RFC 7607 PROPOSED STANDARD BGP · ROUTING
Codification of AS 0 Processing
BGP routers must never originate, propagate or accept AS number 0.
RFC EDITOR
Kumari, W., Bush, R., Schiller, H., and K. Patel, "Codification of AS 0 Processing", RFC 7607, DOI 10.17487/RFC7607, August 2015, <https://www.rfc-editor.org/info/rfc7607>.
BIBTEX
@misc{rfc7607,
series = {Request for Comments},
number = 7607,
howpublished = {RFC 7607},
publisher = {RFC Editor},
doi = {10.17487/RFC7607},
url = {https://www.rfc-editor.org/info/rfc7607},
author = {W. Kumari and R. Bush and H. Schiller and K. Patel},
title = {{Codification of AS 0 Processing}},
pagetotal = 5,
year = 2015,
month = aug,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 7607 Google
Updates: 4271 R. Bush
Category: Standards Track Internet Initiative Japan
ISSN: 2070-1721 H. Schiller
Google
K. Patel
Cisco Systems
August 2015
Codification of AS 0 Processing
Abstract
This document updates RFC 4271 and proscribes the use of Autonomous
System (AS) 0 in the Border Gateway Protocol (BGP) OPEN, AS_PATH,
AS4_PATH, AGGREGATOR, and AS4_AGGREGATOR attributes in the BGP UPDATE
message.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7607.
Copyright Notice
Copyright (c) 2015 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
(http://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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 2
2. Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Normative References . . . . . . . . . . . . . . . . . . 4
5.2. Informative References . . . . . . . . . . . . . . . . . 4
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 4
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Autonomous System 0 was listed in the IANA Autonomous System Number
Registry as "Reserved - May be use [sic] to identify non-routed
networks" ([IANA.AS_Numbers]).
[RFC6491] specifies that AS 0 in a Route Origin Attestation (ROA) is
used to mark a prefix and all its more specific prefixes as not to be
used in a routing context. This allows a resource holder to signal
that a prefix (and the more specifics) should not be routed by
publishing a ROA listing AS 0 as the only origin. To respond to this
signal requires that BGP implementations not accept or propagate
routes containing AS 0.
No clear statement that AS 0 was proscribed could be found in any BGP
specification. This document corrects this omission, most
importantly in the case of the AS_PATH. This represents an update to
the error handling procedures given in Sections 6.2 and 6.3 of
[RFC4271] by specifying the behavior in the presence of AS 0.
At least two implementations discard routes containing AS 0, and this
document codifies this behavior.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Behavior
A BGP speaker MUST NOT originate or propagate a route with an AS
number of zero in the AS_PATH, AS4_PATH, AGGREGATOR, or
AS4_AGGREGATOR attributes.
An UPDATE message that contains the AS number of zero in the AS_PATH
or AGGREGATOR attribute MUST be considered as malformed and be
handled by the procedures specified in [RFC7606].
An UPDATE message that contains the AS number of zero in the AS4_PATH
or AS4_AGGREGATOR attribute MUST be considered as malformed and be
handled by the procedures specified in [RFC6793].
If a BGP speaker receives zero as the peer AS in an OPEN message, it
MUST abort the connection and send a NOTIFICATION with Error Code
"OPEN Message Error" and subcode "Bad Peer AS" (see Section 6 of
[RFC4271]). A router MUST NOT initiate a connection claiming to be
AS 0.
Authors of future protocol extensions that carry the Autonomous
System number are encouraged to keep in mind that AS 0 is reserved
and to provide clear direction on how to handle AS 0.
3. IANA Considerations
The IANA has updated the registry for "16-bit Autonomous System
Numbers" so that the entry for AS 0 is simply "Reserved".
4. Security Considerations
By allowing a Resource Public Key Infrastructure (RPKI) resource
holder to issue a ROA saying that AS 0 is the only valid origin for a
route, we allow them to state that a particular address resource is
not in use. By ensuring that all implementations that see AS 0 in a
route ignore that route, we prevent a malicious party from announcing
routes containing AS 0 in an attempt to hijack those resources.
In addition, by standardizing the behavior upon reception of an
AS_PATH (or AS4_PATH) containing AS 0, this document makes the
behavior better defined.
5. References
5.1. Normative References
[IANA.AS_Numbers]
IANA, "Autonomous System (AS) Numbers",
<http://www.iana.org/assignments/as-numbers>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<http://www.rfc-editor.org/info/rfc6793>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, July 2015,
<http://www.rfc-editor.org/info/rfc7606>.
