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CDN Loop Prevention

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8586.
Authors Stephen Ludin , Mark Nottingham , Nick Sullivan
Last updated 2018-12-20 (Latest revision 2018-10-23)
Replaces draft-cdn-loop-prevention
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tommy Pauly
Shepherd write-up Show Last changed 2018-11-26
IESG IESG state Became RFC 8586 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES. Needs 9 more YES or NO OBJECTION positions to pass.
Responsible AD Alexey Melnikov
Send notices to Patrick McManus <>, Tommy Pauly <>
IANA IANA review state IANA OK - Actions Needed
HTTP Working Group                                              S. Ludin
Internet-Draft                                       Akamai Technologies
Intended status: Standards Track                           M. Nottingham
Expires: April 27, 2019                                           Fastly
                                                             N. Sullivan
                                                        October 24, 2018

                          CDN Loop Prevention


   This specification defines the CDN-Loop request header field for

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

   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 April 27, 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
   ( 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Relationship to Via . . . . . . . . . . . . . . . . . . .   2
     1.2.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  The CDN-Loop Request Header Field . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   In modern deployments of HTTP servers, it is common to interpose
   Content Delivery Networks (CDNs) in front of origin servers to
   improve end-user perceived latency, reduce operational costs, and
   improve scalability and reliability of services.

   Often, more than one CDN is in use by a given origin.  This happens
   for a variety of reasons, such as cost savings, arranging for
   failover should one CDN have issues, or to directly compare their

   As a result, it is not unknown for forwarding CDNs to be configured
   in a "loop" accidentally; because routing is achieved through a
   combination of DNS and forwarding rules, and site configurations are
   sometimes complex and managed by several parties.

   When this happens, it is difficult to debug.  Additionally, it
   sometimes isn't accidental; loops between multiple CDNs be used as an
   attack vector (e.g., see [loop-attack]), especially if one CDN
   unintentionally strips the loop detection headers of another.

   This specification defines the CDN-Loop request header field for HTTP
   to enable secure interoperability of forwarding CDNs.  Having a
   header that is guaranteed not to be modified by other CDNs that are
   used by a shared customer helps give each CDN additional confidence
   that any purpose (debugging, data gathering, enforcement) that they
   use this header for is free from tampering due to how that customer
   configured the other CDNs.

1.1.  Relationship to Via

   HTTP defines the Via header field in [RFC7230], Section 5.7.1 for
   "tracking message forwards, avoiding request loops, and identifying

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   the protocol capabilities of senders along the request/response

   In theory, Via could be used to identify these loops.  However, in
   practice it is not used in this fashion, because some HTTP servers
   use Via for other purposes - in particular, some implementations
   disable some HTTP/1.1 features when the Via header is present.

1.2.  Conventions and Definitions

   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.

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with a list extension, defined in Section 7 of
   [RFC7230], that allows for compact definition of comma-separated
   lists using a '#' operator (similar to how the '*' operator indicates
   repetition).  Additionally, it uses the OWS rule from [RFC7230] and
   the parameter rule from [RFC7231].

2.  The CDN-Loop Request Header Field

   The CDN-Loop request header field is intended to help a Content
   Delivery Network identify when an incoming request has already passed
   through that CDN's servers, to prevent loops.

   CDN-Loop = #cdn-id
   cdn-id   = token *( OWS ";" OWS parameter )

   Conforming Content Delivery Networks SHOULD add a value to this
   header field to all requests they generate or forward (creating the
   header if necessary).

   The token identifies the CDN as a whole.  Chosen token values SHOULD
   be unique enough that a collision with other CDNs is unlikely.
   Optionally, the token can have semicolon-separated key/value
   parameters, to accommodate additional information for the CDN's use.

   As with all HTTP headers defined using the "#" rule, the CDN-Loop
   header can be added to by comma-separating values, or by creating a
   new header field with the desired value.

   For example:

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   CDN-Loop: FooCDN, barcdn; host=""
   CDN-Loop: baz-cdn; abc="123"; def="456", anotherCDN

   Note that the token syntax does not allow whitespace, DQUOTE or any
   of the characters "(),/:;<=>?@[]{}".  See [RFC7230], Section 3.2.6.
   Likewise, note the rules for when parameter values need to be quoted
   in [RFC7231], Section 3.1.1.

   To be effective, intermediaries - including Content Delivery Networks
   - MUST NOT remove this header field, or allow it to be removed (e.g.,
   through configuration) and servers (including intermediaries) SHOULD
   NOT use it for other purposes.

3.  Security Considerations

   The threat model that the CDN-Loop header field addresses is a
   customer who is attempting to attack a service provider by
   configuring a forwarding loop by accident or malice.  For it to
   function, CDNs cannot allow it to be modified by customers (see
   Section 2).

   The CDN-Loop header field can be generated by any client, and
   therefore its contents cannot be trusted.  CDNs who modify their
   behaviour based upon its contents should assure that this does not
   become an attack vector (e.g., for Denial-of-Service).

   It is possible to sign the contents of the header (either by putting
   the signature directly into the field's content, or using another
   header field), but such use is not defined (or required) by this

4.  IANA Considerations

   This document registers the "CDN-Loop" header field in the Permanent
   Message Header Field Names registry.

   o  Header Field Name: CDN-Loop

   o  Protocol: http

   o  Status: standard

   o  Reference: (this document)

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5.  References

5.1.  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,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [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,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

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

5.2.  Informative References

              Chen, J., Jiang, J., Zheng, X., Duan, H., Liang, J., Li,
              K., Wan, T., and V. Paxson, "Forwarding-Loop Attacks in
              Content Delivery Networks", ISBN 1-891562-41-X,
              DOI 10.14722/ndss.2016.23442, February 2016,

Authors' Addresses

   Stephen Ludin
   Akamai Technologies


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   Mark Nottingham


   Nick Sullivan


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