Packet Reordering Metrics
draft-ietf-ippm-reordering-13
The information below is for an old version of the document that is already published as an RFC.
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This is an older version of an Internet-Draft that was ultimately published as RFC 4737.
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| Authors | Al Morton , Gomathi Ramachandran , Stanislav Shalunov , Len Ciavattone , Jerry Perser | ||
| Last updated | 2015-10-14 (Latest revision 2006-05-01) | ||
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draft-ietf-ippm-reordering-13
Network Working Group A.Morton
Internet Draft L.Ciavattone
Document: <draft-ietf-ippm-reordering-13.txt> G.Ramachandran
Category: Standards Track AT&T Labs
S.Shalunov
Internet2
J.Perser
Veriwave
Packet Reordering Metric for IPPM
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
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This document is an Internet-Draft and is subject to all provisions
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo defines metrics to evaluate if a network has maintained
packet order on a packet-by-packet basis. It provides motivations
for the new metrics and discusses the measurement issues, including
the context information required for all metrics. The memo first
defines a reordered singleton, and then uses it as the basis for
sample metrics to quantify the extent of reordering in several
useful dimensions for network characterization or receiver design.
Additional metrics quantify the frequency of reordering and the
Morton, et al. Standards Track exp. November 2006 Page 1
Packet Reordering Metric for IPPM May 2006
distance between separate occurrences. We then define a metric
oriented toward assessing reordering effects on TCP. Several
examples of evaluation using the various sample metrics are
included. An Appendix gives extended definitions for evaluating
order with packet fragmentation.
Contents
Status of this Memo................................................1
Copyright Notice...................................................1
Abstract...........................................................1
1. Conventions Used in this Document...............................3
2. Introduction....................................................4
2.1 Motivation.....................................................4
2.2 Goals and Objectives...........................................5
2.3 Required Context for All Reordering Metrics....................6
3. A Reordered Packet Singleton Metric.............................6
3.1 Metric Name....................................................7
3.2 Metric Parameters..............................................7
3.3 Definition.....................................................8
3.4 Sequence Discontinuity Definition..............................8
3.5 Evaluation of Reordering in Dimensions of Time or Bytes........9
3.6 Discussion.....................................................9
4. Sample Metrics.................................................10
4.1 Reordered Packet Ratio........................................10
4.1.1 Metric Name.................................................10
4.1.2 Metric Parameters...........................................10
4.1.3 Definition..................................................11
4.1.4 Discussion..................................................11
4.2 Reordering Extent.............................................11
4.2.1 Metric Name.................................................11
4.2.2 Notation and Metric Parameters..............................11
4.2.3 Definition..................................................12
4.2.4 Discussion..................................................12
4.3 Reordering Late Time Offset...................................13
4.3.1 Metric Name.................................................13
4.3.2 Metric Parameters...........................................13
4.3.3 Definition..................................................14
4.3.4 Discussion..................................................14
4.4 Reordering Byte Offset........................................15
4.4.1 Metric Name.................................................15
4.4.2 Metric Parameters...........................................15
4.4.3 Definition..................................................15
4.4.4 Discussion..................................................15
4.5 Gaps between multiple Reordering Discontinuities..............16
4.5.1 Metric Names................................................16
4.5.2 Parameters..................................................16
4.5.3 Definition of Reordering Discontinuity......................16
4.5.4 Definition of Reordering Gap................................16
4.5.5 Discussion..................................................17
4.6 Reordering-free Runs..........................................17
4.6.1 Metric Names................................................18
Morton, et al. Standards Track exp. November 2006 Page 2
Packet Reordering Metric for IPPM May 2006
4.6.2 Parameters..................................................18
4.6.3 Definition..................................................18
4.6.4 Discussion and Illustration.................................19
5. Metrics Focused on Receiver Assessment: A TCP-Relevant Metric..19
5.1 Metric Name...................................................20
5.2 Parameter Notation............................................20
5.3 Definitions...................................................20
5.4 Discussion....................................................21
6. Measurement and Implementation Issues..........................21
6.1 Passive Measurement Considerations............................24
7. Examples of Arrival Order Evaluation...........................25
7.1 Example with a Single Packet Reordered........................25
7.2 Example with Two Packets Reordered............................26
7.3 Example with Three Packets Reordered..........................28
7.4 Example with Multiple Packet Reordering Discontinuities.......29
8. Security Considerations........................................29
8.1 Denial of Service Attacks.....................................29
8.2 User Data Confidentiality.....................................30
8.3 Interference with the Metric..................................30
9. IANA Considerations............................................30
10. Normative References..........................................32
11. Informative References........................................33
12. Acknowledgments...............................................35
13. Appendix A Example Implementations in C (Informative).........35
14. Appendix B Fragment Order Evaluation (Informative)............38
14.1 Metric Name..................................................38
14.2 Additional Metric Parameters.................................38
14.3 Definition...................................................38
14.4 Discussion: Notes on Sample Metrics when Evaluating Fragments40
15. Disclaimer and License........................................40
16. Author's Addresses............................................40
Full Copyright Statement..........................................41
Intellectual Property.............................................41
Acknowledgement...................................................42
1. Conventions Used in this Document
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 in [RFC2119]. Although
RFC 2119 was written with protocols in mind, the key words are used
in this document for similar reasons. They are used to ensure the
results of measurements from two different implementations are
comparable, and to note instances when an implementation could
perturb the network.
In this memo, the characters "<=" should be read as "less than or
equal to" and ">=" as "greater than or equal to".
Morton, et al. Standards Track exp. November 2006 Page 3
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2. Introduction
Ordered arrival is a property found in packets that transit their
path, where the packet sequence number increases with each new
arrival and there are no backward steps. The Internet Protocol
[RFC791] [RFC2640] has no mechanisms to assure either packet
delivery or sequencing, and higher layer protocols (above IP) should
be prepared to deal with both loss and reordering. This memo defines
reordering metrics.
A unique sequence identifier carried in each packet, such as an
incrementing consecutive integer message number, establishes the
Source Sequence.
The detection of reordering at the Destination is based on packet
arrival order in comparison with a non-reversing reference value
[Cia03].
This metric is consistent with [RFC2330], and classifies arriving
packets with sequence numbers smaller than their predecessors as
out-of-order, or reordered. For example, if sequentially numbered
packets arrive 1,2,4,5,3, then packet 3 is reordered. This is
equivalent to Paxon's reordering definition in [Pax98], where "late"
packets were declared reordered. The alternative is to emphasize
"premature" packets instead (4 and 5 in the example), but only the
arrival of packet 3 distinguishes this circumstance from packet
loss. Focusing attention on late packets allows us to maintain
orthogonality with the packet loss metric. The metric's construction
is very similar to the sequence space validation for received
segments in [RFC793]. Earlier work to define ordered delivery
includes [Cia00], [Ben99], [Lou01], [Bel02], [Jai02] and [Cia03].
2.1 Motivation
A reordering metric is relevant for most applications, especially
when assessing network support for Real-Time media streams. The
extent of reordering may be sufficient to cause a received packet to
be discarded by functions above the IP layer.
Packet order may change during transfer, and several specific path
characteristics can make reordering more likely.
Examples are:
* When two (or more) paths with slightly differing transfer times
support a single packet stream or flow, then packets traversing
the longer path(s) may arrive out-of-order. Multiple paths may be
used to achieve load balancing, or may arise from route
instability.
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* To increase capacity, a network device designed with multiple
processors serving a single port (or parallel links) may reorder
as a byproduct.
* A layer 2 retransmission protocol that compensates for an error-
prone link may cause packet reordering.
* If for any reason, the packets in a buffer are not serviced in the
order of their arrival, their order will change.
* If packets in a flow are assigned to multiple buffers (following
evaluation of traffic characteristics, for example), and the
buffers have different occupations and/or service rates, then
order will likely change.
When one or more of the above path characteristics are present
continuously, then reordering may be present on a steady-state
basis. The steady-state reordering condition typically causes an
appreciable fraction of packets to be reordered. This form of
reordering is most easily detected by minimizing the spacing between
test packets. Transient reordering may occur in response to network
instability; temporary routing loops can cause periods of extreme
reordering. This condition is characterized by long in-order streams
with occasional instances of reordering, sometimes with extreme
correlation. However, we do not expect packet delivery in a
completely random order, where for example, the last packet or the
first packet in a sample is equally likely to arrive first at the
destination. Thus we expect at least a minimal degree of order in
the packet arrivals, as exhibited in real networks.
The ability to restore order at the destination will likely have
finite limits. Practical hosts have receiver buffers with finite
size in terms of packets, bytes, or time (such as de-jitter
buffers). Once the initial determination of reordering is made, it
is useful to quantify the extent of reordering, or lateness, in all
meaningful dimensions.
