One document matched: draft-ietf-ips-iscsi-impl-guide-05.txt
Differences from draft-ietf-ips-iscsi-impl-guide-04.txt
INTERNET DRAFT Mallikarjun Chadalapaka
draft-ietf-ips-iscsi-impl-guide-05.txt Hewlett-Packard Co.
Editor
Expires
August 2007
iSCSI Corrections and Clarifications
Status of this Memo
By submitting this Internet-Draft, each author represents
that any applicable patent or other IPR claims of which he or
she is aware have been or will be disclosed, and any of which
he or she becomes aware will be disclosed, in accordance with
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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 [RFC2119].
Abstract
iSCSI is a SCSI transport protocol and maps the SCSI family
of application protocols onto TCP/IP. RFC 3720 defines the
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iSCSI protocol. This document compiles the clarifications to
the original protocol definition in RFC 3720 to serve as a
companion document for the iSCSI implementers. This document
updates RFC 3720 and the text in this document supersedes the
text in RFC 3720 when the two differ.
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Table of Contents
1 Definitions and acronyms ...............................5
1.1 Definitions ............................................5
1.2 Acronyms ...............................................5
2 Introduction ...........................................7
3 iSCSI semantics for SCSI tasks .........................8
3.1 Residual handling ......................................8
3.1.1 Overview..............................................8
3.1.2 SCSI REPORT LUNS and Residual Overflow................9
3.2 R2T Ordering ..........................................10
3.3 Model Assumptions for Response Ordering ...............11
3.3.1 Model Description....................................11
3.3.2 iSCSI Semantics with the Interface Model.............12
3.3.3 Current List of Fenced Response Use Cases............12
4 Task Management .......................................14
4.1 Requests Affecting Multiple Tasks .....................14
4.1.1 Scope of affected tasks..............................14
4.1.2 Clarified multi-task abort semantics.................14
4.1.3 Updated multi-task abort semantics...................16
4.1.4 Affected tasks shared across RFC3720 & FastAbort
sessions....................................................18
4.1.5 Implementation considerations........................19
4.1.6 Rationale behind the new semantics...................20
5 Discovery semantics ...................................22
5.1 Error Recovery for Discovery Sessions .................22
5.2 Reinstatement Semantics of Discovery Sessions .........22
5.2.1 Unnamed Discovery Sessions...........................23
5.2.2 Named Discovery Sessions.............................23
5.3 Target PDUs during Discovery ..........................24
6 Negotiation and Others ................................25
6.1 TPGT Values ...........................................25
6.2 SessionType Negotiation ...............................25
6.3 Understanding NotUnderstood ...........................25
6.4 Outstanding Negotiation Exchanges .....................26
7 iSCSI Error Handling and Recovery .....................27
7.1 ITT ...................................................27
7.2 Format Errors .........................................27
7.3 Digest Errors .........................................27
7.4 Message Error Checking ................................28
8 iSCSI PDUs ............................................29
8.1 Asynchronous Message ..................................29
8.2 Reject ................................................29
9 Login/Text Operational Text Keys ......................30
9.1 TaskReporting .........................................30
10 Security Considerations ...............................32
11 IANA Considerations ...................................33
12 References and Bibliography ...........................34
12.1 Normative References.................................34
12.2 Informative References...............................34
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13 Editor's Address ......................................35
14 Acknowledgements ......................................36
15 Full Copyright Statement ..............................37
16 Intellectual Property Statement .......................38
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1 Definitions and acronyms
1.1 Definitions
I/O Buffer - A buffer that is used in a SCSI Read or Write
operation so SCSI data may be sent from or received into
that buffer. For a read or write data transfer to take
place for a task, an I/O Buffer is required on the
initiator and at least one required on the target.
SCSI-Presented Data Transfer Length (SPDTL): SPDTL is the
aggregate data length of the data that SCSI layer
logically "presents" to iSCSI layer for a Data-in or
Data-out transfer in the context of a SCSI task. For a
bidirectional task, there are two SPDTL values - one for
Data-in and one for Data-out. Note that the notion of
"presenting" includes immediate data per the data
transfer model in [SAM2], and excludes overlapping data
transfers, if any, requested by the SCSI layer.
Third-party: A term used in this document to denote nexus
objects (I_T or I_T_L) and iSCSI sessions which reap the
side-effects of actions that take place in the context of
a separate iSCSI session, while being third parties to
the action that caused the side-effects. One example of
a Third-party session is an iSCSI session hosting an
I_T_L nexus to an LU that is reset with an LU Reset TMF
via a separate I_T nexus.
1.2 Acronyms
Acronym Definition
-------------------------------------------------------------
EDTL Expected Data Transfer Length
IANA Internet Assigned Numbers Authority
IETF Internet Engineering Task Force
I/O Input - Output
IP Internet Protocol
iSCSI Internet SCSI
iSER iSCSI Extensions for RDMA
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ITT Initiator Task Tag
LO Leading Only
LU Logical Unit
LUN Logical Unit Number
PDU Protocol Data Unit
RDMA Remote Direct Memory Access
R2T Ready To Transfer
R2TSN Ready To Transfer Sequence Number
RFC Request For Comments
SAM SCSI Architecture Model
SCSI Small Computer Systems Interface
SN Sequence Number
SNACK Selective Negative Acknowledgment - also
Sequence Number Acknowledgement for data
TCP Transmission Control Protocol
TMF Task Management Function
TTT Target Transfer Tag
UA Unit Attention
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2 Introduction
Several iSCSI implementations had been built after [RFC3720] was
published and the iSCSI community is now richer by the resulting
implementation expertise. The goal of this document is to
leverage this expertise both to offer clarifications to the
[RFC3720] semantics and to address defects in [RFC3720] as
appropriate. This document intends to offer critical guidance
to implementers with regard to non-obvious iSCSI implementation
aspects so as to improve interoperability and accelerate iSCSI
adoption. This document, however, does not purport to be an
all-encompassing iSCSI how-to guide for implementers, nor a
complete revision of [RFC3720]. This document instead is
intended as a companion document to [RFC3720] for the iSCSI
implementers.
iSCSI implementers are required to reference [RFC3722] and
[RFC3723] in addition to [RFC3720] for mandatory requirements.
