Transcript Week 4 Network Layer and Routing

Network Layer and
Routing
The Network Layer
 Layer 3 on the OSI reference model
 The layer at which routing occurs!
 It responds to service requests from the
transport layer and issues service
requests to the data link layer. The
network layer addresses messages and
translates logical addresses and names
into physical addresses.
OSI
 In an OSI network there are four significant
architectural entities: hosts, areas, a backbone,
and a domain.
 A domain is any portion of an OSI network that
is under common administrative authority.
Within any OSI domain, one or more areas can
be defined. An area is a logical entity; it is
formed by a set of contiguous routers and the
data links that connect them. All routers in the
same area exchange information about all of
the hosts that they can reach.
OSI & Routing
 Areas are connected to form a
backbone. All routers on the backbone
know how to reach all areas.
Equipment
 A router consists of a computer
networking device that determines the
next network point to which to forward a
data packet toward its destination, a
process known as routing. Routing
occurs at layer 3 of the OSI model.
Routers
 Routing is most commonly associated with the
Internet Protocol, although other less-popular
routed protocols continue in use.
 In the original 1960’s-era of routing, generalpurpose computers served as routers. Although
general-purpose computers can perform
routing, modern high-speed routers are highly
specialised computers, generally with extra
hardware added to accelerate both common
routing functions such as packet forwarding and
specialised functions such as IPSec encryption.
Routers
 A router must be connected to at least
two networks, or it will have nothing to
route. A special variety of router is the
one-armed router used to route packets
in a virtual LAN environment.
 A router creates and/or maintains a
table, called a "routing table" that stores
the best routes to certain network
destinations and the "routing metrics"
associated with those routes.
Switch
 A device that filters and forwards
packets between LAN segments.
Switches operate at the data link layer
(layer 2) and sometimes the network
layer (layer 3) of the OSI model and
therefore support any packet protocol.
LANs that use switches to join segments
are called switched LANs or, in the case
of Ethernet networks, switched Ethernet
LANs.
Network Layer …
 It also determines the route from the source to
the destination computer and manages traffic
problems, such as switching, routing, and
controlling the congestion of data packets.
 The network layer provides the functional and
procedural means of transferring variable length
data sequences from a source to a destination
via one or more networks while maintaining the
quality of service requested by the Transport
layer.
Network Layer…
 The Network layer performs network
routing, flow control, segmentation/desegmentation, and error control
functions. The network layer deals with
transmitting information all the way from
its source to its destination - and
transmitting things from anywhere you
like, to anywhere you like.
Network Layer…
 If you can't contact a place at the
network layer, then you can't contact that
place at all. However, it does it in a very
basic way, without error detection or flow
control.
 The following slide indicates particular
items the network layer addresses.
Network Layer Responsibilities
 Is the network connection orientated or
connectionless? For example, snail mail
is connectionless, because you can send
a letter to someone completely out of the
blue. On the other hand, the telephone
system is connection orientated,
because you have to close to one half
and make a connection before you can
talk to them.
Network Layer Responsibilities
 What are the Global Addresses?
Everybody in the network needs to have
a unique address which determines who
they are. This address will normally be
hierarchical, so you can be “Jane Smith"
to Londoners, or “Jane Smith, London"
to people in the UK, or “Jane Smith,
London, UK" to people anywhere in the
world. On the internet, these addresses
are known as IP Numbers.
Network Layer Responsibilities
 How do you forward a message? This is of
particular interest to mobile applications, where
a user may rapidly move from place to place,
and it must be arranged that his messages
follow him. IPv4 doesn't really allow for this,
though it has been hacked on somewhat at a
later date. Fortunately, the forthcoming IPv6 has
a much better designed solution, which should
make these kinds of applications much
smoother.
Network Layer Protocols
 IP
 IPX
 NW Link
 NetBEIU
Routing-switches
 A switch that also performs routing operations.
Usually a switch operates at layer 2 (the Data
Link layer) of the OSI model while routers
operate at layer 3 (the Network layer).
 Routing switches perform many of the layer 3
functions usually reserved for routers. The
routing is implemented in hardware rather than
software, it is faster. The downside of routing
switches is that they are not as powerful or as
flexible as full-fledged routers.
 Because they perform some layer 3 functions,
routing switches are sometimes called layer-3
switches.
Routing
 Routing within an area is called level 1
routing; routing between areas is called
level 2 routing. An IS that can route only
within areas is known as a level 1 IS.
 A level 1 IS needs to know only about
the other level 1 ISs in its own level 1
area and about the nearest level 2 IS
that it can use to forward traffic out of its
own area.
Routing from Cisco
 To help simplify, Cisco has created route maps
on their routers.
 Over time, Cisco has enhanced its route
redistribution support to improve administrative
control over methods by which routing
information moves between routing domains.
 A route map is a set of instructions that tell the
router how routing information is to be
redistributed between two routing protocols or
between two instances of the same routing
protocol.
Routing from Cisco
 Route maps give network managers
unprecedented control over the ways
that routing information is propagated in
their networks. Redistribution
configuration files that use route maps
are easy to create, understand, and
modify. Using route maps, Cisco users
are able to build larger, more robust,
reliable networks, with better traffic
control than ever before.
IPv4
 IPv4 is version 4 of IP. It was the first version of
the Internet Protocol to be widely deployed, and
forms the basis for the current (as of 2004)
Internet.
 IPv4 addresses are written in dot-decimal
notation. Here's an example: 127.0.0.1.
 IPv4 uses 32-bit addresses, limiting it to
4,294,967,296 unique addresses, many of
which are reserved for special purposes such
as local networks or multicast addresses,
reducing the number of addresses that can be
allocated as public Internet addresses.
IPv4
 As the number of addresses available is
consumed, an IPv4 address shortage
appears to be inevitable in the long run.
 This limitation has helped stimulate the
push towards IPv6, which is currently in
the early stages of deployment, and is
expected to eventually replace IPv4.
IPv6
 IPv6 is version 6 of the Internet Protocol.
IPv6 is intended to replace the previous
standard, IPv4, which only supports up
to about 4 billion (4 × 109) addresses,
whereas IPv6 supports up to about 3.4 ×
1038 addresses.
IPv6
 IPv6 is the second version of the Internet
Protocol to be widely deployed, and is
expected to form the basis for future
expansion of the Internet.
 The compelling reason behind the
formation of IPv6 was lack of address
space, especially in the heavily
populated countries of Asia such as India
and China.
IPv6 Notation
 IPv6 addresses are 128 bits long but are
normally written as eight groups of 4
hexadecimal digits each. For example,
 3ffe:6a88:85a3:08d3:1319:8a2e:0370:7344
IPv6 Notation
 If a 4 digit group is 0000, it may be omitted.
For example:
 3ffe:6a88:85a3:0000:1319:8a2e:0370:7344
 is the same IPv6 address as
3ffe:6a88:85a3::1319:8a2e:0370:7344