1. 18. Multicast Communication
Multicast within a subnet
Reverse path forwarding
IGMP
PIM, SSM
Roch Guerin
(with adaptations from Jon Turner and John DeHart, and material from Kurose and Ross)

2. What Is Network Multicast?8
Delivery of packets to a (sub)set of users
Distinct from broadcast that targets all users
Network multicast means that the network rather than the sender makes copies of the packet
Main goal is to realize more efficient delivery
Fewer packet copies consuming network bandwidth 4 4 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1

3. Multicast Routing Multicast address (Class D)
leading 1110 in first byte (from 224 to 239)
Originated in the classful network architecture of the early internet.
Has been continued to be used in the Classless Inter-Domain Routing (CIDR) era of the Internet.
Identifies content not destinations (a bidirectional channel or group ID)
Users can subscribe to receive content sent to a group address
Users can contribute content by sending it to a group address

4. Multicast Routing (continued)
Two basic issues
Group membership, i.e., joining a group (hosts expresses interest in receiving packets addressed to a given multicast group)
Forwarding packets addressed to a group to all interested hosts
To all subnets with subscribers, to all subscribers in a subnet

5. Multicast Routing (continued)
Components
IGMP: Internet Group Multicast Protocol
protocol for registering host interest in multicast group
Multicast routing protocol(s)
(DVMRP, MOSPF, PIM, SSM, MP-BGP)
build network forwarding state
deliver packets to subnets
Ethernet multicast/broadcast for subnet delivery
IGMP determines what groups are needed in the subnet
Multicast Routing protocol (like PIM) build forwarding state to deliver packets to subnets
Ethernet multicast takes care of delivery within subnet.

6. Internet Group Multicast (RFC 2236)
query
membership report
To forward among subnets, router needs to know about multicast subscribers within subnets
router multicasts membership query packets
hosts respond with membership reports after random delay
suppress duplicate reports; usually just one report per group
If host sees another host report, knows it doesn’t need to send one
Router forwards multicast packets among networks based on membership information
subscriptions timeout if not renewed periodically

7. IGMP Details Message fields
type – query, report, leave group
max response time – max wait time before responding to query
group address – “subject” multicast addr; 0 for “general query”
On each subnet, one router plays role of “Querier”
router with smallest IP address acts as querier
issues periodic general membership queries
sent on “all systems” multicast address:
Hosts send reports to “subject” multicast address
in response to general queries or when first joining a group
Leave group message sent by host when leaving
sent to “all routers” multicast address ( )
querier responds with a group-specific query
verify that no other group members still out there

8. IGMP Snooping (RFC 4541) Objective
Reduce unnecessary traffic in switches
use group membership to limit propagation of multicast packets
learn which hosts are in which group by snooping on IGMP
Suppression of duplicate membership reports by hosts would interfere with snooping
Switches send report packets only to routers, not to hosts
implies switches must be able to detect which ports go to routers
most switches also suppress redundant reports to routers
Forwarding based on group membership
forwarding can use either Ethernet or IP addresses
IP addresses preferred due to inexact multicast address mapping R
Switches use this to reduce unnecessary traffic.
What does the “Forwarding based on …” bullet mean?

9. Multicast Listener Discovery (IPv6) – (1)
Goals
Detects what multicast (and source) addresses have listeners on a given interface/link
Offer filtering capabilities based on including or excluding specific multicast and source addresses
Allow hosts to proactively notify router of changes
MLD overview
Two types of entities: Routers and listeners
Two types of messages: Queries and Reports
Queries are generated periodically or in response to changes on multicast subscriptions
Reports can be generated in response to queries or after local changes in multicast subscriptions
Host suppression has been eliminated from MLDv2, i.e., all hosts respond to a query

10. Multicast Listener Discovery (IPv6) – (2)
Router’s behavior
Election of “querier” if more than one router on a link
Periodically transmit (general) queries
Transmit targeted queries to rapidly validate new state after changes, e.g., after deletions by one listener
Pass state information to (multicast) routing protocol
Listener’s behavior
Generate reports in response to
Timer expiration
Queries
Local changes in interface state

11. Multicast Listener Discovery (IPv6) – (3)
Multicast Listener Queries (from querier to listeners)
General Query: No multicast address specified, asks for general report of multicast interest on link
Multicast Address Specific: Targets information for one multicast address
Multicast Address and Source Specific: Targets information for specific sources for a multicast address
Multicast Listener Report (from listeners to routers)
Include multiple “records”, one per multicast address
A record identifies multicast address, list of sources, and record type
Record type specifies the current state or targeted state change
State can be INCLUDE or EXCLUDE, which determines how the list of sources is to be handled
State change can be TO_INCLUDE or TO_EXCLUDE or specify a change to the list of sources, i.e., sources to be allowed or blocked

12. Multicast in a Subnet
subscriber to :30123
packet to :30123
Within switched subnet, multicasting relies on Ethernet
IP multicast address embedded in Ethernet multicast address
more precisely, low 23 bits are embedded
switches deliver multicast packets to all hosts (simple case)
Receiving application configures multicast socket (UDP)
bind to specific port, join multicast group
Sending app sends UDP packet on multicast socket
use multicast address/port, set TTL to 1

13. Class D to MAC Address Mapping (From IP Multicast to E/N Multicast)
23 bits copied from IP address 01 00 5E
IEEE 802 MAC Address
Multicast bit
Reserved bit
Class D IP address
‘1110’ = Class D indication
Ignored
IP Multicast delivers multicast packets to routers
Routers need to deliver multicast packets to hosts
MAC address used to “encode” IP multicast address
MAC multicast bit set
23 bits (out of 28) copied from IP address to MAC address
Probability of multicast collision? Is it a problem?

