Hi,
Overall, looks good. I have a few comments that are inlined into the
document below.
Thanks.
Derek
INTERNET-DRAFT Nick Duffield (Editor)
draft-ietf-psamp-framework-03.txt AT&T Labs - Research
June 2003
Expires: December 2003
A Framework for Passive Packet Measurement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119.
Abstract
A wide range of traffic engineering and troubleshooting tasks rely
on timely and detailed traffic measurements that can be
consistently interpreted. This document describes a framework for
packet sampling that is (a) general enough to serve as the basis
for a wide range of operational tasks, and (b) needs only a small
set of packet selectors that facilitate ubiquitous deployment in
router interfaces or dedicated measurement devices, even at very
high speeds. The framework also covers reporting and exporting
functions used by the sampling element, and configuration of the
sampling element.
Comments on this document should be addressed to the PSAMP WG
mailing list: psamp@ops.ietf.org
To subscribe: psamp-request@ops.ietf.org, in body: subscribe
Archive: https://ops.ietf.org/lists/psamp/
Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 1]
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0 Contents
1 Motivation ................................................. 3
2 Elements, Terminology, and Architecture .................... 3
3 Requirements ............................................... 6
3.1 Selection Process Requirements ......................... 6
3.2 Reporting Process Requirements ......................... 6
3.3 Export Process Requirements ............................ 7
3.4 Configuration Requirements ............................. 7
4 Packet Selection ............................................ 8
4.1 Packet Selection Terminology ............................ 8
4.2 Packet Selection Operations for a PSAMP ................. 9
4.3 Input Sequence Numbers for Primitive Selectors .......... 11
4.4 Composite Selectors . .................................. 12
4.5 Constraints on the Sampling Frequency ................... 12
4.6 Criteria for Choice of Selection Operations ............ 12
5 Reporting Process .......................................... 13
5.1 Mandatory Contents of Packet Reports (MUST) ............. 13
5.2 Recommended Contents for Packet Reports (SHOULD) ........ 13
5.3 Report Interpretation ................................. 14
7 Parallel Measurement Processes ............................. 15
6 Export Process .............................................. 15
7.1 Collector Destination .................................. 15
7.2 Local Export ........................................... 15
7.3 Reliable vs. Unreliable Transport ...................... 16
7.4 Limiting Delay in Exporting Measurement Packets ........ 16
7.5 Configurable Export Rate Limit ......................... 16
7.6 Congestion-aware Unreliable Transport .................. 17
7.7 Collector-based Rate Reconfiguration ................... 17
7.7.1 Changing the Export Rate and Other Rates ........... 17
7.7.2 Notions of Fairness ................................ 18
7.7.3 Behavior Under Overload and Failure ................ 19
8 Configuration and Management ............................... 19
9 Feasibility and Complexity ................................. 20
9.1 Feasibility ............ ............................... 20
9.1.1 Filtering .......................................... 20
9.1.2 Sampling ........................................... 20
9.1.3 Hashing ............................................ 20
9.1.4 Reporting .......................................... 21
9.1.5 Export ............................................. 21
9.2 Potential Hardware Complexity .......................... 21
10 Applications .............................................. 22
10.1 Baseline Measurement and Drill Down ................... 22
10.2 Passive Performance Measurement ....................... 23
10.3 Troubleshooting ....................................... 23
11 Security Considerations .................................... 24
12 References ................................................ 25
13 Authors' Addresses ........................................ 26
14 Intellectual Property Statement ........................... 27
15 Full Copyright Statement .................................. 27
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1 Motivation
This document describes a framework in which to define a standard
This document describes a framework that defines a standard
set of capabilities for network elements to sample subsets of
packets by statistical and other methods. The framework will
accommodate future work to (i) specify a set of selectors by which
packets are sampled (ii) specify the information that is to be made
available for reporting on sampled packets; (iii) describe a
protocol by which information on sampled packets is reported to
applications; (iv) describe a protocol by which packet selection
and reporting are configured.
The motivation to standardize these capabilities comes from the
need for measurement-based support for network management and
control across multivendor domains. This requires domain wide
consistency in the types of selection schemes available, the manner
in which the resulting measurements are presented, and
consequently, consistency of the interpretation that can be put on
them.
The capabilities are positioned as suppliers of packet samples to
higher level consumers, including both remote collectors and
applications, and on board measurement-based applications. Indeed,
development of the standards within the framework described here
should be open to influence by the requirements of standards in
related IETF WGs, for example, IP Performance Metrics (IPPM)
[PAMM98] and Internet Traffic Engineering (TEWG) [LCTV02].
Conversely, we expect that aspects of this framework not
specifically concerned with the central issue of packet selection
and report formation may be able to leverage work in other
WGs. Potential examples are the format and export of measurement
reports, which may leverage the information model and export
protocols of IP Flow Information Export (IPFIX) [QZCZ03], and work
in congestion aware unreliable transport in the Datagram Congestion
Control Protocol (DCCP) [FHK02], and related work in The Stream
Control Transmission Protocol (SCTP) [SCTP] and [PR-SCTP].
2 Elements, Terminology, and Architecture
This section defines the basic elements of the PSAMP framework. At
the highest level, the architecture comprises observation points
(at which packets are observed), measurement processes (which
select packets and construct reports on them) and export processes
(which export reports to collectors). The full defintions of these
terms now follow.
* Observation Point: the observation point is a location in the
network where a packet stream is observed. Examples are, a line
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to which a probe is attached, a shared medium, such as an
Ethernet-based LAN, a single port of a router, or set of
interfaces (physical or logical) of a router, an embedded
measurement subsystem within an interface.
Observation Point: the observation point is a location in the
network where a packet stream is observed. Examples are (i) a line
to which a probe is attached, (ii) a shared medium, such as an
Ethernet-based LAN, (iii) a single port of a router, or set of
interfaces (physical or logical) of a router or (iv) an embedded
measurement subsystem within an interface.
ADD (i), (ii) ...
* Measurement Process: the combination of a selection process
followed by a reporting process.
