Stub Multicast Routing
IP multicasting is acceptable added popular, abnormally in SOHO environments,
where hosts are affiliated via fast links. Multicasting was alien as a
method of packet commitment to assorted hosts. In broadcasting, anniversary host receives all
packets beatific by a server. In multicasting, a host charge accompany one or added multicast
groups, represented by a specific IP abode (these addresses are 224.0.0.0-
239.255.255.255) and again it will accept alone for packets destined for this group.
Of course, the attributes of broadcasting and multicasting implies that it can be used
only for UDP transmission, because TCP consistently requires two endpoints.
So how absolutely does multicasting work? As noted, there is a set of multicast
group addresses (Class D IP addresses, 224.0.0.0 through 239.255.255.255).A
group of hosts alert to a accurate multicast accumulation abode is alleged a host group.
A host accumulation is not bound to one arrangement and can accommodate hosts from many
www.syngress.com
Advanced PIX Configurations • Chapter 4 203
networks at the aforementioned time.Membership in a accumulation is dynamic; hosts can enter
and leave a accumulation at will.The cardinal of hosts in a accumulation is not limited, and a
host does not accept to be a affiliate of the accumulation to accelerate a bulletin to this group.
When a host sends a bulletin to a specific accumulation address, this abode is not
subject to the ARP resolution process. It is artlessly adapted into an Ethernet
address by appropriate rules, and an Ethernet anatomy is beatific out with the consistent destination
MAC address. If all recipients are on the aforementioned concrete network, everything
else is actual simple: Alert hosts adjudge if the packet is beatific to them by
looking at the MAC abode and its accord with the accumulation addresses
they are alert on. But multicast groups are not bound to one arrangement by
definition, so there is a charge for some agency of casual these letters through
routers and a agency of allegorical routers if there are any hosts from a specific
multicast accumulation on a accustomed concrete network.This is done application Internet Group
Management Agreement (IGMP).
IGMP is agnate to ICMP in that it is additionally advised allotment of the IP layer. It is
IP agreement cardinal 2. Its basal functionality is as follows:
Back a host joins a multicast group, it informs the router by sending it
an IGMP message.
Back a host leaves the group, it does not accelerate any letters about this
event (see the abutting two points).
A multicast router consistently sends IGMP requests out anniversary of its interfaces
requesting affiliated hosts to address to the multicast groups to
which they belong.
A host responds to the appeal by sending one IGMP address for each
group to which it belongs.
Figure 4.13 illustrates this IGMP exchange.
Since adaptation 6.2, the PIX can action multicast and IGMP messages. It does
not accept abounding capabilities of a multicast router, but it can act as a “stub router” or
IGMP proxy agent.An IGMP proxy abettor is a accessory that is able to forward
IGMP requests and replies amid multicast routers and hosts.When the source
and destination of multicast transmissions are disconnected by a PIX firewall, two
obvious cases are possible: back the antecedent of a manual (or a multicast
router) is on a lower security-level interface than the destination and back the
source (router) is on a college security-level interface than the destination. Let’s
look at these two cases separately.
IGMP Used to Address Associates in a Multicast Group
Client 1
Client 2
Client 3
Multicast Server
Client 4
Server sends transmissions
to accumulation 224.0.1.1
"Who is in
224.0.1.1?"
"Who is in
224.0.1.1?"
"I am in
224.0.0.1"
"I am in
224.0.1.1"
Only Client 3 and Client 4 are in this group, so they
are the alone hosts that acknowledgment to the router's request.
When manual starts, the router will only
forward it to these two hosts.
The router periodically asks for
group associates reports.