draft-ietf-ippm-reordering-01.txt   draft-ietf-ippm-reordering-02.txt 
IP Performance Metrics Working Group A.Morton Network Working Group A.Morton
Internet Draft L.Ciavattone Internet Draft L.Ciavattone
Document: <draft-ietf-ippm-reordering-01.txt> G.Ramachandran Document: <draft-ietf-ippm-reordering-02.txt> G.Ramachandran
Category: Standards Track AT&T Labs Category: Standards Track AT&T Labs
S.Shalunov S.Shalunov
Internet2 Internet2
J.Perser J.Perser
Spirent Spirent
Packet Reordering Metric for IPPM Packet Reordering Metric for IPPM
Status of this Memo Status of this Memo
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six months and may be updated, replaced, or made obsolete by other six months and may be updated, replaced, or made obsolete by other
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reference material or to cite them other than as "work in progress." reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
1. Abstract Abstract
This memo defines a simple metric to determine if a network has This memo defines a simple metric to determine if a network has
maintained packet order. It provides motivations for the new metric, maintained packet order. It provides motivations for the new metric,
suggests a metric definition, and discusses the issues associated suggests a metric definition, and discusses the issues associated
with measurement. The memo includes sample metrics to quantify the with measurement. The memo includes sample metrics to quantify the
extent of reordering in several useful dimensions. Some examples of extent of reordering in several useful dimensions. Some examples of
evaluation using the various sample metrics are included. evaluation using the various sample metrics are included.
2. Conventions used in this document 1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2]. document are to be interpreted as described in RFC 2119 [2].
Although RFC 2119 was written with protocols in mind, the key words Although RFC 2119 was written with protocols in mind, the key words
are used in this document for similar reasons. They are used to are used in this document for similar reasons. They are used to
ensure the results of measurements from two different ensure the results of measurements from two different
implementations are comparable, and to note instances when an implementations are comparable, and to note instances when an
implementation could perturb the network. implementation could perturb the network.
3. Introduction 2. Introduction
Ordered delivery is a property of successful packet transfer Ordered delivery is a property of successful packet transfer
attempts, where the packet sequence ascends for each arriving packet attempts, where the packet sequence ascends for each arriving packet
and there are no backward steps. and there are no backward steps.
An explicit sequence number, such as the sending time of each packet An explicit sequence number, such as an incrementing message number
or an incrementing message number carried in each packet establishes or the packet sending time carried in each packet, establishes the
the Source Sequence. Source Sequence.
The presence of reordering at the Destination is based on arrival The presence of reordering at the Destination is based on arrival
order. order.
This metric is consistent with RFC 2330 [3], and classifies arriving This metric is consistent with RFC 2330 [3], and classifies arriving
packets with sequence numbers smaller than their predecessors as packets with sequence numbers smaller than their predecessors as
out-of-order, or reordered. For example, if arriving packets are out-of-order, or reordered. For example, if arriving packets are
numbered 1,2,4,5,3, then packet 3 is reordered. This is equivalent numbered 1,2,4,5,3, then packet 3 is reordered. This is equivalent
to Paxon's reordering definition in [4], where "late" packets were to Paxon's reordering definition in [4], where "late" packets were
declared reordered. The alternative is to emphasize "premature" declared reordered. The alternative is to emphasize "premature"
packets instead (4 and 5 in the example). The metric's construction packets instead (4 and 5 in the example), but only the arrival of
is very similar to the sequence space validation for received packet 3 distinguishes this circumstance from packet loss. Focusing
segments in RFC793 [5]. Earlier work to define ordered delivery attention on late packets allows us to maintain orthogonality with
includes [6], [7] and more ???. 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 and
more ???.
3.1 Motivation 2.1 Motivation
A reordering metric is relevant for most applications, especially A reordering metric is relevant for most applications, especially
when assessing network support for Real-Time media streams. The when assessing network support for Real-Time media streams. The
extent of reordering may be sufficient to cause a received packet to extent of reordering may be sufficient to cause a received packet to
be discarded by functions above the IP layer. be discarded by functions above the IP layer.
Packet order is not expected to change during transfer, but several Packet order is not expected to change during transfer, but several
specific path characteristics can cause their order to change. specific path characteristics can cause their order to change.
Examples are: Examples are:
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buffers have different occupations and/or service rates, then buffers have different occupations and/or service rates, then
order will likely change. order will likely change.
The ability to restore order at the destination will likely have The ability to restore order at the destination will likely have
finite limits. Practical hosts have receiver buffers with finite finite limits. Practical hosts have receiver buffers with finite
size in terms of packets, bytes, or time (such as de-jitter size in terms of packets, bytes, or time (such as de-jitter
buffers). Once the initial determination of reordering is made, it buffers). Once the initial determination of reordering is made, it
is useful to quantify the extent of reordering, or lateness, in all is useful to quantify the extent of reordering, or lateness, in all
meaningful dimensions. meaningful dimensions.
3.2 Goals and Objectives 2.2 Goals and Objectives
The definitions below intend to satisfy the goals of: The definitions below intend to satisfy the goals of:
1. Determining whether or not packet order is maintained. 1. Determining whether or not packet order is maintained.
2. Quantifying the extent (achieving this second goal requires 2. Quantifying the extent (achieving this second goal requires
assumptions of upper layer functions and capabilities to assumptions of upper layer functions and capabilities to
restore order, and therefore several solutions). restore order, and therefore several solutions).
Reordering Metrics MUST: Reordering Metrics MUST:
+ be relevant to one or more known applications + be relevant to one or more known applications
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+ work with Poisson and Periodic test streams + work with Poisson and Periodic test streams
+ work even if the stream has duplicate or lost packets + work even if the stream has duplicate or lost packets
Reordering Metrics SHOULD: Reordering Metrics SHOULD:
+ have concatenating results for segments measured separately + have concatenating results for segments measured separately
+ have simplicity for easy consumption and understanding + have simplicity for easy consumption and understanding
+ have relevance to TCP performance + have relevance to TCP performance
+ have relevance to Real-time application performance + have relevance to Real-time application performance
4. An Ordered Arrival Singleton Metric 3. An Ordered Arrival Singleton Metric
The IPPM framework RFC 2330 [3] gives the definitions of singletons, The IPPM framework RFC 2330 [3] gives the definitions of singletons,
samples, and statistics. samples, and statistics.
