Re: [Bier] ASIC restrictions

Michael Menth <menth@uni-tuebingen.de> Tue, 15 November 2022 22:47 UTC

Message-ID: <9a33f62f-355d-7366-5038-5a329b579cf2@uni-tuebingen.de>
Date: Tue, 15 Nov 2022 23:46:44 +0100
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To: Toerless Eckert <tte@cs.fau.de>, Tony Przygienda <tonysietf@gmail.com>
Cc: Vasilenko Eduard <vasilenko.eduard=40huawei.com@dmarc.ietf.org>, "bier@ietf.org" <bier@ietf.org>
References: <81ff0e3b3bad4e6a892e3aa005aa9e9a@huawei.com> <CA+wi2hN2UpY4ZX51ofWfXDoPu3vW+8zZLtL4LDXrH2sh605Osw@mail.gmail.com> <Y3NllOEy2JdJ2NOZ@faui48e.informatik.uni-erlangen.de>
From: Michael Menth <menth@uni-tuebingen.de>
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Subject: Re: [Bier] ASIC restrictions
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Hi Toerless,

Am 15.11.2022 um 11:10 schrieb Toerless Eckert:
...
> It is quite common though to have replication through lookup of multicast
> tables, such as presented in Steffen/Michaels P4 presentation, but i am not
> too sure, how much post-processing is then still possible. One has to remember that
> every BIER packet copy can have a different bitstring, because of the bit clear
> operation which is specific to each copy. Usually, the only post-processing
> on platforms with multicst table lookup is then limited to some L2 adjacencies.
> If Tofino can actually still do a per-packet rewrite of the bitstring (as required by BIER)
> after such a replication, then that would speak for Tofino. 

This is exactly what we do. Tofino's processing pipeline is divided into 
an ingress ingress and egress part. The operations in the egress part 
are specific for the outgoing port. The behavior is described in a short 
paragraph in VI.A.2.d of the paper:
https://atlas.informatik.uni-tuebingen.de/~menth/papers/Menth22-Sub-2.pdf

> If Tofino can not
> do that, then that would speak for BIER-TE, because BIER-TE does
> only require an ingres clearing of bits, common for all packet copies (except for
> one crazy optional feature).

I second you that this is an advantage of BIER-TE and RBS and its 
variants compared to some other more sclalable alternatives for BIER. 
Some of them have been presented in IETF (under different names, 
possibly only as drafts), but so far they have not yet been intensively 
discussed and understood. They require quite some effort to clear all 
relevant bits. However, this aspect is subject to study and not yet 
documented.

Kind regards

Michael

> 
> Cheers
>      Toerless


-- 
Prof. Dr. habil. Michael Menth
University of Tuebingen
Faculty of Science
Department of Computer Science
Chair of Communication Networks
Sand 13, 72076 Tuebingen, Germany
phone: (+49)-7071/29-70505
fax: (+49)-7071/29-5220
mailto:menth@uni-tuebingen.de
http://kn.inf.uni-tuebingen.de

[Bier] ASIC restrictions Vasilenko Eduard
Re: [Bier] ASIC restrictions Tony Przygienda
Re: [Bier] ASIC restrictions Toerless Eckert
Re: [Bier] ASIC restrictions Tony Przygienda
Re: [Bier] ASIC restrictions Michael Menth