diff --git a/src/core/downloader.c b/src/core/downloader.c
index 2f28bc95..15ef962d 100644
--- a/src/core/downloader.c
+++ b/src/core/downloader.c
@@ -114,19 +114,6 @@ static int downloader_ensure_size ( struct downloader *downloader,
  *
  */
 
-/**
- * Handle start() event received via job control interface
- *
- * @v job		Downloader job control interface
- */
-static void downloader_job_start ( struct job_interface *job ) {
-	struct downloader *downloader = 
-		container_of ( job, struct downloader, job );
-
-	/* Start data transfer */
-	xfer_request_all ( &downloader->xfer );
-}
-
 /**
  * Handle kill() event received via job control interface
  *
@@ -142,7 +129,7 @@ static void downloader_job_kill ( struct job_interface *job ) {
 
 /** Downloader job control interface operations */
 static struct job_interface_operations downloader_job_operations = {
-	.start		= downloader_job_start,
+	.start		= ignore_job_start,
 	.done		= ignore_job_done,
 	.kill		= downloader_job_kill,
 	.progress	= ignore_job_progress,
diff --git a/src/core/hw.c b/src/core/hw.c
index 77b39ba1..a3eb8500 100644
--- a/src/core/hw.c
+++ b/src/core/hw.c
@@ -3,6 +3,7 @@
 #include <string.h>
 #include <errno.h>
 #include <gpxe/refcnt.h>
+#include <gpxe/process.h>
 #include <gpxe/xfer.h>
 #include <gpxe/open.h>
 
@@ -15,6 +16,7 @@
 struct hw {
 	struct refcnt refcnt;
 	struct xfer_interface xfer;
+	struct process process;
 };
 
 static const char hw_msg[] = "Hello world!\n";
@@ -22,6 +24,7 @@ static const char hw_msg[] = "Hello world!\n";
 static void hw_finished ( struct hw *hw, int rc ) {
 	xfer_nullify ( &hw->xfer );
 	xfer_close ( &hw->xfer, rc );
+	process_del ( &hw->process );
 }
 
 static void hw_xfer_close ( struct xfer_interface *xfer, int rc ) {
@@ -30,26 +33,25 @@ static void hw_xfer_close ( struct xfer_interface *xfer, int rc ) {
 	hw_finished ( hw, rc );
 }
 
-static int hw_xfer_request ( struct xfer_interface *xfer,
-			     off_t start __unused, int whence __unused,
-			     size_t len __unused ) {
-	struct hw *hw = container_of ( xfer, struct hw, xfer );
-	int rc;
-
-	rc = xfer_deliver_raw ( xfer, hw_msg, sizeof ( hw_msg ) );
-	hw_finished ( hw, rc );
-	return 0;
-}
-
 static struct xfer_interface_operations hw_xfer_operations = {
 	.close		= hw_xfer_close,
 	.vredirect	= ignore_xfer_vredirect,
-	.request	= hw_xfer_request,
+	.request	= ignore_xfer_request,
 	.seek		= ignore_xfer_seek,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= ignore_xfer_deliver_raw,
 };
 
+static void hw_step ( struct process *process ) {
+	struct hw *hw = container_of ( process, struct hw, process );
+	int rc;
+
+	if ( xfer_ready ( &hw->xfer ) == 0 ) {
+		rc = xfer_deliver_raw ( &hw->xfer, hw_msg, sizeof ( hw_msg ) );
+		hw_finished ( hw, rc );
+	}
+}
+
 static int hw_open ( struct xfer_interface *xfer, struct uri *uri __unused ) {
 	struct hw *hw;
 
@@ -59,6 +61,7 @@ static int hw_open ( struct xfer_interface *xfer, struct uri *uri __unused ) {
 		return -ENOMEM;
 	memset ( hw, 0, sizeof ( *hw ) );
 	xfer_init ( &hw->xfer, &hw_xfer_operations, &hw->refcnt );
+	process_init ( &hw->process, hw_step, &hw->refcnt );
 
