code: public static boolean isResponseLine(final Buffer buffer) throws IOException {
          if (buffer == null || buffer.getReadableBytes() < 7) {
              return false;
          }
  
          final byte a = buffer.getByte(0);
          final byte b = buffer.getByte(1);
          final byte c = buffer.getByte(2);
          final byte d = buffer.getByte(3);
          final byte e = buffer.getByte(4);
          final byte f = buffer.getByte(5);
          final byte g = buffer.getByte(6);
          return a == 'S' && b == 'I' && c == 'P' && d == '/' && e == '2' && f == '.' && g == '0';
      }
docs: Simple method to check whether the supplied buffer could be a SIP
  response line. Hence, does it start with SIP/2.0 or not. Note, if this
  method returns false it does not necessarily mean that this is a request
  line but if you have already determined that that the incoming data could
  be a SIP message (e.g. by using the method
  {@link SipFramer#couldBeSipMessage(Buffer)}) then it is very likely that
  it is a request line.
  
  @return
  @throws IOException
  @throws IndexOutOfBoundsException