- From: Jim Whitehead <ejw@ics.uci.edu>
- Date: Mon, 13 Mar 2000 10:06:51 -0800
- To: Mike Evans <Mike.Evans@jack.see.plym.ac.uk>, w3c-dist-auth@w3.org
- Message-ID: <NDBBIKLAGLCOPGKGADOJGELNCPAA.ejw@ics.uci.edu>
Mike Evans writes: I need to know if any work is currently being conducted into using WebDAV over slow/intermittent connections? From what I have read from the archives, the discussion on this topic seemed to end in 1997, with a small discussion on 'email access to DAV functionality' and 'disconnected operation', but I can't find any outcome to these discussions. Was any decision reached to include/preclude such operation in the spec, or was it decided to leave it to specific impementations to handle in their own way? The final decision was to design the specification in such a way that someone could come back later and add email access if they desired (to date no one has done this). The concern at the time was that some large fraction of the Internet only had intermittent access, and would not be able to access WebDAV functionality if it were limited to HTTP. The designers of WebDAV didn't agree with this assessment, since while the protocol was being developed there was an increasing trend towards higher modem speeds, and high speed access. However, it still seems quite possible to add in email transport for WebDAV operations. One approach is to package up WebDAV requests and responses as message/http MIME messages (defined in the HTTP specification), and email them from client to server. Note that email access and disconnected access aren't the same thing. WebDAV has the potential for being used in a disconnected manner using HTTP. One scenario is for a client to connect to the server, perform a LOCK/PROPFIND/GET sequence, then completely disconnect. The lock stays active during this time, since locks are not associated with network connections. Then, when the client is ready to finish, it reconnects, and does a PUT/PROPPATCH/UNLOCK sequence. I'm not sure what affect an intermittent connection would have. WebDAV, like HTTP, assumes it is running over TCP/IP, which is a reliable transport. An intermittent connection suggests that the connection isn't reliable. I suspect the best place to solve this is below the application layer. To give you some context, my Ph.D work is centred around managing information on the web. Hardly new, I know, but part of the work is based on developing a resource migration mechanism for the web. Before you all switch off, I am currently designing a resource migration protocol that manages the migration process, with WebDAV being used to handle the physical movement of the resource across servers during a migration. The idea is that an agent manages the process on behalf of a client. The client could be a user, or a programmatic client (such as a distributed object). If it's the latter, it will need to send a 'Migrate' message to the agent (which could be on a different host to the client), and wait for the response. What concerns me is the potential delay in receiving this response. The migration process comprises several DAV commands, some authorisation along the way, and the updating of a suitable name service, which could potentially take some time. Waiting for the response may not be a good idea, but I'd hate to go ahead and implement my own asyncronous operation if it conflicts with what has already been developed (and why reinvent the wheel anyway?!) I suspect there is some overlap between Web site migration, and cache replication (in both you are trying to duplicate a Web site) -- there is some discussion of cache replication protocols in this Internet Protocol Journal article: "Web Caching" Geoff Huston http://www.cisco.com/warp/public/759/ipj_2-3/ipj_2-3_webcaching.html Due to the time involved to perform the 'migrate' operation, I think you're right, typical remote procedure call semantics aren't ideal, since there is too large a time between request and response. What you'd like to do is send off the request, and receive an asynchronous event notification once it's completed. Unfortunately, there is currently no generic event notification protocol standard, although plenty of work has been done on this topic (see http://www.cs.caltech.edu/~adam/isen/event-systems.html for a survey of 144 of them). Could anybody inform me, therefore, if any work has been carried out into disconnected operation, as I will gratefully use it. If not, I'll have to come up with my own solution. Clearly, as I'm working on automatic operation of the migration process, I can't use e-mail notification. Well, email doesn't necessarily preclude automatic handling. You could have an email sent to a particular address, and have email to that address handled by a program, not some email user interface. BTW, I was wondering what (if anything) I should do with the migration protocol once it's complete? The protocol _uses_ WebDAV, it doesn't _extend_ it, but it does extend HTTP (minimally), and it does define cross-server operations, so it may be useful to others. Should I write an Internet draft? Would it be useful to this group? If not, who would it be useful for? (the IETF and W3C seem a bit quiet on the resource migration front, and the URN group seems a bit contentious! Would they welcome such a draft?). A detailed description of using WebDAV to perform Web site migration would be a useful document for others to read. Since it doesn't sound like you're adding much protocol mechanism, this would likely end up as an Informational (not standards track) RFC. But, once you've written it up, it would be much easier to see what trajectory it should take. - Jim
Received on Monday, 13 March 2000 13:07:13 UTC