Process-to-Process Delivery: Chapter 23 Process-to-Process Delivery: UDP (Part 1)

23-1 PROCESS-TO-PROCESS DELIVERY The transport layer is responsible for process-to-process delivery—the delivery of a packet, part of a message, from one process to another. Two processes communicate in a client/server relationship. The internet model has three protocols at transport layer : UDP, TCP and SCTP.

Types of data deliveries

Client/Server Paradigm There are several ways to achieve process to process communication. The common one is “Client/ Server Paradigm”. A process on the local host, called client, needs services from a process usually on the remote host, called a server. Both processes have same name. For e.g. To get Day and time from a remote machine, We need a Daytime client process running on local host and a daytime sever process running on a remote machine.

Addressing Whenever we need to deliver something to one specific destination , we need address. At transport layer , we need a transport address called PORT NUMBER, to choose among multiple processes running on the destination host. Destination port number is needed for delivery and source port number is needed for reply. Port numbers are 16-bit integers between 0 and 65536.

The client program defines itself with a port number, chosen randomly by the transport layer software running on the client host called EPHERMERAL(temporary) PORT Number. Server must also define itself with a port number,( not chosen randomly). Internet uses universal port number for servers called WELL-KNOWN (permanent) PORT Number.

Port numbers

Port number Ranges The IANA (Internet Assigned Number Authority) has divided the port numbers into three ranges: Well known Assigned and controlled by IANA. Range varies from 0 to 1023 Registered Not assigned or controlled by IANA, can only be registered with IANA to prevent duplication. Range varies from 1024 to 49,151 Dynamic Neither controlled nor registered. Can be used by any process. They are ephemeral ports. Range varies from 49,152 to 65,535

IP addresses versus port numbers Destination IP address defines the host among the different hosts in the world. After the host has been selected, port number defines one of the processes on the particular host.

Socket address Process to process delivery needs two identifiers, IP address and the port number at each end to make a connection. The combination of an IP address and a port number is called a SOCKET ADDRESS. Transport layer protocol needs a pair of socket addresses: Client socket address and Server socket Address. Four pieces of information are part of IP header and transport layer protocol header. IP header contains IP addresses; UDP or TCP header contains Port number

Multiplexing and demultiplexing At sender, several processes need to send the packets, but there is only one transport layer , so for this many-to-one relationship we require multiplexing. Protocol accepts the message from different processes, differentiated by the assigned port number. After adding header, transport layer passes the packet to network layer. Demultiplexing At receiver, the relationship is one-to-many and for this we require demultiplexing. After error checking and dropping the header, transport layer delivers each message to appropriate process based on the port number

Connection-less Vs Connection-Oriented Transport layer protocols are of two kinds: Connection-less Connection-oriented

Connection-less Protocol Connection-Oriented Protocol Packets are sent without establishing a connection. Packets are not numbered. Packets may be delayed, lost or may arrive out of sequence. No acknowledgement Example protocol: UDP First connection is established between sender and receiver. After data transfer completes, connection is terminated. Example protocol: TCP and SCTP

Reliable Vs Un-reliable Transport layer services can be reliable or un-reliable. Reliable services are achieved by flow control and error control. Reliable services are slow and complex. TCP and SCTP are connection- oriented and reliable. Unreliable services are speedy and simple. UDP is unreliable and connectionless.

Error control

Position of UDP, TCP, and SCTP in TCP/IP suite

23-2 USER DATAGRAM PROTOCOL (UDP) The User Datagram Protocol (UDP) is called a connectionless, unreliable transport protocol. It does not add anything to the services of IP except to provide process-to-process communication instead of host-to-host communication. Provides very limited error checking.

Table 23.1 Well-known ports used with UDP

User datagram format 1. Source Port Number- Port Number used by process running on source host. It is 16 bit long (0 to 65535). If source host is client, port number is temporary port number requested by the process and if source host is server, port number is well known port number. 2. Destination Port Number- Port number use by the process running on destination host. It is also 16 bit long. 3. Length – this field defines the total length of the user datagram (header plus data). UDP length = IP length – IP header’s length 4. Checksum: This field is used to detect the errors over the entire user datagram. UDP checksum includes three sections pseudoheader, UPD header and data coming from application layer.

Pseudoheader for checksum calculation

UDP Operation UDP provides connectionless service. Each user datagram sent is an independent datagram. Data grams are not numbered. No connection establishment and termination which means each datagram can travel on a different path. It is simple and unreliable. No flow control so no window mechanism. Receiver may overflow with incoming messages. No error control except for checksum which means sender does not know if a message has been lost or duplicated. When Receiver detects the error through checksum, user datagram is discarded. To send a message from one process to another ,UDP protocol encapsulates and decapsulates messages in IP datagram.

Uses of UDP Suitable for a process that requires simple request- response communication with little concern for flow and error control. Not used for a process such as FTP(file transfer protocol). Suitable for a process with internal flow and error control mechanisms such as TFTP(trivial file transfer protocol). Suitable for transport protocol for multicasting Useful for management processes such as SNMP. Suitable for some route updating protocols such as RIP(routing information protocol.)