Chapter 3 outline 3.1 Transport-layer 3.5 Connection-oriented - - PowerPoint PPT Presentation
Chapter 3 outline 3.1 Transport-layer 3.5 Connection-oriented services transport: TCP 3.2 Multiplexing and segment structure demultiplexing reliable data transfer flow control 3.3 Connectionless connection
Transport Layer 3-1
3.1 Transport-layer
3.2 Multiplexing and
3.3 Connectionless
3.4 Principles of
3.5 Connection-oriented
segment structure reliable data transfer flow control connection management
3.6 Principles of
3.7 TCP congestion
Transport Layer 3-2
RFCs: 793, 1122, 1323, 2018, 2581 full duplex data:
bi-directional data flow
in same connection
MSS: maximum segment
size connection-oriented:
handshaking (exchange
sender, receiver state before data exchange flow controlled:
sender will not
point-to-point:
reliable, in-order byte
no “message boundaries”
pipelined:
TCP congestion and flow
control set window size send & receive buffers
socket door TCP send buffer TCP receive buffer socket door
segment
application writes data application reads data
Transport Layer
ACK but no payload.
showing TCP header fields including an option, followed by the payload.
Transport Layer
Transport Layer 3-5
byte stream
“number” of first byte in segment’s data ACKs:
seq # of next byte
expected from
cumulative ACK
Q: how receiver handles
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
User types ‘C’ host ACKs receipt
‘C’ host ACKs receipt of ‘C’, echoes back ‘C’
time simple telnet scenario
Transport Layer 3-6
longer than RTT
but RTT varies
too short: premature
timeout
unnecessary
retransmissions
too long: slow reaction
to segment loss
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
Transport Layer 3-7
EstimatedRTT = (1- α)*EstimatedRTT + α*SampleRTT Exponential weighted moving average influence of past sample decreases exponentially fast typical value: α = 0.125
Transport Layer 3-8
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 (milliseconds) SampleRTT Estimated RTT
Transport Layer 3-9
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-β)*DevRTT + β*|SampleRTT-EstimatedRTT| (typically, β = 0.25) Then set timeout interval:
Transport Layer 3-10
3.1 Transport-layer
3.2 Multiplexing and
3.3 Connectionless
3.4 Principles of
3.5 Connection-oriented
segment structure reliable data transfer flow control connection management
3.6 Principles of
3.7 TCP congestion
Transport Layer 3-11
TCP creates rdt
Pipelined segments Cumulative acks TCP uses single
Retransmissions are
timeout events duplicate acks
Initially consider
ignore duplicate acks ignore flow control,
congestion control
Transport Layer 3-12
Create segment with
seq # is byte-stream
start timer if not
expiration interval:
retransmit segment
restart timer
If acknowledges
update what is known to
be acked
start timer if there are
Transport Layer 3-13
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 */
Comment:
cumulatively ack’ed byte Example:
y= 73, so the rcvr wants 73+ ; y > SendBase, so that new data is acked
Transport Layer 3-14
Host A
S e q = 1 , 2 b y t e s d a t a ACK=100
time premature timeout
Host B
Seq=92, 8 bytes data ACK=120 Seq=92, 8 bytes data Seq=92 timeout ACK=120
Host A
Seq=92, 8 bytes data ACK=100
loss
timeout
lost ACK scenario
Host B
Seq=92, 8 bytes data A C K = 1
time
Seq=92 timeout
SendBase = 100 SendBase = 120 SendBase = 120 Sendbase = 100
Transport Layer 3-15
Host A
Seq=92, 8 bytes data ACK=100
loss
timeout
Cumulative ACK scenario
Host B
Seq=100, 20 bytes data A C K = 1 2
time
SendBase = 120
Transport Layer 3-16
Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. # . Gap detected Arrival of segment that partially or completely fills gap
Delayed ACK. Wait up to 500ms for 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 Immediate send ACK, provided that segment starts at lower end of gap
Transport Layer 3-17
Time-out period often
long delay before
resending lost packet Detect lost segments
Sender often sends
many segments back-to- back
If segment is lost,
there will likely be many duplicate ACKs. If sender receives 3
fast retransmit: resend
segment before timer expires
Transport Layer 3-18
Host A timeout Host B
time
resend 2nd segment
Figure 3.37 Resending a segment after triple duplicate ACK
Transport Layer 3-19
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 }
a duplicate ACK for already ACKed segment fast retransmit
Transport Layer 3-20
3.1 Transport-layer
3.2 Multiplexing and
3.3 Connectionless
3.4 Principles of
3.5 Connection-oriented
segment structure reliable data transfer flow control connection management
3.6 Principles of
3.7 TCP congestion
Transport Layer 3-21
– Receiving app. is slow at reading from TCP’s input buffer – this is an App-layer problem, but solved w/ help from Transport layer.
– sending app. blocks on calls to its transport layer
Transport Layer 3-22
spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare
Sender limits unACKed
guarantees receive
buffer doesn’t overflow
Transport Layer 3-23
3.1 Transport-layer
3.2 Multiplexing and
3.3 Connectionless
3.4 Principles of
3.5 Connection-oriented
segment structure reliable data transfer flow control connection management
3.6 Principles of
3.7 TCP congestion
Transport Layer 3-24
establish “connection” before exchanging data segments
initialize TCP variables:
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();
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 3-25
client closes socket: clientSocket.close();
sends TCP FIN control segment to server
FIN, replies with ACK. Closes connection, sends FIN.
client
FIN
server
A C K ACK F I N
close close closed timed wait
Transport Layer 3-26
replies with ACK.
Enters “timed wait” -
will respond with ACK to received FINs
modification, can handle simultaneous FINs.
client
FIN
server
A C K ACK F I N
closing closing closed timed wait closed
Transport Layer 3-27
TCP client lifecycle TCP server lifecycle