RFC - Unknown
(April 1984; No errata)
||RFC Editor Note
RFC 893 (Unknown)
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Network Working Group Samuel J. Leffler
Request for Comments: 893 Michael J. Karels
University of California at Berkeley
Status of this Memo
This RFC discusses the motivation for use of "trailer encapsulations"
on local-area networks and describes the implementation of such an
encapsulation on various media. This document is for information
only. This is NOT an official protocol for the ARPA Internet
A trailer encapsulation is a link level packet format employed by
4.2BSD UNIX (among others). A trailer encapsulation, or "trailer",
may be generated by a system under certain conditions in an effort to
minimize the number and size of memory-to-memory copy operations
performed by a receiving host when processing a data packet.
Trailers are strictly a link level packet format and are not visible
(when properly implemented) in any higher level protocol processing.
This note cites the motivation behind the trailer encapsulation and
describes the trailer encapsulation packet formats currently in use
on 3 Mb/s Experimental Ethernet, 10 Mb/s Ethernet, and 10 Mb/s V2LNI
ring networks .
The use of a trailer encapsulation was suggested by Greg Chesson, and
the encapsulation described here was designed by Bill Joy.
Trailers are motivated by the overhead which may be incurred during
protocol processing when one or more memory to memory copies must be
performed. Copying can be required at many levels of processing,
from moving data between the network medium and the host's memory, to
passing data between the operating system and user address spaces.
An optimal network implementation would expect to incur zero copy
operations between delivery of a data packet into host memory and
presentation of the appropriate data to the receiving process. While
many packets may not be processed without some copying operations,
when the host computer provides suitable memory management support it
may often be possible to avoid copying simply by manipulating the
appropriate virtual memory hardware.
In a page mapped virtual memory environment, two prerequisites are
usually required to achieve the goal of zero copy operations during
packet processing. Data destined for a receiving agent must be
Leffler & Karels [Page 1]
RFC 893 April 1984
aligned on a page boundary and must have a size which is a multiple
of the hardware page size (or filled to a page boundary). The latter
restriction assumes virtual memory protection is maintained at the
page level; different architectures may alter these prerequisites.
Data to be transmitted across a network may easily be segmented in
the appropriate size, but unless the encapsulating protocol header
information is fixed in size, alignment to a page boundary is
virtually impossible. Protocol header information may vary in size
due to the use of multiple protocols (each with a different header),
or it may vary in size by agreement (for example, when optional
information is included in the header). To insure page alignment the
header information which prefixes data destined for the receiver must
be reduced to a fixed size; this is normally the case at the link
level of a network. By taking all (possibly) variable length header
information and moving it after the data segment a sending host may
"do its best" in allowing the receiving host the opportunity to
receive data on a page aligned boundary. This rearrangement of data
at the link level to force variable length header information to
"trail" the data is the substance of the trailer encapsulation.
There are several implicit assumptions in the above argument.
1. The receiving host must be willing to accept trailers. As this
is a link level encapsulation, unless a host to host negotiation
is performed (preferably at the link level to avoid violating
layering principles), only certain hosts will be able to converse,
or their communication may be significantly impaired if trailer
packets are mixed with non-trailer packets.
2. The cost of receiving data on a page aligned boundary should be
comparable to receiving data on a non-page aligned boundary. If
the overhead of insuring proper alignment is too high, the savings
in avoiding copy operations may not be cost effective.
3. The size of the variable length header information should be
significantly less than that of the data segment being
transmitted. It is possible to move trailing information without
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