multicasting
DESCRIPTION
Multicasting. CSE 6590 Winter 2012. Internet Multicast Service Model. 128.59.16.12. 128.119.40.186. multicast group 226.17.30.197. 128.34.108.63. 128.34.108.60. Multicast group concept: use of indirection a host “sends” IP datagrams to multicast group. - PowerPoint PPT PresentationTRANSCRIPT
Multicasting
CSE 6590Winter 2012
1April 21, 2023
Internet Multicast Service Model
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.
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128.119.40.186
128.59.16.12
128.34.108.63
128.34.108.60
multicast group
226.17.30.197
Multicast Groups Class D Internet addresses reserved for multicast:
Host group semantics:o anyone can “join” (receive from) multicast group.o anyone can send to multicast group.o no network-layer identification to hosts of members.
Needed: infrastructure to deliver multicast-addressed datagrams to all hosts that have joined that multicast group.
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Multicast Addressing
• Class D address (see next slide)• Source: unicast IP address S
Receivers: multicast group ID G, a class-D address• Each group is identified by (S, G)• Ethernet broadcast address (all 1’s)• 2 ways of doing IP multicast at the link layer:
– Link-layer (Ethernet) broadcast– Link-layer (Ethernet) multicastBoth cases need filtering at IP layer.
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IPv4 Address Formats
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Multicast Addressing (2)
1. Link-layer (Ethernet) broadcast•IP multicast packet is encapsulated in an Ethernet broadcast frame and transmitted on the bus.•Every host picks up the Ethernet frame and does filtering at the IP layer to decide whether to keep or discard the frame.•Redundant reception by many hosts.2. Link-layer (Ethernet) multicast•Requires a mapping of IP multicast address to an Ethernet multicast address (see next slide).•There are up to 32 IP class-D addresses mapped to the same Ethernet multicast address.•The IP module still has to filter out packets for non-member hosts.
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Mapping from Class D IP adress to Ethernet multicast adress
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Multicast Protocols
Transport layer• UDP• Real-time Transport Protocol (RTP): for multimedia
content• ReSerVation Protocol (RSVP): for bandwidth
reservation in a multicast distribution
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Multicast Protocols (2)
Routing, delivery• On a local network (join/leave):
– Internet Group Management Protocol (IGMP)– Multicast Listener Discovery (MLD): similar to IGMP but for
IPv6
• Intra-domain (routing): – MOSPF, PIM, DVMRP
• Inter-domain (routing): – Multicast Border Gateway Protocol (MBGP)
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Joining a multicast group: 2-step process
• Local: host informs local multicast router of desire to join group: IGMP (Internet Group Management Protocol)
• Wide area: local router interacts with other routers to receive multicast datagram flow– many protocols (e.g., DVMRP, MOSPF, PIM)
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IGMPIGMP
IGMP
wide-areamulticast
routing
IGMP
Internet Group Management Protocol (IGMP)
• RFC 3376 used to exchange multicast group information between hosts & routers on a LAN
• hosts send messages to routers to subscribe to (join) and unsubscribe from (leave) multicast group
• routers check which multicast groups are of interest to which hosts
• IGMP currently at version 3
IGMP (2)
• Router: sends IGMP query at regular intervals– hosts belonging to a multicast group must reply to query if
wishing to join or stay in the group.• Host: sends IGMP report (reply) when application wishes to
join a multicast group.– IP_ADD_MEMBERSHIP socket option– hosts need not explicitly “unsubscribe” when leaving
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query report
IGMP (3)• Router: broadcasts Host
Membership Query message on LAN.
• Host: replies with Host Membership Report message to indicate group membership– randomized delay
before responding– may send multiple times– implicit leave via no
reply to Query
• Group-specific Query• Leave Group message
– Last host replying to Query can send explicit Leave Group message
– Router performs group-specific query to see if any hosts left in group
– Introduced in RFC 2236
• IGMP v3: current version
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Operation of IGMP v1 and v2• IGMPv1
– hosts could join group– routers used timer to
unsubscribe members• IGMPv2 enabled hosts to
unsubscribe • operational model:
– receivers have to subscribe to groups
– sources do not have to subscribe to groups
– any host can send traffic to any multicast group
Problems:• spamming of multicast
groups• establishment of
distribution trees is problematic (source location unknown)
• finding globally unique multicast addresses difficult (duplicate addresses)
IGMP v3
• addresses weaknesses by:– allowing hosts to specify list from which they want
to receive traffic– blocking traffic from other hosts at routers– allowing hosts to block packets from sources that
send unwanted traffic
IGMP Message FormatsMembership Query
• sent by multicast router• three subtypes: general query, group-specific query, group-
and-source specific query
Membership Query Fields
IGMP Message FormatsMembership Report
IGMP Message FormatsGroup Record
IGMP Operation - Joining
• IGMP host wants to make itself known as group member to other hosts and routers on LAN
• IGMPv3 allows hosts to specify wanted or unwanted sources (filtering capabilities)– EXCLUDE mode – all sources except those listed– INCLUDE mode – only from sources listed
IGMP Operation – Keeping Lists Valid
IGMP Operation - Leaving
• host leaves group by sending a leave group message to the all-routers static multicast address– sends a membership report message with EXCLUDE option
and null list of source addresses
• router determines if have any remaining group members using group-specific query message
Group Membership with IPv6
• IGMP defined for IPv4– uses 32-bit addresses
• IPv6 internets need the same functionality.• IGMP functions included in Internet Control
Message Protocol v6 (ICMPv6).– ICMPv6 has functionality of ICMPv4 & IGMP.
