1. 15-744: Computer Networking
L-20 Multicast

2. Multicast Routing Unicast: one source to one destination
Multicast: one source to many destinations
Two main functions:
Efficient data distribution
Logical naming of a group

3. Example Applications Broadcast audio/video Push-based systems
Software distribution
Web-cache updates
Teleconferencing (audio, video, shared whiteboard, text editor)
Multi-player games
Server/service location
Other distributed applications

4. Overview IP Multicast Service Basics Multicast Routing Basics
Overlay Multicast
Reliability
Congestion Control

5. IP Multicast Architecture
Service model
Hosts
Host-to-router protocol (IGMP)
Routers
Multicast routing protocols (various)

6. Multicast – Efficient Data Distribution
Src
Src

7. Multicast Router Responsibilities
Learn of the existence of multicast groups (through advertisement)
Identify links with group members
Establish state to route packets
Replicate packets on appropriate interfaces
Routing entry:
Src, incoming interface
List of outgoing interfaces

8. IP Multicast Service Model (rfc1112)
Each group identified by a single IP address
Groups may be of any size
Members of groups may be located anywhere in the Internet
Members of groups can join and leave at will
Senders need not be members
Group membership not known explicitly
Analogy:
Each multicast address is like a radio frequency, on which anyone can transmit, and to which anyone can tune-in.

9. IP Multicast Addresses
Class D IP addresses –
How to allocated these addresses?
Well-known multicast addresses, assigned by IANA
Transient multicast addresses, assigned and reclaimed dynamically, e.g., by “sdr” program 1
Group ID

10. Multicast Scope Control – Small TTLs
TTL expanding-ring search to reach or find a nearby subset of a group s 1 2 3

11. Multicast Scope Control – Large TTLs
Administrative TTL Boundaries to keep multicast traffic within an administrative domain, e.g., for privacy or resource reasons
The rest of the Internet
TTL threshold set on interfaces to these links, greater than the diameter of the admin. domain
An administrative domain

12. Overview IP Multicast Service Basics Multicast Routing Basics
Overlay Multicast
Reliability
Congestion Control

13. IP Multicast Architecture
Service model
Hosts
Host-to-router protocol (IGMP)
Routers
Multicast routing protocols (various)

14. Multicast Routing
Basic objective – build distribution tree for multicast packets
Multicast service model makes it hard
Anonymity
Dynamic join/leave

15. Shared vs. Source-based Trees
Separate shortest path tree for each sender
DVMRP, MOSPF, PIM-DM, PIM-SM
Shared trees
Single tree shared by all members
Data flows on same tree regardless of sender
CBT, PIM-SM

16. Source-based Trees
Router S
Source R
Receiver R R R S S R

17. Shared Tree
Router S
Source R
Receiver R R RP R S S R

18. Shared vs. Source-Based Trees
Shortest path trees – low delay, better load distribution
More state at routers (per-source state)
Efficient for in dense-area multicast
Shared trees
Higher delay (bounded by factor of 2), traffic concentration
Choice of core affects efficiency
Per-group state at routers
Efficient for sparse-area multicast
Which is better?  extra state in routers is bad!

19. Routing Techniques Flood and prune Link-state multicast protocols
Begin by flooding traffic to entire network
Prune branches with no receivers
Examples: DVMRP, PIM-DM
Unwanted state where there are no receivers
Link-state multicast protocols
Routers advertise groups for which they have receivers to entire network
Compute trees on demand
Example: MOSPF
Unwanted state where there are no senders

20. Routing Techniques Core based protocols
Specify “meeting place” aka core
Sources send initial packets to core
Receivers join group at core
Requires mapping between multicast group address and “meeting place”
Examples: CBT, PIM-SM

21. Distance-Vector Multicast Routing
DVMRP consists of two major components:
A conventional distance-vector routing protocol (like RIP)
A protocol for determining how to forward multicast packets, based on the routing table
DVMRP router forwards a packet if
The packet arrived from the link used to reach the source of the packet (reverse path forwarding check – RPF)
If downstream links have not pruned the tree

