diff --git a/src/srtp/replay.c b/src/srtp/replay.c
new file mode 100644
index 0000000..f6d16d9
--- /dev/null
+++ b/src/srtp/replay.c
@@ -0,0 +1,64 @@
+/**
+ * @file srtp/replay.c  SRTP replay protection
+ *
+ * Copyright (C) 2010 Creytiv.com
+ */
+#include <re_types.h>
+#include <re_mbuf.h>
+#include <re_list.h>
+#include <re_aes.h>
+#include <re_srtp.h>
+#include "srtp.h"
+
+
+enum {
+	SRTP_WINDOW_SIZE = 64
+};
+
+
+void srtp_replay_init(struct replay *replay)
+{
+	if (!replay)
+		return;
+
+	replay->bitmap = 0;
+	replay->lix    = 0;
+}
+
+
+/*
+ * Returns false if packet disallowed, true if packet permitted
+ */
+bool srtp_replay_check(struct replay *replay, uint64_t ix)
+{
+	uint64_t diff;
+
+	if (!replay)
+		return false;
+
+	if (ix > replay->lix) {
+		diff = ix - replay->lix;
+
+		if (diff < SRTP_WINDOW_SIZE) {   /* In window */
+			replay->bitmap <<= diff;
+			replay->bitmap |= 1;     /* set bit for this packet */
+		}
+		else
+			replay->bitmap = 1;
+
+		replay->lix = ix;
+		return true;
+	}
+
+	diff = replay->lix - ix;
+	if (diff >= SRTP_WINDOW_SIZE)
+		return false;
+
+	if (replay->bitmap & (1ULL << diff))
+		return false; /* already seen */
+
+	/* mark as seen */
+	replay->bitmap |= (1ULL << diff);
+
+	return true;
+}