IP Multicast Lecture 3: PIM-SM

IP MulticastLecture 3: PIM-SM

Carl Harris

Communications Network Services

Virginia Tech

PIM ReviewConstructing the shared tree for G

2. Create (*,G) iif=2 oif=3

4. Create (*,G) iif=NULL oif=2

6. Create (*,G) iif=2 oif=4

8. Update (*,G) iif=NULL oif=1,2

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

R1

1. Host R1 sends an IGMPmembership report for group G

3. C sends Join/Prune towards RP for G (i.e. router A) Group=G Join=A, WC, RPTPrune=NULL

R2

5. Host R2 sends an IGMPmembership report for group G

7. B sends Join/Prune Group=G Join=A, WC, RPTPrune=NULL

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Review RPT for G connecting hosts R1 and R2

R2

R1

(*,G) iif=NULL oif=1,2

(*,G) iif=2 oif=3

(*,G) iif=2 oif=4

Multicast Packet ForwardingReview

• When a packet is received by a router from a directly connected source S addressed to a group G:– If neither (S,G) nor (*,G) state exists, the packet

is dropped. We’re now ready to see how this rule must be changed for PIM-SM.

– If (S,G) state exists, its entry timer is refreshed and the packet will be forwarded using the oif list for the (S,G) state.

Multicast Packet ForwardingReview

– If no (S,G) state exists and the packet matches (*,G) state, the (S,G) state is created.

• iif is set to the interface on which the packet was received

• oif is copied from the (*,G) state, excluding the interface on which the packet was received.

PIM-SM Source Discovery

• A source S for group G is discovered when the router receives a packet from S addressed to group G.– A directly connected source is discovered when

the router receives a packet addressed to group G from the source on the connecting interface.

– A distant (non-connected) source is discovered when the router receives a packet from the source on the shared tree for group G.

PIM Register Message

• The Register message is one means by which packets from a source S reach the root of the shared tree for group G.– Register messages are sent as unicast PIM

messages addressed to the RP.– The payload of a Register message is a

complete multicast packet (headers and all).– The RP decapsulates the Register message, and

forwards (or drops) the packet according to prevailing state. No RPF check occurs. Why?

Designated Router

• On every subnet, one router is “elected” as the designated router (DR) for that network.– Only the DR sends Register messages to the

RP, and only for directly connected sources.– Only the DR sends Join/Prune messages toward

the RP or toward a source.– The highest IP address among PIM routers on a

subnet is elected as the DR.

Multicast Packet ForwardingPIM-SM version

• When a packet is received by a PIM router on interface X from any source S addressed to a group G, exactly one of six possible conditions will be met.– Based on which of these conditions is met,

there are two possible outcomes (packet is forwarded, or packet is dropped).

– The router must consider the possible conditions in the order presented here.

Send aRegister

message to theRP if needed.

Forward thepacket tothe oif list

for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no

If needed, set (S,G) SPT-bit

and send(S,G)RPT prune.

Forwarding Flow ChartPIM router processing a packet from source S to group G which has arrived on interface X.

If desired,create (S,G)

and send(S,G)SPT join.

If S is directlyconnected,

create (S,G).

Send aRegister

message to the RP if needed.


for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no






Case 1, receiving via SPT

Case 2, receiving via RPT


create (S,G).

Multicast Packet ForwardingPIM-SM cases 1 and 2

• Case 1:(S,G) state exists and its iif is interface X.– This condition represents the situation in which

the router has joined the SPT for S sending to G, and is receiving packets from that SPT.

• Case 2:(*,G) state exists and its iif is interface X.– This condition represents the situation in which

the router is receiving packets for G via the RPT (shared tree).

PIM ExampleCase 1

R(*,G) iif=3 oif=1,2(S,G) iif=2 oif=1

1

2

3

S G

p

aylo

adPacket arrives oninterface 2 from source S to group G.

(S,G) exists and itsiif matches the interfaceon which the packet wasreceived.

We also have (*,G), but sincewe check case 1 first, we don’tnotice the (*,G) state.

PIM ExampleCase 2

R(*,G) iif=3 oif=1,2

1

2

3

S G

paylo

ad

Packet arrives oninterface 3 from source S to group G. Only (*,G) exists and its

iif matches the interfaceon which the packet wasreceived.

