CS 455/555 Data Communications and Computer Networks – Fall 2005
Introduction to Transport Protocols:
Transmission Control Protocol (TCP)
& User Datagram Protocol (UDP)
Chapters 24 & 25
Reliable Stream transport Service
• Problem: If underlying protocol is
unreliable, how do we guarantee efficient,
reliable delivery?
– Write complex applications that handle
network-related error conditions
– Increase the complexity of the underlying
protocols to include reliable transport
– Develop a general purpose protocol that
guarantees reliable stream delivery and that
removes reliability responsibilities from the
applications and lower-layer protocols.
Introduction
• Internet Protocol (IP) provides “unreliable
datagram service” between hosts. At the IP
layer, a destination address only identifies a
host computer.
– For which user or application?
• Transport protocols provide end-to-end
delivery between endpoints of a connection,
e.g., processes or programs.
End-to-End Communication
Introduction (continued)
• IP datagram service is unreliable in that
datagrams may be
– Lost (usually dropped due to congestion)
– Duplicated
– Delivered out of order
• TCP provides reliable service nevertheless
• Challenge:
– How to provide a reliable transport service over
unreliable (conceptually) technology?
Transport and Network Layers
• Transport Layer Services:
–
–
–
–
Reliable/Unreliable data transfer
Delay guarantees
Bandwidth guarantees
Security
• Transport protocol is built using services from
Network Layer
• Transport services can be constrained by the
services of the underlying network layer protocol
– Delay and bandwidth guarantees
– Unreliable data transfer
Internet Transport Layer
• User Datagram Protocol (UDP) and
Transmission Control Protocol (TCP) are
Internet transport layer protocols
• We refer to layer 4 PDU as a segment (for
both UDP and TCP)
• The Network Layer protocol, IP, is a besteffort delivery service (unreliable).
• UDP and TCP provide application
multiplexing & demultiplexing and data
integrity
Multiplexing/Demultiplexing
Protocol software throughout the layers multiplex
or demultiplex among objects at the next layer.
– At the transport layer level:
• Demultiplexing - delivering segments to the correct
application process
• Multiplexing - gathering data at the source host
from different processes and encapsulation
Demultiplexing
Application
Application
Transport
ICMP
Application
Application
TCP
UDP
IGMP
Network
IP
ARP
Link
RARP
Ethernet
Driver
incoming frame
Application
Demultiplexing Details
1024-5000
FTP
server 21
User process
User process
User process
User process
TCP src port
ICMP
UDP 17
1
IGMP
2
Others
TCP dest
port

source
addr
hdr
cksum
type
IP x0800
AppleTalk
data


TCP
RARP x8035
Novell
discard
9 server

header
6
IP header protocol
x0806
7


ARP
dest
addr
TCP
telnet
23 server
echo
server
Ethernet frame type
dest
addr
source
addr

