multicast addresses

8/12/2019 Multicast Addresses

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Multicast Addresses


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Overview

Multicast is grossly defined as sending a single packetto multiple selected destinations.

1. One-to-many

Radio station broadcast Transmitting news, stock-price

Software updates to multiple hosts

2. Many-to-many Multimedia teleconferencing Online multi-player games

Distributed simulations


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Overview

Without support for multicast

A source needs to send a separate packet with the

identical data to each member of the group

This redundancy consumes more bandwidth Redundant traffic is not evenly distributed, concentrated near the

sending host

Source needs to keep track of the IP address of each

member in the group

Group may be dynamic

To support many-to-many and one-to-many IP

provides an IP-level multicast


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Overview

Basic IP multicast model is many-to-many

based on multicast groups

Each group has its own IP multicast address

Hosts that are members of a group receive copies

of any packets sent to that groups multicast

address

A host can be in multiple groups A host can join and leave groups


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Overview

Using IP multicast to send the identical packet

to each member of the group

A host sends a single copy of the packet addressed

to the groups multicast address

The sending host does not need to know the

individual unicast IP address of each member

Sending host does not send multiple copies of thepacket


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Overview

IPs original many-to-many multicast has been

enhanced with support for a form of one-to-many

multicast

One-to-many multicast Called Source specific multicast (SSM)

A receiving host specifies both a multicast group and a

specific sending host

e.g., radio broadcasts

Many-to-many model

Called Any source multicast (ASM)


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Overview

A host signals its desire to join or leave a multicast

group by communicating with its local router using a

special protocol

In IPv4, the protocol is Internet Group ManagementProtocol (IGMP)

In IPv6, the protocol is Multicast Listener Discovery (MLD)

The router has the responsibility for making

multicast behave correctly with regard to the host


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Multicast Routing

Multicast via unicast

A routers unicast forwarding table indicates the links to beused to forward the unicast packets

To support multicast, a router must additionally havemulticast forwarding tables that indicate, based on multicastaddress, which links to use to forward the multicast packet

Unicast forwarding tables collectively specify a set of paths

Multicast forwarding tables collectively specify a set of trees Multicast distribution trees


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Multicast Routing

For source specific multicast, the multicast

forwarding tables must indicate which links to use

based on the combination of multicast address and

the unicast IP address of the source

Multicast routing is the process by which multicast

distribution trees are determined or, the process by

which the multicast forwarding tables are built.


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Distance-Vector Multicast Routing Protocol (DVMRP)

Distance-vector routing for unicast can be extended to support multicast.

The resulting protocol is called Distance Vector Multicast Routing Protocol In the distance-vector algorithm, each router maintains a table of

, and exchanges a list of

pairs with its directly connected neighbors.

Extending this algorithm to support multi-cast is a two-stage process.

First, we create a broadcast mechanism that allows a packet to be

forwarded to all the networks on the internet.

Second, we need to refine the networks that donot have hosts that

belong to the multicast group.

DVMRP is one of several multicast routing protocols described as flood-

and-prune protocols.


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From the unicast routing table, each router already knows

that shortest path to source S goes through NextHop router N.

When multicast packets are received from S, forward on all

outgoing links (except the one on which the packet arrived),

iff packet is arrived from the link on the shortest path.

Eliminate duplicate broadcast packets by designating one

router as the parent router (relative to S) for each link,

where only the parent router is allowed to forward multicast

packets from that source over the LAN

parent selected as

router that has the shortest path to source S

smallest address to break ties


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Reverse Path Broadcast (RPB) or reverse path forwarding (RPF)

Goal: Prune networks that have no hosts in group G

Step 1: Determine a leaf networkwith no members in G

leaf if the parent router is the only router in the network

each host that is a member of group G periodically announce this fact

over the network Step 2: Propagate no members of G here information up the

shortest-path tree by

The routers augment update sent to

neighbors with set of groups for which this network is interested in

receiving multicast packets.

This is propagated from router to router, so that a given router knows

for what groups it should forward multicast packets

Distance-Vector Multicast


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Protocol Independent Multicast (PIM)

PIM was developed in response to the scaling problems of

earlier multicast routing protocols

Two modes

Sparse mode: PIM-SM

Dense mode: PIM-DM (similar to RPM)

Sparse and dense refer to the proportion of routers that will

want the multicast.

PIM dense mode (PIM-DM) uses a flood-and-prune algorithmlike DVMRP, and suffers from the same scalability problem.