2.2 Goals and Objectives
The definitions below intend to satisfy the goals of:
1. Determining whether or not packet reordering has occurred.
2. Quantifying the degree of reordering. (We define a number of
metrics to meet this goal, because receiving procedures differ
by protocol or application. Since the effects of packet
reordering vary with these procedures, a metric that quantifies
a key aspect of one receiver's behavior could be irrelevant to
a different receiver. If all the metrics defined below are
reported, they give a wide-ranging view of reordering
conditions.)
Reordering Metrics MUST:
Morton, et al. Standards Track exp. November 2006 Page 5
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+ have one or more applications, such as receiver design or network
characterization, and a compelling relevance in the view of the
interested community.
+ be computable "on the fly"
+ work even if the stream has duplicate or lost packets
It is desirable for Reordering Metrics to have one or more of the
following attributes:
+ ability to concatenate results for segments measured separately
to estimate the reordering of an entire path
+ simplicity for easy consumption and understanding
+ relevance to TCP design
+ relevance to Real-time application performance
The current set of metrics meets all the requirements above and
provides all but the concatenation attribute (except in the case
where measurements of path segments exhibit no reordering, and one
may estimate that the complete path composed of these segments would
also exhibit no reordering). However, satisfying these goals
restricts the set of metrics to those that provide some clear
insight into network characterization or receiver design. They are
not likely to be exhaustive in their coverage of reordering effects
on applications, and additional measurements may be possible.
2.3 Required Context for All Reordering Metrics
A critical aspect of all reordering metrics is their inseparable
bond with the measurement conditions. Packet reordering is not well
defined unless the full measurement context is reported. Therefore,
all reordering metric definitions include the following parameters:
1. The "Packet of Type-P" [RFC2330] identifiers for the packet
stream, including the transport addresses for source and
destination, and any other information which may result in different
packet treatments.
2. The stream parameter set for the sending discipline, such as the
parameters unique to Periodic Streams (as in [RFC3432]), TCP-like
Streams (as in [RFC3148]), or Poisson Streams (as in [RFC2330]). The
stream parameters include the packet size, either specified as a
fixed value or as a pattern of sizes (as applicable).
Whenever a metric is reported, it MUST include a description of
these parameters to provide a context for the results.
3. A Reordered Packet Singleton Metric
The IPPM framework [RFC2330] describes the notions of singletons,
samples, and statistics. For easy reference:
By a 'singleton' metric, we refer to metrics that are,
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in a sense, atomic. For example, a single instance of "bulk
throughput capacity" from one host to another might be defined
as a singleton metric, even though the instance involves
measuring the timing of a number of Internet packets.
The evaluation of packet order requires several supporting concepts.
The first is an algorithm (function) that produces a series of
strictly monotonically increasing identifiers applied to packets at
the source to uniquely establish the order of packet transmission
(where a function, g(x), is strictly monotonically increasing if for
any x>y, g(x)>g(y) ). The unique sequence identifier may simply be
an incrementing consecutive integer message number, or sequence
number as used below. The prospect of sequence number roll-over is
discussed in Section 6.
The second supporting concept is a stored value which is the "next
expected" packet number. Under normal conditions, the value of Next
Expected (NextExp) is the sequence number of the previous packet
plus 1 for message numbering (in general, the receiver reproduces
the sender's algorithm and the sequence of identifiers so that the
"next expected" can be determined).
Each packet within a packet stream can be evaluated with this order
singleton metric.
3.1 Metric Name
Type-P-Reordered
3.2 Metric Parameters
+ Src, the IP address of a host
+ Dst, the IP address of a host
+ SrcTime, the time of packet emission from the Source (or wire
time)
+ s, the unique packet sequence number applied at the Source, in
units of messages.
+ NextExp, the Next Expected Sequence number at the Destination, in
units of messages. The stored value in NextExp is determined from
a previously arriving packet.
And optionally:
+ PayloadSize, the number of bytes contained in the information
field and referred to when the SrcByte sequence is based on bytes
transfered.
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+ SrcByte, the packet sequence number applied at the Source, in
units of payload bytes.
3.3 Definition
If a packet s, (sent at time, SrcTime) is found to be reordered by
comparison with the Next Expected value, its Type-P-Reordered =
TRUE; otherwise Type-P-Reordered = FALSE, as defined below:
The value of Type-P-Reordered is defined as TRUE if s < NextExp (the
packet is reordered). In this case, the NextExp value does not
change.
The value of Type-P-Reordered is defined as FALSE if s >= NextExp
(the packet is in-order). In this case, NextExp is set to s+1 for
comparison with the next packet to arrive.
Since the Next Expected value cannot decrease, it provides a non-
reversing order criterion to identify reordered packets.
This definition can also be specified in pseudo-code.
On successful arrival of a packet with sequence number s:
if s >= NextExp then /* s is in-order */
NextExp = s + 1;
Type-P-Reordered = False;
else /* when s < NextExp */
Type-P-Reordered = True
3.4 Sequence Discontinuity Definition
Packets with s > NextExp are a special case of in-order delivery.
This condition indicates a sequence discontinuity, either because of
packet loss or reordering. Reordered packets must arrive for the
sequence discontinuity to be defined as a reordering discontinuity
(see section 4).
We define two different states for in-order packets.
When s = NextExp, the original sequence has been maintained, and
there is no discontinuity present.
When s > NextExp, some packets in the original sequence have not yet
arrived, and there is a sequence discontinuity associated with
packet s. The size of the discontinuity is s - NextExp, equal to
the number of packets presently missing, either reordered or lost.
In pseudo-code:
On successful arrival of a packet with sequence number s:
if s >= NextExp, then /* s is in-order */
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if s > NextExp then
SequenceDiscontinuty = True;
SeqDiscontinutySize = s - NextExp;
else
SequenceDiscontinuty = False;
NextExp = s + 1;
Type-P-Reordered = False;
else /* when s < NextExp */
Type-P-Reordered = True;
SequenceDiscontinuty = False;
Whether any Sequence Discontinuities occur (and their size) is
determined by the conditions causing loss and/or reordering along
the measurement path. Note that a packet could be reordered at one
point, and subsequently lost elsewhere on the path, but this cannot
be known from observations at the Destination.
3.5 Evaluation of Reordering in Dimensions of Time or Bytes
It is possible to use alternate dimensions of time or payload bytes
to test for reordering in the definition of section 3.3, as long as
the SrcTimes and SrcBytes are unique and reliable. Sequence
Discontinuities are easily defined and detected with message
numbering, however, this is not so simple in the dimensions of time
or bytes. This is a detractor for the alternate dimensions because
the Sequence Discontinuity definition plays a key role in the sample
metrics that follow.
It is possible to detect Sequence Discontinuities with payload byte
numbering, but only when the test device knows exactly what value to
assign as NextExp in response to any packet arrival. This is
possible when the complete pattern of payload sizes is stored at the
Destination, or if the size pattern can be generated using a pseudo-
random number generator and a shared seed. If payload size is
constant, byte numbering adds needless complexity over message
numbering.
It may be possible to detect Sequence Discontinuities with Periodic
Streams and Source Time numbering, but there are practical pitfalls
with sending exactly on-schedule and with clock reliability.
The dimensions of time and bytes remain an important basis for
characterizing the extent of reordering, as described in sections
4.3 and 4.4.
3.6 Discussion
Any arriving packet bearing a sequence number from the sequence that
establishes the Next Expected value can be evaluated to determine
whether it is in-order or reordered, based on a previous packet's
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Packet Reordering Metric for IPPM May 2006
arrival. In the case where Next Expected is Undefined (because the
arriving packet is the first successful transfer), the packet is
designated in-order (Type-P-Reordered=FALSE).
This metric assumes re-assembly of packet fragments before
evaluation. In principle, it is possible to use the Type-P-Reordered
metric to evaluate reordering among packet fragments, but each
fragment must contain source sequence information.
See the Appendix on fragment order evaluation for more detail.
If duplicate packets (multiple non-corrupt copies) arrive at the
destination, they MUST be noted and only the first to arrive is
considered for further analysis (copies would be declared reordered
according to the definition above). This requirement has the same
storage implications as earlier IPPM metrics, and follows the
precedent of [RFC2679]. We provide a suggestion to minimize storage
size needed in Section 6 on Measurement and Implementation Issues.
4. Sample Metrics
In this section, we define metrics applicable to a sample of packets
from a single Source sequence number system. When reordering occurs,
it is highly desirable to assert the degree to which a packet is
out-of-order, or reordered with respect other packets. This section
defines several metrics that quantify the extent of reordering in
various units of measure. Each metric highlights a relevant use.
The metrics in the sub-sections below have a network
characterization orientation, but also have relevance to receiver
design where reordering compensation is of interest. We begin with a
simple ratio metric indicating the reordered portion of the sample.
4.1 Reordered Packet Ratio
4.1.1 Metric Name
Type-P-Reordered-Ratio-Stream
4.1.2 Metric Parameters
The parameter set includes Type-P-Reordered singleton parameters,
the parameters unique to Poisson Streams (as in [RFC2330], Periodic
Streams (as in [RFC3432]), or TCP-like Streams (as in [RFC3148]),
packet size or size patterns, and the following:
+ T0, a start time
+ Tf, an end time
+ dT, a waiting time for each packet to arrive, in seconds
Morton, et al. Standards Track exp. November 2006 Page 10
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+ K, the total number of packets in the stream sent from Source to
Destination
+ L, the total number of packets received (arriving between T0 and
Tf+dT) out of the K packets sent. Recall that identical copies
(duplicates) have been removed, so L <= K.
