Routing Protocols

Network-Specific Method
• To reduce the routing table and simplify the searching
process
• Here, instead of having an entry for every destination
host connected to the same physical network, we
• have only one entry that defines the address of the
destination network itself.
• In other words, we treat all hosts connected to the
same network as one single entity.
• For example, if 1000 hosts are attached to the same
network, only one entry exists in the routing
• table instead of 1000.

Host-specific method
• The destination host address is given in the
routing table. The rationale behind this method is
the inverse of the network-specific method.
• Figure 6.5 if the administrator wants all packets
arriving for host B delivered to router R3 instead
of R1, one single entry in the routing table of host
A can explicitly define the route.
• Host-specific routing is used for purposes such as
checking the route or providing security measure

Default Method
• To simplify routing
• In Figure 6.6 host A is connected to a network
with two routers. Router R1 routes the
packets to hosts connected to network N2.
However, for the rest of the Internet, router
• R2 is used. So instead of listing all networks in
the entire Internet, host A can just have one
entry called the default
• (normally defined as network address 0.0.0.0).

Routing Based on Label
• In a connection-oriented network (virtual-
circuit approach), a switch forwards a packet
based on the label attached to a packet.
• Routing is normally based on searching the
contents of a table; switching can be done by
accessing a table using an index.
• Routing involves searching; switching
involves accessing.

MPLS routers
• Some conventional routers in the Internet can
be replaced by MPLS routers that can behave
like a router and a switch. When behaving like
a router, MPLS can forward the packet based
on the destination address; when behaving
like a switch, it can forward a packet based on
the label.(Multiprotocol)

Structure of Router
• four components: input ports,
• output ports,
• the routing processor,
• and the switching fabric,

An input port
• An input port performs the physical and data link
layer functions of the router. The bits are
constructed from the received signal. The packet
is decapsulated from the frame.
• Errors are detected and corrected.
• The packet is ready to be forwarded by the
network layer. In addition to a physical layer
processor and a data link processor, the input
port has buffers (queues) to hold the packets
before they are directed to the switching fabric.

Output Ports
• An output port performs the same functions
as the input port, but in the reverse order.
• First the outgoing packets are queued, then
the packet is encapsulated in a frame, and
finally the physical layer functions are applied
to the frame to create the signal to be sent on
the line.

Routing Processor
• The routing processor performs the functions of
the network layer. The destination address is
used to find the address of the next hop and, at
the same time, the output port number from
which the packet is sent out.
• This activity is sometimes referred to as table
lookup because the routing processor searches
the routing table.
• In the newer routers, this function of the routing
processor is being moved to the input ports to
facilitate and expedite the process.

Switching Fabrics
• The most difficult task in a router is to move
the packet from the input queue to the output
queue. The speed with which this is done
affects the size of the input/output queue and
the overall delay in packet delivery.
• In the past, when a router was actually a
dedicated computer, the memory of the
computer or a bus was used as the switching
fabric.

• The input port stored the packet in memory;
the output port got the packet from the
memory.

Crossbar Switch
• The simplest type of switching fabric is the
crossbar switch .
• A crossbar switch connects n inputs to n
outputs in a grid, using electronic
microswitches at each crosspoint.

Banyan Switch
• More realistic than the crossbar switch is the
banyan switch

• A banyan switch is a multistage switch with
microswitches at each stage that route the
packets based on the output port represented
as a binary string. For n inputs and n outputs,
we have log2(n) stages with n/2 microswitches
at each stage

Batcher-Banyan Switch
• The problem with the banyan switch is the
possibility of internal collision even when two
packets are not heading for the same output
port. We can solve this problem by sorting the
arriving packets based on their destination port.
• K. E. Batcher designed a switch that comes before
the banyan switch and sorts the incoming packets
according to their final destination. The
combination is called the Batcher-banyan switch

• Normally, another hardware module called a
trap is added between the Batcher switch and
the banyan switch. The trap module prevents
duplicate packets (packets with the same
output destination) from passing to the
banyan switch simultaneously. Only one
packet for each destination is allowed at each
tick; if there is more than one, they wait for
the next tick.

Routing Tables
• “An internet is a combination of networks
connected by routers. When a datagram goes
from a source to a destination, it will probably
pass through many routers until it reaches the
router attached to the destination network”

Cost or Metric
• A router receives a packet from a network and passes it
to another network. A router is usually attached to
several networks. When it receives a packet, to which
network should it pass the packet? The decision is
based on optimization:
• Which of the available pathways is the optimum
pathway?
• What is the definition of the term optimum?
• One approach is to assign a cost for passing through a
network.
• We call this cost or a metric.
• High cost can be thought of as something bad; low
cost can be thought of
• something good.

