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12 RFC 4173
IP Storage Working Group Prasenjit Sarkar
Internet Draft IBM
Document: draft-ietf-ips-iscsi-boot-09.txt Duncan Missimer
Category: Standards Track Rhapsody Networks
Constantin Sapuntzakis
Stanford University
27 February 2003
Bootstrapping Clients using the iSCSI Protocol
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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The words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
documents are to be interpreted as described in RFC 2119.
Abstract
iSCSI is a proposed transport protocol for SCSI that operates on top
of TCP. This memo describes a standard mechanism to enable clients
to bootstrap themselves using the iSCSI protocol. The goal of this
standard is to enable iSCSI boot clients to obtain the information to
open an iSCSI session with the iSCSI boot server.
1. Introduction
The Small Computer Systems Interface (SCSI) is a popular family of
protocols for communicating with I/O devices, especially storage
devices. SCSI can be characterized as a request/response messaging
protocol with a standard architecture and componentized command sets
for different device classes.
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iSCSI is a proposed transport protocol for SCSI that operates on top
of TCP. The role of iSCSI is necessitated by the evolution of the
system interconnect from a shared bus to a switched network. IP
networks meet the architectural and performance requirements of
transporting SCSI, paving the way for the iSCSI protocol.
Many diskless clients sometimes bootstrap off remote SCSI devices.
Such diskless entities are lightweight, space-efficient and power-
conserving, and are increasingly popular in various environments.
This memo describes a standard mechanism to enable clients to
bootstrap themselves using the iSCSI protocol. The goal of this
standard is to enable iSCSI boot clients to obtain the information to
open an iSCSI session with the iSCSI boot server. It is possible that
all the information is not available at the very outset, so the memo
describes steps to obtain the information required to bootstrap
clients off an iSCSI boot server.
2. Requirements
1. There must be no restriction of network topology between the iSCSI
boot client and the boot server other than those in effect for
establishing the iSCSI session. Consequently, it is possible for an
iSCSI boot client to boot from an iSCSI boot server behind gateways
or firewalls as long as it is possible to establish an iSCSI session
between the client and the server.
2. The following represents the minimum information required for an
iSCSI boot client to contact an iSCSI boot server: (a) the client's
IP address (IPv6 or IPv4); (b) the server's iSCSI Target Name; and
(c) mandatory iSCSI initiator capability.
The above assumes that the default LUN for the boot process is 0 and
the default port for the iSCSI boot server is the well-known iSCSI
port. However, both may be overridden at the time of configuration.
Additional information may be required at each stage of the boot
process.
3. It is possible for the iSCSI boot client to have none of the above
information or capability on starting.
4. The client should be able to complete boot without user
intervention (for boots that occur during an unattended power-up).
However, there should be a mechanism for the user to input values so
as to bypass stages of the boot protocol.
5. Additional protocol software (for example, DHCP) may be necessary
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if the minimum information required for an iSCSI session is not
provided.
3. Related Work
The Reverse Address Resolution Protocol (RARP) [Finlayson84] through
the extensions defined in the Dynamic RARP (DRARP)) [Brownell96]
explicitly addresses the problem of network address discovery, and
includes an automatic IP address assignment mechanism. The Trivial
File Transfer Protocol (TFTP) [Sollins81] provides for transport of a
boot image from a boot server. BOOTP [Croft85, Reynolds93, Wimer93]
is a transport mechanism for a collection of configuration
information. BOOTP is also extensible, and official extensions have
been defined for several configuration parameters. DHCPv4 [Droms97,
Droms93] and DHCPv6 [Bound02] are standards for hosts to be
dynamically configured in an IP network. The Service Location
Protocol (SLP) provides for location of higher level services
[Guttman99].
4. Software stage
Some iSCSI boot clients may lack the resources to boot up with the
mandatory iSCSI initiator capability. Such boot clients may choose to
obtain iSCSI initiator software from a boot server. Currently, there
are many established protocols that allow such a service to enable
clients to load software images. For example, BOOTP and DHCP servers
have the capability to provide software images on requests from boot
clients.
It is to be noted that this document does not recommend any of the
above protocols, and the final decision of which boot protocol is to
be used to load iSCSI initiator software is left to the discretion of
the implementor.
