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Departamento de
Tecnología Electrónica
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Chapter 2
Transport Layer
Advance Features
Computer Networking:
A Top Down Approach
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
Transport Layer Advanced Features
1
Chapter 2 outline
 2.1 Transport-layer
services
 2.2 Connectionoriented transport:
TCP
 2.3 Principles of
congestion control
 2.4 TCP congestion
control
 2.5 Stream Control
Transmission Protocol
Transport Layer Advanced Features
2
Chapter 2 outline
 2.1 Transport-layer
services
 2.2 Connectionoriented transport:
TCP
 2.3 Principles of
congestion control
 2.4 TCP congestion
control
 2.5 Stream Control
Transmission Protocol
Transport Layer Advanced Features
3
Transport services and protocols
 provide logical communication between app
processes running on different hosts
 transport protocols run in end systems

breaks app messages into segments, passes to
network layer
Host A
Network
Host B
Transport Layer Advanced Features
4
Internet transport-layer protocols
 more than one transport protocol available to apps
 Internet: TCP and UDP
FTP
HTTP
SMTP
DNS
TCP
UDP
IP
TFTP
Application
Transport
Network
Link layer (LLC y MAC)
Physical
Layer
Transport Layer Advanced Features
5
Internet transport-layer
protocols
 more than one
transport
protocol available
to apps

Internet: TCP
and UDP
TCP
UDP
Connection- oriented Non-connectionoriented
Reliable
Unreliable
Segment grouping
Unfragmented
messages
Rcv orders segments User datagram
ACKs and timers
No ACKs
Flow control
No flow control
Congestion control
No congestion control
Transport Layer Advanced Features
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Internet transport-layer
protocols
 Port: identifies application
 Port numbers: http://www.iana.org/assignments/port-
numbers
Application protocol
Port numbers
Transport protocol
FTP
20, 21
TCP
Telnet
23
TCP
SMTP
25
TCP
DNS
53
UDP (TCP (*))
TFTP
69
UDP
HTTP
80
TCP
POP3
110
TCP
RIP
520
UDP
Transport Layer Advanced Features
7
Chapter 2 outline
 2.1 Transport-layer
services
 2.2 Connectionoriented transport:
TCP
 2.3 Principles of
congestion control
 2.4 TCP congestion
control
 2.5 Stream Control
Transmission Protocol
Transport Layer Advanced Features
8
TCP: Overview
 point-to-point:
 one sender, one receiver
 reliable, in-order byte
stream:

no “message boundaries”
 pipelined:
 TCP congestion and flow
control set window size
 send & receive buffers
socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
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
socket
door
overwhelm receiver
segment
Transport Layer Advanced Features
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TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pnter
Options (variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
application
data
(variable length)
Transport Layer Advanced Features
10
TCP seq. #’s and ACKs
Host A
Host B
Client starts
active open
Server is in passive open,
starts connection and
confirms client open
Client confirms
server open
Connection
established
Transport Layer Advanced Features
time
11
Chapter 2 outline
 2.1 Transport-layer
services
 2.2 Connectionoriented transport:
TCP
 2.3 Principles of
congestion control
 2.4 TCP congestion
control
 2.5 Stream Control
Transmission Protocol
Transport Layer Advanced Features
12
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)
Transport Layer Advanced Features
13
Causes/costs of congestion
Host A
 Two tx:



lIN = Sending rate
lOUT = Receiving
rate
C= Router capability
Host B
lout
lin : original data
unlimited shared
output link buffers
 Queue theory
 large delays
when congested
 maximum
achievable
throughput
Transport Layer Advanced Features
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Causes/costs of congestion:
 Supposition: Tx retx lost packets
 Now, lIN is not tx -> higher rate lIN’
 Congestion costs:


more work (retx)
unneeded retransmissions: link carries multiple copies of pkt
Host A
Host B
lout
lin : original data
unlimited shared
output link buffers
Transport Layer Advanced Features
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Causes/costs of congestion
 Two tx and the same dest
 Supposition: congestion in the router between A and dest
 Another cost of congestion:

when a packet is dropped, any upstream transmission capacity
used for that packet was wasted
Host A
lin : original data
lout
l'in : original data, plus
retransmitted data
finite shared output
link buffers
Host B
Transport Layer Advanced Features
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Causes/costs of congestion
 Congestion costs: a summary
 Big delays when the incoming rate to a router is near to the
link capability.
 The tx must retx the lost segments due to the router
overflow.
 Unnecessary BW is used for the retx of duplicated copies of
segments (due to delays).
 Upstream transmission capacity used for a packet is wasted
when it is discarded.
Transport Layer Advanced Features
17
Approaches towards congestion control
Two broad approaches towards congestion control:
Network-assisted
congestion control:
End-end congestion
control:
 no explicit feedback from
network
 congestion inferred from
end-system observed loss,
delay
 approach taken by TCP
 routers provide feedback
to end systems
 E.g. single bit indicating
congestion
Transport Layer Advanced Features
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Chapter 2 outline
 2.1 Transport-layer
services
 2.2 Connectionoriented transport:
TCP
 2.3 Principles of
congestion control
 2.4 TCP congestion
control
 2.5 Stream Control
Transmission Protocol
Transport Layer Advanced Features
19
TCP Congestion Control
Host A
 sender limits
Host B
Host A
Host B
transmission:
timeout
LastByteSent-LastByteAcked 
CongWin
 How does sender
perceive congestion?

