forked from premiere/premiere-libtorrent
396 lines
13 KiB
ReStructuredText
396 lines
13 KiB
ReStructuredText
======================================
|
|
BitTorrent extension for DHT RSS feeds
|
|
======================================
|
|
|
|
:Author: Arvid Norberg, arvid@rasterbar.com
|
|
:Version: 1.0.0
|
|
|
|
.. contents:: Table of contents
|
|
:depth: 2
|
|
:backlinks: none
|
|
|
|
This is a proposal for an extension to the BitTorrent DHT to allow
|
|
for decentralized RSS feed like functionality.
|
|
|
|
The intention is to allow the creation of repositories of torrents
|
|
where only a single identity has the authority to add new content. For
|
|
this repository to be robust against network failures and resilient
|
|
to attacks at the source.
|
|
|
|
The target ID under which the repository is stored in the DHT, is the
|
|
SHA-1 hash of a feed name and the 512 bit public key. This private key
|
|
in this pair MUST be used to sign every item stored in the repository.
|
|
Every message that contain signed items MUST also include this key, to
|
|
allow the receiver to verify the key itself against the target ID as well
|
|
as the validity of the signatures of the items. Every recipient of a
|
|
message with feed items in it MUST verify both the validity of the public
|
|
key against the target ID it is stored under, as well as the validity of
|
|
the signatures of each individual item.
|
|
|
|
As with normal DHT announces, the write-token mechanism is used to
|
|
prevent IP spoof attacks.
|
|
|
|
There are two new proposed messages, ``announce_item`` and ``get_item``.
|
|
Every valid item that is announced, should be stored.
|
|
|
|
terminology
|
|
-----------
|
|
|
|
In this document, a *storage node* refers to the node in the DHT to which
|
|
an item is being announce. A *subscribing node* refers to a node which
|
|
makes look ups in the DHT to find the storage nodes, to request items
|
|
from them.
|
|
|
|
linked lists
|
|
------------
|
|
|
|
Items are chained together in a geneal singly linked list. A linked
|
|
list does not necessarily contain RSS items, and no RSS related items
|
|
are mandatory. However, RSS items will be used as examples in this BEP::
|
|
|
|
key = SHA1(name + key)
|
|
+---------+
|
|
| head | key = SHA1(bencode(item))
|
|
| +---------+ +---------+
|
|
| | next |-------->| item | key = SHA1(bencode(item))
|
|
| | key | | +---------+ +---------+
|
|
| | name | | | next |------->| item |
|
|
| | seq | | | key | | +---------+
|
|
| | ... | | | ... | | | next |--->0
|
|
| +---------+ | +---------+ | | key |
|
|
| sig | | sig | | | ... |
|
|
+---------+ +---------+ | +---------+
|
|
| sig |
|
|
+---------+
|
|
|
|
The ``next`` pointer is at least 20 byte ID in the DHT key space pointing to where the next
|
|
item in the list is announced. The list is terminated with an ID of all zeroes.
|
|
|
|
The ID an items is announced to is determined by the SHA1 hash of the bencoded representation
|
|
of the item iteself. This contains all fields in the item, except the signature.
|
|
The only mandatory fields in an item are ``next``, ``key`` and ``sig``.
|
|
|
|
The ``key`` field MUST match the public key of the list head node. The ``sig`` field
|
|
MUST be the signature of the bencoded representation of ``item`` or ``head`` (whichever
|
|
is included in the message).
|
|
|
|
All subscribers MUST verify that the item is announced under the correct DHT key
|
|
and MUST verify the signature is valid and MUST verify the public key is the same
|
|
as the list-head. If a node fails any of these checks, it must be ignored and the
|
|
chain of items considered terminated.
|
|
|
|
Each item holds a bencoded dictionary with arbitrary keys, except two mandatory keys:
|
|
``next`` and ``key``. The signature ``sig`` is transferred outside of this dictionary
|
|
and is the signature of all of it. An implementation should stora any arbitrary keys that
|
|
are announced to an item, within reasonable restriction such as nesting, size and numeric
|
|
range of integers.
