> > In practice, this is what I would propose: > > * Wireshark dissector made capable of accepting any whole-byte preamble > > length mPackets. > > * mPacket capture devices are made responsible for detecting any frames > > with non-integer preamble, and correct for it by adding 4 bits extra > > preamble at the beginning. That way the dissector never has to realign a > > whole frame on bit level. > > I.e., the packet, as stored in a capture file (pcap, pcapng, whatever), will > have somewhere between 1 and 7 octets of 0x55, followed by an SMD?
Yes. Technically IEEE says a mPacket receiver should accept any number of 0x55 octets. So it could in theory be more than 7 octets. But in real world it would normally be 1-7 octets. Including half-octets... > > * A capture device which has added 4 bits of preamble, shall indicate this > > by setting the “unaligned frame error” bit in epb_flags, to let the > > dissector know that it should ignore the least significant nibble of the > > first preamble byte. > > Why is this necessary? Is the goal here to indicate the on-the-wire preamble > length? If so, why not just write 0x50 rather than 0x55 as the first octet > if the least significant nibble is a padding nibble? Yes, indication of the true on-wire preamble is what I am trying to achieve. I think you are right in that just writing 0x50 as first octet would work. But the dissector would have to deviate slightly from IEEE. A "0" at the beginning would constitute a SFD error. But in practice it should be essentially impossible for the reconciliation sublayer to see anything but "5" at the beginning of a frame. Due to the way the Ethernet PHY layers are specified. So making an exception for the first byte might be acceptable? > > While a mPacket with 4 bits of missing preamble is not actually an error, I > > can think of no other meaningful use for the “unaligned frame error” bit in > > epb_flags for mPackets. So it should be ok to make use of it for this > > purpose? Maybe? > > Unless the “unaligned frame error” is actually intended to indicate > > “dribble error”? (An extra 4 bit nibble at the end of a frame) > And the "unaligned frame error" should perhaps be renamed "misaligned frame > error". > See, for example, this page of an Ethernet adapter manual: > https://manualsdump.com/en/manuals/asante_technologies-asante_maccon_family_ethernet_network_cards_for_the_macintosh/142796/53 Ok! Then I certainly cannot use that flag. Regards, Timmy Brolin ___________________________________________________________________________ Sent via: Wireshark-dev mailing list <wireshark-dev@wireshark.org> Archives: https://www.wireshark.org/lists/wireshark-dev Unsubscribe: https://www.wireshark.org/mailman/options/wireshark-dev mailto:wireshark-dev-requ...@wireshark.org?subject=unsubscribe ___________________________________________________________________________ Sent via: Wireshark-dev mailing list <wireshark-dev@wireshark.org> Archives: https://www.wireshark.org/lists/wireshark-dev Unsubscribe: https://www.wireshark.org/mailman/options/wireshark-dev mailto:wireshark-dev-requ...@wireshark.org?subject=unsubscribe
