[Patch] MdeModulePkg: Update IP4 stack to support point-to-point link with 31-bit mask.

[Fu Siyuan <siyuan.fu@intel.com>]

This patch is to follow RFC3021 which allows to use 31-bit mask
in point-to-point link.
If a 31-bit subnet mask is assigned to a point-to-point link, it
leaves the <Host-number> with only 1 bit.  Consequently, only two
possible addresses may result:
  {<Network-number>, 0} and {<Network-number>, -1}
These addresses have historically been associated with network and
broadcast addresses (see Section 2.2).  In a point-to-point link with
a 31-bit subnet mask, the two addresses above MUST be interpreted as
host addresses.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
---
 MdeModulePkg/Include/Library/NetLib.h             |  6 ++++--
 MdeModulePkg/Library/DxeNetLib/DxeNetLib.c        | 14 ++++++++++----
 MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Common.c | 20 +++++++++++++-------
 3 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/MdeModulePkg/Include/Library/NetLib.h b/MdeModulePkg/Include/Library/NetLib.h
index 4cd4227..b9df46c 100644
--- a/MdeModulePkg/Include/Library/NetLib.h
+++ b/MdeModulePkg/Include/Library/NetLib.h
@@ -414,8 +414,10 @@ NetGetIpClass (
 
   ASSERT if NetMask is zero.
   
-  If all bits of the host address of IP are 0 or 1, IP is also not a valid unicast address.
-
+  If all bits of the host address of IP are 0 or 1, IP is also not a valid unicast address,
+  except when the originator is one of the endpoints of a point-to-point link with a 31-bit
+  mask (RFC3021).
+  
   @param[in]  Ip                    The IP to check against.
   @param[in]  NetMask               The mask of the IP.
 
diff --git a/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c b/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c
index 1ee77fe..b8544b8 100644
--- a/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c
+++ b/MdeModulePkg/Library/DxeNetLib/DxeNetLib.c
@@ -637,7 +637,9 @@ NetGetIpClass (
 
   ASSERT if NetMask is zero.
   
-  If all bits of the host address of IP are 0 or 1, IP is also not a valid unicast address.
+  If all bits of the host address of IP are 0 or 1, IP is also not a valid unicast address,
+  except when the originator is one of the endpoints of a point-to-point link with a 31-bit
+  mask (RFC3021).
 
   @param[in]  Ip                    The IP to check against.
   @param[in]  NetMask               The mask of the IP.
@@ -657,9 +659,13 @@ NetIp4IsUnicast (
   if (Ip == 0 || IP4_IS_LOCAL_BROADCAST (Ip)) {
     return FALSE;
   }
-  
-  if (((Ip &~NetMask) == ~NetMask) || ((Ip &~NetMask) == 0)) {
-    return FALSE;
+
+  if (NetGetMaskLength (NetMask) != 31) {
+    if (((Ip &~NetMask) == ~NetMask) || ((Ip &~NetMask) == 0)) {
+      return FALSE;
+    }
+  } else {
+    return TRUE;
   }
 
   return TRUE;
diff --git a/MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Common.c b/MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Common.c
index 7c7d182..b8160c1 100644
--- a/MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Common.c
+++ b/MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Common.c
@@ -314,16 +314,22 @@ Ip4StationAddressValid (
 
   //
   // Station address can't be subnet broadcast/net broadcast address
+  // RFC3021: In a point-to-point network with 31-bit mask, the <Host-number> only has
+  // 1 bit, only two possible addresses may result:
+  //   {<Network-number>, 0} and {<Network-number>, -1}
+  // The two address above must be interpreted as host addresses.
   //
-  if ((Ip == (Ip & Netmask)) || (Ip == (Ip | ~Netmask))) {
-    return FALSE;
-  }
+  if (Len != 31) {
+    if ((Ip == (Ip & Netmask)) || (Ip == (Ip | ~Netmask))) {
+      return FALSE;
+    }
 
-  NetBrdcastMask = gIp4AllMasks[MIN (Len, Type << 3)];
+    NetBrdcastMask = gIp4AllMasks[MIN (Len, Type << 3)];
 
-  if (Ip == (Ip | ~NetBrdcastMask)) {
-    return FALSE;
+    if (Ip == (Ip | ~NetBrdcastMask)) {
+      return FALSE;
+    }
   }
 
   return TRUE;
-}
\ No newline at end of file
+}
-- 
1.9.5.msysgit.1