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Versions: (draft-scudder-idr-optional-transitive) 00 01 02 03 04 draft-ietf-idr-error-handling

Internet Engineering Task Force                               J. Scudder
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                 E. Chen
Expires: September 27, 2010                                Cisco Systems
                                                          March 26, 2010


         Error Handling for Optional Transitive BGP Attributes
               draft-ietf-idr-optional-transitive-02.txt

Abstract

   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 in the case of optional transitive
   attributes.  This document revises BGP's error-handling rules for
   optional transitive attributes, and provides guidelines for the
   authors of documents defining new optional transitive attributes.  It
   also introduces a new Path Attribute flag, Neighbor-Complete, to
   allow more accurate fault-finding.  Finally, it revises the error
   handling procedures for several existing optional transitive
   attributes.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 27, 2010.

Copyright Notice



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   Copyright (c) 2010 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 BSD License.


1.  Introduction

   According to the base BGP specification [RFC4271], 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 in the case of optional
   transitive attributes whose Partial flag is set; the reason is that
   such attributes may have been propagated without being checked by
   intermediate routers that do not recognize the attribute -- in effect
   the attributes may have been tunneled, and when they do reach a
   router that recognizes and checks them, the session that is reset may
   not be associated with the router that is at fault.  This document
   revises BGP's error-handling rules for optional transitive
   attributes, and provides guidelines for the authors of documents
   defining new optional transitive attributes.  It also revises the
   error handling procedures for several existing optional transitive
   attributes.  Specifically, the error handling procedures of
   [RFC4271], [RFC1997], and [RFC4360] are revised.

   Error handling procedures are not revised if the error can be imputed
   to the direct neighbor.  A new flag, Neighbor-Complete, is introduced
   which, when used, allows the direct neighbor's involvement to be
   determined unequivocally.  Imputation of "blame" to the direct
   neighbor is achieved by checking the Partial flag and the Neighbor-
   Complete flag.  If the Partial flag is clear, or the Neighbor-
   Complete flag is set, the original error handling procedures remain
   in force.

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].




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2.  Neighbor-Complete Flag Bit

   It is desirable to know whether a neighbor recognizes, or does not
   recognize, a given optional transitive attribute.  The Partial Path
   Attribute flag does not provide exactly this information -- it only
   enables the determination that a given neighbor did understand such
   an attribute, if the flag is set to zero.  However, if the flag is
   set to one all that can be concluded is that some BGP speaker in the
   path did not understand the attribute, it cannot be determined
   whether the speaker in question was the neighbor or some other
   speaker.

   To remedy this, we introduce a new Path Attribute Flag to those
   defined in [RFC4271] Section 4.3.  The fifth high-order bit (bit 4)
   of the Attribute Flags octet is the Neighbor-Complete bit.  It
   indicates whether the neighbor that sent the message recognizes the
   attribute (if set to one) or does not recognize it (if set to zero).
   The Neighbor-Complete flag only applies to optional transitive
   attributes.  For other types of attributes the flag MUST be sent as
   zero and ignored when received.

   A BGP speaker MUST set the Neighbor-Complete flag to one when sending
   a recognized, or zero when sending an unrecognized, optional
   transitive path attribute to its neighbor.

   The Neighbor-Complete flag is the equivalent of the Partial flag,
   with two differences.  First, it is reset on a hop-by-hop basis.
   Second, its "polarity" is reversed, with one instead of zero
   indicating that a neighbor does recognize the attribute.  The reason
   for this difference is that during the period while this
   specification is being adopted, some BGP speakers will recognize the
   Neighbor-Complete flag and some will not.  Since the previous
   definition [RFC4271] of bit 4 required it to be sent as zero, the use
   of one to mean "attribute recognized" allows the recipient of such a
   flag to unequivocally determine that a neighbor does recognize the
   given attribute.

   Use of the flag on receipt is discussed in Section 3.


3.  Revision to Base Specification

   Section 6.3 of [RFC4271] is revised as follows.  The paragraphs
   related to "any recognized attribute" and "an optional attribute" do
   not apply to optional transitive attributes received with their
   Partial flag set and Neighbor-Complete flag clear -- an error limited
   to such an attribute SHALL NOT be responded to by sending a
   NOTIFICATION message or resetting the BGP session.  Instead, when



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   such an attribute is determined to be malformed, the UPDATE message
   containing that attribute SHOULD be treated as though all contained
   routes had been withdrawn just as if they had been listed in the
   WITHDRAWN ROUTES field of the UPDATE message, thus causing them to be
   removed from the Adj-RIB-In according to the procedures of [RFC4271].
   In the case of an optional transitive attribute which has no effect
   on route selection or installation, the malformed attribute MAY
   instead be discarded and the UPDATE message continue to be processed.

