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Network Working Group                                  Samuel J. LefflerRequest for Comments: 893                              Michael J. Karels                                    University of California at Berkeley                                                              April 1984Trailer EncapsulationsStatus 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   community.Introduction   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 [1].   The use of a trailer encapsulation was suggested by Greg Chesson, and   the encapsulation described here was designed by Bill Joy.Motivation   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 beLeffler & 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      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                                                [Page 2]

RFC 893                                                       April 1984   Thought has been given to negotiating the user of trailers on a per   host basis using a variant of the Address Resolution Protocol [2]   (actually augmenting the protocol), but at present all systems using   trailers require hosts sharing a network medium to uniformly accept   trailers or never transmit them.  (The latter is easily carried out   at boot time in 4.2BSD without modifying the operating system source   code.)   The second point is (to our knowledge) insignificant.  While a host   may not be able to take advantage of the alignment and size   properties of a trailer packet, it should nonetheless never hamper   it.   Regarding the third point, let us assume the trailing header   information is copied and not remapped, and consider the header   overhead in the TCP/IP protocols as a representative example [3].  If   we assume both the TCP and IP protocol headers are part of the   variable length header information, then the smallest trailer packet   (generated by a VAX) would have 512 bytes of data and 40+ bytes of   header information (plus the trailer header described later).  While   the trailing header could have IP and/or TCP options included this   would normally be rare (one would expect most TCP options, for   example, to be included in the initial connection setup exchange) and   certainly much smaller than 512 bytes.  If the data segment is   larger, the ratio decreases and the expected gain due to fewer copies   on the receiving end increases.  Given the relative overheads of a   memory to memory copy operation and that of a page map manipulation   (including translation buffer invalidation), the advantage is   obvious.   The fourth issue, we believe, is actually a non-issue.  In our   implementation the additional code required to support the trailer   encapsulation amounts to about a dozen lines of code in each link   level "network interface driver".  The resulting performance   improvement more than warrants this minor investment in software.   It should be recognized that modifying the network (and normal link)   level format of a packet in the manner described forces the receiving   host to buffer the entire packet before processing.  Clever   implementations may parse protocol headers as the packet arrives to   find out the actual size (or network level packet type) of an   incoming message.  This allows these implementations to avoid   preallocating maximum sized buffers to incoming packets which it can   recognize as unacceptable.  Implementations which parses the network   level format on the fly are violating layering principles which have   been extolled in design for some time (but often violated in   implementation).  The problem of postponing link level typeLeffler & Karels                                                [Page 3]

RFC 893                                                       April 1984   recognition is a valid criticism.  In the case of network hardware   which supports DMA, however, the entire packet is always received   before processing begins.Trailer Encapsulation Packet Formats   In this section we describe the link level packet formats used on the   3 Mb/s Experimental Ethernet, and 10 Mb/s Ethernet networks as well   as the 10 Mb/s V2LNI ring network.  The formats used in each case   differ only in the format and type field values used in each of the   local area network headers.   The format of a trailer packet is shown in the following diagram.      +----+-------------------------------------------------+----+      | LH |                     data                        | TH |      +----+-------------------------------------------------+----+           ^                    (  ^  )                      ^      LH:         The fixed-size local network header.  For 10 a Mb/s Ethernet,         the 16-byte Ethernet header.  The type field in the header         indicates that both the packet type (trailer) and the length of         the data segment.         For the 10 Mb/s Ethernet, the types are between 1001 and 1010         hexadecimal (4096 and  4112 decimal). The type is calculated as         1000 (hex) plus the number of 512-byte pages of data.  A         maximum  of 16 pages of data may be transmitted in a single         trailer packet (8192 bytes).      data:         The "data" portion of the packet.  This is normally only data         to be delivered to the receiving processes (i.e. it contains no         TCP or IP header information).  Data size is always a multiple         of 512 bytes.      TH:         The "trailer".  This is actually a composition of the original         protocol headers and a fixed size trailer prefix which defines         the type and size         of the trailing data.  The format of a trailer is shown below.   The carats (^) indicate the page boundaries on which the receiving   host would place its input buffer for optimal alignment whenLeffler & Karels                                                [Page 4]

RFC 893                                                       April 1984   receiving a trailer packet.  The link level receiving routine is able   to locate the trailer using the size indicated in the link level   header's type field.  The receiving routine is expected to discard   the link level header and trailer prefix, and remap the trailing data   segment to the front of the packet to regenerate the original network   level packet format.Trailer Format   +----------------+----------------+------~...~----------+   |      TYPE      |  HEADER LENGTH |  ORIGINAL HEADER(S) |   +----------------+----------------+------~...~----------+   Type:        16 bits      The type field encodes the original link level type of the      transmitted packet.  This is the value which would normally be      placed in the link level header if a trailer were not generated.   Header length:       16 bits      The header length field of the trailer data segment.  This      specifies the length in bytes of the following header data.   Original headers: <variable length>      The header information which logically belongs before the data      segment.  This is normally the network and transport level      protocol headers.Summary   A link level encapsulation which promotes alignment properties   necessary for the efficient use of virtual memory hardware facilities   has been described.  This encapsulation format is in use on many   systems and is a standard facility in 4.2BSD UNIX.  The encapsulation   provides an efficient mechanism by which cooperating hosts on a local   network may obtain significant performance improvements.  The use of   this encapsulation technique currently requires uniform cooperation   from all hosts on a network; hopefully a per host negotiation   mechanism may be added to allow consenting hosts to utilize the   encapsulation in a non-uniform environment.Leffler & Karels                                                [Page 5]

RFC 893                                                       April 1984References   [1]  "The Ethernet - A Local Area Network", Version 1.0, Digital   Equipment Corporation, Intel Corporation, Xerox Corporation,   September 1980.   [2]  Plummer, David C., "An Ethernet Address Resolution Protocol",RFC-826,  Symbolics Cambridge Research Center, November 1982.   [3]  Postel, J., "Internet Protocol",RFC-791, USC/Information   Sciences Institute, September 1981.Leffler & Karels                                                [Page 6]