Small Computer Systems Interface protocol over the Internet (iSCSI)

Requirements and Design Considerations

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2002). All Rights Reserved.

Abstract

This document specifies the requirements iSCSI and its related infrastructure should satisfy and the design considerations guiding the iSCSI protocol development efforts. In the interest of timely adoption of the iSCSI protocol, the IPS group has chosen to focus the first version of the protocol to work with the existing SCSI architecture and commands, and the existing TCP/IP transport layer. Both these protocols are widely-deployed and well-understood. The thought is that using these mature protocols will entail a minimum of new invention, the most rapid possible adoption, and the greatest compatibility with Internet architecture, protocols, and equipment.

Conventions used in this document

   This document describes the requirements for a protocol design, but
   does not define a protocol standard.  Nevertheless, the key words
   "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in RFC-2119 [2].

Table of Contents

   1.   Introduction.................................................2
   2.   Summary of Requirements......................................3
   3.   iSCSI Design Considerations..................................7
   3.1. General Discussion...........................................7
   3.2. Performance/Cost.............................................9
   3.3. Framing.....................................................11
   3.4. High bandwidth, bandwidth aggregation.......................13
   4.   Ease of implementation/complexity of protocol...............14
   5.   Reliability and Availability................................15
   5.1. Detection of Data Corruption................................15
   5.2. Recovery....................................................15
   6.   Interoperability............................................16
   6.1. Internet infrastructure.....................................16
   6.2. SCSI........................................................16
   7.   Security Considerations.....................................18
   7.1. Extensible Security.........................................18
   7.2. Authentication..............................................18
   7.3. Data Integrity..............................................19
   7.4. Data Confidentiality........................................19
   8.   Management..................................................19
   8.1. Naming......................................................20
   8.2. Discovery...................................................21
   9.   Internet Accessibility......................................21
   9.1. Denial of Service...........................................21
   9.2. NATs, Firewalls and Proxy servers...........................22
   9.3. Congestion Control and Transport Selection..................22
   10.  Definitions.................................................22
   11.  References..................................................23
   12.  Acknowledgements............................................24
   13.  Author's Addresses..........................................25
   14.  Full Copyright Statement....................................26

1. Introduction

The IP Storage Working group is chartered with developing comprehensive technology to transport block storage data over IP protocols. This effort includes a protocol to transport the Small Computer Systems Interface (SCSI) protocol over the Internet (iSCSI). The initial version of the iSCSI protocol will define a mapping of SCSI transport protocol over TCP/IP so that SCSI storage controllers (principally disk and tape arrays and libraries) can be attached to IP networks, notably Gigabit Ethernet (GbE) and 10 Gigabit Ethernet (10 GbE).

The iSCSI protocol is a mapping of SCSI to TCP, and constitutes a "SCSI transport" as defined by the ANSI T10 document SCSI SAM-2 document [SAM2, p. 3, "Transport Protocols"].

2. Summary of Requirements

The iSCSI standard:

From section 3.2 Performance/Cost:

MUST allow implementations to equal or improve on the current state of the art for SCSI interconnects.

MUST enable cost competitive implementations.

SHOULD minimize control overhead to enable low delay communications.

MUST provide high bandwidth and bandwidth aggregation.

MUST have low host CPU utilizations, equal to or better than current technology.

MUST be possible to build I/O adapters that handle the entire SCSI task.

SHOULD permit direct data placement architectures.

MUST NOT impose complex operations on host software.

MUST provide for full utilization of available link bandwidth.

MUST allow an implementation to exploit parallelism (multiple connections) at the device interfaces and within the interconnect fabric.

From section 3.4 High Bandwidth/Bandwidth Aggregation:

MUST operate over a single TCP connection.

SHOULD support 'connection binding', and it MUST be optional to implement.

From section 4 Ease of Implementation/Complexity of Protocol:

SHOULD keep the protocol simple.

SHOULD minimize optional features.

MUST specify feature negotiation at session establishment (login).

MUST operate correctly when no optional features are negotiated as well as when individual option negotions are unsuccessful.

From section 5.1 Detection of Data Corruption:

MUST support a data integrity check format for use in digest generation.

MAY use separate digest for data and headers.

iSCSI header format SHOULD be extensible to include other data integrity digest calculation methods.

From section 5.2 Recovery:

MUST specify mechanisms to recover in a timely fashion from failures on the initiator, target, or connecting infrastructure.

MUST specify recovery methods for non-idempotent requests.

SHOULD take into account fail-over schemes for mirrored targets or highly available storage configurations.

SHOULD provide a method for sessions to be gracefully terminated and restarted that can be initiated by either the initiator or target.

From section 6 Interoperability:

iSCSI protocol document MUST be clear and unambiguous.

From section 6.1 Internet Infrastructure:

MUST:
-- be compatible with both IPv4 and IPv6
-- use TCP connections conservatively, keeping in mind there may

be many other users of TCP on a given machine.

MUST NOT require changes to existing Internet protocols.

SHOULD minimize required changes to existing TCP/IP implementations.

