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Internet Engineering Task Force                                R. Shakir
Internet-Draft                                                        BT
Intended status: Informational                          November 9, 2014
Expires: May 13, 2015

Operational Requirements for Enhanced Error Handling Behaviour in BGP-4


   BGP-4 is utilised as a key intra- and inter-Autonomous System routing
   protocol in modern IP networks.  The failure modes as defined by the
   original protocol standards are based on a number of assumptions
   around the impact of session failure.  Numerous incidents both in the
   global Internet routing table and within Service Provider networks
   have been caused by strict handling of a single invalid UPDATE
   message causing large-scale failures in one or more Autonomous

   This memo describes the current use of BGP-4 within Service Provider
   networks, and outlines a set of requirements for further work to
   enhance the mechanisms available to a BGP-4 implementation when
   erroneous data is detected.  Whilst this document does not provide
   specification of any standard, it is intended as an overview of a set
   of enhancements to BGP-4 to improve the protocol's robustness to suit
   its current deployment.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 13, 2015.

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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.

Table of Contents

   1.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Role of BGP-4 in Service Provider Networks  . . . . . . .   3
     2.2.  Service Requirements for Amended BGP Error Handling . . .   4
   3.  Classes of Errors within UPDATE Messages  . . . . . . . . . .   6
     3.1.  Characteristics of Session Scope Errors . . . . . . . . .   6
     3.2.  Characteristics of Message Scope Errors . . . . . . . . .   7
     3.3.  Characteristics of Attribute Scope Errors . . . . . . . .   7
     3.4.  Avoiding Session Scope Errors . . . . . . . . . . . . . .   7
     3.5.  Future Attributes introduced to BGP . . . . . . . . . . .   8
   4.  Error Handling for Non-Critical Errors  . . . . . . . . . . .   8
     4.1.  NLRI-level Error Handling Requirements  . . . . . . . . .   8
       4.1.1.  Notifying the Remote Peer of Non-Critical Errors  . .   9
     4.2.  Recovering RIB Consistency following NLRI-level Error
           Handling  . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Error Handling for Critical Errors  . . . . . . . . . . . . .  10
     5.1.  Long-Lived Critical Errors  . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informational References  . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  Problem Statement

   BGP has developed into a key intra- and inter-domain routing
   protocol, deployed within both the Internet and private networks.
   The changing deployments of the protocol have resulted in increased
   demand for robustness of the routing system - with the error handling
   behaviour defined in [RFC4271] having been shown to have caused
   numerous incidents within live network deployments.  This document
   intends to provide an overview of the current deployment cases for
   BGP-4, and define a set of requirements (from the perspective of a
   network operator) for enhancing error handling within the protocol.

2.1.  Role of BGP-4 in Service Provider Networks

   BGP was designed as an inter-autonomous system (AS) routing protocol.
   Many of the error handling mechanisms within the protocol are defined
   in order to be guarantee consistency and correctness of information
   between two neighbouring speakers.  The assumption is made that each
   AS operates with many adjacencies, each propagating a relatively
   small amount of routing information.  Through focusing on information
   consistency, the protocol specification prefers failure of an
   individual routing adjacency to maintaining reachability to all NLRI
   propagated through a particular neighbour, with the expectation that
   alternate, less direct, paths can be selected where a failure occurs.
   These assumptions resulted in the specification made in [RFC4271]
   whereby the receipt of an erroneous UPDATE message is reacted to by
   sending a NOTIFICATION message, and tearing down the adjacency with
   the remote speaker from whom the error was observed.

   BGP's deployments have evolved with the growth of IP-based services.
   Historically, a network would deploy an interior gateway protocol
   (IGP) to carry infrastructure and customer routes, and utilise an
   external gateway protocol (EGP) such as BGP to propagate routes to
   other autonomous systems.  However, within modern deployments to
   ensure route convergence within an AS is within acceptable time
   bounds the amount of information within the IGP has been minimised
   (typically to only infrastructure routes). iBGP is then utilised to
   carry both internal, customer and external routes within an AS.  As
   such, this has resulted in BGP having become an IGP, with traditional
   IGPs providing only reachability between nodes within the AS for
   packet forwarding, and to establish iBGP sessions.  This change in
   role within the overall architecture of an AS has resulted in an
   increased robustness requirement for BGP, with the expectation of a
   similar level of robustness to that of an IGP being set.  The loss of
   an iBGP session can result in significant levels of unreachability
   internally to an AS, especially since there are typically limited
   (when compared to the Internet) signalling and forwarding paths

