CORE                                                        M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: August 30, 2019 January 4, 2020                                          McAfee
                                                              J. Shallow
                                                               NCC Group
                                                       February 26,
                                                            July 3, 2019

        Constrained Application Protocol (CoAP) Hop Limit Hop-Limit Option


   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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Intended Usage  . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2   3
   3.  Hop-Limit Option  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  HTTP-Mapping Considerations . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     4.1.   5
     5.1.  CoAP Response Code  . . . . . . . . . . . . . . . . . . .   4
     4.2.   5
     5.2.  CoAP Option Number  . . . . . . . . . . . . . . . . . . .   5
   5.   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6   8

1.  Introduction

   More and more applications are using the Constrained Application
   Protocol (CoAP) [RFC7252] as a communication protocol between
   involved application agents.  For example,
   [I-D.ietf-dots-signal-channel] specifies how CoAP is used as a
   distributed denial-of-service (DDoS) attack signaling protocol for
   seeking for help from DDoS mitigation providers.  In such contexts, a
   CoAP client can communicate directly with a server or indirectly via

   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), which is inserted in particular
   by on-path proxies. 3).  Also, the document defines a new CoAP
   Response Code (Section 4.1) 5.1) to report loops together with relevant
   diagnostic information to ease troubleshooting.

1.1.  Intended Usage

   The Hop-Limit option has originally been designed for a specific use
   case [I-D.ietf-dots-signal-channel].  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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   Readers should be familiar with the terms and concepts defined in

3.  Hop-Limit Option

   The Hop-Limit option (see Section 4.2) 5.2) 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 rejected using 4.00 (Bad Request) error
   message. handled following the procedure specified
   in Section 5.4.5 of [RFC7252].

   The value of the Hop-Limit option is encoded as an 8-bit unsigned integer
   (see Section 3.2 of [RFC7252]).  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 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 [RFC7252] for caching.  Note that
   loops which involve only such proxies won't be detected.
   Nevertheless, the presence of such proxies won't 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

   The initial Hop-Limit value SHOULD 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 [RFC7252].

      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

   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 SHOULD include includes its information (e.g., proxy name, proxy alias,
   IP address) in the diagnostic payload under the
   conditions detailed in Section 5.5.2 of [RFC7252].  That information
   MUST NOT include any space character.  The information inserted by a
   CoAP proxy can be, for example, a proxy name (e.g.,,
   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 SHOULD prepend prepends its own information in the diagnostic
   payload with a space character used as separator.  Only one
   information per proxy SHOULD 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 [RFC7252]).  If
   not, an intermediate proxy forwards the TBA1 (Hop Limit Reached)
   error message to the next hop without updating the diagnostic

4.  HTTP-Mapping Considerations

   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 [RFC7252].  The implementation guidelines for HTTP/CoAP
   mappings are elaborated in [RFC8075].

   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 [RFC7230]) or CDN-Loop header field [RFC8586] 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 [RFC8075].

   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 Section 3.

   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.

5.  IANA Considerations


5.1.  CoAP Response Code

   IANA is requested to add the following entry to the "CoAP Response
   Codes" sub-registry available at

                  | Code | Description      | Reference |
                  | TBA1 | Hop Limit Reached| [RFCXXXX] |

                        Table 1: CoAP Response Codes

   This document suggests 5.06 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.


5.2.  CoAP Option Number

   IANA is requested to add the following entry to the "CoAP Option
   Numbers" sub-registry available at

         | Number | C | U | N | R | Name             | Reference |
         |  TBA2  |   |   |   |   | Hop-Limit        | [RFCXXXX] |
             C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                  Table 2: CoAP Option Number


6.  Security Considerations

   Security considerations related to CoAP proxying are discussed in
   Section 11.2 of [RFC7252].

   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.


7.  Acknowledgements

   This specification was part of [I-D.ietf-dots-signal-channel].  Many
   thanks to those who reviewed DOTS specifications.

   Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, and Jim
   Schaad for the reviews.


   Carsten Bormann provided the "Intended Usage" text.

8.  References


8.1.  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,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC8075]  Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Guidelines for Mapping Implementations: HTTP to
              the Constrained Application Protocol (CoAP)", RFC 8075,
              DOI 10.17487/RFC8075, February 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.


8.2.  Informative References

              K, R., Boucadair, M., Patil, P., Mortensen, A., and N.
              Teague, "Distributed Denial-of-Service Open Threat
              Signaling (DOTS) Signal Channel Specification", draft-
              ietf-dots-signal-channel-34 (work in progress), January May 2019.

   [RFC8586]  Ludin, S., Nottingham, M., and N. Sullivan, "Loop
              Detection in Content Delivery Networks (CDNs)", RFC 8586,
              DOI 10.17487/RFC8586, April 2019,

Authors' Addresses

   Mohamed Boucadair
   Rennes  35000


   Tirumaleswar Reddy
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071


   Jon Shallow
   NCC Group
   United Kingdom