Subject: RE: [SIP] over-length message

From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 8 Feb 2001 08:18:59 -0500

> -----Original Message-----
> From: Eric Burger [mailto:[email-masked]]
> Sent: Wednesday, February 07, 2001 8:56 PM
> To: [email-masked]
> Subject: RE: [SIP] over-length message
> 
> 
> His point wasn't that it wouldn't fit into a UDP MTU, it's 
> that it might not
> fit into an ATM MTU, for example.

Well, there is no such thing as a UDP MTU. There is an IP maximum packet
size, which is 64k. The MTU is different on each link, and the path MTU is
the minimum of the MTU along a message path. Should a packet arrive on a
link which is in excess of the MTU, it is fragmented. THis is done at the IP
layer, transparent to the applications. So, a proxy simply won't care that a
3k UDP packet arrived over ATM unfragmented, and will depart over ethernet
with an MTU of 1500 bytes, requiring fragmentation. Fragmentation is not a
great thing, as it has known error exponentiation effects, but functionally,
its completely hidden from the proxy. Thus, the problem that has been
described does not exist.

-Jonathan R.
---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (973) 952-5000
http://www.dynamicsoft.com

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---

From: Jonathan Lennox <[email-masked]>
Date: Thu, 8 Feb 2001 13:50:46 -0500 (EST)

> described does not exist.

Except for all the embedded SIP devices which seem to have decided that they
don't need to support IP fragmentation.  This has been a significant
interoperability problem at bakeoffs.

Needless to say, I think this is something that SIP implementors should
beaware of, and fix before considering their embedded devices
production-quality.  I also think that we ought to add a torture test case
where crucial information in the SIP request sits after the 1500 byte mark.

Cc'd to sip-implementors; followups should go there.

-- 
Jonathan Lennox
[email-masked]

---

From: Takumi Ohba <[email-masked]>
Date: Fri, 9 Feb 2001 15:06:53 +0900

Dear all,

Sorry for continuing this topic on this mailing list, but I would like to
confirm one point, which may relate to SIP protocol development.

I understand that SIP over transport protocols using IP has no limit, thanks
to the IP fragmentation function.
So, SIP has no need to worry about message length if it uses IP based
transport protocols, but should use TCP if it wants to send longer message
than the path MTU.
I also understand that supporting IP fragmentation is implementation issue.

On the other hand, SIP cannot send too long message over a transport
protocol that has no fragmentation function, such as MTP or signalling AAL.
However, nobody except me considers that SIP uses such transport protocol at
this stage, and therefore, this is not a problem.  In other word, it is only
a future possible problem.
If we will use such transport protocols, SIP will have to care about message
length and specify something (handling of over length message, or message
segmentation procedures in SIP level).

Is my understanding correct?

Regards,
Takumi Ohba

***************************************************
Takumi OHBA
NTT Network Service Systems Laboratories
tel +81 422 59 4405  fax +81 422 59 3494
e-mail [email-masked]
***************************************************