Net_Chapter_3-software-computer network.pdf

Transport Layer 3-1
Chapter 3
Transport Layer
Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012
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All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved

Transport Layer 3-2
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 TCP congestion control

Transport Layer 3-3
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
application
transport
network
data link
physical
application
transport
network
data link
physical

Transport Layer 3-4
Transport vs. network layer
 network layer:
logical
communication
between hosts
 transport layer:
logical
communication
between processes
 relies on, enhances,
network layer
services
12 kids in Ann’s house
sending letters to 12 kids in
Bill’s house:
 hosts = houses
 processes = kids
 app messages = letters in
envelopes
 transport protocol = Ann
and Bill who demux to in-
house siblings
 network-layer protocol =
postal service
household analogy:

Transport Layer 3-5
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
application
transport
network
data link
physical
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical

Transport Layer 3-6
Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

Transport Layer 3-7
Multiplexing/demultiplexing
process
socket
use header info to deliver
received segments to correct
socket
demultiplexing at receiver:
handle data from multiple
sockets, add transport header
(later used for demultiplexing)
multiplexing at sender:
transport
application
physical
link
network
P2
P1
transport
application
physical
link
network
P4
transport
application
physical
link
network
P3

Transport Layer 3-8
How demultiplexing works
 host receives IP datagrams
 each datagram has source IP
address, destination IP
address
 each datagram carries one
transport-layer segment
 each segment has source,
destination port number
 host uses IP addresses &
port numbers to direct
segment to appropriate
socket
source port # dest port #
32 bits
application
data
(payload)
other header fields
TCP/UDP segment format

3-9
R1 R2
address in a packet
S1
S2
A
B
C
D E
I
H
G
F
message Hsegment Hdatagram
Dst port Dst IP
From P1 To P2
P1
P2

Transport Layer 3-10
Connectionless demux: example
DatagramSocket
serverSocket = new
DatagramSocket
(6428);
transport
application
physical
link
network
P3
transport
application
physical
link
network
P1
transport
application
physical
link
network
P4
DatagramSocket
mySocket1 = new
DatagramSocket
(5775);
DatagramSocket
mySocket2 = new
DatagramSocket
(9157);
source port: 9157
dest port: 6428
source port: 6428
dest port: 9157
source port: ?
dest port: ?
source port: ?
dest port: ?

Connection-oriented demux
 TCP socket identified
by 4-tuple:
 source IP address
 source port number
 dest IP address
 dest port number
 demux: receiver 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

Connection-oriented demux: example
transport
application
physical
link
network
P3
transport
application
physical
link
P4
transport
application
physical
link
network
P2
source IP,port: A,9157
dest IP, port: B,80
source IP,port: B,80
dest IP,port: A,9157
host: IP
address A
host: IP
address C
network
P6
P5
P3
source IP,port: C,5775
dest IP,port: B,80
source IP,port: C,9157
dest IP,port: B,80
three segments, all destined to IP address: B,
dest port: 80 are demultiplexed to different sockets
server: IP
address B

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

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
 UDP use:
 streaming multimedia
apps (loss tolerant, rate
sensitive)
 DNS
 SNMP
 reliable transfer over
UDP:
 add reliability at
application layer
 application-specific error
recovery!

UDP: segment header
32 bits
application
data
(payload)
UDP segment format
length checksum
length, in bytes of
UDP segment,
including header
 no connection
establishment (which can
add delay)
 simple: no connection
state at sender, receiver
 small header size
 no congestion control:
UDP can blast away as
fast as desired
why is there a UDP?

UDP checksum
sender:
 treat segment contents,
including header fields,
as sequence of 16-bit
integers
 checksum: addition
(one’s 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
….
Goal: detect “errors” (e.g., flipped bits) in
transmitted segment

Internet checksum: example
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
Note: when adding numbers, a carryout from the most
significant bit needs to be added to the result

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

Principles of reliable data
transfer
 important in application, transport, link layers
 top-10 list of important networking topics!
 characteristics of unreliable channel will determine
complexity of reliable data transfer protocol (rdt)

transfer

Reliable data transfer: getting started
send
side
receive
side
rdt_send(): called from above,
(e.g., by app.). Passed data to
deliver to receiver upper layer
udt_send(): called by rdt,
to transfer packet over
unreliable channel to receiver
rdt_rcv(): called when packet
arrives on rcv-side of channel
deliver_data(): called by
rdt to deliver data to upper

