Internet DRAFT - draft-zimmermann-tcpm-reordering-detection
draft-zimmermann-tcpm-reordering-detection
TCP Maintenance and Minor Extensions (TCPM) WG A. Zimmermann
Internet-Draft NetApp, Inc.
Intended status: Standards Track L. Schulte
Expires: May 14, 2015 Aalto University
C. Wolff
A. Hannemann
credativ GmbH
November 10, 2014
Detection and Quantification of Packet Reordering with TCP
draft-zimmermann-tcpm-reordering-detection-02
Abstract
This document specifies an algorithm for the detection and
quantification of packet reordering for TCP. In the absence of
explicit congestion notification from the network, TCP uses only
packet loss as an indication of congestion. One of the signals TCP
uses to determine loss is the arrival of three duplicate
acknowledgments. However, this heuristic is not always correct,
notably in the case when paths reorder packets. This results in
degraded performance.
The algorithm for the detection and quantification of reordering in
this document uses information gathered from the TCP Timestamps
Option, the TCP SACK Option and its DSACK extension. When a
reordering event is detected, the algorithm calculates a reordering
extent by determining the number of segments the reordered segment
was late with respect to its position in the sequence number space.
Additionally, the algorithm computes a second reordering extent that
is relative to the amount of outstanding data and thus provides a
better estimation of the reordering delay when other sender state
changes.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 14, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . 5
4. The Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Initialization During Connection Establishment . . . . . 6
4.2. Receiving Acknowledgments . . . . . . . . . . . . . . . . 6
4.3. Receiving Acknowledgment Closing Hole . . . . . . . . . . 7
4.4. Receiving Duplicate Selective Acknowledgment . . . . . . 8
4.5. Reordering Extent Computation . . . . . . . . . . . . . . 8
4.6. Retransmitting Segment . . . . . . . . . . . . . . . . . 9
4.7. Placeholder for Response Algorithm . . . . . . . . . . . 9
4.8. Retransmission Timeout . . . . . . . . . . . . . . . . . 9
5. Protocol Steps in Detail . . . . . . . . . . . . . . . . . . 9
6. Discussion of the Algorithm . . . . . . . . . . . . . . . . . 12
6.1. Reasoning for the Relative Reordering Extent . . . . . . 12
6.2. Calculation of the Relative Reordering Extent . . . . . . 13
6.3. Persistent Reception of Selective Acknowledgments . . . . 13
6.4. Unreliable ACK reception . . . . . . . . . . . . . . . . 15
6.5. Packet Duplication . . . . . . . . . . . . . . . . . . . 15
7. Related Work . . . . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 18
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Appendix A. Changes from previous versions of the draft . . . . 20
A.1. Changes from draft-zimmermann-tcpm-reordering-
detection-01 . . . . . . . . . . . . . . . . . . . . . . 21
A.2. Changes from draft-zimmermann-tcpm-reordering-
detection-00 . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
When the Transmission Control Protocol (TCP) [RFC0793] decides that
the oldest outstanding segment is lost, it performs a retransmission
and changes the sending rate [RFC5681]. This occurs either when the
Retransmission Timeout (RTO) timer expires for a segment [RFC6298],
or when three duplicate acknowledgments (ACKs) for a segment have
been received (Fast Retransmit/Fast Recovery) [RFC5681]. The
assumption behind Fast Retransmit is that non-congestion events that
can cause duplicate ACKs to be generated (packet duplication, packet
reordering and packet corruption) are infrequent. However, a number
of Internet measurement studies have shown that packet reordering is
not a rare phenomenon [Pax97], [BPS99], [BS02], [ZM04], [GPL04],
[JIDKT07] and has negative performance implications on TCP [BA02],
[ZKFP03].
From TCP's perspective, the result of packet reordering on the
forward-path is the reception of out-of-order segments by the TCP
receiver. In response to every received out-of-order segment, the
TCP receiver immediately sends a duplicate ACK. (Note: [RFC5681]
recommends that delayed ACKs not be used when the ACK is triggered by
an out-of-order segment.) The sender side, if the number of
consecutively received duplicate ACKs exceeds the duplicate
acknowledgment threshold (DupThresh), retransmits the first
unacknowledged segment [RFC5681] and continues with a loss recovery
algorithm such as NewReno [RFC6582] or the Selective Acknowledgment
(SACK) based loss recovery [RFC6675]. If a segment arrives due to
reordering more than three segments (the default value of DupThresh
[RFC5681]) too late at the TCP receiver, the sender is not able to
distinguish this reordering event from a segment loss, resulting in
an unnecessary retransmission and rate reduction.
Since DupThresh is defined in segments rather than bytes [RFC5681],
TCP usually quantifies packet reordering in terms of segments.
Informally, the reordering extent [RFC4737] is defined as the maximum
distance in segments between the reception of a reordered segment and
the earliest segment received with a larger sequence number. If a
segment is received in-order, its reordering extent is undefined
[RFC4737].
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Another approach taken by this specification quantifies the
reordering extend for a packet not only through an absolute value,
but also through a measure that is relative to the amount of
outstanding data, in an attempt to approximate a time-based measure.
The presented scheme can thereby easily be adapted to the Stream
Control Transmission Protocol (SCTP) [RFC2960], since SCTP uses
congestion control algorithms similar to TCP.
Overall, this document describes mechanisms to detect reordering on
the forward-path during a TCP connection, and provides these samples
as an input for an additional reaction algorithm.
The remainder of this document is organized as follows. Section 3
provides a high-level description of the packet reordering detection
mechanisms. In Section 4, the algorithm is specified. In Section 5,
each step of the algorithm is discussed in detail. Section 6
provides a discussion of several design decisions of the algorithm.
Section 7 discusses related work. Section 9 discusses security
concerns.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described [RFC2119].
The reader is expected to be familiar with the TCP state variables
given in [RFC0793] (SEG.SEQ, SND.UNA), [RFC5681] (FlightSize), and
[RFC6675] (DupThresh, SACK scoreboard). SND.FACK (forward
acknowledgment) is used to describe the highest sequence number -
plus one - that has been either cumulatively or selectively
acknowledged by the receiver and subsequently seen by the sender
[MM96]. Further, the term 'acceptable acknowledgment' is used as
defined in [RFC0793]. That is, an ACK that increases the
connection's cumulative ACK point by acknowledging previously
unacknowledged data. The term 'duplicate acknowledgment' is used as
defined in [RFC6675], which is different from the definition of
duplicate acknowledgment in [RFC5681].
This specification defines the four TCP sender states 'open',
'disorder', 'recovery', and 'loss' as follows. As long as no
duplicate ACK is received and no segment is considered lost, the TCP
sender is in the 'open' state. Upon the reception of the first
consecutive duplicate ACK, TCP will enter the 'disorder' state.
After receiving DupThresh duplicate ACKs, the TCP sender switches to
the 'recovery' state and executes standard loss recovery procedures
like Fast Retransmit and Fast Recovery [RFC5681]. Upon a
retransmission timeout, the TCP sender enters the 'loss' state. The
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'recovery' state can only be reached by a transition from the
'disorder' state, the 'loss' state can be reached from any other
state.
3. Basic Concepts
The following specification depends on the TCP Timestamps option
[RFC1323], the TCP Selective Acknowledgment (SACK) [RFC2018] option
and the latter's Duplicate Selective Acknowledgment (DSACK) extension
[RFC2883]. The reader is assumed to be familiar with the algorithms
specified in these documents.
Reordering is quantified by an absolute and a relative reordering
extent. If a hole in the SACK scoreboard of the TCP sender is closed
either cumulatively by an acceptable ACK or selectively by a new
SACK, then the absolute reordering extent is computed as the number
of segments in the SACK scoreboard between the sequence number of the
reordered segment and the highest selectively or cumulatively
acknowledged sequence number. The relative reordering extent is then
computed as the ratio between the absolute reordering extent and the
FlightSize stored when entering the 'disorder' state.
If the hole that was closed in the SACK scoreboard corresponds to a
segment that was not retransmitted, or if the retransmission of such
a segment can be determined as a spurious retransmission by means of
the Eifel detection algorithm [RFC3522], then the calculated
reordering extent is immediately valid. Otherwise, if the
verification of the Eifel detection algorithm has not been possible,
the reordering extent is stored for a possibly subsequent DSACK. If
no such DSACK is received in the next two round-trip times (RTTs),
the reordering extents are discarded.
4. The Algorithm
Given that usually both the Nagle algorithm [RFC0896] [RFC1122] and
the TCP Selective Acknowledgment Option [RFC2018] are enabled, a TCP
sender MAY employ the following algorithm to detect and quantify the
current perceived packet reordering in the network.
Without the Nagle algorithm, there is no straight forward way to
accurately calculate the number of outstanding segments in the
network (and, therefore, no good way to derive an appropriate
reordering extent) without adding state to the TCP sender. A TCP
connection that does not employ the Nagle algorithm SHOULD NOT use
this methodology.
If a TCP sender implements the following algorithm, the
implementation MUST follow the various specifications provided in
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Sections 4.1 to 4.8. The algorithm MUST be executed *before* the
Transmission Control Block or the SACK scoreboard have been updated
by another loss recovery algorithm.