Send aRegister

message to the RP if needed.


for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no






Case 4: have (S,G),

wrong iif, SPT not ready

Case 3: have (S,G),

wrong iif, SPT active


create (S,G).

Multicast Packet ForwardingPIM-SM case 3

• Case 3: (S,G) exists, but iif is not interface X. (S,G) has the SPT-bit set.– This covers the situation in which the router has

joined the SPT for S sending to G, but the packet has arrived on the wrong interface.

PIM ExampleCase 3

R(*,G) iif=3 oif=1,2(S,G) iif=2 oif=1 SPT-bit

1

2

3

S G

payl o

a d

Packet arrives oninterface 3 from source S to group G.

(S,G) exists but itsiif does not match the interface on which the packet was received. Thisrouter is receiving packets viathe SPT and thus doesn’t want to receive them via RPT.

We also have (*,G). Sincethe router matches on (S,G)first, and the SPT-bit is set.this (*,G) state is ignoredand the packet is dropped.


• Case 4: (S,G) exists, but iif is not interface X. (S,G) does not have the SPT-bit set.

Note that only the SPT-bit condition differentiates case 4 from case 3.

– This covers the situation in which the router has created (S,G) state, and has sent a join towards S, but has yet to receive packets on that SPT.

• The router must continue to except packets from S to G via the RPT, so it ignores the (S,G) state and uses (*,G) instead.

PIM ExampleCase 4

R(*,G) iif=3 oif=1,2(S,G) iif=2 oif=1

1

2

3

S G

payl o

a d

Packet arrives oninterface 3 from source S to group G.

(S,G) exists but its iif does not match the interface on which the packet was received. This router has sent a join towards S on interface 2,but the lack of the SPT-bit tells usthat the SPT is not deliveringpackets yet.

We also have (*,G). Sincesince the iif matches, and we’renot getting packets from the SPT (no SPT-bit on (S,G)) we accept the packet from the RPT just like case 2.

Send aRegister



for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no






Case 5: Have (*,G), wrong iif.


create (S,G)


• Case 5: (S,G) does not exist. (*,G) does exist, but its iif is not interface X.– This condition represents either a packet

received on the wrong interface, or a previously unknown directly connected source.

– We’ll see shortly that if S is a directly connected source, and if this router is the DR on the network connected to interface X we may need to deliver the packet to the root of the shared tree using a Register message.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example Case 5


(*,G) iif=2 oif=3

(*,G) iif=2 oif=4

S

S G

p

aylo

ad

S G

p

aylo

ad

Here, B receives a packet from S to Gon interface 3. He has only shared treestate (*,G) and 3 is not the right iif. Since S is connected, and assuming B is the DR on the network containing S, he’ll need to send this packet to the root of the shared tree in a Register message.

Over here, router C hassomehow receives a packetfrom S to G on interface 3.Since S is not directlyconnected, and interface 2is not the iif for (*,G), theremust be a forwarding loop,which C suppresses byignoring the packet since itdoesn’t pass RPF.

Send aRegister



for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no






Case 6: no G states


create (S,G)


• Case 6: Neither (S,G) nor (*,G) state exists.– This condition represents a router that has

neither directly connected nor downstream receivers for group G.

– Despite that this router knows of no receivers for G there may be receivers elsewhere in the PIM domain, thus we may need to deliver the packet to the root of the shared tree (if this router is the DR on the network connected to interface X, and if S is directly connected).

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example Case 6

(*,G) iif=NULL oif=2

(*,G) iif=2 oif=3

S

S G

p

aylo

ad

B receives a packet from S to Gon interface 3. He has no states for G.Since S is connected, and assuming B is the DR on the network containing S, he’ll need to send this packet to the root of the shared tree in a Register message.

Router C does have downstream receiversfor group G on interface 3, but B doesn’t know this. When B sends the packet in a Registerto the root of the shared tree (at router A), thepacket will be forwarded down to C, anyway.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example A packet is delivered via the RPT

R2

R1


(*,G) iif=2 oif=3

(*,G) iif=2 oif=4

S

3. A decapulates the Registerpacket and forwards it nativelyaccording to (*,G) state (I.e. on the RP tree).

2. B creates (S,G) iif=3 oif=4 and unicasts the packet to A (the RP for G) in a Register packet.

1. Host S sends a packet destinedfor group G. 4. B and C forward the

packet according to (*,G)state.

