3 @unnumbered Multicast areas
5 NNCP has ability to multicast packets: send single packet to multiple
6 recipients, which also can send it further to others. It can also be
7 called echomail (like in FidoNet networks) or newsgroup (like in Usenet
11 Each multicast group is identified by so-called @strong{area}. Area
12 consists of private/public Curve25519 keypairs for @ref{Encrypted area,
13 packets encryption}, identity (BLAKE2b-256 hash of the public key) and
14 possible subscribers. Areas are created with @command{@ref{nncp-cfgnew}}
17 You can make either file or exec transmissions to the areas. Those
18 ordinary file/exec packets are double wrapped in:
21 @item encrypted packet, securing the actual packet contents from
22 participants not having area's keypairs (but still being able to relay
23 that encrypted packet to the others)
24 @item area packet, containing area's identity, telling that tossing node
25 can should it to the subscribers further
29 Area's message identity (@code{MsgHash}) is the hash of the encrypted
30 packet header. Because the area packet, containing the encrypted packet,
31 is relayed as-is without any modifications, that area message's hash
32 will be the same on each node it reaches.
34 @ref{nncp-toss, Tosser}'s algorithm of processing the area packet is
38 @item check is it known area's identity (@code{AREA}).
39 Fail/skip if it is unknown
40 @item hash encrypted packet's header, getting the @code{MsgHash}
41 @item for each area's subscribers:
43 @item check if that message was already seen (sent or received from)
44 before by the destination node: check existence of
45 @file{SPOOL/NODE/area/AREA/MsgHash} file. Skip that node if
47 @item if subscriber's node is not the one we received the packet
48 from, then create outgoing encrypted packet to it, with that
50 @item create corresponding @file{MsgHash} file
51 @item "rewind" the outer encrypted file to the beginning and repeat
52 the whole cycle again, while all of subscribers will "seen" that
55 Expensive signature verification and shared key computation
56 procedures are skipped in the following cycles -- only symmetric
57 cryptography will be in use, having negligible CPU resource
60 @item check if we have seen that area's message before by looking at
61 @file{SPOOL/SELF/area/AREA/MsgHash}. If so, remove the packet,
62 because it is just a ordinary possible duplicate, finish its processing
63 @item check if we have got corresponding area's private key. If no key
64 exists, then remove the packet, finish its processing -- we just
65 relay it further without being able to read it
66 @item look if area's encrypted packet's sender is known to us. If
67 neither it is known, nor we have @code{allow-unknown} configuration
68 option set for that area, then fail
69 @item otherwise start decryption procedure, possibly ignoring the
70 sender's signature verification if it is unknown
71 @item fed the decrypted contents to the toss-procedure as an ordinary
72 plain packet, receiving files or exec calls
73 @item mark area's message as the seen one, remove the packet, finish
77 Because outgoing packets creation for each subscriber can be time and
78 (disk) resource consuming, we can suddenly fail. It would be bad if we
79 will loose the possibility to retry the multicasting process again. So
80 we have got to save somehow outgoing area's message in permanent
81 storage, while outgoing copies are created. That is why the initial (not
82 relaying) message to the area is sent to the @strong{self} and processed
83 by the @ref{nncp-toss, tosser} to create necessary outgoing message
84 copies. Because message to myself is also encrypted, area's message is
85 encrypted and secured and noone sees plaintext @code{MsgHash}, knowing
86 that you either originated or have that message on the disk.
88 For example we have got 4 nodes participating in the single area and
89 let's send file to that area from the @code{nodeA}:
92 nodeA -> subs: ["nodeB", "nodeD"]
93 nodeB -> subs: ["nodeC", "nodeD", "nodeA"], no keys
94 nodeC -> subs: ["nodeB"]
95 nodeD -> subs: ["nodeA", "nodeB"]
106 $ nncp-file nodelist-20210704.rec.zst area:nodelist-updates:
107 $ nncp-toss -node self
112 @command{@ref{nncp-file}} creates an encrypted packet with area packet and
113 encrypted packet inside it, with our own @code{self} node as a recipient
114 (in the @file{SPOOL/SELF/tx} directory). It also creates the
115 @file{SPOOL/SELF/area/AREA/MsgHash} file.
118 @command{@ref{nncp-toss}} sees @file{tx/} file and "opens" it, applying the
119 area message tossing procedure as described above. That will create
120 outgoing packets in @file{SPOOL/nodeB/tx} and @file{SPOOL/nodeD/tx}
121 directories with @file{SPOOL/nodeB/area/AREA/MsgHash}
122 @file{SPOOL/nodeD/area/AREA/MsgHash} files. Because we already have
123 @file{SPOOL/SELF/area/AREA/MsgHash}, that packet is removed then.
126 When @code{nodeB} receives the encrypted packet, it sees the area one
127 inside. It copies/relays it to the @code{nodeC} and @code{nodeD}. It can
128 not read area's message because it lacks the keys.
131 @code{nodeC} does not relay it to anyone. Just stores
132 @file{nodelist-20210704.rec.zst} in the incoming directory.
135 @code{nodeD} receives packets from both @code{nodeA} and @code{nodeB}.
136 Only one of them processed, and other is ignored because corresponding
137 @file{MsgHash} file will exist.
139 If @code{nodeD} will receive packet from the @code{nodeB} first, it will
140 relay it to the @code{nodeA} also, that will silently remove it when
141 tossing, because it was already seen.
144 When @code{nodeC} sends message to the area, then @code{nodeA} will
145 receive it twice from @code{nodeB} and @code{nodeD}, ignoring one of