how to update task's DelegatedBy and taskuser with webservice

step1: create task

   owner -> user1

   DelegatedBy  -> null

   taskuser  ->  user1 (automatic created)

step2: change owner

   owner -> user2

   DelegatedBy  -> user1(automatic updated)

   taskuser  ->  user2 (automatic updated)

step3: change owner again

   owner -> user3

   DelegatedBy  -> user1(no update)

   taskuser  ->  user3 (automatic updated)  and user2 (manual updated)

it's ok using cod operation for up step.but, there are issues when i do it with webservice 2.0.

1.DelegatedBy is not updated correctly in step2(updated by webservice's userID)

2.taskuser can't be updated for user2 in step3

ps.using task WSDL.

Q:

1.which WSDL is mandotory except for task WSDL