diff --git a/protocols/osi1-2/eth-frame.yml b/protocols/osi1-2/eth-frame.yml
index bc1f03a..000a38c 100644
--- a/protocols/osi1-2/eth-frame.yml
+++ b/protocols/osi1-2/eth-frame.yml
@@ -34,11 +34,15 @@ protocol: &ethframe
               bitfield: yes
               length: 12
     -
-      field: Ethertype (Eth II) or length (IEEE 802.3)
+      field: protocolid
+      desc: Ethertype (Eth II) or length (IEEE 802.3)
       length: 2
+      nextlayer: yes
     -
       field: data
       data: true
     -
-      field: FCS (32-bit CRC)
+      field: CRC
+      desc: FCS (32-bit CRC)
       length: 4
+      optional: yes #most network stacks compute that automatically
diff --git a/structure-schema.yml b/structure-schema.yml
index 47f1a00..ff45779 100644
--- a/structure-schema.yml
+++ b/structure-schema.yml
@@ -28,6 +28,8 @@ mapping:
                 type: bool
               "repeatable":
                 type: bool
+              "nextlayer": #identifies this field as nextlayer field (see nextlayer definition below)
+                type: bool
               "data": #data placeholder for next layer
                 type: bool
               "reference": #reference another protocol directly (e.g. ethpacket->ethframe) 
@@ -39,7 +41,7 @@ mapping:
         type: int 
         range: { max: 7, min: 1 }
         required: yes
-      "nextlayer":
+      "nextlayer": #identifier list for next layer protocols
         type: seq
         sequence:
           - type: map
@@ -50,6 +52,8 @@ mapping:
               "identifier":
                 type: int
                 required: yes
+              "extension": 
+                type: bool
       "extensions": 
         type: seq
         sequence: 
@@ -64,6 +68,13 @@ mapping:
                   "referencedfrom": #referencing field of protocol definition
                     type: str
                     required: yes
+                  "referenceid": #reference id used in protocol definition or extension itself
+                    type: int
+                    required: yes
+                  "before": # standard is after the normal protocol structure, but could be used for vlan in ethernet for example.
+                    type: str
+                  "after": 
+                    type: str
                   "fields":
                     type: seq
                     sequence: *fieldsseq