Review%20of%20Previous%20Lecture

1
Review of Previous Lecture

• Transport-layer services
• Multiplexing and demultiplexing
• Connectionless transport UDP

2
Outline

• Reliable transfer protocols
• rdt1.0 reliable transfer over a reliable channel
• rdt2.0 channel with bit errors
• rdt2.1 sender, handles garbled ACK/NAKs
• rdt2.2 a NAK-free protocol
• rdt3.0 channels with errors and loss
• Pipelined protocols
• Go-back-N
• Selective repeat
• Connection-oriented transport TCP
• Overview and segment structure

3
Principles of Reliable data transfer

• important in app., transport, link layers
• top-10 list of important networking topics!

• characteristics of unreliable channel will
  determine complexity of reliable data transfer
  protocol (rdt)

4
Principles of Reliable data transfer

• important in app., transport, link layers
• top-10 list of important networking topics!

• characteristics of unreliable channel will
  determine complexity of reliable data transfer
  protocol (rdt)

5
Principles of Reliable data transfer

• important in app., transport, link layers
• top-10 list of important networking topics!

• characteristics of unreliable channel will
  determine complexity of reliable data transfer
  protocol (rdt)

6
Reliable data transfer getting started
send side
receive side
7
Reliable data transfer getting started

• Well
• incrementally develop sender, receiver sides of
  reliable data transfer protocol (rdt)
• consider only unidirectional data transfer
• but control info will flow on both directions!
• use finite state machines (FSM) to specify
  sender, receiver

event causing state transition
actions taken on state transition
state when in this state next state uniquely
determined by next event
8
Rdt1.0 reliable transfer over a reliable channel

• underlying channel perfectly reliable
• no bit errors
• no loss of packets
• separate FSMs for sender, receiver
• sender sends data into underlying channel
• receiver read data from underlying channel

rdt_send(data)
rdt_rcv(packet)
Wait for call from below
Wait for call from above
extract (packet,data) deliver_data(data)
packet make_pkt(data) udt_send(packet)
sender
receiver
9
Rdt2.0 channel with bit errors

• underlying channel may flip bits in packet
• recall UDP checksum to detect bit errors
• the question how to recover from errors
• acknowledgements (ACKs) receiver explicitly
  tells sender that pkt received OK
• negative acknowledgements (NAKs) receiver
  explicitly tells sender that pkt had errors
• sender retransmits pkt on receipt of NAK
• new mechanisms in rdt2.0 (beyond rdt1.0)
• error detection
• receiver feedback control msgs (ACK,NAK)
  rcvr-gtsender

10
rdt2.0 FSM specification
rdt_send(data)
receiver
snkpkt make_pkt(data, checksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) isNAK(rcvpkt)
Wait for call from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) isACK(rcvpkt)
L
sender
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
extract(rcvpkt,data) deliver_data(data) udt_send(A
CK)
11
rdt2.0 operation with no errors
rdt_send(data)
snkpkt make_pkt(data, checksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) isNAK(rcvpkt)
Wait for call from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) isACK(rcvpkt)
Wait for call from below
L
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
extract(rcvpkt,data) deliver_data(data) udt_send(A
CK)
12
rdt2.0 error scenario
rdt_send(data)
snkpkt make_pkt(data, checksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) isNAK(rcvpkt)
Wait for call from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) isACK(rcvpkt)
Wait for call from below
L
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
extract(rcvpkt,data) deliver_data(data) udt_send(A
CK)
13
rdt2.0 FSM specification
rdt_send(data)
receiver
snkpkt make_pkt(data, checksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) isNAK(rcvpkt)
Wait for call from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) isACK(rcvpkt)
L
sender
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
extract(rcvpkt,data) deliver_data(data) udt_send(A
CK)
14
rdt2.0 has a fatal flaw!

• What happens if ACK/NAK corrupted?
• sender doesnt know what happened at receiver!
• cant just retransmit possible duplicate
• What to do?
• sender ACKs/NAKs receivers ACK/NAK? What if
  sender ACK/NAK lost?
• retransmit, but this might cause retransmission
  of correctly received pkt!

