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Add ".. -*- coding: utf-8-with-signature -*-" to the first line of each .rst file. This tells emacs to treat the file contents as utf-8, and also to prepend a so-called utf-8 "bom" marker at the beginning of the file. This patch also prepends those markers to each of those files.
136 lines
6.5 KiB
ReStructuredText
136 lines
6.5 KiB
ReStructuredText
.. -*- coding: utf-8-with-signature -*-
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=======================
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Node Keys in Tahoe-LAFS
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=======================
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"Node Keys" are cryptographic signing/verifying keypairs used to
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identify Tahoe-LAFS nodes (client-only and client+server). The private
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signing key is stored in NODEDIR/private/node.privkey , and is used to
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sign the announcements that are distributed to all nodes by the
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Introducer. The public verifying key is used to identify the sending
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node from those other systems: it is displayed as a "Node ID" that looks
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like "v0-abc234xyz567..", which ends with a long base32-encoded string.
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These node keys were introduced in the 1.10 release (April 2013), as
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part of ticket #466. In previous releases, announcements were unsigned,
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and nodes were identified by their Foolscap "Tub ID" (a somewhat shorter
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base32 string, with no "v0-" prefix).
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Why Announcements Are Signed
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----------------------------
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All nodes (both client-only and client+server) publish announcements to
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the Introducer, which then relays them to all other nodes. These
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announcements contain information about the publishing node's nickname,
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how to reach the node, what services it offers, and what version of code
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it is running.
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The new private node key is used to sign these announcements, preventing
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the Introducer from modifying their contents en-route. This will enable
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future versions of Tahoe-LAFS to use other forms of introduction
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(gossip, multiple introducers) without weakening the security model.
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The Node ID is useful as a handle with which to talk about a node. For
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example, when clients eventually gain the ability to control which
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storage servers they are willing to use (#467), the configuration file
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might simply include a list of Node IDs for the approved servers.
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TubIDs are currently also suitable for this job, but they depend upon
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having a Foolscap connection to the server. Since our goal is to move
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away from Foolscap towards a simpler (faster and more portable)
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protocol, we want to reduce our dependence upon TubIDs. Node IDs and
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Ed25519 signatures can be used for non-Foolscap non-SSL based protocols.
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How The Node ID Is Computed
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---------------------------
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The long-form Node ID is the Ed25519 public verifying key, 256 bits (32
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bytes) long, base32-encoded, with a "v0-" prefix appended, and the
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trailing "=" padding removed, like so:
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v0-rlj3jnxqv4ee5rtpyngvzbhmhuikjfenjve7j5mzmfcxytwmyf6q
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The Node ID is displayed in this long form on the node's front Welcome
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page, and on the Introducer's status page. In most other places
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(share-placement lists, file health displays), the "short form" is used
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instead. This is simply the first 8 characters of the base32 portion,
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frequently enclosed in square brackets, like this:
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[rlj3jnxq]
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In contrast, old-style TubIDs are usually displayed with just 6 base32
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characters.
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Version Compatibility, Fallbacks For Old Versions
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-------------------------------------------------
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Since Tahoe-LAFS 1.9 does not know about signed announcements, 1.10
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includes backwards-compatibility code to allow old and new versions to
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interoperate. There are three relevant participants: the node publishing
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an announcement, the Introducer which relays them, and the node
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receiving the (possibly signed) announcement.
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When a 1.10 node connects to an old Introducer (version 1.9 or earlier),
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it sends downgraded non-signed announcements. It likewise accepts
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non-signed announcements from the Introducer. The non-signed
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announcements use TubIDs to identify the sending node. The new 1.10
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Introducer, when it connects to an old node, downgrades any signed
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announcements to non-signed ones before delivery.
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As a result, the only way to receive signed announcements is for all
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three systems to be running the new 1.10 code. In a grid with a mixture
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of old and new nodes, if the Introducer is old, then all nodes will see
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unsigned TubIDs. If the Introducer is new, then nodes will see signed
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Node IDs whenever possible.
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Share Placement
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---------------
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Tahoe-LAFS uses a "permuted ring" algorithm to decide where to place
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shares for any given file. For each potential server, it uses that
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server's "permutation seed" to compute a pseudo-random but deterministic
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location on a ring, then walks the ring in clockwise order, asking each
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server in turn to hold a share until all are placed. When downloading a
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file, the servers are accessed in the same order. This minimizes the
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number of queries that must be done to download a file, and tolerates
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"churn" (nodes being added and removed from the grid) fairly well.
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This property depends upon server nodes having a stable permutation
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seed. If a server's permutation seed were to change, it would
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effectively wind up at a randomly selected place on the permuted ring.
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Downloads would still complete, but clients would spend more time asking
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other servers before querying the correct one.
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In the old 1.9 code, the permutation-seed was always equal to the TubID.
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In 1.10, servers include their permutation-seed as part of their
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announcement. To improve stability for existing grids, if an old server
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(one with existing shares) is upgraded to run the 1.10 codebase, it will
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use its old TubID as its permutation-seed. When a new empty server runs
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the 1.10 code, it will use its Node ID instead. In both cases, once the
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node has picked a permutation-seed, it will continue using that value
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forever.
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To be specific, when a node wakes up running the 1.10 code, it will look
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for a recorded NODEDIR/permutation-seed file, and use its contents if
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present. If that file does not exist, it creates it (with the TubID if
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it has any shares, otherwise with the Node ID), and uses the contents as
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the permutation-seed.
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There is one unfortunate consequence of this pattern. If new 1.10 server
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is created in a grid that has an old client, or has a new client but an
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old Introducer, then that client will see downgraded non-signed
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announcements, and thus will first upload shares with the TubID-based
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permutation-seed. Later, when the client and/or Introducer is upgraded,
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the client will start seeing signed announcements with the NodeID-based
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permutation-seed, and will then look for shares in the wrong place. This
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will hurt performance in a large grid, but should not affect
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reliability. This effect shouldn't even be noticeable in grids for which
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the number of servers is close to the "N" shares.total number (e.g.
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where num-servers < 3*N). And the as-yet-unimplemented "share
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rebalancing" feature should repair the misplacement.
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If you wish to avoid this effect, try to upgrade both Introducers and
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clients at about the same time. (Upgrading servers does not matter: they
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will continue to use the old permutation-seed).
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