Constrained Application Protocol (CoAP) Hop-Limit Option
RFC 8768

Document Type RFC - Proposed Standard (March 2020; No errata)
Last updated 2020-03-20
Replaces draft-boucadair-core-hop-limit
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Stream WG state Submitted to IESG for Publication
Document shepherd Jaime Jimenez
Shepherd write-up Show (last changed 2019-09-10)
IESG IESG state RFC 8768 (Proposed Standard)
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Send notices to Jaime Jimenez <jaime@iki.fi>
IANA IANA review state IANA OK - Actions Needed
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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

Abstract

   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.

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
   https://www.rfc-editor.org/info/rfc8768.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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
   Acknowledgements
   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
      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.

2.  Terminology

   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 [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
   [RFC7252].

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
   option.

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