Next FCL Error SSWG - 8/19/97

Neil Wanamaker ntw at crossroads.com
Mon Aug 18 14:10:56 PDT 1997


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Neil Wanamaker <ntw at crossroads.com>
*
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	The draft document for the Class 3 proposal is to be found on
ftp.network.com in directory FC/AL/incoming as files

	97189R4c.doc   (Word document)
	97189R4c.vsd   (Visio Drawing)

	Plain text is attached to this message for those with difficult
firewalls.

	Neil Wanamaker


>  

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Class 3 Error Detection and Recovery for Sequential Access Devices
Preliminary ANSI T10 Working Document 97-189R4c

Scope
Problems exist in PLDA in detecting and correcting error conditions on =
sequential access devices (tapes). The basic causes of these problems =
are due to the lack of a guaranteed delivery protocol and the implicit =
state information intrinsic to sequential access devices.  More =
specifically, lost frames in FCP can result in FC information units =
being lost.   ULP recovery is not sufficient for a variety of reasons, =
including an inability to detect such errors,  the effort required to =
implement recovery mechanisms, and the extended time required to detect =
and recover from error conditions.
Requirements
An ideal solution will incorporate the following characteristics:
	Provide the ability to recover from lost frames in FCP for sequential =
access devices
	Interoperability with block and sequential access devices
	No or minimal changes to FC-PH and PLDA
	No additional protocol overhead for normal operation=20
	Can be implemented with existing silicon=09
	Don't turn fiber transport errors into tape drive recovery
	Optimize for single sequence errors
	Don't add inefficiencies for multiple-sequence errors

Problem Analysis
On stream and media changer devices there are two classes of commands =
for which it is critical to know whether the command was accepted by =
the target, and then whether successful completion of the command =
occurred.=20
The first class, unique to these devices, are those that alter the =
media state or content in a way that simply re-executing the command =
will not recover the error. These include read/write/position/write =
filemarks (the tape is repositioned past the referenced block(s) or =
files only if the operation started; how far the operation continued is =
critical to proper recovery) and move medium/load/unload medium (which =
may have actually changed the medium in the target). Unfortunately, =
these comprise most of the commands issued during normal operation of =
the subsystem.
The second class, which is not unique to these devices, are those in =
which information is lost if it is presumed sent by the target, but not =
received by the initiator. These commands include request sense and =
read/reset log. Loss of sense data also may affect error recovery from =
failed commands of the aforementioned media move/change class, but it =
may also affect proper error recovery for cached/RAID disk controllers =
as well.
On a parallel SCSI bus, the host adapter has positive confirmation that =
the target accepted the command by the fact that the target requested =
all bytes of the CDB and continued to the next phase without a Restore =
Pointers message. Such confirmation is only implicit in a serial =
protocol by receipt of a response message, such as Transfer_Ready or =
Response. In cases of some commands, this implicit confirmation may =
require a lengthy period of time, during which mechanical movement =
requiring several multiples of E_D_TOV occurs (in FLA environments, =
R_A_TOV may be the appropriate value).  Similarly, the target has =
positive confirmation that the host has accepted sense or log data =
immediately upon completion of the data and status phases; this data =
may now be reset. In a serial environment, this is only implicit by =
receipt of the next command. Note that a change to the target to only =
