--- 1/draft-ietf-idr-error-handling-05.txt 2014-02-14 10:14:38.830056447 -0800 +++ 2/draft-ietf-idr-error-handling-06.txt 2014-02-14 10:14:38.862057222 -0800 @@ -1,73 +1,112 @@ -Internet Engineering Task Force (IETF) J. Scudder -Internet Draft Juniper Networks -Update: 1997, 4271, 4360, 5701 (if approved) E. Chen -Intended Status: Standards Track Cisco Systems -Expires: August 6, 2014 P. Mohapatra - Cumulus Networks +Internet Engineering Task Force E. Chen, Ed. +Internet-Draft Cisco Systems, Inc. +Updates: 1997, 4271, 4360, 4456, 4760, 5701 (if approved)J. Scudder, Ed. +Intended status: Standards Track Juniper Networks +Expires: August 18, 2014 P. Mohapatra + Cumulus Networks, Inc. K. Patel - Cisco Systems - February 5, 2014 + Cisco Systems, Inc. + February 14, 2014 Revised Error Handling for BGP UPDATE Messages - draft-ietf-idr-error-handling-05.txt + draft-ietf-idr-error-handling-06 Abstract According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable as a session reset would impact not only routes with the offending attribute, but also other valid routes exchanged over the session. This document partially revises the error handling for UPDATE messages, and provides guidelines for the authors of documents defining new attributes. Finally, it revises the error handling procedures for a number of existing attributes. -Status of this Memo + This document updates error handling for RFCs 1997, 4271, 4360, 4456, + 4760, and 5701. - This Internet-Draft is submitted to IETF in full conformance with the +Status of This Memo + + This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering - Task Force (IETF), its areas, and its working groups. Note that - other groups may also distribute working documents as Internet- - Drafts. + Task Force (IETF). Note that other groups may also distribute + working documents as Internet-Drafts. The list of current Internet- + Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - The list of current Internet-Drafts can be accessed at - http://www.ietf.org/1id-abstracts.html - - The list of Internet-Draft Shadow Directories can be accessed at - http://www.ietf.org/shadow.html - This Internet-Draft will expire on August 6, 2014. + This Internet-Draft will expire on August 18, 2014. Copyright Notice Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . 3 + 2. Revision to Base Specification . . . . . . . . . . . . . . . 4 + 3. Parsing of NLRI Fields . . . . . . . . . . . . . . . . . . . 6 + 3.1. Inconsistency of Attribute Length Fields . . . . . . . . 6 + 3.2. Syntactic Correctness of NLRI Fields . . . . . . . . . . 7 + 4. Operational Considerations . . . . . . . . . . . . . . . . . 7 + 5. Error Handling Procedures for Existing Attributes . . . . . . 8 + 5.1. ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . 8 + 5.2. AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . 8 + 5.3. NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . 9 + 5.4. MULTI_EXIT_DESC . . . . . . . . . . . . . . . . . . . . . 9 + 5.5. LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . 9 + 5.6. ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . 9 + 5.7. AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . 9 + 5.8. Community . . . . . . . . . . . . . . . . . . . . . . . . 10 + 5.9. Extended Community . . . . . . . . . . . . . . . . . . . 10 + 5.10. IPv6 Address Specific BGP Extended Community Attribute . 10 + 5.11. ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . . 11 + 5.12. CLUSTER_LIST . . . . . . . . . . . . . . . . . . . . . . 11 + 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 + 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 + 9. Normative References . . . . . . . . . . . . . . . . . . . . 12 + Appendix A. Why not discard UPDATE messages? . . . . . . . . . . 12 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 + 1. Introduction According to the base BGP specification [RFC4271], a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable as a session reset would impact not only routes with the offending attribute, but also other valid routes exchanged over the session. In the case of optional transitive attributes, the behavior is especially troublesome and may present a potential security vulnerability. The reason is that such @@ -81,74 +120,75 @@ minimize the impact on routing by a malformed UPDATE message, while maintaining protocol correctness to the extent possible. This can be achieved largely by maintaining the established session and keeping the valid routes exchanged, but removing the routes carried in the malformed UPDATE from the routing system. This document partially revises the error handling for UPDATE messages, and provides guidelines for the authors of documents defining new attributes. Finally, it revises the error handling procedures for a number