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Lecture Topics: 11/27
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Networks
Layered Model
Ethernet
IP
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
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)
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
Protocol Stack
• These layers comprise the protocol stack
• Each layer of the stack adds extra info to the packet
Application layer
Application Data
Transport layer
TCP Header Application Data
TCP Segment
Network layer
IP Header TCP Header Application Data
IP Datagram
Data link layer
Ethernet
Header
IP Header TCP Header Application Data
Ethernet Frame
Ethernet
Checksum
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
4 bytes
Source Address
Type
Ethernet
Header
CRC
IP Header TCP Header Application Data
Ethernet Frame
Ethernet
Checksum
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
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
Carrier Sense
• Nodes can sense when the wire is being used
• A node waits to send if someone else is sending
B wants to send, but
can’t b/c A is sending
A finishes sending
A
B
C
A
B
C
A
B
C
B can send after A is
finished
• What’s the problem?
Collision Detection
• Nodes detect if there are multiple senders
B and C want to send,
but can’t b/c A is
sending
A
B
C
A
B
C
A
B
C
A
B
C
A finishes sending
B and C both send at
the same time
B and C detect that
there was a collision
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
Multiple Collisions
B and C detect a collision and jam Ethernet
A
B
C
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
Minimum Packet Size
• Sender cannot sense a collision if packets are too small
A and B send small packets at the
same time
A and B stop sending before
detecting a collision
A
B
A
B
The packets are corrupted, but
neither sender knows
A
B
• Min packet must be twice the max propagation delay
A
B
A can send for one propagation
delay without B sensing it
A
B
A must send for another
propagation delay before sensing
that B caused a collision
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
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
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
Internet History
• Goal: effective use of existing networks
– 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
Survivability
• Internet approach
– Cheap, failable components
– Stateless routers + self-healing
– Keep routing simple (non-adaptive)
– End to end recovery
• Telephone approach
– Ultra reliable switches
– Self-healing
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
IP Routers
Routers send packet
to next closest point
H
H
H
R
R
R
H
R
R
R
R
H
R
H
H: Hosts
R: Routers
IP Addresses
• IP addresses are 4 bytes (e.g. 128.95.8.137)
– part of the address designates the network (e.g.
128.95)
– the rest of the address designates the machine on
the network (e.g. 8.137)
• 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
How do routers know where
to send packets?
• Forwarding tables at each router
– Network == 128.95, 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?
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
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)