Command Ordering 6-June-03
IPS Mallikarjun Chadalapaka
Internet Draft Rob Elliott
draft-ietf-ips-command-ordering-00.txt Hewlett-Packard Co.
Category: Informational-track
SCSI Command Ordering Considerations with iSCSI
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Status of this Memo
This document is an Internet-Draft and fully conforms to all provi-
sions of Section 10 of [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
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groups may also distribute working documents as Internet-Drafts.
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rial or to cite them except as "work in progress."
The list of Internet-Drafts can be accessed at http://www.ietf.org/
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Abstract
iSCSI is a SCSI transport protocol designed to run on top of TCP. The
iSCSI session abstraction is equivalent to the SCSI I_T nexus, and
the iSCSI session provides an ordered command delivery from the SCSI
initiator to the SCSI target. This document goes into the design
considerations that led to the iSCSI session model as it is defined
today, relates the SCSI command ordering features defined in T10
specifications to the iSCSI concepts, and finally provides guidance
to system designers on how true command ordering solutions can be
built based on iSCSI.
Acknowledgments
We are grateful to the IPS working group whose work defined the iSCSI
protocol. Thanks also to David Black (EMC) who encouraged the publi-
cation of this document. Special thanks are also in order for Randy
Haagens (HP) for his insightful review comments.
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Status of this Memo . . . . . . . . . . . . . . . . . . . . . . . . . 2
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1. Definitions and Acronyms . . . . . . . . . . . . . . . . . . . . . 4
1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of the iSCSI Protocol . . . . . . . . . . . . . . . . . . 6
3.1 Protocol Mapping Description . . . . . . . . . . . . . . . . . 6
3.2 The I_T Nexus Model . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Ordered Command Delivery . . . . . . . . . . . . . . . . . . . 8
3.3.1 Questions . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.2 The Session Guarantee . . . . . . . . . . . . . . . . . . 9
3.3.3 Ordering Onus . . . . . . . . . . . . . . . . . . . . . . 9
3.3.4 Design Intent . . . . . . . . . . . . . . . . . . . . . .10
4. The Command Ordering Scenario . . . . . . . . . . . . . . . . . .10
4.1 SCSI Layer . . . . . . . . . . . . . . . . . . . . . . . . . .10
4.1.1 Command Reference Number (CRN) . . . . . . . . . . . . . .10
4.1.2 Task Attributes . . . . . . . . . . . . . . . . . . . . .10
4.1.3 Auto Contingent Allegiance (ACA) . . . . . . . . . . . . .11
4.1.4 UA Interlock . . . . . . . . . . . . . . . . . . . . . . .11
4.2 iSCSI Layer . . . . . . . . . . . . . . . . . . . . . . . . .11
5. Connection Failure Considerations . . . . . . . . . . . . . . . .12
6. Command Ordering System Considerations . . . . . . . . . . . . . .12
7. Reservation Considerations . . . . . . . . . . . . . . . . . . . .13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .15
9. Security Considerations . . . . . . . . . . . . . . . . . . . . .15
10. References and Bibliography . . . . . . . . . . . . . . . . . . .16
10.1 Normative References . . . . . . . . . . . . . . . . . . . . .16
10.2 Informative References: . . . . . . . . . . . . . . . . . . .16
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .16
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 17
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1. Definitions and Acronyms
1.1 Definitions
- I_T nexus: [SAM2] defines the I_T nexus as a relationship between a
SCSI initiator port and a SCSI target port. [iSCSI] defines an iSCSI
session as the iSCSI representation of an I_T nexus. In the iSCSI
context, the I_T nexus (i.e. the iSCSI session) is a relationship
between an iSCSI initiator's end of the session (SCSI Initiator Port)
and the iSCSI target's Portal Group (SCSI Target Port).
- PDU (Protocol Data Unit): An iSCSI initiator and iSCSI target com-
municate using iSCSI protocol messages. These messages are called
"iSCSI protocol data units" (iSCSI PDUs).