5.2. Informative References
[RFC6491] Manderson, T., Vegoda, L., and S. Kent, "Resource Public
Key Infrastructure (RPKI) Objects Issued by IANA",
RFC 6491, DOI 10.17487/RFC6491, February 2012,
<http://www.rfc-editor.org/info/rfc6491>.
Acknowledgements
The authors wish to thank Elwyn Davies, Enke Chen, Brian Dickson,
Bruno Decraene, Robert Raszuk, Jakob Heitz, Danny McPherson, Chris
Morrow, iLya, John Scudder, Jeff Tantsura, Daniel Ginsburg, and Susan
Hares. Apologies to those we may have missed; it was not
intentional.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States
Email: warren@kumari.net
Randy Bush
Internet Initiative Japan
5147 Crystal Springs
Bainbridge Island, WA 98110
United States
Email: randy@psg.com
Heather Schiller
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States
Email: has@google.com
Keyur Patel
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States
Email: keyupate@cisco.com
AI ELI5
On this page
PROBLEMNo BGP specification clearly said AS 0 was forbidden, yet a ROA listing AS 0 as the only origin is used to say a prefix must not be routed.
MECHANISMA BGP speaker MUST NOT originate or propagate AS 0, UPDATEs carrying it are treated as malformed, and an OPEN claiming AS 0 is rejected.
WHY IT MATTERSRoutes containing AS 0 are ignored everywhere, which stops anyone from hijacking address space that its holder marked as not in use.
§1Why AS 0 Needed a Rule
AS 0 was listed in the IANA registry as reserved and usable to identify non-routed networks. RFC 6491 says that AS 0 in a Route Origin Attestation (ROA) marks a prefix and its more specifics as not to be used in routing, which lets a resource holder signal that a prefix should not be routed.
To respond to that signal, BGP implementations must not accept or propagate routes containing AS 0. The RFC says no clear statement proscribing AS 0 existed in any BGP specification. It fixes that omission, most importantly for the AS_PATH, and codifies what at least two implementations already do: discard such routes. It updates the error handling in Sections 6.2 and 6.3 of RFC 4271.
An owner can say "nobody should use this address" by naming zero as its only user. For that to work, routers must refuse anything that mentions zero.
§2Required BGP Behavior
A BGP speaker MUST NOT originate or propagate a route with AS number zero in the AS_PATH, AS4_PATH, AGGREGATOR or AS4_AGGREGATOR attributes.
- An UPDATE with AS 0 in
AS_PATHorAGGREGATORMUST be considered malformed and handled under RFC 7606. - An UPDATE with AS 0 in
AS4_PATHorAS4_AGGREGATORMUST be considered malformed and handled under RFC 6793. - If a speaker receives zero as the peer AS in an OPEN message, it MUST abort the connection and send a NOTIFICATION with Error Code "OPEN Message Error" and subcode "Bad Peer AS". A router MUST NOT initiate a connection claiming to be AS 0.
Authors of future extensions that carry an AS number are encouraged to remember AS 0 is reserved and say how to handle it.
If a message mentions zero, treat it as broken. If a router says its own number is zero, hang up on it.
§3IANA Registry Change
IANA updated the registry for 16-bit Autonomous System Numbers so that the entry for AS 0 is simply "Reserved", dropping the earlier wording about identifying non-routed networks.
The official list now just says zero is reserved, with no other use.
§4Security Considerations
A Resource Public Key Infrastructure (RPKI) resource holder can issue a ROA naming AS 0 as the only valid origin, stating that an address resource is not in use. Because every implementation that sees AS 0 in a route ignores that route, a malicious party cannot announce routes containing AS 0 to hijack those resources. Standardizing the behavior for an AS_PATH or AS4_PATH with AS 0 also makes it better defined.
Since everyone ignores zero, a bad actor cannot use it to sneak into an address that its owner marked as unused.
§5References
[RFC4271]RFC 4271Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.January 2006[RFC6491]RFC 6491Manderson, T., Vegoda, L., and S. Kent, "Resource Public Key Infrastructure (RPKI) Objects Issued by IANA", RFC 6491, February 2012.February 2012[RFC6793]RFC 6793Vohra, Q. and E. Chen, "BGP Support for Four-Octet Autonomous System (AS) Number Space", RFC 6793, December 2012.December 2012[RFC7606]RFC 7606Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K. Patel, "Revised Error Handling for BGP UPDATE Messages", RFC 7606, July 2015.August 2015