Border Gateway Protocol 4 (BGP-4) Send Hold Timer

Abstract

This document defines the SendHoldTimer session attribute for the Border Gateway Protocol (BGP) Finite State Machine (FSM). A session should be terminated if the TCP receive window is zero for the duration of the Send Hold Timer, in this situation the peer is expected to terminate the connection. For robustness, this document specifies that the local system should also close the connection. This document updates RFC4271.¶

1. Introduction

This document defines the SendHoldTimer session attribute for the Border Gateway Protocol (BGP) [RFC4271] Finite State Machine (FSM) defined in section 8.¶

As BGP runs over TCP [RFC0793] it is possible for hosts in the ESTABLISHED state to encounter a BGP peer that is advertising a TCP Receive Window (RCV.WND) of size zero and thus preventing the local system from sending KEEPALIVE, CEASE, WITHDRAW, UPDATE, or other critical messages across the wire. At the moment of writing, most BGP implementations appear unable to handle this situation in a robust fashion.¶

Not terminating a stuck BGP session can result in Denial Of Service, the subsequent failure to generate and deliver BGP WITHDRAW messages to other BGP peers of the local system is detrimental to all participants of the inter-domain routing system. This phenomena is theorised to have contributed to IP traffic backholing events in global Internet routing system [l3outage]¶

This specification intends to improve this situation by requiring sessions to be terminated if the TCP receive window is zero for the duration of the Send Hold Timer. Through codification of the aforementioned requirement, operators will benefit from consistent behavior across different BGP implementations.¶

BGP speakers following this specification do not exclusively rely on remote systems robustly closing connections, but will also locally close connections.¶

2. Specification of the Send Hold Timer

BGP speakers are implemented following a conceptual model "BGP Finite State Machine" (FSM), which is outlined in section 8 of [RFC4271]. This specification updates the BGP FSM as following:¶

    Event XX: SendHoldTimer_Expires
    Definition : An event generated when the SendHoldTimer expires.
    Status: Mandatory

3. Send Hold Timer Expired Error Handling

If a system does not send and receive successive KEEPALIVE, UPDATE, and/or NOTIFICATION messages within the period specified in the Send Hold Time, then the BGP connection is closed and a log message is emitted.¶

4. 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]¶

While not a BGP implementation, as a result of investigation into this phenomena, the Linux TCP implementation TCP_USER_TIMEOUT option was improved, allowing easier termination of TCP sessions which are not progressing data [TCP_USER_TIMEOUT].¶

5. Acknowledgements

The authors would like to thank William McCall for their helpful review of this document.¶

6. 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 'stuck' peers:¶

• Failure to send BGP messages to a peer implies the peer is operating based on stale routing information.¶
• Failure to disconnect from a 'stuck' peer hinders the local system's ability to construct a non-stale local Routing Information Base (RIB).¶
• Failure to disconnect from a 'stuck' 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.¶

7. IANA Considerations

This document requests IANA to assign a value named "Send Hold Timer Expired" in the "BGP Error (Notification) Codes" sub-registry under the "Border Gateway Protocol (BGP) Parameters" registry.¶

8. References