* Packet Stream: a sequence of packets, each of which was observed
at the observation point. Note that when packets are sampled
from a stream, the selected packets usually do not have common
properties by which they can be distinguished from packets that
have not been selected. Therefore we define here the term
stream instead of flow, which is defined as set of packets with
common properties [QuZC02].
* Packet Content: the union of the following: packet header, packet
payload, encapsulation headers, and link layer headers.
* Observed Packet Stream: the packet stream comprising all packets
observed at the observation point.
* Selection Process: a selection process selects a substream of
packets from the observed packet stream. A selection process
entails the composition of one or more selectors in succession,
acting on each packet in the observed packet stream. When
selectors are composed, the output stream packet issuing from
one selector forms the input packet stream for the succeeding
selector.
* Selector (or selection operation): a configurable packet
selection operation that acts on single packets. It takes as
its input, the content of a single packet from a packet stream,
information derived from the packet's treatment at the
observation point, and selection state that may be maintained
at the observation point. If the packet is selected, this same
information may be considered as the output. Selectors may
change the selection state.
* Composite Selector: an ordered composition of selectors.
* Primitive Selector: a selector that is not a composition of
multiple selectors.
* Selection State: the selection process may maintain state
information for use by the selection process and/or the
reporting process. At a given time, the selection state may
depend on packets observed up that time and/or other variables.
Examples include sequence numbers of packets at the input of
selectors, timestamps, iterators for pseudorandom number
generators, calculated hash values, and indicators of whether a
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packet was selected by a given selector.
* Reporting Process: the creation of a report stream of information
on packets selected by a selection processes, in preparation
for export. The input to a reporting process comprises that
information available to a selection process, for the selected
packets. The report stream contains two distinguished types of
information: packet reports, and report interpretation.
* Packet Reports: a configurable subset of the per packet input to
the reporting process.
* Report Interpretation: subsidiary information relating to one or
more packets, that is used for interpretation of their packet
reports. Examples include configuration parameters of the PSAMP
device, and configuration parameters of the selection and
reporting process.
* Export Process: sends the output of one or more reporting process
to one or more collectors.
* Collector: a collector receives a report stream exported by one
or more measurement processes. In some cases, the entity that
hosts the measurement and/or export process may also serve as
the collector.
* Measurement packets: one or packet reports, and perhaps report
interpretation, are bundled by the export process into a
measurement packet for export to a collector.
Various possibilities for the high level architecture of these
elements is as follows.
= Observation Point, MP = Measurement Process, EP = Export Process
+---------------------+ +------------------+
|Observation Point(s) | | Collector(1) |
|MP(s)--->EP----------+---------------->| |
|MP(s)--->EP----------+-------+-------->| |
+---------------------+ | +------------------+
|
+---------------------+ | +------------------+
|Observation Point(s) | +-------->| Collector(2) |
|MP(s)--->EP----------+---------------->| |
+---------------------+ +------------------+
+---------------------+
|Observation Point(s) |
|MP(s)--->EP---+ |
| | |
|Collector(3)<-+ |
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+---------------------+
3 Requirements
3.1 Selection Process Requirements.
* Ubiquity: The selectors must be simple enough to be implemented
ubiquitously at maximal line rate.
* Applicability: the set of selectors must be rich enough to
support a range of existing and emerging measurement based
applications and protocols. This requires a workable trade-off
between the range of traffic engineering applications and
operational tasks it enables, and the complexity of the set of
capabilities.
* Extensibility: to allow for additional packet selectors to
support future applications.
* Flexibility: to support selection of packets using different
network protocols or encapsulation layers (e.g. IPv4, IPv6,
MPLS, etc), and under packet encryption.
I agree with Benoit that packet encryption will make general PSAMP
difficult, especially since the encryption key will not generally be
available.
* Robust Selection: packet selection MUST be robust w.r.t. attempts
to craft a packet stream from which packets are selected
disproportionately (e.g. to evade selection, or overload the
measurement system).
* Parallel Measurements: multiple independent measurement
processes at the same entity.
* Non-contingency: in order to satisfy the ubiquity requirement,
the selection decision for each packet MUST NOT depend on
future packets. Rather, the selection decision MUST be capable
of being made on the basis of the selection process input up to
and including the packet in question. This excludes selection
functions that require caching of packet for selection
contingent on subsequent packets. See also the timeliness
requirement following.
Selectors are outlined in Section 4, and described in more detail in
the companion document [ZMRD03].
3.2 Reporting Process Requirements
* Transparency: allow transparent interpretation of measurements as
communicated by PSAMP reporting, without any need to obtain
additional information concerning the observed packet stream.
* Robustness to Information Loss: allow robust interpretation of
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measurements with respect to reports missing due to data loss,
e.g. in transport, or within the measurement, reporting or
exporting processes. Inclusion in reporting of information
that enables the accuracy of measurements to be determined.
* Faithfulness: all reported quantities that relate to the packet
treatment MUST reflect the router state and configuration
encountered by the packet at the time it is received by the
measurement process.
* Privacy: selection of the content of packet reports will be
cognizant of privacy and anonymity issues while being
responsive to the needs of measurement applications, and in
accordance with RFC 2804. Full packet capture of arbitrary
packet streams is explicitly out of scope.
A specific reporting processes meeting these requirements, and the
requirement for ubiquity, is described in Section 5.
3.3 Export Process Requirements
* Timeliness: reports on selected packets MUST be made available
to the collector quickly enough to support near real time
applications. Specifically, any report on a packet MUST be
dispatched within 1 second of the time of receipt of the packet
by the measurement process.
* Congestion Avoidance: export of a report stream across a network
MUST be congestion avoiding in compliance with RFC 2914.
* Secure Export:
- confidentiality: the option to encrypt exported data MUST be
provided.
The situation where this may not be necessary is if the collector and
the observation point are not separated by insecure links. For
example, if the collector was part of the router that contained the
observation points, it's unlikely that security is helpful. Instead,
it requires either a general-purpose processor or encryption hardeware,
slowing down packet flow or increasing cost/complexity/power if
installing hardware.