+ R, the ratio of reordered packets to received packets, defined
below
Note that parameter dT is effectively the threshold for declaring a
packet as lost. The IPPM Packet Loss Metric [RFC2680] declines to
recommend a value for this threshold, saying instead that "good
engineering, including an understanding of packet lifetimes, will be
needed in practice."
4.1.3 Definition
Given a stream of packets sent from a Source to a Destination, the
ratio of reordered packets in the sample is
R = (Count of packets with Type-P-Reordered=TRUE) / ( L )
This fraction may be expressed as a percentage (multiply by 100).
Note that in the case of duplicate packets, only the first copy is
used.
4.1.4 Discussion
When the Type-P-Reordered-Ratio-Stream is zero, no further
reordering metrics need be examined for that sample. Therefore, the
value of this metric is its simple ability to summarize the results
for a reordering-free sample.
4.2 Reordering Extent
This section defines the extent to which packets are reordered, and
associates a specific Sequence Discontinuity with each reordered
packet. This section inherits the Parameters defined above.
4.2.1 Metric Name
Type-P-Packet-Reordering-Extent-Stream
4.2.2 Notation and Metric Parameters
Recall that K is the number of packets in the stream at the Source
and L is the number of packets received at the Destination.
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Each packet has been assigned a sequence number, s, a consecutive
integer from 1 to K in the order of packet transmission (at the
source).
Let s[1], s[2], ..., s[L], represent the original sequence numbers
associated with the packets in order of arrival.
s[i] can be thought of as a vector, where the index i is the arrival
position of the packet with sequence number s. In theory, any
Source sequence number could appear in any arrival position, but
this is unlikely in reality.
Consider a reordered packet (Type-P-Reordered=TRUE) with arrival
index i and source sequence number s[i]. There exists a set of
indexes j (1 <= j < i) such that s[j] > s[i].
The new parameters are:
+ i, the index for arrival position, where i-1 represents an
arrival earlier than i.
+ j, a set of one or more arrival indexes, where 1 <= j < i.
+ s[i], the original sequence numbers, s, in order of arrival.
+ e, the Reordering Extent, in units of packets, defined below.
4.2.3 Definition
The reordering extent, e, of packet s[i] is defined to be i-j for
the smallest value of j where s[j] > s[i].
Informally, the reordering extent is the maximum distance, in
packets, from a reordered packet to the earliest packet received
that has a larger sequence number. If a packet is in-order, its
reordering extent is undefined. The first packet to arrive is in-
order by definition, and has undefined reordering extent.
Comment on the definition of extent: For some arrival orders, the
assignment of a simple position/distance as the reordering extent
tends to overestimate the receiver storage needed to restore order.
A more accurate and complex procedure to calculate packet storage
would be to subtract any earlier reordered packets that the receiver
could pass on to the upper layers (see the Byte Offset metric). With
the bias understood, this definition is deemed sufficient,
especially for those who demand "on the fly" calculations.
4.2.4 Discussion
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The packet with index j (s[j], identified in the Definition above)
is the reordering discontinuity associated with packet s at index i
(s[i]). This definition is formalized below.
Note that the K packets in the stream could be some subset of a
larger stream, but L is still the total number of packets received
out of the K packets sent in that subset.
If a receiver intends to restore order, then its buffer capacity
determines its ability to handle packets that are reordered. For
cases with single reordered packets, the extent e gives the number
of packets that must be held in the receiver's buffer while waiting
for the reordered packet to complete the sequence. For more complex
scenarios, the extent may be an overestimate of required storage
(see section 4.4 on Reordering Byte Offset and the Examples
section). Also, if the receiver purges its buffer for any reason,
the extent metric would not reflect this behavior, assuming instead
that the receiver would exhaustively attempt to restore order.
Although reordering extent primarily quantifies the offset in terms
of arrival position, it may also be useful for determining the
portion of reordered packets that can or cannot be restored to order
in a typical receiver buffer based on their arrival order alone (and
without the aid of retransmission).
A sample's reordering extents may be expressed as a histogram, to
easily summarize the frequency of various extents.
4.3 Reordering Late Time Offset
Reordered packets can be assigned offset values indicating their
lateness in terms of buffer time that a receiver must possess to
accommodate them. Offset metrics are calculated only on reordered
packets, as identified by the reordered packet singleton metric in
Section 3.
4.3.1 Metric Name
Type-P-Packet-Late-Time-Stream
4.3.2 Metric Parameters
In addition to the parameters defined for Type-P-Reordered-Ratio-
Stream, we specify:
+ DstTime, the time that each packet in the stream arrives at the
destination, and may be associated with index i, or packet s[i]
+ LateTime(s[i]), the offset of packet s[i] in units of seconds,
defined below
Morton, et al. Standards Track exp. November 2006 Page 13
Packet Reordering Metric for IPPM May 2006
4.3.3 Definition
Lateness in time is calculated using destination times. When
received packet s[i] is reordered, and has a reordering extent e,
then:
LateTime(s[i]) = DstTime(i)-DstTime(i-e)
Alternatively, using similar notation to that of section 4.2, an
equivalent definition is:
LateTime(s[i]) = DstTime(i)-DstTime(j), for min{j|1<=j<i} that
satisfies s[j]>s[i].
4.3.4 Discussion
The offset metrics can help predict whether reordered packets will
be useful in a general receiver buffer system with finite limits.
The limit may be the time of storage prior to a cyclic play-out
instant (as with de-jitter buffers).
Note that the One-way IPDV [RFC3393] gives the delay variation for a
packet w.r.t. the preceding packet in the source sequence. Lateness
and IPDV give an indication of whether a buffer at the destination
has sufficient storage to accommodate the network's behavior and
restore order. When an earlier packet in the Source sequence is
lost, IPDV will necessarily be undefined for adjacent packets, and
LateTime may provide the only way to evaluate the usefulness of a
packet.
In the case of de-jitter buffers, there are circumstances where the
receiver employs loss concealment at the intended play-out time of a
late packet. However, if this packet arrives out of order, the Late
Time determines whether the packet is still useful. IPDV no longer
applies, because the receiver establishes a new play-out schedule
with additional buffer delay to accommodate similar events in the
future (this requires very minimal processing).
The combination of loss and reordering influences the LateTime
metric. If presented with the arrival sequence 1, 10, 5 (where
packets 2, 3, 4, and 6 through 9 are lost), LateTime would not
indicate exactly how "late" packet 5 is from its intended arrival
position. IPDV [RFC3393] would not capture this either, because of
the lack of adjacent packet pairs. Assuming a Periodic Stream
[RFC3432], an expected arrival time could be defined for all
packets, but this is essentially a single-point delay variation
metric (as defined in ITU-T Recommendations [I.356] and [Y.1540]),
and not a reordering metric.
A sample's LateTime results may be expressed as a histogram, to
summarize the frequency of buffer times needed to accommodate
reordered packets and permit buffer tuning on that basis. A CDF with
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buffer time vs. percent of reordered packets accommodated may be
informative.
4.4 Reordering Byte Offset
Reordered packets can be assigned offset values indicating the
storage in bytes that a receiver must possess to accommodate them.
Offset metrics are calculated only on reordered packets, as
identified by the reordered packet singleton metric in Section 3.
4.4.1 Metric Name
Type-P-Packet-Byte-Offset-Stream
4.4.2 Metric Parameters
We use the same parameters defined earlier, including the optional
parameters of SrcByte and PayloadSize, and define:
+ ByteOffset(s[i]), the offset of packet s[i] in bytes
4.4.3 Definition
The Byte stream offset for reordered packet s[i] is the sum of the
payload sizes of packets qualified by the following criteria:
* Arrival prior to the reordered packet, s[i], and
* The send sequence number, s, is greater than s[i].
Packets that meet both these criteria are normally buffered until
the sequence beneath them is complete. Note that these criteria
apply to both in-order and reordered packets.
For reordered packet s[i] with a reordering extent e:
ByteOffset(s[i]) = Sum[qualified packets]
= Sum[PayloadSize(packet at i-1 if qualified),
PayloadSize(packet at i-2 if qualified), ...
PayloadSize(packet at i-e always qualified)]
Using our earlier notation:
ByteOffset(s[i]) =
Sum[payloads of s[j] where s[j]>s[i] and i > j >= i-e]
4.4.4 Discussion
We note that estimates of buffer size due to reordering depend
greatly on the test stream, in terms of the spacing between test
packets and their size, especially when packet size is variable. In
these and other circumstances, it may be most useful to characterize
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offset in terms of the payload size(s) of stored packets, using the
Type-P-packet-Byte-Offset-Stream metric.