In addition, [RFC3721] also contains useful information for
iSCSI implementers. The text in this document, however, updates
and supersedes the text in [RFC3720] and [RFC3721] whenever
there is such a question.
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3 iSCSI semantics for SCSI tasks
3.1 Residual handling
Section 10.4.1 of [RFC3720] defines the notion of "residuals"
and specifies how the residual information should be encoded
into the SCSI Response PDU in Counts and Flags fields. Section
3.1.1 clarifies the intent of [RFC3720] and explains the general
principles. Section 3.1.2 describes the residual handling in
the REPORT LUNS scenario.
3.1.1 Overview
SCSI-Presented Data Transfer Length (SPDTL) is the term this
document uses (see section 1.1 for definition) to represent the
aggregate data length that the target SCSI layer attempts to
transfer using the local iSCSI layer for a task. Expected Data
Transfer Length (EDTL) is the iSCSI term that represents the
length of data that the iSCSI layer expects to transfer for a
task. EDTL is specified in the SCSI Command PDU.
When SPDTL = EDTL for a task, the target iSCSI layer completes
the task with no residuals. Whenever SPDTL differs from EDTL
for a task, that task is said to have a residual.
If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in
the SCSI Response PDU as specified in [RFC3720]. Residual Count
MUST be set to the numerical value of (SPDTL - EDTL).
If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in
the SCSI Response PDU as specified in [RFC3720]. Residual Count
MUST be set to the numerical value of (EDTL - SPDTL).
Note that the Overflow and Underflow scenarios are independent
of Data-in and Data-out. Either scenario is logically possible
in either direction of data transfer.
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3.1.2 SCSI REPORT LUNS and Residual Overflow
This section discusses the residual overflow issues citing the
example of SCSI REPORT LUNS command. Note however that there
are several SCSI commands (e.g. INQUIRY) with ALLOCATION LENGTH
fields following the same underlying rules. The semantics in
the rest of the section apply to all such SCSI commands.
The specification of the SCSI REPORT LUNS command requires that
the SCSI target limit the amount of data transferred to a
maximum size (ALLOCATION LENGTH) provided by the initiator in
the REPORT LUNS CDB. If the Expected Data Transfer Length
(EDTL) in the iSCSI header of the SCSI Command PDU for a REPORT
LUNS command is set to at least as large as that ALLOCATION
LENGTH, the SCSI layer truncation prevents an iSCSI Residual
Overflow from occurring. A SCSI initiator can detect that such
truncation has occurred via other information at the SCSI layer.
The rest of the section elaborates this required behavior.
iSCSI uses the (O) bit (bit 5) in the Flags field of the SCSI
Response and the last SCSI Data-In PDUs to indicate that that an
iSCSI target was unable to transfer all of the SCSI data for a
command to the initiator because the amount of data to be
transferred exceeded the EDTL in the corresponding SCSI Command
PDU (see Section 10.4.1 of [RFC3720]).
The SCSI REPORT LUNS command requests a target SCSI layer to
return a logical unit inventory (LUN list) to the initiator SCSI
layer (see section 6.21 of SPC-3 [SPC3]). The size of this LUN
list may not be known to the initiator SCSI layer when it issues
the REPORT LUNS command; to avoid transfer of more LUN list data
than the initiator is prepared for, the REPORT LUNS CDB contains
an ALLOCATION LENGTH field to specify the maximum amount of data
to be transferred to the initiator for this command. If the
initiator SCSI layer has under-estimated the number of logical
units at the target, it is possible that the complete logical
unit inventory does not fit in the specified ALLOCATION LENGTH.
In this situation, section 4.3.3.6 in [SPC3] requires that the
target SCSI layer "shall terminate transfers to the Data-In
Buffer" when the number of bytes specified by the ALLOCATION
LENGTH field have been transferred.
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Therefore, in response to a REPORT LUNS command, the SCSI layer
at the target presents at most ALLOCATION LENGTH bytes of data
(logical unit inventory) to iSCSI for transfer to the initiator.
For a REPORT LUNS command, if the iSCSI EDTL is at least as
large as the ALLOCATION LENGTH, the SCSI truncation ensures that
the EDTL will accommodate all of the data to be transferred. If
all of the logical unit inventory data presented to the iSCSI
layer - i.e. the data remaining after any SCSI truncation - is
transferred to the initiator by the iSCSI layer, an iSCSI
Residual Overflow has not occurred and the iSCSI (O) bit MUST
NOT be set in the SCSI Response or final SCSI Data-Out PDU.
This is not a new requirement but is already required by the
combination of [RFC3720] with the specification of the REPORT
LUNS command in [SPC3]. If the iSCSI EDTL is larger than the
ALLOCATION LENGTH however in this scenario, note that the iSCSI
Underflow MUST be signaled in the SCSI Response PDU. An iSCSI
Underflow MUST also be signaled when the iSCSI EDTL is equal to
ALLOCATION LENGTH but the logical unit inventory data presented
to the iSCSI layer is smaller than ALLOCATION LENGTH.