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   The volume and nature of the information carried within BGP has also
   changed - it has become the ubiquitous means through which service
   information can be propagated between devices.  For instance, being
   utilised to carry IP/MPLS service information such as Layer 3 IP VPN
   routes [RFC4364] , and Layer 2 Virtual Private LAN Service device
   membership [RFC4761].  Since these extensions to the protocol allow
   signalling of multiple services (represented by address families
   within BGP), and multiple customer topologies (i.e., subsets of
   routes within each address family) via the BGP protocol, the impact
   of session failure is increased.  The tear down of a single BGP
   session can result in a complete outage to all customer services
   signalled via the session, even where the triggering event is related
   to only one service or topology being carried.

   In addition, there has been significant growth in the volume of
   routing information carried in BGP.  In numerous networks, the RIB
   size of individual BGP speakers can be of the order of millions of
   paths.  Particularly large volumes are observed at BGP speakers
   performing aggregation and border roles (such as ASBR, or route
   reflector hierarchies).  This increased volume of routes results not
   only in a significant number of services being impacted during a
   protocol failure, but also increases the time to recovery after re-
   establishing a BGP session.  The time taken to learn, compute and
   distribute new paths increases the impact of failures on services
   carried by the network - adding further weight to the requirement to
   avoid failures, or limit the extent of their impact.  Particularly,
   the impact of individual session failures is increased due to the
   existence of a relatively small number of highly-critical BGP
   sessions within Internet and multi-service network deployments.
   These sessions propagate a high-proportion of the reachability
   information - for instance, providing an Internet AS with the global
   routing table from upstream providers, or providing IP/MPLS Provider
   Edge devices adjacency with route reflector hierarchy providing
   signalling for elements of services connected elsewhere within the
   routing domain.  In both cases, the failure of these sessions can
   result in a significant outage to customer services.

2.2.  Service Requirements for Amended BGP Error Handling

   Alongside the infrastructure requirements outlined above, service
   provider customer requirements continue to evolve.  In particular,
   there are increasing requirements for robustness and fault isolation
   based on:

   o  The increasing reliance on public IP service instead of private
      networks - resulting is requirements for greater availability of
      Internet services.  The diversity of autonomous systems has
      resulted in individual BGP sessions within the Internet carrying

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      more routing information (e.g., IP transit, or large peering
      interconnections), which is originated from more individual
      networks - increasing both the impact of an individual session
      failure, and the number of different sources of error which can
      lead to its failure.  To meet the requirement of high-availability
      Internet services, it is therefore an expectation that the error
      handling behaviour MUST affect only the those routes, or
      autonomous systems, that are are impacted by the erroneous
      messages, rather than all routes received by a particular session,
      such that the maximum service availability is maintained.

   o  The requirement to support multiple services.  In multi-service
      environments such as those that support L3VPNs, multiple customer
      VPNs are isolated from one another, and from other IP environments
      (such as the Internet).  There is an expectation from a service
      perspective therefore that the customer service is within its own
      fault domain (even when carried via a shared set of signalling),
      hence an error on routes or BGP messages related to one VPN SHOULD
      NOT negatively impact other VPNs.  Further to this, an error
      relating to another service (i.e., another address family, such as
      Internet or L2VPN services) SHOULD NOT impact the availability of
      the VPN service.  Both of these principles of fault separation are
      required in order to support multiple services and segregated
      customer infrastructures over a common network infrastructure
      whilst meeting the availability required of them.

   It should be noted that the requirements for fault isolation and
   high-availability do not imply that routing information that is
   potentially erroneous (through being carried in an UPDATE message
   that cannot be parsed for example) is always maintained despite
   questions as to its integrity, particularly as such routing
   information may result in leakage between services - but merely that
   there is a requirement to reconsider the balance between protocol
   correctness, and robustness.

   In addition to these service requirements, an increasing requirement
   to minimise the time taken to recover from incidents exists.  In some
   cases, this may require an operator to compromise on correctness in
   order to maintain integrity of a subset of routing information or
   services.  To meet this requirement, mechanisms allowing an operator
   to ignore all errors or maintain "known good" routing information MAY
   be required.  The implementation of such mechanisms is a business
   consideration of the service provider in question, and MUST consider
   the balance between the risk of incorrectness and the overall impact
   to a network platform.  Such mechanisms are particularly of use where
   lack of routing information violates an operator's policies (e.g.,
   filtering rules distributed via BGP FlowSpec are no longer
   installed), or fault isolation requires significant external liaison

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   (such as contacting a third-party autonomous system to amend or
   filter route announcement).

