Download Group Address

11.1 Introduction
IP multicast refers to the delivery of IP datagrams to a set of hosts in a
group (multicast group). These hosts do not have to be on the same
physical network but locate across the internet.
Membership in an IP multicast group is dynamic. A host may join or leave
at any time. A host can also participate more multicast groups than one.
Members in a multicast group can choose to send/receive/both
messages to the group. Non-members can send to multicast groups.
Multicast within a physical network can allocate a hardware address for
multicast purposes. Multicast across the internet requires multicast
capable routers to deliver multicast datagrams.
This chapter discusses some protocols for IP multicasting.
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11.2 IP Multicast Address and Handling
•
•
•
•
•
Multicast group addresses (class D) range from 224.0.0.0 through to
239.255.255.255.
224.0.0.0 is reserved and 224.0.0.1 is assigned to all IP multicast
groups including routers and hosts. 224.0.0.2 is for all multicast
routers. There are also some well-known groups like well-known
TCP port numbers. Other groups are formed on temporary basis.
They cease to exist when membership reaches zero.
Multicast addresses can only exist in the destination portion of the IP
datagram, they are not valid in source, source route option, etc.
Multicast group members can choose not to send nor receive, send
but not receive, both send and receive IP multicast.
IP multicast groups are managed by specific networks. A host can
choose to join a multicast group within a physical network or across
a number of networks.
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11.3 Internet Group Management Protocol (IGMP)
• RFC1112(v1-1989)/2236(v2-1997)/3376(v3-2002) This is a lower
layer (IP) protocol like ICMP and uses IP to carry messages.
• Multicast hosts and routers use IGMP to communicate group
membership information.
• IGMP has a two fold purpose:
– Local routers periodically poll hosts on the local network with the
address of 224.0.0.1, to determine whether any host remains as
member of a multicast group. Multicast group hosts each will have
a small random delay before responding to the poll message, to
avoid network congestion. Members of the same group will not
respond if a responding message has already been sent.
– Hosts respond IGMP message to the multicast address to declare
their membership. Local multicast routers will arrange necessary
routing. (IP protocol type 2 and TTL=1)
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• Multicast hosts run a three-state mechanism:
• IGMPv2 (RFC2236) message format:
0
1
2
3
4 (byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Type
| Max Resp Time|
Checksum
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Group Address
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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• Type 0x11 is membership query, this can be general (group address=0)
or for a specific group.
Type 0x12 and 0x16 are membership report (response) for v1 and v2.
Type 0x17 is leave group message, this improves the response time for
a router to notice that all members have left a multicast group.
• A Leave group message is multicasted to 224.0.0.2 immediately after
each time a host leaves a multicast group. The querier router will
multicast a type 1 query message to see any host is still left in that
particular group.
• Max response time field is meaningful only in message Type 0x11 and
specifies the maximum allowed time before a response report is sent.
• The Checksum field is applied to the whole IP payload.
• Group Address field is meaningful according to the type field of IGMP
messages.
• Other types of messages and additional code fields are added in
IGMPv3, such as type 0x13 for DVMRP messages.
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11.4 Multicast Message Forwarding
• Apart from multicast capable routers, a tree shaped path must be
formed for the datagrams to be distributed.
• In the case of source-based multicast routing protocols (DVMRP,
MOSPF), a separate tree route is calculated for each source/destination
combination. This can be based on hop counts or link cost.
• A tree route means there is exactly one path from the source to each
group member.
• For shared tree protocols (PIM sparse, CBT), a single tree is built for a
multicast group. One router is selected as the root for the group’s tree.
This is referred to as the Rendezvous Point (RP) or the core.
• Shared tree based algorithms are more scalable and require routers to
maintain less information.
• The following example is from “OSPF Anatomy of an Internet Routing
Protocol”, John Moy, Addison- Wesley, 1999.
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• A multicast network with three sources
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Three source-based trees.
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• One shared tree for three sources, with R4 as the Rendezvous Point.
