1. 6.1. Transport Control Protocol (TCP) It is the most widely used transport protocol in the world. Provides reliable end to end connection between two hosts who want to communicate over an unreliable network. TCP transport data across the networks in blocks called TCP segments. when an application first starts using TCP, the TCP module on the client’s computer and the TCP module on the server’s computer start communicating with each other. 6.1.1 Addressing: It uses ports to define which type of network application from the server, the client want to communicate to.

2. 6.1.2 TCP Segments A TCP connection is a bidirectional octet stream, along which every byte of data sent has a sequence number. TCP PACKET HEADER Field NameBits Source Port16 Destination Port16 Sequence Number32 Acknowledgment Number32 Window16 URG Flag1 ACK Flag1 PSH Flag1 RST Flag1 SYN Flag1 FIN Flag1 Options1 Data

3. 6.1.3 TCP Service Primitives 6.1.3.1 TCP request primitives ACTIVE-OPEN – used to initiate a connection, and results in TCP sending a SYN message. ACTIVE-OPEN-DATA – same like above but can contain data as well. FULL-PASSIVE-OPEN – tell TCP to listen for connection requests from a specified remote sites. UNSPECIFIED-PASSIVE-OPEN – tell TCP to listen for connection requests from any remote sites. CLOSE and ABORT – used to terminate a connection normally and abnormally. SEND – to transfer data over the connection. STATUS – to determine the connection status.

4. 6.1.3 TCP Service Primitives 6.1.3.2 TCP confirm and indication primitives OPEN-SUCCESS confirm– tell an application whether an ACTIVE-OPEN request has succeeded. OPEN-FAILURE confirm– tell an application whether an ACTIVE-OPEN request has failed. OPEN-ID primitive provides the connection identification. STATUS request is met with a STATUS-RESPONSE to obtain the connection status. TERMINATE confirm – tells the application that the connection has ended. CLOSING – tells an application that the remote TCP entity has issued a CLOSE request. DELIVER – to indicate the delivery of the data in across end. ERROR– to indicate if an error has occurred.

5. 6.1.5 Connection States 6.1.5.1 Connection Establishment IDLE CONN SYN (x) SYN (y), ACK (x+1) Client Server A TCP connection is usually established by a TCP client, using a 3-way handshake. SYN o IDLE CONN SYN o SYN(x+1), ACK (y+1)

6. 6.1.5 Connection States 6.1.5.2 Connection Management Each peer includes an acknowledgement and window information with each packet. Sending data causes window to be filled. Acknowledgements from receiver will free up buffers in the senders queue. If data is received out-of-order, the receiver will buffer it till the missing segment is received. 6.1.5.3 Connection Termination Either the client or server may terminate data transmission at any time by sending a packet where the FIN bit is set.

7. 6.1.5 Connection States 6.1.5.4 Crash Handling If the connection is lost because of crash on one peer, any data sent on the other end will cause the half-open connection to be discovered. Hence, the receiving peer sends a packet with the RST flag and a sequence number derived from the incoming packet to signify that the connection is no longer in existence. 6.1.6 Flow Control TCP is a variable size sliding window protocol with time-out and retransmits. Each packet what an acknowledgement contains window, which is the number of octets of data that the receiver is willing to accept, beginning with the sequence number in the acknowledgement.

8. 6.2 TCP Limitations TCP data and control messages have the same message header structure. Hence, for even small amounts of data in each message percentage overhead is very high. The TCP checksum mechanism is very weak, as interface layer has it already, due to which transport layer could well be subtle enough to escape detection with its scheme. It is hard to obtain QoS control over a TCP connection as setting the URG bit in a TCP header does not affect normal flow control.

9. 6.2 User Datagram Protocol It does not maintain an end to end connection with the remote UDP module and hence is called connection less data transmission. It doesn’t provide guaranteed data delivery. It provides multiplexing of information between applications based on port number, and also implements a checksum scheme to check the integrity of the data. Example network applications: NFS, SNMP and so forth. Checksum: It is the method to validate the integrity of UDP data packets during the transmission. Flow Control: In UDP if more UDP packets arrive at the receiving end after the buffer is overrun then ultimately it discards all those packets.

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