1. 2017 session 1 TELE3118: Network Technologies Week 9: Transport Layer Basics, UDP
Slides have been adapted from:
Computer Networking: A Top Down Approach, 7th (global) edition. Jim Kurose, Keith Ross. Pearson, April All material copyright J.F Kurose and K.W. Ross, All Rights Reserved.
Computer Networks, 5th edition. Andrew S. Tanenbaum, David J. Wetherall, Pearson, 2010.
Transport Layer

2. Transport services and protocols
application
transport
network
data link
physical
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
logical end-end transport
application
transport
network
data link
physical
Transport Layer

3. Transport vs. network layer
network layer: logical communication between hosts
transport layer: logical communication between processes
relies on, enhances, network layer services
household analogy:
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
Transport Layer

4. Internet transport-layer protocols
application
transport
network
data link
physical
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
network
data link
physical
network
data link
physical
network
data link
physical
logical end-end transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Transport Layer

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

6. 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
32 bits
source port #
dest port #
other header fields
application
data
(payload)
TCP/UDP segment format
Transport Layer

7. Connectionless demultiplexing
recall: when creating datagram to send into UDP socket, must specify
destination IP address
destination port #
recall: created socket has host-local port #:
DatagramSocket mySocket = new DatagramSocket(12534);
when host receives UDP segment:
checks destination port # in segment
directs UDP segment to socket with that port #
IP datagrams with same dest. port #, but different source IP addresses and/or source port numbers will be directed to same socket at dest
Transport Layer

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

9. 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
Transport Layer

10. Connection-oriented demux: example
application
application
application P4 P5 P6 P3 P2 P3
transport
transport
transport
network
network
network
link
link
link
physical
physical
physical
server: IP address B
source IP,port: B,80
dest IP,port: A,9157
host: IP address C
host: IP address A
source IP,port: C,5775
dest IP,port: B,80
source IP,port: A,9157
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
Transport Layer

11. Connection-oriented demux: example
threaded server
application
application
application P4 P3 P2 P3
transport
transport
transport
network
network
network
link
link
link
physical
physical
physical
server: IP address B
source IP,port: B,80
dest IP,port: A,9157
host: IP address C
host: IP address A
source IP,port: C,5775
dest IP,port: B,80
source IP,port: A,9157
dest IP, port: B,80
source IP,port: C,9157
dest IP,port: B,80
Transport Layer

12. 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!
Transport Layer

13. UDP: segment header why is there a UDP?
length, in bytes of UDP segment, including header
32 bits
source port #
dest port #
length
checksum
why is there a UDP?
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
application
data
(payload)
UDP segment format
Transport Layer

14. Reliable data transfer: principles
sender
receiver
sender
receiver
send pkt0
send pkt0
pkt0
pkt0
rcv pkt0
rcv pkt0
send ack0
send ack0
ack0
ack0
rcv ack0
rcv ack0
send pkt1
pkt1
send pkt1
pkt1 X
loss
rcv pkt1
ack1
send ack1
rcv ack1
send pkt0
pkt2
timeout
resend pkt1
rcv pkt2
ack2
send ack2
pkt1
rcv pkt1
ack1
send ack1
rcv ack1
send pkt0
pkt2
(a) no loss
rcv pkt2
ack2
send ack2
(b) packet loss
Transport Layer

15. rdt: Stop-and-wait operation
sender
receiver
first packet bit transmitted, t = 0
last packet bit transmitted, t = L / R
first packet bit arrives
RTT
last packet bit arrives, send ACK
ACK arrives, send next
packet, t = RTT + L / R
Stop-and-wait is correct, but performance stinks
e.g.: 1 Gbps link, 15 ms prop. delay, 8000 bit packet:
Transport Layer

16. rdt with pipelining 9-packet pipelining increases
sender
receiver
first packet bit transmitted, t = 0
last bit transmitted, t = L / R
first packet bit arrives
RTT
last packet bit arrives, send ACK
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
ACK arrives, send next
packet, t = RTT + L / R
9-packet pipelining increases
utilization by a factor of 3!
Transport Layer

17. 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
Transport Layer

18. 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
when timer expires, retransmit only that unacked packet
Transport Layer