|
|
|
|
skip lists
|
|
----------
|
|
|
|
The ``next`` key stored in the list head and the items is a string of at least length
|
|
20 bytes, it may be any length divisible by 20. Each 20 bytes are the ID of the next
|
|
item in the list, the item 2 hops away, 4 hops away, 8 hops away, and so on. For
|
|
simplicity, only the first ID (1 hop) in the ``next`` field is illustrated above.
|
|
|
|
A publisher of an item SHOULD include as many IDs in the ``next`` field as the remaining
|
|
size of the list warrants, within reason.
|
|
|
|
These skip lists allow for parallelized lookups of items and also makes it more efficient
|
|
to search for specific items. It also mitigates breaking lists missing some items.
|
|
|
|
Figure of the skip list in the first list item::
|
|
|
|
n Item0 Item1 Item2 Item3 Item4 Item5 Item6 Item7 Item8 Item9 Item10
|
|
0 O----->
|
|
20 O------------>
|
|
40 O-------------------------->
|
|
60 O------------------------------------------------------>
|
|
|
|
*n* refers to the byte offset into the ``next`` field.
|
|
|
|
list-head
|
|
---------
|
|
|
|
The list head item is special in that it can be updated, without changing its
|
|
DHT key. This is required to prepend new items to the linked list. To authenticate
|
|
that only the original publisher can update the head, the whole linked list head
|
|
is signed. In order to avoid a malicious node to overwrite the list head with an old
|
|
version, the sequence number ``seq`` must be monotonically increasing for each update,
|
|
and a node hosting the list node MUST not downgrade a list head from a higher sequence
|
|
number to a lower one, only upgrade.
|
|
|
|
The list head's DHT key (which it is announced to) MUST be the SHA1 hash of the name
|
|
(``n``) and ``key`` fields concatenated.
|
|
|
|
Any node MUST reject any list head which is announced under any other ID.
|
|
|
|
messages
|
|
--------
|
|
|
|
These are the messages to deal with linked lists.
|
|
|
|
The ``id`` field in these messages has the same semantics as the standard DHT messages,
|
|
i.e. the node ID of the node sending the message, to maintain the structure of the DHT
|
|
network.
|
|
|
|
The ``token`` field also has the same semantics as the standard DHT message ``get_peers``
|
|
and ``announce_peer``, when requesting an item and to write an item respectively.
|
|
|
|
``nodes`` and ``nodes6`` has the same semantics as in its ``get_peers`` response.
|
|
|
|
requesting items
|
|
................
|
|
|
|
This message can be used to request both a list head and a list item. When requesting
|
|
a list head, the ``n`` (name) field MUST be specified. When requesting a list item the
|
|
``n`` field is not required.
|
|
|
|
.. parsed-literal::
|
|
|
|
{
|
|
"a":
|
|
{
|
|
"id": *<20 byte ID of sending node>*,
|
|
"key": *<64 byte public curve25519 key for this list>*,
|
|
"n": *<list name>*
|
|
"target": *<target-id for 'head' or 'item'>*
|
|
},
|
|
"q": "get_item",
|
|
"t": *<transaction-id>*,
|
|
"y": "q",
|
|
}
|
|
|
|
When requesting a list-head the ``target`` MUST always be SHA-1(*feed_name* + *public_key*).
|
|
``target`` is the target node ID the item was written to.
|
|
|
|
The ``n`` field is the name of the list. If specified, It MUST be UTF-8 encoded string
|
|
and it MUST match the name of the feed in the receiving node.
|
|
|
|
request item response
|
|
.....................
