Once got parsed type, how to project back to original bytes per each field?

[Bingo Zhao]

Say I've got a message proto defined as below:

message Msg{

optional int32 a;

optional string b;

optional bytes payload;

...

}

 and got a bunch of bytes [0x.......] parsed as type Msg. I may get all fields contents from Msg object but I'd just want to project the index:length in original bytes which correlating to specified fields(payload field say). What I'm supposed to do achieving this?