IP Performance Metrics Working P. Chimento Group JHU Applied Physics Lab Internet-Draft J. Ishac Expires: November 18, 2006 NASA Glenn Research Center May 17, 2006 Defining Network Capacity draft-ietf-ippm-bw-capacity-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on November 18, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Measuring capacity is a task that sounds simple, but in reality can be quite complex. In addition, the lack of a unified nomenclature on this subject makes it increasingly difficult to properly build, test, and use techniques and tools built around these constructs. This document provides definitions for the terms 'Capacity' and 'Available Capacity' related to IP traffic traveling between a source and destination in an IP network. By doing so, we hope to provide a Chimento & Ishac Expires November 18, 2006 [Page 1] Internet-Draft Network Capacity May 2006 common framework for the discussion and analysis of a diverse set of current and future estimation techniques. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 Standard or Correctly Formed Packets . . . . . . . . . . . 8 3.2 Other Potential Factors . . . . . . . . . . . . . . . . . 8 3.3 Common Literature Terminology . . . . . . . . . . . . . . 9 3.4 Comparison to Bulk Transfer Capacity (BTC) . . . . . . . . 9 3.5 Type P Packets . . . . . . . . . . . . . . . . . . . . . . 10 3.6 Time and Sampling . . . . . . . . . . . . . . . . . . . . 10 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8.1 Normative References . . . . . . . . . . . . . . . . . . . 15 8.2 Informative References . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . . 17 Chimento & Ishac Expires November 18, 2006 [Page 2] Internet-Draft Network Capacity May 2006 1. Introduction Measuring the capacity of a link or network path is a task that sounds simple, but in reality can be quite complex. Any physical medium requires that information be encoded and, depending on the medium, there are various schemes to convert information into a sequence of signals that are transmitted physically from one location to another. While on some media, the maximum frequency of these signals can be thought of as "capacity", on other media, the signal transmission frequency and the information capacity of the medium (channel) may be quite different. For example, a satellite channel may have a carrier frequency of a few gigahertz, but an information-carrying capacity of only a few hundred kilobits per second. Often similar or identical terms are used to refer to these different applications of capacity, adding to the ambiguity and confusion, and the lack of a unified nomenclature makes it difficult to properly build, test, and use various techniques and tools. We are interested in information-carrying capacity, but even this is not straightforward. Each of the layers, depending on the medium, adds overhead to the task of carrying information. The wired Ethernet uses Manchester coding or 4/5 coding which cuts down considerably on the "theoretical" capacity. Similarly RF (radio frequency) communications will often add redundancy to the coding scheme to implement forward error correction because the physical medium (air) is lossy. This can further decrease the information efficiency. In addition to coding schemes, usually the physical layer and the link layer add framing bits for multiplexing and control purposes. For example, on SONET there is physical layer framing and typically also some layer 2 framing such as HDLC, PPP or even ATM. Aside from questions of coding efficiency, there are issues of how access to the channel is controlled which may affect the capacity also. For example, a multiple-access medium with collision detection, avoidance and recovery mechanisms has a varying capacity from the point of view of the users. This varying capacity depends upon the total number of users contending for the medium, how busy the users are, and bounds resulting from the mechanisms themselves. RF channels are also varying capacity, depending on range, environmental conditions, mobility and shadowing, etc. The important points to derive from this discussion are these: First, capacity is only meaningful when defined relative to a given protocol layer in the network. It is meaningless to speak of "link" capacity Chimento & Ishac Expires November 18, 2006 [Page 3] Internet-Draft Network Capacity May 2006 without qualifying exactly what is meant. Second, capacity is not necessarily fixed, and consequently, a single measure of capacity at whatever layer may in fact provide a skewed picture (either optimistic or pessimistic) of what is actually available. Chimento & Ishac Expires November 18, 2006 [Page 4] Internet-Draft Network Capacity May 2006 2. Definitions In this section, we specify definitions for capacity. We begin by first defining a baseline capacity that is simply tied to the physical properties of the link. Nominal Physical Link Capacity: Or NomCap(L) is the theoretical maximum amount of data that the link can support. For example, an OC-3 link would be capable of roughly 155 Mbps. We stress that this is a measurement at the physical layer and not the network IP layer, which we will define separately. While NomCap(L) is typically constant over time, there are links whose characteristics may allow otherwise, such as