1. (How the routers’ tables are filled in)
Chapter 11
Unicast Routing
Protocols
(RIP, OSPF, BGP)
(How the routers’ tables are filled in)
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2. Popular Unicast Routing protocols
RIP (already covered) – Routing Information Protocol – treats each network the same (assigns the same cost for each network)
OSPF (continue covering) – Open Shortest Path First protocol – assigns a cost for passing through a network based on the type of service required – routes through the network can have different cost – each router would have several tables
BGP – Border Gateway Protocol – is an exterior routing protocol that uses a policy that defines what paths should be chosen
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3. Graphical representation of an internet
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4. OSPF uses Dijkstra’s Shortest Path Algorithm
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5. Dijkstra’s Algorithm Cont..
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6. Dijkstra’s Algorithm Continued
At this point, we would have the least cost path from A to all other nodes
Note: to find the least cost paths from E to all other nodes, Disjkstra’s algorithm has to run again
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7. Answer in Hyphen Format
A-D
A-D-N3
A-D-N3-F
A-D-N3-F-N5
A-N1
A-N1-B
A-N1-B-E
A-N1-B-E-N4
A-N1-C
A-N1-C-N2
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8. How do the OSPF packets travel across the network(s) ??????????????
A re-occurring theme
How do the OSPF packets travel across the network(s) ??????????????
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9. OSPF packets are encapsulated in IP datagrams.
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10. BGP: Border Gateway Protocol
BPG is an inter-autonomous system routing protocol that makes use of path vector routing
For BPG, each routing entry contains (1) destination network, (2) next router (hop) and (3) entire path to reach destination
Path is an ordered list of autonomous systems that the packet should travel through to make it to the destination
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11. Ch 12: Multicast Routing
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12. Put Your Unicast Routing HAT on
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13. Recall Unicasting Vs Multicasting Vs Broadcasting
Unicasting Case:
In unicast routing, the router forwards the received packet through only one of its interfaces.
Both the source and destination addresses are unicast addresses
One-to-One Relationship
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14. Multicasting Case
Packet starts from source, S1, and goes to all destinations belonging to group, G1
In multicast routing, the router may forward the received packet through several of its interfaces.
The source address is unicast and the destination address is a group address (Class D)
Group address define a set of Rx’s
One-to-Many Relationship
Actually, the packet is DUPLICATED at each router – only one copy travels in between any two routers
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15. Broadcasting Case One-to-all case would cause traffic problems
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16. Multicasting versus multiple unicasting
Recall Multicasting
One packet start from source and is duplicated at each router
Only one copy of the packet travels in between any two routers
Packet has a single group address
Multiple Unicast Case
More than one copy of the packet starts from the source
Each copy of the packet has a different destination address
In this case, there could be multiple copies traveling between any two routers
Multicast is more efficient than multiple unicast because in the unicast cast, some links will use more bandwidth in handling more packet copies. Also for the unicast case, there is more delay at the source due to packet duplication
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Lecture

17. MULTICAST ROUTING Now we understand what multicasting is
Now let’s understand how packets are routed in the multicast case
Some Objectives of multicast routing (very complex)
Each Rx of the group must get only one copy of the packet
Rx’s not belonging to the group DO NOT get a copy of the packet
The packet can not visit the same router more than once (no loops)
The route from Tx to various Rx’s must be optimal (shortest path)
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18. MULTICAST TREES
Recall in the RIP/OSPF unicast World, we converted networks to graphs in finding the optimal routes
For graphs, nodes can have both successors and predecessors
In the multicast World, networks are converted to trees
Tree has a hierarchical structure
Each node on a tree has (1) a single parent and (2) zero to multiple off-springs (children)
The root (source or Tx) of the tree is the initial node (has no parent)
A leaf (group members) of a tree has no child
Called Spanning Tree provided all nodes are connected
NOTE: show students difference between graph & tree
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Lecture

