questions about the window size

Section 5.2 of RFC4254 says:

   The maximum amount of data allowed is determined by the maximum
   packet size for the channel, and the current window size, whichever
   is smaller.  The window size is decremented by the amount of data
   sent.  Both parties MAY ignore all extra data sent after the allowed
   window is empty.

This is the data transfer packet:

byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
string data

Since string's are defined by RFC4251 as being "stored as a uint32
containing its length (number of bytes that follow) and zero (= empty
string) or more bytes that are the value of the string" it's unclear
weather or not the uint32 length field contributes to the total or not. Any
ideas?

It is also unclear from this RFC whether or not
SSH_MSG_CHANNEL_EXTENDED_DATA should be decrementing from the window size
or not. Any insight would be appreciated - thanks!