1. Networks and Protocols
WAN Services
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Copyright 2009 Asia Pacific Institute of Information Technology

2. Topic & Structure of the lesson
WAN Services
Application Layer Software

3. Learning Outcomes At the end of this module, YOU should be able to:
Discuss the.

4. Key Terms you must be able to use
If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams:
Analog services
Digital services
Leased lines
Digital subscriber line
Multi-Protocol Label Switching (MPLS)
File Transfer Protocol (FTP)
Trivial File Transfer Protocol (TFTP)
Simple Mail Transfer Protocol (SMTP)
Hypertext Transfer Protocol
World Wide Web (WWW)

5. Main Teaching Points
Repeater

6. Introduction to TCP/IP
Developed for the Advanced Projects Research Agency (ARPA)
Designed to network a wide variety of computing platforms, allowing expensive resources to be shared across the USA
Designed to be fault tolerant in case of nuclear or other war
Originally heavily based on UNIX systems

7. Introduction to TCP/IP
The purpose of TCP/IP
Provides a common communication standard for network devices (e.g. mainframes, PCs, remote devices, telephones, etc.)
Provides a framework for interconnection and interoperation regardless of platform or physical network medium
Where is TCP/IP used?
Basic applications:
Telnet (23) : Remote terminal session

8. Introduction to TCP/IP
FTP (21/20) : File Transfer
SMTP/POP (25/110) : Electronic mail
NFS (uses RPC) : Network File System
More advanced applications:
HTTP (WWW port 80)
A transport for just about everything
TCP/IP overview
The term TCP/IP is used generically to refer to anything and everything related to the specific network (IP) and transport (TCP) layer protocols:

9. Introduction to TCP/IP
UDP IP
ARP
TELNET
FTP
Layered protocols
Early communication software was one big program which did everything

10. Introduction to TCP/IP
Early communication software was one big program which did everything
Difficult to modify and add new functionality
The program was broken down into parts or layers, each layer with very specific functionality
ISO 7 – layer model

11. Introduction to TCP/IP
TCP/IP stack
TCP/IP in relation to the OSI 7 – layer model
Data flow through stack
Data from the application flows down through the layers
Each layer ads its own header information
Each layer multiplexes data from one or more higher places

12. Introduction to TCP/IP
Data from the network flows upward through the layer
Each layer strips off the corresponding layer’s header information
Each layer de-multiplexes information to one or more higher layers
An Ethernet card recognises an Ethernet frame which has its own address in the destination address field
The link layer passes the data to the correct network layer protocol based

13. Introduction to TCP/IP
The link layer passes the data to the correct network layer protocol based on the contents of the TYPE field
The IP layer passes data up to the next layer based on the contents of the protocol field in the IP header
The next layer passes data up to the specific application based on the contents of the Port field in the TCP/UDP header

14. Address Resolution Protocol (ARP)
Addressing issues
TCP/IP is designed for many different types of physical network:
Ethernet
Token Ring
Leased lines
Each has its own format for physical addressing
To run successfully on all existing and future physical networks, IP addressing must be independent of the physical layer

15. Address Resolution Protocol (ARP)
You have no control over the address assigned to your network interface
The manufacturer encodes the address onto the interface (i.e. physical address)
If the card fails and is replaced, the machine’s physical address changes
Addressing problems:
Machines send data to each other using the physical address
We want to send data to another computer’s IP address

16. Address Resolution Protocol (ARP)
The ARP protocol is used to do this
Example of ARP process:
machine A wants to send data to machine B whose IP address is aaa.bb.ccc.ddd
sends a broadcast packet, with 0806 in the type field
Who has IP address aaa.bbb.ccc.ddd?
machine B recognises its own IP address and responds, ‘Hello, that’s me! Here is my hardware address’

17. Address Resolution Protocol (ARP)
machine A now has B’s physical address
The IP frame can now be coded into a properly addressed Ethernet frame

18. Address Resolution Protocol (ARP)
The answer is held in a cache so that the next time A has data for B, it can simply look in the cache for its physical address
Frequently used addresses stay in the cache
Others time-out so as not to waste memory space

19. Internet Control Message Protocol (ICMP)
IP provides best-effort delivery
Delivery problems can be ignored; datagrams can be “thrown away”
ICMP provides error-reporting mechanism
Error detection
Internet layer can detect a variety of errors:
Checksum (header)
Time-To-Live (TTL) expired
No route to destination network
Can’t deliver to destination host (e.g no ARP reply)

20. Internet Control Message Protocol (ICMP)
Internet layer discards datagrams with problems
Error reporting
Some errors can be reported
Router sends message back to source in datagram
Message contains information about problem
Encapsulation in IP datagram
Types of messages
ICMP defines error and informational messages

