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bluesky - Re: the three-services model

bluesky AT lists.ibiblio.org

Subject: Global-Scale Distributed Storage Systems

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  • From: Wei Dai <weidai AT eskimo.com>
  • To: Global-Scale Distributed Storage Systems <bluesky AT franklin.oit.unc.edu>
  • Subject: Re: the three-services model
  • Date: Sat, 17 Feb 2001 20:09:12 -0800


On Sat, Feb 17, 2001 at 09:25:24AM -0800, hal AT finney.org wrote:
> Not all services seem to fit this model. Mojonation does not have user
> friendly names AFAIK. The end result of their publication process is a
> "dinode" consisting of 8 SHA-1 hashes concatenated. One example is:
> http://localhost:4004/Dinode/eNpl0stuglAQBuBXIezRUzkCJogBFOQmKFYsGwNyFO
> VmQUR8-mrF9MLyn8n3ZxbDjq5JjF1QXhyydIi_dQA-4tjxIc0C9DMGncF9gRWJdzoEQ3zh9
> I8CszPNCb28gSxMPnwH0sQiTysci1G6P4f3KgpSDM6xdujlCPPjzH_QIkwMay_BbbYKFUOn
> qBUpvlfmji4NHEvLZIjTePc_qnZSTUs6kTOEbMtH5Lo5n9pz6F32DaLaKCXsYB1pNC-TSlk
> CBiy3B30r-ycSNKjfRusprzPL8xUyO3GRXdcilGI1VJPN6XUebKN5ZFaWtrbUebS3bqqxEU
> 4C_5kfqRnfILKNavoqlxpZbatc7glFbToXy9RdxNuv83ptpMWRPwahOOgrK7UoAqcWtdtGZ
> oLVvEFvbeSDcDp2XEXyILFR6rMr9RN14gtpNGkQeKBldvbib1lwDNv9HZ-FxQyhAAUcZLt_
> Mtt9vgv3BbSVu-I/. (I've broken it up into multiple lines.)
>
> Likewise I don't think Napster or Gnutella have a URL or name for
> persistent addressing of data.
>
> Instead, these systems are search based. When you want to find data, you
> do a search providing various keywords of substrings. Data is published
> on the network with associated meta-data that can include title, author,
> etc. Your searches use this information to find data that you might want.
> This is presented to you and you then choose which files to download.

The three services model doesn't preclude additional services like
keyword-based searching, but clearly for a general purpose storage system,
persistent and secure naming is a key requirement. Also note that the
three services can be designed, implemented, and provided independently of
each other, as long as we standardize the interfaces. So although
MojoNation may not provide a name service, it can still take advantage of
other name services. (Why does it use eight concatenated hashes instead of
just one, btw?)

> I think the trust issue with (a) is present in step 1 in the three
> step model.

The idea is to limit this trust issue to only the name service, so that
you don't have to trust location or storage servers not to modify the
data.

> This is a hard problem which has been much discussed on the
> Freenet list. How do you trust the name to address mapping? Do you
> need to introduce cryptography, a trust model, a PKI? Such systems
> have never been successful, although it might be argued that P2P is the
> "killer ap" for end-user PKI.

I think a distributed trust model is the only thing that can work. But
because the three services can be independent, there can actually be
multiple name services, using different trust models or PKIs (or even
insecure ones that don't use any cryptography).

> These problems exist in Freenet, but they don't seem that severe.
> Data objects in that system are considered fluid and don't have a
> permanent home, flowing through the network. Any system which will avoid
> the slashdot effect (what Adam calls flash crowds) needs to be able to
> migrate and spread data according to demand.

Freenet can't be considered a general purpose storage system either, since
it doesn't provide any reliability. There is no way to make reasonably
sure that if I publish a document on Freenet, it can be accessed from
anywhere, and will be stored for at least a certain amount of time. In
this regard it's even worse than the world wide web.

> As for servers not being
> able to choose what they store, in Freenet it is a feature not a bug,
> as it is considered to add to the censorship resistance of the network.

Perhaps servers not being able to choose what they store can be considered
a feature, but the other side of that coin is that users can't choose
which servers to store their data, which is certainly a bug. If servers
are addressed by content-hash id, as a user I may be forced to store my
data on a server that I know is unreliable or controlled by an attacker.

The advantage of seperating the location service from the
storage/transport service is to allow users to choose which servers to use
for long-term guaranteed storage, while still allowing the data to be
accessed from anywhere, and not preventing other servers from caching
whatever data they deem most profitable.





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