Internet DRAFT - draft-ietf-ippm-6man-pdm-option
draft-ietf-ippm-6man-pdm-option
INTERNET-DRAFT N. Elkins
Inside Products
R. Hamilton
Chemical Abstracts Service
M. Ackermann
Intended Status: Proposed Standard BCBS Michigan
Expires: December 28, 2017 June 26, 2017
IPv6 Performance and Diagnostic Metrics (PDM) Destination Option
draft-ietf-ippm-6man-pdm-option-13
Abstract
To assess performance problems, this document describes optional
headers embedded in each packet that provide sequence numbers and
timing information as a basis for measurements. Such measurements
may be interpreted in real-time or after the fact. This document
specifies the Performance and Diagnostic Metrics (PDM) destination
options extension header. The field limits, calculations, and usage
in measurement of PDM are included in this document.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
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Copyright and License Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
IETF Trust Legal Provisions of 28-dec-2009, Section 6.b(i), paragraph
3: This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Rationale for defined solution . . . . . . . . . . . . . . . 5
1.3 IPv6 Transition Technologies . . . . . . . . . . . . . . . . 6
2 Measurement Information Derived from PDM . . . . . . . . . . . . 6
2.1 Round-Trip Delay . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Server Delay . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Performance and Diagnostic Metrics Destination Option Layout . . 7
3.1 Destination Options Header . . . . . . . . . . . . . . . . . 7
3.2 Performance and Diagnostic Metrics Destination Option . . . 7
3.2.1 PDM Layout . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2 Base Unit for Time Measurement . . . . . . . . . . . . . 10
3.3 Header Placement . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Header Placement Using IPSec ESP Mode . . . . . . . . . . . 11
3.4.1 Using ESP Transport Mode . . . . . . . . . . . . . . . . 11
3.4.2 Using ESP Tunnel Mode . . . . . . . . . . . . . . . . . 12
3.5 Implementation Considerations . . . . . . . . . . . . . . . 12
3.5.1 PDM Activation . . . . . . . . . . . . . . . . . . . . . 12
3.5.2 PDM Timestamps . . . . . . . . . . . . . . . . . . . . . 12
3.6 Dynamic Configuration Options . . . . . . . . . . . . . . . 12
3.7 Information Access and Storage . . . . . . . . . . . . . . . 13
4 Security Considerations . . . . . . . . . . . . . . . . . . . . 13
4.1 Resource Consumption and Resource Consumption Attacks . . . 13
4.2 Pervasive monitoring . . . . . . . . . . . . . . . . . . . . 13
4.3 PDM as a Covert Channel . . . . . . . . . . . . . . . . . . 14
4.4 Timing Attacks . . . . . . . . . . . . . . . . . . . . . . . 14
5 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 15
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 15
6.2 Informative References . . . . . . . . . . . . . . . . . . . 16
Appendix A: Context for PDM . . . . . . . . . . . . . . . . . . . 16
A.1 End User Quality of Service (QoS) . . . . . . . . . . . . . 16
A.2 Need for a Packet Sequence Number (PSN) . . . . . . . . . . 17
A.3 Rationale for Defined Solution . . . . . . . . . . . . . . . 17
A.4 Use PDM with Other Headers . . . . . . . . . . . . . . . . . 17
Appendix B : Timing Considerations . . . . . . . . . . . . . . . . 19
B.1 Timing Differential Calculations . . . . . . . . . . . . . . 19
B.2 Considerations of this time-differential representation . . 20
B.2.1 Limitations with this encoding method . . . . . . . . . 20
B.2.2 Loss of precision induced by timer value truncation . . 21
Appendix C: Sample Packet Flows . . . . . . . . . . . . . . . . . 22
C.1 PDM Flow - Simple Client Server . . . . . . . . . . . . . . 22
C.1.1 Step 1 . . . . . . . . . . . . . . . . . . . . . . . . . 23
C.1.2 Step 2 . . . . . . . . . . . . . . . . . . . . . . . . . 23
C.1.3 Step 3 . . . . . . . . . . . . . . . . . . . . . . . . . 24
C.1.4 Step 4 . . . . . . . . . . . . . . . . . . . . . . . . . 25
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C.1.5 Step 5 . . . . . . . . . . . . . . . . . . . . . . . . . 26
C.2 Other Flows . . . . . . . . . . . . . . . . . . . . . . . . 26
C.2.1 PDM Flow - One Way Traffic . . . . . . . . . . . . . . . 26
C.2.2 PDM Flow - Multiple Send Traffic . . . . . . . . . . . . 28
C.2.3 PDM Flow - Multiple Send with Errors . . . . . . . . . . 29
Appendix D: Potential Overhead Considerations . . . . . . . . . . 30
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
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1 Background
To assess performance problems, measurements based on optional
sequence numbers and timing may be embedded in each packet. Such
measurements may be interpreted in real-time or after the fact.
As defined in RFC2460 [RFC2460], destination options are carried by
the IPv6 Destination Options extension header. Destination options
include optional information that need be examined only by the IPv6
node given as the destination address in the IPv6 header, not by
routers or other "middle boxes". This document specifies the
Performance and Diagnostic Metrics (PDM) destination option. The
field limits, calculations, and usage in measurement of the PDM
destination options extension header are included in this document.
1.1 Terminology
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 [RFC2119].
1.2 Rationale for defined solution
The current IPv6 specification does not provide timing nor a similar
field in the IPv6 main header or in any extension header. The IPv6
Performance and Diagnostic Metrics destination option (PDM) provides
such fields.
Advantages include:
1. Real measure of actual transactions.
2. Ability to span organizational boundaries with consistent
instrumentation.