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• Shared-tree protocols have some problems which prevented their
application:
– Less efficient paths. Longer paths than those of source based trees,
which are calculated on costs and hop counts.
– Traffic concentration. On the RP and the few links around it. This
may create congestion and bandwidth problems.
– Robustness. The failure of the RP will lead to the failure of the
whole group.
– RP selection. Manual or automatic.
• Each multicast router has relevant multicast routing tables for the
multicast groups.
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11.5 Multicast Backbone (Mbone)
• MBONE was created in 1992 to provide multicast service. (first for
IETF meetings and later for some other events.)
• MBONE is the test ground for multicast protocols and applications.
• MBONE is not yet available for public at large, mainly due to the
complexity of routers running multicast facilities.
• MBONE routers are typically Unix Workstations running mrouted
program (an implementation of DVMRP). This is because most
Internet routers do not have multicast functions or having them turned
off.
• The MBONE routers are connected with unicast tunnels, to get
through Internet sections not supporting multicast.
• The core of MBONE runs DVMRP.
• The MBONE tunnels can carry repetitive data traffic in one segment
for many destinations.
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• The construction of many source trees may also pose high traffic
demands.
• MBONE applications can filter unwanted multicast traffic by the
setting a threshold of IP TTL that is allowed through a tunnel. The
initial TTL value for many multicast applications are not 255. A TTL
threshold can stop undesirable multicast traffic.
11.6 Overview of Multicast Routing Protocols
• Source-Based Trees:
– Distance Vector Multicast Routing Protocol (DVMRP)
(Broadcast-and prune)
– Protocol Independent Multicast, Dense Mode (PIM Dense)
(Broadcast-and prune)
– Multicast OSPF, MOSPF (Domainwide reports)
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• Shared Tree:
– PIM Sparse
– CBT (Core Based Trees)
• Broadcast-and-prune protocols for the source-based trees include all
network segments to start with. When the first multicast datagram
arrives at a router, the router may realise whether there is/is not any
member in its leaf. In latter case it will send a prune message to the
upstream router.
• Domainwide reports protocols broadcast their group membership
information throughout the routing domain to establish the source trees
(discussed later with MOSPF).
• DVMRP is used as the main protocol for MBONE. The UNIX
mrouted program implemented and modified some of its aspects.
• DVMRP uses IGMP type 0x13 message as encapsulation. The second
byte of IGMP header indicates the DVMRP message type.
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• DVMRP messages include Probes (for neighbor), Route Reports (for
broadcast-and-prune), tree maintenance and router monitoring
messages.
• DVMRP routers periodically broadcasts to its neighbors a list of
sources and the distance from these sources to the router, this is similar
to RIP.
• A DVMRP router will choose the minimum hop-count router as its
previous hop from the source.
• PIM Dense can be thought of as DVMRP without the routing updates.
• PIM Dense was believed as appropriate when multicast group
members are densely populated throughout the routing domain.
• PIM Sparse is a shared-tree protocol that shares packets with PIM
Dense and can be switched from PIM Dense. It contains unique
messages for the selection of RP, and also DR for each network
segment.
• Both PIM Dense and Sparse have been implemented for some
MBONE edge routing.
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• CBT is not yet deployed in the Internet. It is also directly on IP and
uses DR for each network segment to contact the core of the shared
tree.
• MOSPF will be discussed in the following section.
11.7 MOSPF
• The protocol OSPF itself uses its own multicast mechanism to
communicate LSA and other messages. In order to provide user
multicast, OSPF can be extended to MOSPF.
• MOSPF has been deployed in private internets with OSPF.
• It is easier to have MOSPF in an OSPF unicast routing domain than
DVMRP, since the protocols are built on different principles (Distance
Vector vs. Link State SPF).
• MOSPF introduces a new type of LSA, group-membership-LSA, to
the LSDB for the calculation of multicast paths.
• MOSPF can route around multicast datagrams from routers incapable
of multicast routing.
