diff --git a/src/rlc.cpp b/src/rlc.cpp
index 1a2aa0f..1a05546 100644
--- a/src/rlc.cpp
+++ b/src/rlc.cpp
@@ -37,3 +37,16 @@
 	for (size_t i = 0; i < ARRAY_SIZE(m_v_b); ++i)
 		mark_invalid(i);
 }
+
+int gprs_rlc_v_b::resend_needed(const uint16_t v_a, const uint16_t v_s,
+				const uint16_t mod_sns,
+				const uint16_t mod_sns_half)
+{
+	for (uint8_t bsn = v_a; bsn != v_s; bsn = (bsn + 1) & mod_sns) {
+		uint16_t index = bsn & mod_sns_half;
+		if (is_nacked(index) || is_resend(index))
+			return bsn;
+	}
+
+	return -1;
+}