3. No time synchronization needed between session partners
4. Ability to handle all transport protocols (TCP, UDP, SCTP, etc) in
a uniform way
The PDM provides the ability to determine quickly if the (latency)
problem is in the network or in the server (application). That is,
it is a fast way to do triage. For more information on background
and usage of PDM, see Appendix A.
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1.3 IPv6 Transition Technologies
In the path to full implementation of IPv6, transition technologies
such as translation or tunneling may be employed. It is possible
that an IPv6 packet containing PDM may be dropped if using IPv6
transition technologies. For example, an implementation using a
translation technique (IPv6 to IPv4) which does not support or
recognize the IPv6 Destination Options extension header may simply
drop the packet rather than translating it without the extension
header.
It is also possible that some devices in the network may not
correctly handle multiple IPv6 Extension Headers, including the IPv6
Destination Option. For example, adding the PDM header to a packet
may push the layer 4 information to a point in the packet where it is
not visible to filtering logic, and may be dropped. This kind of
situation is expected to become rare over time.
2 Measurement Information Derived from PDM
Each packet contains information about the sender and receiver. In IP
protocol, the identifying information is called a "5-tuple".
The 5-tuple consists of:
SADDR : IP address of the sender
SPORT : Port for sender
DADDR : IP address of the destination
DPORT : Port for destination
PROTC : Protocol for upper layer (ex. TCP, UDP, ICMP, etc.)
The PDM contains the following base fields:
PSNTP : Packet Sequence Number This Packet
PSNLR : Packet Sequence Number Last Received
DELTATLR : Delta Time Last Received
DELTATLS : Delta Time Last Sent
Other fields for storing time scaling factors are also in the PDM and
will be described in section 3.
This information, combined with the 5-tuple, allows the measurement
of the following metrics:
1. Round-trip delay
2. Server delay
2.1 Round-Trip Delay
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Round-trip *Network* delay is the delay for packet transfer from a
source host to a destination host and then back to the source host.
This measurement has been defined, and the advantages and
disadvantages discussed in "A Round-trip Delay Metric for IPPM"
[RFC2681].
2.2 Server Delay
Server delay is the interval between when a packet is received by a
device and the first corresponding packet is sent back in response.
This may be "Server Processing Time". It may also be a delay caused
by acknowledgements. Server processing time includes the time taken
by the combination of the stack and application to return the
response. The stack delay may be related to network performance. If
this aggregate time is seen as a problem, and there is a need to make
a clear distinction between application processing time and stack
delay, including that caused by the network, then more client based
measurements are needed.
3 Performance and Diagnostic Metrics Destination Option Layout
3.1 Destination Options Header
The IPv6 Destination Options Header is used to carry optional
information that needs to be examined only by a packet's destination
node(s). The Destination Options Header is identified by a Next
Header value of 60 in the immediately preceding header and is defined
in RFC2460 [RFC2460]. The IPv6 Performance and Diagnostic Metrics
Destination Option (PDM) is implemented as an IPv6 Option carried in
the Destination Options Header. The PDM does not require time
synchronization.
3.2 Performance and Diagnostic Metrics Destination Option
3.2.1 PDM Layout
The IPv6 Performance and Diagnostic Metrics Destination Option (PDM)
contains the following fields:
SCALEDTLR: Scale for Delta Time Last Received
SCALEDTLS: Scale for Delta Time Last Sent
PSNTP : Packet Sequence Number This Packet
PSNLR : Packet Sequence Number Last Received
DELTATLR : Delta Time Last Received
DELTATLS : Delta Time Last Sent
PDM has alignment requirements. Following the convention in IPv6,
these options are aligned in a packet so that multi-octet values
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within the Option Data field of each option fall on natural
boundaries (i.e., fields of width n octets are placed at an integer
multiple of n octets from the start of the header, for n = 1, 2, 4,
or 8) [RFC2460].
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The PDM destination option is encoded in type-length-value (TLV)
format as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | ScaleDTLR | ScaleDTLS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN This Packet | PSN Last Received |
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delta Time Last Received | Delta Time Last Sent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
TBD = 0xXX (TBD) [To be assigned by IANA] [RFC2780]
In keeping with RFC2460[RFC2460], the two high order bits of the
Option Type field are encoded to indicate specific processing of the
option; for the PDM destination option, these two bits MUST be set to
00.
The third high order bit of the Option Type specifies whether or not
the Option Data of that option can change en-route to the packet's
final destination.
In the PDM, the value of the third high order bit MUST be 0.
Option Length
8-bit unsigned integer. Length of the option, in octets, excluding
the Option Type and Option Length fields. This field MUST be set to
10.
Scale Delta Time Last Received (SCALEDTLR)
8-bit unsigned integer. This is the scaling value for the Delta Time
Last Received (DELTATLR) field. The possible values are from 0-255.
See Section 4 for further discussion on Timing Considerations and
formatting of the scaling values.
Scale Delta Time Last Sent (SCALEDTLS)
8-bit signed integer. This is the scaling value for the Delta Time
Last Sent (DELTATLS) field. The possible values are from 0 to 255.
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Packet Sequence Number This Packet (PSNTP)
16-bit unsigned integer. This field will wrap. It is intended for
use while analyzing packet traces.
Initialized at a random number and incremented monotonically for each
packet of the session flow of the 5-tuple. The random number
initialization is intended to make it harder to spoof and insert such
packets.
Operating systems MUST implement a separate packet sequence number
counter per 5-tuple.
Packet Sequence Number Last Received (PSNLR)
16-bit unsigned integer. This is the PSNTP of the packet last
received on the 5-tuple.
This field is initialized to 0.
