Advanced Computer Networks: Lecture 31 - Dr. Amir Qayyum
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CS716
Advanced Computer Networks
By Dr. Amir Qayyum
1
Lecture No. 31
2
TCP Segment Header
• 16bit advertised window
– Space remaining in receive window
• 16bit checksum
– Uses IP checksum algorithm
– Computed on header, data, and pseudoheader
• 16bit urgent data pointer
– If URG = 1
– Index of last byte of urgent data in segment
3
TCP Segment Format
• Each connection identified with 4tuple:
– (SrcPort,SrcIPAddr,DsrPort,DstIPAddr)
• Sliding window + flow control
– ACK, SequenceNum, AdvertisedWindow
Data(SequenceNum)
Receiver
Sender
• Flags
Acknowledgment +
AdvertisedWindow
– SYN, FIN, RESET, PUSH, URG, ACK
4
TCP Options – Existing & Proposed
• Negotiate maximum segment size
(MSS)
– Each host suggests a value
– Minimum of two values is chosen
– Prevents IP fragmentation over
first/last hops
5
TCP Options – Existing & Proposed
• Packet timestamp
– Allows RTT calculation for retransmitted
packets
– Extends sequence number space for
identification of stray packets (packets arriving
very late)
• Negotiate advertised window granularity
– Allows larger windows
– Good for routes with large bandwidthdelay
6
products
TCP State Description
•
•
•
•
•
•
•
•
•
•
•
CLOSED
LISTEN
SYN_RCVD
SYN_SENT
ESTABLISHED
CLOSE_WAIT
LAST_ACK
FIN_WAIT_1
FIN_WAIT_2
CLOSING
TIME_WAIT
Disconnected
Waiting for incoming connection
Connection request received
Connection request sent
Ready for data transport
Connection closed by peer
Closed by peer, closed locally, await ACK
Connection closed locally
Closed locally and ACK’d
Closed by both sides “simultaneously”
Wait for network to discard related packets
7
State Transition Diagram
CLOSED
Active open/SYN
Passive open
Close
Close
LISTEN
SYN_RCVD
SYN/SYN + ACK
Send/SYN
SYN/SYN + ACK
ACK
Close/FIN
SYN + ACK/ACK
ESTABLISHED
Close/FIN
FIN/ACK
FIN_WAIT_1
ACK
FIN_WAIT_2
CLOSE_WAIT
AC FIN/ACK
K
+
FI
N
/A
CK
Close/FIN
CLOSING
ACK
FIN/ACK
SYN_SENT
TIME_WAIT
LAST_ACK
Timeout after
two segment
lifetimes
ACK
CLOSED
8
ThinkPairShare
• Describe the path taken
– By a server under normal conditions, and
– By a client under normal conditions,
– Assuming that the client closes the connection first
• Consider the TIME_WAIT state
– What purpose does this state serve ?
– Prove that at least one side of a connection enters this state
before returning to CLOSED
– Explain how both sides might enter this state
9
Sliding Window Implementation
• Sequence numbers are indices into byte stream
• ACK sequence number is actually next byte expected (as
opposed to last byte received)
• Receiver buffers contain
– Data ready for delivery to application until requested
– Outoforder data out to maximum buffer capacity
• Sender buffers contain
– Unacknowledged data
– Unsent data out to maximum buffer capacity
10
Sliding Window
application
• Sender side
LastByteAcked
Window
LastByteSent
max buffer size
TCP
LastByteWritten
time
– Green: sent and acknowledged
– Red: sent (or can be sent) but not acknowledged
– Blue: available but not within send window
11
Sliding Window
• Receiver side
NextByteRead
application
TCP
NextByteExpected
LastByteReceived
Advertised window
max buffer size
time
– Green: received and ready to be delivered
– Red: received and buffered
– Blue: received and discarded
12
Sliding Window Math
Sending application
Receiving application
TCP
TCP
LastByteWritten
LastByteAcked
LastByteRead
LastByteSent
NextByteExpected
• Sending side
– LastByteAcked < =
LastByteSent
– LastByteSent < =
LastByteWritten
– buffer bytes between
LastByteAcked and
LastByteWritten
LastByteRcvd
• Receiving side
– LastByteRead <
NextByteExpected
– NextByteExpected <
= LastByteRcvd +1
– buffer bytes between
NextByteRead and
LastByteRcvd 13
Flow vs Congestion Control
• Flow control prevents buffer overflow at receiver
(only source and destination are relevant)
• Congestion control addresses bandwidth
interactions between distinct packet flows
• TCP provides both
– Flow control based on advertised window
– Congestion control will be discussed later …
14
TCP Flow Control Issues
• Problem: slow receiver application
–
–
–
–
Advertised window goes to 0
Sender cannot send more data
Nondata packets used to update window
Receiver may not spontaneously generate, or update may be
lost
• Solution: smart sender/dumb receiver
– Sender periodically sends a 1byte segment, ignoring advertised
window of 0
– Eventually, window opens
– Sender learns of opening from next ACK of 1byte segment
15