Send aRegister



for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no







create (S,G)

Sending Register Messages

• A register message is sent by a router when a packet from source S to group G on interface X only under the following conditions:– Source S is directly connected, and interface X is

the RPF interface for S.– The router is the DR on the network connected

to interface X,– The Register Suppression Timer is not running.

We find out about how this timer gets set, shortly…

Send aRegister



for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no







create (S,G)

Joining the SPT for (S,G)

• A PIM router with connected receivers for group G may elect to join the shortest path tree (SPT) for a source S any time after it has discovered source S.– Intermediate PIM routers (those without

directly connected receivers) do not initiate the construction of an SPT.

– The decision of whether/when to join the SPT for a source is controlled entirely by the last-hop router for a collection of receivers.


• When a last-hop router is ready to join the SPT for a source S sending to group G– (S,G) state is created; iif is set to the RPF

interface for S, oif is copied from (*,G), excluding the RPF interface for S.

– If S is not directly connected, the router sends a Join/Prune message for (S,G) towards S.

– A router with a connected source S and connected receivers (on interfaces other than that which leads to S) is already on the SPT.


• Until the router has received at least one packet on the from S via the SPT for G, it must continue to accept packets from S on the RPT for G.– Packets received while waiting for the SPT join

to take effect are Case 4. – When the first packet arrives via the SPT, the

router sets (S,G) SPT-bit, and subsequent packets from S to G are Case 1.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM ExampleB joins the SPT for (S,G)

R2

R1


(*,G) iif=2 oif=3

(*,G) iif=2 oif=4

S

1. Host S sends a packet destined for group G.

2. Create (S,G) SPT-bit

4. B encapsulates the packet in a Register packet and unicasts it to the RP for G.

3. B forwards thepacket according to(S,G) state to oif=4.

5. A decapulates the Registerpacket and forwards it nativelyaccording to (*,G) state (I.e. on the RP tree).

6. C forwards thepacket according toits (*,G) state.

B drops the packet whenit arrives on the RP tree.Why?

Send aRegister



for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no







create (S,G)

Pruning (S,G) from the RPT

• When a router is receiving source S for group G via the SPT, it drops those packets from S that arrive on the RPT for G.– After the router has (S,G) SPT-bit state, RPF is

violated by the those packets from S to G that arrive via the RPT for G, so those packets must be ignored.

– The setting of the SPT-bit flag on (S,G) is a trigger event: I am receiving packets from S to G via the SPT, so I want to prune S from the RPT.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM ExampleB prunes (S,G) from the RPT

R2

R1


(*,G) iif=2 oif=3

(*,G) iif=2 oif=4(S,G) iif=3 oif=4

S


2. Set (S,G) SPT-bit

5. B encapsulates the packet in a Register packet and unicasts it to the RP for G.


6. A decapulates the Registerpacket and forwards it nativelyaccording to (*,G) state.


B drops the packet whenit arrives on the RP tree.Why?

3. B sends a (S,G)RPT prunetowards the RP for G.

(S,G)RPT Prune

• The setting of the SPT-bit flag in (S,G) triggers the router to send a Join/Prune message towards the RP for group G.– Group address = G– Join list = NULL– Prune list = {S, RPT}

(S,G)RPT Prune

• When a router receives a Join/Prune message with S in the prune list and the RPT bit set– If (S,G) state exists, the interface on which the

Join/Prune is received is removed from the oif list.

– If (S,G) does not exist and (*,G) state exists,

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example C joins the SPT for (S,G)

R2

R1

(*,G) iif=NULL oif=1,2 (S,G) iif=NULL oif=2 RPT-bit

(*,G) iif=2 oif=3

(*,G) iif=2 oif=4(S,G) iif=3 oif=4 SPT-bit

S

1. Host S sends another packet destined for group G.


3. B encapsulates the packet in a Register packet and unicasts it to the RP for G (router A).

4. A decapulates the Registerpacket and forwards it nativelyaccording to (S,G) state.


6. Create (S,G)iif=1 -- RPF-neighbor for Soif=3 -- copied from (*,G)

7. C sends Join/Prunefor G towards RPF for S.Join=S, Prune=NULL

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example C prunes the RPT for (S,G)

R2

R1

(*,G) iif=NULL oif=1,2(S,G) iif=NULL oif=2 RPT-bit

(*,G) iif=2 oif=3(S,G) iif=1 oif=3

(*,G) iif=2 oif=4 (S,G) iif=3 oif=4,1 SPT-bit

S



4. A decapulates the Registerpacket and forwards it nativelyaccording to (S,G) state.

2. B forwards thepacket according to(S,G) state.

5. C receives the packet on iif=1, sets the SPT-bit for (S,G)and forwards the packet accordingly.

What happens to this packet?

