Network Working Group A. Sabatini
Internet-Draft Broker Communications Inc.
Intended Status: Standards Track .
Expires: February 14, 2013 August 15, 2012
Highly Efficient Selective Acknowledgement (SACK) for TCP
draft-sabatini-tcp-sack-01
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Abstract
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This memo expands on the Selective Acknowledgement Protocol described
in RFC2018 to improve its performance and efficiency while reducing
the delay involved in recovering lost segments. This leads to very
reliable and efficient communications regardless of transit delay or
high levels of lost segments due to noise or congestion. It
introduces a fundamentally new way of looking at Selective
Acknowledgement and uses this concept to improve the performance of
the RFC2018 protocol. This memo proposes an implementation of the
improved SACK and discusses its performance and related issues.
Acknowledgements
Much of the text in this document is taken directly from RFC2018 "TCP
Selective Acknowledgement Options" by M. Mathis, J. Mahdavi, S. Floyd
and A. Romanow and RFC1072 "TCP Extensions for Long-Delay Paths" by
B. Braden and V. Jacobson.
1. Introduction
This revision to the SACK protocol has its roots in a similar, HDLC
based protocol I designed and implemented for secure financial
transactions. That protocol, being designed for use on a worldwide
basis, was born out of the need for a protocol that would handle any
communications environment no matter how noisy or how much delay
(including multiple satellite hops) was in the path. In later years
its properties were found valuable in congestion situations where
packets were dropped.
Multiple packet losses from a window of data can have a catastrophic
effect on TCP throughput. TCP [Postel81] uses a cumulative
acknowledgment scheme in which received segments that are not at the
left edge of the receive window are not acknowledged. This forces
the sender to either wait a round-trip time to find out about each
lost packet, or to unnecessarily retransmit segments which have been
correctly received [Fall95]. With the cumulative acknowledgment
scheme, multiple dropped segments generally cause TCP to lose its
ACK-based clock, reducing overall throughput.
Selective Acknowledgment (SACK) is a strategy which corrects this
behavior in the face of multiple dropped segments. With selective
acknowledgments, the data receiver can inform the sender about all
segments that have arrived successfully, so the sender need
retransmit only the segments that have actually been lost. The
compatible extensions to RFC2018 proposed here enhance the protocol
by changing retransmission from a worst case timer basis to a
deterministic, state driven basis which responds rapidly to link
conditions.
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I propose modifications to the SACK options as proposed in RFC2018.
Specifically, I add a transmit state to each transmitted message and
return that transmit state when each acknowledgement is sent. By
using the returned transmit state I can tell what messages have been
transmitted after the information in the acknowledgement and thus
rebuild the current state of the receiver at the transmitter. I also
propose changes to the way SACK blocks are reported to insure that
the oldest, and thus the most critical, are transmitted expeditiously
without jeopardizing the multiple repetition of SACK information
which gives the current protocol its reliability. Additionally since
the space to store acknowledgements in IPv4 is limited and may not be
able to accommodate all of the acknowledgement pairs, I propose a
method of sending the complete receiver state by sending multiple
acknowledgements when it becomes evident that transmission has
stalled due to loss of multiple ACKs.
The RFC2018 selective acknowledgment extension uses two TCP options.
The first is an enabling option, "SACK-permitted", which may be sent
in a SYN segment to indicate that the SACK option can be used once
the connection is established. This option is extended to both
indicate that this newer version of the protocol is being used and to
establish an initial value for transmit state. The other is the SACK
option itself, which may be sent over an established connection once
permission has been given by SACK-permitted. This has also been
extended to add both the transmit state implicit in the message and
the transmit state that was received at the far end (now called
"Returned State").
The SACK option is to be included in a segment sent from a TCP that
is receiving data to the TCP that is sending that data; we will refer
to these TCP's as the data receiver and the data sender,
respectively. We will consider a particular simplex data flow; any
data flowing in the reverse direction over the same connection can be
treated independently.
