IDR J. Snijders
Internet-Draft Fastly
Updates: 4271 (if approved) B. Cartwright-Cox
Intended status: Standards Track Port 179 Ltd
Expires: 3 November 2024 Y. Qu
Futurewei Technologies
2 May 2024
Border Gateway Protocol 4 (BGP-4) Send Hold Timer
draft-ietf-idr-bgp-sendholdtimer-06
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 connection 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 3 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 . . . . . . . . . . . . . . . . . 6
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 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 connection 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.
4. Send Hold Timer Expired Error Handling
If a system does not send successive KEEPALIVE or UPDATE 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 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.
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 connection. Meanwhile an error
message is logged into the local system.
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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.
9. Acknowledgements
The authors would like to thank William McCall, Theo de Raadt, John
Heasley, Nick Hilliard, Jeffrey Haas, Tom Petch, Susan Hares, Ben
Maddison, and Claudio Jeker 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>.
[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>.
[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>.
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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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