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SubjectRe: IP: optimize as a router not host
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> > Rogier Wolff <r.e.wolff@et.tudelft.nl> writes:
> > >
> > > Suppose a sender sends packets 1,2,3,4 and 5. At the receiving
> > > end you get 1,2,4,5. What you do is you ack that you got packet
> > > 1 and two. When you get 4 and 5 you ack that you got 2 again
> > > and again. When a sender gets these it is supposed to conclude
> > > that packet 3 got lost and try re-sending that.
> >
> > There is a RFC which implements selective reject -- i.e., in the
> > ack, you add some TCP options which tell the sender that you do
> > have packets 4 and 5 (or rather, the sequence numbers for which 4
> > and 5 contained data).
>
> The selective ack RFC never went anywhere because you can use a
> scheme known as fast retransmit instead to avoid pipeline stalls on
> a loss of 1 frame/window or less. What happens is the sender sends 1
> 2 3 4 5 6 the receive gets 1 2 4 5 6. The acks from the receiver
> thus go 1 2 2 2 2 .. seeing 3 acks for an old frame in a row the
> sender now sends frame 3 again immediately, then will get an ack of
> 7 and continue.

This is not entirely accurate. Alan is correct in saying that the
fast retransmit algorithm is used to recover from a single lost
frame per window size. However, quoting from RFC 1323 [Van
Jacobsen, the author of the algorithm is a co-author of RFC 1323]:

There are three fundamental performance problems with the
current TCP over LFN paths:

...

(1) Window Size Limit

...

(2) Recovery from Losses

Packet losses in an LFN can have a catastrophic effect on
throughput. Until recently, properly-operating TCP
implementations would cause the data pipeline to drain with
every packet loss, and require a slow-start action to
recover. Recently, the Fast Retransmit and Fast Recovery
algorithms [Jacobson90c] have been introduced. Their
combined effect is to recover from one packet loss per
window, without draining the pipeline. However, more than
one packet loss per window typically results in a
retransmission timeout and the resulting pipeline drain and
slow start.

Expanding the window size to match the capacity of an LFN
results in a corresponding increase of the probability of
more than one packet per window being dropped. This could
have a devastating effect upon the throughput of TCP over an
LFN. In addition, if a congestion control mechanism based
upon some form of random dropping were introduced into
gateways, randomly spaced packet drops would become common,
possible increasing the probability of dropping more than one
packet per window.

To generalize the Fast Retransmit/Fast Recovery mechanism to
handle multiple packets dropped per window, selective
acknowledgments are required. Unlike the normal cumulative
acknowledgments of TCP, selective acknowledgments give the
sender a complete picture of which segments are queued at the
receiver and which have not yet arrived. Some evidence in
favor of selective acknowledgments has been published
[NBS85], and selective acknowledgments have been included in
a number of experimental Internet protocols -- VMTP
[Cheriton88], NETBLT [Clark87], and RDP [Velten84], and
proposed for OSI TP4 [NBS85]. However, in the non-LFN
regime, selective acknowledgments reduce the number of
packets retransmitted but do not otherwise improve
performance, making their complexity of questionable value.
However, selective acknowledgments are expected to become
much more important in the LFN regime.

RFC-1072 defined a new TCP "SACK" option to send a selective
acknowledgment. However, there are important technical
issues to be worked out concerning both the format and
semantics of the SACK option. Therefore, SACK has been
omitted from this package of extensions. It is hoped that
SACK can "catch up" during the standardization process.


So, as you can see, selective acknowledgment has been disconnected
from the other improvements of RFC 1323 and is now being worked on
by the TCP for Large Windows working group of the IETF (tcplw) and
there is a current draft for sack - draft-ietf-tcplw-sack-00.txt.

Also, please note the following from the draft-ietf-tcplw-sack-00.txt:

4. GENERATING SACK OPTIONS: DATA RECEIVER BEHAVIOR

If the data receiver has received a SACK-Permitted option on the
SYN for this connection, the data receiver MAY elect to generate
SACK options as described below. If the data receiver generates
SACK options under any circumstance, it SHOULD generate them
under all permitted circumstances. If the data receiver has not
received a SACK-Permitted option for a given connection, it MUST
NOT send SACK options on that connection.

If sent at all, SACK options SHOULD be included in all ACKs which
do not ACK the highest sequence number in the data receiver's
queue. In this situation the network has lost or mis-ordered
data, such that the receiver holds non-contiguous data in its
queue. RFC 1122, Section 4.2.2.21, discusses the reasons for
the receiver to send ACKs in response to additional segments
received in this state. The receiver SHOULD send an ACK for
every valid segment that arrives containing new data, and each
of these "duplicate" ACKs SHOULD bear a SACK option.

If the data receiver chooses to send a SACK option, the
following rules apply:

* The first SACK block (i.e., the one immediately following
the kind and length fields in the option) MUST specify the
contiguous block of data containing the segment which
triggered this ACK, unless that segment advanced the
Acknowledgment Number field in the header. This assures
that the ACK with the SACK option reflects the most recent
state change at the data receiver.

* The data receiver SHOULD include as many distinct SACK
blocks as possible in the SACK option. Note that the
maximum available option space may not be sufficient to
report all blocks present in the receiver's queue.

* The SACK option SHOULD be filled out by repeating the most
recently reported SACK blocks (based on first SACK blocks in
previous SACK options) that are not subsets of a SACK block
already included in the SACK option being constructed. This
assures that in normal operation every SACK block is repeated
several times. (At least three times for large-window TCP
implementations [RFC1323]).

It is very important that the SACK option always reports the
block containing the most recently received segment, because
this provides the sender with the most up-to-date information
about the state of the network and the data receiver's queue.


So, if the draft for SACK becomes a standard, then while you are not
required to send a SACK, if you do use SACK, it should be used in
all circumstances (including the loss of a single packet per window
where now you just use the fast retransmit/fast recovery) where it
is permitted. This seems to indicate that SACK is intended as a
replacement for fast retransmit/fast recovery once standardized.

<tex@isc.upenn.edu>








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