Re: [Softwires] The port mapping issue

On 27 March 2013 16:59, Tom Taylor <tom.taylor.stds@gmail.com> wrote:
> The meeting minutes record a disagreement over what port mapping algorithm
> to use. This affects both MAP-E and LW 4over6. As I understand it:
>
> - either of these two technologies will work with either contiguous ports or
> ports scattered according to the GMA algorithm
>
> - the real objection to GMA comes from Alain Durand, who wants to set up
> simple min-port, max-port filters on his network equipment.
>
>
> We all agree that port scattering offers negligible security advantage.
>
> The reason that I heard given for preferring GMA for MAP-E is that it
> eliminates a restriction on the End-User Ipv6 address because the PSID is
> free to range from 0 upwards rather than from some higher number upwards. I
> don't follow this argument for two reasons:
>
> - you now have a restriction that the offset field A must range from 1
> upwards
>
> - the PSID field has an upper limit 2^k-1 imposed by the sharing ratio,
> imposing a further restriction on the End-User IPv6 address value.
>
> Could someone spell out more clearly why the GMA was seen as necessary for
> MAP-E?

GMA codifies (power of 2) logic that translates easily into hardware
based longest match lookups. Furthermore, it is actually trivially
simple to implement on any platform (not hardware restricted thus):
Something like 2 lines of C code, in optimized form. Lastly, it is
very efficient in terms of info encoding: a PSID conveys a code-point
that maps the entire port space or port range. Finally, embedding a
explicit port-range in an IPv6 address is not an option (the N:1 rule
case).

Regards,
Woj.
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