RE: 回复： RE: what is the Maxlength of SIP?

[Herve Jourdain]

Hi,

 

The fact that it was specified to fallback to TCP if too large for a single
UDP datagram is probably to avoid IP fragmentation…

IPv4 has provision for fragmentation, but it costs resources for reassembly,
and thus is not always allowed on routers…

The 200 that was chosen in the specs is an arbitrary number that provides
space for responses larger than the request, if I remember correctly, not
the sum of IP and UDP headers (which would be only 28 bytes in IPv4, with
“standard” headers). I think the figures I gave are for the request, the
response should be able to go up to MTU…

 

Regards,

 

Herve

 

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From: 雨 陈 [mailto:chen.yu26 at yahoo.com.cn] 
Sent: vendredi 25 janvier 2008 07:43
To: Herve Jourdain
Cc: group SIP
Subject: 回复： RE: what is the Maxlength of SIP?

 


Hi, Herve,

Thanks for your answer.It help me a lot
<http://us.i1.yimg.com/us.yimg.com/i/mesg/tsmileys2/01.gif> . Well, I am
still not sure about the follow,

 

I know the MTU is a limit unit on the ethernet . So the number you metioned
"200",is it a sum bytes of  the IP and UDP packet  header ?

 

And you tell me when the SIP is too larger for UDP to send ,it will resort
to TCP, However,I think this issue can be solved below the transport layer,
I mean on the network layer. Why not the carried IP packet be divided into
more smaller ones to transmit, something like fragmentation?

 

Nora

  

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