Chapter 24 Transport Control Protocol TCP

1
Chapter 24 Transport Control Protocol (TCP)

• Layer 4 protocol
• Responsible for reliable end-to-end transmission
• Provides illusion of reliable network to
  application programmers
• no need to worry that data will be lost,
  duplicated or delivered out of order
• Sequence number used to reorder segments

2
Services offered by TCP to application programs

• Connection-oriented service
• application first requests a connection to a
  destination and then use the connection to
  transfer data.
• point-to-point communication
• each TCP connection has exactly two endpoints.
• complete reliability
• TCP guarantees that the data sent across a
  connection will be delivered exactly as sent,
  with no data missing or out of order.
• full duplex communication
• data may flow in either direction at any time.
  TCP can buffer both outgoing and incoming data.

3
TCP services (cont.)

• stream interface
• TCP provides a stream interface in which an
  application sends a continuous sequence of bytes
  across a connection
• TCP does not provide a notion of records.
• reliable connection startup
• when two applications create a connection, both
  must agree to the new connection
• random 32 bit sequence numbers generated by
  sender receiver at beginning of a connection to
  prevent interference from other connections..
• graceful connection shutdown
• TCP guarantees to deliver all transmitted data
  reliably before closing a connection.

4
TCP Characteristics

• end-to-end protocol (fig 24.1)
• TCP provides a connection directly from an
  application on one computer to another
  application on a remote computer  
• Provides virtual connections
• connections achieved in software
• underlying hardware does not provide support for
  connections.
• TCP software modules on the two machines exchange
  messages to achieve the illusions of a
  connection.

5
Reliability through Retransmission (fig 24.2)

• When TCP sends data, the sender handles packet
  loss via a retransmission scheme where both
  sender and receiver participates
• When TCP receives data, it sends an
  acknowledgement back to the sender.
• Whenever TCP sends data, a timer is started.
• Data is retransmitted if the timer expires
  before an acknowledgement arrives
• If acknowledgement packet is lost, receiver may
  receive duplicate packet.
•  

6
How long should TCP wait before retransmitting?

• The delay required for data to reach a
  destination and an acknowledgement to return
  depends on traffic conditions in the internet as
  well as the distance to the destination.
• TCP uses adaptive retransmission (fig 24.3) since
  its retransmission timer changes based on current
  network conditions and delays.
• TCP monitors delays by calculating the round-trip
  time from the time a packet is sent until an
  acknowledgement of the packet is received.
• Ideally, timer is set just long enough to
  determine that a packet was lost

7
Flow Control(fig 24.4)

• handled via a window mechanism such as sliding
  window with changing window sizes
• When a connection is established, each end of the
  connection allocates a buffer to hold incoming
  data, and sends the size of the buffer (window
  advertisement) to the other end.
• As data arrives, the receiver sends the remaining
  buffer size (window size advertisement) along
  with the acknowledgement.
• window size is always measured beyond (relative
  to) the data being acknowledged.
• window size decreases whenever data fills the
  buffer and increases whenever the application
  reads data from the buffer.

8
Three-Way Handshake(fig 24.5)

• used by TCP to guarantee that connections are
  established or terminated reliably by exchanging
  three messages
• necessary and sufficient to ensure unambiguous
  agreement in spite of packet loss (causing
  retransmission) or duplication (due to
  retransmission)
• synchronization (SYN) segment used to create a
  connection,
• finish (FIN) segment used to close a connection

9
Congestion Control

• packet loss or extremely long delays is more
  likely to be caused by congestion (routers
  dropping packets due to overflow of router
  buffers) than hardware failure.
• Retransmission of packets by transport protocols
  can exacerbate the problem by injecting
  additional copies of a message,
• TCP uses packet loss as a measure of congestion,
  and responds by reducing the rate at which it
  retransmit data (router sends source quench
  message when it drops an incoming packet).

10
Congestion Control

• Used by TCP when a message is lost
• Instead of retransmitting enough data to fill
  the receivers buffer window size, TCP starts by
  sending a single message containing data.
• If the acknowledgement arrives, TCP doubles the
  amount of data being sent by sending two
  messages.
• When acknowledgement arrives for these two, TCP
  sends four more, and so on.
• TCPs congestion control scheme helps alleviate
  congestion by backing off quickly.
• By not adding retransmissions to a congested
  network, TCPs congestion control scheme helps
  prevents congestion collapse.

11
TCP Segment (Message) Format(fig 24.6)

• 16 bit source port identifies the application
  program that sent the data
• 16 bit destination port identifies application
  program on the receiving computer.
• 32-bit sequence number of outgoing data in
  message used by receiver to reorder out-of-order
  segments and to compute acknowledgement number
• 32-bit acknowledgement number containing
  sequence number of data that has being received
• 16 bit window specifies buffer space available
  for incoming data
• 16 bit checksum
• data