Network Working Group                                          A.Morton
Internet Draft                                             L.Ciavattone
Document: <draft-ietf-ippm-reordering-10.txt> <draft-ietf-ippm-reordering-11.txt>            G.Ramachandran
Category: Standards Track                                     AT&T Labs

                     Packet Reordering Metric for IPPM

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This memo defines metrics to evaluate if a network has maintained
   packet order on a packet-by-packet basis. It provides motivations
   for the new metrics and discusses the measurement issues, including
   the context information required for all metrics. The memo first
   defines a reordered singleton, and then uses it as the basis for
   sample metrics to quantify the extent of reordering in several
   useful dimensions for network characterization or receiver design.
   Additional metrics quantify the frequency of reordering and the
   distance between separate occurrences. We then define a metric
   oriented toward assessing reordering affects on TCP. Several
   examples of evaluation using the various sample metrics are
   included. An Appendix gives extended definitions for evaluating
   order with packet fragmentation.


   Status of this Memo................................................1
   Copyright Notice...................................................1
   1. Conventions used in this document...............................3
   2. Introduction....................................................3 Introduction....................................................4
   2.1 Motivation.....................................................4
   2.2 Goals and Objectives...........................................5
   2.3 Required Context for All Reordering Metrics....................6
   3. A Reordered Packet Singleton Metric.............................6
   3.1 Metric Name:...................................................7
   3.2 Metric Parameters:.............................................7
   3.3 Definition:....................................................7
   3.4 Sequence Discontinuity Definition..............................8
   3.5 Evaluation of Reordering in Dimensions of Time or Bytes........9
   3.6 Discussion.....................................................9
   4. Sample Metrics.................................................10
   4.1 Reordered Packet Ratio........................................10
   4.1.1 Metric Name:................................................10
   4.1.2 Metric Parameters:..........................................10
   4.1.3 Definition:.................................................10
   4.1.4 Discussion..................................................11
   4.2 Reordering Extent.............................................11
   4.2.1 Metric Name:................................................11
   4.2.2 Parameter Notation:.........................................11 Notation and Metric Parameters:.............................11
   4.2.3 Definition:.................................................12
   4.2.4 Discussion:.................................................12
   4.3 Reordering Late Time Offset...................................13
   4.3.1 Metric Name:................................................13
   4.3.2 Metric Parameters:..........................................13
   4.3.3 Definition:.................................................13
   4.3.4 Discussion..................................................13
   4.4 Reordering Byte Offset........................................14
   4.4.1 Metric Name:................................................14
   4.4.2 Metric Parameters:..........................................14
   4.4.3 Definition:.................................................14
   4.4.4 Discussion..................................................15
   4.5 Gaps between multiple Reordering Discontinuities..............15
   4.5.1 Metric Name:................................................15
   4.5.2 Parameters:.................................................15
   4.5.3 Definition of Reordering Discontinuity:.....................16
   4.5.4 Definition of Reordering Gap:...............................16
   4.5.5 Discussion..................................................16
   4.6 Reordering-free Runs..........................................17
   4.6.1 Metric Name:................................................17
   4.6.2 Parameters:.................................................17
   4.6.3 Definition:.................................................17
   4.6.4 Discussion:.................................................18 Discussion and Illustration:................................18
   5. Metrics Focused on Receiver Assessment: A TCP-Relevant Metric..19
   5.1 Metric Name:..................................................19
   5.2 Parameter Notation:...........................................19
   5.3 Definitions...................................................19
   5.4 Discussion:...................................................20
   6. Measurement and Implementation Issues..........................21
   7. Examples of Arrival Order Evaluation...........................24
   7.1 Example with a single packet reordered........................24
   7.2 Example with two packets reordered............................25
   7.3 Example with three packets reordered..........................27
   7.4 Example with Multiple Packet Reordering Discontinuities.......28
   8. Security Considerations........................................28
   8.1 Denial of Service Attacks.....................................28
   8.2 User data confidentiality.....................................29
   8.3 Interference with the metric..................................29
   9. IANA Considerations............................................29
   10. Normative References..........................................29
   11. Informative References........................................30
   12. Acknowledgments...............................................31
   13. Appendix A Example Implementations in C (Informative).........31
   14. Appendix B Fragment Order Evaluation (Informative)............34
   14.1 Metric Name:.................................................34
   14.2 Additional Metric Parameters:................................34
   14.3 Definition:..................................................34 Definition:..................................................35
   14.4 Discussion: Notes on Sample Metrics when evaluating Fragments36
   15. Author's Addresses............................................36
   Full Copyright Statement..........................................37
   Intellectual Property.............................................37

1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].
   Although RFC 2119 was written with protocols in mind, the key words
   are used in this document for similar reasons.  They are used to
   ensure the results of measurements from two different
   implementations are comparable, and to note instances when an
   implementation could perturb the network.

   In this memo, the characters "<=" should be read as "less than or
   equal to" and ">=" as "greater than or equal to".

2. Introduction

   Ordered arrival is a property found in packets that transit their
   path, where the packet sequence number increases with each new
   arrival and there are no backward steps. The Internet Protocol
   [RFC791] has no mechanisms to assure either packet delivery or
   sequencing, and higher layer protocols (above IP) should be prepared
   to deal with both loss and reordering. This memo defines reordering

   A unique sequence number, such as an incrementing message number
   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 [RFC2330], and classifies
   arriving packets with sequence numbers smaller than their
   predecessors as out-of-order, or reordered. For example, if
   sequentially numbered packets arrive 1,2,4,5,3, then packet 3 is
   reordered. This is equivalent to Paxon's reordering definition in
   [Pax98], where "late" packets were declared reordered. The
   alternative is to emphasize "premature" packets instead (4 and 5 in
   the example), but only the arrival of packet 3 distinguishes this
   circumstance from packet loss. Focusing attention on late packets
   allows us to maintain orthogonality with the packet loss metric. The
   metric's construction is very similar to the sequence space
   validation for received segments in RFC 793 [RFC793]. Earlier work
   to define ordered delivery includes [Cia00], [Ben99], [Lou01],
   [Bel02], [Jai02] and [Cia03].

2.1 Motivation

   A reordering metric is relevant for most applications, especially
   when assessing network support for Real-Time media streams. The
   extent of reordering may be sufficient to cause a received packet to
   be discarded by functions above the IP layer.

   Packet order may change during transfer, and several specific path
   characteristics can make reordering more likely.

   Examples are:
   * When two (or more) paths with slightly differing transfer times
     support a single packet stream or flow, then packets traversing
     the longer path(s) may arrive out-of-order. Multiple paths may be
     used to achieve load balancing, or may arise from route
   * To increase capacity, a network device designed with multiple
     processors serving a single port (or parallel links) may reorder
     as a byproduct.
   * A layer 2 retransmission protocol that compensates for an error-
     prone link may cause packet reordering.

   * If for any reason, the packets in a buffer are not serviced in the
     order of their arrival, their order will change.
   * If packets in a flow are assigned to multiple buffers (following
     evaluation of traffic characteristics, for example), and the
     buffers have different occupations and/or service rates, then
     order will likely change.

   When one or more of the above path characteristics are present
   continuously, then reordering may be present on a steady-state
   basis. The steady-state reordering condition typically causes an
   appreciable fraction of packets to be reordered. This form of
   reordering is most easily detected by minimizing the spacing between
   test packets.  Transient reordering may occur in response to network
   instability; temporary routing loops can cause periods of extreme
   reordering. This condition is characterized by long in-order streams
   with occasional instances of reordering, sometimes with extreme
   correlation. However, we do not expect packet delivery in a
   completely random order, where for example, the last packet or the
   first packet in a sample is equally likely to arrive first at the
   destination. Thus we expect at least a minimal degree of order in
   the packet arrivals, as exhibited in real networks.

