diff --git a/internals.h b/internals.h
index 8e97e8a..f6b747d 100644
--- a/internals.h
+++ b/internals.h
@@ -53,6 +53,14 @@ enum outcome_type {
 
 GNUNET_NETWORK_STRUCT_BEGIN
 
+/**
+ * Every message is prefixed with this header.
+ * 
+ * BEWARE: Introducing a 16bit length field will not work. M+1st price auctions
+ * with private outcome send a n*n*(n-1)*k*128 = n²(n-1)*64 (for a reasonable
+ * k-choice of 512 possible prices) byte message from the seller to all bidders
+ * (decrypt-round). For just two bidders this is already 256k bytes, but 16bit
+ * uint only can address 64k bytes. */
 struct msg_head {
 	uint32_t prot_version GNUNET_PACKED;
 	uint32_t msg_type     GNUNET_PACKED;