Network Working Group A.Morton Internet Draft L.Ciavattone Document: 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, suggests a metric definition, and discusses the issues associated with measurement. The memo includes 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. Sept 2003 Page 1 Packet Reordering Metric for IPPM Mar 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 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. Sept 2003 Page 2 Packet Reordering Metric for IPPM Mar 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] gives the definitions of singletons, samples, and statistics. 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 Src. 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 Src time is used as the sequence number, Morton, et al. Standards Track exp. Sept 2003 Page 3 Packet Reordering Metric for IPPM Mar 2003 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 Src (or wire time) + s, the packet sequence number applied at the Src, in units of messages. + SrcByte, the packet sequence number applied at the Src, in units of payload bytes. + NextExp, the Next Expected Sequence number at the Dst, in units 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: In-order packets have sequence numbers (or Src times) greater than or equal to the value of Next Expected. Each new in-order packet will increase the Next Expected (typically by 1 for message numbering, or the payload size plus 1 for byte numbering). The Next Expected value cannot decrease, thereby specifying non-reversing order as the basis to identify reordered packets. A reordered packet outcome occurs when a single IP packet at the Dst Measurement Point results in the following: The packet has a Src sequence number lower than the Next Expected (NextExp), and therefore the packet is reordered. The Next Expected value does not change on the arrival of this packet. This definition can also be specified in pseudo-code. On successful arrival of a packet with sequence number s: if s >= NextExp, /* s is in-order */ then NextExp = s + PayloadSize + 1; else /* when s < NextExp */ Morton, et al. Standards Track exp. Sept 2003 Page 4 Packet Reordering Metric for IPPM Mar 2003 designate packet s as reordered; The sequence number s may be replaced by SrcTime or SrcByte. When using s (message-based sequence numbering) or Src 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 if 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. If duplicate packets (multiple non-corrupt copies) arrive at the destination, they MUST be noted and only the first to arrive is considered for further analysis (additional copies would be declared reordered according to the definition above). This requirement has the same storage requirements as earlier IPPM metrics, and follows the precedent of RFC 2679. 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 part of a reordering event (defined in the next section). Discontinuities are easiest to detect with message numbering or payload byte numbering where payload size is constant, and may be possible with Periodic Streams and Src Time numbering. 4. Sample Metrics In this section, we define metrics applicable to a sample of packets from a single Src sequence number system. We begin with a simple ratio metric indicating the reordered portion of the sample. When this ratio is zero, no further reordering metrics are needed for that sample. When reordering occurs, it is highly desirable to assert the degree to which a packet is out-of-order, or reordered with respect to a sample of packets. This section defines several metrics that quantify the extent of reordering in various units of measure. Each "extent" metric highlights a relevant application. 4.1 Reordered Packet Ratio 4.1.1 Metric Name: Type-P-Reordered-Ratio-Poisson/Periodic-Stream 4.1.2 Metric Parameters: Morton, et al. Standards Track exp. Sept 2003 Page 5 Packet Reordering Metric for IPPM Mar 2003 The parameter set includes Type-P-Non-Reversing-Order singleton parameters, the parameters unique to Poisson or Periodic Streams, 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 Events and their Extent Note: This section is expected to evolve further as we integrate the various metrics of reordering extent (n-reordering and other distance metrics used in earlier drafts). The co-authors are not yet satisfied with all aspects of this section, and comments are welcome. 4.2.1 Metric Name: Type-P-packet-n-Reordering-Event-Poisson/Periodic-Stream 4.2.2 Parameter Notation: Let n be a positive integer (a parameter). Let k be a positive integer (sample size, the number of packets sent). Let l be a non- negative integer representing the number of packets that were received out of the k packets sent. (Note that there is no relationship between k and l: on one hand, losses can make l less than k; on the other hand, duplicates can make l greater than k.) Assign each sent packet a sequence number, 1 to k. Let s[1], s[2], ..., s[l] be the original sequence numbers of the received packets, in the order of arrival. 