   The ability to restore order at the destination will likely have
   finite limits.  Practical hosts have receiver buffers with finite
   size in terms of packets, bytes, or time (such as de-jitter
   buffers). Once the initial determination of reordering is made, it
   is useful to quantify the extent of reordering, or lateness, in all
   meaningful dimensions.

2.2 Goals and Objectives

   The definitions below intend to satisfy the goals of:

     1. Determining whether or not packet reordering has occurred.
     2. Quantifying the degree of reordering. (We define a number of
        metrics to meet this goal, because receiving procedures differ
        by protocol or application. Since the affects of packet
        reordering vary with these procedures, a metric that quantifies
        a key aspect of one receiver's behavior could be irrelevant to
        a different receiver. If all the metrics defined below are
        reported, they give a wide-ranging view of reordering

   Reordering Metrics MUST:

   +  have one or more applications, such as receiver design or network
      characterization, and a compelling relevance in the working
      group's view.
   +  be computable "on the fly"
   +  work even if the stream has duplicate or lost packets
   It is desirable for Reordering Metrics to have one or more of the
   following attributes:

   +  ability to concatenate results for segments measured separately
      to estimate the reordering of an entire path
   +  simplicity for easy consumption and understanding
   +  relevance to TCP design
   +  relevance to Real-time application performance

   The current set of metrics meet all the requirements above and
   provides all but the concatenation attribute (except in the case
   where segments exhibit no reordering, and one may estimate that the
   segment concatenation would also exhibit this desirable outcome).
   However, satisfying these goals restricts the set of metrics to
   those that provide some clear insight into network characterization
   or receiver design. They are not likely to be exhaustive in their
   coverage of reordering effects on applications, and additional
   measurements may be possible.

2.3 Required Context for All Reordering Metrics

   A critical aspect of all reordering metrics is their inseparable
   bond with the measurement conditions. Packet reordering is not well
   defined unless the full measurement context is reported. Therefore,
   all reordering metric definitions include the following parameters:

   1. The "Packet of Type-P" [RFC2330] identifiers for the packet
   stream, including the transport addresses for source and
   destination, and any other information which may result in different
   packet treatments.

   2. The stream parameter set for the sending discipline, such as the
   parameters unique to Periodic Streams (as in RFC 3432 [RFC3432]),
   TCP-like Streams (as in RFC 3148 [RFC3148]), or Poisson Streams (as
   in RFC 2330 [RFC2330]. The stream parameters include the packet
   size, either specified as a fixed value or as a pattern of sizes (as

   Whenever a metric is reported, it MUST include a description of
   these parameters to provide a context for the results.

3. A Reordered Packet Singleton Metric

   The IPPM framework RFC 2330 [RFC2330] 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
        as a singleton metric, even though the instance involves
        measuring the timing of a number of Internet packets.

   The evaluation of packet order requires several supporting concepts.
   The first is an algorithm (function) that produces a series of
   monotonically increasing identifiers applied to packets at the
   source to uniquely establish the order of packet transmission.  The
   unique sequence identifier may simply be an incrementing integer
   message number, as used below.

   The second supporting concept is a stored value which is the "next
   expected" packet number. Under normal conditions, the value of Next
   Expected (NextExp) is the sequence number of the previous packet
   plus 1 for message numbering (in general, the receiver reproduces
   the sender's algorithm and the sequence of identifiers so that the
   "next expected" can be determined).

   Each packet within a packet stream can be evaluated with this order
   singleton metric.

3.1 Metric Name:


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

   +  s, the unique packet sequence number applied at the Source, in
      units of messages.

   +  NextExp, the Next Expected Sequence number at the Destination, in
      units of messages. The stored value in NextExp is determined from
      a previously arriving packet.

   And optionally:

   +  PayloadSize, the number of bytes contained in the information
      field and referred to when the SrcByte sequence is based on bytes

   +  SrcByte, the packet sequence number applied at the Source, in
      units of payload bytes.

3.3 Definition:

   If a packet s, (sent at time, SrcTime) is found to be reordered by
   comparison with the Next Expected value, its Type-P-Reordered =
   TRUE; otherwise Type-P-Reordered = FALSE, as defined below:

   The value of Type-P-Reordered is defined as TRUE if s < NextExp (the
   packet is reordered). In this case, the NextExp value does not

   The value of Type-P-Reordered is defined as FALSE if s >= NextExp
   (the packet is in-order). In this case, NextExp is set to s+1 for
   comparison with the next packet to arrive.

   Since the Next Expected value cannot decrease, it provides a non-
   reversing order criterion to identify reordered packets.

   This definition can also be specified in pseudo-code.

   On successful arrival of a packet with sequence number s:
        if s >= NextExp then /* s is in-order */
                NextExp = s + 1;
                Type-P-Reordered = False;
        else     /* when s < NextExp */
                Type-P-Reordered = True

3.4 Sequence Discontinuity Definition

   Packets with s > NextExp are a special case of in-order delivery.
   This condition indicates a sequence discontinuity, either because of
   packet loss or reordering. Reordered packets must arrive for the
   sequence discontinuity to be defined as a reordering discontinuity
   (see section 4).

   We define two different states for in-order packets.

   When s = NextExp, the original sequence has been maintained, and
   there is no discontinuity present.

   When s > NextExp, some packets in the original sequence have not yet
   arrived, and there is a sequence discontinuity associated with
   packet s.  The size of the discontinuity is s - NextExp, equal to
   the number of packets presently missing, either reordered or lost.

   In pseudo-code:

   On successful arrival of a packet with sequence number s:
        if s >= NextExp, then /* s is in-order */
                if s > NextExp then
                          SequenceDiscontinuty = True;
                          SeqDiscontinutySize = s - NextExp;
                          SequenceDiscontinuty = False;
                NextExp = s + 1;
                Type-P-Reordered = False;

        else /* when s < NextExp */
                Type-P-Reordered = True;
                SequenceDiscontinuty = False;

   Whether there are any Sequence Discontinuities and their size is
   determined by the conditions causing loss and/or reordering along
   the measurement path. Note that a packet could be reordered at one
   point, and subsequently lost elsewhere on the path, but this cannot
   be known from observations at the Destination.

3.5 Evaluation of Reordering in Dimensions of Time or Bytes

   It is possible to use alternate dimensions of time or payload bytes
   to test for reordering in the definition of section 3.3, as long as
   the SrcTimes and SrcBytes are unique and reliable. Sequence
   Discontinuities are easily defined and detected with message
   numbering, however, this is not so simple in the dimensions of time
   or bytes. This is a detractor for the alternate dimensions because
   the Sequence Discontinuity definition plays a key role in the sample
   metrics that follow.

   It is possible to detect Sequence Discontinuities with payload byte
   numbering, but only when the complete pattern of payload sizes is
   stored at the Destination, or when payload size is constant and then
   the byte numbering adds needless complexity over message numbering.

   It may be possible to detect Sequence Discontinuities with Periodic
   Streams and Source Time numbering, but there are practical pitfalls
   with sending exactly on-schedule and with clock reliability.

   The dimensions of time and bytes remain an important basis for
   characterizing the extent of reordering, as described later.

3.6 Discussion

   Any arriving packet bearing a sequence number from the sequence that
   establishes the Next Expected value can be evaluated to determine
   whether it is in-order or reordered, based on a previous packet's
   arrival. In the case where Next Expected is Undefined (because the
   arriving packet is the first successful transfer), the packet is
   designated in-order (Type-P-Reordered=FALSE).