Example “Optimization”
• For example, if we want to maximize the
throughput in a network, the high throughput
means low cost and the low throughput
means high cost.
• As another example, if we want to minimize
the delay, low delay is low cost and high delay
is high cost.

Static versus Dynamic Routing Tables
• A routing table can be either static or
dynamic.
• A static table is one with manual entries.
• A dynamic table is updated automatically
when there is a change somewhere in the
internet.

Static or Dynamic ??
• Today, an internet needs dynamic routing
tables. The tables need to be updated as soon
as there is a change in the internet. For
instance, they need to be updated when a link
is down, and they need to be updated
whenever a better route has been found.

Routing Protocol
• Routing protocols have been created in response
to the demand for dynamic routing tables. A
• routing protocol is a combination of rules and
procedures that lets routers in the internet
inform each other of changes.
• It allows routers to share whatever they know
about the internet or their neighborhood. The
sharing of information allows a router in Mumbai
to know about the failure of a network in
NewYork.

• The routing protocols also include procedures
for combining information received from
other routers.
• Routing protocols can be either an interior
protocol or an exterior protocol.
• An interior protocol handles intradomain
routing; an exterior protocol handles
interdomain routing.

Intradomain routing and Interdomain
Routing
• An internet is divided into autonomous
systems. An autonomous system (AS) is a
group of networks and routers under the
authority of a single administration.
• Routing inside an autonomous system is
referred to as intra-domain routing.
• Routing between autonomous systems is
referred to as inter-domain routing.

Distance vector routing
• AS, with all routers and networks, as a graph, a
set of nodes and lines (edges) connecting the
nodes.
• A router can normally be represented by a node
and a network by a link connecting two nodes.
• The graph theory used an algorithm called
Bellman-Ford (also called Ford-Fulkerson) for a
while to find the shortest path between nodes in
a graph given the distance between nodes.

A sends four records to its neighbors,
routers B, D, and C.

Count to Infinity
• A problem with distance vector routing is that any
decrease in cost (good news) propagates quickly,
but any increase in cost (bad news) propagates
slowly.
• For a routing protocol to work properly, if a link
is broken (cost becomes infinity), every other
router should be aware of it immediately, but in
distance vector routing, this takes some time. The
problem is referred to as count to infinity. It
takes several updates before the cost for a broken
link is recorded as infinity by all routers.

• Two-Node Loop
• Three-Node Instability

Routing Information Protocol (RIP)
• The Routing Information Protocol (RIP) is an intradomain (interior) routing protocol
used inside an autonomous system. It is a very simple protocol based on distance vector
routing. RIP implements distance vector routing directly with some considerations:
• 1. In an autonomous system, we are dealing with routers and networks (links), what
• was described as a node.
• 2. The destination in a routing table is a network, which means the first column
• defines a network address.
• 3. The metric used by RIP is very simple; the distance is defined as the number of links
(networks) that have to be used to reach the destination. For this reason, the metric in
RIP is called a hop count.
• 4. Infinity is defined as 16, which means that any route in an autonomous system using
RIP cannot have more than 15 hops.
• 5. The next node column defines the address of the router to which the packet is to be
sent to reach its destination.

Fields
• ❑ Command. This 8-bit field specifies the type of
message: request (1) or response (2).
• ❑ Version. This 8-bit field defines the version.
• ❑ Family. This 16-bit field defines the family of the
protocol used. For TCP/IP the value is 2.
• ❑ Network address. The address field defines the address
of the destination network.
• RIP has allocated 14 bytes for this field to be applicable to
any protocol.
• However, IP currently uses only 4 bytes. The rest of the
address is filled with 0s.
• ❑ Distance. This 32-bit field defines the hop count (cost)
from the advertising router to the destination network

Requests and Responses
.
• RIP has two types of messages: request and
response
• Request
• A request message is sent by a router that has
just come up or by a router that has some
time-out entries. A request can ask about
specific entries or all entries

Response
• A response can be either solicited or unsolicited.
A solicited response is sent only in answer to a
request. It contains information about the
destination specified in the corresponding
request.
• An unsolicited response, on the other hand, is
sent periodically, every 30 seconds or when there
is a change in the routing table. The response is
sometimes called an update packet.

RIP Version 2
• RIP version 2 was designed to overcome some
of the shortcomings of version 1.
• The designers of version 2 have not
augmented the length of the message for
each entry.
• They have only replaced those fields in version
1 that were filled with 0s for the TCP/IP
protocol with some new fields.

