Internet Group Management Protocol (IGMP)
Internet Group Management Protocol (IGMP) is a protocol that manages
the membership lists for IP multicasting in a TCP/IP network. IP
multicasting is a process by which a message is transmitted to a
select group of recipients, known as a multicast group. IGMP
maintains the list of members who subscribe to each multicast group.
Managing IP Multicasting
All
of the members of a multicast group listen for IP traffic directed
to a specific multicast IP address and receive the packets
sent to that IP address. However, because multicasting involves
multiple computers, the packets are sent using the unreliable UDP
protocol, which does not guarantee the delivery of the packets to
the multicast group. When multiple computers need to access
information, such as streaming media, an IP address reserved for
multicasting is used. Routers that are configured to process
multicast IP addresses pick up this information and forward it to
all subscribers of the multicast group associated with the multicast
IP address.
For
multicast information to reach its recipients, it is important that
each router in the path of communication supports multicasting.
Windows 2003-based computers can both send and receive IP multicast
traffic.
Address
Resolution Protocol (ARP)
Located in the Internet layer of the TCP/IP suite, Address
Resolution Protocol (ARP) performs address resolution for outgoing
packets. Address resolution is the process by which IP addresses are
mapped to MAC addresses. The network adapters use the MAC address to
determine if a packet is meant for that computer. Without the MAC
address, the network adapters do not know if they are to pass the
data to a higher layer for further processing. As the outgoing
packets in the IP layer are being readied for transmission on the
network, the source and destination MAC addresses must be added.
ARP
Cache
ARP
stores a table containing IP addresses and their corresponding MAC
addresses. The area of memory where this table is stored is referred
to as the ARP cache. The ARP cache for any computer contains the
mappings for only computers and routers that reside on the same
segment.
Physical Address Resolution
ARP
compares every outbound packet’s destination IP address with the ARP
cache to determine the MAC address to which the packet will be sent.
If there is a matching entry, the MAC address is retrieved from the
cache. If not, ARP broadcasts a request for the computer owning the
IP address in question to reply with its MAC address. Next, the
computer with the corresponding IP address adds the initial
computer’s MAC address to its cache and then replies with its own
MAC address. When an ARP reply is received, the ARP cache is updated
with the new information and the packet can then be sent. If the
packet is going to another segment, ARP resolves the MAC address for
the router responsible for that segment, rather than resolving the
address for the final destination computer. The router is then
responsible for either finding the MAC address of the destination or
forwarding the packet to another router.
