diff --git a/multi-node/README.md b/multi-node/README.md
index a3f5fb107..47edbc654 100644
--- a/multi-node/README.md
+++ b/multi-node/README.md
@@ -11,6 +11,8 @@ Design Choice
 =====
 * Does distributed xgboost reply on MPI?
   - Yes, but the dependency is isolated in [sync module](../src/sync/sync.h)
+  - All other parts of code uses interface defined in sync.h
+  - sync_mpi.cpp is a implementation of sync interface
   - Specificially, xgboost reply on MPI protocol that provide Broadcast and AllReduce,
      if there are platform/framework that implements these protocol, xgboost should naturally extends to these platform
 * How is the data distributed?