Re: [Idr] draft-spaghetti-idr-bgp-sendholdtimer - Feedback requested

Jeffrey Haas <jhaas@pfrc.org> Mon, 26 April 2021 00:19 UTC

Return-Path: <jhaas@slice.pfrc.org>
X-Original-To: idr@ietfa.amsl.com
Delivered-To: idr@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 0019A3A08E7 for <idr@ietfa.amsl.com>; Sun, 25 Apr 2021 17:19:52 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.9
X-Spam-Level:
X-Spam-Status: No, score=-1.9 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_BLOCKED=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id anyrRazV5LB3 for <idr@ietfa.amsl.com>; Sun, 25 Apr 2021 17:19:51 -0700 (PDT)
Received: from slice.pfrc.org (slice.pfrc.org [67.207.130.108]) by ietfa.amsl.com (Postfix) with ESMTP id 951533A08DE for <idr@ietf.org>; Sun, 25 Apr 2021 17:19:51 -0700 (PDT)
Received: by slice.pfrc.org (Postfix, from userid 1001) id 026451E45A; Sun, 25 Apr 2021 20:43:02 -0400 (EDT)
Date: Sun, 25 Apr 2021 20:43:02 -0400
From: Jeffrey Haas <jhaas@pfrc.org>
To: "Jakob Heitz (jheitz)" <jheitz@cisco.com>
Cc: Robert Raszuk <robert@raszuk.net>, "idr@ietf. org" <idr@ietf.org>, Ben Cox <ben=40benjojo.co.uk@dmarc.ietf.org>
Message-ID: <20210426004302.GE19004@pfrc.org>
References: <CAL=9YSVy+mvxvAv+maxkUSzPbe0bfnUy-XJJTtcVhi3S3bm=WQ@mail.gmail.com> <20210423212348.GB19004@pfrc.org> <CAOj+MMGH+y-gxSLaakknWSPFLEk9ikkUU1fa=3H0FjkokAbg3w@mail.gmail.com> <20210424004838.GC19004@pfrc.org> <CAOj+MMH5yzpPZjdUcfXV4cxCORqCsQY4X+niBjnwxjPfN-tsJA@mail.gmail.com> <BYAPR11MB3207E4A0BDC3367E21886C55C0439@BYAPR11MB3207.namprd11.prod.outlook.com> <20210425192705.GD19004@pfrc.org> <BYAPR11MB3207F5B28D5C9B402940D197C0439@BYAPR11MB3207.namprd11.prod.outlook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <BYAPR11MB3207F5B28D5C9B402940D197C0439@BYAPR11MB3207.namprd11.prod.outlook.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Archived-At: <https://mailarchive.ietf.org/arch/msg/idr/NypjsVrnLPdm1H9js2esrQkeZ9U>
Subject: Re: [Idr] draft-spaghetti-idr-bgp-sendholdtimer - Feedback requested
X-BeenThere: idr@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Inter-Domain Routing <idr.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/idr>, <mailto:idr-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/idr/>
List-Post: <mailto:idr@ietf.org>
List-Help: <mailto:idr-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/idr>, <mailto:idr-request@ietf.org?subject=subscribe>
X-List-Received-Date: Mon, 26 Apr 2021 00:19:53 -0000

Jakob,

On Sun, Apr 25, 2021 at 11:28:43PM +0000, Jakob Heitz (jheitz) wrote:
> My understanding of TCP zero window is different.
> It is the peer sending 0 in the "window" field of the TCP header
> in the 16 bit word, at offset 14 bytes into the header.
> That means it will not accept any TCP data from you,
> but it is perfectly capable of sending you data and
> accepting your acks for it.

You're quite correct and I was mistaken here.

> If it were not so, then it would not be able to send you keepalives
> and your hold timer would expire.

Eventually, which was my incorrect thinking.

> It does not mean that the data plane is broken.
> If that were the case, you would get either no keepalives
> from the peer or no more acks to your own keepalives.

For my part, I don't believe that "broken data plane" is the main issue
here.  It's simply the fact that since the peer is blocked from sending
updates, we have a flavor of the dreaded "stuck route" problem.

> If forwarding is broken one way from you to your peer, then
> you will no longer get ACKs back from keepalives or other
> BGP packets you send to your peer.

Forwarding for directly attached peers (same subnet) often will behave well
when forwarding for remotely attached peers may fail.  A quirk of various
silicon.  And certainly not a guaranteed behavior.

>  This is not a zero window
> condition. This may be another condition we wish to monitor.
> Note, there are other ways to monitor this condition, e.g. BFD.

I might know a little about that.

-- Jeff