mirror of
https://github.com/tahoe-lafs/tahoe-lafs.git
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62 lines
2.1 KiB
Plaintext
62 lines
2.1 KiB
Plaintext
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"URI Extension Block"
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This block is a serialized dictionary with string keys and string values
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(some of which represent numbers, some of which are SHA-256 hashes). All
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buckets hold an identical copy. The hash of the serialized data is kept in
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the URI.
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The download process must obtain a valid copy of this data before any
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decoding can take place. The download process must also obtain other data
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before incremental validation can be performed. Full-file validation (for
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clients who do not wish to do incremental validation) can be performed solely
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with the data from this block.
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At the moment, this data block contains the following keys (and an estimate
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on their sizes):
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size 5
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segment_size 7
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num_segments 2
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needed_shares 2
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total_shares 3
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codec_name 3
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codec_params 5+1+2+1+3=12
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tail_codec_params 12
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share_root_hash 32 (binary) or 52 (base32-encoded) each
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plaintext_hash
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plaintext_root_hash
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crypttext_hash
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crypttext_root_hash
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Some pieces are needed elsewhere (size should be visible without pulling the
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block, the Tahoe3 algorithm needs total_shares to find the right peers, all
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peer selection algorithms need needed_shares to ask a minimal set of peers).
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Some pieces are arguably redundant but are convenient to have present
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(test_encode.py makes use of num_segments).
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The rule for this data block is that it should be a constant size for all
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files, regardless of file size. Therefore hash trees (which have a size that
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depends linearly upon the number of segments) are stored elsewhere in the
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bucket, with only the hash tree root stored in this data block.
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This block will be serialized as follows:
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assert that all keys match ^[a-zA-z_\-]+$
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sort all the keys lexicographically
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for k in keys:
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write("%s:" % k)
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write(netstring(data[k]))
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Serialized size:
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dense binary (but decimal) packing: 160+46=206
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including 'key:' (185) and netstring (6*3+7*4=46) on values: 231
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including 'key:%d\n' (185+13=198) and printable values (46+5*52=306)=504
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We'll go with the 231-sized block, and provide a tool to dump it as text if
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we really want one.
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