The evaluation of packet order requires several supporting concepts. The evaluation of packet order requires several supporting concepts.
The first is a sequence number applied to packets at the source to The first is a sequence number applied to packets at the source to
uniquely identify the order of packet transmission. The sequence uniquely identify the order of packet transmission. The sequence
number may be established by a simple message number, a byte stream 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 number, or it may be the actual time when each packet departs from
the Src. the Src.
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Expected (NextExp) is the sequence number of the previous packet Expected (NextExp) is the sequence number of the previous packet
(plus 1 for message numbering). In byte stream numbering, NextExp (plus 1 for message numbering). In byte stream numbering, NextExp
is a value 1 byte greater than the last in-order packet sequence is a value 1 byte greater than the last in-order packet sequence
number + payload. If Src time is used as the sequence number, number + payload. If Src time is used as the sequence number,
NextExp is the Src time from the last in-order packet + 1 clock NextExp is the Src time from the last in-order packet + 1 clock
tick. tick.
Each packet within a packet stream can be evaluated for its order Each packet within a packet stream can be evaluated for its order
singleton metric. singleton metric.
4.1 Metric Name: 3.1 Metric Name:
Type-P-Non-Reversing-Order Type-P-Non-Reversing-Order
4.2 Metric Parameters: 3.2 Metric Parameters:
+ Src, the IP address of a host + Src, the IP address of a host
+ Dst, the IP address of a host + Dst, the IP address of a host
+ SrcTime, the time of packet emission from the Src (or wire time) + SrcTime, the time of packet emission from the Src (or wire time)
+ SrcNum, the packet sequence number applied at the Src, in units + s, the packet sequence number applied at the Src, in units of
of messages or bytes. messages.
+ SrcByte, the packet sequence number applied at the Src, in units
of payload bytes.
+ NextExp, the Next Expected Sequence number at the Dst, in units + NextExp, the Next Expected Sequence number at the Dst, in units
of messages, time, or bytes. of messages, time, or bytes.
+ PayloadSize, the number of bytes contained in the information + PayloadSize, the number of bytes contained in the information
field and referred to when the SrcNum sequence is based on byte field and referred to when the SrcByte sequence is based on byte
transfer. transfer.
4.3 Definition: 3.3 Definition:
In-order packets have sequence numbers (or Src times) greater than In-order packets have sequence numbers (or Src times) greater than
or equal to the value of Next Expected. Each new in-order packet or equal to the value of Next Expected. Each new in-order packet
will increase the Next Expected (typically by 1 for message will increase the Next Expected (typically by 1 for message
numbering, or the payload size plus 1 for byte numbering). The Next numbering, or the payload size plus 1 for byte numbering). The Next
Expected value cannot decrease, thereby specifying non-reversing Expected value cannot decrease, thereby specifying non-reversing
order as the basis to identify reordered packets. order as the basis to identify reordered packets.
A reordered packet outcome occurs when a single IP packet at the Dst A reordered packet outcome occurs when a single IP packet at the Dst
Measurement Point results in the following: Measurement Point results in the following:
The packet has a Src sequence number lower than the Next Expected The packet has a Src sequence number lower than the Next Expected
(NextExp), and therefore the packet is reordered. The Next Expected (NextExp), and therefore the packet is reordered. The Next Expected
value does not change on the arrival of this packet. value does not change on the arrival of this packet.
This definition can also be specified in pseudo-code. This definition can also be specified in pseudo-code.
On successful arrival of a packet with sequence number n: On successful arrival of a packet with sequence number s:
if n >= NextExp, /* n is in-order */ if s >= NextExp, /* s is in-order */
then then
NextExp = n + PayloadSize + 1; NextExp = s + PayloadSize + 1;
else /* when n < NextExp */ else /* when s < NextExp */
designate packet n as reordered; designate packet s as reordered;
When using message-based sequence numbering or Src time, The sequence number s may be replaced by SrcTime or SrcByte. When
using s (message-based sequence numbering) or Src time,
PayloadSize=0. PayloadSize=0.
4.4 Discussion 3.4 Discussion
Any arriving packet bearing a sequence number from the sequence that Any arriving packet bearing a sequence number from the sequence that
establishes the Next Expected value can be evaluated to determine if establishes the Next Expected value can be evaluated to determine if
it is in-order, or reordered, based on a previous packet's arrival. it is in-order, or reordered, based on a previous packet's arrival.
In the case where Next Expected is Undefined (because the arriving In the case where Next Expected is Undefined (because the arriving
packet is the first successful transfer), the packet is designated packet is the first successful transfer), the packet is designated
in-order. in-order.
5. Sample Metrics 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 (additional copies would be declared
reordered according to the definition above). This requirement has
the same storage requirements as earlier IPPM metrics, and follows
the precedent of RFC 2679.
It is highly desirable to assert the degree to which a packet is Packets with s > NextExp are a special case of in-order delivery.
out-of-order, or reordered with respect to a sample of packets. This This condition indicates a sequence discontinuity, either because of
section defines several metrics that quantify the extent of packet loss or reordering. Reordered packets must arrive for the
reordering in various units of measure. Each metric highlights a sequence discontinuity to be part of a reordering event (defined in
relevant application. the next section). Discontinuities are easiest to detect with
message numbering or payload byte numbering where payload size is
constant, and may be possible with Periodic Streams and Src Time
numbering.
5.1 n-Reordering 4. Sample Metrics
[Note: This is the 10/2002 definition of n-Reordering. This In this section, we define metrics applicable to a sample of packets
definition focuses on TCP sender and receiver behavior, and in from a single Src sequence number system. We begin with a simple
particular, New Reno TCP behavior when n=3.] ratio metric indicating the reordered portion of the sample. When
this ratio is zero, no further reordering metrics are needed for
that sample.
Metric Name: Type-P-packet-n-reordering-Poisson/Periodic-Stream When reordering occurs, it is highly desirable to assert the degree
to which a packet is out-of-order, or reordered with respect to a
sample of packets. This section defines several metrics that
quantify the extent of reordering in various units of measure. Each
"extent" metric highlights a relevant application.
