multicast routing protocols. the need for multicast routing n routing based on member information...
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Multicast Routing Protocols
The Need for Multicast Routing
Routing based on member information– Whenever a multicast router receives a multicast packet it
checks the group ID of the message and forwards the packet only if there is a member of that group in networks connected to it
Member information exchange– For delivering a multicast packet from the source to the
destination nodes on other networks, multicast routers need to exchange the information they have gathered from the group membership of the hosts directly connected to them
Basic Router Model
Since hosts can send any time to any group, routers must be prepared to receive on all link-layer group addresses– And know when to forward or drop packets
What does a router keep track of?– interfaces leading to receivers– sources when utilizing source distribution trees– prune state depending on the multicast routing protocol (e.g. Dense Mode)
Data Distribution Concepts
Routers maintain state to deliver data down a distribution tree Source trees
– Router keeps (S,G) state so packets can flow from the source to all receivers
– Trades off low delay from source against router state
Data Distribution Concepts
Shared trees– Router keeps (*,G) state so packets flow from the root of the
tree to all receivers– Trades off higher delay from source against less router state
Data Distribution Concepts
How is the tree built?– On demand, in response to data arrival
» Dense-mode protocols (PIM-DM and DVMRP)» MOSPF
– Explicit control– Sparse-mode protocols (PIM-SM and CBT)
Data Distribution Concepts
Building distribution trees requires knowledge of where members are– flood data to find out where members are not (Dense-mode protocols)– flood group membership information (MOSPF), and build tree as data
arrives– send explicit joins and keep join state (Sparse-mode protocols)
Data Distribution Concepts
Construction of source trees requires knowledge of source locations
–In dense-mode protocols you learn them when data arrives (at each depth of the tree)–Same with MOSPF–In sparse-mode protocols you learn them when data arrives on the shared tree (in leaf routers only)
»Ignore since routing based on direction from RP»Pay attention if moving to source tree
Data Distribution Concepts
To build a shared tree you need to know where the core (RP) is– Can be learned dynamically in the routing protocol (Auto-RP,
PIMv2)– Can be configured in the routers
Data Distribution Concepts
Shared trees make sense for– Many low-rate sources– Applications that don’t require low delay– Consistent policy and access control across most participants in a group– When most of the source trees overlap topologically with the shared tree
Multicast Routing Protocols—Characteristics
Types of multicast protocols– Dense-mode
» Flood and prune behavior– Sparse-mode
» Explicit join behavior
Multicast Routing Protocols—Characteristics
Dense-mode protocols–Assumes dense group membership
»Branches that are pruned don’t get data»Pruned branches can later be grafted to reduce join latency
–DVMRP—Distance Vector Multicast Routing Protocol–Dense-mode PIM—Protocol Independent Multicast
Multicast Routing Protocols—Characteristics
Sparse-mode protocols»Assumes group membership is sparsely populated across a large region »Uses either source or shared distribution trees»Explicit join behavior—assumes no one wants the packet unless asked »Joins propagate from receiver to source or Rendezvous Point (Sparse mode PIM) or Core (Core Based Tree)
Intra-Domain Multicast Routing Protocols
Similar to unicast routing protocols - such as Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) protocol -, there should be multicast routing protocols such that multicast routers can determine where to forward multicast messages
Existing multicast protocols:– Distance Vector Multicast Routing Protocol (DVMRP) based on
the RIP unicast routing protocol– Multicast Extensions to OSPF (MOSPF) protocol based on the
OSPF unicast routing protcol– Protocol Independent Multicast Sparse Mode (PIM-SM)
protocol:» performs better when group members are sparsely distributed
Protocol Independent Multicast (PIM) The major proposed (and used) multicast protocols perform well if
group members are densely packed and bandwidth is not a problem However, the fact that DVMRP periodically floods the network and
