TCP/IP Tutorial and Technical Overview - IBM Redbooks

The following figure shows the payload format for routed IP PDUs (Figure 2-8).
Figure 2-8 CPCS-PDU payload format for IP PDUs
Where:
IP PDU Normal IP datagram, starting with the IP header.
LLC A 3-byte LLC header with the format DSAP-SSAP-Ctrl.
For IP data, it is set to 0xAA-AA-03 to indicate the
presence of a SNAP header. The Ctrl field always has the
value 0x03, specifying Unnumbered Information
Command PDU.
OUI The 3-byte Organizationally Unique Identifier (OUI)
identifies an organization that administers the meaning of
the following 2-byte Protocol Identifier (PID). To specify
an EtherType in PID, the OUI has to be set to 0x00-00-00.
PID The Protocol Identifier (PID) field specifies the protocol
type of the following PDU. For IP datagrams, the assigned
EtherType or PID is 0x08-00.
The default MTU size for IP members in an ATM network is discussed in RFC
2225 and defined to be 9180 bytes. The LLC/SNAP header is 8 bytes; therefore,
the default ATM AAL5 PDU size is 9188 bytes. The possible values can be
between zero and 65535. You are allowed to change the MTU size, but then all
members of a LIS must be changed as well in order to have the same value.
RFC 1755 recommends that all implementations should support MTU sizes up to
and including 64 KB.
Chapter 2. Network interfaces 55