Hi, Overall, looks good. I have a few comments that are inlined into the document below. Thanks. Derek This document describes a framework that defines a standardINTERNET-DRAFT Nick Duffield (Editor) draft-ietf-psamp-framework-03.txt AT&T Labs - Research June 2003 Expires: December 2003 A Framework for Passive Packet Measurement Copyright (C) The Internet Society (2003). All Rights Reserved. This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 obsoleted 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. 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. Abstract A wide range of traffic engineering and troubleshooting tasks rely on timely and detailed traffic measurements that can be consistently interpreted. This document describes a framework for packet sampling that is (a) general enough to serve as the basis for a wide range of operational tasks, and (b) needs only a small set of packet selectors that facilitate ubiquitous deployment in router interfaces or dedicated measurement devices, even at very high speeds. The framework also covers reporting and exporting functions used by the sampling element, and configuration of the sampling element. Comments on this document should be addressed to the PSAMP WG mailing list: psamp@ops.ietf.org To subscribe: psamp-request@ops.ietf.org, in body: subscribe Archive: https://ops.ietf.org/lists/psamp/ Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 1] Internet-Draft Passive Packet Measurement June 2003 0 Contents 1 Motivation ................................................. 3 2 Elements, Terminology, and Architecture .................... 3 3 Requirements ............................................... 6 3.1 Selection Process Requirements ......................... 6 3.2 Reporting Process Requirements ......................... 6 3.3 Export Process Requirements ............................ 7 3.4 Configuration Requirements ............................. 7 4 Packet Selection ............................................ 8 4.1 Packet Selection Terminology ............................ 8 4.2 Packet Selection Operations for a PSAMP ................. 9 4.3 Input Sequence Numbers for Primitive Selectors .......... 11 4.4 Composite Selectors . .................................. 12 4.5 Constraints on the Sampling Frequency ................... 12 4.6 Criteria for Choice of Selection Operations ............ 12 5 Reporting Process .......................................... 13 5.1 Mandatory Contents of Packet Reports (MUST) ............. 13 5.2 Recommended Contents for Packet Reports (SHOULD) ........ 13 5.3 Report Interpretation ................................. 14 7 Parallel Measurement Processes ............................. 15 6 Export Process .............................................. 15 7.1 Collector Destination .................................. 15 7.2 Local Export ........................................... 15 7.3 Reliable vs. Unreliable Transport ...................... 16 7.4 Limiting Delay in Exporting Measurement Packets ........ 16 7.5 Configurable Export Rate Limit ......................... 16 7.6 Congestion-aware Unreliable Transport .................. 17 7.7 Collector-based Rate Reconfiguration ................... 17 7.7.1 Changing the Export Rate and Other Rates ........... 17 7.7.2 Notions of Fairness ................................ 18 7.7.3 Behavior Under Overload and Failure ................ 19 8 Configuration and Management ............................... 19 9 Feasibility and Complexity ................................. 20 9.1 Feasibility ............ ............................... 20 9.1.1 Filtering .......................................... 20 9.1.2 Sampling ........................................... 20 9.1.3 Hashing ............................................ 20 9.1.4 Reporting .......................................... 21 9.1.5 Export ............................................. 21 9.2 Potential Hardware Complexity .......................... 21 10 Applications .............................................. 22 10.1 Baseline Measurement and Drill Down ................... 22 10.2 Passive Performance Measurement ....................... 23 10.3 Troubleshooting ....................................... 23 11 Security Considerations .................................... 24 12 References ................................................ 25 13 Authors' Addresses ........................................ 26 14 Intellectual Property Statement ........................... 27 15 Full Copyright Statement .................................. 27 Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 2] Internet-Draft Passive Packet Measurement June 2003 1 Motivation This document describes a framework in which to define a standard set of capabilities for network elements to sample subsets of packets by statistical and other methods. The framework will accommodate future work to (i) specify a set of selectors by which packets are sampled (ii) specify the information that is to be made available for reporting on sampled packets; (iii) describe a protocol by which information on sampled packets is reported to applications; (iv) describe a protocol by which packet selection and reporting are configured. The motivation to standardize these capabilities comes from the need for measurement-based support for network management and control across multivendor domains. This requires domain wide consistency in the types of selection schemes available, the manner in which the resulting measurements are presented, and consequently, consistency of the interpretation that can be put on them. The capabilities are positioned as suppliers of packet samples to higher level consumers, including both remote collectors and applications, and on board measurement-based applications. Indeed, development of the standards within the framework described here should be open to influence by the requirements of standards in related IETF WGs, for example, IP Performance Metrics (IPPM) [PAMM98] and Internet Traffic Engineering (TEWG) [LCTV02]. Conversely, we expect that aspects of this framework not specifically concerned with the central issue of packet selection and report formation may be able to leverage work in other WGs. Potential examples are the format and export of measurement reports, which may leverage the information model and export protocols of IP Flow Information Export (IPFIX) [QZCZ03], and work in congestion aware unreliable transport in the Datagram Congestion Control Protocol (DCCP) [FHK02], and related work in The Stream Control Transmission Protocol (SCTP) [SCTP] and [PR-SCTP]. 2 Elements, Terminology, and Architecture This section defines the basic elements of the PSAMP framework. At the highest level, the architecture comprises observation points (at which packets are observed), measurement processes (which select packets and construct reports on them) and export processes (which export reports to collectors). The full defintions of these terms now follow. * Observation Point: the observation point is a location in the network where a packet stream is observed. Examples are, a line Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 3] Internet-Draft Passive Packet Measurement June 2003 to which a probe is attached, a shared medium, such as an Ethernet-based LAN, a single port of a router, or set of interfaces (physical or logical) of a router, an embedded measurement subsystem within an interface. Observation Point: the observation point is a location in the network where a packet stream is observed. Examples are (i) a line to which a probe is attached, (ii) a shared medium, such as an Ethernet-based LAN, (iii) a single port of a router, or set of interfaces (physical or logical) of a router or (iv) an embedded measurement subsystem within an interface. ADD (i), (ii) ... I agree with Benoit that packet encryption will make general PSAMP difficult, especially since the encryption key will not generally be available.