- SCSI device: A SCSI device is an entity that contains one or more
SCSI ports that are connected to a service delivery subsystem and
supports SCSI application protocols. In the iSCSI context, the SCSI
Device is the component within an iSCSI Node that provides the SCSI
functionality. The SCSI Device Name is defined to be the iSCSI Name
of the node.
- Session: A group of logically related iSCSI connections that link
an initiator with a target form a session (equivalent to a SCSI I-T
nexus). The number of participating iSCSI connections within an iSCSI
session may vary over time. The multiplicity of connections at the
iSCSI level is completely hidden for the SCSI layer - each SCSI port
in an I_T nexus sees only one peer SCSI port across all the connec-
tions of a session.
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1.2 Acronyms
Acronym Definition
--------------------------------------------------------------
ACA Auto Contingent Allegiance
ASC Additional Sense Code
ASCQ Additional Sense Code Qualifier
CRN Command Reference Number
IETF Internet Engineering Task Force
ISID Initiator Session Identifier
ITT Initiator Task Tag
LU Logical Unit
LUN Logical Unit Number
NIC Network Interface Card
PDU Protocol Data Unit
TMF Task Management Function
TSIH Target Session Identifying Handle
SAM-2 SCSI Architecture Model - 2
SAN Storage Area Network
SCSI Small Computer Systems Interface
TCP Transmission Control Protocol
UA Unit Attention
WG Working Group
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2. Introduction
iSCSI is a SCSI transport protocol ([iSCSI]) designed to enable run-
ning SCSI application protocols on TCP/IP networks, including poten-
tially the Internet. Given the size and scope of Internet, iSCSI
thus enables some exciting new SCSI applications. Potential new
application areas for exploiting iSCSI's value include the following.
a) Larger (diameter) Storage Area Networks (SANs) than had
been possible until now.
b) Asynchronous remote mirroring
c) Remote tape vaulting
Each of these applications takes advantage of the practically unlim-
ited geographical distance that iSCSI enables between a SCSI initia-
tor and a SCSI target. In each of these cases, because of the long
delays involved, there is a very high incentive for the initiator to
stream SCSI commands back-to-back without waiting for the SCSI sta-
tus of previous commands. Command streaming may be employed prima-
rily by two classes of applications - while one class may not
particularly care about ordered command execution, the other class
does rely on ordered command execution (i.e. there is an application-
level dependency on the ordering among SCSI commands). As an exam-
ple, cases b) and c) listed earlier clearly require ordered command
execution. A mirroring application does not want the writes to be
committed out of order on the remote SCSI target, so as to preserve
the transactional integrity of the data on that target. To summa-
rize, SCSI command streaming when coupled with the guarantee of
ordered command execution on the SCSI target is extremely valuable
for a critical class of applications in long-latency networks.
This document reviews the various protocol considerations in design-
ing storage solutions that employ SCSI command ordering. This docu-
ment also analyzes and explains the design intent of [iSCSI] with
respect to command ordering.
3. Overview of the iSCSI Protocol
3.1 Protocol Mapping Description
The iSCSI protocol is a mapping of the SCSI remote procedure invoca-
tion model (see [SAM2]) over the TCP protocol.
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SCSI's notion of a task maps to an iSCSI task. Each iSCSI task is
uniquely identified within that I_T nexus by a 32-bit unique identi-
fier called Initiator Task Tag (ITT). The ITT is both an iSCSI iden-
tifier of the task and a classic SCSI task tag.
SCSI commands from the initiator to the target are carried in iSCSI
requests called SCSI Command PDUs. SCSI status back to the initia-
tor is carried in iSCSI responses called SCSI Response PDUs. SCSI
Data-out from the initiator to the target is carried in SCSI Data-Out
PDUs, and the SCSI Data-in back to the initiator is carried in SCSI
Data-in PDUs.
