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Versions: (draft-ietf-idr-optional-transitive) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

Internet Engineering Task Force (IETF)                        J. Scudder
Internet Draft                                          Juniper Networks
Update: 1997, 4271, 4360 (if approved)                           E. Chen
Intended Status: Standards Track                            P. Mohapatra
Expires: May 17, 2012                                           K. Patel
                                                           Cisco Systems
                                                       November 16, 2011


             Revised Error Handling for BGP UPDATE Messages
                  draft-ietf-idr-error-handling-00.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 as 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 several existing 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
   http://www.ietf.org/1id-abstracts.html

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

   This Internet-Draft will expire on May 17, 2012.




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Copyright Notice

   Copyright (c) 2011 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 Simplified 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 as a session reset would
   impact not only routes with the offending attribute, but also other
   valid routes exchanged over the session.  In the case of optional
   transitive attributes, the behavior is especially troublesome and may
   present a potential security vulnerability.  The reason is that such
   attributes may have been propagated without being checked by
   intermediate routers that do not recognize the attributes -- in
   effect the attribute 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.

   The goal for revising the error handling for UPDATE messages is to
   minimize the impact on routing by a malformed UPDATE message, while
   maintaining protocol correctness to the extent possible.  This can be
   achieved largely by maintaining the established session and keeping
   the valid routes exchanged, but removing the routes carried in the
   malformed UPDATE from the routing system.

   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 several existing attributes.  Specifically, the error
   handling procedures of [RFC4271], [RFC1997], and [RFC4360] are
   revised.






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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 RFC 2119 [RFC2119].


2. Revision to Base Specification

   The first paragraph of Section 6.3 of [RFC4271] is revised as
   follows:

   Old Text:

      All errors detected while processing the UPDATE message MUST be
      indicated by sending the NOTIFICATION message with the Error Code
      UPDATE Message Error. The error subcode elaborates on the specific
      nature of the error.

   New text:

      An error detected while processing the UPDATE message for which a
      session reset is specified MUST be indicated by sending the
      NOTIFICATION message with the Error Code UPDATE Message Error.
      The error subcode elaborates on the specific nature of the error.

   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 the following case described in Section 6.3 of
   [RFC4271] is revised

      If any recognized attribute has Attribute Flags that conflict with
      the Attribute Type Code, then the Error Subcode MUST be set to
      Attribute Flags Error.  The Data field MUST contain the erroneous
      attribute (type, length, and value).

   as follows:

      If any attribute has Attribute Flags that conflict with the
      Attribute Type Code, then the error SHOULD be logged, and the
      Attribute Flags MUST be reset to the correct value.  The UPDATE
      message MUST continue to be processed.



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

   When a path attribute in an UPDATE message is determined to be
   malformed, the UPDATE message containing that attribute MUST be
   treated as though all contained routes had been withdrawn just as if
   they had been listed in the WITHDRAWN ROUTES field (or in the
   MP_UNREACH_NLRI attribute [RFC4760bis] if appropriate) 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
   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.  For the sake of brevity, the former
   approach is termed "treat-as-withdraw", and the latter as "attribute
   discard".

   The approach of "treat-as-withdraw" MUST be used for the error
   handling of the cases described in Section 6.3 of [RFC4271] that
   specify a session reset and involve any of the following attributes:
   ORIGIN, AS_PATH, NEXT_HOP, MULTI_EXIT_DISC, and LOCAL_PREF.

   The approach of "attribute discard" MUST be used for the error
   handling of the cases described in Section 6.3 of [RFC4271] that
   specify a session reset and involve any of the following attributes:
   ATOMIC_AGGREGATE and AGGREGATOR.

   When multiple malformed attributes exist in an UPDATE message, if the
   same approach (either "treat-as-withdraw" or "attribute discard") is
   specified for the handling of these malformed attributes, then the
   specified approach MUST be used. Otherwise "treat-as-withdraw" MUST
   be used.

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

   Finally, we observe that in order to use the approach of "treat-as-
   withdraw", the entire NLRI field and/or MP_REACH and MP_UNREACH
   [RFC4760bis] attributes need to be successfully parsed.  If this is
   not possible, the procedures of [RFC4271] continue to apply.
   Alternatively the error handling procedures specified in [RFC4760bis]
   for disabling a particular AFI/SAFI MAY be followed.









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3. Parsing of NLRI Fields

   To facilitate the determination of the NLRI field in an UPDATE with a
   malformed attribute, the MP_REACH or MP_UNREACH attribute (if
   present) SHOULD be encoded as the very first path attribute in an
   UPDATE as recommended by [RFC4760bis].  An implementation, however,
   MUST still be prepared to receive these fields in any position.

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


4. Operational Considerations

   Although the "treat-as-withdraw" error-handling behavior defined in
   Section 2 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.

   When a malformed attribute is indeed detected over an IBGP session,
   we RECOMMEND that routes with the malformed attribute be identified
   and traced back to the ingress router in the network where the routes
   were sourced or received externally, and then a filter be applied on
   the ingress router to prevent the routes from being sourced or
   received.  This will help maintain routing consistency in the
   network.

   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 handled by the "attribute
   discard" instead of the "treat-as-withdraw" approach, it is critical



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   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 a malformed attribute
   to be diagnosed.  At a minimum, such facilities MUST include logging
   an error listing the NLRI involved, and containing the entire
   malformed UPDATE message when such an attribute is detected.  The
   malformed UPDATE message SHOULD be analyzed, and the root cause
   SHOULD be investigated.


5. Error Handling Procedures for Existing Optional 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:

      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 using the approach of "attribute discard".


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 using the approach of "treat-as-withdraw".






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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 using the approach of "treat-as-withdraw".

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


6. IANA Considerations

   This document makes no request of IANA.


7. 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.


8. Acknowledgments

   The authors wish to thank Ron Bonica, Mach Chen, Andy Davidson, Dong
   Jie, Rex Fernando, Joel Halpern, Akira Kato, Miya Kohno, Tony Li,
   Alton Lo, Shin Miyakawa, Tamas Mondal, Jonathan Oddy, Robert Raszuk,
   Yakov Rekhter, Rob Shakir, Naiming Shen, Shyam Sethuram, Ananth
   Suryanarayana, and Kaliraj Vairavakkalai for their observations and
   discussion of this topic, and review of this document.











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9. 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.

   [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.

   [RFC4760bis]
              Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4",
              draft-ietf-idr-rfc4760bis-03.txt, work in progress,
              August 2011.


Appendix A.  Why not discard UPDATE messages?

   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.




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   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
   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.


10. Authors' Addresses

   John G. Scudder
   Juniper Networks

   Email: jgs@juniper.net


   Enke Chen
   Cisco Systems, Inc.

   EMail: enkechen@cisco.com


   Pradosh Mohapatra
   Cisco Systems, Inc.

   EMail: pmohapat@cisco.com




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   Keyur Patel
   Cisco Systems, Inc.

   EMail: keyupate@cisco.com















































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