diff --git a/wsproto/frame_protocol.py b/wsproto/frame_protocol.py
index 7406eac..081f6ed 100644
--- a/wsproto/frame_protocol.py
+++ b/wsproto/frame_protocol.py
@@ -32,7 +32,25 @@ def process(self, data):
 
 
 # RFC6455, Section 5.2 - Base Framing Protocol
-MAX_FRAME_PAYLOAD = 2 ** 64
+
+# Payload length constants
+PAYLOAD_LENGTH_TWO_BYTE = 126
+PAYLOAD_LENGTH_EIGHT_BYTE = 127
+MAX_PAYLOAD_NORMAL = 125
+MAX_PAYLOAD_TWO_BYTE = 2 ** 16
+MAX_PAYLOAD_EIGHT_BYTE = 2 ** 64
+MAX_FRAME_PAYLOAD = MAX_PAYLOAD_EIGHT_BYTE
+
+# MASK and PAYLOAD LEN are packed into a byte
+MASK_MASK = 0x80
+PAYLOAD_LEN_MASK = 0x7f
+
+# FIN, RSV[123] and OPCODE are packed into a single byte
+FIN_MASK = 0x80
+RSV1_MASK = 0x40
+RSV2_MASK = 0x20
+RSV3_MASK = 0x10
+OPCODE_MASK = 0x0f
 
 
 class Opcode(IntEnum):
@@ -78,6 +96,17 @@ class CloseReason(IntEnum):
 )
 
 
+# RFC 6455, Section 7.4.2 - Status Code Ranges
+MIN_CLOSE_REASON = 1000
+MIN_PROTOCOL_CLOSE_REASON = 1000
+MAX_PROTOCOL_CLOSE_REASON = 2999
+MIN_LIBRARY_CLOSE_REASON = 3000
+MAX_LIBRARY_CLOSE_REASON = 3999
+MIN_PRIVATE_CLOSE_REASON = 4000
+MAX_PRIVATE_CLOSE_REASON = 4999
+MAX_CLOSE_REASON = 4999
+
+
 NULL_MASK = struct.pack("!I", 0)
 
 
@@ -143,18 +172,18 @@ def _consume_exactly(self, nbytes):
         return (yield from self._consume_at_most(nbytes))
 
     def _parse_extended_payload_length(self, opcode, payload_len):
-        if opcode.iscontrol() and payload_len > 125:
+        if opcode.iscontrol() and payload_len > MAX_PAYLOAD_NORMAL:
             raise ParseFailed("Control frame with payload len > 125")
-        if payload_len == 126:
+        if payload_len == PAYLOAD_LENGTH_TWO_BYTE:
             data = yield from self._consume_exactly(2)
             (payload_len,) = struct.unpack("!H", data)
-            if payload_len <= 125:
+            if payload_len <= MAX_PAYLOAD_NORMAL:
                 raise ParseFailed(
                     "Payload length used 2 bytes when 1 would have sufficed")
-        elif payload_len == 127:
+        elif payload_len == PAYLOAD_LENGTH_EIGHT_BYTE:
             data = yield from self._consume_exactly(8)
             (payload_len,) = struct.unpack("!Q", data)
-            if payload_len < 2 ** 16:
+            if payload_len < MAX_PAYLOAD_TWO_BYTE:
                 raise ParseFailed(
                     "Payload length used 8 bytes when 2 would have sufficed")
             if payload_len >> 63:
@@ -167,11 +196,11 @@ def _parse_extended_payload_length(self, opcode, payload_len):
     def _parse_header(self):
         # returns a Header object
         (fin_rsv_opcode,) = yield from self._consume_exactly(1)
-        fin = bool(fin_rsv_opcode & 0x80)
-        rsv = (bool(fin_rsv_opcode & 0x40),
-               bool(fin_rsv_opcode & 0x20),
-               bool(fin_rsv_opcode & 0x10))
-        opcode = fin_rsv_opcode & 0x0f
+        fin = bool(fin_rsv_opcode & FIN_MASK)
+        rsv = (bool(fin_rsv_opcode & RSV1_MASK),
+               bool(fin_rsv_opcode & RSV2_MASK),
+               bool(fin_rsv_opcode & RSV3_MASK))
+        opcode = fin_rsv_opcode & OPCODE_MASK
         try:
             opcode = Opcode(opcode)
         except ValueError:
@@ -181,8 +210,8 @@ def _parse_header(self):
             raise ParseFailed("Invalid attempt to fragment control frame")
 
         (mask_len,) = yield from self._consume_exactly(1)
-        has_mask = bool(mask_len & 0x80)
-        payload_len = mask_len & 0x7f
+        has_mask = bool(mask_len & MASK_MASK)
+        payload_len = mask_len & PAYLOAD_LEN_MASK
         payload_len = yield from self._parse_extended_payload_length(
             opcode, payload_len
         )
@@ -233,7 +262,7 @@ def _process_CLOSE_payload(self, data):
             raise ParseFailed("CLOSE with 1 byte payload")
         else:
             (code,) = struct.unpack("!H", data[:2])
-            if code < 1000:
+            if code < MIN_CLOSE_REASON or code > MAX_CLOSE_REASON:
                 raise ParseFailed("CLOSE with invalid code")
             try:
                 code = CloseReason(code)
@@ -242,7 +271,8 @@ def _process_CLOSE_payload(self, data):
             if code in LOCAL_ONLY_CLOSE_REASONS:
                 raise ParseFailed(
                     "remote CLOSE with local-only reason")
-            if not isinstance(code, CloseReason) and code < 3000:
+            if not isinstance(code, CloseReason) and \
+               code <= MAX_PROTOCOL_CLOSE_REASON:
                 raise ParseFailed(
                     "CLOSE with unknown reserved code")
             try:
@@ -366,7 +396,8 @@ def close(self, code=None, reason=None):
         if code is not None:
             payload += struct.pack('!H', code)
             if reason is not None:
-                payload += _truncate_utf8(reason.encode('utf-8'), 123)
+                payload += _truncate_utf8(reason.encode('utf-8'),
+                                          MAX_PAYLOAD_NORMAL - 2)
 
         return self._serialize_frame(Opcode.CLOSE, payload)
 
@@ -410,14 +441,14 @@ def _serialize_frame(self, opcode, payload=b'', fin=True):
 
         payload_length = len(payload)
         quad_payload = False
-        if payload_length <= 125:
+        if payload_length <= MAX_PAYLOAD_NORMAL:
             first_payload = payload_length
             second_payload = None
-        elif payload_length <= 65535:
-            first_payload = 126
+        elif payload_length <= MAX_PAYLOAD_TWO_BYTE:
+            first_payload = PAYLOAD_LENGTH_TWO_BYTE
             second_payload = payload_length
         else:
-            first_payload = 127
+            first_payload = PAYLOAD_LENGTH_EIGHT_BYTE
             second_payload = payload_length
             quad_payload = True