3.  Classes of Errors within UPDATE Messages

   To meet the requirement to provide more targeted error handling,
   errors are therefore classified into the following scopes:

   o  Attribute Scope - in this case, an error can be localised to a
      particular attribute within the message.  For instance, such
      errors may occur when invalid flags are set within an individual
      attribute within a message, which is otherwise well-formed.

   o  Message Scope - errors resulting in the inability to parse a
      single UPDATE message, but not affecting the ability of an
      implementation to parse subsequent BGP messages.  For instance,
      where the overall length of an UPDATE message is correct, but the
      length of a single attribute contained within it is erroneously

   o  Session Scope - where errors occur such that an error in an UPDATE
      message results in the inability to the parse subsequent messages.
      In this case, attribute length errors may result in the inability
      for a BGP implementation to locate the bounds of an UPDATE, and
      hence the subsequent message from a peer.

   For session-scope errors, the error handling approach implemented
   MUST conform with the requirements described in Section 5 of this
   document (generically referred to as "Critical" error handling
   mechanisms).  Session-scope errors requiring Critical error handling
   MUST be the only case whereby the impact of error handling mechanisms
   should be allowed to impact entire BGP sessions between two BGP

   For message- and attribute-level errors, "Non-Critical" error
   handling mechanisms SHOULD be used, which MUST meet the specification
   described in Section 4.  In the case of attribute-scope errors, a BGP
   speaker MUST limit the impact of error-handling mechanisms to the
   NLRI carried within the message, and MAY (where applicable) limit to
   the scope of error handling to the individual attribute.  Where a
   message-scope error occurs, a BGP speaker MUST limit the impact of
   error handling to the NLRI contained within the affected UPDATE.

3.1.  Characteristics of Session Scope Errors

   Based on analysis of existing BGP implementations, and incidents
   within the Internet and private network routing tables, it is
   expected that errors with a session level scope are restricted to:

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   o  UPDATE Message Length errors - where the specified UPDATE message
      length is inconsistent with the sum of the Total Path Attribute
      and Withdrawn Routes length.  These errors relate to message
      packing or framing, and result in cases whereby the NLRI attribute
      cannot be correctly extracted from the message.

   o  Errors parsing the NLRI attribute of an UPDATE message - where the
      contents of the IPv4 Unicast Advertised or Withdrawn Routes
      attributes, or multi-protocol BGP NLRI attributes (MP_REACH_NLRI
      and/or MP_UNREACH_NLRI as defined in [RFC2858]), cannot be
      successfully parsed.

3.2.  Characteristics of Message Scope Errors

   Message scope errors are restricted to those whereby erroneous
   encoding results in the ability to parse and determine the NLRI
   carried by the message - but the carried attributes are invalid.
   These errors (based on existing attributes) are limited to:

   o  Errors where the length of all path attributes contained within
      the UPDATE does not correspond to the total path attribute length.

   o  UPDATE messages missing mandatory attributes, unrecognised non-
      optional attributes, or those that contain duplicate or invalid
      attributes (be they unsupported, or unexpected).

   o  Those messages where the NEXT_HOP, the MP_REACH_NLRI next-hop
      values are missing, zero-length, or invalid for the relevant
      address family.

3.3.  Characteristics of Attribute Scope Errors

   Attribute scope errors are defined to be those that relate to an
   individual attribute (not related to the NLRI) carried by an UPDATE
   message.  Particularly, where:

   o  Zero- or invalid-length errors in path attributes, excluding those
      containing NLRI.

   o  Invalid data or flags are contained in a path attribute that does
      not relate to the NLRI.

3.4.  Avoiding Session Scope Errors

   In order to maximise the number of cases whereby the NLRI attributes
   can be reliably extracted from a received message, where a BGP
   speaker supports multi-protocol extensions, the MP_REACH_NLRI and
   MP_UNREACH_NLRI attributes SHOULD be utilised for all address

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   families (including IPv4 Unicast) and these attributes should be the
   first attribute contained within the UPDATE message.  For these Non-
   Critical errors, the NLRI-targeted error handling requirements
   described in Section 4 should be followed.

