Technical_topic_ASN_1_and_ACN_How_to_add_a_CRC_value_to_an_encoded_packet.md

+# Technical topic: ASN.1 and ACN - How to add a CRC value to an encoded packet
+
+## <span id="The_need" class="mw-headline">The need</span>
+
+If you are using ASN.1 to encode the messages you send over a network
+(as you should\!) you may face the situation where ASN.1 cannot know the
+value of a field before the whole packet is encoded. In space-ground
+links there are two fields of the standard telecommand packets that
+enter this category:
+
+  - a Length field in the packet header (used to know the number of
+    bytes to wait for on the network interface)
+  - a CRC/Checksum field at the end of the packet
+
+In the ASN.1 model you may add a placeholder for these fields but the
+encoder is of course not able to set the value, as it may require to
+compute it with a user-defined algorithm.
+
+Adding manually the fields to the buffer after the encoding phase is
+possible but not trivial, as you need to take care of endianness and
+possibly shift bits here and there. In other words, you loose some
+benefits of ASN.1 by doing things manually again.
+
+Luckily ASN1SCC provides help using a simple API to fill a value in the
+packet after it is encoded. You only need to know where to put the
+value.
+
+## <span id="Solution_using_ASN.1_and_ACN" class="mw-headline">Solution using ASN.1 and ACN</span>
+
+**EDIT** A solution better than the one explained on this page is
+documented in the ASN.1/ACN user manual. Please check this link and
+ignore the rest of this page:
+
+<http://taste.tuxfamily.org/wiki/index.php?title=Technical_topic:_ASN.1_-_An_introduction_to_ACN#encoding_fields_that_depend_on_the_encoding_binary_stream>
+
+## <span id="DEPRECATED_PART" class="mw-headline">DEPRECATED PART</span>
+
+We need two things:
+
+  - to specify the placeholder for the values
+  - to fill in the value after the packet is encoded
+
+The placeholder can be present in the ASN.1 type itself, but since it is
+not a field that is of any relevance to the end user (not application
+semantics) we will rather use ACN to add it.
+
+Assuming a top-level structure in ASN.1 that mimicks a space-to-ground
+telemetry packet (TM):
+
+  
+
+``` 
+       TM-PACKET ::= SEQUENCE {
+        header  TM-HEADER,
+        data    TM-DATA OPTIONAL
+    }
+   with
+    TM-HEADER ::= SEQUENCE {
+        applicationProcessID    INTEGER(0..2047),
+        grouping-flags      TC-HEADER-SEQUENCE-FLAGS,
+        sequence-count      INTEGER(0..16383)
+       }
+```
+
+One of the fields we want to add is at the level of the packet itself:
+we want to add a packet error control (CRC). Additionally we want to add
+at the end of the TM-HEADER structure a field that contains the total
+size of the TM-PACKET record.
+
+  
+We can create a corresponding ACN encoding structure that "adds" the
+missing fields (in bold):
+
+``` 
+       TM-PACKET [] {
+               header                 [],
+               data                   [present-when header.data-field-header-flag],
+               packet-error-control    NULL    [pattern '0000000000000000'B, align-to-next byte]
+       }
+       
+       TM-HEADER [] {
+               data-field-header-flag  BOOLEAN         [],
+               applicationProcessID                    [],
+               grouping-flags                          [],
+               sequence-count                          [],
+               packet-length           NULL    [pattern '0000000000000000'B]
+       }
+```
+
+In your code, you may create the functions that compute the CRC and size
+after the packet is encoded.
+
+Here is the code you can use:
+
+``` 
+      // A buffer to encode your TM
+      static byte encBuff[TM_PACKET_REQUIRED_BYTES_FOR_ACN_ENCODING];
+      BitStream bitStrm;
+      BitStream bitStrm_aux;
+      flag ret;
+      int errCode;
+      int i;
+      //telemetry packet to be encoded
+      static TM_PACKET tm_packet = ....   
+          
+      // initialize bit stream
+      BitStream_Init(&bitStrm, encBuff, TM_PACKET_REQUIRED_BYTES_FOR_ACN_ENCODING);
+          
+      // Encode value using ACN
+      ret = TM_PACKET_ACN_Encode(&tm_packet, &bitStrm, &errCode, TRUE);
+   
+      // First field to add: get the length of the full packet
+      asn1SccSint encoded_data_length = BitStream_GetLength(&bitStrm);
+      
+      // Encode length field 
+      bitStrm_aux.buf = bitStrm.buf;
+      bitStrm_aux.count = bitStrm.count;
+      bitStrm_aux.currentByte = 4;
+      bitStrm_aux.currentBit = 0; 
+      BitStream_EncodeConstraintWholeNumber(&bitStrm_aux, encoded_data_length - 6, 0, 0xFFFF);
+      
+      //encode crc field;
+      bitStrm_aux.buf = bitStrm.buf;
+      bitStrm_aux.count = bitStrm.count;
+      bitStrm_aux.currentByte = bitStrm.currentByte - 2;
+      bitStrm_aux.currentBit = 0; 
+      uint16_t crc = gen_crc16(encBuff, bitStrm.currentByte);  // User-defined CRC computation
+      BitStream_EncodeConstraintWholeNumber(&bitStrm_aux, crc, 0, 0xFFFF);
+      
+```
+
+That's it\!