Subject: [PWE3] Comments on draft-ietf-pwe3-ms-pw-requirements-02.txt

From: Stewart Bryant <[email-masked]>
Date: Mon, 24 Jul 2006 13:19:14 +0100

Here are some comments concerning draft-ietf-pwe3-ms-pw-requirements-02.txt

I have seen one email by Dan Romascanu concerning this draft.

Does anyone else on the list have any comments on this draft?

- Stewart

------------------------------------------------------

Abstract

   This document describes the necessary requirements to allow a service
   provider to extend the reach of pseudo-wires across multiple domains.

SB> Is multi-domain still the right context? It seems that a number
SB> of single domain requirements of MS-PW have emerged since this
SB> work was started.
SB> 
SB> We need to consider whether the addition of single domain to the 
SB> original mult-domain modifies the requirements in any way.
SB> 
SB> This comment applies in multiple places to this draft.
SB> 
SB> Are there any differences with regard to TDM PWs that we
SB> need to capture? 

================================

SB> Should we use  Figure 4 PW switching Reference Model from 
SB> http://www.ietf.org/internet-drafts/draft-ietf-pwe3-ms-pw-arch-01.txt
SB> or is the intent to bridge the gap between RFC3985 and 
SB> draft-ietf-pwe3-ms-pw-arch-01.txt through the use of this diagram
SB> which is closer to RFC3985?

           Native  |<-----------Pseudo Wire----------->|  Native
           Service |                                   |  Service
            (AC)   |    |<-PSN1-->|     |<--PSN2->|    |   (AC)
             |     V    V         V     V         V    V     |
             |     +----+         +-----+         +----+     |
   +----+    |     | PE1|=========| PE2 |=========| PE3|     |    +----+
   |    |----------|........PW1.........|...PW3........|----------|    |
   | CE1|    |     |    |         |     |         |    |     |    |CE2 |
   |    |----------|........PW2.........|...PW4........|----------|    |
   +----+    |     |    |=========|     |=========|    |     |    +----+
        ^          +----+         +-----+         +----+          ^
        |      Provider Edge 1       ^        Provider Edge 3     |
        |                            |                            |
        |                            |                            |
        |                    PW switching point                   |
        |                                                         |
        |                                                         |
        |<------------------- Emulated Service ------------------>|

                 Figure 2: PW switching Reference Model

==========================

   Figure 2 extends this architecture to show a multi-segment case. PE1
   and PE3 provide PWE3 service to CE1 and CE2. These PEs reside in
   different PSN domains, PSN1 and PSN2, respectively. A PSN tunnel
   extends from PE1 to PE2 across PSN1, and a second PSN tunnel extends
   from PE2 to PE3 across PSN2.  PWs are used to connect the Attachment
   circuits (ACs) attached to PE1 to the corresponding ACs attached to
   PE3. Each PW on the tunnel across PSN1 is stitched to a PW in the
   tunnel across PSN2 at PE2 to complete the multi-segment PW (MS-PW)
   between PE1 and PE3. PE2 is therefore the PW switching point and will
   be referred to as the PW switching provider edge (S-PE). 

SB> S-PE not shown in the diagram - I think that we need the new diagram

                |<--------------Pseudo Wire----------->|
                |       Provider         Provider      |
            AC  |    |<----1---->|     |<----2--->|    |  AC
            |   V    V           V     V          V    V  |
            |   +----+     +-----+     +----+     +----+  |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
   |    |-------|.....PW1..........PW2.........PW3.....|-------|    |
   | CE1|   |   |    |     |     |     |    |     |    |  |    |CE2 |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
        ^       +----+     +-----+     +----+     +----+       ^
        |         PE1        PE2        PE3         PE4        |
        |                     ^          ^                     |
        |                     |          |                     |
        |                  PW switching points                 |
        |                                                      |
        |                                                      |
        |<------------------- Emulated Service --------------->|

         Figure 3: PW switching inter provider Reference Model

   Note that although Figure 2 only shows a single S-PE, a PW may
   transit more than one S-PE along its path.