the dynamic activation of additional transponders for a satellite link. The nominal physical link capacity is provided as a means to help distinguish between the commonly used link layer capacities and the remaining definitions for IP layer capacity. As a result, the value of NomCap(L) does not influence the other definitions presented in this document. There are many factors that can reduce the IP information carrying capacity of the link, some of which have already been discussed in the introduction. However, the goal of this document is not to become an exhaustive list of of such factors. Rather, we outline some of the major examples in the following section, thus providing food for thought to those implementing the algorithms or tools that attempt to measure capacity accurately. The remaining definitions are all given in terms of "IP layer bits" in order to distinguish these definitions from the nominal physical capacity of the link. IP layer bits: Eight (8) times the number of octets in all IP packets received, from the first octet of the IP header to the last octet of the IP packet payload, inclusive. We then define a path P of length n as a series of links (L1, L2, ..., Ln) connecting a sequence of nodes (N1, N2, ..., Nn+1). A source, S, and destination, D, reside at N1 and Nn+1 respectively. Furthermore, we define a link L as a special case where the path size is one. The concept of links are consistent with [RFC2330] in that they represent a link-level connection between two nodes, and each node is not necessarily a router. However, the concept of a path differs slightly as intermediate nodes N2...Nn do not need to be routers. They may, for example, be Ethernet switches or other kinds Chimento & Ishac Expires November 18, 2006 [Page 5] Internet-Draft Network Capacity May 2006 of layer 2 or layer 1 devices. IP layer bits are recorded at the destination, D, beginning at time T and ending at a time T+I. Since the definitions are based on averages, the two time parameters, T and I, must accompany any report or estimate of the following values in order for them to remain meaningful. It is not required that the interval boundary points fall between packet arrivals at D. However, boundaries that fall within a packet will invalidate the packets on which they fall. Specifically, the data from the partial packet that is contained within the interval will not be counted. This may artificially bias some of the values, depending on the length of the interval and the amount of data received during that interval. We elaborate on what constitutes correctly received data in the next section. IP Layer Link Capacity: We define the IP Layer link capacity, C(L,T,I), to be the maximum number of IP layer bits that can be transmitted from the source S and correctly received by the destination D over the link L during the interval [T, T+I], divided by I. Using this, we can then extend this notion to an entire path, such that the IP layer path capacity simply becomes that of the link with the smallest capacity along that path. IP Layer Path Capacity: C(P,T,I) = min {1..n} {C(Ln,T,I)} The previous definitions specify a link's capacity, namely the IP information bits that can be transmitted across a link or path should the resource be free of any congestion. Determining how much capacity is available for use on a congested link is potentially much more useful. However, in order to define the available capacity we must first specify how much is being used. IP Layer Link Usage: The average usage of a link L, Used(L,T,I), is the actual number of IP layer bits correctly transmitted from any source over link L during the interval [T, T+I], divided by I. An important distinction between usage and capacity is that Used(L,T,I) is not the maximum amount, but rather, the actual amount of IP bits that are sent. The information transmitted across the link can be generated by any source, including those who may not be directly attached to either side of the link. In addition, each information flow from these sources may share any number (from one to n) of links in the overall path between S and D. Next, we express usage as a fraction of the overall IP layer link capacity. Chimento & Ishac Expires November 18, 2006 [Page 6] Internet-Draft Network Capacity May 2006 Average IP Layer Link Utilization: Util(L,T,I) = ( Used(L,T,I) / C(L,T,I) ) Thus, the utilization now represents the fraction of the capacity that is being used and is a value between zero, meaning nothing is used, and one, meaning the link is fully saturated. Multiplying the utilization by 100 yields the percent utilization of the link. By using the above, we can now define the capacity available over the link as well as the path between S and D. Note that this is essentially the definition in [PDM]. IP Layer Available Link Capacity AvailCap(L,T,I) = C(L,T,I) * ( 1 - Util(L,T,I) ) IP Layer Available Path Capacity AvailCap(P,T,I) = min {1..n} {AvailCap(Ln,T,I)} Since measurements of available capacity are more volatile that that of capacity, it is important that both the time and interval be specified as their values have a great deal of influence on the results. In addition, a sequence of measurements may be beneficial in offsetting the volatility when attempting to characterize available capacity. Chimento & Ishac Expires