_pen
being zero, trr_code refers to a value in the registry defined in
Section 8.1.
trr_pen: The PEN in host byte order to is used in combination with
the reason-code specified in trr_code. When this socket option is
used with setsockopt(), it is an error to use zero as a value for
trr_pen as long as trr_code is not zero.
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When getsockopt() with this socket option is performed on a socket,
which has not received a RST with a diagnostic payload containing a
reason-code and pen, zero is provided as the trr_code and trr_pen.
When setsockopt() with a trr_code and trr_pen of zero is performed,
the special handling of RST segments sent during the ungraceful
termination of the TCP connection is disabled.
6.1.3. Get or Set the Diagnostic Payload as Description
(TCP_RST_REASON_DESC)
Some implementations might not support the sending and receiving of
the reason in description form. In this case this socket option is
not implemented.
Using getsockopt() with the IPPROTO_TCP-level socket option with the
name TCP_RST_REASON_DESC allows the caller to retrieve the reason-
description of the diagnostic payload in the RST segment, which
terminated the corresponding TCP connection.
Using setsockopt() with this socket option allows the caller to
provide a reason-description to be sent as part of the diagnostic
payload when the application triggers the sending of a RST segment by
using close() in combination with the SOL_SOCKET-level socket option
with name SO_LINGER. Providing an empty character array disables the
sending of a reason-description in this case. For accepted sockets,
this socket option is inherited from the listening socket.
7. Security Considerations
[RFC9293] discusses TCP-related security considerations. In
particular, RST-specific attacks and their mitigations are discussed
in Section 3.10.7.3 of [RFC9293].
The following subsections discuss considerations specific to each
encoding format.
7.1. Compact Format
The presence of vendor-specific reason codes may be used to
fingerprint hosts. Such a concern does not apply if the reason codes
are taken from the IANA-maintained registry. Implementers are, thus,
encouraged to register new codes within IANA instead of maintaining
specific registries.
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7.2. Free-description Format
It is RECOMMENDED to control the size of acceptable diagnostic
payload and keep it as brief as possible. The RECOMMENDED acceptable
maximum size of the RST diagnostic payload is 255 octets.
Also, it is RECOMMENDED to avoid leaking privacy-related information
as part of the diagnostic payload (e.g., including a description such
as "user X resets explicitly the connection" is not recommended).
The "reason-description" string, when present, MUST NOT include any
private information that an observer would not otherwise have access
to.
The reason description, when present, MUST NOT be displayed to end
users but is intended to be consumed by applications. Such a
description may carry a malicious message to mislead the end-user.
8. IANA Considerations
8.1. New Registry for TCP Failure Causes
This document requests IANA to create a new registry entitled "TCP
Failure Causes" under the "Transmission Control Protocol (TCP)
Parameters" registry group [IANA-TCP].
Values are taken from the 1-65535 range.
The assignment policy for this registry is "Expert Review"
(Section 4.5 of [RFC8126]). See more guidance at Section 8.2.
The registry is initially populated with the values listed in
Table 2.