• Handling duplicates
• sender adds sequence number to each pkt
• sender retransmits current pkt if ACK/NAK garbled
• receiver discards (doesnt deliver up) duplicate
  pkt

Sender sends one packet, then waits for receiver
response
15
rdt2.1 sender, handles garbled ACK/NAKs
rdt_send(data)
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isNAK(rcvpkt) )
Wait for call 0 from above
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt)
L
L
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isNAK(rcvpkt) )
rdt_send(data)
sndpkt make_pkt(1, data, checksum) udt_send(sndp
kt)
udt_send(sndpkt)
16
rdt2.1 receiver, handles garbled ACK/NAKs
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
has_seq0(rcvpkt)
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(ACK, chksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) corrupt(rcvpkt)
rdt_rcv(rcvpkt) corrupt(rcvpkt)
sndpkt make_pkt(NAK, chksum) udt_send(sndpkt)
sndpkt make_pkt(NAK, chksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) not corrupt(rcvpkt)
has_seq1(rcvpkt)
rdt_rcv(rcvpkt) not corrupt(rcvpkt)
has_seq0(rcvpkt)
sndpkt make_pkt(ACK, chksum) udt_send(sndpkt)
sndpkt make_pkt(ACK, chksum) udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
has_seq1(rcvpkt)
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(ACK, chksum) udt_send(sndpkt)
17
rdt2.1 discussion

• Sender
• seq added to pkt
• two seq. s (0,1) will suffice. Why?
• must check if received ACK/NAK corrupted
• twice as many states
• state must remember whether current pkt has 0
  or 1 seq.

• Receiver
• must check if received packet is duplicate
• state indicates whether 0 or 1 is expected pkt
  seq
• Can receiver know if its last ACK/NAK received OK
  at sender?
• No

18
rdt2.2 a NAK-free protocol

• same functionality as rdt2.1, using NAKs only
• instead of NAK, receiver sends ACK for last pkt
  received OK
• receiver must explicitly include seq of pkt
  being ACKed
• duplicate ACK at sender results in same action as
  NAK retransmit current pkt

19
rdt2.2 sender, receiver fragments
rdt_send(data)
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isACK(rcvpkt,1) )
udt_send(sndpkt)
sender FSM fragment
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt,0)
rdt_rcv(rcvpkt) (corrupt(rcvpkt)
has_seq1(rcvpkt))
L
receiver FSM fragment
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
has_seq1(rcvpkt)
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(ACK1, chksum) udt_send(sndpkt)
20
Outline

• Reliable transfer protocols
• rdt2.1 sender, handles garbled ACK/NAKs
• rdt2.2 a NAK-free protocol
• rdt3.0 channels with errors and loss
• Pipelined protocols
• Go-back-N
• Selective repeat
• Connection-oriented transport TCP
• Overview and segment structure

21
rdt3.0 channels with errors and loss

• New assumption underlying channel can also lose
  packets (data or ACKs)
• checksum, seq. , ACKs, retransmissions will be
  of help, but not enough

• Approach sender waits reasonable amount of
  time for ACK
• retransmits if no ACK received in this time
• if pkt (or ACK) just delayed (not lost)
• retransmission will be duplicate, but use of
  seq. s already handles this
• receiver must specify seq of pkt being ACKed
• requires countdown timer

22
rdt3.0 sender
rdt_send(data)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isACK(rcvpkt,1) )
sndpkt make_pkt(0, data, checksum) udt_send(sndp
kt) start_timer
L
rdt_rcv(rcvpkt)
L
timeout
udt_send(sndpkt) start_timer
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt,1)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
isACK(rcvpkt,0)
stop_timer
stop_timer
timeout
udt_send(sndpkt) start_timer
rdt_rcv(rcvpkt)
L
rdt_send(data)
rdt_rcv(rcvpkt) ( corrupt(rcvpkt)
isACK(rcvpkt,0) )
sndpkt make_pkt(1, data, checksum) udt_send(sndp
kt) start_timer
L
23
rdt3.0 in action
24
rdt3.0 in action
25
Performance of rdt3.0

• rdt3.0 works, but performance stinks
• example 1 Gbps link, 15 ms e-e prop. delay, 1KB
  packet

L (packet length in bits)
8kb/pkt
T


8 microsec
transmit
R (transmission rate, bps)
109 b/sec

• U sender utilization fraction of time sender
  busy sending

• 1KB pkt every 30 msec -gt 33kB/sec thruput over 1
  Gbps link
• network protocol limits use of physical resources!

