|
@@ -15,3 +15,84 @@ interface. This DNS server must be announced in router advertisements (using
|
|
|
on *batman-adv*. If your mesh does not have global IPv6 connectivity, you can setup
|
|
|
your *radvd* not to announce a default route by setting the *default lifetime* to 0;
|
|
|
in this case, the *radvd* is only used to announce the DNS server.
|
|
|
+
|
|
|
+
|
|
|
+What is a good MTU on the mesh-vpn
|
|
|
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
+
|
|
|
+Setting the MTU on the transport interface requires careful consideration, as
|
|
|
+setting it too low will cause excessive fragmentation and setting it too high
|
|
|
+may leave peers with a broken tunnel due to packet loss.
|
|
|
+
|
|
|
+Consider these key values:
|
|
|
+
|
|
|
+- Payload: Allow for the transport of IPv6 packets, by adhering to the minimum MTU
|
|
|
+ of 1280 Byte specified in RFC 2460
|
|
|
+ - and configure `MSS clamping`_ accordingly,
|
|
|
+ - and announce your link MTU via Router Advertisments and DHCP
|
|
|
+
|
|
|
+ .. MSS clamping: http://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.cookbook.mtu-mss.html
|
|
|
+
|
|
|
+- Encapsulation: Account for the overhead created by the configured mesh protocol
|
|
|
+ encapsulating the payload, which is
|
|
|
+ - up to 32 Byte (14 Byte Ethernet + 18 Byte batadv) for batman-adv compat v15 (v2014.0 and later)
|
|
|
+ - up to 28 Byte (14 Byte Ethernet + 14 Byte batadv) for batman-adv compat v14 (v2011.3.0 until and including v2013.4.0)
|
|
|
+
|
|
|
+- PMTU: What MTU does the path between your gateway and each of its peers support?
|
|
|
+
|
|
|
+For reference, the complete MTU stack looks like this:
|
|
|
+
|
|
|
+.. image:: https://cloud.hamburg.freifunk.net/thumbnail/de738ee3ac/1024/MTU%20within%20a%20batman-adv%20network%20v5.png
|
|
|
+
|
|
|
+Minimum MTU
|
|
|
+-----------
|
|
|
+
|
|
|
+Calculcate the minimum transport MTU by adding the encapsulation overhead to the
|
|
|
+minimum payload MTU required. This is the lowest recommended value, since going
|
|
|
+lower would cause unnecessary fragmentation for clients which respect the announced
|
|
|
+link MTU.
|
|
|
+
|
|
|
+Example: Our network currently uses batman-adv v15, it therefore requires up
|
|
|
+to 32 Bytes of encapsulation overhead on top of the minimal link MTU required for
|
|
|
+transporting IPv6.::
|
|
|
+
|
|
|
+ \ 1312 1294 1280 0
|
|
|
+ \---------+-----------------+-------------+----------------------------------+
|
|
|
+ \TAP | batadv v15 | Ethernet | Payload |
|
|
|
+ \-------+-----------------+-------------+----------------------------------+
|
|
|
+ \ ^
|
|
|
+ |
|
|
|
+
|
|
|
+ MTU_LOW = 1280 Byte + 14 Byte + 18 Byte = 1312 Byte
|
|
|
+
|
|
|
+Maximum MTU
|
|
|
+-----------
|
|
|
+
|
|
|
+Calculating the maximum transport MTU is interesting, because it increases the
|
|
|
+throughput, by allowing larger payloads to be transported, but also more difficult
|
|
|
+as you have to take into account the tunneling overhead and each peers PMTU, which
|
|
|
+varies between providers.
|
|
|
+The underlying reasons are mostly PPPoE, Tunneling and IPv6 transition technologies
|
|
|
+like DS-Lite.
|
|
|
+
|
|
|
+Example: The peer with the smallest MTU on your network is behind DS-Lite and can
|
|
|
+transport IPv4 packets up to 1436 Bytes in size. Your tunnel uses IPv4 (20 Byte),
|
|
|
+UDP (8 Byte), Fastd (24 byte) and you require TAP (14 Byte) for Layer 2 (Ethernet)
|
|
|
+Tunneling.::
|
|
|
+
|
|
|
+ 1436 1416 1408 1384 1370 \
|
|
|
+ +-------------------+--------+-----------------------+-------------+------\
|
|
|
+ | IP | UDP | Fastd | TAP | bat\
|
|
|
+ +-------------------+--------+-----------------------+-------------+--------\
|
|
|
+ ^ \
|
|
|
+ |
|
|
|
+
|
|
|
+ MTU_HIGH = 1436 Byte - 20 Byte - 8 Byte - 24 Byte - 14 Byte = 1370 Byte
|
|
|
+
|
|
|
+Conclusion
|
|
|
+----------
|
|
|
+
|
|
|
+Determining the maximum MTU can be a tedious process, especially since the PMTU
|
|
|
+of peers could change at any time. The general recommendation for maximized
|
|
|
+compatibility is therefore the minimum MTU of 1312 Byte, which works well with
|
|
|
+all combinations of IPv4, IPv6, batman-adv compat v14 and v15.
|