+=======+=================================+=======================+
| Value | Description | Specification (if |
| | | available) |
+=======+=================================+=======================+
| 0 | Reserved | ThisDocument |
+-------+---------------------------------+-----------------------+
| 1 | Illegal Option | Section 3.1 of |
| | | [RFC9293] |
+-------+---------------------------------+-----------------------+
| 2 | Desynchronized state | Section 3.5.1 of |
| | | [RFC9293] |
+-------+---------------------------------+-----------------------+
| 3 | New data is received after | Sections 3.6.1 and |
| | CLOSE is called | 3.10.7.1 of [RFC9293] |
+-------+---------------------------------+-----------------------+
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| 4 | ABORT Process | Section 3.10.5 of |
| | | [RFC9293] |
+-------+---------------------------------+-----------------------+
| 5 | Unexpected ACK received by non- | Section 3.10.7 of |
| | synchronized state connection | [RFC9293] |
+-------+---------------------------------+-----------------------+
| 6 | Unexpected SYN in the window | Section 3.10.7 of |
| | | [RFC9293] |
+-------+---------------------------------+-----------------------+
| 7 | Unexpected security compartment | Appendix A.1 of |
| | | [RFC9293] |
+-------+---------------------------------+-----------------------+
| 8 | Malformed Message | ThisDocument |
+-------+---------------------------------+-----------------------+
| 9 | Not Authorized | ThisDocument |
+-------+---------------------------------+-----------------------+
| 10 | Resource Exceeded | ThisDocument |
+-------+---------------------------------+-----------------------+
| 11 | Network Failure | ThisDocument |
+-------+---------------------------------+-----------------------+
| 12 | Reset received from the peer | ThisDocument |
+-------+---------------------------------+-----------------------+
| 13 | Destination Unreachable | ThisDocument |
+-------+---------------------------------+-----------------------+
| 14 | Connection Timeout | ThisDocument |
+-------+---------------------------------+-----------------------+
| 15 | Too much outstanding data | Section 3.6 of |
| | | [RFC8684] |
+-------+---------------------------------+-----------------------+
| 16 | Unacceptable performance | Section 3.6 of |
| | | [RFC8684] |
+-------+---------------------------------+-----------------------+
| 17 | Middlebox interference | Section 3.6 of |
| | | [RFC8684] |
+-------+---------------------------------+-----------------------+
Table 2: Initial TCP Failure Causes
Note that codes in the 8-14 range can be used by service functions
(CGN, firewall, proxy, etc.).
Note to the RFC Editor: Please replace ThisDocument with the RFC
number assigned to this document.
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8.2. Guidelines for the Designated Experts
Criteria that should be applied by the designated experts include
determining whether the proposed registration duplicates existing
entries and whether the registration description is clear and fits
the purpose of this registry.
The designated experts may approve registration once they checked
that the new requested code is not covered by an existing code and if
the provided reasoning to register the new code is acceptable. A
registration request may supply a pointer to a specification where
that code is defined. However, a registration may be accepted even
if no permanent and readily available public specification is
available.
Registration requests are to be sent to rst-diag-review@ietf.org
(mailto:rst-diag-review@ietf.org) and are evaluated within a three-
week review period on the advice of one or more designated experts.
Within the review period, the designated experts will either approve
or deny the registration request, communicating this decision to the
review list and IANA. Denials should include an explanation and, if
applicable, suggestions as to how to make the request successful.
9. References
9.1. Normative References
[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/rfc/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[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/rfc/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/rfc/rfc8174>.
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[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/rfc/rfc8684>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/rfc/rfc9293>.
9.2. Informative References
[IANA-TCP] "Transmission Control Protocol (TCP) Parameters",
<https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml#>.
[Private-Enterprise-Numbers]
"Private Enterprise Numbers",
<https://www.iana.org/assignments/enterprise-numbers>.
[RFC5612] Eronen, P. and D. Harrington, "Enterprise Number for
Documentation Use", RFC 5612, DOI 10.17487/RFC5612, August
2009, <https://www.rfc-editor.org/rfc/rfc5612>.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
April 2011, <https://www.rfc-editor.org/rfc/rfc6146>.
[RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
A., and H. Ashida, "Common Requirements for Carrier-Grade
NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
April 2013, <https://www.rfc-editor.org/rfc/rfc6888>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/rfc/rfc7252>.
[RFC7857] Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
S., and K. Naito, "Updates to Network Address Translation
(NAT) Behavioral Requirements", BCP 127, RFC 7857,
DOI 10.17487/RFC7857, April 2016,
<https://www.rfc-editor.org/rfc/rfc7857>.
[RFC9371] Baber, A. and P. Hoffman, "Registration Procedures for
Private Enterprise Numbers (PENs)", RFC 9371,
DOI 10.17487/RFC9371, March 2023,
<https://www.rfc-editor.org/rfc/rfc9371>.
