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1
3: Transport Layer 3a-1
Chapter 3: Transport LayerChapter goals:❒ understand principles
behind transport layerservices:
❍ multiplexing/demultiplexing
❍ reliable data transfer❍ flow control❍ congestion control
❒ instantiation andimplementation in theInternet
Chapter Overview:❒ transport layer services❒ multiplexing/demultiplexing❒ connectionless transport: UDP❒ principles of reliable data
transfer❒ connection-oriented transport:
TCP❍ reliable transfer❍ flow control❍ connection management
❒ principles of congestion control❒ TCP congestion control
3: Transport Layer 3a-2
Transport services and protocols
❒ provide logical communicationbetween app’ processesrunning on different hosts
❒ transport protocols run inend systems
❒ transport vs network layerservices:
❒ network layer: data transferbetween end systems
❒ transport layer: datatransfer between processes
❍ relies on, enhances, networklayer services
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
2
3: Transport Layer 3a-3
Transport-layer protocols
Internet transport services:❒ reliable, in-order unicast
delivery (TCP)❍ congestion❍ flow control❍ connection setup
❒ unreliable (“best-effort”),unordered unicast ormulticast delivery: UDP
❒ services not available:❍ real-time❍ bandwidth guarantees❍ reliable multicast
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
3: Transport Layer 3a-4
applicationtransportnetwork
M P2applicationtransportnetwork
Multiplexing/demultiplexingRecall: segment - unit of data
exchanged betweentransport layer entities
❍ aka TPDU: transportprotocol data unit
receiver
HtHn
Demultiplexing: delivering received segments to correct app layer processes
segmentsegment M
applicationtransportnetwork
P1M
M MP3 P4
segmentheader
application-layerdata
3
3: Transport Layer 3a-5
Multiplexing/demultiplexing
multiplexing/demultiplexing:❒ based on sender, receiver
port numbers, IP addresses❍ source, dest port #s in
each segment❍ recall: well-known port
numbers for specificapplications
gathering data from multiple app processes, enveloping data with header (later used for demultiplexing)
source port # dest port #
32 bits
applicationdata
(message)
other header fields
TCP/UDP segment format
Multiplexing:
3: Transport Layer 3a-6
Multiplexing/demultiplexing: examples
host A server Bsource port: xdest. port: 23
source port:23dest. port: x
port use: simple telnet app
Web clienthost A
Webserver B
Web clienthost C
Source IP: CDest IP: B
source port: xdest. port: 80
Source IP: CDest IP: B
source port: ydest. port: 80
port use: Web server
Source IP: ADest IP: B
source port: xdest. port: 80
4
3: Transport Layer 3a-7
UDP: User Datagram Protocol [RFC 768]
❒ “no frills,” “bare bones”Internet transportprotocol
❒ “best effort” service, UDPsegments may be:
❍ lost❍ delivered out of order
to app❒ connectionless:
❍ no handshaking betweenUDP sender, receiver
❍ each UDP segmenthandled independentlyof others
Why is there a UDP?❒ no connection
establishment (which canadd delay)
❒ simple: no connection stateat sender, receiver
❒ small segment header❒ no congestion control: UDP
can blast away as fast asdesired
3: Transport Layer 3a-8
UDP: more❒ often used for streaming
multimedia apps❍ loss tolerant❍ rate sensitive
❒ other UDP uses(why?):
❍ DNS❍ SNMP
❒ reliable transfer over UDP:add reliability atapplication layer
❍ application-specificerror recover!
source port # dest port #
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength, in
bytes of UDPsegment,including
header
5
3: Transport Layer 3a-9
UDP checksum
Sender:❒ treat segment contents
as sequence of 16-bitintegers
❒ checksum: addition (1’scomplement sum) ofsegment contents
❒ sender puts checksumvalue into UDP checksumfield
Receiver:❒ compute checksum of
received segment❒ check if computed checksum
equals checksum field value:❍ NO - error detected❍ YES - no error detected.
But maybe errorsnonethless? More later ….
Goal: detect “errors” (e.g., flipped bits) in transmittedsegment
3: Transport Layer 3a-10
Principles of Reliable data transfer❒ important in app., transport, link layers❒ top-10 list of important networking topics!
❒ characteristics of unreliable channel will determinecomplexity of reliable data transfer protocol (rdt)
6
3: Transport Layer 3a-11
Reliable data transfer: getting started
sendside
receiveside
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 packetarrives on rcv-side of channel
deliver_data(): called byrdt to deliver data to upper
3: Transport Layer 3a-12
Reliable data transfer: getting startedWe’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
state1
state2
event causing state transitionactions taken on state transition
state: when in this“state” next state
uniquely determinedby next event
eventactions
7
3: Transport Layer 3a-13
Rdt1.0: reliable transfer over a reliable channel
❒ underlying channel perfectly reliable❍ no bit erros❍ no loss of packets
❒ separate FSMs for sender, receiver:❍ sender sends data into underlying channel❍ receiver read data from underlying channel
3: Transport Layer 3a-14
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❍ human scenarios using ACKs, NAKs?
❒ new mechanisms in rdt2.0 (beyond rdt1.0):❍ error detection❍ receiver feedback: control msgs (ACK,NAK) rcvr->sender
8
3: Transport Layer 3a-15
rdt2.0: FSM specification
sender FSM receiver FSM
3: Transport Layer 3a-16
rdt2.0: in action (no errors)
sender FSM receiver FSM
9
3: Transport Layer 3a-17
rdt2.0: in action (error scenario)
sender FSM receiver FSM
3: Transport Layer 3a-18
rdt2.0 has a fatal flaw!
What happens ifACK/NAK corrupted?
❒ sender doesn’t know whathappened at receiver!
❒ san’t just retransmit:possible duplicate
What to do?❒ sender ACKs/NAKs
receiver’s ACK/NAK? Whatif sender ACK/NAK lost?
❒ retransmit, but this mightcause retransmission ofcorrectly received pkt!
Handling duplicates:❒ sender adds sequence
number to each pkt❒ sender retransmits current
pkt if ACK/NAK garbled❒ receiver discards (doesn’t
deliver up) duplicate pkt
Sender sends one packet, then waits for receiver response
stop and wait
10
3: Transport Layer 3a-19
rdt2.1: sender, handles garbled ACK/NAKs
3: Transport Layer 3a-20
rdt2.1: receiver, handles garbled ACK/NAKs
11
3: Transport Layer 3a-21
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” pkthas 0 or 1 seq. #
Receiver:❒ must check if received
packet is duplicate❍ state indicates whether
0 or 1 is expected pktseq #
❒ note: receiver can notknow if its lastACK/NAK received OKat sender
3: Transport Layer 3a-22
rdt2.2: a NAK-free protocol
❒ same functionality asrdt2.1, using NAKs only
❒ instead of NAK,receiver sends ACK forlast pkt received OK
❍ receiver must explicitlyinclude seq # of pktbeing ACKed
❒ duplicate ACK atsender results in sameaction as NAK:retransmit current pkt
senderFSM
!
12
3: Transport Layer 3a-23
rdt3.0: channels with errors and loss
New assumption:underlying channel canalso lose packets (dataor ACKs)
❍ checksum, seq. #, ACKs,retransmissions will beof help, but not enough
Q: how to deal with loss?❍ sender waits until
certain data or ACKlost, then retransmits
❍ yuck: drawbacks?
Approach: sender waits“reasonable” amount oftime for ACK
❒ retransmits if no ACKreceived in this time
❒ if pkt (or ACK) just delayed(not lost):
❍ retransmission will beduplicate, but use of seq.#’s already handles this
❍ receiver must specify seq# of pkt being ACKed
❒ requires countdown timer
3: Transport Layer 3a-24
rdt3.0 sender
13
3: Transport Layer 3a-25
rdt3.0 in action
3: Transport Layer 3a-26
rdt3.0 in action
14
3: Transport Layer 3a-27
Performance of rdt3.0
❒ rdt3.0 works, but performance stinks❒ example: 1 Gbps link, 15 ms e-e prop. delay, 1KB packet:
Ttransmit = 8kb/pkt10**9 b/sec = 8 microsec
Utilization = U = = 8 microsec30.016 msec
fraction of timesender busy sending = 0.00015
❍ 1KB pkt every 30 msec -> 33kB/sec thruput over 1 Gbps link❍ network protocol limits use of physical resources!
