diff --git a/paramiko/transport.py b/paramiko/transport.py
index 29f59e8..4e2ddbc 100644
--- a/paramiko/transport.py
+++ b/paramiko/transport.py
@@ -22,7 +22,7 @@
 L{BaseTransport} handles the core SSH2 protocol.
 """
 
-import sys, os, string, threading, socket, struct
+import sys, os, string, threading, socket, struct, time
 
 from common import *
 from ssh_exception import SSHException
@@ -154,7 +154,7 @@ class BaseTransport (threading.Thread):
         # /negotiated crypto parameters
         self.expected_packet = 0
         self.active = False
-        self.initial_kex_done = 0
+        self.initial_kex_done = False
         self.write_lock = threading.Lock()	# lock around outbound writes (packet computation)
         self.lock = threading.Lock()		# synchronization (always higher level than write_lock)
         self.channels = { }			# (id -> Channel)
@@ -171,6 +171,9 @@ class BaseTransport (threading.Thread):
         self.received_packets_overflow = 0
         # user-defined event callbacks:
         self.completion_event = None
+        # keepalives:
+        self.keepalive_interval = 0
+        self.keepalive_last = time.time()
         # server mode:
         self.server_mode = 0
         self.server_key_dict = { }
@@ -432,6 +435,8 @@ class BaseTransport (threading.Thread):
         @param bytes: the number of random bytes to send in the payload of the
         ignored packet -- defaults to a random number from 10 to 41.
         @type bytes: int
+
+        @since: fearow
         """
         m = Message()
         m.add_byte(chr(MSG_IGNORE))
@@ -464,6 +469,19 @@ class BaseTransport (threading.Thread):
                 break
         return True
 
+    def set_keepalive(self, interval):
+        """
+        Turn on/off keepalive packets (default is off).  If this is set, after
+        C{interval} seconds without sending any data over the connection, a
+        "keepalive" packet will be sent (and ignored by the remote host).  This
+        can be useful to keep connections alive over a NAT, for example.
+        
+        @param interval: seconds to wait before sending a keepalive packet (or
+        0 to disable keepalives).
+        @type interval: int
+        """
+        self.keepalive_interval = interval
+
     def global_request(self, kind, data=None, wait=True):
         """
         Make a global request to the remote host.  These are normally
@@ -481,6 +499,8 @@ class BaseTransport (threading.Thread):
         request was successful (or an empty L{Message} if C{wait} was
         C{False}); C{None} if the request was denied.
         @rtype: L{Message}
+
+        @since: fearow
         """
         if wait:
             self.completion_event = threading.Event()
@@ -491,6 +511,7 @@ class BaseTransport (threading.Thread):
         if data is not None:
             for item in data:
                 m.add(item)
+        self._log(DEBUG, 'Sending global request "%s"' % kind)
         self._send_message(m)
         if not wait:
             return True
@@ -691,6 +712,13 @@ class BaseTransport (threading.Thread):
         finally:
             self.lock.release()
 
+    def _check_keepalive(self):
+        if (not self.keepalive_interval) or (not self.initial_kex_done):
+            return
+        now = time.time()
+        if now > self.keepalive_last + self.keepalive_interval:
+            self.global_request('keepalive@lag.net', wait=False)
+
     def _py22_read_all(self, n):
         out = ''
         while n > 0:
@@ -698,6 +726,7 @@ class BaseTransport (threading.Thread):
             if self.sock not in r:
                 if not self.active:
                     raise EOFError()
+                self._check_keepalive()
             else:
                 x = self.sock.recv(n)
                 if len(x) == 0:
@@ -720,9 +749,11 @@ class BaseTransport (threading.Thread):
             except socket.timeout:
                 if not self.active:
                     raise EOFError()
+                self._check_keepalive()
         return out
 
     def _write_all(self, out):
+        self.keepalive_last = time.time()
         while len(out) > 0:
             n = self.sock.send(out)
             if n <= 0:
@@ -1156,7 +1187,7 @@ class BaseTransport (threading.Thread):
         self.e = self.f = self.K = self.x = None
         if not self.initial_kex_done:
             # this was the first key exchange
-            self.initial_kex_done = 1
+            self.initial_kex_done = True
         # send an event?
         if self.completion_event != None:
             self.completion_event.set()
@@ -1169,6 +1200,7 @@ class BaseTransport (threading.Thread):
 
     def _parse_global_request(self, m):
         kind = m.get_string()
+        self._log(DEBUG, 'Received global request "%s"' % kind)
         want_reply = m.get_boolean()
         ok = self.check_global_request(kind, m)
         extra = ()
@@ -1186,11 +1218,13 @@ class BaseTransport (threading.Thread):
             self._send_message(msg)
 
     def _parse_request_success(self, m):
+        self._log(DEBUG, 'Global request successful.')
         self.global_response = m
         if self.completion_event is not None:
             self.completion_event.set()
         
     def _parse_request_failure(self, m):
+        self._log(DEBUG, 'Global request denied.')
         self.global_response = None
         if self.completion_event is not None:
             self.completion_event.set()