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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 16 17 18 19 RFC 7606

Internet Engineering Task Force                             E. Chen, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Updates: 1997, 4271, 4360, 4456, 4760,                   J. Scudder, Ed.
         5543, 5701, 6368 (if approved)                 Juniper Networks
Intended status: Standards Track                            P. Mohapatra
Expires: October 23, 2015                               Sproute Networks
                                                                K. Patel
                                                     Cisco Systems, Inc.
                                                          April 21, 2015


             Revised Error Handling for BGP UPDATE Messages
                    draft-ietf-idr-error-handling-19

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, because 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 a number of existing
   attributes.

   This document updates error handling for RFCs 1997, 4271, 4360, 4456,
   4760, 5543, 5701 and 6368.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on October 23, 2015.





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Copyright Notice

   Copyright (c) 2015 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Error-Handling Approaches . . . . . . . . . . . . . . . . . .   4
   3.  Revision to BGP UPDATE Message Error Handling . . . . . . . .   4
   4.  Attribute Length Fields . . . . . . . . . . . . . . . . . . .   6
   5.  Parsing of Network Layer Reachability Information (NLRI)
       Fields  . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Encoding NLRI . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Missing NLRI  . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Syntactic Correctness of NLRI Fields  . . . . . . . . . .   8
     5.4.  Typed NLRI  . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   9
   7.  Error Handling Procedures for Existing Attributes . . . . . .  10
     7.1.  ORIGIN  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.2.  AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.3.  NEXT_HOP  . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.4.  MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . .  11
     7.5.  LOCAL_PREF  . . . . . . . . . . . . . . . . . . . . . . .  11
     7.6.  ATOMIC_AGGREGATE  . . . . . . . . . . . . . . . . . . . .  11



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     7.7.  AGGREGATOR  . . . . . . . . . . . . . . . . . . . . . . .  12
     7.8.  Community . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.9.  ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . .  12
     7.10. CLUSTER_LIST  . . . . . . . . . . . . . . . . . . . . . .  12
     7.11. MP_REACH_NLRI . . . . . . . . . . . . . . . . . . . . . .  13
     7.12. MP_UNREACH_NLRI . . . . . . . . . . . . . . . . . . . . .  13
     7.13. Traffic Engineering path attribute  . . . . . . . . . . .  13
     7.14. Extended Community  . . . . . . . . . . . . . . . . . . .  13
     7.15. IPv6 Address Specific BGP Extended Community Attribute  .  14
     7.16. ATTR_SET  . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Guidance for Authors of BGP Specifications  . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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, because a session reset
   impacts 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.  This is because
   attributes may have been propagated without being checked by
   intermediate routers that don't recognize the attributes.  In effect,
   the attributes may have been tunneled; and when they reach a router
   that recognizes and checks the attributes, the session that is reset
   may not be associated with the router that is at fault.  To make
   matters worse, in such cases although the problematic attributes may
   have originated with a single update transmitted by a single BGP
   speaker, by the time they encounter a router that checks them they
   may have been replicated many times, and thus may cause the reset of
   many peering sessions.  Thus the damage inflicted may be multiplied
   manyfold.

   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.




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   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 a number of existing attributes.  Specifically, the
   error handling procedures of [RFC1997], [RFC4271], [RFC4360],
   [RFC4456], [RFC4760], [RFC5543], [RFC5701], and [RFC6368] are
   revised.

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].

2.  Error-Handling Approaches

   In this document we refer to four different approaches to handle
   errors found in BGP path attributes.  They are as follows (listed in
   order, from the one with the "strongest" action to the one with the
   "weakest" action):

   o  Session reset: This is the approach used throughout the base BGP
      specification [RFC4271], where a NOTIFICATION is sent and the
      session terminated.

   o  AFI/SAFI disable: [RFC4760] specifies a procedure for disabling a
      particular AFI/SAFI (Address Family Identifier and Subsequent
      Address Family Identifier).

   o  Treat-as-withdraw: In this approach, the UPDATE message containing
      the path attribute in question 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 if appropriate) of the UPDATE message, thus causing them
      to be removed from the Adj-RIB-In according to the procedures of
      [RFC4271].

   o  Attribute discard: In this approach the malformed attribute MUST
      be discarded and the UPDATE message continues to be processed.
      This approach MUST NOT be used except in the case of an attribute
      that has no effect on route selection or installation.