Delta Time Last Received (DELTATLR)
A 16-bit unsigned integer field. The value is set according to the
scale in SCALEDTLR.
Delta Time Last Received = (Send time packet n - Receive time packet
n-1)
Delta Time Last Sent (DELTATLS)
A 16-bit unsigned integer field. The value is set according to the
scale in SCALEDTLS.
Delta Time Last Sent = (Receive time packet n - Send time packet n-1)
3.2.2 Base Unit for Time Measurement
A time differential is always a whole number in a CPU; it is the unit
specification -- hours, seconds, nanoseconds -- that determine what
the numeric value means. For PDM, the base time unit is 1 attosecond
(asec). This allows for a common unit and scaling of the time
differential among all IP stacks and hardware implementations.
Note that PDM provides the ability to measure both time differentials
that are extremely small, and time differentials in a
Delay/Disruption Tolerant Networking (DTN) environment where the
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delays may be very great. To store a time differential in just 16
bits that must range in this way will require some scaling of the
time differential value.
One issue is the conversion from the native time base in the CPU
hardware of whatever device is in use to some number of attoseconds.
It might seem this will be an astronomical number, but the conversion
is straightforward. It involves multiplication by an appropriate
power of 10 to change the value into a number of attoseconds. Then,
to scale the value so that it fits into DELTATLR or DELTATLS, the
value is shifted by of a number of bits, retaining the 16 high-order
or most significant bits. The number of bits shifted becomes the
scaling factor, stored as SCALEDTLR or SCALEDTLS, respectively. For
additional information of this process, including examples, please
see Appendix A.
3.3 Header Placement
The PDM Destination Option is placed as defined in RFC2460 [RFC2460].
There may be a choice of where to place the Destination Options
header. If using ESP mode, please see section 3.4 of this document
for placement of the PDM Destination Options header.
For each IPv6 packet header, the PDM MUST NOT appear more than once.
However, an encapsulated packet MAY contain a separate PDM associated
with each encapsulated IPv6 header.
3.4 Header Placement Using IPSec ESP Mode
IPSec Encapsulating Security Payload (ESP) is defined in [RFC4303]
and is widely used. Section 3.1.1 of [RFC4303] discusses placement
of Destination Options Headers.
The placement of PDM is different depending on if ESP is used in
tunnel or transport mode.
In ESP case, no 5-tuple is available, as there are no port numbers.
ESP flow should be identified only by using SADDR, DADDR and PROTOC.
The SPI numbers SHOULD be ignored when considering the flow over
which PDM information is measured.
3.4.1 Using ESP Transport Mode
Note that Destination Options may be placed before or after ESP or
both. If using PDM in ESP transport mode, PDM MUST be placed after
the ESP header so as not to leak information.
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3.4.2 Using ESP Tunnel Mode
Note that Destination Options may be placed before or after ESP or
both in both the outer set of IP headers and the inner set of IP
headers. A tunnel endpoint that creates a new packet may decide to
use PDM independent of the use of PDM of the original packet to
enable delay measurements between the two tunnel endpoints.
3.5 Implementation Considerations
3.5.1 PDM Activation
An implementation should provide an interface to enable or disable
the use of PDM. This specification recommends having PDM off by
default.
PDM MUST NOT be turned on merely if a packet is received with a PDM
header. The received packet could be spoofed by another device.
3.5.2 PDM Timestamps
The PDM timestamps are intended to isolate wire time from server or
host time, but may necessarily attribute some host processing time to
network latency.
RFC2330 [RFC2330] "Framework for IP Performance Metrics" describes
two notions of wire time in section 10.2. These notions are only
defined in terms of an Internet host H observing an Internet link L
at a particular location:
+ For a given IP packet P, the 'wire arrival time' of P at H on L
is the first time T at which any bit of P has appeared at H's
observational position on L.
+ For a given IP packet P, the 'wire exit time' of P at H on L is
the first time T at which all the bits of P have appeared at H's
observational position on L.
This specification does not define the exact H's observing position
on L. That is left for the deployment setups to define. However, the
position where PDM timestamps are taken SHOULD be as close to the
physical network interface as possible. Not all implementations will
be able to achieve the ideal level of measurement.
3.6 Dynamic Configuration Options
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If the PDM destination options extension header is used, then it MAY
be turned on for all packets flowing through the host, applied to an
upper-layer protocol (TCP, UDP, SCTP, etc), a local port, or IP
address only. These are at the discretion of the implementation.
3.7 Information Access and Storage
Measurement information provided by PDM may be made accessible for
higher layers or the user itself. Similar to activating the use of
PDM, the implementation may also provide an interface to indicate if
received
PDM information may be stored, if desired. If a packet with PDM
information is received and the information should be stored, the
upper layers may be notified. Furthermore, the implementation should
define a configurable maximum lifetime after which the information
can be removed as well as a configurable maximum amount of memory
that should be allocated for PDM information.
4 Security Considerations
PDM may introduce some new security weaknesses.
4.1 Resource Consumption and Resource Consumption Attacks
PDM needs to calculate the deltas for time and keep track of the
sequence numbers. This means that control blocks which reside in
memory may be kept at the end hosts per 5-tuple.
A limit on how much memory is being used SHOULD be implemented.
Without a memory limit, any time a control block is kept in memory,
an attacker can try to misuse the control blocks to cause excessive
resource consumption. This could be used to compromise the end host.
PDM is used only at the end hosts and memory is used only at the end
host and not at routers or middle boxes.
4.2 Pervasive monitoring
Since PDM passes in the clear, a concern arises as to whether the
data can be used to fingerprint the system or somehow obtain
information about the contents of the payload.
