Re: [Lsr] Flow Control Discussion for IS-IS Flooding Speed

"Les Ginsberg (ginsberg)" <ginsberg@cisco.com> Wed, 19 February 2020 06:05 UTC

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From: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
To: Tony Li <tony1athome@gmail.com>
CC: "lsr@ietf.org" <lsr@ietf.org>
Thread-Topic: [Lsr] Flow Control Discussion for IS-IS Flooding Speed
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Date: Wed, 19 Feb 2020 06:05:16 +0000
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Subject: Re: [Lsr] Flow Control Discussion for IS-IS Flooding Speed
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Tony –

There is no such assumption.

Transmitter has exact knowledge of how many unacknowledged LSPs have been transmitted on each interface.

Using an algorithm functionally equivalent to the example algorithm in the draft, the transmitter slows down when the neighbor is not acknowledging in a timely manner LSPs sent on that interface.
The reason the neighbor is falling behind is irrelevant.

Maybe the receiver has a per interface queue and the associated line card is overloaded.
Maybe the receiver has a single queue but there are so many LSPs received on other interfaces in the front of the queue that the receiver hasn’t yet processed the ones received on this interface.
Maybe the receiver received the same LSPs on other interfaces and is now so busy sending these LSPs that it has fallen behind on processing its receive queue.
Maybe BGP is consuming high CPU and starving IS-IS…

The transmitter doesn’t care.  It just adjusts the transmission rate based on actual performance.

If all interfaces on the receiver are backed up all the neighbors will slow down their transmission rate.

The TX side flow control is purely based on performance on each interface – there are no implementation requirements imposed or implied as regards the receiver.

    Les


From: Tony Li <tony1athome@gmail.com>
Sent: Tuesday, February 18, 2020 7:10 PM
To: Les Ginsberg (ginsberg) <ginsberg@cisco.com>
Cc: lsr@ietf.org
Subject: Re: [Lsr] Flow Control Discussion for IS-IS Flooding Speed


https://datatracker.ietf.org/doc/draft-ginsberg-lsr-isis-flooding-scale/  advocates for a transmit based flow control where the transmitter monitors the number of unacknowledged LSPs sent on each interface and implements a backoff algorithm to slow the rate of sending LSPs based on the length of the per interface unacknowledged queue.


Les,

This makes the assumption that there is a per-interface queue on the LSP receiver. That has never been the case on any implementation that I’ve ever seen.

Without this assumption or more information, it seems difficult for the LSP transmitter to have enough information about how to proceed.

Tony