SAML 1.1 Browser/Artifact Profile

[David Yu <tsunbonyu@gmail.com>]

Dear SAML experts,
I am working on implementing SAML 1.1 Browser/Artifact Profile.
But I have a question on how to extract the SourceId from artifact.
I am looking for a while and read the document sstc-saml-tech-overview-1.1-

cd.pdf
It just mentioned "On receiving the HTTP message, the Artifact Receiver, on the destination web site, extracts the source-ID."
I do not have idea how to extract the source-ID from a giving artifact.
Is there a standard way to do it or it is up to the artifact provider (Source Site)?
Could someone please provide an example how to do this?

Thank you so much