- integrity: alterations in transit to exported data MUST be
detectable at the collector
- authenticity: authenticity of exported data MUST be
verifiable by the collector in order to detect forged data.
The motivation here is the same as for security in IPFIX
export; see Sections 6.3 and 10 of [QZCZ03].
3.4 Configuration Requirements
* Ease of Configuration: of sampling and export parameters,
e.g. for automated remote reconfiguration in response to
measurements.
* Secure Configuration: the option to configure via protocols that
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prevent unauthorized reconfiguration or eavesdropping on
configuration communications MUST be available. Eavesdropping
on configuration might allow an attacker to gain knowledge that
would be helpful in crafting a packet stream to (for example)
evade subversion, or overload the measurement infrastructure.
Configuration is discussed in Section 8. Feasibility and complexity
of PSAMP operations is discussed in Section 9.
Reuse of existing protocols will be encouraged provided the
protocol capabilities are compatible with the requirements laid out
in this document.
4 Packet Selection
4.1 Packet Selection Terminology.
* Filtering: a filter is a selection operation that selects a
packet deterministically based on the packet content, its
treatment, and functions of these occurring in the selection
state. Examples include match/mask filtering, and hash-based
selection.
* Sampling: a selection operation that is not a filter is called a
sampling operation. This reflects the intuitive notion that if
the selection of a packet cannot be exactly predicted from its
content, there must be some type of sampling taking place.
* Content-independent Sampling: a sampling operation that does not
use packet content (or quantities derived from it) as the basis
for selection is called a content-independent sampling
operation. Examples include systematic sampling, and uniform
pseudorandom sampling driven by a pseudorandom number whose
generation is independent of packet content. Note that in
independent sampling it is not necessary to access the packet
content in order to make the selection decision.
* Content-dependent Sampling: a sampling operation where selection
is dependent on packet content is called a content-dependent
sampling operation. Examples include pseudorandom selection
according to a probability that depends on the contents of a
packet field; note that this is not a filter.
* Emulated Sampling: selection operations in any of the above four
categories may be emulated by operations in the same or another
category for the purposes of implementation. For example,
uniform pseudorandom sampling may be emulated by hash-based
selection, using suitable hash function and hash domain.
I don't agree that uniform pseudorandom sampling may be emulated by
hash-based seletion. A DOS attack that knew the hash-based selection
method could evade it, but not a pseudorandoming sampling scheme.
* Hash-based selection: a filter specified by a hash domain, a hash
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function, and hash range and a hash selection range.
* Hash domain: a subset of the packet content and the packet
treatment, viewed as an N-bit string for some positive integer
N.
* Hash range: a set of M-bit strings for some positive integer M.
* Hash function: a deterministic map from the hash domain into the
hash range.
* Selection range: a subset of the hash range. The packet is
selected if the action of the hash function on the hash domain
for the packet yields a result in the hash selection range.
* Pool size: the size of a set of packets in a packet stream.
* Sample size: the size of a set of packets selected by a sampling
operation.
* Target Sampling Frequency: a configurable sampling frequency in a
sampling operation.
* Attained Sampling Frequency: Given a subset of packets in a stream
input to a sampling operation, the attained sampling frequency is
the ratio of the sample size to the pool size.
4.2 Packet Selection Operations for a PSAMP
A spectrum of packet selection operations is described in detail in
[ZMRD03]. Here we only briefly summarize the meanings for
completeness.
A PSAMP selection process MUST support at least one of the
following selectors.
* Systematic Time Based:
packet selection is triggered at periodic instants separated
by a time called the Spacing. All packets that arrive within a
certain time of the trigger (called the Interval Length) are
selected.
* Systematic Count Based:
similar to systematic time based expect that selection is
reckoned w.r.t. packet count rather than time. Packet
selection is triggered periodically by packet count, a number
of successive packets being selected subsequent to each trigger.
* Uniform Probabilistic: packets are selected independently with
fixed sampling probability p.
* Non-uniform Probabilistic:
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packets are selected independently with probability p that
depends on packet content.
* Probabilistic n-out-of-N:
form each count-based successive block of N packets, n are
selected at random
* Match/Mask Filtering:
Should Match/Mask be inverted to Mask/Match since the mask comes first?
This entails taking the masking portions of the packet
(i.e. taking the bitwise AND with a binary mask) and selecting
the packet if the result falls in a specified range. This
specification doesn't preclude the future definition of a high
level syntax for defining filtering in a concise way (e.g. TCP
port taking a particular value) providing that syntax can be
compiled into the bitwise _expression_.
Match/mask operations SHOULD be available for different
protocol portions of the packet:
o the IP header (excluding options in IPv4, stacked headers in
IPv6)
o transport header
o encapsulation headers (including MPLS label stack, ATOM)
When an entity offers Match/Mask filtering in the selection
process and, in its usual capacity other than in performing
PSAMP functions, identifies or processes information from one
or more of the above protocols, then the information SHOULD be
made available for filtering. For example, when an entity
routes based on destination IP address, that field should be
made available. Conversely, an entity that does not route is
not expected to be able to locate an IP address within a
packet, or make it available for filtering, although it MAY do
so.
* Hash-based Selection:
Hash-based selection will employ one or more hash functions to
be standardized. The hash domain is specified by a bitmaps on
the IP packet header and the IP payload.
When the hash function is sufficiently good, hash-based
selection can be used to emulate uniform random sampling over
the hash domain. The target sampling frequency is then the
ratio of the size of the selection range to the hash range.
Same as above.
Applications of hash-based selection include:
o Trajectory Sampling: all routers use the same hash selector;
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the hash domain includes only portions of the packet that do
not change from hope to hop (e.f. TTL is excluded). Hence
packets are consistently selected in the sense that they
are selected at all routers on their path or none. Reports
also include a second hash (the label hash) that
distinguishes different packets. Reports of a given packet
reaching the collector from different routers can be used to
reconstruct the path taken by the packet. Trajectory
Sampling is proposed in [DuGr01]; further description is
found in [ZMRD03]; some applications are described in
Section 10.
o Consistent Flow Sampling: the hash domain is a flow key. For
a given flow, either all or none of its packets are
sampled. This is accomplished without the need to maintain
flow state.
