--- 1/draft-ietf-bmwg-ippm-delay-00.txt 2006-02-04 22:52:43.000000000 +0100 +++ 2/draft-ietf-bmwg-ippm-delay-01.txt 2006-02-04 22:52:43.000000000 +0100 @@ -1,17 +1,17 @@ Network Working Group G. Almes, Advanced Network & Services Internet Draft S. Kalidindi, Advanced Network & Services -Expiration Date: May 1997 November 1996 +Expiration Date: September 1997 March 1997 A One-way Delay Metric for IPPM - + 1. Status of this Memo This document is an Internet Draft. Internet Drafts are working doc- uments of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute work- ing documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months, and may be updated, replaced, or obsoleted by other documents @@ -24,32 +24,32 @@ munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. 2. Introduction This memo defines a metric for one-way delay of packets across Inter- - net paths. It builds on notions introduced and discussed in the - revised IPPM Framework document (currently ); the reader is assumed to be familiar with that - document. {Comment: The revised document, which is being edited in - parallel with the present document, introduces the notion of 'type-P' - packets, develops some notions of clock uncertainties, develops some - notions of measurement calibration, and develops some techniques use- - ful for statistics.} + net paths. It builds on notions introduced and discussed in the IPPM + Framework document (currently "Framework for IP Provider Metrics" + ); the reader is assumed to be + familiar with that document. + + This memo is intended to be very parallel in structure to a companion + document for Packet Loss (soon to be submitted as "A Packet Loss Met- + ric for IPPM" ). The structure of the memo is as follows: -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 + A 'singleton' analytic metric, called Type-P-One-way-Delay, will be introduced to measure a single observation of one-way delay. + Using this singleton metric, a 'sample', called Type-P-One-way- Delay-Stream, will be introduced to measure a sequence of single- ton delays measured at times taken from a Poisson process. + Using this sample, several 'statistics' of the sample will be defined and discussed. This progression from singleton to sample to statistics, with clear separation among them, is important. {Comment: In fact, it might be @@ -79,24 +79,24 @@ It is outside the scope of this document to say precisely how delay metrics would be applied to specific problems. 2.2. General Issues Regarding Time Whenever a time (i.e., a moment in history) is mentioned here, it is understood to be measured in seconds relative to 0000 UT on 1 January 1900. {Comment: times will thus be commensurate with NTP timestamps [Mills: RFC 1305].} - As described more fully in the (revised) Framework document, there - are four distinct, but related notions of clock uncertainty: + As described more fully in the Framework document, there are four + distinct, but related notions of clock uncertainty: -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 synchronization measures the extent to which two clocks agree on what time it is. For example, the clock on one host might be 5.4 msec ahead of the clock on a second host. accuracy measures the extent to which a given clock agrees with UTC. For example, the clock on a host might be 27.1 msec behind UTC. @@ -116,51 +116,48 @@ 3. A Singleton Definition for One-way Delay 3.1. Metric Name: Type-P-One-way-Delay 3.2. Metric Parameters: + Src, the IP address of a host + Dst, the IP address of a host + T, a time - + First-hop, the IP address of the first hop router on the path from - Src to Dst; this optional parameter defaults to the router one hop - from Src whenever there is in fact only one such router - {Comment: the presence of first-hop is motivated by cases such as - with Merit's NetNow setup, in which a Src on one NAP can reach a Dst - on another NAP by either of several different backbone networks. - Generally, this optional step is useful when several different routes - are possible from Src to Dst and determining the first hop can effec- - tively choose among them. The more flexible loose source route IP + + Path, the path* from Src to Dst; in cases where there is only one + path from Src to Dst, this optional parameter can be omitted + {Comment: the presence of path is motivated by cases such as with + Merit's NetNow setup, in which a Src on one NAP can reach a Dst on + another NAP by either of several different backbone networks. Gener- + ally, this optional parameter is useful only when several different + routes are possible from Src to Dst. Using the loose source route IP option is avoided since it would often artificially worsen the per- formance observed, and since it might not be supported along some paths.} -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 3.3. Metric Units: The value of a type-P-One-way-Delay is either a non-negative real number or an undefined (informally, infinite) number of seconds. 3.4. Definition: For a non-negative real number dT, >>the *Type-P-One-way-Delay* from - Src to Dst at T [via first-hop] is dT<< means that Src sent a type-P - packet [via first-hop] to Dst at time T and that Dst received that - packet at time T+dT. + Src to Dst at T [via path] is dT<< means that Src sent a type-P + packet [via path] to Dst at time T and that Dst received that packet + at time T+dT. - >>The *Type-P-One-way-Delay* from Src to Dst at T [via first-hop] is - undefined (informally, infinite)<< means that Src sent a type-P - packet [via first-hop] to Dst at time T and that Dst did not receive - that packet. + >>The *Type-P-One-way-Delay* from Src to Dst at T [via path] is unde- + fined (informally, infinite)<< means that Src sent a type-P packet + [via path] to Dst at time T and that Dst did not receive that packet. 