Multicasting A message can be unicast, multicast, or

broadcast. Let us clarify these terms as they relate to the Internet

Unicasting

In unicasting, the router forwards the received packet through only one of its interfaces.

Multicasting

In multicasting, the router may forward the received packet through several of its interfaces.

In multicasting communications there is one source and a group of destinations

In broadcasting there is one source but all of other hosts are the destinations

Multicasting and multiple unicasting Multicasting starts with one single packet from the

source that is duplicated by the router The destination address in each packet is the same

for all duplicates In multiple unicasting several packets starts from the

source. Destination address will be different in each packet

There may be multiple copies traveling between two routers---E.g. Email

There will be a delay between packets in MU

Multicasting versus multiple unicasting

Multicast Applications Multicasting has many applications today such

as access to distributed databases information dissemination teleconferencing distance learning

Multicast Routing In multicast routing, each involved router needs to

construct a shortest path tree for each group When a router receives a multicast packet it forwards

to different networksTwo types Source based tree Group shared tree

Source Based Tree In the source-based tree approach, each

router needs to have one shortest path tree for each group.

If the number of groups is m , each router needs to have m shortest path trees , one for each group

Source-based tree approach

Group Shared Tree In the group-shared tree approach, only the core

router, which has a shortest path tree for each group, is involved in multicasting.

Instead of each router having m shortest path tree, only one designated router called the centre core or rendezvous router, takes the responsibility of distributing multicast traffic

Group-shared tree approach

Taxonomy of common multicast protocols

Mulitcast link state routing Multicast link state routing uses the source-

based tree approach. Information about a group comes from

IGMP

Multicast Open Shortest Path First (MOSPF) Multicast Open Shortest Path First protocol is an

extension of OSPF protocol that uses multicast link state routing to create source based tree.

Multicast distance vector routing Multicast distance vector routing uses source based tree But router never makes the routing table When a router receives a multicast packet it forwards the

packet as though it is consulting a routing table.

Flooding A router receives a packet and without

even looking at the destination group address, send it out from every interface except the one from which it was received.

Reverse path forwarding(RPF) In RPF a router forwards only the copy

that has traveled the shortest path from the source to the router.

No loops But duplicate copies may receive

RPF

Figure 15.9 Problem with RPF

Reverse Path Broadcasting(RPB)

Figure 15.10 RPF versus RPB

Reverse path multicating(RPM) RPM adds pruning and grafting to RPB to

create a multicast shortest path tree that supports dynamic membership changes

RPF, RPB, and RPM

Distance Vector Multicast Routing Protocol (DVMRP) Is an implementation of multicast

distance vector routing. It is a source based routing protocol

CBT The Core-Based Tree (CBT) protocol is a

group-shared protocol that uses a core as the root of the tree. The autonomous system is divided into regions and a core (center router or rendezvous router) is chosen for each region.

Every router is informed of the unicast address of the selected rendezvous router

Each router then sends a unicast join message Intermediate router extracts information such as

unicast address of sender and interfaces through which it has passed

When all message received a tree is formed at rendezvous router

Group-shared tree with rendezvous router

Sending a multicast packet to the rendezvous router

In CBT, the source sends the multicast packet (encapsulated in a unicast packet) to the core router.

The core router decapsulates the packet and forwards it to all interested interfaces

If router wants to leave the group it sends a leave message to upstream router.

Difference 1) tree is first made and then pruned but in CBT

initial no tree, joining gradually makes the tree 2) made from the root up but CBT formed from

the leaves down

Host Configuration: BOOTP and DHCP

BOOTP The Bootstrap Protocol (BOOTP) is a client/server

protocol that configures a diskless computer or a computer that is booted for the first time.

BOOTP provides the IP address, subnet mask, the address of a default router, and the address of a name server.

Configuration file

RARP provides only IP address Both client and server should be on

same network

Operations BOOTP server issues a passive open command on UDP

port number 67 and waits for client A BOOTP client issues an active open command on port

number 68 This message is encapsulated in UDP datagram and in

turn encapsulated in IP datagram IP addresses of client and server will be all zeros and all

ones Server responds

Client and server on the same network

Client and server on two different networks

Use of UDP ports

Using TFTP In the reply message server defines the

path name of a file in which the client can find complete booting information.

Client then uses TFTP to obtain the rest of information

Error Control BOOTP requires UDP uses the checksum BOOTP client uses timers and a

retransmission policy if it does not receive the BOOTP reply to a request.

Timers will be set randomly

BOOTP packet format

Operation Code: 8 bit: 1 request 2 reply Hardware type: type of physical network for

Ethernet value is 1 Hardware Length: 8 bit: length of physical address Hop count: 8bit Specifies max no of hops a packet

can travel Transaction ID: 4 byte: integer Number of seconds: 16 bit: elapsed since the

client started to boot Client IP address : 4 byte Your IP address : 4 byte

Server IP address : 4 byte Gateway IP address: 4 byte: IP address of router Client hardware address: Physical address of client Server name: optional 64 byte : contains domain name of the

server Bootfile name: optional 128 byte: full path name of the bootfile Options: 64 byte: only in reply it is used

server uses a number called magic cookie in the format of IP address with the value: 99.130.83.99

Option format

Options for BOOTP

DHCP In BOOTP binding is predetermined The Dynamic Host Configuration Protocol

(DHCP) provides static and dynamic address allocation that can be manual or automatic

Static Address allocation

Backward compatible with BOOTP A DHCP server has a database that

statically binds physical address to IP address

Dynamic Address Allocation Use a second database When a DHCP client requests a

temporary IP address, the DHCP server goes to the pool of available(unused) IP address and assigns an IP address for a negotiable period of time.

First check the static database

Manual and automatic configuration Mapping the IP address to physical

address configuration In BOOTP it is manual In DHCP it has both manual and

automatic

Packet Format

DHCP packet

Flag 1 bit flag – to let the client specify a

forced broadcast reply from the server

Options Tag 53

Options Other option include

51 :Lease time58 : Renewal (T1) time value59 : Rebinding (T2) time value

Transition States DHCP client transitions from one state to

another depending on the message it receives of sends

DHCP transition diagram

Initializing state When the DHCP client first starts, it is in

the initializing state. Client broadcast the DHCPDISCOVER

message Using port 67

Selecting State After sending DHCPDISCOVER message client

goes to selecting state Servers respond with DHCPOFFER

Offers IP address, Lease time Default lease time is 1hour

Client select the offer of one server and send DHCPREQUEST message

Client goes to requesting state

Requesting State The client remains in the requesting

state until it receives a DHCPACK message from the server which creates a binding between the client physical address and its IP address.

After the receipt of DHCPACK client goes to bound state

Bound State Client uses the IP address until the lease

expires When 50% of the lease period is reached

, client sends another DHCPREQUEST to ask for renewal.

Then goes to renewal state

Renewal state Client remains in the renewal state until

one of two events happens1) It can receive a DHCPACK, which renews the lease agreementClient reset the timergoes back to bound state

2) No DHCPACK is received and 87.5 % of the lease time expires, the client goes to rebinding state

Rebinding state The client remains in rebinding state until of three

events happens1) Client receives a DHCPNACK2) The lease time expires3) DHCPACKIn the first two it goes to initializing state and try to

get another IP addressIn the third it goes to bound state

Exchanging messages