Routing at Network Layer - WordPress.com · 2013-05-17 · Network Layer Internet as a...

Department of Computer Science

DCS

COMSATS Institute of Information Technology

Routing at Network Layer

Advanced Computer Networks

Rab Nawaz Jadoon

Assistant Professor

COMSATS University, Lahore

Pakistan

Department of Computer Science 22

IPv6

An IPv6 address is 128 bits long. An IPv6 address consists of 16 bytes (octets).

Hexadecimal Colon Notation To make addresses more readable, IPv6 specifies

hexadecimal colon notation.

128 bits is divided into eight sections, each 2 bytes in length.

Two bytes in hexadecimal notation requires four hexadecimal digits.

Therefore, the address consists of 32 hexadecimal digits, with every four digits separated by a colon,


Network Layer

IPv6 address in binary and hexadecimal colon notation,


Network Layer Abbreviation

IP address in hexadecimal is too long, most of the digits may be zero.

To overcome this problem we can abbreviate the address.

The leading zeros of a section (four digits between two colons) can be omitted.


Network Layer

Example:

Expand the address 0:15::1:12:1213 to its original.


Network Layer

Address space

IPv6 has a much larger address space; 2128

addresses are available.

IPv6 divided the address into several categories.

A few leftmost bits, called the type prefix, in each address define its category.

The type prefix is variable in length.

It is designed such that no code is identical to the first part of any other code.

when an address is given, the type prefix can easily be determined.


Network Layer

Table 19.5 shows the prefix for each typeof address.


Cont…


Network Layer

Unicast Addresses IPv6 defines two types of unicast addresses

Geographically based ( it is left for future use)

Provider-based. (The provider-based address is generally used by a normal host as a unicast address).


Fields for the provider-based address

Type identifier.

This 3-bit field defines the address as a provider-based address.

Registry identifier

This 5-bit field indicates the agency that has registered the address.

Currently three registry centers have been defined.

INTERNIC (code 11000) is the center for North America;

RIPNIC (code 01000) is the center for European registration;

APNIC (code 10100) is for Asian and Pacific countries.


Network Layer Provider identifier.

identifies the provider for Internet access (such as an ISP).

A 16-bit length is recommended for this field.

Subscriber identifier.

When an organization subscribes to the Internet through a provider, it is assigned a subscriber

identification.



IPv6

Subnet identifier.

Each subscriber can have many different subnetworks, and each subnetwork can have an

identifier.


Node identifier.

The last field defines the identity of the node

connected to a subnet.

A length of 48 bits is recommended for this field to make it compatible with the 48-bit link (physical) address used by Ethernet.


Network Layer

Multicast Addresses

Multicast addresses are used to define a group of hosts instead of just one.

A packet sent to a multicast address must be delivered to each member of the group.

A permanent group address is defined by the Internet authorities and can be accessed at all times.

A transient group address, on the other hand, is used only temporarily. E.g. Systems engaged in a teleconference


Network Layer

Anycast Addresses

An anycast address, like a multicast address, also

defines a group of nodes.

An anycast address is delivered to only one of the members of the anycast group.

Reserved Addresses

These addresses start with eight Os (type prefix is

0000 0000).


Reserved Addresses


Example Unspecified address

An unspecified address is used when a host does not know its own address and sends an inquiry to find its

address.

Loopback address

A loop back address is used by a host to test itself

without going into the network.

Compatible address

Used during the transition from IPv4 to IPv6.

Mapped address

A mapped address is also used during transition.


Reserved Addresses

Local Addresses These addresses are used when an organization wants to use IPv6 protocol without being connected to the global Internet. In other words, they provide addressing for private networks.


Transition from IPv4 to IPv6

The transition from IPv4 to IPv6 cannot happen suddenly.

It takes a considerable amount of time before every system in the Internet can move from IPv4 to IPv6.

The transition must be smooth to prevent any problems between IPv4 and IPv6 systems.

Three strategies have been devised by the IETF to help the transition.

Duel Stack

Tunneling

Header Translation


Network Layer

Three transition strategies


Network Layer Dual Stack

It is recommended that all hosts, before migrating completely to version 6, have a dual stack of protocols.

A station must run IPv4 and IPv6 simultaneously until all the Internet uses IPv6.


Network Layer

Tunneling

Tunneling is a strategy used when two computers using IPv6 want to communicate with each other and the packet must pass through a region that uses IPv4.

