Constrained Application Protocol (CoAP) Hop-Limit Option

More and more applications are using the Constrained Application Protocol (CoAP) as a communication protocol between involved application agents. For example, 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 (). Also, the document defines a new CoAP Response Code () to report loops together with relevant diagnostic information to ease troubleshooting ().

The Hop-Limit option has originally been designed for a specific use case . However, its intended usage is general: CoAP proxies that do not have specific knowledge that proxy forwarding loops are avoided in some other way, are expected to 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.

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 when, and only when, they appear in all capitals, as shown here. Readers should be familiar with the terms and concepts defined in .

The Hop-Limit option (see ) is an elective option used to detect and prevent infinite loops when proxies are involved. The option is not repeatable. Therefore, any message carrying multiple Hop-Limit options MUST be handled following the procedure specified in Section 5.4.5 of . The value of the Hop-Limit option is encoded as an unsigned integer (see Section 3.2 of ). This value MUST be between 1 and 255 inclusive. CoAP messages 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 which 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 which does not understand the Hop-Limit option must follow the recommendations in Section 5.7.1 of for caching. Note that loops which 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 which support the Hop-Limit option is involved in the loop. A CoAP proxy which understands the Hop-Limit option MAY be instructed, using a configuration parameter, to insert a Hop-Limit option when relaying a request which do 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 to be 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. 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 messages (i.e., ignore the value of Hop-Limit received in a message). This modification should be done with caution in case proxy-forwarded traffic repeatedly crosses the administrative domain boundary in a loop and so Hop-Limit detection gets broken. Otherwise, a CoAP proxy which understands the Hop-Limit option MUST decrement the value of the option by 1 prior to forwarding it. A CoAP proxy which understands the Hop-Limit option MUST NOT use a stored TBA1 (Hop Limit Reached) error response unless the value of the Hop-Limit option in the presented request is less 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 . 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 less value of the Hop-Limit option is used instead. CoAP messages MUST NOT be forwarded if the Hop-Limit option is set to '0' after decrement. Messages that cannot be forwarded because of exhausted Hop-Limit SHOULD be logged with a TBA1 (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 which detects a loop includes its information in the diagnostic payload under the conditions detailed in Section 5.5.2 of . That information MUST NOT include any space character. The information 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 TBA1 (Hop Limit Reached) error message prepends its own information in the diagnostic payload with a space character used as separator. Only one information per proxy should appear in the diagnostic payload. Doing so allows to limit the size of the TBA1 (Hop Limit Reached) error message, and to ease correlation with hops count. Note that an intermediate proxy prepends its information only if there is enough space as determined by the Path MTU (Section 4.6 of ). If not, an intermediate proxy forwards the TBA1 (Hop Limit Reached) error message to the next hop without updating the diagnostic payload.

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 . The implementation guidelines for HTTP/CoAP mappings are elaborated in . By default, the HTTP-to-CoAP Proxy inserts a Hop-Limit option following the guidelines in . 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 ) or CDN-Loop header field is present in the HTTP request. The HTTP-to-CoAP Proxy uses 508 (Loop Detected) as the HTTP response status code to map TBA1 (Hop Limit Reached). Furthermore, it maps the diagnostic payload of TBA1 (Hop Limit Reached) as per Section 6.6 of . 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 TBA1 (Hop Limit Reached). Moreover, the CoAP-to-HTTP Proxy inserts its information following the guidelines in . When both HTTP-to-CoAP and CoAP-to-HTTP proxies are involved, the loop detection may get broken 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 is requested to add the following entry to the "CoAP Response Codes" sub-registry available at https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#response-codes:

This document suggests 5.08 as a code to be assigned for the new response code. Editorial Note: Please update TBA1 statements within the document with the assigned code.

IANA is requested to add the following entry to the "CoAP Option Numbers" sub-registry available at https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#option-numbers:

Security considerations related to CoAP proxying are discussed in Section 11.2 of . The diagnostic payload of a TBA1 (Hop Limit Reached) error message may leak sensitive information revealing the topology of an administrative domain. To prevent that, a CoAP proxy which 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 TBA1 (Hop Limit Reached) response.

This specification was part of . Many thanks to those who reviewed DOTS specifications. Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, Jim Schaad, and Jaime Jiménez for the reviews. Carsten Bormann provided the "Intended Usage" text.