We have seen that application layer operates as processes that communicate between hosts. Transport layer provides process-to-process delivery of messages. The underlying network layer provides host-to-host delivery of messages, without any reliability guarantees. So, job of transport layer is to take this host-to-host delivery service from the network layer, and provide process-to-process delivery abstraction to the application layer.
Processes send messages via sockets. So, we can also think of the transport layer as providing a data delivery service from socket to socket.
Some terminology: application layer “message” -> transport layer “segment” -> IP/network layer “datagram”.
Main functions of transport layer:
Multiplexing and demultiplexing (mux/demux): several processes on a host may be sending messages. Each application process is assigned a port number. Transport layer takes the application layer message, appends port number, and passes to network layer. At the receiver end, it hands over message to correct application process based on port number. Both TCP and UDP provide this basic mux/demux functionality. In fact, UDP is a barebones transport layer that provides ONLY this functionality.
Reliable data delivery: TCP provides reliable data delivery. That is, the application can be assured that the message will reach the other side (or it will know if lost). TCP provides reliability using several mechanisms, on top of a unreliable network layer.
Congestion control: TCP ensures that data from one application does not swamp all the links and routers on the path. Congestion control is not so much a service to the application as it is a service to the Internet.
What are function that transport layer cannot provide? It cannot provide bandwidth or delay guarantees because the underlying network layer does not provide guarantees. Note that IP layer does not provide security either, but TCP has special mechanisms (SSL) to provide security over unsecure transport. But bandwidth/delay guarantee is harder.
Two main transport protocols: TCP and UDP.
TCP is a connection oriented protocol, i.e., there are a few connection establishment messages before data can be sent from sender to receiver. However, this connection state resides only at end hosts; unlike connections in circuit switching where state is established at all routers on path. The side that initiates connection is called TCP client. The side that responds is called server. Data flow can happen both ways, TCP is full duplex.
Recall from socket programming: both client and server first create sockets. Client sends a connect request to server IP and port. Then, the “TCP handshake” takes place. TCP client sends a SYN packet to server to initiate connection. Server responds with SYNACK. Client then sends SYN ACK ACK (or simply ACK), along with any data possibly. At this point, the accept call at the server returns, i.e., the server accepts the request, a new client socket is created for this connection, and connection is said to be established. Once connection is established, both client and server may send data through the TCP connection via the sockets. This data exchange translates to read/write on sockets.
So when a packet arrives at a host, how does TCP know which client socket to deliver message to? TCP uses 4-tuple (source IP, source port, destination IP, destination port) as the demux key to identify appropriate socket. Why can’t it just use destination IP and port? Because server can have several sockets open for different clients.
How is mux/demux done in UDP? Note that there is no handshake in UDP. Sender and receiver applications create sockets, and fill in the destination port and IP in the socket structures. Then they simply send and receive messages using the socket handle. The source and destination port numbers are embedded in the UDP header. Note that a UDP-based server only has one listening socket, not one per client. When a packet reaches the destination IP address (via network layer), the destination port is looked up to identify the correct socket to deliver the message to. That is, the demux “key” is the 2-tuple (destination IP, destination port).
Why is demux key different for UDP (2-tuple) vs TCP (4-tuple)? Because TCP maintains a separate socket for every client-server communication (connection-oriented), whereas UDP server processes all clients via the same socket. So, next question, why is TCP connection oriented and why does it have to maintain a separate socket for every connection? Because TCP implements reliability, it is easier with separate sockets. For example, when a message arrives at a socket, it is easy to simply see the source IP/port associated with that socket and send ACK. If you want to send a reply to a UDP message, you need to extract the source IP and port from the UDP header for every message.
There is not much else to UDP other than mux/demux. UDP is just barebones transport: mux/demux with light error detection (checksum over header: includes port numbers and checksum (IP addresses are from IP header). Lower header overhead than TCP (UDP-8 bytes, TCP-20 bytes).
Which applications use UDP?
DNS: simple request reply does not justify the overhead of connection setup of TCP.
Multimedia applications: they have more control over what data is sent when. For example, if the voice sample is stale, no point in TCP retransmitting it.