multicast1 instructor: anirban mahanti office: ict 745 email: [email protected] slides are...
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Multicast 1
Multicast
Instructor: Anirban MahantiOffice: ICT 745Email: [email protected]
Slides are adapted from the companion web site of the textbook “Computer Networking: A Top Down Approach Featuring the Internet”, 3rd edition, Jim Kurose, Keith Ross Addison-Wesley, 2005.
Also see – C. Diot et al., “Deployment Issues for the IP Multicast Service and Architecture”, IEEE Network, Vol. 14, No. 1, January/February 2000.
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Multicast 2
Source-duplication versus in-network duplication. (a) source duplication, (b) in-network duplication
(a) (b)
R1
R2
R3 R4
R1
R2
R3 R4
duplicatecreation/transmissionduplicate
duplicate
Broadcast Routing
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Multicast 3
A
B
G
DE
c
F
Reverse Path Forwarding
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Multicast 4
A
B
G
DE
c
F
A
B
G
DE
c
F
(a) Broadcast initiated at A (b) Broadcast initiated at D
Spanning-Tree Broadcast
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Multicast 5
Multicast: one sender to many receivers Multicast: act of sending datagram to multiple
receivers with single “transmit” operation analogy: one teacher to many students
Question: how to achieve multicast
Multicast via unicast source sends N unicast
datagrams, one addressed to each of N receivers
multicast receiver (red)
not a multicast receiver (red)
routersforward unicastdatagrams
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Multicast 6
Multicast: one sender to many receivers Multicast: act of sending datagram to multiple
receivers with single “transmit” operation analogy: one teacher to many students
Question: how to achieve multicast
Network multicast Router actively participate in
multicast, making copies of packets as needed and forwarding towards multicast receivers
Multicastrouters (red) duplicate and forward multicast datagrams
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Multicast 7
Multicast: one sender to many receivers Multicast: act of sending datagram to multiple
receivers with single “transmit” operation analogy: one teacher to many students
Question: how to achieve multicast
Application-layer multicast end systems involved in
multicast copy and forward unicast datagrams among themselves
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Multicast 8
Internet Multicast Service Model
multicast group concept: use of indirection hosts addresses IP datagram to multicast group routers forward multicast datagrams to hosts
that have “joined” that multicast group
128.119.40.186
128.59.16.12
128.34.108.63
128.34.108.60
multicast group
226.17.30.197
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Multicast 9
Multicast groups class D Internet addresses reserved for multicast:
host group semantics:o anyone can “join” (receive) multicast groupo anyone can send to multicast groupo no network-layer identification to hosts of
members needed: infrastructure to deliver mcast-addressed
datagrams to all hosts that have joined that multicast group
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Multicast 10
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)
IGMPIGMP
IGMP
wide-areamulticast
routing
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Multicast 11
IGMP: Internet Group Management Protocol host: sends IGMP report when application joins
mcast group IP_ADD_MEMBERSHIP socket option host need not explicitly “unjoin” group when
leaving router: sends IGMP query at regular intervals
host belonging to a mcast group must reply to query
query report
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Multicast 12
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
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Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting
routers having local mcast group members tree: not all paths between routers used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members
Shared tree Source-based trees
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Approaches for building mcast treesApproaches: source-based tree: one tree per source
shortest path trees reverse path forwarding
group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees
…we first look at basic approaches, then specific protocols adopting these approaches
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Shortest Path Tree
mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm
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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Reverse Path Forwarding
if (mcast datagram received on incoming link on shortest path back to center)
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
• result is a source-specific reverse SPT– may be a bad choice with asymmetric links
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
datagram will be forwarded
LEGENDS: source
datagram will not be forwarded
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Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast
group members no need to forward datagrams down subtree “prune” msgs sent upstream by router with
no downstream group members
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
prune message
LEGENDS: source
links with multicastforwarding
P
P
P
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Shared-Tree: Steiner Tree
Steiner Tree: minimum cost tree connecting all routers with attached group members
problem is NP-complete excellent heuristics exists not used in practice:
computational complexity information about entire network needed monolithic: rerun whenever a router needs
to join/leave
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Center-based trees
single delivery tree shared by all one router identified as “center” of tree to join:
edge router sends unicast join-msg addressed to center router
join-msg “processed” by intermediate routers and forwarded towards center
join-msg either hits existing tree branch for this center, or arrives at center
path taken by join-msg becomes new branch of tree for this router
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Center-based trees: an example
Suppose R6 chosen as center:
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
path order in which join messages generated
LEGEND
21
3
1
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Internet Multicasting Routing: DVMRP
DVMRP: distance vector multicast routing protocol, RFC1075
flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP’s own routing tables
constructed by communicating DVMRP routers no assumptions about underlying unicast initial datagram to mcast group flooded
everywhere via RPF routers not wanting group: send upstream
prune msgs
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DVMRP: continued… soft state: DVMRP router periodically (1
min.) “forgets” branches are pruned: mcast data again flows down unpruned branch downstream router: 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
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PIM: Protocol Independent Multicast
not dependent on any specific underlying unicast routing algorithm (works with all)
two different multicast distribution scenarios :
Dense: group members
densely packed, in “close” proximity.
bandwidth more plentiful
Sparse: # networks with group
members small wrt # interconnected networks
group members “widely dispersed”
bandwidth not plentiful
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Consequences of Sparse-Dense Dichotomy: Dense group membership by
routers assumed until routers explicitly prune
data-driven construction on mcast tree (e.g., RPF)
bandwidth and non-group-router processing profligate
Sparse: no membership until
routers explicitly join receiver- driven
construction of mcast tree (e.g., center-based)
bandwidth and non-group-router processing conservative
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PIM- Dense Mode
flood-and-prune RPF, similar to DVMRP but
underlying unicast protocol provides RPF info for incoming datagram
less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm
has protocol mechanism for router to detect it is a leaf-node router
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PIM - Sparse Mode
center-based approach router sends join msg
to rendezvous point (RP) intermediate routers
update state and forward join
after joining via RP, router can switch to source-specific tree increased performance:
less concentration, shorter paths
R1
R2
R3
R4
R5
R6R7
join
join
join
all data multicastfrom rendezvouspoint
rendezvouspoint
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PIM - Sparse Mode
sender(s): unicast data to RP,
which distributes down RP-rooted tree
RP can extend mcast tree upstream to source
RP can send stop msg if no attached receivers “no one is listening!”
R1
R2
R3
R4
R5
R6R7
join
join
join
all data multicastfrom rendezvouspoint
rendezvouspoint
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Multicast 29
PIM-SM continued …
What if source is located in remote domains? PIM-SM requires group-RP mappings to be
advertised to all PIM-SM domains Use Multicast Source Discovery Protocol,
functionality is similar to BGP Inter-domain multicast to be managed
by Border Gateway Multicast Protocol (BGMP)
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Multicast 30
Deployment Issues
Requirements for wide-scale deployment Easy/transparent install, sender/receiver
group management, unique multicast addresses, robust & reliable transmissions
Issues Router Migration Inter-domain multicast – traffic management? Group management – authorization, billing
policy Address allocation Implementation issues