[asterisk-users] Microsoft Office Communications Server

[Philipp von Klitzing]

Hi!

> What is the logic of them using SIP over TCP? Is this a broad industry
> trend? Or just the latest attempt to get around SIP/NAT issues?

I remember a quote of Henning Schulzrinne where he states that having 
designed SIP with UDP in mind was the biggest mistake he (and Mark 
Handle?) were to be found guilty of. I am not sure if this is what's 
driving Microsoft's decisions, my guess is that this is/was mostly driven 
by security reasons (and the new focus of Microsoft on security aspects).

Cheers, Philipp


* Taken from http://www.faqs.org/rfcs/rfc4168.html:

3.1.  Advantages over UDP

   All the advantages that SCTP has over UDP regarding SIP transport are
   also shared by TCP.  Below, there is a list of the general advantages
   that a connection-oriented transport protocol such as TCP or SCTP has
   over a connection-less transport protocol such as UDP.

   Fast Retransmit: SCTP can quickly determine the loss of a packet,
      because of its usage of SACK and a mechanism that sends SACK
      messages faster than normal when losses are detected.  The result
      is that losses of SIP messages can be detected much faster than
      when SIP is run over UDP (detection will take at least 500 ms, if
      not more).  Note that TCP SACK exists as well, and TCP also has a
      fast retransmit option.  Over an existing connection, this results
      in faster call setup times under conditions of packet loss, which
      is very desirable.  This is probably the most significant
      advantage of SCTP for SIP transport.

   Congestion Control: SCTP maintains congestion control over the entire
      association.  For SIP, this means that the aggregate rate of
      messages between two entities can be controlled.  When SIP is run
      over TCP, the same advantages are afforded.  However, when run
      over UDP, SIP provides less effective congestion control.  This is
      because congestion state (measured in terms of the UDP retransmit
      interval) is computed on a transaction-by-transaction basis,
      rather than across all transactions.  Thus, congestion control
      performance is similar to opening N parallel TCP connections, as
      opposed to sending N messages over one TCP connection.

   Transport-Layer Fragmentation: SCTP and TCP provide transport-layer
      fragmentation.  If a SIP message is larger than the MTU size, it
      is fragmented at the transport layer.  When UDP is used,
      fragmentation occurs at the IP layer.  IP fragmentation increases
      the likelihood of having packet losses and makes NAT and firewall
      traversal difficult, if not impossible.  This feature will become
      important if the size of SIP messages grows dramatically.


* Quote from http://tools.ietf.org/html/draft-jennings-sip-dtls-01:

   There has been considerable discussion of why SIP needs DTLS when we
   have TLS.  This is the wrong question.  The right question is why SIP
   has UDP and TCP (not to mention SCTP).  There are two reasons for
   believing that UDP is likely to be an important protocol in SIP for
   the foreseeable future.

   o  In theory, there is no problem building systems that terminate a
      million TCP connections on a single host.  In practice, the common
      operating systems used for building SIP aggregation devices make
      this impossible.  To date, no one has demonstrated terminating
      over 100k SIP TCP connections to a single host.  Doing that many
      connections with UDP has not been difficult.

   o  If we want to talk about "running code" for SIP, it's UDP.  Unless
      UDP is deprecated for SIP, it is important to provide a reasonable
      level of security for it.