1. COMP/ELEC 429 Introduction to Computer Networks
Multicast Routing
Some slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

2. What applications do you think consume the most bandwidth in the Internet?
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

3. Internet Traffic 1995-2010 Other Video Newsgroups Peer-to-peer DNS
Telnet
FTP
Web
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

4. What would it take to run a live Internet TV station?
Broadcast
Center
Backbone
ISP
A live Internet TV station would be 4Mbps/stream!
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

5. Terminologies What is “unicast”? What is “broadcast?
Cast: to send, to throw
Unicast: to send to a single receiver
Point-to-point communication
Nearly all wide-area Internet traffic is unicast
Web traffic, Video, Music, BitTorrent, SSH, FTP,...
What is “broadcast?
Broadcast: to send everywhere (recall broadcast in local area Ethernet)
What is “multicast”?
In between unicast and broadcast
Each packet is sent to multiple specific receivers
Point-to-Multipoint communication
Useful for: Internet TV/radio, stock price update, video conference ……
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

6. How to Send to Multiple Receivers?
What are the simplest ways?
1: Send a copy of the packet to one receiver at a time until all receivers have it
i.e. use unicast to implement multicast
Doesn’t scale with respect to # receivers
2: Flood a packet throughout the network and have non-receivers discard the packet
i.e. use broadcast to implement multicast
Don’t scale with respect to network size
3: Use IP routers to construct a distribution tree
a.k.a. IP multicast
Many ways to construct tree
4: Peer-to-peer
5: Content Distribution “Network” (Akamai, Limelight, etc.)
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

7. IP Multicast Idea: Use IP routers in distribution tree
Copy data at routers
At most one copy of a data packet per link
Broadcast
Center
Backbone
ISP
Routers compute trees and forward packets along them
LANs use broadcasting
Important: This is not the only way to solve the distirbution problem
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

8. IP Multicast Routing Approaches
Kinds of Trees
Source Specific Trees
Most suitable for single sender
E.g. Internet TV/radio, distance learning, stock quote
Shared Tree
Multiple senders in a group
E.g. Teleconference, multiplayer games, distributed databases
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

9. Source Specific Trees Each source is the root of its own tree
One tree per source
Tree can consists of shortest paths to each receiver 5 7 4 8 6 11 2 10 3 1 13 12
Members of the multicast tree
Sender
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

10. Source Specific Trees Each source is the root of its own tree
One tree per source
Tree can consists of shortest paths to each receiver 5 7 4 8 6 11 2 10 3 1 13 12
Lowest delay can be achieved but
expensive to construct/maintain;
routers need to manage a tree per source
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

11. Shared Tree One tree used by all members in a group5 7 4 8 6 11 2 10 3 1 13 12
Easier to construct/maintain but hard to pick a “good” tree for everyone!
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

12. Multicast Group Address
IPv4 Multicast 28
1110
Multicast Group Address
First octet:
Class D addresses
These are group identifiers
Not specific to an end host
Flat address space
In practice, pick a group address at random, hope no collision
No security in the network layer
Will use “G” to designate an IP multicast group address
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

13. IP Multicast Service Model
[G, data]
R0 joins G
R1 joins G
Rn-1 joins G R0 S R1
Net .
Rn-1
Receivers join a multicast group which is identified by a multicast address (e.g. G)
Sender(s) send data to address G
Network routes data to each of the receivers
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

14. IP Multicast Implementation Issues
How is join implemented?
How is send implemented?
How much information about trees is kept and who keeps it?
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

15. IP Multicast Routing Intra-domain Inter-domain
Distance-vector multicast (DVMRP)
Protocol-Independent Multicast – Sparse Mode (PIM-SM)
Inter-domain
Multicast Source Discovery Protocol (MSDP)
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

16. Distance Vector Multicast Routing Protocol (DVMRP)
An extension to DV routing
Use shortest path DV routes to determine if link is on the source-rooted spanning tree
Three steps in developing DVMRP
Reverse Path Flooding
Reverse Path Broadcasting
Truncated Reverse Path Broadcasting
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

17. Reverse Path Flooding (RPF)
Extension to DV unicast routing
Packet forwarding
If incoming link is shortest path to source
Send on all links except incoming
Packets always take shortest path
assuming delay is symmetric
Issues
Some links (LANs) may receive multiple copies
Every link receives each multicast packet, even if no interested hosts
e:3
c:2
d:3
a:1
b:2 r s
name:hop count to s
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

18. Example
Flooding can cause a given packet to be sent multiple times over the same link
Solution: Called “Reverse Path Broadcasting” S x y a
duplicate packet z b
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

19. Reverse Path Broadcasting (RPB)
Choose parent of each link along reverse shortest path to source
Only parent forward to a link (child link)
Use DV routing update to identify parent
Use the poison reverse DV entry!
If no router is used as parent, then the router with the smallest address becomes parent of link S 5 6 x y
forward only
to child link a
child link of x
for S z b x y S 5 6
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

20. Don’t Really Want to Flood!
This is still a broadcast algorithm – the traffic goes everywhere
Need to “Prune” the tree when there are subtrees with no group members
Solution: Truncated Reverse Path Broadcasting
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

21. Truncated Reverse Path Broadcasting (TRPB)
Extend RPB to eliminate unneeded forwarding
Explicit group joining
Members periodically send “join” requests
If another LAN member has joined (overheard join message), other members do not send join message
Router with no member downstream is removed from tree
Leaf router sends “prune” message to upstream router when no member
Recur S r2 r1
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

22. DVMRP Scaling State requirements:
O(Sources  Groups) active state
Source specific trees
Flood and prune is disruptive if number of receivers is small
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

23. PIM-SM (No more flooding)
Protocol-Independent Multicast – Sparse Mode
Pick a router as the “rendezvous point” for the group
Build a tree rooted at the RP
Union of the unicast paths from members to the RP becomes shared tree
To send, router receiving multicast packet from host tunnels multicast packet to RP, and RP sends it to multicast group
Reduce routing table state from O(S x G) to O(G)
No packet flooding
Note: PIM-DM “Dense Mode” is similar to DVMRP
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

24. PIM-SM Example Group members M2, M3 sends group join (*, G) to RP
M1 multicasts data
Register msg RP M1 M2 M3
control (join) messages
data
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

25. PIM-SM Optimization 1 RP sends source specific join (M1, G) to M1
Eliminates need for tunneling RP M1 M2 M3
control (join) messages
data
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

26. PIM-SM Optimization 2
M2, M3 sends source specific join (M1, G) to M1 instead
Previous tree paths to RP may expire
Eliminates detour
Justified if volume of data is large RP M1 M2
control (join) messages M3
data
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

27. Disadvantages
Potentially sub-optimal delay unless source specific tree optimization is performed
Need good RP selection
Optimal choice (computing topological center) is NP hard
Single point of failure
RP goes out and all packets could be lost until error recovery elects a new RP
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

28. Inter-Domain Multicast
Rendezvous Point
TCP Connection
Intra-Domain
PIM Tree
TCP Connection
Intra-Domain
PIM Tree
Intra-Domain
PIM Tree
Construct multicast tree paths between domains on-demand
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

29. Inter-Domain Multicast with Multicast Source Discovery Protocol (MSDP)
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University

30. Disadvantages with IP Multicast
Complex Protocols
Increase state in routers
Needs universal router support
Cannot incrementally deploy
Poor security control
Hard to multicast data reliably
Hard to perform congestion control on multicast data
Next time we’ll study end-to-end alternatives to IP multicast
T. S. Eugene Ng eugeneng at cs.rice.edu Rice University