Re: [Isis-wg] Encoding inconsistency between ISIS and OSPFv2 extensions for SR

Peter Psenak <ppsenak@cisco.com> Mon, 16 June 2014 07:16 UTC

Return-Path: <ppsenak@cisco.com>
X-Original-To: isis-wg@ietfa.amsl.com
Delivered-To: isis-wg@ietfa.amsl.com
Received: from localhost (ietfa.amsl.com [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id C4E3A1A03B0; Mon, 16 Jun 2014 00:16:57 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -10.152
X-Spam-Level:
X-Spam-Status: No, score=-10.152 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, RP_MATCHES_RCVD=-0.651, SPF_PASS=-0.001, USER_IN_DEF_DKIM_WL=-7.5] autolearn=ham
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 8fv_-2REMnXE; Mon, 16 Jun 2014 00:16:54 -0700 (PDT)
Received: from aer-iport-4.cisco.com (aer-iport-4.cisco.com [173.38.203.54]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 0FFF01A0365; Mon, 16 Jun 2014 00:16:53 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=cisco.com; i=@cisco.com; l=7966; q=dns/txt; s=iport; t=1402903014; x=1404112614; h=message-id:date:from:mime-version:to:subject:references: in-reply-to:content-transfer-encoding; bh=PtZiLx6wqO8p4j7mqZUX3ffgzLscFGcMmvWL+nmcb0U=; b=WNjasWPikiKgyW8Y/1i7TzRotEf0JHehGpbB8XoHxn9qgPe0W9o3rZDc Xek7mpWUe+JGfVhrt+ckpPvyj5j7aBtsjATnDrNjjNw39w0a3MSmv1aEU T55rWIF7zJerifcKhJ4vtHNInMlBdYBrrAUY8V1Hu7dV9sjpU+glngnCp g=;
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-Anti-Spam-Result: AhkFABSZnlOtJssW/2dsb2JhbABag1+qQQEBAQMFAZFnhz0BgSV1hAMBAQEDAQEBATU2Cg0ECxEEAQEBCRYIBwkDAgECARUfCQgGAQwGAgEBiDYIDc5rF4Vjg2CFOgaEPQEDmkOBQ4U3jF6DQjs
X-IronPort-AV: E=Sophos;i="5.01,484,1400025600"; d="scan'208";a="83196453"
Received: from aer-iport-nat.cisco.com (HELO aer-core-4.cisco.com) ([173.38.203.22]) by aer-iport-4.cisco.com with ESMTP; 16 Jun 2014 07:16:52 +0000
Received: from [10.55.51.202] (ams-ppsenak-8719.cisco.com [10.55.51.202]) by aer-core-4.cisco.com (8.14.5/8.14.5) with ESMTP id s5G7Gpdd008339; Mon, 16 Jun 2014 07:16:51 GMT
Message-ID: <539E99E3.5030305@cisco.com>
Date: Mon, 16 Jun 2014 09:16:51 +0200
From: Peter Psenak <ppsenak@cisco.com>
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Thunderbird/24.4.0
MIME-Version: 1.0
To: Xuxiaohu <xuxiaohu@huawei.com>, "isis-wg@ietf.org list" <isis-wg@ietf.org>, OSPF List <ospf@ietf.org>
References: <1FEE3F8F5CCDE64C9A8E8F4AD27C19EE0828073D@NKGEML512-MBS.china.huawei.com> <539AB6E9.3070108@cisco.com> <1FEE3F8F5CCDE64C9A8E8F4AD27C19EE082807D1@NKGEML512-MBS.china.huawei.com> <539AEEE6.3080605@cisco.com> <1FEE3F8F5CCDE64C9A8E8F4AD27C19EE08280DB0@NKGEML512-MBS.china.huawei.com>
In-Reply-To: <1FEE3F8F5CCDE64C9A8E8F4AD27C19EE08280DB0@NKGEML512-MBS.china.huawei.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Archived-At: http://mailarchive.ietf.org/arch/msg/isis-wg/pYJEgNfepJXrJxlSEVOF17Mb8c8
Subject: Re: [Isis-wg] Encoding inconsistency between ISIS and OSPFv2 extensions for SR
X-BeenThere: isis-wg@ietf.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: IETF IS-IS working group <isis-wg.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/isis-wg>, <mailto:isis-wg-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/isis-wg/>
List-Post: <mailto:isis-wg@ietf.org>
List-Help: <mailto:isis-wg-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/isis-wg>, <mailto:isis-wg-request@ietf.org?subject=subscribe>
X-List-Received-Date: Mon, 16 Jun 2014 07:16:58 -0000

