Transcript PowerPoint - DePaul University

Internetworking Technologies
Transmission Control Protocol (TCP)
TDC365 Spring 2001
John Kristoff - DePaul University
1
IP review
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IP provides just enough 'connectedness'
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Global addressing
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Hop-by-hop routing
IP over everything
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Ethernet, ATM, X.25, SONET, etc.
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Lack of state in the network
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Unreliable packet (datagram) switching
TDC365 Spring 2001
John Kristoff - DePaul University
2
TCP key features
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Sequencing
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Byte-stream delivery
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Connection-oriented
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Reliability
TDC365 Spring 2001
John Kristoff - DePaul University
3
TCP feature summary
Provides a completely reliable (no data
duplication or loss), connection-oriented,
full-duplex stream transport service that
allows two application programs to form a
connection, send data in either direction
and then terminate the connection.
TDC365 Spring 2001
John Kristoff - DePaul University
4
Apparent contradiction
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IP offers best-effort (unreliable) delivery
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TCP uses IP
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TCP provides completely reliable transfer
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How is this possible?
TDC365 Spring 2001
John Kristoff - DePaul University
5
Achieving reliability
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Reliable connection startup
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Reliable data transfer
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Sender starts a timer
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Receiver sends ACK when data arrives
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Sender retransmits if timer expires before
ACK is returned
Reliable connection shutdown
TDC365 Spring 2001
John Kristoff - DePaul University
6
Reliability illustrated
TDC365 Spring 2001
John Kristoff - DePaul University
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How long to wait before
retransmitting?
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Time for ACK to return depends on:
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Distance between sender/receiver
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Network traffic conditions
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End system conditions
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Packets can be lost, damaged or fragmented
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Traffic conditions can change rapidly
TDC365 Spring 2001
John Kristoff - DePaul University
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Solving the retransmission
problem
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Keep running average of round trip time
(RTT) for each TCP connection
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Current average (estimate) determines
retransmission timer
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How does each RTT affect the average?
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This is known as adaptive retransmission
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Key to TCP's success
TDC365 Spring 2001
John Kristoff - DePaul University
9
Adaptive retranmission illustrated
TDC365 Spring 2001
John Kristoff - DePaul University
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Flow control
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Match sending rate to receiver rate
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TCP uses a sliding window
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Receiver advertises available buffer space
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Also known as the window
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Sender can transmit a full window size before
receiving an ACK
TDC365 Spring 2001
John Kristoff - DePaul University
11
Window advertisement
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Each ACK carries the current window size
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Called the window advertisement
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Can be zero (a closed window)
Interpretation of window advertisement:
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Receiver: I can accept X octets or less unless I
tell you otherwise
TDC365 Spring 2001
John Kristoff - DePaul University
12
Window advertisement illustrated
TDC365 Spring 2001
John Kristoff - DePaul University
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Another view: the sliding window
TDC365 Spring 2001
John Kristoff - DePaul University
14
Byte stream sequencing
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Each segment contains a sequence number
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Sequencing helps ensure in-order delivery
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TCP sequence numbers are fixed at 32 bits
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Is the byte stream of limited size (232 bytes)?