2. Underlying concepts
In order for a sender to know how to optimally transmit messages to a
receiver the sender must recreate the state of the receiver as of the
last acknowledgement received (which segments have been received and
acknowledged, which segments have not) and then "age" or modify that
state by updating it based upon the messages transmitted since the
state implicit in the acknowledgement was current. In order to do
this the sender must maintain a transmission order list which
contains entries for the segment ranges of each message as it is
sent. We called the index into the transmission order list "Send
State" and transmit this state variable with each message. The
receiver, after correctly receiving the message, saves this value and
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returns it (now called "Returned State") and the list of selectively
acknowledged segments with each acknowledgement. When the sender
receives this information it is then capable of constructing a list
of missing segments by taking its unacknowledged segment range list
and modifying it on the basis of the received selective
acknowledgements and then removing from that list all segments that
have been transmitted since the message which caused the
acknowledgement which is all segments sent with indexes between the
current "send state" and the "receive state" in the acknowledgement
message.
To accommodate the issue of receiving segments out of order at the
receiver, or those packets delayed by alternate routing, the sender
does not instantly update its Current Returned State value from the
incoming ACK (which could trigger a false retransmission) but rather
puts it on a timer queue for a length of time ("Reordering Time")
appropriate to the delay randomness in the arrival path (typically 20
to 100 ms based on media, speed and distance), which when the timer
entry expires, causes the update of the Current Returned State value.
If the updated Current Returned State value shows blocks that remain
unacknowledged after this time out they are assumed to be lost and
they are queued for retransmission.
Thus by transmitting the complete acknowledgement information through
the SACK blocks from the receiver along with an indicator to the
sender as to its state current at the time of the acknowledgement the
sender can accurately recreate the current status of the receiver
assuming all "in flight" messages were received and thus only send
the unacknowledged messages starting with the oldest followed by any
new messages whose transmission is requested.
3. Enhanced Sack-Permitted Option
This document is designed to be an extension of RFC2018 and any
implementation of it must be designed to fall back to handling
RFC2018 when he other paty is not capable of handling the enhanced
protocol.
Although Enhanced SACK is a compatible extension of standard SACK it
is recognized that certain middleware boxes are not RFC compliant as
to extensions and therefore will fail if, as would properly be done,
Enhanced SACK was handled as such. Therefore Enhanced SACK is
transmitted as two options both in the SYN packets as well as data
and ACK packets.
The first option of the pair MUST be the standard SACK option (Kind =
4) if this endpoint desires a SACK session of any kind. The second
four or six byte option may be sent in a SYN by a TCP that has been
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extended to receive (and presumably process) the Enhanced SACK option
in order to indicate its willingness to enter into an Enhanced SACK
session.
This option MUST NOT be transmitted on non-SYN segments in the
current protocol, it is left to future study as to its use for
transmitting long sequences of acknowledgements in one frame.
TCP SACK-Permitted Option:
Kind: 4
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind=4 | Length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TCP Enhanced SACK Setup Option:
Kind: X1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |P|R|P|R| |
| Kind=X1 | Length=6 or 8 |T|T|X|X| Reserved |
| | |O|O|T|T| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
if RXT = 0 not requesting extended state
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send State Token |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
if RXT = 1 requesting extended state
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Send State Token | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Control Bits: 4 bits (from left to right):
PTO: Tokens Permitted, the sender supports the requirements of
this extension
RTO: Request Tokens, sender requests link use the protocol
outlined in this extension
PXT: Extended Tokens Supported
RXT: Request Extended Tokens: sender requests using extended
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tokens.
Reserved: 12 bits
Reserved for future use, must be set to zero
If RTO is set then the Send State immediately follows, 16 bits if RXT
is not set and 24 bits if it is. If necessary the option is padded
with a binary zero byte so that length is an even number. In the
case that one end can only support 16 bit tokens only the right most
16 bits of the extended field is used.
For brevity in this document only the lesser, 16 bit format is shown.
4. SACK Option Format
As with the SYN options, Enhanced SACK information must be
transmitted as a seperate option in order to accomodate non RFC
compliant middleware boxes. By its nature it must precede the TCP
SACK Option.
TCP Enhanced SACK Option:
Kind: X2
Length: 6 (or 8 if extended token)
+--------+--------+
| Kind=X2 | Len=6 |
+--------+--------+--------+--------+
| Send State | Returned State |
+--------+--------+--------+--------+
TCP SACK Option:
Kind: 5
Length: Variable
+--------+--------+
| Kind=5 | Length |
+--------+--------+--------+--------+
| Left Edge of 1st Block |
+--------+--------+--------+--------+
| Right Edge of 1st Block |
+--------+--------+--------+--------+
| |
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/ . . . /
| |
+--------+--------+--------+--------+
| Left Edge of nth Block |
+--------+--------+--------+--------+
| Right Edge of nth Block |
+--------+--------+--------+--------+
The SACK option is to be sent by a data receiver to inform the data
sender of non-contiguous blocks of data that have been received and
queued. The data receiver awaits the receipt of data (perhaps by
means of retransmissions) to fill the gaps in sequence space between
received blocks. When missing segments are received, the data
receiver acknowledges the data normally by advancing the left window
edge in the Acknowledgement Number Field of the TCP header. The SACK
option does not change the meaning or use of the Acknowledgement
Number field.
This option contains a list of some of the blocks of contiguous
sequence space occupied by data that has been received and queued
within the window.
Each contiguous block of data queued at the data receiver is defined
in the SACK option by two 32-bit unsigned integers in network byte
order:
* Left Edge of Block
This is the first sequence number of this block.
* Right Edge of Block
This is the sequence number immediately following the last
sequence number of this block.
Each block represents received bytes of data that are contiguous and
isolated; that is, the bytes just below the block, (Left Edge of
Block - 1), and just above the block, (Right Edge of Block), have not
been received.
A SACK option that specifies n blocks will have a length of 8*n+6
bytes, so the 40 bytes available for TCP options can specify a
maximum of 4 blocks. It is suggested that the Enhanced SACK will
provide the time-stamp information used for RTTM [Jacobson92].
5. Generating Sack Options: Data Receiver Behavior
If the data receiver has received a SACK-Permitted option on the SYN
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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 MUST 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 MUST 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 MUST send an ACK for every valid segment that arrives
containing new data, and each of these "duplicate" ACKs SHOULD bear a
SACK option.
The purpose of the SACK blocks is to recreate the status of the
receiver at the transmitter. To that end the most important
information is (1) new or changed blocks, (2) the second transmission
of new or changed blocks, (3) a complete enumeration of all received
blocks starting from the oldest first.
If the data receiver chooses to send a SACK option, the following
rules apply:
* The data receiver first fills in "Send State" in the option from
the current value of its "Send State". The data receiver then
fills in "Returned State" from its "Saved Send State" which was
set by either the SYN option or the SACK option of the last TCP
packet that contained a value which was logically greater than the
current saved value.
* SACK blocks representing all discontiguous segment ranges
received where those ranges are logically over the Acknowledgement
Number in the TCP header are kept in logically ascending by
segment range list. Additionally a count (a four byte binary for
safety) is maintained in each block which represents the number of
times it has been transmitted. Each time a SACK block is added or
changes (normally by being merged with another entry) the count is
set to zero(0).
* All SACK block slots SHOULD be filled on each each normal ACK
transmitted, starting with those that have the lowest count
(acknowledging the most recently received segments), followed by
those with the next lowest count, and so on until all SACK block
slots are filled. As each SACK block is moved to a slot its count
is incremented by one(1) (thus care needs to be taken on the
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second and subsequent passes to skip those entries currently in
SACK blocks slots). By always starting from the oldest we insure
the most critical have the first chance at receiving a SACK block
slot. SACK blocks are empty ONLY if there are less SACK blocks
outstanding than there are available slots. This methodology
assures a fair number of transmissions to all SACK blocks.
* The receiver receives a Send State from the sender that is
logically greater than any previously seen the receiver must
generate an ACK regardless of whether any SACK blocks have
changed. Note that such a Send State change can come from an ACK
produced by the sender as well as a message.
* The definitions in RFC1022 are changed such that if there are
entries on the SACK block list an ACK ALWAYS goes out in response
to a received data segment.
* To insure that the last added or changed SACK block is
transmitted a second time, if the link goes idle for 2*Reordering
Time the receiver SHOULD send another ACK following the rules
above.
* A timer is maintained based on the timestamp of the oldest SACK
block and is set to RTT*1.25, it is reset each time a SACK block
with a different segment start becomes the oldest SACK block. At
the expiration of this timer, since this and probably other
segments have not been retransmitted to the receiver, the receiver
resets the timer to .25*RTT and again sends sufficient
acknowledgements to completely transmit all current SACK blocks
starting from the one with the logically lowest segment start and
proceeding in ascending sequence. Note that this process is
aborted by any action that changes the oldest SACK block. This
timer is used to assure that in case of a burst error the sender
has enough information to restart properly.
6. Interpreting the Sack Option and Retransmission Strategy: Data
Sender Behavior
As each transmission request from the calling program is processed
and entry is made into the segment queue to handle the request and
its buffering. Entries are removed from the segment queue when the
segment is completely acknowledged either through the Acknowledgement
Number Field of the TCP header passing through the end of that
segment or by a SACK block completing the acknowledgement of that
segment. The segment is also appended to the end of the
retransmission queue and transmission restarted from that segment if
transmission has stopped.
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Before processing the SACK information the Acknowledgement Number
Field of the TCP header is used to eliminate outdated entries from
the segment queue, saved list and retransmission queue before new
information is added. The acknowledgement may split the first entry
in either the segment queue or the retransmission queue in which case
a pseudo entry is created in that queue for the unacknowledged
remainder which additionally points to the saved original entry with
an additional field which is the count of pseudo segments derived
from it, set to one in this case. The Acknowledgement Number Field
of the TCP header may end up eliminating a pseudo entry in which case
the pseudo segment count of the original saved entry is decremented
and if zero the segment is then entirely removed from both the
segment queue and the saved list.
In processing selective acknowledgements the transmitter applies each
SACK block to first the segment queue and then the retransmission
queue. If the SACK block completely acknowledges a segment it is
removed from the segment queue and moved to the saved list with a
count of zero or completely if from the retransmission queue. If the
SACK block completely acknowledges a pseudo segment that segment is
removed and if from the segment queue the pseudo segment count in the
related saved entry is decremented. If the SACK block acknowledges
the beginning or end of a segment in either queue a pseudo entry is
created with the adjusted unacknowledged remainder, if the segment
was on the segment list the original segment is moved to the saved
list and the pseudo count is set to one. If the entry was already a
pseudo segment and this SACK acknowledges the beginning or end of the
segment, the segment limits are adjusted but no other action occurs.
If this entry is already a pseudo entry and the SACK block splits the
segment in two a second pseudo entry is created to handle the right
hand side of the range and, if on the segment list, the pseudo
segment count in the related save list entry is incremented by one.
The original pseudo entry is modified to represent the left hand
range created by the SACK.
The sender maintains a Transmission List which an array of structures
into which the segment start and end addresses of each transmitted
block (be it a primary transmission or a retransmission) is placed.
This list is, for optimal processing, a power of 2 in size and is, at
a minimum, four times as large as the Maximum Number of Segments
Outstanding. As each segment is transmitted the current Transmit
Token modulus the Transmission List size is used as an index into
this structure to store the segment start and end and then the
Transmit Token is incremented by one.
When each each new SACK option is processed, its Returned Token is
checked against the Current Returned Token and the Future Returned
Token List, if logically greater than any of the above, it is
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inserted into the Future Returned Token list in logical order along
with the current time-stamp incremented by the Reordering Time. If
it is the first entry on the list a timer is started for the value
token time stamp minus current time stamp. When the timer expires
the first entry on the Future Returned Token list set as the Current
Returned Token and then it is removed from the list. If there are
more members on the Future Returned Token list the timer is restarted
with a value of the time-stamp in that entry minus the current time-
stamp. A change in the Current Returned Token causes a recreation of
the Retransmission Queue by first copying the Segment Queue and then
removing from it all segments that have been transmitted
subsequently, in a process identical to processing a SACK block
starting at the segment identified by the Current Returned Token from
the Transmission List and continuing through the Transmission List up
to the (but not including) the Transmit Token. The Transmission
Pointer is then set to first incomplete entry in the Retransmission
Queue and transmission restarted if it has stopped.
Another method of implementation which allows quicker retransmission
response at the expense of building the Retransmission Queue a second
time is to retrieve the time stamp of the just receieved Returned
Token if that Returned Token has not previously been seen (by
comparing it with the Current Returned Token and the Future Returned
Token list) and then subtracting from that timestamp the Reordering
Time to allow for out of order messages. This value is then used to
select any entry on the Transmission List with an equal or greater
timestamp. The first version of the Retransmission Queue is created
by copying the Segment Queue and then removing the segments that have
since been retransmitted based on the adjusted timestamp. When the
timer on the Future Returned Token List expires the retransmission
queue is recreated a second time as in the preceeding paragraph.
