SNAP header

The Sub-Network Attachment Point header (SNAP header) is an IEEE-defined layer 2 encapsulation header format for Ethernet and similar network packets. It is used at the logical link control layer of the IEEE 802.3 communications protocol stack, as a trailer to the IEEE-defined LLC header[?], in so-called "LLC/SNAP encapsulation".

In LLC/SNAP encapsulation the SNAP header is preceded by a LLC header that consists of the 3 bytes

 AA AA 03

specifying a U-type LLC frame with SNAP as both the SSAP and DSAP protocols, and the rest of the control field set to zero.

The 5 byte SNAP header itself then follows, in the following format:

• Organization Code (3 bytes)
• Local Code (2 bytes)

For the special case of ARP and IP packet encapsulation, the Organization Code is set to zero, and the EtherType information is stored in the 16-bit Local Code field.

Thus, the LLC/SNAP headers are 8 bytes long in total. This reduces the size of the available payload for protocols such as the Internet Protocol to 1492 bytes, compared to the use of the Ethernet II framing[?].

References:

• J. Postel and J. Reynolds, A Standard for the Transmission of IP Datagrams over IEEE 802 Networks, RFC 1042[?]

External links:

• RFC 1042 (http://www.faqs.org/rfcs/rfc1042.html)