IP within IP is the actual concept of tunneling.


Network Layer

Header Translation Header translation is necessary when the majority of

the Internet has moved to IPv6 but some systems still use IPv4.

The sender wants to use IPv6, but the receiver does not understand IPv6.

Tunneling does not work in this case

The header format must be totally changed through header translation.

The header of the IPv6 packet is convened to an IPv4 header


Header translation strategy

Note

Header translation uses the mapped address to translate an IPv6 address to an IPv4 address.

The 1Pv6 mapped address is changed to an lPv4 address by

extracting the rightmost 32 bits.


INTERNETWORKING

The physical and data link layers of a network operate locally.

Responsible for HOP to HOP delivery.

This internetwork is made of five networks: four LANs and one WAN.

If host A needs to send a data packet to host D, the packet needs to go first from A to R1 (a switch or router), then from R1 to R3, and finally from R3 to host D.


Network Layer

The data packet passes through three links.

In each link, two physical and two data link layers are involved.

When data arrive at interface f1 of R1,

How does R1 know that interface f3 is the outgoing interface?

There is no provision in the data link (or physical) layer to help R1 make the right decision.


Network Layer

Network layer in an internetwork


Network Layer

Functionality of the Network Layer

The general idea of the functionality of the network layer at a source, at a router, and at the destination is,

The network layer at the source is responsible for creating a packet from the data coming from another protocol upper layer)

The network layer is responsible for checking its routing table to find the routing information.

The network layer at the switch or router is responsible for routing the packet.


Network Layer

When a packet arrives, Consult the routing table

Find the interface over which the packet is sent over.

Changes in the packet header regarding routing information.

Then passed to Data Link Layer.

The network layer at the destination, Responsible for address verification;

If the packet is a fragment, The network layer waits until all fragments have arrived.

Then reassembles them and delivers them to transport layer.


Network Layer


Network Layer

Internet as a Connectionless Network

Communication at the network layer in the Internet is connectionless.

The network layer protocol treats each packet independently, with each packet having no relationship to any other packet.

The packets in a message may or may not travel the same path to their destination.


Network Layer

Main reason of connectionless services at the internet,

The reason for this decision is that the Internet is made of so many heterogeneous networks that it is almost impossible to create a connection from the source to the destination without knowing the nature of the networks in advance.


Network Layer

Position of lPv4 in TCP/IP protocol suite

Note:

IPv4 is an unreliable and connectionless datagramprotocol


IPv4 datagram

Datagram

IPv4 datagram format


IPv4 Header

Version (VER). This 4-bit field defines the version of the IPv4 protocol.

Header length (HLEN). This 4-bit field defines the total length of the datagram header

in 4-byte words.

The length of the header is variable (between 20 and 60 bytes).

Services. 8-bit field.

TOS

Differentiated services

Note:

Rest of the information in header you may read it from the book.


IPv4 Header

TOS First 3 bits are called precedence bits.

The next 4 bits are called type of service (TOS) bits.

The last bit is not used.

Precedence Precedence is a 3-bit subfield ranging from 0 (000 in binary)

to 7 (111 in binary).

Used in congestion control

Some datagrams in the Internet are more important than others.

For example, a datagram used for network management is much more urgent and important than a datagram containing optional information for a group.


IPv4 Header

TOS bits

4-bit subfield with each bit having a special meaning.

The bit patterns and their interpretations are given in Table


IPv4 Header

Differentiated Services

In this interpretation, the first 6 bits make up the codepoint subfield, and the last 2 bits are not used.

The codepoint subfield can be used in two different ways.

When the 3 rightmost bits are Os, the 3 leftmost bits are interpreted the same as the precedence bits in the service type interpretation.

When the 3 rightmost bits are not all Os, the 6 bits define 64 services based on the priority assignment by the Internet or local authorities


IPv4 Header

Total length. This is a 16-bit field that defines the total length

(header plus data) of the IPv4 datagram in bytes.

Length of data = total length - header length

Time to live. Today, this field is used mostly to control the

maximum number of hops (routers) visited by the datagram. (e.g 16 hops)

Protocol. This 8-bit field defines the higher-level protocol that

uses the services of the IPv4 layer.


IPv4 Header

An IPv4 datagram can encapsulate data from several higher-level protocols such as TCP, UDP, ICMP, and IGMP.