5. DHCP stage
In order to use an iSCSI boot server, the following pieces of
information are required for an ISCSI boot client.
- The IP address of the iSCSI boot client (IPv4 or IPv6)
- The IP transport endpoint for the iSCSI Target Port for the iSCSI
boot server. If the transport is TCP, for example, this has to
resolve to an IP address and a TCP port number. TCP is currently the
only transport approved for iSCSI.
- The eight-byte LUN structure identifying the Logical Unit within
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the iSCSI boot server.
At boot time, all or none of this information may be stored in the
iSCSI boot client. This section describes techniques for obtaining
the required information via the DHCP stage. Otherwise, if the iSCSI
boot client has all the information, the boot client may proceed
directly to the Boot stage.
An iSCSI boot client which does not know its IP address at power-on
may acquire its IP address via DHCP. An iSCSI boot client which is
capable of using both DHCPv6 and DHCPv4 should first attempt to use
DHCPv6 to obtain its IP address, falling back on DHCPv4 in the event
of failure.
Unless otherwise specified here, DHCP fields such as the client ID
and gateway information are used in an identical way as applications
other than iSCSI do.
A DHCP server (v4 or v6) MAY instruct an iSCSI client how to reach
its boot device. This is done using the variable length DHCP option
named Root Path. The use of the option field is reserved for iSCSI
boot use by prefacing the string with "iscsi:".
The option field consists of an UTF-8 [Yergeau98] string. The string
MUST contain only alphanumberic characters, "." , ":" and "-"; no
other characters are permissible. The string has the following
composition:
"iscsi:"<servername>":"<protocol>":"<port>":"<LUN>":"<targetname>
The fields "servername", "port", "protocol" and "LUN" are OPTIONAL
and should be left blank if there are no corresponding values. The
"targetname" field is not optional and MUST be provided.
The "servername" is the name of iSCSI server and contains either a
valid domain name, a literal IPv4 address, or a literal IPv6 address.
If the "servername" field contains a literal IPv4 address, the IPv4
address MUST be in standard dotted decimal notation as defined in
Section 2.1 of RFC 1123 [Braden89].
If the "servername" field contains an IPv6 address, the address MUST
be represented in the IPv6 address format x.x.x.x.x.x.x.x where the
'x's are the hexadecimal values of the eight 16-bit pieces of the
address. Note that this format representation is specific to iSCSI
boot.
If the "servername" is a domain name, the name MUST be a fully
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qualified domain name (FQDN) and should abide by the rules specified
in Sections 3.1 and 3.5 of RFC 1034 [Mockaopetris87] and the reply
from the host configuration server should contain the Domain Name
Server Option [Alexander93]. It must also be pointed out that the
use of DNS for address translation in enterprise environments must
contain adequate levels of fault tolerance and security.
If the "servername" field contains 4 decimal components, the
"servername" is assumed to be an IPv4 address. If there are more than
4 decimal components or if there is a hexadecimal component, the the
"servername" is assumed to be an IPv6 address. If the least
significant (rightmost) component is an approved domain extension,
then the "servername" field is assumed to be a domain name. If the
"servername" field is left blank, then no default value is assumed in
its place.
The "protocol" field is the decimal representation of the IANA-
approved string for the trasport protocol to be used for iSCSI. If
the protocol field is left bank, the default value is assumed to be
"6" for TCP. The transport protocol MUST have been approved for use
in iSCSI; currently, the only approved protocol is TCP.
The "port" is the decimal representation of the port on which the
iSCSI boot server is listening. If not specified, the port defaults
to the well-known iSCSI port.
The "LUN" field is a hexadecimal representation of the LU number. If
the LUN field is blank, then LUN 0 is assumed. If the LUN field is
not blank, the representation MUST be divided into four groups of
four hexadecimal digits, separated by "-". Digits above 9 may be
either lower or upper case. An example of such a representation
would be 4752-3A4F-6b7e-2F99. For the sake of brevity, at most three
leading zero ("0") digits MAY be omitted in any group of hexadecimal
digits. Thus, the "LUN" representation 6734-9-156f-127 is equivalent
to 6734-0009-156f-0127. Furthermore, trailing groups containing only
the "0" digit MAY be omitted along with the preceding "-". So, the
"LUN" representation 4186-9 is equivalent to 4186-0009-0000-0000.
Other concise representations of the LUN field MUST NOT be used.
Note that SCSI targets are allowed to present different LU numberings
for different SCSI initiators, so that to our knowledge nothing
precludes a SCSI target from exporting several different LUs to
several different SCSI initiators as their respective LUN 0s.
The "targetname" field is an iSCSI Name that is defined by the iSCSI
standard [Satran02] to uniquely identify an iSCSI target.
If the "servername" field is provided by DHCP, then that field is
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used in conjunction with other associated fields to contact the boot
server in the Boot stage (Section 7). However, if the "servername"
field is not provided, then the "targetname" field is then used in
the Discovery Service stage in conjunction with other associated
fields. (Section 6).
6. Discovery Service stage
This stage is required if the DHCP server (v4 or v6) is unaware of
any iSCSI boot servers or if the DHCP server is unable to provide the
minimum information required to connect to the iSCSI boot server
other than the targetname.
The discovery service is based on the SLP protocol [Guttman99,
Bakke02] and is an instantiation of the SLP Service or Directory
Agent.
The iSCSI boot client may have obtained the targetname of the iSCSI
boot server in the DHCP stage (Section 5). In that case, the iSCSI
boot client queries the Discovery Service using query string 1 of the
iSCSI Target Concrete Service Type Template as specified in Section
6.2 of the iSCSI SLP interaction document [Bakke02] to resolve the
targetname to an IP address and port number. Once this is obtained,
the iSCSI boot client proceeds to the Boot stage (Section 7).
It is possible that the port number obtained from the Discovery
Service may conflict with the one obtained from the DHCP service. In
such a case, the implementor has the option to try both port numbers
in the Boot stage.
If the iSCSI boot client does not have any targetname information,
the iSCSI boot client then may query the Discovery Service with query
string 4 of the iSCSI Target Concrete Service Type Template as
specified in Section 6.2 of the iSCSI SLP interaction document
[Bakke02]. In response to this query, the discovery service provides
the boot client with a list of iSCSI boot servers the boot client is
allowed to access.
If the list of iSCSI boot servers is empty, subsequent actions are
left to the discretion of the implementor. Otherwise, the iSCSI boot
client may contact any iSCSI boot server in the list. Moreover, the
order in which iSCSI boot servers are contacted is also left to the
discretion of the implementor.
7. Boot stage
Once the iSCSI boot client has obtained the minimum information to
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open an iSCSI session with the iSCSI boot server, the actual booting
process can start.
The actual sequence of iSCSI commands needed to complete the boot
process is left to the implementor. This was done because of varying
requirements from different vendors and equipment, making it
difficult to specify a common subset of the iSCSI standard that would
be acceptable to everybody.
The iSCSI session established for boot may be taken over by the
booted software in the iSCSI boot client.
8. Security Considerations
The security discussion is centered around securing the communication
involved in the iSCSI boot process.
However, the issue of applying credentials to a boot image loaded
through the iSCSI boot mechanism is outside the scope of this
document. One key difference between the iSCSI boot mechanism and
BOOTP-based image loading is the fact that the identity of a boot
image may not be known when the Boot stage starts. The identity of
certain boot images and their locations are known only after
examining the contents of a boot disk exposed by the iSCSI boot
service. Furthermore, images themselves may recursively load other
images based on both hardware configurations and user input.
Consequently, a practical way to verify loaded boot images is to make
sure that each image loading software verify the image to be loaded
using a mechanism of their choice.
The considerations involved in designing a security architecture for
the iSCSI boot process include configuration, deployment and
provisioning issues apart from typical security considerations.
The software stage SHOULD not be involved in a secure iSCSI boot
process as this would add the additional complexity of trying to
secure the process of loading the software necessary to run the later
stages of iSCSI boot. It is therefore assumed that all the necessary
software is resident on the iSCSI boot client.
In the case where the DHCP stage is necessary, the iSCSI boot client
must contact the DHCP server using IPSEC. Since DHCP server software
is freely available and can be deployed easily, care must be taken to
make sure that the communication is secure. Consequently, pre-shared
keys MUST be avoided in authenticating the IPSEC communication
channel. As public key techniques are not recommended for
authenticating the IPSEC communication channel in the Boot stage, an
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implementation SHOULD avoid these techniques to avoid two different
authentication mechanisms.