4.2.3 Definition 1: Received packet number i (n < i <= l), with Src sequence number s (s[i]), is reordered to the extent n if and only if for all j such that i-n <= j < i we have s[j] > s[i]. Morton, et al. Standards Track exp. Sept 2003 Page 6 Packet Reordering Metric for IPPM Mar 2003 Claim: If by this definition, the extent of 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 presence of n-reordering events for a particular value of n (when actually writing applications that would report the metric, one would probably report it for several values of n, such as 1, 2, 3, 4 -- and maybe a few more consecutive values). Also, this definition does not assign an extent to all reordered packets as defined by the singleton metric, in particular when blocks of successive packets are reordered. (In the arrival sequence s={1,2,3,7,8,9,4,5,6}, packets 4, 5, and 6 are reordered, but only 4 is assigned a reordering extent according to Definition 1.) All reordered packets are assigned a reordering extent by associating them with a specific reordering event, as defined below. 4.2.4 Definition 2: Note: The intent of this section is to assign a reordering extent to all reordered packets, not just the ones identified by Definition 1. This definition is new in this version and needs more study. A packet s[i] that satisfies Definition 1 constitutes an n- Reordering Event with the following characteristics: 1. The maximum value of n that satisfies Definition 1 is the extent of the reordering event. (Extent n is assigned to all packets associated with this event in part 3 below.) 2. The in-order packet arrival defined as beginning the event (having indicated a sequence discontinuity) is s[j] for j that satisfies the following: min{j|1<=j s[i] Typically i-n=j. This aspect of a reordering event is used later in the definition of the gap between successive events. 3. The arrival of any subsequent reordered packets with sequence number s meeting the following condition: s[j] > s[*] > s[i], or (s at beginning of event) > s > (lowest s in the reordering event) are associated with the reordering event first identified by s[i], the sequence discontinuity that starts the event at s[j], and are assigned the same reordering extent, n. >>> Comment on Part 3.: For some arrival orders, the assignment of the same extent to all packets in a reordering event will tend to underestimate their true extent. However, a pure position/distance metric (as appeared in earlier versions of this draft) would tend to Morton, et al. Standards Track exp. Sept 2003 Page 7 Packet Reordering Metric for IPPM Mar 2003 overestimate the receiver storage needed. We need to weigh the value of adding more complexity in this definition against the accuracy it would provide. A more accurate and complex procedure would be to count any additional in-order packets that arrive after a reordering event is identified, and add them to the extent of the first reordered packet (up to some counter limit of interest) for packets in the interval s[i] < s[*] < s[j]. Those who desire "on-the-fly" calculation must assess whether such a procedure is feasible. Discussion: A receiver must perform some amount of "work" to restore order to all packets that are part of an n-reordering event. The extent n gives the number of packets that must be held in the receiver's buffer while waiting for the reordered packets in the sequence. As reordered packets arrive, the amount of work stays the same if they are all part of the same reordering event. If new reordering events occur, the work in terms of buffer size can increase. See Examples section (specific example to be provided). Knowledge of the reordering extent n is particularly useful for determining the portion of reordered packets that can or cannot be restored to order in a typical TCP receiver buffer based on their arrival order alone (and without the aid of retransmission). Important special cases are n=1 and n=3: - For n=1, absence of 1-reordering events means the sequence numbers that the receiver sees are monotonically increasing with respect to the previous arriving packet. - For n=3, a NewReno TCP sender would retransmit 1 packet in response to a 3-reordering event and therefore consider this a loss event for the purposes of congestion control (the sender will half its congestion window). Detecting 3-reordering events is useful for determining the portion of reordered packets that are in fact as good as lost. We note that the definition of n-reordering events cannot predict the exact number of packets unnecessarily retransmitted by a TCP sender under some circumstances, such as closely-spaced reordering events. The definition is less complicated than a TCP implementation where both time and position influence the sender's behavior. A sample's reordering extents may be expressed as a histogram, to easily summarize the frequency of various extents. 