Note: For processing efficiency we believe most people will implement
the Retransmission Queue as additional fields in the Segment Queue.
6.1 Handling last segment problem
If the sender side of the link goes idle for 2*Reordering Time and
there are still unacknowledged segments the sender SHOULD send an ACK
(which would have the updated Send State) so that the receiver may
ultimately detect if the last message is missing and cause it to be
transmitted (the receiver would pass the Send State back as Returned
State and the sender would realize the segment is still outstanding).
6.2 Congestion Control Issues
This document does not attempt to specify in detail the congestion
control algorithms for implementations of TCP with SACK. However,
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the congestion control algorithms present in the de facto standard
TCP implementations MUST be preserved [Stevens94]. This algorithm
eliminates much unnecessary retransmission so is likely to lessen
overall congestion.
Note that the enhanced protocol does not suffer from traditional
congestion collapse even though it is more robust, since it does not
use timers and is rate limited by the tokens. Delayed and lost
messages and ACKS make it slower but do not increase the traffic it
sends significantly.
The use of time-outs as a fall-back mechanism for detecting dropped
packets is unchanged by the SACK option. Because in normal operation
acknowledgements will prevent retransmit timeout, when a retransmit
timeout occurs the data sender SHOULD ignore prior SACK information
in determining which data to retransmit.
Future research into congestion control algorithms may take advantage
of the additional information provided by SACK. One such area for
future research concerns modifications to TCP for a wireless or
satellite environment where packet loss is not necessarily an
indication of congestion.
7. Efficiency and Worst Case Behavior
Although this high efficiency improved SACK option sends more and
larger SACK blocks and more acknowledgements than the previous
version, with an active bi-directional link additional
acknowledgements are often associated with data transmission and thus
not a penalty. If the SACK option needs to be used due to segment
loss then the improved efficiency afforded with this protocol more
than justifies the additional SACK blocks.
The deployment of other TCP options may reduce the number of
available SACK blocks to 2 or even to 1. This will reduce the
redundancy of SACK delivery in the presence of lost ACKs. Even so,
the exposure of TCP SACK in regard to the unnecessary retransmission
of packets is strictly less than the exposure of current
implementations of TCP. The worst-case conditions necessary for the
sender to needlessly retransmit data is discussed in more detail in a
separate document [Floyd96].
Older TCP implementations which do not have the SACK option will not
be unfairly disadvantaged when competing against SACK-capable TCPs.
This issue is discussed in more detail in [Floyd96].
8. Time-stamping
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One pleasant benefit of having a token which is returned by the far
end on a deterministic basis is the easy calculation of round trip
delay. We can save a time stamp along with the segment information
in our transmission order array. This allows us to calculate round
trip delay when we receive our "Returned State" value and use it to
access the time-stamp. Since more than one received message might
have the same "Returned State" value we mark the time-stamp after use
to indicate that the value should not be used again. Note that if an
acknowledgement is lost we will calculate a longer delay than is
accurate therefore we must smooth the returned values, typically
returning the sma llest out of the last N where N is typically four.
9. Data Receiver Reneging
Since the Sender is recreating the state of the Receiver, the data
Receiver MUST NOT discard data in its queue once that data has been
reported in a SACK option. The Receiver is responsible for
allocating enough buffers so that the missing segments within the
window may be properly received and processed. Since enhanced SACK
is event driven the lack of a new event for 2.50*RTT SHOULD trigger a
connection reset to guard agains denial of service attacks.
10. Security Considerations
This document neither strengthens nor weakens TCP's current security
properties.
11. References
[Jacobson88], Jacobson, V. and R. Braden, "TCP Extensions for Long-
Delay Paths", RFC 1072, October 1988.
[Jacobson92] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[Mathis96] Mathis, M., Mahdavi, J., Floyd, S., Romanow, J. "TCP
Selective Acknowleddgement Options", RFC 2018, October 1996.
[Postel81] Postel, J., "Transmission Control Protocol - DARPA
Internet Program Protocol Specification", RFC 793, DARPA, September
1981.
Author's Address
Anthony Sabatini
Broker Communications Inc.
200 West 20th Street
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Suite 1216
New York, NY 10011
Email: draft-sack@tsabatini.com
The author is currently a master's degree candidate at -
Hofstra University
Hempstead, N.Y.
His adviser is Dr. Xiang Fu
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