* Measurement Process: the combination of a selection process followed by a reporting process. * Packet Stream: a sequence of packets, each of which was observed at the observation point. Note that when packets are sampled from a stream, the selected packets usually do not have common properties by which they can be distinguished from packets that have not been selected. Therefore we define here the term stream instead of flow, which is defined as set of packets with common properties [QuZC02]. * Packet Content: the union of the following: packet header, packet payload, encapsulation headers, and link layer headers. * Observed Packet Stream: the packet stream comprising all packets observed at the observation point. * Selection Process: a selection process selects a substream of packets from the observed packet stream. A selection process entails the composition of one or more selectors in succession, acting on each packet in the observed packet stream. When selectors are composed, the output stream packet issuing from one selector forms the input packet stream for the succeeding selector. * Selector (or selection operation): a configurable packet selection operation that acts on single packets. It takes as its input, the content of a single packet from a packet stream, information derived from the packet's treatment at the observation point, and selection state that may be maintained at the observation point. If the packet is selected, this same information may be considered as the output. Selectors may change the selection state. * Composite Selector: an ordered composition of selectors. * Primitive Selector: a selector that is not a composition of multiple selectors. * Selection State: the selection process may maintain state information for use by the selection process and/or the reporting process. At a given time, the selection state may depend on packets observed up that time and/or other variables. Examples include sequence numbers of packets at the input of selectors, timestamps, iterators for pseudorandom number generators, calculated hash values, and indicators of whether a Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 4] Internet-Draft Passive Packet Measurement June 2003 packet was selected by a given selector. * Reporting Process: the creation of a report stream of information on packets selected by a selection processes, in preparation for export. The input to a reporting process comprises that information available to a selection process, for the selected packets. The report stream contains two distinguished types of information: packet reports, and report interpretation. * Packet Reports: a configurable subset of the per packet input to the reporting process. * Report Interpretation: subsidiary information relating to one or more packets, that is used for interpretation of their packet reports. Examples include configuration parameters of the PSAMP device, and configuration parameters of the selection and reporting process. * Export Process: sends the output of one or more reporting process to one or more collectors. * Collector: a collector receives a report stream exported by one or more measurement processes. In some cases, the entity that hosts the measurement and/or export process may also serve as the collector. * Measurement packets: one or packet reports, and perhaps report interpretation, are bundled by the export process into a measurement packet for export to a collector. Various possibilities for the high level architecture of these elements is as follows. = Observation Point, MP = Measurement Process, EP = Export Process +---------------------+ +------------------+ |Observation Point(s) | | Collector(1) | |MP(s)--->EP----------+---------------->| | |MP(s)--->EP----------+-------+-------->| | +---------------------+ | +------------------+ | +---------------------+ | +------------------+ |Observation Point(s) | +-------->| Collector(2) | |MP(s)--->EP----------+---------------->| | +---------------------+ +------------------+ +---------------------+ |Observation Point(s) | |MP(s)--->EP---+ | | | | |Collector(3)<-+ | Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 5] Internet-Draft Passive Packet Measurement June 2003 +---------------------+ 3 Requirements 3.1 Selection Process Requirements. * Ubiquity: The selectors must be simple enough to be implemented ubiquitously at maximal line rate. * Applicability: the set of selectors must be rich enough to support a range of existing and emerging measurement based applications and protocols. This requires a workable trade-off between the range of traffic engineering applications and operational tasks it enables, and the complexity of the set of capabilities. * Extensibility: to allow for additional packet selectors to support future applications. * Flexibility: to support selection of packets using different network protocols or encapsulation layers (e.g. IPv4, IPv6, MPLS, etc), and under packet encryption. The situation where this may not be necessary is if the collector and the observation point are not separated by insecure links. For example, if the collector was part of the router that contained the observation points, it's unlikely that security is helpful. Instead, it requires either a general-purpose processor or encryption hardeware, slowing down packet flow or increasing cost/complexity/power if installing hardware.* Robust Selection: packet selection MUST be robust w.r.t. attempts to craft a packet stream from which packets are selected disproportionately (e.g. to evade selection, or overload the measurement system). * Parallel Measurements: multiple independent measurement processes at the same entity. * Non-contingency: in order to satisfy the ubiquity requirement, the selection decision for each packet MUST NOT depend on future packets. Rather, the selection decision MUST be capable of being made on the basis of the selection process input up to and including the packet in question. This excludes selection functions that require caching of packet for selection contingent on subsequent packets. See also the timeliness requirement following. Selectors are outlined in Section 4, and described in more detail in the companion document [ZMRD03]. 