RFC 893 (rfc893) - Page 2 of 6

Trailer encapsulations

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
      physically copying it, but often implementation constraints and
      the characteristics of the underlying network hardware preclude
      merely remapping the header(s).

      4. The memory to memory copying overhead which is expected to be
      performed by the receiver must be significant enough to warrant
      the added complexity in the both the sending and receiving host
      software.

   The first point is well known and the motivation for this note.


Leffler & Karels