Re: [trill] Order of bits in TRILL bitmaps

Donald Eastlake <d3e3e3@gmail.com> Thu, 21 August 2014 18:46 UTC

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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Thu, 21 Aug 2014 14:45:47 -0400
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Subject: Re: [trill] Order of bits in TRILL bitmaps
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Hi Petr,

On Thu, Aug 21, 2014 at 2:37 AM, Petr Hroudný <petr.hroudny@gmail.com> wrote:
> Hi all,
>
> I'm looking for the WG opinion about the correct order of bits in bitmaps
> specified in RFC7176 sections 2.2.2 & 2.2.5 and 2.2.4 & 2.3.1.
>
> For the first two, the RFC says:
>
>    o  VLAN bit-map: The highest-order bit indicates the VLAN equal to
>       the start VLAN ID, the next highest bit indicates the VLAN equal
>       to start VLAN ID + 1, continuing to the end of the VLAN bit-map
>       field.
>
> I understand this as follows: if only the start VLAN ID is enabled, the
> bitmap should have a value of 0x80.
>
> Nevertheless, I've seen an implementation, where the bitmap reads 0x01.
>
> Which one is correct?

The default in the IETF is big-endian. so the high order bit of a byte
is 0x80. I believe that if you send that byte on Ethernet, you will
see seven zeros followed by a one because that media sends the low
order bit first. In any case, I think 0x80 is correct.

> For the second two, the RFC says:
>
>    o  Capabilities and Header Flags Supported: A bit vector of 32 bits
>       numbered 0 through 31 in network order.
>
> I understand this as follows: if I want to set the FGL-safe bit (bit 1), the
> bitmap should read 0x40 0x00 0x00 0x00.
>
> Nevertheless, I've seen an implementation, where the bitmap reads 0x00,
> 0x00, 0x00, 0x40
>
> Which one is correct?

The "network order" used in the IETF for a 32 bit field is to send
send bytes from left to right. This bit vector is like (well, it is
the same size) a IPv4 address which is also described as having 32
bits numbered from 0 through 31 (see Figure 4 of RFC 791). If you send
the loopback address (127.0.0.1), the "127" byte gets sent first and
the "1" byte last. On a serial media, the "127" byte might be
0b11111110 (how I believe it is on Ethernet) or 0b01111111, but it is
still the first byte transmitted.

So I would say 40-00-00-00 is correct.

> As this might create a serious interop issues, I'm seeking for WG opinion on
> this.

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 155 Beaver Street, Milford, MA 01757 USA
 d3e3e3@gmail.com

>     Thanks, Petr
>
>
>
>
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