QoS & VoIP, Packet fragmentation for Ethernet?

[Randall White]

Hi All,

I am setting up some IP phones at a remote office over a VPN. The VPN is terminated to a PIX 501. Voice quality is good, except when there is a large file transfer over the VPN. Since the PIX doesn't do QoS, I will be putting an ethernet-to-ethernet router on the inside to handle queueing.

My question is, since the uplink speed is low (128k), how can I fragment the packets going to the VPN? I think the serialization delay is killing the VoIP packets. I want to do something like Frame Relay Fragmentation, but on ethernet.

I've thought of limiting MTU, but I only want to do this for traffic outbound to the VPN, not everything going to and from the internet.

Any ideas are appreciated.

Thanks, Randy

[vkapoor5]

Serialization delay is most likely to occur only on low speed links, that is the reason mechanisms like PPP multilink and Frame-relay fragmentation are available for such links which use any of these encapsulations. I guess the PIX will be connected through a router to Internet/VPN. So you can try to use one of these mechanisms depending on the encapsulation you use on that router. Since these mechanisms work on the hardware queue, I don't think you can restrict it to certain flows alone.