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comments to draft-psamp-framework-05.txt


I have some (very late, I know and am sorry) comments to framework-05.txt. Some of these comments have probably been covered and a decision made at previous meetings; if so, please ignore.



* Reporting Process: creates a report stream on packets selected by a selection process, in preparation for export. The input to the reporting process comprises that information available to the selection process per selected packet, specifically: (i) the selected packet’s content; (ii) information derived from the selected packet's treatment at the observation point; (iii) any selection state maintained by the inputting selection process, reflecting any modifications to the selection state made during selection of the packet.

Should the selection state presented to the reporting process be pre-modification instead of post-modification? It's fairly straightforward to go from pre-modification to post-modification, but it may be much harder to go from post-modification to pre-modification. Pre-modification selection state, however, may be difficult to implement (have to keep around copies of all selection state until it is decided whether the packet will be selected or not) and thus should not be a requirement.

* Secure Export: (i) confidentiality: the option to encrypt exported data MUST be provided.

I know these requirements have already been discussed, so I'll just ask a question and make a comment. What sort of encryption are we going to require? This requirement could severely limit the sampling frequency.

(ii) integrity: alterations in transit to exported data MUST be detectable at the collector

What sort of integrity-detecting mechanisms are required? Again, this requirement could severely limit the sampling-frequency.

4.2 PSAMP Packet Selection Operations A spectrum of packet selection operations is described in detail in [ZMRD03]. Here we only briefly summarize the meanings for completeness. A PSAMP selection process MUST support at least one of the following selectors.

To me, it seems that the mask/match filtering is the selector that doesn't belong. Some argument can be made that the others can roughly approximate each other, but not the mask/match. Thus, requiring both a sampling selector and a mask/match or just a sampling selector seems more reasonable.

4.3 Input Sequence Numbers for Primitive Selection Processes. Each instance of a primitive selection process MUST maintain a count of packets presented at its input. The counter value is to be included as a sequence number for selected packets. The sequence numbers are considered as part of the packet's selection state.

What if we are using a compound selector? Do we tack on the sequence numbers for each of the selectors, or just the first one or just the last one? There could be several. From an implementation point of view, only passing the last one is the simplest.

Use of input sequence numbers enables applications to determine the attained frequency at which packets are selected, and hence correctly normalize network usage estimates regardless of loss of information, regardless of whether this loss occurs because of discard of packet reports in the measurement or reporting process (e.g. due to resource contention in the host of these processes), or loss of export packets in transmission or collection. See [RFC3176] for further details. As an example, consider a set of n consecutive packet reports r1, r2, …, rn, selected by a sampling operation and received at a Duffield (Ed.) Expires April 2004 [Page 13]

Internet Draft Packet Selection and Reporting December 2003 collector. Let s1, s2, …, sn be the input sequence numbers reported by the packets. The attained selection frequency, taking into account both packet sampling at the observation point and selection arising from loss in transmission, is R = (n-1)/(sn-s1). (Note R would be 1 if all packet were selected and there were no transmission loss). The attained selection frequency can be used to estimate the number bytes present in a portion of the observed packet stream. Let b1, b2, …, bn be the bytes reported in each of the packets that reached the collector, and set B = b1+b2+,…,+bn. Then the total bytes present in packets in the observed packet stream whose input sequence numbers lie between s1 and sn is estimated by B/R, i.e, scaling up the measured bytes through division by the attained selection frequency. 4.4 Composite Selectors The ability to compose selectors in a selection process SHOULD be provided. The following combinations appear to be most useful for applications: * filtering followed by sampling * sampling followed by filtering Composite selectors are useful for drill down applications. The first component of a composite selector can be used to reduce the load on the second component. In this setting, the advantage to be gained from a given ordering can depend on the composition of the packet stream. 4.5 Constraints on the Sampling Frequency Sampling at full line rate, i.e. with probability 1, is not excluded in principle, although resource constraints may not support it in practice. 4.6 Criteria for Choice of Selection Operations In current practice, sampling has been performed using particular algorithms, including: * pseudorandom independent sampling with probability 1/N; * systematic sampling of every Nth packet. The question arises as to whether both of these should be standardized as distinct selection operations, or whether they can be regarded as different implementations of a single selection operation. Duffield (Ed.) Expires April 2004 [Page 14]