This field specifies the final destination protocol to which the IPv4 datagram is delivered.

this field helps the receiving network layer know to which protocol the data belong


IPv4 Header

The value of this field for each higher-level protocol is shown in Table

Checksum

Source address.

This 32-bit field defines the IPv4 address of the source.

Destination address.

This 32-bit field defines the IPv4 address of the destination.


ICMP Internet Control Message Protocol

IP provides unreliable and connectionless datagram delivery.

The IP protocol is a best-effort delivery service that delivers a datagram from its original source to its final destination. It has two deficiencies:

Lack of error control and lack of assistance mechanisms

No flow control Mechanism

The IP protocol also lacks a mechanism for host and management queries. A host sometimes needs to determine if a router or another

host is alive.


ICMP

The Internet Control Message Protocol (ICMP) has been designed to compensate for the above two deficiencies.

It is a companion to the IP protocol.

Types of Messages ICMP messages are divided into two broad

Categories: Error-reporting messages and Query messages

Error Reporting Messages The error-reporting messages report problems that a router

or a host (destination) may encounter when it processes an IP packet.


ICMP

The query messages

These occur in pairs, help a host or a network manager get specific information from a router or another host.

For example, nodes can discover their neighbors.

Also, hosts can discover and learn about routers on their network, and routers can help a node redirect its messages.


ICMP

Message Format

An ICMP message has an 8-byte header and a variable-size data section.

The first 4 bytes are common to all.

General format of lCMP messages is as under,


ICMP

ICMP Type It defines the type of the message

ICMP Code It specifies the reason for the particular message

type.

Checksum The last common field is the checksum field.

The data section in error messages carries information for finding the original packet that had the error.


ICMP Error Reporting

One of the main responsibilities of ICMP is to report errors.

ICMP does not correct errors--it simply reports them.

Error messages are always sent to the original source because the only information available in the datagram about the route is the source and destination IP addresses.

ICMP uses the source IP address to send the error message to the source (originator) of the datagram.


Types of Error handled by ICMP

Five types of errors are handled:


ICMP

The following are important points about ICMP error messages: No ICMP error message will be generated in

response to a datagram carrying an ICMP error message.

No ICMP error message will be generated for a fragmented datagram that is not the first fragment.

No ICMP error message will be generated for a datagram having a multicast address.

No ICMP error message will be generated for a datagram having a special address such as 127.0.0.0 or 0.0.0.0


ICMP

Note All error messages contain a data section that

includes the IP header of the original datagram plus the first 8 bytes of data in that datagram.

The original datagram header is added to give the original source, which receives the error message, information about the datagram itself.

The 8 bytes of data are included b/c, Provide information about the port numbers (UDP and TCP)

and sequence number (TCP).

ICMP forms an error packet, which is then encapsulated in an IP datagram


ICMP

Contents of data field for the error messages

IP Header 8 BytesRest of

The IP Data

ICMPHeader

IP Header 8 Bytes

ICMPHeader

IP Header 8 BytesIP Header

Received Datagram

ICMP Message

Sent IP Datagram


ICMP Error Messages

Destination Unreachable

When a router cannot route a datagram or a host cannot deliver a datagram, the datagram is discarded and the router or the host sends a destination-unreachable message back to the source host that initiated the datagram.

Source Quench

IP does not have a flow control mechanism embedded in the protocol.

The lack of flow control can create congestion in routers or the destination host.

The source-quench message in ICMP was designed to add a kind of flow control to the IP. When a router or host discards a datagram due to congestion, it sends a source-quench message to the sender of the datagram.


ICMP Error Messages

Source Quench message has two purposes. First, it informs the source that the datagram has

been discarded.

Secondly inform the source should slow down (quench) the sending process.

Time Exceeded When a datagram visits a router, the value of this

field is decremented by 1. When the time-to-live value reaches 0, after decrementing, the router discards the datagram.

Second, a time-exceeded message is also generated when not all fragments that make up a message arrive at the destination host within a certain time limit.


ICMP Error Messages

Parameter Problem If a router or the destination host discovers an

ambiguous or missing value in any field of the datagram, it discards the datagram and sends a parameter-problem message back to the source.

Redirection When a router needs to send a packet destined for

another network, it must know the IP address of the next appropriate router.