19. Two Types of Trees are used for multicasting by protocols:
Source-Based Trees – a single tree is created for each Source-to-Group combination. For example, given M sources and N groups, there would be a maximum of MxN trees
Group-Shared Trees – each group has it’s own tree. Given N groups, there would be a maximum of N trees.
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20. Source-Based Tree
Given a source needs to send a packet to group-1, a certain tree is used
Given the same source needs to send a packet to group-5, a different tree is used
Challenge: (1) determining all source-to-group combinations, (2) each tree needs to be optimal
Two approaches used to create optimal source-based trees:
(1st) An extension to the unicast distance vector routing we covered in regards to RIP – used by DVMRP (Distance Vector Multicasting Routing Protocol)
(2nd) An extension to the unicast link state routing we covered in regards to OSPF – used by MOSPF (Multicast Open Shortest Path First)
Another protocol called PIM-DM (Protocol Independent Multicast – Dense Mode) uses either RIP or OSPF depending on need
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21. Group-Shared Tree
Given a source (source x) needs to send a packet to group-1, a certain tree is used
Given a different source (source y) needs to send a packet to the same group, group-1, the same tree is used
If either source-x or source-y need to send to a different group, the tree would change
Note: the tree changes when the group changes – the tree remains the same for a group regardless if the source changes – the group determines the tree
Two approaches used to create optimal group-shared trees:
(1st) Steiner Tree – the optimal tree has the minimum cost routes like Dijkstra’ algorithm however, instead of it being based around a source node, it is not based around any particular source (very complex and has to re-run every time the topology changes) – not really used by the Internet
(2nd) Rendezvous-Point Tree – a tree is created for each group and a single router is selected as the core or rendezvous point or root of the tree. The CBT (Core-Based Tree Protocol) and PIM-SM (Protocol Independent Multicast – Sparse Mode) use the rendezvous-point tree approach.
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22. Multicast routing protocols
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23. DVMRP
Distance Vector Multicast Routing Protocol –similar to the distance vector routing protocol we covered for the unicast case – next hop scenario.
For DVMRP, the optimal tree is not pre-defined – only the next hop
How do we build a tree using the DVMRP approach ?
Use a modified “flooding” approach
Recall what flooding is: a router sends a copy of a packet out of all of it’s interfaces – all interfaces except the interface the packet came in on
Flooding will cause looping problems (ie. the same packet copy that left the router will re-visit the router)
The flooding is modified to stop the looping problem
How is the flooding modified ??????
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24. DVMRP - How is the flooding modified ??????
Instead of forwarding copies of the packet through all interfaces (except the receiving interface), ONLY FORWARD THE PACKET IF IT CAME IN ON THE SHORTEST PATH
If it comes in on the non-shortest path – drop it
This approach of only forwarding the packet if it comes in on the shortest path is called Reverse Path Forwarding (RPF) – RPF prevents looping
How does the router determines if the packet came in on the shortest path ???
Recall that the unicast routing tables contain the next hop based around the shortest path – the table has destinations, interfaces and next hops en route to destinations
If the router used the packet’s source address (instead of destination address), the router could determine the NEXT HOP and desired INTERFACE to exit en route to the packet’s source address
Punch Line: if the INTERFACE the packet arrived at, is the same INTERFACE the packet needs to take in achieving the shortest path en route to the source address – then the PACKET ARRIVED USING THE SHORTEST PATH – make sense ??
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25. DVMRP Continuing EXAMPLE
A multicast router receives a packet with source address and destination address from interface 2. Should the router discard or forward the packet based on the following unicast table ?
Destination
Interface 1 2 3
SOLUTION: In interpreting the source address of using the default mask, the router would send the packet to network via interface 3 (not interface 2). Recall the packet came in on interface 2 – therefore, the router would DROP the packet (and not forward it)
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26. DVMRP Continuing
What RPF guarantees is: each network will receive a copy of the multicast packet WITHOUT the loop problem
What RPF doesn’t guarantee is: each network will receive ONLY ONE COPY
With the Reverse Path Forwarding approach, networks could received multiple copies (see example below)
In fixing this problem, a policy called Reverse Path Broadcasting (RPB) can be implemented.
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Lecture

27. Reverse Path Broadcasting (RPB)
To eliminate networks (nodes) receiving more than one copy, ONLY THE PARENT HAS THE RIGHT TO FORWARD (this is the RPB policy)
Recall: with a tree, each node has only ONE PARENT
Therefore, if the parent is the only node that can forward, no node should receive duplicates
Policy: the router sends the packet only out of those interfaces for which it is the designated parent.
See the example 
The next question: “How is the parent determined ????”
The router with the shortest path to the source is designated as the PARENT
Recall: because routers share info with their neighbors, they can easily determine which router has the shortest path to the source
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28. RPB creates a shortest path broadcast tree (not multicast tree) from the source to each destination. It guarantees that each destination receives one and only one copy of the packet.
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29. Reverse Path Multicasting (RPM)
Recall RPB broadcast a packet versus multicast
How is multicasting achieved ? (1) the first packet is broadcasted no matter what, (2) the remaining packets are multicasted based on pruning and grafting
Another name for pruning and grafting is Reverse Path Multicasting (RPM)
Based on pruning
Using the IGMP (Internet Group Management Protocol), each PARENT ROUTER holds a membership and knows which groups it is not responsible for.
The PARENT ROUTER sends a “prune message” to it’s upstream router letting the upstream router know NOT to send any packets belonging to certain no-interest groups through that corresponding interface.
That upstream router will do the same to it’s upstream router
This creates a “pruning” effect in that only the packets belonging to a group are forwarded through a particular interface
Based on grafting
Suppose a “leaf” router (a router with NO children) had previously sent a prune message and suddenly realize it NOW INTERESTED in receiving the multicast packet
The leaf router will issue a grafting message upstream and as a result, multicasting will resume
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30. RPM adds pruning and grafting to RPB to create a multicast shortest path tree that supports dynamic membership changes.
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