diff --git a/src/include/gpxe/tcpip_if.h b/src/include/gpxe/tcpip_if.h
index bac39699..ce095efa 100644
--- a/src/include/gpxe/tcpip_if.h
+++ b/src/include/gpxe/tcpip_if.h
@@ -83,7 +83,7 @@ extern void trans_rx ( struct pk_buff *pkb, uint8_t trans_proto,
 extern int trans_tx ( struct pk_buff *pkb, struct tcpip_protocol *tcpip, 
 		      struct sockaddr *dest );
 
-extern uint16_t calc_chksum ( void *data, size_t len );
+extern uint16_t calc_chksum ( void *b, int len );
 
 extern struct tcpip_protocol * find_tcpip_protocol ( uint8_t trans_proto );
 extern struct tcpip_net_protocol * find_tcpip_net_protocol ( sa_family_t sa_family );
diff --git a/src/include/gpxe/udp.h b/src/include/gpxe/udp.h
index e6511160..68034583 100644
--- a/src/include/gpxe/udp.h
+++ b/src/include/gpxe/udp.h
@@ -41,6 +41,23 @@ struct udp_connection;
  *
  */
 struct udp_operations {
+	
+	/**
+	 * Transmit data
+	 *
+	 * @v conn	UDP connection
+	 * @v buf	Temporary data buffer
+	 * @v len	Length of temporary data buffer
+	 *
+	 * The application may use the temporary data buffer to
+	 * construct the data to be sent.  Note that merely filling
+	 * the buffer will do nothing; the application must call
+	 * udp_send() in order to actually transmit the data.  Use of
+	 * the buffer is not compulsory; the application may call
+	 * udp_send() on any block of data.
+	 */
+	void ( * senddata ) ( struct tcp_connection *conn, void *buf,
+			      size_t len );
 	/**
 	 * New data received
 	 *
@@ -69,11 +86,6 @@ struct udp_connection {
 	struct udp_operations *udp_op;
 };
 
-/**
- * List of registered UDP connections
- */
-static LIST_HEAD ( udp_conns );
-
 /**
  * UDP protocol
  */
diff --git a/src/net/tcpip_if.c b/src/net/tcpip_if.c
index 5c5e5962..80d69268 100644
--- a/src/net/tcpip_if.c
+++ b/src/net/tcpip_if.c
@@ -107,30 +107,19 @@ int trans_tx ( struct pk_buff *pkb, struct tcpip_protocol *tcpip,
 /**
  * Calculate internet checksum
  *
- * @v data      Pointer to the data
- * @v len       Length of data to be checksummed
- * @ret chksum  16 bit internet checksum
- *
- * This function calculates the internet checksum (refer RFC1071) for "len"
- * bytes beginning at the location "data"
+ * @v b		Pointer to the data
+ * @v len	Length of data to be checksummed
+ * @ret result	16 bit internet checksum
  */
-uint16_t calc_chksum ( void *data, size_t len ) {
-	register long sum = 0;
-	uint16_t checksum;
-	unsigned short *temp;
-	while ( len > 1 ) {
-		temp = (unsigned short*) data++;
-		sum += *temp;
-		len -= 2;
-	}
-	if ( len > 0 ) {
-		sum += *(unsigned char *)data;
-	}
-	while ( sum >> 16 ) {
-		sum = ( sum & 0xffff ) + ( sum >> 16 );
-	}
-	checksum = ~sum;
-	return checksum;
+uint16_t calc_chksum(void *b, int len) {
+	uint16_t *buf = b, result;
+	uint16_t sum=0;
+	for ( sum = 0; len > 1; len -= 2 ) /* Sum all 16b words */
+		sum += *buf++;
+	if ( len == 1 )                  /* If any stray bytes, */
+		sum += *(unsigned char*)buf;          /* add to sum */
+	sum = (sum >> 16) + (sum & 0xffff);    /* Add the carry */
+	sum += (sum >> 16);                          /* (again) */
+	result = ~sum;             /* Take the one's complement */
+	return result;                      /* Return 16b value */
 }
-
-
diff --git a/src/net/udp.c b/src/net/udp.c
index b84d5161..0fca99a4 100644
--- a/src/net/udp.c
+++ b/src/net/udp.c
@@ -18,6 +18,14 @@
  * UDP protocol
  */
 
+/**
+ * List of registered UDP connections
+ */
+static LIST_HEAD ( udp_conns );
+
+/**
+ * Some utility functions
+ */
 static inline void copy_sockaddr ( struct sockaddr *source, struct sockaddr *dest ) {
 	memcpy ( dest, source, sizeof ( *dest ) );
 }
@@ -98,7 +106,27 @@ int udp_buf_alloc ( struct udp_connection *conn, size_t len ) {
 }
 
 /**
- * Send data via a UDP connection
+ * User request to send data via a UDP connection
+ *
+ * @v conn	UDP connection
+ *
+ * This function allocates buffer space and invokes the function's senddata()
+ * callback. The callback may use the buffer space
+ */
+int udp_senddata ( struct udp_connection *conn ) {
+	conn->tx_pkb = pkb_alloc ( UDP_MAX_TXPKB );
+	if ( conn->tx_pkb == NULL ) {
+		DBG ( "Error allocating packet buffer of length %d\n",
+							UDP_MAX_TXPKB );
+		return -ENOMEM;
+	}
+	conn->udp_op->senddata ( conn, conn->tx_pkb, pkb_len ( conn->tx_pkb ) );
+	return 0;
+}
+		
+
+/**
+ * Transmit data via a UDP connection
  *
  * @v conn      UDP connection
  * @v data      Data to send
@@ -124,7 +152,7 @@ int udp_send ( struct udp_connection *conn, const void *data, size_t len ) {
 
 		/* Reserve space for the headers and copy contents */
 		pkb_reserve ( conn->tx_pkb, UDP_MAX_HLEN );
-		memcpy ( pkb_put ( conn->tx_pkb, len ), data, len );
+		memmove ( pkb_put ( conn->tx_pkb, len ), data, len );
 	}
 
 	/*