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11.7.1 A multicast example
This example
is from “OSPF
Anatomy of an
Internet
Routing
Protocol”,
John Moy,
AddisonWesley, 1999.
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• S1 is a multicast source (eg. for radio) and G1s are the group members.
• When the G1 member on 128.186.4.0/24 tune in, it sends IGMP onto
the network segment. Assume router E is the DR, it originates a groupmembership-LSA, and flood it throughout the MOSPF domain and
inform other routers that 128.186.4.0/24 has a G1 member.
• When router A receives the next G1 packet from S1, it will calculate
the path and forward the packet to the FDDI ring 128.186.3.0. Routers
B, C, D and F will all calculate paths to 128.186.4.0/24, with only D
forwarding the packet to G. Router G will also do the routing
calculation and finally forward the segment on 128.186.4.0.
• When the G1 member of 128.186.5.0/24 tunes in, router F will
originates the LSA labeling itself rather than the network as
destination. This is because the network is a stub network.
• When the G1 member of 128.186.6.0/24 tunes in, router F will not
originates an LSA again and flood the network. It simply forward G1
packets to that network.
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11.7.2 Group-Membership-LSAs and routing calculation
• IGMP Host Membership Report is sent from a group member. IGMP
Membership Queries are sent by the Querier router (normally DR).
Hosts delay Membership Report for a random time, if there are other
replies before its reply, it will not reply.
• If there is no reply after a period of time (leave latency), or Leave
Group messages are received in IGMPv2. The router will realise that
there is no member in its net segment any more.
• A group-membership-LSA reports membership for the multicast IP
address, in the form of router-LSA or network-LSA. Different cases
with stub and multi-homed networks (why?)
• Multicast routers calculate routes on demand, different from unicast.
• The calculation is stored as an entry in the router’s multicast
forwarding cache (multicast routing table), only the router’s position in
the path and next hop is important in relation to a multicast group.
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• The forwarding cache stores two types of information:
– Previous hop (a router or network) from which a matching
multicast datagram is received.
– Next hop (or the router interface, only one) that it should forward
the matching datagram.
– The above ensures that only one copy the datagram reaches the
destination network.
– This cache will be modified when new group-member-LSA
arrives.
• MOSPF routing calculation uses the same Dijkstra’s algorithm for
SPF, as in unicast. The differences are:
– Unicast calculation has the router itself as the root of tree for
datagram forwarding. Each router does a different calculation.
– Multicast calculation always take the data source as the root, all
routers on the path for the same source do the same calculation.
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– Unicast calculation allows equal cost multipath for load balancing,
multicast must prune multipath to avoid delivery of multi copies of
a datagram.
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11.7.3 Hierarchical multicast in MOSPF
• MOSPF uses OSPF areas and hierarchy for routing efficiency.
• The concept of border and internal routers are the same as unicast.
• One disadvantage of hierarchy is suboptimal routes in unicast. In
multicast, this means unnecessary forwarding (which will be discarded
later).
• Multicast group members in nonbackbone areas is advertised by the
area border router into the backbone.
• Internal routers of nonbackbone areas do not know the membership
information within other areas. They can only forward multicast
packets to their border routers (wildcard multicast receivers) in default.
• Border routers forward multicast messages to backbone area first,
before the messages are distributed.
• Route calculation can be difficult between areas, the border routers in
one area (if more than one) may be advertising the same network(s)
due to the route aggregation in summary LSAs.
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• In this case, one router may be dedicated to handle incoming traffic for
a group or some groups of multicast. Other routers discard messages
belong to those groups.
11.7.3 Other issues of MOSPF
• MOSPF can identify non-multicast routers in the network by checking
the router-LSA. Route calculation can skip them.
• When mixing multicast and non-multicast routers in a LAN, a
multicast router should be selected as the DR.
• Inter-working with DVMRP (for MBONE). MOSPF acts like an IGP,
which imports information from and export to DVMRP.
• DVMRP treats the whole MOSPF domain as a LAN segment.
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