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rfc:rfc7606

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.

Chen, et al. Standards Track [Page 1] RFC 7606 Revised Error Handling for BGP August 2015

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
 (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.
 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.

Chen, et al. Standards Track [Page 2] RFC 7606 Revised Error Handling for BGP August 2015

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

Chen, et al. Standards Track [Page 3] RFC 7606 Revised Error Handling for BGP August 2015

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].

Chen, et al. Standards Track [Page 4] RFC 7606 Revised Error Handling for BGP August 2015

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.

Chen, et al. Standards Track [Page 5] RFC 7606 Revised Error Handling for BGP August 2015

        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.

Chen, et al. Standards Track [Page 6] RFC 7606 Revised Error Handling for BGP August 2015

 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

Chen, et al. Standards Track [Page 7] RFC 7606 Revised Error Handling for BGP August 2015

 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).

Chen, et al. Standards Track [Page 8] RFC 7606 Revised Error Handling for BGP August 2015

 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.

Chen, et al. Standards Track [Page 9] RFC 7606 Revised Error Handling for BGP August 2015

 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

Chen, et al. Standards Track [Page 10] RFC 7606 Revised Error Handling for BGP August 2015

 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).

Chen, et al. Standards Track [Page 11] RFC 7606 Revised Error Handling for BGP August 2015

 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".

Chen, et al. Standards Track [Page 12] RFC 7606 Revised Error Handling for BGP August 2015

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

Chen, et al. Standards Track [Page 13] RFC 7606 Revised Error Handling for BGP August 2015

 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.

Chen, et al. Standards Track [Page 14] RFC 7606 Revised Error Handling for BGP August 2015

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

Chen, et al. Standards Track [Page 15] RFC 7606 Revised Error Handling for BGP August 2015

 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.

Chen, et al. Standards Track [Page 16] RFC 7606 Revised Error Handling for BGP August 2015

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>.

Chen, et al. Standards Track [Page 17] RFC 7606 Revised Error Handling for BGP August 2015

 [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>.

Chen, et al. Standards Track [Page 18] RFC 7606 Revised Error Handling for BGP August 2015

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

Chen, et al. Standards Track [Page 19]

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