Xiaohu,

please see inline:

On 6/16/14 04:58 , Xuxiaohu wrote:
> Hi Peter,
>
> Please see my response inline
>
>> -----Original Message-----
>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> Sent: Friday, June 13, 2014 8:31 PM
>> To: Xuxiaohu; isis-wg@ietf.org list; OSPF List
>> Subject: Re: [Isis-wg] Encoding inconsistency between ISIS and OSPFv2
>> extensions for SR
>>
>> Hi Xiaohu,
>>
>> please see inline:
>>
>> On 6/13/14 12:09 , Xuxiaohu wrote:
>>> Hi peter,
>>>
>>>> -----Original Message-----
>>>> From: Isis-wg [mailto:isis-wg-bounces@ietf.org] On Behalf Of Peter
>>>> Psenak
>>>> Sent: Friday, June 13, 2014 4:32 PM
>>>> To: Xuxiaohu; isis-wg@ietf.org list; OSPF List
>>>> Subject: Re: [Isis-wg] Encoding inconsistency between ISIS and OSPFv2
>>>> extensions for SR
>>>>
>>>> Xiaohu,
>>>>
>>>> please see inline:
>>>>
>>>> On 6/13/14 09:51 , Xuxiaohu wrote:
>>>>> Hi all,
>>>>>
>>>>> There are some encoding inconsistencies between OSPFv2 extensions
>>>>> and ISIS
>>>> extensions for SR as follows:
>>>>>
>>>>> 1. In ISIS-SR, the prefix-sid advertisement is piggybacked on the IP
>>>>> reachability
>>>> advertisement. In OSPF-SR, the prefix-sid advertisement is
>>>> piggybacked on OSPF Extended Prefix LSA which is used to advertise
>>>> other attributes associated with the prefix, rather than the
>>>> reachability. IMHO, the OSPF encoding is more flexible since the
>>>> label distribution and the reachability advertisement are
>>>> independent. As a result, the route summary on area boundaries at
>>>> least can be enabled as before. Besides, the prefix-SID sub-TLV can
>>>> be used to advertise a range of prefix/SID pairs (see item2). In
>>>> fact, ISIS allows us to do the same way as OSPF with a much lower
>>>> cost (see section 3 of
>> http://tools.ietf.org/html/draft-xu-isis-global-label-sid-adv-00). Of course, it
>> seems that you co-authors prefer to the piggyback way.
>>>>
>>>> OSPF LSAs that are used to advertise the prefixex are not extensible,
>>>> so we had to define a new LSA for the purpose of advertising a prefix related
>> attributes.
>>>> ISIS is different, as they can add sub-TLVs to existing TLVs.
>>>
>>> I see. For ISIS, you use the piggyback way (piggyback the label/sid
>> advertisement on the reachability advertisement messages). For OSPFv2, you
>> have no way to use the piggyback way anymore, so you defined a new LSA...
>> That's why I said you prefer to the piggyback way. However, I don't think the
>> piggyback way is much worthwhile from the perspective of flexibility and
>> extensibility.
>>>
>>>>>
>>>>> 2. In ISIS-SR, the Prefix-SID Sub-TLV can only be used to advertise
>>>>> an SID for a
>>>> single prefix. And it relays on the SID/Label Binding TLV to
>>>> advertise a range of prefix/SID pairs. In contrast, In OSPF-SR, the
>>>> prefix-sid sub-TLV can be used to specify a range of addresses and
>>>> their associated Prefix SIDs. By the way, in the 4.3.  SID/Label
>>>> Binding sub-TLV. it has the following text: "Range Size: usage is the
>>>> same as described in Section 4.2." Did you co-authors want to propose
>>>> two ways (i.e., prefix-sid sub-TLV and SID-Label Binding sub-TLV) to achieve
>> the same goal (i..e, advertise a range of prefix/SID pairs)?
>>>>
>>>> because in OSPF advertisement of the prefix SID is decoupled from the
>>>> advertisement of prefix reachability, we can afford to advertise the
>>>> range of SIDs in the prefix-SID sub-TLV as such.
>>>
>>> IMHO, the ISIS and OSPFv3 advertisement of the prefix SIDs should be
>>> decoupled from the prefix reachability advertisement as well:)
>>
>> in OSPFv3 case, we have a way to advertise the prefix using the proposed
>> encoding in draft-acee-ospfv3-lsa-extend, but do not advertise the reachability
>> of the prefix - it's call NU-bit (rfc5340, A.4.1.1.)
>
> That's great. BTW, don't you believe the ISIS protocol has provided almost the same capability as the NU-bit (see the following text quoted from RFC5305)?

correct, max-metric means unreachable in ISIS.