21. Internet Control Message Protocol (ICMP)
Error messages:
Source quench
Time exceeded
Destination unreachable
Redirect
Fragmentation required
Informational messages:
Echo request/reply
Address mask request/reply
Router discovery

22. Internet Control Message Protocol (ICMP)
Message Transport
ICMP encapsulated in IP
ICMP messages sent in response to incoming datagrams with problems
ICMP message not sent for ICMP message

23. Internet Control Message Protocol (ICMP)
ICMP and Reachability
An Internet host A, is reachable from another host B, if datagrams can be delivered from A to B
ping program tests reachability – sends datagram from B to A that A echoes back to B
Use ICMP echo request and echo reply messages
Internet layer includes code to reply to incoming ICMP echo request messages

24. Internet Control Message Protocol (ICMP)
Ping sample output (Windows)

25. Internet Protocol (IP)
IP module is central to Internet technology
Routes IP packets between intermediate systems
Fragmentation and re-assembly
Unreliable datagram service
IP address: 32 bit value
Usually written as four octets in dotted decimal notation
Consists of two parts: network number and host number
Three classes of IP address: class A, B, and C
Twenty + RFCs specifying IP

26. Internet Protocol (IP)

27. Internet Protocol (IP)
Ver – format of IP header (e.g. 4 or 6)
IHL – IP header length in 32 bit words
TOS – Type of Service
Total length – bytes in the datagram
Identification – IP packet ID number
Flags – may/don’t/last/more fragment(s)
Fragment Offset – for fragment reassembly
TTL – packet time to live (hops)
Protocol – protocol to give data to

28. Internet Protocol (IP)
Header Checksum – checksum on header only
Source Address – IP address of packet originator
Destination Address – IP address of destination
IP Address Classes

29. Internet Protocol (IP)

30. Internet Protocol (IP)

31. Internet Protocol (IP)
Subnet Mask
You have no control over the network address
Assigned by e.g. RIPE, RIR, etc.
Subnet mask allows you to assign part of the host field of the address to be network number
Allows your network to be divided into interior networks; externally only one network address is sufficient to access your site
Keeps size of external routing tables to a minimum

32. Internet Protocol (IP)
The effect of the Netmask

33. Internet Protocol (IP)
As the number of subnets increases, the number of hosts possible on each subnet decreases
The node number of a host on a given subnet is added to the subnet address to give the complete IP number for the node
E.g. With a subnet mask of and a network number of , Host 1 on this network would have the IP number –

34. Internet Protocol (IP)
Best-effort Delivery
IP is designed to work over all types of network hardware, which may malfunction
So IP datagrams may get lost, may be duplicated, may be delayed, may be delivered out of order, or may be delivered in a corrupt state
We need higher layers of protocol software to deal with these errors

35. User Datagram Protocol (UDP)
Connectionless unreliable datagram delivery service
Adds only port number for multiplexing and delivery to application and checksum
If checksum fails on receipt, packet is simply discarded
If application can’t accept data, packet is discarded]
Low overhead: only 8 bytes of header added
Used for broadcasts
NFS
DNS

36. User Datagram Protocol (UDP)
UDP datagram:
Source port – sending process (reply to this port)
Destination port – receiving process
Length – length of UDP header + data
Checksum – pseudo header + UDP header + data

37. Transmission Control Protocol (TCP)
The transport layer
has no concept of routing
does not know about any intermediate nodes
Reliable delivery
TCP works on top of IP
TCP is connection-oriented. It corrects lost, corrupted, out-of-order and delayed packets
Guaranteed delivery service
Positive acknowledgement with timeout and re-transmission

38. Transmission Control Protocol (TCP)
Sliding window protocol
Full-duplex connection
Used for:
Telnet
FTP
SMTP
There are Twenty RFCs describing TCP
How TCP ensures reliability
Every message has a sequence number. The receiver can tell if a message is missing or out of order

39. Transmission Control Protocol (TCP)
Corrupted messages are detected by means of checksums
TCP acknowledges all correctly received messages (sends ACK to the sender)
TCP header:

40. Transmission Control Protocol (TCP)
Initial sequence numbers are chosen randomly and sent to the other side when the connection is set up
Source port – the sending process
Destination port – the receiving process
Sequence number – sequence number of first byte of data in the segment
Acknowledgement number
Valid if ACK bit is set
Gives sequence number of next data expected to be received

41. Data offset (or HLEN) – number of 32-bit words in TCP header
Flags (or Code Bits)
Window – number of bytes sender will accept
Checksum – pseudo header + TCP header + data
Urgent – points to byte in segment that follows urgent data

42. Quick Review Question

43. Follow Up Assignment

44. Summary of Main Teaching Points

45. Question and Answer Session
Q & A

46. Next Session
Topic and Structure of next session