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!
 use finite state machines (FSM) to specify sender,
receiver
state
1
state
2
event causing state transition
actions taken on state transition
state: when in this
“state” next state
uniquely determined
by next event
event
actions
Reliable data transfer: getting started

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 reads data from underlying channel
Wait for
call from
above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packet,data)
deliver_data(data)
Wait for
call from
below
rdt_rcv(packet)
sender receiver

 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->sender
rdt2.0: channel with bit errors
How do humans recover from “errors”
during conversation?

 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
 feedback: control msgs (ACK,NAK) from receiver to
sender
rdt2.0: channel with bit errors

rdt2.0: FSM specification
Wait for
call from
above
sndpkt = make_pkt(data, checksum)
udt_send(sndpkt)
extract(rcvpkt,data)
deliver_data(data)
udt_send(ACK)
rdt_rcv(rcvpkt) &&
notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) && isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) &&
corrupt(rcvpkt)
Wait for
ACK or
NAK
Wait for
call from
below
sender
receiver
rdt_send(data)


rdt2.0: operation with no errors
Wait for
call from
above
snkpkt = make_pkt(data, checksum)
udt_send(sndpkt)
deliver_data(data)
udt_send(ACK)
rdt_rcv(rcvpkt) &&
notcorrupt(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) &&
corrupt(rcvpkt)
Wait for
ACK or
NAK
Wait for
call from
below
rdt_send(data)


rdt2.0: error scenario
Wait for
call from
above
snkpkt = make_pkt(data, checksum)
udt_send(sndpkt)
deliver_data(data)
udt_send(ACK)
rdt_rcv(rcvpkt) &&
notcorrupt(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) &&
corrupt(rcvpkt)
Wait for
ACK or
NAK
Wait for
call from
below
rdt_send(data)


rdt2.0 has a fatal flaw!
what happens if
ACK/NAK corrupted?
 sender doesn’t know
what happened at
receiver!
 can’t just retransmit:
possible duplicate
handling duplicates:
 sender retransmits
current pkt if ACK/NAK
corrupted
 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

sender receiver
rcv pkt
send ack
send ack
rcv corrupted feedback
resend pkt
send pkt
rcv pkt
pkt
pkt
ack
Duplicate delivery
X
…
deliver upward
deliver upward
dup???

sender receiver
rcv pkt
send ack
send ack
send pkt 0
rcv pkt
pkt 0
pkt 1
ack
sender adds sequence number to each pkt
…
deliver upward
deliver upward
ack
send pkt 1

sender receiver
rcv pkt
send ack
send ack
send pkt 0
rcv pkt
pkt 0
pkt 0
ack
sender adds sequence number to each pkt
…
deliver upward
dup -> discard
ack
resend pkt 0
X
X

rdt2.1: sender, handles garbled ACK/NAKs
Wait for
call 0 from
above
sndpkt = make_pkt(0, data, checksum)
udt_send(sndpkt)
rdt_send(data)
Wait for
ACK or
NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
( corrupt(rcvpkt) ||
isNAK(rcvpkt) )
udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt)
&& notcorrupt(rcvpkt)
&& isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
isNAK(rcvpkt) )
rdt_rcv(rcvpkt)
&& isACK(rcvpkt)
Wait for
call 1 from
above
Wait for
ACK or
NAK 1



Wait for
0 from
below
sndpkt = make_pkt(NAK, chksum)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
not corrupt(rcvpkt) &&
has_seq0(rcvpkt)
rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)
&& has_seq1(rcvpkt)
deliver_data(data)
sndpkt = make_pkt(ACK, chksum)
udt_send(sndpkt)
Wait for
1 from
below
&& has_seq0(rcvpkt)
deliver_data(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) && (corrupt(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
not corrupt(rcvpkt) &&
has_seq1(rcvpkt)
rdt_rcv(rcvpkt) && (corrupt(rcvpkt)
udt_send(sndpkt)
sndpkt = make_pkt(NAK, chksum)
udt_send(sndpkt)
rdt2.1: receiver, handles garbled ACK/NAKs

