COMP3310/6331 – #9
WANs and Layers
Dr Markus Buchhorn: markus.buchhorn@anu.edu.au
Remember this?
App ? App !
An application talks to an operating system
• which talks to the computer hardware
• which talks to a “cable”
• which talks to another computer’s hardware • which talks to that operating system
• which talks to an application over there
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Or another way
The application
Blah, blah, magic, blah…
“the cable”
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Which now becomes
The application
“Communication stuff”
“Network stuff”
“the cable”
And more!!
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Going Wide-Area Networking
• Introducing the Network Layers model! • Why?
– Because it’s good for you! Lots of examinable concepts! • No really, why?
• Many LANs are user-friendly, run at high speed, with large address spaces – Can send information efficiently from A to B
• Why not go global?
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Because: Scaling
• Address tables and management don’t scale globally
– Billions of devices – and every switch needs to store them all?
• Updates take a long time to propagate – Long delays, depending on the paths
– Topology changes happen a lot
• Broadcasts have to be sent globally – E.g. whenever a new device connects – Spanning Tree won’t converge in time
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Because: Traffic control
• LANs organise themselves for simplicity – not optimisation • Spanning Tree doesn’t guarantee the optimal topology
– Sometimes people do know better
• Network traffic costs money, needs to consider politics
S
DS
D
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Because: Which LAN?
• Many LAN choices
– 802.3 Ethernet, 802.11 WiFi, xDSL, 4G, … each fit-for-purpose (wired/wireless)
• Different LANs don’t mix. Mismatched behaviours with different – Address schemes
– Service models (frames, cells, circuits)
– Security models
– Frame sizes
– Performance
– Prioritisation mechanisms
• Don’t want to write applications tuned to different LAN types
• Don’t want to buy boxes that translate between every possible combination – (Many, expensive) single points of failure
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Solving these
• Want to communicate across networks: aka Inter-network • Take the LAN to the WAN
• Scaling problems
– Use a hierarchy of connections, addresses and aggregate/group
• Traffic control
– Optimise routing with more information, and support prioritisation
• Which LAN?
– All of them. Put a common layer across the top
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Fundamental theorem of software engineering
• “We can solve any problem by introducing an extra level of indirection layer”
• “…except the problem of too many levels of indirection layers…” – Attributed to David J Wheeler et al.
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Just one layer?
• Networks have complex requirements – Find a path
– Make connections
– Transfer communications (bits) reliably – Ensure security
– Share bandwidth, globally and diverse paths – Allow for hosts to come/go
– Deal with topology changes
–…
• Networks don’t really care what you’re using it for!
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Why stop at networks?
• Applications need functionality too
– Find/advertise resources
– Connect to other machines • One or many
– Exchange application-specific messages – Adapt to capabilities
• Devices, software, …
– Maintain state of a connection
– Expect sufficient reliability
– Expect trustworthiness
– Maximise performance, minimise delays
• Simplify: let’s modularise/layer things…
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Layers upon layers
• Use layers to divide (allocate) functionality
• Use protocols to exchange information within a layer • User services from lower layers to build on
Host 1
Protocol A
Host 2
App(2)
e.g.
Copper medium and Ethernet protocol
Simple!
App(1)
Physical medium
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Layers upon layers
• Use layers to divide (allocate) functionality
• Use protocols to exchange information within a layer • User services from lower layers to build on
Protocol A
App(2)
Protocol is not Ethernet nor Wifi?
App(1)
WiFi Ethernet
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Layers upon layers
• Use layers to divide (allocate) functionality
• Use protocols to exchange information within a layer • User services from lower layers to build on
Protocol A
Applications offload the networking details
Network layer provides a service, takes care of everything?
App(1)
Protocol N
App(2)
Net(1)
Net(2)
WiFi
Ethernet
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Layers upon layers
Protocol A
App(1)
Protocol N
App(2)
Net(1)
Protocol L
Net(2)
Link(1)
Link(2)
Network layer: transmission, topology, …
Offload the Link details
WiFi
Ethernet
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A “protocol stack”
Protocol A
App(1)
Protocol T
Protocol N
App(2)
Transport (1)
Transport (2)
Net(1)
Protocol L
Net(2)
Link(1)
Link(2)
Offload more: reliability, app-muxing, security, performance, …
Each protocol exchanges Protocol Data Units (PDUs)
Each layer provides services through an API, exchanging Service Data Units (SDUs)
WiFi
Ethernet
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For example…
Web (HTTP)
Browser (1)
TCP
IP
Server (2)
Transport (1)
Transport (2)
Net(1)
Protocol(s) L
Net(2)
Link(1)
Link(2)
WiFi Ethernet
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Hiding link complexity HTTP
Browser (1)
Net(1)
TCP
IP
Server (2)
Transport (1)
Transport (2)
Net(2)
Link(2)
Physical (2)
WiFi Bridge
Air
Ethernet
Copper
Link(1)
Physical (1)
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Hiding path complexity HTTP
Browser (1)
Server (2)
TCP
Transport (1)
Transport (2)
Net(2)
Link(2)
Physical (2)
Net(1)
IP WiFi
Air
20 different LAN links
Ethernet
Copper
Link(1)
Physical (1)
Applications don’t know nor care. Unless there is a performance question.
