1. Chapter 3 Transport Layer
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Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002.

2. Chapter 3 outline 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
3.1 Transport-layer services
3.2 Multiplexing and demultiplexing
3.3 Connectionless transport: UDP
3.4 Principles of reliable data transfer

3. TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 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
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

4. UDP Segment Structure From lecture 8 Application data (message)
32 bits
source port #
dest port #
Length, in
bytes of UDP
segment,
including
header
From lecture 8
length
checksum
Application
data
(message)
UDP segment format

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

6. TCP/UDP Comparison
see RFC 854 & RFC 1323

7. simple telnet scenario
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
Host A
Host B
User
types
‘C’
Seq=42, ACK=79, data = ‘C’
host ACKs
receipt of
‘C’, echoes
back ‘C’
Seq=79, ACK=43, data = ‘C’
host ACKs
receipt
of echoed
‘C’
Seq=43, ACK=80
Answer: TCP spec doesn’t say, - up to implementor
time
simple telnet scenario

8. Sequence #’s Example: A wants to send B file of 500,000 bytes
MSS is 1,000 bytes in data stream; therefore TCP constructs 500 segments
First byte of data stream is numbered 0
1st segment assigned seq # 0
2nd segment assigned seq#1000
3rd segment assigned seq#2000
etc

9. TCP Round Trip Time and 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

10. TCP Round Trip Time and Timeout
EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT
Exponential weighted moving average
influence of past sample decreases exponentially fast
typical value:  = 0.125

11. Example RTT estimation:

12. TCP Round Trip Time and Timeout
Setting the timeout
EstimtedRTT plus “safety margin”
large variation in EstimatedRTT -> larger safety margin
first estimate of how much SampleRTT deviates from EstimatedRTT:
DevRTT = (1-)*DevRTT +
*|SampleRTT-EstimatedRTT|
(typically,  = 0.25)
Then set timeout interval:
TimeoutInterval = EstimatedRTT + 4*DevRTT

13. Chapter 3 outline 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
3.1 Transport-layer services
3.2 Multiplexing and demultiplexing
3.3 Connectionless transport: UDP
3.4 Principles of reliable data transfer

14. TCP reliable data transfer
REVIEW:
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
Q: Why is TCP using a single timer?

15. 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 the “ack” is for a previously unacked segment:
update what is known to be acked
start timer if there are outstanding segments

16. TCP sender (simplified)
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
event: ACK received, with ACK field value of y
if (y > SendBase) {
SendBase = y
if (there are currently not-yet-acknowledged segments) }
} /* end of loop forever */
TCP sender (simplified)
Comment:
SendBase-1: last
cumulatively ack’ed byte
Example:
SendBase-1 = 71; y= 73, so the rcvr wants 73+ ; y > SendBase, so that new data is acked

17. TCP: retransmission scenarios
Host A
Seq=92, 8 bytes data
ACK=100
loss
timeout
lost ACK scenario
Host B X
time
Animation Link
SendBase
= 100

18. TCP retransmission scenarios (more)
Host A
Host B
Host A
Seq=92, 8 bytes data
ACK=100
loss
timeout
Cumulative ACK scenario
Host B X
Seq=100, 20 bytes data
ACK=120
time
Seq=92 timeout
Seq=92, 8 bytes data
Seq=100, 20 bytes data
ACK=100
ACK=120
Sendbase
= 100
Seq=92, 8 bytes data
SendBase
= 120
SendBase
= 120
Seq=92 timeout
ACK=120
SendBase
= 120
premature timeout
time

19. Doubling Timeout Interval
Timeout Interval is Dynamic:
After each timeout event, TCP doubles the amount of time it waits for the next acknowledgement.
Example: Initial timeout TIME = .75 sec.
(segment retransmitted) TIME = 1.5 sec.
(segment retransmitted) TIME = 3 sec.
( This is only when the timeout event is detected.)

20. TCP ACK generation [RFC 1122, RFC 2581]
Event at Receiver
Arrival of in-order segment with
expected seq #. All data up to
expected seq # already ACKed
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
TCP Receiver action
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 startsat lower end of gap
GOOD
BAD

21. Fast Retransmit
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
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.

22. 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 {
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

23. Chapter 3 outline 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
3.1 Transport-layer services
3.2 Multiplexing and demultiplexing
3.3 Connectionless transport: UDP
3.4 Principles of reliable data transfer

24. TCP Flow Control
sender won’t overflow
receiver’s buffer by
transmitting too much,
too fast
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 slow at reading from buffer

25. TCP Flow control: how it works
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow
- guarantees receive buffer
doesn’t overflow
So that TCP does not overflow the buffer…
LastByteRcvd-LastByteRead <= RcvBuffer
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer
= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Variables:
RcvBuffer
LastByteRead
LastByteRcvd

26. Example: File Transfer How does RcvWindow provide flow control service?
SCENARIO
A sends file to B via TCP connection
B allocates A, a receive buffer
B’s app. pr. reads from the buffer
B tells A the amount of spare room it has in the buffer by setting the value of RcvWindow in every segment it sends to A
Initially B sets RcvWindow = RcvBuffer
A keeps track of LastByteSent and LastByteAcked
LastByteSent-LastByteAcked = unacknowledged data in connection
To prevent overflow –
LastByteSent-LastByteAcked <= RcvWindow
PROBLEM
If B’s receive buffer becomes full and RcvWindow = 0, then nothing can be sent to A
As B’s app. pr. empties, host A never knows that some space is free
yikes…HOST A IS BLOCKED!!!
TCP requires A to continually send 1 data byte to B
The buffer will begin to empty and the acknowledgements will contain and Non-zero RcvWindow value
Applet:

27. Chapter 3 outline 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
3.1 Transport-layer services
3.2 Multiplexing and demultiplexing
3.3 Connectionless transport: UDP
3.4 Principles of reliable data transfer

28. TCP Connection Management
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
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();

29. TCP Connection Management (cont.)
client
FIN
server
ACK
close
closed
timed wait
Closing a connection:
client closes socket: clientSocket.close();
Step 1: client end system sends TCP FIN control segment to server
Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN.

30. TCP Connection Management (cont.)
client
server
Step 3: client receives FIN, replies with ACK.
Enters “timed wait” - will respond with ACK to received FINs
Step 4: server, receives ACK. Connection closed.
Note: with small modification, can handle simultaneous FINs.
closing
FIN
ACK
closing
FIN
ACK
timed wait
closed
closed

31. TCP Connection Management (cont)
TCP server
lifecycle
TCP client
lifecycle