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diff --git a/doc/rfc/rfc2498.txt b/doc/rfc/rfc2498.txt new file mode 100644 index 0000000..c59cf8a --- /dev/null +++ b/doc/rfc/rfc2498.txt @@ -0,0 +1,563 @@ + + + + + + +Network Working Group J. Mahdavi +Request for Comments: 2498 Pittsburgh Supercomputing Center +Category: Experimental V. Paxson + Lawrence Berkeley National Laboratory + January 1999 + + + IPPM Metrics for Measuring Connectivity + +Status of this Memo + + This memo defines an Experimental Protocol for the Internet + community. It does not specify an Internet standard of any kind. + Discussion and suggestions for improvement are requested. + Distribution of this memo is unlimited. + +Copyright Notice + + Copyright (C) The Internet Society (1999). All Rights Reserved. + +1. Introduction + + Connectivity is the basic stuff from which the Internet is made. + Therefore, metrics determining whether pairs of hosts (IP addresses) + can reach each other must form the base of a measurement suite. We + define several such metrics, some of which serve mainly as building + blocks for the others. + + This memo defines a series of metrics for connectivity between a pair + of Internet hosts. It builds on notions introduced and discussed in + RFC 2330, the IPPM framework document. The reader is assumed to be + familiar with that document. + + The structure of the memo is as follows: + + + An analytic metric, called Type-P-Instantaneous-Unidirectional- + Connectivity, will be introduced to define one-way connectivity at + one moment in time. + + Using this metric, another analytic metric, called Type-P- + Instantaneous-Bidirectional-Connectivity, will be introduced to + define two-way connectivity at one moment in time. + + Using these metrics, corresponding one- and two-way analytic + metrics are defined for connectivity over an interval of time. + + + + + + + + +Mahdavi & Paxson Experimental [Page 1] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + + + Using these metrics, an analytic metric, called Type-P1-P2- + Interval-Temporal-Connectivity, will be introduced to define a + useful notion of two-way connectivity between two hosts over an + interval of time. + + Methodologies are then presented and discussed for estimating + Type-P1-P2-Interval-Temporal-Connectivity in a variety of + settings. + + Careful definition of Type-P1-P2-Interval-Temporal-Connectivity and + the discussion of the metric and the methodologies for estimating it + are the two chief contributions of the memo. + +2. Instantaneous One-way Connectivity + +2.1. Metric Name: + + Type-P-Instantaneous-Unidirectional-Connectivity + +2.2. Metric Parameters: + + + Src, the IP address of a host + + Dst, the IP address of a host + + T, a time + +2.3. Metric Units: + + Boolean. + +2.4. Definition: + + Src has *Type-P-Instantaneous-Unidirectional-Connectivity* to Dst at + time T if a type-P packet transmitted from Src to Dst at time T will + arrive at Dst. + +2.5. Discussion: + + For most applications (e.g., any TCP connection) bidirectional + connectivity is considerably more germane than unidirectional + connectivity, although unidirectional connectivity can be of interest + for some security applications (e.g., testing whether a firewall + correctly filters out a "ping of death"). Most applications also + require connectivity over an interval, while this metric is + instantaneous, though, again, for some security applications + instantaneous connectivity remains of interest. Finally, one might + not have instantaneous connectivity due to a transient event such as + a full queue at a router, even if at nearby instants in time one does + have connectivity. These points are addressed below, with this + metric serving as a building block. + + + +Mahdavi & Paxson Experimental [Page 2] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + + Note also that we have not explicitly defined *when* the packet + arrives at Dst. The TTL field in IP packets is meant to limit IP + packet lifetimes to 255 seconds (RFC 791). In practice the TTL field + can be strictly a hop count (RFC 1812), with most Internet hops being + much shorter than one second. This means that most packets will have + nowhere near the 255 second lifetime. In principle, however, it is + also possible that packets might survive longer than 255 seconds. + Consideration of packet lifetimes must be taken into account in + attempts to measure the value of this metric. + + Finally, one might assume that unidirectional connectivity is + difficult to measure in the absence of connectivity in the reverse + direction. Consider, however, the possibility that a process on + Dst's host notes when it receives packets from Src and reports this + fact either using an external channel, or later in time when Dst does + have connectivity to Src. Such a methodology could reliably measure + the unidirectional connectivity defined in this metric. + +3. Instantaneous Two-way Connectivity + +3.1. Metric Name: + + Type-P-Instantaneous-Bidirectional-Connectivity + +3.2. Metric Parameters: + + + A1, the IP address of a host + + A2, the IP address of a host + + T, a time + +3.3. Metric Units: + + Boolean. + +3.4. Definition: + + Addresses A1 and A2 have *Type-P-Instantaneous-Bidirectional- + Connectivity* at