One document matched: draft-ietf-ips-iscsi-impl-guide-02.txt
Differences from draft-ietf-ips-iscsi-impl-guide-01.txt
INTERNET DRAFT Mallikarjun Chadalapaka
draft-ietf-ips-iscsi-impl-guide-02.txt Hewlett-Packard Co.
Editor
Expires September 2006
iSCSI Implementer's Guide
Status of this Memo
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Abstract
iSCSI is a SCSI transport protocol and maps the SCSI family
of application protocols onto TCP/IP. RFC 3720 defines the
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 ...............................3
1.1 Definitions ............................................3
1.2 Acronyms ...............................................3
2 Introduction ...........................................5
3 iSCSI semantics for SCSI tasks .........................6
3.1 Residual handling ......................................6
3.1.1 Overview..............................................6
3.1.2 SCSI REPORT LUNS and Residual Overflow................7
3.2 R2T Ordering ...........................................8
3.3 SCSI Protocol Interface Model for Response Ordering ....8
3.3.1 Model Description.....................................9
3.3.2 iSCSI Semantics with the Interface Model..............9
3.3.3 Current List of Fenced Response Use Cases............10
4 Task Management .......................................12
4.1 Requests Affecting Multiple Tasks .....................12
4.1.1 Scope of affected tasks..............................12
4.1.2 Clarified multi-task abort semantics.................12
4.1.3 Updated multi-task abort semantics...................14
4.1.4 Rationale behind the new semantics...................16
5 Discovery semantics ...................................18
5.1 Error Recovery for Discovery Sessions .................18
5.2 Reinstatement Semantics of Discovery Sessions .........18
5.2.1 Unnamed Discovery Sessions...........................19
5.2.2 Named Discovery Sessions.............................19
5.3 TPGT Values ...........................................20
5.4 Session type negotiation ..............................20
6 iSCSI Error Handling and Recovery .....................21
6.1 ITT ...................................................21
6.2 Format Errors .........................................21
6.3 Digest Errors .........................................21
7 iSCSI PDUs ............................................23
7.1 Asynchronous Message ..................................23
8 Login/Text Operational Text Keys ......................24
8.1 FastMultiTaskAbort ....................................24
9 Security Considerations ...............................25
10 IANA Considerations ...................................26
11 References and Bibliography ...........................27
11.1 Normative References.................................27
11.2 Informative References...............................27
12 Editor's Address ......................................28
13 Acknowledgements ......................................29
14 Full Copyright Statement ..............................30
15 Intellectual Property Statement .......................31
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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 took 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 all the noted RFCs 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 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
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.
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
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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 [RFC 3720] 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
MUST be fulfilled by the initiator in the order in which they
were received on a connection.
3.3 SCSI Protocol Interface Model for Response Ordering
Whenever an iSCSI session is composed of multiple connections,
the Response PDUs (task responses or TMF responses) originating
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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 SCSI
protocol interface model is assumed.
3.3.1 Model Description
SCSI protocol layer instructs the SCSI transport layer of a
"Response Fence" associated with the response in question when
the "Send Command Complete" protocol data service (SAM-2, clause
5.4.2) and "Task Management Function Executed" (SAM-2, clause
6.9) service are invoked. The Response Fence flag instructs the
SCSI transport layer that the following conditions must be met
in delivering the response message:
(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 application client on the initiator.
(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 SCSI transport (e.g. iSCSI) layer.
3.3.2 iSCSI Semantics with the Interface Model
The target iSCSI layer MUST do the following on sensing the
"Response Fence" flag associated with a response being handed
down from the target SCSI layer:
a) If it is a single-connection session, no special processing
is required. Standard SCSI Response PDU build 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 (may 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
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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.
Response Fence flag MUST be assumed set by the target SCSI layer
on the following SCSI completion messages handed down to the
target iSCSI layer:
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 mult-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.
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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], i.e. those not complying with this document.
Initiators may assess target compliance to this document via
negotiating for FastMultiTaskAbort (section 8.1) 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 FastMultiTaskAbort
(section 8.1) is negotiated to "Yes".
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 that 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 all currently valid target transfer tags of
the affected tasks to be responded to.
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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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 FastMultiTaskAbort (section 8.1) key to "Yes" 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 Async Message PDU with AsyncEvent=5 as
defined in section 7.1.
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
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".
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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 along with any buffer
allocations for the TTTs intact.
d. MUST generate an Asynchronous Message PDU with AsyncEvent=5
(section 7.1) on:
i) each connection of each third-party session that at
least one affected task is allegiant to, and
ii) each connection except the non-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.
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 once it receives the
associated Nop-Out acknowledgement that the initiator
generated in response to the 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. In the case of iSCSI/iSER, these transfers would be
into tagged buffers with STags not owned by any active tasks.
Step.g specifies an event to free up the resources. A target
may, on an implementation-defined internal timeout, also
choose to drop the connections on which it did not receive the
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expected Nop-Out acknowledgements so as to reclaim the
associated buffer, STag and TTT resources as appropriate.
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
FastMultiTaskAbort 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 FastMultiTaskAbort=Yes is
operational on a session, the clearing effects of reset
operations on "Incomplete TTTs" is "N".
4.1.4 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
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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
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 TPGT Values
SAM-2 and SAM-3 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. A future revision of SAM will be corrected to address
this discrepancy.
5.4 Session type 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.
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6 iSCSI Error Handling and Recovery
6.1 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).
6.2 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
6.3 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).
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7 iSCSI PDUs
7.1 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 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.
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8 Login/Text Operational Text Keys
This section follows the same conventions as section 12 of
[RFC3720].
8.1 FastMultiTaskAbort
Use: LO
Senders: Initiator and Target
Scope: SW
Irrelevant when: SessionType=Discovery
FastMultiTaskAbort=<boolean-value>
Default is No.
Result function is AND.
This key is used to negotiate the updated fast multi-task abort
semantics defined in section 4.1.3. By negotiating this key to
"Yes", an initiator and a target agree that the new semantics
MUST be used in the multi-task TMF handling situations. The
default is to use the [RFC3720] TMF semantics as clarified in
section 4.1.2.
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9 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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10 IANA Considerations
This draft does not have any specific IANA considerations other
than those already noted in [RFC3720].
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11 References and Bibliography
11.1 Normative References
[RFC3720] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka,
M., and E. Zeidner, "Internet Small Computer Systems
Interface (iSCSI)", RFC 3720, April 2004.
[RFC3722] Bakke, M., "String Profile for Internet Small
Computer Systems Interface (iSCSI) Names", RFC 3722, April
2004.
[RFC3723] Aboba, B., Tseng, J., Walker, J., Rangan, V., and
F. Travostino, "Securing Block Storage Protocols over IP",
RFC 3723, April 2004.
[SPC3] T10/1416-D, SCSI Primary Commands-3.
11.2 Informative References
[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.
[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 X3.366-2003, SCSI Architecture Model-2 (SAM-2).
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12 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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13 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), 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), Rob Elliott (T10 liaison, R2T ordering), Joseph
Pittman(TMF scope), Somesh Gupta (multi-task abort
semantics). This document benefited from all these
contributions.
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14 Full Copyright Statement
Copyright (C) The Internet Society (2006). 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 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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15 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
attention any copyrights, patents or patent applications,
or 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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