Tcp 6[1]

CS4254 Outline
Computer Network Architecture and •Transmission Control Protocol
Programming

Dr. Ayman A. Abdel-Hamid
Computer Science Department
Virginia Tech

Transmission Control Protocol (TCP)

TCP © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 1 TCP © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 2

Transport Layer 1/2 Transport Layer 2/2

Process-to-process delivery

Transport Layer Addressing Port Numbers
Addresses •Port numbers are 16-bit integers (0 65,535)
•Data link layer MAC address Servers use well know ports, 0-1023 are privileged
•Network layer IP address Clients use ephemeral (short-lived) ports
•Transport layer Port number (choose among multiple
processes running on destination host) •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

Socket Addressing Multiplexing and Demultiplexing
•Process-to-process delivery needs two identifiers Multiplexing
IP address and Port number Sender side may have
Combination of IP address and port number is called a several processes that
socket address (a socket is a communication endpoint) need to send packets
Client socket address uniquely identifies client process (albeit only 1 transport-
layer protocol)
Server socket address uniquely identifies server process
Demultiplexing
•Transport-layer protocol needs a pair of socket addresses
At receiver side, after
Client socket address error checking and
Server socket address header dropping,
For example, socket pair for a TCP connection is a 4-tuple transport-layer delivers
Local IP address, local port, and each message to
foreign IP address, foreign port appropriate process

Transmission Control Protocol 1/10 Transmission Control Protocol 2/10
•TCP must perform typical transport layer functions:
•Reliable
Segmentation breaks message into packets
End-to-end error control since IP is an unreliable Service requires ACK and performs retransmission
End-to-end flow control to avoid buffer overflow If ACK not received, retransmit and wait a longer time for
Multiplexing and demultiplexing sessions ACK. After a number of retransmissions, will give up
•TCP is [originally described in RFC 793, 1981] How long to wait for ACK? (dynamically compute RTT for
Reliable estimating how long to wait for ACKs, might be ms for LANs or
seconds for WANs)
Connection-oriented virtual circuit
Stream-oriented users exchange streams of data RTT = α * old RTT + (1- α)* new RTT where α usually 90%
Full duplex concurrent transfers can take place in both Most common, Retransmission time = 2* RTT
directions Acknowledgments can be “piggy-backed” on reverse direction
Buffered TCP accepts data and transmits when appropriate data packets or sent as separate packets
(can be overridden with “push”)

•Sequence Numbers •Sending and Receiving buffers
Associated with every byte that it sends Senders and receivers may not produce and consume data at
To detect packet loss, reordering and duplicate removal same speed
2 buffers for each direction (sending and receiving buffer)
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

•Flow Control
•TCP uses a sliding window mechanism for flow control
Tell peer exactly how many bytes it is willing to accept
•Sender maintains 3 pointers for each connection (advertised window sender can not overflow receiver buffer)
Pointer to bytes sent and acknowledged Sender window includes bytes sent but not acknowledged
Pointer to bytes sent, but not yet acknowledged Receiver window (number of empty locations in receiver buffer)
Receiver advertises window size in ACKs
Sender window includes bytes sent but not acknowledged
Sender window <= receiver window (flow control)
Pointer to bytes that cannot yet be sent
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)

•Congestion Control
•Error Control
Mechanisms for detecting corrupted segments, lost segments, TCP assumes the cause of a lost segment is due to congestion
out-of-order segments, and duplicated segments in the network
Tools: checksum (corruption), ACK, and time-out (one time- If the cause of the lost segment is congestion, retransmission of
out counter per segment) the segment does not remove the problem, it actually aggravates
Lost segment or corrupted segment are the same situation: it
segment will be retransmitted after time-out (no NACK in
The network needs to tell the sender to slow down (affects the
TCP)
sender window size in TCP)
Duplicate segment (destination discards)
Actual window size = Min (receiver window size, congestion
Out-of-order segment (destination does not acknowledge,
window size)
until it receives all segments that precede it)
Lost ACK (loss of an ACK is irrelevant, since ACK The congestion window is flow control imposed by the sender
mechanism is cumulative) The advertised window is flow control imposed by the receiver

•Congestion Control
•Full-Duplex
44
send and receive data in both directions.
congestion window size in Kbytes

40
36 Keep sequence numbers and window sizes for each direction
32
28
of data flow
24
Series1
20
16
12
8
4
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Transmission number


TCP Connection Establishment TCP Options
Each SYN can contain TCP options
•MSS Option
maximum segment the maximum amount of data it is
Passive open willing to accept in each TCP segment
Sending TCP uses receiver’s MSS as its MSS
•Window Scale Option
maximum window is 65,535 bytes (corresponding field in
TCP header occupies 16 bits)
it can be scaled (left-shifted) by 0-14 bits providing a
maximum of 65,535 * 214 bytes (one gigabyte)
Option needed for high-speed connections or long delay paths
SYN: Synchronize
In this case, the other side must send the option with its
ACK: Acknowledge
SYN

TCP MSS and output TCP Connection Termination
•TCP MSS is = (interface MTU – fixed sizes of IP and TCP headers (20 bytes))
MSS on an Ethernet (IPv4)= 1460 bytes (1500 (why?) - 40)
•Successful return from write implies you can reuse application buffer

•FIN: Finish
•Step 1 can be sent with data
•Steps 2 and 3 can be combined into 1 segment

State Transition Diagram 1/4 State Transition Diagram 2/4

Typical TCP Typical
states visited TCP
by a TCP states
client visited by
a TCP
server


State Transition Diagram 3/4 State Transition Diagram 4/4
State Description

CLOSED There is no connection.
LISTEN The server is waiting for calls from the client.
SYN-SENT A connection request is sent; waiting for acknowledgment.
SYN-RCVD A connection request is received.
ESTABLISHED Connection is established.
The application has requested the closing of the
FIN-WAIT-1
connection.
FIN-WAIT-2 The other side has accepted the closing of the connection.
TIME-WAIT Waiting for retransmitted segments to die.
CLOSE-WAIT The server is waiting for the application to close.
LAST-ACK The server is waiting for the last acknowledgment.

Can use netstat command to see some TCP states

Packet Exchange TIME_WAIT State
•The end that performs the active close goes through this state
•Duration spent in this state is twice the maximum segment life (2
Piggybacking
feature MSL)
MSL: maximum amount of time any given IP can live in the network
•Every TCP implementation must choose a value for MSL
Recommended value is 2 minutes (traditionally used 30 seconds)
•TIME_WAIT state motives
allow old duplicate segments to expire in the network (relate to connection
incarnation)
TCP will not initiate a new incarnation of a connection that is in
TIME_WAIT state
Send 1-segment
request and receive 1- Implement TCP’s full-duplex connection termination reliably
segment reply The end that performs the active close might have to resend the final
TCP © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 27 TCP
ACK © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 28

TCP Segment Format TCP Header Fields 1/2
•Source Port and Destination Port
Identify processes at ends of the connection
•Control bits
URG urgent (urgent data present)
ACK acknowledgment
PSH push request
Inform receiver TCP to send data to application ASAP
RST reset the connection
SYN synchronize sequence numbers
FIN sender at end of byte stream


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
TCP © Dr. Ayman Abdel-Hamid, CS4254 Spring 2006 31

Tcp 6[1]

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Tcp 6[1]