I scheme (sips) will be discussed, which uses TLS transport. The default SIP port number for TLS transport is port 5061. If TLS is used between two proxies, each proxy may have a certificate allowing mutual authentication. However, if a client does not have a certificate, TLS can be used in conjunction with another authentication mechanism, such as SIP digest, to allow mutual authentication. The SIP use of TLS takes advantage of both the encryption and authenti- cation services. However, the encryption and authentication is only useful on a single hop. If a SIP request takes multiple hops (i.e., includes one or more proxy servers), TLS is not useful for end-to-end authentication. S/MIME encryption, described in Section 3.7 solves this problem. SIP Proxies must support TLS and will likely use TLS for long-lived connections. 2.5.4 SCTP Transport An extension to SIP defines the use of SCTP [9] with SIP to provide reliable stream-based transport with some advantages over TCP transport for a SIP: Understanding the Session Initiation Protocol message-based protocol such as SIP. First, it has built-in message segmentation, SO that individual SIP messages are separated at the transport layer. With TCP, the SIP protocol must use the Content-Length calculation to delineate messages. If a TCP connection is being shared by a number of SIP transactions and dialogs, the "head of line blocking" problem discussed in Section 1.4.3.4 can cause the buffer to contain valid SIP messages that could be processed by the server while the retransmission takes place. Due to its message level delineation, SCTP is able to continue to forward messages to the application layer while simultaneously requesting a retransmission of a dropped message. Note that this is only a problem when multiple applications are multiplexed over a single TCP connection. An example of this is a TCP link between two signaling proxy serv- ers. For a user agent to proxy TCP connection, this is usually not a problem unless the two have many simultaneous dialogs establ