CORE M. Boucadair
Internet-Draft Orange
Intended status: Standards Track T. Reddy
Expires: March 21, 2019 McAfee
J. Shallow
NCC Group
September 17, 2018
Constrained Application Protocol (CoAP) Hop Limit Option
draft-ietf-core-hop-limit-00
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 March 21, 2019.
Copyright Notice
Copyright (c) 2018 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
Boucadair, et al. Expires March 21, 2019 [Page 1]
Internet-Draft CoAP Hop Limit Option September 2018
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 . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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, 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.
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
[RFC2119].
Readers should be familiar with the terms and concepts defined in
[RFC7252].
Within this document, CoAP agent refers to both CoAP client and CoAP
proxy.
Boucadair, et al. Expires March 21, 2019 [Page 2]
Internet-Draft CoAP Hop Limit Option September 2018
3. Hop-Limit Option
Hop-Limit option (see Section 4.2) 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 [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 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
Boucadair, et al. Expires March 21, 2019 [Page 3]
Internet-Draft CoAP Hop Limit Option September 2018
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 [RFC7252].
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 entries 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
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 | | | x | | Hop-Limit | [RFCXXXX] |
+--------+---+---+---+---+------------------+-----------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Table 2: CoAP Option Number
Boucadair, et al. Expires March 21, 2019 [Page 4]
Internet-Draft CoAP Hop Limit Option September 2018
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 review.
7. References
7.1. 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,
<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>.
7.2. Informative References
[I-D.ietf-dots-signal-channel]
Reddy, T., 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-25 (work in progress), September
2018.
Authors' Addresses
Boucadair, et al. Expires March 21, 2019 [Page 5]
Internet-Draft CoAP Hop Limit Option September 2018
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
Boucadair, et al. Expires March 21, 2019 [Page 6]