IP datagrams and ARP requests and replies are transmitted in
standard 802.2 LLC Type 1 Unnumbered Information format, control
code 3, with the DSAP and the SSAP fields of the 802.2 header set
to 170, the assigned global SAP value for SNAP [6]. The 24-bit
Organization Code in the SNAP is zero, and the remaining 16 bits
are the EtherType from Assigned Numbers [7] (IP = 2048, ARP =
2054).
IEEE 802 packets may have a minimum size restriction. When
necessary, the data field should be padded (with octets of zero)
to meet the IEEE 802 minimum frame size requirements. This
padding is not part of the IP datagram and is not included in the
total length field of the IP header.
For compatibility (and common sense) the minimum packet size used
with IP datagrams is 28 octets, which is 20 (minimum IP header) +
8 (LLC+SNAP header) = 28 octets (not including the MAC header).
The minimum packet size used with ARP is 24 octets, which is 20
(ARP with 2 octet hardware addresses and 4 octet protocol
addresses) + 8 (LLC+SNAP header) = 24 octets (not including the
MAC header).
In typical situations, the packet size used with ARP is 32 octets,
which is 28 (ARP with 6 octet hardware addresses and 4 octet
protocol addresses) + 8 (LLC+SNAP header) = 32 octets (not
including the MAC header).
IEEE 802 packets may have a maximum size restriction.
Implementations are encouraged to support full-length packets.
For compatibility purposes, the maximum packet size used with IP
datagrams or ARP requests and replies must be consistent on a
particular network.
Gateway implementations must be prepared to accept full-length
packets and fragment them when necessary.
Host implementations should be prepared to accept full-length
packets, however hosts must not send datagrams longer than 576
octets unless they have explicit knowledge that the destination is
prepared to accept them. A host may communicate its size
preference in TCP based applications via the TCP Maximum Segment
Size option [10].
Datagrams on IEEE 802 networks may be longer than the general
Internet default maximum packet size of 576 octets. Hosts
connected to an IEEE 802 network should keep this in mind when
sending datagrams to hosts not on the same IEEE 802 network. It
may be appropriate to send smaller datagrams to avoid unnecessary
fragmentation at intermediate gateways. Please see [10] for
further information.
