Understanding the network.pdf - Back to Home

eassembled before being handed to the ULP. Additionally, IP does not guarantee
datagram delivery. The Layer 2 transmission devices involved in the transport of
datagrams can discard the Layer 2 frames containing IP datagrams if the adequate
resources needed for transmission are unavailable. In the event that IP datagrams
are lost, it is the responsibility of the ULP of the transmitting host to resend the lost
data.
The following section explains the many components of the IP protocol and its
operation, such as its interaction with the Layer 2 protocols that provide network
address to transmission address translation, the structure of the IP datagram, and
the various elements of network addressing, including IP datagram delivery.
IP Interactions with Layer 2 (ARP, Proxy ARP, and
RARP)
All Layer 3 delivery is dependent upon Layer 2 for actual transport. This being the
case, a mechanism needs to exist for translating Layer 3 network addresses to
Layer 2 transport addresses. The ARP protocol was developed to perform these
translations for IP.
When IP assembles a datagram, it takes the destination address provided by the
ULP and performs a lookup in the host's route table to determine if the destination
is local or remote to itself. If the destination is local, the datagram is delivered
directly to the local host. If the datagram is not local, the datagram is sent to a
network gateway, which takes responsibility to ensure that it is delivered correctly.
In either case, the IP datagram needs to be placed inside a Layer 2 packet for
delivery.
In order to determine the correct Layer 2 destination address, the sending host
must have a mechanism for translating the Layer 2 to Layer 3 address map. It uses
Address Resolution Protocol (ARP), which is a dynamic process, and Reverse
Address Resolution Protocol (RARP), which is an application service, to assist it in
finding the locally connected hosts. ARP and RARP map the different Layer 2
protocol addresses (for example, Ethernet/and Token Ring) to the 32-bit addresses
used by IP to identify the different hosts on the network. Technically, both the Layer
2 and Layer 3 IP addresses both provide the same basic function of host
identification in terms of the data delivery process, but nevertheless, the addresses
are incompatible. So, in order for data to be delivered between locally connected
hosts, a remapping service is needed.
ARP runs as a local process on a host (computer or other network device) connected
to the network. ARP accomplishes two tasks: