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RFC 7606

Proposed STD
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Revised Error Handling for BGP UPDATE Messages

Updates:    1997    4271    4360    4456    4760    5543    5701    6368


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Internet Engineering Task Force (IETF)                      E. Chen, Ed.
Request for Comments: 7606                           Cisco Systems, Inc.
Updates: 1997, 4271, 4360, 4456, 4760,                   J. Scudder, Ed.
         5543, 5701, 6368                               Juniper Networks
Category: Standards Track                                   P. Mohapatra
ISSN: 2070-1721                                         Sproute Networks
                                                                K. Patel
                                                     Cisco Systems, Inc.
                                                             August 2015


             Revised Error Handling for BGP UPDATE Messages

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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7606.

Page 2 
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
   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.

   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
   material may not have granted the IETF Trust the right to allow
   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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Error-Handling Approaches . . . . . . . . . . . . . . . . . .   5
   3.  Revision to BGP UPDATE Message Error Handling . . . . . . . .   5
   4.  Attribute Length Fields . . . . . . . . . . . . . . . . . . .   7
   5.  Parsing of Network Layer Reachability Information (NLRI)
       Fields  . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Encoding NLRI . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Missing NLRI  . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Syntactic Correctness of NLRI Fields  . . . . . . . . . .   9
     5.4.  Typed NLRI  . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .  10
   7.  Error-Handling Procedures for Existing Attributes . . . . . .  11
     7.1.  ORIGIN  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  NEXT_HOP  . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.4.  MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . .  12
     7.5.  LOCAL_PREF  . . . . . . . . . . . . . . . . . . . . . . .  12
     7.6.  ATOMIC_AGGREGATE  . . . . . . . . . . . . . . . . . . . .  12
     7.7.  AGGREGATOR  . . . . . . . . . . . . . . . . . . . . . . .  13
     7.8.  Community . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.9.  ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . .  13
     7.10. CLUSTER_LIST  . . . . . . . . . . . . . . . . . . . . . .  13
     7.11. MP_REACH_NLRI . . . . . . . . . . . . . . . . . . . . . .  14
     7.12. MP_UNREACH_NLRI . . . . . . . . . . . . . . . . . . . . .  14
     7.13. Traffic Engineering Path Attribute  . . . . . . . . . . .  14
     7.14. Extended Community  . . . . . . . . . . . . . . . . . . .  14
     7.15. IPv6 Address Specific BGP Extended Community Attribute  .  15
     7.16. ATTR_SET  . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Guidance for Authors of BGP Specifications  . . . . . . . . .  15
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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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; 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 message 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 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].

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2.  Error-Handling Approaches

   In this document, we refer to four different approaches to handle
   errors found in a BGP UPDATE message.  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: Section 7 of [RFC4760] allows a BGP speaker that
      detects an error in a message for a given AFI/SAFI to optionally
      "ignore all the subsequent routes with that AFI/SAFI received over
      that session".  We refer to this as "disabling a particular AFI/
      SAFI" or "AFI/SAFI disable".

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

   a.  The first paragraph 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.

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          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.)

   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.

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   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 will continue
       to be processed.

   h.  When multiple attribute errors exist in an UPDATE message, if the
       same approach (as described in Section 2) 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 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:

   o  In the first case, the length of the last encountered path
      attribute would cause the Total Attribute Length to be exceeded
      when parsing the enclosed path attributes.

   o  In the second case, fewer than three octets remain (or fewer than
      four octets, if the Attribute Flags field has the Extended Length
      bit set) when beginning to parse the attribute.  That is, this
      case exists if there remains unconsumed data in the path
      attributes but yet insufficient data to 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

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   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
   message 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 message.

   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 message.  When such an UPDATE message 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 message either carries only withdrawn
   routes (either in the Withdrawn Routes field or the MP_UNREACH_NLRI
   attribute) or it advertises reachable routes (either in the Network
   Layer Reachability Information field or the MP_REACH_NLRI attribute).

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   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 Multiprotocol BGP (MP-BGP) capability [RFC4760], it is possible
   for a BGP speaker to advertise support for the given address family
   and subaddress family while still not supporting a particular type of
   NLRI within that AFI/SAFI.

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   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 message received on an Internal BGP (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 suboptimal
   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 an
   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 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

Top      ToC       Page 11 
   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
   the "BGP Path Attributes" registry [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 if 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 are true:

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

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   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 message 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].

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

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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:

   o  The Community attribute SHALL be considered malformed if its
      length is not a non-zero multiple of 4.

   o  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 message SHALL be handled using the approach of "treat-as-
      withdraw".

7.10.  CLUSTER_LIST

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

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

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   o  if received from an internal neighbor, it SHALL be considered
      malformed if its length is not a non-zero multiple of 4.  If
      malformed, the UPDATE message 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; 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 is 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.

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

7.12.  MP_UNREACH_NLRI

   Sections 3 and 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:

   o  The Extended Community attribute SHALL be considered malformed if
      its length is not a non-zero multiple of 8.

   o  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.

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7.15.  IPv6 Address Specific BGP Extended Community Attribute

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

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

   o  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 message 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
   detailed in Section 2.  The "treat-as-withdraw" approach is generally
   preferred and the "session reset" approach is discouraged.  Authors
   of BGP documents are also reminded to review the discussion of
   optional transitive attributes in the first paragraph of the

Top      ToC       Page 16 
   Introduction of this document.  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.  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; 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.

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

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10.  References

10.1.  Normative References

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

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <http://www.rfc-editor.org/info/rfc1997>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <http://www.rfc-editor.org/info/rfc4360>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <http://www.rfc-editor.org/info/rfc4456>.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              DOI 10.17487/RFC4724, January 2007,
              <http://www.rfc-editor.org/info/rfc4724>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <http://www.rfc-editor.org/info/rfc4760>.

   [RFC5543]  Ould-Brahim, H., Fedyk, D., and Y. Rekhter, "BGP Traffic
              Engineering Attribute", RFC 5543, DOI 10.17487/RFC5543,
              May 2009, <http://www.rfc-editor.org/info/rfc5543>.

   [RFC5701]  Rekhter, Y., "IPv6 Address Specific BGP Extended Community
              Attribute", RFC 5701, DOI 10.17487/RFC5701, November 2009,
              <http://www.rfc-editor.org/info/rfc5701>.

Top      ToC       Page 18 
   [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, DOI 10.17487/RFC6368, September 2011,
              <http://www.rfc-editor.org/info/rfc6368>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <http://www.rfc-editor.org/info/rfc6793>.

10.2.  Informative References

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5549]  Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
              Layer Reachability Information with an IPv6 Next Hop",
              RFC 5549, DOI 10.17487/RFC5549, May 2009,
              <http://www.rfc-editor.org/info/rfc5549>.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <http://www.rfc-editor.org/info/rfc6514>.

   [RFC7117]  Aggarwal, R., Ed., Kamite, Y., Fang, L., Rekhter, Y., and
              C. Kodeboniya, "Multicast in Virtual Private LAN Service
              (VPLS)", RFC 7117, DOI 10.17487/RFC7117, February 2014,
              <http://www.rfc-editor.org/info/rfc7117>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
              <http://www.rfc-editor.org/info/rfc7366>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <http://www.rfc-editor.org/info/rfc7432>.

Top      ToC       Page 19 
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.

Authors' Addresses

   Enke Chen (editor)
   Cisco Systems, Inc.

   Email: enkechen@cisco.com


   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