Network Working Group A.Morton Internet Draft L.Ciavattone Document: <draft-ietf-ippm-reordering-03.txt> G.Ramachandran Category: Standards Track AT&T Labs S.Shalunov Internet2 J.Perser Spirent Packet Reordering Metric for IPPM Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or made obsolete by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This memo defines a simple metric to determine if a network has maintained packet order. It provides motivations for the new metric, gives the metric definition, and discusses the issues associated with its measurement. The memo defines additional sample metrics to quantify the extent of reordering in several useful dimensions. Some examples of evaluation using the various sample metrics are included. 1. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [2]. Although RFC 2119 was written with protocols in mind, the key words 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. Morton, et al. Standards Track exp. Dec 2003 Page 1 Packet Reordering Metric for IPPM June 2003 2. Introduction Ordered delivery is a property of successful packet transfer attempts, where the packet sequence ascends for each arriving packet and there are no backward steps. An explicit sequence number, such as an incrementing message number or the packet sending time carried in each packet, establishes the Source Sequence. The presence of reordering at the Destination is based on arrival order. This metric is consistent with RFC 2330 [3], and classifies arriving packets with sequence numbers smaller than their predecessors as out-of-order, or reordered. For example, if arriving packets are 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 declared reordered. The alternative is to emphasize "premature" packets instead (4 and 5 in the example), but only the arrival of packet 3 distinguishes this circumstance from packet loss. Focusing attention on late packets allows us to maintain orthogonality with the packet loss metric. The metric's construction is very similar to the sequence space validation for received segments in RFC793 [5]. Earlier work to define ordered delivery includes [6], [7], [8], [9], [10] and more ???. 2.1 Motivation A reordering metric is relevant for most applications, especially when assessing network support for Real-Time media streams. The extent of reordering may be sufficient to cause a received packet to be discarded by functions above the IP layer. Packet order is not expected to change during transfer, but several specific path characteristics can cause their order to change. Examples are: * When two paths, one with slightly longer transfer time, support a single packet stream or flow, then packets traversing the longer path may arrive out-of-order. Multiple paths may be used to achieve load balancing, or may arise from route instability. * To increase capacity, a network device designed with multiple processors serving a single port may reorder as a byproduct. * A layer 2 retransmission protocol that compensates for an error- prone link may cause packet reordering. * 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 Morton, et al. Standards Track exp. Dec 2003 Page 2 Packet Reordering Metric for IPPM June 2003 buffers have different occupations and/or service rates, then order will likely change. The ability to restore order at the destination will likely have finite limits. Practical hosts have receiver buffers with finite size in terms of packets, bytes, or time (such as de-jitter buffers). Once the initial determination of reordering is made, it is useful to quantify the extent of reordering, or lateness, in all meaningful dimensions. 2.2 Goals and Objectives The definitions below intend to satisfy the goals of: 1. Determining whether or not packet order is maintained. 2. Quantifying the extent (achieving this second goal requires assumptions of upper layer functions and capabilities to restore order, and therefore several solutions). Reordering Metrics MUST: + be relevant to one or more known applications + be computable "on the fly" + work with Poisson and Periodic test streams + work even if the stream has duplicate or lost packets Reordering Metrics SHOULD: + have concatenating results for segments measured separately + have simplicity for easy consumption and understanding + have relevance to TCP performance + have relevance to Real-time application performance 3. An Ordered Arrival Singleton Metric The IPPM framework RFC 2330 [3] describes the notions of singletons, samples, and statistics. For easy reference: By a 'singleton' metric, we refer to metrics that are, in a sense, atomic. For example, a single instance of "bulk throughput capacity" from one host to another might be defined as a singleton metric, even though the instance involves measuring the timing of a number of Internet packets. The evaluation of packet order requires several supporting concepts. The first is a sequence number applied to packets at the source to uniquely identify the order of packet transmission. The sequence number may be established by a simple message number, a byte stream number, or it may be the actual time when each packet departs from the Source. Morton, et al. Standards Track exp. Dec 2003 Page 3 Packet Reordering Metric for IPPM June 2003 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 byte stream numbering, NextExp is a value 1 byte greater than the last in-order packet sequence number + payload. If Source time is used as the sequence number, NextExp is the Src time from the last in-order packet + 1 clock tick. Each packet within a packet stream can be evaluated for its order singleton metric. 