3: Transport Layer 3a-28
Pipelined protocolsPipelining: 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
15
3: Transport Layer 3a-29
Go-Back-NSender:❒ k-bit seq # in pkt header❒ “window” of up to N, consecutive unack’ed pkts allowed
❒ ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”❍ may deceive duplicate ACKs (see receiver)
❒ timer for each in-flight pkt❒ timeout(n): retransmit pkt n and all higher seq # pkts in window
3: Transport Layer 3a-30
GBN: sender extended FSM
16
3: Transport Layer 3a-31
GBN: receiver extended FSM
receiver simple:❒ 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!❍ ACK pkt with highest in-order seq #
3: Transport Layer 3a-32
GBN inaction
17
3: Transport Layer 3a-33
Selective Repeat
❒ receiver individually acknowledges all correctlyreceived pkts
❍ buffers pkts, as needed, for eventual in-order deliveryto upper layer
❒ sender only resends pkts for which ACK notreceived
❍ sender timer for each unACKed pkt❒ sender window
❍ N consecutive seq #’s❍ again limits seq #s of sent, unACKed pkts
3: Transport Layer 3a-34
Selective repeat: sender, receiver windows
18
3: Transport Layer 3a-35
Selective repeat
data from above :❒ if next available seq # in
window, send pkttimeout(n):❒ resend pkt n, restart timerACK(n) in [sendbase,sendbase+N]:
❒ mark pkt n as received❒ if n smallest unACKed pkt,
advance window base tonext unACKed seq #
senderpkt n in [rcvbase, rcvbase+N-1]
❒ send ACK(n)❒ out-of-order: buffer❒ in-order: deliver (also
deliver buffered, in-orderpkts), advance window tonext not-yet-received pkt
pkt n in [rcvbase-N,rcvbase-1]
❒ ACK(n)otherwise:❒ ignore
receiver
3: Transport Layer 3a-36
Selective repeat in action
19
3: Transport Layer 3a-37
Selective repeat: dilemmaExample:❒ seq #’s: 0, 1, 2, 3❒ window size=3
❒ receiver sees nodifference in twoscenarios!
❒ incorrectly passesduplicate data as newin (a)
Q: what relationshipbetween seq # sizeand window size?
1
3: Transport Layer 3b-1
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 (exchangeof control msgs) init’ssender, receiver statebefore data exchange
❒ flow controlled:❍ sender will not
overwhelm receiver
❒ point-to-point:❍ one sender, one receiver
❒ reliable, in-order bytesteam:
❍ no “message boundaries”❒ pipelined:
❍ TCP congestion and flowcontrol set window size
❒ send & receive buffers
socketdoor
TCPsend buffer
TCPreceive buffer
socketdoor
segment
applicationwrites data
applicationreads data
3: Transport Layer 3b-2
TCP segment structure
source port # dest port #
32 bits
applicationdata
(variable length)
sequence numberacknowledgement number
rcvr window sizeptr urgent datachecksum
FSRPAUheadlen
notused
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 willingto accept
countingby bytes of data(not segments!)
