So why they switched to babel ? babel makes some magic with MTU ? I dont see the end of the story. Il 08/ago/2015 04:47, "Jenny Ryan" <[email protected]> ha scritto:
> ---------- Forwarded message ---------- > From: "Marc Juul" <[email protected]> > Date: Aug 6, 2015 9:58 AM > Subject: Why we switched to Babel > To: "Jenny Ryan" <[email protected]> > Cc: > > When people share their internet we use a tunnel to hide their IP (and to > connect them to the mesh through the Internet). This tunnel takes a few > bytes of overhead. All physical connections, whether they're ethernet, wifi > or even an abstraction like a tunnel, have an MTU (Maximum Transfer Unit) > which is the size of the largest packet that can be sent over the > connection. Normally the MTU is 1500 but since the tunnel needs a few bytes > for overhead the MTU of the tunnel will be less than 1500. > > When people connect to the wifi network called peoplesopen.net they will > connect with an MTU of 1500 since this is the default for wifi. We need a > way to tell them that the MTU is actually less than 1500. In an IP network > (which is at layer 3) there is a built-in system for dealing with this. If > any router receives a packet larger than what it can pass on through the > next connection (in this case through the tunnel) it can report back to the > client using the ICMP protocol (a companion protocol to IP, and the > protocol used for ping) that the packet was too big and the client can then > adjust its MTU accordingly. > > For batman-adv, because it is a layer 2 protocol, we don't have this > system available. We tried different tactics such as using DHCP to tell the > clients the MTU they should use, but it turns out that many operating > systems completely ignore this. We tried something called TCP MSS clamping, > which is a bit of a dirty hack, but that only works for TCP, which is a > problem since UDP is widely used for e.g. VOIP, video streaming, gaming, > torrenting, etc. > > Even combining the different tricks we still had a problem where some > operating systems would get in trouble if they tried to send large UDP > packets. We had a moment where we realized that the only types of common > communication that wouldn't work on this mesh would be torrenting and video > streaming from windows computers, and joked about that being a feature > instead of a bug :) but in the end we switched to Babel. > > There was another reason: In batman-adv the MAC address is the identifier > used for each device. It is possible to configure many devices to randomize > their MAC address but it needs the user to do something, so most people > will never know to do it. Having the MAC address as the identifier makes it > pretty easy for anyone to track anyone else as they move about the city, as > long as they know the MAC address of e.g. their phone, which it is easy to > discover if you are ever in the same room with them, just by listening to > network traffic. We had some ideas for how this could be fixed, but the > potential solutions we came up with were never satisfactory. > > For mesh, each time a user connects to a new node they get a new IP > address. In the future we may implement roaming support which will let > people keep their IP as they move around the city, but it will still switch > after e.g. 10 minutes, so tracking people becomes much harder. > > That's a lot of text, but eh it's a complicated issue. It sounds like > you're having many late night conversations. Hope it is enjoyable! > > -- > marc/juul > > > > > _______________________________________________ > Babel-users mailing list > [email protected] > http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/babel-users >
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