3.1 Metric Name: Type-P-Non-Reversing-Order 3.2 Metric Parameters: + Src, the IP address of a host + Dst, the IP address of a host + SrcTime, the time of packet emission from the Source (or wire time) + s, the packet sequence number applied at the Source, in units of messages. + SrcByte, the packet sequence number applied at the Source, in units of payload bytes. + NextExp, the Next Expected Sequence number at the Destination, in units of messages, time, or bytes. + PayloadSize, the number of bytes contained in the information field and referred to when the SrcByte sequence is based on byte transfer. 3.3 Definition: The Type-P-Non-Reversing-Order of a packet is defined as true if s >= NextExp (the packet is in-order). In this case, NextExp is set to s+1. The Type-P-Non-Reversing-Order of a packet is defined as false if s < NextExp (the packet is reordered). In this case, NextExp value does not change. Since the Next Expected value cannot decrease, it represents a non- reversing order that is the basis to identify reordered packets. Morton, et al. Standards Track exp. Dec 2003 Page 4 Packet Reordering Metric for IPPM June 2003 For the alternate sequence dimensions, in-order packets have byte stream numbers or Source times greater than or equal to the value of Next Expected. Each new in-order packet will increase the Next Expected by 1 clock tick for Source times, or the payload size plus 1 for byte numbering. This definition can also be specified in pseudo-code. On successful arrival of a packet with sequence number s: if s >= NextExp, /* s is true, in-order */ then NextExp = s + PayloadSize + 1; else /* when s < NextExp */ /* packet s is false, reordered */ The sequence number s may be replaced by SrcTime or SrcByte. When using s (message-based sequence numbering) or Source time, PayloadSize=0. 3.4 Discussion Any arriving packet bearing a sequence number from the sequence that establishes the Next Expected value can be evaluated to determine whether it is in-order or reordered, based on a previous packet's arrival. In the case where Next Expected is Undefined (because the arriving packet is the first successful transfer), the packet is designated in-order. This metric assumes re-assembly of packet fragments before evaluation. 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 requirements as earlier IPPM metrics, and follows the precedent of RFC 2679. Packets with s > NextExp are a special case of in-order delivery. This condition indicates a sequence discontinuity, either because of packet loss or reordering. Reordered packets must arrive for the sequence discontinuity to be defined as a reordering discontinuity (see next section). Discontinuities are easiest to detect with message numbering or payload byte numbering where payload size is constant (and retransmissions are distinguished), and may be possible with Periodic Streams and Source Time numbering. 4. Sample Metrics In this section, we define metrics applicable to a sample of packets from a single Source sequence number system. We begin with a simple ratio metric indicating the reordered portion of the sample. When Morton, et al. Standards Track exp. Dec 2003 Page 5 Packet Reordering Metric for IPPM June 2003 this ratio is zero, no further reordering metrics are needed for that sample. When reordering occurs, it is highly desirable to assert the degree to which a packet is out-of-order, or reordered with respect 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 use. The metrics in the sub-sections below have a network characterization orientation, but also have relevance to receiver design. 4.1 Reordered Packet Ratio 4.1.1 Metric Name: Type-P-Reordered-Ratio-Stream 4.1.2 Metric Parameters: The parameter set includes Type-P-Non-Reversing-Order singleton parameters, the parameters unique to Poisson or Periodic Streams (as in RFC 2330 and RFC3432), plus the following: + T0, a start time + Tf, an end time + dT, a waiting time for each packet to arrive 4.1.3 Definition: For the packets arriving successfully between T0 and Tf+dT, the ratio of reordered packets in the sample is (Total of Reordered packets) / (Total packets received) This fraction may be expressed as a percentage (multiply by 100%). Note that in the case of duplicate packets, only the first copy is used. 4.2 Reordering Extent This section defines the extent to which packets are reordered, and associates a specific sequence discontinuity with each reordered packet. 