Addendum to SUMMARY: le0: trailer error

From: Roberto Dominijanni (
Date: Sat Jul 23 1994 - 01:36:15 CDT

Since posting a summary of replies to my query about the meaning of the
"le0: trailer error" on July 20, three additional responses were

    Dan Penrod (
    Zenon Dewicki (
    Matthew R. Hofener (

While in agreement with the responses quoted in the summary, they provided
additional information that might be of interest.

David Penrod quotes from "TCP Illustrated Volume 1 - The Protocols" by
W. Richard Stevens:

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 variable-length fields at the beginning
        of the data protion 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.

and goes on to add:

    So that's what it is... an old kludge to speed up TCP/IP performance on
    ethernets. Generally it's not used anymore. It is a flag that can be
    set with the 'ifconfig' command.

Matthew Hofener quotes the Sun "System and Network Administration" manual,
Appendix B, "Error Messages from the Monitor and Boot Program", pg. 812.

        le%d: trailer error
        An incoming packet claimed to have a trailing header but did not.

Again, many thanks to all who responded.

Roberto Dominijanni

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