3.2 Reporting Process Requirements * Transparency: allow transparent interpretation of measurements as communicated by PSAMP reporting, without any need to obtain additional information concerning the observed packet stream. * Robustness to Information Loss: allow robust interpretation of Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 6] Internet-Draft Passive Packet Measurement June 2003 measurements with respect to reports missing due to data loss, e.g. in transport, or within the measurement, reporting or exporting processes. Inclusion in reporting of information that enables the accuracy of measurements to be determined. * Faithfulness: all reported quantities that relate to the packet treatment MUST reflect the router state and configuration encountered by the packet at the time it is received by the measurement process. * Privacy: selection of the content of packet reports will be cognizant of privacy and anonymity issues while being responsive to the needs of measurement applications, and in accordance with RFC 2804. Full packet capture of arbitrary packet streams is explicitly out of scope. A specific reporting processes meeting these requirements, and the requirement for ubiquity, is described in Section 5. 3.3 Export Process Requirements * Timeliness: reports on selected packets MUST be made available to the collector quickly enough to support near real time applications. Specifically, any report on a packet MUST be dispatched within 1 second of the time of receipt of the packet by the measurement process. * Congestion Avoidance: export of a report stream across a network MUST be congestion avoiding in compliance with RFC 2914. * Secure Export: - confidentiality: the option to encrypt exported data MUST be provided. I don't agree that uniform pseudorandom sampling may be emulated by hash-based seletion. A DOS attack that knew the hash-based selection method could evade it, but not a pseudorandoming sampling scheme.- integrity: alterations in transit to exported data MUST be detectable at the collector - authenticity: authenticity of exported data MUST be verifiable by the collector in order to detect forged data. The motivation here is the same as for security in IPFIX export; see Sections 6.3 and 10 of [QZCZ03]. 3.4 Configuration Requirements * Ease of Configuration: of sampling and export parameters, e.g. for automated remote reconfiguration in response to measurements. * Secure Configuration: the option to configure via protocols that Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 7] Internet-Draft Passive Packet Measurement June 2003 prevent unauthorized reconfiguration or eavesdropping on configuration communications MUST be available. Eavesdropping on configuration might allow an attacker to gain knowledge that would be helpful in crafting a packet stream to (for example) evade subversion, or overload the measurement infrastructure. Configuration is discussed in Section 8. Feasibility and complexity of PSAMP operations is discussed in Section 9. Reuse of existing protocols will be encouraged provided the protocol capabilities are compatible with the requirements laid out in this document. 4 Packet Selection 4.1 Packet Selection Terminology. * Filtering: a filter is a selection operation that selects a packet deterministically based on the packet content, its treatment, and functions of these occurring in the selection state. Examples include match/mask filtering, and hash-based selection. * Sampling: a selection operation that is not a filter is called a sampling operation. This reflects the intuitive notion that if the selection of a packet cannot be exactly predicted from its content, there must be some type of sampling taking place. * Content-independent Sampling: a sampling operation that does not use packet content (or quantities derived from it) as the basis for selection is called a content-independent sampling operation. Examples include systematic sampling, and uniform pseudorandom sampling driven by a pseudorandom number whose generation is independent of packet content. Note that in independent sampling it is not necessary to access the packet content in order to make the selection decision. * Content-dependent Sampling: a sampling operation where selection is dependent on packet content is called a content-dependent sampling operation. Examples include pseudorandom selection according to a probability that depends on the contents of a packet field; note that this is not a filter. * Emulated Sampling: selection operations in any of the above four categories may be emulated by operations in the same or another category for the purposes of implementation. For example, uniform pseudorandom sampling may be emulated by hash-based selection, using suitable hash function and hash domain. Should Match/Mask be inverted to Mask/Match since the mask comes first?* Hash-based selection: a filter specified by a hash domain, a hash Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 8] Internet-Draft Passive Packet Measurement June 2003 function, and hash range and a hash selection range. * Hash domain: a subset of the packet content and the packet treatment, viewed as an N-bit string for some positive integer N. * Hash range: a set of M-bit strings for some positive integer M. * Hash function: a deterministic map from the hash domain into the hash range. * Selection range: a subset of the hash range. The packet is selected if the action of the hash function on the hash domain for the packet yields a result in the hash selection range. * Pool size: the size of a set of packets in a packet stream. * Sample size: the size of a set of packets selected by a sampling operation. * Target Sampling Frequency: a configurable sampling frequency in a sampling operation. * Attained Sampling Frequency: Given a subset of packets in a stream input to a sampling operation, the attained sampling frequency is the ratio of the sample size to the pool size. 4.2 Packet Selection Operations for a PSAMP 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. * Systematic Time Based: packet selection is triggered at periodic instants separated by a time called the Spacing. All packets that arrive within a certain time of the trigger (called the Interval Length) are selected. * Systematic Count Based: similar to systematic time based expect that selection is reckoned w.r.t. packet count rather than time. Packet selection is triggered periodically by packet count, a number of successive packets being selected subsequent to each trigger. * Uniform Probabilistic: packets are selected independently with fixed sampling probability p. * Non-uniform Probabilistic: Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 9] Internet-Draft Passive Packet Measurement June 2003 packets are selected independently with probability p that depends on packet content. * Probabilistic n-out-of-N: form each count-based successive block of N packets, n are selected at random * Match/Mask Filtering: Same as above.This entails taking the masking portions of the packet (i.e. taking the bitwise AND with a binary mask) and selecting the packet if the result falls in a specified range. This specification doesn't preclude the future definition of a high level syntax for defining filtering in a concise way (e.g. TCP port taking a particular value) providing that syntax can be compiled into the bitwise _expression_. Match/mask operations SHOULD be available for different protocol portions of the packet: o the IP header (excluding options in IPv4, stacked headers in IPv6) o transport header o encapsulation headers (including MPLS label stack, ATOM) When an entity offers Match/Mask filtering in the