4.2.1 Metric Name: Type-P-packet-Reordering-Extent-Stream Morton, et al. Standards Track exp. Dec 2003 Page 6 Packet Reordering Metric for IPPM June 2003 4.2.2 Parameter Notation: Given a stream of packets sent from a source to a destination, let K be the total number of packets in that stream. Assign each packet a sequence number, a consecutive integer from 1 to K in the order of packet emission. Let L be the total number of packets received out of the K packets sent. Recall that identical copies (duplicates) have been removed, so L<=K. Let s[1], s[2], ..., s[L], represent the original sequence numbers associated with the packets in order of arrival. Consider a reordered packet (as identified in section 3) with arrival index i and source sequence number s[i]. There exists a set of indexes j (1<=j<i) such that s[j] > s[i]. 4.2.3 Definition: The reordering extent, e, of packet s[i] is defined to be i-j for the smallest value of j. Informally, the reordering extent is the maximum distance, in packets, from a reordered packet to the earliest packet received that has a larger sequence number. If a packet is in-order, it's reordering extent is undefined. The first packet to arrive is in- order by definition, and has undefined reordering extent. >>>>>>> Comment on this definition of extent: For some arrival orders, the assignment of a simple position/distance as the reordering extent tends to overestimate the receiver storage needed to restore order. We need to weigh the value of adding more complexity in this definition against the accuracy it would provide. A more accurate and complex procedure to calculate packet storage would be to subtract any earlier reordered packets that the receiver could pass on to the upper layers. Those who desire "on-the-fly" calculation must assess whether such a procedure is feasible. 4.2.4 Discussion: The packet with index j (s[j], identified in the Definition above) is the reordering discontinuity associated with packet with index i (s[i]). This definition is formalized below. Note that the K packets in the stream could be some subset of a larger stream, but L is still the total number of packets received out of the K packets sent in that subset. Morton, et al. Standards Track exp. Dec 2003 Page 7 Packet Reordering Metric for IPPM June 2003 A receiver must possess storage to restore order to packets that are reordered. For cases with single reordered packets, the extent e gives the number of packets that must be held in the receiver's buffer while waiting for the reordered packet to complete the sequence. For more complex scenarios, the extent may be an overestimate of required storage. See Examples section (specific example to be provided). Knowledge of the reordering extent e is particularly useful for determining the portion of reordered packets that can or cannot be restored to order in a typical receiver buffer based on their arrival order alone (and without the aid of retransmission). A sample's reordering extents may be expressed as a histogram, to easily summarize the frequency of various extents. 4.3 Reordering Offset Any reordered packets can be assigned offset values indicating the storage in bytes and lateness in terms of buffer time that a receiver must possess to accommodate them. The various offset metrics are calculated only on reordered packets, as identified by the ordered arrival singleton in section 3. 4.3.1 Metric Name: Type-P-packet-Late-Time-Stream Metric Parameters: In addition to the parameters defined for Type-P- Non-Reversing-Order, we specify: + DstTime, the time that each packet in the stream arrives at Dst Definition: Lateness in time is calculated using Dst times. When received packet i is reordered, and has a reordering extent e, then: LateTime(i) = DstTime(i)-DstTime(i-e) Alternatively, using similar notation to that of section 4.2, an equivalent definition is: LateTime(i) = DstTime(i)-DstTime(j), for min{j|1<=j<i} that satisfies s[j]>s[i], or SrcTime[j]>SrcTime[i]. 4.3.2 Metric Name: Type-P-packet-Byte-Offset-Stream Metric Parameters: We use the same parameters defined above. Definition: Byte stream offset is the sum of the payload sizes of intervening in-order packets between the reordered packet and the discontinuity (including the packet at the discontinuity). For reordered packet i with a reordering extent e: ByteOffset(i) = Sum[in-order packets back to reordering discon.] Morton, et al. Standards Track exp. Dec 2003 Page 8 Packet Reordering Metric for IPPM June 2003 = Sum[PayloadSize(packet at i-1 if in-order), PayloadSize(packet at i-2 if in-order), ... PayloadSize(packet at i-e if in-order)] 4.3.3 Discussion The offset metrics can help predict whether reordered packets will be useful in a general receiver buffer system with finite limits. The limit may be the number of bytes or packets the buffer can store, or the time of storage prior to a cyclic play-out instant (as with de-jitter buffers). Note that the One-way IPDV [11] gives the delay variation for a packet w.r.t. the preceding packet in the source sequence. Lateness and IPDV give an indication of whether a buffer at Dst has sufficient storage to accommodate the network's behavior and restore order. When an earlier packet in the Src sequence is lost, IPDV will necessarily be undefined for adjacent packets, and Late Time may provide the only way to evaluate the usefulness of a packet. In the case of de-jitter buffers, there are circumstances where the receiver employs loss concealment at the intended play-out time of a late packet. However, if this packet arrives out of order, the Late Time determines whether the packet is still useful. IPDV no longer applies, because the receiver establishes a new play-out schedule with additional buffer delay to accommodate similar events in the future - this requires very minimal processing. When packets in the stream have variable sizes, it may be most useful to characterize Offset in terms of the payload size(s) of stored packets (using byte stream numbering). 