Let us discuss fingerprinting of the end host first. It is possible
that seeing the pattern of deltas or the absolute values could give
some information as to the speed of the end host - that is, if it is
a very fast system or an older, slow device. This may be useful to
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the attacker. However, if the attacker has access to PDM, the
attacker also has access to the entire packet and could make such a
deduction based merely on the time frames elapsed between packets
WITHOUT PDM.
As far as deducing the content of the payload, in terms of the
application level information such as web page, user name, user
password and so on, it appears to us that PDM is quite unhelpful in
this regard. Having said that, the ability to separate wire-time
from processing time may potentially provide an attacker with
additional information. It is conceivable that an attacker could
attempt to deduce the type of application in use by noting the server
time and payload length. Some encryption algorithms attempt to
obfuscate the packet length to avoid just such vulnerabilities. In
the future, encryption algorithms may wish to obfuscate the server
time as well.
4.3 PDM as a Covert Channel
PDM provides a set of fields in the packet which could be used to
leak data. But, there is no real reason to suspect that PDM would be
chosen rather than another part of the payload or another Extension
Header.
A firewall or another device could sanity check the fields within the
PDM but randomly assigned sequence numbers and delta times might be
expected to vary widely. The biggest problem though is how an
attacker would get access to PDM in the first place to leak data.
The attacker would have to either compromise the end host or have Man
in the Middle (MitM). It is possible that either one could change
the fields. But, then the other end host would get sequence numbers
and deltas that don't make any sense.
It is conceivable that someone could compromise an end host and make
it start sending packets with PDM without the knowledge of the host.
But, again, the bigger problem is the compromise of the end host.
Once that is done, the attacker probably has better ways to leak
data.
Having said that, if a PDM aware middle box or an implementation
(destination host) detects some number of "nonsensical" sequence
numbers or timing information, it could take action to block,
discard, or alert on this traffic.
4.4 Timing Attacks
The fact that PDM can help in the separation of node processing time
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from network latency brings value to performance monitoring. Yet, it
is this very characteristic of PDM which may be misused to make
certain new type of timing attacks against protocols and
implementations possible.
Depending on the nature of the cryptographic protocol used, it may be
possible to leak the credentials of the device. For example, if an
attacker can see that PDM is being used, then the attacker might use
PDM to launch a timing attack against the keying material used by the
cryptographic protocol.
An implementation may want to be sure that PDM is enabled only for
certain ip addresses, or only for some ports. Additionally, the
implementation SHOULD require an explicit restart of monitoring after
a certain time period (for example for 1 hour), to make sure that PDM
is not accidentally left on after debugging has been done etc.
Even so, if using PDM, a user "Consent to be Measured" SHOULD be a
pre-requisite for using PDM. Consent is common in enterprises and
with some subscription services. The actual content of "Consent to
be Measured" will differ by site but it SHOULD make clear that the
traffic is being measured for quality of service and to assist in
diagnostics as well as to make clear that there may be potential
risks of certain vulnerabilities if the traffic is captured during a
diagnostic session.
5 IANA Considerations
This draft requests an Destination Option Type assignment with the
act bits set to 00 and the chg bit set to 0 from the Destination
Options and Hop-by-Hop Options sub-registry of Internet Protocol
Version 6 (IPv6) Parameters [ref to RFCs and URL below].
http://www.iana.org/assignments/ipv6-parameters/ipv6-
parameters.xhtml#ipv6-parameters-2
Hex Value Binary Value Description Reference
act chg rest
-------------------------------------------------------------------
TBD TBD Performance and [This draft]
Diagnostic Metrics
(PDM)
6 References
6.1 Normative References
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[RFC1122] Braden, R., "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2681] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
Delay Metric for IPPM", RFC 2681, September 1999.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines
For Values In the Internet Protocol and Related Headers", BCP 37, RFC
2780, March 2000.
[RFC4303] Kent, S, "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
6.2 Informative References
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330, May 1998.
[TRAM-TCPM] Trammel, B., "Encoding of Time Intervals for the TCP
Timestamp Option-01", Internet Draft, July 2013. [Work in Progress]
Appendix A: Context for PDM
A.1 End User Quality of Service (QoS)
The timing values in the PDM embedded in the packet will be used to
estimate QoS as experienced by an end user device.
For many applications, the key user performance indicator is response
time. When the end user is an individual, he is generally
indifferent to what is happening along the network; what he really
cares about is how long it takes to get a response back. But this is
not just a matter of individuals' personal convenience. In many
cases, rapid response is critical to the business being conducted.
Low, reliable and acceptable response times are not just "nice to
have". On many networks, the impact can be financial hardship or can
endanger human life. In some cities, the emergency police contact
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system operates over IP; law enforcement, at all levels, use IP
networks; transactions on our stock exchanges are settled using IP
networks. The critical nature of such activities to our daily lives
and financial well-being demand a simple solution to support response
time measurements.
A.2 Need for a Packet Sequence Number (PSN)
While performing network diagnostics of an end-to-end connection, it
often becomes necessary to isolate the factors along the network path
responsible for problems. Diagnostic data may be collected at
multiple places along the path (if possible), or at the source and
destination. Then, in post-collection processing, the diagnostic
data corresponding to each packet at different observation points
must be matched for proper measurements. A sequence number in each
packet provides sufficient basis for the matching process. If need
be, the timing fields may be used along with the sequence number to
ensure uniqueness.
This method of data collection along the path is of special use to
determine where packet loss or packet corruption is happening.
The packet sequence number needs to be unique in the context of the
session (5-tuple).