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Appendix A. Implementation and Experimental Validation in Linux
Questions and concerns have been raised regarding whether RST with
payload affects the normal termination of flows across different
software platforms, operating systems, middleboxes, etc. Even though
Section 3.5.3 of [RFC9293] explicitly allows this behavior, a full
implementation is needed to widely verify if unexpected cases can
happen in the real world.
The overall design in Linux is to pre-allocate a large enough zeroed
buffer, put a reset reason code in the first byte and sent it out to
verify whether the RST with payload can be possibly declined by any
equipment in between two sides and the other side successfully parses
the RST with payload.
A.1. Implementation
The following implementation is accomplished on top of Linux 6.16:
*Payload Attachment*: Allocate a 1000-byte data payload attached to
all generated RST packets.
*Reason Code Encoding*: The first byte of the payload is used to
store a predefined reset reason code that is listed in
include/net/rstreason.h file, while the remainder of the payload
is zero-padded. The reason code is generated by the existing
mechanism called TCP reset reasons
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/
linux.git/commit/?id=d5115a55ffb52).
*Handling of Reset Types*: The implementation distinguishes between
the two primary reset scenarios in tcp_send_active_reset() and
tcp_v4_send_reset() respectively:
* For an *Active Reset*, initiated proactively by the local
system, the payload is placed in the linear area of the socket
buffer (sk_buff).
* For a *Passive Reset*, sent in response to an unexpected or
invalid incoming packet, the payload is stored in the non-
linear (paged) area of the sk_buff.
Complete patch is shown in Figure 6.
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diff --git a/include/net/tcp.h b/include/net/tcp.h
index b3815d104340..0b32257774c8 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -62,6 +62,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
#define MAX_TCP_OPTION_SPACE 40
#define TCP_MIN_SND_MSS 48
#define TCP_MIN_GSO_SIZE (TCP_MIN_SND_MSS - MAX_TCP_OPTION_SPACE)
+#define PAYLOAD_LEN 1000
/*
* Never offer a window over 32767 without using window scaling. Some
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 84d3d556ed80..49250e6bd6a1 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -741,6 +741,7 @@ static bool tcp_v4_ao_sign_reset(const struct sock *sk, struct sk_buff *skb,
static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
enum sk_rst_reason reason)
{
+ u32 len = sizeof(struct tcphdr) + REPLY_OPTIONS_LEN + PAYLOAD_LEN;
const struct tcphdr *th = tcp_hdr(skb);
struct {
struct tcphdr th;
@@ -757,6 +758,7 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
#endif
u64 transmit_time = 0;
struct sock *ctl_sk;
+ char buffer[len];
struct net *net;
u32 txhash = 0;
@@ -786,7 +788,8 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
}
memset(&arg, 0, sizeof(arg));
- arg.iov[0].iov_base = (unsigned char *)&rep;
+ memset(&buffer, 0, len);
+ arg.iov[0].iov_base = (unsigned char *)buffer;
arg.iov[0].iov_len = sizeof(rep.th);
net = sk ? sock_net(sk) : skb_dst_dev_net_rcu(skb);
@@ -911,6 +914,10 @@ static void tcp_v4_send_reset(const struct sock *sk, struct sk_buff *skb,
ctl_sk->sk_mark = 0;
ctl_sk->sk_priority = 0;
}
+ memcpy(buffer, (char *)&rep, arg.iov[0].iov_len);
+ /* put rst reason into the first byte in payload */
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+ buffer[arg.iov[0].iov_len] = reason;
+ arg.iov[0].iov_len += PAYLOAD_LEN;
ip_send_unicast_reply(ctl_sk, sk,
skb, &TCP_SKB_CB(skb)->header.h4.opt,
ip_hdr(skb)->saddr, ip_hdr(skb)->daddr,
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index b616776e3354..c07dd009a0de 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3628,12 +3628,14 @@ void tcp_send_fin(struct sock *sk)
