From 51c6548631dab543ecbb903e8644b73c54e5bdcd Mon Sep 17 00:00:00 2001
From: Sylvain Rochet <gradator@gradator.net>
Date: Mon, 19 Oct 2020 00:26:14 +0200
Subject: [PATCH] PPP: allow negotiating MRU with PPP_MRU config option

PPP_MRU is now free to be used for what it should have been. Now using
it at PPP init stage to set the wanted MRU value, triggering a MRU
negotiation at the LCP phase.

I doubt anyone needs it anyway, but, well, at least it is fixed and the
MRU/MTU config mess is cleaned.

And while we are at it, better document PPP MRU config values.
---
 src/include/netif/ppp/ppp_opts.h | 18 +++++++++++++++---
 src/netif/ppp/lcp.c              |  2 +-
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/include/netif/ppp/ppp_opts.h b/src/include/netif/ppp/ppp_opts.h
index 8fe4a3ee..3d82cbf7 100644
--- a/src/include/netif/ppp/ppp_opts.h
+++ b/src/include/netif/ppp/ppp_opts.h
@@ -495,21 +495,33 @@
  */
 
 /**
- * PPP_MRU: Default MRU
+ * PPP_MRU: MRU value we want to negotiate (peer MTU)
+ *
+ * It only affects PPPoS because PPPoE value is derived from the
+ * Ethernet interface MTU and PPPoL2TP have a separate setting.
  */
 #ifndef PPP_MRU
 #define PPP_MRU                         1500
 #endif
 
 /**
- * PPP_MAXMRU: Normally limit MRU to this (pppd default = 16384)
+ * PPP_MAXMRU: Normally limit peer MRU to this
+ *
+ * This is the upper limit value to which we set our interface MTU.
+ * If the peer sends a larger number, we will just ignore it as we
+ * are not required to maximize the use of the peer capacity.
+ *
+ * It only affects PPPoS because PPPoE value is derived from the
+ * Ethernet interface MTU and PPPoL2TP have a separate setting.
  */
 #ifndef PPP_MAXMRU
 #define PPP_MAXMRU                      1500
 #endif
 
 /**
- * PPP_MINMRU: No MRUs below this
+ * PPP_MINMRU: No peer MRUs below this
+ *
+ * Peer must be able to receive at least our minimum MTU.
  */
 #ifndef PPP_MINMRU
 #define PPP_MINMRU                      128
diff --git a/src/netif/ppp/lcp.c b/src/netif/ppp/lcp.c
index eefdd314..9c550520 100644
--- a/src/netif/ppp/lcp.c
+++ b/src/netif/ppp/lcp.c
@@ -373,7 +373,7 @@ static void lcp_init(ppp_pcb *pcb) {
 
     BZERO(wo, sizeof(*wo));
     wo->neg_mru = 1;
-    wo->mru = PPP_DEFMRU;
+    wo->mru = PPP_MRU;
     wo->neg_asyncmap = 1;
     wo->neg_magicnumber = 1;
     wo->neg_pcompression = 1;