One document matched: draft-ietf-ippm-reordering-05.txt
Differences from draft-ietf-ippm-reordering-04.txt
Network Working Group A.Morton
Internet Draft L.Ciavattone
Document: <draft-ietf-ippm-reordering-05.txt> G.Ramachandran
Category: Standards Track AT&T Labs
S.Shalunov
Internet2
J.Perser
Consultant
Packet Reordering Metric for IPPM
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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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. 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. Additional metrics quantify the frequency of
reordering and the distance between separate occurrences. We then
define metrics with a receiver analysis orientation. Several
examples of evaluation using the various sample metrics are
included. An Appendix gives extended definitions for evaluating
order with packet fragmentation.
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 RFC 2119 [2].
Although RFC 2119 was written with protocols in mind, the key words
Morton, et al. Standards Track exp. August 2004 Page 1
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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.
2. Introduction
Ordered delivery is a property of successful packet transfer
attempts, where the packet sequence ascends for each arriving packet
and there are no backward steps.
An explicit sequence number, such as an incrementing message number
or the packet sending time carried in each packet, 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.
This metric is consistent with RFC 2330 [3], 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 [4], 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 [5]. Earlier work to define ordered delivery
includes [6], [7], [8], [9], [10] and [11.
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 is not expected to change during transfer, but several
specific path characteristics can cause order to change.
Examples are:
* When two paths, one with slightly longer transfer time, support a
single packet stream or flow, then packets traversing the longer
path may arrive out-of-order. Multiple paths may be used to
achieve load balancing, or may arise from route instability.
* To increase capacity, a network device designed with multiple
processors serving a single port may reorder as a byproduct.
* A layer 2 retransmission protocol that compensates for an error-
prone link may cause packet reordering.
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* 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. Measurements most easily detect this form of reordering when
the spacing between packets is minimized. Transient reordering may
occur in response to network instability; temporary routing loops
can cause periods of extreme reordering.
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 order is maintained.
2. Quantifying the extent (achieving this second goal requires
assumptions of upper layer functions and capabilities to
restore order, and therefore several solutions).
Reordering Metrics MUST:
+ be relevant to one or more known applications
+ be computable "on the fly"
+ work with Poisson and Periodic test streams
+ work even if the stream has duplicate or lost packets
Reordering Metrics SHOULD:
+ have concatenating results for segments measured separately
+ have simplicity for easy consumption and understanding
+ have relevance to TCP performance
+ have relevance to Real-time application performance
3. A Reordered Packet Singleton Metric
The IPPM framework RFC 2330 [3] describes the notions of singletons,
samples, and statistics. For easy reference:
By a 'singleton' metric, we refer to metrics that are,
in a sense, atomic. For example, a single instance of "bulk
throughput capacity" from one host to another might be defined
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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 a sequence number applied to packets at the source to
uniquely identify the order of packet transmission. The sequence
number may be established by a simple message number, a byte stream
number, or it may be the actual time when each packet departs from
the Source.
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).
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 packet sequence number applied at the Source, in units of
messages.
+ SrcByte, the packet sequence number applied at the Source, in
units of payload bytes.
+ NextExp, the Next Expected Sequence number at the Destination, in
units of messages, time, or bytes.
+ PayloadSize, the number of bytes contained in the information
field and referred to when the SrcByte sequence is based on byte
transfer.
3.3 Definition:
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.
The value of Type-P-Reordered is defined as true if s < NextExp (the
packet is reordered). In this case, NextExp value does not change.
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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, /* s is in-order */
then
NextExp = s + 1;
Type-P-Reordered = False;
else /* when s < NextExp */
Type-P-Reordered = True
We note that when s = NextExp, the original sequence has been
maintained, and there is no discontinuity present.
3.4 Evaluation in Time or Byte Order
For the alternate sequence dimensions, in-order packets have byte
stream numbers or Source times greater than or equal to the value of
NextExp. Each new in-order packet will increase the NextExp SrcTime
plus 1 clock tick when using Source times, or to SrcByte plus the
payload size plus 1 for byte numbering. In the pseudo-code above,
SrcByte (or SrcTime) replaces the sequence number s, and we have:
if SrcByte >= NextExp, /* packet is in-order */
then
NextExp = SrcByte + PayloadSize + 1;
When using Source time, PayloadSize=0 (or a fixed time increment, if
using a reliable periodic packet source).
3.5 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
arrival. In the case where Next Expected is Undefined (because the
arriving packet is the first successful transfer), the packet is
designated in-order.
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
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storage implications as earlier IPPM metrics, and follows the
precedent of RFC 2679.
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 next section). Discontinuities are easiest to detect with
message numbering or payload byte numbering where payload size is
constant (and retransmissions are distinguished), and may be
possible with Periodic Streams and Source Time numbering.
4. Sample Metrics
In this section, we define metrics applicable to a sample of packets
from a single Source sequence number system. We begin with a simple
ratio metric indicating the reordered portion of the sample. When
this ratio is zero, no further reordering metrics are needed for
that sample.
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 "extent"
metric highlights a relevant use.
The metrics in the sub-sections below have a network
characterization orientation, but also have relevance to receiver
design.
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 or Periodic Streams (as in RFC 2330
and RFC3432), plus the following:
+ T0, a start time
+ Tf, an end time
+ dT, a waiting time for each packet to arrive
4.1.3 Definition:
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For the packets arriving successfully between T0 and Tf+dT, the
ratio of reordered packets in the sample is
(Total of Reordered packets) / (Total packets received)
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.
4.2 Reordering Extent
This section defines the extent to which packets are reordered, and
associates a specific sequence discontinuity with each reordered
packet.
4.2.1 Metric Name:
Type-P-packet-Reordering-Extent-Stream
4.2.2 Parameter Notation:
Given a stream of packets sent from a source to a destination, let K
be the total number of packets in that stream.
Assign each packet a sequence number, a consecutive integer from 1
to K in the order of packet emission.
Let L be the total number of packets received out of the K packets
sent. Recall that identical copies (duplicates) have been removed,
so L<=K.
Let s[1], s[2], ..., s[L], represent the original sequence numbers
associated with the packets in order of arrival.
Consider a reordered packet (as identified in section 3) 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].
4.2.3 Definition:
The reordering extent, e, of packet s[i] is defined to be
i-j for the smallest value of j.
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 this 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
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to restore order. We need to weigh the value of adding more
complexity in this definition against the accuracy it would provide.
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.
Those who desire "on-the-fly" calculation must assess whether such a
procedure is feasible.
4.2.4 Discussion:
The packet with index j (s[j], identified in the Definition above)
is the reordering discontinuity associated with packet with 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.
A receiver must possess storage to restore order to 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 Examples section (specific
example to be provided).
Knowledge of the reordering extent e is particularly 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 Offset
Any reordered packets can be assigned offset values indicating the
storage in bytes and lateness in terms of buffer time that a
receiver must possess to accommodate them. The various offset
metrics are calculated only on reordered packets, as identified by
the ordered arrival singleton in section 3.
4.3.1 Metric Name: Type-P-packet-Late-Time-Stream
Metric Parameters: In addition to the parameters defined for Type-P-
Reordered, we specify:
+ DstTime, the time that each packet in the stream arrives at Dst
Definition: Lateness in time is calculated using Dst times. When
received packet i is reordered, and has a reordering extent e, then:
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LateTime(i) = DstTime(i)-DstTime(i-e)
Alternatively, using similar notation to that of section 4.2, an
equivalent definition is:
LateTime(i) = DstTime(i)-DstTime(j), for min{j|1<=j<i} that
satisfies s[j]>s[i], or SrcTime[j]>SrcTime[i].
4.3.2 Metric Name: Type-P-packet-Byte-Offset-Stream
Metric Parameters: We use the same parameters defined above.
Definition: Byte stream offset is the sum of the payload sizes of
intervening in-order packets between the reordered packet and the
discontinuity (including the packet at the discontinuity).
For reordered packet i with a reordering extent e:
ByteOffset(i) = Sum[in-order packets back to reordering discon.]
= Sum[PayloadSize(packet at i-1 if in-order),
PayloadSize(packet at i-2 if in-order), ...
PayloadSize(packet at i-e if in-order)]
4.3.3 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 number of bytes or packets the buffer can
store, or the time of storage prior to a cyclic play-out instant (as
with de-jitter buffers).
Note that the One-way IPDV [12] 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 Dst has
sufficient storage to accommodate the network's behavior and restore
order. When an earlier packet in the Src sequence is lost, IPDV will
necessarily be undefined for adjacent packets, and Late Time 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.
When packets in the stream have variable sizes, it may be most
useful to characterize Offset in terms of the payload size(s) of
stored packets (using byte stream numbering).
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4.4 Gaps between multiple Reordering Discontinuities
4.4.1 Metric Name:
Type-P-packet-Reordering-Gap-Stream
4.4.2 Parameters:
No new parameters.
4.4.3 Definition of Reordering Discontinuity:
All reordered packets are associated with a packet at a reordering
discontinuity, defined as the in-order packet arrival s[j] at the
minimum value of j (1<=j<i) for which s[j]> s[i].
Recall that i - e = min(j). Subsequent reordered packets may be
associated with the same s[j], or with a different discontinuity.
This definition is used in the definition of the Reordering Gap,
below.
4.4.4 Definition of Reordering Gap:
A reordering gap is the distance between successive reordering
discontinuities. Type-P-packet-Reordering-Gap-Stream assigns a value
to (all) packets in a stream.
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(j') = (j') - (j)
Otherwise:
The Type-P-packet-Reordering-Gap-Stream for the packet is 0.
Gaps may also be expressed in time:
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GapTime(j') = DstTime(j') - DstTime(j)
4.4.5 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.
4.5 Reordering-free Runs
This section defines a metric based on a count of consecutive
packets between reordered packets.
4.5.1 Metric Name:
Type-P-packet-Reordering-Free-Run-Stream
4.5.2 Parameters:
No new parameters.
4.5.3 Definition:
As packets in a sample arrive at the Destination, the count of
packets to the next reordered packet is a Reordering-Free run. Note
that the minimum run-length is one according to this definition. A
pseudo code example follows:
r = 0; /* r is the run counter */
n = 0; /* n 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 squared sum of the run counters */
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 = 1;
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n++;
}
Each arrival of a reordered packet yields a new count in the Run
vector. Long runs accompany periods where order was maintained,
while short runs indicate frequent, or multi-packet reordering.
4.5.4 Discussion:
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 accumulator.
The percent of packets in order is 100*a/p
The average in order run length is a/n
The q counter gives an indication of variation of the in order runs
from the average by comparing q/a to a/n ((q/a)/(a/n))
For example for 36 packets with 3 runs of 11 in-order packets we
have:
p = 36
n = 3
a = 33
q = 3 * (11*11) = 363
ave io = 11
q/a = 11
(q/a)/ave = 1.0
For 36 packets with 3 runs, 2 of length 1 and one of length 31
p = 36
n = 3
a = 33
q = 1 + 1 + 961 = 963
ave io = 11
q/a = 29.18
(q/a)/ave = 2.65
5. Metric Related to Receiver Assessment
5.1 A TCP-Relevant Metric
5.1.1 Metric Name:
Type-P-packet-n-Reordering-Stream
5.1.2 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
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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.
5.1.3 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 4
is n-reordered, with n=3.)
Definition 2: The degree of n-reordering of the sample is m/l.
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.
5.1.4 Discussion:
The degree of n-reordering may be expressed as a percentage, in
which case the number from definition 2 is multiplied by 100.
Knowledge of n-reordering is particularly useful for determining the
portion of reordered packets that can or cannot be restored to order
in a typical TCP receiver buffer based on their arrival order alone
(and without the aid of retransmission).
Important special cases are n=1 and n=3:
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- 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
half its congestion window). Detecting instances of 3-reordering is
useful for determining the portion of reordered packets that are in
fact as good as lost.
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. The definition is less complicated than a TCP
implementation where both time and position influence the sender's
behavior.
A packet's n-reordering is sometimes equal to its reordering extent,
e. n-reordering is different in the following ways:
1. n is a count of *adjacent* early packets.
2. Some reordered packets may not be n-reordered, but will have e
(see the examples).
6. Measurement Issues
The results of tests will be dependent on the time interval between
measurement packets (both at the Src, and during transport where
spacing may change). Clearly, packets launched infrequently (e.g.,
1 per 10 seconds) are unlikely to be reordered.
Test streams may prefer to use a periodic sending interval so that a
known temporal bias is maintained, also bringing simplified results
analysis (as described in RFC 3432 [13]). In this case, the periodic
sending interval should be chosen to reproduce the closest Src
packet spacing expected. Of course, packet spacing is likely to vary
as the stream traverses the test path.
<<<<Ed.Note: Is this sufficient? It is a very important
consideration.
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.
Assuming that the necessary sequence information (sequence number
and/or source time stamp) is included in the packet payload
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(possibly in application headers such as RTP), packet sequence may
be evaluated in a passive measurement arrangement. Also, it is
possible to evaluate sequence at a single point along a path, since
the usual need for synchronized Src and Dst Clocks may be relaxed to
some extent.
When the Src sequence is based on byte stream, or payload numbering,
care must be taken to avoid declaring retransmitted packets
reordered. The additional reference of Src Time is one way to avoid
this ambiguity.
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,
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.
In practice, there may be limited ability to determine reordering
extent, because the storage for previous packets may be limited.
Saving only packets that indicate discontinuities (and their arrival
positions) will reduce storage volume. When discarding all stream
information beyond a threshold packet count, the reordering extent
or degree of n-reordering may need to be expressed as greater than
the threshold value, and Gap calculations would not be possible.
The requirement to ignore duplicate packets also requires storage.
Here, tracking the sequence numbers of missing packets may minimize
storage. Missing packets may eventually be declared lost, or
reordered if they arrive. The missing packet list and the largest
sequence number received thus far are sufficient information to
determine if a packet is a duplicate.
Morton, et al. Standards Track exp. August 2004 Page 15
Packet Reordering Metric for IPPM February 2004
7. Examples of Arrival Order Evaluation
This section provides some examples to illustrate how the non-
reversing order criterion works, and the value of viewing reordering
in both the dimensions of time 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.
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.
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:
Using the notation <s[1], ..., s[i], ..., s[L]>, the received
packets are represented as:
Morton, et al. Standards Track exp. August 2004 Page 16
Packet Reordering Metric for IPPM February 2004
\/
s = 1, 2, 3, 5, 6, 7, 8, 4, 9, 10
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
/\
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 also indicate
unusual behavior for Packet 4.
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 defined to be
4-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.
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:
Morton, et al. Standards Track exp. August 2004 Page 17
Packet Reordering Metric for IPPM February 2004
when j=5, 7 > 5, so the reordering extent is 1 or more.
when j=4, but 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 4.2.4. So Packet 6 is associated with the same
reordering discontinuity as Packet 5, at Packet 7.
Following the definitions of section 5.1, Packet 5 is defined to be
1-reordered, but Packet 6 is not qualified 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.
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
Morton, et al. Standards Track exp. August 2004 Page 18
Packet Reordering Metric for IPPM February 2004
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 qualified
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 4.2.4, Definition 2, we find that the minimum j=4 for all
three packets. They are all associated with the reordering
discontinuity at Packet 7.
This example shows again that the n-reordering definition identifies
a single Packet (4) with a sufficient degree of reordering to result
in one unnecessary packet retransmission by the New Reno TCP sender.
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 metric indicates 5 and 6 are reordered, and they are
all associated with a single reordering discontinuity).
Table 4 Example with Packets Multiple 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, 1, 1, 2, 3, 4, 5, 6, 1, 2, 3, 4, ...
n = 1 2 3
end r counts = 5 1 6
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(j') = (j') - (j)
Gap(11) = (11) - (4) = 7
Morton, et al. Standards Track exp. August 2004 Page 19
Packet Reordering Metric for IPPM February 2004
We also have three reordering-free runs of lengths 5, 1, and 6.
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 are 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 (Src) to
another host (Dst) through intervening networks. This method could
be abused for denial of service attacks directed at Dst and/or the
intervening network(s).
Administrators of Src, Dst, and the intervening network(s) should
establish bilateral or multi-lateral agreements 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.
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 Dst 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.
To discourage the kind of interference mentioned above, packet
interference checks, such as cryptographic hash, may be used.
9. IANA Considerations
Since this metric does not define a protocol or well-known values,
there are no IANA considerations in this memo.
Morton, et al. Standards Track exp. August 2004 Page 20
Packet Reordering Metric for IPPM February 2004
10. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework
for IP Performance Metrics", RFC 2330, May 1998.
4 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.
5 Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
Obtain via: http://www.rfc-editor.org/rfc/rfc793.txt
6 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
7 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.