   An example of an attribute which has no effect on route selection or
   installation is the AGGREGATOR attribute.

   A document which specifies an optional transitive attribute MUST
   provide specifics regarding what constitutes an error for that
   attribute and how that error is to be handled.

   Note that the revised error handling only applies when an individual
   optional transitive attribute is received with its Partial flag set
   and Neighbor-Complete flag clear and deemed to be erroneous.  In the
   event that an UPDATE message is deemed to be malformed in any other
   way then the procedures of [RFC4271] MUST be applied.  This is
   likewise the case if an optional transitive attribute is received
   whose Partial flag is not set or whose Neighbor-Complete flag is set
   -- this is because the detected error can be imputed to the direct
   peer.

   Examples of errors which would continue to be treated according to
   the procedures of [RFC4271] include the cases where the Total
   Attribute Length is inconsistent with the message length, or where
   there is more than one attribute with a given type code.  Also,
   implicit in the foregoing paragraph is the fact that if due to an
   error, including those in an optional transitive attribute, the other
   attributes of the UPDATE message cannot be correctly parsed, then the
   procedures of [RFC4271] continue to apply.

   In the specific case of incorrect path attribute flags -- i.e., a
   path attribute that is known by its type code to be Optional and
   Transitive but whose flags are not set accordingly -- the behavior
   specified by [RFC4271] SHALL be followed.  (Consider that in the case
   of such an error, the "tunneling" argument given above does not
   apply, by definition.)

   Finally, we observe that in order to treat an UPDATE as though all
   contained routes had been withdrawn as discussed above, 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 way beyond the scope of this
   document and therefore, the procedures of [RFC4271] would continue to



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   apply.


4.  Operational Considerations

   Although the "treat as withdraw" error-handling behavior defined in
   Section 3 makes every effort to preserve BGP's correctness, we note
   that if an UPDATE received on an IBGP session is subjected to this
   treatment, inconsistent routing within the affected Autonomous System
   may result.  The consequences of inconsistent routing can include
   long-lived forwarding loops and black holes.  While lamentable, this
   issue is expected to be rare in practice, and more importantly is
   seen as less problematic than the session-reset behavior it replaces.

   Even if inconsistent routing does not arise, the "treat as withdraw"
   behavior can cause either complete unreachability or sub-optimal
   routing for the destinations whose routes are carried in the affected
   UPDATE message.

   Note that "treat as withdraw" is different from discarding an UPDATE
   message.  The latter violates the basic BGP principle of incremental
   update, and could cause invalid routes to be kept.  (See also
   Appendix A.)

   For any malformed attribute which is discarded instead of the
   containing UPDATE being treated as a withdraw as discussed in
   Section 3, it is critical to consider the potential impact of doing
   so.  In particular, if the attribute in question has or may have an
   effect on route selection or installation, the presumption is that
   discarding it is unsafe, unless careful analysis proves otherwise.
   The analysis should take into account the tradeoff between preserving
   connectivity and potential side effects.

   Because of these potential issues, a BGP speaker MUST provide
   debugging facilities to permit issues caused by malformed optional
   transitive attributes to be diagnosed.  At a minimum, such facilities
   SHOULD include logging an error when such an attribute is detected.


5.  Error Handling Procedures for Existing Optional Transitive
    Attributes

5.1.  AGGREGATOR

   The error handling of [RFC4271] is revised as follows:

   The AGGREGATOR attribute SHALL be considered malformed if any of the
   following applies:



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   o  Its length is not 6 (when the "4-octet AS number capability" is
      not advertised to, or not received from the peer [RFC4893]).

   o  Its length is not 8 (when the "4-octet AS number capability" is
      both advertised to, and received from the peer).

   An UPDATE message with a malformed AGGREGATOR attribute SHALL be
   handled as follows.  If its Partial flag is set and its Neighbor-
   Complete flag is clear, either the attribute MUST be discarded or the
   UPDATE containing it treated as a withdraw as discussed in Section 3.
   Otherwise (i.e. if its Partial flag is clear or its Neighbor-Complete
   flag is set), the procedures of [RFC4271] MUST be followed with
   respect to an Optional Attribute Error.

5.2.  Community

   The error handling of [RFC1997] is revised as follows:

   The Community attribute SHALL be considered malformed if its length
   is not a nonzero multiple of 4.

