1. Lecture 3 Dr. Richard Spillman Fall 2012
NETWORKS
Lecture 3
Dr. Richard Spillman
Fall 2012

2. OUTLINE
Application
Transport Layer

3. Application

4. Email Systems Connects users through e-mail servers Mail Message
Sender’s PC
Mail
Server
Receiver’s PC
Message

5. Mail Servers Act like a postal service Deliver mail to one another
Report delivery failures
-- hawaii.edu is the mail host
Hold incoming mail until user logs in
Mail servers also called: MTA
Mail Transfer Agent

6. Sending Outgoing Mail Simple Mail Transfer Protocol (SMTP) SMTP SMTP
TCP/IP host-host mail standard (RFC 821)
SMTP servers listen on port 25
Simple but effective, and widely used:
For PC to send to mail host
For one mail host to send to another
SMTP
SMTP

7. Mail Downloading Standards
POP (RFC 1081)
Post Office Protocol
Downloading mail only
Widely implemented
IMAP (RFC 1203)
Internet Message Access Protocol
Newer standard
Do mailbox management tasks beyond downloading
Can leave mail on the server and manage it there

8. ftp FTP server stores files Client logs into FTP server
Client program sends command to get a file
FTP server sends the file with error correction
FTP Server
1. Login
2. FTP “get” Command
3. Download

9. telnet Traditional Terminal-Host communication
Log into host computer (server)
You can run programs there, such as clients
Processing is done on the server
Uses Internet to connect to host (server)
can access host from anywhere on the Internet
Telnet software on client PC
Manages login, use, and disconnections

10. Summary
The application layer adds a header that provides the necessary information to run the application across the network
Physical
Link
Network
Transport
Application
http, ftp, smtp, telnet, . . .

11. Transport Layer
The Transport Layer Gives Platform Independence and Reliability
Two computers do not have to be of the same platform type
A PC user does not even know what kind of computer the webserver is

12. Transport Functions Transport Layer can provide:
vendor independence (uniform network interface)
endpoint addressing (ports)
reliability
flow control
connections
fragmentation and reassembly
But it is not involved in routing – determining where the two computers are actually located

13. Function Details 1 Synchronization and acknowledgement.
Initially, when the transmitter makes contact with the receiver it makes a unique connection.
The transmitter thus knows that the receiver is on-line, and willing to receive data.
Acknowledgements and retransmissions.
This allows the receiver to send back acknowledgements which tell the transmitter that the data segments have been received correctly.
If no acknowledgements have been received, the transmitter can either resend the data, or can assume that the receiver has crashed and that the connection is to be terminated.
Flow control.
This allows the receiver to tell the transmitter that it cannot receive any more data at present.
This typically happens when the receiver has filled-up its receiving buffer.

14. Function Details 2 Windowing.
This is where the transmitter and the receiver agree on a window size when the connection is initially made.
The window then defines the number of data segments that can be sent before the transmitter must wait for an acknowledgement from the receiver.
Multiple connections onto a single data stream.
The transport layer takes data from one or more applications; it then marks them with a unique connection number and segment number.
At the receiver these can be demultiplexed to the correct application program.
Reordering of data segments.
All the data segments that are transmitted are marked with a sequence number.
Thus if any are delivered in the incorrect order, or if any of them are missing, the receiver can easily reorder them or discard segments if one or more are missing.

15. Transport Layer Protocols
The functions of the Transport Layer are implement by one of two common protocols
TCP – Transmission Control Protocol
UDP – User Datagram Protocol
Each adds a different type of header to the data + application package header

16. TCP Operation
When a host wishes to make a connection, TCP sends out a request message to the destination machine that contains unique numbers called a socket number, and a port number.
The port number has a value which is associated with the application (for example a TELNET connection has the port number 23 and an FTP connection has the port number 21).
The message is then passed to the IP layer, which assembles a datagram for transmission to the destination.
When the destination host receives the connection request, it returns a message containing its own unique socket number and a port number.
The socket number and port number thus identify the virtual connection between the two hosts.
After the connection has been made the data can flow between the two hosts (called a data stream).
Sending Port Number
Receiving Port Number
Data Transfer between Ports