4.4 Gaps between multiple Reordering Discontinuities 4.4.1 Metric Name: Type-P-packet-Reordering-Gap-Stream 4.4.2 Parameters: No new parameters. 4.4.3 Definition of Reordering Discontinuity: All reordered packets are associated with a packet at a reordering discontinuity, defined as the in-order packet arrival s[j] at the minimum value of j (1<=j<i) for which s[j]> s[i]. Morton, et al. Standards Track exp. Dec 2003 Page 9 Packet Reordering Metric for IPPM June 2003 Recall that i - e = min(j). Subsequent reordered packets may be associated with the same s[j], or with a different discontinuity. This definition is used in the definition of the Reordering Gap, below. 4.4.4 Definition of Reordering Gap: A reordering gap is the distance between successive reordering discontinuities. Type-P-packet-Reordering-Gap-Stream assigns a value to (all) packets in a stream. If: The packet s[j'] is found to be a reordering discontinuity, based on the arrival of reordered packet s[i'] with extent e', and an earlier reordering discontinuity s[j], based on the arrival of reordered packet s[i] with extent e is detected, and i' > i, and there are no reordering discontinuities between j and j', then the Reordering Gap for packet s[j'] is the difference between the arrival positions the reordering discontinuities, as shown below: Gap(j') = (j') - (j) Otherwise: The Type-P-packet-Reordering-Gap-Stream for the packet is 0. Gaps may also be expressed in time: GapTime(j') = DstTime(j') - DstTime(j) 4.4.5 Discussion When separate reordering discontinuities can be distinguished, then a count may also be reported (along with the discontinuity description, such as the number of reordered packets associated with that discontinuity and their extents and offsets). The Gaps between a sample's reordering discontinuities may be expressed as a histogram, to easily summarize the frequency of various gaps. Reporting the mode, average, range, etc. may also summarize the distributions. The Gap metric may help to correlate the frequency of reordering discontinuities with their cause. Morton, et al. Standards Track exp. Dec 2003 Page 10 Packet Reordering Metric for IPPM June 2003 4.5 Reordering-free Runs This section defines a metric based on a count of consecutive in- order packet arrivals. 4.5.1 Metric Name: Type-P-packet-Reordering-Free-Run-Stream 4.5.2 Parameters: No new parameters. 4.5.3 Definition: As packets in a sample arrive at Dst, the count of packets to the next reordered packet is a Reordering-Free run. Note that the minimum run-length is one according to this definition. A pseudo code example follows: r = 0; /* r is the run counter */ z = 1; /* z is the index for storing different runs */ Run[*]; /* a vector of run-lengths */ while(packets arrive with sequence number s) { if (s >= NextExp) /* s is in-order */ then r++; else /* s is reordered */ Run[z]=r; r = 1; z++; } 4.5.4 Discussion: Each arrival of a reordered packet yields a new count in the Run vector. Long runs accompany periods where order was maintained, while short runs indicate frequent, or multi-packet reordering. 5. Metric Related to Receiver Assessment 5.1 A TCP-Relevant Metric 5.1.1 Metric Name: Type-P-packet-n-Reordering-Stream 5.1.2 Parameter Notation: Morton, et al. Standards Track exp. Dec 2003 Page 11 Packet Reordering Metric for IPPM June 2003 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. 5.1.3 Definitions Definition 1: Received packet number i (n < i <= l), with source sequence number s[i], is n-reordered if and only if for all j such that i-n <= j < i, s[j] > s[i]. Claim: If by this definition, a packet's reordering is n and 0 < n' < n, then the packet is also reordered to the n' extent. Note: This definition is illustrated by C code in Appendix A. It determines the n-reordering for a value of n=3 (when actually writing applications that would report the metric, one would probably report it for several values of n, such as 1, 2, 3, 4 -- and maybe a few more consecutive values). This definition does not assign an n to all reordered packets as defined by the singleton metric, in particular when blocks of successive packets are reordered. (In the arrival sequence s={1,2,3,7,8,9,4,5,6}, packets 4, 5, and 6 are reordered, but only 4 is n-reordered, with n=3.) Definition 2: The degree of n-reordering of the sample is m/l. Definition 3: The degree of "monotonic reordering" of the sample is its degree of 1-reordering. Definition 4: A sample is said to have no reordering if its degree of n-reordering is 0. 