time T if address A1 has Type-P-Instantaneous- + Unidirectional-Connectivity to address A2 and address A2 has Type-P- + Instantaneous-Unidirectional-Connectivity to address A1. + +3.5. Discussion: + + An alternative definition would be that A1 and A2 are fully connected + if at time T address A1 has instantaneous connectivity to address A2, + and at time T+dT address A2 has instantaneous connectivity to A1, + where T+dT is when the packet sent from A1 arrives at A2. This + definition is more useful for measurement, because the measurement + + + +Mahdavi & Paxson Experimental [Page 3] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + + can use a reply from A2 to A1 in order to assess full connectivity. + It is a more complex definition, however, because it breaks the + symmetry between A1 and A2, and requires a notion of quantifying how + long a particular packet from A1 takes to reach A2. We postpone + discussion of this distinction until the development of interval- + connectivity metrics below. + +4. One-way Connectivity + +4.1. Metric Name: + + Type-P-Interval-Unidirectional-Connectivity + +4.2. Metric Parameters: + + + Src, the IP address of a host + + Dst, the IP address of a host + + T, a time + + dT, a duration + {Comment: Thus, the closed interval [T, T+dT] denotes a time + interval.} + +4.3. Metric Units: + + Boolean. + +4.4. Definition: + + Address Src has *Type-P-Interval-Unidirectional-Connectivity* to + address Dst during the interval [T, T+dT] if for some T' within [T, + T+dT] it has Type-P-instantaneous-connectivity to Dst. + +5. Two-way Connectivity + +5.1. Metric Name: + + Type-P-Interval-Bidirectional-Connectivity + +5.2. Metric Parameters: + + + A1, the IP address of a host + + A2, the IP address of a host + + T, a time + + dT, a duration + {Comment: Thus, the closed interval [T, T+dT] denotes a time + interval.} + + + + + +Mahdavi & Paxson Experimental [Page 4] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + +5.3. Metric Units: + + Boolean. + +5.4. Definition: + + Addresses A1 and A2 have *Type-P-Interval-Bidirectional-Connectivity* + between them during the interval [T, T+dT] if address A1 has Type-P- + Interval-Unidirectional-Connectivity to address A2 during the + interval and address A2 has Type-P-Interval-Unidirectional- + Connectivity to address A1 during the interval. + +5.5. Discussion: + + This metric is not quite what's needed for defining "generally + useful" connectivity - that requires the notion that a packet sent + from A1 to A2 can elicit a response from A2 that will reach A1. With + this definition, it could be that A1 and A2 have full-connectivity + but only, for example, at time T1 early enough in the interval [T, + T+dT] that A1 and A2 cannot reply to packets sent by the other. This + deficiency motivates the next metric. + +6. Two-way Temporal Connectivity + +6.1. Metric Name: + + Type-P1-P2-Interval-Temporal-Connectivity + +6.2. Metric Parameters: + + + Src, the IP address of a host + + Dst, the IP address of a host + + T, a time + + dT, a duration + {Comment: Thus, the closed interval [T, T+dT] denotes a time + interval.} + +6.3. Metric Units: + + Boolean. + +6.4. Definition: + + Address Src has *Type-P1-P2-Interval-Temporal-Connectivity* to + address Dst during the interval [T, T+dT] if there exist times T1 and + T2, and time intervals dT1 and dT2, such that: + + + + + +Mahdavi & Paxson Experimental [Page 5] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + + + T1, T1+dT1, T2, T2+dT2 are all in [T, T+dT]. + + T1+dT1 <= T2. + + At time T1, Src has Type-P1 instantanous connectivity to Dst. + + At time T2, Dst has Type-P2 instantanous connectivity to Src. + + dT1 is the time taken for a Type-P1 packet sent by Src at time T1 + to arrive at Dst. + + dT2 is the time taken for a Type-P2 packet sent by Dst at time T2 + to arrive at Src. + +6.5. Discussion: + + This metric defines "generally useful" connectivity -- Src can send a + packet to Dst that elicits a response. Because many applications + utilize different types of packets for forward and reverse traffic, + it is possible (and likely) that the desired responses to a Type-P1 + packet will be of a different type Type-P2. Therefore, in this + metric we allow for different types of packets in the forward and + reverse directions. + +6.6. Methodologies: + + Here we sketch a class of methodologies for estimating Type-P1-P2- + Interval-Temporal-Connectivity. It is a class rather than a single + methodology because the particulars will depend on the types P1 and + P2. + +6.6.1. Inputs: + + + Types P1 and P2, addresses A1 and A2, interval [T, T+dT]. + + N, the number of packets to send as probes for determining + connectivity. + + W, the "waiting time", which bounds for how long it is useful to + wait for a reply to a packet. + Required: W <= 255, dT > W. + +6.6.2. Recommended values: + + dT = 60 seconds. + W = 10 seconds. + N = 20 packets. + + + + + + + + + + + +Mahdavi & Paxson Experimental [Page 6] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + +6.6.3. Algorithm: + + + Compute N *sending-times* that are randomly, uniformly distributed + over [T, T+dT-W]. + + At each sending time, transmit from A1 a well-formed packet of + type P1 to A2. + + Inspect incoming network traffic to A1 to determine