MUST be designed to allow future substitution of SCTP (for TCP) as an IP transport protocol with minimal changes to iSCSI protocol operation, protocol data unit (PDU) structures and formats.

From section 6.2 SCSI:

Any feature SAM2 requires in a valid transport mapping MUST be specified by iSCSI.

MUST specify strictly ordered delivery of SCSI commands over an iSCSI session between an initiator/target pair.

The command ordering mechanism SHOULD seek to minimize the amount of communication necessary across multiple adapters doing transport off-load.

MUST specify for each feature whether it is OPTIONAL, RECOMMENDED or REQUIRED to implement and/or use.

MUST NOT require changes to the SCSI-3 command sets and SCSI client code except except where SCSI specifications point to "transport dependent" fields and behavior.

SHOULD track changes to SCSI and the SCSI Architecture Model.

MUST be capable of supporting all SCSI-3 command sets and device types.

SHOULD support ACA implementation.

MUST allow for the construction of gateways to other SCSI transports

MUST reliably transport SCSI commands from the initiator to the target.

MUST correctly deal with iSCSI packet drop, duplication, corruption, stale packets, and re-ordering.

From section 7.1 Extensible Security:

SHOULD require minimal configuration and overhead in the insecure operation.

MUST provide for strong authentication when increased security is required.

SHOULD allow integration of new security mechanisms without breaking backwards compatible operation.

From section 7.2 Authentication:

MAY support various levels of authentication security.

MUST support private authenticated login.

iSCSI authenticated login MUST be resilient against attacks.

MUST support data origin authentication of its communications; data origin authentication MAY be optional to use.

From section 7.3 Data Integrity:

SHOULD NOT preclude use of additional data integrity protection protocols (IPSec, TLS).

From section 7.4 Data Confidentiality:

MUST provide for the use of a data encryption protocol such as TLS or IPsec ESP to provide data confidentiality between iSCSI endpoints

From section 8 Management:

SHOULD be manageable using standard IP-based management protocols.

iSCSI protocol document MUST NOT define the management architecture for iSCSI, or make explicit references to management objects such as MIB variables.

From section 8.1 Naming:

MUST support the naming architecture of SAM-2. The means by which an iSCSI resource is located MUST use or extend existing Internet standard resource location methods.

MUST provide a means of identifying iSCSI targets by a unique identifier that is independent of the path on which it is found.

The format for the iSCSI names MUST use existing naming authorities.

An iSCSI name SHOULD be a human readable string in an international character set encoding.

Standard Internet lookup services SHOULD be used to resolve iSCSI names.

SHOULD deal with the complications of the new SCSI security architecture.

iSCSI naming architecture MUST address support of SCSI 3rd party operations such as EXTENDED COPY.

From section 8.2 Discovery:

MUST have no impact on the use of current IP network discovery techniques.

MUST provide some means of determining whether an iSCSI service is available through an IP address.

SCSI protocol-dependent techniques SHOULD be used for further discovery beyond the iSCSI layer.

MUST provide a method of discovering, given an IP end point on its well-known port, the list of SCSI targets available to the requestor. The use of this discovery service MUST be optional.

From section 9 Internet Accessability.

SHOULD be scrutinized for denial of service issues and they should be addressed.

From section 9.2 Firewalls and Proxy Servers

SHOULD allow deployment where functional and optimizing middle- boxes such as firewalls, proxy servers and NATs are present.

use of IP addresses and TCP ports SHOULD be firewall friendly.

From section 9.3 Congestion Control and Transport Selection

MUST be a good network citizen with TCP-compatible congestion control (as defined in [RFC2914]).

iSCSI implementations MUST NOT use multiple connections as a means to avoid transport-layer congestion control.

3. iSCSI Design Considerations

3.1. General Discussion

Traditionally, storage controllers (e.g., disk array controllers, tape library controllers) have supported the SCSI-3 protocol and have been attached to computers by SCSI parallel bus or Fibre Channel.

The IP infrastructure offers compelling advantages for volume/ block-oriented storage attachment. It offers the opportunity to take advantage of the performance/cost benefits provided by competition in the Internet marketplace. This could reduce the cost of storage network infrastructure by providing economies arising from the need to install and operate only a single type of network.

In addition, the IP protocol suite offers the opportunity for a rich array of management, security and QoS solutions. Organizations may initially choose to operate storage networks based on iSCSI that are independent of (isolated from) their current data networks except for secure routing of storage management traffic. These organizations anticipated benefits from the high performance/cost of IP equipment and the opportunity for a unified management architecture. As security and QoS evolve, it becomes reasonable to build combined networks with shared infrastructure; nevertheless, it is likely that sophisticated users will choose to keep their storage sub-networks isolated to afford the best control of security and QoS to ensure a high-performance environment tuned to storage traffic.