PIM does not depend on any particular unicast routingit can

be used with any unicast routing protocol


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PIM-SM Routers join the multicast distribution tree using PIM protocol

messages known as Join messages.

PIM-SM assigns to each group a special router known as the

rendezvous point (RP)

Acts as the central relay between source and group

A multicast forwarding tree is built as a result of routers sendingJoin messages to the RP.

Two types of tree are constructed:

a shared tree, which may be used by all senders,

a source-specific tree, which may be used only by a specific sending host.

The normal mode of operation creates the shared tree first,

followed by one or more source-specific trees.

The default is to have only one tree for a group.


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PIM-SMRP

R3 R2 R4

R1 R5

Join

R4 sends Join to RP and

joins shared tree

RP

R3 R2 R4

R1 R5

Join

R5 sends Join to RP and joins sharedtree: R2 does not forward Join to RP

since it knows link (RP, R2) has been apart of the shared tree

Rendezvous pointRP =

Shared tree

Source-specific tree for source R1

A Join message must pass through some sequence of routers before reaching the RP (e.g., R2).

Each router along the path looks at the Join and creates a forwarding table entry for the shared

tree, called a (*, G) entry (* meaning all senders).

To create the forwarding table entry, it looks at the interface on which the Join arrived and marks

that interface as one on which it should forward data packets for this group. It then determines which interface it will use to forward the Join toward the RP.

This will be the only acceptable interface for incoming packets sent to this group.

It then forwards the Join toward the RP.

Eventually, the message arrives at the RP, completing the construction of the tree branch.


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Source R1 tunnels the multicast packetto RP, which forwards it along theshared tree to R4 and R5

suppose a host wishes to send a message to the

group.

So, it constructs a packet with the appropriatemulticast group address as its destination and sends it

to a router on its local network known as the

designated router (DR).

Suppose the DR is R1 in Figure.

Instead of simply forwarding the multicast packet, R1

tunnels it to the RP.

That is, R1 encapsulates the multicast packet inside a

PIM Register message and sends to the unicast IP

address of the RP.

RP finds an IP packet inside, addressed to the

multicast address of this group.

The RP sends it out onto the shared tree of which the

RP is the root.


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PIM-SM: optimization

(e.g., when there is a lot of data traffic to the group)

1. Avoid overhead incurred by tunneling from source

to RP

Join message sent by the RP to the sending host is specific to that sender.

Thus, the effect of the new Join is to create a sender-specific state in the

routers between the identified source and the RP. This is referred to as (S, G) state, since it applies to one sender to one

group, and contrasts with the (*, G) state that was installed between the

receivers and the RP that applies to all senders.

RP

R3 R2 R4

R1 R5

JoinRP builds source-specific tree to R1 bysending Join to R1


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2. Avoid the increased path length (or tree depth) dueto transmission relay via RP

The next possible optimization is to replace the entire shared tree with asource-specific tree.

This is desirable because the path from sender to receiver via the RP mightbe significantly longer than the shortest possible path.

The router at the down-stream end of the tree (eg: R4) sends a source-

specific Join toward the source. Since it is the shortest path toward the source, the routers along the way

create (S, G) state for this tree, and the result is a tree that has its root atthe source, rather than the RP.

RP

R3 R2 R4

R1 R5

JoinJoin

R4 and R5 build source-specific tree to R1 bysending Joins to R1


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Note on PIM

PIM is protocol independent in terms of unicast

routing protocol independent

All of its mechanisms for building and maintaining trees

take advantage of unicast routing without depending onany particular unicast routing protocol.

The formation of trees is entirely determined by the paths

that Join messages follow, which is determined by thechoice of shortest paths made by unicast routing.


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Inter-domain Multicast

The PIM-SM protocol is typically not used across domains,

only within a domain.

To extend multicast across domains using PIM-SM, Multicast

Source Discovery Protocol (MSDP) was created.

MSDP is used to connect different domainseach running

PIM-SM internally, with its own RPsby connecting the RPs of

the different domains.


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MSDP (Multicast source discovery protocol)

Each RP has one or more MSDP

peer RPs in other domains. Each pair of MSDP peers is

connected by a TCP connection

over which the MSDP protocol

runs.

All the MSDP peers for a givenmulticast group form a loose

mesh that is used as a broadcast

network.

MSDP messages are broadcast

through the mesh of peer RPsusing the reverse path broadcast

algorithm.


