cRTP without TCP Compression - Possible??

[pborelli]

I have configured cRTP on a PPP (in fact, Multilink PPP) link. I noticed that, when the command

"ip rtp header-compression"

is issued, the command

"ip tcp header-compression"

is automatically added by the router.

What makes me worry is that, as this link transports both voice and data (and much data, in fact), the TCP compression will take much of my CPU. And, as I need header compression only for voice packets, which are NOT TCP, I'd like to disable the TCP header compression. But, for my surprise, when I disable it, using the command

"no ip tcp header-compression"

the RTP compression is removed too!

Is there a way to have cRTP on a PPP link WITHOUT TCP compression?

Before going to PPP, I was using Frame Relay with RTP compression, and it was NOT necessary to have TCP compression too.

This is a link between a 2620 and a 7206, both with 12.2(1). The rate is 512k.

I'd thank for ANY input.

Paulo.

[bstremp]

The docs at http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt6/qcfcrtp.htm#36050 don’t seem to indicate tcp is required with rtp header compression. It could be an IOS bug. Check with TAC.

[msolak]

Hi Paulo,

i think the cRTP feature is only for IP/UDP/RTP header.

Regards

Mehmet

[bphelps]

cRTP "compresses" both RTP UDP packets and all TCP packets. You can't tell cRTP to stop compressing TCP packets. It does it all. You might accurately call it cRTP-TCP, or even more accurately call it truncated-RTP-TCP since it doesn't actually compress the packets but merely removes most of the header info, reassembling the chopped up packets on the recieving router.

[pborelli]

Thank you for the explanation. But why can't RTP header compression be configured independently of TCP?? As most of the link will be used by data packets, there'll be much more demand for the CPU, which is not desired (mainly because TCP compression will not be so efficient as RTP compression, since data packets are much bigger than voice packets). What do you think?

[bphelps]

Yeah, but it doesn't work that way, though you made a good point.

You could use two serial cards and route all voip traffic through one and data through the other, applying crtp on only one card.

[pborelli]

but in this case, I wouldn't be able to share data and voice on the same BW... What I think is weird is: why the router needs TCP compression to perform RTP compression??