Added textual description of ECN-CE indicating congestion events.

draft-ietf-quic-recovery.md

@@ -417,6 +417,11 @@ to accelerate loss recovery.  The receiver SHOULD send an immediate ACK
 when it receives a new packet which is not one greater than the
 largest received packet number.
 
+Also, reception of an packet marked as ECN Congestion Experience (ECN-CE)
+SHOULD be acknowledged more quickly to quicker react to congesiton events.
+Additional ECN-CE marks received during the same recovery period does 
+not need to be immediately acknowledged. See {{congestion-ecn}}.   
+
 As an optimization, a receiver MAY process multiple packets before
 sending any ACK frames in response.  In this case they can determine
 whether an immediate or delayed acknowledgement should be generated
@@ -625,7 +630,9 @@ Pseudocode for OnPacketSent follows:
 
 ### On Ack Receipt
 
-When an ack is received, it may acknowledge 0 or more packets.
+When an ack (ACK or ACK_ECN frame) is received, it may acknowledge 0 
+or more packets. Reception of ACK_ECN frame requires that the receiver
+check if ECN indicates a congestion event, see {{congestion-ecn}}.
 
 Pseudocode for OnAckReceived and UpdateRtt follow:
 
@@ -642,8 +649,8 @@ Pseudocode for OnAckReceived and UpdateRtt follow:
 
     DetectLostPackets(ack.largest_acked_packet)
     SetLossDetectionAlarm()
-    // Detect if ACK_ECN frame indicates ECN marking
-    if (ack is of type ACK_ECN):
+    // Detect if ACK_ECN frame indicates ECN-CE marking
+    if (Frame type is of type ACK_ECN):
        OnPacketsMarked(ack.ce_counter)
 
 
@@ -678,7 +685,7 @@ newly acked packet.
 
 If this is the first acknowledgement following RTO, check if the smallest newly
 acknowledged packet is one sent by the RTO, and if so, inform congestion control
-of a verified RTO, similar to F-RTO {{?RFC5682}}
+of a verified RTO, similar to F-RTO {{?RFC5682}}.
 
 Pseudocode for OnPacketAcked follows:
 
@@ -883,13 +890,31 @@ QUIC hosts MUST NOT send packets if they would increase bytes_in_flight
 the packet is a probe packet sent after the TLP or RTO alarm fires, as described
 in {{tlp}} and {{rto}}.
 
+## Explicit Congestion Notification {#congestion-ecn}
+
+If ECN {!RFC3168} has been verified to work for the current path QUIC 
+will use the ECN Congestion Experienced (ECN-CE) IP packet marking as a 
+signal of congestion as a complement to paket loss. This document 
+specifies to use the classical ECN-CE response, i.e. the same response 
+as for packet loss. However, there exist potenatial for future 
+experimentation in using other response functions as discussed in 
+{!RFC8311}.
+
+The ACK_ECN frame defined in {{QUIC-TRANSPORT}} does not provide 
+information on which of the newely acknowledged packets that 
+was marked with ECN-CE. Therefore, it will be assumed that 
+the congestion event starts at the highest acknowledged packet
+number.  
+
 ## Slow Start
 
-QUIC begins every connection in slow start and exits slow start upon
-loss. QUIC re-enters slow start anytime the congestion window is less
-than sshthresh, which typically only occurs after an RTO.
-While in slow start, QUIC increases the congestion window by the
-number of acknowledged bytes when each ack is processed.
+QUIC begins every connection in slow start and exits slow start upon 
+loss or reception of ECN-CE packet markings. QUIC re-enters slow start 
+anytime the congestion window is less than sshthresh, which typically 
+only occurs after an RTO. While in slow start, QUIC increases the 
+congestion window by the number of acknowledged bytes when each ack is 
+processed. 
+
 
 ## Congestion Avoidance
 
@@ -901,18 +926,20 @@ window and sets the slow start threshold to the new congestion window.
 
 ## Recovery Period
 
-Recovery is a period of time beginning with detection of a lost packet.
-Because QUIC retransmits stream data and control frames, not packets,
-it defines the end of recovery as a packet sent after the start of
-recovery being acknowledged.  This is slightly different from TCP's
-definition of recovery ending when the lost packet that started
-recovery is acknowledged.
+Recovery is a period of time beginning with detection of a lost packet 
+or the reception of an ECN-CE mark. Because QUIC retransmits stream data 
+and control frames, not packets, it defines the end of recovery as a 
+packet sent after the start of recovery being acknowledged. This is 
+slightly different from TCP's definition of recovery ending when the 
+lost packet that started recovery is acknowledged.
+
+During recovery, the congestion window is not increased or decreased. As 
+such, multiple lost packets and/or ECN-CE marks only decrease the 
+congestion window once as long as they're lost before exiting recovery. 
+This causes QUIC to decrease the congestion window multiple times if 
+retransmisions are lost, but limits the reduction to once per round 
+trip. 
 
-During recovery, the congestion window is not increased or decreased.
-As such, multiple lost packets only decrease the congestion window once as
-long as they're lost before exiting recovery. This causes QUIC to decrease
-the congestion window multiple times if retransmisions are lost, but limits
-the reduction to once per round trip.
 
 ## Tail Loss Probe
 
@@ -980,6 +1007,12 @@ kLossReductionFactor (default 0.5):
 Variables required to implement the congestion control mechanisms
 are described in this section.
 
+ack_ce_cntr:
+: The ACK_ECN counter for ECN-CE marks previously processed. Used to 
+  determine when one or more packet acknowledged by the ACK_ECN frame
+  was marked with ECN-CE. 
+
+
 bytes_in_flight:
 : The sum of the size in bytes of all sent packets that contain at least
   one retransmittable frame, and have not been acked or declared
@@ -1009,6 +1042,7 @@ variables as follows:
    bytes_in_flight = 0
    end_of_recovery = 0
    ssthresh = infinite
+   ack_ce_cntr = 0
 ~~~
 
 ### On Packet Sent
@@ -1048,19 +1082,20 @@ acked_packet from sent_packets.
 ### On Packets Marked
 
 Invoked by an increment in the number of CE marked packets, as 
-indicated by a newly received ACK_ECN frame. The variable ack_ce_counter 
-is used to check if packets are recently CE marked. 
+indicated by a newly received ACK_ECN frame when compared to
+ack_ce_cntr.   
 
 ~~~
    OnPacketsMarked(ce_counter):
-      if (end_of_recovery < largest_acked_packet && ce_counter > ack_ce_cntr):
-        // Start a new congestion epoch
-        end_of_recovery = largest_sent_packet
-        congestion_window *= kMarkReductionFactor
-        congestion_window = max(congestion_window, kMinimumWindow)
-        ssthresh = congestion_window
-        // update ack_ce_cntr
+      if (ce_counter > ack_ce_cntr):
+		// update ack_ce_cntr
         ack_ce_cntr = ce_counter
+		if (!InRecovery(largest_acked_packet)):
+		  // Start a new congestion epoch
+		  end_of_recovery = largest_sent_packet
+          congestion_window *= kMarkReductionFactor
+          congestion_window = max(congestion_window, kMinimumWindow)
+          ssthresh = congestion_window
 ~~~