clear sense/log data on receipt of a command other than request sense =
or read/reset log would eliminate this problem.
In summary, the errors that are of concern are where FCP information =
units are lost in transit between an FCP initiator and target.  The =
cause for such loss is not specific, but is assumed to be cases where a =
link level connection is maintained between the target and initiator, =
and some number of FCP IU's are dropped.  Other cases are either =
handled by PLDA through existing methods, or may be generally =
classified as unrecoverable and treated in a fashion similar to a SCSI =
bus reset.
In order to meet the defined requirements, any proposed solution must =
enable the initiator to make the following determinations:
	An error condition occurred (an FCP IU is expected and not received, =
or not responded to)
	If FCP_CMND, was it received by the target
	If FCP_DATA, was it received or sent by target
	If FCP XFER_RDY or FCP_RSP, was it sent by target
Note that the solution must work in a Class 3 environment, preferably =
with no change to existing hardware.
Tools For Solution
The tools prescribed in FC-PH for FC-2 recovery are the Read Exchange =
Status  (RES), and Read Sequence Status (RSS) Extended Link Services, =
and the Abort Sequence (ABTS) Basic Link Service.
We have identified several functions providing some of the needed =
functionality,  these are listed below, along with deficiencies we have =
identified in each of the existing mechanisms.  We are proposing =
additional ELS functions to provide the required functionality.
RES  (Read Exchange Status) provides some of the required =
functionality; its function is to inquire of the status of an operation =
during and for some period of time after its life. Unfortunately, in =
several  of the cases of interest, the  RX_ID is unknown to the =
exchange initiator. In these cases, the initiator must use an RX_ID of =
0xFFFF, which, combined with the  FC_PH wording  that "...the Responder =
destination N_PORT would use RX_ID and ignore the OX_ID", means that if =
the Responder had not received the command frame, the RES would be =
rejected, and if the Responder had received the command and sent an =
FCP_RSP response frame, the RES would be rejected,  in both cases with =
the same reason code; only in the case where the command was in process =
but no FCP_RSP response frame had been sent by the Responder would a =
useful response be sent. Real implementations appear to search for the =
S_ID - OX_ID  pair when the RX_ID is set to 0xFFFF in the RES request, =
and this behavior needs to become required. =20
Further, even if this change is implemented, in the case of a =
non-transfer command, it is impossible to detect the difference between =
a command that was never received and a command whose response was lost =
unless the target retains ESB information for a period of RA_TOV after =
the exchange is closed.
A third issue is the fact that the standard only requires that the =
Responder to the RES need only provide information on sequences =
received. Provision of status on sequences transmitted is optional.
A final issue is the fact that many existing silicon implementations =
hide sequence information from the firmware. Provision of information =
about SEQ_IDs, SEQ_CNTs, etc., is not available, and SEQ_IDs may not be =
unique within exchanges. This means that it is not possible to provide =
sequence-level information sufficient to perform error recovery.
Similar arguments apply to the use of the RSS (Read Sequence Status), =
though the wording of the applicable section  indicates that "...the =
Responder destination N_PORT may use RX_ID and ignore the OX_ID..."
ABTS, while recommended in FC-PH for use in polling for sequence =
delivery in Class 3, is always interpreted as an abort of the exchange =
in FC-PLDA, and is therefore not useful for this purpose.  In addition, =
 the ABTS requires the use of sequence identification and delivery =
information which is not made visible to firmware in some existing =
silicon implementations.