3.5. Discussion: Type-P-One-way-Delay is a relatively simple analytic metric, and one that we believe will afford effective methods of measurement. The following issues are likely to come up in practice: + Since delay values will often be as low as the 100 usec to 10 msec range, it will be important for Src and Dst to synchronize very closely. GPS systems afford one way to achieve synchronization to @@ -176,73 +173,77 @@ large (and the packet is yet to arrive at Dst). As noted by Mah- davi and Paxson, simple upper bounds (such as the 255 seconds the- oretical upper bound on the lifetimes of IP packets [Postel: RFC 791]) could be used, but good engineering, including an under- standing of packet lifetimes, will be needed in practice. {Com- ment: Note that, for many applications of these metrics, the harm in treating a large delay as infinite might be zero or very small. A TCP data packet, for example, that arrives only after several multiples of the RTT may as well have been lost.} -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 + As with other 'type-P' metrics, the value of the metric may depend on such properties of the packet as protocol, (UDP or TCP) port number, size, and arrangement for special treatment (as with IP precedence or with RSVP). + + If the packet is duplicated along the path (or paths!) so that + multiple non-corrupt copies arrive at the destination, then the + packet is counted as received, and the first copy to arrive deter- + mintes the packet's one-way delay. 3.6. Methodologies: As with other Type-P-* metrics, the detailed methodology will depend on the Type-P (e.g., protocol number, UDP/TCP port number, size, precedence). Generally, for a given Type-P, the methodology would proceed as fol- lows: + Arrange that Src and Dst are synchronized; that is, that they have clocks that are very closely synchronized with each other and each fairly close to the actual time. + At the Src host, select Src and Dst IP addresses, and form a test packet of Type-P with these addresses. Any 'padding' portion of the packet needed only to make the test packet a given size should be filled with randomized bits to avoid a situation in which the measured delay is lower than it would otherwise be due to compres- sion techniques along the path. - + Optionally, select a first-hop router IP address and arrange for - Src to send the packet to that router. {Comment: This could be - done, for example, by installing a temporary host-route for Dst in - Src's routing table.} + + Optionally, select a specific path and arrange for Src to send the + packet to that path. {Comment: This could be done, for example, + by installing a temporary host-route for Dst in Src's routing + table.} + At the Dst host, arrange to receive the packet. + At the Src host, place a timestamp in the prepared Type-P packet, - and send it towards Dst [via first-hop]. + and send it towards Dst [via path]. + If the packet arrives within a reasonable period of time, take a timestamp as soon as possible upon the receipt of the packet. By subtracting the two timestamps, an estimate of one-way delay can be computed. Error analysis of a given implementation of the method must take into account the closeness of synchronization between Src and Dst. If the delay between Src's timestamp and the actual sending of the packet is known, then the estimate could be adjusted by subtracting this amount; uncertainty in this value must be taken into account in error analysis. Similarly, if the delay between the actual receipt of the packet and Dst's timestamp is known, then the estimate could be adjusted by subtracting this amount; uncertainty in this value must be taken into account in error analysis. -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 + If the packet fails to arrive within a reasonable period of time, the one-way delay is taken to be undefined (informally, infinite). Note that the threshold of 'reasonable' here is a parameter of the methodology. {Comment: or should it be a parameter of the met- ric?} - Issues such as the packet format, the means by which the first-hop is + Issues such as the packet format, the means by which the path is ensured, the means by which Dst knows when to expect the test packet, and the means by which Src and Dst are synchronized are outside the scope of this document. {Comment: We plan to document elsewhere our own work in describing such more detailed implementation techniques and we encourage others to as well.} 3.7. Errors and Uncertainties: The description of any specific measurement method should include an accounting and analysis of various sources of error/uncertainty. The @@ -267,21 +268,21 @@ tion, accuracy, resolution, and skew mentioned in the Introduction, we note the following: + Any error in the synchronization between the source clock and the dest clock will contribute to error in the delay measurement. We say that the source clock and the dest clock have a synchroniza- tion error of Tsynch if the source clock is Tsynch ahead of the dest clock. Thus, if we know the value of Tsynch exactly, we could correct for clock synchronization by adding Tsynch to the uncorrected value of Tdest-Tsource. -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 + The accuracy of a clock is important only in identifying the time at which a given delay was measured. Accuracy, per se, has no importance to the accuracy of the measurement of delay. This is because, when computing delays, we are interested only in the dif- ferences between clock values. + The resolution of a clock adds to uncertainty about any time mea- sured with it. Thus, if the source clock has a resolution of 10 msec, then this adds 10 msec of uncertainty to any time value mea- sured with it. We will denote the resolution of the source clock @@ -317,66 +318,65 @@ To the extent that the difference between wire time and host time is accurately known, this knowledge can be used to correct for host time measurements and the corrected value more accurately estimates the desired (wire time) metric. To the extent, however, that the difference between wire time and host time is uncertain, this uncertainty must be accounted for in an analysis of a given measurement method. We denote by Hsource an -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 upper bound on the uncertainty in the difference between wire time and host time on the Src host, and similarly define Hdest for the Dst host. We then note that these problems introduce a total uncertainty of Hsource+Hdest. This estimate of total wire-vs-host uncertainty should be included in the error/uncertainty analysis of any measure- ment implementation. 