selection process and, in its usual capacity other than in performing PSAMP functions, identifies or processes information from one or more of the above protocols, then the information SHOULD be made available for filtering. For example, when an entity routes based on destination IP address, that field should be made available. Conversely, an entity that does not route is not expected to be able to locate an IP address within a packet, or make it available for filtering, although it MAY do so. * Hash-based Selection: Hash-based selection will employ one or more hash functions to be standardized. The hash domain is specified by a bitmaps on the IP packet header and the IP payload. When the hash function is sufficiently good, hash-based selection can be used to emulate uniform random sampling over the hash domain. The target sampling frequency is then the ratio of the size of the selection range to the hash range. Pre-selection? Every packet, even if it isn't selected, gets a unique (subject ot wraparound) sequence number? Thus, the sampled packets will have holes in the count. If so, how can the collector determine that packets have been dropped from the sequence number?Applications of hash-based selection include: o Trajectory Sampling: all routers use the same hash selector; Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 10] Internet-Draft Passive Packet Measurement June 2003 the hash domain includes only portions of the packet that do not change from hope to hop (e.f. TTL is excluded). Hence packets are consistently selected in the sense that they are selected at all routers on their path or none. Reports also include a second hash (the label hash) that distinguishes different packets. Reports of a given packet reaching the collector from different routers can be used to reconstruct the path taken by the packet. Trajectory Sampling is proposed in [DuGr01]; further description is found in [ZMRD03]; some applications are described in Section 10. o Consistent Flow Sampling: the hash domain is a flow key. For a given flow, either all or none of its packets are sampled. This is accomplished without the need to maintain flow state. Some applications need to calculate packet hashes for purposes other than selection (e.g. the label hash in Trajectory Sampling). This can be achieved by placing a calculated hash in the selection state, and setting the selection range to be the whole of the hash range. * Router State Filtering: This class of filters selects a packet on based on the following conditions, combined with the AND, OR or NOT operators: o Ingress interface at which packet arrives equals a specified value o Egress interface to which packet is routed to equals a specified value o Origin AS equals a specified value or lies within a given range. o Destination AS equals a specified value or lies within a given range o Packet violated acl on the router o Failed rpf o Failed rsvp o No route found for the packet Router architectural considerations may preclude some information concerning the packet treatment, e.g routing state, being available at line rate for selection of packets. However, if selection not based on routing state has reduced down from line rate, subselection based on routing state may be feasible. 4.3 Input Sequence Numbers for Primitive Selectors. Each instance of a primitive selector MUST maintain a count of packets presented at its input. The counter value is to be included Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 11] Internet-Draft Passive Packet Measurement June 2003 as a sequence number for selected packets. This enables applications to determine the attained frequency at which packets are selected, and hence correctly normalize network usage estimates regardless of loss of information, whether this occurs because of discard of packet reports in the measurement or reporting process (e.g. due to resource contention), or loss of measurement packets in transmission or collection; see [PPM01]. The sequence numbers are considered as part of the packet's selection state. 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. 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: - contents of different packets are statistically independent - correlations between contents of different packets decay at a specified rate - contents of certain fields within the same packet are Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 12] Internet-Draft Passive Packet Measurement June 2003 significantly variable and exhibit small cross correlation (b) the desired reference sampling model, e.g., one of: - sample packets with long term probability 1/N - sample packets independent with probability 1/N (c) the set of possible alternatives and implementations, e.g., one of: - pseudorandom independent sampling with probability 1/N - systematic sampling with period N - hash-based sampling with target probability 1/N (d) the tolerance for error in the applications that use the measurements. 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: (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. The reporting process MUST be able to include the following in each packet report, as a configurable option: (ii) some number of contiguous bytes from the start of the packet. Some devices may not have the resource capacity or functionality to provide more detailed 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. 5.2 Recommended Contents for Packet Reports (SHOULD) The reporting process SHOULD provide for the inclusion in packet Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 13] Internet-Draft Passive Packet Measurement June 2003 reports of the following information, inclusion any or all being configurable as a option. (iii) fields relating to the following protocols used in the packet, specifically: IPv4, IPV6, transport protocols, MPLS, ATOM. (iv) packet treatment, including: - identifiers for any input and output interfaces of the observation point that were traversed by the packet - source and destination AS (v) selection state associated with the packet, including: - timestamps - hashes, where calculated. The specific fields will include those set out as requirements for IPFIX [QZCZ03], with modifications appropriate to reporting on single packets rather than flows. When an entity that hosts a reporting process and, in its usual capacity other than performing PSAMP functions, identifies or process one or more of the above fields, then the contents of each such field(s) SHOULD be made available for optional reporting. For example, when a device routes based on destination IP address, that field should be made available. Conversely, an entity that does not route is not expected to be able to locate an IP address within a packet, or make it available for reporting, although it MAY do so. 5.3 Report Interpretation Information for use in report interpretation MUST include (i) configuration parameters of the selectors of the packets reported on; (ii) format of the packet reports (iii) configuration parameters and state information of the network element; (iv) indication of the inherent accuracy of the reported quantities, e.g., of timestamps; (v) identifiers for observation point, measurement process, and export process. The requirements for robustness and transparency are motivations for including report interpretation in the report stream. Inclusion makes the report stream self-defining. The PSAMP framework excludes reliance on an alternative model in which interpretation is recovered out of band. This latter approach is not robust with respect to undocumented changes in selector configuration, and may give rise to future architectural problems for network management Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 14] Internet-Draft Passive Packet Measurement June 2003 systems to coherently manage both configuration and data collection. It is not envisaged that all report interpretation be included in every packet report. Many of the quantities listed above are expected to be relatively static; they could be communicated periodically, and upon change. To conserve network bandwidth and resources at the collector, the measurement packets may be compressed before export. Compression is expected to be quite effective since the sampled packets may share many fields in common, e.g. if a filter focuses on packets with certain values in particular header fields. Using compression, however, could impact the timeliness of reports. Any consequent delay MUST not violate the timeliness requirement for availability of packet reports at the collector. 6 Parallel Measurement Processes Because of the increasing number of distinct measurement applications, with varying requirements, it is desirable to set up parallel measurement processes on a stream of packets. A PSAMP device SHOULD support more than one independently configurable measurement process. The measurement process may have an exclusive export process, or may share it with other measurement processes. Each of the parallel measurement processes SHOULD be independent. However, resource constraints may prevent complete reporting on a packet selected by multiple selection processes. In this case, reporting for the packet MUST be complete for at least one measurement process; other measurement processes need only report that they selected the packet. The priority amongst measurement processes to report packets MUST be configurable. It is not proposed to standardize the number of parallel measurement processes. 7 Export Process 7.1 Collector Destination When exporting to a remote collector, the collector is identified by IP address and port number. 7.2 Local Export The report stream may be directly exported to on-board measurement based applications, for example those that form composite statistics from more than one packet. Local export may be presented through an interface direct to the higher level applications, i.e., through an API, rather than employing the transport used for off-board export. Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 15] Internet-Draft Passive Packet Measurement June 2003 A possible example of local export could be that packets selected by the PSAMP measurement process serve as the input for the IPFIX protocol, which then forms flow records out of the stream of selected packets. Note that IPFIX being still developed; this is given only as a possible example. 7.3 Reliable vs. Unreliable Transport The export of the report stream does not require reliable export. On the contrary, retransmission of lost measurement packets consumes additional network resources and requires maintenance of state by the export process. As such, the export process would have to be able to receive and process acknowledgments, and to store unacknowledged data. Furthermore, the entity that hosts the export process may not possess its own network address (for example an embedded measurement subsystem in an interface) at which to receive acknowledgments. These requirements would be a significant impediment to having ubiquitous support PSAMP. Instead, it is proposed that the export process support unreliable export. Sequence numbers on the measurement packets would indicate when loss has occurred, and the analysis of the collected measurement data can account for this loss. In some sense, packet loss becomes another form of sampling (albeit a less desirable, and less controlled, form of sampling). 7.4 Limiting Delay in Exporting Measurement Packets The export process may queue the report stream in order to export multiple reports in a single measurement packet. Any consequent delay MUST still allow for timely availability of packet reports at the collector. 7.5 Configurable Export Rate Limit The export process MUST be able to limit its export rate; otherwise it could overload the network and/or the collector. Note this problem would be exacerbated if using reliable transport mode, since any lost packets would be retransmitted, thereby imposing an additional load on the network. At times, the reporting process may generate new reports or report interpretation faster than the allowed export rate. In this situation, the export process MUST discard the excess reports rather than transmitting them to the collector. Sequence numbers reported for selector input enable correction for lost reports. An additional sequence number for dispatched measurement packets enables the collector to determine the degree of loss in transmission. Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 16] Internet-Draft Passive Packet Measurement June 2003 There are two options for a configurable rate limit. First, if the transport protocol has a configurable rate limit, that can be used. The second option is to limit the rate at which measurement packets are supplied to the transport protocol. A candidate for implementation of rate limiting is the leaky bucket, with tokens corresponding e.g. to bytes or packets. The export rate limit MUST be configurable per export process. Note that since congestion loss can occur at any link on the export path, it is not sufficient to limit rate simply as a function of the bandwidth of the interface out of which export takes place. 7.6 Congestion-aware Unreliable Transport Exported measurement traffic competes for resources with other Internet transfers. Congestion-aware export is important to ensure that the measurement packets do not overwhelm the capacity of the network or unduly degrade the performance of other applications, while making good use of available bandwidth resources. Choice of transport for PSAMP has to be made under the following constraints: (i) IESG has mandated that all transport in new protocols must be congestion aware (ii) reliable transport is too onerous for general entities that support PSAMP (see Section 7.3) (iii) there currently exists no IETF standardized unreliable congestion-aware transport In the absence of an existing IETF standardized unreliable congestion-aware protocol, PSAMP will provisionally nominate the reliable congestion aware transport protocol TCP as the interim transport protocol for export. From the preceding arguments, TCP is unsatisfactory for final standardization in PSAMP. In the meantime, the PSAMP WG will evaluate (at least) the following alternatives for congestion aware unreliable transport, as they become available, with a view to selecting one of them and discarding TCP: (i) unreliable transport protocols adopted in the future by the IPFIX WG, (ii) the Datagram Congestion Control Protocol (DCCP); currently under development; see [FHK02] (iii) The Stream Control Transmission Protocol (SCTP) under development [SCTP]. SCTP is by default reliable, but has the capability to operate in unreliable and partially reliable modes [PR-SCTP]. See [D03] for description of its potential use in flow export. Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 17] Internet-Draft Passive Packet Measurement June 2003 (iv) collector-based rate reconfiguration, described below. 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 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. 7.7.1 Changing the Export Rate and Other Rates Forcing the export process to discard excess reports is an effective control under short term congestion. Alternatively, the selection process could be reconfigured to select fewer packets, or the reporting process could be reconfigured to send smaller reports on each selected packet. This may be a more appropriate reaction to long-term congestion. In some cases, a collector may receive measurement packets due to more than one export process, and could decide to reduce the export or other rates associated with one export process rather than another, in order to prioritize the measurement data. This type of flexibility is valuable for network operators that collect measurement data from multiple locations to drive multiple applications. 7.7.2 Notions of Fairness In some cases, it may be reasonable to allow the collector to have flexibility in deciding how aggressively to respond to congestion. For example, the exporting entity and the collector may have a very small round-trip time relative to other traffic. Conventional TCP-friendly congestion control would allocate a very large share of the bandwidth to the PSAMP export traffic. Instead, the collector could apply an algorithm that reacts more aggressively to congestion to give a larger share of the bandwidth to other traffic (with larger RTTs). In other cases, the measurement packets may require a larger share of the bandwidth than other flows. For example, consider a link that carries tens of thousands of flows, including some non Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 18] Internet-Draft Passive Packet Measurement June 2003 TCP-friendly DoS attack traffic. Restricting the PSAMP traffic to a fair share allocation may be too restrictive, and might limit the collection of the data necessary to diagnose the DoS attack which overloads links over which measurement packets are carried. In order to maintain report collection during periods of congestion, PSAMP report streams may claim more than a fair share of link bandwidth, provided the number of report streams in competition with fair sharing traffic is limited. The collector could also employ policies that allocate bandwidth in certain proportions amongst different measurement processes. 7.7.3 Behavior Under Overload and Failure The congestion control algorithm has to be robust to severe overload or complete loss of connectivity between the exporting entity and the collector, and also to the failure of exporting entity or the collector. For example, in a scenario where the collector is unable to reconfigure the export rate because of loss of reverse (collector to exporting entity) connectivity, it is desirable for the exporting entity to reduce the export rate autonomously. Similarly, if no measurement packets reach the collector because of loss of forward connectivity, the collector should not react to this by increasing the export rate. This problem may be solved through periodic heartbeat packets in both directions (i.e., measurement packets in the forward direction, configuration refresh messages in the reverse direction). This allows each side to detect a loss in connectivity or outright failure and to react appropriately. 8 Configuration and Management A key requirement for PSAMP is the easy reconfiguration of the parameters of the measurement process: those for selection, packet reports and export. Examples are (i) support of measurement based applications that want to drill-down on traffic detail in real-time; (ii) collector-based rate reconfiguration. To facilitate reconfiguration and retrieval of parameters, they are to reside in a Management Information Base (MIB). Mandatory configuration, capabilities and monitoring objects will cover all minimum required (MUST) PSAMP functionality. Secondary objects will cover the recommended PSAMP functionality (SHOULD), and MUST be provided only when such functionality is offered by an entity. Such PSAMP functionality includes configuration of offered selectors, composite selectors, multiple measurement processes, and report format including the choice of fields to be reported. For further details concerning the PSAMP MIB, see [DRC03]. Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 19] Internet-Draft Passive Packet Measurement June 2003 PSAMP requires a uniform mechanism with which to access and configure the MIB. SNMP access MUST be provided by the entity hosting the MIB. 9 Feasibility and Complexity In order for PSAMP to be supported across the entire spectrum of networking equipment, it must be simple and inexpensive to implement. One can envision easy-to-implement instances of the mechanisms described within this draft. Thus, for that subset of instances, it should be straightforward for virtually all system vendors to include them within their products. Indeed, sampling and filtering operations are already realized in available equipment. Here we give some specific arguments to demonstrate feasibility and comment on the complexity of hardware implementations. We stress here that the point of these arguments is not to favor or recommend any particular implementation, or to suggest a path for standardization, but rather to demonstrate that the set of possible implementations is not empty. 9.1 Feasibility 9.1.1 Filtering Filtering consists of a small number of mask (bit-wise logical), comparison and range (greater than) operations. Implementation of at least a small number of such operations is straightforward. For example, filters for security access control lists (ACLs) are widely implemented. This could be as simple as an exact match on certain fields, or involve more complex comparisons and ranges. 9.1.2 Sampling Sampling based on either counters (counter set, decrement, test for equal to zero) or range matching on the hash of a packet (greater than) is possible given a small number of selectors, although there may be some differences in ease of implementation for hardware vs. software platforms. 9.1.3 Hashing Hashing functions vary greatly in complexity. Execution of a small number of sufficient simple hash functions is implementable at line rate. Concerning the input to the hash function, hop-invariant IP header fields (IP address, identification) and TCP/UDP header fields (port numbers, TCP sequence number) drawn from the first 40 bytes of the packet have been found to possess a considerable variability; see [DuGr01]. Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 20] Internet-Draft Passive Packet Measurement June 2003 9.1.4 Reporting The simplest packet report would duplicate the first n bytes of the packet. However, such an uncompressed format may tax the bandwidth available to the reporting process for high sampling rates; reporting selected fields would save on this bandwidth. Thus there is a trade-off between simplicity and bandwidth limitations. 