5.1.4 Discussion: The degree of n-reordering may be expressed as a percentage, in which case the number from definition 2 is multiplied by 100. Knowledge of n-reordering is particularly useful for determining the portion of reordered packets that can or cannot be restored to order in a typical TCP receiver buffer based on their arrival order alone (and without the aid of retransmission). Important special cases are n=1 and n=3: Morton, et al. Standards Track exp. Dec 2003 Page 12 Packet Reordering Metric for IPPM June 2003 - For n=1, absence of 1-reordering means the sequence numbers that the receiver sees are monotonically increasing with respect to the previous arriving packet. - For n=3, a NewReno TCP sender would retransmit 1 packet in response to an instance of 3-reordering and therefore consider this packet lost for the purposes of congestion control (the sender will half its congestion window). Detecting instances of 3-reordering is useful for determining the portion of reordered packets that are in fact as good as lost. We note that the definition of n-reordering cannot predict the exact number of packets unnecessarily retransmitted by a TCP sender under some circumstances, such as cases with closely-spaced reordered singletons. The definition is less complicated than a TCP implementation where both time and position influence the sender's behavior. A sample's n-reordering may be expressed as a histogram, to summarize the frequency for each value of n. 6. Measurement Issues The results of tests will be dependent on the time interval between measurement packets (both at the Src, and during transport where spacing may change). Clearly, packets launched infrequently (e.g., 1 per 10 seconds) are unlikely to be reordered. Test streams may prefer to use a periodic sending interval so that a known temporal bias is maintained, also bringing simplified results analysis (as described in RFC 3432 [12]). In this case, the periodic sending interval should be chosen to reproduce the closest Src packet spacing expected. Of course, packet spacing is likely to vary as the stream traverses the test path. <<<<Ed.Note: Is this sufficient? It is a very important consideration. The Non-reversing order criterion and all metrics described above remain valid and useful when a stream of packets experiences packet loss, or both loss and reordering. In other words, losses alone do not cause subsequent packets to be declared reordered. Assuming that the necessary sequence information (sequence number and/or source time stamp) is included in the packet payload (possibly in application headers such as RTP), packet sequence may be evaluated in a passive measurement arrangement. Also, it is possible to evaluate sequence at a single point along a path, since the usual need for synchronized Src and Dst Clocks may be relaxed to some extent. When the Src sequence is based on byte stream, or payload numbering, care must be taken to avoid declaring retransmitted packets Morton, et al. Standards Track exp. Dec 2003 Page 13 Packet Reordering Metric for IPPM June 2003 reordered. The additional reference of Src Time is one way to avoid this ambiguity. Since this metric definition may use sequence numbers with finite range, it is possible that the sequence numbers could reach end-of- range and roll over to zero during a measurement. By definition, the Next Expected value cannot decrease, and all packets received after a roll-over would be declared reordered. Sequence number roll-over can be avoided by using combinations of counter size and test duration where roll-over is impossible (and sequence is reset to zero at the start). Also, message-based numbering results in slower sequence consumption. There may still be cases where methodological mitigation of this problem is desirable (e.g., long- term testing). The elements of mitigation are: 1. There must be a test to detect if a roll-over has occurred. It would be nearly impossible for the sequence numbers of successive packets to jump by more than half the total range, so these large discontinuities are designated as roll-over. 2. All sequence numbers used in computations are represented in a sufficiently large precision. The numbers have a correction applied (equivalent to adding a significant digit) whenever roll-over is detected. 3. Reordered packets coincident with sequence numbers reaching end- of-range must also be detected for proper application of correction factor. In practice, there may be limited ability to determine reordering extent, because the storage for previous packets may be limited. Saving only packets that indicate discontinuities (and their arrival positions) will reduce storage volume. When discarding all stream information beyond a threshold packet count, the reordering extent or degree of n-reordering may need to be expressed as greater than the threshold value, and Gap calculations would not be possible. The requirement to ignore duplicate packets also requires storage. Here, tracking the sequence numbers of missing packets may minimize storage. Missing packets may eventually be declared lost, or reordered if they arrive. The missing packet list and the largest sequence number received thus far are sufficient information to determine if a packet is a duplicate. 