In view of these difficulties, we are proposing the addition of the =
Read Exchange Concise (REC), a new ELS which returns only the =
information required for error recovery. =20
Additionally,  there needs to be a mechanism for requesting =
retransmission of information  that was not received at the =
destination. We are proposing the addition of the System Retransmission =
Request (SRR),  a new ELS which provides a mechanism for  a sequence =
recipient to request retransmission of a missing IU. This is modified =
|from previous proposals by requesting this on the basis of  IU type and =
 relative offset, rather than by SEQ_ID and SEQ_CNT.=20
Proposed Solution
A method is proposed where the initiator determines the state of an =
exchange and initiates appropriate recovery.  A timer is used in =
conjunction with internal driver state information to determine if a =
target response is overdue,  indicating that frames or sequences may =
have been lost.  The initiator will then request exchange information =
|from the target  from which it can be determined if corrective action =
is necessary.  The initiator may then request that the target =
retransmit an IU, or provide early indication to the ULP that an error =
has occurred.
The timer is based on the maximum frame propagation delivery time =
through the fabric.  This is significantly less than typical ULP time =
out values, providing the capability to detect and correct errors =
before ULP actions take effect.  The suggested time out for FLA =
environments is twice R_A_TOV (currently 2 seconds).=20
 The method of determining target sequence state is by using the REC =
extended link service.  A target device supporting this ELS must =
maintain a limited amount of exchange context until the next command is =
received, making this appropriate for devices not supporting queueing.  =
The REC ELS Request and Reply sequences  are described later in this =
document.
Details of the recovery mechanism follow. For illustrative purposes, an =
FCP exchange is discussed; the mechanisms would apply to any FC-4, with =
the appropriate changes to the IU names and protocols. In order to =
reduce verbosity, the term "initiator" is used to denote Exchange =
Originator, and "target" to denote Exchange Responder.
After (2 x R_A_TOV) with no reply sequence received to the FCP_CMND_IU:
Issue REC for the exchange containing the FCP_CMND. The REC is issued =
in a new exchange.  If there is no ACC or LS_RJT response to the REC =
within 2*R_A_TOV, send ABTS to abort the exchange containing the REC, =
followed by an RRQ if the ABTS is ACCepted.  The REC shall  be retried =
at a rate not to exceed once per 2*R_A_TOV for at least 3 times.  If =
none of the RECs receives a response,  the initiator shall report an =
error condition to the ULP.
If the response is an LS_RJT, with a reason code indicating that the =
function is not supported,  as is required in PLDA for block devices, =
treat the target as a disk or other device not supporting this proposal =
and allow normal ULP recovery to occur.
 If the FCP_CMND was not received by the target (i.e., the initiator =
receives an LS_RJT for the REC, with a reason code indicating that the =
OX_ID is unknown), send ABTS to abort the original sequence/exchange, =
followed by an RRQ if the ABTS is ACCepted. Resend  the command (using =
a new OX_ID).=20
The target shall retain exchange information until the next command has =
been received.  In this way, the initiator may determine the difference =
between a command that was never received and one whose reply =
sequence(s) were lost.
If the ACC for an REC indicates that the FCP_CMND was received by the =
target, and that no reply sequence has been sent, the command is in =
process and no recovery is needed at this time.  At intervals of =
2*R_A_TOV the REC shall be retransmitted.  This is to ensure that no =
reply sequences have been lost. If at any time,  there is no reply to =
the REC,  an ABTS is sent for the REC, followed by an RRQ if the ABTS =
is ACCepted, and the REC is retried as specified above.
If the ACC for an REC indicates that an FCP_XFER_RDY was sent by the =
target (by indicating that the initiator holds sequence initiative, and =
that the exchange is not complete), but not received by the initiator, =
issue an SRR Extended Link Service (see below for details) frame to =
request retransmission of the FCP_XFER_RDY (R_CTL =3D data descriptor). =
 The target retransmits the FCP_XFER_RDY in a new sequence, and =
containing the appropriate Relative Offset. When the FCP_XFER_RDY is =
successfully received, the data is sent, and the operation continues =
normally. No error is reported to the ULP, though the error counters in =
the LESB should be updated.  If the SRR receives a LS_RJT,  perform =
sequence error recovery as documented in PLDA section 9.1, 9.3.
If an ACC for and REC indicates that data was sent by the target, but no=
t successfully received by the initiator ( by indicating a data sent =
count greater than the initiator has successfully received), issue an =
SRR Extended Link Service frame to request retransmission of the =
FCP_DATA   (R_CTL =3D Solicited Data) that was not successfully =
received. The initiator passes the Relative Offset of the next data =
requested. The target retransmits the data in a new sequence. The =
received data is delivered to the ULP, and no error is reported  If the =
target responds to the SRR with an LS_RJT and a reason code indicating =
that the function could not be performed, the target shall present an =
FCP_RSP IU with an appropriate error status (e.g., Sense key 4, ASC/ASQ =
of 48/00 (Initiator Detected Error message received)).
If an ACC for an REC indicates that an FCP_RSP sequence was sent by the =
target, but not received by the initiator (E_STAT indicates the =
exchange is complete), issue an SRR Extended Link Service frame to =
request retransmission of the FCP_RSP IU. The target retransmits the =
FCP_RSP in a new sequence.  The response is delivered to the ULP, and =
no error is reported. If the SRR receives a LS_RJT,  perform sequence =
error recovery as documented in PLDA section 9.1, 9.3.
If the ACC for an REC indicates that an FCP_DATA sequence was sent by =
the initiator, but not received by the target (the data received count =
in the REC response is smaller than the initiator sent, and the target =
indicates that he does not hold sequence initiative), the responder =
issues an SRR Extended Link Service frame to request retransmission of =
the FCP_XFER_RDY.  As documented in PLDA Sec. 9.2, the target discards =
the sequence in error, but does not initiate any recovery action.  When =
the ACC is received for the SRR, the target transmits an FCP_XFER_RDY =
with the appropriate Relative Offset parameter, and the initiator =
responds with the requested data.  The operation should complete with =
no error indication to the ULP.
It is the responsibility of the initiator to determine the appropriate =
action (retry, allow ULP time out, or return status to ULP) required =
based on the information determined by REC and other internal state.  =
As described in PLDA, the target does not initiate recovery action.

SRR Basic Link Service=20
The SRR (System Resend Request) Extended link service frame follows the =
rules for extended link services as defined in FC-PH Rev 4.3, Section =
23.1.  A new Link Service command code in R_CTL needs to be added to =
FC_PH. The next available value is 0001 0011b.
In the event that the target cannot accept this request, the target =
shall present a check condition as if it had not responded to an =
Initiator Detected Error with a Restore Pointers message (i.e., Sense =
Key =3D 4, ASC/ASQ =3D 48/00). The target shall not reject requests for =
retransmission of  FCP_XFER_RDY or FCP_RSP frames unless the SRR is not =
supported.=20
The SRR payload and reject codes are defined below.  The Accept does =
not require a payload.  The direction flag indicates to the target that =
the initiator is requesting sequence data transfer to (0) or from (1) =
the target.  All other fields are as defined in FC-PH.



                          Item

Size
Bytes
OX_ID
2
RX_ID
2
Relative Offset
4
R_CTL for IU
1
Direction (1 =3D I->T, 2 =3D T->I)
1
		SRR  Payload

Encoded Value
LS_RJT Reason code explanation
0x00052A00
Can't resend requested sequence
Reserved

		SRR LS_RJT Reason Codes
Read Exchange Concise (REC)
The REC Links Service request Sequence requests an N_Port to return =
exchange information for the RX_ID or OX_ID originated by the S_ID =
specified in the Payload of the request Sequence.  The specification of =
OX_ID and RX_ID may be useful or required information for the =
destination N_Port to locate the status information requested. A =
Responder destination N_Port would use the RX_ID and ignore the OX_ID, =
unless the RX_ID was undetermined (i.e., RX_ID =3D 0xffff). An =
Originator N_Port  would use the OX_ID and ignore the RX_ID.  This =
function provides the N_Port transmitting the request with information =
regarding the current status of the Exchange specified.
If the destination N_Port of the REC request determines that the =
SEQ_ID, Originator S_ID , OX_ID, or RX_ID are inconsistent, then it =
shall reply with an LS_RJT Sequence with a reason code that it is =
unable to perform the command request.=20
Protocol:=20
	Read Exchange Concise request Sequence
	Accept (ACC) reply Sequence
Format: FT_1
Addressing:
The S_ID field designates the source N_Port requesting the Exchange =
information. The D_ID field designates the destination N_Port to which =
the request is being made.
Payload:
The format of the Payload is shown in the following table. The Payload =
shall include an Association Header for the Exchange if the destination =
N_Port requires X_ID reassignment.
 REC Payload
Item=20
Size
-Bytes
Hex '13000000'
4
Reserved
1
Exchange Originator S_ID
3
OX_ID
2
RX_ID
2
Association Header
(optionally required)
32
Reply Link Service Sequence

	Service Reject (LS_RJT)
		Signifies rejection of the REC command.
	ACC
		Signifies that the N_Port has transmitted the requested data.
- Accept payload:
- The format of the Accept Payload is shown in the table below. The =
format of the Concise Exchange Status is specified in below.
=20
Note that for a sequence to be reported as received, the entire =
sequence must have been successfully received. For a sequence to be =
reported as transmitted, the entire sequence must have been =
successfully transmitted.
 REC Accept Payload
Item
Size
-Bytes
Hex '02000000'
4
Concise Exchange Status (see 24.8.xx)
N
Association Header
(optionally required)
32

 Concise Exchange Status

Item
Size
-Bytes
OX_ID
2
RX_ID
2
Originator Address Identifier (High order byte - reserved)
4
Responder Address Identifier (High order byte - reserved)
4
Data received count
4
Data transmitted count
4
E_STAT
4



=20
Crossroads Systems Inc.	1	Brian R. Smith


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