Mapping SCSI over IP also provides:

-- Extended distance ranges
-- Connectivity to "carrier class" services that support IP

The following applications for iSCSI are contemplated:

      -- Local storage access, consolidation, clustering and pooling (as
         in the data center)
      -- Network client access to remote storage (eg. a "storage service
         provider")
      -- Local and remote synchronous and asynchronous mirroring between
         storage controllers
      -- Local and remote backup and recovery

iSCSI will support the following topologies:

      -- Point-to-point direct connections
      -- Dedicated storage LAN, consisting of one or more LAN segments
      -- Shared LAN, carrying a mix of traditional LAN traffic plus
         storage traffic
      -- LAN-to-WAN extension using IP routers or carrier-provided "IP
         Datatone"
      -- Private networks and the public Internet

IP LAN-WAN routers may be used to extend the IP storage network to the wide area, permitting remote disk access (as for a storage utility), synchronous and asynchronous remote mirroring, and remote

   backup and restore (as for tape vaulting).  In the WAN,  using TCP
   end-to-end avoids the need for specialized equipment for protocol
   conversion, ensures data reliability, copes with network congestion,
   and provides retransmission strategies adapted to WAN delays.

The iSCSI technology deployment will involve the following elements:

   (1)  Conclusion of a complete protocol standard and supporting
        implementations;
   (2)  Development of Ethernet storage NICs and related driver and
        protocol software; [NOTE: high-speed applications of iSCSI are
        expected to require significant portions of the iSCSI/TCP/IP
        implementation in hardware to achieve the necessary throughput.]
   (3)  Development of compatible storage controllers; and
   (4)  The likely development of translating gateways to provide
        connectivity between the Ethernet storage network and the Fibre
        Channel and/or parallel-bus SCSI domains.
   (5)  Development of specifications for iSCSI device management such
        as MIBs, LDAP or XML schemas, etc.
   (6)  Development of management and directory service applications to
        support a robust SAN infrastructure.

Products could initially be offered for Gigabit Ethernet attachment, with rapid migration to 10 GbE. For performance competitive with alternative SCSI transports, it will be necessary to implement the performance path of the full protocol stack in hardware. These new storage NICs might perform full-stack processing of a complete SCSI task, analogous to today's SCSI and Fibre Channel HBAs, and might also support all host protocols that use TCP (NFS, CIFS, HTTP, etc).

The charter of the IETF IP Storage Working Group (IPSWG) describes the broad goal of mapping SCSI to IP using a transport that has proven congestion avoidance behavior and broad implementation on a variety of platforms. Within that broad charter, several transport alternatives may be considered. Initial IPS work focuses on TCP, and this requirements document is restricted to that domain of interest.

3.2. Performance/Cost

In general, iSCSI MUST allow implementations to equal or improve on the current state of the art for SCSI interconnects. This goal breaks down into several types of requirement:

Cost competitive with alternative storage network technologies:

In order to be adopted by vendors and the user community, the iSCSI protocol MUST enable cost competitive implementations when compared to other SCSI transports (Fibre Channel).

Low delay communication:

Conventional storage access is of a stop-and-wait remote procedure call type. Applications typically employ very little pipelining of their storage accesses, and so storage access delay directly impacts performance. The delay imposed by current storage interconnects, including protocol processing, is generally in the range of 100 microseconds. The use of caching in storage controllers means that many storage accesses complete almost instantly, and so the delay of the interconnect can have a high relative impact on overall performance. When stop-and-wait IO is used, the delay of the interconnect will affect performance. The iSCSI protocol SHOULD minimize control overhead, which adds to delay.

Low host CPU utilization, equal to or better than current technology:

For competitive performance, the iSCSI protocol MUST allow three key implementation goals to be realized:

   (1)  iSCSI MUST make it possible to build I/O adapters that handle an
        entire SCSI task, as alternative SCSI transport implementations
        do.
   (2)  The protocol SHOULD permit direct data placement ("zero-copy"
        memory architectures, where the I/O adapter reads or writes host
        memory exactly once per disk transaction.
   (3)  The protocol SHOULD NOT impose complex operations on the host
        software, which would increase host instruction path length
        relative to alternatives.

Direct data placement (zero-copy iSCSI):

Direct data placement refers to iSCSI data being placed directly "off the wire" into the allocated location in memory with no intermediate copies. Direct data placement significantly reduces the memory bus and I/O bus loading in the endpoint systems, allowing improved performance. It reduces the memory required for NICs, possibly reducing the cost of these solutions.

This is an important implementation goal. In an iSCSI system, each of the end nodes (for example host computer and storage controller) should have ample memory, but the intervening nodes (NIC, switches) typically will not.

High bandwidth, bandwidth aggregation:

The bandwidth (transfer rate, MB/sec) supported by storage controllers is rapidly increasing, due to several factors:

1. Increase in disk spindle and controller performance; 2. Use of ever-larger caches, and improved caching algorithms; 3. Increased scale of storage controllers (number of supported

spindles, speed of interconnects).

The iSCSI protocol MUST provide for full utilization of available link bandwidth. The protocol MUST also allow an implementation to exploit parallelism (multiple connections) at the device interfaces and within the interconnect fabric.

The next two sections further discuss the need for direct data placement and high bandwidth.