4. A Definition for Samples of One-way Delay Given the singleton metric Type-P-One-way-Delay, we now define one particular sample of such singletons. The idea of the sample is to - select a particular binding of the parameters Src, Dst, first-hop, - and Type-P, then define a sample of values of parameter T. The means - for defining the values of T is to select a beginning time T0, a - final time Tf, and an average rate lambda, then define a pseudo- - random Poisson arrival process of rate lambda, whose values fall - between T0 and Tf. The time interval between successive values of T - will then average 1/lambda. + select a particular binding of the parameters Src, Dst, path, and + Type-P, then define a sample of values of parameter T. The means for + defining the values of T is to select a beginning time T0, a final + time Tf, and an average rate lambda, then define a pseudo-random + Poisson arrival process of rate lambda, whose values fall between T0 + and Tf. The time interval between successive values of T will then + average 1/lambda. 4.1. Metric Name: Type-P-One-way-Delay-Stream 4.2. Metric Parameters: + Src, the IP address of a host + Dst, the IP address of a host - + First-hop, the IP address of the first hop router on the path from - Src to Dst; this optional parameter defaults to the router one hop - from Src whenever there is in fact only one such router + + Path, the path* from Src to Dst; in cases where there is only one + path from Src to Dst, this optional parameter can be omitted + T0, a time + Tf, a time + lambda, a rate in reciprocal seconds 4.3. Metric Units: A sequence of pairs; the elements of each pair are: + T, a time, and + dT, either a non-negative real number or an undefined number of seconds. The values of T in the sequence are monotonic increasing. Note that T would be a valid parameter to Type-P-One-way-Delay, and that dT would be a valid value of Type-P-One-way-Delay. -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 4.4. Definition: Given T0, Tf, and lambda, we compute a pseudo-random Poisson process beginning at or before T0, with average arrival rate lambda, and end- ing at or after Tf. Those time values greater than or equal to T0 and less than or equal to Tf are then selected. At each of the times in this process, we obtain the value of Type-P-One-way-Delay at this time. The value of the sample is the sequence made up of the result- ing pairs. If there are no such pairs, the sequence is @@ -389,42 +389,42 @@ fully specified. Pseudo-random number generators of good quality will be needed to achieve the desired qualities. The sample is defined in terms of a Poisson process both to avoid the effects of self-synchronization and also capture a sample that is statistically as unbiased as possible. {Comment: there is, of course, no claim that real Internet traffic arrives according to a Poisson arrival process.} All the singleton Type-P-One-way-Delay metrics in the sequence will - have the same values of Src, Dst, [first-hop,] and Type-P. + have the same values of Src, Dst, [path,] and Type-P. Note also that, given one sample that runs from T0 to Tf, and given new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf, the subsequence of the given sample whose time values fall between T0' and Tf' are also a valid Type-P-One-way-Delay-Stream sample. 4.6. Methodologies: The methodologies follow directly from: + the selection of specific times, using the specified Poisson arrival process, and + the methodologies discussion already given for the singleton Type- P-One-way-Delay metric. Care must, of course, be given to correctly handle out-of-order arrival of test packets; it is possible that the Src could send one test packet at TS[i], then send a second one (later) at TS[i+1], while the Dst could receive the second test packet at TR[i+1], and then receive the first one (later) at TR[i]. -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 4.7. Errors and Uncertainties: In addition to sources of errors and uncertainties associated with methods employed to measure the singleton values that make up the sample, care must be given to analyze the accuracy of the Poisson arrival process of the wire-time of the sending of the test packets. Problems with this process could be caused by either of several things, including problems with the pseudo-random number techniques used to generate the Poisson arrival process, or with jitter in the @@ -457,21 +457,21 @@ > Then the 50th percentile would be 110 msec, since 90 msec and 100 msec are smaller and 110 msec and 'undefined' are larger. 5.2. Type-P-One-way-Delay-Median Given a Type-P-One-way-Delay-Stream, the median of all the dT values in the Stream. In computing the median, undefined values are treated as infinitely large. -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 As noted in the Framework document, the median differs from the 50th percentile only when the sample contains an even number of values, in which case the mean of the two central values is used. Example: suppose we take a sample and the results are: Stream2 = < @@ -502,21 +502,21 @@ Thanks also to Sean Shapira for several useful suggestions. 8. References G. Almes, W. Cerveny, P. Krishnaswamy, J. Mahdavi, M. Mathis, and V. Paxson, "Framework for IP Provider Metrics", Internet Draft , July 1996. J. Postel, "Internet Protocol", RFC 791, September 1981. -ID One-way Delay Metric November 1996 +ID One-way Delay Metric March 1997 D. Mills, "Network Time Protocol (v3)", RFC 1305, April 1992. 9. Authors' Addresses Guy Almes Advanced Network & Services, Inc. 200 Business Park Drive Armonk, NY 10504 USA