the fact that MOSPF sends group membership information over the links, make these protocols not efficient in cases where group members are sparsely distributed among regions and the bandwidth is not plentiful
To address these issues, PIM contains two protocols: PIM-DM and PIM-SM
Although these two algorithms belong to PIM and they share similar control messages, they are essentially two different protocols
PIM-Sparse Mode (PIM-SM)
PIM-SM has a key difference with existing dense-mode protocols (DVMRP, MOSPF)
In PIM-SM protocol routers need to explicitly announce their will for receiving multicast messages of multicast groups, while dense-mode protocols assumes that all routers need to receive multicast messages unless they explicitly send a prune message
Sparse Mode Protocol Independent Multicast (PIM-SM) I. Phase
Sender
Receiver
Legend
router
IP connection
group member host
non member host
rendezvous point (RP)
data flow
Sparse Mode Protocol Independent Multicast (PIM-SM) II. Phase
Sender
Receiver
Legend
router
IP connection
SPT branch
group member
non member
rendezvous point (RP)
data flow
Explicit join model– Receivers join to the Rendezvous Point (RP)– Senders register with the RP– Data flows down the shared tree and goes only
to places that need the data from the sources– Last hop routers can join source tree if the data rate warrants by sending
joins to the source RPF check for the shared tree uses the RP RPF check for the source tree
uses the source
Sparse Mode PIM
Only one RP is chosen for a particular group RP statically configured or dynamically learned (Auto-RP, PIM v2 candidate RP
advertisements) Data forwarded based on the source state (S, G)
if it exists, otherwise use the shared state (*, G) Draft: draft-ietf-idmr-pim-sm-specv2-00.txt Draft: draft-ietf-idmr-pim-arch-04.txt
Sparse Mode PIM
Sparse Mode PIM Example
Receiver 1
B
E
A D
Source
C
Receiver 2
RP
Link
Data
Control
Sparse Mode PIM Example
Receiver 1
B
E
A D
Source Receiver 1 Joins Group GC Creates (*, G) State, Sends(*, G) Join to the RP
C
Receiver 2
RP
Join
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source RP Creates (*, G) State
C
Receiver 2
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source Source Sends DataA Sends Registers to the RP
C
Receiver 2
Register
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source RP de-encapsulates RegistersForwards Data Down the Shared TreeSends (S,G) Join Towards the Source
C
Receiver 2
Join Join
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source RP Sends Register-Stop OnceData Arrives Natively
C
Receiver 2
Register-Stop
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source C Sends (S, G) Joins to Join theShortest Path (SPT) Tree
C
Receiver 2
(S, G) Join
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source When C Receives Data Natively,It Sends Prunes Up the RP tree forthe Source. RP Deletes (S, G) OIF andSends Prune Towards the Source
C
Receiver 2
(S, G) RP Bit Prune
(S, G) Prune
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source New Receiver 2 JoinsE Creates State and Sends (*, G) Join
C
Receiver 2
(*, G) PIM Join
(*, G) IGMP Join
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source C Adds Link Towards E to the OIFList of Both (*, G) and (S, G)Data from Source Arrives at E
C
Receiver 2
Sparse Mode PIM Example
Receiver 1
B
E
A RP D
Source
New Source Starts SendingD Sends Registers, RP Sends JoinsRP Forwards Data to Receiversthrough Shared Tree
C
Receiver 2
Source 2(*, G) PIM Register
Multicast Routing Protocols Comparison
Multicast Routing Protocol Unicast Protocol Dependence
DVMRP RIP
MOSPF OSPF
PIM-SM -
Source Specific Protocol Independent Multicast (PIM-SS)
Only one S source can send data to an (S,G) channel, where G is a multicast address
In such a way the problem of global assignment of the multicast address is eliminated, since the used addresses are local considering to the actual sender
Each hosts are responsible to use different multicast addresses The distribution tree according to an (S,G) SSM channel is
always rooted in the S source, in such a way RP-based shared trees are not necessary
ASM and SSM routing
Broadcast MembershipMOSPF
Problem of learning group membership
Flood and PruneDVMRP
Rendezvous MechanismPIM-SM
Intra-domain Multicast
MSDP Peering
Inter-domain Multicast
BBC, 2005
Multicast, state-of-the-art