Some applications need to calculate packet hashes for purposes
other than selection (e.g. the label hash in Trajectory
Sampling). This can be achieved by placing a calculated hash
in the selection state, and setting the selection range to be
the whole of the hash range.
* Router State Filtering:
This class of filters selects a packet on based on the following
conditions, combined with the AND, OR or NOT operators:
o Ingress interface at which packet arrives equals a specified
value
o Egress interface to which packet is routed to equals a
specified value
o Origin AS equals a specified value or lies within a given
range.
o Destination AS equals a specified value or lies within a given
range
o Packet violated acl on the router
o Failed rpf
o Failed rsvp
o No route found for the packet
Router architectural considerations may preclude some
information concerning the packet treatment, e.g routing
state, being available at line rate for selection of
packets. However, if selection not based on routing state has
reduced down from line rate, subselection based on routing
state may be feasible.
4.3 Input Sequence Numbers for Primitive Selectors.
Each instance of a primitive selector MUST maintain a count of
packets presented at its input. The counter value is to be included
Pre-selection? Every packet, even if it isn't selected, gets a unique
(subject ot wraparound) sequence number? Thus, the sampled packets
will have holes in the count. If so, how can the collector determine
that packets have been dropped from the sequence number?
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as a sequence number for selected packets. This enables
applications to determine the attained frequency at which packets
are selected, and hence correctly normalize network usage estimates
regardless of loss of information, whether this occurs because of
discard of packet reports in the measurement or reporting process
(e.g. due to resource contention), or loss of measurement packets
in transmission or collection; see [PPM01]. The sequence numbers
are considered as part of the packet's selection state.
4.4 Composite Selectors
The ability to compose selectors in a selection process SHOULD be
provided. The following combinations appear to be most useful for
applications:
* filtering followed by sampling
* sampling followed by filtering
Composite selectors are useful for drill down applications. The
first component of a composite selector can be used to reduce the
load on the second component. In this setting, the advantage to be
gained from a given ordering can depend on the composition of the
packet stream.
4.5 Constraints on the Sampling Frequency
Sampling at full line rate, i.e. with probability 1, is not
excluded in principle, although resource constraints may not
support it in practice.
4.6 Criteria for Choice of Selection Operations
In current practice, sampling has been performed using particular
algorithms, including:
- pseudorandom independent sampling with probability 1/N;
- systematic sampling of every Nth packet.
The question arises as to whether both of these should be
standardized as distinct selection operations, or whether they can
be regarded as different implementations of a single selection
operation.
To determine the answer to this question, we need to consider
(a) measured or assumed statistical properties of the packet
stream, e.g., one or more of the following:
- contents of different packets are statistically independent
- correlations between contents of different packets decay
at a specified rate
- contents of certain fields within the same packet are
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significantly variable and exhibit small cross correlation
(b) the desired reference sampling model, e.g., one of:
- sample packets with long term probability 1/N
- sample packets independent with probability 1/N
(c) the set of possible alternatives and implementations, e.g.,
one of:
- pseudorandom independent sampling with probability 1/N
- systematic sampling with period N
- hash-based sampling with target probability 1/N
(d) the tolerance for error in the applications that use the
measurements.
We can say that a given alternative from (c) reproduces a reference
model (b) for the applications if the results obtained using them
are sufficiently accurate in (d) for traffic satisfying an assumed
statistical properties in (a). Clearly, application to evaluate
methods in (c) requires developing agreement on the relevant
properties in (a), (b) and (d).
Example: systematic sampling with period N will not count the
occurrence of closely space packets (less than N counts apart) from
the same flow. Thus for applications that are concerned with the
joint statistics of multiple packets within flows, systematic
sampling may not reproduce the results obtained with random
sampling sufficiently accurately.
5 Reporting Process
5.1 Mandatory Contents of Packet Reports (MUST)
The reporting process MUST include the following in each packet report:
(i) the input sequence number(s) of any sampling operation
that acted on the packet in the instance of a measurement
process of which the reporting process is a component.
The reporting process MUST be able to include the following
in each packet report, as a configurable option:
(ii) some number of contiguous bytes from the start of the
packet.
Some devices may not have the resource capacity or functionality to
provide more detailed reports that those in (i) and (ii)
above. Using this minimum required reporting functionality, the
reporting process places the burden of interpretation on the collector,
or on applications that it supplies.
5.2 Recommended Contents for Packet Reports (SHOULD)
The reporting process SHOULD provide for the inclusion in packet
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reports of the following information, inclusion any or all being
configurable as a option.
(iii) fields relating to the following protocols
used in the packet, specifically: IPv4, IPV6, transport
protocols, MPLS, ATOM.
(iv) packet treatment, including:
- identifiers for any input and output interfaces of the
observation point that were traversed by the packet
- source and destination AS
(v) selection state associated with the packet, including:
- timestamps
- hashes, where calculated.
The specific fields will include those set out as requirements for
IPFIX [QZCZ03], with modifications appropriate to reporting on
single packets rather than flows.
When an entity that hosts a reporting process and, in its usual
capacity other than performing PSAMP functions, identifies or
process one or more of the above fields, then the contents of each
such field(s) SHOULD be made available for optional reporting. For
example, when a device routes based on destination IP address, that
field should be made available. Conversely, an entity that does
not route is not expected to be able to locate an IP address within
a packet, or make it available for reporting, although it MAY do
so.
5.3 Report Interpretation
Information for use in report interpretation MUST include (i)
configuration parameters of the selectors of the packets reported
on; (ii) format of the packet reports (iii) configuration
parameters and state information of the network element; (iv)
indication of the inherent accuracy of the reported quantities,
e.g., of timestamps; (v) identifiers for observation point,
measurement process, and export process.
The requirements for robustness and transparency are motivations
for including report interpretation in the report stream. Inclusion
makes the report stream self-defining. The PSAMP framework
excludes reliance on an alternative model in which interpretation
is recovered out of band. This latter approach is not robust with
respect to undocumented changes in selector configuration, and may
give rise to future architectural problems for network management
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systems to coherently manage both configuration and data collection.