The LUN LIST LENGTH field in the logical unit inventory (first
field in the inventory) is not affected by truncation of the
inventory to fit in ALLOCATION LENGTH; this enables a SCSI
initiator to determine that the received inventory is incomplete
by noticing that the LUN LIST LENGTH in the inventory is larger
than the ALLOCATION LENGTH that was sent in the REPORT LUNS CDB.
A common initiator behavior in this situation is to re-issue the
REPORT LUNS command with a larger ALLOCATION LENGTH.
3.2 R2T Ordering
Section 10.8 in [RFC3720] says the following:
The target may send several R2T PDUs. It, therefore, can have
a number of pending data transfers. The number of outstanding
R2T PDUs are limited by the value of the negotiated key
MaxOutstandingR2T. Within a connection, outstanding R2Ts MUST
be fulfilled by the initiator in the order in which they were
received.
The quoted [RFC3720] text was unclear on the scope of
applicability - either per task, or across all tasks on a
connection - and may be interpreted as either. This section is
intended to clarify that the scope of applicability of the
quoted text is a task. No R2T ordering relationship - either in
generation at the target or in fulfilling at the initiator -
across tasks is implied. I.e., outstanding R2Ts within a task
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MUST be fulfilled by the initiator in the order in which they
were received on a connection.
3.3 Model Assumptions for Response Ordering
Whenever an iSCSI session is composed of multiple connections,
the Response PDUs (task responses or TMF responses) originating
in the target SCSI layer are distributed onto the multiple
connections by the target iSCSI layer according to iSCSI
connection allegiance rules. This process generally may not
preserve the ordering of the responses by the time they are
delivered to the initiator SCSI layer. Since ordering is not
expected across SCSI responses anyway, this approach works fine
in the general case. However to address the special cases where
some ordering is desired by the SCSI layer, the following
"Response Fence" semantics are defined with respect to handling
SCSI response messages as they are handed off from the SCSI
protocol layer to the iSCSI layer.
3.3.1 Model Description
Target SCSI protocol layer hands off the SCSI response messages
to the target iSCSI layer by invoking the "Send Command
Complete" protocol data service ([SAM2], clause 5.4.2) and "Task
Management Function Executed" ([SAM2], clause 6.9) service. On
receiving the SCSI response message, iSCSI layer exhibits the
Response Fence behavior for certain SCSI response messages
(section 3.3.3 describes the specific instances where the
semantics must be realized). Whenever the Response Fence
behavior is required for a SCSI response message, the target
iSCSI layer MUST ensure that the following conditions are met in
delivering the response message to the initiator iSCSI layer:
(1) Response with Response Fence MUST chronologically be
delivered after all the "preceding" responses on the
I_T_L nexus, if the preceding responses are delivered at
all, to the initiator iSCSI layer.
(2) Response with Response Fence MUST chronologically be
delivered prior to all the "following" responses on the
I_T_L nexus.
The "preceding" and "following" notions refer to the order of
hand-off of a response message from the target SCSI protocol
layer to the target iSCSI layer.
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3.3.2 iSCSI Semantics with the Interface Model
Whenever the TaskReporting key (section 9.1) is negotiated to
ResponseFence or FastAbort for an iSCSI session and the Response
Fence behavior is required for a SCSI response message, the
target iSCSI layer MUST perform the actions described in this
section for that session.:
a) If it is a single-connection session, no special processing
is required. Standard SCSI Response PDU build and dispatch
process happens.
b) If it is a multi-connection session, target iSCSI layer
takes note of last-sent and unacknowledged StatSN on each
of the connections in the iSCSI session, and waits for
acknowledgement (SHOULD solicit for acknowledgement by way
of a Nop-In) of each such StatSN to clear the fence. SCSI
response with the Response Fence flag must be sent to the
initiator only after receiving acknowledgements for each of
the unacknowledged StatSNs.
c) Target iSCSI layer must wait for an acknowledgement of the
SCSI Response PDU that carried the response which the
target SCSI layer marked with the Response Fence flag. The
fence must be considered cleared after receiving the
acknowledgement.
d) All further status processing for the LU is resumed only
after clearing the fence. If any new responses for the
I_T_L nexus are received from the SCSI layer before the
fence is cleared, those Response PDUs must be held and
queued at the iSCSI layer until the fence is cleared.
3.3.3 Current List of Fenced Response Use Cases
This section lists the fenced response use cases that iSCSI
implementations must comply with. However, this is not an
exhaustive enumeration. It is expected that as SCSI protocol
specifications evolve, the specifications will specify when
response fencing is required on a case-by-case basis.
Whenever the TaskReporting key (section 9.1) is negotiated to
ResponseFence or FastAbort for an iSCSI session, target iSCSI
layer MUST assume that Response Fence flag is set by the target
SCSI layer on the following SCSI completion messages handed down
to it:
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1. The first completion message carrying the UA after the
multi-task abort on issuing and third-party sessions.
2. The TMF Response carrying the multi-task TMF Response on
the issuing session.
3. The completion message indicating ACA establishment on the
issuing session.
4. The first completion message carrying the ACA ACTIVE status
after ACA establishment on issuing and third-party
sessions.
5. The TMF Response carrying the Clear ACA response on the
issuing session.
6. The response to a PERSISTENT RESERVE OUT/PREEMPT AND ABORT
command
Note: Due to the absence of ACA-related fencing requirements in
[RFC3720], initiator implementations SHOULD NOT use ACA on
multi-connection iSCSI sessions to targets complying only with
[RFC3720]. Initiators which want to employ ACA on multi-
connection iSCSI sessions SHOULD first assess response fencing
behavior via negotiating for ResponseFence or FastAbort values
for the TaskReporting (section 9.1) key.