9.1.5 Export Ease of exporting measurement packets depends on the system architecture. Most systems should be able to support export by insertion of measurement packets, even through the software path. 9.2 Potential Hardware Complexity We now comment on the complexity of possible hardware implementations. Achieving low constants for performance while minimizing hardware resources is, of course, a challenge, especially at very high clock frequencies. Most of these operations, however, are very basic and their implementations very well understood; in fact, the average ASIC designer simply uses canned library instances of these operations rather than design them from scratch. In addition, networking equipment generally does not need to run at the fastest clock rates, further reducing the effort required to get reasonably efficient implementations. Simple bit-wise logical operations are easy to implement in hardware. Such operations (NAND/NOR/XNOR/NOT) directly translate to four-transistor gates. Each bit of a multiple-bit logical operation is completely independent and thus can be performed in parallel incurring no additional performance cost above a single bit operation. Comparisons (EQ/NEQ) take O(lg(M)) stages of logic, where M is the number of bits involved in the comparison. The lg(M) is required to accumulate the result into a single bit. Greater than operations, as used to determine whether a hash falls in a selection range, are a determination of the most significant not-equivalent bit in the two operands. The operand with that most-significant-not-equal bit set to be one is greater than the other. Thus, a greater than operation is also an O(lg(M)) stages of logic operation. Optimized implementations of arithmetic operations are also O(lg(M)) due to propagation of the carry bit. Setting a counter is simply loading a register with a state. Such an operation is simple and fast O(1). Incrementing or decrementing a counter is a read, followed by an arithmetic operation followed by a store. Making the register dual-ported does take additional Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 21] Internet-Draft Passive Packet Measurement June 2003 space, but it is a well-understood technique. Thus, the increment/decrement is also an O(lg(M)) operation. Hashing functions come in a variety of forms. The computation involved in a standard Cyclic Redundancy Code (CRC) for example are essentially a set of XOR operations, where the intermediate result is stored and XORed with the next chunk of data. There are only O(1) operations and no log complexity operations. Thus, a simple hash function, such as CRC or generalizations thereof, can be implemented in hardware very efficiently. At the other end of the range of complexity, the MD5 function uses a large number of bit-wise conditional operations and arithmetic operations. The former are O(1) operations and the latter are O(lg(M)). MD5 specifies 256 32b ADD operations per 16B of input processed. Consider processing 10Gb/sec at 100MHz (this processing rate appears to be currently available). This requires processing 12.5B/cycle, and hence at least 200 adders, a sizeable number. Because of data dependencies within the MD5 algorithm, the adders cannot be simply run in parallel, thus requiring either faster clock rates and/or more advanced architectures. Thus selection hashing functions as complex as MD5 may be precluded from ubiquitous use at full line rate. This motivates exploring the use of selection hash functions with complexity somewhere between that of MD5 and CRC. However, identification hashing with MD5 on only selected packets is feasible at a sufficiently low sampling frequency. 10 Applications We first describe several representative operational applications that require traffic measurements at various levels of temporal and spatial granularity. Some of the goals here appear similar to those of IPFIX, at least in the broad classes of applications supported. However, there are two major differences: - PSAMP aims for ubiquitous deployment of packet measurement, including devices that are not expected to support IPFIX. This offers broader reach for existing applications. - PSAMP can support new applications through the type of packet selectors that it supports 10.1 Baseline Measurement and Drill Down Packet sampling is ideally suited to determine the composition of the traffic across a network. The approach is to enable measurement on a cut-set of the network links such that each packet entering the network is seen at least once, for example, on all ingress links. Unfiltered sampling with a relatively low frequency establishes baseline measurements of the network traffic. Reports include packet attributes of common interest: source and destination address and port numbers, prefix, protocol number, type Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 22] Internet-Draft Passive Packet Measurement June 2003 of service, etc. Traffic matrices are indicated by reporting source and destination AS matrices. Absolute traffic volumes are estimated by renormalizing the sampled traffic volumes through division by either the target sampling frequency, or the attained sampling frequency (as derived by interface packet counters included in the report stream) Suppose an operator or a measurement based application detects an interesting subset of a packet stream, as identified by a particular packet attribute. Real-time drill-down to that subset is achieved by instantiating a new measurement process on the same packet stream from which the subset was reported. The selection process of the new measurement process filters according to the attribute of interest, and composes with sampling if necessary to manage the frequency of packet selection. 10.2 Passive Performance Measurement Hash-based sampling enables the tracking of the performance experience by customer traffic, customers identified by a list of source or destination prefixes, or by ingress or egress interfaces. Operational uses include the verification of Service Level Agreements (SLAs), and troubleshooting following a customer complaint. In this application, Trajectory Sampling is enabled at all network ingress and egress interfaces. The label hash is used to match up ingress and egress samples. Rates of loss in transit between ingress and egress are estimated from the proportion of trajectories for which no egress report is received. Note loss of customer packets is distinguishable from loss of packet reports through use of report sequence numbers. Assuming synchronization of clock between different entities, delay of customer traffic across the network may also be measured. Extending hash-selection to all interfaces in the network would enable attribution of poor performance to individual network links. 