Transcript Multicasting and Multicast Routing Protocols

Introduction to Multicast
Routing Protocols
Shortest Path Trees
• Optimal Routing: Shortest Path Trees
– The process of intradomain routing
attempts to find the shortest path
between a router and another destinations
resulting in shortest path tree (SPT)
– The root of the tree is the source and the
leafs are the potential destinations
– However, the number of trees and the
formation of the trees in unicasting and
multicast routing are different
Shortest Path Trees in Unicast
• In unicast routing, when a router receives a
packet to forward, it needs to find the
shortest path to the destination of the packet
• The next-hop entry in the routing table
corresponding to the destination is the start of
the shortest path tree
• Each line of the routing table is a shortest
path; the whole routing table is a shortest path
• Each router needs only one shortest path tree
to forward a packet; however, each router has
its own shortest path tree
Unicast Routing (cont.)
Shortest Path Trees in Multicast
• In a multicast routing, a multicast packet may
have destinations in more than one network
• Forwarding of a single packet to members of
a group requires a shortest path tree
• If we have n groups, we may need n shortest
path trees
• Two approaches have been used to solve the
problem: source based trees and groupshared trees
Concept of Multicast Routing
• The concept of multicast routing can be
described from the broadcast routing in
the network
• The broadcast routing algorithm can be
based on different strategies such as
uncontrolled flooding and controlled
flooding
Uncontrolled Flooding
• A router receives a broadcast packet and sends it out
from every interface except the one from which it
was received
• However, uncontrolled flooding create loops which
means a packet that has left the router may come
back again from another interface or the same
interface and be forwarded again
• If a router is connected to more than two other
routers, it will create and forward multiple copies of
the broadcast packet, each of which will create
multiple copies of itself at other routers with more
than two neighbors, and so on  Broadcast Storm
Controlled Flooding
• The key to avoiding a broadcast storm is for a
router to choose when to flood a packet and
when not to flood a packet
• In sequence-number-controlled flooding, a
source puts its address and sequence number
into a broadcast packet
• Each router maintains a list of the source
address and sequence number of each
broadcast packet it has already received,
duplicated, and forwarded
Controlled Flooding (cont.)
• When a router receives a broadcast
packet, it first checks whether the
packet is in the list
• If so, the packet is dropped; if not, the
packet is duplicated and forwarded to
all the neighbors (The Gnutella protocol
uses this strategy to broadcast queries
in its overlay network at the application
layer level)
Reverse Path Forwarding (RPF)
• RPF is another controlled flooding strategy
• In RPF, a router forwards only the copy
arrived on the link that is on its own
shortest path back to the source of packet
• To find this copy, A router consults its
unicast routing table as though it wants to
send a packet to the source address
• Since the routing table tells the router
the next hop router address, if the packet
has just come from the address defined it
means the packet has traveled the
shortest path from the source
Reverse Path Forwarding (RPF)
(cont.)
Spanning-Tree Broadcast
• While sequence-number-controlled flooding
and RPF avoid broadcast storms, they don’t
completely avoid the transmission of
redundant broadcast packets
• The solution is spanning tree—a tree that
contains each and every node in a graph
• When a source node wants to send a
broadcast packet, it sends the packet out on
all of the incident links that belong to the
spanning tree
Spanning-Tree Broadcast (cont.)
• A node receiving the packet then forwards
the packet to all its neighbors in the spanning
tree
Construction of a Spanning Tree
• The main complexity associated with the
spanning-tree approach is the creation and
maintenance of it
• One simple algorithm is using center-based
approach in which a center node is defined
and routers then send unicast tree-join
message addressed to the center node
• A tree-join message is forwarded using
unicast routing until it either arrives at a
router that already belongs to the spanning
tree or arrives at the center
Construction of a Spanning Tree (cont.)
• In either case, the path that the treejoin message has followed defines the
branch of the spanning tree between
the edge router that initiated the treejoin message and the center
• One can think of this new path as being
grafted onto the existing spanning tree
Construction of a Spanning Tree (cont.)