22. Example Topology G G S G

23. Broadcast with TruncationG G S G

24. Prune G G
Prune (s,g)
Prune (s,g) S G

25. Graft G G G
Report (g)
Graft (s,g)
Graft (s,g) S G

26. Steady State G G G S G

27. Overview IP Multicast Service Basics Multicast Routing Basics
Overlay Multicast
Reliability
Congestion Control

28. Supporting Multicast on the Internet
Application ?
At which layer should multicast be implemented? ?
In the hourglass Internet architecture,
IP is the compatibility layer in the Internet architecture.
All hosts must implement IP
Two choices
multicast at IP
or application: only a subset, customizability
One important architecture question is, at which layer should multicast be implemented.
The convention wisdom has been to support multicast in the IP layer for efficiency and performance reasons. However, more than 10 years since this is proposed, it still has not been widely deployed.
This paper revisits this question with emphasis on Internet evaluation. In particular, we show that multicast at the application layer can be efficient compared to IP Multicast. IP
Network
Internet architecture

29. IP Multicast Highly efficient Good delay MIT Berkeley UCSD CMU routers
In the IP Multicast architecture,
- routers replicate multicast pkts inside the network to all receivers
- motivation: highly efficiency
- highly efficient: single pkt that traverse each physical link
- delay is good from source to all receivers
routers
end systems
multicast flow
Highly efficient
Good delay

30. End System Multicast Overlay Tree MIT1 MIT Berkeley MIT2 UCSD CMU1 CMU
Recently, we and others have advocated for an alternative architecture, where all multicast functionality, including pkt replication and group management are pushed to end systems.
- We call this architecture End System Multicast
- In this architecture, end system organize themselves into an overlay tree root at the source
- data is sent along the overlay tree.
- It is an overlay in the sense that each link in the overlay tree corresponds to a physical path in the underlying network
CMU
CMU2
UCSD
MIT1
MIT2
CMU2
Overlay Tree
Berkeley
CMU1

31. Potential Benefits Over IP Multicast
Quick deployment
All multicast state in end systems
Computation at forwarding points simplifies support for higher level functionality
MIT1
In ESM, there is no additional support from the routers except unicast, so the deployment can be immediate.
In IP Multicast, routers need to maintain per-group state.
- This causes concerns regards to scalability to number of groups.
- In ESM, all multicast state is pushed to end systems.
Finally, computation at end systems can potentially simplify support for higher-layer functionality,
- such as congestion control and reliability,
- as well as application-specific customization, such as XXX.
These high-layer functionality is harder to achieve in IP Multicast because
- the data splitting points are at the routers and they are constraint in computation.
MIT
Berkeley
MIT2
UCSD
CMU1
CMU
CMU2

32. Concerns with End System Multicast
Self-organize recipients into multicast delivery overlay tree
Must be closely matched to real network topology to be efficient
Performance concerns compared to IP Multicast
Increase in delay
Bandwidth waste (packet duplication)
However, there are several concerns with ESM.
- In the absence of support from the routers, there is a challenge to coordinate among end systems to construct efficient overlay tree.
Even with an efficient overlay tree, …
- there can be an increase in delay because data can travel through multiple overlay hops, as shown for MIT2. In IP Multicast, data is sent directly along the forwarding path.
- moreover, there can be waste in bandwidth because duplicated pkts can traverse the same physical links, as shown in links near UCSD. In IP Multicast, a single pkt traverse each physical link at most once.
MIT2
Berkeley
MIT1
UCSD
CMU2
CMU1
IP Multicast
MIT2
Berkeley
MIT1
CMU1
CMU2
UCSD
End System Multicast

33. Overview IP Multicast Service Basics Multicast Routing Basics
Overlay Multicast
Reliability
Congestion Control

34. Implosion S S R1 R1 R2 R2 R3 R4 R3 R4 Packet 1 is lost
All 4 receivers request a resend S
Resend request S 1 2 R1 R1 R2 R2 R3 R4 R3 R4

