RFC8768: Constrained Application Protocol (CoAP) Hop-Limit Option

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Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 8768                                        Orange
Category: Standards Track                                     T. Reddy.K
ISSN: 2070-1721                                                   McAfee
                                                              J. Shallow
                                                              March 2020

        Constrained Application Protocol (CoAP) 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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Intended Usage
   2.  Terminology
   3.  Hop-Limit Option
   4.  Debugging and Troubleshooting
   5.  HTTP Mapping Considerations
   6.  IANA Considerations
     6.1.  CoAP Response Code
     6.2.  CoAP Option Number
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

1.  Introduction

   More and more applications are using the Constrained Application
   Protocol (CoAP) [RFC7252] 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).

1.1.  Intended Usage

   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

   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

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 properties of the Hop-Limit option are shown in Table 1.  The
   formatting of this table follows the one used in Table 4 of [RFC7252]
   (Section 5.10).  The C, U, N, and R columns indicate the properties
   Critical, Unsafe, NoCacheKey, and Repeatable defined in Section 5.4
   of [RFC7252].  None of these properties is marked for the Hop-Limit

    | Number | C | U | N | R | Name      | Format | Length | Default |
    | 16     |   |   |   |   | Hop-Limit | uint   | 1      | 16      |

                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 [RFC7252].

   The value of the Hop-Limit option is encoded as an unsigned integer
   (see Section 3.2 of [RFC7252]).  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 [RFC7252] 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

   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 [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 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.

4.  Debugging and Troubleshooting

   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 [RFC7252].  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 [RFC7252]).  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.

5.  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 5.08 (Hop Limit Reached).  Furthermore, it maps
   the diagnostic payload of 5.08 (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 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

6.  IANA Considerations

6.1.  CoAP Response Code

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

                 | Code | Description       | Reference |
                 | 5.08 | Hop Limit Reached | RFC 8768  |

                       Table 2: CoAP Response Codes

6.2.  CoAP Option Number

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

                    | Number | Name      | Reference |
                    | 16     | Hop-Limit | RFC 8768  |

                       Table 3: CoAP Option Number

7.  Security Considerations

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

   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.

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, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

              Reddy, T., Boucadair, M., Patil, P., Mortensen, A., and N.
              Teague, "Distributed Denial-of-Service Open Threat
              Signaling (DOTS) Signal Channel Specification", Work in
              Progress, Internet-Draft, draft-ietf-dots-signal-channel-
              41, 6 January 2020, <https://tools.ietf.org/html/draft-

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


   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.

Authors' Addresses

   Mohamed Boucadair
   35000 Rennes

   Email: mohamed.boucadair@orange.com

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

   Email: kondtir@gmail.com

   Jon Shallow
   United Kingdom

   Email: supjps-ietf@jpshallow.com