Cannot use VJ compression with pppol2tp plugin

[lano1106]

In the file pppd/plugins/pppol2tp/pppol2tp.c

there is this code:

/* Setup option defaults. Compression options are disabled! */

modem = 0;

lcp_allowoptions[0].neg_accompression = 1;
lcp_wantoptions[0].neg_accompression = 0;

lcp_allowoptions[0].neg_pcompression = 1;
lcp_wantoptions[0].neg_pcompression = 0;

and I have been wondering why no matter what options I was trying to tweak, I couldn't make my side of PPP link use VJ:

Feb 26 10:31:00 whippet2 pppd[72589]: sent [IPCP ConfReq id=0x1 <addr 0.0.0.0>] Feb 26 10:31:00 whippet2 pppd[72589]: rcvd [IPCP ConfReq id=0x1 <compress VJ 0f 01> <addr 10.255.255.0>] Feb 26 10:31:00 whippet2 pppd[72589]: sent [IPCP ConfRej id=0x1 <compress VJ 0f 01>] Feb 26 10:31:00 whippet2 pppd[72589]: rcvd [IPCP ConfNak id=0x1 <addr 198.18.193.208>] Feb 26 10:31:00 whippet2 pppd[72589]: sent [IPCP ConfReq id=0x2 <addr 198.18.193.208>] Feb 26 10:31:00 whippet2 pppd[72589]: rcvd [IPCP ConfReq id=0x2 <addr 10.255.255.0>] Feb 26 10:31:00 whippet2 pppd[72589]: sent [IPCP ConfAck id=0x2 <addr 10.255.255.0>] Feb 26 10:31:00 whippet2 pppd[72589]: rcvd [IPCP ConfAck id=0x2 <addr 198.18.193.208>]

Any idea why this restriction is in place? The other side was willing to use compression with me.

thx,

[lano1106]

I have found a part of the answer. In L2TP RFC 2661, section 8.1, it is written the following:

If VJ compression is used, and neither PPP reliable delivery nor sequence numbers are enabled, each lost packet results in a 1 in 2**16 chance of a TCP segment being forwarded with incorrect contents [RFC1144]. Where the combination of the packet loss rate with this statistical exposure is unacceptable, TCP header compression SHOULD NOT be used.

May I suggest that a reference to this be added to the comment in the code above?

[lano1106]

I still have to dig more the project code to figure out if VJ compression match pcompression or accompression and if the same reasoning applies to both pppd compressions...

[paulusmack]

pcompression and accompression operate at a lower level than VJ compression and are not related to it.