Re: [mpls] Mode negotiation for PSC

Eric, it is a little bit confusing.

See [JR] inline...

________________________________
From : "Eric Osborne (eosborne)" <eosborne@cisco.com>
Sent : 2013-08-17 05:49:16 ( +09:00 )
To : Loa Andersson <loa@pi.nu>
Cc : mpls@ietf.org <mpls@ietf.org>
Subject : Re: [mpls] Mode negotiation for PSC

Hi Loa-

See inline

> -----Original Message-----
> From: Loa Andersson [mailto:loa@pi.nu]
> Sent: Monday, August 12, 2013 1:27 AM
> To: Eric Osborne (eosborne)
> Cc: mpls@ietf.org
> Subject: Re: [mpls] Mode negotiation for PSC
>
> Eric,
>
>
>
> On 2013-08-08 14:34, Eric Osborne (eosborne) wrote:
>
>
> > Example of method 2
> > -------------------
> > Both Node A and Node Z announce two things - Capabilities and Mask.
> > Capabilities is a bitmap of the capabilities that are supported.
> > Mask is a mask of don't-care bits against the Capabilities string. A
> 0 in Mask means "I don't care if we do this capability or not", and a 1
> means "we must (or must not) agree on this capability in order to come
> up".
> >
> > A.capabilities = 00000
> > A.mask = 00000
>
> Why is this not
>
> A.capabilities = 11111
> A.mask = 00000
>

Althought I didn't put it in my email, a capabilties of 1 and mask of 0 is illegal. This is partly because it makes no sense ("I require this capability but I don't care if we do it or not") and partly because there's a case which doesn't converge right:
[JR] The meaning of cap=11111 and mask=00000 would be "I am capable of all five things but I don't care if we do it or not"

If I have 11111/00000 on one side and 00000/00000 on the other I end up with

A.cap = 11111, A.mask = 00000
Z.cap = 00000, Z.mask = 00000

res = (111111 & 00000) ^ (00000 & 00000)
res = 11111 ^ 00000 = 11111
[JR] The answer for (11111 & 00000) should be 00000. And the final result of 00000 ^ 00000 will be 00000.

and if res != 0 then the nodes can't converge on a common subset, even though there are 2^5 combinations that would work for both nodes.

[JR] Now, two ends will operate without any capabilities.

eric

> ?
>
> /Loa
>
> >
> > This says "I am capable of supporting all capabilities and I don't
> care if we do any of them or not"
>
>
> --
>
>
> Loa Andersson email: loa@mail01.huawei.com
> Senior MPLS Expert loa@pi.nu
> Huawei Technologies (consultant) phone: +46 739 81 21 64
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