TCP IP Illustrated

the crc field is a cyclic redundancy check a

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Unformatted text preview: at detects errors in the rest of the frame. (This is also called the FCS or frame check sequence.) There is a minimum size for 802.3 and Ethernet frames. This minimum requires that the data portion be at least 38 bytes for 802.3 or 46 bytes for Ethernet. To handle this, pad bytes are inserted to assure that the frame is long enough. We'll encounter this minimum when we start watching packets on the wire. In this text we'll display the Ethernet encapsulation when we need to, because this is the most commonly used form of encapsulation. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (2 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm Figure 2.1 IEEE 802.2/802.3 encapsulation (RFC 1042) and Ethernet encapsulation (RFC 894). 2.3 Trailer Encapsulation RFC 893 [Leffler and Karels 1984] describes another form of encapsulation used on Ethernets, called trailer encapsulation. It was an experiment with early BSD systems on DEC VAXes that improved performance by rearranging the order of the fields in the IP datagram. "The variablelength fields at the beginning of the data portion of the Ethernet frame (the IP header and the TCP header) were moved to the end (right before the CRC). This allows the data portion of the frame to be mapped to a hardware page, saving a memory-to-memory copy when the data is copied in the kernel. TCP data that is a multiple of 512 bytes in size can be moved by just manipulating the kernel's page tables. Two hosts negotiated the use of trailer encapsulation using an extension of ARP. Different Ethernet frame type values are defined for these frames. Nowadays trailer encapsulation is deprecated, so we won't show any examples of it. Interested readers are referred to RFC 893 and Section 11.8 of [Leffler et al. 1989] for additional details. 2.4 SLIP: Serial Line IP file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (3 of 11...
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