1. Multicast on the Internet
COSC 6590
11/23/2018

2. Addressing Class D address Ethernet broadcast address (all 1’s)
IP multicast using
Link-layer (Ethernet) broadcast
Link-layer (Ethernet) multicast
Both cases need filtering at IP layer.
Source: unicast IP address S
Receivers: multicast group ID G
Each group is identified by (S, G)

3. IPv4 Address Formats

4. Mapping from Class D IP adress to Ethernet multicast adress

5. Reverse Path Forwarding
rely on router’s knowledge of unicast shortest path from it to sender
each router has simple forwarding behavior:
if (mcast datagram received on incoming link on shortest path back to center)
then flood datagram onto all outgoing links
else ignore datagram
Notes:
3.3 Network Layer: Multicast Routing Algorithms

6. Reverse Path Forwarding (2)
Building a loop-free broadcast tree
No knowledge of group membership

7. Reverse Path ForwardingB G D E c F

8. Spanning-Tree BroadcastF
(a) Broadcast initiated at A
(b) Broadcast initiated at D

9. Internet Multicast Service Model
multicast
group
multicast group concept: use of indirection
a host “sends” IP datagrams to multicast group
routers forward multicast datagrams to hosts that have “joined” that multicast group
Notes:
The Internet multicast service model and addressing/group management ideas have their foundation in the PhD thesis of S. Deering: “Multicast Routing in a Datagram Network,” Dept. of Computer Science, Stanford University, 1991.
3.1 Network Layer: Introduction

10. Multicast groups class D Internet addresses reserved for multicast:
host group semantics:
anyone can “join” (receive from) multicast group
anyone can send to multicast group
no network-layer identification to hosts of members
needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group
Notes:
3.1 Network Layer: Introduction

11. Joining a mcast group: two-step process
local: host informs local mcast router of desire to join group: IGMP (Internet Group Management Protocol)
wide area: local router interacts with other routers to receive mcast datagram flow
many protocols (e.g., DVMRP, MOSPF, PIM)
IGMP
IGMP
wide-area
multicast
routing
Notes:
3.2 Network Layer: Multicast Addressing and Group Management
IGMP

12. IGMP: Internet Group Management Protocol
host: sends IGMP report when application joins mcast group
IP_ADD_MEMBERSHIP socket option
hosts need not explicitly “unjoin” group when leaving
router: sends IGMP query at regular intervals
hosts belonging to a mcast group must reply to query
Notes:
3.2 Network Layer: Multicast Addressing and Group Management
query
report

13. IGMP router: Host Membership Query msg broadcast on LAN to all hosts
host: Host Membership Report msg to indicate group membership
randomized delay before responding
implicit leave via no reply to Query
group-specific Query
Leave Group msg
last host replying to Query can send explicit Leave Group msg
router performs group-specific query to see if any hosts left in group
Introduced in RFC 2236
IGMP v3: current version
Notes:
RFC-1112: S. Deering, “Host Extension for IP Multicasting,” August 1989
RFC 2236: R. Fenner, “Internet Group Management Protocol, Version 2”, November 1997.
B. Cain, S. Deering, A. Thyagarajan, “Internet Group Management Protocol, Version 3,” work in progress, draft-ietf-idmr-igmp-v3-00.txt
3.2 Network Layer: Multicast Addressing and Group Management

14. Internet Group Management Protocol: Summary
For membership management.
Between a host on a subnet (Ethernet) and the router for the subnet.
The router periodically broadcast an IGMP host-membership query message on its subnet.
A host subscribes to a group replies by multicasting a host-membership report message.
Note: feedback implosion  uses a random timer.
The report is sent 3 times (for reliability).
IGMP-1: hosts send no report  leaving the group
IGMP-2: hosts send explicit host-membership leave messages to reduce leave latency.

15. Truncated Broadcasting
Extension of Reverse Path Forwarding
No members of a group on a subnet  leaf router will not forward packets of this group to the subnet (pruning).
But does not reduce traffic in the core network
More efficient multicast routing is needed!!!

