Modeling the Internet from the scratch: Link-layer, LAN, Switch - Code snippet: Switch Forwarding

swch.go

// Forward receives id of port it receives frame, address of sender
// and frame to send to receiver. Based on id and address it determines whether to
// broadcast frame or forward it to others, otherwise just discard frame.
func (s *Switch) Forward(incoming Id, frame na.Frame) error {
    s.Table.Update(incoming, frame.Src)
    frm, err := s.frmEnc.Encode(frame)
    if err != nil {
        return err
    }
    
    entry, ok := s.Table.LookupByAddr(frame.Dest)
    if !ok {
        return s.broadcastExcept(incoming, frm)
    }
    
    if entry.Incoming.Equal(incoming) {
        log.Printf("discard frame from id: %s, src: %s, dest: %s\n", incoming, frame.Src, frame.Dest)
        return nil
    }
    
    p := s.PortList[entry.Incoming]
    if err := p.Transmit(frm); err != nil {
        return err
    }
    return nil
}