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, 34 insertions, 0 deletions

diff --git a/drivers/ppposeq/ppposeq.h b/drivers/ppposeq/ppposeq.h
new file mode 100644
index 00000000..ecdbdfb1
--- /dev/null
+++ b/drivers/ppposeq/ppposeq.h
@@ -0,0 +1,34 @@
+#ifndef __PPPOSEQ_H
+#define __PPPOSEQ_H
+
+#include <linux/if_pppox.h>
+
+/*
+ * ppposeq - PPP over a SEQPACKET AF_PPPOX socket.
+ *
+ * Replaces the pty + ppp_async transport for userspace PPP terminators
+ * such as sstp. A pty is a byte stream, so frame boundaries are lost in
+ * the tty flip buffer and have to be rebuilt with HDLC escape+FCS framing.
+ * Here the socket itself is the ppp endpoint and each datagram carries
+ * exactly one ppp frame, so no framing is needed on either side.
+ *
+ *	fd = socket(AF_PPPOX, SOCK_SEQPACKET, PX_PROTO_OSEQ);
+ *	connect(fd, &sa, sizeof(sa));		// registers the channel
+ *	ioctl(fd, PPPIOCGCHAN, &idx);
+ *	chan = open("/dev/ppp"); ioctl(chan, PPPIOCATTCHAN, &idx);
+ *	// frames flow over fd with send()/recv()
+ *
+ * Only the protocol number is new; everything else uses the common
+ * AF_PPPOX and PPPIOC* interfaces.
+ */
+
+#ifndef PX_PROTO_OSEQ
+#define PX_PROTO_OSEQ	3
+#endif
+
+struct sockaddr_ppposeq {
+	__kernel_sa_family_t sa_family;		/* AF_PPPOX */
+	unsigned int	sa_protocol;		/* PX_PROTO_OSEQ */
+} __attribute__((packed));
+
+#endif