Internet Engineering Task Force (IETF) E. Chen, Ed.Internet-DraftRequest for Comments: 7606 Cisco Systems, Inc. Updates: 1997, 4271, 4360, 4456, 4760, J. Scudder, Ed. 5543, 5701, 6368(if approved)Juniper NetworksIntended status:Category: Standards Track P. MohapatraExpires: October 23, 2015ISSN: 2070-1721 Sproute Networks K. Patel Cisco Systems, Inc.April 21,August 2015 Revised Error Handling for BGP UPDATE Messagesdraft-ietf-idr-error-handling-19Abstract 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 isundesirable,undesirable because a session reset would impact not only routes with the offendingattribute,attribute but alsoother,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, 5543,57015701, and 6368. Status of This Memo ThisInternet-Draftissubmitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documentsan Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF).Note that other groups may also distribute working documents as Internet-Drafts. The listIt represents the consensus ofcurrent Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents validthe IETF community. It has received public review and has been approved fora maximumpublication 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 ofsix monthsthis document, any errata, and how to provide feedback on it may beupdated, replaced, or obsoleted by other documentsobtained atany 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.http://www.rfc-editor.org/info/rfc7606. 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. 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 HandlingError-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 . . . . . . . . . . . . . . . . . . . .1112 7.7. AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . 12 7.8. Community . . . . . . . . . . . . . . . . . . . . . . . . 12 7.9. ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . . 12 7.10. CLUSTER_LIST . . . . . . . . . . . . . . . . . . . . . .1213 7.11. MP_REACH_NLRI . . . . . . . . . . . . . . . . . . . . . . 13 7.12. MP_UNREACH_NLRI . . . . . . . . . . . . . . . . . . . . . 13 7.13. Traffic Engineeringpath attributePath Attribute . . . . . . . . . . . 13 7.14. Extended Community . . . . . . . . . . . . . . . . . . .1314 7.15. IPv6 Address Specific BGP Extended Community Attribute . 14 7.16. ATTR_SET . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Guidance for Authors of BGP Specifications . . . . . . . . .1415 9.IANASecurity Considerations . . . . . . . . . . . . . . . . . . .. .15 10.Security Considerations . . . . . . . . . . . . . . . .References . . .15 11. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . 1612.10.1. Normative References . . . . . . . . . . . . . . . . . . 16 10.2. Informative References . . . . . . .16 12.1. Normative References .. . . . . . . . . . 17 Acknowledgements . . . . . . .16 12.2. Informative References. . . . . . . . . . . . . . . . .1718 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .1718 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 isundesirable,undesirable because a session reset impacts not only routes with the offendingattribute,attribute but alsoother,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;andwhen 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 suchcasescases, 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 manytimes,times and thus may cause the reset of many peering sessions.ThusThus, 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 UPDATEmessage,message while maintaining protocol correctness to the extent possible. This can be achieved largely by maintaining the established session and keeping the valid routesexchanged,exchanged but removing the routes carried in the malformed UPDATE message from the routing system. This document partially revises the error handling for UPDATEmessages,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 thisdocumentdocument, we refer to four different approaches to handle errors found in a BGPpath attributes.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]specifiesallows a BGP speaker that detects an error in aproceduremessage fordisablingaparticulargiven AFI/SAFI(Address Family Identifier and Subsequent Address Family Identifier).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 thisapproachapproach, 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 of [RFC4271] in a number of ways. SeealsoSection 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. 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 thetreat-as-withdraw"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. 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(either "session reset", "treat-as-withdraw" or "attribute discard")(as described in Section 2) is specified for the handling of these malformed attributes, then the specified approach MUST be used.OtherwiseOtherwise, 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 furtherbelow,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 lengthvalues.values: o In the"overrun"first case,as the enclosed path attributes are parsed,the length of the last encountered path attribute would cause the Total Attribute Length to beexceeded. In the "underrun" case, asexceeded when parsing the enclosed pathattributes are parsed, afterattributes. o In thelast successfully-parsed attribute,second case, fewer than three octetsremain, orremain (or fewer than four octets, if the Attribute Flags field has the Extended Length bitset -- thatset) 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 singleminimum-sizedminimum- sized path attribute. In either of these cases, an error condition exists and thetreat-as-withdraw"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.(OfOf the path attributes considered in this specification, only AS_PATH and ATOMIC_AGGREGATE may validly have an attribute length ofzero.)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 anUPDATE.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 anUPDATE.UPDATE message. When such an UPDATE message is received, we observe that the NLRI field can be determined using the"Message Length", "WithdrawnMessage Length, Withdrawn RouteLength"Length, and"TotalTotal AttributeLength"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")(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 Routesfield,field or the MP_UNREACH_NLRIattribute),attribute) or it advertises reachable routes (either in the Network Layer Reachability Informationfield,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 UPDATEmessage,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 than32,32. 