Problem with WiFi
- B is sending to A, C wants to send to D
- As C receives B’s packets, carrier sense (see CSMA) would prevent it from sending to D, even though wouldn’t interfere
- CTS would help
- C hears RTS from B, but not CTS from A
- C knows its transmission wouldn’t interfere with A
- C is safe to transmit to D
Impact on Higher Layer Protocols
- Wireless and mobility change path properties
- Wireless: Higher packet loss, not from congestion
- Mobility: transient disruptions and changes in RTT
- Logically, impact should be minimal
- Best-effort service model remains unchanged
- TCP and UDP can and do run over wireless, mobile
- But, performance is affected
- TCP treats packet loss as a sign of congestion
- TCP tries to estimate RTT to drive retransmissions
- TCP does not perform well on out-of-order packets
- Internet not designed with these issues in mind