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Frame Format and MAC Level Issues Connected: An Internet Encyclopedia
Frame Format and MAC Level Issues

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Frame Format and MAC Level Issues

Frame Format and MAC Level Issues

For all hardware types

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.


Next: IEEE 802.2 Details

Connected: An Internet Encyclopedia
Frame Format and MAC Level Issues

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