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 than128),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, forexampleexample, MCAST-VPN [RFC6514],MCAST-VPLS [RFC7117]MCAST- VPLS [RFC7117], and EVPN [RFC7432] have NLRI that are typed. Since supported type values within the address family are not expressed in theMP-BGPMultiprotocol BGP (MP-BGP) capability [RFC4760], it is possible for a BGP speaker to advertise support for the given address family andsub-addresssubaddress family while still not supporting a particular type of NLRI within thatAFI/ SAFI.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 message received on anIBGPInternal 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 receivedexternally,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 orsub-optimalsuboptimal 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 incrementalupdate,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 NLRIinvolved,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 thatattribute discard"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 specifyattribute discard"attribute discard" in this document do not affect route selection by default, inprincipleprinciple, 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.(InIn general, as long as such policies are only applied to external BGP sessions, correctness issues are not expected toarise.)arise. 7.Error HandlingError-Handling Procedures for Existing Attributes In the following subsections, we elaborate on the conditions for error-checking various pathattributes,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 thiswriting,writing that were not defined witherror-handlingerror handling consistent with Section8,8 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 DistinguisherLabels)Labels), and 29 (BGP-LS Attribute) do haveerror-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)(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 not1,1 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 isencountered,encountered or if it contains a malformed segment. A segment is considered malformed if any of the followingobtains:are true: o There is anoverrun,overrun where thepath segment lengthPath Segment Length field of the last segment encountered would cause the Attribute Length to be exceeded. o There is anunderrun,underrun where after the lastsuccessfully-parsed segment,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). o It has apath segment lengthPath 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","treat-as-withdraw" but MAY follow thesession reset"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 asfollows.follows: oIfif the LOCAL_PREF attribute is received from an external neighbor, it SHALL be discarded using the approach of "attributediscard",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"."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". 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-octet4-octet AS numbercapability"capability is not advertisedto,to or not received from the peer [RFC6793]). o Its length is not 8 (when the"4-octet4-octet AS numbercapability"capability is both advertisedto,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 anonzeronon-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 asfollows.follows: oIfif the ORIGINATOR_ID attribute is received from an external neighbor, it SHALL be discarded using the approach of "attributediscard",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"."treat-as- withdraw". 7.10. CLUSTER_LIST The error handling of [RFC4456] is revised asfollows.follows: oIfif the CLUSTER_LIST attribute is received from an external neighbor, it SHALL be discarded using the approach of "attributediscard",discard"; or o if received from an internal neighbor, it SHALL be considered malformed if its length is not anonzeronon-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 theNLRI, and thusNLRI; 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 inuseuse, then the next hop would have to have a length of 4 or 16.SectionSections 3 andSection5 provide further discussion of the handling of this attribute. 7.12. MP_UNREACH_NLRISectionSections 3 andSection5 discuss the handling of this attribute. 7.13. Traffic Engineeringpath attributePath 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 anonzeronon-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. 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 anonzeronon-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"."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. Thetreat-as-withdraw"treat-as-withdraw" approach is generallypreferred.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 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 isunsafe,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 beenterminated, whileterminated; with thisspecificationspecification, 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 insimilarsimilar, though not as obviouslyexploitable vulnerabilities. [RFC7366]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)thus to discard messages that don't verify. 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. 12.10. References12.1.10.1. Normative References [IANA-BGP-ATTRS] IANA, "BGP Path Attributes",<http://www.iana.org/assignments/ bgp-parameters/bgp-parameters.xhtml#bgp-parameters-2>.<http://www.iana.org/assignments/bgp-parameters>. [RFC1997]Chandrasekeran,Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, DOI 10.17487/RFC1997, August1996.1996, <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, March1997.1997, <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, January2006.2006, <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, February2006.2006, <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, April2006.2006, <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, January2007.2007, <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, January2007.2007, <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, May2009.2009, <http://www.rfc-editor.org/info/rfc5543>. [RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended Community Attribute", RFC 5701, DOI 10.17487/RFC5701, November2009.2009, <http://www.rfc-editor.org/info/rfc5701>. [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, September2011.2011, <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, December2012. 12.2.2012, <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, January2008.2008, <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, May2009.2009, <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, February2012.2012, <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, February2014.2014, <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, September2014.2014, <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, February2015.2015, <http://www.rfc-editor.org/info/rfc7432>. 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