8 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.
9 J.Bellardo and S.Savage, "Measuring Packet Reordering,"
Proceedings of the ACM SIGCOMM Internet Measurement Workshop
2002, November 6-8, Marseille, France.
10 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.
11 L.Ciavattone, A.Morton, and G.Ramachandran, "Standardized Active
Measurements on a Tier 1 IP Backbone," IEEE Communications Mag.,
pp 90-97, June 2003.
12 Demichelis, C., and Chimento, P., "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393, November
2002.
13 Raisanen, V., Grotefeld, G., and Morton, A., "Network performance
measurement with periodic streams", RFC 3432, November 2002.
Morton, et al. Standards Track exp. August 2004 Page 21
Packet Reordering Metric for IPPM February 2004
12. Acknowledgments
The authors would like to acknowledge many helpful discussions with
Matt Mathis, Jon Bennett, and Matt Zekauskas. We gratefully
acknowledge the foundation laid by the authors of the IP performance
Framework [3].
13. Appendix A (informative)
Two example c-code implementations of reordering definitions follow:
Example 1 n-reordering ============================================
#include <stdio.h>
#define MAX_N 100
#define min(a, b) ((a) < (b)? (a): (b))
#define loop(x) ((x) >= 0? x: x + MAX_N)
/*
* 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.
*/
int
read_sequence_number()
{
int res, rc;
rc = scanf("%d\n", &res);
if (rc == 1) return res;
else return EOF;
}
int
main()
{
int m[MAX_N]; /* We have m[j-1] == number
of
* j-reordered packets. */
int ring[MAX_N]; /* Last sequence numbers
seen. */
int r = 0; /* Ring pointer for next
write. */
int l = 0; /* Number of sequence
numbers read. */
Morton, et al. Standards Track exp. August 2004 Page 22
Packet Reordering Metric for IPPM February 2004
int s; /* Last sequence number
read. */
int j;
for (j = 0; j < MAX_N; j++) m[j] = 0;
for (; (s = read_sequence_number()) != EOF; l++, r = (r+1) %
MAX_N) {
for (j=0; j<min(l, MAX_N) && s<ring[loop(r-j-1)];
j++) m[j]++;
ring[r] = s;
}
for (j = 0; j < MAX_N && 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 < MAX_N) printf("no %d-reordering\n", j+1);
else printf("only up to %d-reordering is handled\n", MAX_N);
exit(0);
}
Example 2 singleton and n-reordering comparison =================
#include <stdio.h>
#define MAX_N 100
#define min(a, b) ((a) < (b)? (a): (b))
#define loop(x) ((x) >= 0? x: x + MAX_N)
/* Global counters */
int receive_packets=0; /* number of recieved */
int reorder_packets=0; /* number of reordered packets */
/* 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
{
Morton, et al. Standards Track exp. August 2004 Page 23
Packet Reordering Metric for IPPM February 2004
static int ring[MAX_N]; /* 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) % MAX_N;
for (j=0; j<min(l, MAX_N) && 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 += testorder2(packet);
}
printf("Received packets = %d, Reordered packets = %d\n",
receive_packets, reorder_packets);
exit(0);
}
13. Appendix on fragment order evaluation
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.
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.
13.1 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).
Morton, et al. Standards Track exp. August 2004 Page 24
Packet Reordering Metric for IPPM February 2004
+ 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.
13.2 Definition:
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 draft
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#){
Morton, et al. Standards Track exp. August 2004 Page 25
Packet Reordering Metric for IPPM February 2004
/* 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;
}
else{
Reordering = True; /* fragment reordered */
}
}
if (s>=NextExp AND MoreFrag==1 AND s < FragSeq#){
/* case where a late fragment arrived */
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.
13.3 Notes on Sample Metrics
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.
14. Author's Addresses
Al Morton
AT&T Labs
Morton, et al. Standards Track exp. August 2004 Page 26
Packet Reordering Metric for IPPM February 2004
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
Room C4 - 2B29
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
200 Business Park Drive, Suite 307
Armonk, NY 10504
Phone: + 1 914 765 1182
EMail: <shalunov@internet2.edu>
Jerry Perser
Consultant
Calabasas, CA 91302 USA
Phone: + 1
EMail: <jerry@perser.org>
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Packet Reordering Metric for IPPM February 2004
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