Message Format
❑ Route tag. This field carries information such as the
autonomous system number. It can be used to enable RIP to
receive information from an interdomain routing protocol.
❑ Subnet mask. This is a 4-byte field that carries the subnet
mask (or prefix). This means that RIP2 supports classless ID
addressing and CIDR.
❑ Next-hop address. This field shows the address of the next
hop. This is particularly useful if two autonomous systems
share a network (a backbone, for example). Then the message
can define the router, in the same autonomous system or
another autonomous system, to which the packet next goes.

• RIPv1 uses classful addressing.
• The only entry in the message format is the
network address (with a default mask). RIPv2
adds one field for the subnet mask, which can
be used to define a network prefix length.
This means that in this version, we can use
classless addressing. A group of networks can
be combined into one prefix and advertised
collectively.
Classless Addressing

Authentication
• Authentication is added to protect the message
against unauthorized advertisement. No new fields
are added to the packet; instead, the first entry of the
message is set aside for authentication information.
• To indicate that the entry is authentication
information and not routing information, the value of
FFFF16 is entered in the family field
• The second field, the authentication type, defines the
protocol used for authentication, and the third field
contains the actual authentication data

Multicasting
• Version 1 of RIP uses broadcasting to send RIP
messages to every neighbor.
• In this way, all the routers on the network
receive the packets, as well as the hosts.
• RIP version 2, on the other hand, uses the all-
router multicast address to send the RIP
messages only to RIP routers in the network.

Encapsulation
• RIP messages are encapsulated in UDP user
datagrams.
• A RIP message does not include a field that
indicates the length of the message.
• This can be determined from the UDP packet.
The well-known port assigned to RIP in UDP is
port 520.
• RIP uses the services of UDP on well-known
port 520.

Open Shortest Path First (OSPF)
protocol
• The Open Shortest Path First (OSPF) protocol
is an intradomain routing protocol based on
link state routing. Its domain is also an
autonomous system

• To handle routing efficiently and in a timely
manner, OSPF divides an autonomous system
into areas.
• An area is a collection of networks, hosts, and
routers all contained within an autonomous
system.
• An autonomous system can be divided into
many different areas. All networks inside an
area must be connected.

• Routers inside an area flood the area with
routing information
• At the border of an area, special routers called
area border routers summarize the
information about the area and send it to
other areas.
• Among the areas inside an autonomous
system is a special area called the backbone;

• The routers inside the backbone are called the
backbone routers.
• Backbone router can also be an area border
router.

• If, because of some problem, the connectivity
between a backbone and an area is
• broken, a virtual link between routers must be
created by the administration to allow
continuity of the functions of the backbone as
the primary area.
• Each area has an area identification. The area
identification of the backbone is zero.

Metric
• The OSPF protocol allows the administrator to
assign a cost, called the metric, to each route.
The metric can be based on a type of service
(minimum delay, maximum throughput, and
so on).
• As a matter of fact, a router can have multiple
routing tables, each based on a different type
of service.

• Types of Links
• In OSPF terminology, a connection is called a
link. Four types of links have been defined:
point-to-point, transient, stub, and virtual

• Point-to-Point Link
• A point-to-point link connects two routers
without any other host or router in between.
In other words, the purpose of the link
(network) is just to connect the two routers.
An example of this type of link is two routers
connected by a telephone line or
• a T-line. There is no need to assign a network
address to this type of link.

Transient Link
• A transient link is a network with several routers
attached to it. The data can enter through any of the
routers and leave through any router. All LANs and
some WANs with two or more routers are of this
type. In this case, each router has many neighbors.
• For example, consider the Ethernet Router A has
routers B, C, D, and E as neighbors. Router B has
routers A, C, D, and E as neighbors. If we want to
show the neighborhood relationship in this situation.

Stub Link
• A stub link is a network that is connected to only one
router. The data packets enter the network through
this single router and leave the network through this
same router.
• This is a special case of the transient network. We can
show this situation using the router as a node and
using the designated router for the network.
• However, the link is only one directional, from the
router to the network.

Virtual Link
• When the link between two routers is broken,
the administration may create a virtual link
between them using a longer path that
probably goes through several routers

Graphical Representation of Network
• Small AS with seven networks and six routers.
• Two of the networks are point-to-point networks.
• We use symbols such as N1 and N2 for transient and stub
networks.
• There is no need to assign an identity to a point-to-point
network.
• The figure also shows the graphical representation of the AS as
seen by OSPF.
• We have used color nodes for the routers and shaded nodes for
the networks (represented by designated routers).
• However, OSPF sees both as nodes. Note that we have three
stub networks.

