Transcript Chapter3_5th_Aug_2009

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
Transport Layer
3-1
Transport services and protocols
 provide logical communication
between app processes
running on different hosts
 transport protocols run in
end systems
 more than one transport
protocol available to apps
 Internet: TCP and UDP
application
transport
network
data link
physical
application
transport
network
data link
physical
Transport Layer
3-2
Process
socket
Port 9876
UDP
Process
socket
Port 9999
UDP
2: Application Layer
3
Process
welcome
socket
Port 6789
socket
Process
socket
Port 9999
Port 6789
TCP
TCP
Host S
Host C
2: Application Layer
4
Transport vs. network layer
 network layer: delivers packets between hosts
 transport layer: delivers packets between
processes
 relies on & enhances network layer services
Transport Layer
3-5
Internet transport-layer protocols
 reliable, in-order
delivery (TCP)



connection setup
congestion control
flow control
 unreliable, unordered
delivery: UDP

no-frills extension of
“best-effort” IP
 services not available:
 delay guarantees
 bandwidth guarantees
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physicalnetwork
network
data link
physical
data link
physical
network
data link
physical
application
transport
network
data link
physical
Transport Layer
3-6
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
Transport Layer
3-7
Multiplexing/demultiplexing
Multiplexing at send host:
gathering data from multiple
sockets, enveloping data with
header (later used for
demultiplexing)
Demultiplexing at rcv host:
delivering received segments
to correct socket
= socket
application
transport
network
link
= process
P3
P1
P1
application
transport
network
P2
P4
application
transport
network
link
link
physical
host 1
physical
host 2
physical
host 3
Transport Layer
3-8
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
 UDP: host uses port numbers
to direct segment to
appropriate socket
 TCP: 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
Transport Layer
3-9
Connectionless demultiplexing
 When host receives UDP segment:


checks destination port number in segment
directs UDP segment to socket with that port
number
Transport Layer 3-10
Connection-oriented demux
 TCP socket (connection)
identified by 4-tuple:




source IP address
source port number
dest IP address
dest port number
 receiving 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
Transport Layer
3-11
Connection-oriented demux
(cont)
P1
P4
P5
P2
P6
P1P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
SP: 9157
client
IP: A
DP: 80
S-IP: A
D-IP:C
SP: 9157
server
IP: C
DP: 80
S-IP: B
D-IP:C
Client
IP:B
Transport Layer 3-12
Connection-oriented demux:
Threaded Web Server
P1
P2
P4
P1P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
SP: 9157
client
IP: A
DP: 80
S-IP: A
D-IP:C
SP: 9157
server
IP: C
DP: 80
S-IP: B
D-IP:C
Client
IP:B
Transport Layer 3-13
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
Transport Layer 3-14
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
Transport Layer 3-15
UDP: more
 often used for streaming
multimedia apps
 loss tolerant
 rate sensitive
32 bits
Length, in source port #
bytes of UDP
length
segment,
including
 reliable transfer over UDP:
header
add reliability at
application layer
 application-specific
error recovery!
dest port #
checksum
Application
data
(message)
UDP segment format
Transport Layer 3-16
UDP checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment
Sender:
Receiver:
 treat segment contents
 compute checksum of
as sequence of 16-bit
integers
 checksum: addition (1’s
complement sum) of
segment contents
 sender puts checksum
value into UDP checksum
field
received segment
 check if computed checksum
equals checksum field value:
 NO - error detected
 YES - no error detected.
But maybe errors
nonetheless? More later
….
Transport Layer 3-17
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
wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
Transport Layer 3-18
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
Transport Layer 3-19
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)
Transport Layer 3-20
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)
Transport Layer 3-21
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)
Transport Layer 3-22
Reliable data transfer: getting started
We’ll:
 incrementally develop sender, receiver sides of
reliable data transfer protocol (rdt)
 consider only unidirectional data transfer
 but control info will flow on both directions!
The book uses finite state machines (FSM) to
specify sender, receiver
Transport Layer 3-23
Rdt1.0: reliable transfer over a reliable channel
 underlying channel perfectly reliable
no bit errors
 no loss of packets
 no duplicates
 first-in-first-out