A.3 Rationale for Defined Solution
One of the important functions of PDM is to allow you to quickly
dispatch the right set of diagnosticians. Within network or server
latency, there may be many components. The job of the diagnostician
is to rule each one out until the culprit is found.
How PDM fits into this diagnostic picture is that PDM will quickly
tell you how to escalate. PDM will point to either the network area
or the server area. Within the server latency, PDM does not tell
you if the bottleneck is in the IP stack or the application or buffer
allocation. Within the network latency, PDM does not tell you which
of the network segments or middle boxes is at fault.
What PDM does tell you is whether the problem is in the network or
the server.
A.4 Use PDM with Other Headers
For diagnostics, one my want to use PDM with other headers (L2, L3,
etc). For example, if PDM is used is by a technician (or tool)
looking at a packet capture, within the packet capture, they would
have available to them the layer 2 header, IP header (v6 or v4), TCP,
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UCP, ICMP, SCTP or other headers. All information would be looked
at together to make sense of the packet flow. The technician or
processing tool could analyze, report or ignore the data from PDM, as
necessary.
For an example of how PDM can help with TCP retransmit problems,
please look at Appendix C.
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Appendix B : Timing Considerations
B.1 Timing Differential Calculations
The time counter in a CPU is a binary whole number, representing a
number of milliseconds (msec), microseconds (usec) or even
picoseconds (psec). Representing one of these values as attoseconds
(asec) means multiplying by 10 raised to some exponent. Refer to this
table of equalities:
Base value = # of sec = # of asec 1000s of asec
--------------- ------------- ------------- -------------
1 second 1 sec 10**18 asec 1000**6 asec
1 millisecond 10**-3 sec 10**15 asec 1000**5 asec
1 microsecond 10**-6 sec 10**12 asec 1000**4 asec
1 nanosecond 10**-9 sec 10**9 asec 1000**3 asec
1 picosecond 10**-12 sec 10**6 asec 1000**2 asec
1 femtosecond 10**-15 sec 10**3 asec 1000**1 asec
For example, if you have a time differential expressed in
microseconds, since each microsecond is 10**12 asec, you would
multiply your time value by 10**12 to obtain the number of
attoseconds. If you time differential is expressed in nanoseconds,
you would multiply by 10**9 to get the number of attoseconds.
The result is a binary value that will need to be shortened by a
number of bits so it will fit into the 16-bit PDM DELTA field.
The next step is to divide by 2 until the value is contained in just
16 significant bits. The exponent of the value in the last column of
of the table is useful here; the initial scaling factor is that
exponent multiplied by 10. This is the minimum number of low-order
bits to be shifted-out or discarded. It represents dividing the time
value by 1024 raised to that exponent.
The resulting value may still be too large to fit into 16 bits, but
can be normalized by shifting out more bits (dividing by 2) until the
value fits into the 16-bit DELTA field. The number of extra bits
shifted out is then added to the scaling factor. The scaling factor,
the total number of low-order bits dropped, is the SCALEDTL value.
For example: say an application has these start and finish timer
values (hexadecimal values, in microseconds):
Finish: 27C849234 usec (02:57:58.997556)
-Start: 27C83F696 usec (02:57:58.957718)
========== ========= ===============
Difference 9B9E usec 00.039838 sec or 39838 usec
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To convert this differential value to binary attoseconds, multiply
the number of microseconds by 10**12. Divide by 1024**4, or simply
discard 40 bits from the right. The result is 36232, or 8D88 in hex,
with a scaling factor or SCALEDTL value of 40.
For another example, presume the time differential is larger, say
32.311072 seconds, which is 32311072 usec. Each microsecond is 10**12
asec, so multiply by 10**12, giving the hexadecimal value
1C067FCCAE8120000. Using the initial scaling factor of 40, drop the
last 10 characters (40 bits) from that string, giving 1C067FC. This
will not fit into a DELTA field, as it is 25 bits long. Shifting the
value to the right another 9 bits results in a DELTA value of E033,
with a resulting scaling factor of 49.
When the time differential value is a small number, regardless of the
time unit, the exponent trick given above is not useful in
determining the proper scaling value. For example, if the time
differential is 3 seconds and you want to convert that directly, you
would follow this path:
3 seconds = 3*10**18 asec (decimal)
= 29A2241AF62C0000 asec (hexadecimal)
If you just truncate the last 60 bits, you end up with a delta value
of 2 and a scaling factor of 60, when what you really wanted was a
delta value with more significant digits. The most precision with
which you can store this value in 16 bits is A688, with a scaling
factor of 46.
B.2 Considerations of this time-differential representation
There are a few considerations to be taken into account with this
representation of a time differential. The first is whether there are
any limitations on the maximum or minimum time differential that can
be expressed using method of a delta value and a scaling factor. The
second is the amount of imprecision introduced by this method.
B.2.1 Limitations with this encoding method
The DELTATLS and DELTATLR fields store only the 16 most-significant
bits of the time differential value. Thus the range, excluding the
scaling factor, is from 0 to 65535, or a maximum of 2**16-1. This
method is further described in [TRAM-TCPM].
The actual magnitude of the time differential is determined by the
scaling factor. SCALEDTLR and SCALEDTLS are 8-bit unsigned integers,
so the scaling factor ranges from 0 to 255. The smallest number that
can be represented would have a value of 1 in the delta field and a
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value of 0 in the associated scale field. This is the representation
for 1 attosecond. Clearly this allows PDM to measure extremely small
time differentials.
On the other end of the scale, the maximum delta value is 65535, or
FFFF in hexadecimal. If the maximum scale value of 255 is used, the
time differential represented is 65535*2**255, which is over 3*10**55
years, essentially, forever. So there appears to be no real
limitation to the time differential that can be represented.
