06-12-2012, 06:41 PM
Network Layer: Address Mapping, Error Reporting, and Multicasting
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INTRODUCTION
In Chapter 20 we discussed the Internet Protocol (IP) as the main protocol at the network
layer. IP was designed as a best-effort delivery protocol, but it lacks some features such
as flow control and error control. It is a host-to-host protocol using logical addressing.
To make IP more responsive to some requirements in today's intemetworking, we need
the help of other protocols.
We need protocols to create a mapping between physical and logical addresses.
IP packets use logical (host-to-host) addresses. These packets, however, need to be encapsulated
in a frame, which needs physical addresses (node-to-node). We will see that a
protocol called ARP, the Address Resolution Protocol, is designed for this purpose. We
sometimes need reverse mapping-mapping a physical address to a logical address. For
example, when booting a diskless network or leasing an IP address to a host. Three protocols
are designed for this purpose: RARP, BOOTp, and DHCP.
Lack of flow and error control in the Internet Protocol has resulted in another protocol,
ICMP, that provides alerts. It reports congestion and some types of errors in the
network or destination host.
IP was originally designed for unicast delivery, one source to one destination. As the
Internet has evolved, the need for multicast delivery, one source to many destinations, has
increased tremendously. IGMP gives IP a multicast capability.
In this chapter, we discuss the protocols ARP, RARP, BOOTP, DHCP, and IGMP
in some detail. We also discuss ICMPv6, which will be operational when IPv6 is operational.
ICMPv6 combines ARP, ICMP, and IGMP in one protocol.
ADDRESS MAPPING
An internet is made of a combination of physical networks connected by internetworking
devices such as routers. A packet starting from a source host may pass through several
different physical networks before finally reaching the destination host. The hosts and
routers are recognized at the network level by their logical (IP) addresses.
However, packets pass through physical networks to reach these hosts and routers.
At the physical level, the hosts and routers are recognized by their physical addresses.
Mapping Logical to Physical Address: ARP
Anytime a host or a router has an IP datagram to send to another host or router, it has the
logical (IP) address of the receiver. The logical (IP) address is obtained from the DNS
(see Chapter 25) if the sender is the host or it is found in a routing table (see Chapter 22)
if the sender is a router. But the IP datagram must be encapsulated in a frame to be able to
pass through the physical network. This means that the sender needs the physical address
of the receiver. The host or the router sends an ARP query packet. The packet includes
the physical and IP addresses of the sender and the IP address of the receiver. Because
the sender does not know the physical address of the receiver, the query is broadcast
over the network (see Figure 21.1).
Every host or router on the network receives and processes the ARP query packet,
but only the intended recipient recognizes its IP address and sends back an ARP response
packet. The response packet contains the recipient's IP and physical addresses. The packet
is unicast directly to the inquirer by using the physical address received in the query
packet.
Cache Memory
Using ARP is inefficient if system A needs to broadcast an ARP request for each IP
packet it needs to send to system B. It could have broadcast the IP packet itself. ARP
can be useful if the ARP reply is cached (kept in cache memory for a while) because a
system normally sends several packets to the same destination. A system that receives
an ARP reply stores the mapping in the cache memory and keeps it for 20 to 30 minutes
unless the space in the cache is exhausted. Before sending an ARP request, the system
first checks its cache to see if it can find the mapping.