Internetchecksum
(as in UDP)
2
3: Transport Layer 3b-3
TCP seq. #’s and ACKsSeq. #’s:
❍ byte stream“number” of firstbyte in segment’sdata
ACKs:❍ seq # of next byte
expected fromother side
❍ cumulative ACKQ: how receiver handles
out-of-order segments❍ A: TCP spec doesn’t
say, - up toimplementor
Host A Host B
Seq=42, ACK=79, data = ‘C’
Seq=79, ACK=43, data = ‘C’
Seq=43, ACK=80
Usertypes
‘C’
host ACKsreceipt
of echoed‘C’
host ACKsreceipt of‘C’, echoes
back ‘C’
timesimple telnet scenario
3: Transport Layer 3b-4
TCP: reliable data transfer
simplified sender, assuming
waitfor
event
waitfor
event
event: data received from application above
event: timer timeout for segment with seq # y
event: ACK received,with ACK # y
create, send segment
retransmit segment
ACK processing
•one way data transfer•no flow, congestion control
3
3: Transport Layer 3b-5
TCP:reliabledatatransfer
00 sendbase = initial_sequence number 01 nextseqnum = initial_sequence number 0203 loop (forever) { 04 switch(event) 05 event: data received from application above 06 create TCP segment with sequence number nextseqnum 07 start timer for segment nextseqnum 08 pass segment to IP 09 nextseqnum = nextseqnum + length(data) 10 event: timer timeout for segment with sequence number y 11 retransmit segment with sequence number y 12 compue new timeout interval for segment y 13 restart timer for sequence number y 14 event: ACK received, with ACK field value of y 15 if (y > sendbase) { /* cumulative ACK of all data up to y */ 16 cancel all timers for segments with sequence numbers < y 17 sendbase = y 18 } 19 else { /* a duplicate ACK for already ACKed segment */ 20 increment number of duplicate ACKs received for y 21 if (number of duplicate ACKS received for y == 3) { 22 /* TCP fast retransmit */ 23 resend segment with sequence number y 24 restart timer for segment y 25 } 26 } /* end of loop forever */
SimplifiedTCPsender
3: Transport Layer 3b-6
TCP ACK generation [RFC 1122, RFC 2581]
Event
in-order segment arrival, no gaps,everything else already ACKed
in-order segment arrival, no gaps,one delayed ACK pending
out-of-order segment arrivalhigher-than-expect seq. #gap detected
arrival of segment that partially or completely fills gap
TCP Receiver action
delayed ACK. Wait up to 500msfor next segment. If no next segment,send ACK
immediately send singlecumulative ACK
send duplicate ACK, indicating seq. #of next expected byte
immediate ACK if segment startsat lower end of gap
4
3: Transport Layer 3b-7
TCP: retransmission scenariosHost A
Seq=92, 8 bytes data
ACK=100
loss
tim
eout
time lost ACK scenario
Host B
X
Seq=92, 8 bytes data
ACK=100
Host A
Seq=100, 20 bytes data
ACK=100
Seq=
92 t
imeo
ut
time premature timeout,cumulative ACKs
Host B
Seq=92, 8 bytes data
ACK=120
Seq=92, 8 bytes data
Seq=
100
tim
eout
ACK=120
3: Transport Layer 3b-8
TCP Flow Controlreceiver: explicitly
informs sender of(dynamically changing)amount of free bufferspace
❍ RcvWindow field inTCP segment
sender: keeps the amountof transmitted,unACKed data less thanmost recently receivedRcvWindow
sender won’t overrunreceiver’s buffers by
transmitting too much, too fast
flow control
receiver buffering
RcvBuffer = size or TCP Receive Buffer
RcvWindow = amount of spare room in Buffer
5
3: Transport Layer 3b-9
TCP Round Trip Time and Timeout
Q: how to set TCPtimeout value?
❒ longer than RTT❍ note: RTT will vary
❒ too short: prematuretimeout
❍ unnecessaryretransmissions
❒ too long: slow reactionto segment loss
Q: how to estimate RTT?❒ SampleRTT: measured time from
segment transmission until ACKreceipt
❍ ignore retransmissions,cumulatively ACKed segments
❒ SampleRTT will vary, wantestimated RTT “smoother”
❍ use several recentmeasurements, not justcurrent SampleRTT
3: Transport Layer 3b-10
TCP Round Trip Time and Timeout
EstimatedRTT = (1-x)*EstimatedRTT + x*SampleRTT
❒ Exponential weighted moving average❒ influence of given sample decreases exponentially fast❒ typical value of x: 0.1
Setting the timeout❒ EstimtedRTT plus “safety margin”❒ large variation in EstimatedRTT -> larger safety margin
Timeout = EstimatedRTT + 4*Deviation
Deviation = (1-x)*Deviation + x*|SampleRTT-EstimatedRTT|
6
3: Transport Layer 3b-11
TCP Connection Management
Recall: TCP sender, receiverestablish “connection”before exchanging datasegments
❒ initialize TCP variables:❍ seq. #s❍ buffers, flow control
info (e.g. RcvWindow)❒ client: connection initiator Socket clientSocket = new
Socket("hostname","portnumber");
❒ server: contacted by client Socket connectionSocket =
welcomeSocket.accept();
Three way handshake:Step 1: client end system
sends TCP SYN controlsegment to server
❍ specifies initial seq #
Step 2: server end systemreceives SYN, replies withSYNACK control segment
❍ ACKs received SYN❍ allocates buffers❍ specifies server->
receiver initial seq. #
3: Transport Layer 3b-12
TCP Connection Management (cont.)
