Multicast traffic lies between the extremes of unicast (one
source, one destination) and broadcast (one source, all destinations).
Multicast is a “one source, many destinations” method
of traffic distribution, meaning that the destinations needing to
receive the information from a particular source receive the traffic
stream.
IP network destinations (clients) do not often communicate directly
with sources (servers), so the routers between source and destination
must be able to determine the topology of the network from the unicast
or multicast perspective to avoid routing traffic haphazardly. The
multicast router must find multicast sources on the network, send
out copies of packets on several interfaces, prevent routing loops,
connect interested destinations with the proper source, and keep the
flow of unwanted packets to a minimum. Standard multicast routing
protocols provide most of these capabilities.
This chapter contains the following topics. For more information
about multicast, see the JUNOS Multicast Protocols Configuration Guide. For configuration
instructions, see Configuring a Multicast Network.
To understand multicast routing, you must be familiar with the
terms defined in Table 155. See Figure 67 for a general view of some
of the elements commonly used in an IP multicast network architecture.
Table 155: Multicast
Terms
Term
Definition
administrative scoping
Multicast routing strategy that limits the routers and interfaces
used to forward a multicast packet by reserving a range of multicast
addresses.
Auto-RP
Cisco multicast routing protocol that allows sparse-mode routing
protocols to find rendezvous points (RPs) within a routing domain.
bootstrap router (BSR)
Multicast mechanism that allows routers running PIM sparse mode
to find rendezvous points (RPs) within a routing domain.
branch
Part of a multicast network that is formed when a leaf subnetwork
is joined to the multicast distribution tree. Branches with no interested
receivers are pruned from the tree so that multicast packets are no
longer replicated on the branch.
broadcast routing protocol
Protocol that distributes traffic from a particular source to
all destinations.
dense mode
Multicast routing mode appropriate for LANs with many interested
receivers.
Designated Router (DR)
Router on a subnet that is selected to control multicast routes
for the sources and receivers on the subnet. When more than one multicast-enabled
router is located on a subnet, the selected DR is the router with
the highest priority. If the DR priorities match, the router with
the highest IP address is selected as the DR.
The source’s DR sends PIM register messages from the source
network to the rendezvous point (RP). The receiver’s DR sends
PIM join and PIM prune messages from the receiver network toward the
RP.
Distributed multicast routing protocol that dynamically generates
IP multicast distribution trees using reverse-path multicasting (RPM)
to forward multicast traffic to downstream interfaces.
distribution tree
Path linking multicast receivers (listeners) to sources. The
root of the tree is at the source, and the branches connect subnetworks
of interested receivers (leaves). Multicast packets are replicated
only where a distribution tree branches. To shorten paths to a source
at the edge of a network, sparse mode multicast protocols can use
a shared distribution tree located more centrally
in the network backbone.
downstream interface
Interface on a multicast router that is leading toward the receivers.
You can configure all the logical interfaces except one as downstream
interfaces.
group address
Multicast destination address. A multicast network uses the
Class D IP address of a logical group of multicast receivers
to identify a destination. IP multicast packets have a multicast group
address as the destination address and a unicast source address.
Internet Group Management Protocol (IGMP)
Multicast routing protocol that runs between receiver hosts
and routers to determine whether group members are present. Services
Routers support IGMPv1, IGMPv2, and IGMPv3.
leaf
IP subnetwork that is connected to a multicast router and that
includes at least one host interested in receiving IP multicast packets.
The router must send a copy of its multicast packets out on each interface
with a leaf, and its action is unaffected by the number of leaves
on the interface.
listener
Another name for a receiver in a multicast network.
multicast routing protocol
Protocol that distributes traffic from a particular source to
only the destinations needing to receive it. Typical multicast routing
protocols are the Distance Vector Multicast Routing Protocol (DVMRP)
and Protocol Independent Multicast (PIM).
Multicast Source Discovery Protocol (MSDP)
Multicast routing protocol that connects multicast routing domains
and allows them to find rendezvous points (RPs).
Pragmatic General Multicast (PGM)
Special protocol layer for multicast traffic that can be used
between the IP layer and the multicast application to add reliability
to multicast traffic.
Protocol Independent Multicast (PIM) protocol
Protocol-independent multicast routing protocol that can be
used in either sparse or dense mode. In sparse mode, PIM routes to
multicast groups that might span WANs and interdomain Internets. In
dense mode, PIM is a flood-and-prune protocol.
pruning
Removing from a multicast distribution tree branches that no
longer include subnetworks with interested hosts. Pruning ensures
that packets are replicated only as needed.
reverse-path forwarding (RPF)
Multicast routing strategy that allows a router to receive packets
through an interface if it is the same interface a unicast packet
uses as the shortest path back to the source.
rendezvous point (RP)
Core router operating as the root of a shared distribution tree
in a multicast network.
Session Announcement Protocol (SAP)
Multicast routing protocol used with other multicast protocols—typically
Session Description Protocol (SDP)—to handle session conference
announcements.