3.5.  Future Attributes introduced to BGP

   Where attributes are introduced by future extensions to the BGP
   protocol error handling behaviour SHOULD be assumed to be be at a
   message- or attribute-scope, unless otherwise specified within the
   per-extension memo, or the attribute relates directly to carrying
   NLRI.  It is recommended that authors of future BGP extensions SHOULD
   specify the error handling behaviour required on a per-attribute
   error basis.

4.  Error Handling for Non-Critical Errors

4.1.  NLRI-level Error Handling Requirements

   When a Non-Critical error is detected within an UPDATE message a BGP
   speaker MUST NOT send a NOTIFICATION message to the remote neighbour.
   Instead, the NLRI contained within the message SHOULD be considered
   as being withdrawn by the neighbour (referred to as treat-as-
   withdraw), until they are updated by a subsequent UPDATE message.
   Where defined is acceptable by the relevant memo, for the specific-
   case of attribute-scope errors, the erroneous attribute MAY be
   discarded by an implementation.  This attribute-discard approach MUST
   only be used for attributes that do not impact best-path selection
   within an implementation.  An operator SHOULD consider the impact of
   implementing policies considering such attributes as part of the
   route selection algorithm, such that operator configuration does not
   result in unexpected consequences should such an attribute be

   Network operators SHOULD recognise that where treat-as-withdraw
   behaviour is implemented black-holing or looping of traffic may occur
   in the period between the NLRI being treated as withdrawn, and
   subsequent updates, dependent upon the routing topology.  It SHOULD
   be noted that such periods of RIB inconsistency (where one speaker
   has advertised a prefix, which has had treat-as-withdraw applied to
   it by the receiving speaker) may be relatively long lived, based on
   situations such as an erroneous implementation at the receiver, or
   the error occurring within an optional-transitive attribute not
   examined by the direct neighbour.  In order to allow operators to
   select sessions on which this risk of inconsistency is acceptable, an
   implementation SHOULD provide means by which Non-Critical error
   handling can be disabled on a per-session basis.

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   Since the Non-Critical error handling required within this section
   results in no NOTIFICATION message being transmitted, the fact that
   an error has occurred, and there may be inconsistency between the
   local and remote BGP speaker MUST be flagged to the network operator
   through standard operational interfaces (e.g., SNMP, syslog).  The
   information highlighted MUST include the NLRI identified to be
   contained within the error message, and SHOULD contain a exact copy
   of the received message for further analysis.

4.1.1.  Notifying the Remote Peer of Non-Critical Errors

   In order that the operator of the BGP speaker from whom an erroneous
   UPDATE message has been advertised is aware of the fact that some
   NLRI advertised to the remote speaker have been considered invalid, a
   BGP speaker SHOULD support mechanisms to report the occurrence of
   Non-Critical error handling to the remote speaker.  The receiving
   speaker SHOULD transmit the NLRI contained within the erroneous
   message to the advertising speaker.  An exact copy of the received
   UPDATE message SHOULD also be sent.

   The exchange of such information related to events occurring as a
   result of BGP messages is not currently supported by any extension to
   the protocol.  Clearly, where the two speakers reside within the same
   administrative domain, shared logging information can be utilised to
   identify the root cause of errors.  However, in many cases these
   devices reside within separate administrative domains (e.g., are
   ASBRs for Internet or private networks).  In this case, mechanisms
   allowing transmission in-band to the BGP session SHOULD be utilised
   (e.g., the OPERATIONAL message described in
   [I-D.ietf-idr-operational-message]).  Such an in-band channel is
   preferred based on the BGP session representing a pre-established
   trusted source which is related to a specific BGP-speaking device
   within a network.  It is expected that the overall system scalability
   of a BGP speaker is improved through utilising the existing channel,
   rather than incurring overhead for maintaining many additional
   sessions for relatively infrequent messaging events when errors
   occur.  However, the extensions providing such a channel MUST
   consider their impact to base BGP protocol functions such as the
   transmission of UPDATE or KEEPALIVE messages, and SHOULD limit the
   volume of messaging to direct reactions to Non-Critical errors
   occurring.  These considerations SHOULD be made in order to ensure
   that no compromise is made to the security, scalability and
   robustness of BGP.  Where additional BGP monitoring information that
   is not suitable to be carried in-band is required, out-of-band
   mechanisms such as the BMP protocol described in [I-D.ietf-grow-bmp]
   could be utilised to provide further information relating to
   erroneous messages.