Parameter Notation: Let n be a positive integer (a parameter). Let 4.1 Reordered Packet Ratio
k be a positive integer (sample size, the number of packets sent).
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. Let s[1], ...,
s[l] be the original sequence numbers of the received packets, in
the order of arrival (duplicates are possible).
Definition 1: Received packet number i (n < i <= l) is called n- 4.1.1 Metric Name:
reordered if and only if for all j such that i-n <= j < i we have
s[j] > s[i].
Note: This definition is illustrated by C code in Appendix A. It Type-P-Reordered-Ratio-Poisson/Periodic-Stream
computes n-reordering for a particular value of n (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).
Claim: If a packet is n-reordered and 0 < n' < n, then the packet is 4.1.2 Metric Parameters:
also n'-reordered.
Let m be the number of n-reordered packets in the sample. The parameter set includes Type-P-Non-Reversing-Order singleton
parameters, the parameters unique to Poisson or Periodic Streams,
plus the following:
Definition 2: The degree of n-reordering of the sample is m/(l-n). + T0, a start time
Definition 3: The degree of reordering of the sample is its degree + Tf, an end time
of 1-reordering.
<<<<Ed.Note - Def. 3 is no longer true using Definition 1. Blocks of
reordered packets are not classified in/out-of order equivalently by
singleton metric in section 4. See the examples in Table 2 and 3 in
section 7. It appears that packets with 1-reordering and higher may
be a subset of the reordered packets as designated by the singleton,
and this is TBD.
<<<<Ed.Note - Need to add a short subsection to define the metrics + dT, a waiting time for each packet to arrive
on "proportion of reordered packets in the sample".
Definition 4: A sample is said to have no reordering if its degree 4.1.3 Definition:
of reordering is 0.
Discussion: For the packets arriving successfully between T0 and Tf+dT, the
ratio of reordered packets in the sample is
The degree of n-reordering may be expressed as a percentage, in (Total of Reordered packets) / (Total packets received)
which case the number from definition 2 is multiplied by 100.
For a given sample, the number of n-reordered packets is the number This fraction may be expressed as a percentage (multiply by 100%).
of packets that would be considered as good as lost by a receiver Note that in the case of duplicate packets, only the first copy is
that uses a buffer of n packets to correct reordering. used.
Important special cases are n=1 and n=3: 4.2 Reordering Events and their Extent
Note: This section is expected to evolve further as we integrate the
various metrics of reordering extent (n-reordering and other
distance metrics used in earlier drafts). The co-authors are not yet
satisfied with all aspects of this section, and comments are
welcome.
- For n=1, absence of 1-reordering means the sequence numbers that 4.2.1 Metric Name:
the receiver sees are monotonically increasing with respect to the
previous arriving packet.
- For n=3, a NewReno TCP sender would retransmit 3-reordered packets Type-P-packet-n-Reordering-Event-Poisson/Periodic-Stream
and therefore consider 3-reordering a loss event for the purposes of
congestion control (the sender will half its congestion window). 3-
reordering is useful for determining the portion of reordered
packets that are in fact as good as lost.
n-reordering is particularly useful for determining the portion of 4.2.2 Parameter Notation:
reordered packets which 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).
5.2 Reordering Offset Let n be a positive integer (a parameter). Let k be a positive
integer (sample size, the number of packets sent). 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.
Any packet whose sequence number causes the Next Expected value to Let s[1], s[2], ..., s[l] be the original sequence numbers of the
increment by more than the usual increment indicates a discontinuity received packets, in the order of arrival.
in the sequence. From this point on, any packets with sequence
number less than the Next Expected value can be assigned Offset
values indicating their position (in packets or bytes) and lateness
in terms of time of arrival with respect to a sequence
discontinuity. The various Offset metrics are calculated only on
reordered packets, as defined in section 4.
5.2.1 Metric Name: Type-P-packet-Position-Offset-Poisson/Periodic- 4.2.3 Definition 1:
Stream
Metric Parameters: In addition to the parameters defined for Type-P- Received packet number i (n < i <= l), with Src sequence number s
Non-Reversing-Order, we specify: (s[i]), is reordered to the extent n if and only if for all j such
that i-n <= j < i we have s[j] > s[i].
+ DstOrder, numerical order in which each packet in the stream Claim: If by this definition, the extent of a packet's reordering is
arrives at Dst n and 0 < n' < n, then the packet is also reordered to the n'
extent.
Definition: Reordered packets are associated with a specific Note: This definition is illustrated by C code in Appendix A. It
sequence discontinuity by determining which earlier packet's determines the presence of n-reordering events for a particular
sequence number skipped over them. We calculate all expressions of value of n (when actually writing applications that would report the
Offset with respect to that packet. Position Offset is calculated metric, one would probably report it for several values of n, such
from a Dst Order number assigned to each packet on arrival: as 1, 2, 3, 4 -- and maybe a few more consecutive values).
Also, this definition does not assign an extent 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 assigned a reordering extent according to Definition 1.) All
reordered packets are assigned a reordering extent by associating
them with a specific reordering event, as defined below.
Position Offset = 4.2.4 Definition 2:
DstOrder(reordered packet)-DstOrder(packet at discontinuity) Note: The intent of this section is to assign a reordering extent to
all reordered packets, not just the ones identified by Definition 1.
This definition is new in this version and needs more study.
Using the notation of Section 5.1, an equivalent definition is: A packet s[i] that satisfies Definition 1 constitutes an n-
The Position Offset of Reordered Packet i is m = i-j, for Reordering Event with the following characteristics:
min{j|1<=j<i} that satisfies s[j]> s[i].
A sample's position offset may be expressed as a histogram, to 1. The maximum value of n that satisfies Definition 1 is the extent
easily summarize the extent and frequency of various offsets. of the reordering event. (Extent n is assigned to all packets
associated with this event in part 3 below.)
5.2.2 Metric Name: Type-P-packet-Late-Time-Poisson/Periodic-Stream 2. The in-order packet arrival defined as beginning the event
(having indicated a sequence discontinuity) is s[j] for j that
satisfies the following:
min{j|1<=j<i} for which s[j]> s[i]
Typically i-n=j. This aspect of a reordering event is used later in
the definition of the gap between successive events.