4.3 Reordering Offset Morton, et al. Standards Track exp. Sept 2003 Page 8 Packet Reordering Metric for IPPM Mar 2003 Any packet whose sequence number causes the Next Expected value to increment by more than the usual increment indicates a discontinuity in the sequence. From this point on, any reordered packets can be assigned Offset values indicating the storage in bytes and lateness in terms of buffer time that a receiver must possess to accommodate them. The various Offset metrics are calculated only on reordered packets, as identified by the ordered arrival singleton in section 3. 4.3.1 Metric Name: Type-P-packet-Late-Time-Poisson/Periodic-Stream Metric Parameters: In addition to the parameters defined for Type-P- 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 packet i is reordered, and part of a reordering event with n extent (assuming j=i-n): LateTime(i) = DstTime(i)-DstTime(i-n) Alternatively, using similar notation to that of section 4.2, an equivalent definition is: LateTime(i) = DstTime[i]-DstTime[j], for min{j|1<=js[i], or SrcTime[j]>SrcTime[i]. 4.3.2 Metric Name: Type-P-packet-Byte-Offset-Poisson/Periodic-Stream Metric Parameters: We use the same parameters defined above. Definition: Byte stream offset is the sum of the payload sizes of all intervening packets between the reordered packet and the discontinuity (including the packet at the discontinuity). For reordered packet i ByteOffset(i) = Sum[in-order packets to start of the reordering event] = Sum[PayloadSize(packet at i-1), PayloadSize(packet at i-2), ... PayloadSize(packet at i-n)] Alternatively, if all payload sizes are equal: ByteOffset(i) = n * PayloadSize where n is the reordering extent. >>>>Comment: Previous comments on the accuracy of extent n apply here as well. 4.3.3 Discussion The Offset metrics can predict whether reordered packets will be 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 Morton, et al. Standards Track exp. Sept 2003 Page 9 Packet Reordering Metric for IPPM Mar 2003 the time of storage prior to a cyclic play-out instant (as with de- jitter buffers). Note that the One-way IPDV [9] gives the delay variation for a packet w.r.t. the preceding packet in the source sequence. Lateness 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 Events 4.4.1 Metric Name: Type-P-packet-Reordering-Event-Gap-Poisson/Periodic-Stream 4.4.2 Parameters: No new parameters. 4.4.3 Definition: A reordering event with extent n is detected according to section 4.2 with the arrival of packet s[i]. The next reordering event with extent n' is detected at packet i', and there are no reordering events between i and i'. The Reordering Event Gap is the difference between the arrival positions the packets, as shown below (assuming j=i-n): Gap(i) = (i'-n') - (i-n) Gaps may also be expressed in time: GapTime(i) = DstTime(i'-n') - DstTime(i-n) Morton, et al. Standards Track exp. Sept 2003 Page 10 Packet Reordering Metric for IPPM Mar 2003 The Gaps between a sample's reordering events may be expressed as a histogram, to easily summarize the frequency of various extents. 4.4.4 Discussion When separate reordering events can be distinguished, then an event count may also be reported (along with the event description, such as the number of reordered packets and their extents or offsets). The distribution of various metrics may also be reported and summarized by the mode, average, range, histogram, etc. The Gap metric may help to correlate the frequency of reordering events with their cause. 5. Measurement Issues The results of tests will be dependent on the time interval between 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 [10]). In this case, the periodic sending interval should be chosen to reproduce the closest Src packet spacing expected. <<<, 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 n=1, 7<=J<8, and 8 > 4, so the reordering extent is 1 or more. when n=2, 6<=J<8, and 7 > 4, so the reordering extent is 2 or more. when n=3, 5<=J<8, and 6 > 4, so the reordering extent is 3 or more. when n=4, 4<=J<8, and 5 > 4, so the reordering extent is 4 or more. when n=5, 3<=J<8, but 3 < 4, and 4 is the maximum extent. Further, we can compute the Late Time (210-148=62ms using DstTime) compared to packet 5's arrival. If Dst has a de-jitter buffer that holds more than 4 packets, or at least 62 ms storage, packet 4 may be useful. Note that 1-way delay and IPDV also indicate unusual behavior for packet 4. If all packets contained 100 byte payloads, then Byte Offset is equal to 400 bytes. 