diff --git a/src/rlc.cpp b/src/rlc.cpp
index acd4169..c52417d 100644
--- a/src/rlc.cpp
+++ b/src/rlc.cpp
@@ -58,7 +58,7 @@
 	m_v_b.reset();
 }
 
-int gprs_rlc_dl_window::resend_needed()
+int gprs_rlc_dl_window::resend_needed() const
 {
 	for (uint16_t bsn = v_a(); bsn != v_s(); bsn = mod_sns(bsn + 1)) {
 		if (m_v_b.is_nacked(bsn) || m_v_b.is_resend(bsn))
diff --git a/src/rlc.h b/src/rlc.h
index ee86c2a..14d2082 100644
--- a/src/rlc.h
+++ b/src/rlc.h
@@ -303,7 +303,7 @@
 	const uint16_t distance() const;
 
 	/* Methods to manage reception */
-	int resend_needed();
+	int resend_needed() const;
 	int mark_for_resend();
 	void update(BTS *bts, char *show_rbb, uint16_t ssn,
 			uint16_t *lost, uint16_t *received);