Re: [PATCH 1/1] ip.7: Add "special and reserved addresses" section

[Seth David Schoen <schoen@loyalty.org>]

John Gilmore writes:

> > > > Are there network devices that are neither point-to-point, nor do they
> > > > have broadcast support?
> 
> It seems to me that cellular radio systems are neither point-to-point,
> nor do they support broadcast.  And they keep moving further and further
> toward being IP-based (e.g. phone calls on your LTE phone use SIP over
> UDP over IP over cellular!).  But given the proprietary nature of most
> cellphone radio chips and their firmware, it's possible that Linux has
> never built a fully capable network interface to talk to them.  I don't
> think we should attempt to rule it out, though.

I found in include/uapi/linux/if_link.h the following classification
about this:

   IFF_LOOPBACK, IFF_BROADCAST and IFF_POINTOPOINT are
   more not changeable by user. They describe link media
   characteristics and set by device driver.

   Comments:
   - Combination IFF_BROADCAST|IFF_POINTOPOINT is invalid
   - If neither of these three flags are set;
     the interface is NBMA.

https://en.wikipedia.org/wiki/Non-broadcast_multiple-access_network

The likeliest example there in terms of running IP directly over such a
network would presumably be ATM, which had a whole series of RFCs about
how to make this work, and which is supported by the Linux kernel (e.g.
net/atm/clip.c, for "Classical IP over ATM" following RFC 1577).  That
includes, for example, a non-broadcast-based ARP mechanism!

I'll try to figure out what a Linux ATM interface does about IP broadcasts.
It seems like the RFC 1577 instruction is to still allow IP broadcasts
but accept that they won't be mapped onto link-layer broadcasts:

   ATM does not support broadcast addressing, therefore there are no
   mappings available from IP broadcast addresses to ATM broadcast
   services. Note: this lack of mapping does not restrict members from
   transmitting or receiving IP datagrams specifying any of the four
   standard IP broadcast address forms as described in [8].  Members,
   upon receiving an IP broadcast or IP subnet broadcast for their LIS,
   MUST process the packet as if addressed to that station.

In other IP-over-ATM methods it appears that broadcasting is emulated by
a lower layer.

https://www.cse.wustl.edu/~jain/cis788-97/ftp/ip_over_atm/index.html#3