cos.writeRawVarint32(fullLength);
// write header
cos.writeRawVarint32(HEADER_TAG);
cos.writeRawVarint32(headerLength);
header.writeTo(cos);
// write protobuf body length and body
cos.writeRawVarint32(PROTOBUF_BODY_TAG);
cos.writeRawVarint32(protoBodyLength);
msg.pBody.writeTo(cos);