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6. Error Handling

This section provides an update to RFC 4271 [RFC4271] with respect to the error conditions noted here and their handling.

Given that the two-octet AS numbers dominate during the transition and are carried in the AS_PATH attribute by an OLD BGP speaker, in this document the "attribute discard" approach is chosen to handle a malformed AS4_PATH attribute.

Similarly, as the AS4_AGGREGATOR is just informational, the "attribute discard" approach is chosen to handle a malformed AS4_AGGREGATOR attribute.

The AS4_PATH attribute and AS4_AGGREGATOR attribute MUST NOT be carried in an UPDATE message between NEW BGP speakers. A NEW BGP speaker that receives the AS4_PATH attribute or the AS4_AGGREGATOR attribute in an UPDATE message from another NEW BGP speaker MUST discard the path attribute and continue processing the UPDATE message. This case SHOULD be logged locally for analysis.

In addition, the path segment types AS_CONFED_SEQUENCE and AS_CONFED_SET [RFC5065] MUST NOT be carried in the AS4_PATH attribute of an UPDATE message. A NEW BGP speaker that receives these path segment types in the AS4_PATH attribute of an UPDATE message from an OLD BGP speaker MUST discard these path segments, adjust the relevant attribute fields accordingly, and continue processing the UPDATE message. This case SHOULD be logged locally for analysis.

The AS4_PATH attribute in an UPDATE message SHALL be considered malformed under the following conditions:

  • the attribute length is not a multiple of two or is too small (i.e., less than 6) for the attribute to carry at least one AS number, or
  • the path segment length in the attribute is either zero or is inconsistent with the attribute length, or
  • the path segment type in the attribute is not one of the types defined: AS_SEQUENCE, AS_SET, AS_CONFED_SEQUENCE, and AS_CONFED_SET.

A NEW BGP speaker that receives a malformed AS4_PATH attribute in an UPDATE message from an OLD BGP speaker MUST discard the attribute and continue processing the UPDATE message. The error SHOULD be logged locally for analysis.

The AS4_AGGREGATOR attribute in an UPDATE message SHALL be considered malformed if the attribute length is not 8.

A NEW BGP speaker that receives a malformed AS4_AGGREGATOR attribute in an UPDATE message from an OLD BGP speaker MUST discard the attribute and continue processing the UPDATE message. The error SHOULD be logged locally for analysis.

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