[ipxe-devel] [RESEND PATCH] [dhcp] Extract timing parameters out to header and document

Alex Williamson alex.williamson at redhat.com
Mon Feb 9 19:52:19 UTC 2015


iPXE uses DHCP timeouts loosely based on values recommended by the
specification, but often abbreviated to reduce timeouts for reliable
and/or simple network topologies.  Previous attempts to change the
defaults to more spec-compliant values have met resistance and
apathy, therefore this patch simply tries to extract the timing
parameters to a config file and document them.  The resulting default
iPXE behavior is exactly the same, but downstreams are now afforded
the opportunity to implement spec compliant behavior via config file
overrides.

I believe the following overrides defined in config/local/general.h
provide sufficiently spec compliant DHCP timeouts:

/*
 * PXE spec defines timeouts of 4, 8, 16, 32 seconds
 */
#undef DHCP_DISC_START_TIMEOUT_SEC
#define DHCP_DISC_START_TIMEOUT_SEC	4
#undef DHCP_DISC_END_TIMEOUT_SEC
#define DHCP_DISC_END_TIMEOUT_SEC	32

/*
 * Elapsed time used for early break waiting for ProxyDHCP, this therefore
 * needs to be less than the cumulative time for the first 2 timeouts.
 */
#undef DHCP_DISC_PROXY_TIMEOUT_SEC
#define DHCP_DISC_PROXY_TIMEOUT_SEC	11

/*
 * Approximate PXE spec requirement using minimum timeout (0.25s) for
 * timeouts of 0.25, 0.5, 1, 2, 4
 */
#undef DHCP_REQ_START_TIMEOUT_SEC
#define DHCP_REQ_START_TIMEOUT_SEC	0
#undef DHCP_REQ_END_TIMEOUT_SEC
#define DHCP_REQ_END_TIMEOUT_SEC	4

/*
 * Same as normal request phase, except non-fatal, so we extend the timer
 * to 8 and set the early timeout to an elapsed time value that causes a
 * break after the 4 second timeout.
 */
#undef DHCP_PROXY_START_TIMEOUT_SEC
#define DHCP_PROXY_START_TIMEOUT_SEC	0
#undef DHCP_PROXY_END_TIMEOUT_SEC
#define DHCP_PROXY_END_TIMEOUT_SEC	8
#undef DHCP_REQ_PROXY_TIMEOUT_SEC
#define DHCP_REQ_PROXY_TIMEOUT_SEC	7

/*
 * Same as above, retry each server using standard timeouts, extended by
 * one so that we can increment to the next before a timer induced failure.
 */
#undef PXEBS_START_TIMEOUT_SEC
#define PXEBS_START_TIMEOUT_SEC		0
#undef PXEBS_END_TIMEOUT_SEC
#define PXEBS_END_TIMEOUT_SEC		8
#undef PXEBS_MAX_TIMEOUT_SEC
#define PXEBS_MAX_TIMEOUT_SEC		7

Signed-off-by: Alex Williamson <alex.williamson at redhat.com>
---

Michael, does this look reasonable or can you suggest a better
mechanism for downstreams to tune DHCP timeouts for customers on
topologies that require something more spec compliant without forking
the code?  I know you don't want to change the default user
experience so allowing build-time tuning via a local config header
seemed like a compromise and I hope the additional documentation helps
make the change worthwhile.  Thanks,

Alex

 src/config/general.h    |   61 +++++++++++++++++++++++++++++++++++++++++++++++
 src/include/ipxe/dhcp.h |   11 +-------
 src/net/udp/dhcp.c      |   31 ++++++++++++++----------
 3 files changed, 80 insertions(+), 23 deletions(-)

diff --git a/src/config/general.h b/src/config/general.h
index 5392034..bead2ac 100644
--- a/src/config/general.h
+++ b/src/config/general.h
@@ -182,6 +182,67 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #undef	GDBUDP			/* Remote GDB debugging over UDP
 				 * (both may be set) */
 