   This metric assumes re-assembly of packet fragments before
   evaluation. In principle, it is possible to use the Type-P-Reordered
   metric to evaluate reordering among packet fragments, but each
   fragment must contain source sequence information.
   See the Appendix on fragment order evaluation for more detail.

   If duplicate packets (multiple non-corrupt copies) arrive at the
   destination, they MUST be noted and only the first to arrive is
   considered for further analysis (copies would be declared reordered
   according to the definition above). This requirement has the same
   storage implications as earlier IPPM metrics, and follows the
   precedent of RFC 2679. We provide a suggestion to minimize storage
   size needed in the section on Measurement and Implementation Issues.

4. Sample Metrics

   In this section, we define metrics applicable to a sample of packets
   from a single Source sequence number system. When reordering occurs,
   it is highly desirable to assert the degree to which a packet is
   out-of-order, or reordered with respect other packets. This section
   defines several metrics that quantify the extent of reordering in
   various units of measure. Each metric highlights a relevant use.

   The metrics in the sub-sections below have a network
   characterization orientation, but also have relevance to receiver
   design where reordering compensation is of interest. We begin with a
   simple ratio metric indicating the reordered portion of the sample.

4.1 Reordered Packet Ratio

4.1.1 Metric Name:


4.1.2 Metric Parameters:

   The parameter set includes Type-P-Reordered singleton parameters,
   the parameters unique to Poisson Streams (as in RFC 2330 [RFC2330],
   Periodic Streams (as in RFC 3432 [RFC3432]), or TCP-like Streams (as
   in RFC 3148 [RFC3148]), packet size or size patterns, and the

   + T0, a start time

   + Tf, an end time

   + dT, a waiting time for each packet to arrive

   + K,  the total number of packets in the stream sent from Source to

   + L,  the total number of packets received (arriving between T0 and
     Tf+dT) out of the K packets sent. Recall that identical copies
     (duplicates) have been removed, so L <= K.

4.1.3 Definition:

   Given a stream of packets sent from a Source to a Destination, the
   ratio of reordered packets in the sample is
   (Count of packets with Type-P-Reordered=TRUE) / ( L )

   This fraction may be expressed as a percentage (multiply by 100).
   Note that in the case of duplicate packets, only the first copy is

4.1.4 Discussion

   When the Type-P-Reordered-Ratio-Stream is zero, no further
   reordering metrics need be examined for that sample. Therefore, the
   value of this metric is its simple ability to summarize the results
   for a reordering-free sample.

4.2 Reordering Extent

   This section defines the extent to which packets are reordered, and
   associates a specific Sequence Discontinuity with each reordered
   packet. This section inherits the Parameters defined above.

4.2.1 Metric Name:


4.2.2 Notation and Metric Parameters:

   Recall that K is the number of packets in the stream at the Source
   and L is the number of packets received at the Destination.

   Each packet has been assigned a sequence number, s, a consecutive
   integer from 1 to K in the order of packet transmission (at the

   Let s[1], s[2], ..., s[L], represent the original sequence numbers
   associated with the packets in order of arrival.

   s[i] can be thought of as a vector, where the index i is the arrival
   position of the packet with sequence number s.  In theory, any
   Source sequence number could appear in any arrival position, but
   this is unlikely in reality.

   Consider a reordered packet (Type-P-Reordered=TRUE) with arrival
   index i and source sequence number s[i]. There exists a set of
   indexes j (1 <= j < i) such that s[j] > s[i].

   The new parameters are:

   + i,     the index for arrival position, where i-1 represents an
     arrival earlier than i.

   + j,     a set of one or more arrival indexes,  where 1 <= j < i.

   + s[i],  the original sequence numbers, s, in order of arrival.

   + e,     the Reordering Extent, defined below.

4.2.3 Definition:

   The reordering extent, e, of packet s[i] is defined to be i-j for
   the smallest value of j where s[j] > s[i].

   Informally, the reordering extent is the maximum distance, in
   packets, from a reordered packet to the earliest packet received
   that has a larger sequence number.  If a packet is in-order, its
   reordering extent is undefined. The first packet to arrive is in-
   order by definition, and has undefined reordering extent.

   Comment on the definition of extent:  For some arrival orders, the
   assignment of a simple position/distance as the reordering extent
   tends to overestimate the receiver storage needed to restore order.
   A more accurate and complex procedure to calculate packet storage
   would be to subtract any earlier reordered packets that the receiver
   could pass on to the upper layers (see the Byte Offset metric). With
   the bias understood, this definition is deemed sufficient,
   especially for those who demand "on the fly" calculations.

4.2.4 Discussion:

   The packet with index j (s[j], identified in the Definition above)
   is the reordering discontinuity associated with packet s at index i
   (s[i]). This definition is formalized below.

   Note that the K packets in the stream could be some subset of a
   larger stream, but L is still the total number of packets received
   out of the K packets sent in that subset.

   If a receiver intends to restore order, then its buffer capacity
   determines its ability to handle packets that are reordered. For
   cases with single reordered packets, the extent e gives the number
   of packets that must be held in the receiver's buffer while waiting
   for the reordered packet to complete the sequence. For more complex
   scenarios, the extent may be an overestimate of required storage
   (see section 4.4 on Reordering Byte Offset and the Examples

   Although reordering extent primarily quantifies the offset in terms
   of arrival position, it may also be useful for determining the
   portion of reordered packets that can or cannot be restored to order
   in a typical receiver buffer based on their arrival order alone (and
   without the aid of retransmission).

   A sample's reordering extents may be expressed as a histogram, to
   easily summarize the frequency of various extents.

4.3 Reordering Late Time Offset

   Reordered packets can be assigned offset values indicating their
   lateness in terms of buffer time that a receiver must possess to
   accommodate them. Offset metrics are calculated only on reordered
   packets, as identified by the reordered packet singleton metric in
   Section 3.

4.3.1 Metric Name:


4.3.2 Metric Parameters:

   In addition to the parameters defined for Type-P-Reordered-Ratio-
   Stream, we specify:

   +  DstTime, the time that each packet in the stream arrives at the
     destination, and may be associated with index i, or packet s[i]

   +  LateTime(s[i]), the offset of packet s[i] in time, defined below

4.3.3 Definition:

   Lateness in time is calculated using destination times. When
   received packet s[i] is reordered, and has a reordering extent e,

   LateTime(s[i]) = DstTime(i)-DstTime(i-e)

   Alternatively, using similar notation to that of section 4.2, an
   equivalent definition is:

   LateTime(s[i]) = DstTime(i)-DstTime(j), for min{j|1<=j<i} that
   satisfies s[j]>s[i].

4.3.4 Discussion

   The offset metrics can help predict whether reordered packets will
   be useful in a general receiver buffer system with finite limits.
   The limit may be the time of storage prior to a cyclic play-out
   instant (as with de-jitter buffers).

   Note that the One-way IPDV [RFC3393] gives the delay variation for a
   packet w.r.t. the preceding packet in the source sequence. Lateness
   and IPDV give an indication of whether a buffer at the destination
   has sufficient storage to accommodate the network's behavior and
   restore order. When an earlier packet in the Source sequence is
   lost, IPDV will necessarily be undefined for adjacent packets, and
   LateTime may provide the only way to evaluate the usefulness of a

   In the case of de-jitter buffers, there are circumstances where the
   receiver employs loss concealment at the intended play-out time of a
   late packet. However, if this packet arrives out of order, the Late
   Time determines whether the packet is still useful. IPDV no longer
   applies, because the receiver establishes a new play-out schedule
   with additional buffer delay to accommodate similar events in the
   future (this requires very minimal processing).