26
rdt3.0 stop-and-wait operation
sender
receiver
first packet bit transmitted, t 0
last packet bit transmitted, t L / R
first packet bit arrives
RTT
last packet bit arrives, send ACK
ACK arrives, send next packet, t RTT L / R
27
Outline

• Reliable transfer protocols
• rdt2.1 sender, handles garbled ACK/NAKs
• rdt2.2 a NAK-free protocol
• rdt3.0 channels with errors and loss
• Pipelined protocols
• Go-back-N
• Selective repeat
• Connection-oriented transport TCP
• Overview and segment structure

28
Pipelined protocols

• Pipelining sender allows multiple, in-flight,
  yet-to-be-acknowledged pkts
• range of sequence numbers must be increased
• buffering at sender and/or receiver

• Two generic forms of pipelined protocols
  go-Back-N, selective repeat

29
Pipelining increased utilization
sender
receiver
first packet bit transmitted, t 0
last bit transmitted, t L / R
first packet bit arrives
RTT
last packet bit arrives, send ACK
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
ACK arrives, send next packet, t RTT L / R
Increase utilization by a factor of 3!
30
Go-Back-N

• Sender
• k-bit seq in pkt header
• window of up to N, consecutive unacked pkts
  allowed

• ACK(n) ACKs all pkts up to, including seq n -
  cumulative ACK
• may deceive duplicate ACKs (see receiver)
• Single timer for all in-flight pkts
• timeout(n) retransmit pkt n and all higher seq
  pkts in window
• How many states will the FSM have?

31
GBN sender extended FSM
rdt_send(data)
if (nextseqnum lt baseN) sndpktnextseqnum
make_pkt(nextseqnum,data,chksum)
udt_send(sndpktnextseqnum) if (base
nextseqnum) start_timer nextseqnum
else refuse_data(data)
L
base1 nextseqnum1
timeout
start_timer udt_send(sndpktbase) udt_send(sndpkt
base1) udt_send(sndpktnextseqnum-1)
rdt_rcv(rcvpkt) corrupt(rcvpkt)
L
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
base getacknum(rcvpkt)1 If (base
nextseqnum) stop_timer else start_timer
32
GBN receiver extended FSM
default
udt_send(sndpkt)
rdt_rcv(rcvpkt) notcurrupt(rcvpkt)
hasseqnum(rcvpkt,expectedseqnum)
L
Wait
extract(rcvpkt,data) deliver_data(data) sndpkt
make_pkt(expectedseqnum,ACK,chksum) udt_send(sndpk
t) expectedseqnum
expectedseqnum1 sndpkt
make_pkt(expectedseqnum,ACK,chksum)

• ACK-only always send ACK for correctly-received
  pkt with highest in-order seq
• may generate duplicate ACKs
• need only remember expectedseqnum
• out-of-order pkt
• discard (dont buffer) -gt no receiver buffering!
• Re-ACK pkt with highest in-order seq

33
GBN inaction
34
Selective Repeat

• receiver individually acknowledges all correctly
  received pkts
• buffers pkts, as needed, for eventual in-order
  delivery to upper layer
• sender only resends pkts for which ACK not
  received
• sender timer for each unACKed pkt
• sender window
• N consecutive seq s
• again limits seq s of sent, unACKed pkts

35
Selective repeat sender, receiver windows

• What happened to the first two yellow packets?
• There is also a mistake here (group discussion)

36
Selective repeat

• pkt n in rcvbase, rcvbaseN-1
• send ACK(n)
• out-of-order buffer
• in-order deliver (also deliver buffered,
  in-order pkts), advance window to next
  not-yet-received pkt
• pkt n in rcvbase-N,rcvbase-1
• ACK(n) (earlier ack lost)
• otherwise
• ignore

• data from above
• if next available seq in window, send pkt
• timeout(n)
• resend pkt n, restart timer
• ACK(n) in sendbase,sendbaseN
• mark pkt n as received
• if n smallest unACKed pkt, advance window base to
  next unACKed seq

37
Selective repeat in action
38
Selective repeat dilemma

• Example seq s 0, 1, 2, 3
• window size3
• receiver sees no difference in two scenarios!
• incorrectly passes duplicate data as new in (a)
• Q How to fix this?
• Q what relationship between seq size and
  window size?
• What about GBN ?

