MTU for Mesh-VPN
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 Advertisements and DHCP
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 batman-adv).
PMTU: What MTU does the path between your gateway and each of its peers support?
For reference, the complete MTU stack looks like this:
Example for Minimum MTU
Calculate 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 | batman-adv v15 | Ethernet | Payload |
\-------+-----------------+-------------+----------------------------------+
\ ^
|
MTU_LOW = 1280 Byte + 14 Byte + 18 Byte = 1312 Byte
Example for 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
Tables for Different VPN Providers
VPN Protocol Overhead (IPv4)
Overhead of the VPN protocol layers in bytes on top of an Ethernet frame.
fastd |
WireGuard |
|
---|---|---|
IPv4 |
20 |
20 |
UDP |
8 |
8 |
Protocol |
24 |
32 |
TAP |
14 |
/ |
Sum |
66 |
60 |
Intermediate Layer Overhead
Overhead of additional layers on top of the VPN packet needed for different VPN providers.
fastd |
WireGuard |
|
---|---|---|
IPv6 |
/ |
40 |
vxlan |
/ |
16 |
Ethernet |
/ |
14 |
Batman v15 |
18 |
18 |
Ethernet |
14 |
14 |
Sum |
32 |
102 |
Minimum MTU
Calculation of different derived MTUs based on a 1280 byte payload to avoid fragmentation.
Suggestions:
This configuration is only suggested for fastd.
For WireGuard, this configuration is unsuitable. To obtain a 1280 byte payload with our protocol stack (see below), the Ethernet frame payload would be 1442 bytes long (for IPv4). As we assume that the WAN network might have a (worst case) MTU of only 1436 (with DSLite), this packet would be too long for the WAN network.
fastd |
WireGuard |
|
---|---|---|
max unfragmented payload* |
1280 |
1280 |
intermediate layer overhead |
32 |
102 |
VPN MTU** |
1312 |
1382 |
protocol overhead (IPv4) |
66 |
60 |
min acceptable WAN MTU (IPv4) |
1378 |
1442 |
min acceptable WAN MTU (IPv6) |
1398 |
1462 |
* Maximum size of payload going into the bat0 interface, that will not be fragmented by batman.
** This is the MTU that is set in the site.conf.
Maximum MTU
Calculation of different derived MTUs based on a maximum WAN MTU of 1436.
Suggestions:
This configuration can be used for fastd.
For WireGuard, this is the recommended configuration. batman-adv will fragment larger packets transparently to avoid packet loss.
fastd |
WireGuard |
|
---|---|---|
min acceptable WAN MTU (IPv4) |
1436 |
1436 |
protocol overhead (IPv4) |
66 |
60 |
VPN MTU** |
1370 |
1376 |
intermediate layer overhead |
32 |
102 |
max unfragmented payload* |
1338 |
1274 |
min acceptable WAN MTU (IPv6) |
1398 |
1462 |
* Maximum size of payload going into the bat0 interface, that will not be fragmented by batman.
** This is the MTU that is set in the site.conf.
Suggested MSS Values
It is highly advised to use MSS clamping for TCP on the gateways/supernodes in order to avoid the fragmentation mechanism of batman whenever possible. Especially on small embedded devices, fragmentation costs performance.
As batmans fragmentation is transparent to the TCP layer, clamping the MSS
automatically to the PMTU does not work. Instead, the MSS must be specified
explicitly. In iptables, this is done via -j TCPMSS --set-mss X
,
whereby X
is the desired MSS.
Since the MSS is specified in terms of payload of a TCP packet, the MSS is different for IPv4 and IPv6. Here are some examples for different max unfragmented payloads:
max unfragmented payload |
1274 |
1280 |
1338 |
1354 |
---|---|---|---|---|
suggested MSS (IPv4, -40 bytes) |
1234 |
1240 |
1298 |
1314 |
suggested MSS (IPv6, -60 bytes) |
1214 |
1220 |
1278 |
1294 |
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 an MTU of 1312 bytes for fastd and 1376 bytes for WireGuard.