14. IPv6 Multicast to MAC Address Mapping
An IPv6 multicast address consists of 16 octets of the form FF:flgs+scop:X(3):…:X(16)
flgs denotes flags with 0000 for well-known multicast addresses, and 0001 for transient multicast addresses
scop identifies the scope of the multicast group, with five defined values: 0001 (node-local), 0010 (link-local), 0101 (site-local), 1000 (organization-local), and 1110 (global)
Note that the “meaning” of a well-known multicast address is independent of its scope
An IPv6 multicast address is mapped to a 48-bit MAC address of the form 33:33:X(13):X(14):X(15):X(16)
IEEE 802 MAC Address
: :X(13):X(14):X(15):X(16)
IPv6 multicast address
FF : flags,scope : group ID 8 4
112
X(3):…:X(13):X(14):X(15):X(16) 8 8 32

15. Multicast Issues in Larger NetworksB R2 R1
IGMP is not always enough
suppose a multicast has members in subnets A and C, but not B
routers will not forward packets across B
Multiple paths among subnets can produce multicast duplicates
need to restrict routing of each multicast group to a tree
requires routers to maintain forwarding information for each multicast group

16. Reverse Path ForwardingC B R2 R1 F E R6 R5
unicast path to A
multicast from A
Efficient distribution of multicast packets requires that packets be forwarded along a tree
in principle, any tree joining subnets can be used and each multicast can use a different tree
Internet generally uses reverse of unicast routes
Shortest path tree to sender (not necessarily symmetric)
so routing based on combination of (source adr, multicast adr)
means multicast with many senders requires more routing state
more efficient to use shared tree through “rendezvous point”

17. Shortest Path Tree vs. Minimum Spanning Tree
The two are NOT the same. Why not?
SPT: shortest path from root to EACH node
MST: minimal TOTAL weight
Shortest Path Tree
(from A)
Minimum Spanning Tree A 50 10 A 50 10

18. Current Multicast Routing Protocols
Protocol Independent Multicast (PIM)
Two different modes of operation based on the distribution of subscribed receivers, i.e., dense vs. sparse
Flood and prune for dense groups
Build targeted (shortest-path) tree for sparse groups
Receivers need to explicitly join per source/group
Sparse mode (PIM-SM) is the more commonly deployed
Source Specific Multicast (SSM)
Basically PIM-SM but assumes single known sender
This affords significant simplifications in both protocol operation and packet forwarding
The emerging solution of choice in practice
Well-matched to service model of most multicast applications, e.g., video broadcast typically comes from one or a few well-know servers

19. Multicast Using Shared Tree (PIM)B R2 R1 F E R6 R5
Rendezvous Point
multicast tree links
denotes subnet’s Designated Router *
Rendez-vous point (RP) serves as multicast tree root
different groups can have different RPs, routers can determine assigned RP for each multicast address
When Designated Router (DR) discovers new subscriber
sends join message to RP, creating multicast state along path
Sending hosts just send packets to multicast address
initially, DR may forward to RP inside unicast packet

20. Joining a Multicast Group
Rendezvous Point * C R2 R6 B A D
IGMP Report
join
DR on new leaf node’s subnet observes IGMP report
Join packets sent on shortest unicast path to RP
multicast routing state added at each hop (graft new branch to tree)
Packets delivered to new leaf along new tree path
packets from leaf also use this path

21. Non-Subscriber Sending to Group
Rendezvous Point * C R2 R6 B A D
multicast packet
encapsulated in unicast packet
join
Non-subscribing sender’s packets sent to RP by DR
RP extracts multicast packet and forwards on shared tree
RP may initiate join on source’s multicast tree
adds multicast routing state along path, merges with RP tree
when native packets reach RP, it tells DR to stop encapsulating

22. Switch to More Direct Path
Rendezvous Point * C R2 R6 B A D
prune
join
Router in tree may choose to switch to more direct path
sends join back towards source
after packets arrive on direct path, prune links in original path from source
Note: expensive for multicasts with many senders

23. More About PIM (RFC 4601)
PIM stands for Protocol Independent Multicast
PIM can use routing data from any routing protocol
e.g., unicast routing state, or multicast BGP
PIM generally used only within single routing domain
RFC omits essential details for inter-domain routing
e.g., how are RPs coordinated among different domains
BGP extensions may eventually enable inter-domain multicast
Routers must be able to determine RP for any address
can use statically configured mapping based on address ranges
or, a PIM domain can use a Bootstrap Router (BSR) to distribute information used to map a multicast address to an RP
domain has a defined set of RPs distributed by BSR
PIM routers use a hash function to select an RP in this set
e.g., if N candidate RPs, hash(multicast address) mod N selects one