Border Gateway Protocol (BGP)
• Border Gateway Protocol (BGP) is an
interdomain routing protocol using path
vector routing. It first appeared in 1989 and
has gone through four versions.
• Types of Autonomous Systems

Types of Autonomous Systems
• Stub AS
• A stub AS has only one connection to another AS.
The interdomain data traffic in a stub
• AS can be either created or terminated in the AS.
• The hosts in the AS can send data traffic to other
ASs. The hosts in the AS can receive data coming
from hosts in other ASs.
• Data traffic, however, cannot pass through a stub
AS. A stub AS is either a source or a sink. A good
example of a stub AS is a small corporation or a
small local ISP.

Multihomed AS
• A multihomed AS has more than one connection to
other ASs, but it is still only a source or sink for data
traffic. It can receive data traffic from more than one
AS.
• It can send data traffic to more than one AS, but there
is no transient traffic.
• It does not allow data coming from one AS and going
to another AS to pass through.
• A good example of a multihomed AS is a large
corporation that is connected to more than one
regional or national AS that does not allow transient
traffic.

Transit AS
• A transit AS is a multihomed AS that also
allows transient traffic.
• Good examples of transit ASs are national and
international ISPs (Internet backbones).

• CIDR
• BGP uses classless interdomain routing
addresses. In other words, BGP uses a prefix,
to define a destination address.
• The address and the number of bits (prefix
length) are used in updating messages.

Attributes
• The path was presented as a list of
autonomous systems, but is, in fact, a list of
attributes. Each attribute gives some
information about the path.
• The list of attributes helps the receiving router
make a better decision when applying its
policy

• Attributes are divided into two broad
categories: well-known and optional. A
wellknown attribute is one that every BGP
router must recognize.
• An optional attribute is one that needs not
be recognized by every router.

BGP Sessions
• The exchange of routing information between
two routers using BGP takes place in a session.
• A session is a connection that is established
between two BGP routers only for the sake of
exchanging routing information. To create a
reliable environment, BGP uses the services of
TCP.

External and Internal BGP
• The E-BGP session is used to exchange
information between two speaker nodes
belonging to two different autonomous
systems.
• The IBGP session, on the other hand, is used
to exchange routing information between two
routers inside an autonomous system.

Packet Format
• All BGP packets share the same common header
• ❑ Marker. The 16-byte marker field is reserved
for authentication.
• ❑ Length. This 2-byte field defines the length of
the total message including the header.
• ❑ Type. This 1-byte field defines the type of the
packet. As we said before, we have four types,
and the values 1 to 4 define those types.

Open Message
• To create a neighborhood relationship, a
router running BGP opens a TCP connection
with a neighbor and sends an open message.
• If the neighbor accepts the neighborhood
relationship, it responds with a keepalive
message, which means that a relationship has
been established between the two routers

• The fields of the open message are as follows:
• ❑ Version. This 1-byte field defines the version of
BGP. The current version is 4.
• ❑ My autonomous system. This 2-byte field
defines the autonomous system number.
• ❑ Hold time. This 2-byte field defines the
maximum number of seconds that can elapse
until one of the parties receives a keepalive or
update message from the other. If a router does
not receive one of these messages during the
hold time period, it considers the other party
dead.

• BGP identifier. This 4-byte field defines the router
that sends the open message. The router usually
uses one of its IP addresses (because it is unique)
for this purpose.
• Option length. The open message may contain
some option parameters. In this case, this 1-byte
field defines the length of the total option
parameters. If there are no option parameters,
the value of this field is zero.
• Option parameters. If the value of the option
parameter length is not zero, it means that there
are some option parameters.

Update Message
• The update message is the heart of the BGP
protocol. It is used by a router to withdraw
destinations that have been advertised
previously, announce a route to a new
destination, or both.
• Note that BGP can withdraw several
destinations that were advertise before, but it
can only advertise one new destination in a
single update message.

Keepalive Message
• The routers (called peers in BGP parlance)
running the BGP protocols exchange keepalive
messages regularly (before their hold time
expires) to tell each other that they are alive..

Notification Message
• A notification message is sent by a router
whenever an error condition is detected or a
router wants to close the connection.

Encapsulation
• BGP messages are encapsulated in TCP
segments using the well-known port 179.
• This means that there is no need for error
control and flow control.
• When a TCP connection is opened, the
exchange of update, keepalive, and
notification messages is continued until a
notification message of type cease is sent.

Routing Protocols

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Routing Protocols