The presence of Constrained Application Protocol (CoAP) proxies may lead to infinite forwarding loops, which is undesirable. To prevent and detect such loops, this document specifies the Hop-Limit CoAP option.

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More and more applications are using the Constrained Application Protocol (CoAP) [RFC 7252] as a communication protocol between application agents. For example, [DOTS-SIG-CHANNEL] specifies how CoAP is used as a signaling protocol between domains under distributed denial-of-service (DDoS) attacks and DDoS mitigation providers. In such contexts, a CoAP client can communicate directly with a server or indirectly via proxies. When multiple proxies are involved, infinite forwarding loops may be experienced (e.g., routing misconfiguration, policy conflicts). To prevent such loops, this document defines a new CoAP option, called Hop-Limit (Section 3). Also, the document defines a new CoAP Response Code (Section 6.1) to report loops together with relevant diagnostic information to ease troubleshooting (Section 4).

The Hop-Limit option was originally designed for a specific use case [DOTS-SIG-CHANNEL]. However, its intended usage is general:

• New CoAP proxies MUST implement this option and have it enabled by default.

Note that this means that a server that receives requests both via proxies and directly from clients may see otherwise identical requests with and without the Hop-Limit option included; servers with internal caching will therefore also want to implement this option, since understanding the Hop-Limit option will improve caching efficiency.

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here. Readers should be familiar with the terms and concepts defined in [RFC 7252].

The properties of the Hop-Limit option are shown in Table 1. The formatting of this table follows the one used in Table 4 of 5.10. The C, U, N, and R columns indicate the properties Critical, Unsafe, NoCacheKey, and Repeatable defined in Section 5.4 of RFC 7252. None of these properties is marked for the Hop-Limit option. Table 1: CoAP Hop-Limit Option Properties The Hop-Limit option (Section 6.2) is an elective option used to detect and prevent infinite loops of CoAP requests when proxies are involved. The option is not repeatable. Therefore, any request carrying multiple Hop-Limit options MUST be handled following the procedure specified in Section 5.4.5 of RFC 7252. The value of the Hop-Limit option is encoded as an unsigned integer (see Section 3.2 of RFC 7252). This value MUST be between 1 and 255 inclusive. CoAP requests received with a Hop-Limit option set to '0' or greater than '255' MUST be rejected by a CoAP server/proxy using 4.00 (Bad Request). The Hop-Limit option is safe to forward. That is, a CoAP proxy that does not understand the Hop-Limit option should forward it on. The option is also part of the cache key. As such, a CoAP proxy that does not understand the Hop-Limit option must follow the recommendations in Section 5.7.1 of RFC 7252 for caching. Note that loops that involve only such proxies will not be detected. Nevertheless, the presence of such proxies will not prevent infinite loop detection if at least one CoAP proxy that supports the Hop-Limit option is involved in the loop. A CoAP proxy that understands the Hop-Limit option SHOULD be instructed, using a configuration parameter, to insert a Hop-Limit option when relaying a request that does not include the Hop-Limit option. The initial Hop-Limit value should be configurable. If no initial value is explicitly provided, the default initial Hop-Limit value of 16 MUST be used. This value is chosen so that in the majority of cases, it is sufficiently large to guarantee that a CoAP request would not be dropped in networks when there were no loops, but not so large as to consume CoAP proxy resources when a loop does occur. The value is still configurable to accommodate unusual topologies. Lower values should be used with caution and only in networks where topologies are known by the CoAP client (or proxy) inserting the Hop-Limit option. Because forwarding errors may occur if inadequate Hop-Limit values are used, proxies at the boundaries of an administrative domain MAY be instructed to remove or rewrite the value of Hop-Limit carried in received requests (i.e., ignore the value of Hop-Limit received in a request). This modification should be done with caution in case proxy-forwarded traffic repeatedly crosses the administrative domain boundary in a loop, rendering ineffective the efficacy of loop detection through the Hop-Limit option. Otherwise, a CoAP proxy that understands the Hop-Limit option MUST decrement the value of the option by 1 prior to forwarding it. A CoAP proxy that understands the Hop-Limit option MUST NOT use a stored 5.08 (Hop Limit Reached) error response unless the value of the Hop-Limit option in the presented request is smaller than or equal to the value of the Hop-Limit option in the request used to obtain the stored response. Otherwise, the CoAP proxy follows the behavior in Section 5.6 of RFC 7252.

• Note: If a request with a given value of Hop-Limit failed to reach a server because the hop limit is exhausted, then the same failure will be observed if a smaller value of the Hop-Limit option is used instead.

CoAP requests MUST NOT be forwarded if the Hop-Limit option is set to '0' after decrement. Requests that cannot be forwarded because of exhausted Hop-Limit SHOULD be logged with a 5.08 (Hop Limit Reached) error response sent back to the CoAP peer. It is RECOMMENDED that CoAP implementations support means to alert administrators about loop errors so that appropriate actions are undertaken.