|
|
|
|
This is the format of a response of a list head:
|
|
|
|
.. parsed-literal::
|
|
|
|
{
|
|
"r":
|
|
{
|
|
"head":
|
|
{
|
|
"key": *<64 byte public curve25519 key for this list>*,
|
|
"next": *<20 bytes item ID>*,
|
|
"n": *<name of the linked list>*,
|
|
"seq": *<monotonically increasing sequence number>*
|
|
},
|
|
"sig": *<curve25519 signature of 'head' entry (in bencoded form)>*,
|
|
"id": *<20 byte id of sending node>*,
|
|
"token": *<write-token>*,
|
|
"nodes": *<n * compact IPv4-port pair>*,
|
|
"nodes6": *<n * compact IPv6-port pair>*
|
|
},
|
|
"t": *<transaction-id>*,
|
|
"y": "r",
|
|
}
|
|
|
|
This is the format of a response of a list item:
|
|
|
|
.. parsed-literal::
|
|
|
|
{
|
|
"r":
|
|
{
|
|
"item":
|
|
{
|
|
"key": *<64 byte public curve25519 key for this list>*,
|
|
"next": *<20 bytes item ID>*,
|
|
...
|
|
},
|
|
"sig": *<curve25519 signature of 'item' entry (in bencoded form)>*,
|
|
"id": *<20 byte id of sending node>*,
|
|
"token": *<write-token>*,
|
|
"nodes": *<n * compact IPv4-port pair>*,
|
|
"nodes6": *<n * compact IPv6-port pair>*
|
|
},
|
|
"t": *<transaction-id>*,
|
|
"y": "r",
|
|
}
|
|
|
|
A client receiving a ``get_item`` response MUST verify the signature in the ``sig``
|
|
field against the bencoded representation of the ``item`` field, using the ``key`` as
|
|
the public key. The ``key`` MUST match the public key of the feed.
|
|
|
|
The ``item`` dictionary MAY contain arbitrary keys, and all keys MUST be stored for
|
|
items.
|
|
|
|
announcing items
|
|
................
|
|
|
|
The message format for announcing a list head:
|
|
|
|
.. parsed-literal::
|
|
|
|
{
|
|
"a":
|
|
{
|
|
"head":
|
|
{
|
|
"key": *<64 byte public curve25519 key for this list>*,
|
|
"next": *<20 bytes item ID>*,
|
|
"n": *<name of the linked list>*,
|
|
"seq": *<monotonically increasing sequence number>*
|
|
},
|
|
"sig": *<curve25519 signature of 'head' entry (in bencoded form)>*,
|
|
"id": *<20 byte node-id of origin node>*,
|
|
"target": *<target-id as derived from public key and name>*,
|
|
"token": *<write-token as obtained by previous request>*
|
|
},
|
|
"y": "q",
|
|
"q": "announce_item",
|
|
"t": *<transaction-id>*
|
|
}
|
|
|
|
The message format for announcing a list item:
|
|
|
|
.. parsed-literal::
|
|
|
|
{
|
|
"a":
|
|
{
|
|
"item":
|
|
{
|
|
"key": *<64 byte public curve25519 key for this list>*,
|
|
"next": *<20 bytes item ID>*,
|
|
...
|
|
},
|
|
"sig": *<curve25519 signature of 'item' entry (in bencoded form)>*,
|
|
"id": *<20 byte node-id of origin node>*,
|
|
"target": *<target-id as derived from item dict>*,
|
|
"token": *<write-token as obtained by previous request>*
|
|
},
|
|
"y": "q",
|
|
"q": "announce_item",
|
|
"t": *<transaction-id>*
|
|
}
|
|
|
|
A storage node MAY reject items and heads whose bencoded representation is
|
|
greater than 1024 bytes.
|
|
|
|
re-announcing
|
|
-------------
|
|
|
|
In order to keep feeds alive, subscriber nodes SHOULD help out in announcing
|
|
items they have downloaded to the DHT.
|
|
|
|
Every subscriber node SHOULD store items in long term storage, across sessions,
|
|
in order to keep items alive for as long as possible, with as few sources as possible.
|
|
|
|
Subscribers to a feed SHOULD also announce items that they know of, to the feed.
|
|
Since a feed may have many subscribers and many items, subscribers should re-announce
|
|
items according to the following algorithm.