SB> Again fig does not show S-PE

==============================

SB> So Access/Metro is in the body but not in the abstract/intro

       -vi. Pseudo-Wires in Access and Metro Networks: Service providers
            are looking to extend PWs to access and metro networks. The
            prime motive is cost reduction in capital and operation
            expenses.
==============================

6.1.1. Architecture

        -i. If MS-PWs are tunneled, across a PSN that only supports SS-
            PWs, then only the requirements of [PWE3-REQ] apply to that
            PSN. The fact that the overlay is carrying MS-PWs MUST be
            transparent to the routers in the PSN.

       -ii. The PWs MUST remain transparent to the P-routers. A P-router
            is not an S-PE or an T-PE from the MS-PW architecture
            viewpoint. P-routers provide transparent PSN transport for
            PWs and MUST not have any knowledge of the PWs traversing
            them.

      -iii. The MS-PWs MUST use the same encapsulation modes specified
            for SS-PWs.

       -iv. The MS-PWs MUST be composed of SS-PWs.

        -v. A MS-PW MUST be able to pass across PSNs of all technologies
            specified by PWE3 for SS-PWs. When crossing from one PSN
            technology to another, an S-PE must provide the necessary
            PSN interworking functions in that case.

       -vi. Both directions of a PW segment MUST terminate on the same
            S-PE/T-PE.

      -vii. S-PEs MAY only support switching PWs of the same PW type. In
            this case, the PW type is transparent to the S-PE in the
            forwarding plane, except for functions needed to provide for
            interworking between different PSN technologies.

     -viii. Solutions MAY provide a way prioritize the setup and
            maintenance process among PWs.

SB> When we worked on the corresponding arch draft we concluded tha
SB> whether a PW was MS or SS should be transparent to the T-PE
SB> to allow backwards compatibility at the T-PE. I think that 
SB> this needs to be captured here.

==================================

6.4.4. Routing

       -ii. The MS-PW path MUST have the ability to be dynamically setup
            between the T-PEs by provisioning only the T-PEs.

SB> ... and the converse?

======================================

6.5.4. Routing

   Following are requirements associated with dynamic route selection
   for a MS-PW:

SB> Route selection or path selection? Surely path selection?

       -ii. The PW Routing function MUST support re-routing around
            failures that occur between the statically configured
            segment endpoints. This may be done by choosing another PSN
            tunnel between the two segment endpoints or setting up an
            alternative tunnel.

SB> This sounds like we are going to define a routing protocol? Surely
SB> we are at most going to place requirements on existing
SB> routing protocols.

========================

SB> I think that we need a congestion considerations section to
SB> preempt discussion on that topic.

=======================

Dan Romascanu's comment also needs to be addressed

http://www1.ietf.org/mail-archive/web/pwe3/current/msg08298.html

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---

From: [email-masked]
Date: Mon, 24 Jul 2006 14:54:23 +0200

stewart

o) section 6.1.1 states

> The MS-PWs MUST be composed of SS-PWs.

while the definition of MS-PW states

>     - Multi-Segment Pseudo Wire (MS-PW).  A static or dynamically
>       configured set of two or more contiguous PW segments that behave
>       and function as a single point-to-point PW. Each end of a MS-PW
>       by definition MUST terminate on a T-PE.
>
>     - PW Segment. A part of a single-segment or multi-segment PW, which
>       is set up between two PE devices, T-PEs and/or S-PEs.

there is a difference between requirement and the proposed def.

i would say that MS-PWs MUST be composed of PW segments (instead) 

o) section 6.1.1 states

>            PSN interworking functions in that case.

what is meant here by "the necessary PSN interworking fuctions" ? not sure 
to see rationale wrt to Fig 2.

o) section 6.1.1 states

>            interworking between different PSN technologies.

first sentence, is this a requirement or a limitation that the 
architecture shall take into account;

why PW Type is only transparent to S-PE is that case ?

PSN tunnel being terminated on S-PEs, why is there a need for interworking 
at that level ?

- d.

Stewart Bryant <[email-masked]>
24/07/2006 14:19
 
        To:     [email-masked], Luca Martini 
<[email-masked]>, Matthew BOCCI/GB/ALCATEL@ALCATEL, pwe3 
<[email-masked]>
        cc: 
        Subject:        [PWE3] Comments on 
draft-ietf-pwe3-ms-pw-requirements-02.txt

Here are some comments concerning 
draft-ietf-pwe3-ms-pw-requirements-02.txt

- Stewart

Abstract

6.1.1. Architecture

6.4.4. Routing

6.5.4. Routing

========================

=======================