ICMP Query Messages

Query Messages

In addition to error reporting, ICMP can diagnose some network problems.

four different pairs of messages,

Encapsulation of lCMP query messages


ICMP Query Messages

Echo Request and Reply

The echo-request and echo-reply messages are designed for diagnostic purpose.

echo-request and echo-reply messages determines whether two systems (hosts or routers) can communicate with each other.

Today, most systems provide a version of the ping command that can create a series (instead of just one) of echo-request and echo-reply messages, providing statistical information.


ICMP Query Messages

Timestamp Request and Reply

Two machines (hosts or routers) can use the timestamp request and timestamp reply messages to determine the round-trip time needed for an IP datagram to travel between them.

Also used for synchronizing the clocks between hosts.

Address-Mask Request and Reply

To obtain its mask, a host sends an address-mask-request message to a router on the LAN.

Address-mask-reply message, providing the necessary mask for the host.


ICMP Query Messages

Router Solicitation and Advertisement A host that wants to send data to a host on another

network needs to know the address of routers connected to its own network.

Also, the host must know if the routers are alive and functioning.

A host can broadcast (or multicast) a router-solicitation message.

The router or routers that receive the solicitation message broadcast their routing information using the router-advertisement message.

Note that when a router sends out an advertisement, it announces not only its own presence but also the presence of all routers on the network of which it is aware.


Debugging Tools

C:\Documents and Settings\rabnawaz>ping 172.16.2.69

Pinging 172.16.2.69 with 32 bytes of data:

Reply from 172.16.2.69: bytes=32 time=60ms TTL=128Reply from 172.16.2.69: bytes=32 time<1ms TTL=128Reply from 172.16.2.69: bytes=32 time<1ms TTL=128Reply from 172.16.2.69: bytes=32 time<1ms TTL=128

Ping statistics for 172.16.2.69:Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:Minimum = 0ms, Maximum = 60ms, Average = 15ms

•Debugging Tools•Ping•Traceroute


Internet Group Management Protocol

IGMP

The IP protocol can be involved in two types of communication: Unicasting

One source and destination

Multicasting. One source multiple destinations

IGMP is not a multicasting routing protocol; it is a protocol that manages group membership.

In any network, there are one or more multicast routers.

The IGMP protocol gives the multicast routers information about the membership status of hosts (routers) connected to the network.


Group Management

A multicast router may receive thousands of multicast packets every day for different groups.

If a router has no knowledge about the membership status of the hosts, it must broadcast all these packets.

This creates a lot of traffic and consumes bandwidth. A better solution is to keep a list of groups in the

network for which there is at least one loyal member.

IGMP helps the multicast router create and update this list.


IGMP

Note

IGMP is a group management protocol. It helps a multicast router create and update a list of loyal members related to each router interface.


IGMP

IGMP Messages

IGMPv2 has three types of messages:

Query

Membership report

Leave report.

There are two types of query messages:

general and special


IGMP

Message Format

Type.

This 8-bit field defines the type of message,


IGMP

Maximum Response Time.

This 8-bit field defines the amount of time in which a query must be answered.

The value is in tenths of a second; for example, if the value is 100, it means 10 s.

Checksum.

This is a 16-bit field carrying the checksum. The checksum is calculated over the 8-byte message.

Group Address

The value of this field is 0 for a general query message.

The value defines the groupid (multicast address of the group) in the special query,


IGMP Operation

IGMP Operation

IGMP operates locally.

A multicast router connected to a network has a list of multicast addresses of the groups with at least one loyal member in that network.

List of groups having loyal members

To another network

To another network

To another network

Network


IGMP Operation

For each group, there is one router that has the duty of distributing the multicast packets destined for that group. Router R distributes packets with the multicast

address of 225.70.8.20.

A host or multicast router can have membership in a group.

When a host has member ship, it means that one of its processes (an application program) receives multicast packets from some group.

When a router has membership, it means that a network connected to one of its other interfaces receives these multicast packets.


IGMP Operation

Joining a Group A host or a router can join a group.

A host maintains a list of processes that have membership in a group.

When a process wants to join a new group, It sends its request to the host.

The host adds the name of the process and the name of the requested group to its list.

If this is the first entry for this particular group, the host sends a membership report message.

Else

No problem (don’t send membership report)


IGMP Operation

In IGMP, a membership report is sent twice, one after the other.

Leaving a Group

When a host sees that no process is interested in a specific group, it sends a leave report.

