The Network Header
Page 18-9

The Network Header

There are essentially two requirements for the any to any switching transformation function to
address the network header fields:
• Network Address to MAC Address Mapping
In every protocol there is a mechanism for mapping global network wide addresses to the
MAC addresses required in the local broadcast domain.
• Frame Size Requirements of the Media
Different media have different minimum and maximum frame sizes leading to the issues
of padding insertion/stripping and fragmentation/reassembly or maximum frame size
negotiation protocols at the network level.

Address Mapping

There are almost as many ways to map a global network level address to a local subnetwork
MAC address as there are routing protocols. These may or may not be affected by any to any
switching.
Some may construct MAC addresses algorithmically, for example, DECNET model. Some may
involve table lookups with an additional protocol to build and maintain these tables, for
example, the IP/ARP model. Others may involve some form of building the network address
around the MAC address as in the IPX model.
In all cases these mechanisms are susceptible, without good design and forethought, to the
problem of canonical versus non-canonical representation of addresses in the network header
area.

Address Mapping in IP: ARP

To map a 32-bit IP network address into the MAC address of a locally connected station a
router uses the Address Resolution Protocol (ARP) to build an ARP Table. The router broad-
casts a request containing the IP address in the body of the frame. The station with that IP
address responds with its MAC address in the body of an ARP reply frame. The router inserts
these two addresses in its ARP table and can then use the MAC address received to transmit
any frames addressed to that IP address.
Since a router can have interfaces to Ethernet ports (canonical MAC addresses) and FDDI and
Token Ring (non-canonical MAC addresses), it is crucial that the router keeps track of what
media type it receives on each port.
If IP ARP were defined such that all MAC addresses, when conveyed in the body of an ARP,
were in canonical format, switching would be easy. A router, when taking an address from
the ARP table and using it as the destination MAC address on an Ethernet port would use the
address as is. If sending to FDDI or Token Ring it would bit swap the address to non-canoni-
cal format as required by the media.
MAC Header RIF Encapsulation Network Header Data