diff --git a/src/arch/i386/interface/pxe/pxe_tftp.c b/src/arch/i386/interface/pxe/pxe_tftp.c
index 97077d37..715a0b61 100644
--- a/src/arch/i386/interface/pxe/pxe_tftp.c
+++ b/src/arch/i386/interface/pxe/pxe_tftp.c
@@ -113,12 +113,6 @@ static int pxe_tftp_xfer_deliver_iob ( struct xfer_interface *xfer __unused,
 	/* Calculate new buffer position */
 	pxe_tftp.offset += len;
 
-	/* Mildly ugly hack; assume that the first non-zero seek
-	 * indicates the block size.
-	 */
-	if ( pxe_tftp.blksize == 0 )
-		pxe_tftp.blksize = pxe_tftp.offset;
-
 	/* Record maximum offset as the file size */
 	if ( pxe_tftp.max_offset < pxe_tftp.offset )
 		pxe_tftp.max_offset = pxe_tftp.offset;
@@ -265,10 +259,12 @@ PXENV_EXIT_t pxenv_tftp_open ( struct s_PXENV_TFTP_OPEN *tftp_open ) {
 
 	/* Wait for OACK to arrive so that we have the block size */
 	while ( ( ( rc = pxe_tftp.rc ) == -EINPROGRESS ) &&
-		( pxe_tftp.blksize == 0 ) ) {
+		( pxe_tftp.max_offset == 0 ) ) {
 		step();
 	}
+	pxe_tftp.blksize = xfer_window ( &pxe_tftp.xfer );
 	tftp_open->PacketSize = pxe_tftp.blksize;
+	DBG ( " blksize=%d", tftp_open->PacketSize );
 
 	/* EINPROGRESS is normal; we don't wait for the whole transfer */
 	if ( rc == -EINPROGRESS )
@@ -571,6 +567,7 @@ PXENV_EXIT_t pxenv_tftp_get_fsize ( struct s_PXENV_TFTP_GET_FSIZE
 		step();
 	}
 	tftp_get_fsize->FileSize = pxe_tftp.max_offset;
+	DBG ( " fsize=%d", tftp_get_fsize->FileSize );
 
 	/* EINPROGRESS is normal; we don't wait for the whole transfer */
 	if ( rc == -EINPROGRESS )
diff --git a/src/net/udp/tftp.c b/src/net/udp/tftp.c
index b262b108..889362a1 100644
--- a/src/net/udp/tftp.c
+++ b/src/net/udp/tftp.c
@@ -986,11 +986,29 @@ static void tftp_xfer_close ( struct xfer_interface *xfer, int rc ) {
 	tftp_done ( tftp, rc );
 }
 
+/**
+ * Check flow control window
+ *
+ * @v xfer		Data transfer interface
+ * @ret len		Length of window
+ */
+static size_t tftp_xfer_window ( struct xfer_interface *xfer ) {
+	struct tftp_request *tftp =
+		container_of ( xfer, struct tftp_request, xfer );
+
+	/* We abuse this data-xfer method to convey the blocksize to
+	 * the caller.  This really should be done using some kind of
+	 * stat() method, but we don't yet have the facility to do
+	 * that.
+	 */
+	return tftp->blksize;
+}
+
 /** TFTP data transfer interface operations */
 static struct xfer_interface_operations tftp_xfer_operations = {
 	.close		= tftp_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
-	.window		= unlimited_xfer_window,
+	.window		= tftp_xfer_window,
 	.alloc_iob	= default_xfer_alloc_iob,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,