Transmission control protocol _

TCP 1
Computer Network
Transmission Control Protocol (TCP)

TCP 2
Transport Layer Addressing
Addresses
Transport layer adds Port number (choose among multiple
processes running on destination host)

TCP 3
Port Numbers
•Port numbers are 16-bit integers (0  65,535)
Servers use well know ports, 0-1023 are privileged
Clients use ephemeral (short-lived) ports
•Internet Assigned Numbers Authority (IANA) maintains a list of
port number assignment
Well-known ports (0-1023)  controlled and assigned by
IANA
Registered ports (1024-49151)  IANA registers and lists
use of ports as a convenience (49151 is ¾ of 65536)
Dynamic ports (49152-65535)  ephemeral ports
For well-known port numbers, see /etc/services on a UNIX or
Linux machine

TCP 4
Socket Addressing
•Process-to-process delivery needs two identifiers
IP address and Port number
Combination of IP address and port number is called a
socket address (a socket is a communication endpoint)
Client socket address uniquely identifies client process
Server socket address uniquely identifies server process
•Transport-layer protocol needs a pair of socket addresses
Client socket address
Server socket address
For example, socket pair for a TCP connection is a 4-tuple
Local IP address, local port, and
foreign IP address, foreign port

TCP 5
Multiplexing and Demultiplexing
Multiplexing
Sender side may have
several processes that
need to send packets (but
only 1 transport-layer
protocol)
Demultiplexing
At receiver side, after
error checking and
header dropping,
transport-layer delivers
each message to
appropriate process

TCP 6
Transmission Control Protocol 1/10
•TCP must perform typical transport layer functions:
Segmentation  breaks message into packets
End-to-end error control since IP is an unreliable Service
End-to-end flow control  to avoid buffer overflow
Multiplexing and demultiplexing sessions
•TCP is [originally described in RFC 793, 1981]
Reliable
Connection-oriented  virtual circuit
Stream-oriented  users exchange streams of data
Full duplex  concurrent transfers can take place in both
directions
Buffered  TCP accepts data and transmits when appropriate
(can be overridden with “push”)

TCP 7
•Reliable
requires ACK and performs retransmission
If ACK not received, retransmit and wait a longer time for
ACK. After a number of retransmissions, will give up
How long to wait for ACK?
Most common, Retransmission time = 2* RTT

TCP 8
•Sequence Numbers
Associated with every byte that it sends
To detect packet loss, reordering and duplicate removal
Two fields are used sequence number and acknowledgment
number. Both refer to byte number and not segment number
Sequence number for each segment is the number of the first
byte carried in that segment
The ACK number denotes the number of the next byte that
this party expects to receive (cumulative)
If an ACK number is 5643  received all bytes from beginning up
to 5642
This acknowledges all previous bytes as received error-free

TCP 9
•Sending and Receiving buffers
Senders and receivers may not produce and consume data at
same speed
2 buffers for each direction (sending and receiving buffer)

TCP 10
•TCP uses a sliding window mechanism for flow control
•Sender maintains 3 pointers for each connection
Pointer to bytes sent and acknowledged
Pointer to bytes sent, but not yet acknowledged
Sender window includes bytes sent but not acknowledged
Pointer to bytes that cannot yet be sent

TCP 11
•Flow Control
Tell peer exactly how many bytes it is willing to accept
(advertised window  sender can not overflow receiver buffer)
Sender window includes bytes sent but not acknowledged
Receiver window (number of empty locations in receiver buffer)
Receiver advertises window size in ACKs
Sender window <= receiver window (flow control)
Sliding sender window (without a change in receiver’s advertised
window)
Expanding sender window (receiving process consumes data faster
than it receives  receiver window size increases)
Shrinking sender window (receiving process consumes data more
slowly than it receives  receiver window size reduces)
Closing sender window (receiver advertises a window of zero)

TCP 12
•Congestion Control
TCP assumes the cause of a lost segment is due to congestion
in the network
If the cause of the lost segment is congestion, retransmission of
the segment does not remove the problem, it actually aggravates
it
The network needs to tell the sender to slow down (affects the
sender window size in TCP)
Actual window size = Min (receiver window size, congestion
window size)
The congestion window is flow control imposed by the sender
The advertised window is flow control imposed by the receiver

TCP 13
•Error Control
Mechanisms for detecting corrupted segments, lost segments,
out-of-order segments, and duplicated segments
Tools: checksum (corruption), ACK, and time-out (one time-
out counter per segment)
Lost segment or corrupted segment are the same situation:
segment will be retransmitted after time-out (no NACK in
TCP)
Duplicate segment (destination discards)
Out-of-order segment (destination does not acknowledge,
until it receives all segments that precede it)
Lost ACK (loss of an ACK is irrelevant, since ACK
mechanism is cumulative)

Reserved TCP Port Numbers
Port number: Service:
0 Reserved
7 Echo
17 Quote of the day
21 FTP
23 TELNET
25 SMTP
37 Time
79 Finger
80 HTTP
119 NNTP

It is connection oriented protocol so follows
following
• Provides flow and error control
• receiver can immediately handle any packet it receives
• receiver can never be overwhelmed with incoming packets.
• Three way handshaking : connection establishment , data
transfer , connection termination
Transmission control protocol Process
08/31/24 15
Dr. Prasanna Singh CN Unit-4
CSBS

Transmission control protocol _

Establishing a TCP Connection
• The 3-way handshake
• Guarantee that both sides are ready for connection
• Allows both sides to agree on initial sequence numbers
Receive data
ACK=
x+1,seq=y
Site 1 Network Site 2
Send data
with seqno=x
Send
ACK= y+1
Receive ACK
Receive ACK

•How much data should be sent by sender to the receiver
•So that Receiver can process it …. How much data should be sent by
sender without getting acknowledgement
Data Transfer( Pushing or pulling)
08/31/24 18
CSBS

Data Transfer at the transport layer
08/31/24 19
CSBS

Closing a TCP Connection
• Applications should close a connection when they have no
more data to transmit
• Connection can be closed in either one or both directions
• Site 1 finishes transmitting data and waits for ACK from site 2
• Site 1 transmits a segment with the FIN bit set
• Site 2 acknowledges the FIN segment
• Site 2 notifies the application that no more data is coming
• Data can still be transmitted from site 2 to site 1
• Site 1 will still receive and acknowledge data from site 2
• Eventually, site 2 will finish transmitting and close its connection
• Both endpoints delete record of the connection

Closing a TCP Connection (cont)
Site 1 Network Site 2
Send FIN
seq=x
Receive FIN
Send ACK
x+1
Receive ACK (inform application)
(app closes
connection)

TCP 23
TCP Header Fields 1/2
•Source Port and Destination Port
Identify processes at ends of the connection
•Control bits

TCP 24
TCP Header Fields 2/2
•Sequence Number: position of the data in the sender’s byte stream
•Acknowledgment Number: position of the byte that the source
expects to receive next (valid if ACK bit set)
•Header Length: header size in 32-bit units. Value ranges from [5-15]
•Window: advertised window size in bytes
•Urgent
defines end of urgent data (or “out-of-band”) data and start of normal data
Added to sequence number (valid only if URG bit is set)
•Checksum: 16-bit CRC (Cyclic Redundancy Check) over header
and data
•Options: up to 40 bytes of options

Transmission control protocol _

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