Re: [Simple] Questions regarding MSRP Relay drafts

[Paul Kyzivat <pkyzivat at cisco.com>]

Smile Wang wrote:
> Hi,all
>
> I have a doubt about the draft-ietf-simple-msrp-relays-05.txt.
>
> In section 3 the second example about a message being re-chunked
>
> through two relays.
>
> Alice           a.example.org(relay1)  b.example.net(relay2)         Bob
>
>      |                     |                    |                     |
>
>      |--- SEND 1-3 ------->|                    |                     |
>
>      |<-- 200 OK ----------|                    |  (slow link)        |
>
>      |--- SEND 4-7 ------->|--- SEND 1-5 ------>|                     |
>
>      |<-- 200 OK ----------|<-- 200 OK ---------|--- SEND 1-3 ------->|
>
>      |--- SEND 8-10 ------>|--- SEND 6-10 ----->|                ....>|
>
>      |<-- 200 OK ----------|<-- 200 OK ---------|                  ..>|
>
>      |                     |                    |<-- 200 OK ----------|
>
>      |                     |                    |<-- REPORT 1-3 ------|
>
>      |                     |<-- REPORT 1-3 -----|--- SEND 4-7 ------->|
>
>      |<-- REPORT 1-3 ------|                    |                 ...>|
>
>      |                     |                    |<-- REPORT 4-7 ----->|
>
>      |                     |<-- REPORT 4-7 -----|--- SEND 8-10 ------>|
>
>      |<-- REPORT 4-7 ------|                    |                  ..>|
>
>      |                     |                    |<-- 200 OK ----------|
>
>      |                     |<-- REPORT done-----|<-- REPORT done -----|
>
>      |<-- REPORT done -----|                    |                     |
>
>      |                     |                    |                     |
>
>  
>
> Why relay1 receive the part of message(1-3,4-7,8-10) not direct send to 
> relay2,but re-chunked 1-5,6-10?

There is no requirement that this rechunking be done. Relay1 could, if 
it wished, do as you suggest.

I suppose if the outbound link is congested, so that the relay receives 
multiple chunks that it has not been able to send, it might as well 
combine them.

> I consider that MSRP is app layer protocol which 
> don’t take
>
> into account the things that transfer layer to do.And MSRP use TCP or 
> TSL which
>
> send the stream data,unlike UDP which data length exceed MTU must be split.

Its a little more complicated than that. Several MSRP sessions may be 
multiplexed over the same link. In that case, it may become necessary to 
break a large message into chunks so that other messages (or parts of 
them) may be interleaved.

	Paul

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