From 1627a4286479f43763bc17bbf5d7eb830aec4f67 Mon Sep 17 00:00:00 2001
From: Jonathan Hui <jonhui@google.com>
Date: Thu, 14 Feb 2019 11:33:39 -0800
Subject: [PATCH] [mpl] fix bug in comparing sequence numbers (#3588)

Fix serial number arithmetic due to value wrap.
---
 src/core/net/ip6_mpl.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/core/net/ip6_mpl.cpp b/src/core/net/ip6_mpl.cpp
index b4ee3b39c..ec47f449c 100644
--- a/src/core/net/ip6_mpl.cpp
+++ b/src/core/net/ip6_mpl.cpp
@@ -158,12 +158,14 @@ otError Mpl::UpdateSeedSet(uint16_t aSeedId, uint8_t aSequence)
         {
             // have existing entries for aSeedId
 
-            if (aSequence == mSeedSet[i].GetSequence())
+            int8_t diff = static_cast<int8_t>(aSequence - mSeedSet[i].GetSequence());
+
+            if (diff == 0)
             {
                 // already received, drop message
                 ExitNow(error = OT_ERROR_DROP);
             }
-            else if (insert == NULL && aSequence < mSeedSet[i].GetSequence())
+            else if (insert == NULL && diff < 0)
             {
                 // insert in order of sequence
                 insert = &mSeedSet[i];