17. TCP Reliability Networks are typically unreliable
routers may discard packets if they are overworked (congested)
packets may arrive at the destination out of order, if they travel via different routes through the Internet
physical transmission medium may corrupt data (e.g. electromagnetic interference)
The TCP protocol provides for error correction and detection in order to deal with:
lost packets
packets delivered out of order
duplicate packets
corrupted packets

18. Lost Packets
When a host receives a packet, it replies with the acknowledgement number set to the value of the next sequence number it expects to see
Timeout and retransmission
assume a host has sent a packet of data, and is waiting for an acknowledgement from its peer that has a matching (or greater) sequence number
if that acknowledgement does not arrive within a timeout period, the host will retransmit the packet
If the packet was not lost, just delayed, then there will be two copies of it delivered
No problem, because remote end discards duplicates!

19. Corrupted Packets
The TCP protocol uses a checksum to verify data integrity
A checksum is similar to a hash value
The checksum value is calculated and attached to the data packet before it is sent
At the receiving end, the checksum value is calculated and compared to the value that was sent
If they do not match then the packet is not acknowledged
After the time out period, the packet is sent again

20. Acknowledgement Number Options (variable length)
TCP Header
The TCP protocol adds a header to the data + application header package
TCP
app
data
32 bits
Source Port #
Destination Port #
Sequence Number
Acknowledgement Number
Head
Length
Reserved U A P R S F
Receiver Window Size
CheckSum
Urgent Data Pointer
Options (variable length)

21. End Point Addressing Every computer has a unique address
on the Internet this is the computer’s IP address
But, a single computer can run many applications
e.g. a server in an organization may run a:
web server
mail server
USENET news server
more ...
We need to have an address not only for each computer, but for each network application running on that computer
each application is addressed by port number
Both clients and servers use ports
clients open new ports when creating a connection
servers listen on ports, for incoming connections

22. What is a Port? Port Numbers
Provides unique address for each service on a host
i.e. port# = transport layer address
Range from
Services run on “well-known” or “privileged” ports
Clients connect from non-privileged or dynamic ports
Internet Assigned Numbers Authority (IANA) tracks port assignments
Source Port #
Destination Port #
TCP Header

23. Server Ports
Applications monitor input activity looking for their port number
Server
TELNET server
listens for port 23
Input TCP packets
with port number
FTP server
listens for port 21
HTTP server
listens for port 80
SMTP server
listens for port 161

24. Common TCP Ports Port Service Comment Port Service Comment
1 TCPmux 7 echo
9 discard Null 11 systat Users
13 daytime 15 netstat
17 qotd Quote 18 msp Message send protocol
19 chargen ttytst source 21 ftp
23 telnet 25 smtp Mail
37 time Timserver 39 rlp Resource location
42 nameserver IEN whois Nicname
53 domain DNS 57 mtp Deprecated
67 bootps BOOTP server 67 bootps
68 bootpc BOOTP client 69 tftp
70 gopher Internet Gopher 77 rje Netrjs
79 finger 80 www WWW HTTP
87 link Ttylink 88 kerberos Kerberos v5
95 supdup hostnames
102 iso-tsap ISODE 105 csnet-ns CSO name server
107 rtelnet Remote Telnet 109 pop2 POP version 2
110 pop3 POP version sunrpc
113 auth Rap ID 115 sftp
117 uucp-path nntp USENET
123 ntp Network Timel 137 netbios-ns NETBIOS Name Service
138 netbios-dgm NETBIOS 139 netbios-ssn NETBIOS session
143 imap snmp SNMP
162 snmp-trap SNMP trap 163 cmip-man ISO management over IP
178 nextstep NeXTStep bgp BGP
191 prospero irc Internet Relay Chat
199 smux SNMP Multiplexer 201 at-rtmp AppleTalk routing

25. Socket = IP address + port number
Sockets
Socket = IP address + port number
:80
Client and server each need to open a socket.
The server opens a socket and begins listening.
The client opens a socket and sends to the server’s open socket.