Chapter 24 Chapter 24 Transport Control Protocol Transport Control Protocol

(TCP)(TCP)Layer 4 protocol Responsible for reliable end-to-end

transmissionProvides 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

Services offered by TCP to Services offered by TCP to application programsapplication 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.

TCP services (cont.)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.

TCP CharacteristicsTCP 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.

Reliability through Reliability through Retransmission (fig 24.2)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.  

How long should TCP wait How long should TCP wait before retransmitting?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

Flow Control(fig 24.4) 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.

Three-Way HandshakeThree-Way Handshake(fig 24.5)(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

Congestion ControlCongestion 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).

Congestion ControlCongestion Control Used by TCP when a message is lost Instead of retransmitting enough data to fill the receiver’s

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.

TCP’s congestion control scheme helps alleviate congestion by backing off quickly.

By not adding retransmissions to a congested network, TCP’s congestion control scheme helps prevents congestion collapse.

TCP Segment (Message) FormatTCP Segment (Message) Format(fig 24.6)(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