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IETF RFC 9687
Last modified on Wednesday, November 6th, 2024
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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/RFC 9687.
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 [RFC 4271].
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 [RFC 2119] [RFC 8174] 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 [RFC 9293], 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 [RFC 4271].
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 [RFC 4271] 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 [RFC 4271] 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 [RFC 4271].
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 [RFC 4271] 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 [RFC 4271] 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 [RFC 9687] 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
[RFC 4273]).
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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC 4271, January 2006,
<https://www.rfc-editor.org/info/RFC 4271>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC 9293, August 2022,
<https://www.rfc-editor.org/info/RFC 9293>.
10.2. Informative References
[bgpzombies]
Fontugne, R., "BGP Zombies", April 2019,
<https://labs.ripe.net/author/romain_fontugne/bgp-
zombies/>.
[RFC 4273] Haas, J., Ed. and S. Hares, Ed., "Definitions of Managed
Objects for BGP-4", RFC 4273, DOI 10.17487/RFC 4273,
January 2006, <https://www.rfc-editor.org/info/RFC 4273>.
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
RFC TOTAL SIZE: 14628 bytes
PUBLICATION DATE: Wednesday, November 6th, 2024
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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