1. Lecture Topics: 11/27
Networks
Layered Model
Ethernet IP

2. What is a network? A network allows computers to communicate
There are lots of different views of a network
System Area Network (computers in the same room)
Local Area Network (computers in the same building)
Wide Area Network (computers on the same planet)
All basically the same idea: one computer sends another computer a message

3. What is a protocol?
A protocol is a common standard that determines how two machines communicate
Many different types of protocols
low level (transfer bits between two connected machines, e.g. Ethernet)
middle level (transfer bits between two unconnected machines, e.g. IP)
high level (do something useful, e.g. HTTP get a web page)

4. OSI Layered Network Model
Layering of protocols is good
enforces modularity (easier to make changes)
layers don’t need to know details of higher or lower layers
Physical layer
getting raw bits from one node to another
coaxial, fiber optics, wireless
Data link layer
groups bits together to transmit between nodes on a network
Ethernet, PPP, SLIP
Network layer
transmits packets between networks (IP)
Transport layer
sequencing and acknowledgment of received data (UPD,TCP)
Application layer
telnet, Internet Explorer

5. Protocol Stack These layers comprise the protocol stack
Each layer of the stack adds extra info to the packet
Application layer
Application Data
IP Header
TCP Header
Ethernet
Header
Checksum
Ethernet Frame
IP Datagram
TCP Segment
Transport layer
Network layer
Data link layer

6. Ethernet Packets Ethernet header includes
the network address of the source and destination
hardwired and globally unique
the type of the packet (e.g. IP or ARP)
Ethernet checksum used to detect errors
is a hash of the entire packet
when a packet is received the hash is checked with the CRC
6 bytes
Destination Address
Source Address
Type
4 bytes
CRC
Ethernet
Header
IP Header
Application Data
TCP Header
Ethernet
Checksum
Ethernet Frame

7. Ethernet
Protocol defining how to send packets within a LAN (local area network)
Developed at Xerox PARC in early ’70s
Packet based, distributed administration
A broken node does not affect the network
CSMA/CD (Carrier Sense, Multiple Access, Collision Detection)
Basic idea:
Wait for line to become quiet
While transmitting, check if someone else was transmitting
If someone else was transmitting, stop and wait before retransmitting

8. Multiple Access All nodes are connected to the same shared wire
All nodes receive all packets
A destination address distinguishes
addresses are 48 bits and are globally unique
Example: If A sends a packet, it is received by both B and C A B C

9. Carrier Sense Nodes can sense when the wire is being used
A node waits to send if someone else is sending A B C
B wants to send, but can’t b/c A is sending A B C
A finishes sending A B C
B can send after A is finished
What’s the problem?

10. Collision Detection Nodes detect if there are multiple senders A B C
B and C want to send, but can’t b/c A is sending
A finishes sending
B and C both send at the same time
B and C detect that there was a collision

11. After Collision Detection
B and C jam the Ethernet to make sure all nodes see the collision A B C
B and C wait a random amount of time before sending again A B C
C happens to wait less time and sends first A B C
B waits for C to finish and then sends A B C

12. Multiple Collisions B and C detect a collision and jam Ethernet
B and C wait a random amount of time Random[0,WaitTime] A B C
B and C happen to collide again A B C
B and C double the amount of time to wait Random[0,2*WaitTime] A B C A B C
B happens to send first A B C
C waits for B to finish and then sends A B C

13. Minimum Packet Size
Sender cannot sense a collision if packets are too small A B
A and B send small packets at the same time
A and B stop sending before detecting a collision A B
The packets are corrupted, but neither sender knows A B
Min packet must be twice the max propagation delay A B A B
A can send for one propagation delay without B sensing it
A must send for another propagation delay before sensing that B caused a collision

14. Determining Minimum Size
The minimum packet size depends on
length of the Ethernet (1 km)
speed of the signal (km/sec)
bits sent per second (Mbits/sec)
For 10Mbit Ethernet, the minimum packet size is 64 bytes

15. Maximum Packet Size Ethernet uses a maximum packet size of 1500
only for fairness, so nodes don’t have to wait indefinitely to send a packet

16. Ethernet Evaluation
Ethernet had many key ideas that have spread to other layers of the protocol
decentralized control
exponential backoff
best effort—no guarantees that a packet would be delivered
There are other data link protocols, why did Ethernet win?
biggest competitor was token rings,
allowed bandwidth to be shared fairly
one machine’s failure caused the entire network to fail

17. Internet History Goal: effective use of existing networks Other goals:
minimal support from underlying networks
packet switching
routers connecting networks
Other goals:
survive hardware failure
support multiple types of apps
run on wide variety of networks
distributed management of resources
cost-effective
low cost host attachment
Internet hugely successful because of it can run on anything

18. Survivability Internet approach Telephone approach
Cheap, failable components
Stateless routers + self-healing
Keep routing simple (non-adaptive)
End to end recovery
Telephone approach
Ultra reliable switches
Self-healing

19. IP
Ethernet allows packets to be sent between nodes on the same network
Internet Protocol (IP) allows packets to be sent between nodes on different networks
Key idea: some machines (i.e. routers) are on multiple networks
Ethernet is still used to send packets between routers

20. Routers send packet to next closest point
IP Routers
Routers send packet to next closest point H R H R H H R R R R R H R
H: Hosts
R: Routers H

21. IP Addresses IP addresses are 4 bytes (e.g. 128.95.8.137)
part of the address designates the network (e.g )
the rest of the address designates the machine on the network (e.g )
A router sends the packet to the correct network, and the network sends the packet to the correct node
this greatly reduces the information a router must store

22. How do routers know where to send packets?
Forwarding tables at each router
Network == , then send to router X
Original Internet: manual update
Now: automatic update based on “cost”
exchange tables with neighbors
use neighbor with smallest hop count
what if node says zero cost to everywhere?

23. Setting up Routing Tables
Graph theory to compute “shortest path”
routers = nodes
links = edges
delay, hops = cost
Need dynamic computation to adapt to changes in topology
if a router goes down, might need to find another path

24. IP Routing Protocols There are two basic kinds of routing protocols
Distance Vector
exchange routing tables with neighbors
no one knows complete topology
now used between admin domains (i.e. AT&T and MCI
Link state
tell everyone who your neighbors are
everyone computes shortest path
now used within admin domains (within AT&T)