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int idr BGP AS 0 AS_PATH AS-PATH 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.

Autonomous System 0 was listed in the IANA Autonomous System Number Registry as "Reserved - May be use [sic] to identify non-routed networks" (). 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 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.

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 .

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 . 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 . 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 ). 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.

The IANA has updated the registry for "16-bit Autonomous System Numbers" so that the entry for AS 0 is simply "Reserved".

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.

According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable, because a session reset would impact not only routes with the offending attribute, but also other, valid routes exchanged over the session. This document partially revises the error handling for UPDATE messages and provides guidelines for the authors of documents defining new attributes. Finally, it revises the error handling procedures for a number of existing attributes. This document updates error handling for RFCs 1997, 4271, 4360, 4456, 4760, 5543, 5701 and 6368. IANA

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.