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
“expected” pkt should
have seq # of 0 or 1
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

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

sender receiver
rcv pkt1
send ACK
send ACK
send pkt1
send pkt0
rcv pkt0
Pkt 0
Pkt 1
Ack 0
No bit error in RDT 2.2
…
deliver 1st pkt upward
Ack 1
deliver 2nd pkt upward
send ACK
send pkt0
rcv pkt0
Pkt 0
Ack 0
deliver 3rd pkt upward

sender receiver
rcv pkt
send ACK
send ACK
send pkt1
send pkt0
rcv pkt
Pkt 0
Pkt 1
Ack 0
Corrupted packet handling in RDT 2.2
X
…
deliver upward
Discard corrupted packet
Ack 0

rdt2.2: sender, receiver fragments
Wait for
call 0 from
above
udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) &&
isACK(rcvpkt,1) )
rdt_rcv(rcvpkt)
&& isACK(rcvpkt,0)
Wait for
ACK
0
sender FSM
fragment
&& has_seq1(rcvpkt)
deliver_data(data)
sndpkt = make_pkt(ACK1, chksum)
udt_send(sndpkt)
Wait for
0 from
below
rdt_rcv(rcvpkt) &&
(corrupt(rcvpkt) ||
has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSM
fragment


sender receiver
so far so good ?
send ACK
send pkt
send pkt
rcv pkt
Pkt 0
Pkt 1
Ack 0
packet loss in RDT 2.2
X
…
deliver upward
waits and waits
...
...
...
waits and waits

rdt3.0: channels with errors and loss
new assumption:
underlying channel
can also lose packets
(data, 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 seq. #’s
already handles this
 receiver must specify seq
# of pkt being ACKed
 requires countdown timer

rdt3.0 sender
udt_send(sndpkt)
start_timer
rdt_send(data)
Wait
for
ACK0
rdt_rcv(rcvpkt) &&
isACK(rcvpkt,1) )
Wait for
call 1 from
above
udt_send(sndpkt)
start_timer
rdt_send(data)
rdt_rcv(rcvpkt)
&& isACK(rcvpkt,0)
rdt_rcv(rcvpkt) &&
isACK(rcvpkt,0) )
rdt_rcv(rcvpkt)
&& isACK(rcvpkt,1)
stop_timer
stop_timer
udt_send(sndpkt)
start_timer
timeout
udt_send(sndpkt)
start_timer
timeout
rdt_rcv(rcvpkt)
Wait for
call 0from
above
Wait
for
ACK1

rdt_rcv(rcvpkt)




sender receiver
rcv pkt1
rcv pkt0
send ack0
send ack1
send ack0
rcv ack0
send pkt0
send pkt1
rcv ack1
send pkt0
rcv pkt0
pkt0
pkt0
pkt1
ack1
ack0
ack0
(a) no loss
sender receiver
rcv pkt1
rcv pkt0
send ack0
send ack1
send ack0
rcv ack0
send pkt0
send pkt1
rcv ack1
send pkt0
rcv pkt0
pkt0
pkt0
ack1
ack0
ack0
(b) packet loss
pkt1
X
loss
pkt1
timeout
resend pkt1
rdt3.0 in action

rdt3.0 in action
rcv pkt1
send ack1
(detect duplicate)
pkt1
sender receiver
rcv pkt1
rcv pkt0
send ack0
send ack1
send ack0
rcv ack0
send pkt0
send pkt1
rcv ack1
send pkt0
rcv pkt0
pkt0
pkt0
ack1
ack0
ack0
(c) ACK loss
ack1
X
loss
pkt1
timeout
resend pkt1
rcv pkt1
send ack1
(detect duplicate)
pkt1
sender receiver
rcv pkt1
send ack0
rcv ack0
send pkt1
send pkt0
rcv pkt0
pkt0
ack0
(d) premature timeout/ delayed ACK
pkt1
timeout
resend pkt1
ack1
send ack1
send pkt0
rcv ack1
pkt0
ack1
ack0
send pkt0
rcv ack1 pkt0
rcv pkt0
send ack0
ack0
rcv pkt0
send ack0
(detect duplicate)