3.  Revision to BGP UPDATE Message Error Handling

   This specification amends [RFC4271] Section 6.3 in a number of ways.
   See also Section 7 for treatment of specific path attributes.

   a.  The first paragraph is revised as follows:



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          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.

   b.  Error handling for the following case 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.

   c.  Attribute Flag error handling is revised as follows:

          Old Text:

             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).

          New Text:

             If the value of either the Optional or Transitive bits in
             the Attribute Flags is in conflict with their specified
             values, then the attribute MUST be treated as malformed and
             the treat-as-withdraw approach used, unless the
             specification for the attribute mandates different handling
             for incorrect Attribute Flags.

   d.  If any of the well-known mandatory attributes are not present in
       an UPDATE message, then "treat-as-withdraw" MUST be used.  (Note
       that [RFC4760] reclassifies NEXT_HOP as what is effectively
       discretionary.)






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   e.  "Treat-as-withdraw" MUST be used for the cases that specify a
       session reset and involve any of the attributes ORIGIN, AS_PATH,
       NEXT_HOP, MULTI_EXIT_DISC, or LOCAL_PREF.

   f.  "Attribute discard" MUST be used for any of the cases that
       specify a session reset and involve ATOMIC_AGGREGATE or
       AGGREGATOR.

   g.  If the MP_REACH_NLRI attribute or the MP_UNREACH_NLRI [RFC4760]
       attribute appears more than once in the UPDATE message, then a
       NOTIFICATION message MUST be sent with the Error Subcode
       "Malformed Attribute List".  If any other attribute (whether
       recognized or unrecognized) appears more than once in an UPDATE
       message, then all the occurrences of the attribute other than the
       first one SHALL be discarded and the UPDATE message continue to
       be processed.

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

   i.  The Withdrawn Routes field MUST be checked for syntactic
       correctness in the same manner as the NLRI (network layer
       reachability information) field.  This is discussed further
       below, and in Section 5.3.

   j.  Finally, we observe that in order to use the approach of "treat-
       as-withdraw", the entire NLRI field and/or the MP_REACH_NLRI and
       MP_UNREACH_NLRI attributes need to be successfully parsed -- what
       this entails is discussed in more detail in Section 5.  If this
       is not possible, the procedures of [RFC4271] and/or [RFC4760]
       continue to apply, meaning that the "session reset" approach (or
       the "AFI/SAFI disable" approach) MUST be followed.

4.  Attribute Length Fields

   There are two error cases in which the Total Attribute Length value
   can be in conflict with the enclosed path attributes, which
   themselves carry length values.  In the "overrun" case, as the
   enclosed path attributes are parsed, the length of the last
   encountered path attribute would cause the Total Attribute Length to
   be exceeded.  In the "underrun" case, as the enclosed path attributes
   are parsed, after the last successfully-parsed attribute, fewer than
   three octets remain, or fewer than four octets, if the Attribute
   Flags field has the Extended Length bit set -- that is, there remains
   unconsumed data in the path attributes but yet insufficient data to



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   encode a single minimum-sized path attribute.  In either of these
   cases, an error condition exists and the treat-as-withdraw approach
   MUST be used (unless some other, more severe error is encountered
   dictating a stronger approach), and the Total Attribute Length MUST
   be relied upon to enable the beginning of the NLRI field to be
   located.

   For all path attributes other than those specified as having an
   attribute length that may be zero, it SHALL be considered a syntax
   error for the attribute to have a length of zero.  (Of the path
   attributes considered in this specification, only AS_PATH and
   ATOMIC_AGGREGATE may validly have an attribute length of zero.)