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Each RP knows the sources in its own domain

because it receives a Register message whenever a

new source arises.

Each RP periodically uses MSDP to broadcast Source

Active messages to its peers, giving the IP address of

the source, the multicast group address, and the IP

address of the originating RP.

When an RP receives a multicast from the source,the RP uses its shared tree to forward the multicast

to the receivers in its domain.


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MSDP operation:

(a) The source SR sends a Register to its domains RP, RP1; then RP1 sends a source-specific Join to

SR, and an MSDP Source Active to its MSDP peer in Domain B, RP2; then RP2 sends a source-

specific Join to SR.

(b) As a result, RP1 and RP2 are in the source-specific tree for source SR.


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Multiprotocol Label Switching

MPLS


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MPLS Used to improve the performance of the Internet.

Multiprotocol label switching (MPLS), combines some of the

properties of virtual circuits with the flexibility and robustness

of datagrams.

It relies on IP addresses and IP routing protocols to do its job.

Forwards packets using short, fixed length labels with local

scope

MPLS - Capabilities:

To enable IP capabilities on devices that do not have the capability to

forward IP datagrams in the normal manner;

To forward IP packets along explicit routes: routes precalculatedseparate to IP protocol

To support certain types of virtual private network services.


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Three applications of MPLS

Destination-Based Forwarding

Explicit Routing

Virtual Private Networks and Tunnels


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1. Destination-Based Forwarding

Routers R3 and R4 has one connected network, with prefixes 18.1.1/24 and18.3.3/24.

R1 and R2 have routing tables that indicate which outgoing interface each router

would use when forwarding packets to one of those two networks.

When MPLS is enabled on a router, the router allocates a label for each prefix in

its routing table, and advertises both the label and the prefix that it represents to

its neighboring routers.

This advertisement is carried in the Label Distribution Protocol.


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Destination Based Forwarding

MPLS attaching labels with IP datagrams

Router allocates a label for each prefix in its

routing table


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Advertisement of label and prefixes to neighbors via Label

Distribution Protocol (LDP)

Attach corresponding label to all packets sent to that router for that

prefix

R2 allocates labels and advertises bindings to R1.


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Other routers store these labels as remote

labels in their routing table along with prefixes

R1 stores the received labels in a table.


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Other routers store these labels as remote labels intheir routing table along with prefixes

R3 advertises another binding, and R2 stores the received label in a table.


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Destination Based Forwarding Example

Suppose a packet destined to 10.1.1.5 arrives at R1

R1 matches the prefix 10.1.1, attaches label 15 to packet, send to R2 R1 acts as Label Edge Router (LER)

an LER performs a complete IP lookup on arriving IP packets, and thenapplies labels to them


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MPLS Achievements

Fixed-length label lookup instead of variablelength IP prefix lookup Simpler to implement exact match instead of longest

match algorithm

Only forwarding algorithm is changed Any standard routing algorithm may still be used

Packets will follow the path selected by IP route


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How to Attach Labels to Packets ?

Depends upon type of link carrying packets When IP packets carried as complete frames

Label is inserted as a shim between layer 2 header and layer 3

header

Like on most link types: Ethernet, Token Ring, PPP When the switch function is an MPLS LSR

Label need to be in a place where switch can use it

label switching routers (LSRs)devices that run IP control

protocols but use the label-switching forwarding algorithm.

In ATM, labels are inserted in ATM cell header (combination of VPI

and VCI).


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An MPLS Network

Mixture of conventional IP routers, label edgerouters (LER) and ATM switches (as LSRs)

All are using the same routing protocols

Reduction in number of adjacencies that each

router must maintain Greatly reduce amount of work of each router

Edge routers have a full view of completetopology of the network

Edge routers pick a new path in case of node failure


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MPLS Example of MPLS Network

Conventional Network Overlay of virtual circuits

Each router connect to other by a virtual circuit


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MPLS Example of MPLS Network

MPLS Network

Routers peer directly with LSRs

No virtual circuits interconnecting routers

Each router has only one adjacency


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2. Explicit Routing

MPLS provides a convenient way to add capabilities similarto source routing to IP networks Explicit routing

It is not the real source of the packet that picks the routedifferent from source routing.

One of the routers inside the service providers networkdoes this

Application

Traffic Engineering: refers to the task of ensuring that

sufficient resources are available in a network to meetthe demands placed on it.