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Initial sequence numbers (ISN) are
exchanged at connection startup
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ACKs acknowledgement cumulative bytes
TDC365 Spring 2001
John Kristoff - DePaul University
15
TCP segment format
TDC365 Spring 2001
John Kristoff - DePaul University
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Application multiplexing
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Separation of functionality from IP
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In the beginning TCP and IP were one
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No unused bits to encode application process
Cannot use OS dependent quality
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Process ID
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Task number
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Job name
Semantics must work on all end systems
TDC365 Spring 2001
John Kristoff - DePaul University
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Port numbers
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Separation of functionality from IP
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In the beginning TCP and IP were one
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No unused bits to encode application process
Cannot use OS dependent quality
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Process ID
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Task number
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Job name
Semantics must work on all end systems
TDC365 Spring 2001
John Kristoff - DePaul University
18
TCP ports
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Each application assigned a unique integer
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Server
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Follows a standard (e.g. Well know ports)
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Uses a well known port number
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Usually uses lower port numbers
Client
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Obtains unused port from protocol software
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Usually uses high numbered ports
TDC365 Spring 2001
John Kristoff - DePaul University
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TCP connection startup
• Uses a three message exchange
• AKA 3-way handshake
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Necessary and sufficient for unambiguous
and reliable startup
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Sequence number exchange is primary goal
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Can also exchange other parameters
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e.g. maximum segment size
TDC365 Spring 2001
John Kristoff - DePaul University
20
3-way handshake illustrated
TDC365 Spring 2001
John Kristoff - DePaul University
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TCP connection shutdown
TDC365 Spring 2001
John Kristoff - DePaul University
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Congestion
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Flow control
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Congestion control
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Avoidance
TDC365 Spring 2001
John Kristoff - DePaul University
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Useful terms before proceding
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Maximum segment size (MSS)
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Window
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Retransmission
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Timer
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Delayed ACK
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Duplicate ACK
TDC365 Spring 2001
John Kristoff - DePaul University
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Congestion window
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Sender based flow control
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Rather than by a window advertisement
Sender uses min(cwnd, advertised window)
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This is the transmission window
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Infer network conditions and adjust
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Use timers, ACKs and network feedback
TDC365 Spring 2001
John Kristoff - DePaul University
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TCP segment retransmission
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TCP starts timer after sending a segment
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If ACK returns, reset timer
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If not, retransmit and set (timer = timer x 2)
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This is backoff
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Can't retransmit forever, error may occur
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Timer is the estimate of round trip time (RTT)
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Current, running average of RTT
TDC365 Spring 2001
John Kristoff - DePaul University
26
Round trip time (RTT)
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TCP measures RTT
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TCP uses estimated RTT to calculate timers
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If ACKs return quickly, timers are short
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If loss occurs, recovery is quicker
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The converse is also true
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RTT too high = throughput suffers
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RTT too low = unnecessary retransmits
TDC365 Spring 2001
John Kristoff - DePaul University
27
TCP slow start
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Initialize cwnd to 1 MSS
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Increase cwnd by 1
MSS for every ACK
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Not really that slow
TDC365 Spring 2001
John Kristoff - DePaul University
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TCP congestion avoidance
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When timer expires (loss?), slow down!
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Set ssthresh = (transmission window x ½)
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Set cwnd = 1
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Transmit min(cwnd, transmission window)
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Slow start until transmission window = sshtresh
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Thereafter, increase cwnd by 1/cwnd per ACK
TDC365 Spring 2001
John Kristoff - DePaul University
29
Congestion avoidance illustrated
TDC365 Spring 2001
John Kristoff - DePaul University
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TCP fast retransmit
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If 3 or more duplicate ACKs are received
before timer has expired
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Sender assumes loss
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Assumes out of order packets had enough time
to be reassembled
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Retransmit without waiting for timer to expire
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Enter TCP fast recovery
TDC365 Spring 2001
John Kristoff - DePaul University
31
TCP fast recovery
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Sender assumes data is still flowing
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Due to the reception of duplicate ACKs
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Loss was probably a temporary event
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Go into congestion avoidance, not slow start
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Just cut cwnd in half
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Resume additive increase (1/cwnd) per ACK
TDC365 Spring 2001
John Kristoff - DePaul University
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Other mechanisms
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Selective acknowledgements (SACK)
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Traffic shaping
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ACK pacing
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AQM (FIFO and RED and variants)
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ECN, ICMP source quench, back pressure
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Fair queueing
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Class/uality of service (CoS/QoS)
TDC365 Spring 2001
John Kristoff - DePaul University
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Congestion collapse
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When network load increases and less actual work
gets done due to packet drops
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Fragmented packets are especially bad
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As retransmissions increase, goodput drops
TDC365 Spring 2001
John Kristoff - DePaul University
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