B.2.2 Loss of precision induced by timer value truncation
As PDM specifies the DELTATLR and DELTATLS values as 16-bit unsigned
integers, any time the precision is greater than those 16 bits, there
will be truncation of the trailing bits, with an accompanying loss of
precision in the value.
Any time differential value smaller than 65536 asec can be stored
exactly in DELTATLR or DELTATLS, because the representation of this
value requires at most 16 bits.
Since the time differential values in PDM are measured in
attoseconds, the range of values that would be truncated to the same
encoded value is 2**(Scale)-1 asec.
For example, the smallest time differential that would be truncated
to fit into a delta field is
1 0000 0000 0000 0000 = 65536 asec
This value would be encoded as a delta value of 8000 (hexadecimal)
with a scaling factor of 1. The value
1 0000 0000 0000 0001 = 65537 asec
would also be encoded as a delta value of 8000 with a scaling factor
of 1. This actually is the largest value that would be truncated to
that same encoded value. When the scale value is 1, the value range
is calculated as 2**1 - 1, or 1 asec, which you can see is the
difference between these minimum and maximum values.
The scaling factor is defined as the number of low-order bits
truncated to reduce the size of the resulting value so it fits into a
16-bit delta field. If, for example, you had to truncate 12 bits, the
loss of precision would depend on the bits you truncated. The range
of these values would be
0000 0000 0000 = 0 asec
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to
1111 1111 1111 = 4095 asec
So the minimum loss of precision would be 0 asec, where the delta
value exactly represents the time differential, and the maximum loss
of precision would be 4095 asec. As stated above, the scaling factor
of 12 means the maximum loss of precision is 2**12-1 asec, or 4095
asec.
Compare this loss of precision to the actual time differential. The
range of actual time differential values that would incur this loss
of precision is from
1000 0000 0000 0000 0000 0000 0000 = 2**27 asec or 134217728 asec
to
1111 1111 1111 1111 1111 1111 1111 = 2**28-1 asec or 268435455 asec
Granted, these are small values, but the point is, any value between
these two values will have a maximum loss of precision of 4095 asec,
or about 0.00305% for the first value, as encoded, and at most
0.001526% for the second. These maximum-loss percentages are
consistent for all scaling values.
Appendix C: Sample Packet Flows
C.1 PDM Flow - Simple Client Server
Following is a sample simple flow for the PDM with one packet sent
from Host A and one packet received by Host B. The PDM does not
require time synchronization between Host A and Host B. The
calculations to derive meaningful metrics for network diagnostics are
shown below each packet sent or received.
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C.1.1 Step 1
Packet 1 is sent from Host A to Host B. The time for Host A is set
initially to 10:00AM.
The time and packet sequence number are saved by the sender
internally. The packet sequence number and delta times are sent in
the packet.
Packet 1
+----------+ +----------+
| | | |
| Host | ----------> | Host |
| A | | B |
| | | |
+----------+ +----------+
PDM Contents:
PSNTP : Packet Sequence Number This Packet: 25
PSNLR : Packet Sequence Number Last Received: -
DELTATLR : Delta Time Last Received: -
SCALEDTLR: Scale of Delta Time Last Received: 0
DELTATLS : Delta Time Last Sent: -
SCALEDTLS: Scale of Delta Time Last Sent: 0
Internally, within the sender, Host A, it must keep:
Packet Sequence Number of the last packet sent: 25
Time the last packet was sent: 10:00:00
Note, the initial PSNTP from Host A starts at a random number. In
this case, 25. The time in these examples is shown in seconds for
the sake of simplicity.
C.1.2 Step 2
Packet 1 is received at Host B. Its time is set to one hour later
than Host A. In this case, 11:00AM
Internally, within the receiver, Host B, it must note:
Packet Sequence Number of the last packet received: 25
Time the last packet was received : 11:00:03
Note, this timestamp is in Host B time. It has nothing whatsoever to
do with Host A time. The Packet Sequence Number of the last packet
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received will become PSNLR which will be sent out in the packet sent
by Host B in the next step. The time last received will be used to
calculate the DELTALR value to be sent out in the packet sent by Host
B in the next step.
C.1.3 Step 3
Packet 2 is sent by Host B to Host A. Note, the initial packet
sequence number (PSNTP) from Host B starts at a random number. In
this case, 12. Before sending the packet, Host B does a calculation
of deltas. Since Host B knows when it is sending the packet, and it
knows when it received the previous packet, it can do the following
calculation:
Sending time : packet 2 - receive time : packet 1
The result of this calculation is called: Delta Time Last Received
(DELTATLR)
Note, both sending time and receive time are saved internally in Host
B. They do not travel in the packet. Only the Delta is in the
packet.
Assume that within Host B is the following:
Packet Sequence Number of the last packet received: 25
Time the last packet was received: 11:00:03
Packet Sequence Number of this packet: 12
Time this packet is being sent: 11:00:07
Now a delta value to be sent out in the packet can be calculated.
DELTATLR becomes:
4 seconds = 11:00:07 - 11:00:03 = 3782DACE9D900000 asec
This is the derived metric: Server Delay. The time and scaling
factor must be converted; in this case, the time differential is
DE0B, and the scaling factor is 2E, or 46 in decimal. Then, these
values, along with the packet sequence numbers will be sent to Host A
as follows:
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Packet 2
+----------+ +----------+
| | | |
| Host | <---------- | Host |
| A | | B |
| | | |
+----------+ +----------+
PDM Contents:
PSNTP : Packet Sequence Number This Packet: 12
PSNLR : Packet Sequence Number Last Received: 25
DELTATLR : Delta Time Last Received: DE0B (4 seconds)
SCALEDTLR: Scale of Delta Time Last Received: 2E (46 decimal)
DELTATLS : Delta Time Last Sent: -
SCALEDTLS: Scale of Delta Time Last Sent: 0
The metric left to be calculated is the Round-Trip Delay. This will
be calculated by Host A when it receives Packet 2.