Performance of rdt3.0
 rdt3.0 is correct, but performance stinks
 e.g.: 1 Gbps link, 15 ms prop. delay, 8000 bit packet:
 U sender: utilization – fraction of time sender busy sending
U
sender =
.008
30.008
= 0.00027
L / R
RTT + L / R
=
 if RTT=30 msec, 1KB pkt every 30 msec: 33kB/sec thruput
over 1 Gbps link
 network protocol limits use of physical resources!
Dtrans =
L
R
8000 bits
109 bits/sec
= = 8 microsecs

rdt3.0: stop-and-wait operation
first packet bit transmitted, t0 = 0
sender receiver
RTT
last packet bit transmitted, t1 = L / R
first packet bit arrives
last packet bit arrives, send ACK
ACK arrives, send next
packet, t = RTT + L / R
U
sender =
.008
30.008
= 0.00027
L / R
RTT + L / R
=

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

Pipelining: increased utilization
first packet bit transmitted, t = 0
sender receiver
RTT
last bit transmitted, t = L / R
first packet bit arrives
last packet bit arrives, send ACK
ACK arrives, send next
packet, t = RTT + L / R
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
3-packet pipelining increases
utilization by a factor of 3!
U
sender =
.0024
30.008
= 0.00081
3L / R
RTT + L / R
=

Pipelined protocols: overview
Go-back-N:
 sender can have up to
N unacked packets in
pipeline
 receiver only sends
cumulative ack
 doesn’t ack packet if
there’s a gap
 sender has timer for
oldest unacked packet
 when timer expires,
retransmit all unacked
packets
Selective Repeat:
 sender can have up to N
unack’ed packets in
pipeline
 rcvr sends individual ack
for each packet
 sender maintains timer
for each unacked packet
retransmit only that
unacked packet

3-51
GBN , SR : no loss
send pkt0
send pkt1
send pkt2
send pkt3
(wait)
sender receiver
rcv ack0, send pkt4
rcv ack1, send pkt5
rcv pkt4, deliver, send ack4
0 1 2 3 4 5 6 7 8
sender window (N=4)
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
rcv ack2, send pkt6
rcv ack3, send pkt7
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8

3-52
GBN , SR - Last packet loss
send pkt0
send pkt1
send pkt2
send pkt3
(wait)
sender receiver
0 1 2 3 4 5 6 7 8
sender window (N=4)
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
re-send pkt3
0 1 2 3 4 5 6 7 8
pkt 3 timeout
loss
X
0 1 2 3 4 5 6 7 8

GBN - Loss in consecutive transmissions
send pkt0
send pkt1
send pkt2
send pkt3
(wait)
sender receiver
rcv pkt3, discard, resend ack1
rcv ack0
rcv ack1
pkt 2 timeout
resend pkt2
resend pkt3
Xloss
ignore duplicate ACK 1
0 1 2 3 4 5 6 7 8
sender window (N=4)
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
rcv ack2
rcv ack3
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8

Pipelined protocols: overview
Go-back-N:
 sender can have up to
N unacked packets in
pipeline
 receiver only sends
cumulative ack
 doesn’t ack packet if
there’s a gap
 sender has timer for
oldest unacked packet
retransmit all unacked
packets
Selective Repeat:
 sender can have up to N
unack’ed packets in
pipeline
 rcvr sends individual ack
for each packet
 sender maintains timer
for each unacked packet
retransmit only that
unacked packet

3-55
GBN - cumulative ack
send ...
send ...
...
sender receiver
Have packets 0, 1, 2, 3, 4, and
5 all been successfully
received in order by the
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 #
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt)
&& notcurrupt(rcvpkt)
&& hasseqnum(rcvpkt,expectedseqnum)
deliver_data(data)
sndpkt = make_pkt(expectedseqnum,ACK,chksum)
udt_send(sndpkt)
expectedseqnum++
expectedseqnum=1
sndpkt =
make_pkt(expectedseqnum,ACK,chksum)

GBN: receiver extended FSM

Selective repeat in action
send pkt0
send pkt1
send pkt2
send pkt3
(wait)
sender receiver
receive pkt0, send ack0
receive pkt3, buffer,
send ack3
rcv ack0, send pkt4
rcv ack1, send pkt5
pkt 2 timeout
send pkt2
Xloss
send ack4
send ack5
rcv pkt2; deliver pkt2,
pkt3, pkt4, pkt5; send ack2
record ack3 arrived
0 1 2 3 4 5 6 7 8
sender window (N=4)
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
record ack4 arrived
record ack4 arrived
Q: what happens when ack2 arrives?