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4 Task Management
4.1 Requests Affecting Multiple Tasks
This section clarifies and updates the original text in section
10.6.2 of [RFC3720]. The clarified semantics (section 4.1.2)
are a superset of the protocol behavior required in the original
text and all iSCSI implementations MUST support the new
behavior. The updated semantics (section 4.1.3) on the other
hand are mandatory only when the new key TaskReporting (section
9.1) is negotiated to "FastAbort".
4.1.1 Scope of affected tasks
This section defines the notion of "affected tasks" in multi-
task abort scenarios. Scope definitions in this section apply
to both the clarified protocol behavior (section 4.1.2) and the
updated protocol behavior (section 4.1.3).
ABORT TASK SET: All outstanding tasks for the I_T_L nexus
identified by the LUN field in the ABORT TASK SET TMF
Request PDU.
CLEAR TASK SET: All outstanding tasks in the task set for
the LU identified by the LUN field in the CLEAR TASK SET
TMF Request PDU. See [SPC3] for the definition of a "task
set".
LOGICAL UNIT RESET: All outstanding tasks from all
initiators for the LU identified by the LUN field in the
LOGICAL UNIT RESET Request PDU.
TARGET WARM RESET/TARGET COLD RESET: All outstanding tasks
from all initiators across all LUs to which the TMF-issuing
session has access to on the SCSI target device hosting the
iSCSI session.
Usage: an "ABORT TASK SET TMF Request PDU" in the preceding text
is an iSCSI TMF Request PDU with the "Function" field set to
"ABORT TASK SET" as defined in [RFC3720]. Similar usage is
employed for other scope descriptions.
4.1.2 Clarified multi-task abort semantics
All iSCSI implementations MUST support the protocol behavior
defined in this section as the default behavior. The execution
of ABORT TASK SET, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET
WARM RESET, and TARGET COLD RESET TMF Requests consists of the
following sequence of actions in the specified order on the
specified party.
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The initiator iSCSI layer:
a. MUST continue to respond to each TTT received for the
affected tasks.
b. Should receive any responses that the target may provide
for some tasks among the affected tasks (may process them
as usual because they are guaranteed to have
chronologically originated prior to the TMF response).
c. Should receive the TMF Response concluding all the tasks in
the set of affected tasks.
The target iSCSI layer:
a. MUST wait for responses on currently valid target transfer
tags of the affected tasks from the issuing initiator. MAY
wait for responses on currently valid target transfer tags
of the affected tasks from third-party initiators.
b. MUST wait (concurrent with the wait in Step.a) for all
commands of the affected tasks to be received based on the
CmdSN ordering. SHOULD NOT wait for new commands on
third-party affected sessions - only the instantiated tasks
have to be considered for the purpose of determining the
affected tasks. In the case of target-scoped requests
(i.e. TARGET WARM RESET and TARGET COLD RESET), all the
commands that are not yet received on the issuing session
in the command stream however can be considered to have
been received with no command waiting period - i.e. the
entire CmdSN space up to the CmdSN of the task management
function can be "plugged".
c. MUST propagate the TMF request to and receive the response
from the target SCSI layer.
d. MUST address the Response Fence flag on the TMF Response on
issuing session as defined in 3.3.2.
e. MUST address the Response Fence flag on the first post-TMF
Response on third-party sessions as defined in 3.3.2. If
some tasks originate from non-iSCSI I_T_L nexuses then the
means by which the target ensures that all affected tasks
have returned their status to the initiator are defined by
the specific non-iSCSI transport protocol(s).
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Implementation note: Technically, the TMF servicing is complete
in Step.d. Data transfers corresponding to terminated tasks may
however still be in progress on third-party iSCSI sessions even
at the end of Step.e. TMF Response MUST NOT be sent by the
target iSCSI layer before the end of Step.d, and MAY be sent at
the end of Step.d despite these outstanding data transfers until
after Step.e.
4.1.3 Updated multi-task abort semantics
Protocol behavior defined in this section MUST be implemented by
all iSCSI implementations complying with this document.
Protocol behavior defined in this section MUST be exhibited by
iSCSI implementations on an iSCSI session when they negotiate
the TaskReporting (section 9.1) key to "FastAbort" on that
session. The execution of ABORT TASK SET, CLEAR TASK SET,
LOGICAL UNIT RESET, TARGET WARM RESET, and TARGET COLD RESET TMF
Requests consists of the following sequence of actions in the
specified order on the specified party.
The initiator iSCSI layer:
a. MUST NOT send any more Data-Out PDUs for affected tasks on
the issuing connection of the issuing iSCSI session once
the TMF is sent to the target.
b. Should receive any responses that the target may provide
for some tasks among the affected tasks (may process them
as usual because they are guaranteed to have
chronologically originated prior to the TMF response).
c. MUST respond to each Async Message PDU with AsyncEvent=5 as
defined in section 8.1.
d. Should receive the TMF Response concluding all the tasks in
the set of affected tasks.
The target iSCSI layer:
a. MUST wait for all commands of the affected tasks to be
received based on the CmdSN ordering on the issuing
session. SHOULD NOT wait for new commands on third-party
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affected sessions - only the instantiated tasks have to be
considered for the purpose of determining the affected
tasks. In the case of target-scoped requests (i.e. TARGET
WARM RESET and TARGET COLD RESET), all the commands that
are not yet received on the issuing session in the command
stream can be considered to have been received with no
command waiting period - i.e. the entire CmdSN space up to
the CmdSN of the task management function can be "plugged".
b. MUST propagate the TMF request to and receive the response
from the target SCSI layer.
c. MUST leave all active "affected TTTs" (i.e. active TTTs
associated with affected tasks) valid.
d. MUST send an Asynchronous Message PDU with AsyncEvent=5
(section 8.1) on:
i) each connection of each third-party session to which at
least one affected task is allegiant if
TaskReporting=FastAbort is operational on that third-
party session, and
ii) each connection except the issuing connection of the
issuing session that has at least one allegiant affected
task.