   The combination of loss and reordering influences the LateTime
   metric. If presented with the arrival sequence 1, 10, 5 (where
   packets 2, 3, 4, and 6 through 9 are lost), LateTime would not
   indicate exactly how "late" packet 5 is from its intended arrival
   position. IPDV [RFC3393] would not capture this either, because of
   the lack of adjacent packet pairs.  Assuming a Periodic Stream
   [RFC3432], an expected arrival time could be defined for all
   packets, but this is essentially a single-point delay variation
   metric (as defined in ITU-T Recommendations [I.356] and [Y.1540]),
   and not a reordering metric.

   A sample's LateTime results may be expressed as a histogram, to
   summarize the frequency of buffer times needed to accommodate
   reordered packets and permit buffer tuning on that basis. A CDF with
   buffer time vs. percent of reordered packets accommodated may be

4.4 Reordering Byte Offset

   Reordered packets can be assigned offset values indicating the
   storage in bytes that a receiver must possess to accommodate them.
   Offset metrics are calculated only on reordered packets, as
   identified by the reordered packet singleton metric in Section 3.

4.4.1 Metric Name:


4.4.2 Metric Parameters:

   We use the same parameters defined earlier, including the optional
   parameters of SrcByte and PayloadSize, and define:

   +  ByteOffset(s[i]), the offset of packet s[i] in bytes

4.4.3 Definition:

   The Byte stream offset for reordered packet s[i] is the sum of the
   payload sizes of packets qualified by the following criteria:

   * Arrival prior to the reordered packet, s[i], and
   * The send sequence number, s, is greater than s[i].

   Packets that meet both these criteria are normally buffered until
   the sequence beneath them is complete. Note that these criteria
   apply to both in-order and reordered packets.

   For reordered packet s[i] with a reordering extent e:
   ByteOffset(s[i]) = Sum[qualified packets]
                    = Sum[PayloadSize(packet at i-1 if qualified),
                        PayloadSize(packet at i-2 if qualified), ...
                        PayloadSize(packet at i-e always qualified)]

   Using our earlier notation:
   ByteOffset(s[i]) =
               Sum[payloads of s[j] where s[j]>s[i] and i > j >= i-e]

4.4.4 Discussion

   We note that estimates of buffer size due to reordering depend on
   greatly on the test stream, in terms of the spacing between test
   packets and their size, especially when packet size is variable. In
   these and other circumstances, it may be most useful to characterize
   offset in terms of the payload size(s) of stored packets, using the
   Type-P-packet-Byte-Offset-Stream metric.

   The byte offset metric can help predict whether reordered packets
   will be useful in a general receiver buffer system with finite
   limits.  The limit is expressed as the number of bytes the buffer
   can store.

   A sample's ByteOffset results may be expressed as a histogram, to
   summarize the frequency of buffer lengths needed to accommodate
   reordered packets and permit buffer tuning on that basis. A CDF with
   buffer size vs. percent of reordered packets accommodated may be

4.5 Gaps between multiple Reordering Discontinuities

4.5.1 Metric Name:


4.5.2 Parameters:

   We use the same parameters defined earlier, but add the convention
   that index i' is greater than i, likewise j' > j, and define:

   +  Gap(s[j']), the Reordering Gap of packet s[j'] in units of
      integer messages
   +  GapTime(s[j']), the Reordering Gap of packet s[j'] in units of

4.5.3 Definition of Reordering Discontinuity:

   All reordered packets are associated with a packet at a reordering
   discontinuity, defined as the in-order packet s[j] that arrived at
   the minimum value of j (1<=j<i) for which s[j]> s[i].

   Note that s[j] will have been found to cause a sequence
   discontinuity, where s > NextExp when evaluated with the reordered
   singleton metric as described in section 3.4.

   Recall that i - e = min(j). Subsequent reordered packets may be
   associated with the same s[j], or with a different discontinuity.
   This fact is used in the definition of the Reordering Gap, below.

4.5.4 Definition of Reordering Gap:

   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.


      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

   Gap(s[j'])    =   (j')  -  (j)


   The Type-P-packet-Reordering-Gap-Stream for the packet is 0.

   Gaps may also be expressed in time:

   GapTime(s[j']) = DstTime(j') - DstTime(j)

4.5.5 Discussion
   When separate reordering discontinuities can be distinguished, then
   a count may also be reported (along with the discontinuity
   description, such as the number of reordered packets associated with
   that discontinuity and their extents and offsets). The Gaps between
   a sample's reordering discontinuities may be expressed as a
   histogram, to easily summarize the frequency of various gaps.
   Reporting the mode, average, range, etc. may also summarize the

   The Gap metric may help to correlate the frequency of reordering
   discontinuities with their cause. Gap lengths are also informative
   to receiver designers, revealing the period of reordering
   discontinuities. The combination of reordering gaps and extent
   reveals whether receivers will be required to handle cases of
   overlapping reordered packets.

4.6 Reordering-free Runs

   This section defines a metric based on a count of consecutive in-
   order packets between reordered packets.

4.6.1 Metric Name:


4.6.2 Parameters:

   We use the same parameters defined earlier, and define the

   + r, the run counter

   + x, the number of runs, also the number of reordered packets

   + a, the accumulator of in-order packets

   + p, the number of packets (when the stream is complete, p=(x+a)=L)

   + q, the sum of the squares of the runs counted

4.6.3 Definition:

   As packets in a sample arrive at the Destination, the count of in-
   order packets between reordered packets is a Reordering-Free run.
   Note that the minimum run-length is zero according to this
   definition. A pseudo code example follows:

   r = 0; /* r is the run counter */
   x = 0; /* x is the number of runs */
   a = 0; /* a is the accumulator of in order packets */
   p = 0; /* p is the number of packets */
   q = 0; /* q is the sum of the squares of the runs counted */

   while(packets arrive with sequence number s)
        if (s >= NextExp) /* s is in-order */
                then r++;
        else    /* s is reordered */
                q+= r*r;
                r = 0;

   Each in-order arrival increments the run counter and the accumulator
   of in order packets, each reordered packet resets the run counter
   after adding it to the sum of the squared lengths.

   Each arrival of a reordered packet yields a new run count.  Long
   runs accompany periods where order was maintained, while short runs
   indicate frequent, or multi-packet reordering.

   The percent of packets in-order is 100*a/p

   The average Reordering-Free run length is a/x

   The q counter gives an indication of variation of the Reordering-
   Free runs from the average by comparing q/a to a/x  ((q/a)/(a/x))

4.6.4 Discussion and Illustration:

   Type-P-packet-Reordering-Free-Run-Stream parameters give a brief
   summary of the stream's reordering characteristics including the
   average reordering-free run length, and the variation of run
   lengths, therefore a key application of this metric is network

   For 36 packets with 3 runs of 11 in-order packets we have:
      p = 36
      x = 3
      a = 33
      q = 3 * (11*11) = 363
      ave reordering-free run = 11
      q/a = 11
      (q/a)/(a/x) = 1.0

   For 36 packets with 3 runs, 2 runs of length 1 and one of length 31
      p = 36
      x = 3
      a = 33
      q = 1 + 1 + 961 = 963
      ave reordering-free run = 11
      q/a = 29.18
      (q/a)/(a/x) = 2.65

   The variability in run length is prominent in the difference between
   the q values (sum of the squared run lengths) and comparing average
   run length to the (q/a)/(a/x) ratios (equals 1 when all runs are the
   same length).