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Role of each layer
• Each consumes services (functionality) from the layer below
• Each offers services (functionality) to layer above
• Which functions belong in each??
Application Presentation
Session Transport
Network
Link, Physical
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Reference Protocol Stacks
An OSI (ISO+ITU-T/CCITT) reference – great standard – almost never used
Layer
Name
Function
7
Application
Deliver functionality
6
Presentation
Convert information for application needs
5
Session
Combine diverse communications, maintain state
4
Transport
Ensure end-to-end performance
3
Network
Send packets over multiple links
2
Link
Transmit Frames
1
Physical
Modulation and encoding of bits
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In reality
• There are protocols that span layers
– For internetworking, some knowledge has to move up and down layers
• There are layers that have sublayers – E.g. Ethernet has MAC and LLC
• Logical Link Control; adds payload-muxing, flow-control and extra error-handling • There are people who think about this classification too much
– It’s not a rule
• But as a concept, and to guide protocol design – it’s very useful
– Think about what functionality you need/offer, and where it should be
– Success: Internet end-to-end principle – smart edges, dumb core (rules, not state)
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Internet “Reference” protocol Stack
Layer
Name
Function
7
Application
Deliver functionality (and the presentation/session)
4
Transport
Ensure end-to-end performance
3
Network
Send packets over multiple links
1,2
Physical, Link
Transmit Frames
Layering considered harmful… (rfc3439)
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Naming
Layer
What it transports (Protocol Data Unit)
How they connect
Application
Messages/Data
Proxy, gateway
Transport
Segments/Datagrams
Network
Packets (!!)
Router
Link
Frames (cells, circuits)
Switch, Bridge
Physical
Bits
Hub (repeater)
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Real world implementation
• Every protocol has a dictionary for what it sends – Every layer has a ‘payload’ to transmit
• Every payload is passed to a lower layer and is encapsulated – Think of a letter in an envelope
– Add headers (and trailers), maybe encrypt, compress, segment
– Repeat till it’s sent
IP
TCP HTTP
Ethernet 802.3
Preamble
Start of Frame
MAC dest
MAC src
802.1Q tag [opt]
Type / Length
Payload
Checksum
7 byte
1 byte
6 byte
6 byte
4 byte
2 byte
42-1500 byte
4 byte
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“Demuxing” the “encapsulated”
• Host/Receiver ultimately gets the (whole) message • Need to pass it to the process that needs it
– Which one???
IP
TCP HTTP
Ethernet 802.3
• Every layer has a ‘key’ about its payload (in its header) – Ethernet has an address, and Ether-type
– IP has an address and Transport-type (tcp vs udp)
– TCP has a port number
– Applications have message keys (e.g. http url’s)
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Meanwhile, out on the network
• Many devices watch traffic
– To learn, to report, to manage – How does it scale?
• Ideally don’t hold up the traffic
– Minimal packet inspection
– Only read the layer (headers) they are responsible for forwarding • Link, network
• Sometimes we need more, e.g. security, priority
– Deep Packet Inspection, in real-time • Right down to the inner payload
• Significant load, delays
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Security, Priority
• Having DPI is considered harmful
• Lack of DPI can be considered negligent
• Good design works out how much to unpack, and when • Firewalls and DMZs and VLANs and VPNs and …
• We’ll come back to this a few times
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Actual Protocol Stack?
Layer
Name
Function
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Money
Actually decides what happens
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Religion
Arguing for the sake of it
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Politics
Stomps on what you’d like to do
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Application
Deliver functionality
6
Presentation
Convert information for application needs
5
Session
Combine diverse communications, maintain state
4
Transport
Ensure end-to-end performance
3
Network
Send packets over multiple links
2
Link
Transmit Frames
1
Physical
Modulation and encoding of bits
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