If there are multiple affected LUs (say due to a target
reset), then one Async Message PDU MUST be sent for each
such LU on each connection that has at least one allegiant
affected task. The LUN field in the Asynchronous Message
PDU MUST be set to match the LUN for each such LU.
e. MUST address the Response Fence flag on the TMF Response on
issuing session as defined in 3.3.2.
f. MUST address the Response Fence flag on the first post-TMF
Response on third-party sessions as defined in 3.3.2. If
some tasks originate from non-iSCSI I_T_L nexuses then the
means by which the target ensures that all affected tasks
have returned their status to the initiator are defined by
the specific non-iSCSI transport protocol(s).
g. MUST free up the affected TTTs (and STags, if applicable)
and the corresponding buffers, if any, once it receives
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each associated Nop-Out acknowledgement that the initiator
generated in response to each Async Message.
Implementation note: Technically, the TMF servicing is complete
in Step.e. Data transfers corresponding to terminated tasks may
however still be in progress even at the end of Step.f. TMF
Response MUST NOT be sent by the target iSCSI layer before the
end of Step.e, and MAY be sent at the end of Step.e despite
these outstanding Data transfers until Step.g. Step.g specifies
an event to free up any such resources that may have been
reserved to support outstanding data transfers.
4.1.3.1 Clearing effects update
Appendix F.1 of [RFC3720] specifies the clearing effects of
target and LU resets on "Incomplete TTTs" as "Y". This meant
that a target warm reset or a target cold reset or an LU reset
would clear the active TTTs upon completion. The
TaskReporting=FastAbort (section 9.1) semantics defined by this
section however do not guarantee that the active TTTs are
cleared by the end of the reset operations. In fact, the new
semantics are designed to allow clearing the TTTs in a "lazy"
fashion after the TMF Response is delivered. Thus, when
TaskReporting=FastAbort is operational on a session, the
clearing effects of reset operations on "Incomplete TTTs" is
"N".
4.1.4 Affected tasks shared across RFC3720 & FastAbort sessions
If an iSCSI target implementation is capable of supporting
TaskReporting=FastAbort functionality (section 9.1), it may end
up in a situation where some sessions have TaskReporting=RFC3720
operational (RFC3720 sessions) while some other sessions have
TaskReporting=FastAbort operational (FastAbort sessions) even
while accessing a shared set of affected tasks (section 4.1.1).
If the issuing session is a RFC3720 session, iSCSI target
implementation is FastAbort-capable and third-party affected
session is a FastAbort session, the following behavior SHOULD be
exhibited by the iSCSI target layer:
a. Between steps c and d of target behavior in section 4.1.2,
send an Asynchronous Message PDU with AsyncEvent=5 (section
8.1) on each connection of each third-party session to
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which at least one affected task is allegiant. If there
are multiple affected LUs, then send one Async Message PDU
for each such LU on each connection that has at least one
allegiant affected task. When sent, the LUN field in the
Asynchronous Message PDU MUST be set to match the LUN for
each such LU.
b. After step e of target behavior in section 4.1.2, free up
the affected TTTs (and STags, if applicable) and the
corresponding buffers, if any, once each associated Nop-Out
acknowledgement is received that the third-party initiator
generated in response to each Async Message sent in step a.
If the issuing session is a FastAbort session, iSCSI target
implementation is FastAbort-capable and third-party affected
session is a RFC3720 session, the following behavior MUST be
exhibited by the iSCSI target layer: Asynchronous Message PDUs
MUST NOT be sent on the third-party session to prompt the
FastAbort behavior.
If the third-party affected session is a FastAbort session and
issuing session is a FastAbort session, initiator in the third-
party role MUST respond to each Async Message PDU with
AsyncEvent=5 as defined in section 8.1. Note that an initiator
MAY thus receive these Async Messages on a third-party affected
session even if the session is a single-connection session.
4.1.5 Implementation considerations
Both in clarified semantics (section 4.1.2) and updated
semantics (section 4.1.3), there may be outstanding data
transfers even after the TMF completion is reported on the
issuing session. In the case of iSCSI/iSER [iSER], these would
be tagged data transfers for STags not owned by any active
tasks. Whether or not real buffers support these data transfers
is implementation-dependent. However, the data transfers
logically MUST be silently discarded by the target iSCSI layer
in all cases. A target MAY, on an implementation-defined
internal timeout, also choose to drop the connections on which
it did not receive the expected Data-out sequences (section
4.1.2) or Nop-Out acknowledgements (section 4.1.3) so as to
reclaim the associated buffer, STag and TTT resources as
appropriate.
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4.1.6 Rationale behind the new semantics
There are fundamentally three basic objectives behind the
semantics specified in section 4.1.2 and section 4.1.3.
1. Maintaining an ordered command flow I_T nexus abstraction
to the target SCSI layer even with multi-connection
sessions.
o Target iSCSI processing of a TMF request must maintain
the single flow illusion. Target behavior in Step.b
of section 4.1.2 and Step.a of section 4.1.3
correspond to this objective.
