Transport Layer
TCP and UDP
IS250
Spring 2010
chuang@ischool.berkeley.edu
Transport Layer Functions
1.
2.
3.
4.
5.
Addressing (ports)
Data integrity (error detection)
Reliable data transport
Flow control
Congestion control
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TCP Transmission Rate
 Q: Why limit ourselves to send only four packets at a time?
 Q: how much data can be sent before ACK received?
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Flow Control
Queue
Producer
Stream
of messages
Consumer
Flow control
(“Slow down please!”)
Note: We don’t want packet to travel the entire
network only to be dropped at the destination.
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4
Flow Control: Two Approaches
 Stop-and-go
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 Sliding Window
5
Sliding Window
 Recipient explicitly
requests lower send rate
by specifying window size
(or MaxUnackedPackets)
 Source stops sending
when number of
unacknowledged data
equal to window size
window_size
acknowledged
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sent
can be sent outside window
6
TCP Header: Flow Control
0
16
TCP Header
Source Port Number (16)
31
Destination Port Number (16)
Sequence Number (32)
Acknowledgement Number (32)
Hdr Len
Reserved (6) Flags (6)
(4)
TCP Checksum (16)
Window Size (16)
Urgent Pointer (16)
Options (if any)
Padding
Data
…
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TCP Flow Control
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Throughput as Function of
Window Size
Sender
Receiver
Time
Throughput =
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Window Size
Roundtrip Time
9
Transport Layer Functions
1.
2.
3.
4.
5.
Addressing (ports)
Data integrity (error detection)
Reliable data transport
Flow control
Congestion control
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10
Network Congestion
 If link is congested
- Router queue fills up
- Drops packets
 Source does not receive ACK
- Resends packets
- Makes congestion worse
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TCP Congestion Control
 Use packet drop as indicator of congestion
 Do not send all data to receiver at once
 Voluntary source-imposed policy (RFC 2581)
-
slow start (SS)
congestion avoidance (CA)
fast retransmission
fast recovery
 TCP Tahoe: SS + CA + fast retransmission
 TCP Reno: all four
 Other variants: TCP SACK, TCP Vegas, TCP
Westwood, …
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TCP Congestion + Flow Control
window_size
acknowledged
sent
can be sent outside window
 Control transmission rate by setting window size
 Window size set to be smaller of:
- rwnd: receiver window (flow control)
- cwnd: congestion window (congestion control)
win = min(rwnd, cwnd)
 rwnd set by receiver
 Question: how does sender set cwnd?
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Congestion Window Size
cwnd
Congestion
Avoidance
Slow Start
Time
 TCP congestion control is an algorithm for
sender to adaptively adjust window size
 At steady state, cwnd oscillates around the
optimal window size
14
Slow Start
 Whenever starting traffic on a new connection,
or whenever increasing traffic after congestion
was experienced:
- Set cwnd =1 (one segment)
- Each time a segment is acknowledged increment
cwnd by one (cwnd++).
 Does Slow Start increment slowly? Not really.
In fact, the increase of cwnd is exponential
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Slow Start Example
 The congestion
window size grows
very rapidly
cwnd = 1
segment 1
ACK for segmen
cwnd = 2
segment 2
segment 3
ACK for segmen
 TCP slows down
the increase of
cwnd when
cwnd >=
ssthresh
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cwnd = 4
t1
ts 2 + 3
segment 4
segment 5
segment 6
segment 7
ACK for segmen
ts 4+5+6+7
cwnd = 8
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Congestion Avoidance
 Slows down “Slow Start”
- If cwnd > ssthresh then
each time a segment is acknowledged increment
cwnd by 1/cwnd (cwnd += 1/cwnd).
 So cwnd is increased by one only if all
segments have been acknowledged.
 (We will learn later how to set ssthresh)
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Slow Start/Congestion
Avoidance Example
 Assume that ssthresh = 8
14
cw n d = 1
cw n d = 2
cw n d = 4
10
cw n d = 8
8
6
ssthresh
4
2
cw n d = 9
6
t=
4
t=
2
t=
0
0
t=
Cwnd (in segments)
12
Roundtrip times
cw n d = 1 0
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TCP Congestion Control Pseudocode
Initially:
cwnd = 1;
ssthresh = infinite;
New ack received:
if (cwnd < ssthresh)
/* Slow Start*/
cwnd = cwnd + 1;
else
/*Congestion Avoidance*/
cwnd = cwnd + 1/cwnd;
Timeout:
/* Multiplicative decrease */
ssthresh = 0.5 * win;
cwnd = 1;
win = min(cwnd, rwnd);
while (next < unack + win)
transmit next packet;
SEQ #
unack
next
win
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The big picture
cwnd
Timeout
Congestion
Avoidance
Slow Start
1
Time
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Cumulative and Duplicate
ACKs
 TCP uses
cumulative ACK
 ACK N means all
bytes up to N-1
have been received
segment 1
cw n d = 1
ACK 2
cw n d = 2
t
ACK for segmen
1
segment 2
segment 3
ACK 4
cw n d = 4
ACK 3
segment 4
segment 5
 Duplicate ACKs may
be due to
- packets
reordering
- lost packet
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segment 6
segment 7
cwnd = 2
cw n d = 8
ACK 4
ACK 4
ACK 4
ts 4+5+6+7
ACK for segmen
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Fast Retransmit/Fast
Recovery
cwnd
Congestion
Avoidance
Slow Start
1
 Retransmit after 3 duplicated ACKs
Time
- Don’t want for timeout
 No need to slow start again
- halve cwnd
 At steady state, cwnd oscillates around the
optimal window size.
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TCP Congestion Control
Shortcomings
 “Fairness criterion”
- Is “equal division” of resources always desirable?
 Estimating congestion by retransmission is
flawed for wireless links
 Depends on accurate implementation -cheating possible
 Application can avoid congestion control by
using UDP
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