39
Outline

• Reliable transfer protocols
• Pipelined protocols
• Selective repeat
• Connection-oriented transport TCP
• Overview and segment structure
• Reliable data transfer
• Flow control
• Connection management

40
TCP Overview RFCs 793, 1122, 1323, 2018, 2581

• point-to-point
• one sender, one receiver
• reliable, in-order byte steam
• no message boundaries
• pipelined
• TCP congestion and flow control set window size
• send receive buffers

• full duplex data
• bi-directional data flow in same connection
• MSS maximum segment size
• connection-oriented
• handshaking (exchange of control msgs) inits
  sender, receiver state before data exchange
• flow controlled
• sender will not overwhelm receiver

41
TCP segment structure
URG urgent data (generally not used)
counting by bytes of data (not segments!)
ACK ACK valid
PSH push data now (generally not used)
bytes rcvr willing to accept
RST, SYN, FIN connection estab (setup,
teardown commands)
Internet checksum (as in UDP)
Compared w/ the header of UDP, what is missing?
42
TCP seq. s and ACKs

• Seq. s
• byte stream number of first byte in segments
  data
• ACKs
• seq of next byte expected from other side
• cumulative ACK
• Q how receiver handles out-of-order segments
• A TCP spec doesnt say, - up to implementor

Host B
Host A
User types C
Seq42, ACK79, data C
host ACKs receipt of C, echoes back C
Seq79, ACK43, data C
host ACKs receipt of echoed C
Seq43, ACK80
simple telnet scenario
43
TCP Round Trip Time and Timeout

• Q how to estimate RTT?
• SampleRTT measured time from segment
  transmission until ACK receipt
• ignore retransmissions
• SampleRTT will vary, want estimated RTT
  smoother
• average several recent measurements, not just
  current SampleRTT

• Q how to set TCP timeout value?
• longer than RTT
• but RTT varies
• too short premature timeout
• unnecessary retransmissions
• too long slow reaction to segment loss

44
TCP Round Trip Time and Timeout
EstimatedRTT (1- ?)EstimatedRTT ?SampleRTT

• Exponential weighted moving average
• influence of past sample decreases exponentially
  fast
• typical value ? 0.125

45
Example RTT estimation
46
TCP Round Trip Time and Timeout

• Setting the timeout
• EstimtedRTT plus safety margin
• large variation in EstimatedRTT -gt larger safety
  margin
• first estimate of how much SampleRTT deviates
  from EstimatedRTT

DevRTT (1-?)DevRTT
?SampleRTT-EstimatedRTT (typically, ? 0.25)
Then set timeout interval
TimeoutInterval EstimatedRTT 4DevRTT
47
Outline

• Reliable transfer protocols
• Pipelined protocols
• Selective repeat
• Connection-oriented transport TCP
• Overview and segment structure
• Reliable data transfer
• Flow control
• Connection management

48
TCP reliable data transfer

• TCP creates rdt service on top of IPs unreliable
  service
• Pipelined segments
• Cumulative acks
• TCP uses single retransmission timer

• Retransmissions are triggered by
• timeout events
• duplicate acks
• Initially consider simplified TCP sender
• ignore duplicate acks
• ignore flow control, congestion control

49
TCP sender events

• timeout
• retransmit segment that caused timeout
• restart timer
• Ack rcvd
• If acknowledges previously unacked segments
• update what is known to be acked
• start timer if there are outstanding segments
• Difference from GBN?

