DSCP (Differentiated Services Code Point) Bleaching

[Adrian Fretwell]

Some of you may have noticed that the default FusionPBX installation includes some Iptables rules to set the DSCP byte to 46 (Expedited Forwarding) for outgoing RTP packets and 26 for outgoing SIP packets in the IPv4 header.

Yealink phones also set these same QOS (Quality of Service) tags.

This DSCP tagging is done to give SIP and especially RTP a priority route over the network.

I have noticed that here in the UK these DSCP values, more often than not, get set to zero or "bleached" as the packets traverse the internet between customer and data centre.

I can see the DSCP tags set correctly as packets leave my premises on the fibre, but they are all set to zero when they arrive at the DC. Same in the other direction set correctly when packets leave the server in the DC, but set to zero when they arrive at the destination.

I have found several routers at customers sites that set DSCP to zero before placing the packet on the WAN side.

Does anyone have any more insights in to this or any suggestions how we can better ensure end to end QOS for VoIP?

 

[ad5ou]

There is no QOS on the public internet. The only way for QOS settings to survive across WAN links is via MPLS or dedicated links.

Generally the best you can do is have a good router at the edge and enough bandwidth to handle normal usage.

 

[Adrian Fretwell]

There is no QOS on the public internet. The only way for QOS settings to survive across WAN links is via MPLS or dedicated links.

Generally the best you can do is have a good router at the edge and enough bandwidth to handle normal usage.

I thought that was the case but I had to ask the question to be sure. What I don't understand is why zero out DSCP on the public internet, why not just ignore it but leave it alone?

 

[ad5ou]

The general Internet may not have QOS, but most ISP's and backbone providers have QOS in the same equipment passing general internet packets along with who knows what. Internal management traffic, MPLS traffic, P2P circuits, etc