diff --git a/src/arch/i386/interface/pxe/pxe_tftp.c b/src/arch/i386/interface/pxe/pxe_tftp.c
index aab376e8..f4801bad 100644
--- a/src/arch/i386/interface/pxe/pxe_tftp.c
+++ b/src/arch/i386/interface/pxe/pxe_tftp.c
@@ -71,6 +71,17 @@ static void pxe_tftp_close ( struct pxe_tftp_connection *pxe_tftp, int rc ) {
 	pxe_tftp->rc = rc;
 }
 
+/**
+ * Check flow control window
+ *
+ * @v pxe_tftp		PXE TFTP connection
+ * @ret len		Length of window
+ */
+static size_t pxe_tftp_xfer_window ( struct pxe_tftp_connection *pxe_tftp ) {
+
+	return pxe_tftp->blksize;
+}
+
 /**
  * Receive new data
  *
@@ -128,6 +139,8 @@ static int pxe_tftp_xfer_deliver ( struct pxe_tftp_connection *pxe_tftp,
 static struct interface_operation pxe_tftp_xfer_ops[] = {
 	INTF_OP ( xfer_deliver, struct pxe_tftp_connection *,
 		  pxe_tftp_xfer_deliver ),
+	INTF_OP ( xfer_window, struct pxe_tftp_connection *,
+		  pxe_tftp_xfer_window ),
 	INTF_OP ( intf_close, struct pxe_tftp_connection *, pxe_tftp_close ),
 };
 
@@ -167,19 +180,19 @@ static int pxe_tftp_open ( uint32_t ipaddress, unsigned int port,
 	/* Reset PXE TFTP connection structure */
 	memset ( &pxe_tftp, 0, sizeof ( pxe_tftp ) );
 	intf_init ( &pxe_tftp.xfer, &pxe_tftp_xfer_desc, NULL );
+	if ( blksize < TFTP_DEFAULT_BLKSIZE )
+		blksize = TFTP_DEFAULT_BLKSIZE;
+	pxe_tftp.blksize = blksize;
 	pxe_tftp.rc = -EINPROGRESS;
 
 	/* Construct URI string */
 	address.s_addr = ipaddress;
 	if ( ! port )
 		port = htons ( TFTP_PORT );
-	if ( blksize < TFTP_DEFAULT_BLKSIZE )
-		blksize = TFTP_DEFAULT_BLKSIZE;
-	snprintf ( uri_string, sizeof ( uri_string ),
-		   "tftp%s://%s:%d%s%s?blksize=%zd",
-		   sizeonly ? "size" : "",
-		   inet_ntoa ( address ), ntohs ( port ),
-		   ( ( filename[0] == '/' ) ? "" : "/" ), filename, blksize );
+	snprintf ( uri_string, sizeof ( uri_string ), "tftp%s://%s:%d%s%s",
+		   sizeonly ? "size" : "", inet_ntoa ( address ),
+		   ntohs ( port ), ( ( filename[0] == '/' ) ? "" : "/" ),
+		   filename );
 	DBG ( " %s", uri_string );
 
 	/* Open PXE TFTP connection */
diff --git a/src/include/ipxe/tftp.h b/src/include/ipxe/tftp.h
index 38be0d4d..aecafa2a 100644
--- a/src/include/ipxe/tftp.h
+++ b/src/include/ipxe/tftp.h
@@ -80,6 +80,4 @@ union tftp_any {
 	struct tftp_oack	oack;
 };
 
-extern void tftp_set_request_blksize ( unsigned int blksize );
-
 #endif /* _IPXE_TFTP_H */
diff --git a/src/net/udp/tftp.c b/src/net/udp/tftp.c
index a6c64b4a..d686aac9 100644
--- a/src/net/udp/tftp.c
+++ b/src/net/udp/tftp.c
@@ -287,24 +287,6 @@ static int tftp_presize ( struct tftp_request *tftp, size_t filesize ) {
 	return 0;
 }
 
-/**
- * TFTP requested blocksize
- *
- * This is treated as a global configuration parameter.
- */
-static unsigned int tftp_request_blksize = TFTP_MAX_BLKSIZE;
-
-/**
- * Set TFTP request blocksize
- *
- * @v blksize		Requested block size
- */
-void tftp_set_request_blksize ( unsigned int blksize ) {
-	if ( blksize < TFTP_DEFAULT_BLKSIZE )
-		blksize = TFTP_DEFAULT_BLKSIZE;
-	tftp_request_blksize = blksize;
-}
-
 /**
  * MTFTP multicast receive address
  *
@@ -345,6 +327,7 @@ static int tftp_send_rrq ( struct tftp_request *tftp ) {
 	const char *path;
 	size_t len;
 	struct io_buffer *iobuf;
+	size_t blksize;
 
 	/* Strip initial '/' if present.  If we were opened via the
 	 * URI interface, then there will be an initial '/', since a
@@ -370,6 +353,11 @@ static int tftp_send_rrq ( struct tftp_request *tftp ) {
 	if ( ! iobuf )
 		return -ENOMEM;
 
+	/* Determine block size */
+	blksize = xfer_window ( &tftp->xfer );
+	if ( blksize > TFTP_MAX_BLKSIZE )
+		blksize = TFTP_MAX_BLKSIZE;
+
 	/* Build request */
 	rrq = iob_put ( iobuf, sizeof ( *rrq ) );
 	rrq->opcode = htons ( TFTP_RRQ );
@@ -378,8 +366,8 @@ static int tftp_send_rrq ( struct tftp_request *tftp ) {
 	if ( tftp->flags & TFTP_FL_RRQ_SIZES ) {
 		iob_put ( iobuf, snprintf ( iobuf->tail,
 					    iob_tailroom ( iobuf ),
-					    "blksize%c%d%ctsize%c0", 0,
-					    tftp_request_blksize, 0, 0 ) + 1 );
+					    "blksize%c%zd%ctsize%c0",
+					    0, blksize, 0, 0 ) + 1 );
 	}
 	if ( tftp->flags & TFTP_FL_RRQ_MULTICAST ) {
 		iob_put ( iobuf, snprintf ( iobuf->tail,