The preferred handling of data packets from an important application can be jeopardized by extremely long data packets from other applications. This may be the case, for example, when IP telephony and FTP data transfer are simultaneously active on the WAN connection.
FTP transfer uses quite large data packets of 1500 bytes, while the Voice-over-IP connection packets are of 24 bytes net and are sent in relatively frequent intervals. If FTP packets are present in the send queue of the device at the moment when a VoIP packet is to be transmitted, the VoIP packet cannot be sent until the line is free again. Depending on the connection’s transmission rate, this can lead to a noticeable delay in voice transmission.
This disruptive behavior can be offset if data packets not belonging to the preferred QoS connection do not exceed a certain length. For example, the FTP connection only sends packets small enough to ensure that the time-critical VoIP connection can deliver packets at the necessary frequency and without delay. For TCP-secured FTP transfers, delays are not critical.
There are two different ways to influence the length of a packet:
- The device can inform the users of the data connection that they should only send data packets up to a certain length. This is done by forcing a suitable PMTU (Path Maximum Transmission Unit) at the sending end in a process referred to as "PMTU reduction".
PMTU reduction can be operated in both the transmit and receive directions. For the transmit direction, the senders in their own LAN are set to a smaller packet size by PMTU reduction; for the receive direction, the senders in the WAN, e.g. Web or FTP servers on the Internet, are also instructed to deliver a smaller packet size.
If the data connection is already established when the VoIP connection starts, the senders very quickly regulate the packet length down to the permissible value. When a data connection is established with a VoIP connection already in place, the maximum permissible packet length is negotiated directly during the connection handshake.
Important: The reduced packet length on the data connection is still used even after the VoIP connection is terminated and until the sender re-checks the PMTU value.
- The device itself is able to split packets due for transmission but which are above an adjustable maximum size (e.g. 256 bytes) into smaller units. This method referred to as "fragmentation" is not supported by all servers on the Internet, because processing fragmented packets is considered a security risk, so it is often turned off. This may cause disruptions when downloading data or when transferring websites, for example. This method is therefore recommended only for connections that do not involve unknown Internet servers, such as for the direct connection of branches to a central office over a VPN connection that does not simultaneously run Internet traffic.