3.3. Framing

Framing refers to the addition of information in a header, or the data stream to allow implementations to locate the boundaries of an iSCSI protocol data unit (PDU) within the TCP byte stream. There are two technical requirements driving framing: interfacing needs, and accelerated processing needs.

A framing solution that addresses the "interfacing needs" of the iSCSI protocol will facilitate the implementation of a message-based upper layer protocol (iSCSI) on top of an underlying byte streaming protocol (TCP). Since TCP is a reliable transport, this can be accomplished by including a length field in the iSCSI header. Finding the protocol frame assumes that the receiver will parse from the beginning of the TCP data stream, and never make a mistake (lose alignment on packet headers).

The other technical requirement for framing, "accelerated processing", stems from the need to handle increasingly higher data rates in the physical media interface. Two needs arise from higher data rates:

(1) LAN environment - NIC vendors seek ways to provide "zero-copy"

methods of moving data directly from the wire into application buffers.

(2) WAN environment- the emergence of high bandwidth, high latency,

low bit error rate physical media places huge buffer requirements on the physical interface solutions.

First, vendors are producing network processing hardware that offloads network protocols to hardware solutions to achieve higher data rates. The concept of "zero-copy" seeks to store blocks of data in appropriate memory locations (aligned) directly off the wire, even when data is reordered due to packet loss. This is necessary to drive actual data rates of 10 Gigabit/sec and beyond.

Secondly, in order for iSCSI to be successful in the WAN arena it must be possible to operate efficiently in high bandwidth, high delay networks. The emergence of multi-gigabit IP networks with latencies in the tens to hundreds of milliseconds presents a challenge. To fill such large pipes, it is necessary to have tens of megabytes of outstanding requests from the application. In addition, some protocols potentially require tens of megabytes at the transport layer to deal with buffering for reassembly of data when packets are received out-of-order.

In both cases, the issue is the desire to minimize the amount of memory and memory bandwidth required for iSCSI hardware solutions.

Consider that a network pipe at 10 Gbps x 200 msec holds 250 MB. [Assume land-based communication with a spot half way around the world at the equator. Ignore additional distance due to cable routing. Ignore repeater and switching delays; consider only a speed-of-light delay of 5 microsec/km. The circumference of the globe at the equator is approx. 40000 km (round-trip delay must be considered to keep the pipe full). 10 Gb/sec x 40000 km x 5 microsec/km x B / 8b = 250 MB]. In a conventional TCP implementation, loss of a TCP segment means that stream processing MUST stop until that segment is recovered, which takes at least a time of <network round trip> to accomplish. Following the example above, an implementation would be obliged to catch 250 MB of data into an anonymous buffer before resuming stream processing; later, this data would need to be moved to its proper location. Some proponents of iSCSI seek some means of putting data directly where it belongs, and avoiding extra data movement in the case of segment drop. This is a key concept in understanding the debate behind framing methodologies.

The framing of the iSCSI protocol impacts both the "interfacing needs" and the "accelerated processing needs", however, while including a length in a header may suffice for the "interfacing needs", it will not serve the direct data placement needs. The framing mechanism developed should allow resynchronization of packet boundaries even in the case where a packet is temporarily missing in the incoming data stream.

3.4. High bandwidth, bandwidth aggregation

At today's block storage transport throughput, any single link can be saturated by the volume of storage traffic. Scientific data applications and data replication are examples of storage applications that push the limits of throughput.

Some applications, such as log updates, streaming tape, and replication, require ordering of updates and thus ordering of SCSI commands. An initiator may maintain ordering by waiting for each update to complete before issuing the next (a.k.a. synchronous updates). However, the throughput of synchronous updates decreases inversely with increases in network distances.

For greater throughput, the SCSI task queuing mechanism allows an initiator to have multiple commands outstanding at the target simultaneously and to express ordering constraints on the execution of those commands. The task queuing mechanism is only effective if the commands arrive at the target in the order they were presented to the initiator (FIFO order). The iSCSI standard must provide an ordered transport of SCSI commands, even when commands are sent along different network paths (see Section 5.2 SCSI). This is referred to as "command ordering".

The iSCSI protocol MUST operate over a single TCP connection to accommodate lower cost implementations. To enable higher performance storage devices, the protocol should specify a means to allow operation over multiple connections while maintaining the behavior of a single SCSI port. This would allow the initiator and target to use multiple network interfaces and multiple paths through the network for increased throughput. There are a few potential ways to satisfy the multiple path and ordering requirements.

A popular way to satisfy the multiple-path requirement is to have a driver above the SCSI layer instantiate multiple copies of the SCSI transport, each communicating to the target along a different path. "Wedge" drivers use this technique today to attain high performance. Unfortunately, wedge drivers must wait for acknowledgement of completion of each request (stop-and-wait) to ensure ordered updates.

Another approach might be for iSCSI protocol to use multiple instances of its underlying transport (e.g. TCP). The iSCSI layer would make these independent transport instances appear as one SCSI transport instance and maintain the ability to do ordered SCSI command queuing. The document will refer to this technique as "connection binding" for convenience.