6. C sends an(S,G) RPT prunetowards the RP.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example SPT for (S,G) connecting hosts R1 and R2

R2

R1

(*,G) iif=NULL oif=1,2(S,G) iif=NULL oif=NULL RPT-bit


(*,G) iif=2 oif=4(S,G) iif=3 oif=4,1 SPT-bit

S

Register Suppression

• When the oif list for an (S,G) state on the RP is null, the RP doesn’t want to receive (S,G) register packets.– the oif list is null because there are no

downstream receivers for G or because all downstream receivers have pruned S from the RPT for G.

– the contents of the register packet won’t be forwarded, so there’s no need to get them in the first place.


• The RP for G unicasts a Register-Stop message to the source of an (S,G) Register message when the oif for (S,G) is null.– The source of the Register packet is always the

DR for the network containing S.– The Register-Stop message causes the DR to

set the Register Suppression Timer for (S,G). Even if the DR has no downstream receivers

for group G, he’ll create (S,G) oif=null so he’ll have a place to store this timer.

Send aRegister



for this state.

begin

Does(S,G)exist

?

Does(*,G)exist

?

Is it(S,G)iif=X

?

Is it(*,G)iif=X

?

Is it(S,G)

SPT-bit?

Drop thepacket.

end

no

no

yes yes

no

no yes

yes yes

no







create (S,G)


• When the DR receives a packet addresed to group G from a directly connected source S, a Register packet is sent to the RP only if the Register Suppression Timer for (S,G) is not running.– When the timer expires, the first hop router for

S resumes sending Register packets.– If (S,G) is (still) fully pruned from the RPT, the

RP will (again) send a Register-Stop to the source of the Register packet.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM Example RP sends a Register-Stop to B

R2

R1

(*,G) iif=NULL oif=1,2(S,G) iif=NULL oif=NULL RPT-bit



S



4. A unicasts a Register-Stop to B.

2. B and C forward thepacket according to(S,G) state.

5. B sets the Register SuppressionTimer for (S,G).

RP Joins the SPT for (S,G)

• When the RP receives a Register packet for from source S to group G it may elect to join the SPT for source S.– The RP acts like a last-hop router and a first-hop

router.– More efficient when there are intermediate routers

between the first hop router for S and the RP that have existing (S,G) state.

– Reduces overhead on the RP -- unlike a Register packet, a native (S,G) packet requires no decapsulation.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM ExampleRP joins the SPT for (S,G)

R2

R1

(*,G) iif=NULL oif=1,2(S,G) iif=NULL oif=2 RPT-bit

(*,G) iif=2 oif=3


S



3. B encapsulates the packet in a Register message and unicasts it to the RP for G (router A).

4. A decapulates the Registerpacket and it is forwarded nativelyaccording to existing state.

5. Update (S,G) set iif=1.6. Send Join/Prune

towards RPF neighbor forS. Group=G, Join=S, Prune=NULL.

Register Supression (Again)

• When the RP has (S,G) SPT-bit state, it doesn’t want to receive Register packets in which the payload is a multicast packet from source S to group G.– SPT-bit is set because the RP is receiving

packets from S to G natively on the shortest path tree.

– The RP unicasts a Register-Stop to the DR for S whenever it receives an (S,G) Register packet while (S,G) SPT-bit state exists.

B

A

C1

2

3

1 2

1

2

3

4

RP(G) = A

PIM ExampleRP sends a Register-Stop to B

R2

R1

(*,G) iif=NULL oif=1,2(S,G) iif=1 oif=2 RPT-bit

(*,G) iif=2 oif=3



S5. A unicasts a Register-Stopto B. B sets theRegister Suppressiontimer for (S,G).

3. B encapsulates the packet in a Register message and unicasts it to the RP for G (router A).

2. B forwards thepacket according to(S,G) state.

What happens if the Register message arrives first?

4. A receives native packet from B on interface 1 and sets SPT bit for (S,G). The packetis then forwarded according toexisting state.

End of Lecture 3

IP Multicast Lecture 3: PIM-SM

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