5. Metrics Focused on Receiver Assessment: A TCP-Relevant Metric

   This section describes a metric that conveys information associated
   with the affect of reordering on TCP.  However, in order to infer
   anything about TCP performance, the test stream MUST bear a close
   resemblance to the TCP sender of interest. RFC 3148 [RFC3148] lists
   the specific aspects of congestion control algorithms that must be
   specified. Further, RFC 3148 recommends that Bulk Transfer Capacity
   metrics SHOULD have instruments to distinguish three cases of packet
   reordering (in section 3.3). The sample metrics defined above
   satisfy the requirements to classify packets that are slightly or
   grossly out-of-order. The metric in this section adds the capability
   to estimate whether reordering might cause the DUP-ACK threshold to
   be exceeded causing the Fast Retransmit algorithm to be invoked.
   Additional TCP Kernel Instruments are summarized in [Mat03].

5.1 Metric Name:


5.2 Parameter Notation:

   Let n be a positive integer (a parameter).  Let k be a positive
   integer equal to the number of packets sent (sample size).  Let l be
   a non-negative integer representing the number of packets that were
   received out of the k packets sent.  (Note that there is no
   relationship between k and l: on one hand, losses can make l less
   than k; on the other hand, duplicates can make l greater than k.)
   Assign each sent packet a sequence number, 1 to k, in order of
   packet emission.

   Let s[1], s[2], ..., s[l] be the original sequence numbers of the
   received packets, in the order of arrival.

5.3 Definitions

   Definition 1: Received packet number i (n < i <= l), with source
   sequence number s[i], is n-reordered if and only if for all j such
   that i-n <= j < i, s[j] > s[i].

   Claim: If by this definition, a packet's reordering is n and 0 < n'
   < n, then the packet is also reordered to the n' extent.

   Note: This definition is illustrated by C code in Appendix A.  It
   determines the n-reordering for a value of n=3 (when actually
   writing applications that would report the metric, one would
   probably report it for several values of n, such as 1, 2, 3, 4 --
   and maybe a few more consecutive values).

   This definition does not assign an n to all reordered packets as
   defined by the singleton metric, in particular when blocks of
   successive packets are reordered. (In the arrival sequence
   s={1,2,3,7,8,9,4,5,6}, packets 4, 5, and 6 are reordered, but only
   packet 4 is n-reordered, with n=3.)

   Definition 2: The degree of n-reordering of the sample is m/l, where
   m is the number of n-reordered packets in the sample.

   Definition 3: The degree of "monotonic reordering" of the sample is
   its degree of 1-reordering.

   Definition 4: A sample is said to have no reordering if its degree
   of n-reordering is 0.

5.4 Discussion:

   The degree of n-reordering may be expressed as a percentage, in
   which case the number from Definition 2 is multiplied by 100.

   The n-reordering metric is helpful for matching the duplicate ACK
   threshold setting to a given path.  For example, if a path exhibits
   no more than 5-reordering, a DUP-ACK threshold of 6 may avoid
   unnecessary retransmissions.

   Important special cases are n=1 and n=3:

   - For n=1, absence of 1-reordering means the sequence numbers that
   the receiver sees are monotonically increasing with respect to the
   previous arriving packet.

   - For n=3, a NewReno TCP sender would retransmit 1 packet in
   response to an instance of 3-reordering and therefore consider this
   packet lost for the purposes of congestion control (the sender will
   half its congestion window, see [RFC2581]). 3 is default threshold
   for SCTP [RFC2960], and the future Datagram Congestion Control
   Protocol (DCCP).

   A sample's n-reordering may be expressed as a histogram, to
   summarize the frequency for each value of n.

   We note that the definition of n-reordering cannot predict the exact
   number of packets unnecessarily retransmitted by a TCP sender under
   some circumstances, such as cases with closely-spaced reordered
   singletons. Both time and position influence the sender's behavior.

   A packet's n-reordering designation is sometimes equal to its
   reordering extent, e. n-reordering is different in the following

   1. n is a count of early packets with consecutive arrival positions
   at the receiver.

   2. Reordered packets (Type-P-Reordered=TRUE) may not be n-reordered,
   but will have an extent, e (see the examples).

6. Measurement and Implementation Issues

   The results of tests will be dependent on the time interval between
   measurement packets (both at the Source, and during transport where
   spacing may change).  Clearly, packets launched infrequently (e.g.,
   1 per 10 seconds) are unlikely to be reordered.

   In order to gauge the reordering for an application according to the
   metrics defined in this memo, it is RECOMMENDED to use the same
   sending pattern as the application of interest. In any case, the
   exact method of packet generation MUST be reported with the
   measurement results, including all stream parameters.

   +  To make inferences about applications that use TCP, it is
      REQUIRED to use TCP-like Streams as in [RFC3148]

   +  For real-time applications, it is RECOMMENDED to use Periodic
      Streams as in [RFC3432]

   It is acceptable to report the metrics of Sections 3 and 4 with
   other IPPM metrics using Poisson Streams [RFC2330]. Poisson streams
   represent an "unbiased sample" of network performance for packet
   loss and delay metrics. However, it would be incorrect to make
   inferences about the application categories above using reordering
   metrics measured with Poisson streams.

   Test stream designers may prefer to use a periodic sending interval
   so that a known temporal bias is maintained, also bringing
   simplified results analysis (as described in [RFC3432]). In this
   case, it is RECOMMENDED that the periodic sending interval should be
   chosen to reproduce the closest Source packet spacing expected.
   Testers must recognize that streams sent at the link speed
   serialization limit MUST have limited duration and MUST consider
   packet loss as an indication that the stream has caused congestion,
   and suspend further testing.

   When intending to compare independent measurements of reordering, it
   is RECOMMENDED to use the same test stream parameters in each
   measurement system.

   Packet lengths might also be varied to attempt to detect instances
   of parallel processing (they may cause steady state reordering). For
   example, a line-speed burst of the longest (MTU-length) packets
   followed by a burst of the shortest possible packets may be an
   effective detecting pattern.  Other size patterns are possible.

   The non-reversing order criterion and all metrics described above
   remain valid and useful when a stream of packets experiences packet
   loss, or both loss and reordering. In other words, losses alone do
   not cause subsequent packets to be declared reordered.

   Assuming that the necessary sequence information (message number) is
   included in the packet payload (possibly in application headers such
   as RTP), reordering metrics may be evaluated in a passive
   measurement arrangement.  Also, it is possible to evaluate order at
   any point along a Source-Destination path, recognizing that
   intermediate measurements may differ from those made at the
   Destination (where the reordering affect on applications can be

   It is possible to apply these metrics to evaluate reordering in a
   TCP sender's stream. In this case, the Source sequence numbers would
   be based on byte stream, or segment numbering. Since the stream may
   include retransmissions due to loss or reordering, care must be
   taken to avoid declaring retransmitted packets reordered. The
   additional sequence reference of s or SrcTime helps to avoid this
   ambiguity, or the optional TCP timestamp field [RFC1323].

   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

   3. Reordered packets coincident with sequence numbers reaching end-
   of-range must also be detected for proper application of correction

   Ideally, the test instrument would have the ability to use all
   earlier packets at any point in the test stream. In practice there
   will be limited ability to determine reordering extent, due to the
   storage requirements for previous packets. Saving only packets that
   indicate discontinuities (and their arrival positions) will reduce
   storage volume.

   Another solution is to use a sliding history window of packets,
   where the window size would be determined by an upper bound on the
   useful reordering extent. This bound could be several packets or
   several seconds-worth of packets, depending on the intended
   analysis. When discarding all stream information beyond the window,
   the reordering extent or degree of n-reordering may need to be
   expressed as greater than the window length if the reordering
   discontinuity information has been discarded, and Gap calculations
   would not be possible.

