diff -r 823aea72606b include/flounder/flounder_support_ump.h
--- a/include/flounder/flounder_support_ump.h	Tue Jan 17 11:31:21 2012 -0800
+++ b/include/flounder/flounder_support_ump.h	Wed Jan 18 11:08:52 2012 -0800
@@ -31,7 +31,7 @@
 struct flounder_ump_state {
     struct ump_chan chan;
 
-    ump_index_t sent_id;   ///< Sequence number of next message to be sent
+    ump_index_t next_id;   ///< Sequence number of next message to be sent
     ump_index_t seq_id;    ///< Last sequence number received from remote
     ump_index_t ack_id;    ///< Last sequence number acknowledged by remote
     ump_index_t last_ack;  ///< Last acknowledgement we sent to remote
@@ -57,7 +57,7 @@
 
 /// Computes (from seq/ack numbers) whether we can currently send on the channel
 static inline bool flounder_stub_ump_can_send(struct flounder_ump_state *s) {
-    return (ump_index_t)(s->sent_id - s->ack_id) < s->chan.max_send_msgs;
+    return (ump_index_t)(s->next_id - s->ack_id) <= s->chan.max_send_msgs;
 }
 
 #define ENABLE_MESSAGE_PASSING_TRACE 1
@@ -69,13 +69,13 @@
 #if ENABLE_MESSAGE_PASSING_TRACE
     trace_event_raw((((uint64_t)0xEA)<<56) | 
                     ((uint64_t)s->chan.sendid << 12) | 
-                    (s->sent_id & 0xffff));
+                    (s->next_id & 0xffff));
 #endif // ENABLE_MESSAGE_PASSING_TRACE
     assert(s->chan.sendid != 0);
     assert(msgtype < (1 << FL_UMP_MSGTYPE_BITS)); // check for overflow
     ctrl->header = ((uintptr_t)msgtype << UMP_INDEX_BITS) | (uintptr_t)s->seq_id;
     s->last_ack = s->seq_id;
-    s->sent_id++;
+    s->next_id++;
 }
 
 /// Process a "control" word
diff -r 823aea72606b lib/barrelfish/flounder_support.c
--- a/lib/barrelfish/flounder_support.c	Tue Jan 17 11:31:21 2012 -0800
+++ b/lib/barrelfish/flounder_support.c	Wed Jan 18 11:08:52 2012 -0800
@@ -358,7 +358,7 @@
 
 void flounder_stub_ump_state_init(struct flounder_ump_state *s, void *binding)
 {
-    s->sent_id = 0;
+    s->next_id = 1;
     s->seq_id = 0;
     s->ack_id = 0;
     s->last_ack = 0;