1. CS4470 Computer Networking Protocols
6/10/2018
CS Computer Networking Protocols
6. Internetworking
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Network technologies Motivation of Internetworking
Internet and routers
IP addressing
ARP
6. Internetworking CS470

3. Network technologies Ethernet FDDI Run over different cables
6/10/2018
Network technologies
Ethernet
Run over different cables
Twisted-pair Ethernet: 10Base-T
Fast Ethernet: 100Base-TX, 100Base-FX
Gigabit Ethernet: 1000baseX, 1000baseT
Physical layers are different
FDDI
Ring based token network
Run on fiber
FDDI has since been effectively obsolesced by fast Ethernet
6. Internetworking CS470

4. Network technologies PPP (point-to-point protocol)
A data link protocol commonly used in establishing a direct connection between two networking nodes
For both router-router leased line connection and the dial-up host-router connection.
Common leased line services
DS1(T1), DS3(T3)
Over SONET (Synchronized Optical Network): OC1, OC12, OC24, OC36, OC48
6. Internetworking CS470

5. Motivation of internetworking
Why internetworking
There are different network technologies
Ethernet, Token ring, ATM, Wireless LAN, …
Connect multiple physical networks into a large, uniform communication system
Challenges
Heterogeneity
Scalability
6. Internetworking CS470

6. Challenges of internetworking
Heterogeneity
Users on one type of network should be able to communicate with users on other types of networks
Establishing connectivity between hosts on two different networks may require traversing several other different networks
Each network has its own addressing scheme, media access control protocols, service model,…
6. Internetworking CS470

7. Challenges of internetworking
Scalability
The Internet has roughly doubled in size each year for 20 years
Routing
How to find an efficient path through a network with millions/billions of nodes
Addressing
How to provide suitable identifiers for all nodes
6. Internetworking CS470

8. Bridges and switches
Can bridges and switches be used to interconnect different networks?
Bridges and switches only know frames
Different networks use incompatible frame format
A frame created for one network technology cannot be transmitted on a network that uses a different technology
6. Internetworking CS470

9. Interconnect heterogeneous networks
Hardware
Routers: interconnect physical networks, also called gateways
Software
IP protocol
Provides universal service
Universal service
A user on any computer in any networks can talk to any other user
6. Internetworking CS470

10. What is an internetwork
Directly connected or switched network
Such a network uses one technology, such as Ethernet, ATM, Token ring
Also called physical network
Internetwork
A logical network built on a collection of physical networks
Routers are the nodes that connect the physical networks
6. Internetworking CS470

11. A simple internetwork router host FDDI Token Ring H5 H4 H6 H3 H2 H1 H8
Ethernet
Point-to-Point
Link (e.g., ISDN)
router
host
6. Internetworking CS470

12. Software Achieve universal service Complexity of the software
Software is needed on computers and routers
Complexity of the software
Deal with different frame format
Routing
how to find an efficient (loop free) path through a network with millions/billions of nodes
Addressing
how to provide suitable identifiers for all the nodes
Does the MAC address work?
6. Internetworking CS470

13. MAC addresses don’t work
Flat structure
Not efficient to forward data based on MAC addresses
Take space to store
Different in different networks
Hierarchical addresses preferred
An address is divided into two parts
Part 1: identify the network
Part2: identify the computer
Make forwarding efficient
6. Internetworking CS470

14. IP addresses Specified by Internet Protocol When we address a datagram
Sending host puts destination internet address in datagram
Destination address can be interpreted by any intermediate router
Routers examine address and forward datagram on to the destination
All of these addresses are virtual
they are defined in software, not hardware
6. Internetworking CS470

15. IP Addresses Each host is assigned a 32-bit number
Unique across entire Internet
Dotted decimal notation:
Each IP address is divided into a prefix and a suffix
Prefix identifies network to which computer is attached
No two networks can be assigned the same network number
Suffix identifies computer within that network
No two computers on the same network can have the same suffix, but computers on different networks can have the same suffix
Address format makes routing efficient
6. Internetworking CS470