if a + successful reply is received. The particulars of doing so are + dependent on types P1 & P2, discussed below. If any successful + reply is received, the value of the measurement is "true". At + this point, the measurement can terminate. + + If no successful replies are received by time T+dT, the value of + the measurement is "false". + +6.6.4. Discussion: + + The algorithm is inexact because it does not (and cannot) probe + temporal connectivity at every instant in time between [T, T+dT]. + The value of N trades off measurement precision against network + measurement load. The state-of-the-art in Internet research does not + yet offer solid guidance for picking N. The values given above are + just guidelines. + +6.6.5. Specific methodology for TCP: + + A TCP-port-N1-port-N2 methodology sends TCP SYN packets with source + port N1 and dest port N2 at address A2. Network traffic incoming to + A1 is interpreted as follows: + + + A SYN-ack packet from A2 to A1 with the proper acknowledgement + fields and ports indicates temporal connectivity. The measurement + terminates immediately with a value of "true". {Comment: if, as a + side effect of the methodology, a full TCP connection has been + established between A1 and A2 -- that is, if A1's TCP stack + acknowledges A2's SYN-ack packet, completing the three-way + handshake -- then the connection now established between A1 and A2 + is best torn down using the usual FIN handshake, and not using a + RST packet, because RST packets are not reliably delivered. If + the three-way handshake is not completed, however, which will + occur if the measurement tool on A1 synthesizes its own initial + SYN packet rather than going through A1's TCP stack, then A1's TCP + stack will automatically terminate the connection in a reliable + fashion as A2 continues transmitting the SYN-ack in an attempt to + establish the connection. Finally, we note that using A1's TCP + stack to conduct the measurement complicates the methodology in + that the stack may retransmit the initial SYN packet, altering the + number of probe packets sent.} + + + + +Mahdavi & Paxson Experimental [Page 7] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + + + A RST packet from A2 to A1 with the proper ports indicates + temporal connectivity between the addresses (and a *lack* of + service connectivity for TCP-port-N1-port-N2 - something that + probably should be addressed with another metric). + + An ICMP port-unreachable from A2 to A1 indicates temporal + connectivity between the addresses (and again a *lack* of service + connectivity for TCP-port-N1-port-N2). {Comment: TCP + implementations generally do not need to send ICMP port- + unreachable messages because a separate mechanism is available + (sending a RST). However, RFC 1122 states that a TCP receiving an + ICMP port-unreachable MUST treat it the same as the equivalent + transport-level mechanism (for TCP, a RST).} + + An ICMP host-unreachable or network-unreachable to A1 (not + necessarily from A2) with an enclosed IP header matching that sent + from A1 to A2 *suggests* a lack of temporal connectivity. If by + time T+dT no evidence of temporal connectivity has been gathered, + then the receipt of the ICMP can be used as additional information + to the measurement value of "false". + + {Comment: Similar methodologies are needed for ICMP Echo, UDP, etc.} + +7. Acknowledgments + + The comments of Guy Almes, Martin Horneffer, Jeff Sedayao, and Sean + Shapira are appreciated. + +8. Security Considerations + + As noted in RFC 2330, active measurement techniques, such as those + defined in this document, can be abused for denial-of-service attacks + disguised as legitimate measurement activity. Furthermore, testing + for connectivity can be used to probe firewalls and other security + mechnisms for weak spots. + +9. References + + [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC + 1812, June 1995. + + [RFC1122] Braden, R., Editor, "Requirements for Internet Hosts -- + Communication Layers", STD, 3, RFC 1122, October 1989. + + [RFC2330] Paxson, V., Almes, G., Mahdavi, J. and M. Mathis, + "Framework for IP Performance Metrics", RFC 2330, May + 1998. + + [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September + 1981. + + + +Mahdavi & Paxson Experimental [Page 8] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + +10. Authors' Addresses + + Jamshid Mahdavi + Pittsburgh Supercomputing Center + 4400 5th Avenue + Pittsburgh, PA 15213 + USA + + EMail: mahdavi@psc.edu + + + Vern Paxson + MS 50A-3111 + Lawrence Berkeley National Laboratory + University of California + Berkeley, CA 94720 + USA + + Phone: +1 510/486-7504 + EMail: vern@ee.lbl.gov + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Mahdavi & Paxson Experimental [Page 9] + +RFC 2498 IPPM Metrics for Measuring Connectivity January 1999 + + +11. Full Copyright Statement + + Copyright (C) The Internet Society (1999). 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. + + This document and the information contained herein is provided on an + "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING + TASK FORCE DISCLAIMS 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. + + + + + + + + + + + + + + + + + + + + + + + + +Mahdavi & Paxson Experimental [Page 10] + |