7. Examples of Arrival Order Evaluation This section provides some examples to illustrate how the non- reversing order criterion works, and the value of viewing reordering in both the dimensions of time and position. Throughout this section, we will refer to packets by their source sequence number, except where noted. So "Packet 4" refers to the packet with source sequence number 4, and the reader should refer to Morton, et al. Standards Track exp. Dec 2003 Page 14 Packet Reordering Metric for IPPM June 2003 the tables in each example to determine packet 4's arrival index number, if needed. Table 1 gives a simple case of reordering, where one packet is reordered, Packet 4. Packets are listed according to their arrival, and message numbering is used. Table 1 Example with Packet 4 Reordered, Sending order(SrcNum@Src): 1,2,3,4,5,6,7,8,9,10 s Src Dst Dst Byte Late @Dst NextExp Time Time Delay IPDV Order Offset Time 1 1 0 68 68 1 2 2 20 88 68 0 2 3 3 40 108 68 0 3 5 4 80 148 68 -82 4 6 6 100 168 68 0 5 7 7 120 188 68 0 6 8 8 140 208 68 0 7 4 9 60 210 150 82 8 400 62 9 9 160 228 68 0 9 10 10 180 248 68 0 10 Each column gives the following information: s Packet sequence number at the Source. NextExp The value of NextExp when the packet arrived(before update). SrcTime Packet time stamp at the Source, ms. DstTime Packet time stamp at the Destination, ms. Delay 1-way delay of the packet, ms. IPDV IP Packet Delay Variation, ms IPDV = Delay(SrcNum)-Delay(SrcNum-1) DstOrder Order in which the packet arrived at the Destination. Byte Offset The Byte Offset of a reordered packet, in bytes. LateTime The lateness of a reordered packet, in ms. We can see that when Packet 4 arrives, NextExp=9, and it is declared reordered. We compute the extent of reordering as follows: Using the notation <s[1], ..., s[i], ..., s[L]>, the received packets are represented as: \/ s = 1, 2, 3, 5, 6, 7, 8, 4, 9, 10 i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 /\ when j=7, 8 > 4, so the reordering extent is 1 or more. when j=6, 7 > 4, so the reordering extent is 2 or more. when j=5, 6 > 4, so the reordering extent is 3 or more. when j=4, 5 > 4, so the reordering extent is 4 or more. Morton, et al. Standards Track exp. Dec 2003 Page 15 Packet Reordering Metric for IPPM June 2003 when j=3, but 3 < 4, and 4 is the maximum extent, e=4 (assuming there are no earlier sequence discontinuities, as in this example). Further, we can compute the Late Time (210-148=62ms using DstTime) compared to Packet 5's arrival. If the receiver has a de-jitter buffer that holds more than 4 packets, or at least 62 ms storage, Packet 4 may be useful. Note that 1-way delay and IPDV also indicate unusual behavior for Packet 4. If all packets contained 100 byte payloads, then Byte Offset is equal to 400 bytes. Following the definitions of section 5.1, Packet 4 is defined to be 4-reordered. Table 2 Example with Packets 5 and 6 Reordered, Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10 s Src Dst Dst Byte Late @Dst NextExp Time Time Delay IPDV Order Offset Time 1 1 0 68 68 1 2 2 20 88 68 0 2 3 3 40 108 68 0 3 4 4 60 128 68 0 4 7 5 120 188 68 -22 5 5 8 80 189 109 41 6 100 1 6 8 100 190 90 -19 7 100 2 8 8 140 208 68 0 8 9 9 160 228 68 0 9 10 10 180 248 68 0 10 Table 2 shows a case where Packets 5 and 6 arrive just behind Packet 7, so both 5 and 6 are reordered. The Late times (189-188=1, 190- 188=2) are small. Using the notation <s[1], ..., s[i], ..., s[l]>, the received packets are represented as: \/ \/ s = 1, 2, 3, 4, 7, 5, 6, 8, 9, 10 i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 /\ /\ Considering Packet 5 first: when j=5, 7 > 5, so the reordering extent is 1 or more. when j=4, but 4 < 5, so 1 is its maximum extent, and e=1. Considering Packet 6 next: when j=6, 5 < 6, the extent is not yet defined. when j=5, 7 > 6, so the reordering extent is i-j=2 or more. when j=4, 4 < 6, and we find 2 is its maximum extent, and e=2. Morton, et al. Standards Track exp. Dec 2003 Page 16 Packet Reordering Metric for IPPM June 2003 We can also associate each of these reordered packets with a reordering discontinuity. We find the minimum j=5 (for both packets) according to Section 4.2.4. So Packet 6 is associated with the same reordering discontinuity as Packet 5, at Packet 7. Following the definitions of section 5.1, Packet 5 is defined to be 1-reordered, but Packet 6 is not qualified n-reordered. A hypothetical sender/receiver pair may retransmit Packet 5 unnecessarily, since it is 1-reordered (in agreement with the singleton metric). Though Packet 6 may not be unnecessarily retransmitted, the receiver cannot advance Packet 7 to the higher layers until after Packet 6 arrives. Therefore, the singleton metric correctly determined that Packet 6 is reordered. Table 3 Example with Packets 4, 5, and 6 reordered Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11 s Src Dst Dst Byte Late @Dst NextExp Time Time Delay IPDV Order Offset Time 1 1 0 68 68 1 2 2 20 88 68 0 2 3 3 40 108 68 0 3 7 4 120 188 68 -88 4 8 8 140 208 68 0 5 9 9 160 228 68 0 6 10 10 180 248 68 0 7 4 11 60 250 190 122 8 400 62 5 11 80 252 172 -18 9 400 64 6 11 100 256 156 -16 10 400 68 11 11 200 268 68 0 11 The case in Table 3 is where three packets