An Investigation into Transport Protocols and Data Transport ...

5.3. Network Aid in Congestion Detection 98
router should mark the TCP packets with a Congestion Experienced (CE)
bit pattern and then process it normally. Upon receipt of a packet containing
the ECN codepoints, the TCP receiver should echo back the same bit pattern
in its acks. Should the received packet contain a CE-bit marked, the TCP
receiver should also set an internal flag to indicate that all subsequent acks
should also have the CE-bit set to 1.
When the TCP sender receives a CE marked ack, it should immediately
reduce the transmission rate according to congestion control principles. However,
the signaling of many CE packets must be handled such that TCP reacts
at most once per window (or approximately once every RTT) to prevent
successive reductions in cwnd [Flo94a].
The TCP sender, after reducing its transmission rate as a consequence
of a CE packet, marks the Congestion Window Reduced (CWR) flag in all
subsequent data packets on the first bit of the ECN codepoint. The TCP
receiver will only stop sending CE marked acks once it has received a packet
with the flag CWR.
The use of this field requires that all routers, switches and firewalls must
not modify packets with ECN-bits unless the node is under congestion. Also,
nodes that do not implement ECN should still pass ECN bits unaltered and
should not consider packets with ECN bits set as malformed and drop the
packets.
ECN has the obvious advantages in avoiding unnecessary packet drops
due to lack of memory. It also avoids the excessive delays necessary for
duplicate acknowledgments and retransmission timeouts. Indeed, experimental
validations of ECN have shown that TCP achieves moderately better
throughput [H.K98, SA00]. It also enables the efficient usage of bandwidth on
the forward path of the data transfer and hence improves the responsiveness