C.1.4 Step 4
Packet 2 is received at Host A. Remember, its time is set to one
hour earlier than Host B. Internally, it must note:
Packet Sequence Number of the last packet received: 12
Time the last packet was received : 10:00:12
Note, this timestamp is in Host A time. It has nothing whatsoever to
do with Host B time.
So, now, Host A can calculate total end-to-end time. That is:
End-to-End Time = Time Last Received - Time Last Sent
For example, packet 25 was sent by Host A at 10:00:00. Packet 12 was
received by Host A at 10:00:12 so:
End-to-End time = 10:00:12 - 10:00:00 or 12 (Server and Network RT
delay combined). This time may also be called total Overall Round-
Trip Time (RTT) which includes Network RTT and Host Response Time.
This derived metric we will call Delta Time Last Sent (DELTATLS)
Round trip delay can now be calculated. The formula is:
Round trip delay = (Delta Time Last Sent - Delta Time Last Received)
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Or:
Round trip delay = 12 - 4 or 8
Now, the only problem is that at this point all metrics are in Host A
only and not exposed in a packet. To do that, we need a third packet.
Note: this simple example assumes one send and one receive. That
is done only for purposes of explaining the function of the PDM. In
cases where there are multiple packets returned, one would take the
time in the last packet in the sequence. The calculations of such
timings and intelligent processing is the function of post-processing
of the data.
C.1.5 Step 5
Packet 3 is sent from Host A to Host B.
+----------+ +----------+
| | | |
| Host | ----------> | Host |
| A | | B |
| | | |
+----------+ +----------+
PDM Contents:
PSNTP : Packet Sequence Number This Packet: 26
PSNLR : Packet Sequence Number Last Received: 12
DELTATLR : Delta Time Last Received: 0
SCALEDTLS: Scale of Delta Time Last Received 0
DELTATLS : Delta Time Last Sent: A688 (scaled value)
SCALEDTLR: Scale of Delta Time Last Received: 30 (48 decimal)
To calculate Two-Way Delay, any packet capture device may look at
these packets and do what is necessary.
C.2 Other Flows
What has been discussed so far is a simple flow with one packet sent
and one returned. Let's look at how PDM may be useful in other
types of flows.
C.2.1 PDM Flow - One Way Traffic
The flow on a particular session may not be a send-receive paradigm.
Let us consider some other situations. In the case of a one-way
flow, one might see the following:
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Note: The time is expressed in generic units for simplicity. That
is, these values do not represent a number of attoseconds,
microseconds or any other real units of time.
Packet Sender PSN PSN Delta Time Delta Time
This Packet Last Recvd Last Recvd Last Sent
=====================================================================
1 Server 1 0 0 0
2 Server 2 0 0 5
3 Server 3 0 0 12
4 Server 4 0 0 20
What does this mean and how is it useful?
In a one-way flow, only the Delta Time Last Sent will be seen as
used. Recall, Delta Time Last Sent is the difference between the
send of one packet from a device and the next. This is a measure of
throughput for the sender - according to the sender's point of view.
That is, it is a measure of how fast is the application itself (with
stack time included) able to send packets.
How might this be useful? If one is having a performance issue at
the client and sees that packet 2, for example, is sent after 5 time
units from the server but takes 10 times that long to arrive at the
destination, then one may safely conclude that there are delays in
the path other than at the server which may be causing the delivery
issue of that packet. Such delays may include the network links,
middle-boxes, etc.
Now, true one-way traffic is quite rare. What people often mean by
"one-way" traffic is an application such as FTP where a group of
packets (for example, a TCP window size worth) is sent, then the
sender waits for acknowledgment. This type of flow would actually
fall into the "multiple-send" traffic model.
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C.2.2 PDM Flow - Multiple Send Traffic
Assume that two packets are sent for each ACK from the server. For
example, a TCP flow will do this, per RFC1122 [RFC1122] Section-
4.2.3.
Packet Sender PSN PSN Delta Time Delta Time
This Packet Last Recvd Last Recvd Last Sent
=====================================================================
1 Server 1 0 0 0
2 Server 2 0 0 5
3 Client 1 2 20 0
4 Server 3 1 10 15
How might this be used?
Notice that in packet 3, the client has a value of Delta Time Last
received of 20. Recall that Delta Time Last Received is the Send
time of packet 3 - receive time of packet 2. So, what does one know
now? In this case, Delta Time Last Received is the processing time
for the Client to send the next packet.
How to interpret this depends on what is actually being sent.
Remember, PDM is not being used in isolation, but to supplement the
fields found in other headers. Let's take some examples:
1. Client is sending a standalone TCP ACK. One would find this by
looking at the payload length in the IPv6 header and the TCP
Acknowledgement field in the TCP header. So, in this case, the
client is taking 20 units to send back the ACK. This may or may not
be interesting.
2. Client is sending data with the packet. Again, one would find
this by looking at the payload length in the IPv6 header and the TCP
Acknowledgement field in the TCP header. So, in this case, the
client is taking 20 units to send back data. This may represent
"User Think Time". Again, this may or may not be interesting, in
isolation. But, if there is a performance problem receiving data at
the server, then taken in conjunction with RTT or other packet timing
information, this information may be quite interesting.
Of course, one also needs to look at the PSN Last Received field to
make sure of the interpretation of this data. That is, to make
sure that the Delta Last Received corresponds to the packet of
interest.