>
> "...If a prefix is advertised
>     with a metric larger then MAX_PATH_METRIC (0xFE000000, see paragraph
>     3.0), this prefix MUST NOT be considered during the normal SPF
>     computation.  This allows advertisement of a prefix for purposes
>     other than building the normal IP routing table...".
>
>>>
>>>> No, we do not define two ways to achieve the same thing. Binding TLV
>>>> is used for a different purpose and the same usage is only applicable
>>>> to the Range semantics, not to the whole Binding TLV.
>>>
>>> Does that mean the Binding sub-TLV in the OSPF-SR could not be used to
>> advertise a range of prefix/sid pairs while the binding sub-TLV in the ISIS-SR
>> could?
>>
>> Binding TLV in OSPF is only used to advertise a "LSP path" local to the advertising
>> router, it's not used for anything else. YOu can still advertise a single "LSP path"
>> for range of prefixes.
>
> Don't you believe it's better for the Binding TLV in ISIS to be used to advertise a LSP as well?

Binding TLV in ISIS can be used to advertise "LSP path" as well as SRMS 
mappings.

>
>> In ISIS, due to the need to decouple prefix reachability from SID advertisement,
>> Binding TLV is used for SR Mapping Server (SRMS) adevrtisement on top of what
>> it is used in OSPF (in OSPF SRMS advertisements are using the Prefix/SID
>> sub-TLV).
>
> To decouple prefix reachability from SID advertisement, why not consider the approach of using the MAX_PATH_METRIC trick (see section 3 of http://tools.ietf.org/html/draft-xu-isis-global-label-sid-adv-00)?

it's a matter of choice. Authors of ISIS draft choose the cleaner way IMHO.

regards,
Peter

>
> Best regards,
> Xiaohu
>
>>>>> 6. In ISIS-SR, the prefix-SID sub-TLV doesn't contain the MT-ID
>>>>> field since the
>>>> MT-ID field is already contained in the parent TLV of the prefix-SID
>>>> sub-TLV. In OSPF, the MT-ID field is contained in the Prefix SID
>>>> Sub-TLV since the parent TLV of the prefix-sid sub-TLV doesn't
>>>> contain that MT-ID field. IMHO, it's better to contain the MT-ID in
>>>> the parent prefix-specific TLV of the prefix-SID sub-TLV. In other
>>>> words, why not contain the MT-ID in the OSPF Extended Prefix TLV,
>>>> instead of the prefix-sid sub-TLV (see section 3 of
>> http://tools.ietf.org/html/draft-xu-ospf-global-label-sid-adv-00)?
>>>>
>>>> no, we do not want to put the MT-ID in the OSPF Extended Prefix TLV.
>>>> The reason is that attributes are MT specific, not the prefix itself - e.g.
>>>> you may want to advertise different metrics for the same prefix in
>>>> different topologies, not the same prefix twice.
>>>
>>> Make the prefix-sid as a sub-TLV of the Multi-Topology sub-TLV?
>>
>> no, we don't want to end up with sub-sub-TLVs right from the beginning.
>>
>> regards,
>> Peter
>>
>>>
>>> Best regards,
>>> Xiaohu
>>>
>>>> regards,
>>>> Peter
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>
>>>>> Anyway, although it is unavoidable for us to define extensions to
>>>>> both ISIS and
>>>> OSPF for the same thing due to the fact that both protocols have been
>>>> widely used, could we try our best to keep the encodings of ISIS and
>>>> OSPF as consistent as possible for the same functionality? In this
>>>> way, once someone has read the ISIS extension draft, he or she can
>>>> easily think of what has been done in the OSPF extension draft accordingly,
>> and vice verse.
>>>>>
>>>>> Best regards,
>>>>> Xiaohu
>>>>>
>>>>> _______________________________________________
>>>>> Isis-wg mailing list
>>>>> Isis-wg@ietf.org
>>>>> https://www.ietf.org/mailman/listinfo/isis-wg
>>>>> .
>>>>>
>>>>
>>>> _______________________________________________
>>>> Isis-wg mailing list
>>>> Isis-wg@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/isis-wg
>>> .
>>>
>
> .
>