Closing a connection:client closes socket:
clientSocket.close();
Step 1: client end systemsends TCP FIN controlsegment to server
Step 2: server receivesFIN, replies with ACK.Closes connection, sendsFIN.
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
7
3: Transport Layer 3b-13
TCP Connection Management (cont.)
Step 3: client receives FIN,replies with ACK.
❍ Enters “timed wait” -will respond with ACKto received FINs
Step 4: server, receivesACK. Connection closed.
Note: with smallmodification, can handlysimultaneous FINs.
client
FIN
server
ACK
ACK
FIN
closing
closing
closedti
med
wai
t
closed
3: Transport Layer 3b-14
TCP Connection Management (cont)
TCP clientlifecycle
TCP serverlifecycle
8
3: Transport Layer 3b-15
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!
3: Transport Layer 3b-16
Causes/costs of congestion: scenario 1
❒ two senders, tworeceivers
❒ one router,infinite buffers
❒ no retransmission
❒ large delayswhen congested
❒ maximumachievablethroughput
9
3: Transport Layer 3b-17
Causes/costs of congestion: scenario 2
❒ one router, finite buffers❒ sender retransmission of lost packet
3: Transport Layer 3b-18
Causes/costs of congestion: scenario 2❒ always: (goodput)❒ “perfect” retransmission only when loss:
❒ retransmission of delayed (not lost) packet makes larger(than perfect case) for same
�in
�out=
�in
�out>�
in�out
“costs” of congestion:❒ more work (retrans) for given “goodput”❒ unneeded retransmissions: link carries multiple copies of pkt
10
3: Transport Layer 3b-19
Causes/costs of congestion: scenario 3❒ four senders❒ multihop paths❒ timeout/retransmit
�in
Q: what happens asand increase ?�
in
3: Transport Layer 3b-20
Causes/costs of congestion: scenario 3
Another “cost” of congestion:❒ when packet dropped, any “upstream transmission
capacity used for that packet was wasted!
11
3: Transport Layer 3b-21
Approaches towards congestion control
End-end congestioncontrol:
❒ no explicit feedback fromnetwork
❒ congestion inferred fromend-system observed loss,delay
❒ approach taken by TCP
Network-assistedcongestion control:
❒ routers provide feedbackto end systems
❍ single bit indicatingcongestion (SNA,DECbit, TCP/IP ECN,ATM)
❍ explicit rate sendershould send at
Two broad approaches towards congestion control:
3: Transport Layer 3b-22
Case study: ATM ABR congestion control
ABR: available bit rate:❒ “elastic service”❒ if sender’s path
“underloaded”:❍ sender should use
available bandwidth❒ if sender’s path
congested:❍ sender throttled to
minimum guaranteedrate
RM (resource management)cells:
❒ sent by sender, interspersedwith data cells
❒ bits in RM cell set by switches(“network-assisted”)
❍ NI bit: no increase in rate(mild congestion)
❍ CI bit: congestionindication
❒ RM cells returned to sender byreceiver, with bits intact
12
3: Transport Layer 3b-23
Case study: ATM ABR congestion control
❒ two-byte ER (explicit rate) field in RM cell❍ congested switch may lower ER value in cell❍ sender’ send rate thus minimum supportable rate on path
❒ EFCI bit in data cells: set to 1 in congested switch❍ if data cell preceding RM cell has EFCI set, sender sets CI
bit in returned RM cell
3: Transport Layer 3b-24
TCP Congestion Control❒ end-end control (no network assistance)❒ transmission rate limited by congestion window
size, Congwin, over segments:
❒ w segments, each with MSS bytes sent in one RTT:
throughput = w * MSS
RTT Bytes/sec
Congwin
13
3: Transport Layer 3b-25
TCP congestion control:
❒ two “phases”❍ slow start❍ congestion avoidance
❒ important variables:❍ Congwin
❍ threshold: definesthreshold between twoslow start phase,congestion controlphase
❒ “probing” for usablebandwidth:
❍ ideally: transmit as fastas possible (Congwin aslarge as possible)without loss
❍ increase Congwin untilloss (congestion)
❍ loss: decrease Congwin,then begin probing(increasing) again
3: Transport Layer 3b-26
TCP Slowstart
❒ exponential increase (perRTT) in window size (not soslow!)