Example Explicit Routing


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Example Explicit Routing Often called a fish network because of its shape

The routers R1 and R2 form the tail; R7 is at the head

Suppose the route for traffic from R1 to R7 follows the path R1-R3-R6-R7

Route for traffic from R2 to R7 is through R2-R3-R4-R5-R7

This is to make good use of capacity available along two distinct paths

R1-to-R7 traffic has one forwarding equivalence class, and the R2-to-R7

traffic constitutes a second FEC (a group of IP packets which are forwardedin the same manner).

This is difficult in normal IP routing


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Explicit Routing

Has a capability called fast reroute Fast reroute a capability to make networks more resilient in the

case of failure

Possible to precalculate a path (backup path) betweenrouter A to router B that explicitly avoids a certain link L

Can significantly reduce the time taken to reroute packetsaround a point of failure

There are a range of algorithms that routers can use tocalculate explicit routes automatically.

CSPF (Constrained Shortest Path First): Most commonalgorithm to calculate explicit routes, which is like the link-state algorithms

3 Virtual Private Networks


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3. Virtual Private Networks

Virtual Private Network

A group of connected networks

Connections may be over multiple networks not belonging to the group

(e.g. over the Internet)

E.g., joining the networks of several branches of a company into a

private internetwork

C

A B

MK L

C

A BM

K L


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Tunneling

IP Tunnel

Virtual point-to-point link between an arbitrarily

connected pair of nodes

10.0.0.1

Network

1

Network

2Internetwork

R1 R2

IP Tunnel

IP Dest = 2.xIP Payload

IP Dest = 10.0.0.1IP Dest = 2.xIP Payload

IP Dest = 2.xIP Payload


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Tunneling

Advantages of tunneling Transparent transmission of packets over heterogeneous

networks The data carried may not even be IP messages!

Only need to change relevant routers (end points) Coupled with encryption, gives you a secure private

internetwork. End-points of tunnels may have features not available inother Internet routers.

Multicast Local Addresses

Disadvantages Increases packet size Processing time needed to encapsulate and decapsulate

packets Management at tunnel-aware routers


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The router at the entrance of the tunnel wraps the data to be tunneled in an IP

header (the tunnel header), which represents the address of the router at the

far end of the tunnel, and sends the data like any other IP packet.

The receiving router receives the packet with its own address in the header,

strips the tunnel header, and finds the data that was tunneled, which is thenprocessed.

Exactly what it does with that data depends on what it is.

For example, if it is another IP packet, it would then be forwarded on like a

normal IP packet.

Figure : An ATM circuit is emulated by a tunnel.

Forwarding ATM Cells in MPLS VPN


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An MPLS tunnel is similar to IP tunnel, except that the tunnel

header consists of an MPLS header rather than an IP header.

Assume we want to tunnel ATM cells from one router to anotheracross a network of MPLS-enabled routers.

The goal is to emulate an ATM virtual circuit, ie,

Cells arrive at the entrance, or head, of the tunnel on a certain input port

with a certain VCI Cells should leave the tail end of the tunnel on a certain output port and

a different VCI.

This can be accomplished by configuring the head and tail

routers as follows:

The head router needs to be configured with the incoming port, the

incoming VCI, the demultiplexing label for this emulated circuit, and the

address of the tunnel end router;

The tail end router needs to be configured with the outgoing port, the

outgoing VCI, and the demultiplexing label.




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1. An ATM cell arrives on the

designated input port with the its

VCI value (101 in this eg.)

2. The head router attaches the de-multiplexing label to inform the

end-point router what to do with

the packet.

3. The head router then attaches a

second label, which is the tunnel

label that will get the packet to thetail router.

4. Routers between the head and tail

forward the packet using only the

tunnel label.

5. The tail router removes the tunnel

label, finds the demultiplexinglabel, and recognizes the emulated

circuit.

6. The tail router modifies the ATM

VCI to the correct value (202 in this

case) and sends it out the correct

port.

Layer 3 VPNs


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The previous example has a stack of two labels

You can have larger stacks of labels in the header.

MPLS is also used to support layer 3 VPNs. The packet being carried is an IP packet

Hence the name layer 3 VPNs

A common service provider (see picture next ..)

Has many customers

Each customer has many sites

These sites are linked with tunnels to appear to be one large

Internetwork

Each customer can only reach its own sites The customer is isolated from the rest of the Internet and from other

customers

Customer A cant sent packets directly to customer B, and vice versa.

Customer A can even use IP addresses that have also been used by customer B.

Layer 3 VPN s


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multicast addresses

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