4.1. Initialization During Connection Establishment
After the completion of the TCP connection establishment, the
following state variables MUST be initialized in the TCP transmission
control block:
(C.1) The variable Dsack, which indicates whether a DSACK has been
received so far, and the data structure Samples, which stores
the computed reordering extents, MUST be initialized as:
Dsack = false
Samples = []
(C.2) If the TCP Timestamps option [RFC1122] has been negotiated,
then the variable Timestamps MUST be activated and the data
structure Retrans_TS, which stores the value of the TSval
field of the retransmissions sent during Fast Recovery, MUST
be initialized. Additionally, the data structure
Retrans_Dsack MAY be used in order to detect reordering longer
than RTT with Timestamps and DSACK:
Timestamps = true
Retrans_TS = []
Retrans_Dsack = []
Otherwise, the Timestamps-based detection SHOULD be
deactivated:
Timestamps = false
4.2. Receiving Acknowledgments
For each received ACK that either a) carries SACK information, *or*
b) is a full ACK that terminates the current Fast Recovery procedure,
*or* c) is an acceptable ACK that is received immediately after a
duplicate ACK, execute steps (A.1) to (A.4), otherwise skip to step
(A.4).
(A.1) If a) the ACK carries new SACK information, *and* b) the SACK
scoreboard is empty (i.e., the TCP sender has received no SACK
information from the receiver), then the TCP sender MUST save
the amount of current outstanding data:
FlightSizePrev = FlightSize
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(A.2) If the received ACK either a) cumulatively acknowledges at
most SMSS bytes, *or* b) selectively acknowledges at most SMSS
bytes in the sequence number space in the SACK scoreboard,
then:
The TCP sender MUST execute steps (S.1) to (S.4)
(A.3) If a) Timestamps == false *and* b) the received ACK carries a
DSACK option [RFC2883] and the segment identified by the DSACK
option can be marked according to step (A.1) to (A.4) of
[RFC3708] as a valid duplicate, then:
The TCP sender MUST execute steps (D.1) to (D.3)
(A.4) The TCP sender MUST terminate the processing of the ACK by
this algorithm and MUST continue with the default processing
of the ACK.
4.3. Receiving Acknowledgment Closing Hole
(S.1) If (a) the newly cumulatively or selectively acknowledged
segment SEG is a retransmission *and* b) both equations Dsack
== false and Timestamps == false hold, then the TCP sender
MUST skip to step (A.4).
(S.2) Compute the relative and absolute reordering extent ReorExtR,
ReorExtA:
The TCP sender MUST execute steps (E.1) to (E.4)
(S.3) If a) the newly acknowledged segment SEG was not retransmitted
before *or* b) both equations Timestamps == true and
Retrans_TS[SEG.SEQ] > ACK.TSecr hold, i.e., the ACK
acknowledges the original transmission and not a
retransmission, then hand over the reordering extents to an
additional reaction algorithm:
The TCP sender MUST execute step (P)
(S.4) If a) the previous step (S.3) was not executed *and* b) both
equations Dsack == true and Timestamps == false hold, save the
reordering extents for the newly acknowledged segment SEG for
at least two RTTs:
Samples[SEG.SEQ].ReorExtR = ReorExtR
Samples[SEG.SEQ].ReorExtA = ReorExtA
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(S.5) If a) the newly acknowledged segment SEG was retransmitted
before exactly once *and* b) both equations Dsack == true and
Timestamps == true hold *and* c) Retrans_TS[SEG.SEQ] ==
ACK.TSecr, then save FlightSizePrev for this segment in order
to be calculate the metrics in case a DSACK arrives, i.e.
reordering delay is greater than RTT:
Retrans_Dsack[SEG.SEQ] = FlightSizePrev
4.4. Receiving Duplicate Selective Acknowledgment
(D.1) If no DSACK has been received so far, the sender MUST set:
Dsack = true
(D.2) If a) the previous step (D.1) was not executed *and* a
reordering extent was calculated for the segment SEG
identified by the DSACK option, then the TCP sender MUST
restore the values of the variables ReorExtR and ReorExtA and
delete the corresponding entries in the data structure:
ReorExtR = Samples[SEG.SEQ].ReorExtR
ReorExtA = SAMPLES[SEG.SEQ].ReorExtA
(D.3) If a) step (D.1) was not executed *and* b) FlightSizePrev was
saved in step (S.4) for the segment, then the TCP sender MUST
calculate the reordering extent for the segment with the E
series of steps by using the FlightSizePrev saved for this
segment and afterwards delete the corresponding entries:
FlightSizePrev_saved = Retrans_Dsack[SEG.SEQ]
(D.4) Hand the new reordering extents over to an additional reaction
algorithm:
The TCP sender SHOULD execute step (P)
4.5. Reordering Extent Computation
(E.1) SEG.SEQ is the sequence number of the newly cumulatively or
selectively acknowledged segment SEG.
(E.2) SND.FACK is the highest either cumulatively or selectively
acknowledged sequence number so far plus one.
