1. Chapter 22 Network Layer: Delivery, Forwarding, and Routing
Part 4
BGP and Multicasting

2. Path Vector Routing
Distance vector and link state routing are intradomain routing protocols used inside an autonomous system
Distance vector and link state routing protocols are not suitable for interdomain routing because of scalability
There is a need for a third routing protocol which we call path vector routing.
Path vector routing proved to be useful for interdomain routing.
Both of
these routing protocols become intractable when the domain of operation becomes
large. Distance vector routing is subject to instability if there are more than a few hops
in the domain of operation. Link state routing needs a huge amount of resources to calculate
routing tables. It also creates heavy traffic because of flooding

3. Path Vector Routing is similar to distance vector routing
Assuming that there is one node in each AS that acts as on behalf of the entire AS : Speaker Node
Speaker node creates a routing table and advertises it speaker nodes in the neighboring ASs
advertising the path, not the metric of the nodes

4. Path Vector Routing
The difference between the distance vector routing and path vector routing can be compared to the difference between a national map. A national map can tell us the road to each city and the distance to be travelled if we choose a particular route; an international map can tell us which cities exist in each country and which countries should be passed before reaching that city.

5. Path Vector Routing
At the beginning, each speaker node can know only the reachable of nodes inside its
autonomous system.
Node A1 is the speaker node for AS1, B1 for AS2, C1 for AS3, and Dl for AS4.
Node A1 creates an initial table that shows A1 to A5 are located in AS1 and can be
reached through it. Node B1 advertises that B1 to B4 are located in AS2 and can be
reached through Bl. And so on.
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.

6. Figure 22.31 Stabilized tables for three autonomous systems

7. Path Vector Routing Loop prevention Policy routing
The instability of distance vector routing and creation of loops can be avoided in path vector routing
When a router receives a message, it checks to see if its autonomous system is in the path list to the destination
If it is, looping is involved and the message is ignored
Policy routing
Can be easily implemented through path vector routing
If one of the autonomous systems listed in the path is against its policy, it can ignore that path and that destination. It does not update its routing table with this path, and it does not send this message to its neighbors.

8. Path Vector Routing Optimum path
The optimum path is the path that fits the organization.
we chose the path that had the smaller number of autonomous systems.
For example, a path from AS4 to ASI can be AS4-AS3-AS2-AS1, or it can be AS4-AS3-ASI

9. Border Gateway Protocol (BGP)
Border Gateway Protocol is an interdomain routing protocol using path vector routing
Path Vector Routing
Each entry in the routing table contains the destination network, the next router, and the path to reach the destination
The path is usually defined as an ordered list of autonomous systems that a packet should travel through to reach the destination

10. Type of Autonomous System
Stub AS
has only one connection to another AS
The hosts in the AS can send or receive data traffic from other ASs.
A stub AS is either a source or a sink.
Multihomed AS
has more than one connection to other Ass
It can receive and send data traffic to more than one AS.
no transient traffic.
Transit AS
is a multihomed AS that also allows transient traffic.
ex) national and international ISPs

11. BGP (cont’d) Path attributes
Well-known attributes: every BGP router must recognize
well-known mandatory : ORIGIN (RIP, OSPF, and so on), AS-PATH, NEXT_HOP
well-known discretionary : must be recognized by each router; but is not required to be included in every update message
Optional attributes
Optional transitive : must be passed to the next router by the router that has not implemented this attribute
Optional nontransitive : must be discarded if the receiving router has not implemented this attribute

12. BGP (cont’d) BGP Session External and Internal BGP
The exchange of routing information between two routers using BGP takes place in a session.
Use of services of TCP
Referred to as semi-permanent connections
External and Internal BGP

13. MULTICAST ROUTING PROTOCOLS
In Unicast, the router forwards the received packet through only one of its interfaces.
In Multicasting, the router forwards the received packet through only one of its interfaces
In Broadcasting, The router may forward the received packet through several of its interfaces.

14. Multicast Applications
Access to Distributed Databases
Information Dissemination
Dissemination of News
Teleconferencing
Distance Learning

15. Multicasting versus multiple unicasting
In Multicasting:
The packet is duplicated by the routers.
The destination address is between (class D address) which is the Multicast groups IP addresses.
In Multiple unicasting
The source sends multiple copies of the same packet, each with a different unicast destination address.
Example: sending an to a group
Emulation of multicasting through multiple unicasting is not efficient (use more BW) &may create long delays, particularly with a large group.

16. IP Multicast addresses
IP Multicasting only supports UDP as higher layer
There is no multicast TCP !
IP Multicast addresses
It is class D address- from to
Reserved IP Multicast address is to
Examples of special and reserved Class D addresses, e.g,

17. Multicast Groups
The set of receivers for a multicast transmission is called a multicast group (identified by multicast address)
A user that wants to receive multicast transmissions joins the corresponding multicast group, and becomes a member of that group
Every host (more precisely: interface) can join and leave a multicast group dynamically
no access control
Every IP datagram send to a multicast group is transmitted to all members of the group
no security, no “floor control”
Sender does not need to be a member of the group
After a user joins, the network builds the necessary routing paths so that the user receives the data sent to the multicast group

18. Multicast Routing Optimal Routing: Shortest Path Trees
The Goal is to connect all multicast group members by the tree.
Optimal Routing: Shortest Path Trees
The root of the tree is the source, and the leaves
are the potential destinations. The path from the
root to each destination is the shortest path.
Tow approaches to find it:
Unicast Routing
Multicast Routing
Source-Based Tree
Group-Shared Tree.

19. Shortest Path Trees: Unicast
The router has a shortest path tree to reach all destination. (The whole routing table is a shortest path tree).

20. Shortest Path Trees: Multicast
Difficulties in handling multicast traffic is due to:
A multicast packet may have destinations in more than one network.
If we have n groups, we may need n shortest path trees.
Can be solved using:
Source-based trees
Group-shared trees.

21. Source-Based Tree
The shortest path tree for a group defines the next hop for each network that has loyal member(s) for that group.
We have 5 groups in the domain: G1, G2, G3, G4, and G5. At the moment G1 has loyal members in 4 networks, G2 in 3, G3 in 2, G4 in 2, and G5 in 2.
If R1 receives a packet destined to G1
It sends a copy to R2 & to R4 (all members in G1

22. Group-Shared Tree
In the source-based tree approach, each router needs to have one shortest path tree for each group.
The core has m shortest path trees and the rest of the have none.
If a router receives a multicast packet, it encapsulates the packet in a unicast packet and sends it to the core router.
The core router removes the multicast packet from its capsule, and consults its routing table to route the packet. (sends it as Multicast)