This can also be returned by radius via the SN-PPP-Data-Compression attribute which also allows specifying various combinations of compression types. Add the “no” keyword as follows to turn off PPP data compression: “no ppp data-compression protocols”.
During PPP negotiation, the PDSN advertises the compression types it can support, and the mobile device will choose the compression type it wants, and PDSN will ensue to do the same. Below is an example of the protocol exchange for both IP header and data compression types. For PPP data compression, the bold black shows the PDSN-initiated negotiation while the red shows the mobile-initiated negotiation. Some of the remaining lines cover the IP Header compression negotiation. The accounting request confirms that MPPC data compression and VJ Header compression were negotiated in this case:
INBOUND>>>>> 19:59:31:619 Eventid:24900(6) RADIUS ACCOUNTING Rx PDU, from 192.168.50.200:1813 to 192.168.50.151:32793 (20) PDU-dict=starent-vsa1 Code: 5 (Accounting-Response) Id: 3 Length: 20 Authenticator: 6B C1 C5 D7 3D 6A 51 CD 62 32 24 67 58 34 B9 A5
In the case of PPP data compression for MPPC and STAC, not all PPP packets will be compressed, but only packets greater than a specific size. The minimum compression size can be specified as follows in the subscriber profile:
ppp min-compression-size <size>
(For Deflate, all packets are compressed.) In the case where packet size meets the minimum size for compression, the source and destination addresses of the packets will NOT be displayed by default in monitor subscriber traces, because the data is still compressed. In order to display them, turn on “19 User L3”, which will show output from the sessmgr process uncompressing them. This is the only situation where this option will give you more data than you would otherwise already get by default. You will see the same packets displayed twice, and one of the instances (depending on direction) will have the complete data. Here is an example of this for ICMP request and response packets that were large enough to be compressed:
It should be noted that PPP data compression should be used with care. Some mobile devices are not the robust in implementing data compression. Also, data compression uses lots of CPU resources, so if many subscribers are doing it, the chassis CPU usage should be monitored with “show cpu table”.
Imported from Starent Networks Knowledgebase Article # 10500