Sender
Send pkt
Receiver
Rcv pkt
Send pkt
Rcv pkt
Send pkt
Rcv pkt
Transport Layer 3-24
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) from receiver
to sender
Transport Layer 3-25
rdt2.0 in action
Sender
Send pkt
Rcv ACK
Send pkt
Rcv ACK
Send pkt
Receiver
Rcv pkt
Send ACK
Rcv pkt
Send ACK
Rcv pkt
Send ACK
Rcv ACK
Without error
Sender
Receiver
Send pkt
Rcv pkt
Send ACK
Rcv ACK
Send pkt
x
Rcv NAK
Re-send pkt
Rcv pkt
Dct errors
Send NAK
Rcv pkt
Send ACK
Rcv ACK
With errors
Transport Layer 3-26
rdt2.0 has a fatal flaw!
What happens if
ACK/NAK corrupted?
 sender doesn’t know what
happened at receiver!
Remedy:
 sender retransmits current
pkt if ACK/NAK garbled
 problem: may introduce
duplicates
Handling duplicates:
 sender retransmits current
pkt if ACK/NAK garbled
 sender adds sequence
number to each pkt
 receiver discards (doesn’t
deliver up) duplicate pkt
stop and wait
Sender sends one packet,
then waits for receiver
response
Transport Layer 3-27
rdt2.1 in action
Sender
Send pkt0
Rcv ACK0
Send pkt1
Rcv ACK1
Send pkt0
Receiver
Rcv pkt0
Send ACK
Rcv pkt1
Send ACK
Rcv pkt0
Send ACK
Rcv ACK0
Without error
Sender
Receiver
Send pkt0
Rcv pkt0
Send ACK
Rcv ACK
Send pkt1
x
Rcv NAK
Re-send pkt1
Rcv garbled
ACK/NAK
Re-send pkt1
x
Rcv garbled
pkt
Send NAK
Rcv pkt1
Send ACK
Rcv pkt1
Discard pkt1
With errors
Transport Layer 3-28
rdt2.1: discussion
Sender:
 seq # added to pkt
 two seq. #’s (0,1) will
suffice.
 must check if
received ACK/NAK
corrupted
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
Transport Layer 3-29
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
Transport Layer 3-30
rdt3.0 in action
Transport Layer 3-31
rdt3.0 in action
Transport Layer 3-32
Performance of rdt3.0
 rdt3.0 works, but performance stinks
sender
receive
RTT
Transport Layer 3-33
Pipelined protocols
Pipelining: sender allows multiple, “in-flight”, yet-tobe-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
Transport Layer 3-34
Pipelining: increased utilization
sender
receiver
first packet bit transmitted,
last bit transmitted
RTT
first packet bit arrives
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
Transport Layer 3-35
Pipelining Protocols
Go-back-N: overview
 sender: up to N
unACKed pkts in
pipeline
 receiver: only sends
cumulative ACKs

doesn’t ACK pkt if
there’s a gap
 sender: has timer for
oldest unACKed pkt

Selective Repeat: overview
 sender: up to N unACKed
packets in pipeline
 receiver: ACKs individual
pkts
 sender: maintains timer
for each unACKed pkt

if timer expires: retransmit
only unACKed packet
if timer expires:
retransmit all unACKed
packets
Transport Layer 3-36
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 the first pkt in window
 timeout(n): retransmit pkt n and all higher seq # pkts in window

