Border Gateway Protocol 4 (BGP-4) Send Hold Timer
draft-ietf-idr-bgp-sendholdtimer-07

IDR                                                          J. Snijders
Internet-Draft                                                    Fastly
Updates: 4271 (if approved)                            B. Cartwright-Cox
Intended status: Standards Track                            Port 179 Ltd
Expires: 7 November 2024                                           Y. Qu
                                                  Futurewei Technologies
                                                              6 May 2024

           Border Gateway Protocol 4 (BGP-4) Send Hold Timer
                  draft-ietf-idr-bgp-sendholdtimer-07

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 session 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 session and not solely rely on the remote system for session
   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 7 November 2024.

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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.  SendHoldTimer - Changes to RFC 4271 . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . .   5
   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  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     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 session can result in Denial Of
   Service, and the subsequent failure to generate and deliver BGP
   WITHDRAW and UPDATE messages to other BGP peers 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 sessions 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 sessions, but will also locally close
   blocked sessions.

2.  Example of a problematic scenario

   In implementations lacking the concept of a SendHoldTimer, a
   malfunctioning or overwhelmed remote peer 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
   peer 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 peer that is advertising a TCP Receive Window (RCV.WND) of size
   zero.  This 0 window prevents the local system from sending
   KEEPALIVE, CEASE, WITHDRAW, UPDATE, or any other critical BGP
   messages across the network socket to the remote peer.

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

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

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3.  SendHoldTimer - Changes to RFC 4271

   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 session are added
   to Section 8, before "The state session attribute indicates the
   current state of the BGP FSM":

      9) SendHoldTimer

      10) 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 to each peer independently.

3.2.  Timer Event: SendHoldTimer_Expires

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

   Event XX1: 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:

   Old Text:
      If the local system receives a KEEPALIVE message (KeepAliveMsg
      (Event 26)), the local system:

      -  restarts the HoldTimer and

      -  changes its state to Established.

   New Text:

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      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.":

      If the SendHoldTimer_Expires (Event XX1), 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 cases the
      SendHoldTimer is stopped.

      The SendHoldTimer is stopped following any transition out of the
      Established state.

3.4.  Changes to BGP Timers

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

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   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 session 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 session 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-session 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.

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6.  Operational Considerations

   When the local system recognizes a remote peer 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 session 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 session.  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 addresses the vulnerability of a BGP speaker to a
   potential attack whereby a BGP peer can pretend to be unable to
   process BGP messages and in doing so create a scenario where the
   local system is poisoned with stale routing information.

   There are three detrimental aspects to the problem of not robustly
   handling blocked peers:

   *  Failure to send BGP messages to a peer implies the peer is
      operating based on stale routing information.

   *  Failure to disconnect from a blocked peer hinders the local
      system's ability to construct a non-stale local Routing
      Information Base (RIB).

   *  Failure to disconnect from a blocked peer hinders the local
      system's ability to inform other BGP peers with current network
      reachability information.

   In other respects, this specification does not change BGP's security
   characteristics.

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.

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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, and Claudio Jeker for their helpful review of
   this document.

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>.

   [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>.

   [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>.

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   [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>.

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

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

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

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