Commits on Mar 20, 2024

1. Recvmsg: return packet info (dst addr and nw i/f) …

   This extends `ddsrt_recvmsg` to not only return the packet's source
   address, but also its destination address and the interface on which it
   was received.  If the operating system does not provide this
   information, they can be set to "unknown".
   
   The additional information is valuable because, at least on Linux, one
   sometimes receives multicast packets from an interface on which one did
   not join the multicast group for reasons I am yet to discover.  This
   allows detecting such cases.  It also makes it possible to make a more
   informed guess about which interface is the most suitable choice for
   communicating with a node X, and it also seems likely that receiving a
   multicast from X is a good indicator that sending a multicast to node X
   will work.
   
   This commit does not use the information other than for writing it in
   the trace.  Current implementation is verified to work on macOS, Linux
   and Windows for IPv4 and IPv6.
   
   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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2. Split up ddsi_addrset_from_locatorlists …

   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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3. Ignore distant addresses if nearby ones exist …

   Before this commit, the decision which unicast locators to include was
   to accept all distant locators if they were routable addresses and we
   had a network interface that looked like it could handle them.  The
   "DontRoute" configuration option would change this and made it ignore
   all distant locators.
   
   The first turns out to be problematic in practice because quite often
   there are no suitable routes defined in the networking stack, but
   defaulting to not allowing any routing is also problematic.
   
   What this commit does is twofold:
   
   * Firstly, it uses the new received packet information to associate the
     address with the network interface over which it was received (if
     possible) instead of the first seemingly suitable one;
   
   * Secondly, it only uses distant addresses when it has been established
     that no nearby addresses are in the advertised set.  This heuristic
     probably better fits the use cases.
   
   What it perhaps should also do is reject the address if we know what
   interface it arrived on and that interface is not enabled in the
   configuration.
   
   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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4. Try multicast on loopback when flag not set …

   For IPv4 on Linux, the common case seems to be that the loopback
   interface is perfectly capable of handling multicast but the interface
   usually don't indicate this.  This commit adds a run-time test by
   sending a small multicast packet to itself in this specific case.
   
   For IPv6 there is some other complication, the test says the network is
   unreachable.  Perhaps correctly, perhaps there's a trick I haven't found
   yet.  Either way, if errs on the safe side, so that's fine.
   
   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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5. Remove participant index limit in documentation …

   There is no simple hard limit (other than 2^32-2).
   
   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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6. Make "allow multicast" per-interface …

   This adds an "allow_multicast" attribute to the network interface
   configuration and uses the old General/AllowMulticast as the default
   value.  This way, both backwards compatibility and easy global
   configuration are supported.
   
   This incidentally addresses a slew of bugs in multi-network
   configurations, but it does make things rather more complicated.
   
   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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7. Ignore SPDP from non-selected interfaces …

   Ignore SPDP messages that:
   
   * Provably arrived over a network interface we didn't enable
   
   * Were multicast over a network interface on which did not allow
     multicast discovery.
   
   For an example why this is useful: on Linux, running two proceses one allowing multicast
   over loopback but not Ethernet, and one allowing it over Ethernet but not over loopback,
   both processes do receive the multicast message despite not having joined the multicast
   group on the interface it is sent on.  With this commit, those SPDP messages are ignored
   and the two processes do not discover each other.
   
   The direct reason for this change is to make it possible to have discovery over Ethernet
   only with unicast messages, while using relying on multicast over loopback.  This turns
   out to be a rather important set-up, and is even the new default in ROS 2.
   
   Signed-off-by: Erik Boasson <eb@ilities.com>

   

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