(E.3) The TCP sender MUST compute the absolute reordering extent
ReorExtA as
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ReorExtA = (SND.FACK - SEG.SEQ) / SMSS
(E.4) The TCP sender MUST compute the relative reordering extent
ReorExtR as
ReorExtR= ReorExtA * (SMSS / FlightSizePrev)
4.6. Retransmitting Segment
If the TCP Timestamps option [RFC1323] is used to detect packet
reordering, the TCP sender must save the TCP Timestamps option of all
retransmitted segments during Fast Recovery.
(RET) If a) a segment SEG is retransmitted during Fast Recovery,
*and* b) the equation Timestamps = true holds, the TCP sender
MUST save the value of the TSval field of the retransmitted
segment:
Retrans_TS[SEG.SEQ] = SEG.TSval
4.7. Placeholder for Response Algorithm
(P) This is a placeholder for an additional reaction algorithm
that takes further action using the results of this algorithm,
for example, the adjustment of the DupThresh based on relative
and absolute reordering extent ReorExtR and ReorExtA.
4.8. Retransmission Timeout
The expiration of the retransmission timer should be interpreted as
an indication of a change in path characteristics, and the TCP sender
should consider all saved reordering extents as outdated and delete
them.
(RTO) If an retransmission timeout (RTO) occurs, a TCP sender SHOULD
reset the following variables:
Samples = []
Retrans_TS = []
FlightSizePrev = 0
5. Protocol Steps in Detail
The reception of an ACK represents the starting point for the
detection scheme above. For each received SACK, DSACK or acceptable
ACK that prompts the TCP sender to enter the 'disorder' state, to
remain in the 'disorder' state or to leave either the 'disorder' or
'recovery' states towards the 'open' state, steps (A.1) to (A.4) are
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performed. All other received ACKs are not relevant for the
detection of packet reordering and can be ignored. If the TCP sender
changes from the 'open' to the 'disorder' state due to the reception
of a duplicate ACK (i.e., the SACK scoreboard is empty and an ACK
arrives carrying new SACK information), the current amount of
outstanding data, FlightSize, is stored for the subsequent
calculation of the relative reordering extent (step (A.1)).
Whenever a received acceptable ACK or SACK closes a hole in the
sequence number space of the SACK scoreboard either partially or
completely, this is an indication of packet reordering in the network
(step (A.2)). The prerequisite for an accurate quantification of the
reordering is that only one segment is newly acknowledged (maximum
SMSS bytes of data). If more than one segment per ACK is
acknowledged, either by reordering on the reverse path or the loss of
ACKs, the order in which the segments have been received by the TCP
receiver is no longer accurately determinable so that in this case a
reordering extent is not calculated. Finally, if the received ACK
carries a DSACK option that identifies a segment that was
retransmitted only once, then this is sufficient to conclude
reordering (step (A.3)), so that a previously calculated reordering
extent can be passed to another algorithm (steps (D.3) and (P)).
With just the information provided by the ACK field or SACK
information above SND.UNA, the TCP sender is unable to distinguish
whether the ACK that finally acknowledges retransmitted data (either
cumulatively or selectively) was sent in response to the original
segment or a retransmission of the segment. This is described as the
retransmission ambiguity problem in [KP87]. Therefore, the detection
and quantification of reordering depends on other means to
distinguish between acknowledgments for transmission and
retransmission to detect if a retransmission was spurious. If
neither a DSACK has been received (Dsack == false) so far nor the TCP
Timestamps option has been enabled on connection establishment
(Timestamps == false) then there is no possibility for the TCP sender
to identify spurious retransmissions. Hence, the processing of the
received ACK by the detection algorithm must be terminated for
retransmitted segments (step (S.1)). Otherwise, if the segment that
corresponds to the closed hole in the sequence number space of the
SACK scoreboard has not been retransmitted or the retransmission can
be identified by the Eifel detection algorithm [RFC3522] as a
spurious retransmission, the previously calculated reordering extent
is valid (step (S.2)) and an additional reaction algorithm can be
executed (steps (S.3) and (P)).
For the use of the Eifel detection it is necessary to store the TCP
Timestamps option of all retransmissions sent during Fast Recovery
(step (Ret)). However, if the use of the Eifel detection algorithm
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is not possible (Timestamps == false), the extent of a possible
reordering is stored for the possibility of a subsequent arrival of a
DSACK (step (P.4)). If no such DSACK is received in the next two
round-trip times, the reordering extent is discarded. Since the
DSACK extension is not negotiated during connection establishment
[RFC2883], the reordering extent is only stored if a DSACK was
previously received for the TCP connection (DSACK == true, step
(D.1)).
Regardless of whether packet reordering is detected by using the
SACK-based methodology, the DSACK-based methodology, or the TCP
Timestamps option, quantification of the reordering will always be
done when closing a hole in the sequence number space of the SACK
scoreboard (step (A.2), step (P.2)). The only difference is the time
of detection, which is in the case of DSACK-based methodology at
least one RTT after the time of the quantification. The absolute
reordering extent ReorExtA results from the number of segments in the
SACK scoreboard between the sequence number of the newly acknowledged
segment and the highest either cumulatively or selectively
acknowledged sequence number so far plus one (SND.FACK) (step (E.3)).
