Re: [Lsr] Concerns with draft-chunduri-lsr-isis-preferred-path-routing

Uma Chunduri <uma.chunduri@huawei.com> Tue, 17 July 2018 12:57 UTC

Return-Path: <uma.chunduri@huawei.com>
X-Original-To: lsr@ietfa.amsl.com
Delivered-To: lsr@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 9A324130F06; Tue, 17 Jul 2018 05:57:10 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.901
X-Spam-Level:
X-Spam-Status: No, score=-1.901 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, 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 HRJQM-iC0kMc; Tue, 17 Jul 2018 05:57:08 -0700 (PDT)
Received: from huawei.com (lhrrgout.huawei.com [185.176.76.210]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 04934130E48; Tue, 17 Jul 2018 05:57:08 -0700 (PDT)
Received: from LHREML711-CAH.china.huawei.com (unknown [172.18.7.106]) by Forcepoint Email with ESMTP id 6D470547ECF77; Tue, 17 Jul 2018 13:57:03 +0100 (IST)
Received: from SJCEML703-CHM.china.huawei.com (10.208.112.39) by LHREML711-CAH.china.huawei.com (10.201.108.34) with Microsoft SMTP Server (TLS) id 14.3.399.0; Tue, 17 Jul 2018 13:57:05 +0100
Received: from SJCEML521-MBX.china.huawei.com ([169.254.1.107]) by SJCEML703-CHM.china.huawei.com ([169.254.5.145]) with mapi id 14.03.0399.000; Tue, 17 Jul 2018 05:56:59 -0700
From: Uma Chunduri <uma.chunduri@huawei.com>
To: "Ketan Talaulikar (ketant)" <ketant@cisco.com>, "lsr@ietf.org" <lsr@ietf.org>
CC: "spring@ietf.org" <spring@ietf.org>
Thread-Topic: Concerns with draft-chunduri-lsr-isis-preferred-path-routing
Thread-Index: AdQdvIF2mtuL/a9RTae0GqOPQrjRDAADEYXA
Date: Tue, 17 Jul 2018 12:56:59 +0000
Message-ID: <25B4902B1192E84696414485F5726854135F192C@sjceml521-mbx.china.huawei.com>
References: <5a180e3dc86047beb3991ad8b1d6d54d@XCH-ALN-008.cisco.com>
In-Reply-To: <5a180e3dc86047beb3991ad8b1d6d54d@XCH-ALN-008.cisco.com>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
x-originating-ip: [10.124.182.105]
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-CFilter-Loop: Reflected
Archived-At: <https://mailarchive.ietf.org/arch/msg/lsr/XXJHO9tlw4E0VY9tbLgagCaT2JI>
Subject: Re: [Lsr] Concerns with draft-chunduri-lsr-isis-preferred-path-routing
X-BeenThere: lsr@ietf.org
X-Mailman-Version: 2.1.27
Precedence: list
List-Id: Link State Routing Working Group <lsr.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/lsr>, <mailto:lsr-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/lsr/>
List-Post: <mailto:lsr@ietf.org>
List-Help: <mailto:lsr-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/lsr>, <mailto:lsr-request@ietf.org?subject=subscribe>
X-List-Received-Date: Tue, 17 Jul 2018 12:57:17 -0000

Hi Ketan,


In-line [Uma]:
--
Uma C.

-----Original Message-----
From: Ketan Talaulikar (ketant) [mailto:ketant@cisco.com] 
Sent: Tuesday, July 17, 2018 7:13 AM
To: Uma Chunduri <uma.chunduri@huawei.com>om>; lsr@ietf.org
Cc: spring@ietf.org
Subject: Concerns with draft-chunduri-lsr-isis-preferred-path-routing

Hi Uma,

I would like share more context on the concerns that I raised on this proposal in LSR WG yesterday where we could not complete our discussion on the mike due to time constraints.

IGPs were originally invented for topology computation and then route programming based on the SPT computed. We've extended IGPs to carry/flood information and this includes information that were meant for various applications. IGPs always do distribute topology computation - that is the core principle.

The PPR proposal takes IGPs into the area of flooding p2p paths and then setting up forwarding state along the path - essentially introducing path provisioning capabilities into it. Essentially adding a new functionality that is NOT distributed topology computation.

For clarity, I could summarize the PPR as follows (please correct if wrong):
- Someone (head-end or controller) computes a SR Path which is expressed as a SID List (it's a list of EROs just like in RSVP-TE - loose or strict)
- The head-end floods this SR Path (and its EROs) into the IGP domain so all routers in the area get the P2P paths computed by all head-ends
- Each router then must look at every such SR Path flooded by every router and examine if it is part of the ERO list; if so then it needs to program the forwarding state for that PPR id (aka label)
- The headend can then just look at this like a "tunnel" and do something like IGP shortcut to the destination behind it

This is picking EROs from RSVP-TE and putting them into IGPs for flooding p2p path state pervasively. Consider the kind of flooding scale and challenges when all these SR Paths go to every router irrespective of whether they need/use it. Then on top of that, we are proposing IGPs to program a local cross-connect if they are on that SR Path. My question is, why not just use RSVP-TE in this case? RSVP-TE does signalling but it does it only on the nodes that matter for a specific LSP. 

[Uma]:  This helps in deployments, who is seeking source routing paradigm, but SID stack on the packet is unsustainable. This statement is applicable for both MPLS and IPv6 case. 
                 Coming to the EROs in IGP - it was there in SR drafts, including as working group draft for 3 years.  But what completely lacked was how to use those. There are significant differences in the format and importantly usage to solve various issues including hardware 
                Compatibilities of various line cards (and hence available paths), MTU and line rate issues. I don't think you can use RSVP-TE to solve these SR issues.

This is called SR but it introduces a forwarding state on each of the hops (i.e. the PPR label cross-connect) - something different from SR architecture. 

[Uma]:  You already introduced per path state in various cases (binding sids to local policy, flex-algo).  This has been discussed lately as part of re-chartering discussion. 
                 This thread discusses that in detail and I fully concur what Dave said here https://www.ietf.org/mail-archive/web/spring/current/msg03794.html