Transport Layer: UDP and TCP - UMass Amherst

80 Pages·2015·1.91 MB·English

by  V. Arun

Transport Layer: UDP and TCP
CS491G: Computer Networking Lab
V. Arun
Slides adapted from Kurose and Ross
Transport Layer 3-1
Transport Layer: Outline
1 transport-layer 4 connection-oriented
services transport: TCP
2 multiplexing and §  segment structure
demultiplexing §  reliable data transfer
§  flow control
3 connectionless
§  connection management
transport: UDP
5 principles of congestion
control
6 TCP congestion control
Transport Layer 3-2
Transport services and protocols
application
transport
provide logical communication
v  network
data lin k
between app processes
physical
running on different hosts
transport protocols run in
v 
end systems
§  send side: breaks app
messages into segments,
passes to network layer
§  recv side: reassembles
application
segments into messages,
transport
network
passes to app layer
data lin k
physical
more than one transport
v 
protocol available to apps
§  Internet: TCP and UDP
Transport Layer 3-3
Transport vs. network layer
network layer: logical
v 
household analogy:
communication
between hosts 12 kids in Ann’s house sending
letters to 12 kids in Bill’s
house:
hosts = houses
transport layer: logical v 
v 
processes = kids
communication v 
app messages = letters in
between processes v 

envelopes
§ relies on and enhances
transport protocol = Ann
v 
network layer services and Bill who demux to in-
house siblings
network-layer protocol =
v 
postal service

Transport Layer 3-4
Internet transport-layer protocols
application
reliable, in-order
v  transport
network
delivery (TCP) data lin k
physical
network
§  congestion control network data lin k
data lin k physical
physical
§  flow control
network
data lin k
§  connection setup physical

network
unreliable, unordered
data lin k
v 
physical

delivery: UDP network
data lin k
physical
§  no-frills extension of network
data lin k application
“best-effort” IP physical transport
network
network
data lin k
data lin k
services not available: physical
v  physical
§  delay guarantees
§  bandwidth guarantees
Transport Layer 3-5
Transport Layer: Outline
1 transport-layer 4 connection-oriented
services transport: TCP
2 multiplexing and §  segment structure
demultiplexing §  reliable data transfer
§  flow control
3 connectionless
§  connection management
transport: UDP
5 principles of congestion
control
6 TCP congestion control
Transport Layer 3-6
Multiplexing/demultiplexing
multiplexing at sender:
demultiplexing at receiver:
handle data from multiple
sockets, add transport header use header info to deliver
(later used for demultiplexing) received segments to correct
socket
application
application P1 P2 application
socket
P3 P4
transport
process
network
transport transport
link
network network
physica l
link link
physica l physica l
Transport Layer 3-7
How demultiplexing works
host receives IP datagrams
v  32 bits
§  each datagram has source

source port # dest port #
and destination IP address
§  each datagram carries one

other header fields
transport-layer segment
§  each segment has source
and destination port number
application
host uses IP addresses & data
v 

(payload)
port numbers to direct
segment to right socket

TCP/UDP segment format
Transport Layer 3-8
Connectionless demultiplexing
recall: created socket has recall: when creating
v  v 
host-local port #: datagram to send into
UDP socket, must specify
DatagramSocket mySocket1
= new DatagramSocket(12534);
§  destination IP address

§  destination port #
when host receives UDP IP datagrams with same
v 
segment: dest. (IP, port), but different
source IP addresses and/
§  checks destination IP and
or source port numbers
port # in segment
will be directed to same
§  directs UDP segment to
socket
socket bound to that
(IP,port)
Transport Layer 3-9
Connectionless demux: example
DatagramSocket
serverSocket = new
DatagramSocket
DatagramSocket
DatagramSocket
mySocket2 = new
mySocket1 = new
DatagramSocket (6428);
DatagramSocket
(9157); (5775);
application

application application
P1
P3 P4
transport
transport transport
network
network network
link
link link
physica l
physica l physica l
source port: 6428 source port: ?
dest port: 9157 dest port: ?
source port: 9157 source port: ?
dest port: 6428 dest port: ?
Transport Layer 3-10