When a router sees that none of the networks connected to its interfaces is interested in a specific group, it sends a leave report about that group.

When a multicast router receives a leave report, it cannot immediately purge that group from its list because the report comes from just one host or router; there may be other hosts or routers that are still interested in that group.


IGMP Operation

To make sure, the router sends a special query message and inserts the groupid, or multicast address, related to the group.

The router allows a specified time for any host or router to respond.

Monitoring Membership

Consider the situation in which there is only one host interested in a group, but the host is shut down or removed from the system.

The multicast router will never receive a leave report.

How is this handled?

The multicast router is responsible for monitoring all the hosts or routers in a LAN to see if they want to continue their membership in a group.


IGMP Operation

The router periodically (by default, every 125 s) sends a general query message.


Network Layer


Routing

Delivery

Delivery refers to the way a packet is handled by the underlying networks under the control of the network layer.

Forwarding

Forwarding refers to the way a packet is delivered to the next station.

Routing

Routing refers to the way routing tables are created to help in forwarding.


Routing

Direct and indirect delivery

Direct Delivery Indirect and Direct Delivery



Forwarding Forwarding requires a host or a router to have a

routing table.

When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination.

Forwarding Techniques Several techniques can make the size of the routing

table manageable, Next-Hop Method vs Route Method

Network-Specific vs Host specific method



Host-specific versus network-specific method



Default Method

Host A is connected to a network with two routers.

Router R1 routes the packets to hosts connected to network N2.

So instead of listing all networks in the entire Internet, host A can just have one entry called the default



Routing Table

Static Routing Table

(A static routing table contains information entered manually)

Dynamic Routing Table

A dynamic routing table is updated periodically by using one of the dynamic routing protocols such as RIP, OSPF, or BGP.

Automatic updating of routing tables.


Unicast routing protocols

Routing tables need to be updated when a router is down, and they need to be updated whenever a better route has been found. 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 routing protocols also include procedures for combining information received from other routers.



Optimization

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: optimum route selection.

One approach is to assign a cost for passing through a network, called metric.

The metric assigned to each network depends on the type of protocol.


Routing

Intra- and Interdomain Routing

Today, an internet can be so large that one routing protocol cannot handle the task of updating the routing tables of all routers.

For this reason, an internet is divided into autonomous systems (AS).

An autonomous system (AS) is a group of networks and routers under the authority of a single administration.



Intradomain

Routing inside an autonomous system is referred to as intradomain routing.

Interdomain

Routing between autonomous systems is referred to as interdomain routing.


Autonomous system


Routing Protocols

Popular routing protocols


Distance Vector Routing


In distance vector routing, the least-cost route between any two nodes is the route with minimum distance.

Each node maintains a vector (table) of minimum distances to every node.

Routing tables maintains the next hop entry to forward the packet.



----C

---A-

-AA-A

---A-

C--C-



In distance vector routing, each node shares its routing table with its immediate neighbors periodically and when there is a change.

Initialization



Table Updating



When to Share?

Periodic Update

A node sends its routing table, normally every 30 s, in a periodic update.

Triggered Update

Anytime there is a change in its routing table.



Two-Node Loop Instability

X 6 AX-6-AX-2-

X-6-AX-ὲ-

X-10-A X-6-A

X-10-A X-14-A

X-ὲ- X-ὲ-



Some solution

Defining infinity

Split Horizon

Split Horizon and Poison Reverse


RIP

RIP (Routing Information Protocol)

An Interdomain routing protocol used inside an autonomous system.

Based on distance vector routing.

RIP implements distance vector routing directly with some consideration

In an autonomous system, we are dealing with routers and networks (links). The routers have routing tables; networks do not.

The destination in a routing table is a network, which means the first column defines a network address.


RIP

The metric in RIP is called a hop count.

Infinity is defined as 16, which means that any route in an autonomous system using RIP cannot have more than 15 hops.

The next-node column defines the address of the router to which the packet is to be sent to reach its destination.


RIP

Example of a domain using RIP


Link State Routing

In link state routing, each node in the domain has the entire topology of the domain. The node can use Dijkstra's algorithm to build a

routing table.


Link State Routing

Each node uses the same topology to create a routing table, but the routing table for each node is unique because the calculations are based on different interpretations of the topology.

This is analogous to a city map.

Each person may have the same map, each needs to take a different route to reach her specific destination.