5.  Parsing of Network Layer Reachability Information (NLRI) Fields

5.1.  Encoding NLRI

   To facilitate the determination of the NLRI field in an UPDATE with a
   malformed attribute:

   o  The MP_REACH_NLRI or MP_UNREACH_NLRI attribute (if present) SHALL
      be encoded as the very first path attribute in an UPDATE.

   o  An UPDATE message MUST NOT contain more than one of the following:
      non-empty Withdrawn Routes field, non-empty Network Layer
      Reachability Information field, MP_REACH_NLRI attribute, and
      MP_UNREACH_NLRI attribute.

   Since older BGP speakers may not implement these restrictions, an
   implementation MUST still be prepared to receive these fields in any
   position or combination.

   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.

5.2.  Missing NLRI

   [RFC4724] specifies an End-of-RIB message ("EoR") that can be encoded
   as an UPDATE message that contains only a MP_UNREACH_NLRI attribute
   that encodes no NLRI (it can also be a completely empty UPDATE
   message in the case of the "legacy" encoding).  In all other well-
   specified cases, an UPDATE either carries only withdrawn routes
   (either in the Withdrawn Routes field, or the MP_UNREACH_NLRI



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   attribute), or it advertises reachable routes (either in the Network
   Layer Reachability Information field, or the MP_REACH_NLRI
   attribute).

   Thus, if an UPDATE message is encountered that does contain path
   attributes other than MP_UNREACH_NLRI and doesn't encode any
   reachable NLRI, we cannot be confident that the NLRI have been
   successfully parsed as Section 3 (j) requires.  For this reason, if
   any path attribute errors are encountered in such an UPDATE message,
   and if any encountered error specifies an error-handling approach
   other than "attribute discard", then the "session reset" approach
   MUST be used.

5.3.  Syntactic Correctness of NLRI Fields

   The NLRI field or Withdrawn Routes field SHALL be considered
   "syntactically incorrect" if either of the following are true:

   o  The length of any of the included NLRI is greater than 32,

   o  When parsing NLRI contained in the field, the length of the last
      NLRI found exceeds the amount of unconsumed data remaining in the
      field.

   Similarly, the MP_REACH_NLRI or MP_UNREACH_NLRI attribute of an
   update SHALL be considered to be incorrect if any of the following
   are true:

   o  The length of any of the included NLRI is inconsistent with the
      given AFI/SAFI (for example, if an IPv4 NLRI has a length greater
      than 32 or an IPv6 NLRI has a length greater than 128),

   o  When parsing NLRI contained in the attribute, the length of the
      last NLRI found exceeds the amount of unconsumed data remaining in
      the attribute.

   o  The attribute flags of the attribute are inconsistent with those
      specified in [RFC4760].

   o  The length of the MP_UNREACH_NLRI attribute is less than 3, or the
      length of the MP_REACH_NLRI attribute is less than 5.

5.4.  Typed NLRI

   Certain address families, for example MCAST-VPN [RFC6514], MCAST-VPLS
   [RFC7117] and EVPN [RFC7432] have NLRI that are typed.  Since
   supported type values within the address family are not expressed in
   the MP-BGP capability [RFC4760], it is possible for a BGP speaker to



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   advertise support for the given address family and sub-address family
   while still not supporting a particular type of NLRI within that AFI/
   SAFI.

   A BGP speaker advertising support for such a typed address family
   MUST handle routes with unrecognized NLRI types within that address
   family by discarding them, unless the relevant specification for that
   address family specifies otherwise.

6.  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.

   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.

   Section 8 mentions that attribute discard should not be used in cases
   where "the attribute in question has or may have an effect on route
   selection."  Although all cases that specify attribute discard in



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   this document do not affect route selection by default, in principle
   routing policies could be written that affect selection based on such
   an attribute.  Operators should take care when writing such policies
   to consider the possible consequences of an attribute discard.  (In
   general, as long as such policies are only applied to external BGP
   sessions, correctness issues are not expected to arise.)