3.2 The I_T Nexus Model
In the iSCSI model, the SCSI I_T nexus maps directly to the iSCSI
session which is an iSCSI protocol abstraction spanning one or more
TCP connections. The iSCSI protocol defines the semantics in order
to realize one logical flow of bidirectional communication on the I_T
nexus potentially spanning multiple TCP connections (as many as
2^16). The multiplicity of iSCSI connections is thus completely con-
tained at the iSCSI layer, while the SCSI layer is presented with a
single I_T nexus even in a multi-connection session. A session
between a pair of given iSCSI nodes is identified by the session
identifier (SSID) and each connection within a given session is
uniquely identified by a connection identifier (CID) in iSCSI. The
SSID itself has two components - Initiator Session Identifier (ISID)
and a Target Session Identifying Handler (TSIH) - each identifying
one end of the same session.
There are four crucial functional facets of iSCSI that together
present this single logical flow abstraction to the SCSI layer even
with an iSCSI session spanning across multiple iSCSI connections.
a) Ordered command delivery: A sequence of SCSI commands that
is striped across all the connections in the session is
"reordered" by the target iSCSI layer into an identical
sequence based on a Command Sequence Number (CmdSN) that is
unique across the session. The goal is to achieve band-
width aggregation from multiple TCP connections, but to
still make it appear to the target SCSI layer as if all the
commands had travelled in one flow.
b) Connection allegiance: All the PDU exchanges for a SCSI
Command, up to and including the SCSI Response PDU for the
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Command, are required to flow on the same iSCSI connection
at any given time. This again is intended to hide the
multi-connection nature of a session because the SCSI layer
on either side will never see the PDU contents out of order
(e.g., status cannot bypass read data for an initiator).
c) Task set management function handling: [iSCSI] specifies an
ordered sequence of steps for the iSCSI layer on the SCSI
target in handling the two SCSI task management functions
(TMFs) that manage SCSI task sets. The two TMFs are ABORT
TASK SET that aborts all active tasks in a session and CLEAR
TASK SET that clears the tasks in the task set. The goal of
the sequence of steps is to guarantee that the initiator
receives the SCSI Response PDUs of all unaffected tasks
before the TMF Response itself arrives, regardless of the
number of connections in the iSCSI session. This opera-
tional model is again intended to preserve the single flow
abstraction to the SCSI layer.
d) Immediate task management function handling: Even when a
TMF request is marked as "immediate" (i.e. only has a posi-
tion in the command stream, but does not consume a CmdSN),
[iSCSI] defines semantics that require the target iSCSI
layer to ensure that the TMF request is executed as if the
commands and the TMF request were all flowing on a single
logical channel. This ensures that the TMF request will act
on tasks that it was meant to manage.
The following sections will analyze the "Ordered command delivery"
aspect in more detail, since command ordering is the focus of this
document.
3.3 Ordered Command Delivery
3.3.1 Questions
There has been a lot of debate on this particular aspect in the IPS
Working Group. Most of the debate was centered on two specific ques-
tions -
a) What should be the command ordering behavior required of
iSCSI implementations in the presence of transport errors
(such as TCP checksum escapes, leading to iSCSI digest fail-
ures)?
b) Should [iSCSI] require both initiators and targets to use
ordered command delivery?
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3.3.2 The Session Guarantee
The final disposition of question a) in section 3.3.1 was reflected
in [RFC3347], "iSCSI MUST specify strictly ordered delivery of SCSI
commands over an iSCSI session between an initiator/target pair, even
in the presence of transport errors.". Stated differently, an iSCSI
digest failure, or an iSCSI connection termination must not cause the
iSCSI layer on a target to allow executing the commands in an order
different from that intended (as indicated by the CmdSN order) by the
initiator. This design choice is enormously helpful in building
storage systems and solutions that can now always assume command
ordering to be a service characteristic of an iSCSI substrate.
