08 - COMS 525 Internet Protocols and TCPIP - TCP 1.pptx
1. 1
Transmission Control Protocol
K. PALANIVEL
Systems Analyst, Computer Centre
Pondicherry University, Puducherry – 605014.
LECTURE 8
COMS 525: TCPIP
COURSE
TOPIC
Part - I
2. Overview
TCP = Transmission Control Protocol
Connection-oriented protocol
Provides a reliable unicast end-to-end byte stream
over an unreliable internetwork.
TCP
IP Internetwork
Byte
Stream
Byte
Stream
TCP
3. Connection-Oriented
Before any data transfer, TCP establishes a connection:
One TCP entity is waiting for a connection (“server”)
The other TCP entity (“client”) contacts the server
The actual procedure for setting up connections is more complex.
Each connection is full duplex
CLIENT SERVER
waiting for
connection
request
Data Transer
4. Reliable
• Byte stream is broken up into chunks which are called
seg-ments
• Receiver sends acknowledgements (ACKs) for
segments
• TCP maintains a timer. If an ACK is not received in
time, the segment is retransmitted
• Detecting errors
• TCP has checksums for header and data. Segments
with invalid checksums are discarded
• Each byte that is transmitted has a sequence number
5. Byte Stream Service
To the lower layers, TCP handles data in blocks, the segments.
To the higher layers TCP handles data as a sequence of bytes
and does not identify boundaries between bytes
So: Higher layers do not know about the beginning and end of
segments !
TCP
Application
1. write 100 bytes
2. write 20 bytes
queue of
bytes to be
transmitted TCP
queue of
bytes that
have been
received
Application
1. read 40 bytes
2. read 40 bytes
3. read 40 bytes
Segments
6. TCP Format
IP header TCP header TCP data
Sequence number (32 bits)
DATA
20 bytes 20 bytes
0 15 16 31
Source Port Number Destination Port Number
Acknowledgement number (32 bits)
window size
header
length
0 Flags
Options (if any)
TCP checksum urgent pointer
20
bytes
• TCP segments have a 20 byte header with >= 0 bytes of data.
7. TCP header fields
Port Number:
◦ A port number identifies the endpoint of a connection.
◦ A pair <IP address, port number> identifies one endpoint
of a connection.
◦ Two pairs <client IP address, server port number> and
<server IP address, server port number> identify a TCP
connection.
TCP
IP
Applications
23 104
80
Ports:
TCP
IP
Applications
7 16
80 Ports:
8. TCP header fields
Sequence Number (SeqNo):
Sequence number is 32 bits long.
So the range of SeqNo is
0 <= SeqNo <= 232
-1 4.3 Gbyte
Each sequence number identifies a byte in the byte
stream
Initial Sequence Number (ISN) of a connection is set
during connection establishment for ISN ?
9. TCP header fields
Acknowledgement Number (AckNo):
Acknowledgements are piggybacked, i.e., a segment
from A -> B can contain an acknowledgement for a data
sent in the B -> A direction
Q: Why is piggA hosts uses the AckNo field to send
acknowledgements. (If a host sends an AckNo in a
segment it sets the “ACK flag”)
The AckNo contains the next SeqNo that a hosts wants to
receive .
Example: The acknowledgement for a segment with
sequence numbers 0-1500 is AckNo=1501y backing good ?
10. TCP header fields
Acknowledge Number (cont’d)
TCP uses the sliding window flow protocol to regulate
the flow of traffic from sender to receiver
TCP uses the following variation of sliding window:
no NACKs (Negative ACKnowledgement)
only cumulative ACKs
Example:
Assume: Sender sends two segments with “1..1500” and
“1501..3000”, but receiver only gets the second
segment.
In this case, the receiver cannot acknowledge the second
packet. It can only send AckNo=1
11. TCP header fields
• Header Length ( 4bits):
– Length of header in 32-bit words
– Note that TCP header has variable length (with
minimum 20 bytes)
12. TCP header fields
Flag bits:
◦ URG: Urgent pointer is valid
If the bit is set, the following bytes contain an urgent
message in the range:
SeqNo <= urgent message <= SeqNo+urgent pointer
◦ ACK: Acknowledgement Number is valid
◦ PSH: PUSH Flag
Notification from sender to the receiver that the receiver
should pass all data that it has to the application.
Normally set by sender when the sender’s buffer is empty
13. TCP header fields
Flag bits:
◦ RST: Reset the connection
The flag causes the receiver to reset the connection
Receiver of a RST terminates the connection and
indicates higher layer application about the reset
◦ SYN: Synchronize sequence numbers
Sent in the first packet when initiating a connection
◦ FIN: Sender is finished with sending
Used for closing a connection
Both sides of a connection must send a FIN
14. TCP Header Fields
Window Size:
Each side of the connection advertises the window
size
Window size is the maximum number of bytes that a
receiver can accept.