   An UPDATE message with a malformed Community attribute SHALL be
   handled as follows.  If its Partial flag is set and its Neighbor-
   Complete flag is clear, the update containing it MUST be treated as a
   withdraw as discussed in Section 3.  Otherwise (i.e. if its Partial
   flag is clear or its Neighbor-Complete flag is set), the procedures
   of [RFC4271] MUST be followed with respect to an Optional Attribute
   Error.

5.3.  Extended Community

   The error handling of [RFC4360] is revised as follows:

   The Extended Community attribute SHALL be considered malformed if its
   length is not a nonzero multiple of 8.

   An UPDATE message with a malformed Extended Community attribute SHALL
   be handled as follows.  If its Partial flag is set and its Neighbor-
   Complete flag is clear, the update containing it MUST be treated as a
   withdraw as discussed in Section 3.  Otherwise (i.e. if its Partial
   flag is clear or its Neighbor-Complete flag is set), the procedures
   of [RFC4271] MUST be followed with respect to an Optional Attribute
   Error.

   Note that a BGP speaker MUST NOT treat an unrecognized Extended
   Community Type or Sub-Type as an error.





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6.  Security Considerations

   This specification addresses the vulnerability of a BGP speaker to a
   potential attack whereby a distant attacker can generate a malformed
   optional transitive attribute that is not recognized by intervening
   routers (which thus propagate the attribute unchecked) but that
   causes session resets when it reaches routers that do recognize the
   given attribute type.

   In other respects, this specification does not change BGP's security
   characteristics.


7.  Acknowledgements

   The authors wish to thank Ron Bonica, Andy Davidson, Dong Jie, Rex
   Fernando, Joel Halpern, Akira Kato, Miya Kohno, Alton Lo, Shin
   Miyakawa, Jonathan Oddy, Robert Raszuk, Yakov Rekhter, Rob Shakir,
   Ananth Suryanarayana, and Kaliraj Vairavakkalai for their
   observations and discussion of this topic.  The Neighbor-Complete
   flag was introduced as the result of helpful discussion with Jie Dong
   and Mach Chen.


8.  IANA Considerations

   IANA is requested to establish and maintain a registry of BGP Path
   Attribute Flags.  Flags one through four are defined in [RFC4271].
   Flag five is defined in Section 2 of this document.  Future
   allocations are to be made according to the IETF Standards Action
   policy [RFC5226].


9.  References

9.1.  Normative References

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

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

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

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.



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   [RFC4893]  Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
              Number Space", RFC 4893, May 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

9.2.  Informative References

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


Appendix A.  Why not discard UPDATES?

   A commonly asked question is "why not simply discard the UPDATE
   message instead of treating it like a withdraw?  Isn't that safer and
   easier?"  The answer is that it might be easier, but it would
   compromise BGP's correctness so is unsafe.  Consider the following
   example of what might happen if UPDATE messages carrying bad
   attributes were simply discarded:

                                  AS1--AS2
                                   \   /
                                    \ /
                                    AS3

   o  AS1 prefers to reach AS3 directly, and advertises its route to
      AS2.

   o  AS2 prefers to reach AS3 directly, and advertises its route to
      AS1.

   o  Connections AS3-AS1 and AS3-AS2 fail simultaneously.

   o  AS1 switches to prefer AS2's route, and sends an update message
      which includes a withdraw of its previous announcement.  The
      withdraw is bundled with some advertisements.  It includes a bad
      attribute.  As a result, AS2 ignores the message.

   o  AS2 switches to prefer AS1's route, and sends an update message
      which includes a withdraw of its previous announcement.  The
      withdraw is bundled with some advertisements.  It includes a bad
      attribute.  As a result, AS1 ignores the message.

   The end result is that AS1 forwards traffic for AS3 towards AS2, and
   AS2 forwards traffic for AS3 towards AS1.  This is a permanent (until



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   corrected) forwarding loop.

   Although the example above discusses route withdraws, we observe that
   in BGP the announcement of a route also withdraws the route
   previously advertised.  The implicit withdraw can be converted into a
   real withdraw in a number of ways; for example, the previously-
   announced route might have been accepted by policy, but the new
   announcement might be rejected by policy.  For this reason, the same
   concerns apply even if explicit withdraws are removed from
   consideration.


Authors' Addresses

   John G. Scudder
   Juniper Networks

   Email: jgs@juniper.net


   Enke Chen
   Cisco Systems

   Email: enkechen@cisco.com



























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