Subject: RE: [PWE3] MPLS PID

From: Shahram Davari <[email-masked]>
Date: Thu, 27 Mar 2003 07:06:49 -0800

>I don't feel obligated to answer nonsense questions.  No one has
>proposed putting a CRC in the PID.

First of all I meant computing CRC over the first 20 byte of payload header
and comparing it to the suspected IP CRC in octets 11 and 12 of the payload.
If they match then it is IPv4, if not it is something else (similar procedure for IPv6)

Secondly I am proposing it now. Is there a problem? I feel my procedure is far superior to just checking 4 bits to decide whether the payload is IP or not. 

>
>Curtis
>
>ps - "requiring [..] not to produce a valid CRC" doesn't even parse in
>english so your question is complete nonsense.

It makes perfect sense if you understand my proposal. I think you need to talk to Alia, since she could parse it correctly :)

-Shahram

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From: Eric Rosen <[email-masked]>
Date: Thu, 27 Mar 2003 13:26:02 -0500

Shahram> First  of all  I meant  computing  CRC over  the first  20 byte  of
Shahram> payload header and  comparing it to the suspected  IP CRC in octets
Shahram> 11 and 12 of the payload. If  they match then it is IPv4, if not it
Shahram> is something else (similar procedure for IPv6)

Shahram> Secondly  I am  proposing it  now. Is  there a  problem? I  feel my
Shahram> procedure is far superior to just checking 4 bits to decide whether
Shahram> the payload is IP or not. 

Certainly it's  not superior in  terms of the  amount of computation  or the
number of memory accesses required ;-)

Architecturally,  it doesn't  seem  superior  either.  It  is  based on  the
assumption that the one's complement sum of the first 20 bytes of a non-IPv4
payload will not be zero.  And it doesn't seem to handle IPv6.

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From: Curtis Villamizar <[email-masked]>
Date: Thu, 27 Mar 2003 13:06:56 -0500

In message <[email-masked].
ca>, Shahram Davari writes:
> Secondly I am proposing it now. Is there a problem? I feel my procedure is fa
> r superior to just checking 4 bits to decide whether the payload is IP or not
> . 

It would be a lot more efficient to just put in a PID that states what
the payload type is rather than CRC the header.  This way if it isn't
IPv4 or IPv6, we know what it is.

I don't think we should jam the PID into 4 bits.  Two or four bytes
would be a lot more reasonable (using ethertype for example).

Curtis