+/*
+ * DHCP and PXE Boot Server timeout parameters
+ *
+ * Initial and final timeout for DHCP discovery
+ *
+ * The PXE spec indicates discover request are sent 4 times, with timeouts
+ * of 4, 8, 16, 32 seconds.  iPXE by default uses 1, 2, 4, 8.
+ */
+#define DHCP_DISC_START_TIMEOUT_SEC	1
+#define DHCP_DISC_END_TIMEOUT_SEC	10
+
+/*
+ * ProxyDHCP offers are given precedence by continue to wait for them after
+ * a valid DHCPOFFER is received.  We'll wait through this timeout for it.
+ * The PXE spec indicates waiting through the 4 & 8 second timeouts, iPXE
+ * by default stops after 2.
+ */
+#define DHCP_DISC_PROXY_TIMEOUT_SEC	2
+
+/*
+ * Per the PXE spec, requests are also tried 4 times, but at timeout intervals
+ * of 1, 2, 3, 4 seconds.  To adapt this to an exponential backoff timer, we
+ * can either do 1, 2, 4, 8, ie. 4 retires with a longer interval or start at
+ * 0 (0.25s) for 0.25, 0.5, 1, 2, 4, ie. one extra try and shorter initial
+ * timeouts.  iPXE by default does a combination of both, starting at 0 and
+ * going through the 8 second timeout.
+ */
+#define DHCP_REQ_START_TIMEOUT_SEC	0
+#define DHCP_REQ_END_TIMEOUT_SEC	10
+
+/*
+ * A ProxyDHCP offer without PXE options also goes through a request phase
+ * using these same parameters, but note the early break below.
+ */
+#define DHCP_PROXY_START_TIMEOUT_SEC	0
+#define DHCP_PROXY_END_TIMEOUT_SEC	10
+
+/*
+ * A ProxyDHCP request timeout should not induce a failure condition, so we
+ * always want to break before the above set of timers expire.  The iPXE
+ * default value of 2 breaks at the first timeout after 2 seconds, which will
+ * be after the 2 second timeout.
+ */
+#define DHCP_REQ_PROXY_TIMEOUT_SEC	2
+
+/*
+ * Per the PXE spec, a PXE boot server request is also be retried 4 times
+ * at timeouts of 1, 2, 3, 4.  iPXE uses the same timeouts as discovery,
+ * 1, 2, 4, 8, but will move on to the next server if available after an
+ * elapsed time greater than 3 seconds, therefore effectively only sending
+ * 3 tries at timeouts of 1, 2, 4.
+ */
+#define PXEBS_START_TIMEOUT_SEC		1
+#define PXEBS_END_TIMEOUT_SEC		10
+
+/*
+ * Increment to the next PXE Boot server, if available, after this this much
+ * time has elapsed.
+ */
+#define PXEBS_MAX_TIMEOUT_SEC		3
+
 #include <config/named.h>
 #include NAMED_CONFIG(general.h)
 #include <config/local/general.h>
diff --git a/src/include/ipxe/dhcp.h b/src/include/ipxe/dhcp.h
index bcfb85c..d1dc5bc 100644
--- a/src/include/ipxe/dhcp.h
+++ b/src/include/ipxe/dhcp.h
@@ -18,6 +18,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <ipxe/uuid.h>
 #include <ipxe/netdevice.h>
 #include <ipxe/uaccess.h>
+#include <config/general.h>
 
 struct interface;
 struct dhcp_options;
@@ -639,16 +640,6 @@ struct dhcphdr {
  */
 #define DHCP_MIN_LEN 552
 
-/** Timeouts for sending DHCP packets */
-#define DHCP_MIN_TIMEOUT ( 1 * TICKS_PER_SEC )
-#define DHCP_MAX_TIMEOUT ( 10 * TICKS_PER_SEC )
-
-/** Maximum time that we will wait for ProxyDHCP responses */
-#define PROXYDHCP_MAX_TIMEOUT ( 2 * TICKS_PER_SEC )
-
-/** Maximum time that we will wait for Boot Server responses */
-#define PXEBS_MAX_TIMEOUT ( 3 * TICKS_PER_SEC )
-
 /** Settings block name used for DHCP responses */
 #define DHCP_SETTINGS_NAME "dhcp"
 
diff --git a/src/net/udp/dhcp.c b/src/net/udp/dhcp.c
index 04fad04..3527c44 100644
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -171,8 +171,9 @@ struct dhcp_session_state {
 	void ( * expired ) ( struct dhcp_session *dhcp );
 	/** Transmitted message type */
 	uint8_t tx_msgtype;
-	/** Apply minimum timeout */
-	uint8_t apply_min_timeout;
+	/** Timeout parameters */
+	uint8_t min_timeout_sec;
+	uint8_t max_timeout_sec;
 };
 
 static struct dhcp_session_state dhcp_state_discover;
@@ -272,9 +273,8 @@ static void dhcp_set_state ( struct dhcp_session *dhcp,
 	dhcp->state = state;
 	dhcp->start = currticks();
 	stop_timer ( &dhcp->timer );
-	dhcp->timer.min_timeout =
-		( state->apply_min_timeout ? DHCP_MIN_TIMEOUT : 0 );
-	dhcp->timer.max_timeout = DHCP_MAX_TIMEOUT;
+	dhcp->timer.min_timeout = state->min_timeout_sec * TICKS_PER_SEC;
+	dhcp->timer.max_timeout = state->max_timeout_sec * TICKS_PER_SEC;
 	start_timer_nodelay ( &dhcp->timer );
 }
 