in sequence have long transit times (Packets with s = 4,5,and 6). Delay, Late time, and Byte Offset capture this very well, and indicate variation in reordering extent, while IPDV indicates that the spacing between packets 4,5,and 6 has changed. The histogram of Reordering extents (e) would be: Bin 1 2 3 4 5 6 7 Frequency 0 0 0 1 1 1 0 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. Morton, et al. Standards Track exp. Dec 2003 Page 17 Packet Reordering Metric for IPPM June 2003 when n=2, 6<=j<8, and 9 > 4, so the packet is 2-reordered. when n=3, 5<=j<8, and 8 > 4, so the packet is 3-reordered. when n=4, 4<=j<8, and 7 > 4, so the packet is 4-reordered. when n=5, 3<=j<8, but 3 < 4, and 4 is the maximum n-reordering. Considering packet 5[9] next: when n=1, 8<=j<9, but 4 < 5, so the packet at i=9 is not qualified as n-reordered. We find the same to for Packet 6. We now consider whether reordered Packets 5 and 6 are associated with the same reordering discontinuity as Packet 4. Using the test of Section 4.2.4, Definition 2, we find that the minimum j=4 for all three packets. They are all associated with the reordering discontinuity at Packet 7. This example shows again that the n-reordering definition identifies a single Packet (4) with a sufficient degree of reordering to result in one unnecessary packet retransmission by the New Reno TCP sender. Also, the reordered arrival of Packets 5 and 6 will allow the receiver process to pass Packets 7 through 10 up the protocol stack (the singleton metric indicates 5 and 6 are reordered, and they are all associated with a single reordering discontinuity). Table 4 Example with Packets Multiple Reordering Discontinuities Sending order(s @Src): 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16 Discontinuity Discontinuity |---------Gap---------| s = 1, 2, 3, 6, 7, 4, 5, 8, 9, 10, 12, 13, 11, 14, 15, 16 i = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 r = 1, 2, 3, 4, 5, 1, 1, 2, 3, 4, 5, 6, 1, 2, 3, 4, ... Run[] counts = 5 1 6 Packet 4 has extent e=2, Packet 5 has extent e=3, and Packet 11 has e=2. There are two different reordering discontinuities at Packet 6 (where j=4) and Packet 12 (where j=11). According to the definition of Reordering Gap Gap(j') = (j') - (j) Gap(11) = (11) - (4) = 7 We also have three reordering-free runs of lengths 5, 1, and 6. The differences between these two multiple-event metrics are evident here. Gaps are the distance between sequence discontinuities that are subsequently defined as reordering discontinuities, while reordering-free runs are capture the distance between reordered packets. Morton, et al. Standards Track exp. Dec 2003 Page 18 Packet Reordering Metric for IPPM June 2003 8. Security Considerations 8.1 Denial of Service Attacks This metric requires a stream of packets sent from one host (Src) to another host (Dst) through intervening networks. This method could be abused for denial of service attacks directed at Dst and/or the intervening network(s). Administrators of Src, Dst, and the intervening network(s) should establish bilateral or multi-lateral agreements regarding the timing, size, and frequency of collection of sample metrics. Use of this method in excess of the terms agreed between the participants may be cause for immediate rejection or discard of packets or other escalation procedures defined between the affected parties. 8.2 User data confidentiality Active use of this method generates packets for a sample, rather than taking samples based on user data, and does not threaten user data confidentiality. Passive measurement must restrict attention to the headers of interest. Since user payloads may be temporarily stored for length analysis, suitable precautions MUST be taken to keep this information safe and confidential. 8.3 Interference with the metric It may be possible to identify that a certain packet or stream of packets is part of a sample. With that knowledge at Dst and/or the intervening networks, it is possible to change the processing of the packets (e.g. increasing or decreasing delay) that may distort the measured performance. It may also be possible to generate additional packets that appear to be part of the sample metric. These additional packets are likely to perturb the results of the sample measurement. To discourage the kind of interference mentioned above, packet interference checks, such as cryptographic hash, may be used. 9. IANA Considerations Since this metric does not define a protocol or well-known values, there are no IANA considerations in this memo. 10. References 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. Morton, et al. Standards Track exp. Dec 2003 Page 19 Packet Reordering Metric for IPPM June 2003 3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework for IP Performance Metrics", RFC 2330, May 1998. 4 V.Paxson, "Measurements and Analysis of End-to-End Internet Dynamics," Ph.D. dissertation, U.C. Berkeley, 1997, ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.ps.gz. 5 Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. Obtain via: http://www.rfc-editor.org/rfc/rfc793.txt 6 L.Ciavattone and A.Morton, "Out-of-Sequence Packet Parameter Definition (for Y.1540)", Contribution number T1A1.3/2000-047, October 30, 2000. ftp://ftp.t1.org/pub/t1a1/2000-A13/0a130470.doc 7 J.C.R.Bennett, C.Partridge, and N.Shectman, "Packet Reordering is Not Pathological Network Behavior," IEEE/ACM Transactions on Neteworking, vol.7, no.6, pp.789-798, December 1999. 