24. IGMPv3 (RFC 3376)
Adds support for source-specific multicast using “filters”
include-filter specifies a list of acceptable senders
multicast packets are delivered to socket only if sender is on list
exclude-filter specifies a list of blocked senders
hosts have “interface filters” derived from socket filters
not supported by java multicast sockets; linux does support it
Queries and membership reports are extended to handle “filtered multicasts”
new source-specific query that includes a sender list for target destination address
reports can cover multiple groups and includes a filter per group
IGMPv3 ignores membership reports from other hosts
so all IGMPv3 hosts in a multicast group will respond to a query
Can operate in “compatibility mode” with older versions

25. Source Specific Multicast (SSM)
RFC 3569 (overview); RFC 4607 (specifications)
Basic motivations
In most practical instances, e.g., audio/video distribution, senders are well-known
Much complexity arises from “any sender” support (transition from RP tree to sender specific tree) – 19 pages for RFC 4607 vs. 150 pages for PIM-SM RFC

26. Source Specific Multicast (SSM)
Deployment problems faced by current multicast solutions
Access control enforcement
I want to know who I am receiving data from
Allocation and collision of multicast addresses (in IPv4)
Multicast address is now qualified by sender, i.e., <S1,G>≠<S2,G>
SSM defines a new (more limited) multicast service model
Hosts must subscribe to <S,G>
Directly builds shortest path tree rooted at S
The multicast group is defined by the combination S+G
Multicast group is in range 232/8 for IPv4 and FF3x::/32 for IPv6 (currently limited to FF3x::/96, which facilitates mapping to MAC addresses), where x is a valid scope identifier

27. Which Trees are Best? vs
Reverse path forwarding routes multicast packets on shortest paths
tends to copy packets near the source
copying near destination reduces # of “long-distance packets”
Best multicast route is a Steiner tree
Steiner tree for a vertex subset S is shortest subtree that includes all vertices in S
problem is NP-complete, so no efficient algorithm known for finding optimal solution
good approximations based on minimum spanning tree

28. Other Issues with IP Multicast
Separate protocol needed to reserve network capacity
RSVP protocol (RFC 2205) can be used for this purpose
but cannot select route based on required network capacity
QoS routing has been studied but never implemented
No “private” multicast groups
no way to get short-term exclusive use of a multicast address
no way to limit subscribers or limit who can send to an address
makes it awkward to use for multi-party teleconferencing
privacy requires encryption
Largely unavailable in public internet
Some remaining technical issues for inter-domain extensions
no economic benefit to network providers
multicast senders benefit as they need not duplicate traffic
but ISPs lose money if senders reduce internet access link rate

29. Exercise
Receiver
Consider the network to the right that uses PIM as its multicast routing protocol. All links have unit cost. A host in a subnet of router J wants to receive packets sent to multicast address MG. What multicast tree is constructed as a result, and how is it constructed?
Rendez-vous Point A B C D E F G H I J K

30. Excercise
Receiver
Consider the network to the right that uses PIM as its multicast routing protocol. All links have unit cost. A host in a subnet of router J wants to receive packets sent to multicast address MG. What multicast tree is constructed as a result, and how is it constructed?
Router J receives a multicast group report that includes MG. This triggers a JOIN message sent towards the RP for MG, i.e., F. The message is sent on the shortest path to F and triggers the creation of forwarding state for MG, i.e., with a next hop of J at G, a next hop of G at H, and a next hop of H at F. (Are there other possible shortest paths?)
Rendez-vous Point A B C D E F G H I J K

31. Excercise
Host S on a subnet connected to router A sends a packet to multicast address MG. How and where is the packet forwarded? What possible tree extension would this trigger? S A B C D E F G H I J K

32. Excercise
Host S on a subnet connected to router A sends a packet to multicast address MG. How and where is the packet forwarded? What possible tree extension would this trigger, and what would be the benefit of that extension?
The multicast packet is encapsulated in a unicast packet addressed to F by router A (it knows the ID of the RP for MG). The packet is forwarded on the shortest path to F (A-C-F). Router F decapsulates the packet and sends it on the shortest path tree that was previously created by the JOIN from J S A B C D E F G H I J K

33. Exercise
Host S on a subnet connected to router A sends a packet to multicast address MG. How and where is the packet forwarded? What possible tree extension would this trigger, and what would be the benefit of that extension?
The RP (router F) will likely issue a JOIN message destined to S. The message is sent on the shortest path to S and creates forwarding state for MG at the routers it traverses (at C pointing to F, and at A pointing to C). Once this extension is in place, it will allow A to forwards multicast packets natively (no unicast encapsulation), and conversely won’t require any decapsulation operation at F that will continue forwarding the packets natively (towards H). S A B C D E F G H I J K