Note that by taking the position that an iSCSI session always guaran-
tees command ordering, [iSCSI] was indirectly implying that the prin-
cipal reason for the multi-connection iSCSI session abstraction was
to allow ordered bandwidth aggregation for an I_T nexus. In deploy-
ment models where this cross-connection ordering mandated by [iSCSI]
is deemed expensive, a serious consideration should be given to
deploying multiple single-connection sessions in stead.
3.3.3 Ordering Onus
The final resolution of b) in section 3.3.1 by the iSCSI protocol
designers was in favor of not requiring the initiators to use com-
mand ordering always. This resolution is reflected in dropping the
mandatory ACA usage requirement on the initiators, and allowing an
ABORT TASK TMF to plug a command hole etc., since these are con-
scious choices an initiator makes in favor of not using ordered com-
mand delivery. The net result can be discerned by a careful reader
of [iSCSI] - the onus of ensuring ordered command delivery is always
on the iSCSI targets, while the initiators may or may not utilize
command ordering. iSCSI targets being the servers in the client-
server model do not really attempt to establish whether or not a cli-
ent (initiator) intends to take advantage of command ordering ser-
vice, but instead simply always provide the guaranteed delivery
service. The rationale here is that there are inherent SCSI and
application-level dependencies as we shall see in building a command
ordered solution that are beyond the scope of [iSCSI], to mandate or
even discern the intent with respect to the usage of command order-
ing.
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3.3.4 Design Intent
To summarize the design intent of [iSCSI] -
The service delivery subsystem (see [SAM2]) abstraction pro-
vided by an iSCSI session is guaranteed to have the intrinsic
property of ordered delivery of commands to the target SCSI
layer under all conditions. Consequently, the guarantee of the
ordered command delivery is across the entire I_T nexus span-
ning all the LUs that the nexus is authorized to access. It is
the initiator's discretion to whether or not make use of this
property.
4. The Command Ordering Scenario
A storage systems designer working with SCSI and iSCSI has to con-
sider the following protocol features in SCSI and iSCSI layers, each
of which has a role to play in realizing the command ordering goal.
4.1 SCSI Layer
The SCSI application layer has several tools to enforce ordering.
4.1.1 Command Reference Number (CRN)
CRN is an ordered sequence number which when enabled for a device
server, increments by one for each I_T_L nexus (see [SAM2]). The one
notable drawback with CRN is that there is no SCSI-generic way (such
as through mode pages) to enable or disable the CRN feature. [SAM2]
also leaves the usage semantics of CRN for the SCSI transport proto-
col, such as iSCSI, to specify. [iSCSI] chose not to support the CRN
feature for various reasons.
4.1.2 Task Attributes
SAM-2 defines the following four task attributes - SIMPLE, ORDERED,
HEAD OF QUEUE, and ACA. Each task to an LU may be assigned an
attribute. [SAM2] defines the ordering constraints that each of
these attributes conveys to the device server that is servicing the
task. In particular, judicious use of ORDERED and SIMPLE attributes
applied to a stream of pipelined commands could convey the precise
execution schema for the commands that the initiator issues, pro-
vided the commands are received in the same order on the target.
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4.1.3 Auto Contingent Allegiance (ACA)
ACA is an LU-level condition that is triggered when a command (with
the NACA bit set to 1) completes with CHECK CONDITION. When ACA is
triggered, it prevents all commands other than those with the ACA
attribute from executing until the CLEAR ACA task management func-
tion is executed, while blocking all the other tasks already in the
task set. See [SAM2] for the detailed semantics of ACA. Since ACA
is closely tied to the notion of a task set, one would ideally have
to select the scope of the task set (by setting the TST bit to 1 in
the control mode page of the LU) to be per-initiator in order to pre-
vent command failures in one I_T_L nexus from impacting other I_T_L
nexuses through ACA.