7.  Error Handling Procedures for Existing Attributes

   In the following subsections, we elaborate on the conditions for
   error-checking various path attributes, and specify what approach(es)
   should be used to handle malformations.  It is possible that
   implementations may apply other error checks not contemplated here.
   If so, the error handling approach given here should generally be
   applied.

   This section addresses all path attributes that are defined at the
   time of this writing, that were not defined with error-handling
   consistent with Section 8, and that are not marked as "deprecated" in
   [IANA-BGP-ATTRS].  Attributes 17 (AS4_PATH), 18 (AS4_AGGREGATOR), 22
   (PMSI_TUNNEL), 23 (Tunnel Encapsulation Attribute), 26 (AIGP), 27 (PE
   Distinguisher Labels) and 29 (BGP-LS Attribute) do have error-
   handling consistent with Section 8 and thus are not further discussed
   herein.  Attributes 11 (DPA), 12 (ADVERTISER), 13 (RCID_PATH /
   CLUSTER_ID), 19 (SAFI Specific Attribute), 20 (Connector Attribute),
   21 (AS_PATHLIMIT) and 28 (BGP Entropy Label Capability Attribute) are
   deprecated and thus are not further discussed herein.

7.1.  ORIGIN

   The attribute is considered malformed if its length is not 1, or it
   has an undefined value [RFC4271].

   An UPDATE message with a malformed ORIGIN attribute SHALL be handled
   using the approach of "treat-as-withdraw".

7.2.  AS_PATH

   An AS_PATH is considered malformed if an unrecognized segment type is
   encountered, or if it contains a malformed segment.  A segment is
   considered malformed if any of the following obtains:

   o  There is an overrun, where the path segment length field of the
      last segment encountered would cause the Attribute Length to be
      exceeded.

   o  There is an underrun, where after the last successfully-parsed
      segment, there is only a single octet remaining (that is, there is



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      not enough unconsumed data to provide even an empty segment
      header).

   o  It has a path segment length field of zero.

   An UPDATE message with a malformed AS_PATH attribute SHALL be handled
   using the approach of "treat-as-withdraw".

   [RFC4271] also says that an implementation optionally "MAY check
   whether the leftmost ... AS in the AS_PATH attribute is equal to the
   autonomous system number of the peer that sent the message".  A BGP
   implementation SHOULD also handle routes that violate this check
   using "treat-as-withdraw", but MAY follow the session reset behavior
   if configured to do so.

7.3.  NEXT_HOP

   The attribute is considered malformed if its length is not 4
   [RFC4271].

   An UPDATE message with a malformed NEXT_HOP attribute SHALL be
   handled using the approach of "treat-as-withdraw".

7.4.  MULTI_EXIT_DISC

   The attribute is considered malformed if its length is not 4
   [RFC4271].

   An UPDATE message with a malformed MULTI_EXIT_DISC attribute SHALL be
   handled using the approach of "treat-as-withdraw".

7.5.  LOCAL_PREF

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

   o  If the LOCAL_PREF attribute is received from an external neighbor,
      it SHALL be discarded using the approach of "attribute discard",
      or

   o  if received from an internal neighbor, it SHALL be considered
      malformed if its length is not equal to 4.  If malformed, the
      UPDATE SHALL be handled using the approach of "treat-as-withdraw".

7.6.  ATOMIC_AGGREGATE

   The attribute SHALL be considered malformed if its length is not 0
   [RFC4271].




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   An UPDATE message with a malformed ATOMIC_AGGREGATE attribute SHALL
   be handled using the approach of "attribute discard".

7.7.  AGGREGATOR

   The error conditions specified in [RFC4271] for the attribute are
   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 [RFC6793]).

   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".

7.8.  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".

