I want to parse  application layer protocols from network trace using Google protocol buffer and replay the trace (I am using  python). I need suggestions to automatically generate protocol message description (in .proto file) from a network trace.
So you want to reconstruct what .proto messages were being passed over the application-layer protocol?
This isn't as easy as it sounds.  First, .proto messages can't be sent raw over the wire, as the receiver needs to know how long they are.  They need to be encapsulated somehow, maybe in an HTTP POST or with a raw 4-byte size prepended.  I don't know what it would be for your application, but you'll need to deal with that.
Second, you can't reconstruct the full .proto from the messages alone.  You only get tag numbers and types, not names.  In addition, you will lose information about submessages - submessages and plain strings are encoded identically (you could probably tell which is which by eyeballing them, but I don't think you could do it automatically).  You also will never know about optional items that never got sent.  But you could parse the buffer without the proto and get some reasonable data (ints, repeated strings, and such).
Third, you need to reconstruct the application byte stream from the pcap log.  I'm not sure how to do that, but I suspect there are tools that would do that for you.
Google protocol buffer for parsing Text and Binary protocol messages in network trace (PCAP)