It is not envisaged that all report interpretation be included
in every packet report. Many of the quantities listed above are
expected to be relatively static; they could be communicated
periodically, and upon change.
To conserve network bandwidth and resources at the collector, the
measurement packets may be compressed before export. Compression
is expected to be quite effective since the sampled packets may
share many fields in common, e.g. if a filter focuses on packets
with certain values in particular header fields. Using compression,
however, could impact the timeliness of reports. Any consequent
delay MUST not violate the timeliness requirement for availability
of packet reports at the collector.
6 Parallel Measurement Processes
Because of the increasing number of distinct measurement
applications, with varying requirements, it is desirable to set up
parallel measurement processes on a stream of packets. A PSAMP
device SHOULD support more than one independently configurable
measurement process. The measurement process may have an exclusive
export process, or may share it with other measurement processes.
Each of the parallel measurement processes SHOULD be
independent. However, resource constraints may prevent complete
reporting on a packet selected by multiple selection processes. In
this case, reporting for the packet MUST be complete for at least
one measurement process; other measurement processes need only
report that they selected the packet. The priority amongst
measurement processes to report packets MUST be configurable.
It is not proposed to standardize the number of parallel
measurement processes.
7 Export Process
7.1 Collector Destination
When exporting to a remote collector, the collector is identified
by IP address and port number.
7.2 Local Export
The report stream may be directly exported to on-board measurement
based applications, for example those that form composite
statistics from more than one packet. Local export may be presented
through an interface direct to the higher level applications, i.e.,
through an API, rather than employing the transport used for
off-board export.
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A possible example of local export could be that packets selected
by the PSAMP measurement process serve as the input for the IPFIX
protocol, which then forms flow records out of the stream of
selected packets. Note that IPFIX being still developed; this is
given only as a possible example.
7.3 Reliable vs. Unreliable Transport
The export of the report stream does not require reliable
export. On the contrary, retransmission of lost measurement packets
consumes additional network resources and requires maintenance of
state by the export process. As such, the export process would have
to be able to receive and process acknowledgments, and to store
unacknowledged data. Furthermore, the entity that hosts the export
process may not possess its own network address (for example an
embedded measurement subsystem in an interface) at which to receive
acknowledgments. These requirements would be a significant
impediment to having ubiquitous support PSAMP.
Instead, it is proposed that the export process support unreliable
export. Sequence numbers on the measurement packets would indicate
when loss has occurred, and the analysis of the collected
measurement data can account for this loss. In some sense, packet
loss becomes another form of sampling (albeit a less desirable, and
less controlled, form of sampling).
7.4 Limiting Delay in Exporting Measurement Packets
The export process may queue the report stream in order to export
multiple reports in a single measurement packet. Any consequent
delay MUST still allow for timely availability of packet reports
at the collector.
7.5 Configurable Export Rate Limit
The export process MUST be able to limit its export rate; otherwise
it could overload the network and/or the collector. Note this
problem would be exacerbated if using reliable transport mode,
since any lost packets would be retransmitted, thereby imposing an
additional load on the network.
At times, the reporting process may generate new reports or report
interpretation faster than the allowed export rate. In this
situation, the export process MUST discard the excess reports
rather than transmitting them to the collector. Sequence numbers
reported for selector input enable correction for lost reports. An
additional sequence number for dispatched measurement packets
enables the collector to determine the degree of loss in
transmission.
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There are two options for a configurable rate limit. First, if the
transport protocol has a configurable rate limit, that can be used.
The second option is to limit the rate at which measurement packets
are supplied to the transport protocol. A candidate for
implementation of rate limiting is the leaky bucket, with tokens
corresponding e.g. to bytes or packets.
The export rate limit MUST be configurable per export process. Note
that since congestion loss can occur at any link on the export
path, it is not sufficient to limit rate simply as a function of
the bandwidth of the interface out of which export takes place.
7.6 Congestion-aware Unreliable Transport
Exported measurement traffic competes for resources with other
Internet transfers. Congestion-aware export is important to ensure
that the measurement packets do not overwhelm the capacity of the
network or unduly degrade the performance of other applications,
while making good use of available bandwidth resources.
Choice of transport for PSAMP has to be made under the following
constraints:
(i) IESG has mandated that all transport in new protocols must be
congestion aware
(ii) reliable transport is too onerous for general entities that
support PSAMP (see Section 7.3)
(iii) there currently exists no IETF standardized unreliable
congestion-aware transport
In the absence of an existing IETF standardized unreliable
congestion-aware protocol, PSAMP will provisionally nominate the
reliable congestion aware transport protocol TCP as the interim
transport protocol for export. From the preceding arguments, TCP
is unsatisfactory for final standardization in PSAMP. In the
meantime, the PSAMP WG will evaluate (at least) the following
alternatives for congestion aware unreliable transport, as they
become available, with a view to selecting one of them and
discarding TCP:
(i) unreliable transport protocols adopted in the future by the
IPFIX WG,
(ii) the Datagram Congestion Control Protocol (DCCP); currently
under development; see [FHK02]
(iii) The Stream Control Transmission Protocol (SCTP) under
development [SCTP]. SCTP is by default reliable, but has the
capability to operate in unreliable and partially reliable modes
[PR-SCTP]. See [D03] for description of its potential use in flow
export.
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(iv) collector-based rate reconfiguration, described below.
7.7 Collector-based Rate Reconfiguration
Since collector-based rate reconfiguration is a new proposal, this
draft will discuss it in some detail.
The collector can detect congestion loss along the path from the
exporting device to the collector by observing packet loss,
manifest as gaps in the sequence numbers, or the absence of packets
for a period of time. The server can run an appropriate
congestion-control algorithm to compute a new export rate limit,
then reconfigure the export process with the new rate. This is an
attractive alternative to requiring the export process to receive
acknowledgment packets. Implementing the congestion control
algorithm in the collector has the added advantages of flexibility
in adapting the sending rate and the ability to incorporate new
congestion-control algorithms as they become available.