2. Maintaining a single ordered response flow I_T nexus
abstraction to the initiator SCSI layer even with multi-
connection sessions when one response (i.e. TMF response)
could imply the status of other unfinished tasks from the
initiator's perspective.
o Target must ensure that the initiator does not see
"old" task responses (that were placed on the wire
chronologically earlier than the TMF Response) after
seeing the TMF response. Target behavior in Step.d of
section 4.1.2 and Step.e of section 4.1.3 correspond
to this objective.
o Whenever the result of a TMF action is visible across
multiple I_T_L nexuses, [SAM2] requires the SCSI
device server to trigger a UA on each of the other
I_T_L nexuses. Once an initiator is notified of such
an UA, the application client on the receiving
initiator is required to clear its task state (clause
5.5 in [SAM2]) for the affected tasks. It would thus
be inappropriate to deliver a SCSI Response for a task
after the task state is cleared on the initiator, i.e.
after the UA is notified. The UA notification
contained in the first SCSI Response PDU on each
affected Third-party I_T_L nexus after the TMF action
thus MUST NOT pass the affected task responses on any
of the iSCSI sessions accessing the LU. Target
behavior in Step.e of section 4.1.2 and Step.f of
section 4.1.3 correspond to this objective.
3. Draining all active TTTs corresponding to affected tasks
in a deterministic fashion.
o Data-out PDUs with stale TTTs arriving after the tasks
are terminated can create a buffer management problem
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even for traditional iSCSI implementations, and is
fatal for the connection for iSCSI/iSER
implementations. Either the termination of affected
tasks should be postponed until the TTTs are retired
(as in Step.a of section 4.1.2), or the TTTs and the
buffers should stay allocated beyond task termination
to be deterministically freed up later (as in Step.c
and Step.g of section 4.1.3).
The only other notable optimization is the plugging. If all
tasks on an I_T nexus will be aborted anyway (as with a target
reset), there is no need to wait to receive all commands to plug
the CmdSN holes. Target iSCSI layer can simply plug all missing
CmdSN slots and move on with TMF processing. The first
objective (maintaining a single ordered command flow) is still
met with this optimization because target SCSI layer only sees
ordered commands.
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5 Discovery semantics
5.1 Error Recovery for Discovery Sessions
The negotiation of the key ErrorRecoveryLevel is not required
for Discovery sessions - i.e. for sessions that negotiated
"SessionType=Discovery" - because the default value of 0 is
necessary and sufficient for Discovery sessions. It is however
possible that some legacy iSCSI implementations might attempt to
negotiate the ErrorRecoveryLevel key on Discovery sessions.
When such a negotiation attempt is made by the remote side, a
compliant iSCSI implementation MUST propose a value of 0 (zero)
in response. The operational ErrorRecoveryLevel for Discovery
sessions thus MUST be 0. This naturally follows from the
functionality constraints [RFC3720] imposes on Discovery
sessions.
5.2 Reinstatement Semantics of Discovery Sessions
Discovery sessions are intended to be relatively short-lived.
Initiators are not expected to establish multiple Discovery
sessions to the same iSCSI Network Portal (see [RFC3720]). An
initiator may use the same iSCSI Initiator Name and ISID when
establishing different unique sessions with different targets
and/or different portal groups. This behavior is discussed in
Section 9.1.1 of [RFC3720] and is, in fact, encouraged as
conservative reuse of ISIDs. ISID RULE in [RFC3720] states that
there must not be more than one session with a matching 4-tuple:
<InitiatorName, ISID, TargetName, TargetPortalGroupTag>. While
the spirit of the ISID RULE applies to Discovery sessions the
same as it does for Normal sessions, note that some Discovery
sessions differ from the Normal sessions in two important
aspects:
Because [RFC3720] allows a Discovery session to be
established without specifying a TargetName key in the
Login Request PDU (let us call such a session an "Unnamed"
Discovery session), there is no Target Node context to
enforce the ISID RULE.
Portal Groups are defined only in the context of a Target
Node. When the TargetName key is NULL-valued (i.e. not
specified), the TargetPortalGroupTag thus cannot be
ascertained to enforce the ISID RULE.
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The following sections describe the two scenarios - Named
Discovery sessions and Unnamed Discovery sessions - separately.
5.2.1 Unnamed Discovery Sessions
For Unnamed Discovery sessions, neither the TargetName nor the
TargetPortalGroupTag is available to the targets in order to
enforce the ISID RULE. So the following rule applies.
UNNAMED ISID RULE: Targets MUST enforce the uniqueness of the
following 4-tuple for Unnamed Discovery sessions:
<InitiatorName, ISID, NULL, TargetAddress>. The following
semantics are implied by this uniqueness requirement.
Targets SHOULD allow concurrent establishment of one Discovery
session with each of its Network Portals by the same initiator
port with a given iSCSI Node Name and an ISID. Each of the
concurrent Discovery sessions, if established by the same
initiator port to other Network Portals, MUST be treated as
independent sessions - i.e. one session MUST NOT reinstate the
other.
A new Unnamed Discovery session that has a matching
<InitiatorName, ISID, NULL, TargetAddress> to an existing
discovery session MUST reinstate the existing Unnamed Discovery
session. Note thus that only an Unnamed Discovery session may
reinstate an Unnamed Discovery session.
5.2.2 Named Discovery Sessions
For a Named Discovery session, the TargetName key is specified
by the initiator and thus the target can unambiguously ascertain
the TargetPortalGroupTag as well. Since all the four elements
of the 4-tuple are known, the ISID RULE MUST be enforced by
targets with no changes from [RFC3720] semantics. A new session
with a matching <InitiatorName, ISID, TargetName,
TargetPortalGroupTag> thus will reinstate an existing session.
Note in this case that any new iSCSI session (Discovery or
Normal) with the matching 4-tuple may reinstate an existing
Named Discovery iSCSI session.