3. The arrival of any subsequent reordered packets with sequence
number s meeting the following condition:
s[j] > s[*] > s[i], or
(s at beginning of event) > s > (lowest s in the reordering event)
are associated with the reordering event first identified by s[i],
the sequence discontinuity that starts the event at s[j], and are
assigned the same reordering extent, n.
>>>
Comment on Part 3.: For some arrival orders, the assignment of the
same extent to all packets in a reordering event will tend to
underestimate their true extent. However, a pure position/distance
metric (as appeared in earlier versions of this draft) would tend to
overestimate the receiver storage needed. 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 would be to count any
additional in-order packets that arrive after a reordering event is
identified, and add them to the extent of the first reordered packet
(up to some counter limit of interest) for packets in the interval
s[i] < s[*] < s[j].
Those who desire "on-the-fly" calculation must assess whether such a
procedure is feasible.
Discussion:
A receiver must perform some amount of "work" to restore order to
all packets that are part of an n-reordering event. The extent n
gives the number of packets that must be held in the receiver's
buffer while waiting for the reordered packets in the sequence. As
reordered packets arrive, the amount of work stays the same if they
are all part of the same reordering event. If new reordering events
occur, the work in terms of buffer size can increase. See Examples
section (specific example to be provided).
Knowledge of the reordering extent n 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:
- For n=1, absence of 1-reordering events 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 a 3-reordering event and therefore consider this a loss
event for the purposes of congestion control (the sender will half
its congestion window). Detecting 3-reordering events is useful for
determining the portion of reordered packets that are in fact as
good as lost.
We note that the definition of n-reordering events cannot predict
the exact number of packets unnecessarily retransmitted by a TCP
sender under some circumstances, such as closely-spaced reordering
events. The definition is less complicated than a TCP implementation
where both time and position influence the sender's behavior.
A sample's reordering extents may be expressed as a histogram, to
easily summarize the frequency of various extents.
4.3 Reordering Offset
Any packet whose sequence number causes the Next Expected value to
increment by more than the usual increment indicates a discontinuity
in the sequence. From this point on, 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-Poisson/Periodic-Stream
Metric Parameters: In addition to the parameters defined for Type-P- Metric Parameters: In addition to the parameters defined for Type-P-
Non-Reversing-Order, we specify: Non-Reversing-Order, we specify:
+ DstTime, the time that each packet in the stream arrives at Dst + DstTime, the time that each packet in the stream arrives at Dst
Definition: Lateness in time is calculated using Dst times. Definition: Lateness in time is calculated using Dst times. When
packet i is reordered, and part of a reordering event with n extent
(assuming j=i-n):
Late Time = LateTime(i) = DstTime(i)-DstTime(i-n)
DstTime(reordered packet)-DstTime(packet at discontinuity)
Using similar notation to that of Section 5.1, an equivalent Alternatively, using similar notation to that of section 4.2, an
definition is: equivalent definition is:
The Late Time of Reordered Packet i is t = DstTime[i]-DstTime[j], LateTime(i) = DstTime[i]-DstTime[j], for min{j|1<=j<i} that
for min{j|1<=j<i} that satisfies s[j]>s[i], or satisfies s[j]>s[i], or SrcTime[j]>SrcTime[i].
SrcTime[j]>SrcTime[i].
5.2.3 Metric Name: Type-P-packet-Byte-Offset-Poisson/Periodic-Stream 4.3.2 Metric Name: Type-P-packet-Byte-Offset-Poisson/Periodic-Stream
Metric Parameters: We use the same parameters defined above. Metric Parameters: We use the same parameters defined above.
Definition: Byte stream offset is the sum of the payload sizes of Definition: Byte stream offset is the sum of the payload sizes of
all intervening packets between the reordered packet and the all intervening packets between the reordered packet and the
discontinuity (including the packet at the discontinuity). discontinuity (including the packet at the discontinuity).
When reordered packet has DstOrder=m For reordered packet i
Byte Offset = Sum[PayloadSize(packet, DstOrder=m-1), ByteOffset(i) = Sum[in-order packets to start of the reordering
PayloadSize(packet, DstOrder=m-2), ... event]
PayloadSize(packet at discontinuity)] = Sum[PayloadSize(packet at i-1),
PayloadSize(packet at i-2), ...
PayloadSize(packet at i-n)]
5.2.4 Discussion Alternatively, if all payload sizes are equal:
ByteOffset(i) = n * PayloadSize where n is the reordering extent.
>>>>Comment: Previous comments on the accuracy of extent n apply
here as well.
4.3.3 Discussion
The Offset metrics can predict whether reordered packets will be The Offset metrics can predict whether reordered packets will be
useful in a general, but limited receiver buffer system. The limit useful in a general receiver buffer system with finite limits. The
may be the number of bytes or packets the buffer can store, or the limit may be the number of bytes or packets the buffer can store, or
time of storage prior to a cyclic play-out instant (as with de- the time of storage prior to a cyclic play-out instant (as with de-
jitter buffers). jitter buffers).
Note that the One-way IPDV [8] gives the delay variation for a Note that the One-way IPDV [9] gives the delay variation for a
packet w.r.t. the preceding packet in the source sequence. Lateness packet w.r.t. the preceding packet in the source sequence. Lateness
and IPDV give an indication of whether a buffer at Dst has and IPDV give an indication of whether a buffer at Dst has
sufficient storage to accommodate the network's behavior and restore sufficient storage to accommodate the network's behavior and restore
order. When an earlier packet in the Src sequence is lost, IPDV will order. When an earlier packet in the Src sequence is lost, IPDV will
necessarily be undefined for adjacent packets, and Late Time may necessarily be undefined for adjacent packets, and Late Time may
provide the only way to evaluate the usefulness of a packet. provide the only way to evaluate the usefulness of a packet.
In the case of de-jitter buffers, there are circumstances where the In the case of de-jitter buffers, there are circumstances where the
receiver employs loss concealment at the intended play-out time of a receiver employs loss concealment at the intended play-out time of a
late packet. However, if this packet arrives out of order, the Late late packet. However, if this packet arrives out of order, the Late
Time determines whether the packet is still useful. IPDV no longer Time determines whether the packet is still useful. IPDV no longer
applies, because the receiver establishes a new play-out schedule applies, because the receiver establishes a new play-out schedule
with additional buffer delay to accommodate similar events in the with additional buffer delay to accommodate similar events in the
future - this requires very minimal processing. future - this requires very minimal processing.