16. Multicast Routing Approaches
Minimum cost trees
Minimum Steiner trees
Shortest path trees
Source-based trees
Core-based trees
…we first look at basic approaches, then specific protocols adopting these approaches

17. Minimum Steiner Trees
Steiner Tree: minimum cost tree connecting all routers with attached group members
problem is NP-complete
excellent heuristics exist
not used in practice:
computational complexity
information about entire network needed
monolithic: rerun whenever a router needs to join/leave
Notes:
1. See L. Wei and D. Estrin, “A Comparison of multicast trees and algorithms,” TR USC-CD , Dept. Computer Science, University of California, Sept 1993 for a comparison of heuristic approaches.
3.3 Network Layer: Multicast Routing Algorithms

18. Shortest Path Tree
mcast forwarding tree: tree of shortest path routes from source to all receivers
Dijkstra’s algorithm
S: source
LEGEND R1 2 R4
router with attached
group member 1 R2 5
router with no attached
group member 3 4
Notes:
3.3 Network Layer: Multicast Routing Algorithms R5 6
link used for forwarding,
i indicates order link
added by algorithm R3 i R6 R7

19. Internet Multicasting Routing: DVMRP
DVMRP: distance vector multicast routing protocol, RFC1075
flood and prune: reverse path forwarding, source-based tree
initial datagram to mcast group flooded everywhere via RPF
routers not wanting the mcast data: send prune msgs to upstream neighbors
Notes:
D. Waitzman, S. Deering, C. Partridge, “Distance Vector Multicast Routing Protocol,” RFC 1075, Nov The version of DVMRP in use today is considerably enhanced over the RFC1075 spec.
A more up-to-date “work-in-progress” defines a version 3 of DVMRP: T. Pusateri, “Distance Vector Multicast Routing Protocol,” work-in-progress, draft-ietf-idmr-v3-05.ps
3.4 Network Layer: Internet Multicast Routing Algorithms

20. DVMRP Example S: source LEGEND R1 2 R4 router with attached 1
group member 1 R2 5
router with no attached
group member 3 4
Notes:
3.3 Network Layer: Multicast Routing Algorithms R5 6
link used for forwarding,
i indicates order link
added by algorithm R3 i R6 R7

21. DVMRP (2)
soft state: DVMRP router periodically (1 min.) “forgets” that branches are pruned:
mcast data again flows down unpruned branches
downstream routers: reprune or else continue to receive data
routers can quickly regraft to tree
following IGMP join at leaf
odds and ends
commonly implemented in commercial routers
Mbone routing done using DVMRP
Works well in small autonomous domains
Notes:
1. See for a (slightly outdatet) list of multicast capable routers (supporting DVMPR as well as other protocols) from various vendors.
2. ftp://parcftp.xerox.com/pub/net-research/ipmulti for circa 1996 public copy “mrouted” v3.8 of DVMRP routing software for various workstation routing platforms.
3.4 Network Layer: Internet Multicast Routing Algorithms

22. DVMRP: Summary Distance Vector Multicast Routing Protocol
Leaf router sends a prune message to neighbouring routers when there is no group member on the subnet.
Intermediate routers perform pruning whenever possible.
Flooding and pruning are repeated periodically, when the current state times out.
Between flooding rounds, a host can re-join a group by sending a graft message.
Intermediate routers propagates the graft message upstream until the path is re-connected.

23. MBone Multicast backbone of the Internet
Not all routers support multicast routing protocols and IGMP.
Connecting multicast-capable routers using (virtual) IP tunnels

24. MOSPF Extends OSPF for multicasting.
Every router has the complete topology of its autonomous system.
A receiver joins a multicast group by exchanging IGMP messages with its end-router.
The end-router broadcasts the presence of this destination (group membership) to the whole network.
Each router maintains a group membership table [S,G, <d1, d2, …>]
A sender simply sends data packets as they are available.
Each router uses the network topology, the group membership table, and the multicast group ID in the data packets to compute the route(s) to the destination(s).

25. Core-Based Trees CBT, PIM-SM, PIM-DM
Purpose: to reduce the amount of routing info stored at routers when a multicast group has a large number of members and multiple senders.
A multicast group requires a core (rendez-vous point).
Receivers “join” the (shortest-path) tree rooted at the core  only one tree per multicast group (used for multiple senders).
Sources send multicast data to the core, which then multicasts the data to the tree.

26. References
Multicasting on the Internet and Its Applications, Sanjoy Paul, Kluwer Academic Publishers, 1998.