Multicast Routing Algorithm
• As mentioned that two approaches have been
used to solve the problem: source based trees
and group-shared trees, both of which we
have studied in the context of broadcast
routing
• The two approaches differ according to
whether a single group-shared tree is used to
distribute the traffic for all senders in the
group, or whether a source-specific routing
tree is constructed for each individual sender
Group-Shared Tree
• In this approach, instead of each router having m
shortest path trees, only one designated router
called the center core or rendezvous router takes
the responsibility of distributing multicast traffic
as a center-based approach
• The core has m shortest path trees in its routing
table but the rest of the routers in the domain
have none
• If a router receives a multicast packet, it
encapsulates the packet in a unicast packet and
sends it to the core
• The core router removes the multicast packet from
its capsule, and consults its routing table to route
the packet
Group-Shared Tree
Source-Based Tree
• In the source-based tree approach, a
multicast routing tree is constructed for
each source in the multicast group
• In practice, an RPF algorithm (with source
node x) is used to construct a multicast
forwarding tree for multicast originating at
source x
• However, the RPF broadcast algorithm
requires a bit of modifying for use in
multicast by introducing pruning and
grafting
Reverse Path Forwarding: example
S: source
R1
LEGEND
router with attached
group member
R4
R2
R5
R3
R6
R7
router with no attached
group member
datagram will be
forwarded
datagram will not be
forwarded
Pruning in PRF
• In pruning, a multicast router that receives
multicast packets and has no attached node
joined to that group will send a prune message
to its upstream router
• If a router receives prune message from each
of its downstream routers, then it can stop
sending multicast packets for this group
through that interface
• If this router receives prune messages from
all down stream routers, it in turn sends a
prune message to its upstream router
Reverse Path Forwarding: pruning
• forwarding tree contains subtrees with no multicast
group members
– no need to forward datagrams down subtree
– “prune” msgs sent upstream by router with no
downstream group members
LEGEND
S: source
R1
router with attached
group member
R4
R2
P
R5
R3
R6
P
R7
P
router with no attached
group member
prune message
links with multicast
forwarding
Grafting in RPF
• What if a leaf router has sent a prune
message but later on realizes that one
of its node is again interested in
receiving the multicast packet
• It can send a graft message which
forces the upstream router to resume
sending the multicast message
Multicast Routing Protocols
in the Internet
• During the last few decades, several multicast
routing protocols in the Internet have emerged
• Some of these protocols are extensions of unicast
routing protocols; some are totally new
Multicast Link State Routing
• Multicast link state routing is a direct
extension of unicast routing and uses a
source-based tree approach
• Each router advertises every group
which has any loyal member on the link
and the information about the group
comes from IGMP
• When a router receives all
advertisements, it creates n (n is the
number of groups) topologies, from
which n shortest path trees are made
using Dijkstra’s algorithm
Multicast Link State Routing
(cont.)
• The only problem with this protocol is
the time and space needed to create
and save the many shortest path trees
• The solution is to create the trees only
when needed and the result can be
cached in case there are additional
packets for that destinations
Multicast Open Shortest Path First
(MOSPF)
• MOSPF protocol is an extension of the OSPF
protocol that uses multicast link state routing
to create source-based trees
• The protocol requires a new link state update
packet to associate the unicast address of a
host with the group address
• This packet is called the group- membership
LSA
• MOSPF is a data-driven protocol; the first
time an MOSPF router sees a datagram with a
given source and group address
Distance Vector Multicast Routing
(DVMRP)
• DVMRP implements source-based trees with
RPF, pruning, and grafting
• DVMRP uses a distance vector algorithm that
allows each router to compute the outgoing
link (next hop) that is on its shortest path
back to each possible source
• The information is then used in the RPF
algorithm as discussed
• In addition to computing next-hop
information, DVMRP also computes a list of
dependent downstream routers for pruning
purposes
Distance Vector Multicast Routing
(DVMRP) (cont.)
• A DVMRP prune message contains a
prune lifetime (a default value is two
hours) that indicates how long a pruned
branch will remain pruned before being
automatically restored
• DVMRP graft messages are sent by a
router to its upstream neighbor to
force a previously pruned branch to be
added back on to the multicast tree
Core-Based Tree (CBT) Protocol
• The CBT protocol is a group-shared protocol
that uses a core as the root of the tree
• The autonomous system is divided into regions
an a core (center router or rendezvous
router) is chosen for each region
• There is a procedure in this protocol
– Selecting the Rendezvous Router
– Formation of the Tree
– Sending Multicast Packets
Formation of the Tree
• After the rendezvous point is selected, every router
is informed of the unicast address of the selected
router
• Each router then sends a unicast join message to
show that it wants to join the group
• This message passes through all routes that are
located between the sender and the rendezvous
router
• Each intermediate router extracts the necessary
information from the message, such as the unicast
address of the sender and the interface, and
forwards the message to the next router in the path
• When the rendezvous router has received all join
messages from every member of the group, the tree
is formed
Formation of the Tree (cont.)
• Now every router knows its upstream router (the router that
leads to the root) and the downstream router (the router that
leads to the leaf)
• If a router wants to leave the group, it sends a leave message to
its upstream router
• The upstream router removes the links to that router from the
tree and forwards the message to its upstream router, and so on
Sending Multicast Packets
• After formation of the tree, any source can send a
multicast packet to all members of the group
• It simply sends the packet to the rendezvous router,
using the unicast address of the rendezvous router;
the rendezvous router distributes the packet to all
members of the group
Summary to CBT
•
In CBT, a packet is sent from the source to
members of the group following this
procedure:
1.
