Internet Engineering Task Force (IETF)                          S. Ludin
Request for Comments: 8586                           Akamai Technologies
Category: Standards Track                                  M. Nottingham
ISSN: 2070-1721                                                   Fastly
                                                             N. Sullivan
                                                              April 2019

           Loop Detection in Content Delivery Networks (CDNs)


   This document defines the CDN-Loop request header field for HTTP.
   CDN-Loop addresses an operational need that occurs when an HTTP
   request is intentionally forwarded between Content Delivery Networks
   (CDNs), but is then accidentally or maliciously re-routed back into
   the original CDN causing a non-terminating loop.  The new header
   field can be used to identify the error and terminate the loop.

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

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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   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 . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   In modern deployments of HTTP servers, it is common to interpose
   Content Delivery Networks (CDNs) in front of origin servers to
   improve latency perceived by end users, 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 direct comparison of the
   CDNs' services.

   As a result, it is possible 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 can 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 HTTP request header field to
   help detect such attacks and accidents among forwarding CDNs that
   have implemented it; the header field may not be modified by their

1.1.  Relationship to Via

   HTTP defines the Via header field in Section 5.7.1 of [RFC7230] for
   "tracking message forwards, avoiding request loops, and identifying
   the protocol capabilities of senders along the request/response

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RFC 8586                   CDN Loop Detection                 April 2019

   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 a token (OWS), uri-host, and port
   rules 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 detect loops.

   CDN-Loop  = #cdn-info
   cdn-info  = cdn-id *( OWS ";" OWS parameter )
   cdn-id    = ( uri-host [ ":" port ] ) / pseudonym
   pseudonym = token

   The cdn-id identifies the CDN using either a hostname under its
   control or a pseudonym.  Hostnames are preferred, to help avoid
   accidental collisions.  If a pseudonym is used, unintentional
   collisions are more likely, and therefore values should be carefully
   chosen to prevent them; for example, using a well-known value (such
   as the recognized name of the CDN in question), or a generated value
   with enough entropy to make collisions unlikely (such as a UUID

   Optionally, cdn-info can have semicolon-separated key/value
   parameters to accommodate additional information for the CDN's use.

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

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

   For example:

   GET /image.jpg HTTP/1.1
   Host: cdn-customer.example
   User-Agent: ExampleBrowser/5
   CDN-Loop: foo123.foocdn.example, barcdn.example; trace="abcdef"
   CDN-Loop: AnotherCDN; abc=123; def="456"

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

   The effectiveness of this mechanism relies on all intermediaries
   preserving the header field, since removing (or allowing it to be
   removed, e.g., by customer configuration) would prevent downstream
   CDNs from using it to detect looping.  In general, unknown header
   fields are not removed by intermediaries, but there may be a need to
   add CDN-Loop to an implementation's list of header fields that are
   not to be removed under any circumstances.  The header field SHOULD
   NOT be used for other purposes.

3.  Security Considerations

   The threat model that the CDN-Loop header field addresses is a
   customer who is attacking a service provider by configuring a
   forwarding loop by accident or malice.  For it to function, CDNs
   cannot allow customers to modify or remove it in their configuration
   (see Section 2).

   Note that a CDN that allows customers to remove or modify the
   CDN-Loop header field (i.e., they do not implement this
   specification) remains an attack vector against both implementing and
   non-implementing CDNs.

   A CDN's use of the CDN-Loop header field might expose its presence.
   For example, if CDN A is configured to forward its requests to CDN B
   for a given origin, CDN B's presence can be revealed if it behaves
   differently based upon the presence of the CDN-Loop header field.

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

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   It is possible to sign the contents of the header field (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 specification.

   Depending on how it is used, CDN-Loop can expose information about
   the internal configuration of the CDN; for example, the number of
   hops inside the CDN, and the hostnames of nodes.

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: RFC 8586

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,

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RFC 8586                   CDN Loop Detection                 April 2019

   [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", February 2016,

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,

Authors' Addresses

   Stephen Ludin
   Akamai Technologies


   Mark Nottingham


   Nick Sullivan


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