of existing attributes. Specifically, the - error handling procedures of [RFC4271], [RFC1997], [RFC4360] and - [RFC5701] are revised. + error handling procedures of, [RFC1997], [RFC4271], [RFC4360], + [RFC4456], [RFC4760] and [RFC5701] are revised. -1.1. Specification of Requirements +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. Revision to Base Specification The first paragraph of Section 6.3 of [RFC4271] is revised as follows: Old Text: All errors detected while processing the UPDATE message MUST be indicated by sending the NOTIFICATION message with the Error Code - UPDATE Message Error. The error subcode elaborates on the specific - nature of the error. + UPDATE Message Error. The error subcode elaborates on the + specific nature of the error. New text: An error detected while processing the UPDATE message for which a session reset is specified MUST be indicated by sending the NOTIFICATION message with the Error Code UPDATE Message Error. The error subcode elaborates on the specific nature of the error. The error handling of the following case described in Section 6.3 of [RFC4271] remains unchanged: - If the Withdrawn Routes Length or Total Attribute Length - is too large (i.e., if Withdrawn Routes Length + Total Attribute - Length + 23 exceeds the message Length), then the Error Subcode - MUST be set to Malformed Attribute List. + If the Withdrawn Routes Length or Total Attribute Length is too + large (i.e., if Withdrawn Routes Length + Total Attribute Length + + 23 exceeds the message Length), then the Error Subcode MUST be set + to Malformed Attribute List. The error handling of the following case described in Section 6.3 of [RFC4271] is revised If any recognized attribute has Attribute Flags that conflict with the Attribute Type Code, then the Error Subcode MUST be set to Attribute Flags Error. The Data field MUST contain the erroneous attribute (type, length, and value). as follows: - If the "Optional bit" or the "Transitive bit" in the Attribute - Flags for an attribute conflicts with the Attribute Type Code, - then the error SHOULD be logged, and the conflicting bit in the - Attribute Flags MUST be reset to the correct value. The UPDATE - message MUST continue to be processed. + If any recognized attribute has Attribute Flags that conflict with + the Attribute Type Code, then the attribute MUST be treated as + malformed and the treat-as-withdraw approach (see below) used, + unless the specification for the attribute mandates different + handling for incorrect Attribute Flags. - The error handling of all other cases described in Section 6.3 of - [RFC4271] that specify a session reset is revised as follows. + The error handling of all other cases involving path attributes as + described in Section 6.3 of [RFC4271] that specify a session reset is + revised as follows. When a path attribute (other than the MP_REACH_NLRI attribute [RFC4760] or the MP_UNREACH_NLRI attribute [RFC4760]) in an UPDATE message is determined to be malformed, the UPDATE message containing that attribute MUST be treated as though all contained routes had been withdrawn just as if they had been listed in the WITHDRAWN ROUTES field (or in the MP_UNREACH_NLRI attribute if appropriate) of the UPDATE message, thus causing them to be removed from the Adj-RIB- In according to the procedures of [RFC4271]. In the case of an attribute which has no effect on route selection or installation, the @@ -172,55 +212,108 @@ ATOMIC_AGGREGATE and AGGREGATOR. If the MP_REACH_NLRI attribute or the MP_UNREACH_NLRI 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 appears more than once in an UPDATE message, then all the occurrences of the attribute other than the first one SHALL be discarded and the UPDATE message continue to be processed. - When multiple malformed attributes exist in an UPDATE message, if the + When multiple attribute errors exist in an UPDATE message, if the same approach (either "session reset", or "treat-as-withdraw" or "attribute discard") is specified for the handling of these malformed attributes, then the specified approach MUST be used. Otherwise the approach with the strongest action MUST be used following the order of "session reset", "treat-as-withdraw" and "attribute discard" from the strongest to the weakest. A document which specifies a new attribute MUST provide specifics regarding what constitutes an error for that attribute and how that error is to be handled. Finally, we observe that in order to use the approach of "treat-as- withdraw", the entire NLRI field and/or the MP_REACH_NLRI and MP_UNREACH_NLRI attributes need to be successfully parsed. If this is not possible, the procedures of [RFC4271] continue to apply. Alternatively the error handling procedures specified in [RFC4760] - for disabling a particular AFI/SAFI MAY be followed. + for disabling a particular AFI/SAFI MAY be followed. One notable + case where it would be not possible to successfully parse the NLRI is + if the NLRI field is found to be "syntactically incorrect" (see + Section 3.2). It can be seen that therefore, this part of [RFC4271] + Section 6.3 necessarily continues to apply: + + The NLRI field in the UPDATE message is checked for syntactic + validity. If the field is syntactically incorrect, then the Error + Subcode MUST be set to Invalid Network Field. + + Furthermore, this document extends RFC 4271 by mandating that the + Withdrawn Routes field SHALL be checked for syntactic correctness in + the same manner as the NLRI field. 