Border Gateway Protocol 4 (BGP-4) Send Hold Timer
draft-ietf-idr-bgp-sendholdtimer-14

IDR                                                          J. Snijders
Internet-Draft                                                    Fastly
Updates: 4271 (if approved)                            B. Cartwright-Cox
Intended status: Standards Track                                Port 179
Expires: 30 January 2025                                           Y. Qu
                                                               Futurewei
                                                            29 July 2024

           Border Gateway Protocol 4 (BGP-4) Send Hold Timer
                  draft-ietf-idr-bgp-sendholdtimer-14

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 RFC4271.

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.

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 https://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 30 January 2025.

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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
   2.  Example of a problematic scenario . . . . . . . . . . . . . .   3
   3.  Changes to RFC 4271 - SendHoldTimer . . . . . . . . . . . . .   4
     3.1.  Session Attributes  . . . . . . . . . . . . . . . . . . .   4
     3.2.  Timer Event: SendHoldTimer_Expires  . . . . . . . . . . .   4
     3.3.  Changes to the FSM  . . . . . . . . . . . . . . . . . . .   4
     3.4.  Changes to BGP Timers . . . . . . . . . . . . . . . . . .   6
   4.  Send Hold Timer Expired Error Handling  . . . . . . . . . . .   6
   5.  Implementation Considerations . . . . . . . . . . . . . . . .   6
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Implementation status - RFC EDITOR: REMOVE BEFORE
           PUBLICATION . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

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

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   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
   blackholing 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, but will also locally
   close blocked connections.

2.  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.  This 0 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 major issue arising from a BGP speaker being unable to send a BGP
   message to a given remote speaker is that as a result that speaker
   subsequently is operating based on stale 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.

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   This document provides a mechanism for BGP implementations to detect
   whether the TCP socket to a BGP speaker is progressing (data is being
   transmitted), or persisting in a stalled state.  In case of stalled
   state, the BGP connection can be terminated.

3.  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 follows:

3.1.  Session Attributes

   The following optional session attributes for each connection are
   added to Section 8, before "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

   The SendHoldTime determines how long a BGP speaker will stay in
   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.

3.2.  Timer Event: SendHoldTimer_Expires

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

   NEW

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

3.3.  Changes to the FSM

   The following changes are made to section 8.2.2 in [RFC4271].

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

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   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 in "Established
   State", after the paragraph which 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.
   |  

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   |     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 cases 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.

3.4.  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 5 for suggested
   |  default values.

4.  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.

5.  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 are unaware of the necessity of 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

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   Implementations MAY make the value of SendHoldTime configurable,
   either globally or on a per-peer basis, within the constraints set
   out in Section 3.4 above.

   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.

6.  Operational Considerations

   When the local system recognizes a remote speaker is not processing
   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 documents suggests that an attempt to send a
   NOTIFICATION message with the "Send Hold Timer Expired" error code is
   still made, if doing so will not delay closing the BGP connection.
   Meanwhile an error message is logged into the local system.

   Other mechanisms can be used as well, for example BGP speakers SHOULD
   provide this reason as part of their operational state; e.g.
   bgpPeerLastError in the BGP MIB [RFC4273].

7.  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.

8.  IANA Considerations

   IANA has registered code 8 for "Send Hold Timer Expired" in the "BGP
   Error (Notification) Codes" sub-registry under the "Border Gateway
   Protocol (BGP) Parameters" registry.

9.  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.

10.  References

10.1.  Normative References

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   [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)",
              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/>.

   [BIRD]     Kubecova, K., "BIRD Internet Routing Daemon", October
              2023, <https://gitlab.nic.cz/labs/bird/-/commit/
              bcf2327425d4dd96f381b87501cccf943bed606e>.

   [frr]      Lamparter, D., "bgpd: implement SendHoldTimer", May 2022,
              <https://github.com/FRRouting/frr/pull/11225>.

   [neo-bgp]  Cartwright-Cox, B., "What does bgp.tools support", August
              2022, <https://bgp.tools/kb/bgp-support>.

   [openbgpd] Jeker, C., "bgpd send side hold timer", December 2020,
              <https://marc.info/?l=openbsd-tech&m=160820754925261&w=2>.

   [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>.

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Appendix A.  Implementation status - RFC EDITOR: REMOVE BEFORE
             PUBLICATION

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   *  OpenBGPD [openbgpd]

   *  FRRouting [frr]

   *  neo-bgp (bgp.tools) [neo-bgp]

   *  BIRD [BIRD]

   Patches to recognize error code 8 were merged into OpenBSD's and the-
   tcpdump-group's tcpdump implementations.

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

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   Yingzhen Qu
   Futurewei Technologies
   Santa Clara,
   United States
   Email: yingzhen.ietf@gmail.com

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