D-Link DES-3250TG User Manual page 42

D-Link DES-3250TG Standalone Layer 2 Switch
the network will contain a database of computer names and their corresponding IP and MAC addresses.
The user's computer will send a query to the name database computer, and the database computer will
send a response. Both the query and the response are very short. There is no need to divide the query
or response between multiple packets, so the complexity of TCP is not required. If there is no response
to the query after a period of time, the query can simply be resent.
The User Datagram Protocol (UDP) is designed for communications that do not require division among
multiple packets and subsequent reassembly. UDP does not keep track of what is sent.
UDP uses port numbers in a way that is directly analogous to TCP. There are well-known UDP port
numbers for servers that use UDP.
Figure 5- 11. UDP Packet Header
The UDP header is shorter than a TCP header. UDP also uses a checksum to verify that data is received
uncorrupted.
The Internet Control Message Protocol (ICMP) is also a simplified protocol used for error messages and
messages used by TCP/IP. ICMP, like UDP, processes messages that will fit into a single packet. ICMP
does not, however use ports because its messages are processed by the network software.
Internet Group Management Protocol (IGMP)
End users that want to receive multicast packets must be able to inform nearby routers that they want
to become a multicast group member of the group these packets are being sent to. The Internet Group
Management Protocol (IGMP) is used by multicast routers to maintain multicast group membership.
IGMP is also used to coordinate between multiple multicast routers that may be present on a network
by electing one of the multicast routers as the 'querier'. This router then keep track of the membership
of multicast groups that have active members on the network. IGMP is used to determine whether the
router should forward multicast packets it receives to the subnetworks it is attached to or not. A
multicast router that has received a multicast packet will check to determine if there is at least one
member of a multicast group that has requested to receive multicast packets from this source. If there
is one member, the packet is forwarded. If there are no members, the packet is dropped.
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