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Versions: (draft-boucadair-core-hop-limit) 00 01 02 03

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


        Constrained Application Protocol (CoAP) Hop Limit Option
                      draft-ietf-core-hop-limit-03

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 30, 2019.

Copyright Notice

   Copyright (c) 2019 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Hop-Limit Option  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  CoAP Response Code  . . . . . . . . . . . . . . . . . . .   4
     4.2.  CoAP Option Number  . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

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

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].






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3.  Hop-Limit Option

   The Hop-Limit option (see Section 4.2) 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.

   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 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
   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



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   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
      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 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 [RFC7252].  That information MUST NOT include any
   space character.

   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.

4.  IANA Considerations

4.1.  CoAP Response Code

   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:

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

                        Table 1: CoAP Response Codes




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   This document suggests 5.06 as a code to be assigned for the new
   response code.

      Editorial Note: Please update TBA1 statements within the document
      with the assigned code.

4.2.  CoAP Option Number

   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:

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

                  Table 2: CoAP Option Number

5.  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 response.

6.  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.

7.  References

7.1.  Normative References







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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [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>.

7.2.  Informative References

   [I-D.ietf-dots-signal-channel]
              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-28 (work in progress), January
              2019.

Authors' Addresses

   Mohamed Boucadair
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com


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

   Email: kondtir@gmail.com


   Jon Shallow
   NCC Group
   United Kingdom

   Email: jon.shallow@nccgroup.com




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