19. Go-Back-N: sender 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 receive duplicate ACKs (see receiver)
timer for oldest in-flight pkt
timeout(n): retransmit packet n and all higher seq # pkts in window
Transport Layer

20. 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
Transport Layer

21. Selective repeat: sender, receiver windows
Transport Layer

22. TCP: Overview RFCs: 793,1122,1323, 2018, 2581 point-to-point:
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 steam:
no “message boundaries”
pipelined:
TCP congestion and flow control set window size
Transport Layer

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

24. TCP seq. numbers, ACKs sequence numbers:
source port #
dest port #
sequence number
acknowledgement number
checksum
rwnd
urg pointer
outgoing segment from sender
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
window size N
sender sequence number space
source port #
dest port #
sequence number
acknowledgement number
checksum
rwnd
urg pointer
incoming segment to sender
sent
ACKed
sent, not-yet ACKed
(“in-flight”)
usable
but not
yet sent
not
usable A
Transport Layer

25. TCP seq. numbers, ACKs simple telnet scenario Host A Host B User types
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
simple telnet scenario
Transport Layer

26. 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
Transport Layer

27. TCP round trip time, timeout
EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT
exponential weighted moving average
influence of past sample decreases exponentially fast
typical value:  = 0.125
RTT: gaia.cs.umass.edu to fantasia.eurecom.fr
RTT (milliseconds)
sampleRTT
EstimatedRTT
time (seconds)
Transport Layer

28. TCP round trip time, timeout
timeout interval: EstimatedRTT plus “safety margin”
large variation in EstimatedRTT -> larger safety margin
estimate SampleRTT deviation from EstimatedRTT:
DevRTT = (1-)*DevRTT +
*|SampleRTT-EstimatedRTT|
(typically,  = 0.25)
TimeoutInterval = EstimatedRTT + 4*DevRTT
estimated RTT
“safety margin”
* Check out the online interactive exercises for more examples:
Transport Layer

29. 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
let’s initially consider simplified TCP sender:
ignore duplicate acks
ignore flow control, congestion control
Transport Layer

30. TCP sender events: timeout: data rcvd from app:
retransmit segment that caused timeout
restart timer
ack rcvd:
if ack acknowledges previously unacked segments
update what is known to be ACKed
start timer if there are still unacked segments
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
Transport Layer

31. TCP sender (simplified)
create segment, seq. #: NextSeqNum
pass segment to IP (i.e., “send”)
NextSeqNum = NextSeqNum + length(data)
if (timer currently not running)
start timer
data received from application above L
wait
for
event
NextSeqNum = InitialSeqNum
SendBase = InitialSeqNum
retransmit not-yet-acked segment with smallest seq. #
start timer
timeout
if (y > SendBase) {
SendBase = y
/* SendBase–1: last cumulatively ACKed byte */
if (there are currently not-yet-acked segments)
start timer
else stop timer }
ACK received, with ACK field value y
Transport Layer

32. TCP: retransmission scenarios
Host A
Host B
Host A
Host B
SendBase=92
Seq=92, 8 bytes of data
Seq=92, 8 bytes of data
Seq=100, 20 bytes of data
timeout
timeout
ACK=100 X
ACK=100
ACK=120
Seq=92, 8 bytes of data
Seq=92, 8
bytes of data
SendBase=100
SendBase=120
ACK=100
ACK=120
SendBase=120
lost ACK scenario
premature timeout
Transport Layer

33. TCP: retransmission scenarios
Host A
Host B
timeout
Seq=92, 8 bytes of data
Seq=100, 20 bytes of data
ACK=100 X
ACK=120
Seq=120, 15 bytes of data
cumulative ACK
Transport Layer

34. 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 starts at lower end of gap
Transport Layer

35. TCP fast retransmit if sender receives 3 ACKs for same data
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.
TCP fast retransmit
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
(“triple duplicate ACKs”),
Transport Layer

36. TCP fast retransmit X fast retransmit after sender
Host A
Host B
timeout
Seq=92, 8 bytes of data
Seq=100, 20 bytes of data X
ACK=100
ACK=100
ACK=100
ACK=100
Seq=100, 20 bytes of data
fast retransmit after sender
receipt of triple duplicate ACK
Transport Layer