When packets in the stream have variable sizes, it may be most When packets in the stream have variable sizes, it may be most
useful to characterize Offset in terms of the payload size(s) of useful to characterize Offset in terms of the payload size(s) of
stored packets (using byte stream numbering). stored packets (using byte stream numbering).
For a sample of packets in a stream, results may be reported as a 4.4 Gaps between multiple Reordering Events
ratio of reordered packets to total packets sent by the source
during the test. If separate reordering events can be distinguished,
then an event count may also be reported (along with the event
description, such as the number of reordered packets and their
offsets). The distribution of various Offset metrics may also be
reported and summarized as average, range, etc.
6. Measurement Issues 4.4.1 Metric Name:
Type-P-packet-Reordering-Event-Gap-Poisson/Periodic-Stream
4.4.2 Parameters:
No new parameters.
4.4.3 Definition:
A reordering event with extent n is detected according to section
4.2 with the arrival of packet s[i]. The next reordering event with
extent n' is detected at packet i', and there are no reordering
events between i and i'.
The Reordering Event Gap is the difference between the arrival
positions the packets, as shown below (assuming j=i-n):
Gap(i) = (i'-n') - (i-n)
Gaps may also be expressed in time:
GapTime(i) = DstTime(i'-n') - DstTime(i-n)
The Gaps between a sample's reordering events may be expressed as a
histogram, to easily summarize the frequency of various extents.
4.4.4 Discussion
When separate reordering events can be distinguished, then an event
count may also be reported (along with the event description, such
as the number of reordered packets and their extents or offsets).
The distribution of various metrics may also be reported and
summarized by the mode, average, range, histogram, etc.
The Gap metric may help to correlate the frequency of reordering
events with their cause.
5. Measurement Issues
The results of tests will be dependent on the time interval between The results of tests will be dependent on the time interval between
measurement packets (both at the Src, and during transport where measurement packets (both at the Src, and during transport where
spacing may change). Clearly, packets launched infrequently (e.g., spacing may change). Clearly, packets launched infrequently (e.g.,
1 per 10 seconds) are unlikely to be reordered. 1 per 10 seconds) are unlikely to be reordered.
Test streams may prefer to use a periodic sending interval so that a Test streams may prefer to use a periodic sending interval so that a
known temporal bias is maintained, also bringing simplified results known temporal bias is maintained, also bringing simplified results
analysis [Ref to npmps]. In this case, the periodic sending interval analysis (as described in RFC 3432 [10]). In this case, the periodic
should be chosen to reproduce the closest Src packet spacing sending interval should be chosen to reproduce the closest Src
expected. packet spacing expected.
<<<<Ed.Note: Need to expand this further, it is a very important <<<<Ed.Note: Need to expand this further, it is a very important
consideration. consideration.
The Non-reversing order criterion remains valid and useful when a The Non-reversing order criterion and all metrics described above
stream of packets experiences packet loss, or both loss and remain valid and useful when a stream of packets experiences packet
reordering. In other words, losses alone do not cause subsequent loss, or both loss and reordering. In other words, losses alone do
packets to be declared reordered. not cause subsequent packets to be declared reordered.
Assuming that the necessary sequence information (sequence number Assuming that the necessary sequence information (sequence number
and/or source time stamp) is included in the packet payload and/or source time stamp) is included in the packet payload
(possibly in application headers such as RTP), packet sequence may (possibly in application headers such as RTP), packet sequence may
be evaluated in a passive measurement arrangement. Also, it is be evaluated in a passive measurement arrangement. Also, it is
possible to evaluate sequence at a single point along a path, since 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 the usual need for synchronized Src and Dst Clocks may be relaxed to
some extent. some extent.
When the Src sequence is based on byte stream, or payload numbering, When the Src sequence is based on byte stream, or payload numbering,
care must be taken to avoid declaring retransmitted packets out-of- care must be taken to avoid declaring retransmitted packets
sequence. The additional reference of Src Time is one way to avoid reordered. The additional reference of Src Time is one way to avoid
this ambiguity. this ambiguity.
Since this metric definition may use sequence numbers with finite Since this metric definition may use sequence numbers with finite
range, it is possible that the sequence numbers could reach end-of- range, it is possible that the sequence numbers could reach end-of-
range and roll over to zero during a measurement. By definition, range and roll over to zero during a measurement. By definition,
the Next Expected value cannot decrease, and all packets received the Next Expected value cannot decrease, and all packets received
after a roll-over would be declared out-of-sequence. Sequence after a roll-over would be declared reordered. Sequence number
number roll-over can be avoided by using combinations of counter roll-over can be avoided by using combinations of counter size and
size and test duration where roll-over is impossible (and sequence test duration where roll-over is impossible (and sequence is reset
is reset to zero at the start). Also, message-based numbering to zero at the start). Also, message-based numbering results in
results in slower sequence consumption. There may still be cases slower sequence consumption. There may still be cases where
where methodological mitigation of this problem is desirable (e.g., methodological mitigation of this problem is desirable (e.g., long-
long-term testing). The elements of mitigation are: term testing). The elements of mitigation are:
1. There must be a test to detect if a roll-over has occurred. It 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 would be nearly impossible for the sequence numbers of successive
packets to jump by more than half the total range, so these large packets to jump by more than half the total range, so these large
discontinuities are designated as roll-over. discontinuities are designated as roll-over.
2. All sequence numbers used in computations are represented in a 2. All sequence numbers used in computations are represented in a
sufficiently large precision. The numbers have a correction applied sufficiently large precision. The numbers have a correction applied
(equivalent to adding a significant digit) whenever roll-over is (equivalent to adding a significant digit) whenever roll-over is
detected. detected.
3. Out-of-order packets coincident with sequence numbers reaching 3. Reordered packets coincident with sequence numbers reaching end-
end-of-range must also be detected for proper application of of-range must also be detected for proper application of correction
correction factor. factor.
