update thingA/uri-extension docs

docs/thingA.txt

@@ -1,5 +1,5 @@
 
-We need a new name for this intentionally-vague block of data.
+"URI Extension Block"
 
 This block is a bencoded dictionary. All buckets hold an identical copy. The
 hash of the serialized data is kept in the URI.
@@ -10,25 +10,25 @@ before incremental validation can be performed. Full-file validation (for
 clients who do not wish to do incremental validation) can be performed solely
 with the data from this block.
 
-At the moment, this data block contains the following keys:
+At the moment, this data block contains the following keys (and an estimate
+on their sizes):
 
- size
- segment_size
- num_segments
- needed_shares
- total_shares
+ size                5
+ segment_size        7
+ num_segments        2
+ needed_shares       2
+ total_shares        3
 
- codec_name
- codec_params
- tail_codec_params
+ codec_name          3
+ codec_params        5+1+2+1+3=12
+ tail_codec_params   12
 
- share_root_hash
+ share_root_hash     32 (binary) or 52 (base32-encoded) each
  fileid
  plaintext_root_hash
  verifierid
  crypttext_root_hash
 
-
 Some pieces are needed elsewhere (size should be visible without pulling the
 block, the Tahoe3 algorithm needs total_shares to find the right peers, all
 peer selection algorithms need needed_shares to ask a minimal set of peers).
@@ -43,3 +43,20 @@ files, regardless of file size. Therefore hash trees (which have a size that
 depends linearly upon the number of segments) are stored elsewhere in the
 bucket, with only the hash tree root stored in this data block.
 
+This block will be serialized as follows:
+
+ assert that all keys match ^[a-zA-z_\-]+$
+ sort all the keys lexicographically
+ for k in keys:
+  write("%s:" % k)
+  write(netstring(data[k]))
+
+
+Serialized size:
+
+ dense binary (but decimal) packing: 160+46=206
+ including 'key:' (185) and netstring (6*3+7*4=46) on values: 231
+ including 'key:%d\n' (185+13=198) and printable values (46+5*52=306)=504
+
+We'll go with the 231-sized block, and provide a tool to dump it as text if
+we really want one.