• data rcvd from app
• Create segment with seq
• seq is byte-stream number of first data byte in
  segment
• start timer if not already running (think of
  timer as for oldest unacked segment)
• expiration interval TimeOutInterval

50
TCP sender(simplified)
NextSeqNum InitialSeqNum
SendBase InitialSeqNum loop (forever)
switch(event) event
data received from application above
create TCP segment with sequence number
NextSeqNum if (timer currently
not running) start timer
pass segment to IP
NextSeqNum NextSeqNum length(data)
event timer timeout
retransmit not-yet-acknowledged segment with
smallest sequence number
start timer event ACK
received, with ACK field value of y
if (y gt SendBase)
SendBase y if (there are
currently not-yet-acknowledged segments)
start timer
/ end of loop forever /

• Comment
• SendBase-1 last
• cumulatively acked byte
• Example
• SendBase-1 71y 73, so the rcvrwants 73
  y gt SendBase, sothat new data is acked

51
TCP retransmission scenarios
Host A
Host B
Seq92, 8 bytes data
Seq100, 20 bytes data
ACK100
ACK120
Seq92, 8 bytes data
Sendbase 100
SendBase 120
ACK120
Seq92 timeout
SendBase 100
SendBase 120
premature timeout
52
TCP retransmission scenarios (more)
SendBase 120
53
TCP ACK generation RFC 1122, RFC 2581
TCP Receiver action Delayed ACK. Wait up to
500ms for next segment. If no next segment, send
ACK Immediately send single cumulative ACK,
ACKing both in-order segments Immediately send
duplicate ACK, indicating seq. of next
expected byte Immediate send ACK, provided
that segment startsat lower end of gap
Event at Receiver Arrival of in-order segment
with expected seq . All data up to expected seq
already ACKed Arrival of in-order segment
with expected seq . One other segment has ACK
pending Arrival of out-of-order
segment higher-than-expect seq. . Gap
detected Arrival of segment that partially or
completely fills gap
54
Fast Retransmit

• Time-out period often relatively long
• long delay before resending lost packet
• Detect lost segments via duplicate ACKs.
• Sender often sends many segments back-to-back
• If segment is lost, there will likely be many
  duplicate ACKs.

• If sender receives 3 ACKs for the same data, it
  supposes that segment after ACKed data was lost
• fast retransmit resend segment before timer
  expires

55
Fast retransmit algorithm
event ACK received, with ACK field value of y
if (y gt SendBase)
SendBase y
if (there are currently not-yet-acknowledged
segments) start
timer
else increment count
of dup ACKs received for y
if (count of dup ACKs received for y 3)
resend segment with
sequence number y

a duplicate ACK for already ACKed segment
fast retransmit
56
Outline

• Flow control
• Connection management
• Congestion control

57
TCP Flow Control

• receive side of TCP connection has a receive
  buffer

• speed-matching service matching the send rate to
  the receiving apps drain rate

• app process may be slow at reading from buffer

58
TCP Flow control how it works

• Rcvr advertises spare room by including value of
  RcvWindow in segments
• Sender limits unACKed data to RcvWindow
• guarantees receive buffer doesnt overflow

• (Suppose TCP receiver discards out-of-order
  segments)
• spare room in buffer
• RcvWindow
• RcvBuffer-LastByteRcvd - LastByteRead

59
TCP Connection Management

• Three way handshake
• Step 1 client host sends TCP SYN segment to
  server
• specifies initial seq
• no data
• Step 2 server host receives SYN, replies with
  SYNACK segment
• server allocates buffers
• specifies server initial seq.
• Step 3 client receives SYNACK, replies with ACK
  segment, which may contain data

• Recall TCP sender, receiver establish
  connection before exchanging data segments
• initialize TCP variables
• seq. s
• buffers, flow control info (e.g. RcvWindow)
• client connection initiator
• server contacted by client

What about if the client does not send SYN ACK
60
TCP Connection Management Closing

• Step 1 client end system sends TCP FIN control
  segment to server
• Step 2 server receives FIN, replies with ACK.
  Closes connection, sends FIN.
• Step 3 client receives FIN, replies with ACK.
• Enters timed wait - will respond with ACK to
  received FINs
• Step 4 server, receives ACK. Connection closed.
  
