1Spring Semester 2009, Dept. of Computer Science, Technion

Internet NetworkingInternet Networkingrecitation #7recitation #7

DVMRPDVMRP

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Internet Networking

IntroductionIntroduction

When we discuss multicast routing protocols 3 issues should be addressed: How hosts can join a multicast group (usually performed

by IGMP). How routers distribute between them information about

registered multicast subscriptions. How a router performs routing of a multicast packet.

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Internet Networking

A simple solutionA simple solution

Consider the following solution: multicast packets are flooded from a source to all the routers in the AS.

Advantages: Simplicity.

Disadvantages: Packets are unnecessarily received by all routers. Routers receive duplicate packets.

• A filtering mechanism should be employed. Each router should remember what packets it already forwarded.

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Internet Networking

Avoiding the need to rememberAvoiding the need to remember

To avoid the need to remember, a packet is accepted only if it arrives on the port that corresponds to the shortest path from S. Can it be done in Distance Vector routing?

• Symmetric paths are assumed. Duplicate packets are not avoided.

A

B DC

E F

An accepted packet

Sent but not accepted packet

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Internet Networking

Reverse Path ForwardingReverse Path Forwarding

To avoid duplications, a packet is sent to a neighbor only if it will be accepted.

A router sends a packet to a neighbor only if it is on the shortest path from it to the source. The router needs to know which of its neighbors use it as

a “next hop” to the source. This information is advertised to the router as a regular

route report having a cost of infinity.

• This technique is called “Poison Reverse”.

RPF guarantees that every router receives every packet exactly once.

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Internet Networking

Reverse Path ForwardingReverse Path Forwarding

A

B DC

E F

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Internet Networking

PruningPruning

Problem: Flooding still occurs throughout the AS.

Solution: Flood & Prune method. The first multicast packet from a source S is propagated to

all the network nodes (flooding). When a leaf router (at the specific tree) receives a multicast

message and doesn’t have group members for it, it sends PRUNE message to its father node.

When an intermediate router gets PRUNE messages from all its children then it sends PRUNE message to its father node.

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Internet Networking

Pruning examplePruning example

B

before pruning after pruning

A

DC

E F

PRUNE

PRUNE

A

DC

E F

BB

Only C and E are group members.

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Internet Networking

Re-joining the TreeRe-joining the Tree

After a period of time the PRUNE effect vanishes and the messages are flooded again. Provides robustness to topology changes. Each PRUNE message has a lifetime value associated with it. The lifetime of a PRUNE message sent by a node to an

upstream node must be no more than the minimum of the remaining lifetimes of the PRUNE messages received from its downstream nodes.

Send an explicit JOIN request, which will propagate upwards. A JOIN message must be acknowledged to ensure the

reception of the message. Used only to undo the effect of a PRUNE message.

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Internet Networking

RPF with pruning - summaryRPF with pruning - summary

Advantages: Simplicity Robustness

Disadvantages: Packets are flooded to the whole AS on a periodic basis. All routers must keep state on a per-group and per-source

basis. In principle each JOIN and PRUNE message for a group

must be sent per-source. Does not scale for large multicast network.

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Internet Networking

DVMRP ProtocolDVMRP Protocol

Protocol for multicast routing inside of ASs that use Distance Vector Routing (e.g. RIP). Defined in RFC 1075. Revised by Internet Draft: draft-ietf-idmr-dvmrp-v3-11.

• May become an RFC in the future.

Uses IGMP-like messages for exchanging multicast information between routers.

Based on RPF and flood & prune algorithms.

Suitable for dense multicast trees.

Uses its own routing tables. Allows the multicast routes to be independent of the unicast routes.

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Internet Networking

DVMRP Forwarding TableDVMRP Forwarding Table

Represents the local router’s understanding of the shortest path delivery tree for each (source, group) pair.

Example:a prune message has been sent to the upstream router

the router has received a prune message from a

downstream router.

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Internet Networking

TunnelingTunneling

A method for sending datagram between routers separated by gateways that do not support multicast.

Acts as a virtual network between two routers.

Example: Host on net 1 wants to send a multicast message to a host on net 2

Internet with no support

for multicastR1 R2

net 1 net 2

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Internet Networking

TunnelingTunneling

Tunneling is done by encapsulating the original multicast datagram with an unicast IP datagram.

The source and the destination of the unicast IP packets are the end point of the tunnel.

The encapsulation of the datagram is done by the source.

The destination address in the unicast header is the address of the next router which supports multicast (it is considered the egress of the tunnel).

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Internet Networking

TunnelingTunneling

Example:

Source:R1 Dest:R2 Protocol: IP in IP

Source:S Dest:G Protocol: UDP

UDP header and data

Internet with no support

for multicastR1 R2S

member of G