[dhcp] Extract timing parameters out to config/dhcp.h

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.  Extract the DHCP timing parameters
to config/dhcp.h 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.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

3 files changed, 106 insertions, 23 deletions

diff --git a/src/config/dhcp.h b/src/config/dhcp.h
new file mode 100644
index 00000000..21e863b4
--- /dev/null
+++ b/src/config/dhcp.h
@@ -0,0 +1,87 @@
+#ifndef CONFIG_DHCP_H
+#define CONFIG_DHCP_H
+
+/** @file
+ *
+ * DHCP configuration
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <config/defaults.h>
+
+/*
+ * 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
+//#define DHCP_DISC_START_TIMEOUT_SEC	4	/* as per PXE spec */
+//#define DHCP_DISC_END_TIMEOUT_SEC	32	/* as per PXE spec */
+
+/*
+ * 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
+//#define DHCP_DISC_PROXY_TIMEOUT_SEC	11	/* as per PXE spec */
+
+/*
+ * 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
+//#define DHCP_REQ_END_TIMEOUT_SEC	4	/* as per PXE spec */
+
+/*
+ * 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
+//#define DHCP_PROXY_END_TIMEOUT_SEC	8	/* as per PXE spec */
+
+/*
+ * 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
+//#define DHCP_REQ_PROXY_TIMEOUT_SEC	7	/* as per PXE spec */
+
+/*
+ * 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
+//#define PXEBS_START_TIMEOUT_SEC	0	/* as per PXE spec */
+//#define PXEBS_END_TIMEOUT_SEC		8	/* as per PXE spec */
+
+/*
+ * Increment to the next PXE Boot server, if available, after this
+ * this much time has elapsed.
+ */
+#define PXEBS_MAX_TIMEOUT_SEC		3
+//#define PXEBS_MAX_TIMEOUT_SEC		7	/* as per PXE spec */
+
+#include <config/local/dhcp.h>
+
+#endif /* CONFIG_DHCP_H */
diff --git a/src/include/ipxe/dhcp.h b/src/include/ipxe/dhcp.h
index bcfb85cc..a35794bd 100644
--- a/src/include/ipxe/dhcp.h
+++ b/src/include/ipxe/dhcp.h
@@ -639,16 +639,6 @@ 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 04fad04c..7a1a7c49 100644
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -44,6 +44,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <ipxe/dhcppkt.h>
 #include <ipxe/dhcp_arch.h>
 #include <ipxe/features.h>
+#include <config/dhcp.h>
 
 /** @file
  *
@@ -171,8 +172,9 @@ 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 +274,8 @@ 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 +416,7 @@ 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 +432,8 @@ 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 +449,8 @@ 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 +587,8 @@ 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 +673,7 @@ 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 +689,8 @@ 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 +815,7 @@ 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 +837,8 @@ 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,
 };
 
 /****************************************************************************