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
 limits seq #s of sent, unACKed pkts

3-59
Performance comparison between GBN and SR
send pkt0
send pkt1
send pkt2
send pkt3
(wait)
GBN sender GBN receiver
Xloss
receive ack3, send pkt4
Xloss
Xloss
All ACKs except the last one are lost

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

TCP: Overview 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) inits
sender, receiver state
before data exchange
 flow controlled:
 sender will not
overwhelm receiver
 point-to-point:
 one sender, one receiver
 reliable, in-order byte
stream:
 no “message
boundaries”
 pipelined:
 TCP congestion and flow
control set window size

TCP segment structure
32 bits
application
data
(variable length)
sequence number
acknowledgement number
receive window
Urg data pointer
checksum
F
S
R
P
A
U
head
len
not
used
options (variable length)
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
# bytes
rcvr willing
to accept
counting
by bytes
of data
(not segments!)
Internet
checksum
(as in UDP)

TCP seq. numbers, ACKs
sequence numbers:
byte stream “number” of
first byte in segment’s
data
acknowledgements:
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 implementor source port # dest port #
sequence number
checksum
rwnd
urg pointer
incoming segment to sender
A
sent
ACKed
sent, not-
yet ACKed
(“in-
flight”)
usable
but not
yet sent
not
usable
window size
N
sender sequence number space
sequence number
checksum
rwnd
urg pointer
outgoing segment from sender

User
types
‘C’
host ACKs
receipt
of echoed
‘C’
host ACKs
receipt of
‘C’, echoes
back ‘C’
simple telnet scenario
Host B
Host A
Seq=42, ACK=79, data = ‘C’
Seq=79, ACK=43, data = ‘C’
Seq=43, ACK=80

sends ”Hello, Tom.”
Host B
Host A
Seq=100, ACK=200
Seq=X1, ACK=X2
Seq=X3, ACK=X4
sends ”Hello, Alice.”
sends ACK

TCP round trip time, timeout
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
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

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

TCP reliable data transfer
 TCP creates rdt service
on top of IP’s unreliable
service
 pipelined segments
 cumulative acks
 single retransmission
timer
 retransmissions
triggered by:
 timeout events
 duplicate acks

TCP: retransmission scenarios
lost ACK scenario
Host B
Host A
Seq=92, 8 bytes of data
ACK=100
X
timeout
ACK=100
premature timeout
Host B
Host A
ACK=100
Seq=92, 8
bytes of data
timeout
ACK=120
ACK=120
SendBase=100
SendBase=120
SendBase=120
SendBase=92

TCP: retransmission scenarios
X
cumulative ACK
Host B
Host A
ACK=100
timeout
ACK=120

TCP 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 same data
(“triple duplicate ACKs”),
resend unacked
segment with smallest
seq #
 likely that unacked
segment lost, so don’t
wait for timeout
TCP fast retransmit
(“triple duplicate ACKs”),

X
fast retransmit after sender
receipt of triple duplicate ACK
Host B
Host A
ACK=100
timeout
ACK=100
ACK=100
ACK=100
TCP fast retransmit

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

TCP flow control
application
process
TCP socket
receiver buffers
TCP
code
IP
code
application
OS
receiver protocol stack
application may
remove data from
TCP socket buffers ….
… slower than TCP
receiver is delivering
(sender is sending)
from sender
receiver controls sender, so
sender won’t overflow
receiver’s buffer by transmitting
too much, too fast
flow control

TCP flow control
buffered data
free buffer space
rwnd
RcvBuffer
TCP segment payloads
to application process
 receiver “advertises” free
buffer space by including
rwnd value in TCP header
of receiver-to-sender
segments
 RcvBuffer size set via
socket options (typical default
is 4096 bytes)
 many operating systems
autoadjust RcvBuffer
 sender limits amount of
unacked (“in-flight”) data to
receiver’s rwnd value
 guarantees receive buffer
will not overflow
receiver-side buffering

