TCP RST Diagnostic Paylaod

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Table of Contents

1. Introduction

A TCP connection [I-D.ietf-tcpm-rfc793bis] 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., CGN [RFC6888], NAT64 [RFC6146], firewall) for several reasons. Typically, a NAT function can generate an RST segment to notify the peers 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 a 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 [I-D.ietf-tcpm-rfc793bis] for RST segments.¶

This document does not change the conditions under which an RST segment is generated (Section 3.5.2 of [I-D.ietf-tcpm-rfc793bis]).¶

The generic procedure for processing an RST segment is specified in Section 3.5.3 of [I-D.ietf-tcpm-rfc793bis]. Only the deviations from that procedure to insert and validate an enclosed diagnostic payload is provided in Section 3.¶

The first version of the specification is meant to discuss 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 at Table 1. However, the authoritative source to retrieve the full list of error codes is the IANA-maintained registry Section 4.1.¶

2. Terminology

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 [I-D.ietf-tcpm-rfc793bis].¶

3. RST Diagnostic Payload

In order to unambiguously identify an RST diagnostic payload that is compliant with the present specification, the payload MUST use the I-JSON message format [RFC7493]. The following parameters are defined:¶

rc:

Stands for "Reason Code". This parameter takes a value from an available registry such as the "TCP Failure Causes" registry (Section 4.1).¶

pen:

Includes a Private Enterprise Number [Private-Enterprise-Numbers]. This attribute is included when the reason code is not taken from the IANA-maintained registry (Section 4.1), but from a vendor-specific registry.¶

rd:

Stands for "Reason Description". It includes a brief description of the reason code. This parameter SHOULD NOT be included if a reason code is supplied. This parameter is useful only for codes that are not yet registered or for application-specific codes.¶

At least "rc" and "rd" parameters MUST be included in an RST diagnostic payload. It is RECOMMENDED to omit "pen" if a reason code from the IANA-maintained registry (Section 4.1) fits the reset case.¶

Malformed RST diagnostic payload messages MUST be silently ignored by the receiver.¶

A peer that receives a valid diagnostic payload may pass that information to the local application in addition to the information (MUST-12) described in Section 3.6 of [I-D.ietf-tcpm-rfc793bis]. That information may also be logged locally, unless a local policy specifies otherwise.¶

Figure 1 depicts an example of an RST diagnostic payload that is generated to inform the peer that the connection is reset because an ACK was received while the connection is still in the LISTEN state.¶

{
 "rc": 2
}

Figure 2 shows an example of an RST diagnostic payload that includes a free description to report a case that is not covered yet by the IANA-maintained registry (Section 4.1).¶

{
 "rd": "brief human-readable description"
}

An RST diagnostic payload may also be reset by an on-path service function. For example, the following diagnostic payload is returned by a NAT uppon expiry of the mapping entry to which the TCP connection is bound (Figure 3).¶

{
 "rc": 8
}

Figure 4 illustrates the example of 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.¶

{
 "rc": 1234
 "pen": 32473
}

Figure 4 uses the Enterprise Number 32473 defined for documentation use [RFC5612].¶

4. IANA Considerations

5. Security Considerations

[I-D.ietf-tcpm-rfc793bis] discusses TCP-related security considerations. RST-specific attacks and their mitigations are discussed in Section 3.10.7.3 of [I-D.ietf-tcpm-rfc793bis].¶

In addition to these considerations, it is RECOMMENDED to control the size of acceptable diagnostic payload and keep it as brief as possible. 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).¶

6. Acknowledgements

TBC.¶

Author's Address