7.7.1 Changing the Export Rate and Other Rates
Forcing the export process to discard excess reports is an
effective control under short term congestion. Alternatively, the
selection process could be reconfigured to select fewer packets, or
the reporting process could be reconfigured to send smaller reports
on each selected packet. This may be a more appropriate reaction to
long-term congestion. In some cases, a collector may receive
measurement packets due to more than one export process, and could
decide to reduce the export or other rates associated with one
export process rather than another, in order to prioritize the
measurement data. This type of flexibility is valuable for network
operators that collect measurement data from multiple locations to
drive multiple applications.
7.7.2 Notions of Fairness
In some cases, it may be reasonable to allow the collector to have
flexibility in deciding how aggressively to respond to congestion.
For example, the exporting entity and the collector may have a very
small round-trip time relative to other traffic. Conventional
TCP-friendly congestion control would allocate a very large share
of the bandwidth to the PSAMP export traffic. Instead, the
collector could apply an algorithm that reacts more aggressively to
congestion to give a larger share of the bandwidth to other traffic
(with larger RTTs).
In other cases, the measurement packets may require a larger share
of the bandwidth than other flows. For example, consider a link
that carries tens of thousands of flows, including some non
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TCP-friendly DoS attack traffic. Restricting the PSAMP traffic to
a fair share allocation may be too restrictive, and might limit the
collection of the data necessary to diagnose the DoS attack which
overloads links over which measurement packets are carried. In
order to maintain report collection during periods of congestion,
PSAMP report streams may claim more than a fair share of link
bandwidth, provided the number of report streams in competition
with fair sharing traffic is limited. The collector could also
employ policies that allocate bandwidth in certain proportions
amongst different measurement processes.
7.7.3 Behavior Under Overload and Failure
The congestion control algorithm has to be robust to severe
overload or complete loss of connectivity between the exporting
entity and the collector, and also to the failure of exporting
entity or the collector. For example, in a scenario where the
collector is unable to reconfigure the export rate because of loss
of reverse (collector to exporting entity) connectivity, it is
desirable for the exporting entity to reduce the export rate
autonomously. Similarly, if no measurement packets reach the
collector because of loss of forward connectivity, the collector
should not react to this by increasing the export rate. This
problem may be solved through periodic heartbeat packets in both
directions (i.e., measurement packets in the forward direction,
configuration refresh messages in the reverse direction). This
allows each side to detect a loss in connectivity or outright
failure and to react appropriately.
8 Configuration and Management
A key requirement for PSAMP is the easy reconfiguration of the
parameters of the measurement process: those for selection, packet
reports and export. Examples are (i) support of measurement based
applications that want to drill-down on traffic detail in
real-time; (ii) collector-based rate reconfiguration.
To facilitate reconfiguration and retrieval of parameters, they are
to reside in a Management Information Base (MIB). Mandatory
configuration, capabilities and monitoring objects will cover all
minimum required (MUST) PSAMP functionality.
Secondary objects will cover the recommended PSAMP functionality
(SHOULD), and MUST be provided only when such functionality is
offered by an entity. Such PSAMP functionality includes
configuration of offered selectors, composite selectors, multiple
measurement processes, and report format including the choice of
fields to be reported. For further details concerning the PSAMP MIB,
see [DRC03].
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PSAMP requires a uniform mechanism with which to access and
configure the MIB. SNMP access MUST be provided by the entity
hosting the MIB.
9 Feasibility and Complexity
In order for PSAMP to be supported across the entire spectrum of
networking equipment, it must be simple and inexpensive to
implement. One can envision easy-to-implement instances of the
mechanisms described within this draft. Thus, for that subset of
instances, it should be straightforward for virtually all system
vendors to include them within their products. Indeed, sampling and
filtering operations are already realized in available equipment.
Here we give some specific arguments to demonstrate feasibility and
comment on the complexity of hardware implementations. We stress
here that the point of these arguments is not to favor or recommend
any particular implementation, or to suggest a path for
standardization, but rather to demonstrate that the set of possible
implementations is not empty.
9.1 Feasibility
9.1.1 Filtering
Filtering consists of a small number of mask (bit-wise logical),
comparison and range (greater than) operations. Implementation of
at least a small number of such operations is straightforward. For
example, filters for security access control lists (ACLs) are
widely implemented. This could be as simple as an exact match on
certain fields, or involve more complex comparisons and ranges.
9.1.2 Sampling
Sampling based on either counters (counter set, decrement, test for
equal to zero) or range matching on the hash of a packet (greater
than) is possible given a small number of selectors, although there
may be some differences in ease of implementation for hardware
vs. software platforms.
9.1.3 Hashing
Hashing functions vary greatly in complexity. Execution of a small
number of sufficient simple hash functions is implementable at line
rate. Concerning the input to the hash function, hop-invariant IP
header fields (IP address, identification) and TCP/UDP header
fields (port numbers, TCP sequence number) drawn from the first 40
bytes of the packet have been found to possess a considerable
variability; see [DuGr01].
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9.1.4 Reporting
The simplest packet report would duplicate the first n bytes of the
packet. However, such an uncompressed format may tax the bandwidth
available to the reporting process for high sampling rates;
reporting selected fields would save on this bandwidth. Thus there
is a trade-off between simplicity and bandwidth limitations.
9.1.5 Export
Ease of exporting measurement packets depends on the system
architecture. Most systems should be able to support export
by insertion of measurement packets, even through the software
path.
9.2 Potential Hardware Complexity
We now comment on the complexity of possible hardware
implementations. Achieving low constants for performance while
minimizing hardware resources is, of course, a challenge,
especially at very high clock frequencies. Most of these
operations, however, are very basic and their implementations very
well understood; in fact, the average ASIC designer simply uses
canned library instances of these operations rather than design
them from scratch. In addition, networking equipment generally does
not need to run at the fastest clock rates, further reducing the
effort required to get reasonably efficient implementations.
