Deprecation of AS_SET and AS_CONFED_SET in BGP
Extracted elements (21)
AS_SETs are being prohibited because they blur the semantics of the origin AS for aggregated prefixes, which prevents authentication of route origin in RPKI-based security technologies and can cause reachability problems. Analysis of historical Internet routing data shows AS_SET aggregation is very seldom used in practice.
Consistent brief aggregation (truncating the AS_PATH at the desired origin AS rather than generating an AS_SET) is recommended because brief aggregation without it can yield an unstable origin AS that varies depending on which contributing routes are present, breaking RPKI-ROV.
Not advertising the aggregate prefix back to contributing ASes avoids data-plane forwarding loops that arise when brief aggregation is used, since AS_PATH loop detection no longer prevents contributing ASes from installing the less-specific aggregate route.
When AS_SET aggregation does occur in practice, it is often used incorrectly: the most common case is a single AS in the AS_SET, the same AS often appears in both the AS_SEQUENCE and the AS_SET, and reserved AS numbers appear with some frequency.
An explicit operator configuration override is permitted for the prohibition on advertising AS_SET/AS_CONFED_SET, to accommodate transition phases where legacy peers or aggregation policies require continued use.
This document updates RFC 4271 by deprecating AS_SET (Type 1) origination and RFC 5065 by deprecating AS_CONFED_SET (Type 4) origination; it also obsoletes RFC 6472 (BCP 172), advancing that BCP recommendation to a standards-track prohibition.
An aggregate prefix SHOULD NOT be announced to the contributing ASes; instead, more specific prefixes SHOULD be announced to each contributing AS, excluding any learned from that contributing AS.
BGP speakers MUST NOT advertise BGP UPDATE messages containing AS_SETs or AS_CONFED_SETs in the AS_PATH, unless explicitly configured by a network operator to do otherwise (e.g., during a transition phase).
If the resulting AS_PATH is truncated from the otherwise expected result of BGP AS_PATH aggregation, the ATOMIC_AGGREGATE Path Attribute SHOULD be attached, consistent with RFC 4271 Section 5.1.6.
Network operators MUST use consistent brief aggregation as described in Section 5.2; in consistent brief aggregation, AGGREGATOR and ATOMIC_AGGREGATE Path Attributes are included but the AS_PATH does not have AS_SET or AS_CONFED_SET path segment types.
Per RFC 4632 Rule #2, a router generating an aggregate route MUST discard packets that match the aggregate route but not any of the more-specific routes (i.e., the next hop for the aggregate route should be the null destination).
To ensure a consistent BGP origin AS for aggregate routes, the implementation MUST be configured to truncate the AS_PATH after the right-most instance of the desired origin AS for the aggregate. This is called 'consistent brief' BGP aggregation.
Upon reception of BGP UPDATE messages containing AS_SETs or AS_CONFED_SETs in the AS_PATH or AS4_PATH, BGP speakers MUST use the 'treat-as-withdraw' error handling behavior per RFC 7606.
AS_CONFED_SET (Type 4 AS_PATH segment) is an unordered set of Member AS Numbers in the local confederation that contributing prefixes have traversed, used within a confederation during aggregation. This document deprecates its origination.
AS_SET (Type 1 AS_PATH segment) is an unordered set of Autonomous Systems that contributing prefixes in an aggregate have traversed, created by a router performing route aggregation. This document deprecates its origination.
Brief aggregation refers to BGP AS_PATH aggregation per RFC 4271 Section 9.2.2.2 where any resulting AS_SETs are discarded and the ATOMIC_AGGREGATE attribute is attached; the resulting AS_PATH is the longest leading common sequence of tuples from contributing routes.
Consistent brief aggregation is a form of brief aggregation where the AS_PATH is truncated after the right-most instance of the desired origin AS, ensuring a stable and unambiguous origin AS for RPKI-ROV purposes and avoiding AS_SET generation.
The ATOMIC_AGGREGATE Path Attribute is attached to an aggregate BGP route when AS_SETs are dropped or the AS_PATH is truncated, signaling that the route was aggregated and some AS path information may have been omitted.
Brief aggregation without consistent origin AS control can result in unpredictable origin ASes, requiring resource holders to register ROAs for each potential origin AS and complicating RPKI-ROV validation for aggregate prefixes.
Deprecating AS_SET and AS_CONFED_SET simplifies BGP and reduces the attack surface; removing related code from implementations is beneficial. New BGP security technologies such as RPKI, BGPsec, and ASPA benefit greatly when AS_SETs are not used.
RPKI-ROV never allows a route with AS_SET to be considered Valid; BGPsec does not support AS_SETs; and ASPA-based AS_PATH verification always considers a route with AS_SET as Invalid and ineligible for route selection.