35. Retransmission Re-transmitter How to retransmit Problem: Exposure
Options: sender, other receivers
How to retransmit
Unicast, multicast, scoped multicast, retransmission group, …
Problem: Exposure

36. Exposure S S R1 R1 R2 R2 R3 R4 R3 R4 Packet 1 does not reach R1;
Receiver 1 requests a resend
Packet 1 resent to all 4 receivers
Resend request S S
Resent packet 1 2 1 1 R1 R1 R2 R2 R3 R4 R3 R4

37. Ideal Recovery Model S S R1 R1 R2 R2 R3 R4 R3 R4
Packet 1 reaches R1 but is lost before reaching other Receivers
Only one receiver sends NACK to the nearest S or R with packet
Repair sent only to those that need packet S
Resend request S
Resent packet 1 2 1 R1 R1 1 R2 R2 R3 R4 R3 R4

38. Scalable Reliable Multicast (SRM)
Originally designed for wb
Receiver-reliable
NACK-based
Every member may multicast NACK or retransmission

39. SRM Request Suppression
Packet 1 is lost; R1 requests resend to Source and Receivers
Packet 1 is resent; R2 and R3 no longer have to request a resend
Resend request S
Resent packet S X 1 2 1 R1 R1 X R2 R2 X
Delay varies by distance R3 R3

40. Deterministic Suppression
Time d d
data 2d d d d
nack
repair
= Sender 3d
= Repairer d 4d
= Requestor
Delay = C1dS,R

41. SRM Star Topology X S S R2 R3 R4 R2 R3 R4 Delay is same length
Packet 1 is lost; All Receivers request resends
Packet 1 is resent to all Receivers
Resend request S
Resent packet S X 1 2 1 R2 R3 R4 R2 R3 R4
Delay is same length

42. SRM: Stochastic Suppression
Time 2d d
data 1 d
repair
session msg d 2
NACK d 3
= Sender
Delay = U[0,D2] dS,R
= Repairer
= Requestor

43. SRM (Summary) NACK/Retransmission suppression
Delay before sending
Delay based on RTT estimation
Deterministic + Stochastic components
Periodic session messages
Full reliability
Estimation of distance matrix among members

44. Overview IP Multicast Service Basics Multicast Routing Basics
Overlay Multicast
Reliability
Congestion Control

45. Multicast Congestion Control
What if receivers have very different bandwidths?
Send at max?
Send at min?
Send at avg?
100Mb/s
100Mb/s R S R
1Mb/s
???Mb/s
1Mb/s R R
56Kb/s

46. Video Adaptation: RLM Receiver-driven Layered Multicast
Layered video encoding
Each layer uses its own mcast group
On spare capacity, receivers add a layer
On congestion, receivers drop a layer
Join experiments used for shared learning

47. Layered Media Streams R1 joins layer 1, joins layer 2 joins layer 3 R2
10Mbps
512Kbps
128Kbps
R2 join layer 1,
join layer 2
fails at layer 3
R3 joins layer 1,
fails at layer 2

48. Drop Policies for Layered Multicast
Priority
Packets for low bandwidth layers are kept, drop queued packets for higher layers
Requires router support
Uniform (e.g., drop tail, RED)
Packets arriving at congested router are dropped regardless of their layer
Which is better?

49. RLM Intuition

50. RLM Intuition Uniform Priority RLM approaches optimal operating point
Better incentives to well-behaved users
If oversend, performance rapidly degrades
Clearer congestion signal
Allows shared learning
Priority
Can waste upstream resources
Hard to deploy
RLM approaches optimal operating point
Uniform is already deployed
Assume no special router support

51. RLM Join Experiment
Receivers periodically try subscribing to higher layer
If enough capacity, no congestion, no drops  Keep layer (& try next layer)
If not enough capacity, congestion, drops  Drop layer (& increase time to next retry)
What about impact on other receivers?

52. Join Experiments
Layer 4 3 2 1
Time

53. RLM Scalability? What happens with more receivers?
Increased frequency of experiments?
More likely to conflict (false signals)
Network spends more time congested
Reduce # of experiments per host?
Takes longer to converge
Receivers coordinate to improve behavior