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IETF RFC 8586
Last modified on Wednesday, April 24th, 2019
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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/RFC 8586.
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.
Ludin, et al. Standards Track PAGE 1 
RFC 8586 CDN Loop Detection April 2019
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 [RFC 7230] for
"tracking message forwards, avoiding request loops, and identifying
the protocol capabilities of senders along the request/response
chain."
Ludin, et al. Standards Track PAGE 2
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC 5234] with a list extension, defined in Section 7 of
[RFC 7230], 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 [RFC 7230] and the parameter rule from [RFC 7231].
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
[RFC 4122]).
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).
Ludin, et al. Standards Track PAGE 3
RFC 8586 CDN Loop Detection April 2019
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
[RFC 7230]. Likewise, note the rules for when parameter values need
to be quoted in Section 3.1.1 of [RFC 7231].
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).
Ludin, et al. Standards Track PAGE 4
RFC 8586 CDN Loop Detection April 2019
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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC 5234, January 2008,
<https://www.rfc-editor.org/info/RFC 5234>.
[RFC 7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC 7230, June 2014,
<https://www.rfc-editor.org/info/RFC 7230>.
[RFC 7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC 7231, June 2014,
<https://www.rfc-editor.org/info/RFC 7231>.
Ludin, et al. Standards Track PAGE 5
RFC 8586 CDN Loop Detection April 2019
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
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>.
[RFC 4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC 4122, July 2005,
<https://www.rfc-editor.org/info/RFC 4122>.
Authors' Addresses
Stephen Ludin
Akamai Technologies
Email: sludin@akamai.com
Mark Nottingham
Fastly
Email: mnot@fastly.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
Ludin, et al. Standards Track PAGE 6
RFC TOTAL SIZE: 12560 bytes
PUBLICATION DATE: Wednesday, April 24th, 2019
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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