



TCP Maintenance and Minor Extensions                        M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: 19 March 2026                                             Nokia
                                                                 J. Xing
                                                                 Tencent
                                                       15 September 2025


                       TCP RST Diagnostic Payload
             draft-boucadair-tcpm-rst-diagnostic-payload-12

Abstract

   This document specifies a diagnostic payload format 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.

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/.

   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 19 March 2026.




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Copyright Notice

   Copyright (c) 2025 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  RST Diagnostic Payload  . . . . . . . . . . . . . . . . . . .   4
   4.  Some Examples . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  RST Diagnostic Payload CBOR Key Values  . . . . . . . . .   7
     5.2.  New Registry for TCP Failure Causes . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Implementation and Experimental Validation in
           Linux . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.1.  Implementation  . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  Experimental Validation . . . . . . . . . . . . . . . . .  15
       A.2.1.  Functional Verification . . . . . . . . . . . . . . .  15
       A.2.2.  Compatibility Verification  . . . . . . . . . . . . .  15
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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]).




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   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
   closure.  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 a format of the
   diagnostic payload that is returned in an RST segment.  Returning
   such data 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)."

   This document does not change the conditions under which an RST
   segment is generated (Section 3.5.2 of [RFC9293]).

   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.
   Section 4 provides a set of examples to illustrate the use of TCP RST
   diagnostic payloads.

   This document specifies the format 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 5.2).

      Design note: Other alternate encoding designs may be considered
      (TLV, Plain text, etc.); each has their own pros and cons, mainly:
      amplification impact, need or not of a kernel library and
      availability of such library (if needed), impact of conversion on
      CPU, integration with traffic visualisation tools.  The encoding
      will be updated to reflect the WG consensus.

   Investigation based on some major CGN vendors revealed that RSTs with
   data are not discarded and are translated according to any matching
   mapping entry.  Moreover, implementation and experimental validation
   in Linux are detailed in Appendix A.








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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].

3.  RST Diagnostic Payload

   The RST diagnostic payload MUST be encoded using Concise Binary
   Object Representation (CBOR) Sequence [RFC8742].  The Concise Data
   Definition Language (CDDL) [RFC8610] for the diagnostic payload is
   shown in Figure 1.

   ; This defines an array, the elements of which are to be used
   ; in a CBOR Sequence. There is exactly one occurrence.
   diagnostic-payload = [magic-cookie, reason]
   ; Magic cookie to identify a payload that follows this specification
   magic-cookie = 12345
   ; Reset reason details:
   reason= {
     ? reason-code: uint,
     ? pen:uint,
     ? reason-description: tstr,
   }
   ; Map Keys
   reason-code = 1
   pen = 2
   reason-description = 3

             Figure 1: Structure of the RST Diagnostic Payload

   The RST diagnostic payload comprises a magic cookie that is used to
   unambiguously identify an RST payload that follows this
   specification.  It MUST be set to the RFC number to be assigned to
   this document.

      Note to the RFC Editor: Please replace "12345" with the RFC number
      assigned to this document.

   All parameters in the reason component of an RST diagnostic payload
   are mapped to their CBOR key values as specified in Section 5.1.  The
   description of these parameters is as follows:




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   reason-code:  This parameter takes a value from an available registry
      such as the "TCP Failure Causes" registry (Section 5.2).

   pen:  Includes a Private Enterprise Number
      [Private-Enterprise-Numbers].  This parameter MAY be included when
      the reason code is not taken from the IANA-maintained registry
      (Section 5.2), but from a vendor-specific registry.

   reason-description:  Includes a brief description of the reset reason
      encoded as UTF-8 [RFC3629].  This parameter MUST NOT be included
      if a reason code is supplied.  This parameter is useful only for
      reset reasons that are not yet registered or for application-
      specific reset reasons.

