io 49170 RTP/AVP 0 Chapter 5 Just as e-mail messages can carry more than one attachment, SIP mes- sages can carry several bodies. For instance, Laura may send an INVITE with two bodies: a session description and her photo. That way, Bob's UA can display her photo on the screen while Bob is alerted. Transport Layer We learned previously that SIP is an application layer protocol. Therefore, it makes use of transport layer protocols to transmit requests and responses. The behavior of any application layer protocol varies with the type of transport used. If it's reliable, the application layer protocol builds a message and delivers it to the transport layer, fully expecting that the mes- sage will arrive at the destination. The application layer does not know how the transport layer accomplishes its delivery; it just knows that the task is performed. How is it performed? Typically, the transport layer will retransmit the message until the other end receives it and sends back some type of acknowledgement message. These retransmissions are transparent to the application layer. On the other hand, if an application layer protocol makes runs on top of an unreliable transport layer protocol such as UDP, it cannot assume deliv- ery. Therefore, application layer retransmissions have to be implemented. They are typically implemented as follows. The application layer protocol builds a message and passes it to the transport layer. Should it fail to receive a confirmation of reception from the destination in a certain period of time, it will build the same message again and pass it to the transport again. By utilizing application layer time-outs with its retransmissions, an application layer protocol can still exploit unreliable transport mechanisms. Let us now see how SIP works over both types of transports. INVITE Transactions Because INVITE transactions involve a three-way handshake and an ACK request, they require different handling than any other transaction. There- fore, SIP entities treat INVITEs and ACKs in different way