The byte offset metric can help predict whether reordered packets
will be useful in a general receiver buffer system with finite
limits. The limit is expressed as the number of bytes the buffer
can store.
A sample's ByteOffset results may be expressed as a histogram, to
summarize the frequency of buffer lengths needed to accommodate
reordered packets and permit buffer tuning on that basis. A CDF with
buffer size vs. percent of reordered packets accommodated may be
informative.
4.5 Gaps between multiple Reordering Discontinuities
4.5.1 Metric Names
Type-P-Packet-Reordering-Gap-Stream
Type-P-Packet-Reordering-GapTime-Stream
4.5.2 Parameters
We use the same parameters defined earlier, but add the convention
that index i' is greater than i, likewise j' > j, and define:
+ Gap(s[j']), the Reordering Gap of packet s[j'] in units of
integer messages
and the OPTIONAL parameter:
+ GapTime(s[j']), the Reordering Gap of packet s[j'] in units of
seconds
4.5.3 Definition of Reordering Discontinuity
All reordered packets are associated with a packet at a reordering
discontinuity, defined as the in-order packet s[j] that arrived at
the minimum value of j (1<=j<i) for which s[j]> s[i].
Note that s[j] will have been found to cause a sequence
discontinuity, where s > NextExp when evaluated with the reordered
singleton metric as described in section 3.4.
Recall that i - e = min(j). Subsequent reordered packets may be
associated with the same s[j], or with a different discontinuity.
This fact is used in the definition of the Reordering Gap, below.
4.5.4 Definition of Reordering Gap
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A reordering gap is the distance between successive reordering
discontinuities. The Type-P-Packet-Reordering-Gap-Stream metric
assigns a value for Gap(s[j']) to (all) packets in a stream (and a
value for GapTime(s[j']), when reported).
If:
The packet s[j'] is found to be a reordering discontinuity, based
on the arrival of reordered packet s[i'] with extent e', and
an earlier reordering discontinuity s[j], based on the arrival of
reordered packet s[i] with extent e was already detected, and
i' > i, and
there are no reordering discontinuities between j and j',
then the Reordering Gap for packet s[j'] is the difference between
the arrival positions the reordering discontinuities, as shown
below:
Gap(s[j']) = (j') - (j)
Gaps MAY also be expressed in time:
GapTime(s[j']) = DstTime(j') - DstTime(j)
Otherwise:
Gap(s[j]) (and GapTime(s[j']) ) for packet s[j] is 0.
4.5.5 Discussion
When separate reordering discontinuities can be distinguished, then
a count may also be reported (along with the discontinuity
description, such as the number of reordered packets associated with
that discontinuity and their extents and offsets). The Gaps between
a sample's reordering discontinuities may be expressed as a
histogram, to easily summarize the frequency of various gaps.
Reporting the mode, average, range, etc. may also summarize the
distributions.
The Gap metric may help to correlate the frequency of reordering
discontinuities with their cause. Gap lengths are also informative
to receiver designers, revealing the period of reordering
discontinuities. The combination of reordering gaps and extent
reveals whether receivers will be required to handle cases of
overlapping reordered packets.
4.6 Reordering-free Runs
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This section defines a metric based on a count of consecutive in-
order packets between reordered packets.
4.6.1 Metric Names
Type-P-Packet-Reordering-Free-Run-x-numruns-Stream
Type-P-Packet-Reordering-Free-Run-q-squruns-Stream
Type-P-Packet-Reordering-Free-Run-p-numpkts-Stream
Type-P-Packet-Reordering-Free-Run-a-accpkts-Stream
4.6.2 Parameters
We use the same parameters defined earlier, and define the
following:
+ r, the run counter
+ x, the number of runs, also the number of reordered packets
+ a, the accumulator of in-order packets
+ p, the number of packets (when the stream is complete, p=(x+a)=L)
+ q, the sum of the squares of the runs counted
4.6.3 Definition
As packets in a sample arrive at the Destination, the count of in-
order packets between reordered packets is a Reordering-Free run.
Note that the minimum run-length is zero according to this
definition. A pseudo code example follows:
r = 0; /* r is the run counter */
x = 0; /* x is the number of runs */
a = 0; /* a is the accumulator of in-order packets */
p = 0; /* p is the number of packets */
q = 0; /* q is the sum of the squares of the runs counted */
while(packets arrive with sequence number s)
{
p++;
if (s >= NextExp) /* s is in-order */
then r++;
a++;
else /* s is reordered */
q+= r*r;
r = 0;
x++;
}
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Each in-order arrival increments the run counter and the accumulator
of in-order packets, each reordered packet resets the run counter
after adding it to the sum of the squared lengths.
Each arrival of a reordered packet yields a new run count. Long
runs accompany periods where order was maintained, while short runs
indicate frequent, or multi-packet reordering.
The percent of packets in-order is 100*a/p
The average Reordering-Free run length is a/x
The q counter gives an indication of variation of the Reordering-
Free runs from the average by comparing q/a to a/x ((q/a)/(a/x))
4.6.4 Discussion and Illustration
Type-P-packet-Reordering-Free-Run-Stream parameters give a brief
summary of the stream's reordering characteristics including the
average reordering-free run length, and the variation of run
lengths, therefore a key application of this metric is network
evaluation.
For 36 packets with 3 runs of 11 in-order packets we have:
p = 36
x = 3
a = 33
q = 3 * (11*11) = 363
ave reordering-free run = 11
q/a = 11
(q/a)/(a/x) = 1.0
For 36 packets with 3 runs, 2 runs of length 1 and one of length 31
p = 36
x = 3
a = 33
q = 1 + 1 + 961 = 963
ave reordering-free run = 11
q/a = 29.18
(q/a)/(a/x) = 2.65
The variability in run length is prominent in the difference between
the q values (sum of the squared run lengths) and comparing average
run length to the (q/a)/(a/x) ratios (equals 1 when all runs are the
same length).
5. Metrics Focused on Receiver Assessment: A TCP-Relevant Metric
This section describes a metric that conveys information associated
with the effect of reordering on TCP. However, in order to infer
anything about TCP performance, the test stream MUST bear a close
resemblance to the TCP sender of interest. [RFC3148] lists the
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specific aspects of congestion control algorithms that must be
specified. Further, RFC 3148 recommends that Bulk Transfer Capacity
metrics SHOULD have instruments to distinguish three cases of packet
reordering (in section 3.3). The sample metrics defined above
satisfy the requirements to classify packets that are slightly or
grossly out-of-order. The metric in this section adds the capability
to estimate whether reordering might cause the DUP-ACK threshold to
be exceeded causing the Fast Retransmit algorithm to be invoked.
Additional TCP Kernel Instruments are summarized in [Mat03].
5.1 Metric Name
Type-P-Packet-n-Reordering-Stream
5.2 Parameter Notation
Let n be a positive integer (a parameter). Let k be a positive
integer equal to the number of packets sent (sample size). Let l be
a non-negative integer representing the number of packets that were
received out of the k packets sent. (Note that there is no
relationship between k and l: on one hand, losses can make l less
than k; on the other hand, duplicates can make l greater than k.)
Assign each sent packet a sequence number, 1 to k, in order of
packet emission.
Let s[1], s[2], ..., s[l] be the original sequence numbers of the
received packets, in the order of arrival.
5.3 Definitions
Definition 1: Received packet number i (n < i <= l), with source
sequence number s[i], is n-reordered if and only if for all j such
that i-n <= j < i, s[j] > s[i].
Claim: If by this definition, a packet's reordering is n and 0 < n'
< n, then the packet is also reordered to the n' extent.
Note: This definition is illustrated by C code in Appendix A. It
determines the n-reordering for a value of n=3 (when actually
writing applications that would report the metric, one would
probably report it for several values of n, such as 1, 2, 3, 4 --
and maybe a few more consecutive values).
This definition does not assign an n to all reordered packets as
defined by the singleton metric, in particular when blocks of
successive packets are reordered. (In the arrival sequence
s={1,2,3,7,8,9,4,5,6}, packets 4, 5, and 6 are reordered, but only
packet 4 is n-reordered, with n=3.)
Definition 2: The degree of n-reordering of the sample is m/l, where
m is the number of n-reordered packets in the sample.
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Definition 3: The degree of "monotonic reordering" of the sample is
its degree of 1-reordering.
Definition 4: A sample is said to have no reordering if its degree
of n-reordering is 0.
5.4 Discussion
The degree of n-reordering may be expressed as a percentage, in
which case the number from Definition 2 is multiplied by 100.
The n-reordering metric is helpful for matching the duplicate ACK
threshold setting to a given path. For example, if a path exhibits
no more than 5-reordering, a DUP-ACK threshold of 6 may avoid
unnecessary retransmissions.
Important special cases are n=1 and n=3:
- For n=1, absence of 1-reordering means the sequence numbers that
the receiver sees are monotonically increasing with respect to the
previous arriving packet.
