diff --git a/lib/include/srslte/upper/pdcp_entity_base.h b/lib/include/srslte/upper/pdcp_entity_base.h
index 8236e89bf..9a688e7fc 100644
--- a/lib/include/srslte/upper/pdcp_entity_base.h
+++ b/lib/include/srslte/upper/pdcp_entity_base.h
@@ -40,6 +40,8 @@ namespace srslte {
 #define PDCP_PDU_TYPE_PDCP_STATUS_REPORT 0x0
 #define PDCP_PDU_TYPE_INTERSPERSED_ROHC_FEEDBACK_PACKET 0x1
 
+// Maximum supported PDCP SDU size is 9000 bytes.
+// See TS 38.323 v15.2.0, section 4.3.1
 #define PDCP_MAX_SDU_SIZE 9000
 
 typedef enum {