❒ loss event: timeout (TahoeTCP) and/or or threeduplicate ACKs (Reno TCP)
initialize: Congwin = 1for (each segment ACKed) Congwin++until (loss event OR CongWin > threshold)
Slowstart algorithmHost A
one segment
RTT
Host B
time
two segments
four segments
14
3: Transport Layer 3b-27
TCP Congestion Avoidance
/* slowstart is over *//* Congwin > threshold */Until (loss event) { every w segments ACKed: Congwin++ }threshold = Congwin/2Congwin = 1perform slowstart
Congestion avoidance
1
1: TCP Reno skips slowstart (fast recovery) after three duplicate ACKs
3: Transport Layer 3b-28
TCP FairnessFairness goal: if N TCP
sessions share samebottleneck link, eachshould get 1/N of linkcapacity
TCP congestionavoidance:
❒ AIMD: additiveincrease,multiplicativedecrease
❍ increase window by 1per RTT
❍ decrease window byfactor of 2 on lossevent
AIMD
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
15
3: Transport Layer 3b-29
Why is TCP fair?Two competing sessions:❒ Additive increase gives slope of 1, as throughout increases❒ multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughput
Conn
ect i
on 2
thr
ough
put
congestion avoidance: additive increaseloss: decrease window by factor of 2
congestion avoidance: additive increaseloss: decrease window by factor of 2
3: Transport Layer 3b-30
TCP latency modeling
Q: How long does it take toreceive an object from aWeb server after sendinga request?
❒ TCP connection establishment❒ data transfer delay
Notation, assumptions:❒ Assume one link between
client and server of rate R❒ Assume: fixed congestion
window, W segments❒ S: MSS (bits)❒ O: object size (bits)❒ no retransmissions (no loss,
no corruption)Two cases to consider:❒ WS/R > RTT + S/R: ACK for first segment in
window returns before window’s worth of datasent
❒ WS/R < RTT + S/R: wait for ACK after sendingwindow’s worth of data sent
16
3: Transport Layer 3b-31
TCP latency Modeling
Case 1: latency = 2RTT + O/R Case 2: latency = 2RTT + O/R+ (K-1)[S/R + RTT - WS/R]
K:= O/WS
3: Transport Layer 3b-32
TCP Latency Modeling: Slow Start
❒ Now suppose window grows according to slow start.❒ Will show that the latency of one object of size O is:
RS
RSRTTP
RORTTLatency P )12(2 ����
���
����
where P is the number of times TCP stalls at server:
}1,{min �� KQP
- where Q is the number of times the server would stall if the object were of infinite size.
- and K is the number of windows that cover the object.
17
3: Transport Layer 3b-33
TCP Latency Modeling: Slow Start (cont.)
RTT
initiate TCPconnection
requestobject
first window= S/R
second window= 2S/R
third window= 4S/R
fourth window= 8S/R
completetransmissionobject
delivered
time atclient
time atserver
Example:
O/S = 15 segments
K = 4 windows
Q = 2
P = min{K-1,Q} = 2
Server stalls P=2 times.
3: Transport Layer 3b-34
TCP Latency Modeling: Slow Start (cont.)
RS
RSRTTPRTT
RO
RSRTT
RSRTT
RO
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18
3: Transport Layer 3b-35
Chapter 3: Summary
❒ principles behindtransport layer services:
❍ multiplexing/demultiplexing❍ reliable data transfer❍ flow control❍ congestion control
❒ instantiation andimplementation in the Internet
❍ UDP❍ TCP
Next:❒ leaving the network
“edge” (applicationtransport layer)
❒ into the network “core”
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