 	/* Attach parent interface, mortalise self, and return */
 	xfer_plug_plug ( &hw->xfer, xfer );
diff --git a/src/core/posix_io.c b/src/core/posix_io.c
index 6cefbf7b..3b5660e4 100644
--- a/src/core/posix_io.c
+++ b/src/core/posix_io.c
@@ -191,6 +191,7 @@ static int posix_find_free_fd ( void ) {
 		if ( ! posix_fd_to_file ( fd ) )
 			return fd;
 	}
+	DBG ( "POSIX could not find free file descriptor\n" );
 	return -ENFILE;
 }
 
@@ -226,13 +227,11 @@ int open ( const char *uri_string ) {
 	if ( ( rc = xfer_open_uri ( &file->xfer, uri_string ) ) != 0 )
 		goto err;
 
-	/* Request data */
-	if ( ( rc = xfer_request_all ( &file->xfer ) ) != 0 )
-		goto err;
-
 	/* Wait for open to succeed or fail */
 	while ( list_empty ( &file->data ) ) {
 		step();
+		if ( file->rc == 0 )
+			break;
 		if ( file->rc != -EINPROGRESS ) {
 			rc = file->rc;
 			goto err;
@@ -241,6 +240,7 @@ int open ( const char *uri_string ) {
 
 	/* Add to list of open files.  List takes reference ownership. */
 	list_add ( &file->list, &posix_files );
+	DBG ( "POSIX opened %s as file %d\n", uri_string, fd );
 	return fd;
 
  err:
diff --git a/src/core/xfer.c b/src/core/xfer.c
index f2783f5b..54377c70 100644
--- a/src/core/xfer.c
+++ b/src/core/xfer.c
@@ -35,6 +35,8 @@
 void xfer_close ( struct xfer_interface *xfer, int rc ) {
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 
+	DBGC ( xfer, "XFER %p->%p close\n", xfer, dest );
+
 	dest->op->close ( dest, rc );
 	xfer_unplug ( xfer );
 	xfer_put ( dest );
@@ -52,7 +54,14 @@ int xfer_vredirect ( struct xfer_interface *xfer, int type, va_list args ) {
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 	int rc;
 
+	DBGC ( xfer, "XFER %p->%p redirect\n", xfer, dest );
+
 	rc = dest->op->vredirect ( dest, type, args );
+
+	if ( rc != 0 ) {
+		DBGC ( xfer, "XFER %p<-%p redirect: %s\n", xfer, dest,
+		       strerror ( rc ) );
+	}
 	xfer_put ( dest );
 	return rc;
 }
@@ -89,21 +98,19 @@ int xfer_request ( struct xfer_interface *xfer, off_t offset, int whence,
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 	int rc;
 
+	DBGC ( xfer, "XFER %p->%p request %s+%ld %zd\n", xfer, dest,
+	       whence_text ( whence ), offset, len );
+
 	rc = dest->op->request ( dest, offset, whence, len );
+
+	if ( rc != 0 ) {
+		DBGC ( xfer, "XFER %p<-%p request: %s\n", xfer, dest,
+		       strerror ( rc ) );
+	}
 	xfer_put ( dest );
 	return rc;
 }
 
-/**
- * Request all data
- *
- * @v xfer		Data transfer interface
- * @ret rc		Return status code
- */
-int xfer_request_all ( struct xfer_interface *xfer ) {
-	return xfer_request ( xfer, 0, SEEK_SET, ~( ( size_t ) 0 ) );
-}
-
 /**
  * Seek to position
  *
@@ -116,11 +123,33 @@ int xfer_seek ( struct xfer_interface *xfer, off_t offset, int whence ) {
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 	int rc;
 
+	DBGC ( xfer, "XFER %p->%p seek %s+%ld\n", xfer, dest,
+	       whence_text ( whence ), offset );
+
 	rc = dest->op->seek ( dest, offset, whence );
+
+	if ( rc != 0 ) {
+		DBGC ( xfer, "XFER %p<-%p seek: %s\n", xfer, dest,
+		       strerror ( rc ) );
+	}
 	xfer_put ( dest );
 	return rc;
 }
 