6. Examples of Arrival Order Evaluation
7. Examples of Order Evaluation
This section provides some examples to illustrate how the non- This section provides some examples to illustrate how the non-
reversing order criterion works, and the value of viewing reordering reversing order criterion works, and the value of viewing reordering
in both the dimensions of time and position. in both the dimensions of time and position.
Table 1 gives a simple case of reordering, where one packet (the Table 1 gives a simple case of reordering, where one packet (the
packet with SrcNum=4) arrives out-of-order. Packets are arranged packet with s=4) arrives out-of-order. Packets are arranged
according to their arrival, and message numbering is used. according to their arrival, and message numbering is used.
Table 1 Example with Packet 4 Reordered, Table 1 Example with Packet 4 Reordered,
Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10 Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10
SrcNum Src Dst Dst Posit. Late s Src Dst Dst Byte Late
@Dst NextExp Time Time Delay IPDV Order Offset Time @Dst NextExp Time Time Delay IPDV Order Offset Time
1 1 0 68 68 1 1 1 0 68 68 1
2 2 20 88 68 0 2 2 2 20 88 68 0 2
3 3 40 108 68 0 3 3 3 40 108 68 0 3
5 4 80 148 68 -82 4 5 4 80 148 68 -82 4
6 6 100 168 68 0 5 6 6 100 168 68 0 5
7 7 120 188 68 0 6 7 7 120 188 68 0 6
8 8 140 208 68 0 7 8 8 140 208 68 0 7
4 9 60 210 150 82 8 4 62 4 9 60 210 150 82 8 400 62
9 9 160 228 68 0 9 9 9 160 228 68 0 9
10 10 180 248 68 0 10 10 10 180 248 68 0 10
Each column gives the following information: Each column gives the following information:
SrcNum Packet sequence number at the Source. S Packet sequence number at the Source.
NextExp The value of NextExp when the packet arrived(before NextExp The value of NextExp when the packet arrived(before
update). update).
SrcTime Packet time stamp at the Source, ms. SrcTime Packet time stamp at the Source, ms.
DstTime Packet time stamp at the Destination, ms. DstTime Packet time stamp at the Destination, ms.
Delay 1-way delay of the packet, ms. Delay 1-way delay of the packet, ms.
IPDV IP Packet Delay Variation, ms IPDV IP Packet Delay Variation, ms
IPDV = Delay(SrcNum)-Delay(SrcNum-1) IPDV = Delay(SrcNum)-Delay(SrcNum-1)
DstOrder Order in which the packet arrived at the Destination. DstOrder Order in which the packet arrived at the Destination.
Posit.Offset The Position Offset of an out-of-order packet. Byte Offset The Byte Offset of a reordered packet, in bytes.
LateTime The lateness of an out-of-order packet, ms. LateTime The lateness of a reordered packet, in ms.
We can see that when packet 4 arrives, NextExp=9, and it is declared We can see that when packet 4 arrives, NextExp=9, and it is declared
reordered. Further, we can compute the Offset of packet 4 in terms reordered. We compute the extent of the reordering event as follows:
of position (8-4=4 using DstOrder) and Late Time (210-148=62ms using
DstTime) compared to packet 5's arrival. If Dst 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 500 bytes.
In the notation of n-reordering, <s[1], ..., s[i], ..., s[l]> the Using the notation <s[1], ..., s[i], ..., s[l]>, the received
received packets are represented as: packets are represented as:
\/ \/
s = 1, 2, 3, 5, 6, 7, 8, 4, 9, 10 s = 1, 2, 3, 5, 6, 7, 8, 4, 9, 10
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
/\ /\
when n=1, 7<=J<8, and 8 > 4, so the packet at i=8 is 1-reordered. when n=1, 7<=J<8, and 8 > 4, so the reordering extent is 1 or more.
when n=2, 6<=J<8, and 7 > 4, so the packet at i=8 is 2-reordered. when n=2, 6<=J<8, and 7 > 4, so the reordering extent is 2 or more.
when n=3, 5<=J<8, and 6 > 4, so the packet at i=8 is 3-reordered. when n=3, 5<=J<8, and 6 > 4, so the reordering extent is 3 or more.
when n=4, 4<=J<8, and 5 > 4, so the packet at i=8 is 4-reordered. when n=4, 4<=J<8, and 5 > 4, so the reordering extent is 4 or more.
when n=5, 3<=J<8, but 3 < 4, no more reordering. when n=5, 3<=J<8, but 3 < 4, and 4 is the maximum extent.
We note that the Position Offset is equal to the Max(n) with n- Further, we can compute the Late Time (210-148=62ms using DstTime)
reordering. compared to packet 5's arrival. If Dst 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.
Table 2 Example with Packets 5 and 6 Reordered, Table 2 Example with Packets 5 and 6 Reordered,
Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10 Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10
SrcNum Src Dst Dst Posit. Late s Src Dst Dst Byte Late
@Dst NextExp Time Time Delay IPDV Order Offset Time @Dst NextExp Time Time Delay IPDV Order Offset Time
1 1 0 68 68 1 1 1 0 68 68 1
2 2 20 88 68 0 2 2 2 20 88 68 0 2
3 3 40 108 68 0 3 3 3 40 108 68 0 3
4 4 60 128 68 0 4 4 4 60 128 68 0 4
7 5 120 188 68 -22 5 7 5 120 188 68 -22 5
5 8 80 189 109 41 6 1 1 5 8 80 189 109 41 6 100 1
6 8 100 190 90 -19 7 2 2 6 8 100 190 90 -19 7 100 2
8 8 140 208 68 0 8 8 8 140 208 68 0 8
9 9 160 228 68 0 9 9 9 160 228 68 0 9
10 10 180 248 68 0 10 10 10 180 248 68 0 10
Table 2 shows a case where packets 5 and 6 arrive just behind packet Table 2 shows a case where packets 5 and 6 arrive just behind packet
7, so both 5 and 6 are declared out-of-order. Their positional 7, so both 5 and 6 are reordered. The Late times (189-188=1, 190-
offsets (6-5=1 and 7-5=2, using DstOrder again) and Late times (189- 188=2) are small.
188=1, 190-188=2) are small.
