School of Computing Science Simon Fraser University

1
School of Computing Science Simon Fraser
University

• CMPT 371 Data Communications and Networking
• Chapter 3 Transport Layer

2
Chapter 3 Transport Layer

• Our goals
• understand principles behind transport layer
  services
• multiplexing/demultiplexing
• reliable data transfer
• flow control
• congestion control
• learn about transport layer protocols in the
  Internet
• UDP
• TCP

3
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Principles of congestion control
• 3.7 TCP congestion control

4
Transport services and protocols

• provide logical communication between app
  processes running on different hosts
• transport protocols run in end systems
• send side breaks app messages into segments,
  passes to network layer
• rcv side reassembles segments into messages,
  passes to app layer
• more than one transport protocol available to
  apps
• Internet TCP and UDP

5
Transport vs. network layer

• Household analogy
• 12 kids sending letters to 12 kids
• processes kids
• app messages letters in envelopes
• hosts houses
• transport protocol Ann and Bill
• network-layer protocol postal service

• network layer logical communication between
  hosts
• transport layer logical communication between
  processes
• relies on, enhances, network layer services

6
Internet transport-layer protocols

• reliable, in-order delivery (TCP)
• congestion control
• flow control
• connection setup
• unreliable, unordered delivery UDP
• no-frills extension of best-effort IP
• services not available
• delay guarantees
• bandwidth guarantees

7
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Principles of congestion control
• 3.7 TCP congestion control

8
Multiplexing/demultiplexing
delivering received segments to correct socket
gathering data from multiple sockets, enveloping
data with header (later used for demultiplexing)
process
socket
application
P4
application
application
P1
P2
P3
P1
transport
transport
transport
network
network
network
link
link
link
physical
physical
physical
host 3
host 1
host 2
9
How demultiplexing works

• host receives IP datagrams
• each datagram has source IP address, destination
  IP address
• each datagram carries 1 transport-layer segment
• each segment has source, destination port number
• host uses IP addresses port numbers to direct
  segment to appropriate socket

32 bits
source port
dest port
other header fields
application data (message)
TCP/UDP segment format
10
Connectionless demux

• DatagramSocket serverSocket new
  DatagramSocket(6428)

client IP A
Client IPB
server IP C

• UDP socket identified by (dst IP, dst Port) ?
• datagrams with different src IPs and/or src ports
  are directed to same socket

11
Connection-oriented demux

• TCP socket identified by 4-tuple
• source IP address
• source port number
• dest IP address
• dest port number
• recv host uses all four values to direct segment
  to appropriate socket

• Server host may support many simultaneous TCP
  sockets
• each socket identified by its own 4-tuple
• Web servers have different sockets for each
  connecting client
• non-persistent HTTP will have different socket
  for each request

12
Connection-oriented demux (cont)
S-IP B
D-IPC
SP 9157
Client IPB
DP 80
server IP C
S-IP A
S-IP B
D-IPC
D-IPC
13
Connection-oriented demux Threaded Web Server
P4
S-IP B
D-IPC
SP 9157
Client IPB
DP 80
server IP C
S-IP A
S-IP B
D-IPC
D-IPC
14
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Principles of congestion control
• 3.7 TCP congestion control

15
UDP User Datagram Protocol RFC 768

• no frills, bare bones Internet transport
  protocol
• best effort service, UDP segments may be
• lost
• delivered out of order to app
• connectionless
• no handshaking between UDP sender, receiver
• each UDP segment handled independently of others

• Why is there a UDP?
• no connection establishment (which can add delay)
• simple no connection state at sender, receiver
• small segment header
• no congestion control UDP can blast away as fast
  as desired

16
UDP more

• often used for streaming multimedia apps
• loss tolerant
• rate sensitive
• other UDP uses
• DNS
• SNMP
• reliable transfer over UDP add reliability at
  application layer
• application-specific error recovery!

32 bits
source port
dest port
Length, in bytes of UDP segment, including header
checksum
length
Application data (message)
UDP segment format
17
UDP checksum

• Goal detect errors (e.g., flipped bits) in
  transmitted segment

• Sender
• treat segment contents as sequence of 16-bit
  integers
• checksum addition (1s complement sum) of
  segment contents
• sender puts checksum value into UDP checksum
  field

• Receiver
• compute checksum of received segment
• check if computed checksum equals checksum field
  value
• NO - error detected
• YES - no error detected. But maybe errors
  nonetheless? More later .