Transport Layer 3-37
GBN: receiver
 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 (don’t buffer) -> no receiver buffering!
 Re-ACK pkt with highest in-order seq #
Transport Layer 3-38
GBN in
action
Transport Layer 3-39
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
Transport Layer 3-40
Selective repeat: sender, receiver windows
Transport Layer 3-41
Selective repeat
sender
data from above :
receiver
pkt n in [rcvbase, rcvbase+N-1]
 if next available seq # in
 send ACK(n)
timeout(n):
 in-order: deliver (also
window, send pkt
 resend pkt n, restart timer
ACK(n) in [sendbase,sendbase+N1]:
 mark pkt n as received
 if n smallest unACKed pkt,
advance window base to
next unACKed seq #
 out-of-order: buffer
deliver buffered, in-order
pkts), advance window to
next not-yet-received pkt
pkt n in
[rcvbase-N,rcvbase-1]
 ACK(n)
otherwise:
 ignore
Transport Layer 3-42
Selective repeat in action
Transport Layer 3-43
Selective repeat:
dilemma
Example:
 seq #’s: 0, 1, 2, 3
 window size=3
 receiver sees no
difference in two
scenarios!
 incorrectly passes
duplicate data as new
in (a)
Q: what relationship
between seq # size
and window size?
Transport Layer 3-44
Two-way communication
 Two-way communication, say, between A and B
 Run two GBN’s (or SR’s)
 In one of them, A is sender, B receiver
 In the other, B is sender, A receiver
 A and B each send both data packets and ACKs
 Piggybacking
Transport Layer 3-45
Example
Sender
Receiver
Sender
Receiver
pkt0
ACK0
ACK4
pkt5
pkt1
ACK1
pkt2
ACK2
ACK5
pkt6
ACK6
pkt7
ACK7
Transport Layer 3-46
Piggybacking
Sender
Receiver
Send pkt0,4
Send pkt5,0
Send pkt1,5
Send pkt6,1
Send pkt2,6
Send pkt7,2
Send ACK7
ACK(n): ACKs all pkts up to seq #
n
Transport Layer 3-47
Alternative ACK(n) in GBN
Sender
Receiver
Sender
Receiver
Send pkt0
Send pkt0
Send ACK1
Send ACK0
Send pkt1
Send pkt1
Send ACK2
Send ACK1
Send pkt2
Send pkt2
Send ACK2
ACK(n): ACKs all pkts up to seq # n
Send ACK3
ACK(n): ACKs all pkts up to seq # n-1
Transport Layer 3-48
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
Transport Layer 3-49
TCP: Overview
 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
socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
RFCs: 793, 1122, 1323, 2018, 2581
 full duplex data:
 bi-directional data flow
in same connection
 MSS: maximum segment
size
 connection-oriented:
 handshaking (exchange
of control msgs) init’s
sender, receiver state
before data exchange
 flow controlled:
 sender will not
socket
door
overwhelm receiver
segment
Transport Layer 3-50
TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pointer
Options (variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
application
data
(variable length)
Transport Layer 3-51
TCP seq. #’s and ACKs
Seq. #’s:
 byte stream
“number” of first
byte in segment’s
data
ACKs:
 seq # of next byte
expected from
other side
 cumulative ACK
Q: how receiver handles
out-of-order segments
 A: TCP spec doesn’t
say, - up to
implementer
Host A
User
types
‘C’
Host B
host ACKs
receipt of
‘C’, echoes
back ‘C’
host ACKs
receipt
of echoed
‘C’
simple telnet scenario
time
Transport Layer 3-52
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
Transport Layer 3-53
TCP reliable data transfer
 TCP creates rdt
service on top of IP’s
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
Transport Layer 3-54
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
TimeoutInterval = EstimatedRTT + 4*DevRTT
Transport Layer 3-55
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 > SendBase) {
SendBase = y
if (there are currently not-yet-acknowledged segments)
start timer
}
} /* end of loop forever */
TCP
sender
(simplified)
Comment:
• SendBase-1: last
cumulatively
ACKed byte
Example:
• SendBase-1 = 71;
y= 73, so the rcvr
wants 73+ ;
y > SendBase, so
that new data is
ACKed
Transport Layer 3-56
TCP: retransmission scenarios
Host A
X
loss
Sendbase
= 100
SendBase
= 120
SendBase
= 100
time
SendBase
= 120
lost ACK scenario
Host B
Seq=92 timeout
Host B
Seq=92 timeout
timeout
Host A
time
premature timeout
Transport Layer 3-57
TCP retransmission scenarios (more)
timeout
Host A
Host B
X
loss
SendBase
= 120
time
Cumulative ACK scenario
Transport Layer 3-58
TCP ACK generation
[RFC 1122, RFC 2581]
Event at Receiver
TCP Receiver action
Arrival of in-order segment with
expected seq #. All data up to
expected seq # already ACKed
Delayed ACK. Wait up to 500ms
for next segment. If no next segment,
send ACK
Arrival of in-order segment with
expected seq #. One other
segment has ACK pending
Immediately send single cumulative
ACK, ACKing both in-order segments
Arrival of out-of-order segment
higher-than-expect seq. # .
Gap detected
Immediately send (duplicate) ACK,
indicating seq. # of next expected byte
Arrival of segment that
partially or completely fills gap
Immediate send ACK, provided that
segment starts at lower end of gap
Transport Layer 3-59
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-toback
if segment is lost, there
will likely be many
duplicate ACKs for that
segment
 If sender receives 3
duplicate ACKs (i.e., a
total of 4 ACKs) for
same data, it assumes
that segment after
ACKed data was lost:

fast retransmit: resend
segment before timer
expires
Transport Layer 3-60
Host A
seq # x1
seq # x2
seq # x3
seq # x4
seq # x5
Host B
X
ACK x1
ACK x1
ACK x1
ACK x1
timeout
triple
duplicate
ACKs
time
Transport Layer 3-61
Fast retransmit algorithm:
event: ACK received, with ACK field value of y
if (y > SendBase) {
SendBase = y
if (there are currently not-yet-acknowledged segments)
start timer
}
else /* y < SendBase */
{ 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
Transport Layer 3-62
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
Transport Layer 3-63
TCP Flow Control
 receive side of TCP
connection has a
receive buffer:
(currently)
TCP data application
IP
unused buffer
(in buffer)
process
datagrams
space
flow control
sender won’t overflow
receiver’s buffer by
transmitting too much,
too fast
 speed-matching
service: matching
send rate to receiving
application’s drain rate
 app process may be
slow at reading from
buffer
Transport Layer 3-64
TCP Flow control: how it works
(currently)
TCP data application
IP
unused buffer
(in buffer)
process
datagrams
space
rwnd
RcvBuffer
(suppose TCP receiver
discards out-of-order
segments)
 unused buffer space:
 receiver: advertises
unused buffer space by
including rwnd value in
segment header
 sender: limits # of
unACKed bytes to rwnd

guarantees receiver’s
buffer doesn’t overflow
= rwnd
= RcvBuffer-[LastByteRcvd LastByteRead]
Transport Layer 3-65
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
Transport Layer 3-66
Process
welcome
socket
Port 6789
socket
Process
socket
Port 9999
Port 6789
TCP
TCP
Host S
Host C
2: Application Layer
67
TCP Connection Management
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
Socket clientSocket = new
Socket("hostname","port
number");
 server: contacted by client
Socket connectionSocket =
welcomeSocket.accept();
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

Transport Layer 3-68
Three-way handshak
client
server
• x, y randomly
chosen. Why?
•The third
segment may
carry data
Transport Layer 3-69
TCP Connection Management (cont.)
Closing a connection:
client closes socket:
clientSocket.close();
client
close
Step 1: client sends TCP
close
FIN, replies with ACK.
Closes connection, sends
FIN.
timed wait
FIN control segment to
server
Step 2: server receives
server
closed
Transport Layer 3-70
TCP Connection Management (cont.)
Step 3: client receives FIN,
replies with ACK.

client
server
closing
Enters “timed wait” will respond with ACK
to received FINs
closing
Step 4: server, receives
Note: with small
modification, can handle
simultaneous FINs.
timed wait
ACK. Connection closed.
closed
closed
Transport Layer 3-71
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
Transport Layer 3-72
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)
 a top-10 problem!
Transport Layer 3-73
Approaches towards congestion control
two broad approaches towards congestion control:
end-end congestion
control:
 no explicit feedback from
network
 congestion inferred from
end-system observed loss,
delay
 approach taken by TCP
network-assisted
congestion control:
 routers provide feedback
to end systems
 single bit indicating
congestion
 explicit rate sender
should send at
Transport Layer 3-74
TCP congestion control:
 goal: TCP sender should transmit as fast as possible,
but without congesting network