Internet Draft Packet Selection and Reporting December 2003 To determine the answer to this question, we need to consider (a) measured or assumed statistical properties of the packet stream, e.g., one or more of the following: (i) contents of different packets are statistically independent (ii) correlations between contents of different packets decay at a specified rate (iii) contents of certain fields within the same packet are significantly variable and exhibit small cross correlation (b) the desired reference sampling model, e.g., one of: (i) sample packets with long term probability 1/N (ii) sample packets independent with probability 1/N (c) the set of possible alternatives and implementations, e.g., one of: (i) pseudorandom independent sampling with probability 1/N (ii) systematic sampling with period N (iii) hash-based sampling with target probability 1/N (d) the tolerance for error in the applications that use the collected packet reports. We can say that a given alternative from (c) reproduces a reference model (b) for the applications if the results obtained using them are sufficiently accurate in (d) for traffic satisfying an assumed statistical properties in (a). Clearly, application to evaluate methods in (c) requires developing agreement on the relevant properties in (a), (b) and (d). Example: systematic sampling with period N will not count the occurrence of closely space packets (less than N counts apart) from the same flow. Thus for applications that are concerned with the joint statistics of multiple packets within flows, systematic sampling may not reproduce the results obtained with random sampling sufficiently accurately. 5. Reporting Process 5.1 Mandatory Contents of Packet Reports (MUST) The reporting process MUST include the following in each packet report: Duffield (Ed.) Expires April 2004 [Page 15]

Internet Draft Packet Selection and Reporting December 2003 (i) the input sequence number(s) of any sampling operation that acted on the packet in the instance of a measurement process of which the reporting process is a component.

Should there be an additional sequence number on the packet reports themselves? Without a sequence number on the packet reports, it's possible for the reports to be misinterpreted. For example, if a mask/match selector is either configured incorrectly or the hit rate on it is under what is suspected, it's hard to tell that from report packets being dropped just from the primitive selector sequence number. Hash-based selection can have the same problem. Tacking on an extra counter here isn't much work from an implementation standpoint, compared to everything else we are requiring.

The reporting process MUST be able to include the following in each packet report, as a configurable option: (ii) a basic report on the packet, i.e., some number of contiguous bytes from the start of the packet, including the packet header (which includes link layer, network layer and other encapsulation headers) and some subsequent bytes of the packet payload. Some devices hosting reporting processes may not have the resource capacity or functionality to provide more detailed packet reports that those in (i) and (ii) above. Using this minimum required reporting functionality, the reporting process places the burden of interpretation on the collector, or on applications that it supplies. On the other hand, some devices may have the capability to provide extended packet reports (see Section 5.2 below). These devices may exercise the option not to provide basic reports.

7.7 Collector-based Rate Reconfiguration Since collector-based rate reconfiguration is a new proposal, this draft will discuss it in some detail. The collector can detect congestion loss along the path from the exporting device to the collector by observing packet loss, manifest as gaps in the sequence numbers, or the absence of packets

Just to reiterate, gaps in the selector sequence numbers may or may not indicate loss between the exporting device and the collector. We are assuming we can determine the fraction of packets that will be selected, based on the selector. Our assumptions may be incorrect.

for a period of time. The server can run an appropriate congestion-
control algorithm to compute a new export rate limit, then reconfigure the export process with the new rate. This is an attractive alternative to requiring the export process to receive acknowledgment packets. Implementing the congestion control algorithm in the collector has the added advantages of flexibility in adapting the sending rate and the ability to incorporate new congestion-control algorithms as they become available.

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