In the case that the reordering delay is longer than RTT, the
reordering can not be detected by timestamps or DSACK alone, but both
algorithms are needed: when a packet is retransmitted, but no
reordering could be detected when it was acknowledged, then it might
be possible that a DSACK arrives for this packet. Then, the
reordering extent was longer than RTT and the reordering extent has
to be calculated at the point in time the DSACK arrives (step D.3).
Therefore, we save the FlightSizePrev for a retransmitted segment
when it is acked and no reordering is detected (step S.5).
It is worth noting that the absolute reordering extent includes all
segments (bytes) between the closed hole and the highest acknowledged
sequence number so far, i.e., it also includes segments (bytes) that
are not selectively acknowledged. The reason is that if packet
reordering is considered from a temporal perspective, it is
irrelevant whether there are lost segments or not. The important
fact is that the lost segments have been sent after the delayed
segment and before the highest acknowledged segment, which is
expressed by the metric. In step (E.4), the relative reordering
extent ReorExtR is then calculated by the ratio between the absolute
reordering extent ReorExtA and the amount of outstanding data stored
by step (A.1).
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6. Discussion of the Algorithm
The focus of the following discussion is on the quantification of
reordering by the relative reordering extent and to elaborate on
possible sources of error, which may lead to an inaccurate detection
and quantification of reordering in the network.
6.1. Reasoning for the Relative Reordering Extent
A problem that arises with the way of quantifying reordering solemnly
by the absolute reordering extent is that even in the presence of
constant reordering, reordering extents may vary if the transmission
rate of the TCP sender changes. Therefore, by using a DupThresh that
directly reflects the measured reordering extent, spurious
retransmissions cannot be fully avoided.
The following example illustrates this issue. Assume a path with a
reordering probability of 1%, a reordering delay of 20 ms, and a
bottleneck bandwidth of 3 Mb/s. Because segments that are delayed by
reordering arrive 20 ms too late, the TCP receiver can receive a
maximum of ((20 * 3 * 10^3) / 8) = 7500 bytes out-of-order before the
reordered segment arrives. Hence, with a Sender Maximum Segment Size
(SMSS) of 1460 bytes, the largest possible reordering extent is close
to 5 segments. If the bottleneck bandwidth changes from 3 Mb/s to 4
Mb/s, the maximum reordering extent will increase to 7 segments,
although the reordering delay remains constant.
This simple example shows that even with constant reordering,
spurious retransmissions cannot be completely avoided if the
DupThresh directly reflects the reordering extent. On the other
hand, the reordering extent and the resulting DupThresh can sometimes
also be much too high and do not correspond to the actual packet
reordering present on the path. For example, a slow start overshoot
[Hoe96], [MM96], [MSMO97] at the end of slow start might induce such
a problem.
An obvious solution to the problem would be to quantify packet
reordering not by calculating a reordering extent, but by using the
reordering late time offset [RFC4737]. Since the reordering late
time offset is not specified in segments but captures the difference
between the expected and actual reception time of a reordered
segment, this way of quantifying reordering is independent of the
current transmission rate. Disadvantages of this approach are
however a higher complexity and a worse integration into the TCP
specification, since an implementation would require additional
timers, whereas TCP itself is self-clocked.
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6.2. Calculation of the Relative Reordering Extent
Generally, the characteristics of a relative reordering extent should
be that if packet reordering on a path is constant in terms of rate
and delay, the relative reordering extent should also be constant,
regardless of the current transmission rate of the TCP sender. The
scheme proposed in this document is to calculate the relative
reordering by getting the ratio between absolute reordering (the
amount of data the reordered segment was received too late) and the
amount of outstanding data stored when TCP sender was entering the
'disorder' state (the maximum amount of data a reordered segment can
be received too late). Therefore, the relative reordering extent
expresses the portion of currently outstanding data that is
selectively acknowledged before the reordered segment is cumulatively
acknowledged. If the transmission rate changes, the absolute
reordering extent changes as well, but together with the amount of
outstanding data, and hence the relative reordering extent stays
constant.
A characteristic of the calculation of the relative reordering extent
on the basis of currently outstanding amount of data is that the
FlightSize reflects the bandwidth-delay-product and not the
transmission rate. As a consequence, the relative reordering extent
is not independent of the RTT. If the RTT of the communication path
changes, the amount of outstanding data changes as well, but the
absolute reordering extent remains constant. Hence, the relative
reordering extent adapts. In principle it is possible to design an
algorithm to compute the relative reordering extent independently of
the RTT and to reflect only the characteristics of packet reordering
of the path. But since the calculation would be far from trivial and
introducing more complexity, this is considered to be future
research.