The benefits of PDM are that such information is now available in a
uniform manner for all applications and all protocols without
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extensive changes required to applications.
C.2.3 PDM Flow - Multiple Send with Errors
Let us now look at a case of how PDM may be able to help in a case of
TCP retransmission and add to the information that is sent in the TCP
header.
Assume that three packets are sent with each send from the server.
From the server, this is what is seen.
Pkt Sender PSN PSN Delta Time Delta Time TCP Data
This Pkt LastRecvd LastRecvd LastSent SEQ Bytes
=====================================================================
1 Server 1 0 0 0 123 100
2 Server 2 0 0 5 223 100
3 Server 3 0 0 5 333 100
The client, however, does not receive all the packets. From the
client, this is what is seen for the packets sent from the server.
Pkt Sender PSN PSN Delta Time Delta Time TCP Data
This Pkt LastRecvd LastRecvd LastSent SEQ Bytes
=====================================================================
1 Server 1 0 0 0 123 100
2 Server 3 0 0 5 333 100
Let's assume that the server now retransmits the packet. (Obviously,
a duplicate acknowledgment sequence for fast retransmit or a
retransmit timeout would occur. To illustrate the point, these
packets are being left out.)
So, then if a TCP retransmission is done, then from the client, this
is what is seen for the packets sent from the server.
Pkt Sender PSN PSN Delta Time Delta Time TCP Data
This Pkt LastRecvd LastRecvd LastSent SEQ Bytes
=====================================================================
1 Server 4 0 0 30 223 100
The server has resent the old packet 2 with TCP sequence number of
223. The retransmitted packet now has a PSN This Packet value of 4.
The Delta Last Sent is 30 - the time between sending the packet with
PSN of 3 and this current packet.
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Let's say that packet 4 is lost again. Then, after some amount of
time (RTO) then the packet with TCP sequence number of 223 is resent.
From the client, this is what is seen for the packets sent from the
server.
Pkt Sender PSN PSN Delta Time Delta Time TCP Data
This Pkt LastRecvd LastRecvd LastSent SEQ Bytes
=====================================================================
1 Server 5 0 0 60 223 100
If now, this packet arrives at the destination, one has a very good
idea that packets exist which are being sent from the server as
retransmissions and not arriving at the client. This is because the
PSN of the resent packet from the server is 5 rather than 4. If we
had used TCP sequence number alone, we would never have seen this
situation. The TCP sequence number in all situations is 223.
This situation would be experienced by the user of the application
(the human being actually sitting somewhere) as a "hangs" or long
delay between packets. On large networks, to diagnose problems such
as these where packets are lost somewhere on the network, one has to
take multiple traces to find out exactly where.
The first thing is to start with doing a trace at the client and the
server. So, we can see if the server sent a particular packet and
the client received it. If the client did not receive it, then we
start tracking back to trace points at the router right after the
server and the router right before the client. Did they get these
packets which the server has sent? This is a time consuming
activity.
With PDM, we can speed up the diagnostic time because we may be able
to use only the trace taken at the client to see what the server is
sending.
Appendix D: Potential Overhead Considerations
One might wonder as to the potential overhead of PDM. First, PDM is
entirely optional. That is, a site may choose to implement PDM or
not as they wish. If they are happy with the costs of PDM vs. the
benefits, then the choice should be theirs.
Below is a table outlining the potential overhead in terms of
additional time to deliver the response to the end user for various
assumed RTTs.
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Bytes RTT Bytes Bytes New Overhead
in Packet Per Millisec in PDM RTT
=====================================================================
1000 1000 milli 1 16 1016.000 16.000 milli
1000 100 milli 10 16 101.600 1.600 milli
1000 10 milli 100 16 10.160 .160 milli
1000 1 milli 1000 16 1.016 .016 milli
Below are some examples of actual RTTs for packets traversing large
enterprise networks. The first example is for packets going to
multiple business partners.
Bytes RTT Bytes Bytes New Overhead
in Packet Per Millisec in PDM RTT
=====================================================================
1000 17 milli 58 16 17.360 .360 milli
The second example is for packets at a large enterprise customer
within a data center. Notice that the scale is now in microseconds
rather than milliseconds.
Bytes RTT Bytes Bytes New Overhead
in Packet Per Microsec in PDM RTT
=====================================================================
1000 20 micro 50 16 20.320 .320 micro
As with other diagnostic tools, such as packet traces, a certain
amount of processing time will be required to create and process PDM.
Since PDM is lightweight (has only a few variables), we expect the
processing time to be minimal.
Acknowledgments
The authors would like to thank Keven Haining, Al Morton, Brian
Trammel, David Boyes, Bill Jouris, Richard Scheffenegger, and Rick
Troth for their comments and assistance. We would also like to thank
Tero Kivinen and Jouni Korhonen for their detailed and perceptive
reviews.
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Authors' Addresses
Nalini Elkins
Inside Products, Inc.
36A Upper Circle
Carmel Valley, CA 93924
United States
Phone: +1 831 659 8360
Email: nalini.elkins@insidethestack.com
http://www.insidethestack.com
Robert M. Hamilton
Chemical Abstracts Service
A Division of the American Chemical Society
2540 Olentangy River Road
Columbus, Ohio 43202
United States
Phone: +1 614 447 3600 x2517
Email: rhamilton@cas.org
http://www.cas.org
Michael S. Ackermann
Blue Cross Blue Shield of Michigan
P.O. Box 2888
Detroit, Michigan 48231
United States
Phone: +1 310 460 4080
Email: mackermann@bcbsm.com
http://www.bcbsm.com
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