Tony Rogvall <tony(at)rogvall(dot)se>
Standards Track

EEP 34: Extended basic packet options for decode_packet #

Abstract #

This EEP describes new basic packet options that is used by the gen_tcp which are also present, and the same, as in the erlang:decode_packet.

Rationale #

Current packet options used in erlang:decode_packet cover a range of packet types. The basic ones are {packet,0}, {packet,1}, {packet,2}, and {packet,4}. On outbound traffic from gen_tcp these options prefix the packets with N extra bytes containing an integer in big endian format containing the size of the data.

When communicating with end points implemented by other party it is not always possible to suggest that packet length is present in big endian format or that it is 4 bytes. Today with 64-bit machine showing up it may even be possible that we soon find protocols sending plain machine dependent 64-bit words as packet length descriptors.

New packet types #

This EEP suggest to extend the packet bytes to the range 0-8. Note that the internal maximum packet size is not affected by this EEP, only the format of the packet size indicator.

Furthermore a negative range is suggested to signal little endian formatted size indicator in range -1 .. -8. {packet,-1} is equivalent with {packet,1}. The number of packets bytes prefixed is thus abs(PBytes) where PBytes is in range -8 .. 8.

This EEP also suggest a fixed size packet mode, expressed as {packet, {size,N}}. This mode acts much like {packet,0} in regards to packet bytes, no packet bytes are used. The difference is that, in {active,true} or {active,once} mode, while {packet,0} collects any data available, the {packet,{size,N}} mode collects exactly N bytes before passing it on to the “owner” process. Suggested implementation minimum limitations for N is unsigned 16-bit, resulting in a minimum size of 1 and and maximum size of 65535. A packet size of less than 1 should always result in a badarg error.

Summary #

The packet types suggest by this EEP are:

  • {packet,P}
    for integer P in range -8 .. 8. This is an extension to the existing integer packet types.

  • {packet,{size,N}}
    N is in range > 0 and the maximum N is implementation dependent but never less then 65535.

Backward compatibility #

The author of this EEP has implemented this proposal in the Erlang/OTP standard git release, and has not found any backwards compatibility problems. The files affected by this proposal implementation are: inet_drv.c, packet_parser.c, packet_parser.h and erl_bif_port.c.

Copyright #

This document has been placed in the public domain.