[SIPForum-discussion] SIP using TCP or UDP?

Fri Jan 19 21:15:42 UTC 2007

I think RFC 3261 defines that the payload should be over 1300 bytes before
falling back to TCP unless the MTU size is explicitly known.  In an IMS
implementation, I am fairly sure that you will be pushing the edge of the
1300 byte limit, or the 1500 byte MTU size, once you start adding in the
codecs supported, long FQDNs, privacy,  multiple hops and elements adding
themselves in the via.

I would say listen to the standard, have it start out as UDP, if you push
the limit then fall back to TCP.  I have found that in networks today TCP
support for SIP is fairly spotty.  Support of RFC3261 TCP fallback
mechanisms even spottier, but it is the agreed upon way of handling these
situations.  Otherwise you're stuck with RFC2543, and use UDP unless
explicitly stated and just fragment the packet which the terminating UA
would most probably reject.

Regards,

Rommel

  _____  

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Prasad MNR
Sent: Sunday, January 14, 2007 8:09 PM
To: raman kumar
Cc: discussion at sipforum.org; rezaul.kabir at gmail.com
Subject: Re: [SIPForum-discussion] SIP using TCP or UDP?

Hi,
The following is a quote from the book "Understanding Sip" by Alan B.
Johnston.

"UDP provides the simplest transport for user agents and servers and allows
them to operate without transport layer state. however UDP offers no
congestion control. A series of lost packets on a heavily loaded IP link can
cause retransmissions, which in turn produce more lost packets and can push
the link into congestion collapse. Also UDP may only be used for SIP when
the message (and its response) is known to be less than the MTU size of the
ip network. For simple SIP message, this is not a problem. however for large
messages containing multiple message bodies and large header fields, this
can be a problem. In this case, TCP must be used, since SIP does not support
fragmentation at the SIP layer." 

I think answers Madan's question and also explains why Raman has suggested
to use UDP if its a local network. I may be wrong at this point of time as I
have just started the journey of learning SIP.

Regards, 
Prasad MNR 

On 1/15/07, raman kumar <ramank24 at gmail.com> wrote: 

Razual has explained the right reson. To use the power of UDP
connectionaless protocol the
timer mechanism is inbuilt in the sip 

On 14/01/07, Rezaul Kabir <rezaulkabir at gmail.com> wrote:
> I have not heard any such restrictions on the size of packet. But UDP
> is used by default and if you closely look at RTC 3261 Section 
> 13.3.1.4 you will notice that it talks about a timer to retransmit
> packets and similar example is available in the rfc in several areas.
> This behavior of retransmission clearly indicates that it is desirable 
> to use non-reliable transport such as UDP to send the packets.
> According to RFC3263 Section 4.1 is the Sip Uri does not specify any
> transport then UDP should be selected. Also by RFC2543 UDP is
> mandatory if no transport is defined and for good interpretability.
> TCP should only be used if one is thinking of using SIPS/TLS OR the
> Sip Uri specifies TCP as transport.
>
>
> ---- 
> Rezaul Kabir
>
>
> On 1/13/07, madan kumar <mada2k at gmail.com> wrote:
> > i far as i know udp is used when the packet size is less than 1200 bytes

> and
> > TCP is used when it exceeds 1200 bytes ..... do any of u have a
> explanation
> > for this
> >
> >
> > On 1/10/07, raman kumar < ramank24 at gmail.com <mailto:ramank24 at gmail.com>
> wrote:
> > > when using the sip at the back-end use the UDP
> > > When using in the local premises use TCP
> > >
> > >
> > >
> > > On 26/12/06, Ramachandran < rgunasekaran at velankani.com> wrote:
> > > > Dear all,
> > > >
> > > >
> > > >
> > > >  I am not very sure about the following, 
> > > >
> > > >
> > > >
> > > > What is ideal transport protocol for SIP with IMS deployment?
> > > >
> > > > When to use SIP using UDP? 
> > > >
> > > >
> > > >
> > > > Thanks,
> > > >
> > > > Ram
> > > >
> > > >
> > > >
> > > >
> > > >
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