Link State Routing

If there are changes in any point in the network, The topology must be updated for each node.

Building Routing Tables In link state routing, four sets of actions are required

Routing table showing the least-cost node to every other node. Creation of the link state packet (LSP).

Flooding of LSPs

Formation of a shortest path tree for each node.

Calculation of a routing table based on the shortest path tree.


Link State Routing

LSPs are generated on two occasions:

When there is a change in the topology of the domain.

On a periodic basis.

The period is much longer than Distance vector.

Flooding of LSPs

The creating node sends a copy of the LSP out of each interface.

Older LSPs are discarded, new are kept. This is done through sequence numbers included in LSP.

Flood the information out of each interface except the one from which the packet arrives.

This ensures that flooding stop somewhere in domain.


Link State Routing

Formation of Shortest Path Tree: Dijkstra Algorithm

After receiving all LSPs, each node will have a copy of the whole topology.

A shortest path tree is needed.

A tree is a graph of nodes and links; one node is called the root.

A shortest path tree is a tree in which the path between the root and every other node is the shortest.


Link State Routing


Link State Routing

Calculation of Routing Table from Shortest Path Tree

Each node uses the shortest path tree protocol to construct its routing table.

The routing table shows the cost of reaching each node from the root.


OSPF

Open shortest Path First An intradomain routing protocol based on link state

routing.

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.

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.


OSPF

Among the areas inside an autonomous system is a special area called the backbone;

All the areas inside an autonomous system must be connected to the backbone.

the backbone serves as a primary area.

The routers inside the backbone are called the backbone routers.

a backbone router can also be an area border router.

Each area has an area identification.

The area identification of the backbone is zero.


OSPF

Area in Autonomous system


OSPF

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).

Types of Links In OSPF terminology, a connection is called a link.

Four types of links, Point to point

Transient

Stub

virtual


OSPF

Point to point link

4 4

Transient link

A

C

B

E D


OSPF

If we want to show the neighborhood of previous figure the relationship in this situation,

A

C

B

E

D

Unrealistic representation


OSPF

Realistic representation

A

C

B

E

D

Designated Router


OSPF

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.


OSPF

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.


Interdomain Routing

Path Vector Routing

In path vector routing, we assume that there is one node called speaker node in each autonomous system that acts on behalf of the entire autonomous system.

The speaker node in an AS creates a routing table and advertises it to speaker nodes in the neighboring ASs.

The idea is the same as for distance vector routing except that only speaker nodes in each AS can communicate with each other.


Path Vector Routing

The advertisement is also different in this case,

A speaker node advertises the path, not the metric of the nodes, in its autonomous system or other autonomous systems.

Initialization

At the beginning, each speaker node can know only the reachability of nodes inside its autonomous system.


Path Vector Routing

Initial routing tables in path vector routing

Speaker NodeA1 for AS1B1 for AS2C1 for AS3D1 for As4


Path Vector Routing

Sharing In path vector routing, a speaker in an autonomous

system shares its table with immediate neighbors.

When a speaker node receives a two-column table from a neighbor, it updates its own table by adding the nodes that are not in its routing table and adding its own autonomous system and the autonomous system that sent the table.

According to the figure, if router A1 receives a packet for nodes A3, it knows that the path is in AS1 (the packet is at home);

But if it receives a packet for D1, it knows that the packet should go from AS 1, to AS2, and then to AS3.


Path Vector Routing

Stabilized tables for three autonomous systems


BGP

Border Gateway Protocol

Border Gateway Protocol (BGP) is an interdomain routing protocol using path vector routing.

It first appeared in 1989 and has gone through four versions.

We can divide autonomous systems into three categories:

Stub

Multihomed

Transit.


BGP

Stub AS A stub AS has only one connection to another AS.

Multihomed AS. A multihomed AS has more than one connection to

other Ass.

It does not allow data coming from one AS and going to another AS to pass through.

It does not allow transient traffic.

Transient AS A transit AS is a multihomed AS that also allows

transient traffic.

E.g. national and international ISPs (Internet backbones).


BGP Session

External and Internal BGP

BGP can have two types of sessions:

External BGP (E-BGP)

The E-BGP session is used to exchange information between two speaker nodes belonging to two different autonomous systems.

Internal BGP (I-BGP)

It is used to exchange routing information between two routers inside an autonomous system.

A1B1AS1 AS2

--- I-BGPE-BGP

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