6.3. Persistent Reception of Selective Acknowledgments
Especially on paths with a high bandwidth-delay-product, it is
possible that even with a minor packet reordering, several segments
in a single window of data are delayed. If, in addition, the
sequence numbers of those segments are widely spaced in the sequence
number space and the delay caused by packet reordering is
sufficiently high, this might lead to a constant reception of out-of-
order data. Hence, for each received segment, regardless of whether
a hole in the sequence number space of the receive window is closed
or not, an ACK is sent that carries SACK information. From TCP
sender's perspective, this persistent receiving of new SACK
information leads to the situation that the TCP sender enters the
'disorder' state when receiving the first SACK and never leaves it
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again during the connection lifetime if no segment is lost in
between.
In case of the above reordering detection and quantification scheme,
the persistent reception of SACK blocks causes the amount of
outstanding data, which is stored when the TCP sender enters the
'disorder' state, to never be updated, since FlightSize is only saved
in step (A.1) when the SACK scoreboard is empty. If the transmission
rate of the TCP sender, and therefore also the maximum amount of data
a reordered segment can be received too late, changes significantly
during its stay in the 'disorder' state, the actual amount of
reordering is not accurately determined by the relative reordering
extent. A decrease of the transmission rate would result in an
overestimation of the reordering extent and vice versa.
A simple solution to the problem would be to store the maximum offset
in terms of sequence number space by which a reordered segment can be
received too late only when entering the 'disorder' state, but
individually for every potentially reordered segment, that is, for
every hole in the sequence number space of the SACK scoreboard.
(Note: The maximum offset in terms of sequence number space by which
a reordered segment can be received too late is strictly speaking the
amount of data that have been transmitted later than the reordered
segment. This amount of data can only be expressed by FlightSize
within the 'open' state and not within the 'disorder' state, since
the cumulative ACK point may not advance).
The problem with this simple idea is that for a new hole in the SACK
scoreboard, it is not possible to determine whether it is a result of
packet reordering or loss, and therefore it results in increased
memory usage (to store the amount of data for each hole).
Additionally, the packet reordering would be inaccurately quantified
if the transmission rate changes significantly for a short amount of
time. For example, if the amount of outstanding data is low when
entering the 'disorder' state is entered, the execution of Careful
Extended Limited Transmit (as described in
[I-D.zimmermann-tcpm-reordering-reaction] [RFC4653]) leads to a
significant short-term change of the transmission rate. When the
amount of data by which the reordering segment can be delayed is
determined individually for every new hole, it leads to an
overestimation of the relative reordering extent, since the maximum
amount of data possible is 'artificially' reduced by Careful Extended
Limited Transmit.
A solution to this problem is to store the maximum offset in terms of
sequence number space by which a reordered segment can be received
too late not for every segment individually (which does not guarantee
an accurate calculation of the relative reordering extent) but only
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sufficiently often, e.g., once per RTT. The identification of what
frequency would be adequate, though, is neither trivial nor
universally applicable, since a concrete solution depends on the
transmission behavior of the used TCP in the 'disorder' state and
whether it is more beneficial for an additional reordering response
algorithm to over- or underestimate the packet reordering on the
path. If, for example, TCP-aNCR
[I-D.zimmermann-tcpm-reordering-reaction] is used as additional
reordering response algorithm, the maximum offset in terms of
sequence number space by which a reordered segment can be received
too late is not only stored when entering the 'disorder' state but
also updated every RTT (every cwnd worth of data transmitted without
a loss) while the TCP sender stays in the 'disorder' state.
6.4. Unreliable ACK reception
ACK loss and ACK reordering are a cause for inaccuracies in samples.
6.5. Packet Duplication
Although the problem of packet duplication in today's Internet
[JIDKT07], [MMMR08] is negligible, it may happen in rare cases that
segments on the path to the TCP receiver are duplicated. If a
segment is duplicated on the path, the first incoming segment causes
the receiver to send either an acceptable ACK or a SACK, depending on
whether the segment is the next expected one or not. Each subsequent
identical segment then causes either a duplicate ACK or a DSACK,
respectively, depending on whether the DSACK extension [RFC3708] is
implemented or not.
If by a combination of packet loss and packet duplication the case
occurs that a Fast Retransmit for a lost segment is duplicated on the
path, the TCP sender is not able to distinguish this from packet
reordering. The first received ACK closes a hole in the sequence
number space of the SACK scoreboard, while the second received ACK is
a valid DSACK. Although both cases are indistinguishable from a
theoretical point of view, the TCP sender can take measures to ensure
as far as possible that the DSACK received was not the result of
packet duplication.