4.1.4 UA Interlock
When UA interlock is enabled, the logical unit does not clear any
standard Unit Attention condition reported with autosense and in
addition, establishes a Unit Attention condition when a task is ter-
minated with one of BUSY, TASK SET FULL, or RESERVATION CONFLICT sta-
tuses. This so-called "interlocked UA" is cleared only when the
device server executes an explicit REQUEST SENSE ([SPC3]) command
from the same initiator. From a functionality perspective, the scope
of UA interlock today is slightly different from ACA's because it
enforces ordering behavior for completion statuses other than CHECK
CONDITION, but otherwise conceptually has the same design intent as
ACA. On the other hand, ACA is somewhat more sophisticated because
it allows special "cleanup" tasks (ones with ACA attribute) to exe-
cute when ACA is active. One of the principal reasons UA interlock
came into being was that SCSI designers wanted a command ordering
feature without the side effects of using the aforementioned TST bit
in the control mode page.
4.2 iSCSI Layer
As noted in section 3.2 and section 3.3, the iSCSI protocol enforces
and guarantees ordered command delivery per iSCSI session using the
CmdSN, and this is an attribute of the SCSI transport layer. Note
further that any command ordering solution that seeks to realize
ordering from the initiator SCSI layer to the target SCSI layer would
be of practical value only when the command ordering is guaranteed by
the SCSI transport layer. In other words, the related SCSI applica-
tion layer protocol features such as ACA etc. are based on the
premise of an ordered SCSI transport. Thus iSCSI's command ordering
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is the last piece in completing the puzzle of building solutions that
rely on ordered command execution, by providing the crucial guaran-
tee that all the commands handed to the initiator iSCSI layer will be
transported and handed to the target SCSI layer in the same order.
5. Connection Failure Considerations
[iSCSI] mandates that when an iSCSI connection fails, the active
tasks on that connection must be terminated if not recovered within a
certain negotiated time limit. When an iSCSI target does terminate
some subset of tasks due to iSCSI connection dynamics, there is a
danger that the SCSI layer would simply move on to the next tasks
waiting to be processed and execute them out-of-order unbeknownst to
the initiator SCSI layer. To preclude this danger, [iSCSI] further
mandates the following -
a) The tasks terminated due to the connection failure must be
internally terminated by the iSCSI target "as if" due to a CHECK
CONDITION. While this particular completion status is never com-
municated back to the initiator, the "as if" is still meaningful
and required because if the initiator were using ACA as the com-
mand ordering mechanism of choice, a SCSI-level ACA will be trig-
gered due to this mandatory CHECK CONDITION. This addresses the
aforementioned danger.
b) After the tasks are terminated due to the connection failure,
the iSCSI target must report a Unit Attention condition on the
next command processed on any connection for each affected I_T_L
nexus of that session. This is required because if the initiator
were using UA interlock as the command ordering mechanism of
choice, a SCSI-level UA will trigger a UA-interlock. This again
addresses the aforementioned danger. iSCSI targets must report
this UA with the status of CHECK CONDITION, and the ASC/ASCQ value
of 47h/7Fh ("SOME COMMANDS CLEARED BY ISCSI PROTOCOL EVENT").
6. Command Ordering System Considerations
In general, command ordering is automatically enforced if targets and
initiators comply with the iSCSI specification. However, listed
below are certain additional related implementation considerations
for the iSCSI initiators and targets to take note of.
a) Even when all iSCSI and SCSI command ordering considerations
earlier noted in this draft were applied, it is beneficial for
iSCSI initiators to proactively avoid scenarios that would other-
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wise lead to out-of-order command execution. This is simply
because the SCSI command ordering features such as UA interlock
are likely to be costlier in performance when they are allowed to
be triggered. [iSCSI] provides enough guidance on how to imple-
ment this proactive detection of PDU ordering errors.
