18.1.4 Falsely Indicating ECN-Capability

18.1.4 Falsely Indicating ECN-Capability

This change falsely marks a packet as being ECN-capable. The packet may have been sent by an ECN-capable transport, or it may have been sent by a transport that does not support ECN.

If the packet later encounters moderate congestion at an ECN-capable router, the router could set the CE codepoint instead of dropping the packet. If the transport protocol does not actually support ECN, the transport will never receive this congestion indication and will not respond by reducing its sending rate. The potential consequences of falsely indicating ECN-capability are discussed further in Section 19 below.

If the packet never later encounters congestion at an ECN-capable router, the first of these two changes will have no effect, except for possibly interfering with the transport protocol's use of the ECN nonce. However, the last change will have the effect of providing false congestion reports to monitoring devices along the path. If the transport protocol supports ECN, this change could also have consequences at the transport level, by combining falsely indicating ECN-capability with falsely reporting congestion. For an ECN-capable transport, this would cause the transport to unnecessarily respond to congestion. In this particular case, the router falsely changing the ECN field could have dropped the packet instead. Thus, for this case of an ECN-capable transport, the consequences of this change to the ECN field are no worse than dropping the packet.