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Traffic Shaping and VPNs

The following discussions pertain primarily to ALTQ shaping. Limiters will work fine with VPNs as they would with any other interface and rules. Only the ALTQ shaper requires special consideration.

Traffic shaping with VPNs is a tricky topic because VPN traffic is considered separate from, but also a part of, the WAN traffic through which it also flows. If WAN is 10 Mbit/s, then the VPN can also use 10Mbit/s, but there is not actually 20Mbit/s of bandwidth to consider, only 10Mbit/s. As such, methods of shaping that focus more on prioritization than bandwidth are more reliable, such as PRIQ or in some cases, CBQ.

If all traffic inside the VPN must be prioritized by the firewall, then it is enough to consider only the VPN traffic itself directly on WAN, rather than attempting to queue traffic on the VPN separately. In these cases, use a floating rule on WAN to match the VPN traffic itself. The exact type of traffic varies depending on the type of VPN. IPsec and PPTP traffic on WAN can both be prioritized by the shaper wizard, and these rules can be used as an example to match other protocols.

OpenVPN

With OpenVPN, multiple interfaces exist on the operating system, one per VPN. This can make shaping easier in some cases. Features of OpenVPN can also make it easier to shape traffic on WAN and ignore the tunnel itself.

Shaping inside the tunnel

If multiple classes of traffic are carried on the tunnel, then prioritization must be done to the traffic inside the tunnel. In order for the wizard to consider the traffic in this way, the VPN must be assigned as its own interface in the GUI. To accomplish this, assign it as described in Interface assignment and configuration, and then use the shaper wizard as if it were a separate WAN interface, and classify the traffic as usual.

Shaping outside the tunnel (passtos)

If the primary concern is shaping VoIP traffic over a VPN, another choice to consider is the passtos option in OpenVPN, called Type-of-Service in the OpenVPN client or server options. This option copies the TOS bit from the inner packet to the outer packet of the VPN. Thus, if the VoIP traffic has the TOS (DSCP) portion of the packet header set, then the OpenVPN packets will also have the same value.

This option is more useful for signaling intermediate routers about the QoS needs, however. Though the DSCP option on firewall rules can match based on TOS bits, as described in Diffserv Code Point, such matching would have to occur in the packet creating a firewall state, and not on specific packets flowing through that state.

Note

Because this option tells OpenVPN to copy data from the inner packet to the outer packet, it does expose a little information about the type of traffic crossing the VPN. Whether or not the information disclosure, though minor, is worth the risk for the gains offered by proper packet prioritization depends on the needs of the network environment.

IPsec

IPsec is presented to the operating system on a single interface no matter how many tunnels are configured and no matter which WANs are used by the tunnels. This makes shaping IPsec traffic difficult, especially when trying to shape traffic inside one particular IPsec tunnel.

The IPsec interface is also not possible to use on its own as an interface with the wizard. Floating rules can match and queue traffic on the IPsec interface, but in most cases only inbound traffic will be queued as expected. Actual results may vary.