Simple bit-wise logical operations are easy to implement in
hardware. Such operations (NAND/NOR/XNOR/NOT) directly translate
to four-transistor gates. Each bit of a multiple-bit logical
operation is completely independent and thus can be performed in
parallel incurring no additional performance cost above a single
bit operation.
Comparisons (EQ/NEQ) take O(lg(M)) stages of logic, where M is the
number of bits involved in the comparison. The lg(M) is required
to accumulate the result into a single bit.
Greater than operations, as used to determine whether a hash falls
in a selection range, are a determination of the most significant
not-equivalent bit in the two operands. The operand with that
most-significant-not-equal bit set to be one is greater than the
other. Thus, a greater than operation is also an O(lg(M)) stages
of logic operation. Optimized implementations of arithmetic
operations are also O(lg(M)) due to propagation of the carry bit.
Setting a counter is simply loading a register with a state. Such
an operation is simple and fast O(1). Incrementing or decrementing
a counter is a read, followed by an arithmetic operation followed
by a store. Making the register dual-ported does take additional
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space, but it is a well-understood technique. Thus, the
increment/decrement is also an O(lg(M)) operation.
Hashing functions come in a variety of forms. The computation
involved in a standard Cyclic Redundancy Code (CRC) for example are
essentially a set of XOR operations, where the intermediate result
is stored and XORed with the next chunk of data. There are only
O(1) operations and no log complexity operations. Thus, a simple
hash function, such as CRC or generalizations thereof, can be
implemented in hardware very efficiently.
At the other end of the range of complexity, the MD5 function uses
a large number of bit-wise conditional operations and arithmetic
operations. The former are O(1) operations and the latter are
O(lg(M)). MD5 specifies 256 32b ADD operations per 16B of input
processed. Consider processing 10Gb/sec at 100MHz (this processing
rate appears to be currently available). This requires processing
12.5B/cycle, and hence at least 200 adders, a sizeable
number. Because of data dependencies within the MD5 algorithm, the
adders cannot be simply run in parallel, thus requiring either
faster clock rates and/or more advanced architectures. Thus
selection hashing functions as complex as MD5 may be precluded from
ubiquitous use at full line rate. This motivates exploring the use
of selection hash functions with complexity somewhere between that
of MD5 and CRC. However, identification hashing with MD5 on only
selected packets is feasible at a sufficiently low sampling frequency.
10 Applications
We first describe several representative operational applications
that require traffic measurements at various levels of temporal and
spatial granularity. Some of the goals here appear similar to those
of IPFIX, at least in the broad classes of applications
supported. However, there are two major differences:
- PSAMP aims for ubiquitous deployment of packet measurement,
including devices that are not expected to support IPFIX. This
offers broader reach for existing applications.
- PSAMP can support new applications through the type of packet
selectors that it supports
10.1 Baseline Measurement and Drill Down
Packet sampling is ideally suited to determine the composition of
the traffic across a network. The approach is to enable measurement
on a cut-set of the network links such that each packet entering
the network is seen at least once, for example, on all ingress
links. Unfiltered sampling with a relatively low frequency
establishes baseline measurements of the network traffic. Reports
include packet attributes of common interest: source and
destination address and port numbers, prefix, protocol number, type
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of service, etc. Traffic matrices are indicated by reporting source
and destination AS matrices. Absolute traffic volumes are estimated
by renormalizing the sampled traffic volumes through division by
either the target sampling frequency, or the attained sampling
frequency (as derived by interface packet counters included in the
report stream)
Suppose an operator or a measurement based application detects an
interesting subset of a packet stream, as identified by a
particular packet attribute. Real-time drill-down to that subset is
achieved by instantiating a new measurement process on the same
packet stream from which the subset was reported. The selection
process of the new measurement process filters according to the
attribute of interest, and composes with sampling if necessary to
manage the frequency of packet selection.
10.2 Passive Performance Measurement
Hash-based sampling enables the tracking of the performance
experience by customer traffic, customers identified by a list of
source or destination prefixes, or by ingress or egress
interfaces. Operational uses include the verification of Service
Level Agreements (SLAs), and troubleshooting following a customer
complaint.
In this application, Trajectory Sampling is enabled at all network
ingress and egress interfaces. The label hash is used to match up
ingress and egress samples. Rates of loss in transit between
ingress and egress are estimated from the proportion of
trajectories for which no egress report is received. Note loss of
customer packets is distinguishable from loss of packet reports
through use of report sequence numbers. Assuming synchronization of
clock between different entities, delay of customer traffic across
the network may also be measured.
Extending hash-selection to all interfaces in the network would
enable attribution of poor performance to individual network links.
10.3 Troubleshooting
PSAMP can also be used to diagnose problems whose occurrence is
evident from aggregate statistics, per interface utilization and
packet loss statistics. These statistics are typically moving
averages over relatively long time windows, e.g., 5 minutes, and
serve as a coarse-grain indication of operational health of the
network. The most common method of obtaining such measurements are
through the appropriate SNMP MIBs (MIB-II and vendor-specific
MIBs.)
Suppose an operator detects a link that is persistently overloaded
and experiences significant packet drop rates. There is a wide
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range of potential causes: routing parameters (e.g., OSPF link
weights) that are poorly adapted to the traffic matrix, e.g.,
because of a shift in that matrix; a denial of service attack or a
flash crowd; a routing problem (link flapping). In most cases,
aggregate link statistics are not sufficient to distinguish between
such causes, and to decide on an appropriate corrective action. For
example, if routing over two links is unstable, and the links flap
between being overloaded and inactive, this might be averaged out
in a 5 minute window, indicating moderate loads on both links.
Baseline PSAMP measurement the congested link, as described in
Section 10.1, enables measurements that are fine grained in both
space and time. The operator has to be able to determine how many
bytes/packets are generated for each source/destination address,
port number, and prefix, or other attributes, such as protocol
number, MPLS forwarding equivalence class (FEC), type of service,
etc. This allows to pinpoint precisely the nature of the offending
traffic. For example, in the case of a DDoS attack, the operator
would see a significant fraction of traffic with an identical
destination address.