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow control

TCP 3-way handshake
SYNbit=1, Seq=x
choose init seq num, x
send TCP SYN msg
ESTAB
SYNbit=1, Seq=y
ACKbit=1; ACKnum=x+1
choose init seq num, y
send TCP SYNACK
msg, acking SYN
ACKbit=1, ACKnum=y+1
received SYNACK(x)
indicates server is live;
send ACK for SYNACK;
this segment may contain
client-to-server data
received ACK(y)
indicates client is live
SYNSENT
ESTAB
SYN RCVD
client state
LISTEN
server state
LISTEN

TCP: closing a connection
 client, server each close their side of connection
 send TCP segment with FIN bit = 1
 respond to received FIN with ACK
 on receiving FIN, ACK can be combined with own FIN

FIN_WAIT_2
CLOSE_WAIT
FINbit=1, seq=y
ACKbit=1; ACKnum=y+1
ACKbit=1; ACKnum=x+1
wait for server
close
can still
send data
can no longer
send data
LAST_ACK
CLOSED
TIMED_WAIT
timed wait
for 2*max
segment lifetime
CLOSED
TCP: closing a connection
FIN_WAIT_1 FINbit=1, seq=x
can no longer
send but can
receive data
clientSocket.close()
Host A’s state state
ESTAB
ESTAB

Chapter 3 outline
3.1 transport-layer
services
demultiplexing
3.3 connectionless
transport: UDP
data transfer
transport: TCP
 flow 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!
Principles of congestion control

TCP congestion control:
 approach: sender increases transmission rate
(window size), probing for usable bandwidth, until
loss occurs

TCP Slow Start
 when connection begins,
increase rate
exponentially until first
loss event:
 initially cwnd = 1 MSS
 double cwnd every RTT
 done by incrementing
cwnd for every ACK
received
 summary: initial rate is
slow but ramps up
exponentially fast
Host A
RTT
Host B
time

Q: when should the
exponential
increase switch to
linear?
A: when cwnd gets
to 1/2 of its value
before timeout.
Implementation:
 variable ssthresh
 on loss event, ssthresh
is set to 1/2 of cwnd just
before loss event
TCP: switching from slow start to CA

TCP: detecting, reacting to loss
 loss indicated by timeout:
 cwnd set to 1 MSS;
 window then grows exponentially (as in slow start)
to threshold, then grows linearly
 loss indicated by 3 duplicate ACKs: TCP RENO
 dup ACKs indicate network capable of delivering
some segments
 cwnd is cut in half window then grows linearly
 TCP Tahoe always sets cwnd to 1 (timeout or 3
duplicate acks)

TCP congestion control: additive increase
multiplicative decrease
 approach: sender increases transmission rate
(window size), probing for usable bandwidth, until
loss occurs
 additive increase: increase cwnd by 1 MSS every
RTT until loss detected
 multiplicative decrease: cut cwnd in half after loss
cwnd:
TCP
sender
congestion
window
size
AIMD saw tooth
behavior: probing
for bandwidth
additively increase window size …
…. until loss occurs (then cut window in half)
time

Summary: TCP Congestion Control
timeout
ssthresh = cwnd/2
cwnd = 1 MSS
dupACKcount = 0
retransmit missing segment

cwnd > ssthresh
congestion
avoidance
cwnd = cwnd + MSS (MSS/cwnd)
dupACKcount = 0
transmit new segment(s), as allowed
new ACK
.
dupACKcount++
duplicate ACK
fast
recovery
cwnd = cwnd + MSS
duplicate ACK
ssthresh= cwnd/2
cwnd = ssthresh + 3
dupACKcount == 3
timeout
ssthresh = cwnd/2
cwnd = 1
dupACKcount = 0
ssthresh= cwnd/2
cwnd = ssthresh + 3
dupACKcount == 3
cwnd = ssthresh
dupACKcount = 0
New ACK
slow
start
timeout
ssthresh = cwnd/2
cwnd = 1 MSS
dupACKcount = 0
cwnd = cwnd+MSS
dupACKcount = 0
new ACK
dupACKcount++
duplicate ACK

cwnd = 1 MSS
ssthresh = 64 KB
dupACKcount = 0
New
ACK!
New
ACK!
New
ACK!

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