3. Parsing of NLRI Fields To facilitate the determination of the NLRI field in an UPDATE with a malformed attribute, the MP_REACH_NLRI or MP_UNREACH_NLRI attribute (if present) SHALL be encoded as the very first path attribute in an UPDATE. An implementation, however, MUST still be prepared to receive these fields in any position. If the encoding of [RFC4271] is used, the NLRI field for the IPv4 unicast address family is carried immediately following all the attributes in an UPDATE. When such an UPDATE is received, we observe that the NLRI field can be determined using the "Message Length", "Withdrawn Route Length" and "Total Attribute Length" (when they are consistent) carried in the message instead of relying on the length of individual attributes in the message. +3.1. Inconsistency of Attribute Length Fields + + There are two error cases in which the Total Attribute Length value + can be in conflict with the enclosed path attributes, which + themselves carry length values. In the "overrun" case, as the + enclosed path attributes are parsed, the length of the last + encountered path attribute would cause the Total Attribute Length to + be exceeded. In the "underrun" case, as the enclosed path attributes + are parsed, after the last successfully-parsed attribute, fewer than + three bytes remain, or fewer than four bytes, if the Attribute Flags + field has the Extended Length bit set -- that is, there remains + unconsumed data in the path attributes but yet insufficient data to + encode a single minimum-sized path attribute. In either of these + cases an error condition exists and the treat-as-withdraw approach + MUST be used (unless some other, more severe error is encountered + dictating a stronger approach), and the Total Attribute Length MUST + be relied upon to enable the beginning of the NLRI field to be + located. + +3.2. 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 or MP_UNREACH 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. + 4. Operational Considerations Although the "treat-as-withdraw" error-handling behavior defined in Section 2 makes every effort to preserve BGP's correctness, we note that if an UPDATE received on an IBGP session is subjected to this treatment, inconsistent routing within the affected Autonomous System may result. The consequences of inconsistent routing can include long-lived forwarding loops and black holes. While lamentable, this issue is expected to be rare in practice, and more importantly is seen as less problematic than the session-reset behavior it replaces. @@ -295,25 +388,25 @@ handled using the approach of "treat-as-withdraw". 5.5. LOCAL_PREF The attribute is considered malformed if its length is not 4 [RFC4271]. An UPDATE message with a malformed LOCAL_PREF attribute SHALL be handled as follows: - o using the approach of "attribute discard" if the UPDATE message - is received from an external neighbor, or + o using the approach of "attribute discard" if the UPDATE message is + received from an external neighbor, or - o using the approach of "treat-as-withdraw" if the UPDATE message - is received from an internal neighbor. + o using the approach of "treat-as-withdraw" if the UPDATE message is + received from an internal neighbor. In addition, if the attribute is present in an UPDATE message from an external neighbor, the approach of "attribute discard" SHALL be used to handle the unexpected attribute in the message. 5.6. ATOMIC_AGGREGATE The attribute SHALL be considered malformed if its length is not 0 [RFC4271]. @@ -322,21 +415,21 @@ 5.7. 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 [RFC4893]). + 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". 