data

IP

data

(Ethernet frame types in hex, others in
decimal)
CRC
TCP Connections
• Virtual
• An ordered pair of endpoints
– An endpoint is an ordered pair containing
• An IP address
• A TCP port number
• This information is kept by the OS
– netstat
Port Numbers
FTP client,
port 1184
FTP protocol
FTP server,
port 21
TCP
TCP protocol
TCP
IP
IP protocol
IP
Ethernet protocol
Ethernet
Driver
Ethernet
Driver
• TCP and UDP use port numbers to identify the
applications at each end of a communication.
• How are these port numbers chosen?
How port numbers are chosen
• Port numbers are managed by the Internet Assigned
Numbers Authority (IANA)
– http://www.iana.org/assignments/port-numbers
• Servers are known by their well-known port
number
e.g. FTP
TCP port 21
Telnet TCP port 23
Biff
UDP port 512
• Clients use ephemeral port numbers
(“ephemeral” means “short-lived”)
• Most (but not all) TCP/IP implementations allocate
ephemeral 1024-5000
How port numbers are chosen
• The port numbers are divided into three ranges:
– the Well Known Ports,
– the Registered Ports,
– and the Dynamic and/or Private Ports.
• The Well Known Ports are those from 0 through
1023.
• The Registered Ports are those from 1024 through
49151
• The Dynamic and/or Private Ports are those from
49152 through 65535
Well-known port numbers
• Can be found in /etc/services on Unix system
[codd:~] [9:57am] -> grep telnet /etc/services
telnet
23/tcp
[codd:~] [9:57am] -> grep daytime /etc/services
daytime
13/tcp
daytime
13/udp
[codd:~] [9:58am] -> grep echo /etc/services
echo
7/tcp
echo
7/udp
• Are independent on UDP vs. TCP, but when same service is
offered, same number is used
Reserved Ports
• On Unix systems, ports between 1 and 1023
are reserved ports
• Only a superuser process can assign itself a
reserved port
• This fact is used by some applications
(e.g. rlogin) to implement security between
client and server.
– not very secure however, in era of PC’s and
workstations!!!
Some Sample Services
• There are a few standard, simple services that
almost every TCP/IP implementation provides.
• Three examples:
TCP UDP
Name port port RFC Description
echo 7 7 862 server echoes what client sends
discard 9 9 863 server discards what client sends
daytime13 13 867 server returns time/date in ascii
UDP
• Adds very little to IP:
– Multiplexing/demultiplexing
– Error checking
• Some applications are better suited for UDP
because:
–
–
–
–
–
No connection establishment
No connection state
Small packet overhead
Unregulated send rate
Multicast
Applications and Transport Protocols
Application
Application-Layer
Protocol
Underlying Transport
Protocol
Email
SMTP
TCP
Remote Terminal Access
Telnet
TCP
Web
HTTP
TCP
File Transfer
FTP
TCP
Remote File Server
NFS
Typically UDP
Streaming Multimedia
Proprietary
Typically UDP
Network Management
SNMP
Typically UDP
Routing Protocol
RIP
Typically UDP
Name Translation
DNS
Typically UDP
TCP
• Transmission Control Protocol (TCP) is
most widely used transport protocol
• Provides reliable data delivery by using IP
unreliable datagram delivery
• Compensates for loss, delay, duplication
and similar problems in Internet
components
• Reliable delivery is high-level, familiar
model for construction or applications
TCP
• TCP is protocol (rules, formats), not a piece of
software
• “specifies the format of data and acknowledgements
that two computers exchange in order to achieve
reliable transfer, as well as the procedures the
computers use to ensure that data arrives correctly”
• Determines how hosts should recover from errors
like lost or duplicate packets
• Details initiation and completion procedures for a
TCP stream transfer between 2 hosts
TCP Services
• Characteristics of services provided to applications:
– connection-oriented:
• Connection state resides entirely on end system
• end nodes must agree before transfer can begin
– Point-to-point:
• TCP uses port to identify the destination user process within a host
• After port numbers are assigned on both ends, transfer may begin
• Does not allow multicasting
– complete reliability: no data loss or out-of-order
– Provides full duplex communication:
• concurrent transfer in both ways
• appears as 2 independent streams
– Stream interface:
• data sent to or received from TCP in streams of bits,
divided into octets
• unstructured datastream: does not honor “structured”
streams of data (like records in a file)
• buffered transfer - protocol SW free to divide streams
into packets regardless of how the application sends
– reliable connection startup:
• Virtual Circuit connection - contact made; negotiate for
reliability options; inform applications of ready status
– graceful connection shutdown;
• all data guaramteed to be delivered before closing the
connection
TCP “Service” Summary
• Transport layer makes data transmission transparent to
the upper layers
• Responsible for end-to-end delivery, segmentation and
concatenation
• TCP provides a service to the application layer which
is:
– connection-oriented: connection established before
communication takes place, terminated when done, through
3-way handshakes.
– byte-stream: no record boundaries, no structure, just a
sequence of bytes.
TCP “Service” Summary (cont.)
– reliable:
•
•
•
•
no loss: lost packets are retransmitted
no corruption: checksums insure integrity
ordered: data delivered in the order it was received
no duplicates
– flow controlled: fast sender will not overwhelm
slow receiver, or waste bandwidth on the network,
entire network cooperates to avoid congestion.
– full-duplex: every connection is bi-directional.