@@ -415,7 +415,7 @@ static void dhcp_discovery_rx ( struct dhcp_session *dhcp,
 	/* If we can't yet transition to DHCPREQUEST, do nothing */
 	elapsed = ( currticks() - dhcp->start );
 	if ( ! ( dhcp->no_pxedhcp || dhcp->proxy_offer ||
-		 ( elapsed > PROXYDHCP_MAX_TIMEOUT ) ) )
+		 ( elapsed > DHCP_DISC_PROXY_TIMEOUT_SEC * TICKS_PER_SEC ) ) )
 		return;
 
 	/* Transition to DHCPREQUEST */
@@ -431,7 +431,8 @@ static void dhcp_discovery_expired ( struct dhcp_session *dhcp ) {
 	unsigned long elapsed = ( currticks() - dhcp->start );
 
 	/* Give up waiting for ProxyDHCP before we reach the failure point */
-	if ( dhcp->offer.s_addr && ( elapsed > PROXYDHCP_MAX_TIMEOUT ) ) {
+	if ( dhcp->offer.s_addr &&
+	     ( elapsed > DHCP_DISC_PROXY_TIMEOUT_SEC * TICKS_PER_SEC ) ) {
 		dhcp_set_state ( dhcp, &dhcp_state_request );
 		return;
 	}
@@ -447,7 +448,8 @@ static struct dhcp_session_state dhcp_state_discover = {
 	.rx			= dhcp_discovery_rx,
 	.expired		= dhcp_discovery_expired,
 	.tx_msgtype		= DHCPDISCOVER,
-	.apply_min_timeout	= 1,
+	.min_timeout_sec	= DHCP_DISC_START_TIMEOUT_SEC,
+	.max_timeout_sec	= DHCP_DISC_END_TIMEOUT_SEC,
 };
 
 /**
@@ -584,7 +586,8 @@ static struct dhcp_session_state dhcp_state_request = {
 	.rx			= dhcp_request_rx,
 	.expired		= dhcp_request_expired,
 	.tx_msgtype		= DHCPREQUEST,
-	.apply_min_timeout	= 0,
+	.min_timeout_sec	= DHCP_REQ_START_TIMEOUT_SEC,
+	.max_timeout_sec	= DHCP_REQ_END_TIMEOUT_SEC,
 };
 
 /**
@@ -669,7 +672,7 @@ static void dhcp_proxy_expired ( struct dhcp_session *dhcp ) {
 	unsigned long elapsed = ( currticks() - dhcp->start );
 
 	/* Give up waiting for ProxyDHCP before we reach the failure point */
-	if ( elapsed > PROXYDHCP_MAX_TIMEOUT ) {
+	if ( elapsed > DHCP_REQ_PROXY_TIMEOUT_SEC * TICKS_PER_SEC ) {
 		dhcp_finished ( dhcp, 0 );
 		return;
 	}
@@ -685,7 +688,8 @@ static struct dhcp_session_state dhcp_state_proxy = {
 	.rx			= dhcp_proxy_rx,
 	.expired		= dhcp_proxy_expired,
 	.tx_msgtype		= DHCPREQUEST,
-	.apply_min_timeout	= 0,
+	.min_timeout_sec	= DHCP_PROXY_START_TIMEOUT_SEC,
+	.max_timeout_sec	= DHCP_PROXY_END_TIMEOUT_SEC,
 };
 
 /**
@@ -810,7 +814,7 @@ static void dhcp_pxebs_expired ( struct dhcp_session *dhcp ) {
 	/* Give up waiting before we reach the failure point, and fail
 	 * over to the next server in the attempt list
 	 */
-	if ( elapsed > PXEBS_MAX_TIMEOUT ) {
+	if ( elapsed > PXEBS_MAX_TIMEOUT_SEC * TICKS_PER_SEC ) {
 		dhcp->pxe_attempt++;
 		if ( dhcp->pxe_attempt->s_addr ) {
 			dhcp_set_state ( dhcp, &dhcp_state_pxebs );
@@ -832,7 +836,8 @@ static struct dhcp_session_state dhcp_state_pxebs = {
 	.rx			= dhcp_pxebs_rx,
 	.expired		= dhcp_pxebs_expired,
 	.tx_msgtype		= DHCPREQUEST,
-	.apply_min_timeout	= 1,
+	.min_timeout_sec	= PXEBS_START_TIMEOUT_SEC,
+	.max_timeout_sec	= PXEBS_END_TIMEOUT_SEC,
 };
 
 /****************************************************************************




More information about the ipxe-devel mailing list