Module 8: Link Layer Multicast

Question 1

mutlicast routing approaches

Answer 1

kinds of trees
-source specific trees
-shared trees

Question 2

source specific trees

Answer 2

Question 3

advantages and disavantages of source specific trees

Answer 3

great performance but difficult to construct and maintain because you have to maintain a tree for every content source

Question 4

how does a shared tree solve the disadvantage of a source specific tree?

Answer 4

one tree is used by all members in a group

easier to construct/maintain but hard to pick “good” trees for everyone

Question 5

steiner tree (shared tree)

Answer 5

minimum weighted tree connecting only the multicast members
finding steiner tree is NP hard
heuristics are known to approximate

Question 6

what can we compute instead of steiner?

Answer 6

minimum spanning tree

Question 7

multicast and layering

Answer 7

at different layers
data link layer: ethernet
network layer: IP
application: end system

Question 8

What is the idea of data link multicast?

Answer 8

Question 9

IP Multicast

Answer 9

Question 10

What are the problems with IP multicast?

Answer 10

Question 11

what are some protocols for IP level multicast routing?

Answer 11

Question 12

Explain Distance Vector Multicast Routing Protocol

Answer 12

extension to DV
uses shortest path DV routes to determine if the ink is on the source-rooted spanning tree

3 steps:
reverse path flooding
reverse path broadcasting
truncated reverse path broadcasting

Question 13

reverse path flooding

Answer 13

Question 14

what are issues with RPF?

Answer 14

you may have duplicate transmissions
fix through enhancement: reverse path broadcasting

Question 15

reverse path broadcasting

Answer 15

Question 16

to make this more efficient, we need to…

Answer 16

prune!

Question 17

explain membership pruning

Answer 17

Question 18

single sender multicast SSM

Answer 18

Question 19

what’s wrong with SSM?

Answer 19

ok for broadcast apps
not good for meeting apps with multiple sources

what to do for multiple sources? –core based trees (supporting multiple senders in a single tree)

Question 20

CBT

Answer 20

pick a rendezvous point for the group called the core (shared tree)

unicast packet to core and bounce it back to multicast group

reduce routing table state from O(s x G) to O(G)

called in RP (rendezvous point)

Question 21

how do you build a CBT?

Answer 21

Question 22

what are the disadvantages of CBT?

Answer 22

sub optimal delay
single point of failure
small, local groups with non-local core

Question 23

explain multicast reliability

Answer 23

Question 24

what is nack implosion?

Answer 24

when a packet is lost all receivers int he sub-tree originated at the link where the packet is lost send NACKs
solutions? tolerate failures, erasure coding, inform parent only