8 D.Loguinov and H.Radha, "Measurement Study of Low-bitrate Internet Video Streaming", Proceedings of the ACM SIGCOMM Internet Measurement Workshop 2001 November 1-2, 2001, San Francisco, USA. 9 J.Bellardo and S.Savage, "Measuring Packet Reordering," Proceedings of the ACM SIGCOMM Internet Measurement Workshop 2002, November 6-8, Marseille, France. 10 S.Jaiswal et al., "Measurement and Classification of Out-of- Sequence Packets in a Tier-1 IP Backbone," Proceedings of the ACM SIGCOMM Internet Measurement Workshop 2002, November 6-8, Marseille, France. 11 Demichelis, C., and Chimento, P., "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)", RFC 3393, November 2002. 12 Raisanen, V., Grotefeld, G., and Morton, A., "Network performance measurement with periodic streams", RFC 3432, November 2002. 12. Acknowledgments The authors would like to acknowledge many helpful discussions with Matt Mathis, Jon Bennett, and Matt Zekauskas. We gratefully acknowledge the foundation laid by the authors of the IP performance Framework [3]. 13. Appendix A (informative) Two example c-code implementations of reordering definitions follow: Morton, et al. Standards Track exp. Dec 2003 Page 20 Packet Reordering Metric for IPPM June 2003 Example 1 n-reordering ============================================ #include <stdio.h> #define MAX_N 100 #define min(a, b) ((a) < (b)? (a): (b)) #define loop(x) ((x) >= 0? x: x + MAX_N) /* * Read new sequence number and return it. Return a sentinel value of EOF * (at least once) when there are no more sequence numbers. In this example, * the sequence numbers come from stdin; in an actual test, they would come * from the network. */ int read_sequence_number() { int res, rc; rc = scanf("%d\n", &res); if (rc == 1) return res; else return EOF; } int main() { int m[MAX_N]; /* We have m[j-1] == number of * j-reordered packets. */ int ring[MAX_N]; /* Last sequence numbers seen. */ int r = 0; /* Ring pointer for next write. */ int l = 0; /* Number of sequence numbers read. */ int s; /* Last sequence number read. */ int j; for (j = 0; j < MAX_N; j++) m[j] = 0; for (; (s = read_sequence_number()) != EOF; l++, r = (r+1) % MAX_N) { for (j=0; j<min(l, MAX_N) && s<ring[loop(r-j-1)]; j++) m[j]++; ring[r] = s; Morton, et al. Standards Track exp. Dec 2003 Page 21 Packet Reordering Metric for IPPM June 2003 } for (j = 0; j < MAX_N && m[j]; j++) printf("%d-reordering = %f%%\n", j+1, 100.0*m[j]/(l- j-1)); if (j == 0) printf("no reordering\n"); else if (j < MAX_N) printf("no %d-reordering\n", j+1); else printf("only up to %d-reordering is handled\n", MAX_N); exit(0); } Example 2 singleton and n-reordering comparison ================= #include <stdio.h> #define MAX_N 100 #define min(a, b) ((a) < (b)? (a): (b)) #define loop(x) ((x) >= 0? x: x + MAX_N) /* Global counters */ int receive_packets=0; /* number of recieved */ int reorder_packets=0; /* number of reordered packets */ /* function to test if current packet has been reordered * returns 0 = not reordered * 1 = reordered */ int testorder1(int seqnum) // Al { static int NextExp = 1; int iReturn = 0; if (seqnum >= NextExp) { NextExp = seqnum+1; } else { iReturn = 1; } return iReturn; } int testorder2(int seqnum) // Stanislav { static int ring[MAX_N]; /* Last sequence numbers seen. */ static int r = 0; /* Ring pointer for next write. */ int l = 0; /* Number of sequence numbers read. */ int j; int iReturn = 0; l++; r = (r+1) % MAX_N; Morton, et al. Standards Track exp. Dec 2003 Page 22 Packet Reordering Metric for IPPM June 2003 for (j=0; j<min(l, MAX_N) && seqnum<ring[loop(r-j-1)]; j++) iReturn = 1; ring[r] = seqnum; return iReturn; } int main(int argc, char *argv[]) { int i, packet; for (i=1; i< argc; i++) { receive_packets++; packet = atoi(argv[i]); reorder_packets += testorder2(packet); } printf("Received packets = %d, Reordered packets = %d\n", receive_packets, reorder_packets); exit(0); } 13. Author's Addresses Al Morton AT&T Labs Room D3 - 3C06 200 Laurel Ave. South Middletown, NJ 07748 USA Phone +1 732 420 1571 <acmorton@att.com> Len Ciavattone AT&T Labs Room C4 - 2B29 200 Laurel Ave. South Middletown, NJ 07748 USA Phone +1 732 420 1239 <lencia@att.com> Gomathi Ramachandran AT&T Labs Room C4 - 3D22 200 Laurel Ave. South Middletown, NJ 07748 USA Phone +1 732 420 2353 <gomathi@att.com> Stanislav Shalunov Internet2 200 Business Park Drive, Suite 307 Armonk, NY 10504 Phone: + 1 914 765 1182 EMail: <shalunov@internet2.edu> Morton, et al. Standards Track exp. Dec 2003 Page 23 Packet Reordering Metric for IPPM June 2003 Jerry Perser Spirent Communications 26750 Agoura Road Calabasas, CA 91302 USA Phone: + 1 818 676 2300 EMail: <jerry.perser@spirentcom.com> Full Copyright Statement "Copyright (C) The Internet Society (date). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Morton, et al. Standards Track exp. Dec 2003 Page 24
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