   At least one of "reason-code" and "reason-description" parameters
   MUST be included in an RST diagnostic payload.  The "pen" parameter
   MUST be omitted if a reason code from the IANA-maintained registry
   (Section 5.2) fits the reset case.

   Malformed RST diagnostic payload messages that include the magic
   cookie MUST be silently ignored by the receiver.

   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.

   Per Section 3.6 of [RFC9293], one or more RST segments can be sent to
   reset a connection.  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.

4.  Some Examples

   To ease readability, the CBOR diagnostic notation (Section 8 of
   [RFC8949]) with the parameter names rather than their CBOR key values
   in Section 5.1 is used in Figures 3, 4, 5, and 6.

   Figure 2 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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      19 3039 # unsigned(12345)
      A1    # map(1)
         01 # unsigned(1)
         02 # unsigned(2)

      Figure 2: Example of an RST Diagnostic Payload with Reason Code
                              (CBOR Encoding)

   Figure 3 depicts the same RST diagnostic payload as the one shown in
   Figure 2 but following the CBOR diagnostic notation.

      [
        12345,
        {
          1: 2
        }
      ]

      Figure 3: Example of an RST Diagnostic Payload with Reason Code
                           (Diagnostic Notation)

   Figure 4 shows an example of an RST diagnostic payload that includes
   a free description to report a case that is not covered by an
   appropriate code from the IANA-maintained registry (Section 5.2).

      [
        12345,
        {
          3: "brief human-readable description"
        }
      ]

         Figure 4: Example of an RST Diagnostic Payload with Reason
                     Description (Diagnostic Notation)

   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 5).

      [
        12345,
        {
          1: 8
        }
      ]





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          Figure 5: Example of an RST Diagnostic Payload to Report
                  Connection Timeout (Diagnostic Notation)

   Figure 6 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.

      [
        12345,
        {
          1: 1234,
          2: 32473
        }
      ]

      Figure 6: Example of an RST Diagnostic Payload to Report Vendor-
                 Specific Reason Code (Diagnostic Notation)

   Figure 6 uses the Enterprise Number 32473 defined for documentation
   use [RFC5612].

5.  IANA Considerations

5.1.  RST Diagnostic Payload CBOR Key Values

   IANA is requested to create a new registry titled "RST Diagnostic
   Payload CBOR Key Values" under the "Transmission Control Protocol
   (TCP) Parameters" registry group [IANA-TCP].

   The key value MUST be an integer in the 1-255 range.

   The assignment policy for this registry is "IETF Review" (Section 4.8
   of [RFC8126]).

   The structure of this subregistry and the initial values are provided
   in Table 1.















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   +====================+==========+=================+================+
   |   Parameter Name   | CBOR Key | CBOR Major Type |   Reference    |
   |                    |          |  & Information  |                |
   +====================+==========+=================+================+
   |    reason-code     |    1     |    0 unsigned   | [ThisDocument] |
   +--------------------+----------+-----------------+----------------+
   |        pen         |    2     |    0 unsigned   | [ThisDocument] |
   +--------------------+----------+-----------------+----------------+
   | reason-description |    3     |  3 text string  | [ThisDocument] |
   +--------------------+----------+-----------------+----------------+

                        Table 1: Initial CBOR Keys

5.2.  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]).

   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.

   The registry is initially populated with the values listed in
   Table 2.

    +=======+=================================+=======================+
    | Value | Description                     | Specification (if     |
    |       |                                 | available)            |
    +=======+=================================+=======================+
    |   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 he 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
   (Carrier Grade NAT (CGN), firewall, proxy, etc.).

6.  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].





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   In addition to these considerations, 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 presence of vendor-specific reason codes (Section 3) 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.

   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.

7.  References

7.1.  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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   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

   [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>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8742>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8949>.

   [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>.

7.2.  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", May 2020,
              <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>.




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   [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>.

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).



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      *  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 7.

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);



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@@ -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 */
+       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 7: 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, 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 for the comments.  Thanks also to Li Jinghui
   for the discussion.

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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