To ease debugging and troubleshooting, the CoAP proxy that detects a loop includes an identifier for itself in the diagnostic payload under the conditions detailed in Section 5.5.2 of RFC 7252. That identifier MUST NOT include any space character (ASCII value 32). The identifier inserted by a CoAP proxy can be, for example, a proxy name (e.g., p11.example.net), proxy alias (e.g., myproxyalias), or IP address (e.g., 2001:db8::1). Each intermediate proxy involved in relaying a 5.08 (Hop Limit Reached) error message prepends its own identifier in the diagnostic payload with a space character used as separator. Only one identifier per proxy should appear in the diagnostic payload. This approach allows the limiting of the size of the 5.08 (Hop Limit Reached) error message, eases the correlation with hops count, and detects whether a proxy was involved in the forwarding of the 5.08 (Hop Limit Reached) error message. Note that an intermediate proxy prepends its identifier only if there is enough space as determined by the Path MTU (Section 4.6 of RFC 7252). If not, an intermediate proxy forwards the 5.08 (Hop Limit Reached) error message to the next hop without updating the diagnostic payload. An intermediate proxy MUST NOT forward a 5.08 (Hop Limit Reached) error message if it detects that its identifier is included in the diagnostic payload. Such messages SHOULD be logged and appropriate alerts sent to the administrators.

This section focuses on the HTTP mappings specific to the CoAP extension specified in this document. As a reminder, the basic normative requirements on HTTP/CoAP mappings are defined in Section 10 of RFC 7252. The implementation guidelines for HTTP/CoAP mappings are elaborated in [RFC 8075]. By default, the HTTP-to-CoAP Proxy inserts a Hop-Limit option following the guidelines in Section 3. The HTTP-to-CoAP Proxy may be instructed by policy to insert a Hop-Limit option only if a Via (Section 5.7.1 of RFC 7230) or CDN-Loop header field [RFC 8586] is present in the HTTP request. The HTTP-to-CoAP Proxy uses 508 (Loop Detected) as the HTTP response status code to map 5.08 (Hop Limit Reached). Furthermore, it maps the diagnostic payload of 5.08 (Hop Limit Reached) as per Section 6.6 of RFC 8075. By default, the CoAP-to-HTTP Proxy inserts a Via header field in the HTTP request if the CoAP request includes a Hop-Limit option. The CoAP-to-HTTP Proxy may be instructed by policy to insert a CDN-Loop header field instead of the Via header field. The CoAP-to-HTTP Proxy maps the 508 (Loop Detected) HTTP response status code to 5.08 (Hop Limit Reached). Moreover, the CoAP-to-HTTP Proxy inserts its information following the guidelines in Section 4. When both HTTP-to-CoAP and CoAP-to-HTTP proxies are involved, the loop detection may break if the proxy-forwarded traffic repeatedly crosses the HTTP-to-CoAP and CoAP-to-HTTP proxies. Nevertheless, if the loop is within the CoAP or HTTP legs, the loop detection is still functional.

IANA has registered the following entry in the "CoAP Response Codes" subregistry available at <https://www.iana.org/assignments/core-parameters>: Table 2: CoAP Response Codes

IANA has registered the following entry in the "CoAP Option Numbers" subregistry available at <https://www.iana.org/assignments/core-parameters>: Table 3: CoAP Option Number

Security considerations related to CoAP proxying are discussed in Section 11.2 of RFC 7252. A CoAP endpoint can probe the topology of a network into which it is making requests by tweaking the value of the Hop-Limit option. Such probing is likely to fail if proxies at the boundaries of that network rewrite the value of Hop-Limit carried in received requests (see Section 3). The diagnostic payload of a 5.08 (Hop Limit Reached) error message may leak sensitive information revealing the topology of an administrative domain. To prevent that, a CoAP proxy that is located at the boundary of an administrative domain MAY be instructed to strip the diagnostic payload or part of it before forwarding on the 5.08 (Hop Limit Reached) response.

[RFC2119]

S. Bradner, "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>.

[RFC7230]

R. Fielding, and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.

[RFC7252]

Z. Shelby, K. Hartke, and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.

[RFC8075]

A. Castellani, S. Loreto, A. Rahman, T. Fossati, and E. Dijk, "Guidelines for Mapping Implementations: HTTP to the Constrained Application Protocol (CoAP)", RFC 8075, DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.

[RFC8174]

B. Leiba, "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>.

[DOTS-SIG-CHANNEL]

T Reddy, M Boucadair, P Patil, A Mortensen, and N Teague, "Distributed Denial-of-Service Open Threat Signaling (DOTS) Signal Channel Specification", Internet-Draft draft-ietf-dots-signal-channel-41, January 2020,
<https://tools.ietf.org/html/draft-ietf-dots-signal-channel-41>.

[RFC8586]

S. Ludin, M. Nottingham, and N. Sullivan, "Loop Detection in Content Delivery Networks (CDNs)", RFC 8586, DOI 10.17487/RFC8586, April 2019,
<https://www.rfc-editor.org/info/rfc8586>.

This specification was part of [DOTS-SIG-CHANNEL]. Many thanks to those who reviewed DOTS specifications. Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, Jim Schaad, Jaime Jiménez, Roni Even, Scott Bradner, Thomas Fossati, Radia Perlman, Éric Vyncke, Suresh Krishnan, Roman Danyliw, Barry Leiba, Christer Holmberg, Benjamin Kaduk, and Adam Roach for their review and comments. Carsten Bormann provided the "Intended Usage" text.

Mohamed Boucadair

Tirumaleswar Reddy.K

Jon Shallow