- For n=3, a NewReno TCP sender would retransmit 1 packet in
response to an instance of 3-reordering and therefore consider this
packet lost for the purposes of congestion control (the sender will
halve its congestion window, see [RFC2581]). Three is default
threshold for Stream Control Transport Protocol (SCTP) [RFC2960],
and the Datagram Congestion Control Protocol (DCCP) [RFC4340] when
used with Congestion Control ID 2: TCP-like Congestion Control
[RFC4341].
A sample's n-reordering may be expressed as a histogram, to
summarize the frequency for each value of n.
We note that the definition of n-reordering cannot predict the exact
number of packets unnecessarily retransmitted by a TCP sender under
some circumstances, such as cases with closely-spaced reordered
singletons. Both time and position influence the sender's behavior.
A packet's n-reordering designation is sometimes equal to its
reordering extent, e. n-reordering is different in the following
ways:
1. n is a count of early packets with consecutive arrival positions
at the receiver.
2. Reordered packets (Type-P-Reordered=TRUE) may not be n-reordered,
but will have an extent, e (see the examples).
6. Measurement and Implementation Issues
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The results of tests will be dependent on the time interval between
measurement packets (both at the Source, and during transport where
spacing may change). Clearly, packets launched infrequently (e.g.,
1 per 10 seconds) are unlikely to be reordered.
In order to gauge the reordering for an application according to the
metrics defined in this memo, it is RECOMMENDED to use the same
sending pattern as the application of interest. In any case, the
exact method of packet generation MUST be reported with the
measurement results, including all stream parameters.
+ To make inferences about applications that use TCP, it is
REQUIRED to use TCP-like Streams as in [RFC3148]
+ For real-time applications, it is RECOMMENDED to use Periodic
Streams as in [RFC3432]
It is acceptable to report the metrics of Sections 3 and 4 with
other IPPM metrics using Poisson Streams [RFC2330]. Poisson streams
represent an "unbiased sample" of network performance for packet
loss and delay metrics. However, it would be incorrect to make
inferences about the application categories above using reordering
metrics measured with Poisson streams.
Test stream designers may prefer to use a periodic sending interval
so that a known temporal bias is maintained, also bringing
simplified results analysis (as described in [RFC3432]). In this
case, it is RECOMMENDED that the periodic sending interval should be
chosen to reproduce the closest Source packet spacing expected.
Testers must recognize that streams sent at the link speed
serialization limit MUST have limited duration and MUST consider
packet loss as an indication that the stream has caused congestion,
and suspend further testing.
When intending to compare independent measurements of reordering, it
is RECOMMENDED to use the same test stream parameters in each
measurement system.
Packet lengths might also be varied to attempt to detect instances
of parallel processing (they may cause steady state reordering). For
example, a line-speed burst of the longest (MTU-length) packets
followed by a burst of the shortest possible packets may be an
effective detecting pattern. Other size patterns are possible.
The non-reversing order criterion and all metrics described above
remain valid and useful when a stream of packets experiences packet
loss, or both loss and reordering. In other words, losses alone do
not cause subsequent packets to be declared reordered.
Since this metric definition may use sequence numbers with finite
range, it is possible that the sequence numbers could reach end-of-
range and roll over to zero during a measurement. By definition,
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the Next Expected value cannot decrease, and all packets received
after a roll-over would be declared reordered. Sequence number
roll-over can be avoided by using combinations of counter size and
test duration where roll-over is impossible (and sequence is reset
to zero at the start). Also, message-based numbering results in
slower sequence consumption. There may still be cases where
methodological mitigation of this problem is desirable (e.g., long-
term testing). The elements of mitigation are:
1. There must be a test to detect if a roll-over has occurred. It
would be nearly impossible for the sequence numbers of successive
packets to jump by more than half the total range, so these large
discontinuities are designated as roll-over.
2. All sequence numbers used in computations are represented in a
sufficiently large precision. The numbers have a correction applied
(equivalent to adding a significant digit) whenever roll-over is
detected.
3. Reordered packets coincident with sequence numbers reaching end-
of-range must also be detected for proper application of correction
factor.
Ideally, the test instrument would have the ability to use all
earlier packets at any point in the test stream. In practice there
will be limited ability to determine reordering extent, due to the
storage requirements for previous packets. Saving only packets that
indicate discontinuities (and their arrival positions) will reduce
storage volume.
Another solution is to use a sliding history window of packets,
where the window size would be determined by an upper bound on the
useful reordering extent. This bound could be several packets or
several seconds worth of packets, depending on the intended
analysis. When discarding all stream information beyond the window,
the reordering extent or degree of n-reordering may need to be
expressed as greater than the window length if the reordering
discontinuity information has been discarded, and Gap calculations
would not be possible.
The requirement to ignore duplicate packets also mandates storage.
Here, tracking the sequence numbers of missing packets may minimize
storage size. Missing packets may eventually be declared lost, or
reordered if they arrive. The missing packet list and the largest
sequence number received thus far (NextExp - 1) are sufficient
information to determine if a packet is a duplicate (assuming a
manageable storage size for packets that are missing due to loss).
It is important to note that practical IP networks also have limited
ability to "store" packets, even when routing loops appear
temporarily. Therefore, the maximum storage for reordering metrics
(and their complexity) would only approach the number packets in the
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sample, K, when the sending time for K packets is small with respect
to the network's largest possible transfer time. Another possible
limitation on storage is the maximum length of the sequence number
field, assuming that most test streams do not exhaust this length in
practice.
Last, we note that determining reordering extents and gaps is tricky
when there are overlapped or nested events. Test instrument
complexity and reordering complexity are directly correlated.
6.1 Passive Measurement Considerations
As with other IPPM metrics, the definitions have been constructed
primarily for Active measurements.
Assuming that the necessary sequence information (message number) is
included in the packet payload (possibly in application headers such
as RTP), reordering metrics may be evaluated in a passive
measurement arrangement. Also, it is possible to evaluate order at
any point along a Source-Destination path, recognizing that
intermediate measurements may differ from those made at the
Destination (where the reordering effect on applications can be
inferred).
It is possible to apply these metrics to evaluate reordering in a
TCP sender's stream. In this case, the Source sequence numbers would
be based on byte stream, or segment numbering. Since the stream may
include retransmissions due to loss or reordering, care must be
taken to avoid declaring retransmitted packets reordered. The
additional sequence reference of s or SrcTime helps to avoid this
ambiguity in active measurement, or the optional TCP timestamp field
[RFC1323] in passive measurement.
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7. Examples of Arrival Order Evaluation
This section provides some examples to illustrate how the non-
reversing order criterion works, how n-reordering works in
comparison, and the value of quantifying reordering in all the
dimensions of time, bytes, and position.
Throughout this section, we will refer to packets by their source
sequence number, except where noted. So "Packet 4" refers to the
packet with source sequence number 4, and the reader should refer to
the tables in each example to determine packet 4's arrival index
number, if needed.
7.1 Example with a Single Packet Reordered
Table 1 gives a simple case of reordering, where one packet is
reordered, Packet 4. Packets are listed according to their arrival,
and message numbering is used. All packets contain PayloadSize=100
bytes, with SrcByte=(s x 100)-99 for s=1,2,3,4,...
Table 1 Example with Packet 4 Reordered,
Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10
s Src Dst Dst Byte Late
@Dst NextExp Time Time Delay IPDV Order Offset Time
1 1 0 68 68 1
2 2 20 88 68 0 2
3 3 40 108 68 0 3
5 4 80 148 68 -82 4
6 6 100 168 68 0 5
7 7 120 188 68 0 6
8 8 140 208 68 0 7
4 9 60 210 150 82 8 400 62
9 9 160 228 68 0 9
10 10 180 248 68 0 10
Each column gives the following information:
s Packet sequence number at the Source.
NextExp The value of NextExp when the packet arrived(before
update).
SrcTime Packet time stamp at the Source, ms.
DstTime Packet time stamp at the Destination, ms.
Delay 1-way delay of the packet, ms.
IPDV IP Packet Delay Variation, ms
IPDV = Delay(SrcNum)-Delay(SrcNum-1)
DstOrder Order in which the packet arrived at the Destination.
Byte Offset The Byte Offset of a reordered packet, in bytes.
LateTime The lateness of a reordered packet, in ms.
We can see that when Packet 4 arrives, NextExp=9, and it is declared
reordered. We compute the extent of reordering as follows:
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Using the notation <s[1], ..., s[i], ..., s[L]>, the received
packets are represented as:
\/
s = 1, 2, 3, 5, 6, 7, 8, 4, 9, 10
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
/\
Applying the definition of Type-P-packet-Reordering-Extent-Stream:
when j=7, 8 > 4, so the reordering extent is 1 or more.
when j=6, 7 > 4, so the reordering extent is 2 or more.
when j=5, 6 > 4, so the reordering extent is 3 or more.
when j=4, 5 > 4, so the reordering extent is 4 or more.
when j=3, but 3 < 4, and 4 is the maximum extent, e=4 (assuming
there are no earlier sequence discontinuities, as in this example).
