Friday, June 10, 2011

Transport Layer

Stream Control Transmission Protocol (SCTP) is a new relaiable, message oriented transport layer protocol. SCTP, however , is mostly designed for internet applications that have recently been introduced. Those new applications, such as IUA(ISDN over IP), M2UA and M3UA (telephony signaling), H.248(media gateway control), H323 (IP telephony), and SIP (IP telephony), need a more sophisticated service than TCP can provide. SCTP provides this enchanced performance and reliability. We briefly compare UDP, TCP, and SCTP.

UDP is a message-oriented protocol. A process delivers a message to UDP, which is encapsulated in a user datagram and sent over the network. UPD conserves the message boundaries; each message is independent of any other message. This is a desirable feature when are dealing with applications such as IP telphony and transmission of real-time data, as we will see later in the text. However, UDP is unreliable; the sender cannot know the destiny of message sent. A message can be lost, duplicated, or received out of order. UDP also lacks soome other features, such as congestion control and flow control, needed for a friendly transport layer protocol.

TCP is a byte oriented protocol. It receives a message or messages from a porcess, stores them as a stream of bytes, and sends them in segments. There is no preservation of the message boundaries. However, TCP is a reliable protocol. The duplicate segments are detected, the lost segments are resent, and the bytes are delivered to the end process in order. TCP also has congesiton control and flow control mechanisms.

SCTP combines the best features of UDP and TCP. SCTP is a reliable message oriented protocol. It preserves the message boundaries and at the same time detects lost data, duplicate data, and out of order data. It also has congestion control and flow control mechanisms. Later we will see that SCTP has other innovative features unavailable in UDP and TCP.

UDP Service : Process to Process Communication. UDP is Connectionless and unreliable transport.

TCP Services :
- Process to Process Communication.
- Stream Delivery Service.
- Full Duplex Communication.
- Connection Oriented Service.
- Reliable Service.

SCTP Services :
- Process to Process Communication.
- Multiple Streams.
- Multihoming.
- Full Duplex Communication.
- Connection Oriented Services.
- Reliable Service.