The TCP/IP Protocol Suite: TCP

Derek Bridge

Department of Computer Science,
University College Cork

The TCP/IP Protocol Suite: TCP

Aims:

• to know what a protocol is
• to know what packet switching is
• to know how TCP achieves reliable data transmission

Protocols

• A communication protocol is an agreement that specifies how two computers will exchange messages
• It will specify:
  • the exact format of the messages
  • the meaning of the messages, and
  • the conditions under which a message should be sent
• Programmers must ensure that the software that runs on the devices that are connected to the network follow the rules of the protocol: so that they send the right messages at the right time in the right format

The TCP/IP Protocol Suite

The Internet uses several dozen protocols, referred to collectively as the TCP/IP Protocol Suite or just TCP/IP in honour of the two key protocols:

The Tranmission Control Protocol (TCP)

This protocol is about reliable data transmission

The Internet Protocol (IP)

This protocol is about routing

The Layers of the TCP/IP Protocol Suite

Circuit-Switching versus Packet-Switching

• The telephone network is a circuit-switched network
  • When you make a call, a circuit is dedicated to you for the whole duration
• But,
  • your use of this circuit excludes others, and
  • the failure of any portion terminates communication

Circuit-Switching versus Packet-Switching

• The Internet is a packet-switched network
  • When two devices communicate, no specific path is allocated to them
  • The data is chopped up into packets
  • Each packet is sent separately by the best available route
• Since the packets may not take the same path,
  • some may get lost, or
  • they may arrive out of order
• But, the weaknesses of circuit-switching are overcome

The Transmission Control Protocol (TCP)

• TCP specifies how to establish and terminate a 'connection' (using hand-shaking messages)
• It also specifies how to take a stream of data and divide it into packets, called TCP segments
• The segments are transmitted separately, and TCP specifies how to achieve a level of reliability:
  • how to cope if segments arrive out of order (or if duplicates arrive)
  • how to cope if segments get lost (including load management), and
  • how to cope if segments get corrupted

TCP segments

Each TCP segment will consist of two parts:

• a header, which includes:
  • the destination port number
  • a sequence number
  • a checksum
• the data (or payload)

Class exercise: How does this help the recipient cope when segments arrive out of order? What about duplicate segments?

Why segments might get lost

• A connection or a device such as a router may fail
• A router may become overrun
• For another reason, see the lecture on IP

TCP's sliding window acknowledgment system

• When a segment reaches its ultimate destination, the recipient device sends an acknowledgment
• If the sender does not receive an acknowledgment, it retransmits the segment

Class exercise:Why is this not sufficient? (Two reasons)

TCP's sliding window acknowledgment system

• When the sender sends a segment, it starts a timer
• If an acknowledgment is received before the timer expires, all is OK
• If no acknowledgment is received before the timer expires, a segment may have been lost, in which case it is retransmitted (with a new timer)

Class exercise: We've seen how to handle duplicates. But why might they occur? (Two reasons)

TCP's sliding window acknowledgement system

• The acknowledgment messages will contain the segment's sequence number. Why?
• But they may also contain information about how busy the recipient is. Why?

Why segments might get corrupted

E.g.

• Lightning can cause random electrical signals to appear on wires
• Electrical signals on wires can become distorted when the wire passes through a strong magnetic field

TCP's checksums

• Sender:
  • Compute a sum based on the data you wish to send
  • Include this sum in the header
• Recipient:
  • Compute the same sum based on the data you have received
  • Compare it with the checksum you have received (in the header)
  • If they are the same, all is well. (Really?)
  • If they are different, the data has been corrupted.
    Class exercise:So then what will the recipient do?

Checksums

Suppose all seems well: your checksum is correct.

Class exercise: Give examples of ways the data could be corrupted without affecting the checksum

The User Datagram Protocol (UDP)

• TCP's reliability comes at a cost in time and bandwidth
• UDP is an alternative used for applications that cannot afford or do not need TCP's reliability
• In UDP, there are no connections, there are no acknowledgments, and checksums are optional

TCP versus UDP

Class exercise: TCP or UDP?

• Sending an email
• Your browser requests a web page
• The web server sends back the web page
• Downloading a music file to playback later
• Streaming a radio show to listen to now