For this purpose, step (A.3) of the above detection method checks via
the steps (A.1) to (A.4) of [RFC3708] whether the segment identified
by the DSACK option is marked as a valid duplicate. Unfortunately,
the steps of [RFC3708] do not check that more DSACKs have been
received than retransmissions have been sent, which is a
characteristic of suffering both packet reordering and packet
duplication at the same time. By simply counting the received
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DSACKs, for example, as additional step (A.5) in [RFC3708], this
corner case can be covered as well.
7. Related Work
Because of retransmission ambiguity problem [KP87], which describes
TCP sender's inability to to distinguish whether the first acceptable
ACK that arrives after a retransmit was sent in response to the
original transmit or the retransmit, two different approaches can
generally be taken to detect and quantify packet reordering. First,
for transmissions (non-retransmitted segments), the detection is
usually conducted by detecting a closed hole in sequence number space
of the SACK scoreboard. Second, for retransmissions, the detection
of packet reordering is accompanied by the detection of the spurious
Fast Retransmits.
Within the IETF, several proposals have been published in the RFC
series to detect and quantify packet reordering. With [RFC4737] the
IPPM Working Group [IPPM] defines several metrics to evaluate whether
a network path has maintained packet order on a packet-by-packet
basis. [RFC4737] gives concrete, well-defined metrics, along with a
methodology for applying the metric to a generic packet stream. The
metric discussed in this document has a different purpose from the
IPPM metrics; this document discusses a TCP specific reordering
metric calculated on the TCP sender's SACK scoreboard.
Besides the IPPM work, several other proposals have been developed to
detect spurious retransmissions with TCP. The Eifel detection
algorithm [RFC3522] uses the TCP Timestamps option to determine
whether the ACK for a given retransmit is for the original
transmission or a retransmission. The F-RTO scheme [RFC5682]
slightly alters TCP's sending pattern immediately following a
retransmission timeout to indicate whether the retransmitted segment
was needed. Finally, the DSACK-based algorithm [RFC3708] uses the
TCP SACK option [RFC2018] with the DSACK extension [RFC2883] to
identify unnecessary retransmissions. The mechanism for detecting
packet reordering outlined in this document rely on the detection
schemes of those documents (except F-RTO that only works for spurious
retransmits triggered by TCP's retransmission timer), although they
do not provide metrics for the reordering extent whereas the
algorithm described in this document does.
RR-TCP [ZKFP03] describes a reordering detection and quantification
scheme that is also based on holes in the sequence number space of
the SACK scoreboard and the reception of DSACKs. For every hole in
the SACK scoreboard, RR-TCP calculates a reordering extent. If the
segment was retransmitted before an ACK was received, it waits for a
DSACK that proves that the segment was spuriously retransmitted. The
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reordering sample in such a case is the mean between the sample
calculated due to the hole in the sequence number space and the
sample calculated in responding to the received DSACK.
The Linux kernel [Linux] implements a reordering detection based on
SACK, DSACK and TCP Timestamps option as well. The detection and
quantification of non-retransmitted segments with SACK or for
retransmitted segments with TCP Timestamps option operates much like
the scheme described in this document, with the exception of the
DSACK detection. First, Linux does not store any information (e.g.,
reordering extent) below the cumulative ACK point, so that DSACKs
below the cumulative ACK point are ignored (for the purpose for
reordering quantification). Second, Linux also does not store any
information about a possible reordering event when a hole in the
sequence number space of the SACK scoreboard is closed. Therefore,
for a DSACK reporting a duplicate above the cumulative ACK, Linux
needs to approximate the reordering on arrival of a DSACK by the
distance between the DSACK and the highest selectively acknowledged
segment.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
The described algorithm neither improves nor degrades the current
security of TCP, since this document only detects and quantifies
reordering and does not change the TCP behavior. General security
considerations for SACK based loss recovery are outlined in
[RFC6675].
10. Acknowledgments
The authors thank the flowgrind [Flowgrind] authors and contributors
for their performance measurement tool, which give us a powerful tool
to analyze TCP's congestion control and loss recovery behavior in
detail.
11. References
11.1. Normative References
[I-D.zimmermann-tcpm-reordering-reaction]
Zimmermann, A., Schulte, L., Wolff, C., and A. Hannemann,
"An adaptive Robustness of TCP to Non-Congestion Events",
draft-zimmermann-tcpm-reordering-reaction-01 (work in
progress), November 2013.
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[MM96] Mathis, M. and J. Mahdavi, "Forward Acknowledgement:
Refining TCP Congestion Control", ACM SIGCOMM 1996
Proceedings, in ACM Computer Communication Review 26 (4),
pp. 281-292, October 1996.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1323] Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, July 2000.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
for TCP", RFC 3522, April 2003.
[RFC3708] Blanton, E. and M. Allman, "Using TCP Duplicate Selective
Acknowledgement (DSACKs) and Stream Control Transmission
Protocol (SCTP) Duplicate Transmission Sequence Numbers
(TSNs) to Detect Spurious Retransmissions", RFC 3708,
February 2004.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
11.2. Informative References
[BA02] Blanton, E. and M. Allman, "On Making TCP More Robust to
Packet Reordering", ACM Computer Communication Review
vol.32, no. 1, pp. 20-30, January 2002.
