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Connecting Devices
and Multi-Homed Machines
Layer 1 (Physical) Devices
Repeater:
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Extends distances by repeating a signal
Any slight variations in the carrier wave for individual
bits is corrected when the carrier wave is reproduced
Hub:
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As above, but re-creates the carrier wave on multiple
ports
Hubs do not decide whether or not to copy data, they
do it (necessary or unnecessary)
All ports are part of the same collision domain
With a hub as the centre of the star, any 2 hosts can have
frame collisions
Layer 2 (Data Link) Devices
Layer 2 switch:
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Serves the same purpose as a hub
Can dynamically determine if a frame should be placed on a port
(and which one)
The data link layer (e.g. MAC) address is used to make this
determination
A table of MAC addresses and corresponding ports is built using
incoming frames
Each LAN segment (port) becomes its own collision domain
Only 2 hosts on the same LAN segment can have frame collisions
Layer 2 bridge
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As above, but the connected networks do not necessarily have
to be of the same type
Layer 2/3 Devices
Broadband (Cable/DSL) or Wireless Router
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Primarily operates as a layer 2 switch / access point
However, these routers often have additional features (some of
which are in layer 3):
Dynamic host configuration (DHCP) – IP address assignment for
network hosts, for example
DNS capability (local or distributed) – Provides naming of hosts
inside the network
IP masquerading – The router can use one IP given by a broadband
provider, but allow all of its hosts to use different IPs inside the
network
Layer 3 Switch
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Uses layer 3 routing to determine a path for packets
Once a path is found, subsequent packets are switched
This switching typically occurs on layer 2
These devices will be discussed in more detail later
Layer 3 (Network) Devices
Layer 3 Bridge
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A bridge that uses network layer addresses (e.g. IP) in its
forwarding database, instead of data link layer addresses (e.g.
MAC)
This type of bridge more readily allows different types of network
to be joined, since they need not share an address type
Cannot handle multiple paths effectively/efficiently: a host is
either on a given port or it is not
Router
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Determines routes for each packet using network layer
addresses (e.g. IP)
Can connect any type of network together
Is capable of determining preferred paths where multiple paths
exist
Routers
What is a Router Made of?
A router has many of the same
components as your computer:
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CPU
Memory
I/O Interfaces (mostly network interfaces)
Operating System
Routers Through History
Gateways:
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A computer with installed software to forward packets
These are obsolete, but were common in the early days of
ARPANet
Routers:
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A computer with specialized hardware and operating system
designed for forwarding packets
Switching Routers:
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A computer with specialized hardware (switching fabric) that
allows packets to be forwarded directly in hardware
The specialized hardware is, in many respects, similar to that of
a switch (e.g. ATM switch)
Router Hardware
Input buffers (one for each network interface):
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Used to store incoming packets before they are processed
Routing processor:
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This is often software running on a CPU which:
Maintains and exchanges routing data with other routers
Controls the switching fabric to forward packets
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With high-end routers, each network interface may have a local
routing processor (for forwarding) so that each can forward the
packets in its own input buffer independently
Switching fabric:
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A network of connections between network interfaces (and their
input and output buffers)
Output buffers (one for each network interface):
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Used to store outgoing packets after they are processed, but
before the network is available for transmission
Routers: Network Interfaces
Often, routers have modularized network
interfaces
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One can add/remove/replace network interfaces as
needs change
Some routers can accept network interface modules
of different types (e.g. Ethernet, Token Ring)
Each network interface would have its own:
Input buffer
Output buffer
Routing processor (in high-end routers)
Routers: Input Buffers
The incoming packets of a network interface are placed
in input buffers
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These are banks of very high speed memory for packet queuing
prior to processing
The packet is stored here until the routing processor is available
The network interface may have a routing processor,
which would:
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… have a copy of the forwarding table (to prevent concurrent
access)