November 18, 2006 [Page 7] Internet-Draft Network Capacity May 2006 3. Discussion 3.1 Standard or Correctly Formed Packets The definitions in this document specify that IP packets must be received correctly. The IPPM framework recommends a set of criteria for such standard-formed packet in section 15 of [RFC2330]. However, it is inadequate for use with this document. Thus, we outline our own criteria below while pointing out any variations or similarities to [RFC2330]. First, data that is in error at layers below IP and cannot be properly passed to the IP layer should not be counted. For example, wireless media often has a considerably larger error rate than wired media, resulting in a reduction in IP Link Capacity. In accordance with the framework, packets that fail validation of the IP header should be discarded. Specifically, the requirements in [RFC1812] section 5.2.2 on IP header validation should be checked, which includes a valid length, checksum, and version field. The framework specifies further restrictions, requiring that any transport header be checked for correctness and that any packets with IP options be ignored. However, the definitions in this document are concerned with the traversal of IP layer bits. As a result, data from the higher layers is not required to be valid or understood as they are simply regarded as part of the IP packet. The same holds true for IP options. Valid IP fragments should also be counted as they expend the resources of a link even though assembly of the full packet may not be possible. The framework differs in this area, discarding IP fragments. In summary, any IP packet that can be properly processed should be included in these calculations. 3.2 Other Potential Factors The base definitions make no mention of hardware duplication of packets. While hardware duplication has no impact on the nominal capacity, it can impact the IP link layer capacity. For example, consider a link which can normally carry a capacity of 2X on average. However, the link has developed a syndrome where it duplicates every incoming packet. The link would still technically carry a capacity of 2X, however the link has a effective capacity of X or lower, depending on framing overhead to send the duplicates, etc. Thus, a value for C(L,T,I) and AvailCap(L,T,I) will reflect the duplication with the lower value. IP encapsulation does not impact the definitions as all IP header and Chimento & Ishac Expires November 18, 2006 [Page 8] Internet-Draft Network Capacity May 2006 payload bits should be counted regardless of content. However, different sized IP packets can lead to a variation in the amount of overhead needed at the lower layers to transmit the data, thus altering the overall IP link layer capacity. Should the link happen to employ a compression scheme such as ROHC [RFC3095] or V.44 [V44], some of the original bits are not transmitted across the link. However, the inflated (not compressed) number of IP-layer bits should be counted. 3.3 Common Literature Terminology Certain terms are often used to characterize specific aspects of the presented definitions. The link with the smallest capacity is commonly referred to as the "narrow link" of a path. Also, the value of n that satisfies AvailCap(P,T,I), is often referred to as the "tight link" within a path. So, while Ln may have a very large capacity, the overall congestion level on the link makes it the likely bottleneck of a connection. Conversely, a link that has the smallest capacity may not be a bottleneck should it be lightly loaded in relation to the rest of the path. Also, common literature often overloads the term "bandwidth" to refer to what we have described as capacity in this document. For example, when inquiring about the bandwidth of a 802.11b link, a network engineer will likely answer with 11 Mbps. However, an electrical engineer may answer with 25 MHz, and an end user may tell you that his observed bandwidth is 8 Mbps. In contrast, the term capacity is not quite as overloaded and is an appropriate term that better reflects what is actually being measured. 3.4 Comparison to Bulk Transfer Capacity (BTC) Bulk Transfer Capacity (BTC) [RFC3148] provides a distinct perspective on path capacity that differs from the definitions in this document in several fundamental ways. First, BTC operates at the transport layer, gauging the amount of capacity available to an application that wishes to send data. Only unique data is measured, meaning header and retransmitted data are not included in the calculation. In contrast, IP layer link capacity includes the IP header and is indifferent to the uniqueness of the data contained within the packet payload (Hardware duplication of packets is an anomaly addressed in the previous section). Second, BTC utilizes a single congestion aware transport connection, such as TCP, to obtain measurements. As a result, BTC implementations react strongly to different path characteristics, topologies, and distances. Since these differences can affect the control loop (propagation delays, segment reordering, etc), the reaction is further dependent on the Chimento & Ishac Expires November 18, 2006 [Page 9] Internet-Draft Network Capacity May 2006 algorithms being employed for the measurements. For example, consider a single event where a link suffers a large duration of bit errors. The event could cause IP layer packets to be discarded, and the lost packets would reduce the IP layer link capacity. However, the same event and subsequent losses would trigger loss recovery for a BTC measurement resulting in the retransmission of data and a potentially reduced sending rate. Thus, a measurement of BTC does not correspond to any of the definitions in this document. Both techniques are useful in exploring the characteristics of a network path, but from different perspectives. 