10.3 Troubleshooting PSAMP can also be used to diagnose problems whose occurrence is evident from aggregate statistics, per interface utilization and packet loss statistics. These statistics are typically moving averages over relatively long time windows, e.g., 5 minutes, and serve as a coarse-grain indication of operational health of the network. The most common method of obtaining such measurements are through the appropriate SNMP MIBs (MIB-II and vendor-specific MIBs.) Suppose an operator detects a link that is persistently overloaded and experiences significant packet drop rates. There is a wide Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 23] Internet-Draft Passive Packet Measurement June 2003 range of potential causes: routing parameters (e.g., OSPF link weights) that are poorly adapted to the traffic matrix, e.g., because of a shift in that matrix; a denial of service attack or a flash crowd; a routing problem (link flapping). In most cases, aggregate link statistics are not sufficient to distinguish between such causes, and to decide on an appropriate corrective action. For example, if routing over two links is unstable, and the links flap between being overloaded and inactive, this might be averaged out in a 5 minute window, indicating moderate loads on both links. Baseline PSAMP measurement the congested link, as described in Section 10.1, enables measurements that are fine grained in both space and time. The operator has to be able to determine how many bytes/packets are generated for each source/destination address, port number, and prefix, or other attributes, such as protocol number, MPLS forwarding equivalence class (FEC), type of service, etc. This allows to pinpoint precisely the nature of the offending traffic. For example, in the case of a DDoS attack, the operator would see a significant fraction of traffic with an identical destination address. In certain circumstances, precise information about the spatial flow of traffic through the network domain is required to detect and diagnose problems and verify correct network behavior. In the case of the overloaded link, it would be very helpful to know the precise set of paths that packets traversing this link follow. This would readily reveal a routing problem such as a loop, or a link with a misconfigured weight. More generally, complex diagnosis scenarios can benefit from measurement of traffic intensities (and other attributes) over a set of paths that is constrained in some way. For example, if a multihomed customer complains about performance problems on one of the access links from a particular source address prefix, the operator should be able to examine in detail the traffic from that source prefix which also traverses the specified access link towards the customer. While it is in principle possible to obtain the spatial flow of traffic through auxiliary network state information, e.g., by downloading routing and forwarding tables from routers, this information is often unreliable, outdated, voluminous, and contingent on a network model. For operational purposes, a direct observation of traffic flow is more reliable, as it does not depend on any such auxiliary information. For example, if there was a bug in a router's software, direct observation would allow to diagnose the effect of this bug, while an indirect method would not. 11 Security Considerations. Security considerations are addressed in: - Section 3.1: item Robust Selection Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 24] Internet-Draft Passive Packet Measurement June 2003 - Section 3.3: item Secure Export - Section 3.4: item Secure Configuration 12 References [B88] R.T. Braden, A pseudo-machine for packet monitoring and statistics, in Proc ACM SIGCOMM 1988 [ClPB93] K.C. Claffy, G.C. Polyzos, H.-W. Braun, Application of Sampling Methodologies to Network Traffic Characterization, Proceedings of ACM SIGCOMM'93, San Francisco, CA, USA, September 13-17, 1993 [DRC03] T. Dietz, D. Romascanu, B. Claise, Definitions of Managed Objects for Packet Sampling, Internet Draft, draft-ietf-psamp-mib-00.txt, work in progress, June 2003. [D03] M. Djernaes, Cisco Systems NetFlow Services Export Version 9 Transport, Internet Draft, draft-djernaes-netflow-9-transport-00.txt, work in progress, February 2003 [DuGr01] N. G. Duffield and M. Grossglauser, Trajectory Sampling for Direct Traffic Observation, IEEE/ACM Trans. on Networking, 9(3), . 280-292, June 2001. [FHK02] S. Floyd, M. Handley. E. Kohler, Problem Statement for DCCP, Internet Draft draft-ietf-dccp-problem-00.txt, work in progress, October 2002. [LCTV02] W.S. Lai, B.Christian, R.W. Tibbs, S. Van den Berghe, A Framework for Internet Traffic Engineering Measurement, Internet Draft draft-ietf-tewg-measure-05.txt, work in progress, February 2003. [PPM01] P. Phaal, S. Panchen, N. McKee, InMon Corporation's sFlow: A Method for Monitoring Traffic in Switched and Routed Networks, RFC 3176, September 2001 [PAMM98] V. Paxson, G. Almes, J. Mahdavi, M. Mathis, Framework for IP Performance Metrics, RFC 2330, May 1998 [QZCZ03] J. Quittek, T. Zseby, B. Claise, S. Zander, Requirements for IP Flow Information Export, Internet Draft draft-ietf-ipfix-reqs-10.txt, work in progress, June 2003. [SPSJTKS01] A. C. Snoeren, C. Partridge, L. A. Sanchez, C. E. Jones, F. Tchakountio, S. T. Kent, W. T. Strayer, Hash-Based IP Traceback, Proc. ACM SIGCOMM 2001, San Diego, CA, September 2001. [RFC2960] Stewart, R. (ed.) "Stream Control Transmission Protocol", Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 25] Internet-Draft Passive Packet Measurement June 2003 RFC 2960, October 2000 [PR-SCTP] Stewart, R, "SCTP Partial Reliability Extension", Internet Draft, draft-stewart-tsvwg-prsctp-01.txt, work in progress, June 2003. [ZMRD03] T. Zseby, M. Molina, F. Raspall, N.G. Duffield, Sampling and Filtering Techniques for IP Packet Selection, Internet Draft draft-ietf-psamp-sample-tech-01.txt, work in progress, March 2003. 13 Authors' Addresses Derek Chiou Avici Systems 101 Billerica Ave North Billerica, MA 01862 Phone: +1 978-964-2017 Email: dchiou@avici.com Benoit Claise Cisco Systems De Kleetlaan 6a b1 1831 Diegem Belgium Phone: +32 2 704 5622 Email: bclaise@cisco.com Nick Duffield AT&T Labs - Research Room B-139 180 Park Ave Florham Park NJ 07932, USA Phone: +1 973-360-8726 Email: duffield@research.att.com Albert Greenberg AT&T Labs - Research Room A-161 180 Park Ave Florham Park NJ 07932, USA Phone: +1 973-360-8730 Email: albert@research.att.com Matthias Grossglauser School of Computer and Communication Sciences EPFL 1015 Lausanne Switzerland Email: matthias.grossglauser@epfl.ch Peram Marimuthu Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 26] Internet-Draft Passive Packet Measurement June 2003 Cisco Systems 170, W. Tasman Drive San Jose, CA 95134 Phone: (408) 527-6314 Email: peram@cisco.com Jennifer Rexford AT&T Labs - Research Room A-169 180 Park Ave Florham Park NJ 07932, USA Phone: +1 973-360-8728 Email: jrex@research.att.com Ganesh Sadasivan Cisco Systems 170 W. Tasman Drive San Jose, CA 95134 Phone: (408) 527-0251 Email: gsadasiv@cisco.com 14 Intellectual Property Statement AT&T Corporation may own intellectual property applicable to this contribution. The IETF has been notified of AT&T's licensing intent for the specification contained in this document. See http://www.ietf.org/ietf/IPR/ATT-GENERAL.txt for AT&T's IPR statement. 15 Full Copyright Statement Copyright (C) The Internet Society (2003). 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 implementation 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. 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