Subject: RE: [Sip] MAJOR Open Issue #184: Message size limitations

From: [email-masked]
Date: Fri, 28 Dec 2001 08:15:11 -0700

Hmmm.  I'd say all your implementations that support reassembling
fragmented SIP risk problems if there is packet loss.  They are making up
for non-compliant devices that are fragmenting the SIP messages (see bis-05
section 7.5).  SIP over unreliable transports should be in one datagram to
maintain the reliability mechanisms built into SIP.  If you fragment the
message, the reliability may be lost.

John Hearty
Level3

> -----Original Message-----
> From: Juha Heinanen [mailto:[email-masked]]
> Sent: Friday, December 28, 2001 1:18 AM
> To: [email-masked]
> Subject: Re: [Sip] MAJOR Open Issue #184: Message size limitations
> 
> 
> i have nothing against introducing an error response for truncated UDP
> messages, but whatever the new spec will say should not make existing
> implementations that support large udp packets non-compliant.
> 
> i have in active use half a dozen different sip phone, 
> proxies and media
> gateway controllers and all of them today send and receive 
> sip messages
> in udp packets that are larger than 1500 bytes.  their network driver
> simply ip fragments and reassembles a packet if it doesn't fit into a
> 1500 byte ethernet frame.  almost a year ago i found one ip phone that
> didn't support reception of framented udp packets, but that 
> was promptly
> fixed, when i reported the issue.
> 
> if the new version of the spec will say (as jonathan 
> suggested) "if the
> request is larger than 1500 bytes, you MUST use TCP to send it", then
> such a statement would make all off-the-shelf sip entities that are
> currently in use in my network non-compliant, which would be 
> a VERY bad
> idea.
> 
> -- juha
> 
> 
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_______________________________________________
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---

From: Juha Heinanen <[email-masked]>
Date: Fri, 28 Dec 2001 17:42:30 +0200

> message, the reliability may be lost.

the bit-05 text you refer to has word SHOULD in it.  if we now turn it
to MUST, we are breaking several existing implementations from major sip
vendors.  we should thus keep SHOULD in the text as suggested this
morning by jari.

-- juha

---

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 28 Dec 2001 10:45:17 -0500

The bigger problem with fragmentation is that it simply doesn't work 
through nats, firewalls, or who-knows-what-else kind of packet 
inspecting beasts one finds in networks these days.

I would still prefer "MUST" use TCP for packets larger than 1500; your 
existing devices will be non-compliant (good motivation to fix it) but 
will still function; I think its fair to say that you still need to be 
able to receive UDP packets larger than 1500.

I am having some second thoughts about the truncated response error 
code; I think there may be some interactions with SIP specifics that 
make this more complex than I thought. I'm still thinking about it, and 
will post separately.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.            72 Eagle Rock Avenue
Chief Scientist                         First Floor
dynamicsoft                             East Hanover, NJ 07936
[email-masked]                 FAX: (973) 952-5050
http://www.jdrosen.net                  PH:  (973) 952-5000
http://www.dynamicsoft.com

---

From: Michael Thomas <[email-masked]>
Date: Fri, 28 Dec 2001 13:20:40 -0800 (PST)

> able to receive UDP packets larger than 1500.

   Something that needs to be brought up here is that
   1500 is a fictitious number. Strictly speaking, it's
   more like 256 or so for IP4 and 1280 for IP6. I'm
   not sure we want to go with the IP4 strictness, but
   those are your only guarantees with PMTU.

	     Mike

---

From: Dean Willis <[email-masked]>
Date: Mon, 31 Dec 2001 11:07:41 -0600

eh, the smallest v4 MTU implementation I've seen was 576 . . . which 
was common on old PPP and SLIP implementations. But even so, it can be 
challenging to fit into 576  . . .

--
Dean

On Friday 28 December 2001 03:20 pm, Michael Thomas wrote:
>  > I would still prefer "MUST" use TCP for packets larger than 1500;
>  > your existing devices will be non-compliant (good motivation to
>  > fix it) but will still function; I think its fair to say that you
>  > still need to be able to receive UDP packets larger than 1500.
>