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4.2.  Recovering RIB Consistency following NLRI-level Error Handling

   In order to recover consistency of Adj-RIBs following Non-Critical
   error handling, a means by which a validation and recovery of
   consistency can be achieved SHOULD be provided to an operator.  This
   functionality MAY be provided through extension of the ROUTE-REFRESH
   [RFC2918] mechanism - providing means to identify the beginning and
   end of a replay of the entire Adj-RIB-Out of the advertising speaker
   (as per the suggestion in [I-D.ietf-idr-bgp-enhanced-route-refresh]).

   As Non-Critical error handling is localised to the NLRI contained
   within the erroneous UPDATE message, a targeted recovery mechanism
   MAY be provided allowing a speaker to request re-advertisement of a
   particular subset of the Adj-RIB-Out. Where such targeted refresh
   functions are available, they SHOULD be preferred to mechanisms
   requesting re-advertisement of the whole Adj-RIB-Out based on their
   more limited use of CPU and network resources.

   A BGP speaker may automatically trigger recovery mechanisms such as
   those described in this section following the receipt of an erroneous
   UPDATE message identified as Non-Critical to expedite recovery.  It
   SHOULD be noted that if automatic recovery mechanisms trigger only
   re-advertisement of an identical erroneous message, they may be
   ineffective.  Additionally, where the best-path to be advertised by
   remote speaker changes, this will be advertised directly, without a
   requirement for a request from the receiver.  However, in some cases,
   RIB consistency recovery mechanisms may prompt alternate UPDATE
   message packing, and hence allow quicker recovery.  Where such
   automatic mechanisms are implemented, those focused on smaller sets
   of NLRI SHOULD be preferred over those requesting the entire RIB.  In
   addition, such mechanisms SHOULD have dampening mechanisms to ensure
   that their impact to computational and network resources is limited.

5.  Error Handling for Critical Errors

   Critical error handling MUST be used where session-scope errors
   occur.  In such cases, a NOTIFICATION message MUST be sent to the
   remote peer.  In order to limit the impact to network operation,
   during such events the mechanisms applied MUST allow for the paths
   NLRI received from the remote speaker to continue to be utilised
   during the session reset and re-establishment.  It is envisaged that
   this requirement may be met through extension of the BGP Graceful
   Restart mechanism ([RFC4724]) to be triggered by NOTIFICATION
   messages indicating the occurrence of a Critical error.  Such an
   extension allows a restart of the TCP and BGP sessions between two
   speakers, in a similar manner to the current session restart
   behaviour triggered by a NOTIFICATION message.  In order to maximise
   the level of re-initialisation which occurs during such a restart

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   triggered by a Critical error, BGP speakers MAY re-initialise memory
   structures related to the RIB where possible.

   Where such a restart event occurs, the continued liveliness of the
   remote device MAY be verified by BGP KEEPALIVE packets or other OAM
   functions such as Bidirectional Forwarding Detection ([RFC5880]).  If
   the observed Critical BGP error is indicative of a wider device
   failure of the remote speaker, it is expected that a BGP sessions
   will not re-establish correctly.  By default, each BGP speaker SHOULD
   maintain a limited time window in which session restart is expected
   in order to mitigate this possibility.

   When a Critical error occurs, the network operator MUST be made aware
   of its occurrence through local logging mechanisms (e.g., SNMP traps
   or syslog).  The BGP speaker receiving an UPDATE message identified
   as a Critical error MUST log its occurrence and a copy of the UPDATE
   message.  Where a inter-device messaging mechanism is implemented (as
   discussed in Section Section 4.1) a copy of the erroneous UPDATE
   message SHOULD be transmitted to the remote speaker upon session-re-
   establishment (or via a separate session if implemented).  Both BGP
   speakers MUST indicate to an operator the cause of a session restart
   was a Critical error in an UPDATE message.

   Since repeated critical errors (and session restarts) may have an
   impact in overall device scaling if Critical error handling does not
   resolve the failure condition, a BGP speaker MAY choose to revert to
   the session tear down behaviour described in the base BGP
   specification.  This reversion SHOULD only be utilised after a number
   of attempts which MUST be controllable by the network operator.
   Where a session is shut down, the implementation MAY utilise a back-
   off from session restart attempts (as per the IdleHoldTimer described
   in the BGP FSM [RFC4271]).  Where reversion to tearing down the BGP
   session is performed, a speaker SHOULD limit the impact of
   withdrawing prefixes from downstream speakers where possible.  It is
   envisaged that this can be achieved by utilising a mechanism such as
   the BGP Graceful Shutdown procedure as described in

