Watch the Mark Knopfler online guitar lesson by Jeff McErlain from Kings of Tone
Mark Knopfler's basic tone is pretty simple, he would use effects but they seemed to be more tune specific. If I want to channel him with pedals, a decent overdrive and a compressor will do just fine. Compressors are tricky pedals to use and I find that I favor them at lower volumes. This is because when an amp is cranked it compresses naturally, so I am trying to emulate that with the compressor. I would recommend the MXR dyna-comp as a good choice and it's a classic.