Upload
samantha-weaver
View
226
Download
1
Embed Size (px)
DESCRIPTION
Multicasting
Citation preview
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.
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
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
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
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
Reverse path multicating(RPM) RPM adds pruning and grafting to RPB to
create a multicast shortest path tree that supports dynamic membership changes
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
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
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
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
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
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
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
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
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
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