Further, we can compute the Late Time (210-148=62ms using DstTime)
compared to Packet 5's arrival. If the receiver has a de-jitter
buffer that holds more than 4 packets, or at least 62 ms storage,
Packet 4 may be useful. Note that 1-way delay and IPDV indicate
unusual behavior for Packet 4. Also, if Packet 4 had arrived at
least 62ms earlier, it would have been in-order in this example.
If all packets contained 100 byte payloads, then Byte Offset is
equal to 400 bytes.
Following the definitions of section 5.1, Packet 4 is designated 4-
reordered.
7.2 Example with Two Packets Reordered
Table 2 Example with Packets 5 and 6 Reordered,
Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10
s Src Dst Dst Byte Late
@Dst NextExp Time Time Delay IPDV Order Offset Time
1 1 0 68 68 1
2 2 20 88 68 0 2
3 3 40 108 68 0 3
4 4 60 128 68 0 4
7 5 120 188 68 -22 5
5 8 80 189 109 41 6 100 1
6 8 100 190 90 -19 7 100 2
8 8 140 208 68 0 8
9 9 160 228 68 0 9
10 10 180 248 68 0 10
Table 2 shows a case where Packets 5 and 6 arrive just behind Packet
7, so both 5 and 6 are reordered. The Late times (189-188=1, 190-
188=2) are small.
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Using the notation <s[1], ..., s[i], ..., s[l]>, the received
packets are represented as:
\/ \/
s = 1, 2, 3, 4, 7, 5, 6, 8, 9, 10
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
/\ /\
Considering Packet 5 first:
when j=5, 7 > 5, so the reordering extent is 1 or more.
when j=4, we have 4 < 5, so 1 is its maximum extent, and e=1.
Considering Packet 6 next:
when j=6, 5 < 6, the extent is not yet defined.
when j=5, 7 > 6, so the reordering extent is i-j=2 or more.
when j=4, 4 < 6, and we find 2 is its maximum extent, and e=2.
We can also associate each of these reordered packets with a
reordering discontinuity. We find the minimum j=5 (for both packets)
according to Section 4.2.3. So Packet 6 is associated with the same
reordering discontinuity as Packet 5, the Reordering Discontinuity
at Packet 7.
This is a case where reordering extent e would over-estimate the
packet storage required to restore order. Only one packet storage is
required (to hold Packet 7), but e=2 for Packet 6.
Following the definitions of section 5, Packet 5 is designated 1-
reordered, but Packet 6 is not designated n-reordered.
A hypothetical sender/receiver pair may retransmit Packet 5
unnecessarily, since it is 1-reordered (in agreement with the
singleton metric). Though Packet 6 may not be unnecessarily
retransmitted, the receiver cannot advance Packet 7 to the higher
layers until after Packet 6 arrives. Therefore, the singleton metric
correctly determined that Packet 6 is reordered.
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7.3 Example with Three Packets Reordered
Table 3 Example with Packets 4, 5, and 6 reordered
Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11
s Src Dst Dst Byte Late
@Dst NextExp Time Time Delay IPDV Order Offset Time
1 1 0 68 68 1
2 2 20 88 68 0 2
3 3 40 108 68 0 3
7 4 120 188 68 -88 4
8 8 140 208 68 0 5
9 9 160 228 68 0 6
10 10 180 248 68 0 7
4 11 60 250 190 122 8 400 62
5 11 80 252 172 -18 9 400 64
6 11 100 256 156 -16 10 400 68
11 11 200 268 68 0 11
The case in Table 3 is where three packets in sequence have long
transit times (Packets with s = 4,5,and 6). Delay, Late time, and
Byte Offset capture this very well, and indicate variation in
reordering extent, while IPDV indicates that the spacing between
packets 4,5,and 6 has changed.
The histogram of Reordering extents (e) would be:
Bin 1 2 3 4 5 6 7
Frequency 0 0 0 1 1 1 0
Using the notation <s[1], ..., s[i], ..., s[l]>, the received
packets are represented as:
s = 1, 2, 3, 7, 8, 9,10, 4, 5, 6, 11
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11
We first calculate the n-reordering. Considering Packet 4 first:
when n=1, 7<=j<8, and 10> 4, so the packet is 1-reordered.
when n=2, 6<=j<8, and 9 > 4, so the packet is 2-reordered.
when n=3, 5<=j<8, and 8 > 4, so the packet is 3-reordered.
when n=4, 4<=j<8, and 7 > 4, so the packet is 4-reordered.
when n=5, 3<=j<8, but 3 < 4, and 4 is the maximum n-reordering.
Considering packet 5[9] next:
when n=1, 8<=j<9, but 4 < 5, so the packet at i=9 is not designated
as n-reordered. We find the same to for Packet 6.
We now consider whether reordered Packets 5 and 6 are associated
with the same reordering discontinuity as Packet 4. Using the test
of Section 4.2.3, we find that the minimum j=4 for all three
packets. They are all associated with the reordering discontinuity
at Packet 7.
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This example shows again that the n-reordering definition identifies
a single Packet (4) with a sufficient degree of n-reordering that
might cause one unnecessary packet retransmission by the New Reno
TCP sender (with DUP-ACK threshold=3 or 4). Also, the reordered
arrival of Packets 5 and 6 will allow the receiver process to pass
Packets 7 through 10 up the protocol stack (the singleton Type-P-
Reordered = TRUE for Packets 5 and 6, and they are all associated
with a single reordering discontinuity).
7.4 Example with Multiple Packet Reordering Discontinuities
Table 4 Example with Multiple Packet Reordering Discontinuities
Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16
Discontinuity Discontinuity
|---------Gap---------|
s = 1, 2, 3, 6, 7, 4, 5, 8, 9, 10, 12, 13, 11, 14, 15, 16
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16
r = 1, 2, 3, 4, 5, 0, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, ...
number of runs,n = 1 2 3
end r counts = 5 0 5
(these values are computed after the packet arrives)
Packet 4 has extent e=2, Packet 5 has extent e=3, and Packet 11 has
e=2. There are two different reordering discontinuities, one at
Packet 6 (where j=4) and one at Packet 12 (where j'=11).
According to the definition of Reordering Gap
Gap(s[j']) = (j') - (j)
Gap(Packet 12) = (11) - (4) = 7
We also have three reordering-free runs of lengths 5, 0, and 5.
The differences between these two multiple-event metrics are evident
here. Gaps are the distance between sequence discontinuities that
are subsequently defined as reordering discontinuities, while
reordering-free runs capture the distance between reordered packets.
8. Security Considerations
8.1 Denial of Service Attacks
This metric requires a stream of packets sent from one host (source)
to another host (destination) through intervening networks. This
method could be abused for denial of service attacks directed at
destination and/or the intervening network(s).
Administrators of source, destination, and the intervening
network(s) should establish bilateral or multi-lateral agreements
Morton, et al. Standards Track exp. November 2006 Page 29
Packet Reordering Metric for IPPM May 2006
regarding the timing, size, and frequency of collection of sample
metrics. Use of this method in excess of the terms agreed between
the participants may be cause for immediate rejection or discard of
packets or other escalation procedures defined between the affected
parties.
8.2 User Data Confidentiality
Active use of this method generates packets for a sample, rather
than taking samples based on user data, and does not threaten user
data confidentiality. Passive measurement must restrict attention to
the headers of interest. Since user payloads may be temporarily
stored for length analysis, suitable precautions MUST be taken to
keep this information safe and confidential. In most cases, a
hashing function will produce a value suitable for payload
comparisons.
8.3 Interference with the Metric
It may be possible to identify that a certain packet or stream of
packets is part of a sample. With that knowledge at the destination
and/or the intervening networks, it is possible to change the
processing of the packets (e.g. increasing or decreasing delay) that
may distort the measured performance. It may also be possible to
generate additional packets that appear to be part of the sample
metric. These additional packets are likely to perturb the results
of the sample measurement. The likely consequences of packet
injection are that the additional packets would be declared
duplicates, or that the original packets would be seen as duplicates
(if they arrive after the corresponding injected packets) causing
invalid measurements on the injected packets.
The requirements for data collection resistance to interference by
malicious parties and mechanisms to achieve such resistance are
available in other IPPM memos. A set of requirements for a data
collection protocol can be found in [RFC3763], and a protocol
specification for the One-Way Active Measurement Protocol (OWAMP) is
in [RFCyyyy]. The security considerations sections of the two OWAMP
documents are extensive and should be consulted for additional
details.
9. IANA Considerations
Metrics defined in this memo are designed to be registered in the
IANA IPPM METRICS REGISTRY as described in initial version of the
registry [RFC4148].
