BIP 144: Segregated Witness (Peer Services) #272

  86 | +* '''Rationale for the 0x01 flag byte in between''': this will allow us to easily add more extra non-committed data to transactions (like txouts being spent, ...). It can be interpreted as a bitvector.
  87 | +
  88 | +=== Handshake ===
  89 | +A new message 'havewitness' is sent after receiving 'verack' to
  90 | +indicate that a node can provide witness if requested (similar to
  91 | +'sendheaders') (Note: it might be better to signal this with a services bit in the version message)

NicolasDorier commented at 5:37 AM on January 7, 2016:

Yes, please, consider using the services bit. Implementation wise, a client wanting to connect to a segwit supported node, don't want the need to wait for a magic timeout period to know if the node support segwit before trying the next one.

Using a message 'havewitness' will make peer discovery magnitude time longer.