This is an additional patch against the Nautilus6-patched rp-l2tp
(rp-l2tp.tgz obtained from http://software.nautilus6.org/packages/rp-l2tp/
on 2006-01-13) to support large frame that cannot be read from pty
at one time.

/***********************************************************************
*
* async-pppd.c
*
* An LNS handler which starts pppd attached to a PTY in
* asynchronous mode.
*
* Copyright (C) 2002 by Roaring Penguin Software Inc.
*
* This software may be distributed under the terms of the GNU General
* Public License, Version 2, or (at your option) any later version.
*
* LIC: GPL
*
***********************************************************************/

--- rp-l2tp-0.4.nautilus6/handlers/async-pppd.c	2005-03-26 08:41:42.000000000 +0900
+++ rp-l2tp-0.4/handlers/async-pppd.c	2006-01-20 18:58:03.000000000 +0900
@@ -67,6 +67,8 @@
     pid_t pid;			/* PID of child PPPD process */
     int fd;			/* File descriptor for event-handler loop */
     EventHandler *event;	/* Event handler */
+    unsigned char buf[2 * 4096];/* Buffer for PPP to L2TP tunnel */
+    size_t buflen;
 };
 
 static int handle_lac_opts(EventSelector *es, l2tp_opt_descriptor *desc, char const *value);
@@ -304,42 +306,74 @@
 readable(EventSelector *es, int fd, unsigned int flags, void *data)
 {
     unsigned char real_buf[4096+EXTRA_HEADER_ROOM];
-    unsigned char buf[2*4096];
+    unsigned char *buf;
     unsigned char *p, *q, c;
     int escaped = 0;
     int n, l = 0;
     l2tp_session *ses = (l2tp_session *) data;
     int iters = 5;
+    struct slave *sl;
+    unsigned char *sync, *scan, *end;
+
+    if (ses == NULL) {
+	(void)read(fd, real_buf, sizeof(real_buf));
+	return;
+    }
+    sl = ses->private;
 
     /* It seems to be better to read in a loop than to go
        back to select loop.  However, don't loop forever, or
        we could have a DoS potential */
     while(iters--) {
-	n = read(fd, buf, sizeof(buf));
-
-	/* TODO: Check this.... */
-	if ((n <= 4) || (buf[0] != HDLC_SYNC) || (buf[n-1] != HDLC_SYNC))
+	n = read(fd, sl->buf + sl->buflen, sizeof(sl->buf) - sl->buflen);
+	if (n < 0)
 	    return;
+	scan = sl->buf + sl->buflen;
+	sl->buflen += n;
+	sync = sl->buf;
+	end = sl->buf + sl->buflen;
+	while (sync < end) {
+	    if ((scan = memchr(scan, HDLC_SYNC, end - scan)) == NULL) {
+		if (sl->buf != sync) {
+		    sl->buflen = end - sync;
+		    memmove(sl->buf, sync, sl->buflen);
+		} else if (sl->buflen == sizeof(sl->buf)) {
+		    sl->buflen = 0;
+		}
+		return;
+	    }
+	    if (*sync != HDLC_SYNC) {
+		/* discard out of sync data */
+		sync = scan++;
+		continue;
+	    }
+	    buf = sync + 1;
+	    n = scan - buf;
+	    sync = scan++;
+
+	    l = 0;
+	    /* EXTRA_HEADER_ROOM bytes extra space for l2tp header */
+	    q = real_buf+EXTRA_HEADER_ROOM;
+	    for (p = buf; p < buf + n; p++) {
+		c = *p;
+		if (escaped) {
+		    escaped = 0;
+		    *q++ = c ^ HDLC_XOR;
+		    l++;
+		} else if (c == HDLC_ESC)
+		    escaped = 1;
+		else {
+		    *q++ = c;
+		    l++;
+		}
+	    }
 
-	if (!ses) continue;
+	    if (l <= 4)
+		continue;
 
-	/* EXTRA_HEADER_ROOM bytes extra space for l2tp header */
-	q = real_buf+EXTRA_HEADER_ROOM;
-	for (p = buf + 1; p < buf + n - 1; p++) {
-	    c = *p;
-	    if (escaped) {
-		escaped = 0;
-		*q++ = c ^ HDLC_XOR;
-		l++;
-	    } else if (c == HDLC_ESC)
-		escaped = 1;
-	    else {
-		*q++ = c;
-		l++;
-	    }
+	    /* Chop off framing and FCS bytes */
+	    l2tp_dgram_send_ppp_frame(ses, real_buf+EXTRA_HEADER_ROOM+2, l-4);
 	}
-	/* Chop off framing and FCS bytes */
-	l2tp_dgram_send_ppp_frame(ses, real_buf+EXTRA_HEADER_ROOM+2, l-4);
     }
 }
 
@@ -366,6 +400,7 @@
     if (!sl) return -1;
     sl->ses = ses;
     sl->es = es;
+    sl->buflen = 0;
 
     /* Get pty */
     if (pty_get(&m_pty, &s_pty) < 0) {