   The requirement to ignore duplicate packets also mandates storage.
   Here, tracking the sequence numbers of missing packets may minimize
   storage size. Missing packets may eventually be declared lost, or
   reordered if they arrive. The missing packet list and the largest
   sequence number received thus far (NextExp - 1) are sufficient
   information to determine if a packet is a duplicate (assuming a
   manageable storage size for packets that are missing due to loss).

   It is important to note that practical IP networks also have limited
   ability to "store" packets, even when routing loops appear
   temporarily. Therefore, the storage for reordering metrics (and
   their complexity) would only approach the number packets in the
   sample, K, when the sending time for K packets is small with respect
   to the network's largest possible transfer time.

   Last, we note that determining reordering extents and gaps is tricky
   when there are overlapped or nested events. Test instrument
   complexity and reordering complexity are directly correlated.

7. Examples of Arrival Order Evaluation

   This section provides some examples to illustrate how the non-
   reversing order criterion works, how n-reordering works in
   comparison, and the value of quantifying reordering in all the
   dimensions of time, bytes, and position.

   Throughout this section, we will refer to packets by their source
   sequence number, except where noted.  So "Packet 4" refers to the
   packet with source sequence number 4, and the reader should refer to
   the tables in each example to determine packet 4's arrival index
   number, if needed.

7.1 Example with a single packet reordered

   Table 1 gives a simple case of reordering, where one packet is
   reordered, Packet 4. Packets are listed according to their arrival,
   and message numbering is used. All packets contain PayloadSize=100
   bytes, with SrcByte=(s x 100)-99 for s=1,2,3,4,...

   Table 1 Example with Packet 4 Reordered,
   Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10
   s            Src     Dst                     Dst     Byte    Late
   @Dst NextExp Time    Time    Delay   IPDV    Order   Offset  Time
    1     1       0      68      68              1
    2     2      20      88      68       0      2
    3     3      40     108      68       0      3
    5     4      80     148      68     -82      4
    6     6     100     168      68       0      5
    7     7     120     188      68       0      6
    8     8     140     208      68       0      7
    4     9      60     210     150      82      8      400     62
    9     9     160     228      68       0      9
   10    10     180     248      68       0     10

   Each column gives the following information:

   s        Packet sequence number at the Source.
   NextExp  The value of NextExp when the packet arrived(before
   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:

   s = 1, 2, 3, 5, 6, 7, 8, 4, 9, 10
   i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

   Applying the definition of Type-P-packet-Reordering-Extent-Stream:
   when j=7, 8 > 4, so the reordering extent is 1 or more.
   when j=6, 7 > 4, so the reordering extent is 2 or more.
   when j=5, 6 > 4, so the reordering extent is 3 or more.
   when j=4, 5 > 4, so the reordering extent is 4 or more.
   when j=3, but 3 < 4, and 4 is the maximum extent, e=4 (assuming
   there are no earlier sequence discontinuities, as in this example).

   Further, we can compute the Late Time (210-148=62ms using DstTime)
   compared to Packet 5's arrival.  If the receiver has a de-jitter
   buffer that holds more than 4 packets, or at least 62 ms storage,
   Packet 4 may be useful. Note that 1-way delay and IPDV indicate
   unusual behavior for Packet 4. Also, if Packet 4 had arrived at
   least 62ms earlier, it would have been in-order in this example.

   If all packets contained 100 byte payloads, then Byte Offset is
   equal to 400 bytes.

   Following the definitions of section 5.1, Packet 4 is designated 4-

7.2 Example with two packets reordered

   Table 2 Example with Packets 5 and 6 Reordered,
   Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10
   s            Src     Dst                     Dst     Byte    Late
   @Dst NextExp Time    Time    Delay   IPDV    Order   Offset  Time
    1     1       0      68      68              1
    2     2      20      88      68       0      2
    3     3      40     108      68       0      3
    4     4      60     128      68       0      4
    7     5     120     188      68     -22      5
    5     8      80     189     109      41      6      100     1
    6     8     100     190      90     -19      7      100     2
    8     8     140     208      68       0      8
    9     9     160     228      68       0      9
   10    10     180     248      68       0     10

   Table 2 shows a case where Packets 5 and 6 arrive just behind Packet
   7, so both 5 and 6 are reordered. The Late times (189-188=1, 190-
   188=2) are small.

   Using the notation <s[1], ..., s[i], ..., s[l]>, the received
   packets are represented as:

                      \/ \/
   s = 1, 2, 3, 4, 7, 5, 6, 8, 9, 10
   i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
                      /\ /\

   Considering Packet 5 first:
   when j=5, 7 > 5, so the reordering extent is 1 or more.
   when j=4, we have 4 < 5, so 1 is its maximum extent, and e=1.

   Considering Packet 6 next:
   when j=6, 5 < 6, the extent is not yet defined.
   when j=5, 7 > 6, so the reordering extent is i-j=2 or more.
   when j=4, 4 < 6, and we find 2 is its maximum extent, and e=2.

   We can also associate each of these reordered packets with a
   reordering discontinuity. We find the minimum j=5 (for both packets)
   according to Section 4.2.3. So Packet 6 is associated with the same
   reordering discontinuity as Packet 5, the Reordering Discontinuity
   at Packet 7.

   This is a case where reordering extent e would over-estimate the
   packet storage required to restore order. Only one packet storage is
   required (to hold Packet 7), but e=2 for Packet 6.

   Following the definitions of section 5, Packet 5 is designated 1-
   reordered, but Packet 6 is not designated n-reordered.

   A hypothetical sender/receiver pair may retransmit Packet 5
   unnecessarily, since it is 1-reordered (in agreement with the
   singleton metric). Though Packet 6 may not be unnecessarily
   retransmitted, the receiver cannot advance Packet 7 to the higher
   layers until after Packet 6 arrives. Therefore, the singleton metric
   correctly determined that Packet 6 is reordered.

7.3 Example with three packets reordered

   Table 3 Example with Packets 4, 5, and 6 reordered
   Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11
   s            Src     Dst                     Dst     Byte    Late
   @Dst NextExp Time    Time    Delay   IPDV    Order   Offset  Time
    1    1        0      68      68              1
    2    2       20      88      68       0      2
    3    3       40     108      68       0      3
    7    4      120     188      68     -88      4
    8    8      140     208      68       0      5
    9    9      160     228      68       0      6
   10   10      180     248      68       0      7
    4   11       60     250     190     122      8      400     62
    5   11       80     252     172     -18      9      400     64
    6   11      100     256     156     -16     10      400     68
   11   11      200     268      68       0     11

   The case in Table 3 is where three packets in sequence have long
   transit times (Packets with s = 4,5,and 6). Delay, Late time, and
   Byte Offset capture this very well, and indicate variation in
   reordering extent, while IPDV indicates that the spacing between
   packets 4,5,and 6 has changed.

   The histogram of Reordering extents (e) would be:

   Bin         1  2  3  4  5  6  7
   Frequency   0  0  0  1  1  1  0

   Using the notation <s[1], ..., s[i], ..., s[l]>, the received
   packets are represented as:

   s = 1, 2, 3, 7, 8, 9,10, 4, 5, 6, 11
   i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11

   We first calculate the n-reordering. Considering Packet 4 first:
   when n=1, 7<=j<8, and 10> 4, so the packet is 1-reordered.
   when n=2, 6<=j<8, and 9 > 4, so the packet is 2-reordered.
   when n=3, 5<=j<8, and 8 > 4, so the packet is 3-reordered.
   when n=4, 4<=j<8, and 7 > 4, so the packet is 4-reordered.
   when n=5, 3<=j<8, but 3 < 4, and 4 is the maximum n-reordering.