X
loss
loss event = timeout or 3
duplicate acks
TCP sender reduces rate
(CongWin) after loss
event
timeout

X
time
lost ACK
scenario
time
Resending a segment
after triple duplicate
ACK
Transport Layer Advanced Features
20
TCP congestion control:
additive increase,
multiplicative decrease (AIMD)
 Approach: increase transmission rate (window size),
Saw tooth
behavior: probing
for bandwidth
congestion window size
probing for usable bandwidth, until loss occurs
 additive increase: increase CongWin by 1 MSS
every RTT until loss detected
 multiplicative decrease: cut CongWin in half after
loss
congestion
window
24 Kbytes
16 Kbytes
8 Kbytes
time
time
Transport Layer Advanced Features
21
TCP Congestion Control: details
 three phases:
 Roughly,
rate =

CongWin
Bytes/sec
RTT
 When connection begins,
CongWin = 1 MSS


Example: MSS = 500 bytes & RTT
= 200 msec
initial rate = 20 kbps


slow start (SS)
Congestion avoidance (CA):
e.g. AIMD
Fast recovery (FR)
 First two are compulsory in
TCP, while the last one is
recommendable
Transport Layer Advanced Features
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TCP Slow Start
 available bandwidth may
Host B
RTT
be >> MSS/RTT
 When connection begins,
increase rate
exponentially fast until
first loss event
 initial rate is slow but
ramps up exponentially
fast
Host A
time
Transport Layer Advanced Features
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Congestion avoidance
 After 3 dup ACKs:
 CongWin is cut in half
 window then grows linearly
 But after timeout event:
 CongWin instead set to 1
MSS;
 window then grows
exponentially
 to a threshold, then grows
linearly
Philosophy:
 3 dup ACKs indicates
network capable of
delivering some segments
 timeout indicates a “more
alarming” congestion scenario
Transport Layer Advanced Features
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Summary: TCP Congestion Control

When CongWin is below Threshold, sender in slow-start phase,
window grows exponentially.

When CongWin is above Threshold, sender is in congestion-avoidance
phase, window grows linearly.

When a triple duplicate ACK occurs, Threshold set to CongWin/2 and
CongWin set to Threshold.

When timeout occurs, Threshold set to CongWin/2 and CongWin is
set to 1 MSS.
Note: TCP version varies the way
congestion control is applied. For
example, TCP Tahoe always reduces
Congestion Window to its initial
value after a loss event. Meanwhile,
in TCP Reno, all the congestion
phases take place.
Transport Layer Advanced Features
25
Chapter 2 outline
 2.1 Transport-layer
services
 2.2 Connectionoriented transport:
TCP
 2.3 Principles of
congestion control
 2.4 TCP congestion
control
 2.5 Stream Control
Transmission Protocol
Transport Layer Advanced Features
26
TCP and UDP limitations
 TCP limitations:
 TCP keeps strict order: head-of-line blocking may be
a problem (data flow blocked until recovering a lost
segment)
 Byte-oriented nature of TCP: must use PSH to
ensure data goes to app
 No multi-home IP hosts
 Relatively vulnerable to some attacks (SYN flooding)
 UDP limitations
 Not reliable
 No data order
 No congestion control
 Solution: SCTP
Transport Layer Advanced Features
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Stream Control Transport Protocol
 SCTP
 Stream Control Transport Protocol, RFC 2960
 Initially created by SIGTRAN group for PSTN
 SCTP enhancements
 Multi-homed host support: redundancy & efficient flow
 Multiple flows in one association: solves head-of-line
blocking TCP problem
Host A
Host B
App 1
SCTP Port = 100
App 2
SCTP Port = 200
-------------------------------OS
NI-1
NI-2
-------------------------------OS
NI
NI-3
IP3 = 10.1.61.11
IP2 = 160.15.82.20
IP Network
IP = 128.33.6.12
IP1 = 161.10.8.221
Transport Layer Advanced Features
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SCTP features
 SCTP features
 Connection oriented
 Concepts
• Endpoints: SCTP endpoint is a list of addresses, same
ports
• Association  multiple possible IPs
• {[160.15.82.20, 161.10.8.221, 10.1.61.11:100]}:
[128.33.6.12:200]} (two endpoints, one association – Figure -)
Host A
Host B
App 1
SCTP Port = 100
App 2
SCTP Port = 200
-------------------------------OS
NI-1
NI-2
-------------------------------OS
NI
NI-3
IP3 = 10.1.61.11
IP2 = 160.15.82.20
IP Network
IP = 128.33.6.12
IP1 = 161.10.8.221
Transport Layer Advanced Features
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SCTP header
 SCTP header
 First 32 bit words  SCTP common header (similar to UDP
header)
 Verification tag: distinguish between associations; prevent
from attacks
 Chunks: building blocks
Transport Layer Advanced Features
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SCTP header
 SCTP chunks
 Blocks with a 32-bit multiple length
 Different types
• Control (e.g., INIT, INIT-ACK, COOKIE-ECHO, COOKIEACK – four-way handshake -)
• Data (e.g. DATA)
Transport Layer Advanced Features
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