diff --git a/py_gen/templates/generic_util.py b/py_gen/templates/generic_util.py
index cd62057..89b158d 100644
--- a/py_gen/templates/generic_util.py
+++ b/py_gen/templates/generic_util.py
@@ -79,50 +79,60 @@
     known field lengths. This class supports efficiently reading
     fields sequentially and is intended to be used recursively by the
     parsers of child objects which will implicitly update the offset.
+
+    buf: buffer object
+    start: initial position in the buffer
+    length: number of bytes after start
+    offset: distance from start
     """
-    def __init__(self, buf):
+    def __init__(self, buf, start=0, length=None):
         self.buf = buf
+        self.start = start
+        if length is None:
+            self.length = len(buf) - start
+        else:
+            self.length = length
         self.offset = 0
 
     def read(self, fmt):
         st = struct.Struct(fmt)
-        if self.offset + st.size > len(self.buf):
+        if self.offset + st.size > self.length:
             raise loxi.ProtocolError("Buffer too short")
-        result = st.unpack_from(self.buf, self.offset)
+        result = st.unpack_from(self.buf, self.start+self.offset)
         self.offset += st.size
         return result
 
     def read_all(self):
-        buf = buffer(self.buf, self.offset)
-        self.offset += len(buf)
-        return str(buf)
+        s = self.buf[(self.start+self.offset):(self.start+self.length)]
+        assert(len(s) == self.length - self.offset)
+        self.offset = self.length
+        return s
 
     def peek(self, fmt, offset=0):
         st = struct.Struct(fmt)
-        if self.offset + offset + st.size > len(self.buf):
+        if self.offset + offset + st.size > self.length:
             raise loxi.ProtocolError("Buffer too short")
-        result = st.unpack_from(self.buf, self.offset + offset)
+        result = st.unpack_from(self.buf, self.start + self.offset + offset)
         return result
 
     def skip(self, length):
-        if self.offset + length > len(self.buf):
+        if self.offset + length > self.length:
             raise loxi.ProtocolError("Buffer too short")
         self.offset += length
 
     def skip_align(self):
-        new_offset = (self.offset + 7) / 8 * 8
-        if new_offset > len(self.buf):
+        new_offset = ((self.start + self.offset + 7) / 8 * 8) - self.start
+        if new_offset > self.length:
             raise loxi.ProtocolError("Buffer too short")
         self.offset = new_offset
 
     def is_empty(self):
-        return self.offset == len(self.buf)
+        return self.offset == self.length
 
-    # Used when parsing variable length objects which have external length
-    # fields (e.g. the actions list in an OF 1.0 packet-out message).
+    # Used when parsing objects that have their own length fields
     def slice(self, length):
-        if self.offset + length > len(self.buf):
+        if self.offset + length > self.length:
             raise loxi.ProtocolError("Buffer too short")
-        buf = OFReader(buffer(self.buf, self.offset, length))
+        reader = OFReader(self.buf, self.start + self.offset, length)
         self.offset += length
-        return buf
+        return reader