l2tp: Add protocol field decompression
When Protocol Field Compression (PFC) is enabled, the "Protocol" field in PPP packet will be received without leading 0x00. See section 6.5 in RFC 1661 for details. So let's decompress protocol field if needed, the same way it's done in drivers/net/ppp/pptp.c. In case when "nopcomp" pppd option is not enabled, PFC (pcomp) can be negotiated during LCP handshake, and L2TP driver in kernel will receive PPP packets with compressed Protocol field, which in turn leads to next error: Protocol Rejected (unsupported protocol 0x2145) because instead of Protocol=0x0021 in PPP packet there will be Protocol=0x21. This patch unwraps it back to 0x0021, which fixes the issue. Sending the compressed Protocol field will be implemented in subsequent patch, this one is self-sufficient. Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org> Signed-off-by: David S. Miller <davem@davemloft.net>
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