mirror of
https://github.com/tahoe-lafs/tahoe-lafs.git
synced 2024-12-21 05:53:12 +00:00
62 lines
2.1 KiB
Plaintext
62 lines
2.1 KiB
Plaintext
|
|
"URI Extension Block"
|
|
|
|
This block is a serialized dictionary with string keys and string values
|
|
(some of which represent numbers, some of which are SHA-256 hashes). All
|
|
buckets hold an identical copy. The hash of the serialized data is kept in
|
|
the URI.
|
|
|
|
The download process must obtain a valid copy of this data before any
|
|
decoding can take place. The download process must also obtain other data
|
|
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 (and an estimate
|
|
on their sizes):
|
|
|
|
size 5
|
|
segment_size 7
|
|
num_segments 2
|
|
needed_shares 2
|
|
total_shares 3
|
|
|
|
codec_name 3
|
|
codec_params 5+1+2+1+3=12
|
|
tail_codec_params 12
|
|
|
|
share_root_hash 32 (binary) or 52 (base32-encoded) each
|
|
plaintext_hash
|
|
plaintext_root_hash
|
|
crypttext_hash
|
|
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).
|
|
Some pieces are arguably redundant but are convenient to have present
|
|
(test_encode.py makes use of num_segments).
|
|
|
|
The rule for this data block is that it should be a constant size for all
|
|
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.
|