void tcp_send_active_reset(struct sock *sk, gfp_t priority,
enum sk_rst_reason reason)
{
+ u32 len = MAX_TCP_HEADER + PAYLOAD_LEN;
+ char payload[PAYLOAD_LEN];
struct sk_buff *skb;
TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
/* NOTE: No TCP options attached and we never retransmit this. */
- skb = alloc_skb(MAX_TCP_HEADER, priority);
+ skb = alloc_skb(len, priority);
if (!skb) {
NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
return;
@@ -3641,8 +3643,13 @@ void tcp_send_active_reset(struct sock *sk, gfp_t priority,
/* Reserve space for headers and prepare control bits. */
skb_reserve(skb, MAX_TCP_HEADER);
+ skb_put(skb, PAYLOAD_LEN);
tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
TCPHDR_ACK | TCPHDR_RST);
+ memset(payload, 0, PAYLOAD_LEN);
+ payload[0] = reason;
+ skb_store_bits(skb, 0, payload, PAYLOAD_LEN);
+ TCP_SKB_CB(skb)->end_seq += PAYLOAD_LEN;
tcp_mstamp_refresh(tcp_sk(sk));
/* Send it off. */
if (tcp_transmit_skb(sk, skb, 0, priority))
Figure 6: Complete Patch
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A.2. Experimental Validation
To ensure a thorough evaluation, a multi-layered experimental
methodology was designed, progressing from basic functional checks to
complex, real-world compatibility and stability tests. The whole
implementation has been deployed in Tencent's production environment
for almost six months.
A.2.1. Functional Verification
The basic functionality test is using iperf or iperf3 to construct a
normal termination senario. The tcpdump tool with -X option
effectively helps to show the [RST+] flag and the 1000-byte payload,
confirming that the kernel correctly generated and transmitted the
augmented RST packets.
Two servers, designated as Client A and Server B. The test is
conducted as following:
1. Start the iperf3 server on Server B (iperf3 -s).
2. Initiate a connection from Client A to Server B (iperf3 -c
[IP_of_B]).
3. After the connection is established, one of the iperf3 processes
is terminated using the kill command, triggering the kernel to
send an RST packet.
4. Simultaneously, tcpdump is run on either host to capture the
reset packet using the filter: 'tcp[tcpflags] & tcp-rst != 0' -X
-nn -vv -S.
A.2.2. Compatibility Verification
*Hardwares and Kernels*: Tests were conducted on various Linux
distributions (e.g., Ubuntu or CentOS) with different kernel
versions. The physical hosts were equipped with a range of
network interface cards (NICs), including Intel i40e, ixgbe, and
Mellanox mlx5.
*Virtualization*: The mechanism was tested in a virtualized
environment where the VM used a virtio_net driver and the host
employed DPDK to redirect packets in the host.
*Middleboxes*: Tests were performed with Layer 4 (L4) and Layer 7
(L7) gateways placed between the client and server to verify
correct packet parsing and forwarding.
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*Wide Area Network (WAN)*: The setup was tested over long-haul
international links to simulate complex conditions, including
China-to-Singapore (RTT > 30ms) and China-to-Germany (RTT >
200ms).
In conclusion, across all complex environment tests, the RST packets
with payloads were successfully received by the peer. No instances
of packets being dropped or mishandled by intermediate middleboxes,
gateways, or diverse hardware and software configurations were
observed.
Acknowledgments
The "diagnostic payload" name is inspired by Section 5.5.2 of
[RFC7252] that was cited by Carsten Bormann in the tcpm mailing list.
Thanks to Jon Shallow, Neal Cardwell, Lars Eggert, Eric Dumazet, Rick
Jones, Yoshifumi Nishida, Li Jinghui, Gleb Smirnoff, and Michael
Tuexen for the reveiw and comments.
Contributors
Michael Tüxen
Email: tuexen@fh-muenster.de
Authors' Addresses
Mohamed Boucadair
Orange
Email: mohamed.boucadair@orange.com
Tirumaleswar Reddy
Nokia
India
Email: kondtir@gmail.com
Jason Xing
Tencent
Email: kerneljasonxing@gmail.com
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