HTTP Working Group                                              S. Ludin
Internet-Draft                                       Akamai Technologies
Intended status: Informational                             M. Nottingham
Expires: December 29, 2018                                        Fastly
                                                             N. Sullivan
                                                           June 27, 2018

                          CDN Loop Prevention


   This specification defines the CDN-Loop request header field for

Status of This Memo

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   This Internet-Draft will expire on December 29, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The CDN-Loop Request Header Field . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   In modern deployments of HTTP servers, it is common to interpose
   Content Delivery Networks (CDNs) 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 any one server; 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 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.

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

   This specification defines the CDN-Loop request header field for
   HTTP, to address this shortcoming.

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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].

3.  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:

   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.

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   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.

4.  Security Considerations

   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

5.  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)

6.  References

6.1.  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,

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   [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, <>.

6.2.  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


   Mark Nottingham


   Nick Sullivan


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