3-1 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: connectionless transport TCP: connection- oriented transport TCP congestion control
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3-1
Transport LayerOur 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: connectionless
transport TCP: connection-oriented
transport TCP congestion control
3-2
Outline
Transport-layer services
Multiplexing and demultiplexing
Connectionless transport: UDP
Principles of reliable data transfer
Connection-oriented transport: TCP segment structure reliable data transfer flow control connection
management
Principles of congestion control
TCP congestion control
3-3
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) init’s sender, receiver state before data exchange
flow controlled: sender will not
overwhelm receiver
point-to-point: one sender, one
receiver
reliable, in-order byte steam
pipelined: TCP congestion and
flow control set window size
send & receive buffers
socketdoor
T C Psend buffer
T C Preceive buffer
socketdoor
segm ent
applicationwrites data
applicationreads data
3-4
TCP segment structure
source port # dest port #
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
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)
The head length is in terms of 32-bit words
TCP supports “byte stream” service
3-5
…Which is emulated by TCP segments
3-6
3-7
TCP seq. #’s and ACKsSeq. #’s:
byte stream “number” of first byte in segment’s data
ACKs: seq # of next byte
expected from other side
cumulative ACKQ: how receiver handles
out-of-order segments A: TCP spec doesn’t
say, - up to implementor
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’, echoesback ‘C’
timesimple telnet scenario
Sequence numbers
3-8
Initial Sequence Numbers
3-9
TCP Slide Window
How much data can a TCP sender have outstanding in the network?
How much data should TCP retransmit when an error occurs? Just selectivly repeat the missing data?
How does the TCP sender avoid overrunning the receiver’s buffers?
3-10
TCP Slide Window
3-11
3-12
TCP Round Trip Time and TimeoutQ: 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
3-13
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )*EstimatedRTT + *SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially
fast typical value: = 0.125
3-14
Example RTT estimation:RTT: gaia.cs.umass.edu to fantasia.eurecom.fr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
3-15
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus “safety margin”
large variation in EstimatedRTT -> larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT:
TimeoutInterval = EstimatedRTT + 4*DevRTT
DevRTT = (1-)*DevRTT + *|SampleRTT-EstimatedRTT|
(typically, = 0.25)
Then set timeout interval:
3-16
Outline
Transport-layer services
Multiplexing and demultiplexing
Connectionless transport: UDP
Principles of reliable data transfer
Connection-oriented transport: TCP segment structure reliable data transfer flow control connection
management
Principles of congestion control
TCP congestion control
3-17
TCP reliable data transfer
TCP creates rdt service on top of IP’s unreliable service
Pipelined segments Cumulative acks TCP uses single
retransmission timer Flavor of “Go-back-
N”
Retransmissions are triggered by: timeout events duplicate acks
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 */
Comment:• SendBase-1: last cumulatively ack’ed byteExample:• SendBase-1 = 71;y= 73, so the rcvrwants 73+ ;y > SendBase, sothat new data is acked
3-20
TCP: retransmission scenarios
Host A
Seq=100, 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92, 8 bytes data
ACK=120
Seq=92, 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92, 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92, 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
3-21
TCP retransmission scenarios (more)
Host A
Seq=92, 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100, 20 bytes data
ACK=120
time
SendBase= 120
3-22
TCP ACK generation [RFC 1122, RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq #. All data up toexpected seq # already ACKed
Arrival of in-order segment withexpected seq #. One other segment has ACK pending
Arrival of out-of-order segmenthigher-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 single cumulative ACK, ACKing both in-order segments
Immediately send duplicate ACK, indicating seq. # of next expected byte
Immediately send ACK, provided thatsegment starts at lower end of gap
3-23
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
3-24
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 { increment count of dup ACKs received for y if (count of dup ACKs received for y == 3) { resend segment with sequence number y }
Fast retransmit algorithm:
a duplicate ACK for already ACKed segment
fast retransmit
3-25
Outline
Transport-layer services
Multiplexing and demultiplexing
Connectionless transport: UDP
Principles of reliable data transfer
Connection-oriented transport: TCP segment structure reliable data transfer flow control connection
management
Principles of congestion control
TCP congestion control
3-26
TCP Flow Control
receive side of TCP connection has a receive buffer:
speed-matching service: matching the send rate to the receiving app’s drain rate app process may be