Subject: Re: [mpls] Last Call: <draft-ietf-mpls-in-udp-04.txt> (Encapsulating MPLS in UDP) to Proposed Standard
From: joel jaeggli <[email-masked]>
To: Joe Touch <[email-masked]>, "[email-masked]" <[email-masked]>
Cc: "[email-masked]" <[email-masked]>, IETF discussion list <[email-masked]>
Date: Mon, 27 Jan 2014 10:48:43 -0800
List-Id: Multi-Protocol Label Switching WG <mpls.ietf.org>

On 1/27/14, 8:48 AM, Joe Touch wrote:
> Those same mechanisms have provided hardware checksum support for a very long time. 

The new header and the payload are actually in different parts of the
forwarding complex until they hit the output queue, you can't checksum
data you don't have.

> Joe
> 
>> On Jan 27, 2014, at 8:40 AM, Stewart Bryant <[email-masked]> wrote:
>>
>>> On 24/01/2014 19:15, Joe Touch wrote:
>>>
>>>> This eliminates the "expands the reach of MPLS argument".
>>>>
>>>> First UDP checksums:
>>>>
>>>>   The UDP checksum is at the beginning of the payload.  Please see
>>>> http://www.ietf.org/mail-archive/web/mpls/current/msg11279.html
>>>>   This makes filling in a new UDP checksum infeasible on most high end
>>>>   hardware.
>>>
>>> That argument would make sense if most hardware wasn't store-and-forward on a per-packet basis.
>> They may be store and forward, but most of the high end designs
>> use multiple grades of memory putting the packet in "slow memory"
>> and providing a snapshot of the header in "fast memory" to the
>> forwarder. Thus although the whole packet is in the system, it
>> it is not accessible to the engine that would need to calculate the
>> c/s.
>>
>> Stewart
>