Using the notation <s[1], ..., s[i], ..., s[l]>, the received
packets are represented as:
In the notation of n-reordering, the received packets are
represented as:
\/ \/ \/ \/
s = 1, 2, 3, 4, 7, 5, 6, 8, 9, 10 s = 1, 2, 3, 4, 7, 5, 6, 8, 9, 10
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
/\ /\ /\ /\
Considering packet 5[6] first: Considering packet 5[6] first:
when n=1, 5<=J<6, and 7 > 5, so the packet at i=6 is 1-reordered. when n=1, 5<=J<6, and 7 > 5, so the reordering extent is 1 or more.
when n=2, 4<=J<6, but 4 < 5, same for all earlier packets. when n=2, 4<=J<6, but 4 < 5, and 1 is the maximum extent.
Considering packet 6[7] next: Considering packet 6[7] next:
when n=1, 6<=J<7, and 5 < 6, so the packet at I=7 is not n-reordered when n=1, 6<=J<7, and 5 < 6, so the packet at i=7 does not have its
for any n, even though: own reordering extent, and must be part of the same reordering event
when N=2, 5<=J<7, and 7 > 6, as packet 5[6]. Using the test of Section 4.2.4, Definition 2, we
because n-reordering requires s[j]>s[i] find that the condition is met for packet 6[7]:
for all j such that i-n <= j < i (see Definition 1 in section 5.1).
s[i] < s < s[i-n]
5[6] < 6[7] < 7[5]
A hypothetical sender/receiver pair may retransmit packet 5[8] A hypothetical sender/receiver pair may retransmit packet 5[8]
unnecessarily, since it is 1-reordered (in agreement with the unnecessarily, since it is reordered with extent n=1(in agreement
singleton metric). However, the receiver cannot advance packet 7[5] with the singleton metric). However, the receiver cannot advance
to the higher layers until after packet 6[7] arrives. Therefore, the packet 7[5] to the higher layers until after packet 6[7] arrives.
singleton metric correctly determined that 6[7] is reordered, and Therefore, the singleton metric correctly determined that 6[7] is
the n-reordering metric indicates that the hypothetical receiver can reordered, and both packets are part of a 1-reordering event.
deal with its arrival efficiently (no unnecessary retransmission).
Table 3 Example with Packets 4, 5, and 6 reordered Table 3 Example with Packets 4, 5, and 6 reordered
Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10,11 Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11
SrcNum Src Dst Dst Posit. Late s Src Dst Dst Byte Late
@Dst NextExp Time Time Delay IPDV Order Offset Time @Dst NextExp Time Time Delay IPDV Order Offset Time
1 1 0 68 68 1 1 1 0 68 68 1
2 2 20 88 68 0 2 2 2 20 88 68 0 2
3 3 40 108 68 0 3 3 3 40 108 68 0 3
7 4 120 188 68 -68 4 7 4 120 188 68 -68 4
8 8 140 208 68 0 5 8 8 140 208 68 0 5
9 9 160 228 68 0 6 9 9 160 228 68 0 6
10 10 180 248 68 0 7 10 10 180 248 68 0 7
4 11 60 250 190 122 8 4 62 4 11 60 250 190 122 8 400 62
5 11 80 252 172 -18 9 5 64 5 11 80 252 172 -18 9 400 64
6 11 100 256 156 -16 10 6 68 6 11 100 256 156 -16 10 400 68
11 11 200 268 68 0 11 11 11 200 268 68 0 11
The case in Table 3 is where three packets in sequence have long The case in Table 3 is where three packets in sequence have long
transit times (packets with SrcNum 4,5,and 6). Delay, Late time, and transit times (packets with s = 4,5,and 6). Delay, Late time, and
Position Offset capture this very well, and indicate variation in Byte Offset capture this very well, and indicate variation in
reordering extent, while IPDV indicates that the spacing between reordering extent, while IPDV indicates that the spacing between
packets 4,5,and 6 has changed. packets 4,5,and 6 has changed.
The histogram of Position Offsets would be: The histogram of Reordering extents (n) would be:
Bin 1 2 3 4 5 6 7 Bin 1 2 3 4 5 6 7
Frequency 0 0 0 1 1 1 0 Frequency 0 0 0 3 0 0 0
In the notation of n-reordering, the received packets are Using the notation <s[1], ..., s[i], ..., s[l]>, the received
represented as: packets are represented as:
s = 1, 2, 3, 7, 8, 9,10, 4, 5, 6, 11 s = 1, 2, 3, 7, 8, 9,10, 4, 5, 6, 11
i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11 i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11
Considering packet 4[8] first: Considering packet 4[8] first:
when n=1, 7<=J<8, and 10> 4, so the packet at i=8 is 1-reordered. when n=1, 7<=J<8, and 10> 4, so the reordering extent is 1 or more.
when n=2, 6<=J<8, and 9 > 4, so the packet at i=8 is 2-reordered. when n=2, 6<=J<8, and 9 > 4, so the reordering extent is 2 or more.
when n=3, 5<=J<8, and 8 > 4, so the packet at i=8 is 3-reordered. when n=3, 5<=J<8, and 8 > 4, so the reordering extent is 3 or more.
when n=4, 4<=J<8, and 7 > 4, so the packet at i=8 is 4-reordered. when n=4, 4<=J<8, and 7 > 4, so the reordering extent is 4 or more.
when n=5, 3<=J<8, but 3 < 4, same for all earlier packets. when n=5, 3<=J<8, but 3 < 4, and 4 is the maximum extent.
Considering packet 5[9] next: Considering packet 5[9] next:
when n=1, 8<=J<9, but 4 < 5, so the packet at i=9 does not have its
own reordering extent, and must be part of the same reordering event
as packet 4[8]. Using the test of Section 4.2.4, Definition 2, we
find that the condition is met for both packets 5[9] and 6[10]:
when n=1, 8<=J<9, and 4 < 5, so the packet at I=9 is not n-reordered s[i] < s < s[i-n]
4[8] < 5[9] < 7[4]
4[8] < 6[10]< 7[4]
This example shows again that the n-reordering definition identifies This example shows again that the n-reordering event definition
a single packet (SrcNum=4) with a sufficient degree of reordering to identifies a single event (s=4) with a sufficient degree of
result in one unnecessary packet retransmission by the New Reno TCP reordering to result in one unnecessary packet retransmission by the
sender. Also, the delayed arrival of SrcNum=5 and SrcNum=6 will New Reno TCP sender. Also, the reordered arrival of packets s=5 and
allow the receiver process to pass Src packets 7 through 10 up the s=6 will allow the receiver process to pass packets 7 through 10 up
protocol stack (the singleton metric indicates 5 and 6 are the protocol stack (the singleton metric indicates 5 and 6 are
reordered). reordered, and they are all part of one reordering event).