5.1.  Long-Lived Critical Errors

   Where Critical error handling mechanisms are required to be utilised,
   significant impact to an operator's network or services may still be
   experienced.  In order to allow an operator to avoid such scenarios:

   o  An implementation MAY provide functionality whereby all future
      Critical errors result in UPDATE messages being discarded.  Such
      functionality MUST be disabled by default, and SHOULD be
      configurable on a per-address-family basis.  An operator MUST

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      consider such mechanisms as a tool of last-resort to maintain
      service for a subset of NLRI, whilst the root cause of a such
      errors is investigated and resolved.  This MAY be achieved by
      filtering erroneous NLRI at an upstream peer.

   o  Provide means by which a the restart timer for Graceful Restart
      can be configured to be a long period (order of days, or weeks)
      such that a critical failure can be resolved whilst maintaining
      operation for a subset of NLRI.  This restart period MUST be
      configured separately to standard graceful-restart timers and MUST
      be configurable per-address-family.  Long-lived restart mechanisms
      MAY be configurable to be utilised by default.  An operator MUST
      configure the impact to forwarding correctness of such
      configuration, based on the expected rate of change of NLRI within
      a particular <AFI,SAFI>.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   The requirements outlined in this document provide mechanisms which
   limit the forwarding impact of the response to an error in a BGP
   UPDATE message.  This is of benefit to the security of a BGP speaker.
   Without these mechanisms, where erroneous UPDATE messages relating to
   a single NLRI entry can be propagated to a BGP speaker, all other
   NLRI carried via the same session are affected by the resulting
   session tear-down.  This may result in a means by which an AS can be
   isolated from particular routing domains (such as the Internet)
   should an UPDATE message be propagated via targeted specific paths.
   It is envisaged by reducing the impact of the reaction of the
   receiving speaker to these messages, the isolation can be constrained
   to specific sets of NLRI, or a specific topology.

   A number of the mechanisms meeting the requirements specified within
   the document (particularly those relating to operational monitoring)
   may raise further security concerns.  Such concerns will be addressed
   during the specification of these mechanisms.

8.  Acknowledgements

   Many thanks are extended to Bruno Decraene and David Freedman for
   their numerous detailed reviews, and significant contribution towards
   the refinement of the requirements in this document.

   In addition, the author would like to thank the following network
   operators for their insight, and valuable input into defining the

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   requirements for a variety of deployments of BGP: Shane Amante, Colin
   Bookham, Rob Evans, Wes George, Tom Hodgson, Sven Huster, Jonathan
   Newton, Neil McRae, Thomas Mangin, Tom Scholl and Ilya Varlashkin.
   Many thanks are extended to Jeff Haas, Wim Hendrickx, Tony Li, Alton
   Lo, Keyur Patel, John Scudder, Adam Simpson and Robert Raszuk for
   their expertise relating to implementations of the BGP protocol.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2858]  Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
              "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.

   [RFC2918]  Chen, E., "Route Refresh Capability for BGP-4", RFC 2918,
              September 2000.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

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

   [RFC4761]  Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
              (VPLS) Using BGP for Auto-Discovery and Signaling", RFC
              4761, January 2007.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

9.2.  Informational References

              Chen, E., Mohapatra, P., and K. Patel, "Revised Error
              Handling for BGP Updates from External Neighbors", draft-
              chen-ebgp-error-handling-01 (work in progress), September

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              Francois, P., Decraene, B., Pelsser, C., Patel, K., and C.
              Filsfils, "Graceful BGP session shutdown", draft-ietf-
              grow-bgp-gshut-06 (work in progress), August 2014.

              Scudder, J., Fernando, R., and S. Stuart, "BGP Monitoring
              Protocol", draft-ietf-grow-bmp-07 (work in progress),
              October 2012.

              Patel, K., Chen, E., and B. Venkatachalapathy, "Enhanced
              Route Refresh Capability for BGP-4", draft-ietf-idr-bgp-
              enhanced-route-refresh-10 (work in progress), June 2014.

              Freedman, D., Raszuk, R., and R. Shakir, "BGP OPERATIONAL
              Message", draft-ietf-idr-operational-message-00 (work in
              progress), March 2012.

Author's Address

   Rob Shakir
   BT plc.
   pp. C3L,
   BT Centre,
   81, Newgate Street,
   London.  EC1A 7AJ

   Email: rob.shakir@bt.com
   URI:   http://www.bt.com/

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