sstp: add ppposeq transport to avoid userspace HDLC framingsstp-ppposeq

A pty is a byte stream, so the tty flip buffer merges frames written
back to back and sstp has to re-delimit them with async HDLC escaping
and a CRC-16 FCS. On a 1452-byte payload that is ~3600 ns per frame,
most of it spent on the FCS.

PPPOSEQ is a pppox protocol whose socket is the ppp endpoint itself,
so one datagram is one frame and no framing is needed at all. The
same payload takes ~380 ns per frame, about 9 times less. Requires
kernel 2.6.37, the first with PX_MAX_PROTO 3, whose remaining slot
it claims.
Supported kernels are from 2.6.37 to 7.2.

The new ppp-mode option selects the transport; auto, the default,
falls back to async when the module is unavailable, so hosts with
prebuilt kernels are unaffected.

PPP_SYNC is removed, being disabled and unfixable over a pty: frame
boundaries cannot be recovered from the stream, and coalescing cannot
be prevented since frames arrive from the network stack.

1 files changed, 1 insertions, 0 deletions

diff --git a/README.md b/README.md
index 2ce25b5f..a78cb145 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,7 @@ Useful build options:
 * BUILD_PPTP_DRIVER=TRUE builds the PPTP kernel module.
 * BUILD_IPOE_DRIVER=TRUE builds the IPoE kernel module.
 * BUILD_VLAN_MON_DRIVER=TRUE builds the VLAN monitoring kernel module.
+* BUILD_PPPOSEQ_DRIVER=TRUE builds the PPPoSEQ kernel module.
 * BUILD_DRIVER_ONLY=TRUE builds only the selected kernel modules.
 * KDIR=/path/to/kernel/build sets the kernel build directory.
 * RADIUS=FALSE omits RADIUS support.