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The Internet Transport Protocols

Sub-topic outline:

• The Internet makes use of two transport layer protocols over IP:

1. Transmission Control Protocol (TCP)

• Connection-oriented

• Provides a reliable end-to-end stream over an unreliable internetwork.

1. User Datagram Protocol (UDP)

• Connectionless

• Simple and efficient, but no guarantee on reliability.

The Transport Control Protocol (TCP)

• When an application wants to send messages out:

• It gives message to the TCP transport entity (a piece of software)

• The TCP entity breaks the message into smaller pieces

• Passes the pieces to IP to send out as separate IP packets.

• Waits for acknowledgements for all pieces, and re-transmit if time-out.

The Transport Control Protocol (TCP)

• When the packets arrive at the destination:

• The TCP entity takes the separate packets from IP – perhaps out of order.

• Puts the different pieces together

• Gives the reconstructed message to the appropriate application.

• Gives IP an acknowledgment message to send back to source.

• IP do not guarantee reliable delivery (eg. handle lost or out-of-sequence packets), so TCP needs to handle it.

The TCP Service Model

• Both sending and receiving TCP entities create points called sockets.

• Each socket has an IP number (32-bits for IPv4) and a 16-bit local number, called a port.

• This 48-bit number forms the address for the connection.

• Some particular well-known ports are reserved for standard application services. Eg.

• FTP: port 21

• TELNET: port 23

• HTTP: port 80

The TCP Protocol

• TPDU are sent as segments, consisting of:

• a fixed 20-byte header part

• a variable-size header part

• zero or more data bytes

• The size of the segments is limited by:

1. The maximum size of the IP packets
2. The maximum transfer unit (MTU) of the network.

The TCP Protocol

• TCP uses a sliding window protocol

• When a sender transmit a segment, it starts a timer

• When a segment arrives at a destination, the receiver sends back a segment with acknowledgment

• If sender times-out, the same segment is re-sent.

The TCP Segment Header

Figure 6-24 p526

• Source and Destination ports – address for the end points of the connection

• Sequence and Acknowledgement numbers – to ensure reliable delivery

• Header length – how many 32-bit words in the header – needed because Options field is variable.

• 6 bits reserved – still unused today

• 6 control flag bits

• Window size – for flow control using a sliding window

• Checksum

• Urgent Pointer – indicates how much interrupt signal data are coming

• Options – to specify other parameters not included in the normal header – eg. maximum segment size the receiver is willing to accept.

TCP Connection Management

• Connection Establishment: 3-way handshake:

• Uses the SYN bit flag set to 1 for segments to establish connection.

• Connection Termination: 2-way handshake

• Uses the FIN bit flag set to 1 for segments to terminate connections.

• Uses timers to avoid the two army problem – if no response from other party, the connection is released regardless.

TCP Congestion Control

• More appropriate to deal with congestion at the transport layer rather than the network layer – since the senders have the most power to control the flow.

• Two types of congestions: network and receiver

• TCP software at the sender keeps two windows:

1. A receiver window indicated by the receiver on how much it can take, and
2. A congestion window kept by the sender for congestion in the network.

TCP Timer Management

• Timers are used for segments sent out and awaiting acknowledgments – but how long to set the timeouts?

• Much harder problem than the timers used for DLL – the possible round-trip times are spread over a much wider range.

• TCP uses a variable (RTT) for each connection – continually adjust this variable based on detected network performance.

User Datagram Protocol (UDP)

• An alternative to TCP for transporting IP packets

• Doesn’t involve the overheads of making a connection

• Doesn’t provide reliability, but much more efficient.

• Used when the data sent are not critical (eg, ICMP echo packets used in pings).

• UDP header: Fig 6-34 p542

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