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5.3 Target PDUs during Discovery
Targets SHOULD NOT send any responses other than a Text Response
and Logout Response on a Discovery session, once in full feature
phase.
Implementation Note: A target may simply drop the connection in
a Discovery session when it would have requested a Logout via an
Async Message on Normal sessions.
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6 Negotiation and Others
6.1 TPGT Values
[SAM2] and [SAM3] specifications incorrectly note in their
informative text that TPGT value should be non-zero, although
[RFC3720] allows the value of zero for TPGT. This section is to
clarify that zero value is expressly allowed as a legal value
for TPGT. This discrepancy currently stands corrected in
[SAM4].
6.2 SessionType Negotiation
During the Login phase, the SessionType key is offered by the
initiator to choose the type of session it wants to create with
the target. The target may accept or reject the offer.
Depending on the type of the session, a target may decide on
resources to allocate and the security to enforce etc. for the
session. If the SessionType key is thus going to be offered as
"Discovery", it SHOULD be offered in the initial Login request
by the initiator.
6.3 Understanding NotUnderstood
[RFC3720] defines NotUnderstood as a valid answer during a
negotiation text key exchange between two iSCSI nodes.
NotUnderstood has the reserved meaning that the sending side did
not understand the key semantics. This section seeks to clarify
that NotUnderstood is a valid answer for both declarative and
negotiated keys. The general iSCSI philosophy is that
comprehension precedes processing for any iSCSI key. A proposer
of an iSCSI key, negotiated or declarative, in a text key
exchange MUST thus be able to properly handle a NotUnderstood
response.
The proper way to handle a NotUnderstood response varies
depending on the lineage and type of the key. All keys defined
in [RFC3720] MUST be supported by all compliant implementations;
a NotUnderstood answer on any of the [RFC3720] keys therefore
MUST be considered a protocol error and handled accordingly.
For all other later keys, a NotUnderstood answer concludes the
negotiation for a negotiated key whereas for a declarative key,
a NotUnderstood answer simply informs the declarer of lack of
comprehension by the receiver. In either case, a NotUnderstood
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answer always requires that the protocol behavior associated
with that key be not used within the scope of the key
(connection/session) by either side.
6.4 Outstanding Negotiation Exchanges
There was some uncertainty around the number of outstanding
Login Response PDUs on a connection. [RFC3720] offers the
analogy of SCSI linked commands to Login and Text negotiations
in sections 5.3 and 10.10.3 respectively, but does not make it
fully explicit. This section is to offer a clarification in
this regard.
There MUST NOT be more than one outstanding Login Request or
Login Response or Text Request or Text Response PDU on an iSCSI
connection. An outstanding PDU in this context is one that has
not been acknowledged by the remote iSCSI side.
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7 iSCSI Error Handling and Recovery
7.1 ITT
Section 10.19 in [RFC3720] mentions this in passing but noted
here again for making it obvious since the semantics apply to
the initiators in general. An ITT value of 0xffffffff is
reserved and MUST NOT be assigned for a task by the initiator.
The only instance it may be seen on the wire is in a target-
initiated NOP-In PDU (and in the initiator response to that PDU
if necessary).
7.2 Format Errors
Section 6.6 of [RFC3720] discusses format error handling. This
section elaborates on the "inconsistent" PDU field contents
noted in [RFC3720].
All initiator-detected PDU construction errors MUST be
considered as format errors. Some examples of such errors are:
- NOP-In with a valid TTT but an invalid LUN
- NOP-In with a valid ITT (i.e. a NOP-In response) and also a
valid TTT
- SCSI Response PDU with Status=CHECK CONDITION, but
DataSegmentLength = 0
7.3 Digest Errors
Section 6.7 of [RFC3720] discusses digest error handling. It
states that "No further action is necessary for initiators if
the discarded PDU is an unsolicited PDU (e.g., Async, Reject)"
on detecting a payload digest error. This is incorrect.
An Asynchronous Message PDU or a Reject PDU carries the next
StatSN value on an iSCSI connection, advancing the StatSN. When
an initiator discards one of these PDUs due to a payload digest
error, the entire PDU including the header MUST be discarded.
Consequently, the initiator MUST treat the exception like a loss
of any other solicited response PDU - i.e. it MUST use one of
the following options noted in [RFC3720]:
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a) Request PDU retransmission with a status SNACK.
b) Logout the connection for recovery and continue the
tasks on a different connection instance.
c) Logout to close the connection (abort all the commands
associated with the connection).
7.4 Message Error Checking
There has been some uncertainty on the extent to which incoming
messages have to be checked for protocol errors, beyond what is
strictly required for processing the inbound message. This
section addresses that question.
Unless [RFC3720] or this draft requires it, an iSCSI
implementation is not required to do an exhaustive protocol
conformance checking on an incoming iSCSI PDU. The iSCSI
implementation especially is not required to double-check the
remote iSCSI implementation's conformance to protocol
requirements.
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8 iSCSI PDUs
8.1 Asynchronous Message
This section defines additional semantics for the Asynchronous
Message PDU defined in section 10.9 of [RFC3720] using the same
conventions.
The following new legal value for AsyncEvent is defined:
5: all active tasks for LU with matching LUN field in the Async
Message PDU are being terminated.
The receiving initiator iSCSI layer MUST respond to this Message
by taking the following steps in order.
i) Stop Data-Out transfers on that connection for all active
TTTs for the affected LUN quoted in the Async Message
PDU.
ii) Acknowledge the StatSN of the Async Message PDU via a
Nop-Out PDU with ITT=0xffffffff (i.e. non-ping flavor),
while copying the LUN field from Async Message to Nop-
Out.
