Reducing the packet length

The length reduction of the data packets is defined with a firewall rule, which observes the following conditions:

The reduction affects to all packets that are sent to the interface and do not match the rule.
Reduction applies globally to all packets on the interface; there is no exception of certain protocols.

Important: For devices with VoIP functions that were already integrated or added in with a software option, fragmentation and PMTU reduction can be set separately for SIP calls.

The reduction in the length of data packets is set in LANconfig when defining the QoS rule:

When configuring a new rule in the firewall from the command line, the reduction is set in the following location whereby parameter “P” is for PMTU reduction (Path MTU, MTU = Maximum Transmission Unit) and “F” for the fragment size: Setup > IP-Router > Firewall > Rule-List

Important: PMTU reduction and fragmentation are always related to the physical connection. Specification of the parameter "W" for the WAN transmission direction is therefore not required here and is ignored, if available.

The following example shows a setting for Voice-over-IP telephony:

Policy	Source	Destination	Action	Protocol
VOIP	IP addresses of the IP phones in the LAN, all ports	IP addresses of the IP phones in the LAN, all ports	%Qcds32 %Fpt256	UDP

This rule sets the minimum send and receive bandwidth to 32 kbps while enforcing the reduction of the PMTU of sent packets to 256 bytes. For the TCP connections, the maximum segment size of the local workstation is set to 216, so that the server sends packets of a maximum of 256 bytes in size (PMTU reduction in the transmit direction).