Transport Protocols. Internet has broadly two types of transport protocols as shown in figure below:

Transcription

1 Transport Protocols Internet has broadly two types of transport protocols as shown in figure below: Connectionless Protocol- UDP (User Datagram Protocol) Features: i. It is connectionless. ii. Provides a way for applications to send encapsulated IP datagram without having to establish a connection between sender and receiver. Advantage of using UDP: i. Simple, and uses minimum overhead ii. Used for sending small messages where reliability is not required. iii. Sending a small message using UDP take muchless interaction between sender and receiving than TCP or SCTP.UDP Frame format UDP Frame Format:

2 UDP transmits segments whose header is 8 byte. Header follows the data as shown in figure. The fields are: Source Port(2 byte) and Destination Port (2 byte): forming a total of 4 bytes serve to identify source and destination machines. Both these ports are 16 bit long means that the prt number can be anything from 0 to When the source/ destination host is a server then the port number in most cases is well known port number. If source/destination machine is a client the port number in moset case is the ephemeral(temporary) port number it has received in the request packet. Length: It is alos a 16-bit long and define the length of the user datagram, header plus data. In fact the total length is much less for UDP. This field is not much necessary as the network layer protocol IP has a length field. The UDP length can be calculated from IP packet as: Checksum: UDP length = IP length IP header s length Calculation of UDP checksum is a little different. The checksum used the following fields for calculation: i. Pseudoheader ii. UDP header iii. Data coming from application layer Note:

3 If pseudoheader (IP header) is not included, user datagram arrives safe and sound If pseudoheader is corrupted, the datagram may arrive at wrong destination Checksum calculation in user datagram is optional. If the checksum is not computed, it is stored as a 0, since by a happy coincidence of one s complement arithmetic a true computed 0 is stored as all 1s. However, turning it off is foolish unless the quality of the data does not matter Calculation of Checksum: Important UDP port numbers As you know every computer or device on the Internet must have a unique number assigned to it called the IP address. This IP address is used to recognize your particular computer out of the millions of other computers connected to the Internet. When information is sent over the Internet to your computer how does your computer accept that information? It accepts that information by using TCP or UDP ports. The port numbers are divided into three ranges: I. Well Known Ports; these are from 0 through 1023, DCCP Well Known ports SHOULD NOT be used without IANA registration.

4 II. III. Registered Ports ; these are from 1024 through 49151, DCCP Registered ports SHOULD NOT be used without IANA registration Dynamic and/or Private Ports: These ports are from through A value of 0 in the port numbers registry below indicates that no port has been allocated. UDP Port Description 39 Remote location Protocol 42 Internet Name server 50 Remote mail checking protocol 67 BOOTP, Bootstratp protocol server 68 BOOTP, Bootstratp protocol client 69 TFTP (Trivial File Transfer Protocol) 53 DNS 69 TFTP 123 NTP(Network time protocol) 153 SGMP (Simple Gateway Monitoring protocol) 161 SNMP Poll 162 SNMP Trap 434 Mobile IP Agent 623 IPMI to CiMC 514 Syslog 546/547 DHCPv6 client/server 1812/1813 Radius UDP Operation: The port numbers serve as mailboxes that applications can rent to receive the incoming mails/packets. The transport layer adds the port numbers. Without the port numbers the transport layer will nt know what to do with each incoming packets. That means, with port numbers, the transport layer delivers the embedded segment to the correct application/process. i. The UDP is connectionless; means there is no need for connection establishment and connection release.

5 ii. iii. iv. There is no error and flow control since the transmission is unreliable. To send message from one process to another, UDP Encapsulates and decapsulates messages in an IP datagram. Queuing: In UDP queues are associated with ports. At the client side when a process starts, it requests a port number from OS. Some implementation create both incoming and outgoing queue associated with each process; other implementation have only incoming queue associated with each process Message for Client: When a message arrives from a client, UDP check to see if an incoming queue has been created for the port number specified in the destination port number field of the user datagram If there is such a queue, UDP sends the received user datagram to the end of queue If there is no such queue created, the datagram is dropped and asks the ICMP to send a port unreachable message to the server All incoming message for one client program, whether from same or from different server, are sent to same queue. If the incoming queue overflows, UDP drops the datagram and ask ICMP to send port unreachable message to be sent to server.

6 Message for Server Site: At the server side, the server ask for incoming and outgoing port, using a well-known port, when it starts running. The queue remains open as long as the server is running. When a message arrives for the server, UDP checks to see if an incoming queue has been created for the port number specified in the destination field of user datagram. If queue is ther, UDP sends the received datagram to the end of queue, else it rejects the datagram and asks the ICMP to send an error port unreachable. All incoming message for one client program, whether from same or from different server, are sent to same queue. If the incoming queue overflows, UDP drops the datagram and ask ICMP to send port unreachable message to be sent to server. When server want to respond to the client, it sends a message to outgoing queue, using source number specified in the request. UDP removes the message one by one, after adding the UDP header, delivers them to IP. An outgoing queue can overflow, If this happens, the operating system asks the server to wait before sending any more messages. Question What is user datagram protocol? Explain the UDP header. What is pseudoheader in UDP datagram. How is the UDP checksum calculated? Explain the features of UDP Explain the queuing in UDP process Reference: 1. Data and Communication Networking by Behrouz Forouzan 2. Computer Networks by A Tenenbaum