Internet Engineering Task Force (IETF)                       J. Snijders
Request for Comments: 9687                                        Fastly
Updates: 4271                                          B. Cartwright-Cox
Category: Standards Track                                       Port 179
ISSN: 2070-1721                                                    Y. Qu
                                                              Futurewei
                                                          November 2024


          Border Gateway Protocol 4 (BGP-4) Send Hold Timer

Abstract

  This document defines the SendHoldTimer, along with the
  SendHoldTimer_Expires event, for the Border Gateway Protocol (BGP)
  Finite State Machine (FSM).  Implementation of the SendHoldTimer
  helps overcome situations where a BGP connection is not terminated
  after the local system detects that the remote system is not
  processing BGP messages.  This document specifies that the local
  system should close the BGP connection and not solely rely on the
  remote system for connection closure when the SendHoldTimer expires.
  This document updates RFC 4271.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9687.

Copyright Notice

  Copyright (c) 2024 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
  (https://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 Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
  2.  Requirements Language
  3.  Example of a Problematic Scenario
  4.  Changes to RFC 4271 - SendHoldTimer
    4.1.  Session Attributes
    4.2.  Timer Event: SendHoldTimer_Expires
    4.3.  Changes to the FSM
    4.4.  Changes to BGP Timers
  5.  Send Hold Timer Expired Error Handling
  6.  Implementation Considerations
  7.  Operational Considerations
  8.  Security Considerations
  9.  IANA Considerations
  10. References
    10.1.  Normative References
    10.2.  Informative References
  Acknowledgements
  Authors' Addresses

1.  Introduction

  This document defines the SendHoldTimer, along with the
  SendHoldTimer_Expires event, for the Border Gateway Protocol (BGP)
  Finite State Machine (FSM) defined in Section 8 of [RFC4271].

  Failure to terminate a blocked BGP connection can result in network
  reachability issues, and the subsequent failure to generate and
  deliver BGP UPDATE messages to another BGP speaker of the local
  system is detrimental to all participants of the inter-domain routing
  system.  This phenomena is thought to have contributed to IP traffic
  packet loss events in the global Internet routing system
  [bgpzombies].

  This specification intends to improve this situation by requiring
  that BGP connections be terminated if the local system has detected
  that the remote system cannot possibly have processed any BGP
  messages for the duration of the SendHoldTime.  Through
  standardization of the aforementioned requirement, operators will
  benefit from consistent behavior across different BGP
  implementations.

  BGP speakers following this specification do not rely exclusively on
  remote systems closing blocked connections; they also locally close
  blocked connections.

2.  Requirements Language

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.

3.  Example of a Problematic Scenario

  In implementations lacking the concept of a SendHoldTimer, a
  malfunctioning or overwhelmed remote speaker may cause data on the
  BGP socket in the local system to accumulate ad infinitum.  This
  could result in forwarding failure and traffic loss, as the
  overwhelmed speaker continues to utilize stale routes.

  An example fault state: as BGP runs over TCP [RFC9293], it is
  possible for a BGP speaker in the Established state to encounter a
  BGP speaker that is advertising a TCP Receive Window (RCV.WND) of
  size zero.  The size zero of this window prevents the local system
  from sending KEEPALIVE, UPDATE, or any other critical BGP messages
  across the network socket to the remote speaker.

  Generally BGP implementations have no visibility into lower-layer
  subsystems such as TCP or the speaker's current Receive Window size,
  and there is no existing BGP mechanism for such a blocked connection
  to be recognized.  Hence BGP implementations are not able to handle
  this situation in a consistent fashion.

  The primary issue that arises when a BGP speaker is unable to send a
  BGP message to a remote speaker is that the affected speaker may end
  up operating with outdated routing information.  Failure of the BGP
  speaker to send (and thus the remote speaker to receive) BGP messages
  on a single BGP session can negatively impact the ability of an
  entire autonomous system (or even a group of autonomous systems) to
  converge.

4.  Changes to RFC 4271 - SendHoldTimer

  BGP speakers are implemented following a conceptual model "BGP Finite
  State Machine" (FSM), which is outlined in Section 8 of [RFC4271].
  This specification adds a BGP timer, SendHoldTimer, and updates the
  BGP FSM as indicated in the following subsections.

4.1.  Session Attributes

  The following optional session attributes for each connection are
  added to the list in Section 8 of [RFC4271] appearing just prior to
  "The optional session attributes support different features of the
  BGP functionality that have implications for the BGP FSM state
  transitions":

  NEW

  |     14) SendHoldTimer
  |
  |     15) SendHoldTime

  SendHoldTime determines how long a BGP speaker will stay in the
  Established state before the TCP connection is dropped because no BGP
  messages can be transmitted to its peer.  A BGP speaker can configure
  the value of the SendHoldTime for each peer independently.

4.2.  Timer Event: SendHoldTimer_Expires

  Another timer event is added to Section 8.1.3 of [RFC4271] as
  follows:

  NEW

  |     Event 29: SendHoldTimer_Expires
  |
  |        Definition: An event generated when the SendHoldTimer
  |        expires.
  |
  |        Status: Optional

4.3.  Changes to the FSM

  The following changes are made to Section 8.2.2 of [RFC4271].

  In "OpenConfirm State", the handling of Event 26 is revised as
  follows:

  OLD

  |  If the local system receives a KEEPALIVE message (KeepAliveMsg
  |  (Event 26)), the local system:
  |
  |     -  restarts the HoldTimer and
  |
  |     -  changes its state to Established.

  NEW

  |  If the local system receives a KEEPALIVE message (KeepAliveMsg
  |  (Event 26)), the local system:
  |
  |     -  restarts the HoldTimer,
  |
  |     -  starts the SendHoldTimer if the SendHoldTime is non-zero,
  |        and
  |
  |     -  changes its state to Established.

