Network Working Group J. Mahdavi
Request for Comments: 2678 Pittsburgh Supercomputing Center
Obsoletes: 2498 V. Paxson
Category: Standards Track Lawrence Berkeley National Laboratory
IPPM Metrics for Measuring Connectivity
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1999). All Rights Reserved.
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 Standards Track [Page 1]RFC 2678 IPPM Metrics for Measuring Connectivity September 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
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 Connectivity2.1. Metric Name:
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:
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.
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 Standards Track [Page 2]RFC 2678 IPPM Metrics for Measuring Connectivity September 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