• ICMPv6 includes group-membership query and group-membership report messages as IGMP.
IGMP: 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.IGMP-3: filtering and blocking
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Multicast Routing
Shortest Path Tree• Multicast cast forwarding tree: tree of shortest path routes
from source to all receivers.– Dijkstra’s algorithm.
R1
R2
R3
R4
R5
R6 R7
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6
3 4
5
i
router with attachedgroup member
router with no attachedgroup member
link used for forwarding,i indicates order linkadded by algorithm
LEGENDS: source
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MOSPF• Extends OSPF for multicasting.• Every router has the complete topology of its own network.• A receiver joins a multicast group G by exchanging IGMP
messages with its end-router R.• The end-router R broadcasts its group membership to the
whole network in the form (G, R).• Every router in the network maintains a group membership
table with each entry being a tuple [S, G, <R1, R2, …>].• 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).
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Reverse Path Forwarding
• Building a loop-free broadcast tree• No knowledge of group membership
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Reverse Path Forwarding (2)
if (multicast datagram received on incoming link on shortest path back to sender)
then flood datagram onto all outgoing links else ignore datagram
rely on router’s knowledge of unicast shortest path from it to sender
each router has simple forwarding behavior:
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Reverse Path Forwarding: Example
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A
B
G
DE
c
F
Spanning-Tree Broadcast
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A
B
G
DE
c
F
A
B
G
DE
c
F
(a) Broadcast initiated at A (b) Broadcast initiated at D
Internet Multicasting Routing: DVMRP
• DVMRP: distance vector multicast routing protocol, RFC1075.
• Flood and prune: reverse path forwarding, source-based tree.– initial datagram to multicast group is flooded
everywhere via RPF– routers not wanting the multicast data: send
prune messages to upstream neighbors
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DVMRP Example
R1
R2
R3
R4
R5
R6 R7
21
6
3 4
5
i
router with attachedgroup member
router with no attachedgroup member
link used for forwarding,i indicates order linkadded by algorithm
LEGENDS: source
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How DVMRP Fits into Multicasting
• IGMP - used for discovery of hosts in multicast groups, messages exchanged between neighbors
• DVMRP - routes multicast datagrams within an AS
• MBGP (Multicast BGP) - routes multicast datagrams between ASs
DVMRP Details• Soft state: DVMRP router periodically (1 min.) “forgets” that
branches are pruned: – multicast data again flows down unpruned branches.– downstream routers: reprune or else continue to receive
data.• Routers can quickly re-graft to tree following an IGMP join at
a leaf router by sending a “graft” message upstream.• Deployment:
– commonly implemented in commercial routers.– Mbone routing done using DVMRP.
• Works well in small autonomous domains.
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DVMRP Prune
• Sent from routers receiving multicast traffic for which they have no downstream group members
• “Prunes” the tree created by DVMRP• Stops needless data from being sent
DVMRP Graft• Used after a branch has been pruned back• Sent upstream by a router that has a host that joins a
multicast group• Goes from router to router until a router active on the
multicast group is reached• Sent for the following cases
– A new host member joins a group– A new dependent router joins a pruned branch– A dependent router restarts on a pruned branch– If a Graft Ack is not received before the timeout
DVMRP Graft ACK
• Used to acknowledge receipt of Graft message• Allows sending downstream router to know
upstream router has received and processed its request
• If not received within 5 sec. Graft message sent again
DVMRP Issues
• Hop count used as metric• Scalability issues
– Performs periodic flooding– Maintains its own routing table
• Older versions lack pruning• Even though it is a main part of MBone,
MBone is being phased out
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 leaf router can re-join a group by
sending a graft message upstream.• Intermediate routers propagates the graft message upstream
until the path is re-connected.
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MBone
• Multicast backbone of the Internet.• Not all routers support multicast routing protocols and IGMP.• Connecting multicast-capable routers using (virtual) IP
tunnels.• Was a long-running experimental approach to enabling
multicast between sites through the use of tunnels.• No longer operational.
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Reading
• Section 19.1, Stallings
More references:• Multicasting on the Internet and Its Applications, Sanjoy Paul,
Kluwer Academic Publishers, 1998, chapters 2, 4, 5.• Computer Networking: A Top-Down Approach, 5th edition,
Kurose and Ross.
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