Session Description Protocol (SDP)
Session directory protocol that advertise multimedia conference
sessions and communicates setup information to participants who want
to join the session.
shortest-path tree (SPT)
Multicast routing strategy for sparse mode multicast protocols.
SPT uses a shared distribution tree rooted in the network backbone
to shorten paths to sources at the edge of a network.
source-specific multicast (SSM)
Service that allows a client to receive multicast traffic directly
from the source, without the help of a rendezvous point (RP).
sparse mode
Multicast routing mode appropriate for WANs with few interested
receivers.
unicast routing protocol
Protocol that distributes traffic from one source to one destination.
upstream interface
Interface on a multicast router that is leading toward the source.
To minimize bandwidth use, configure only one upstream interface on
a router receiving multicast packets.
Multicast Architecture
Multicast-capable routers replicate packets on the multicast
network, which has exactly the same topology as the unicast network
it is based on. Multicast routers use a multicast routing protocol
to build a distribution tree that connects receivers (also called
listeners) to sources.
Upstream and Downstream Interfaces
A single upstream interface on the router leads toward the source
to receive multicast packets. The downstream interfaces on the router
lead toward the receivers to transmit packets. A router can have as
many downstream interfaces as it has logical interfaces, minus 1.
To prevent looping, the router's upstream interface must never receive
copies of its own downstream multicast packets.
Subnetwork Leaves and Branches
On a multicast router, each subnetwork of hosts that includes
at least one interested receiver is a leaf on the multicast distribution
tree (see Figure 67). The router
must send out a copy of the IP multicast packet on each interface
with a leaf. When a new leaf subnetwork joins the tree, a new branch
is built so that the router can send out replicated packets on the
interface. The number of leaves on an interface does not affect the
router. The action is the same for one leaf or a hundred.
A branch that no longer has leaves is pruned from the distribution
tree. No multicast packets are sent out on a router interface leading
to an IP subnetwork with no interested hosts. Because packets are
replicated only where the distribution tree branches, no link ever
carries a duplicate flow of packets.
In IP multicast networks, traffic is delivered to multicast
groups based on an IP multicast group address instead of a unicast
destination address. The groups determine the location of the leaves,
and the leaves determine the branches on the multicast network.
Figure 67: Multicast Elements in an
IP Network
Multicast IP Address Ranges
Multicast uses the Class D IP address range (224.0.0.0 through 239.255.255.255). Multicast addresses usually have
a prefix length of /32, although other prefix lengths are
allowed. Multicast addresses represent logical groupings of receivers
and not physical collections of devices, and can appear only as the
destination in an IP packet, never as the source address.
Notation for Multicast Forwarding States
The multicast forwarding state in a router is usually represented
by one of the following notations:
(S,G) notation—S refers to the unicast IP address
of the source for the multicast traffic and G refers to the particular
multicast group IP address for which S is the source. All multicast
packets sent from this source have S as the source address and G as
the destination address.
(*, G) notation—The asterisk (*) is a wildcard for
the address of any multicast application source sending to group G.
For example, if two sources are originating exactly the same content
for multicast group 224.1.1.2, a router can use (*, 224.1.1.2) to represent the state of a router forwarding traffic from both
sources to the group.
Dense and Sparse Routing Modes
To keep packet replication to a minimum, multicast routing protocols
use the two primary modes shown in Table 156.
Caution:
A common multicast guideline is not to run dense mode
on a WAN under any circumstances.
Table 156: Primary Multicast Routing Modes
Multicast Mode
Description
Appropriate Network for Use
Dense mode
Network is floodedwith traffic on all
possible branches, then pruned back as branches explicitly (by message)
or implicitly (time-out silence) eliminate themselves.
LANs—Networks in which all possible subnets are likely
to have at least one receiver.
Sparse mode
Network establishes and sends packets only on branches that
have at least one leaf indicating (by message) a need for the traffic.
WANs—Network in which very few of the possible receivers
require packets from this source.
Strategies for Preventing Routing Loops
Routing loops are disastrous in multicast networks because of
the risk of repeatedly replicated packets, which can overwhelm a network.
One of the complexities of modern multicast routing protocols is the
need to avoid routing loops, packet by packet, much more rigorously
than in unicast routing protocols. Three multicast strategies—reverse-path
forwarding (RPF), shortest-path tree (SPT), and administrative scoping—help
prevent routing loops by defining routing paths in different ways.
Reverse-Path Forwarding for Loop Prevention
The router's multicast forwarding state runs more logically
based on the reverse path, from the receiver back to the root of the
distribution tree. In reverse-path forwarding (RPF), every multicast
packet received must pass an RPF check before it can be replicated
or forwarded on any interface. When it receives a multicast packet
on an interface, the router verifies that the source address in the multicast IP packet is the destinationaddress for a unicast IP packet back to the source.