Maximum window size is 216
-1= 65535 bytes
TCP Checksum:
TCP checksum covers over both TCP header and TCP
data (also covers some parts of the IP header)
Urgent Pointer:
Only valid if URG flag is set
16. TCP Header Fields
Options:
NOP is used to pad TCP header to multiples of 4 bytes
Maximum Segment Size
Window Scale Options
Increases the TCP window from 16 to 32 bits, I.e., the
window size is interpreted differently: What is the different
interpretation ?
This option can only be used in the SYN segment (first
segment) during connection establishment time
Timestamp Option
Can be used for roundtrip measurements
17. Connection Management in TCP
• Opening a TCP Connection
• Closing a TCP Connection
• Special Scenarios
• State Diagram
18. TCP Connection Establishment
TCP uses a three-way handshake to open a connection:
(1) ACTIVE OPEN: Client sends a segment with
SYN bit set *
port number of client
initial sequence number (ISN) of client
(2) PASSIVE OPEN: Server responds with a segment with
SYN bit set *
initial sequence number of server
ACK for ISN of client
(3) Client acknowledges by sending a segment with:
ACK ISN of server (* counts as one byte)
20. TCP Connection Termination
Each end of the data flow must be shut down
independently (“half-close”)
If one end is done it sends a FIN segment. This means
that no more data will be sent
Four steps involved:
(1) X sends a FIN to Y (active close)
(2) Y ACKs the FIN,
(at this time: Y can still send data to X)
(3) and Y sends a FIN to X (passive close)
(4) X ACKs the FIN.
21. TCP States
State Description
CLOSED No connection is active or pending
LISTEN The server is waiting for an incoming call
SYN RCVD A connection request has arrived; wait for Ack
SYN SENT The client has started to open a connection
ESTABLISHED Normal data transfer state
FIN WAIT 1 Client has said it is finished
FIN WAIT 2 Server has agreed to release
TIMED WAIT Wait for pending packets (“2MSL wait state”)
CLOSING Both Sides have tried to close simultanesously
CLOSE WAIT Server has initiated a release
LAST ACK Wait for pending packets
22. TCP States in “Normal” Connection Lifetime
SYN (SeqNo = x)
SYN (SeqNo = y, AckNo = x + 1 )
(AckNo = y + 1 )
SYN_SENT
(active open)
SYN_RCVD
ESTABLISHED
ESTABLISHED
FIN_WAIT_1
(active close)
LISTEN
(passive open)
FIN (SeqNo = m)
CLOSE_WAIT
(passive close)
(AckNo = m+ 1 )
FIN (SeqNo = n )
(AckNo = n+1)
LAST_ACK
FIN_WAIT_2
TIME_WAIT
CLOSED
23. TCP State Transition Diagram: Opening A Connection
CLOSED
LISTEN
SYN RCVD SYN SENT
ESTABLISHED
active open
send: SYN
recv: SYN, ACK
send: ACK
recv: SYN
send: SYN, ACK
recvd: ACK
send: . / .
recv:
RST
Application sends data
send: SYN
simultaneous open
recv: SYN
send: SYN, ACK
close or
timeout
passive open
send: . / .
recvd: FIN send: FIN
send:
FIN
24. TCP State Transition Diagram: Closing A Connection
FIN_WAIT_1
FIN_WAIT_2
ESTABLISHED
recv: FIN
send: ACK
recv: ACK
send: . / .
recvd: ACK
send: . / .
recv:
FIN, ACK
send: ACK
active close
send: FIN
TIME_WAIT
CLOSING
recv: FIN
send: ACK
CLOSED
Timeout
(2 MSL)
CLOSE_WAIT
LAST_ACK
passive close
recv: FIN
send: ACK
application
closes
send: FIN
recv: ACK
send: . / .
25. 2MSL(Maximum Segment Lifetime) Wait State
2MSL Wait State = TIME_WAIT
When TCP does an active close, and sends the final ACK,
the connection must stay in in the TIME_WAIT state for
twice the maximum segment lifetime.
2MSL= 2 * Maximum Segment Lifetime
Why?
TCP is given a chance to resent the final ACK. (Server
will timeout after sending the FIN segment and resend the
FIN)
The MSL is set to 2 minutes or 1 minute or 30 seconds.
26. Resetting Connections
• Resetting connections is done by setting the RST flag
• When is the RST flag set?
– Connection request arrives and no server process is
waiting on the destination port
– Abort (Terminate) a connection
Causes the receiver to throw away buffered data.
Receiver does not acknowledge the RST segment
