Internet Engineering Task Force Integrated Services WG
INTERNET-DRAFT Scott Shenker
draft-ietf-intserv-charac-00.txt XEROX PARC
14 November 1995
Expires: ?/?/96
Specification of General Characterization Parameters
Status of this Memo
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This document is a product of the Integrated Services working group
of the Internet Engineering Task Force. Comments are solicited and
should be addressed to the working group's mailing list at int-
serv@isi.edu and/or the author(s).
Abstract
This memo defines general characterization parameters for network
elements supporting enhanced qualities of service. General
characterization parameters are those that are not specific to a
particular quality of service.
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Introduction
This memo defines general, or service-independent, characterization
parameters for network elements. General characterization parameters
are those that are not specific to a particular quality of service.
A discussion of how these parameters fit into an integrated services
architecture can be found in [1]. Please refer to that document for
definitions and additional information about the specification of
qualities of service within the IP protocol family.
The specifications of the various qualities of service ([2-4])
describe the associated characterization parameters that must be
exported. However, there are some quantities of interest that are
not specific to a particular quality of service. These "general"
characterizations are described in this document.
Characterization parameters associated with a particular service are
attached to a service_name. The service-name associated with general
characterizations is 0. We describe these general characterization
parameters below.
Number of IS Hops
IS stands for "integrated services aware". An integrated services
aware network element is one that conforms to the various
requirements described in this document and the documents [1-4] (it
need not offer those services, but if it does it supports and
characterizes the services in conformance with the relevant
specification). The composition rule is to increment the counter by
one. This quantity, when composed end-to-end, informs the endpoint
of the number of Integrated-Services aware network elements
traversed along the path from sender to receiver. The parameter_name
for this field is 1. The characterization parameter may be
represented as a single 16-bit unsigned integer in network byte
order.
Number of IP Hops
The local parameter is the number of IP hops between the last (i.e.,
upstream) IS network element and this one. Observe that since the
network element may not be aware of its neighbor on the the upstream
path, *the local parameter must be provided to the network element by
the setup protocol* (possibly by tracking the TTL value).
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The composition rule is additive. This quantity, when composed end-
to-end, informs the endpoint of the number of IP network elements
("hops") traversed along the path from sender to receiver. The
parameter_name for local parameter is 2. The parameter_name for the
composed quantity is 3. These characterization parameters may be
represented as 16-bit unsigned integers in network byte order.
Comparison of the end-to-end composition of the two previous
characterization values will yield the number of non-IS network
elements along the path.
Bandwidth
The local parameter is the bandwidth of the network element. For
links, this value would be the bandwidth of the link. The
composition rule is to take the minimum of the network element's
value and the previously composed value. This quantity, when
composed end-to-end, informs the endpoint of the minimal bandwidth
link along the path from sender to receiver. The parameter_name for
the bandwidth of the network element's link is 4. The parameter_name
for the composed minimal bandwidth along the path is 5.
These values are measured in bytes per second, and can range from 1
byte per second to as large as 40 terabytes per second (or about what
is believed to be the maximum theoretical bandwidth of a single
strand of fiber). Clearly, particularly for large bandwidths, only
the first few digits are significant and so the use of floating point
representations, accurate to at least 0.1% is encouraged. These
characterizations of bandwidth can be represented by floating point
numbers in single-precision IEEE floating point format.
NOTE: If the number of IS hops is less than the number of IP hops,
end systems should treat this value as a hint rather than a
reliable value.
Latency
The local parameter is the latency of the link associated with the
network element, where the latency is defined to be the minimal
possible packet delay along the path taken by the flow. This delay
may result from "speed-of-light" propagation delay, or from packet
processing limitations, or both. The composition rule is additive.
This quantity, when composed end-to-end, informs the endpoint of the
minimal packet delay along the path from sender to receiver. The
parameter_name for the latency of the network element's link is 6.
The parameter_name for the cumulative latency along the path is 7.
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The delays are measured in units of one microsecond. An individual
element can advertise a delay value between 1 and 2**28 (somewhat
over two minutes) and the total delay added across all elements can
range as high as (2**32)-1. Should the sum of the different elements
delay exceed (2**32)-1, the end-to-end advertised delay should be
(2**32)-1.
Note that while the granularity of measurement is microseconds, a
conforming element is free to measure delays more loosely. The
minimum requirement is that the element estimate its delay accurately
to the nearest 100 microsecond granularity. Elements that can
measure more accurately are, of course, encouraged to do so.
NOTE: If the number of IS hops is less than the number of IP hops,
end systems should treat this value as a hint rather than a
promised value.
NOTE: Measuring in milliseconds is not acceptable, because if the
minimum delay value is a millisecond, a path with several hops
will lead to a composed delay of at least several milliseconds,
which is likely to be misleading.
The characterization parameters may be represented as 32-bit unsigned
integers in network byte order.
NOTE: There are some subnet technologies where determining this
minimal delay is difficult. For instance, the speed-of-light
delays on an ethernet bridged via satellite with another ethernet
vary by several orders of magnitude. The exported values should
be conservative estimates of the delays. Any additional delays
(that is, delays larger than this minimal amount) must be
considered part of the variable delays which are described by
characterizations specific to the individual services. For
example, in predictive service the maximal delay experienced going
from one network element to the next should be the delay bound
plus the latency.
MTU
The local characterization parameter is the MTU, where the MTU of a
network element is defined to be the maximum transmission unit the
network element can accommodate without fragmentation. The
composition rule is to take the minimum of the network element's MTU
and the previously composed value. This quantity, when composed end-
to-end, informs the endpoint of the maximum transmission unit that
can traverse the path from sender to receiver without fragmentation.
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The parameter_name for the MTU of the network element's link is 8.
The parameter_name for the composed MTU along the path is 9.
NOTE: If the number of IS hops is less than the number of IP hops,
end systems should treat this value as a hint rather than a
promised value.
Service Availability
The INT SERV working group is still developing proposals for handling
heterogeneity. When that work is complete, a set of requirement
parameters will be defined.
Security Considerations
Security considerations are not discussed in this memo.
References
[1] S. Shenker and J. Wroclawski. "Network Element Service
Specification Template", Internet Draft, June 1995, <draft-ietf-
intserv-svc-template-01.txt>
[2] S. Shenker, C. Partridge, and J. Wroclawski. "Specification of
Controlled Delay Quality of Service", Internet Draft, ?? 1995,
<draft-ietf-intserv-control-del-svc-02.txt>
[3] S. Shenker and C. Partridge. Specification of Guaranteed Quality
of Service", Internet Draft, ?? 1995, <draft-ietf-intserv-
guaranteed-svc-03.txt>
[4] S. Shenker, C. Partridge, B. Davie, and L. Breslau.
"Specification of Predictive Quality of Service", Internet Draft, ??
1995, <draft-ietf-intserv-predictive-svc-01.txt>
Author's Address:
Scott Shenker
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94304-1314
shenker@parc.xerox.com
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415-812-4840
415-812-4471 (FAX)
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