The source, which may or may not be part of the
tree, encapsulates the multicast packet inside a
unicast packet with the unicast destination
address of the core and sends it to the core
2. The core decapsulates the unicast packet and
forwards it to all interested interfaces
3. Each router that receives the multicast packet,
in turn, forwards it to all interested interfaces
Protocol Independent Multicast
(PIM)
• PIM is the name given to two
independent multicast routing protocols:
PIM, Dense Mode (PIM-DM) and PIM,
Sparse Mode (PIM-SM)
PIM-DM
• PIM-DM is used when there is a possibility
that each router is involved in multicasting
(dense mode)
• In this environment, the use of a protocol
that broadcasts the packet is justified
because almost all routers are involved in the
process
• PIM-DM is a source-based tree routing
protocol that uses RPF and pruning/grafting
strategies for multicasting
• Its operation is like DVMRP; however, it does
not depend on a specific unicating protocol
PIM-SM
• PIM-SM is used when there is a slight possibility that
each router is involved in multicasting (sparse mode)
• In this environment, the use of a protocol that
broadcasts the packet is not justified; a protocol
such as CBT that uses a group-shared tree is more
appropriate
• PIM-SM is a group-shared tree routing protocol that
has a rendezvous point (RP) as the source of the tree
• Its operation is like CBT; however, it is simpler
because it does not require acknowledgement from a
join message
• In addition, it creates a backup set of RPs for each
region to cover RP failures
PIM-SM (cont.)
• One of the characteristics of PIM-SM
is that it can switch from a groupshared tree strategy to a source-based
tree strategy when necessary
• This can happen if there is a dense area
of activity far from the RP. That area
can be more efficiently handled with a
source-based tree strategy instead of a
group-shared tree strategy
How to Evaluate a
Multicast Routing Protocol?
• Scalability. What is the amount of state required
in the routers by a multicast routing protocol? How
does the amount of state change as the number of
groups, or number of senders in a group, change?
• Reliance on underlying routing. To what extent
does a multicast protocol rely on information
maintained by an underlying unicast routing
protocol? We have seen solutions that range from
reliance on one specific underlying unicast routing
protocol (MOSPF), to a solution that is completely
independent (PIM), to a solution that implements
much of the same distance vector functionality
(DVMRP)
How to Evaluate a
Multicast Routing Protocol? (cont.)
• Excess (un-needed) traffic received. We
have seen solutions where a router receives
data only if it has an attached node in the
multicast group (MOSPF, PIM-SM) to solution
where the default is for a router to receive
all traffic for all multicast groups (DVMRP,
PIM-DM)
• Traffic concentration. The group-shared tree
approach tends to concentrate traffic on a
smaller number of links (those in the single
tree), whereas source-specific trees tend to
distribute multicast traffic more evenly
Delivery of Multicast Packets at
Data Link Layer
• Most LANs support physical multicast addressing
and Ethernet is one of them
• An Ethernet physical address is 48 bits long. If
the first 25 bits in an Ethernet address are
00000001 00000000 01011110 0, this identifies a
physical multicast address for the TCP/IP
protocol
• The remaining 23 bits can be used to define a
group by which the multicast router extracts the
lease significant 23 bits of a multicast IP address
and inserts them into multicast Ethernet address
Mapping class D to Ethernet physical address
LAN Switches with Multicast
Packets
• Switches that cannot understand multicast addresses
will broadcast traffic sent to a multicast group to all
the members of a LAN; in this case the system's
network card (and operating system) has to filter the
packets sent to multicast groups they are not
subscribed to
• There are switches that listen to multicast traffic
and maintain a state table of which network systems
are subscribed to a given multicast group
• This table is then used to forward traffic destined to
a given group only to a limited set of hosts (ports);
this is done through the use of IGMP snooping
LAN Switches with Multicast
Packets (cont.)
• Additionally, some switches with layer 3
capabilities can act as an IGMP querier
• In networks where there is no router
present (or enabled) to act as a
multicast router a switch might be used
to generate the needed IGMP messages
to get users to subscribe to multicast
traffic
WLAN with Multicast Packets
• An 802.11 wireless local area network will
handle multicast traffic differently,
depending on the configuration of 802.11
power-save mode, DTIM (Delivery Traffic
Indication Message), and beacon interval
settings
• If power-save mode is disabled, then access
points will deliver multicast traffic after each
beacon (default interval = 100ms, but it can
be adjusted)
WLAN with Multicasting (cont.)
• If power-save mode is enabled, the access
point will deliver multicast traffic after each
DTIM, which by default is every 1,2, or 3
beacon intervals in most access points
• As a result, the DTIM and beacon interval
settings should be adjusted for optimum
performance when implementing multicast in
wireless networks
Network with No Multicast Support
• Most WANs do not support physical multicast
address, thus to send a multicast packet through
these networks, a process called tunneling is used
• In tunneling, the multicast packet is encapsulated
in a unicast packet and sent through the network,
where it emerges from the other side as a
multicast packet