In certain circumstances, precise information about the spatial
flow of traffic through the network domain is required to detect
and diagnose problems and verify correct network behavior. In the
case of the overloaded link, it would be very helpful to know the
precise set of paths that packets traversing this link follow. This
would readily reveal a routing problem such as a loop, or a link
with a misconfigured weight. More generally, complex diagnosis
scenarios can benefit from measurement of traffic intensities (and
other attributes) over a set of paths that is constrained in some
way. For example, if a multihomed customer complains about
performance problems on one of the access links from a particular
source address prefix, the operator should be able to examine in
detail the traffic from that source prefix which also traverses the
specified access link towards the customer.
While it is in principle possible to obtain the spatial flow of
traffic through auxiliary network state information, e.g., by
downloading routing and forwarding tables from routers, this
information is often unreliable, outdated, voluminous, and
contingent on a network model. For operational purposes, a direct
observation of traffic flow is more reliable, as it does not depend
on any such auxiliary information. For example, if there was a bug
in a router's software, direct observation would allow to diagnose
the effect of this bug, while an indirect method would not.
11 Security Considerations.
Security considerations are addressed in:
- Section 3.1: item Robust Selection
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- Section 3.3: item Secure Export
- Section 3.4: item Secure Configuration
12 References
[B88] R.T. Braden, A pseudo-machine for packet monitoring and
statistics, in Proc ACM SIGCOMM 1988
[ClPB93] K.C. Claffy, G.C. Polyzos, H.-W. Braun, Application of
Sampling Methodologies to Network Traffic Characterization,
Proceedings of ACM SIGCOMM'93, San Francisco, CA, USA, September
13-17, 1993
[DRC03] T. Dietz, D. Romascanu, B. Claise, Definitions of Managed
Objects for Packet Sampling, Internet Draft,
draft-ietf-psamp-mib-00.txt, work in progress, June 2003.
[D03] M. Djernaes, Cisco Systems NetFlow Services Export Version 9
Transport, Internet Draft,
draft-djernaes-netflow-9-transport-00.txt, work in progress,
February 2003
[DuGr01] N. G. Duffield and M. Grossglauser, Trajectory Sampling for
Direct Traffic Observation, IEEE/ACM Trans. on Networking, 9(3), .
280-292, June 2001.
[FHK02] S. Floyd, M. Handley. E. Kohler, Problem Statement for
DCCP, Internet Draft draft-ietf-dccp-problem-00.txt, work in
progress, October 2002.
[LCTV02] W.S. Lai, B.Christian, R.W. Tibbs, S. Van den Berghe, A
Framework for Internet Traffic Engineering Measurement, Internet
Draft draft-ietf-tewg-measure-05.txt, work in progress, February
2003.
[PPM01] P. Phaal, S. Panchen, N. McKee, InMon Corporation's
sFlow: A Method for Monitoring Traffic in Switched and Routed
Networks, RFC 3176, September 2001
[PAMM98] V. Paxson, G. Almes, J. Mahdavi, M. Mathis, Framework for
IP Performance Metrics, RFC 2330, May 1998
[QZCZ03] J. Quittek, T. Zseby, B. Claise, S. Zander, Requirements
for IP Flow Information Export, Internet Draft
draft-ietf-ipfix-reqs-10.txt, work in progress, June 2003.
[SPSJTKS01] A. C. Snoeren, C. Partridge, L. A. Sanchez, C. E. Jones,
F. Tchakountio, S. T. Kent, W. T. Strayer, Hash-Based IP Traceback,
Proc. ACM SIGCOMM 2001, San Diego, CA, September 2001.
[RFC2960] Stewart, R. (ed.) "Stream Control Transmission Protocol",
Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 25]
Internet-Draft Passive Packet Measurement June 2003
RFC 2960, October 2000
[PR-SCTP] Stewart, R, "SCTP Partial Reliability Extension",
Internet Draft, draft-stewart-tsvwg-prsctp-01.txt, work in
progress, June 2003.
[ZMRD03] T. Zseby, M. Molina, F. Raspall, N.G. Duffield, Sampling
and Filtering Techniques for IP Packet Selection, Internet Draft
draft-ietf-psamp-sample-tech-01.txt, work in progress, March 2003.
13 Authors' Addresses
Derek Chiou
Avici Systems
101 Billerica Ave
North Billerica, MA 01862
Phone: +1 978-964-2017
Email: dchiou@avici.com
Benoit Claise
Cisco Systems
De Kleetlaan 6a b1
1831 Diegem
Belgium
Phone: +32 2 704 5622
Email: bclaise@cisco.com
Nick Duffield
AT&T Labs - Research
Room B-139
180 Park Ave
Florham Park NJ 07932, USA
Phone: +1 973-360-8726
Email: duffield@research.att.com
Albert Greenberg
AT&T Labs - Research
Room A-161
180 Park Ave
Florham Park NJ 07932, USA
Phone: +1 973-360-8730
Email: albert@research.att.com
Matthias Grossglauser
School of Computer and Communication Sciences
EPFL
1015 Lausanne
Switzerland
Email: matthias.grossglauser@epfl.ch
Peram Marimuthu
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Cisco Systems
170, W. Tasman Drive
San Jose, CA 95134
Phone: (408) 527-6314
Email: peram@cisco.com
Jennifer Rexford
AT&T Labs - Research
Room A-169
180 Park Ave
Florham Park NJ 07932, USA
Phone: +1 973-360-8728
Email: jrex@research.att.com
Ganesh Sadasivan
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
Phone: (408) 527-0251
Email: gsadasiv@cisco.com
14 Intellectual Property Statement
AT&T Corporation may own intellectual property applicable to this
contribution. The IETF has been notified of AT&T's licensing intent
for the specification contained in this document. See
http://www.ietf.org/ietf/IPR/ATT-GENERAL.txt for AT&T's IPR
statement.
15 Full Copyright Statement
Copyright (C) The Internet Society (2003). 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.
Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 27]
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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
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Duffield (Ed.) draft-ietf-psamp-framework-03.txt [Page 28]
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