COMP 361, Fall 2000

1
TCP/UPD Protocols
2
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.
3
Relationship Between TCP and Other Protocols

• TCP on one computer uses IP to communicate with
  TCP on another computer

4
Achieving Reliability

• Reliable data transmission
• Reliable connection setup
• Reliable connection shutdown

5
Reliable Data Transmission

• Positive Acknowledgement
• Receiver returns short message when data arrives
• Called an acknowledgement
• Retransmission
• Sender starts timer whenever message is
  transmitted
• If timer expires before acknowledgement arrives,
  sender retransmits message
• Use Go-BackN or Selective Repeat protocols
• Weve seen this before!

6
Retransmission Illustrated
7
How Long Should TCP Wait Before Retransmitting?

• Time for ACK to arrive depends on
• Distance to destination
• Current traffic conditions
• Multiple connections can be open simultaneously
• Traffic conditions change rapidly

8
Important Point
The delay required for data to reach a
destination and an acknowledgement to return
depends on traffic in the internet as well as the
distance to the destination. Since it allows
multiple application programs to communicate with
multiple destinations concurrently, TCP must
handle a variety of delays that can change
rapidly.
9
Solving the Retransmission Problem

• Keep estimate of round trip time on each
  connection
• Use current estimate to set retransmission timer
• Known as adaptive retransmission
• Key to TCPs success

10
Adaptive Retransmission

• Timeout depends on current round-trip estimate

11
Round-Trip Time Measurements

• Retransmission timer is set to a Retransmission
  Timeout (RTO) value
• RTO is calculated based on the RTT measurements
• EstimateRTT_n1 a RTT (1 a)
  EstimateRTT_n
• The new value of EstimateRTT_n1 is weighted
  combination of the previous EstimateRTT_n and the
  real measured value RTT.
• For example, (it will put more weight on the
  actual measurement very typical)
• EstimateRTT_n1 7/8 RTT 1/8 EstimateRTT_n

12
Round-Trip Time Measurements

• Retransmission timer, RTO, should be set so that
  it expires only on rare occasions.
• It is natural to set the RTO to be equal to
  EstimateRTT_n1 plus some margin
• This margin should be large when there is a lot
  of fluctuation in the EstimateRTT values should
  be small when there is little fluctuation.
• TCP uses the following formula
• RTO_n1 EstimateRTT_n1 4RTTvariance_n1

13
Round-Trip Time Measurements

• The RTTvariance is an estimate of how much RTT
  typically deviates from EstimateRTT
• It is calculated as follows
• RTTvariance_n1 b (RTT EstimateRTT_n1)
  (1-b) RTTvariance_n
• TCP uses b 0.25

14
Round-Trip Time Measurements
15
Karn's Algorithm

• If an ACK for a retransmitted segment is
  received, the sender cannot tell if the ACK
  belongs to the original or the retransmission.

• Karn' Algorithm
• Don't update EstimateRTT on any segments that
  have been retransmitted.
• Each time when TCP retransmits, it sets
• RTO_n1 max ( 2RTO_n, 64) (exponential backoff)

16
Connection setup/release
17
TCP connection management

• TCP is a connection oriented protocol
• first establish a connection
• then send data
• connection establishment three-way handshake

18
TCP segment structure
A
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Startup and Shutdown

• Connection Startup
• Must be reliable
• Connection Shutdown
• Must be graceful
• Difficult

20
Why Startup/Shutdown is Difficult

• Segments can be
• lost
• duplicated
• delayed
• delivered out of order
• either side can crash
• either side can reboot
• Need to avoid duplicate shutdown message from
  affecting later connection

21
TCPs Startup Solution

• Use three-message exchange
• Known as the 3-way handshake
• Necessary and sufficient for unambiguous,
  reliable startup
• SYN messages used for connection establishment

22
3-Way Handshake Illustrated
23
TCPs Shutdown Illustrated
24
Sequence numbers

• Segments are labeled with a sequence number
• Protects from out-of-order delivery
• 32-bit number
• Initial Sequence Numbers (ISNs) must be exchanged
  at TCP connection establishment

25
More Complete Illustration of the 3-Way Handshake
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Simple Solution (which has problems)

• CR (SeqNo x)
• Connection Request, A wants to start with SeqNo
  x
• ACK (SeqNo y)
• Acknowledge request, B wants to start with SeqNo
  y
• DATA (SeqNo x)
• Data transmission with SeqNo x

27
Problems with Two-Way Handshake

• B responds to CR(SeqNo z), an old duplicate
  connection requests from A
• In the shown scenario, A believes that the ACK is
  for the connection request CR(SeqNo x)

• Result A starts to send data with Sequence x. B
  will throw the data away since it expects SeqNo
  z

28
Connection Termination

• A connection release should involve both sides of
  the connection (otherwise data is lost)
• Here B should wait after Disconnection Request
  (DR) is sent until all data has arrived

29
Connection Termination in 4 steps

• An elegant way to terminate connections is to
  have each end shut down independently
  (half-close)
• If one end wants to shut down, it sends a DR
  message
• Four steps involved
• (1) A sends a DR to B (active close)
• (2) B ACKs the DR, (at this time B can still
  send data to A)
• (3) and B sends a DR to A (passive close)
• (4) A ACKs the DR

30
Connection Termination in 4 steps

• To account for packet losses, a timer is needed
  to limit the waiting time of a side

31
Connection Termination

• B disconnects after a timeout if it does not
  receive an ACK for its DR
• Note B knows that both sides have sent a DR
  packet hence B knows that no more data is sent

32
Connection Termination

• A has a retransmission timer for sending the
  first DR

33
TCP Flow/Congestion Control
34
TCP flow/congestion control
35
TCP Flow Control

• receiver explicitly informs sender of
  (dynamically changing) amount of free buffer
  space
• RcvWindow field in TCP segment
• sender keeps the amount of transmitted, unACKed
  data less than most recently received RcvWindow

sender wont overrun receivers buffers
by transmitting too much, too fast
RcvBuffer size or TCP Receive Buffer RcvWindow
amount of spare room in Buffer
receiver buffering
36
Window Advertisement Decoupling of Ack and flow
control

• Each acknowledgement carries new window
  information
• Called window advertisement
• Can be zero (called closed window)
• Interpretation I have received up through X and
  can take Y more bytes (credit-based flow control)

37
Window Advertisement
38
Congestion
39
TCP Congestion Control

• also makes use of window congestion window
• in case of congestion decrease window size
• if not increase window size
• rapidly upto some threshold
• then more slowly

40
How does TCP handle congestion?

• congestion window
• sender uses min (congWin, recvrWin)
• Mechanism
• exponential growth (slow start)
• threshold
• linear growth

41
TCP Slowstart
Host A
Host B
one segment
RTT
initialize Congwin 1 for (each segment ACKed)
Congwin until (loss event OR
CongWin gt threshold)
two segments
four segments

• exponential increase (per RTT) in window size
  (not so slow!)

42
TCP Congestion Avoidance
Congestion avoidance
/ slowstart is over / / Congwin gt
threshold / Until (loss event) every w
segments ACKed Congwin threshold
Congwin/2 Congwin 1 perform slowstart
43
TCP Congestion Avoidance