IANA is asked to register the following metrics in the IANA-IPPM-
METRICS-REGISTRY-MIB (where RFC xxxx is replaced with the number of
this memo):
ietfReorderedSingleton OBJECT-IDENTITY
Morton, et al. Standards Track exp. November 2006 Page 30
Packet Reordering Metric for IPPM May 2006
STATUS current
DESCRIPTION
"Type-P-Reordered"
REFERENCE
"Reference RFC xxxx, section 3"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderedPacketRatio OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Reordered-Ratio-Stream"
REFERENCE
"Reference RFC xxxx, section 4.1"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingExtent OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-Extent-Stream"
REFERENCE
"Reference RFC xxxx, section 4.2"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingLateTimeOffset OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Late-Time-Stream"
REFERENCE
"Reference RFC xxxx, section 4.3"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingByteOffset OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Byte-Offset-Stream"
REFERENCE
"Reference RFC xxxx, section 4.4"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingGap OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-Gap-Stream"
REFERENCE
"Reference RFC xxxx, section 4.5"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingGapTime OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-GapTime-Stream"
REFERENCE
Morton, et al. Standards Track exp. November 2006 Page 31
Packet Reordering Metric for IPPM May 2006
"Reference RFC xxxx, section 4.5"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingFreeRunx OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-Free-Run-x-numruns-Stream"
REFERENCE
"Reference RFC xxxx, section 4.6"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingFreeRunq OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-Free-Run-q-squruns-Stream"
REFERENCE
"Reference RFC xxxx, section 4.6"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingFreeRunp OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-Free-Run-p-numpkts-Stream"
REFERENCE
"Reference RFC xxxx, section 4.6"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfReorderingFreeRuna OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-Reordering-Free-Run-a-accpkts-Stream"
REFERENCE
"Reference RFC xxxx, section 4.6"
::= { ianaIppmMetrics nn } -- IANA assigns nn
ietfnReordering OBJECT-IDENTITY
STATUS current
DESCRIPTION
"Type-P-Packet-n-Reordering-Stream"
REFERENCE
"Reference RFC xxxx, section 5"
::= { ianaIppmMetrics nn } -- IANA assigns nn
10. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
Obtain via: http://www.rfc-editor.org/rfc/rfc791.txt
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
Obtain via: http://www.rfc-editor.org/rfc/rfc2119.txt
Morton, et al. Standards Track exp. November 2006 Page 32
Packet Reordering Metric for IPPM May 2006
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and Mathis, M.,
"Framework for IP Performance Metrics", RFC 2330, May
1998.
Obtain via: http://www.rfc-editor.org/rfc/rfc2330.txt
[RFC2460] Deering, S. and Hinden, R., "Internet Protocol Version 6
(IPv6) Specification", RFC 2460, December 1998.
Obtain via: http://www.rfc-editor.org/rfc/rfc2460.txt
[RFC3148] Mathis, M. and Allman, M., "A Framework for Defining
Empirical Bulk Transfer Capacity Metrics", RFC 3148, July
2001.
Obtain via: http://www.rfc-editor.org/rfc/rfc3148.txt
[RFC3432] Raisanen, V., Grotefeld, G., and Morton, A., "Network
performance measurement with periodic streams", RFC 3432,
November 2002.
Obtain via: http://www.rfc-editor.org/rfc/rfc3432.txt
[RFC3763] Shalunov, S. and Teitelbaum, B., "One-way Active
Measurement Protocol (OWAMP) Requirements", RFC 3763,
April 2004.
Obtain via: http://www.rfc-editor.org/rfc/rfc3763.txt
[RFC4148] Stephan, E., "IP Performance Metrics (IPPM) Metrics
Registry", RFC 4148, BCP 108, August 2005.
Obtain via: http://www.rfc-editor.org/rfc/rfc4148.txt
[RFCyyyy] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and
Zeckauskas, M., "One-way Active Measurement Protocol
(OWAMP) Requirements", RFC yyyy, 2006.
Obtain via: http://www.rfc-editor.org/rfc/rfcyyyy.txt
11. Informative References
[Bel02] J.Bellardo and S.Savage, "Measuring Packet Reordering,"
Proceedings of the ACM SIGCOMM Internet Measurement
Workshop 2002, November 6-8, Marseille, France.
[Ben99] J.C.R.Bennett, C.Partridge, and N.Shectman, "Packet
Reordering is Not Pathological Network Behavior,"
IEEE/ACM Transactions on Networking, vol.7, no.6, pp.789-
798, December 1999.
[Cia00] L.Ciavattone and A.Morton, "Out-of-Sequence Packet
Parameter Definition (for Y.1540)", Contribution number
T1A1.3/2000-047, October 30, 2000.
ftp://ftp.t1.org/pub/t1a1/2000-A13/0a130470.doc
Morton, et al. Standards Track exp. November 2006 Page 33
Packet Reordering Metric for IPPM May 2006
[Cia03] L.Ciavattone, A.Morton, and G.Ramachandran, "Standardized
Active Measurements on a Tier 1 IP Backbone," IEEE
Communications Mag., pp 90-97, June 2003.
[I.356] ITU-T Recommendation I.356, "B-ISDN ATM layer cell
transfer performance", March 2000.
[Jai02] S.Jaiswal et al., "Measurement and Classification of Out-
of-Sequence Packets in a Tier-1 IP Backbone," Proceedings
of the ACM SIGCOMM Internet Measurement Workshop 2002,
November 6-8, Marseille, France.
[Lou01] D.Loguinov and H.Radha, "Measurement Study of Low-bitrate
Internet Video Streaming", Proceedings of the ACM SIGCOMM
Internet Measurement Workshop 2001 November 1-2, 2001,
San Francisco, USA.
[Mat03] M. Mathis, J Heffner and R Reddy, "Web100: Extended TCP
Instrumentation for Research, Education and Diagnosis",
ACM Computer Communications Review, Vol 33, Num 3, July
2003. http://www.web100.org/docs/mathis03web100.pdf
[Pax98] V.Paxson, "Measurements and Analysis of End-to-End
Internet Dynamics," Ph.D. dissertation, U.C. Berkeley,
1997, ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.ps.gz.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
Obtain via: http://www.rfc-editor.org/rfc/rfc793.txt
[RFC1323] Jacobson, V., Braden, R., and Borman, D., "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2581] Allman, M., Paxson, V., and Stevens, W., "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2679] Almes, G., Kalidindi, S., and Zekauskas, M., "A One-way
Delay Metric for IPPM", RFC 2679, September 1999.
Obtain via: http://www.rfc-editor.org/rfc/rfc2679.txt
[RFC2680] Almes, G., Kalidindi, S., and Zekauskas, M., "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999.
Obtain via: http://www.rfc-editor.org/rfc/rfc2680.txt
[RFC2960] Stewart, R., et al., "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3393] Demichelis, C., and Chimento, P., "IP Packet Delay
Variation Metric for IP Performance Metrics (IPPM)", RFC
3393, November 2002.
Morton, et al. Standards Track exp. November 2006 Page 34
Packet Reordering Metric for IPPM May 2006
[RFC4340] Kohler, E., Handley, M. and Floyd, S., "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March
2006.
[RFC4341] Floyd, S. and Kohler, E., "Profile for Datagram
Congestion Control Protocol (DCCP) Congestion Control ID
2: TCP-like Congestion Control", RFC 4341, March 2006.
[TBABAJ02] T. Banka, A. A. Bare, A. P. Jayasumana, "Metrics for
Degree of Reordering in Packet Sequences", Proc. 27th
IEEE Conference on Local Computer Networks, Tampa, FL,
Nov. 2002.
[Y.1540] ITU-T Recommendation Y.1540, "Internet protocol data
communication service - IP packet transfer and
availability performance parameters", December 2002.
12. Acknowledgments
The authors would like to acknowledge many helpful discussions with
Matt Zekauskas, Jon Bennett (who authored the sections on
Reordering-Free Runs), and Matt Mathis. We thank David Newman, Henk
Uijterwaal, Mark Allman, Vern Paxson, and Phil Chimento for their
reviews and suggestions, and Michal Przybylski for sharing
implementation experiences with us on the ippm-list. Anura
Jayasumana and Nischal Piratla brought in recent work-in-progress
[TBABAJ02]. We gratefully acknowledge the foundation laid by the
authors of the IP performance Framework [RFC2330].
13. Appendix A Example Implementations in C (Informative)
Two example c-code implementations of reordering definitions follow:
Example 1 n-reordering ============================================
#include <stdio.h>
#define MAXN 100
#define min(a, b) ((a) < (b)? (a): (b))
#define loop(x) ((x) >= 0? x: x + MAXN)
/*
* Read new sequence number and return it. Return a sentinel value
* of EOF (at least once) when there are no more sequence numbers.
* In this example, the sequence numbers come from stdin;
* in an actual test, they would come from the network.
