[ippm] comments on draft-ietf-ippm-bw-capacity-03.txt
Matthew J Zekauskas <matt@internet2.edu> Wed, 08 November 2006 04:34 UTC
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Subject: [ippm] comments on draft-ietf-ippm-bw-capacity-03.txt
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The IPPM Capacity draft authors have asked us to last call the document. As part of that, I re-read the draft, and have some comments, which I already sent to the authors. I mentioned this in the WG meeting today, and repeat them here in case they stir up any other comments. [chair hat off][except, maybe, for the security considerations section at the end...] --Matt -=-=- Nits ---- p3 "even ATM", strike the "even" "affect the capacity also"; I don't like the dangling also, how about dropping it or "also affect the capacity". [I also didn't look up the right thing to do with the "also :)] "roughly 155 Mbps": maybe we should be more precise. Slightly More Substantial ------------------------- I found the whole definitions section a bit backwards. I don't have a problem with the definitions themselves, but rather the sequence in which requirements to understand the definitions generally follow the definitions. I think i'd reorganize section 2. Here's one cut... use it (or modify) if you think it's better. ---snip--- 2. Definitions In this section, we specify definitions for capacity. We begin by first defining "link" and "path" in our context, and then defining a baseline capacity that is simply tied to the physical properties of the link. 2.1 Paths and Links To properly define capacity, we need a notion of link and path that is broader, but yet still consistent with, the definitions in the IPPM Framework document [RFC2330]. In particular, we need to be able to have a link between two network devices that are not necessarily IP-aware. (Consider, for example, an Ethernet switch where the router uplink is 1000baseT, but hosts are connected using 100baseT.) A link L connects two network devices, called nodes. Nodes could be hosts, routers, Ethernet switches, or any other device where input links could have different characteristics than the output link. It is not useful in this context to disambiguate layer 1 circuits into component parts (such as connections between two dense wave division multiplexing nodes). We 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. 2.2 Definition: Nominal Physical Link Capacity The nominal physical link capacity, NomCap(L), is the theoretical maximum amount of data that the link L 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. 2.3 Capacity at the IP Layer 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. 2.3.1 Definition: IP Layer Bits IP layer bits are defined as 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. {mjz this is another "seems backwards" thing, but I guess IP does define everything in terms of octets...} 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. 2.3.2 Definition: 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. 2.3.3 Definition: 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. 2.3.4 Definition: IP Layer Link Usage The average usage of a link L, Used(L,T,I), is the actual number of IP layer bits from any source, correctly received 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 {mjz: inserted "sent" below, since that is important, as in 3.1} of IP bits sent that are correctly received. 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. 2.3.5 Definition: 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]. 2.3.6 Definition: IP Layer Available Link Capacity AvailCap(L,T,I) = C(L,T,I) * ( 1 - Util(L,T,I) ) 2.3.7 Definition: IP Layer Available Path Capacity AvailCap(P,T,I) = min {1..n} {AvailCap(Ln,T,I)} Since measurements of available capacity are more volatile than 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. ---snip--- The notion of "sent and correctly received" is central to all of these definitions, and I wonder if it shouldn't be moved up to section 2. Similarly the notion of time is central, and I think I would also move that up in section 3. the duplicate discussion was also a bit confusing. 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. depending on framing overhead to send the duplicates, etc. Since all the definitions are based on SENT bits that are correctly received, duplicates are not counted, either in usage or capacity. Thus, a value for C(L,T,I) and AvailCap(L,T,I) will reflect the duplication, reporting X instead of 2X. Security considerations: while I believe what you wrote is technically correct, we've always given advice to those that might actually implement something to report the definitions. I've added words mostly stolen from 2330. This document specifies definitions regarding IP traffic traveling between a source and destination in an IP network. Since they are only definitions, they do not raise any security issues and do not have a direct impact on the networking protocol suite. That said, it should be recognized that actually measuring these quantities by conducting Internet measurements can raise both security and privacy concerns. Active techniques, in which traffic is injected into the network, can be abused for denial-of-service attacks disguised as legitimate measurement activity. If recognizable, the active techniques themselves could lend themselves to malicous manipulation. Passive techniques, in which existing traffic is examined, can expose the contents of Internet traffic to unintended recipients. Thus any specification of measurement techniques must include a corresponding discussion of security considerations. References: The reference to 2119 is dangling, and should be deleted. I think 1812, at least, needs to be "normative" (although you could argue about whether normative is necessary for an informational draft too). You need to understand the IP header validation requirements. You could make the ref informative by importing the text, and then including a reference to note where it came from. 2330, maybe, as well, since the document talks about differences from that document (although I'm less positive it's needed; I *think* the doc stands alone w/o reading 2330). _______________________________________________ ippm mailing list ippm@ietf.org https://www1.ietf.org/mailman/listinfo/ippm
- [ippm] comments on draft-ietf-ippm-bw-capacity-03… Matthew J Zekauskas
- Re: [ippm] comments on draft-ietf-ippm-bw-capacit… Joseph Kopena