Q: how to find rate just below congestion level
 decentralized: each TCP sender sets its own rate,
based on implicit feedback:
 ACK: segment received (a good thing!), network not
congested, so increase sending rate
 lost segment: assume loss due to congested
network, so decrease sending rate
Transport Layer 3-75
TCP congestion control: bandwidth probing
 “probing for bandwidth”: increase transmission rate
on receipt of ACK, until eventually loss occurs, then
decrease transmission rate

continue to increase on ACK, decrease on loss (since available
bandwidth is changing, depending on other connections in
network)
sending rate
ACKs being received,
so increase rate
X
X loss, so decrease rate
X
X
TCP’s
“sawtooth”
behavior
X
time
 Q: how fast to increase/decrease?
 details to follow
Transport Layer 3-76
TCP Congestion Control: overview
 three “phases”
 slow start
 congestion avoidance
 fast recovery
 important variables:
 cwnd
 ssthresh (slow start threshold)
 send window size = min(cwnd,rwnd)
 cwnd is dynamic, function of
perceived network congestion
 cwnd is ajusted by the
congestion control algorithm
Transport Layer 3-77
TCP congestion control FSM: overview
cwnd > ssthresh
slow
start
loss:
timeout
congestion
avoidance
loss:
timeout
loss:
timeout
loss:
3dupACK
new ACK
loss:
3dupACK
fast
recovery
Transport Layer 3-78
TCP Slowstart
Host A
• init cwnd = 1 MSS
• increase cwnd by 1MSS
for each segment ACKed
until loss event or when
threshold reached
RTT
Slowstart algorithm
Host B
 cwnd doubles per RTT
(starts at a slow rate
but increases fast)
 loss event: timeout or
three duplicate ACKs
time
2: Transport Layer 4
79
TCP Congestion Avoidance
 when cwnd > ssthresh
grow cwnd linearly
 increase cwnd by 1
MSS per RTT
 implementation:
increase cwnd by
MSS/(cwnd /MSS)
for each ACK received
 ssthresh: one half of
cwnd when last loss
event occurred
2: Transport Layer 4
80
Segment loss event
ssthresh = cwnd/2
 timeout:
 cut cwnd to 1

enter slowstart
 3 duplicate ACKs (Tahoe):
 same as timeout
 3 duplicate ACKs (Reno):
 cwnd = cwnd/2 + 3

enter fast recovery
TCP congestion control FSM: details
duplicate ACK
dupACKcount++
L
cwnd = 1 MSS
ssthresh = 64 KB
dupACKcount = 0
slow
start
timeout
ssthresh = cwnd/2
cwnd = 1 MSS
dupACKcount = 0
retransmit missing segment
dupACKcount == 3
ssthresh= cwnd/2
cwnd = ssthresh + 3
retransmit missing segment
new ACK
cwnd = cwnd+MSS
dupACKcount = 0
transmit new segment(s),as allowed
cwnd > ssthresh
L
timeout
ssthresh = cwnd/2
cwnd = 1 MSS
dupACKcount = 0
retransmit missing segment
timeout
ssthresh = cwnd/2
cwnd = 1
dupACKcount = 0
retransmit missing segment
new ACK
cwnd = cwnd + MSS (MSS/cwnd)
dupACKcount = 0
transmit new segment(s),as allowed
.
congestion
avoidance
duplicate ACK
dupACKcount++
New ACK
cwnd = ssthresh
dupACKcount = 0
dupACKcount == 3
ssthresh= cwnd/2
cwnd = ssthresh + 3
retransmit missing segment
fast
recovery
duplicate ACK
cwnd = cwnd + MSS
transmit new segment(s), as allowed
Transport Layer 3-82
cwnd window size (in segments)
Popular “flavors” of TCP
TCP Reno
ssthresh
ssthresh
TCP Tahoe
Transmission round
Transport Layer 3-83
Summary: TCP Congestion Control
 when cwnd < ssthresh, sender in slow-start
phase, window grows exponentially.
 when cwnd > ssthresh, sender is in congestion-
avoidance phase, window grows linearly.
 when triple duplicate ACK occurs, cut cwnd in
half (cwnd/2 + 3)
 when timeout occurs, cwnd set to 1 MSS.
 ssthresh = cwnd/2 on loss event
Transport Layer 3-84