b) The whole notion of command streaming does of course assume
that the target in question supports command queueing. An iSCSI
target desirous of supporting command ordering solutions should
ensure that the SCSI layer on the target supports command queu-
ing. Especially the remote backup (tape vaulting) applications
that iSCSI enables make a compelling case that tape devices should
give a very serious consideration to supporting command queuing,
at least when used in conjunction with iSCSI.
c) An iSCSI target desirous of supporting high-performance com-
mand ordering solutions that involve specifying a description of
execution schema should ensure that the SCSI layer on the target
in fact does support the ORDERED and SIMPLE task attributes.
d) There is some consideration of expanding the scope of UA
interlock to encompass CHECK CONDITION status and thus make it the
only required command ordering functionality of implementations to
build command ordering solutions. Until this is resolved in T10,
the currently defined semantics of UA interlock and ACA warrant
implementing both features by iSCSI targets desirous of support-
ing command ordering solutions.
7. Reservation Considerations
[iSCSI] describes a "principle of conservative reuse" that encour-
ages iSCSI initiators to reuse the same ISIDs (see section 3.2) to
various SCSI target ports, in order to present the same SCSI initia-
tor port name to those target ports. This is in fact a very crucial
implementation consideration that must be complied with. [SPC3] man-
dates the SCSI targets to associate persistent reservations and the
related registrations with the SCSI initiator port names whenever
they are required by the SCSI transport protocol. Since [iSCSI]
requires the mandatory SCSI initiator port names based on ISIDs,
iSCSI targets are required to work off the SCSI initiator port names
and thus indirectly the ISIDs in enforcing the persistent reserva-
tions.
This fact has the following implications for the implementations.
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a) If a persistent reservation/registration is intended to be
used across multiple SCSI ports of a SCSI device, the initiator
iSCSI implementation must use the same ISID across associated
iSCSI sessions connecting to different iSCSI target portal groups
of the SCSI device.
b) If a persistent reservation/registration is intended to be
used across the power loss of a SCSI target, the initiator iSCSI
implementation must use the same ISIDs as before in re-establish-
ing the associated iSCSI sessions upon subsequent reboot in order
to rely on the persist through power loss capability.
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8. IANA Considerations
This document does not have any IANA considerations.
9. Security Considerations
This document does not have any security considerations.
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10. References and Bibliography
10.1 Normative References
[iSCSI] J. Satran et. al. draft-ietf-ips-iscsi-20.txt (work in
progress)
[RFC790] J. Postel, ASSIGNED NUMBERS, September 1981.
[RFC793] TRANSMISSION CONTROL PROTOCOL, DARPA INTERNET PROGRAM
PROTOCOL SPECIFICATION, September 1981.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revi-
sion 3", RFC 2026, October 1996.
[RFC2119] Bradner, S. "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] T. Narten, and H. Avestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs.", RFC2434, October
1998.
[SAM] ANSI X3.270-1998, SCSI-3 Architecture Model (SAM).
[SAM2] T10/1157D, SCSI Architecture Model - 2 (SAM-2).
[SBC] NCITS.306-1998, SCSI-3 Block Commands (SBC).
10.2 Informative References:
[RFC3347] M. Krueger et. al., "iSCSI Requirements and Design
Considerations"
[SPC3]T10/1416-D, SCSI Primary Commands-3.
11. Authors' Addresses
Mallikarjun Chadalapaka
Hewlett-Packard Company
8000 Foothills Blvd.
Roseville, CA 95747-5668, USA
Phone: +1.916.785.5621
E-mail: cbm@rose.hp.com
Rob Elliott
Hewlett-Packard Company
MC 150801
PO Box 692000
Houston, TX 77269-2000 USA
Phone: +1.281.518.5037
E-mail: elliott@hp.com
Comments may be sent to Mallikarjun Chadalapaka.
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Internet-Draft
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This is an older version of an Internet-Draft that was ultimately published as RFC 3783.
Expired & archived
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