*
*/
Morton, et al. Standards Track exp. November 2006 Page 35
Packet Reordering Metric for IPPM May 2006
int
read_sequence_number()
{
int res, rc;
rc = scanf("%d\n", &res);
if (rc == 1) return res;
else return EOF;
}
int
main()
{
int m[MAXN]; /* We have m[j-1] == number of
* j-reordered packets. */
int ring[MAXN]; /* Last sequence numbers seen. */
int r = 0; /* Ring pointer for next write. */
int l = 0; /* Number of sequence numbers read. */
int s; /* Last sequence number read. */
int j;
for (j = 0; j < MAXN; j++) m[j] = 0;
for (;(s = read_sequence_number())!= EOF;l++,r=(r+1)%MAXN) {
for (j=0; j<min(l, MAXN)&&s<ring[loop(r-j-1)];j++) m[j]++;
ring[r] = s;
}
for (j = 0; j < MAXN && m[j]; j++)
printf("%d-reordering = %f%%\n", j+1, 100.0*m[j]/(l-j-1));
if (j == 0) printf("no reordering\n");
else if (j < MAXN) printf("no %d-reordering\n", j+1);
else printf("only up to %d-reordering is handled\n", MAXN);
exit(0);
}
/* Example 2 singleton and n-reordering comparison =======
Author: Jerry Perser 7-2002 (mod by acm 12-2004)
Compile: $ gcc -o jpboth file.c
Usage: $ jpboth 1 2 3 7 8 4 5 6 (pkt sequence given on cmdline)
Note to cut/pasters: line 59 may need repair
*/
#include <stdio.h>
#define MAXN 100
#define min(a, b) ((a) < (b)? (a): (b))
#define loop(x) ((x) >= 0? x: x + MAXN)
/* Global counters */
int receive_packets=0; /* number of received */
int reorder_packets_Al=0; /* num reordered pkts (singleton) */
Morton, et al. Standards Track exp. November 2006 Page 36
Packet Reordering Metric for IPPM May 2006
int reorder_packets_Stas=0; /* num reordered pkts(n-reordering)*/
/* function to test if current packet has been reordered
* returns 0 = not reordered
* 1 = reordered
*/
int testorder1(int seqnum) // Al
{
static int NextExp = 1;
int iReturn = 0;
if (seqnum >= NextExp) {
NextExp = seqnum+1;
} else {
iReturn = 1;
}
return iReturn;
}
int testorder2(int seqnum) // Stanislav
{
static int ring[MAXN]; /* Last sequence numbers seen. */
static int r = 0; /* Ring pointer for next write */
int l = 0; /* Number of sequence numbers read. */
int j;
int iReturn = 0;
l++;
r = (r+1) % MAXN;
for (j=0; j<min(l, MAXN) && seqnum<ring[loop(r-j-1)]; j++)
iReturn = 1;
ring[r] = seqnum;
return iReturn;
}
int main(int argc, char *argv[])
{
int i, packet;
for (i=1; i< argc; i++) {
receive_packets++;
packet = atoi(argv[i]);
reorder_packets_Al += testorder1(packet); // singleton
reorder_packets_Stas += testorder2(packet); //n-reord.
}
printf("Received packets = %d, Singleton Reordered = %d, n-
reordered = %d\n", receive_packets, reorder_packets_Al,
reorder_packets_Stas );
exit(0);
}
Reference
Morton, et al. Standards Track exp. November 2006 Page 37
Packet Reordering Metric for IPPM May 2006
ISO/IEC 9899:1999 (E), as amended by ISO/IEC 9899:1999/Cor.1:2001
(E). Also published as:
The C Standard: Incorporating Technical Corrigendum 1, British
Standards Institute, ISBN: 0-470-84573-2, Hardcover, 558 pages,
September 2003.
14. Appendix B Fragment Order Evaluation (Informative)
Section 3 stated that fragment re-assembly is assumed prior to order
evaluation, but that similar procedures could be applied prior to
re-assembly. This appendix gives definitions and procedures to
identify reordering in a packet stream that includes fragmentation.
14.1 Metric Name
The Metric retains the same name, Type-P-Reordered, but additional
parameters are required.
This Appendix assumes that the device that divides a packet into
fragments send them according to ascending fragment offset. Early
Linux OS sent fragments in reverse order, so this possibility is
worth checking.
14.2 Additional Metric Parameters
+ MoreFrag, the state of the More Fragments Flag in the IP header
+ FragOffset, the offset from the beginning of a fragmented packet,
in 8 octet units (also from the IP header).
+ FragSeq#, the sequence number from the IP header of a fragmented
packet currently under evaluation for reordering. When set to
zero, fragment evaluation is not in progress.
+ NextExpFrag, the Next Expected Fragment Offset at the
Destination, in 8 octet units. Set to zero when fragment
evaluation is not in progress.
The packet sequence number, s, is assumed to be the same as the IP
header sequence number. Also, the value of NextExp does not change
with the in-order arrival of fragments. NextExp is only updated when
a last fragment or a complete packet arrives.
Note that packets with missing fragments MUST be declared lost, and
the Reordering status of any fragments that do arrive MUST be
excluded from sample metrics.
14.3 Definition
Morton, et al. Standards Track exp. November 2006 Page 38
Packet Reordering Metric for IPPM May 2006
The value of Type-P-Reordered is typically false (the packet is in-
order) when
* the sequence number s >= NextExp,
* AND the fragment offset FragOffset >= NextExpFrag
However, it more efficient to define reordered conditions exactly,
and designate Type-P-Reordered as False otherwise.
The value of Type-P-Reordered is defined as True (the packet is
reordered) under the conditions below. In these cases, the NextExp
value does not change.
Case 1: if s < NextExp
Case 2: if s < FragSeq#
Case 3: if s>= NextExp AND s = FragSeq# AND FragOffset < NextExpFrag
This definition can also be illustrated in pseudo-code. A version of
the code follows, and some simplification may be possible. A
challenging aspect surrounds the housekeeping for the new
parameters.
NextExp=0;
NextExpFrag=0;
FragSeq#=0;
while(packets arrive with s, MoreFrag, FragOffset)
{
if (s>=NextExp AND MoreFrag==0 AND s>=FragSeq#){
/* a normal packet or last frag of an in-order packet arrived
*/
NextExp = s+1;
FragSeq# = 0;
NextExpFrag = 0;
Reordering = False;
}
if (s>=NextExp AND MoreFrag==1 AND s>FragSeq#>=0){
/* a fragment of a new packet arrived, possibly with a
higher sequence number than the current fragmented packet */
FragSeq# = s;
NextExpFrag = FragOffset+1;
Reordering = False;
}
if (s>=NextExp AND MoreFrag==1 AND s==FragSeq#){
/* a fragment of the "current packet s" arrived */
if (FragOffset >= NextExpFrag){
NextExpFrag = FragOffset+1;
Reordering = False;
}
Morton, et al. Standards Track exp. November 2006 Page 39
Packet Reordering Metric for IPPM May 2006
else{
Reordering = True; /* fragment reordered */
}
}
if (s>=NextExp AND MoreFrag==1 AND s < FragSeq#){
/* case where a late fragment arrived,
for illustration only, redundant with else below */
Reordering = True;
}
else { /* when s < NextExp, or MoreFrag==0 AND s < FragSeq# */
Reordering = True;
}
}
A working version of the code would include a check to ensure that
all fragments of a packet arrive before using the Reordered status
further, such as in sample metrics.
14.4 Discussion: Notes on Sample Metrics when Evaluating Fragments
All fragments with the same Source Sequence Number are assigned the
same Source Time.
Evaluation with byte stream numbering may be simplified if the
fragment offset is simply added to the SourceByte of the first
packet (with fragment offset = 0), keeping the 8 octet units of the
offset in mind.
15. Disclaimer and License
Regarding this entire document or any portion of it (including the
pseudo-code and C code), the authors make no guarantees and are not
responsible for any damage resulting from its use. The authors
grant irrevocable permission to anyone to use, modify, and
distribute it in any way that does not diminish the rights of anyone
else to use, modify, and distribute it, provided that redistributed
derivative works do not contain misleading author or version
information. Derivative works need not be licensed under similar
terms.
16. Author's Addresses
Al Morton
AT&T Labs
Room D3 - 3C06
200 Laurel Ave. South
Middletown, NJ 07748 USA
Phone +1 732 420 1571
EMail: <acmorton@att.com>
Len Ciavattone
AT&T Labs
Morton, et al. Standards Track exp. November 2006 Page 40
Packet Reordering Metric for IPPM May 2006
Room A2 - 4G06
200 Laurel Ave. South
Middletown, NJ 07748 USA
Phone +1 732 420 1239
EMail: <lencia@att.com>
Gomathi Ramachandran
AT&T Labs
Room C4 - 3D22
200 Laurel Ave. South
Middletown, NJ 07748 USA
Phone +1 732 420 2353
EMail: <gomathi@att.com>
Stanislav Shalunov
Internet2
1000 Oakbrook DR STE 300
Ann Arbor, MI 48104
+1 734 995 7060
EMail: <shalunov@internet2.edu>
Jerry Perser
Veriwave
USA
Phone:
EMail: <jperser@veriwave.com>
Full Copyright Statement
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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This document and the information contained herein are provided on
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Morton, et al. Standards Track exp. November 2006 Page 41
Packet Reordering Metric for IPPM May 2006
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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Morton, et al. Standards Track exp. November 2006 Page 42