Re: [mpls-tp] [PWE3] Proposal of using GAL for PW

["Shahram Davari" <[e-mail]>]

Giles,

I don’t want yet another VVCV type. If you are not using CW then just use RAL or TTL=1. Besides you proposal only works for MPLS-TP and not MPLS, where there is a Entropy label below PW label.

Thx
SD

From: Giles Heron [mailto:[e-mail]]
Sent: Thursday, July 01, 2010 11:10 AM
To: Shahram Davari; Tom Nadeau; Luca Martini; Andy Malis
Cc: [e-mail]; [e-mail]; HUANG Feng F; [e-mail]
Subject: Re: [mpls-tp] [PWE3] Proposal of using GAL for PW

Hi Shahram,

I didn’t say CW was only for identifying OAM messages - I said adding it just to enable occasional OAM messages was overkill.

The CW has half a dozen uses (the first 3 of which were the “original” ones):
1) enabling small PWE payloads over Ethernet links
2) carrying L2 flags where the L2 header is stripped (e.g. FR)
3) sequence numbering
4) fragmentation (RFC4623).  Stole a couple of spare CW bits.
5) avoiding PWE packets aliasing IP where ECMP implementations “walk the stack” and then look at the first nibble after the stack (nice side effect)
6) OAM indicator for in-band VCCV.  Stole a spare CW bit.

So let’s consider the Ethernet PWE case over MPLS-TP

1) Ethernet PWE packets are by definition larger than the minimum Ethernet payload
2) There are no L2 flags in Ethernet
3) Sequence numbering is rarely used - and isn’t needed in the MPLS-TP case
4) I’m not aware of anyone implementing fragmentation for Ethernet PWE
5) There’s no ECMP when you’re doing MPLS-TP
6) the only one that applies (hence my comment)

As for the parsing thing that seems a bit odd to me.  Surely VCCV only tells you that the payload is a PWE rather than IP?  It doesn’t tell you what sort of PWE it is.

Sure, CW would help interop if everyone had one.  But with Ethernet PWE the history is that nobody ever used them so I’m not sure we make our lives any easier by mandating them now.

As for 1588 (and anything else we might try to squeeze into VCCV) that’s another question.  I’d think we’re more likely to carry 1588 over Ethernet over PWE, or over VCCV (and VCCV can be carried by mechanisms other than the CW).

So the key argument for mandating CW would seem to be ensuring that OAM traffic follows the same path as data traffic.  In the TP case I’d expect to see that behaviour anyway (as any intermediate hops will label switch without looking deep enough into the packet to spot the VCCV identifier - whether that identifier is CW, router alert, TTL, or GAL).

Giles

On 01/07/2010 18:13, "Shahram Davari" <[e-mail]> wrote:
Giles,

CW is not just for identifying OAM messages. It normalizes the packet format and makes the job of parsers much simpler. It allows you to identify the payload type without knowing the PW label context. It also improves interoperability and could also simplify many other applications such as 1588 over MPLS.

Regards,
Shahram


From: [e-mail] [mailto:[e-mail]] On Behalf Of Giles Heron
Sent: Thursday, July 01, 2010 6:22 AM
To: Tom Nadeau; Luca Martini; Andy Malis
Cc: [e-mail]; [e-mail]; HUANG Feng F; [e-mail]
Subject: Re: [mpls-tp] [PWE3] Proposal of using GAL for PW

Not sure I agree.

Many CPs have deployed PWs with no CW.   Adding a CW to all packets just to enable occasional OAM messages seems like overkill.

But the downside of adding GAL is that it’s a fourth OAM mode for PWEs (back to your point about interoperability).  Too many options!

Giles

On 01/07/2010 12:14, "Tom Nadeau" <[e-mail]> wrote:

    I  agree with Andy’s assertion. This service provider’s experience is that making the CW mandatory going forward (and hopefully retrofitting existing PW protocol specs) would improve implementation interoperability.

    --Tom



On 6/30/10 11:22 PM, "Luca Martini" <[e-mail]> wrote:
Andy,

I have to disagree that there was any consensus about this issue.
If anything , there was consensus that there is no written statement that we must  to use the CW in MPLS-TP.

At the end we needed more input from service providers that have deployed PWs.  The point is not whether there is hardware support for the CW, but whether we even want to use it in many cases where it adds absolutely no value. For example ATM PWs in cell mode , where it add almost 10% overhead with no benefit. Another case where the CW is not useful is the ethernet PW without network link load balancing, where we add 4 bytes to every packet just to occasionally send a status , or OAM message.

I would like to propose update the rfc5586 to allow the use of the GAL in PWs without the CW.

This makes the use of the GAL very symmetric among PWs and MPLS-TP LSPs. This makes it easy to process by hardware based implementations.

Luca


Andrew G. Malis wrote:

Larry and Feng,

This issue has previously been discussed at length by the working
group, both at the Anaheim meeting and by email, for example in emails
with the subject line "Possible Contradiction re use of GAL in
pwe3-static-pw-status". There was rough consensus that for MPLS-TP
applications and/or when PW OAM is desired, PW implementations are
mature enough (it has been 10 years now, after all) that the time has
come to require the implementation of the CW for all PWs, including
Ethernet.

Cheers,
Andy

On Wed, Jun 30, 2010 at 6:34 AM, HUANG Feng F
<[e-mail]> <mailto:[e-mail]>  wrote:



it is reasonable to support GAL in MPLS-TP PW OAM, it is more generic, because CW is an option RFC4448 for Ethernet over MPLS.

4.6.  The Control Word

xxxx


The features that the control word provides may not be needed for a
  given Ethernet PW.  For example, ECMP may not be present or active on
  a given MPLS network, strict frame sequencing may not be required,
  etc.  If this is the case, the control word provides little value and
  is therefore optional.  Early Ethernet PW implementations have been
  deployed that do not include a control word or the ability to process
  one if present.  To aid in backwards compatibility, future
  implementations MUST be able to send and receive frames without the
  control word present.
xxxx



B.R.
Feng Huang


-----Original Message-----
From: [e-mail] [mailto:[e-mail]] On Behalf Of Larry
Sent: 2010年6月30日 17:38
To: [e-mail]; [e-mail]
Cc: [e-mail]
Subject: [PWE3] Proposal of using GAL for PW

Dear all:

    In section 4.2 in RFC5586, it is defined that GAL MUST NOT be used with PWs in MPLS-TP. The PWE3 control word [RFC4385] MUST be present when the ACH is used to realize the associated control channel.
    In real application, a lot of MPLS and MPLS-TP equipments do not support control word. It is proposed to use the GAL to identify associated control channel in PW layer.

Best regards,

                Han Li

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Han Li, Ph.D
China Mobile Research Institute
Unit 2, 28 Xuanwumenxi Ave, Xuanwu District, Beijing 100053, China
Fax: [phone]
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