7.9.  ORIGINATOR_ID

   The error handling of [RFC4456] is revised as follows.

   o  If the ORIGINATOR_ID attribute is received from an external
      neighbor, it SHALL be discarded using the approach of "attribute
      discard", or

   o  if received from an internal neighbor, it SHALL be considered
      malformed if its length is not equal to 4.  If malformed, the
      UPDATE SHALL be handled using the approach of "treat-as-withdraw".

7.10.  CLUSTER_LIST

   The error handling of [RFC4456] is revised as follows.






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   o  If the CLUSTER_LIST attribute is received from an external
      neighbor, it SHALL be discarded using the approach of "attribute
      discard", or

   o  if received from an internal neighbor, it SHALL be considered
      malformed if its length is not a nonzero multiple of 4.  If
      malformed, the UPDATE SHALL be handled using the approach of
      "treat-as-withdraw".

7.11.  MP_REACH_NLRI

   If the Length of Next Hop Network Address field of the MP_REACH
   attribute is inconsistent with that which was expected, the attribute
   is considered malformed.  Since the next hop precedes the NLRI field
   in the attribute, in this case it will not be possible to reliably
   locate the NLRI, and thus the "session reset" or "AFI/SAFI disable"
   approach MUST be used.

   "That which was expected", while somewhat vague, is intended to
   encompass the next hop specified for the Address Family Identifier
   and Subsequent Address Family Identifier fields and potentially
   modified by any extensions in use.  For example, if [RFC5549] is in
   use then the next hop would have to have a length of 4 or 16.

   Section 3 and Section 5 provide further discussion of the handling of
   this attribute.

7.12.  MP_UNREACH_NLRI

   Section 3 and Section 5 discuss the handling of this attribute.

7.13.  Traffic Engineering path attribute

   We note that [RFC5543] does not detail what constitutes
   "malformation" for the Traffic Engineering path attribute.  A future
   update to that specification may provide more guidance.  In the
   interim, an implementation that determines (for whatever reason) that
   an UPDATE message contains a malformed Traffic Engineering path
   attribute MUST handle it using the approach of "treat-as-withdraw".

7.14.  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.





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   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.

7.15.  IPv6 Address Specific BGP Extended Community Attribute

   The error handling of [RFC5701] is revised as follows:

   The IPv6 Address Specific Extended Community attribute SHALL be
   considered malformed if its length is not a nonzero multiple of 20.

   An UPDATE message with a malformed IPv6 Address Specific Extended
   Community attribute SHALL be handled using the approach of "treat-as-
   withdraw".

   Note that a BGP speaker MUST NOT treat an unrecognized IPv6 Address
   Specific Extended Community Type or Sub-Type as an error.

7.16.  ATTR_SET

   The final paragraph of Section 5 of [RFC6368] is revised as follows:

      Old Text:

         An UPDATE message with a malformed ATTR_SET attribute SHALL be
         handled as follows.  If its Partial flag is set and its
         Neighbor-Complete flag is clear, the UPDATE is treated as a
         route withdraw as discussed in [OPT-TRANS-BGP].  Otherwise
         (i.e., Partial flag is clear or Neighbor-Complete is set), the
         procedures of the BGP-4 base specification [RFC4271] MUST be
         followed with respect to an Optional Attribute Error.

      New Text:

         An UPDATE message with a malformed ATTR_SET attribute SHALL be
         handled using the approach of "treat as withdraw".

   Furthermore, the normative reference to [OPT-TRANS-BGP] in [RFC6368]
   is removed.

8.  Guidance for Authors of BGP Specifications

   A document that specifies a new BGP attribute MUST provide specifics
   regarding what constitutes an error for that attribute and how that
   error is to be handled.  Allowable error-handling approaches are
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   preferred.  The document SHOULD also provide consideration of what
   debugging facilities may be required to permit issues caused by a
   malformed attribute to be diagnosed.

   For any malformed attribute that is handled by the "attribute
   discard" instead of the "treat-as-withdraw" approach, 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.

   Authors can refer to Section 7 for examples.