5.8. Community The error handling of [RFC1997] is revised as follows: @@ -368,89 +461,116 @@ considered malformed if its length is nonzero and is not a multiple of 20. An UPDATE message with a malformed IPv6 Address Specific Extended Community attribute SHALL be handled using the approach of "treat-as- withdraw". Note that a BGP speaker MUST NOT treat an unrecognized IPv6 Address Specific Extended Community Type or Sub-Type as an error. +5.11. ORIGINATOR_ID + + The error handling of [RFC4456] is revised as follows. + + o If the ORIGINATOR_ID attribute is received from an external + neighbor, it SHALL be discarded using the approach of "attribute + discard", or + + o if received from an internal neighbor, it SHALL be considered + malformed if its length is not equal to 4. If malformed, the + UPDATE SHALL be handled using the approach of "treat-as-withdraw". + +5.12. 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 + + o if received from an internal neighbor, it SHALL be considered + malformed if its length is not a multiple 4. If malformed, the + UPDATE SHALL be handled using the approach of "treat-as-withdraw". + 6. IANA Considerations This document makes no request of IANA. 7. Security Considerations This specification addresses the vulnerability of a BGP speaker to a potential attack whereby a distant attacker can generate a malformed optional transitive attribute that is not recognized by intervening routers (which thus propagate the attribute unchecked) but that causes session resets when it reaches routers that do recognize the given attribute type. In other respects, this specification does not change BGP's security characteristics. -8. Acknowledgments +8. Acknowledgements The authors wish to thank Juan Alcaide, Ron Bonica, Mach Chen, Andy Davidson, Bruno Decraene, Dong Jie, Rex Fernando, Joel Halpern, Akira Kato, Miya Kohno, Tony Li, Alton Lo, Shin Miyakawa, Tamas Mondal, - Jonathan Oddy, Robert Raszuk, Yakov Rekhter, Rob Shakir, Naiming - Shen, Shyam Sethuram, Ananth Suryanarayana, Kaliraj Vairavakkalai and - Lili Wang for their observations and discussion of this topic, and - review of this document. + Jonathan Oddy, Robert Raszuk, Yakov Rekhter, Eric Rosen, Rob Shakir, + Naiming Shen, Shyam Sethuram, Ananth Suryanarayana, Kaliraj + Vairavakkalai and Lili Wang for their observations and discussion of + this topic, and review of this document. 9. Normative References [RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, August 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, February 2006. - [RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS - Number Space", RFC 4893, May 2007. - - [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an - IANA Considerations Section in RFCs", BCP 26, RFC 5226, - May 2008. + [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route + Reflection: An Alternative to Full Mesh Internal BGP + (IBGP)", RFC 4456, April 2006. [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, - "Multiprotocol Extensions for BGP-4", RFC 4760, - January 2007. + "Multiprotocol Extensions for BGP-4", RFC 4760, January + 2007. - [RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended - Community Attribute", RFC 5701, November 2009. + [RFC5701] Rekhter, Y., "IPv6 Address Specific BGP Extended Community + Attribute", RFC 5701, November 2009. + + [RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet + Autonomous System (AS) Number Space", RFC 6793, December + 2012. Appendix A. Why not discard UPDATE messages? A commonly asked question is "why not simply discard the UPDATE message instead of treating it like a withdraw? Isn't that safer and easier?" The answer is that it might be easier, but it would compromise BGP's correctness so is unsafe. Consider the following example of what might happen if UPDATE messages carrying bad attributes were simply discarded: AS1 ---- AS2 \ / \ / \ / AS3 + Figure 1 + o AS1 prefers to reach AS3 directly, and advertises its route to AS2. o AS2 prefers to reach AS3 directly, and advertises its route to AS1. o Connections AS3-AS1 and AS3-AS2 fail simultaneously. o AS1 switches to prefer AS2's route, and sends an update message which includes a withdraw of its previous announcement. The @@ -468,30 +588,30 @@ Although the example above discusses route withdraws, we observe that in BGP the announcement of a route also withdraws the route previously advertised. The implicit withdraw can be converted into a real withdraw in a number of ways; for example, the previously- announced route might have been accepted by policy, but the new announcement might be rejected by policy. For this reason, the same concerns apply even if explicit withdraws are removed from consideration. -10. Authors' Addresses +Authors' Addresses - John G. Scudder + Enke Chen (editor) + Cisco Systems, Inc. + + Email: enkechen@cisco.com + John G. Scudder (editor) Juniper Networks Email: jgs@juniper.net - Enke Chen - Cisco Systems, Inc. - - EMail: enkechen@cisco.com - Pradosh Mohapatra Cumulus Networks, Inc. - EMail: pmohapat@cumulusnetworks.com + Email: pmohapat@cumulusnetworks.com + Keyur Patel Cisco Systems, Inc. - EMail: keyupate@cisco.com + Email: keyupate@cisco.com