Modeling HTTP messages

[ShimonSim]

I am working with some specification that use HTTP request as base and requere some fields or specify that some fields are optional.
How can I show it in UML?
I need
1) show that Header must be before body
2) show which fields are requered and with are optional
3) he order of the fields is optional but there is some preferences. Can I show it?

Thank you,
Shimon.

[thomaskilian]

Use a constraint for 1)
Probably you could use a naming convention for 2)
I don't really understand what you mean by 3)

[ShimonSim]

Thanks it should be enough for me to start.