If the outgoing interface found in the unicast routing table
is the same interface that the multicast packet was received on, the
packet passes the RPF check. Multicast packets that fail the RPF check
are dropped, because the incoming interface is not on the shortest
path back to the source. Routers can build and maintain separate tables
for RPF purposes.
Shortest-Path Tree for Loop Prevention
The distribution tree used for multicast is rooted at the source
and is the shortest-path tree (SPT), but this path can be long if
the source is at the periphery of the network. Providing a shared tree on the backbone as the distribution tree locates
the multicast source more centrally in the network. Shared distribution
trees with roots in the core network are created and maintained by
a multicast router operating as a rendezvous point (RP), a feature
of sparse mode multicast protocols.
Administrative Scoping for Loop Prevention
Scoping limits the routers and interfaces that can forward a
multicast packet. Multicast scoping is administrative in the sense that a range of multicast addresses is reserved for
scoping purposes, as described in RFC 2365, Administratively
Scoped IP Multicast. Routers at the boundary must filter
multicast packets and ensure that packets do not stray beyond the
established limit.
Multicast Protocol Building Blocks
Multicast is not a single protocol, but a collection of protocols
working together to form trees, prune branches, locate sources and
groups, and prevent routing loops:
Distance Vector Multicast Routing Protocol (DVMRP) and
Protocol Independent Multicast (PIM) operate between routers. PIM
can operate in dense mode and sparse mode.
Three versions of the Internet Group Management Protocol
(IGMP) run between receiver hosts and routers.
Several other routing mechanisms and protocols enhance
multicast networks by providing useful functions not included in other
protocols. These include the bootstrap router (BSR) mechanism, Auto-RP
protocol, Multicast Source Discovery Protocol (MSDP), Session Announcement
Protocol (SAP) and Session Discovery Protocol (SDP), and Pragmatic
General Multicast (PGM) protocol.
Dense-mode-only protocol that uses the flood-and-prune or implicit
join method to deliver traffic everywhere and then determine where
the uninterested receivers are. DVRMP uses source-based distribution
trees in the form (S,G) and builds its own multicast routing tables
for RPF checks.
Not appropriate for large-scale Internet use.
PIM dense mode
Sends an implicit join message, so routers
use the flood-and-prune method to deliver traffic everywhere and then
determine where the uninterested receivers are.
PIM dense mode uses source-based distribution trees in the form
(S,G), and also supports sparse-dense mode, with mixed sparse and
dense groups. Both PIM modes use unicast routing information for RPF
checks.
Most promising multicast protocol in use for LANs.
PIM sparse mode
Sends an explicit join message, so routers
determine where the interested receivers are and send join messages
upstream to their neighbors, building trees from receivers to a rendezvous
point (RP) router, which is the initial source of multicast group
traffic.
PIM sparse mode builds distribution trees in the form (*,G),
but migrates to an (S,G) source-based tree if that path is shorter
than the path through the RP router for a particular multicast group's
traffic. Both PIM modes use unicast routing information for RPF checks.
Most promising multicast protocol in use for WANs.
PIM source-specific multicast (SSM)
Enhancement to PIM sparse mode that allows a client to receive
multicast traffic directly from the source, without the help of a
rendezvous point (RP).
Used with IGMPv3 to create a shortest-path tree between receiver
and source.
IGMPv1
The original protocol defined in RFC 1112, Host Extensions
for IP Multicasting. IGMPv1 sends an explicit join message
to the router, but uses a time-out to determine when hosts leave a
group.
IGMPv2
Defined in RFC 2236, Internet Group Management Protocol,
Version 2. Among other features, IGMPv2 adds an explicit
leave message to the join message.
Used by default.
IGMPv3
Defined in RFC 3376, Internet Group Management Protocol,
Version 3. Among other features, IGMPv3 optimizes support
for a single source of content for a multicast group, or source-specific multicast (SSM).
Used with PIM SSM to create a shortest-path tree between receiver
and source.
BSR
Auto-RP
Allow sparse-mode routing protocols to find rendezvous points
(RPs) within the routing domain (autonomous system, or AS). RP addresses
can also be statically configured.
MSDP
Allows groups located in one multicast routing domain to find
rendezvous points (RPs) in other routing domains. MSDP is not used
on an RP if all receivers and sources are located in the same routing
domain.
Typically runs on the same router as PIM sparse mode rendezvous
point (RP).
Not appropriate if all receivers and sources are located in
the same routing domain.
SAP and SDP
Display multicast session names and correlate the names with
multicast traffic. SDP is a session directory protocol that advertises
multimedia conference sessions and communicates setup information
to participants who want to join the session. A client commonly uses
SDP to announce a conference session by periodically multicasting
an announcement packet to a well-known multicast address and port
using SAP.
PGM
Special protocol layer for multicast traffic that can be used
between the IP layer and the multicast application to add reliability
to multicast traffic. PGM allows a receiver to detect missing information
in all cases and request replacement information if the receiver application
requires it.