In the context of the secure iSCSI boot process, the reply from the
DHCP server in the DHCP stage MUST include the servername in IPv4 (or
IPv6) format to avoid reliance on a DNS server (for resolving names)
or a SLP server (to look up targetnames). This reduces the number of
entities involved in the secure iSCSI boot process.
The final communication between the iSCSI boot client and the boot
server in the Boot stage is secured with the help of IPSEC. The
communication must adhere to the recommendations in the main iSCSI
draft [Satran02] for the choice of certification, authentication and
encryption algorithms. However, since the iSCSI boot client is
likely to have a dynamic IP address, pre-shared keys MUST be avoided
in authenticating the IPSEC communication channel.
The mechanism for securing the iSCSI boot process SHOULD not be
enabled by default so as to avoid the configuration, deployment and
provisioning requirements of the secure boot process.
Another point to be noted is that if a boot image inherits an iSCSI
session from a previously loaded boot image, the boot image also
inherts the security properties of the iSCSI session.
Acknowledgments
We wish to thank John Hufferd for taking the initiative to form the
iSCSI boot team. We also wish to thank Doug Otis, Julian Satran,
Bernard Aboba, David Robinson, Mark Bakke and Mallikarjun Chadalapaka
for helpful suggestions and pointers regarding the draft document.
Normative References
[Alexander93] Alexander, S., and R. Droms, "DHCP Options and BOOTP
Vendor
Extensions", RFC 2132, Lachman Technology, Inc., Bucknell
University, October 1993.
[Bakke02] Bakke, M., et al. "Finding iSCSI Targets and Name Servers
using SLP", Work in Progress, March 2002.
[Bound02] Bound, J., Canney, M., and Perkins, C., "Dynamic Host
Configuration
Protocol for IPv6", Work in Progress, June 2002.
[Braden89] Braden, R., "Requirements for Internet Hosts - Application
and Support", RFC 1123, October 1989.
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[Bradner96] Bradner, S., "The Internet Standards Process --
Revision 3", RFC 2026, October 1996.
[Bradner97] Bradner, S. "Key Words for use in RFCs to indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
[Croft85] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)",
RFC 951,
Stanford and SUN Microsystems, September 1985.
[Droms93] Droms, D., "Interoperation between DHCP and BOOTP" RFC
1534,
Bucknell University, October 1993.
[Droms97] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997.
[Droms01] Droms, R., Arbaugh, W., "Authentication for DHCP Messages",
RFC 3118, June 2001.
[Guttman99] Guttman, E., Perkins, C., Verizades, J., Day, M.,
"Service Location Protocol v2", RFC 2608, June 1999.
[Mockaopetris87] Mockaopertis, P., "Domain Names - Concepts and
Facilities", RFC 1034, November 1987.
[Reynolds93] Reynolds, J., "BOOTP Vendor Information Extensions", RFC
1497,
USC/Information Sciences Institute, August 1993.
[Satran02] Satran, J. et al., "iSCSI", Work in Progress, September
2002.
[Yergeau98] Yergeau, F., "UTF-8: A Transformation Format for
ISO-10646", RFC 2279, January 1998.
[Wimer93] Wimer, W., "Clarifications and Extensions for the Bootstrap
Protocol", RFC 1532, Carnegie Mellon University, October 1993.
Informative References
[Brownell96] Brownell, D, "Dynamic RARP extensions for Automatic
Network Adress
Acquisition", RFC 1931, SUN Microsystems, April 1996.
[Finlayson84] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A
Reverse
Address Resolution Protocol", RFC 903, Stanford, June 1984.
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[Sollins81] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783,
NIC,
June 1981.
Authors' Addresses
Prasenjit Sarkar
IBM Almaden Research Center
650 Harry Road
San Jose, CA 95120, USA
Phone: +1 408 927 1417
Email: psarkar@almaden.ibm.com
Duncan Missimer
Rhapsody Networks
3450 W Warren Avenue,
Fremont, CA 94538, USA
Phone: +1 510 743 3095
Email: dmissimer@rhapsodynetworks.com
Constantine Sapuntzakis
Stanford University
353 Serra Hall #406
Stanford, CA 94306, USA
Phone: +1 650 520 0205
Email: csapuntz@stanford.edu
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