  The following paragraph is added to Section 8.2.2 of [RFC4271] in
  "Established State", after the paragraph that ends "unless the
  negotiated HoldTime value is zero":

  NEW

  |  If the SendHoldTimer_Expires (Event 29) occurs, the local system:
  |
  |     -  (optionally) sends a NOTIFICATION message with the BGP Error
  |        Code "Send Hold Timer Expired" if the local system can
  |        determine that doing so will not delay the following actions
  |        in this paragraph,
  |
  |     -  logs an error message in the local system with the BGP Error
  |        Code "Send Hold Timer Expired",
  |
  |     -  releases all BGP resources,
  |
  |     -  sets the ConnectRetryTimer to zero,
  |
  |     -  drops the TCP connection,
  |
  |     -  increments the ConnectRetryCounter by 1,
  |
  |     -  (optionally) performs peer oscillation damping if the
  |        DampPeerOscillations attribute is set to TRUE, and
  |
  |     -  changes its state to Idle.
  |
  |     Each time the local system sends a BGP message, it restarts the
  |     SendHoldTimer unless the SendHoldTime value is zero or the
  |     negotiated HoldTime value is zero, in which case the
  |     SendHoldTimer is stopped.
  |
  |     The SendHoldTimer is stopped following any transition out of
  |     the Established state as part of the "release all BGP
  |     resources" action.

4.4.  Changes to BGP Timers

  Section 10 of [RFC4271] summarizes BGP timers.  This document adds
  another optional BGP timer: SendHoldTimer.

  NEW

  |  SendHoldTime is an FSM attribute that stores the initial value for
  |  the SendHoldTimer.  If SendHoldTime is non-zero, then it MUST be
  |  greater than the value of HoldTime; see Section 6 of [RFC9687] for
  |  suggested default values.

5.  Send Hold Timer Expired Error Handling

  If the local system does not send any BGP messages within the period
  specified in SendHoldTime, then a NOTIFICATION message with the "Send
  Hold Timer Expired" Error Code MAY be sent and the BGP connection
  MUST be closed.  Additionally, an error MUST be logged in the local
  system, indicating the "Send Hold Timer Expired" Error Code.

6.  Implementation Considerations

  Due to the relative rarity of the failure mode that this
  specification is designed to address, and also the fact that network
  operators may be unfamiliar with the formal specification of BGP
  fault detection mechanisms such as HoldTimer, it is likely that a
  large number of operators will be unaware of the need for an
  additional mechanism such as SendHoldTimer.

  Accordingly, it is RECOMMENDED that implementations of this
  specification enable SendHoldTimer by default, without requiring
  additional configuration of the BGP-speaking device.

  The default value of SendHoldTime for a BGP connection SHOULD be the
  greater of:

  *  8 minutes or

  *  2 times the negotiated HoldTime

  Implementations MAY make the value of SendHoldTime configurable,
  either globally or on a per-peer basis, within the constraints set
  out in Section 4.4.

  The subcode for NOTIFICATION message "Send Hold Timer Expired" is set
  to 0 and is not used; no additional data is to be appended to the end
  of a "Send Hold Timer Expired" NOTIFICATION message.

7.  Operational Considerations

  When the local system recognizes that a remote speaker has not
  processed any BGP messages for the duration of the SendHoldTime, it
  is likely that the local system will not be able to inform the remote
  peer through a NOTIFICATION message as to why the connection is being
  closed.  This document suggests that an attempt to send a
  NOTIFICATION message with the "Send Hold Timer Expired" Error Code
  still be made, if doing so will not delay closing the BGP connection.
  Meanwhile, an error message is logged in the local system.

  Other mechanisms can be used as well, for example, BGP speakers
  SHOULD provide this reason ("Send Hold Timer Expired") as part of
  their operational state (for example, bgpPeerLastError in the BGP MIB
  [RFC4273]).

8.  Security Considerations

  This specification does not change BGP's security characteristics.
  Implementing the BGP SendHoldTimer as specified in this document will
  enhance network resilience by terminating connections with
  malfunctioning or overwhelmed remote peers.

9.  IANA Considerations

  IANA has registered value 8 for "Send Hold Timer Expired" in the "BGP
  Error (Notification) Codes" registry within the "Border Gateway
  Protocol (BGP) Parameters" registry group.

10.  References

10.1.  Normative References

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

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

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  [RFC9293]  Eddy, W., Ed., "Transmission Control Protocol (TCP)",
             STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
             <https://www.rfc-editor.org/info/rfc9293>.

10.2.  Informative References

  [bgpzombies]
             Fontugne, R., "BGP Zombies", April 2019,
             <https://labs.ripe.net/author/romain_fontugne/bgp-
             zombies/>.

  [RFC4273]  Haas, J., Ed. and S. Hares, Ed., "Definitions of Managed
             Objects for BGP-4", RFC 4273, DOI 10.17487/RFC4273,
             January 2006, <https://www.rfc-editor.org/info/rfc4273>.

Acknowledgements

  The authors would like to thank William McCall, Theo de Raadt, John
  Heasley, Nick Hilliard, Jeffrey Haas, Tom Petch, Susan Hares, Keyur
  Patel, Ben Maddison, Claudio Jeker, and John Scudder for their
  helpful review of this document.

Authors' Addresses

  Job Snijders
  Fastly
  Amsterdam
  Netherlands
  Email: [email protected]


  Ben Cartwright-Cox
  Port 179 Ltd
  London
  United Kingdom
  Email: [email protected]


  Yingzhen Qu
  Futurewei Technologies
  San Jose, CA 95131
  United States of America
  Email: [email protected]

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