• Note with small modification, can handle
  simultaneous FINs

client
server
closing
FIN
ACK
closing
FIN
ACK
timed wait
closed
closed
61
TCP Connection Management (cont)
TCP server lifecycle
TCP client lifecycle
62
Outline

• Flow control
• Connection management
• Congestion control

63
Principles of Congestion Control

• Congestion
• informally too many sources sending too much
  data too fast for network to handle
• different from flow control!
• manifestations
• lost packets (buffer overflow at routers)
• long delays (queueing in router buffers)
• Reasons
• Limited bandwidth, queues
• Unneeded retransmission for data and ACKs

64
Approaches towards congestion control
Two broad approaches towards congestion control

• Network-assisted congestion control
• routers provide feedback to end systems
• single bit indicating congestion (SNA, DECbit,
  TCP/IP ECN, ATM)
• explicit rate sender should send at

• End-end congestion control
• no explicit feedback from network
• congestion inferred from end-system observed
  loss, delay
• approach taken by TCP

65
TCP Congestion Control

• end-end control (no network assistance)
• sender limits transmission
• LastByteSent-LastByteAcked
• ? CongWin
• Roughly,
• CongWin is dynamic, function of perceived network
  congestion

• How does sender perceive congestion?
• loss event timeout or 3 duplicate acks
• TCP sender reduces rate (CongWin) after loss
  event
• three mechanisms
• AIMD
• slow start
• conservative after timeout events

66
TCP AIMD
additive increase increase CongWin by 1 MSS
every RTT in the absence of loss events probing

• multiplicative decrease cut CongWin in half
  after loss event

Long-lived TCP connection
67
TCP Slow Start

• When connection begins, increase rate
  exponentially fast until first loss event

• When connection begins, CongWin 1 MSS
• Example MSS 500 bytes RTT 200 msec
• initial rate 20 kbps
• available bandwidth may be gtgt MSS/RTT
• desirable to quickly ramp up to respectable rate

68
TCP Slow Start (more)

• When connection begins, increase rate
  exponentially until first loss event
• double CongWin every RTT
• done by incrementing CongWin for every ACK
  received
• Summary initial rate is slow but ramps up
  exponentially fast

69
Refinement (more)

• Q When should the exponential increase switch to
  linear?
• A When CongWin gets to 1/2 of its value before
  timeout.

14
12

10
8
(segments)
congestion window size
6
4
threshold
2

• Implementation
• Variable Threshold
• At loss event, Threshold is set to 1/2 of CongWin
  just before loss event

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Transmission round

70
Refinement
Philosophy

• 3 dup ACKs indicates network capable of
  delivering some segments
• timeout before 3 dup ACKs is more alarming

• After 3 dup ACKs
• CongWin is cut in half
• window then grows linearly
• But after timeout event
• Enter slow start
• CongWin instead set to 1 MSS
• window then grows exponentially
• to a threshold, then grows linearly

71
Summary TCP Congestion Control

• When CongWin is below Threshold, sender in
  slow-start phase, window grows exponentially.
• When CongWin is above Threshold, sender is in
  congestion-avoidance phase, window grows
  linearly.
• When a triple duplicate ACK occurs, Threshold set
  to CongWin/2 and CongWin set to Threshold.
• When timeout occurs, Threshold set to CongWin/2
  and CongWin is set to 1 MSS.

72
TCP Fairness

• Fairness goal if K TCP sessions share same
  bottleneck link of bandwidth R, each should have
  average rate of R/K

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Why is TCP fair?

• Two competing sessions
• Additive increase gives slope of 1, as throughout
  increases
• multiplicative decrease decreases throughput
  proportionally

R
equal bandwidth share
loss decrease window by factor of 2
congestion avoidance additive increase
Connection 2 throughput
loss decrease window by factor of 2
congestion avoidance additive increase
Connection 1 throughput
R
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Fairness (more)

• Fairness and parallel TCP connections
• nothing prevents app from opening parallel
  connections between 2 hosts.
• Web browsers do this
• Example link of rate R supporting 9 connections
  
• new app asks for 1 TCP, gets rate R/10
• new app asks for 11 TCPs, gets R/2 !

• Fairness and UDP
• Multimedia apps often do not use TCP
• do not want rate throttled by congestion control
• Instead use UDP
• pump audio/video at constant rate, tolerate
  packet loss
• Research area TCP friendly