… lookup the destination address in this forwarding table, to
determine the correct output port
… configure the switching fabric to forward the packet to the
correct output buffer
Low-end routers would share one routing processor
Routers: Routing Processors
Routing processors have two functions:
1. Maintain and exchange routing data with
other routers in the network
Often this involves computing the forwarding
table from data received by other routers
2. Use the forwarding table data to
configure the switching fabric to forward
the packet to the correct output port
Routers: Routing Processors
A routing processor is software which executes
on a CPU:
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Off-the-shelf CPU
These are very inexpensive
However, the performance of these CPUs is low since they
are not optimized for the types of operations a router
typically needs to perform
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Application-Specific Integrated Circuit (ASIC)
These are expensive to design (time and money)
They are optimized for typical routing operations
High-end routers use these to achieve higher performance
levels
Routers: Switching Fabric
Switching fabric’s job is to move packets
from the input buffer into the correct output
buffer
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The routing processor determines the correct
output port, using the forwarding table
Routers: Switching Fabric
Switching fabric comes in 3 major types:
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In-memory switching fabric:
The packets are input into the routing processor’s
memory, and output into the correct output buffer
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Bus-based switching fabric:
The packets move along a shared bus (similar to a
network bus) to the correct output buffer
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Crossbar switching fabric:
The packets move along a grid of redundant buses
If any bus fails, alternate paths exist so that
forwarding can continue
Routers: Output Buffers
The switching fabric gets the packet to the
right output port
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However, that port’s network may not be
immediately available
The packets are stored in the output buffer
until the network is available
Router Performance
Several methods to improve router performance
have been discussed:
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Use application-specific integrated circuits
Optimized for routing operations
Include much routing functionality otherwise executed as
software (in memory)
Many routing functions can execute in parallel, adding new
functionality without decreasing throughput
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Use efficient switching fabric
Bus or crossbar-based switching fabrics reduce the need for
in-memory processing
MPLS
Multi-Protocol Label Switching
MPLS
MPLS is another way to improve router
performance
Label switching tries to leverage some of the
performance of virtual circuit switched networks
(e.g. ATM)
Packets are assigned a label upon entering an
MPLS network
This label is used (instead of the IP address) for
making forwarding decisions
MPLS Labels
An MPLS label is an arbitrary value
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This value is typically a numeric identifier
However, labels could also be the frequency (i.e. colour) of
light used in multi-mode optical fibre
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The label can change from one label-switching router
(LSR) to the next
The label must only be unique for the sending and
receiving router
IP addresses, in contrast, are usually unique across the
network
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A value could even be chosen to help the routing
processor choose the correct output port
MPLS: Simplified
MPLS LSR
MPLS LSR
E-Mail Server
User
MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
Web Server
MPLS: Simplified
MPLS LSR
MPLS LSR
E-Mail Server
User
MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
• Here, the label is shown as colour
• Notice the simplicity of the router’s job:
• Red: Up
• Blue: Right
Web Server
MPLS: Simplified
MPLS LSR
MPLS LSR
E-Mail Server
User
MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
• Notice that two labels can be directed
down the same link
Web Server
MPLS: Label Values
31
31
MPLS LSR
MPLS LSR
E-Mail Server
User
7
15
7
MPLS LSR
MPLS LSR
MPLS LSR
47
MPLS LSR
• Notice that label values are not
globally unique
• Each pair of routers agrees on a label
Web Server
MPLS Packets
MPLS adds a small pre-header to the start of any IPv4
(or IPv6, IPX, etc.) packet
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In other words, between the data link and network headers
20 bits
Label
3 bits
Class of Service
1 bit
Stack
8 bits
Hop Limit
The label value
The QoS class of the packet attached (e.g. discardable?)
Is there a stack of labels?
The hop limit, copied from/to the IP header
MPLS and ATM
LSRs can be ATM-enabled
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An LSR can forward a packet (as cells) through an
ATM network
This can be for any number of hops through the ATM network
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In this situation the source and destination ATM
switches must be LSRs
Other switches in between can be normal ATM switches,
however
The source LSR will use AAL segmentation to send the cells
on the ATM network using a VPI/VCI for the destination LSR
The destination LSR will extract the packet and continue
transmission using MPLS