TFTP Server?

OK, stuck again. I can't test it because I can't get it to parse right in the IRE. I'm missing something dumb here, but am going cross-eyed trying to find it. I'll post what I have and come back to it later after I clear my head.

(updated with some comments and a little visual cleanup)

  
 when CLIENT_ACCEPTED {  
     log local0. "Client connected from [IP::client_addr]"  
  binary scan [UDP::payload] xc opcode  
  log local0. "Opcode is $opcode"  
   
 if { $opcode == 1 } {  
  binary scan [UDP::payload] xxa* string  
  log local0. "String is $string"  
  set stringlist [split $string \000]  
  set file [lindex $stringlist 0]  
  set mode [lindex $stringlist 1]  
  log local0. "File is $file, Mode is $mode"  
   
 if { $mode != "octet" && $mode != "netascii" } {  
   
 reject  
   
 } else {  
   
 log local0. "Send the file!"  
  
 set block_id 1 
 set block_start 0 
 set block_len 512 
 set last 0 
  
 while { $last < 1 } { 
  
 log local0. "Block number $block_id" 
  
 set data [binary format xcSa* 3  $block_id [string range $::tftp_file $block_start $block_end]] 
  
  test for the last block - is it less than 512 bytes? If so, set a flag 
 if { [string length [string range $::tftp_file $block_start $block_end]] < 512 } then { 
  
 set last 1  
 } 
  
 log local0. "data = $data" 
  
  send the data 
  
 UDP::respond $data 
  
  increment block_id and shift the pointers to the 
  next block of data to send 
  
 incr block_id 
 block_start = block_len 
 set block_len = block_len + 512 
  
 } 
  
  We should be breaking out of the while loop now 
  The last block should have been sent. 
  
 } 
 }  
   
 } else { reject }  
  }