9.  IANA Considerations

   This document makes no request of IANA.

10.  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.  Since the intervening routers do not recognize the
   attribute, they propagate it without checking it.  When the malformed
   attribute arrives at a router that does recognize the given attribute
   type, that router resets the session over which it arrived.  Since
   significant fan-out can occur between the attacker and the routers
   that do recognize the attribute type, this attack could potentially
   be particularly harmful.

   The improved error handling of this specification could in theory
   interact badly with some now-known weaker cryptographic mechanisms
   should such be used in future to secure BGP.  For example, if a
   (fictional) mechanism that did not supply data integrity was used, an
   attacker could manipulate ciphertext in any attempt to change or
   observe how the receiver reacts.  Absent this specification, the BGP
   session would have been terminated, while with this specification the
   attacker could make potentially many attempts.  While such a
   confidentiality-only mechanism would not be defined today, we have in
   the past seen mechanism definitions that result in similar though not
   as obviously exploitable vulnerabilities.  [RFC7366] The approach
   recommended today to avoid such issues is to prefer use of
   Authenticated Encryption with Additional Data (AEAD) ciphers
   [RFC5116] and (thus) to discard messages that don't verify.





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

11.  Acknowledgements

   The authors wish to thank Juan Alcaide, Deniz Bahadir, Ron Bonica,
   Mach Chen, Andy Davidson, Bruno Decraene, Stephen Farrell, Rex
   Fernando, Jeff Haas, Chris Hall, Joel Halpern, Dong Jie, Akira Kato,
   Miya Kohno, Warren Kumari, Tony Li, Alton Lo, Shin Miyakawa, Tamas
   Mondal, Jonathan Oddy, Tony Przygienda, Robert Raszuk, Yakov Rekhter,
   Eric Rosen, Shyam Sethuram, Rob Shakir, Naiming Shen, Adam Simpson,
   Ananth Suryanarayana, Kaliraj Vairavakkalai, Lili Wang and Ondrej
   Zajicek for their observations and discussion of this topic, and
   review of this document.

12.  References

12.1.  Normative References

   [IANA-BGP-ATTRS]
              "BGP Path Attributes", <http://www.iana.org/assignments/
              bgp-parameters/bgp-parameters.xhtml#bgp-parameters-2>.

   [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.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              January 2007.

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





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   [RFC5543]  Ould-Brahim, H., Fedyk, D., and Y. Rekhter, "BGP Traffic
              Engineering Attribute", RFC 5543, May 2009.

   [RFC5701]  Rekhter, Y., "IPv6 Address Specific BGP Extended Community
              Attribute", RFC 5701, November 2009.

   [RFC6368]  Marques, P., Raszuk, R., Patel, K., Kumaki, K., and T.
              Yamagata, "Internal BGP as the Provider/Customer Edge
              Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)",
              RFC 6368, September 2011.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793, December
              2012.

12.2.  Informative References

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC5549]  Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
              Layer Reachability Information with an IPv6 Next Hop", RFC
              5549, May 2009.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, February 2012.

   [RFC7117]  Aggarwal, R., Kamite, Y., Fang, L., Rekhter, Y., and C.
              Kodeboniya, "Multicast in Virtual Private LAN Service
              (VPLS)", RFC 7117, February 2014.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, September 2014.

   [RFC7432]  Sajassi, A., Aggarwal, R., Bitar, N., Isaac, A., Uttaro,
              J., Drake, J., and W. Henderickx, "BGP MPLS-Based Ethernet
              VPN", RFC 7432, February 2015.

Authors' Addresses

   Enke Chen (editor)
   Cisco Systems, Inc.

   Email: enkechen@cisco.com





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   John G. Scudder (editor)
   Juniper Networks

   Email: jgs@juniper.net


   Pradosh Mohapatra
   Sproute Networks

   Email: mpradosh@yahoo.com


   Keyur Patel
   Cisco Systems, Inc.

   Email: keyupate@cisco.com



































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