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Created October 31, 2020 12:28
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Modeling the Internet from the scratch: Link-layer, LAN, Switch - Code snippet: Switch Forwarding
// 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
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