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Chapter 3
Transport Layer
Computer Networking: A Top Down Approach Featuring the Internet , 3 rd^ edition. Jim Kurose, Keith Ross Addison-Wesley, July
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Chapter 3: Transport Layer
Our goals:
understand
principles behind
transport layer
services:
(^) multiplexing/demulti plexing (^) reliable data transfer (^) flow control (^) congestion control
learn about transport
layer protocols in the
Internet:
(^) UDP: connectionless transport (^) TCP: connection-oriented transport (^) TCP congestion control
Transport services and
protocols
(^) 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
applicatio n transport network data link physical
applicatio n transport network data link physical
network data link physical
network data link physical
network data link physical
network data link data linknetwork^ physical physical
logical end-end transport
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 sending letters
to 12 kids
processes = kids
app messages =
letters in envelopes
hosts = houses
transport protocol =
Ann and Bill
network-layer protocol
= postal service
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of
reliable data transfer
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
Multiplexing/demultiplexing
application
transport
network
link
physical
P1 application transport
network
link
physical
application
transport
network
link
physical
P3 (^) P1^ P2 P
host 1 host 2^ host 3
= socket = process
delivering received segments to correct socket
Demultiplexing at rcv host: gathering data from multiple sockets, enveloping data with header (later used for demultiplexing)
Multiplexing at send host:
Connectionless
demultiplexing
Create sockets with port
numbers:
DatagramSocket mySocket1 = new DatagramSocket(99111);
DatagramSocket mySocket2 = new DatagramSocket(99222);
UDP socket identified by
two-tuple:
(dest IP address, dest port number)
When host receives
UDP segment:
(^) checks destination port number in segment (^) directs UDP segment to socket with that port number
IP datagrams with
different source IP
addresses and/or
source port numbers
directed to same socket
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428);
Client IP:B
P
client IP: A
P3^ P1P
server IP: C
SP: 6428 DP: 9157
SP: 9157 DP: 6428
SP: 6428 DP: 5775
SP: 5775 DP: 6428
SP provides “return address”
Connection-oriented demux
(cont)
Client IP:B
P
client IP: A
P4 P2^ P
server IP: C
SP: 9157 DP: 80
SP: 9157 DP: 80
P5 P6^ P
D-IP:C
S-IP: A D-IP:C
S-IP: B
SP: 5775 DP: 80
D-IP:C
S-IP: B
Connection-oriented demux:
Threaded Web Server
Client IP:B
P
client IP: A
P2^ P
server IP: C
SP: 9157 DP: 80
SP: 9157 DP: 80
P4 P
D-IP:C
S-IP: A D-IP:C
S-IP: B
SP: 5775 DP: 80
D-IP:C
S-IP: B
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
Why is there a UDP?
(^) no connection establishment (which can add delay) (^) simple: no connection state at sender, receiver (^) small segment header (^) no congestion control: UDP can blast away as fast as desired
UDP: more
(^) often used for streaming
multimedia apps (^) loss tolerant (^) rate sensitive
other UDP uses
DNS
SNMP
(^) reliable transfer over
UDP: add reliability at application layer (^) application-specific error recovery!
source port # dest port #
32 bits
Application data (message)
UDP segment format
length checksum
Length, in bytes of UDP segment, including header
Internet Checksum Example
Note
When adding numbers, a carryout from the
most significant bit needs to be added to
the result
Example: add two 16-bit integers
wraparound
sum checksum
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of
reliable data transfer
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
