Transcript 14th of February

ATM, Infrastructure
Getting around the world if you’re a
network packet
How those layers work
 Networks have up to 7 (8!) Layers:
•
•
•
•
•
•
Application: software
Presentation: translation
Session: start and stop comms sessions
Transport: make connection good enough
Network: routing and switching
Data Link (2): Link control and access control
(IEEE 802.2 LLC and MAC)
• Physical: the wire, radio, etc.
Where they happen
 In the main computer software
• Application, Presentation, Transport, some
routing
 In the computer’s hardware
• Data Link (LLC and MAC), Physical
 In the LAN or WAN
• Routing, Transport, Session
 In the global network
• Routing, switching, big time!
From Physical to Network
 Data link controlled by network card,
accesses physical network.
• Sharing (“multiplexing”)
• Collision detection
 Hardware address (eg Ethernet/MAC) has to
be mapped onto IP number!
• ARP server - software on computers that
announces and communicates IP numbers on
LAN
MAC/ARP is important, can go
wrong!
 Computers only occasionally announce
their IP address
 Switches need MAC to know it to send to
right place - re-connections!
 Hubs can make it hard to detect
disconnections
 Wireless systems need to switch and act
like hubs, often.
Going Local, Going Internet
 If it’s a local machine, ARP tells me the MAC
address: send it there (can tell from the IP
number and subnet mask)
 If it’s not a local machine:
• Work out which gateway/router to send it to (IP
number)
• Work out the MAC address of the gateway
(ARP!)
• Send it THERE!
Routing
 Each computer can route a little: sending to
the main routers on the LAN.
 When a main router receives a packet, it
does exactly the same thing the other
computers do:
• Work out which router to send it to
• Send it via the MAC address (not always
ethernet)!
More Routing (IMPORTANT!)
 Packets bounce from one network to
another: Internetworking
 Often don’t have enough bandwidth to
transmit all the packets on:
• Send those packets first that are important
• Throw away less important packets when you
have too much to do!
 Quality of Service: making things work
appropriately
Even More Routing
 Need to communicate which route to go,
and make sure things don’t go in circles
(RIP, BGP, and more)
 The routing rules determine how the
internet behaves. Very iffy in IPv4.
• Video needs to get through fast
• Audio musn’t vary too much in speed
• Important stuff MUST get through!
 How do you decide - they’re all alike!
Quality of Service (QoS)
 Bandwidth: how fast
 Latency: how long (not the same as how
fast)
 Packet Loss: how often you drop the
information.
 Bandwidth, Packet Loss, Latency variation:
how constant is all this?
The Big Problem with IP
 Packets all look alike - really need to
analyze ‘em to work out what you should do
with them. Often just let them all go
through, or all die!
 Packets all different sizes - hard to predict
how to fit them all together in the right
order to make things efficient.
The Telco Solution: ATM
 Asynchronous Transfer Mode
• Chop up all the information into the same-sized
little box: 53 bytes CELLS!
• Forget source and destination: label by what
path the cell takes
• Set up path depending on what the user wants:
QoS
• Simple decisions to keep it moving: Only THREE
layers!
IP vs ATM
 IP: I want to go from 1131 Barclay St to 105
W. Broadway. Route Route Route.
 ATM: I’m going on path 10943
• 10943 = Take a left out of my door, turn right
onto Thurlow, Turn left on Nelson, Right on
Burrard, take the bridge, Turn right on
Broadway.
• “Fabric” remembers the path from the path
numbers.
QoS and ATM
 I ask for how fast I want to go, where I want
to go, how long I want to take going there,
how much I’m willing to put up with
variations
 Fabric finds the path that matches my
needs
 ATM switches make sure the right cells, just
going on the path number, get
switched/dumped in the right way.
IP over ATM
 Big IP pipes are carried by defining virtual
IP networks over ATM
 Other protocols can also be carried.
 A lot of the “broadcast” mode of IP is
missing though. IP lets me send to more
than one place at once.
 ATM can be awfully expensive, but uses
links way more efficiently.
Other QoS stuff
Older systems like Frame: “synchronous” version of
ATM. TDMA is one of these! Big for older satellite
comms, links to remote parts of Canada, and used for
communication with Mars by JPL and others. QoS big
for space stuff!
 RSVP: Resource reSerVation Protocol: trying to tell
the routers how to treat packets
 TOS: oldest and dumbest: simple way of labelling
certain packets as more important than others.

Why should I care?
 Each jitter in video, or weirdness in sound
for IP Telephony or Voice mode in
Messenger, etc, is due to the QoS problems
of IPv4.
 Mail loops, denial of service attacks, etc:
cases where we can’t make sure the
important information gets through
 Bad QoS wastes links, time, money: how
much am I willing to pay?
Big Pipes
 Network QoS problems often solved by
making pipes bigger: if everything gets
through fast, everything is fine
• Bit like saying we can turn the entire country
into big highways. REALLY big highways.
• What happens if you can’t afford big?
• What happens if you can’t afford ATM?
• What happens if you DIE without QoS?