   Considering packet 5[9] next:
   when n=1, 8<=j<9, but 4 < 5, so the packet at i=9 is not designated
   as n-reordered. We find the same to for Packet 6.

   We now consider whether reordered Packets 5 and 6 are associated
   with the same reordering discontinuity as Packet 4.  Using the test
   of Section 4.2.3, we find that the minimum j=4 for all three
   packets. They are all associated with the reordering discontinuity
   at Packet 7.

   This example shows again that the n-reordering definition identifies
   a single Packet (4) with a sufficient degree of n-reordering that
   might cause one unnecessary packet retransmission by the New Reno
   TCP sender (with DUP-ACK threshold=3 or 4). Also, the reordered
   arrival of Packets 5 and 6 will allow the receiver process to pass
   Packets 7 through 10 up the protocol stack (the singleton Type-P-
   Reordered = TRUE for Packets 5 and 6, and they are all associated
   with a single reordering discontinuity).

7.4 Example with Multiple Packet Reordering Discontinuities

   Table 4 Example with Multiple Packet Reordering Discontinuities
   Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16

          Discontinuity         Discontinuity
   s = 1, 2, 3, 6, 7, 4, 5, 8, 9, 10, 12, 13, 11, 14, 15, 16
   i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16

   r = 1, 2, 3, 4, 5, 0, 0, 1, 2,  3,  4,  5,  0,  1,  2,  3, ...
   number of runs,n = 1  2                     3
   end r counts =     5  0                     5
   (these values are computed after the packet arrives)

   Packet 4 has extent e=2, Packet 5 has extent e=3, and Packet 11 has
   e=2. There are two different reordering discontinuities, one at
   Packet 6 (where j=4) and one at Packet 12 (where j'=11).

   According to the definition of Reordering Gap
   Gap(s[j']) = (j') - (j)
   Gap(Packet 12) = (11) - (4) = 7

   We also have three reordering-free runs of lengths 5, 0, and 5.

   The differences between these two multiple-event metrics are evident
   here.  Gaps are the distance between sequence discontinuities that
   are subsequently defined as reordering discontinuities, while
   reordering-free runs capture the distance between reordered packets.

8. Security Considerations

8.1 Denial of Service Attacks

   This metric requires a stream of packets sent from one host (source)
   to another host (destination) through intervening networks.  This
   method could be abused for denial of service attacks directed at
   destination and/or the intervening network(s).

   Administrators of source, destination, and the intervening
   network(s) should establish bilateral or multi-lateral agreements
   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

8.2 User data confidentiality

   Active use of this method generates packets for a sample, rather
   than taking samples based on user data, and does not threaten user
   data confidentiality. Passive measurement must restrict attention to
   the headers of interest. Since user payloads may be temporarily
   stored for length analysis, suitable precautions MUST be taken to
   keep this information safe and confidential. In most cases, a
   hashing function will produce a value suitable for payload

8.3 Interference with the metric

   It may be possible to identify that a certain packet or stream of
   packets is part of a sample. With that knowledge at the destination
   and/or the intervening networks, it is possible to change the
   processing of the packets (e.g. increasing or decreasing delay) that
   may distort the measured performance.  It may also be possible to
   generate additional packets that appear to be part of the sample
   metric. These additional packets are likely to perturb the results
   of the sample measurement.

   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.

10. Normative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.
              Obtain via:

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.
              Obtain via:

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and Mathis, M.,
              "Framework for IP Performance Metrics", RFC 2330, May
              Obtain via:
   [RFC3148]  Mathis, M. and Allman, M., "A Framework for Defining
              Empirical Bulk Transfer Capacity Metrics", RFC 3148, July
              Obtain via:

   [RFC3432]  Raisanen, V., Grotefeld, G., and Morton, A., "Network
              performance measurement with periodic streams", RFC 3432,
              November 2002.

11. Informative References

   [Bel02]    J.Bellardo and S.Savage, "Measuring Packet Reordering,"
              Proceedings of the ACM SIGCOMM Internet Measurement
              Workshop 2002, November 6-8, Marseille, France.

   [Ben99]    J.C.R.Bennett, C.Partridge, and N.Shectman, "Packet
              Reordering is Not Pathological Network Behavior,"
              IEEE/ACM Transactions on Networking, vol.7, no.6, pp.789-
              798, December 1999.

   [Cia00]    L.Ciavattone and A.Morton, "Out-of-Sequence Packet
              Parameter Definition (for Y.1540)", Contribution number
              T1A1.3/2000-047, October 30, 2000.

   [Cia03]    L.Ciavattone, A.Morton, and G.Ramachandran, "Standardized
              Active Measurements on a Tier 1 IP Backbone," IEEE
              Communications Mag., pp 90-97, June 2003.

   [I.356]    ITU-T Recommendation I.356, "B-ISDN ATM layer cell
              transfer performance", March 2000.

   [Jai02]    S.Jaiswal et al., "Measurement and Classification of Out-
              of-Sequence Packets in a Tier-1 IP Backbone," Proceedings
              of the ACM SIGCOMM Internet Measurement Workshop 2002,
              November 6-8, Marseille, France.

   [Lou01]    D.Loguinov and H.Radha, "Measurement Study of Low-bitrate
              Internet Video Streaming", Proceedings of the ACM SIGCOMM
              Internet Measurement Workshop 2001 November 1-2, 2001,
              San Francisco, USA.

   [Mat03]    M. Mathis, J Heffner and R Reddy, "Web100: Extended TCP
              Instrumentation for Research, Education and Diagnosis",
              ACM Computer Communications Review, Vol 33, Num 3, July

   [Pax98]    V.Paxson, "Measurements and Analysis of End-to-End
              Internet Dynamics," Ph.D. dissertation, U.C. Berkeley,

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.
              Obtain via:

   [RFC1323]  Jacobson, V., Braden, R., and Borman, D., "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC2581]  Allman, M., Paxson, V., and Stevens, W., "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC2960]  Stewart, R., et al., "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [RFC3393]  Demichelis, C., and Chimento, P., "IP Packet Delay
              Variation Metric for IP Performance Metrics (IPPM)", RFC
              3393, November 2002.

   [TBABAJ02] T. Banka, A. A. Bare, A. P. Jayasumana, "Metrics for
              Degree of Reordering in Packet Sequences", Proc. 27th
              IEEE Conference on Local Computer Networks, Tampa, FL,
              Nov. 2002.

   [Y.1540]   ITU-T Recommendation Y.1540, "Internet protocol data
              communication service - IP packet transfer and
              availability performance parameters", December 2002.

12. Acknowledgments

   The authors would like to acknowledge many helpful discussions with
   Matt Zekauskas, Jon Bennett (who authored the sections on
   Reordering-Free Runs), and Matt Mathis. We thank David Newman, Henk
   Uijterwaal, Mark Allman, Vern Paxson, and Phil Chimento for their
   reviews and suggestions, and Michal Przybylski for sharing
   implementation experiences with us on the ippm-list. Anura
   Jayasumana and Nischal Piratla brought in recent work-in-progress. work-in-progress
   [TBABAJ02]. We gratefully acknowledge the foundation laid by the
   authors of the IP performance Framework [RFC2330].

