FastRetranFirstData.seq - Restart T3-rtx timer if the endpoint is retransmitting the first outstanding DATA chunk
To chech that the endpoint should restart the T3-rtx timer if the
endpoint is retransmitting the first outstanding DATA chunk sent
to that address.
./FastRetranFirstData.seq [-tooloption ...] -pkt ./FastRetranFirstData.def
-tooloption : v6eval tool option
See Also: ../common/STD_PKT_COMMON.def
../common/SCTP_COMMON.def
Association is established between endpoint A and B. Arrange data
in endpoint B such that messages is sent to endpoint A.
Endpoint A Endpoint B
(ESTABLISHED) (ESTABLISHED)
<---------------- DATA 1
<---------------- DATA 2
<---------------- DATA 3
<---------------- DATA 4
SACK ----------------->
(For DATA 2)
SACK ----------------->
(For DATA 2 and DATA 3)
SACK ----------------->
(For DATA 2 to DATA 4)
<---------------- DATA 1
SACK ----------------->
(For DATA 1 to DATA 4)
<---------------- DATA 5
SACK ----------------->
TEST DESCRIPTION:
1. Check A: the initial cwnd before DATA transmission is set to 4380.
2. From endpoint B, send valid DATA message 4 times to endpoint A,
when there is association between them.
3. From endpoint A, receive DATA message 4 times.
4. From endpoint A, send SACK of the 2nd DATA messages to endpoint
B, report the 1st message is missing.
5. From endpoint A, send SACK of the 2nd, 3rd DATA messages to
endpoint B, report the 1st message is missing.
6. From endpoint A, send SACK of the 2nd, 3rd and 4th DATA messages
to endpoint B, report the 1st message is missing.
7. Check B: The 1st DATA messages is send again immediately.
8. Check C: Congestion control window is 6000 now.
None
RFC 4960
7.2.4. Fast Retransmit on Gap Reports
In the absence of data loss, an endpoint performs delayed
acknowledgement. However, whenever an endpoint notices a hole in the
arriving TSN sequence, it SHOULD start sending a SACK back every time
a packet arrives carrying data until the hole is filled.
Whenever an endpoint receives a SACK that indicates that some TSNs
are missing, it SHOULD wait for two further miss indications (via
subsequent SACKs for a total of three missing reports) on the same
TSNs before taking action with regard to Fast Retransmit.
Miss indications SHOULD follow the HTNA (Highest TSN Newly
Acknowledged) algorithm. For each incoming SACK, miss indications
are incremented only for missing TSNs prior to the highest TSN newly
acknowledged in the SACK. A newly acknowledged DATA chunk is one not
previously acknowledged in a SACK. If an endpoint is in Fast
Recovery and a SACK arrives that advances the Cumulative TSN Ack
Point, the miss indications are incremented for all TSNs reported
missing in the SACK.
When the third consecutive miss indication is received for a TSN(s),
the data sender shall do the following:
1) Mark the DATA chunk(s) with three miss indications for
retransmission.
2) If not in Fast Recovery, adjust the ssthresh and cwnd of the
destination address(es) to which the missing DATA chunks were
last sent, according to the formula described in Section 7.2.3.
3) Determine how many of the earliest (i.e., lowest TSN) DATA chunks
marked for retransmission will fit into a single packet, subject
to constraint of the path MTU of the destination transport
address to which the packet is being sent. Call this value K.
Retransmit those K DATA chunks in a single packet. When a Fast
Retransmit is being performed, the sender SHOULD ignore the value
of cwnd and SHOULD NOT delay retransmission for this single
packet.
4) Restart the T3-rtx timer only if the last SACK acknowledged the
lowest outstanding TSN number sent to that address, or the
endpoint is retransmitting the first outstanding DATA chunk sent
to that address.
5) Mark the DATA chunk(s) as being fast retransmitted and thus
ineligible for a subsequent Fast Retransmit. Those TSNs marked
for retransmission due to the Fast-Retransmit algorithm that did
not fit in the sent datagram carrying K other TSNs are also
marked as ineligible for a subsequent Fast Retransmit. However,
as they are marked for retransmission they will be retransmitted
later on as soon as cwnd allows.
6) If not in Fast Recovery, enter Fast Recovery and mark the highest
outstanding TSN as the Fast Recovery exit point. When a SACK
acknowledges all TSNs up to and including this exit point, Fast
Recovery is exited. While in Fast Recovery, the ssthresh and
cwnd SHOULD NOT change for any destinations due to a subsequent
Fast Recovery event (i.e., one SHOULD NOT reduce the cwnd further
due to a subsequent Fast Retransmit).
Note: Before the above adjustments, if the received SACK also
acknowledges new DATA chunks and advances the Cumulative TSN Ack
Point, the cwnd adjustment rules defined in Section 7.2.1 and Section
7.2.2 must be applied first.
A straightforward implementation of the above keeps a counter for
each TSN hole reported by a SACK. The counter increments for each
consecutive SACK reporting the TSN hole. After reaching 3 and
starting the Fast-Retransmit procedure, the counter resets to 0.
Because cwnd in SCTP indirectly bounds the number of outstanding
TSN's, the effect of TCP Fast Recovery is achieved automatically with
no adjustment to the congestion control window size.