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 Morton, et al. Standards Track exp. Sept 2003 Page 13 Packet Reordering Metric for IPPM Mar 2003 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 , 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[6] first: when n=1, 5<=J<6, and 7 > 5, so the reordering extent is 1 or more. when n=2, 4<=J<6, but 4 < 5, and 1 is the maximum extent. Considering packet 6[7] next: when n=1, 6<=J<7, and 5 < 6, so the packet at i=7 does not have its own reordering extent, and must be part of the same reordering event as packet 5[6]. Using the test of Section 4.2.4, Definition 2, we find that the condition is met for packet 6[7]: s[i] < s < s[i-n] 5[6] < 6[7] < 7[5] A hypothetical sender/receiver pair may retransmit packet 5[8] unnecessarily, since it is reordered with extent n=1(in agreement with the singleton metric). However, the receiver cannot advance packet 7[5] to the higher layers until after packet 6[7] arrives. Therefore, the singleton metric correctly determined that 6[7] is reordered, and both packets are part of a 1-reordering event. 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 -68 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 Morton, et al. Standards Track exp. Sept 2003 Page 14 Packet Reordering Metric for IPPM Mar 2003 reordering extent, while IPDV indicates that the spacing between packets 4,5,and 6 has changed. The histogram of Reordering extents (n) would be: Bin 1 2 3 4 5 6 7 Frequency 0 0 0 3 0 0 0 Using the notation , 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 Considering packet 4[8] first: when n=1, 7<=J<8, and 10> 4, so the reordering extent is 1 or more. when n=2, 6<=J<8, and 9 > 4, so the reordering extent is 2 or more. when n=3, 5<=J<8, and 8 > 4, so the reordering extent is 3 or more. when n=4, 4<=J<8, and 7 > 4, so the reordering extent is 4 or more. when n=5, 3<=J<8, but 3 < 4, and 4 is the maximum extent. Considering packet 5[9] next: when n=1, 8<=J<9, but 4 < 5, so the packet at i=9 does not have its own reordering extent, and must be part of the same reordering event as packet 4[8]. Using the test of Section 4.2.4, Definition 2, we find that the condition is met for both packets 5[9] and 6[10]: s[i] < s < s[i-n] 4[8] < 5[9] < 7[4] 4[8] < 6[10]< 7[4] This example shows again that the n-reordering event definition identifies a single event (s=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 s=5 and s=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 part of one reordering event). 7. Security Considerations 7.1 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 Morton, et al. Standards Track exp. Sept 2003 Page 15 Packet Reordering Metric for IPPM Mar 2003 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. 7.2 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. 7.3 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. 8. IANA Considerations Since this metric does not define a protocol or well-known values, there are no IANA considerations in this memo. 9. References 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. 3 Paxson, V., Almes, G., Mahdavi, J., and Mathis, M., "Framework for IP Performance Metrics", RFC 2330, May 1998. 4 V.Paxson, "Measurements and Analysis of End-to-End Internet Dynamics," Ph.D. dissertation, U.C. Berkeley, 1997, ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.ps.gz. 5 Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. Obtain via: http://www.rfc-editor.org/rfc/rfc793.txt Morton, et al. Standards Track exp. Sept 2003 Page 16 Packet Reordering Metric for IPPM Mar 2003 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 Demichelis, C., and Chimento, P., "IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)", RFC 3393, November 2002. 10 Raisanen, V., Grotefeld, G., and Morton, A., "Network performance measurement with periodic streams", RFC 3432, November 2002. 11. Acknowledgments The authors would like to acknowledge many helpful discussions with Matt Mathis and Jon Bennett. We gratefully acknowledge the foundation laid by the authors of the IP performance Framework [3]. 12. Appendix A (informative) Two example c-code implementations of reordering definitions follow: Example 1 n-reordering ============================================ #include #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() Morton, et al. Standards Track exp. Sept 2003 Page 17 Packet Reordering Metric for IPPM Mar 2003 { 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 #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 */ Morton, et al. Standards Track exp. Sept 2003 Page 18 Packet Reordering Metric for IPPM Mar 2003 /* 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; for (j=0; j Len Ciavattone AT&T Labs Room C4 - 2B29 200 Laurel Ave. South Middletown, NJ 07748 USA Phone +1 732 420 1239 Gomathi Ramachandran AT&T Labs Room C4 - 3D22 200 Laurel Ave. South Middletown, NJ 07748 USA Phone +1 732 420 2353 Stanislav Shalunov Internet2 200 Business Park Drive, Suite 307 Armonk, NY 10504 Phone: + 1 914 765 1182 EMail: Jerry Perser Spirent Communications 26750 Agoura Road Calabasas, CA 91302 USA Phone: + 1 818 676 2300 EMail: Full Copyright Statement "Copyright (C) The Internet Society (date). 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