3.5 Type P Packets Note that these definitions do not make mention of "Type P" packets, while other IPPM definitions do. We could add the packet type as an extra parameter. This would have the effect of defining a large number of quantities, relative to the QoS policies that a given network or concatenation of networks may have in effect in the path. It would produce metrics such as "estimated EF IP Link/Path Capacity" or "estimated EF IP Link Utilization". Such metrics may indeed be useful. For example, this would yield something like the sum of the capacities of all the QoS classes defined along the path as the link or path capacity. The breakdown then gives the user an analysis of how the link or path capacity (or at least the "tight link" capacity) is allocated among classes. These QoS-based capacities become difficult to measure on a path if there are different capacities defined per QoS class on different links in the path. Possibly the best way to approach this would be to measure each link in a path individually, and then combine the information from individual links. 3.6 Time and Sampling We must emphasize the importance of time in the basic definitions of these quantities. We know that traffic on the Internet is highly variable across all time scales. This argues that the time and length of measurements are critical variables in reporting available capacity measurements. The closer to "instantaneous" a metric is, the more important it is to have a plan for sampling the metric over a time period that is sufficiently large. By doing so, we allow valid statistical inferences to be made from the measurements. An obvious pitfall here is sampling in a way that causes bias. For example, a situation where the sampling frequency is a multiple of the frequency of an underlying condition. Chimento & Ishac Expires November 18, 2006 [Page 10] Internet-Draft Network Capacity May 2006 4. Conclusion In this document, we have defined a set of quantities related to the capacity of links in an IP network. In these definitions, we have tried to be as clear as possible and take into account various characteristics that links can have. The goal of these definitions is to enable researchers who propose capacity metrics to relate those metrics to these definitions and to evaluate those metrics with respect to how well they approximate these quantities. In addition, we have pointed out some key auxiliary parameters and opened a discussion of issues related to valid inferences from available capacity metrics. Chimento & Ishac Expires November 18, 2006 [Page 11] Internet-Draft Network Capacity May 2006 5. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. Chimento & Ishac Expires November 18, 2006 [Page 12] Internet-Draft Network Capacity May 2006 6. Security Considerations This document specifies definitions regarding IP traffic traveling between a source and destination in an IP network. These definitions do not raise any security issues and do not have a direct impact on the networking protocol suite. Chimento & Ishac Expires November 18, 2006 [Page 13] Internet-Draft Network Capacity May 2006 7. Acknowledgments The authors would like to acknowledge Mark Allman, Patrik Arlos, Matt Mathis, Al Morton, and Stanislav Shalunov for their suggestions, comments, and reviews. We also thank members of the IETF IPPM Mailing List for their discussions and feedback on this document. Chimento & Ishac Expires November 18, 2006 [Page 14] Internet-Draft Network Capacity May 2006 8. References 8.1 Normative References 8.2 Informative References [PDM] Dovrolis, C., Ramanathan, P., and D. Moore, "Packet Dispersion Techniques and a Capacity Estimation Methodology", IEEE/ACM Transactions on Networking 12(6): 963-977, December 2004. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis, "Framework for IP Performance Metrics", RFC 2330, May 1998. [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [RFC3148] Mathis, M. and M. Allman, "A Framework for Defining Empirical Bulk Transfer Capacity Metrics", RFC 3148, July 2001. [V44] ITU Telecommunication Standardization Sector (ITU-T) Recommendation V.44, "Data Compression Procedures", November 2000. Chimento & Ishac Expires November 18, 2006 [Page 15] Internet-Draft Network Capacity May 2006 Authors' Addresses Phil Chimento JHU Applied Physics Lab 11100 Johns Hopkins Road Laurel, Maryland 20723-6099 USA Phone: +1-240-228-1743 Fax: +1-240-228-0789 Email: Philip.Chimento@jhuapl.edu Joseph Ishac NASA Glenn Research Center 21000 Brookpark Road Cleveland, Ohio 44135 USA Phone: +1-216-433-6587 Fax: +1-216-433-8705 Email: jishac@grc.nasa.gov Chimento & Ishac Expires November 18, 2006 [Page 16] Internet-Draft Network Capacity May 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Chimento & Ishac Expires November 18, 2006 [Page 17]