The goal of prioritizing VoIP traffic is to not delay the transmission of VoIP traffic by moving it ahead of larger packets or less time sensitive packets.  This is done within a router that implements Priority Queuing.  The router places VoIP packets in the “priority queue” and transmits them before other packets that may have arrived at the router before the VoIP packets arrived.  The router can identify the markings on the VoIP packets, places them in the priority queue, and then transmits them first.

Reply Cloud tests for this as part of the Network Assessment for a LAN with results being rendered in the Firewall test results grid.  This test is only available for locations utilizing a Reply Sidekick device.  Not available in Windows or Mac desktop application feature set due to OS security restrictions.

Test Methodology

The Reply packet prioritization test performs two tests.  The first test is a baseline test where all packets are sent with DSCP 0x00 marking.  The second test sends large 1500-byte packets with DSCP 0x00 marking and small 128-byte packets with DSCP 0xEF marking representing the VoIP packets.  All packets in both tests are numbered and transmitted in sequence from the sender which is the Reply Sidekick.

During the second test, the expected result is that the router (running a Priority Queuing algorithm) will place the 128-byte 0xEF marked packets in the priority queue and transmit the packets ahead of the larger 1500-byte 0x00 marked packets.  If those “VoIP” packets arrive out of order or behind the 0x00 marked packets at the Reply Cloud or Carrier test receiver (ReplyPoint Server) the assumption is that prioritization is not being utilized. 

In a normal case where a SIP Client has a private IP address and NAT is performed by a CPE edge router, only the IP address in the IP header is replaced by the NAT operation.  In the example that would be 10.4.12.35 -> 155.166.40.93 being replaced in the IP header.  With SIP-ALG = OFF, the original private IP address will still be seen all the way end-to-end in the SIP INVITE message at the receiving HPBX.  With SIP-ALG = ON, the ALG Gateway (frequently also the CPE edge router) will also place its address into the SIP INVITE as the VIA, From, and Contact address.

Remedy

There are several ways to prioritize VoIP traffic with your router:

1. Quality of Service (QoS) settings: Many routers have QoS settings that allow you to prioritize certain types of traffic, such as VoIP, over other types of traffic. You can typically access these settings through the router’s web-based interface.  It is common for many routers to have a “VoIP Prioritization” setting predefined and visible.
2. Port forwarding: If your VoIP service uses a specific port, you can set up port forwarding to prioritize traffic on that port.
3. Traffic shaping: You can use traffic shaping to limit the bandwidth available to certain types of traffic, such as file-sharing or streaming, and allocate more bandwidth to VoIP traffic.
4. Virtual LAN (VLAN): If your router supports VLANs, you can create a separate VLAN for your VoIP traffic and assign it higher priority than other types of traffic.
5. Differentiated Services Code Point (DSCP): Some routers allow you to set a DSCP value for different types of traffic, which can be used to prioritize certain types of traffic

It’s important to note that these techniques may not work for all routers and may require some configuration to set up. If you’re not comfortable making these changes yourself, you may want to consider consulting with a network administrator or contacting your router’s manufacturer for assistance.