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Versions: 00 01 draft-ietf-idr-error-handling

Network Working Group                                            E. Chen
Internet Draft                                              P. Mohapatra
Updates: 4271 (if approved)                                     K. Patel
Intended Status: Standards Track                           Cisco Systems
Expiration Date: March 9, 2011                         September 8, 2010


     Revised Error Handling for BGP Updates from External Neighbors
                 draft-chen-ebgp-error-handling-00.txt


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Abstract

   In this document we partially revise the error handling of an UPDATE
   message from an external BGP neighbor.  The essence of the revision
   is to avoid resetting an external BGP session by using the "treat-
   as-withdraw" approach when the whole NLRI field of a malformed UPDATE
   message can be parsed.


1. Introduction

   The base BGP specification [RFC4271] requires that a BGP session be
   reset when an UPDATE message containing a malformed attribute is
   received.  This behavior is undesirable in the case of optional
   transitive attributes as has been discussed and revised in [OPT-
   TRANS].

   However, there are other situations where the behavior is also
   undesirable, but are outside the scope of [OPT-TRANS].  For example,
   there have been a few occurrences in the field where the AS-PATH
   attribute is malformed for a small number of routes.  Resetting the
   BGP session would impact all the other valid routes in these cases.

   Our goal is to minimize the scope of the network that is affected by
   a malformed UPDATE message, and also to limit the impact to only the
   routes involved.  The constrain is that the protocol correctness must
   not be violated.

   In this document we partially revise the error handling of an UPDATE
   message from an external BGP neighbor.  The essence of the revision
   is to avoid resetting an external BGP session by using the "treat-
   as-withdraw" approach specified in [OPT-TRANS] when the whole NLRI
   field of a malformed UPDATE message can be parsed.


1.1. Specification of Requirements

   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].











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2. Revision to Base Specification

   The revised error handling specified in this section is applicable
   only for processing an UPDATE message from an external BGP neighbor.

   The error handling of the following case described in Section 6.3 of
   [RFC4271] remains unchanged:

      If the Withdrawn Routes Length or Total Attribute Length
      is too large (i.e., if Withdrawn Routes Length + Total Attribute
      Length + 23 exceeds the message Length), then the Error Subcode
      MUST be set to Malformed Attribute List.

   The error handling of all other cases described in Section 6.3 of
   [RFC4271] that specify a sesion reset is conditionally revised as
   follows.

   If a path attribute in an UPDATE message from an external BGP
   neighbor is determined to be malformed, the message containing that
   attribute SHOULD be treated as though all contained routes had been
   withdrawn ("treat-as-withdraw") when the whole NLRI field in the
   message can be parsed.

   One exception is that the "attribute discard" approach [OPT-TRANS]
   SHOULD be used to handle a malformed optional transitive attribute
   for which the "attribute discard" approach is specified.

   A BGP speaker MUST provide debugging facilities to permit issues
   caused by malformed UPDATE messages to be diagnosed.  At a minimum,
   such facilities SHOULD include logging an error when such an
   attribute is detected.  The malformed UPDATE message SHOULD be
   analyzed, and the root cause SHOULD be investigated.


3. Parsing of NLRI Fields

   As described in [OPT-TRANS], we observe that in order to use the
   "treat-as-withdraw" approach for a malformed UPDATE, the NLRI field
   and/or MP_REACH and MP_UNREACH [RFC4760] attributes need to be
   successfully parsed.  If this were not possible, the UPDATE would
   necessarily be malformed in some other way beyond the scope of this
   document and therefore, the procedures of [RFC4271] would continue to
   apply.

   To facilitate the determination of the NLRI field in an UPDATE with
   malformed attributes,  we strongly RECOMMEND that the MP_REACH or
   MP_UNREACH attribute (if present) be encoded as the very first path
   attribute in an UPDATE.



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   Traditionally the NLRIs for the IPv4 unicast address family are
   carried immediately following all the attributes in an UPDATE
   [RFC4271].  When such an UPDATE is received, we observe that the NLRI
   field can be determined using the "Message Length" and the "Total
   Attribute Length" (when they are consistent) carried in the message
   instead of relying on the length of individual attributes in the
   message.

   Furthermore, it is observed that the NLRIs for the IPv4 unicast
   address family can also be carried in the MP_REACH attribute of an
   UPDATE when the IPV4 unicast address family capability is shared
   (i.e., both advertised and received) over a BGP session.  For the
   same sake of better debugging and fault handling, we also RECOMMEND
   that the MP_REACH attribute be used and be placed as the very first
   path attribute in an UPDATE in this case.


4. Discussion

   As discussed in [OPT-TRANS], the approach of "treat-as-withdraw" is
   not always safe to use. In the case of internal BGP sessions, the
   resolution of recursive nexthops can result in forwarding loops and
   blakholes when the BGP speakers inside a network have inconsistent
   routing information.

   Depending on the network topology, the routing table, routes
   involved, and whether "tunnels" are used inside a network, the
   approach of "treat-as-withdraw" may work for internal BGP sessions
   only in some specific cases.  Thus it may be deployed for internal
   BGP sessions only as a temporary measure to stop continuous session
   flaps due to malformed UPDATE messages.  Such deployment must be
   carefully evaluated on a case-by-case basis.


5. IANA Considerations

   This document makes no request of IANA.














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6. Security Considerations

   TBD


7. Acknowledgments

   We would like to thank Robert Raszuk, Naiming Shen and Tony Li for
   their review and discussions.


8. References


8.1. Normative References

   [RFC4271]   Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
               Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
               2006.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.


8.2. Informative References

   [RFC4760]   Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
               "Multiprotocol Extensions for BGP-4", RFC 4760,
               January 2007.

   [OPT-TRANS] Scudder, J. and E. Chen, "Error Handling for Optional
               Transitive BGP Attributes", Work in Progress, March 2010.


9. Authors' Addresses

   Enke Chen
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134

   EMail: enkechen@cisco.com


   Pradosh Mohapatra
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134



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   EMail: pmohapat@cisco.com


   Keyur Patel
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95134

   EMail: keyupate@cisco.com










































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