The iSCSI protocol SHOULD support connection binding, and it MUST be optional to implement.

In the presence of connection binding, there are two ways to assign features to connections. In the symmetric approach, all the connections are identical from a feature standpoint. In the asymmetric model, connections have different features. For example, some connections may be used primarily for data transfers whereas others are used primarily for SCSI commands.

Since the iSCSI protocol must support the case where there was only one transport connection, the protocol must have command, data, and status travel over the same connection.

In the case of multiple connections, the iSCSI protocol must keep the command and its associated data and status on the same connection (connection allegiance). Sending data and status on the same connection is desirable because this guarantees that status is received after the data (TCP provides ordered delivery). In the case where each connection is managed by a separate processor, allegiance decreases the need for inter-processor communication. This symmetric approach is a natural extension of the single connection approach.

An alternate approach that was extensively discussed involved sending all commands on a single connection and the associated data and status on a different connection (asymmetric approach). In this scheme, the transport ensures the commands arrive in order. The protocol on the data and status connections is simpler, perhaps lending itself to a simpler realization in hardware. One disadvantage of this approach is that the recovery procedure is different if a command connection fails vs. a data connection. Some argued that this approach would require greater inter-processor communication when connections are spread across processors.

The reader may reference the mail archives of the IPS mailing list between June and September of 2000 for extensive discussions on symmetric vs asymmetric connection models.

4. Ease of implementation/complexity of protocol

Experience has shown that adoption of a protocol by the Internet community is inversely proportional to its complexity. In addition, the simpler the protocol, the easier it is to diagnose problems. The designers of iSCSI SHOULD strive to fulfill the requirements of the creating a SCSI transport over IP, while keeping the protocol as simple as possible.

In the interest of simplicity, iSCSI SHOULD minimize optional features. When features are deemed necessary, the protocol MUST specify feature negotiation at session establishment (login). The iSCSI transport MUST operate correctly when no optional features are negotiated as well as when individual option negotiations are unsuccessful.

5. Reliability and Availability

5.1. Detection of Data Corruption

There have been several research papers that suggest that the TCP checksum calculation allows a certain number of bit errors to pass undetected [10] [11].

In order to protect against data corruption, the iSCSI protocol MUST support a data integrity check format for use in digest generation.

The iSCSI protocol MAY use separate digests for data and headers. In an iSCSI proxy or gateway situation, the iSCSI headers are removed and re-built, and the TCP stream is terminated on either side. This means that even the TCP checksum is removed and recomputed within the gateway. To ensure the protection of commands, data, and status the iSCSI protocol MUST include a CRC or other digest mechanism that is computed on the SCSI data block itself, as well as on each command and status message. Since gateways may strip iSCSI headers and rebuild them, a separate header CRC is required. Two header digests, one for invariant portions of the header (addresses) and one for the variant portion would provide protection against changes to portions of the header that should never be changed by middle boxes (eg, addresses).

The iSCSI header format SHOULD be extensible to include other digest calculation methods.

5.2. Recovery

The SCSI protocol was originally designed for a parallel bus transport that was highly reliable. SCSI applications tend to assume that transport errors never happen, and when they do, SCSI application recovery tends to be expensive in terms of time and computational resources.

iSCSI protocol design, while placing an emphasis on simplicity, MUST lead to timely recovery from failure of initiator, target, or connecting network infrastructure (cabling, data path equipment such as routers, etc).

iSCSI MUST specify recovery methods for non-idempotent requests, such as operations on tape drives.

The iSCSI protocol error recover mechanism SHOULD take into account fail-over schemes for mirrored targets or highly available storage configurations that provide paths to target data through multiple "storage servers". This would provide a basis for layered technologies like high availability and clustering.

The iSCSI protocol SHOULD also provide a method for sessions to be gracefully terminated and restarted that can be initiated by either the initiator or target. This provides the ability to gracefully fail over an initiator or target, or reset a target after performing maintenance tasks such as upgrading software.

6. Interoperability

It must be possible for initiators and targets that implement the required portions of the iSCSI specification to interoperate. While this requirement is so obvious that it doesn't seem worth mentioning, if the protocol specification contains ambiguous wording, different implementations may not interoperate. The iSCSI protocol document MUST be clear and unambiguous.

6.1. Internet infrastructure

The iSCSI protocol MUST:

-- be compatible with both IPv4 and IPv6.
-- use TCP connections conservatively, keeping in mind there may

be many other users of TCP on a given machine.

The iSCSI protocol MUST NOT require changes to existing Internet protocols and SHOULD minimize required changes to existing TCP/IP implementations.

iSCSI MUST be designed to allow future substitution of SCTP (for TCP) as an IP transport protocol with minimal changes to iSCSI protocol operation, protocol data unit (PDU) structures and formats. Although not widely implemented today, SCTP has many design features that make it a desirable choice for future iSCSI enhancement.

6.2. SCSI

In order to be considered a SCSI transport, the iSCSI standard must comply with the requirements of the SCSI Architecture Model [SAM-2] for a SCSI transport. Any feature SAM2 requires in a valid transport mapping MUST be specified by iSCSI. The iSCSI protocol document MUST specify for each feature whether it is OPTIONAL, RECOMMENDED or REQUIRED to implement and/or use.

