Re: [mpls] how does it work if ....

Stewart Bryant <stewart.bryant@gmail.com> Mon, 29 March 2021 13:15 UTC

Return-Path: <stewart.bryant@gmail.com>
X-Original-To: mpls@ietfa.amsl.com
Delivered-To: mpls@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 507123A11D3 for <mpls@ietfa.amsl.com>; Mon, 29 Mar 2021 06:15:45 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.098
X-Spam-Level:
X-Spam-Status: No, score=-2.098 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (2048-bit key) header.d=gmail.com
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 Sl4vwE2gxjwq for <mpls@ietfa.amsl.com>; Mon, 29 Mar 2021 06:15:40 -0700 (PDT)
Received: from mail-wr1-x433.google.com (mail-wr1-x433.google.com [IPv6:2a00:1450:4864:20::433]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id A18573A11CE for <mpls@ietf.org>; Mon, 29 Mar 2021 06:15:40 -0700 (PDT)
Received: by mail-wr1-x433.google.com with SMTP id v11so12804108wro.7 for <mpls@ietf.org>; Mon, 29 Mar 2021 06:15:40 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:subject:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to; bh=4mJqmwcWIfdFqA2tGyJGSVdGd4gRN1uqALj2xnUIK8k=; b=IA41mbOM193uYWpW4MZm52+5uEQ3JiLm/tWgL7/hIGhba/W0gxX9h/1kRtHf1RYAqr 9+hcROVdsKjReRkbwMabvEr5fLwsX1aYCXSLnAq6AxsOZnfb6GOvwH6LL3ibPkv1qZij 15glYlcfhZBmXC1HPQwpqq9pIZKapdcMHVFPyAvEoIwpxjM/OPnAeEGbTe+Xyjyk1o3I EjYYQZ3yuwEeOuzC8gW9TOPn+NcxViBrv8QvKT3Qo+oCgCws3l97Wqd4zFMs6vhRNd6i OmvipvrpU5w2mlZnFYuD3qsrD3aV8/rcaWMgICkMW/8yg9gBEEv+7mIkfa7H6xqA0sAT HNzA==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:subject:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to; bh=4mJqmwcWIfdFqA2tGyJGSVdGd4gRN1uqALj2xnUIK8k=; b=kXL6brHoIEbjNNfDfMy94//WYHiKBKk2Yu5xUc/hbLIqXYXbkz6XQnHSnt0uFOfFm7 0RqbuYFUgPk069FKhA91b/CN6tAlCC4ncFacgp4Uc4vpKjh27hgECRlytqkxjjXbSG6+ 9lqpIxWaoKWPLgs4bnfhXcJi7DelGTYRMIGo64nWVS+c09JytiGoMPPdCiXWkwpmNWQz moCwhxOu15lvB9QWnsaQAjqNTNGxXStsXJc5ckxgebBytEcslQIRrG3n851yTJb8GfaN FI2VRZ7fw3UJYY9A4oyi5L32QBtIlF8Jhnbi0Yn7IUppi2j5SIHrKYBuw9PMmgoGMexE oT9Q==
X-Gm-Message-State: AOAM530m9RvUs7lTNPyKA0T8D6krVK4Vro1s38DNfLw3kTkD5c1Pq4IB WltkBEFKb77/a2KDAQR9oTFihicC4eI=
X-Google-Smtp-Source: ABdhPJwV3dZCMa2NuT+A6DiFUb7aaa8ouUT1/gK0Yhxwv4Fh3/B+y8w5x+b/y4zRyMjld/R8n4N+yg==
X-Received: by 2002:a5d:6152:: with SMTP id y18mr28660107wrt.255.1617023737078; Mon, 29 Mar 2021 06:15:37 -0700 (PDT)
Received: from [192.168.8.102] ([148.252.133.242]) by smtp.gmail.com with ESMTPSA id a17sm24627863wmj.9.2021.03.29.06.15.36 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Mon, 29 Mar 2021 06:15:36 -0700 (PDT)
Content-Type: text/plain; charset=utf-8
Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.120.23.2.4\))
From: Stewart Bryant <stewart.bryant@gmail.com>
In-Reply-To: <b01b9787-be76-0de4-de48-54eb2881ae27@pi.nu>
Date: Mon, 29 Mar 2021 14:15:34 +0100
Cc: Stewart Bryant <stewart.bryant@gmail.com>, "mpls@ietf.org" <mpls@ietf.org>, Kireeti Kompella <kireeti.kompella@gmail.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <7F38FC87-6FF5-4347-8677-C384A47E7B3D@gmail.com>
References: <1c87c917-5860-8efe-c640-7bd5cd548190@pi.nu> <D1E90F58-1E68-4B0D-8092-6EB5EEA8783C@gmail.com> <a2b22460-39b8-882e-7e88-26c0a8dbb8d8@pi.nu> <E5688771-7A75-4E36-AB20-0BDB992333B9@gmail.com> <b01b9787-be76-0de4-de48-54eb2881ae27@pi.nu>
To: loa Andersson <loa@pi.nu>
X-Mailer: Apple Mail (2.3608.120.23.2.4)
Archived-At: <https://mailarchive.ietf.org/arch/msg/mpls/fK3yGixatUvhUk9Hyup6kFA4gfE>
Subject: Re: [mpls] how does it work if ....
X-BeenThere: mpls@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Multi-Protocol Label Switching WG <mpls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/mpls>, <mailto:mpls-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/mpls/>
List-Post: <mailto:mpls@ietf.org>
List-Help: <mailto:mpls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/mpls>, <mailto:mpls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Mon, 29 Mar 2021 13:15:45 -0000


> On 26 Mar 2021, at 11:06, Loa Andersson <loa@pi.nu> wrote:
> 
> Stewart,
> 
> I understand evbery single piece yuor argument, but can't get my head around the it ll in the end.
> 
> You say:
> 
>   You only pop the label to see the xSPL if you are the destination.
> 
> So my conclusion is that if you don't pop the top label you can't see the info encoded in the xSPL, unless you scan the stack without popping the top label.  Crossing the Rubicon!
> 
> I guess that this is fine, but that seems to indicate that you have to scan every label stack that passes a node. And when you scan how do you know when to stop, it might be that the first xSPL that you encounter is the important one, it might be deeper down.
> 
> Wouldn't it be better to have something in the top label that tells you where you need to look and what you ar looking for?
> 
> I'd say that this is a FEC.
> 
> /Loa
> 


Loa

If you are going to use a FEC, you might as well use a FEC and only a FEC, though the downside of a FEC is the need to advertise the FEC. That is not an issue with something like RSVP, a reduced flooding scope PPR, SDN etc, where scope of knowledge/flooding is limited to the nodes that need to know, but with the general fooding label distribution methods increases the number of labels to flood, and worse allocate.

A lot of routers scan the whole stack anyway so we know you could scan to find an xSPL somewhere in the stack.

The alternative is to have a scan one below implicit action which would allow you to have a scheme where you looked at ToS if you knew no better, but look one more below if you understood the game.

So I was thinking of a top label with the meaning "Go to Foo", and an SPL below that indicating “using Bar” so the pair had the meaning “Go to Foo using the information you get from Bar”. So the label pair becomes a compound label at ToS.

We can create all sorts of syntax for this, and I am proposing to write a draft examining  some of the possibilities and issues.

So bottom line: I am thinking of using a label group rather than searching the stack in the hope of finding something.

- Stewart