[coap] add support for CoAP header exceeding 255 bytes (#3210)

include/openthread/coap.h

@@ -165,7 +165,7 @@ typedef enum otCoapOptionContentFormat {
     OT_COAP_OPTION_CONTENT_FORMAT_JWS          = 101 ///< application/json-web-signature
 } otCoapOptionContentFormat;
 
-#define OT_COAP_HEADER_MAX_LENGTH 128 ///< Max CoAP header length (bytes)
+#define OT_COAP_HEADER_MAX_LENGTH 512 ///< Max CoAP header length (bytes)
 
 /**
  * This structure represents a CoAP header.
@@ -183,7 +183,7 @@ typedef struct otCoapHeader
         } mFields;                                 ///< Structure representing a CoAP base header
         uint8_t mBytes[OT_COAP_HEADER_MAX_LENGTH]; ///< The raw byte encoding for the CoAP header
     } mHeader;                                     ///< The CoAP header encoding
-    uint8_t      mHeaderLength;                    ///< The CoAP header length (bytes)
+    uint16_t     mHeaderLength;                    ///< The CoAP header length (bytes)
     uint16_t     mOptionLast;                      ///< The last CoAP Option Number value
     uint16_t     mFirstOptionOffset;               ///< The byte offset for the first CoAP Option
     uint16_t     mNextOptionOffset;                ///< The byte offset for the next CoAP Option

src/core/coap/coap_header.cpp

@@ -249,7 +249,7 @@ otError Header::AppendOption(const Option &aOption)
     memcpy(cur, aOption.mValue, aOption.mLength);
     cur += aOption.mLength;
 
-    mHeaderLength += static_cast<uint8_t>(cur - buf);
+    mHeaderLength += static_cast<uint16_t>(cur - buf);
     mOptionLast = aOption.mNumber;
 
 exit:

src/core/coap/coap_header.hpp

@@ -402,7 +402,7 @@ public:
      * @returns The header length in bytes.
      *
      */
-    uint8_t GetLength(void) const { return mHeaderLength; }
+    uint16_t GetLength(void) const { return mHeaderLength; }
 
     /**
      * This method sets a default response header based on request header.