18
Internet Checksum Example

• Note
• When adding numbers, a carryout from the most
  significant bit needs to be added to the result
• Example add two 16-bit integers

1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1
0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0
1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1
1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1
0 0 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0
1 1
wraparound
sum
checksum
19
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Principles of congestion control
• 3.7 TCP congestion control

20
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)

21
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)

22
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)

23
Reliable data transfer getting started
send side
receive side
24
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
25
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
26
rdt2.0 channel with bit errors

• underlying channel may flip bits in packet
• 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

27
rdt2.0
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)
Wait for call from below
L
sender
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
Stop-and-wait protocol Sender sends one packet,
then waits for receiver response
extract(rcvpkt,data) deliver_data(data) udt_send(A
CK)
28
rdt2.0 has a fatal flaw!

• What happens if ACK/NAK corrupted?
• sender doesnt know what happened at receiver!
• Cant we just retransmit?
• NO. possible duplicate, receiver wont know
  whether it is a new or re-transmitted 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

29
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)
30
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)
31
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
• note receiver can not know if its last ACK/NAK
  received OK at sender

32
rdt2.2 a NAK-free protocol

• same functionality as rdt2.1, using ACKs 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

33
rdt3.0 channels with errors and loss

• Packets (data or ACKs) can be lost
• checksum, seq. , ACKs, retransmissions will be
  of help, but not enough
• How do we handle lost packets?
• sender waits reasonable amount of time for ACK
• requires countdown timer
• 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

34
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
35
rdt3.0 in action
36
rdt3.0 in action
37
Performance of rdt3.0

• U sender utilization fraction of time sender
  busy sending

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
38
Performance of rdt3.0 example

• example 1 Gbps link, 15 ms e-e prop. delay, 1KB
  packet

• 1KB pkt every 30 msec -gt 33kB/sec ( 0.264 Mbps)
  throughput over 1 Gbps link
• Very poor performance for high-speed links
• network protocol limits use of physical
  resources!
• Suggestions?

39
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

40
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!
41
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 receive duplicate ACKs (see receiver)
• timer for each in-flight pkt
• timeout(n) retransmit pkt n and all higher seq
  pkts in window
• i.e., go back to n

42
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)
rdt_rcv(rcvpkt) notcorrupt(rcvpkt)
base getacknum(rcvpkt)1 If (base
nextseqnum) stop_timer else start_timer
43
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,
  simpler!
• Re-ACK pkt with highest in-order seq (duplicate
  ACK)

44
GBN inaction
Go back to 2
Window size, N 4
45
Go-Back-N

• Do you see potential problems with GBN?
• Consider high-speed links with long delays
• (called large bandwidth-delay product pipes)
• GBN can fill that pipe by having large N ?
• many unACKed pkts could be in the pipe
• A single lost pkt could cause a re-transmission
  of a huge number (up to N) of pkts ? waste of
  bandwidth
• Solutions??

46
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

47
Selective repeat sender, receiver windows
48
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)
• 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-1
• mark pkt n as received
• if n sendbase, advance window base to next
  unACKed seq

Why does receiver ACK pkts in the previous window?
49
Selective repeat in action
50
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Principles of congestion control
• 3.7 TCP congestion control

51
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

52
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)
53
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 implementer

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
54
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

55
TCP sender events

• 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

• 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

56
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 /
57
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
58
TCP retransmission scenarios (more)
SendBase 120
59
TCP Round Trip Time and Timeout

• If TCP timeout is
• too short premature timeout ? unnecessary
  retransmissions
• too long slow reaction to segment loss
• Q how to set TCP timeout value?
• Based on RTT, but RTT itself varies with time!
• We need to estimate current RTT
• RTT Estimation
• 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

60
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

61
Example RTT estimation
62
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
63
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

64
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
65
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

66
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

67
TCP Connection Management opening

• Application
• client Socket clientSocket new
  Socket("hostname","port number")
• server when contacted by client Socket
  connectionSocket welcomeSocket.accept()
• TCP 3-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

A. SYN Flood DoS attack
Q. How would a hacker exploit TCP 3-way handshake
to bring a server down?
68
TCP Connection Management closing

• Application
• client closes socket clientSocket.close()
• TCP
• Step 1 client end system sends TCP FIN 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 may need to re-send ACK to
  received FINs
• Step 4 server, receives ACK. Connection closed

69
TCP Connection Management
TCP server lifecycle
TCP client lifecycle
70
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Congestion control
• 3.7 TCP congestion control

71
Congestion Control

• Congestion sources send too much data for
  network to handle
• different from flow control, which is e2e
• Congestion results in
• lost packets (buffer overflow at routers)
• more work (retransmissions) for given goodput
• long delays (queueing in router buffers)
• Premature (unneeded) retransmissions
• Waste of upstream links capacity
• Pkt traversed several links, then dropped at
  congested router

