Constrained Application Protocol (CoAP) Hop Limit Option

More and more applications are using Constrained Application Protocol (CoAP) as a communication protocol between involved application agents. For example, specifies how CoAP is used as a distributed denial-of-service (DDoS) attack signaling protocol seeking for help from 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. To prevent such loops, this document defines a new CoAP option, called Hop-Limit, which is inserted in particular by on-path proxies. Also, the document defines a new CoAP Response Code to report loops together with relevant diagnostic information to ease troubleshooting.

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 . Readers should be familiar with the terms and concepts defined in . Within this document, CoAP agent refers to both CoAP client and CoAP proxy.

Hop-Limit option (see ) is an elective option used to detect and prevent infinite loops when proxies are involved. Only one single instance of the option is allowed in a message. Therefore, any message carrying multiple Hop-Limit option instances MUST be rejected using 4.00 (Bad Request) error message. The value of the Hop-Limit option is encoded as an 8-bit 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 agent 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. If a CoAP proxy receives a request which does not include a Hop-Limit option, it SHOULD insert a Hop-Limit option when relaying the request to a next hop (absent explicit policy/configuration otherwise). 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 agent 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, each intermediate proxy, which understands the Hop-Limit option, involved in the handling of a CoAP message MUST decrement the Hop-Limit option value by 1 prior to forwarding upstream if this parameter exists. 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 message sent back to the CoAP peer. It is RECOMMENDED that CoAP agents 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 SHOULD include its information (e.g., proxy name, proxy alias, IP address) in the diagnostic payload under the conditions detailed in Section 5.5.2 of . Each intermediate proxy involved in relaying a TBA1 (Hop Limit Reached) error message SHOULD prepend 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.

IANA is requested to add the following entries to the "CoAP Response Codes" sub-registry available at https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#response-codes:

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 response.

This specification was part of . Many thanks to those who reviewed DOTS specifications. Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, and Jim Schaad for the review.