+/**
+ * Test to see if interface is ready to accept data
+ *
+ * @v xfer		Data transfer interface
+ * @ret rc		Return status code
+ *
+ * This test is optional; the data transfer interface may wish that it
+ * does not yet wish to accept data, but cannot prevent attempts to
+ * deliver data to it.
+ */
+int xfer_ready ( struct xfer_interface *xfer ) {
+	return xfer_seek ( xfer, 0, SEEK_CUR );
+}
+
 /**
  * Allocate I/O buffer
  *
@@ -132,7 +161,13 @@ struct io_buffer * xfer_alloc_iob ( struct xfer_interface *xfer, size_t len ) {
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 	struct io_buffer *iobuf;
 
+	DBGC ( xfer, "XFER %p->%p alloc_iob %zd\n", xfer, dest, len );
+
 	iobuf = dest->op->alloc_iob ( dest, len );
+
+	if ( ! iobuf ) {
+		DBGC ( xfer, "XFER %p<-%p alloc_iob failed\n", xfer, dest );
+	}
 	xfer_put ( dest );
 	return iobuf;
 }
@@ -148,7 +183,15 @@ int xfer_deliver_iob ( struct xfer_interface *xfer, struct io_buffer *iobuf ) {
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 	int rc;
 
+	DBGC ( xfer, "XFER %p->%p deliver_iob %zd\n", xfer, dest,
+	       iob_len ( iobuf ) );
+
 	rc = dest->op->deliver_iob ( dest, iobuf );
+
+	if ( rc != 0 ) {
+		DBGC ( xfer, "XFER %p<-%p deliver_iob: %s\n", xfer, dest,
+		       strerror ( rc ) );
+	}
 	xfer_put ( dest );
 	return rc;
 }
@@ -165,7 +208,15 @@ int xfer_deliver_raw ( struct xfer_interface *xfer,
 	struct xfer_interface *dest = xfer_get_dest ( xfer );
 	int rc;
 
+	DBGC ( xfer, "XFER %p->%p deliver_raw %p+%zd\n", xfer, dest,
+	       data, len );
+
 	rc = dest->op->deliver_raw ( dest, data, len );
+
+	if ( rc != 0 ) {
+		DBGC ( xfer, "XFER %p<-%p deliver_raw: %s\n", xfer, dest,
+		       strerror ( rc ) );
+	}
 	xfer_put ( dest );
 	return rc;
 }
diff --git a/src/include/gpxe/xfer.h b/src/include/gpxe/xfer.h
index 71b69dc5..e6c896c3 100644
--- a/src/include/gpxe/xfer.h
+++ b/src/include/gpxe/xfer.h
@@ -112,6 +112,20 @@ enum seek_whence {
 	SEEK_CUR,
 };
 
+/**
+ * Describe seek basis
+ *
+ * @v whence		Basis for new position
+ */
+static inline __attribute__ (( always_inline )) const char *
+whence_text ( int whence ) {
+	switch ( whence ) {
+	case SEEK_SET:	return "SET";
+	case SEEK_CUR:	return "CUR";
+	default:	return "INVALID";
+	}
+}
+
 extern struct xfer_interface null_xfer;
 extern struct xfer_interface_operations null_xfer_ops;
 
@@ -121,8 +135,8 @@ extern int xfer_vredirect ( struct xfer_interface *xfer, int type,
 extern int xfer_redirect ( struct xfer_interface *xfer, int type, ... );
 extern int xfer_request ( struct xfer_interface *xfer, off_t offset,
 			  int whence, size_t len );
-extern int xfer_request_all ( struct xfer_interface *xfer );
 extern int xfer_seek ( struct xfer_interface *xfer, off_t offset, int whence );
+extern int xfer_ready ( struct xfer_interface *xfer );
 extern struct io_buffer * xfer_alloc_iob ( struct xfer_interface *xfer,
 					   size_t len );
 extern int xfer_deliver_iob ( struct xfer_interface *xfer,