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

               Loop Detection in Content Delivery Networks (CDNs)

    Abstract

       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
       https://www.rfc-editor.org/info/rfc8586.

    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
       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
       customers.

    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
       chain."

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       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",
       "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.

       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
       [RFC4122]).

       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,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234,
                  DOI 10.17487/RFC5234, January 2008,
                  <https://www.rfc-editor.org/info/rfc5234>.

       [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,
                  <https://www.rfc-editor.org/info/rfc7230>.

       [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,
                  <https://www.rfc-editor.org/info/rfc7231>.

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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, <https://www.rfc-editor.org/info/rfc8174>.

    5.2.  Informative References

       [loop-attack]
                  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,
                  <http://www.icir.org/vern/papers/cdn-loops.NDSS16.pdf>.

       [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
                  Unique IDentifier (UUID) URN Namespace", RFC 4122,
                  DOI 10.17487/RFC4122, July 2005,
                  <https://www.rfc-editor.org/info/rfc4122>.

    Authors' Addresses

       Stephen Ludin
       Akamai Technologies

       Email: sludin@akamai.com

       Mark Nottingham
       Fastly

       Email: mnot@fastly.com

       Nick Sullivan
       Cloudflare

       Email: nick@cloudflare.com

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