|
|
|
|
.. parsed-literal::
|
|
|
|
1. pick one random item (*i*) from the local repository (except
|
|
items already announced this round)
|
|
2. If all items in the local repository have been announced
|
|
2.1 terminate
|
|
3. look up item *i* in the DHT
|
|
4. If fewer than 8 nodes returned the item
|
|
4.1 announce *i* to the DHT
|
|
4.2 goto 1
|
|
|
|
This ensures a balanced load on the DHT while still keeping items alive
|
|
|
|
timeouts
|
|
--------
|
|
|
|
Items SHOULD be announced to the DHT every 30 minutes. A storage node MAY time
|
|
out an item after 60 minutes of no one announcing it.
|
|
|
|
A storing node MAY extend the timeout when it receives a request for it. Since
|
|
items are immutable, the data doesn't go stale. Therefore it doesn't matter if
|
|
the storing node no longer is in the set of the 8 closest nodes.
|
|
|
|
RSS feeds
|
|
---------
|
|
|
|
For RSS feeds, following keys are mandatory in the list item's ``item`` dictionary.
|
|
|
|
ih
|
|
The torrent's info hash
|
|
|
|
size
|
|
The size (in bytes) of all files the torrent
|
|
|
|
n
|
|
name of the torrent
|
|
|
|
example
|
|
.......
|
|
|
|
This is an example of an ``announce_item`` message:
|
|
|
|
.. parsed-literal::
|
|
|
|
{
|
|
"a":
|
|
{
|
|
"item":
|
|
{
|
|
"key": "6bc1de5443d1a7c536cdf69433ac4a7163d3c63e2f9c92d
|
|
78f6011cf63dbcd5b638bbc2119cdad0c57e4c61bc69ba5e2c08
|
|
b918c2db8d1848cf514bd9958d307",
|
|
"info-hash": "7ea94c240691311dc0916a2a91eb7c3db2c6f3e4",
|
|
"size": 24315329,
|
|
"n": "my stuff",
|
|
"next": "c68f29156404e8e0aas8761ef5236bcagf7f8f2e"
|
|
}
|
|
"sig": *<signature>*
|
|
"id": "b46989156404e8e0acdb751ef553b210ef77822e",
|
|
"target": "b4692ef0005639e86d7165bf378474107bf3a762"
|
|
"token": "23ba"
|
|
},
|
|
"y": "q",
|
|
"q": "announce_item",
|
|
"t": "a421"
|
|
}
|
|
|
|
Strings are printed in hex for printability, but actual encoding is binary.
|
|
|
|
Note that ``target`` is in fact SHA1 hash of the same data the signature ``sig``
|
|
is the signature of, i.e.::
|
|
|
|
d9:info-hash20:7ea94c240691311dc0916a2a91eb7c3db2c6f3e43:key64:6bc1de5443d1
|
|
a7c536cdf69433ac4a7163d3c63e2f9c92d78f6011cf63dbcd5b638bbc2119cdad0c57e4c61
|
|
bc69ba5e2c08b918c2db8d1848cf514bd9958d3071:n8:my stuff4:next20:c68f29156404
|
|
e8e0aas8761ef5236bcagf7f8f2e4:sizei24315329ee
|
|
|
|
(note that binary data is printed as hex)
|
|
|
|
RSS feed URI scheme
|
|
--------------------
|
|
|
|
The proposed URI scheme for DHT feeds is:
|
|
|
|
.. parsed-literal::
|
|
|
|
magnet:?xt=btfd:*<base16-curve25519-public-key>* &dn= *<feed name>*
|
|
|
|
Note that a difference from regular torrent magnet links is the **btfd**
|
|
versus **btih** used in regular magnet links to torrents.
|
|
|
|
The *feed name* is mandatory since it is used in the request and when
|
|
calculating the target ID.
|
|
|
|
rationale
|
|
---------
|
|
|
|
The reason to use curve25519_ instead of, for instance, RSA is compactness. According to
|
|
http://cr.yp.to/, curve25519 is free from patent claims and there are open implementations
|
|
in both C and Java.
|
|
|
|
.. _curve25519: http://cr.yp.to/ecdh.html
|
|
|