[BPS99] Bennett, J., Partridge, C., and N. Shectman, "Packet
reordering is not pathological network behavior", IEEE/ACM
Transactions on Networking vol. 7, no. 6, pp. 789-798,
December 1999.
[BS02] Bellardo, J. and S. Partridge, "Measuring Packet
Reordering", Proceedings of the 2nd ACM SIGCOMM Workshop
on Internet Measurment (IMW'02) pp. 97-105, November 2002.
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[Flowgrind]
"Flowgrind Home Page", <http://www.flowgrind.net>.
[GPL04] Gharai, L., Perkins, C., and T. Lehman, "Packet
Reordering, High Speed Networks and Transport Protocol
Performance", Proceedings of the 13th IEEE International
Conference on Computer Communications and Networks
(ICCCN'04) pp. 73-78, October 2004.
[Hoe96] Hoe, J., "Improving the Start-up Behavior of a Congestion
Control Scheme for TCP", Proceedings of the Conference on
Applications, Technologies, Architectures, and Protocols
for Computer Communication (SIGCOMM'96) pp. 270-280,
August 1996.
[IPPM] "IP Performance Metrics (IPPM) Working Group",
<http://www.ietf.org/html.charters/ippm-charter.html>.
[JIDKT07] Jaiswal, S., Iannaccone, G., Diot, C., Kurose, J., and D.
Towsley, "Measurement and Classification of Out-of-
Sequence Packets in a Tier-1 IP Backbone", IEEE/ACM
Transactions on Networking vol. 15, no. 1, pp. 54-66,
February 2007.
[KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", ACM SIGCOMM
Computer Communication Review vol. 17, no. 5, pp. 2-7,
November 1987.
[Linux] "The Linux Project", <http://www.kernel.org>.
[MMMR08] Mellia, M., Meo, M., Muscariello, L., and D. Rossi,
"Passive analysis of TCP anomalies", Computer Networks
vol. 52, no. 14, pp. 2663-2676, October 2008.
[MSMO97] Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The
Macroscopic Behavior of the TCP Congestion Avoidance
Algorithm", ACM SIGCOMM Computer Communication Review vol.
27, no. 3, pp. 67-82, July 1997.
[Pax97] Paxson, V., "End-to-End Internet Packet Dynamics", IEEE/
ACM Transactions on Networking vol. 7, no.3, pp. 277-292,
June 1997.
[RFC0896] Nagle, J., "Congestion control in IP/TCP internetworks",
RFC 896, January 1984.
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[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC4653] Bhandarkar, S., Reddy, A., Allman, M., and E. Blanton,
"Improving the Robustness of TCP to Non-Congestion
Events", RFC 4653, August 2006.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
November 2006.
[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP", RFC 5682,
September 2009.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, April 2012.
[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP", RFC
6675, August 2012.
[ZKFP03] Zhang, M., Karp, B., Floyd, S., and L. Peterson, "RR-TCP:
A Reordering-Robust TCP with DSACK", Proceedings of the
11th IEEE International Conference on Network Protocols
(ICNP'03) pp. 95-106, November 2003.
[ZM04] Zhou, X. and P. Mieghem, "Reordering of IP Packets in
Internet", Lecture Notes in Computer Science vol. 3015,
pp. 237-246, April 2004.
Appendix A. Changes from previous versions of the draft
This appendix should be removed by the RFC Editor before publishing
this document as an RFC.
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A.1. Changes from draft-zimmermann-tcpm-reordering-detection-01
o Moved reasoning for relative reordering extent to discussion
o Extended algorithm for calculation of reordering extents greater
than RTT (steps C.2, S.5 and D.3)
o Remove reverse-path reordering from intro
A.2. Changes from draft-zimmermann-tcpm-reordering-detection-00
o Improved the wording throughout the document.
o Replaced and updated some references.
Authors' Addresses
Alexander Zimmermann
NetApp, Inc.
Sonnenallee 1
Kirchheim 85551
Germany
Phone: +49 89 900594712
Email: alexander.zimmermann@netapp.com
Lennart Schulte
Aalto University
Otakaari 5 A
Espoo 02150
Finland
Phone: +358 50 4355233
Email: lennart.schulte@aalto.fi
Carsten Wolff
credativ GmbH
Hohenzollernstrasse 133
Moenchengladbach 41061
Germany
Phone: +49 2161 4643 182
Email: carsten.wolff@credativ.de
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Arnd Hannemann
credativ GmbH
Hohenzollernstrasse 133
Moenchengladbach 41061
Germany
Phone: +49 2161 4643 134
Email: arnd.hannemann@credativ.de
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