16. IP addresses and MAC Addresses
32-bit IP address:
network-layer address
used to get datagram to destination network
MAC (or LAN or physical or Ethernet) address:
used to get frame from one interface to another physically-connected interface (same network)
6. Internetworking CS470

17. Resolving Addresses Hardware only recognizes MAC addresses
IP only uses IP addresses
Consequence: software needed to perform translation
Known as address resolution
6. Internetworking CS470

18. Address Resolution Protocol
Layer 2 protocol
Given
A locally-connected network, N
IP address C of computer on N
Find
Hardware address for C
Technique
Address Resolution Protocol
6. Internetworking CS470

19. ARP is local to a network
One computer can resolve the address of another computer only if both computers attach to the same network
A computer never resolves the address of a computer on a remote network!
6. Internetworking CS470

20. ARP is local to a network (cont.)
A sends data to B
A uses B’s IP address
A resolves B’s IP address to B’s hardware address
A sends the frame to B directly
6. Internetworking CS470

21. ARP is local to a network (cont.)
A sends data to F
A uses F’s IP address
A determines that the data must travel through R1
A resolves R1’s hardware address
A sends the frame to R1 directly
It’s the responsibility of R1 to send the frame to F
6. Internetworking CS470

22. Address Resolution Protocol (ARP)
Keep mappings in table
Table entry contains pair of addresses for one computer
IP address
Hardware address
Build table automatically as needed
6. Internetworking CS470

23. ARP Table Lookup
Use a simple list containing IP address and hardware address for each host on net
Search on IP address and extract corresponding hardware address
Note that all IP addresses have same prefix; can save space by dropping prefix
6. Internetworking CS470

24. ARP Table Lookup
Sequential search may be prohibitively expensive (O(n))
Can use indexing or hashing for O(1) lookup
Indexing - use hostid part of IP address as list (array) index
Hashing - use hashing function on hostid to generate list index
6. Internetworking CS470

25. ARP on your computer Open a command window
StartRuncmd
Display the arp table on your compuer
Under MS-DOS, type arp –a
Clear the arp table
Under MS-DOS, type arp –d *
6. Internetworking CS470

26. ARP Table How to get the table in the first place?
Address resolution with message exchange
Each computer on the network participates in address resolution by agreeing to answer the resolution requests for its address
A computer that needs to resolve an address sends a broadcast request on the network.
All machines receive the request and examine the requested address
If the request matches a computer’s address, the computer responds
6. Internetworking CS470

27. ARP Message Exchange Example
W sends a broadcast message asking for Y’s MAC address
Everyone hears the message
Only Y responds with a Unicast message (still broadcast on bus)
6. Internetworking CS470

28. ARP protocol example 1 (in a LAN)
A wants to send datagram to B
B’s MAC address not in A’s ARP table. A
1A-2F-BB AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN B
6. Internetworking CS470

29. ARP protocol example 1 (in a LAN)
A broadcasts ARP query datagram, containing B's IP address
Dest MAC address = FF-FF-FF-FF-FF-FF
all machines on LAN receive ARP query
B receives ARP datagram, replies to A with its (B's) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ARP is “plug-and-play”:
nodes create their ARP tables without intervention from net administrator
6. Internetworking CS470

30. ARP protocol example 2 (to another LAN)
A sends a datagram from A to B via R, assume A knows B’s IP address A R B
6. Internetworking CS470

31. ARP protocol example 2 (to another LAN)
Two types of nodes
Host: one adapter, IP address and one ARP table
Router: one adapter, one IP Address and one ARP table for each interface
Router has
Two interfaces
Two IP addresses
Two adapters
Two ARP tables
6. Internetworking CS470

32. ARP protocol example 2 (to another LAN)
A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for
A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram
A’s adapter sends frame
R’s adapter receives frame
R removes IP datagram from Ethernet frame, sees its destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B
6. Internetworking CS470