8. Security Considerations [mostly borrowed from npmps] 7. Security Considerations
8.1 Denial of Service Attacks 7.1 Denial of Service Attacks
This metric requires a stream of packets sent from one host (Src) to This metric requires a stream of packets sent from one host (Src) to
another host (Dst) through intervening networks. This method could another host (Dst) through intervening networks. This method could
be abused for denial of service attacks directed at Dst and/or the be abused for denial of service attacks directed at Dst and/or the
intervening network(s). intervening network(s).
Administrators of Src, Dst, and the intervening network(s) should Administrators of Src, Dst, and the intervening network(s) should
establish bilateral or multi-lateral agreements regarding the establish bilateral or multi-lateral agreements regarding the
timing, size, and frequency of collection of sample metrics. Use of timing, size, and frequency of collection of sample metrics. Use of
this method in excess of the terms agreed between the participants this method in excess of the terms agreed between the participants
may be cause for immediate rejection or discard of packets or other may be cause for immediate rejection or discard of packets or other
escalation procedures defined between the affected parties. escalation procedures defined between the affected parties.
8.2 User data confidentiality 7.2 User data confidentiality
Active use of this method generates packets for a sample, rather Active use of this method generates packets for a sample, rather
than taking samples based on user data, and does not threaten user than taking samples based on user data, and does not threaten user
data confidentiality. Passive measurement must restrict attention to data confidentiality. Passive measurement must restrict attention to
the headers of interest. Since user payloads may be temporarily the headers of interest. Since user payloads may be temporarily
stored for length analysis, suitable precautions MUST be taken to stored for length analysis, suitable precautions MUST be taken to
keep this information safe and confidential. keep this information safe and confidential.
8.3 Interference with the metric 7.3 Interference with the metric
It may be possible to identify that a certain packet or stream of 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 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 intervening networks, it is possible to change the processing of the
packets (e.g. increasing or decreasing delay) that may distort the packets (e.g. increasing or decreasing delay) that may distort the
measured performance. It may also be possible to generate measured performance. It may also be possible to generate
additional packets that appear to be part of the sample metric. additional packets that appear to be part of the sample metric.
These additional packets are likely to perturb the results of the These additional packets are likely to perturb the results of the
sample measurement. sample measurement.
To discourage the kind of interference mentioned above, packet To discourage the kind of interference mentioned above, packet
interference checks, such as cryptographic hash, may be used. interference checks, such as cryptographic hash, may be used.
9. IANA Considerations 8. IANA Considerations
Since this metric does not define a protocol or well-known values, Since this metric does not define a protocol or well-known values,
there are no IANA considerations in this memo. there are no IANA considerations in this memo.
10. References 9. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996. 9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement 2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", RFC 2119, March 1997.
3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework 3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework
for IP Performance Metrics", RFC 2330, May 1998. for IP Performance Metrics", RFC 2330, May 1998.
skipping to change at line 681 skipping to change at line 814
3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework 3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework
for IP Performance Metrics", RFC 2330, May 1998. for IP Performance Metrics", RFC 2330, May 1998.
4 V.Paxson, "Measurements and Analysis of End-to-End Internet 4 V.Paxson, "Measurements and Analysis of End-to-End Internet
Dynamics," Ph.D. dissertation, U.C. Berkeley, 1997, Dynamics," Ph.D. dissertation, U.C. Berkeley, 1997,
ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.ps.gz. ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.ps.gz.
5 Postel, J., "Transmission Control Protocol", STD 7, RFC 793, 5 Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981. September 1981.
Obtain via: http://www.rfc-editor.org/rfc/rfc793.txt Obtain via: http://www.rfc-editor.org/rfc/rfc793.txt
6 L.Ciavattone and A.Morton, "Out-of-Sequence Packet Parameter 6 L.Ciavattone and A.Morton, "Out-of-Sequence Packet Parameter
Definition (for Y.1540)", Contribution number T1A1.3/2000-047, Definition (for Y.1540)", Contribution number T1A1.3/2000-047,
October 30, 2000. ftp://ftp.t1.org/pub/t1a1/2000-A13/0a130470.doc 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 7 J.C.R.Bennett, C.Partridge, and N.Shectman, "Packet Reordering is
Not Pathological Network Behavior," IEEE/ACM Transactions on Not Pathological Network Behavior," IEEE/ACM Transactions on
Neteworking, vol.7, no.6, pp.789-798, December 1999. Neteworking, vol.7, no.6, pp.789-798, December 1999.
8 Demichelis, C., and Chimento, P., "IP Packet Delay Variation 8 D.Loguinov and H.Radha, "Measurement Study of Low-bitrate
Metric for IPPM", work in progress. Internet Video Streaming"' Proceedings of the ACM SIGCOMM
Internet Measurement Workshop 2001 November 1-2, 2001, San
Francisco, USA.
9 Demichelis, C., and Chimento, P., "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393, November
2002.
10 Raisanen, V., Grotefeld, G., and Morton, A., "Network performance
measurement with periodic streams", RFC 3432, November 2002.
11. Acknowledgments 11. Acknowledgments
The authors would like to acknowledge the helpful discussions with The authors would like to acknowledge many helpful discussions with
Matt Mathis and Jon Bennett. We gratefully acknowledge the Matt Mathis and Jon Bennett. We gratefully acknowledge the
foundation laid by the authors of the IP performance Framework [3]. foundation laid by the authors of the IP performance Framework [3].
12. Appendix A (informative) 12. Appendix A (informative)
Two example c-code implementations of reordering definitions follow: Two example c-code implementations of reordering definitions follow:
Example 1 n-reordering ============================================ Example 1 n-reordering ============================================
#include <stdio.h> #include <stdio.h>
 End of changes. 

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