13. Appendix A Example Implementations in C (Informative)

   Two example c-code implementations of reordering definitions follow:

   Example 1  n-reordering ============================================

   #include <stdio.h>

   #define MAXN   100

   #define min(a, b) ((a) < (b)? (a): (b))
   #define loop(x) ((x) >= 0? x: x + MAXN)

    * Read new sequence number and return it. Return a sentinel value
    * of EOF (at least once) when there are no more sequence numbers.
    * In this example, the sequence numbers come from stdin;
    * in an actual test, they would come from the network.
           int     res, rc;
           rc = scanf("%d\n", &res);
           if (rc == 1) return res;
           else return EOF;

           int     m[MAXN];       /* We have m[j-1] == number of
                                            * j-reordered packets. */
           int     ring[MAXN];    /* Last sequence numbers seen. */
           int     r = 0;          /* Ring pointer for next write. */
           int     l = 0;        /* Number of sequence numbers read. */
           int     s;              /* Last sequence number read. */
           int     j;

           for (j = 0; j < MAXN; j++) m[j] = 0;
           for (;(s = read_sequence_number())!= EOF;l++,r=(r+1)%MAXN) {
             for (j=0; j<min(l, MAXN)&&s<ring[loop(r-j-1)];j++) m[j]++;
             ring[r] = s;
           for (j = 0; j < MAXN && m[j]; j++)
             printf("%d-reordering = %f%%\n", j+1, 100.0*m[j]/(l-j-1));
           if (j == 0) printf("no reordering\n");
           else if (j < MAXN) printf("no %d-reordering\n", j+1);
           else printf("only up to %d-reordering is handled\n", MAXN);

   /* Example 2   singleton and n-reordering comparison =======
      Author:  Jerry Perser 7-2002 (mod by acm 12-2004)
      Compile: $ gcc -o jpboth file.c
      Usage:   $ jpboth 1 2 3 7 8 4 5 6 (pkt sequence given on cmdline)
      Note to cut/pasters: line 59 may need repair
      #include <stdio.h>

      #define MAXN   100
      #define min(a, b) ((a) < (b)? (a): (b))
      #define loop(x) ((x) >= 0? x: x + MAXN)

      /* Global counters */
      int receive_packets=0;       /* number of received */
      int reorder_packets_Al=0;    /* num reordered pkts (singleton) */
      int reorder_packets_Stas=0; /* num reordered pkts(n-reordering)*/

      /* function to test if current packet has been reordered
       * returns 0 = not reordered
       *         1 = reordered
      int testorder1(int seqnum)   // Al
           static int NextExp = 1;
           int iReturn = 0;

           if (seqnum >= NextExp) {
                   NextExp = seqnum+1;
           } else {
                   iReturn = 1;
           return iReturn;

      int testorder2(int seqnum)   // Stanislav
           static int  ring[MAXN];    /* Last sequence numbers seen. */
           static int  r = 0;         /* Ring pointer for next write */
           int   l = 0;          /* Number of sequence numbers read. */
           int   j;
           int  iReturn = 0;

           r = (r+1) % MAXN;
           for (j=0; j<min(l, MAXN) && seqnum<ring[loop(r-j-1)]; j++)
                       iReturn = 1;
           ring[r] = seqnum;
           return iReturn;
      int main(int argc, char *argv[])
           int i, packet;
           for (i=1; i< argc; i++) {
                packet = atoi(argv[i]);
                reorder_packets_Al += testorder1(packet); // singleton
                reorder_packets_Stas += testorder2(packet); //n-reord.
           printf("Received packets = %d, Singleton Reordered = %d, n-
   reordered = %d\n",  receive_packets, reorder_packets_Al,
   reorder_packets_Stas );


   ISO/IEC 9899:1999 (E), as amended by ISO/IEC 9899:1999/Cor.1:2001
   (E). Also published as:

   The C Standard: Incorporating Technical Corrigendum 1, British
   Standards Institute, ISBN: 0-470-84573-2, Hardcover, 558 pages,
   September 2003.

14. Appendix B Fragment Order Evaluation (Informative)

   Section 3 stated that fragment re-assembly is assumed prior to order
   evaluation, but that similar procedures could be applied prior to
   re-assembly.  This appendix gives definitions and procedures to
   identify reordering in a packet stream that includes fragmentation.

14.1 Metric Name:

   The Metric retains the same name, Type-P-Reordered, but additional
   parameters are required.

   This Appendix assumes that the device that divides a packet into
   fragments send them according to ascending fragment offset. Early
   Linux OS sent fragments in reverse order, so this possibility is
   worth checking.

14.2 Additional Metric Parameters:

   +  MoreFrag, the state of the More Fragments Flag in the IP header

   +  FragOffset, the offset from the beginning of a fragmented packet,
      in 8 octet units (also from the IP header).

   +  FragSeq#, the sequence number from the IP header of a fragmented
      packet currently under evaluation for reordering. When set to
      zero, fragment evaluation is not in progress.

   +  NextExpFrag, the Next Expected Fragment Offset at the
      Destination, in 8 octet units. Set to zero when fragment
      evaluation is not in progress.

   The packet sequence number, s, is assumed to be the same as the IP
   header sequence number. Also, the value of NextExp does not change
   with the in-order arrival of fragments. NextExp is only updated when
   a last fragment or a complete packet arrives.

   Note that packets with missing fragments MUST be declared lost, and
   the Reordering status of any fragments that do arrive MUST be
   excluded from sample metrics.

14.3 Definition:

   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


   while(packets arrive with s, MoreFrag, FragOffset)
   if (s>=NextExp AND MoreFrag==0 AND s>=FragSeq#){
        /* a normal packet or last frag of an in-order packet arrived
        NextExp = s+1;
        FragSeq# = 0;
        NextExpFrag = 0;
        Reordering = False;
   if (s>=NextExp AND MoreFrag==1 AND s>FragSeq#>=0){
        /* a fragment of a new packet arrived, possibly with a
        higher sequence number than the current fragmented packet */
        FragSeq# = s;
        NextExpFrag = FragOffset+1;
        Reordering = False;
   if (s>=NextExp AND MoreFrag==1 AND s==FragSeq#){
        /* a fragment of the "current packet s" arrived */
        if (FragOffset >= NextExpFrag){
                NextExpFrag = FragOffset+1;
                Reordering = False;
                Reordering = True; /* fragment reordered  */
   if (s>=NextExp AND MoreFrag==1 AND s < FragSeq#){
        /* case where a late fragment arrived,
           for illustration only, redundant with else below */
        Reordering = True;
   else { /* when s < NextExp, or MoreFrag==0 AND s < FragSeq# */
        Reordering = True;

   A working version of the code would include a check to ensure that
   all fragments of a packet arrive before using the Reordered status
   further, such as in sample metrics.

14.4 Discussion: Notes on Sample Metrics when evaluating Fragments

   All fragments with the same Source Sequence Number are assigned the
   same Source Time.

   Evaluation with byte stream numbering may be simplified if the
   fragment offset is simply added to the SourceByte of the first
   packet (with fragment offset = 0), keeping the 8 octet units of the
   offset in mind.

15. Author's Addresses

   Al Morton
   AT&T Labs
   Room D3 - 3C06
   200 Laurel Ave. South
   Middletown, NJ 07748 USA
   Phone  +1 732 420 1571
   EMail: <>

   Len Ciavattone
   AT&T Labs
   Room C2 A2 - 3D02 4G06
   200 Laurel Ave. South
   Middletown, NJ 07748 USA
   Phone  +1 732 420 1239
   EMail: <>

   Gomathi Ramachandran
   AT&T Labs
   Room C4 - 3D22
   200 Laurel Ave. South
   Middletown, NJ 07748 USA
   Phone  +1 732 420 2353
   EMail: <>

   Stanislav Shalunov
   200 Business Park Drive, Suite 307
   Armonk, NY 10504
   Phone: + 1 914 765 1182
   1000 Oakbrook DR STE 300
   Ann Arbor, MI 48104
   +1 734 995 7060
   EMail: <>

   Jerry Perser
   Calabasas, CA 91302  USA
   Phone: + 1
   EMail: <>

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