[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>

1 files changed, 87 insertions, 0 deletions

diff --git a/src/config/dhcp.h b/src/config/dhcp.h
new file mode 100644
index 000000000..21e863b4f
--- /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 */