72
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

73
Chapter 3 outline

• 3.1 Transport-layer services
• 3.2 Multiplexing and demultiplexing
• 3.3 Connectionless transport UDP
• 3.4 Principles of reliable data transfer

• 3.5 Connection-oriented transport TCP
• segment structure
• reliable data transfer
• flow control
• connection management
• 3.6 Principles of congestion control
• 3.7 TCP congestion control

74
TCP congestion control Approach

• Approach probe for usable bandwidth in network
• increase transmission rate until loss occurs then
  decrease
• Additive increase, multiplicative decrease (AIMD)

Saw tooth behavior probing for bandwidth
Rate (CongWin)
time
75
TCP Congestion Control

• Sender keeps a new variable, Congestion Window
  (CongWin), and limits unacked bytes to
• LastByteSent - LastByteAcked ? min CongWin,
  RcvWin
• For our discussion assume RcvWin is large enough
• Sending Rate as function of CongWind is roughly
  
• Ignore loss and transmission delay
• Rate CongWind/RTT (bytes/sec)
• So, rate and CongWind are somewhat synonymous

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TCP Congestion Control

• Congestion occurs at routers (inside the network)
  
• Routers do not provide any feedback for TCP
• How can TCP infer congestion?
• From its symptoms timeout or duplicate acks
• Define loss event timeout or 3 duplicate acks
• TCP decreases its CongWin (rate) after a loss
  event
• TCP Congestion Control Algorithm three
  components
• AIMD additive increase, multiplicative decrease
• slow start
• Reaction to timeout events

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AIMD

• additive increase (congestion avoidance phase)
• increase CongWin by 1 MSS every RTT until loss
  detected
• TCP increases CongWin by MSS x (MSS/CongWin) for
  every ACK received
• Ex. MSS 1,460 bytes and CongWin 14,600 bytes
• With every ACK, CongWin is increased by 146 bytes
  
• multiplicative decrease
• cut CongWin in half after loss

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TCP Slow Start

• When connection begins, CongWin 1 MSS
• Example MSS 500 bytes RTT 200 msec
• initial rate CongWin/RTT 20 kbps
• available bandwidth may be gtgt MSS/RTT
• desirable to quickly ramp up to respectable rate
• Slow start
• When connection begins, increase rate
  exponentially fast until first loss event. How
  can we do that?
• double CongWin every RTT. How?
• Increment CongWin by 1 MSS for every ACK received

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TCP Slow Start (contd)

• Increment CongWin by 1 MSS for every ACK
• Summary initial rate is slow but ramps up
  exponentially fast

Host A
Host B
one segment
RTT
two segments
four segments
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Reaction to a Loss event

• TCP Tahoe (Old)
• Threshold CongWin / 2
• Set CongWin 1
• Slow start till threshold
• Then Additive Increase // congestion
  avoidance
• TCP Reno (most current TCP implementations)
• If 3 dup acks // fast
  retransmit
• Threshold CongWin / 2
• Set CongWin Threshold // fast recovery
• Additive Increase
• Else //
  timeout
• Same as TCP Tahoe

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Reaction to a Loss event (contd)
3 dup acks

• Why differentiate between 3 dup acks and timeout?
  
• 3 dup ACKs indicates network capable of
  delivering some segments
• timeout indicates a more alarming congestion
  scenario

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TCP Congestion Control Summary

• Initially
• Threshold is set to large value (65 Kbytes), has
  not effect
• CongWin 1 MSS
• Slow Start (SS) CongWin grows exponentially
• till a loss event occurs (timeout or 3 dup ack)
  or reaches Threshold
• Congestion Avoidance (CA) CongWin grows
  linearly
• 3 duplicate ACK occurs
• Threshold CongWin/2 CongWin Threshold CA
• Timeout occurs
• Threshold CongWin/2 CongWin 1 MSS SS
  till Threshold

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TCP throughput

• Whats the average throughout of TCP as a
  function of window size and RTT?
• Ignore slow start
• Let W be the window size when loss occurs.
• When window is W, throughput is W/RTT
• Just after loss, window drops to W/2, throughput
  to W/2RTT
• Average throughout 0.75 W/RTT

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TCP Futures

• Throughput in terms of loss rate (homework)
• Example 1500 byte segments, 100ms RTT, want 10
  Gbps throughput
• Requires loss rate to be
• L 2?10-10 (wow!)
• New versions of TCP for high-speed needed!

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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 cnctions
• 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

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Chapter 3 Summary

• principles behind transport layer services
• multiplexing, demultiplexing
• reliable data transfer
• flow control
• congestion control
• instantiation and implementation in the Internet
• UDP
• TCP

• Next
• leaving the network edge (application,
  transport layers)
• into the network core