8.2 Reject
Section 10.17.1 of [RFC3720] specifies the Reject reason code of
0x0b with an explanation of "Negotiation Reset". At this point,
we do not see any legitimate iSCSI protocol use case for using
this reason code. Thus reason code 0x0b MUST be considered as
deprecated and MUST NOT be used by any new implementations.
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9 Login/Text Operational Text Keys
This section follows the same conventions as section 12 of
[RFC3720].
9.1 TaskReporting
Use: LO
Senders: Initiator and Target
Scope: SW
Irrelevant when: SessionType=Discovery
TaskReporting=<list-of-values>
Default is RFC3720.
Result function is AND.
This key is used to negotiate the task completion reporting
semantics from the SCSI target. Following table describes the
semantics an iSCSI target MUST support for respective negotiated
key values. Whenever this key is negotiated, at least the
RFC3720 and ResponseFence values MUST be offered as options by
the negotiation originator.
+--------------+------------------------------------------+
| Name | Description |
+--------------+------------------------------------------+
| RFC3720 | RFC 3720-compliant semantics. Response |
| | fencing is not guaranteed and fast |
| | completion of multi-task aborting is not |
| | supported |
+--------------+------------------------------------------+
| ResponseFence| Response Fence (section 3.3.1) semantics |
| | MUST be supported in reporting task |
| | completions |
+--------------+------------------------------------------+
| FastAbort | Updated fast multi-task abort semantics |
| | defined in section 4.1.3 MUST be |
| | supported. Support for Response Fence is|
| | implied - i.e. section 3.3.1 semantics |
| | MUST be supported as well |
+--------------+------------------------------------------+
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When TaskReporting is not negotiated to FastAbort, the default
behavior is to use the [RFC3720] TMF semantics as clarified in
section 4.1.2.
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10 Security Considerations
This document does not introduce any new security considerations
other than those already noted in [RFC3720]. Consequently, all
the iSCSI-related security text in [RFC3723] is also directly
applicable to this document.
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11 IANA Considerations
This draft does not have any specific IANA considerations.
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12 References and Bibliography
12.1 Normative References
[RFC3720] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka,
M., and E. Zeidner, "Internet Small Computer Systems
Interface (iSCSI)", RFC 3720, April 2004.
[RFC3721] Bakke, M., Hafner, J., Hufferd, J., Voruganti, K.,
and M. Krueger, "Internet Small Computer Systems Interface
(iSCSI) Naming and Discovery", RFC 3721, April 2004.
[SPC3] T10/1416-D, SCSI Primary Commands-3.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
12.2 Informative References
[RFC3723] Aboba, B., Tseng, J., Walker, J., Rangan, V., and
F. Travostino, "Securing Block Storage Protocols over IP",
RFC 3723, April 2004.
[RFC3722] Bakke, M., "String Profile for Internet Small
Computer Systems Interface (iSCSI) Names", RFC 3722, April
2004.
[iSER] Ko, M., Chadalapaka, M., Elzur, U., Shah, H., Thaler,
P., J. Hufferd, "iSCSI Extensions for RDMA", IETF Internet
Draft draft-ietf-ips-iser-04.txt (work in progress), June
2005.
[RFC2119] Bradner, S. "Key Words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SAM2] ANSI INCITS 366-2003, SCSI Architecture Model-2 (SAM-
2).
[SAM3] ANSI INCITS 402-2005, SCSI Architecture Model-3 (SAM-
3).
[SAM4] T10 Project: 1683-D, SCSI Architecture Model-4 (SAM-
4), Work in Progress.
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13 Editor's Address
Mallikarjun Chadalapaka
Hewlett-Packard Company
8000 Foothills Blvd.
Roseville, CA 95747-5668, USA
Phone: +1-916-785-5621
E-mail: cbm@rose.hp.com
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14 Acknowledgements
The IP Storage (ips) Working Group in the Transport Area of
IETF has been responsible for defining the iSCSI protocol
(apart from a host of other relevant IP Storage protocols).
The editor acknowledges the contributions of the entire
working group.
The following individuals directly contributed to identifying
[RFC3720] issues and/or suggesting resolutions to the issues
clarified in this document: David Black (REPORT LUNS/overflow
semantics, ACA semantics, TMF semantics), Gwendal Grignou
(TMF scope), Mike Ko (digest error handling for Asynchronous
Message), Dmitry Fomichev (reserved ITT), Bill Studenmund
(residual handling, discovery semantics), Ken Sandars
(discovery semantics), Bob Russell (discovery semantics),
Julian Satran (discovery semantics, TMF semantics), Rob
Elliott (T10 liaison, R2T ordering), Joseph Pittman(TMF
scope), Somesh Gupta (multi-task abort semantics), Eddy
Quicksall (message error checking), Paul Koning (message
error checking). This document benefited from all these
contributions.
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15 Full Copyright Statement
Copyright (C) The IETF Trust (2007). This document is
subject to the rights, licenses and restrictions contained in
BCP 78, and except as set forth therein, the authors retain
all their rights.
This document and the information contained herein are
provided on an "AS IS" basis and THE CONTRIBUTOR, THE
ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY),
THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE
USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
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16 Intellectual Property Statement
The IETF takes no position regarding the validity or scope of
any Intellectual Property Rights or other rights that might
be claimed to pertain to the implementation or use of the
technology described in this document or the extent to which
any license under such rights might or might not be
available; nor does it represent that it has made any
independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can
be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and
any assurances of licenses to be made available, or the
result of an attempt made to obtain a general license or
permission for the use of such proprietary rights by
implementers or users of this specification can be obtained
from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its
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other proprietary rights that may cover technology that may
be required to implement this standard. Please address the
information to the IETF at ietf-ipr@ietf.org.
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