Crawling SharePoint sites using the SPS3 protocol handler

When you setup your content sources in a Microsoft Office SharePoint Server (MOSS 2007), you have a few options to choose from: SharePoint Sites, Web Sites, File Shares, Exchange Public Folders and Business Data. When you use the SharePoint Sites option, you're instructing the indexer to crawl a WSS web front end and you will use sps3:// as the prefix for your start address. This tells the crawler to use a SharePoint-specific protocol handler to enumerate the content and then grab the actual items from the SharePoint server.

A common question here is whether this uses some sort of RPC call into the SharePoint Web Front End (WFE) server. The answer is "no". People asking the question are usually trying to configure the firewalls between a indexer and a MOSS WFE and need to know what TCP/IP ports they need to open. You should be fine with just HTTP (or HTTPS, if your portal requires that). The SPS3 protocol handler uses a web services call (using HTTP/SOAP) to enumerate the content and then uses regular HTTP GET requests to get to the actual items. Crawling using the SPS3 protocol handler requires no RPC calls or direct database access to the target farm. That's the main reason why this type of crawling is supported over WAN links and has a good tolerance to latency.
If you want to confirm this, configure two separate MOSS farms and have one crawl the other:
Configure a new content source using Central Administration, Shared Services, Search Settings, Content Sources, Add Content Source.

Specify SharePoint sites as the type and use SPS3://servername as the start address
Start a full crawl
If you have any network monitoring hardware or software, you will notice that one the first things the crawler will do is use the "Portal Crawl" web service at http://servername/_vti_bin/spscrawl.asmx. The methods in this web service are EnumerateBucket, EnumerateFolder, GetBucket, GetItem and GetSite. It is interesting to see how both "Enumerate" methods will basically return just an "ID" and a "LastModified" datetime, hinting at how SharePoint can do incremental content crawls via this protocol handler... If you just point your browser to that URL yourself, you can find the additional information about the web service, including sample SOAP calls and the WSDL (as you get with any .NET web service). At this point, I could not find much detail on this web service beyond the actual class definition for Microsoft.Office.Server.Search.Internal.Protocols.SPSCrawl.
Here a few pointers to documention that will help you understand the big picture:

There is an overview of this way content sources and protocol handlers work at http://technet2.microsoft.com/Office/en-us/library/f32cb02e-e396-46c5-a65a-e1b045152b6b1033.mspx

You can find some more detailed information and a nice diagram on what a protocol handler does at http://msdn2.microsoft.com/en-us/library/ms974315.aspx

There is also the description of the web services call used at http://msdn2.microsoft.com/en-us/library/ms583576.aspx

Source : http://blogs.technet.com/