Document difference between snd.nxt and snd.last.

[utcp] / README

diff --git a/README b/README
index 6e14a31e447be74c50b7d9f5119361400490b49f..077f958bf55d24975e325982823ef2b830ac84ee 100644 (file)
--- a/README
+++ b/README
@@ -58,11 +58,12 @@ INVARIANTS
 ----------
 
 - snd.una: the sequence number of the first byte we did not receive an ACK for
-- snd.nxt: the sequence number of the first byte after the last one we ever sent
+- snd.nxt: the sequence number of the first byte after the last packet we sent (due to retransmission, this may go backwards)
 - snd.wnd: the number of bytes we have left in our (UTCP/application?) input buffer
+- snd.last: the sequence number of the last byte that was enqueued in the TCP stream (increases only monotonically)
 
 - rcv.nxt: the sequence number of the first byte after the last one we passed up to the application
-- rcv.wnd: the number of bytes the receives has left in its input buffer (may be more/less than our send buffer size)
+- rcv.wnd: the number of bytes the receiver has left in its input buffer (may be more/less than our send buffer size)
 
 - The only packets that do not have ACK set must either have SYN or RST set
 - Only packets received with rcv.nxt <= hdr.seq <= rcv.nxt + rcv.wnd are valid, drop others.
@@ -178,19 +179,23 @@ RETRANSMIT
 RECEIVE PACKET
 --------------
 
-- Drop invalid packets:
-  - Invalid flags or state
-  - ACK always set
-  - hdr.seq not within our receive window
-  - hdr.ack ahead of snd.nxt
-- Handle RST packets
-- Advance snd.una?
-  - reset conn timer if so
-  - remove ACKed data from send buffer
-- If snd.una == snd.nxt, clear rtrx and conn timer
-- Process state changes due to SYN
-- Send new data to application
-- Process state changes due to FIN
+1 Drop invalid packets:
+  a Invalid flags or state
+  b ACK always set
+  c hdr.seq not within our receive window
+  d hdr.ack ahead of snd.nxt or behind snd.una
+2 Handle RST packets
+3 Advance snd.una?
+  a reset conn timer if so
+  b check if our SYN or FIN has been acked
+  c check if any data been acked
+    - remove ACKed data from send buffer
+    - increase cwnd
+  d no advance? NewReno
+4 If snd.una == snd.nxt, clear rtrx and conn timer
+5 Process state changes due to SYN
+6 Send new data to application
+7 Process state changes due to FIN
 
 CONGESTION AVOIDANCE
 --------------------