Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0

John Scudder <jgs@juniper.net> Tue, 15 December 2020 21:48 UTC

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From: John Scudder <jgs@juniper.net>
To: "Jakob Heitz (jheitz)" <jheitz=40cisco.com@dmarc.ietf.org>
CC: Christoph Loibl <c@tix.at>, "idr@ietf.org" <idr@ietf.org>
Thread-Topic: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0
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Subject: Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0
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Agreed.

—John

On Dec 15, 2020, at 4:08 PM, Jakob Heitz (jheitz) <jheitz=40cisco.com@dmarc.ietf.org<mailto:jheitz=40cisco.com@dmarc.ietf.org>> wrote:

[External Email. Be cautious of content]


There is a queue from rtr-B BGP to rtr-A BGP. This queue stretches through the TCP implementations on both routers. That means it consists of the TCP send buffer in rtr-B and the TCP receive buffer in rtr-A, whatever might be in the forwarding buffers on both routers and maybe some IPC queues and whatever else. rtr-A BGP only knows what it is pulling from its receive socket. BGP does not know or care what wnd=0 means. As long as it can keep pulling from its receive socket, it cannot expire its hold timer. An important check that a BGP implementation must do before expiring its hold timer is: "is my TCP receive socket empty?".

Regards,
Jakob.

-----Original Message-----
From: Christoph Loibl <c@tix.at<mailto:c@tix.at>>
Sent: Tuesday, December 15, 2020 12:58 PM
To: John Scudder <jgs@juniper.net<mailto:jgs@juniper.net>>
Cc: Jakob Heitz (jheitz) <jheitz@cisco.com<mailto:jheitz@cisco.com>>; idr@ietf.org<mailto:idr@ietf.org>; Robert Raszuk <robert@raszuk.net<mailto:robert@raszuk.net>>
Subject: Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0

John,

Thanks for answering my question in more detail. Maybe I was unclear (but reading your email I think we are talking about the same).

On 15.12.2020, at 21:00, John Scudder <jgs@juniper.net<mailto:jgs@juniper.net>> wrote:

I think you are talking about this scenario. I’ll copy the example from Rob’s message cited above:

 rtr-A                   rtr-B
 (congested c-p)         (uncongested c-p)
 send window: >0         send window: 0
 recv window: 0          recv window: >0

In this case we expect:
a) rtr-B does not send any BGP packet (KEEPALIVE/UPDATE/NOTIFICATION)
to rtr-A in normal operating circumstances.
b) rtr-A does not expect any KEEPALIVE/UPDATE packets from rtr-B. The
session remains established even if no packet is received in the
holdtime.
c) rtr-A continues to send KEEPALIVE packets to rtr-B.


The part I have a problem to understand is b). It is clear that rtr-A will not receive any packets from rtr-B because rtr-B cannot send them (send window: 0). But does "rtr-A does not expect any KEEPALIVE/UPDATE packets from rtr-B” mean that rtr-A has essentially suspended its hold-timer until it is ready to receive new messages and opens up its recv window? If yes, why? I would expect timers to run independently of the transport protocol.

Cheers Christoph

--
Christoph Loibl
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