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
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   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: job@fastly.com


   Ben Cartwright-Cox
   Port 179 Ltd
   London
   United Kingdom
   Email: ben@benjojo.co.uk


   Yingzhen Qu
   Futurewei Technologies
   San Jose, CA 95131
   United States of America
   Email: yingzhen.ietf@gmail.com







ERRATA