   The SCSI Architectural Model [SAM-2] indicates an expectation that
   the SCSI  transport provides ordering of commands on an initiator
   target-LUN granularity.  There has been much discussion on the IPS
   reflector and in working group meetings regarding the means to ensure
   this ordering.  The rough consensus is that iSCSI MUST specify
   strictly ordered delivery of SCSI commands over an iSCSI session
   between an initiator/target pair, even in the presence of transport
   errors.  This command ordering mechanism SHOULD seek to minimize the
   amount of communication necessary across multiple adapters doing
   transport off-load.  If an iSCSI implementation does not require
   ordering it can instantiate multiple sessions per initiator-target
   pair.

iSCSI is intended to be a new SCSI "transport" [SAM2]. As a mapping of SCSI over TCP, iSCSI requires interaction with both T10 and IETF. However, the iSCSI protocol MUST NOT require changes to the SCSI-3 command sets and SCSI client code except where SCSI specifications point to "transport dependent" fields and behavior. For example, changes to SCSI documents will be necessary to reflect lengthier iSCSI target names and potentially lengthier timeouts. Collaboration with T10 will be necessary to achieve this requirement.

The iSCSI protocol SHOULD track changes to SCSI and the SCSI Architecture Model.

The iSCSI protocol MUST be capable of supporting all SCSI-3 command sets and device types. The primary focus is on supporting 'larger' devices: host computers and storage controllers (disk arrays, tape libraries). However, other command sets (printers, scanners) must be supported. These requirements MUST NOT be construed to mean that iSCSI must be natively implementable on all of today's SCSI devices, which might have limited processing power or memory.

ACA (Auto Contingent Allegiance) is an optional SCSI mechanism that stops execution of a sequence of dependent SCSI commands when one of them fails. The situation surrounding it is complex - T10 specifies ACA in SAM2, and hence iSCSI must support it and endeavor to make sure that ACA gets implemented sufficiently (two independent interoperable implementations) to avoid dropping ACA in the transition from Proposed Standard to Draft Standard. This implies iSCSI SHOULD support ACA implementation.

The iSCSI protocol MUST allow for the construction of gateways to other SCSI transports, including parallel SCSI [SPI-X] and to SCSI FCP[FCP, FCP-2]. It MUST be possible to construct "translating" gateways so that iSCSI hosts can interoperate with SCSI-X devices; so that SCSI-X devices can communicate over an iSCSI network; and so that SCSI-X hosts can use iSCSI targets (where SCSI-X refers to parallel SCSI, SCSI-FCP, or SCSI over any other transport). This requirement is implied by support for SAM-2, but is worthy of emphasis. These are true application protocol gateways, and not just bridge/routers. The different standards have only the SCSI-3 command set layer in common. These gateways are not mere packet forwarders.

The iSCSI protocol MUST reliably transport SCSI commands from the initiator to the target. According to [SAM-2, p. 17.] "The function of the service delivery subsystem is to transport an error-free copy of the request or response between the sender and the receiver" [SAM-2, p. 22]. The iSCSI protocol MUST correctly deal with iSCSI packet drop, duplication, corruption, stale packets, and re-ordering.

7. Security Considerations

In the past, directly attached storage systems have implemented minimal security checks because the physical connection offered little chance for attack. Transporting block storage (SCSI) over IP opens a whole new opportunity for a variety of malicious attacks. Attacks can take the active form (identity spoofing, man-in-the- middle) or the passive form (eavesdropping).

7.1. Extensible Security

The security services required for communications depends on the individual network configurations and environments. Organizations are setting up Virtual Private Networks(VPN), also known as Intranets, that will require one set of security functions for communications within the VPN and possibly many different security functions for communications outside the VPN to support geographically separate components. The iSCSI protocol is applicable to a wide range of internet working environments that may employ different security policies. iSCSI MUST provide for strong authentication when increased security is required. The protocol SHOULD require minimal configuration and overhead in the insecure operation, and allow integration of new security mechanisms without breaking backwards compatible operation.

7.2. Authentication

The iSCSI protocol MAY support various levels of authentication security, ranging from no authentication to secure authentication using public or private keys.

The iSCSI protocol MUST support private authenticated login.

Authenticated login aids the target in blocking the unauthorized use of SCSI resources. "Private" authenticated login mandates protected identity exchange (no clear text passwords at a minimum). Since block storage confidentiality is considered critical in enterprises and many IP networks may have access holes, organizations will want to protect their iSCSI resources.

The iSCSI authenticated login MUST be resilient against attacks since many IP networks are vulnerable to packet inspection.

In addition, the iSCSI protocol MUST support data origin authentication of its communications; data origin authentication MAY be optional to use. Data origin authentication is critical since IP networks are vulnerable to source spoofing, where a malicious third party pretends to send packets from the initiator's IP address. These requirements should be met using standard Internet protocols such as IPsec or TLS. The endpoints may negotiate the authentication method, optionally none.

7.3. Data Integrity

The iSCSI protocol SHOULD NOT preclude use of additional data integrity protection protocols (IPSec, TLS).

7.4. Data Confidentiality

Block storage is used for storing sensitive information, where data confidentiality is critical. An application may encrypt the data blocks before writing them to storage - this provides the best protection for the application. Even if the storage or communications are compromised, the attacker will have difficulty reading the data.

In certain environments, encryption may be desired to provide an extra assurance of confidentiality. An iSCSI implementation MUST provide for the use of a data encryption protocol such as TLS or IPsec ESP to provide data confidentiality between iSCSI endpoints.

8. Management

iSCSI implementations SHOULD be manageable using standard IP-based management protocols. However, the iSCSI protocol document MUST NOT define the management architecture for iSCSI within the network infrastructure. iSCSI will be yet another resource service within a complex environment of network resources (printers, file servers, NAS, application servers, etc). There will certainly be efforts to design how the "block storage service" that iSCSI devices provide is integrated into a comprehensive, shared model, network management environment. A "network administrator" (or "storage administrator") will desire to have integrated applications for assigning user names, resource names, etc. and indicating access rights. iSCSI devices presumably will want to interact with these integrated network management applications. The iSCSI protocol document will not attempt to solve that set of problems, or specify means for devices to provide management agents. In fact, there should be no mention of MIBs or any other means of managing iSCSI devices as explicit references in the iSCSI protocol document, because management data and protocols change with the needs of the environment and the business models of the management applications.

8.1. Naming

Whenever possible, iSCSI MUST support the naming architecture of SAM-2. Deviations and uncertainties MUST be made explicit, and comments and resolutions worked out between ANSI T10 and the IPS working group.

The means by which an iSCSI resource is located MUST use or extend existing Internet standard resource location methods. RFC 2348 [12] specifies URL syntax and semantics which should be sufficiently extensible for the iSCSI resource.

The iSCSI protocol MUST provide a means of identifying an iSCSI storage device by a unique identifier that is independent of the path on which it is found. This name will be used to correlate alternate paths to the same device. The format for the iSCSI names MUST use existing naming authorities, to avoid creating new central administrative tasks. An iSCSI name SHOULD be a human readable string in an international character set encoding.

Standard Internet lookup services SHOULD be used to resolve names. For example, Domain Name Services (DNS) MAY be used to resolve the <hostname> portion of a URL to one or multiple IP addresses. When a hostname resolves to multiple addresses, these addresses should be equivalent for functional (possibly not performance) purposes. This means that the addresses can be used interchangeably as long as performance isn't a concern. For example, the same set of SCSI targets MUST be accessible from each of these addresses.

   An iSCSI device naming scheme MUST interact correctly with the
   proposed SCSI security architecture [99-245r9].  Particular attention
   must be directed to the proxy naming architecture defined by the new
   security model.  In this new model,  a host is identified by an
   Access ID, and SCSI Logical Unit Numbers (LUNs) can be mapped in a
   manner that gives each AccessID a unique LU map.  Thus, a given LU
   within a target may be addressed by different LUNs.

The iSCSI naming architecture MUST address support of SCSI 3rd party operations such as EXTENDED COPY. The key issue here relates to the naming architecture for SCSI LUs - iSCSI must provide a means of passing a name or handle between parties. iSCSI must specify a means of providing a name or handle that could be used in the XCOPY command and fit within the available space allocated by that command. And it must be possible, of course, for the XCOPY target (the third party) to de-reference the name to the correct target and LU.

8.2. Discovery

iSCSI MUST have no impact on the use of current IP network discovery techniques. Network management platforms discover IP addresses and have various methods of probing the services available through these IP addresses. An iSCSI service should be evident using similar techniques.

The iSCSI specifications MUST provide some means of determining whether an iSCSI service is available through an IP address. It is expected that iSCSI will be a point of service in a host, just as SNMP, etc are points of services, associated with a well known port number.

SCSI protocol-dependent techniques SHOULD be used for further discovery beyond the iSCSI layer. Discovery is a complex, multi- layered process. The SCSI protocol specifications provide specific commands for discovering LUs and the commands associated with this process will also work over iSCSI.

The iSCSI protocol MUST provide a method of discovering, given an IP end point on its well-known port, the list of SCSI targets available to the requestor. The use of this discovery service MUST be optional.

Further discovery guidelines are outside the scope of this document and may be addressed in separate Informational documents.

9. Internet Accessibility

9.1. Denial of Service

As with all services, the denial of service by either incorrect implementations or malicious agents is always a concern. All aspects of the iSCSI protocol SHOULD be scrutinized for potential denial of service issues, and guarded against as much as possible.

9.2. NATs, Firewalls and Proxy servers

NATs (Network Address Translator), firewalls, and proxy servers are a reality in today's Internet. These devices present a number of challenges to device access methods being developed for iSCSI. For example, specifying a URL syntax for iSCSI resource connection allows an initiator to address an iSCSI target device both directly and through an iSCSI proxy server or NAT. iSCSI SHOULD allow deployment where functional and optimizing middle-boxes such as firewalls, proxy servers and NATs are present.

The iSCSI protocol's use of IP addressing and TCP port numbers MUST be firewall friendly. This means that all connection requests should normally be addressed to a specific, well-known TCP port. That way, firewalls can filter based on source and destination IP addresses, and destination (target) port number. Additional TCP connections would require different source port numbers (for uniqueness), but could be opened after a security dialogue on the control channel.

It's important that iSCSI operate through a firewall to provide a possible means of defending against Denial of Service (DoS) assaults from less-trusted areas of the network. It is assumed that a firewall will have much greater processing power for dismissing bogus connection requests than end nodes.

9.3. Congestion Control and Transport Selection

The iSCSI protocol MUST be a good network citizen with proven congestion control (as defined in [RFC2914]). In addition, iSCSI implementations MUST NOT use multiple connections as a means to avoid transport-layer congestion control.

10. Definitions

Certain definitions are offered here, with references to the original document where applicable, in order to clarify the discussion of requirements. Definitions without references are the work of the authors and reviewers of this document.

Logical Unit (LU): A target-resident entity that implements a device model and executes SCSI commands sent by an application client [SAM- 2, sec. 3.1.50, p. 7].

   Logical Unit Number (LUN): A 64-bit identifier for a logical unit
   [SAM-2, sec. 3.1.52, p. 7].
   
   SCSI Device:  A device that is connected to a service delivery
   subsystem and supports a SCSI application protocol [SAM-2, sec.
   3.1.78, p. 9].

Service Delivery Port (SDP): A device-resident interface used by the application client, device server, or task manager to enter and retrieve requests and responses from the service delivery subsystem. Synonymous with port (SAM-2 sec. 3.1.61) [SAM-2, sec. 3.1.89, p. 9].

Target: A SCSI device that receives a SCSI command and directs it to one or more logical units for execution [SAM-2 sec. 3.1.97, p. 10].

Task: An object within the logical unit representing the work associated with a command or a group of linked commands [SAM-2, sec. 3.1.98, p. 10].

Transaction: A cooperative interaction between two objects, involving the exchange of information or the execution of some service by one object on behalf of the other [SAM-2, sec. 3.1.109, p. 10].

11. References

   1.   Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.
   
   2.   Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.
   
   3.   [SAM-2] ANSI NCITS.  Weber, Ralph O., editor.  SCSI Architecture
        Model -2 (SAM-2).  T10 Project 1157-D.  rev 23, 16 Mar 2002.
   
   4.   [SPC-2] ANSI NCITS.  Weber, Ralph O., editor.  SCSI Primary
        Commands   2 (SPC-2).  T10 Project 1236-D.  rev 20, 18 July
        2001.
   
   5.   [CAM-3] ANSI NCITS.  Dallas, William D., editor.  Information
        Technology - Common Access Method - 3 (CAM-3)).  X3T10 Project
        990D.  rev 3, 16 Mar 1998.
   
   6.   [99-245r8] Hafner, Jim.  A Detailed Proposal for Access
        Controls.  T10/99-245 revision 9, 26 Apr 2000.
   
   7.   [SPI-X] ANSI NCITS.  SCSI Parallel Interface - X.
   
   8.   [FCP] ANSI NCITS.  SCSI-3 Fibre Channel Protocol [ANSI
        X3.269:1996].
   
   9.   [FCP-2] ANSI NCITS.  SCSI-3 Fibre Channel Protocol - 2
        [T10/1144-D].
   
   10.  Paxon, V. End-to-end internet packet dynamics, IEEE Transactions
        on Networking 7,3 (June 1999) pg 277-292.

11. Stone J., Partridge, C. When the CRC and TCP checksum disagree, ACM Sigcomm (Sept. 2000).

   12.  Malkin, G. and A. Harkin, "TFTP Blocksize Option", RFC 2348, May
        1998.
   
   13.  Floyd, S., "Congestion Control Principles", BCP 14, RFC 2914,
        September 2000.

12. Acknowledgements

Special thanks to Julian Satran, IBM and David Black, EMC for their extensive review comments.

13. Author's Addresses

Address comments to:

   Marjorie Krueger
   Hewlett-Packard Corporation
   8000 Foothills Blvd
   Roseville, CA 95747-5668, USA
   Phone: +1 916 785-2656
   EMail: marjorie_krueger@hp.com
   
   Randy Haagens
   Hewlett-Packard Corporation
   8000 Foothills Blvd
   Roseville, CA 95747-5668, USA
   Phone: +1 916 785-4578
   EMail: Randy_Haagens@hp.com

Costa Sapuntzakis
Stanford University
353 Serra Mall Dr #407
Stanford, CA 94305
Phone: 650-723-2458
EMail: csapuntz@stanford.edu

   Mark Bakke
   Cisco Systems, Inc.
   6450 Wedgwood Road
   Maple Grove, MN 55311
   Phone: +1 763 398-1054
   EMail: mbakke@cisco.com

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