Module 8: Link Layer Multicast Flashcards
mutlicast routing approaches
kinds of trees
-source specific trees
-shared trees
source specific trees
advantages and disavantages of source specific trees
great performance but difficult to construct and maintain because you have to maintain a tree for every content source
how does a shared tree solve the disadvantage of a source specific tree?
one tree is used by all members in a group
easier to construct/maintain but hard to pick “good” trees for everyone
steiner tree (shared tree)
minimum weighted tree connecting only the multicast members
finding steiner tree is NP hard
heuristics are known to approximate
what can we compute instead of steiner?
minimum spanning tree
multicast and layering
at different layers
data link layer: ethernet
network layer: IP
application: end system
What is the idea of data link multicast?
IP Multicast
What are the problems with IP multicast?
what are some protocols for IP level multicast routing?
Explain Distance Vector Multicast Routing Protocol
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
reverse path flooding
what are issues with RPF?
you may have duplicate transmissions
fix through enhancement: reverse path broadcasting
reverse path broadcasting
to make this more efficient, we need to…
prune!
explain membership pruning
single sender multicast SSM
what’s wrong with SSM?
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)
CBT
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)
how do you build a CBT?
what are the disadvantages of CBT?
sub optimal delay
single point of failure
small, local groups with non-local core
explain multicast reliability
what is nack implosion?
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
