INTERNET DRAFT Vivek Kashyap
<draft-ietf-ipoib-dhcp-over-infiniband-01.txt> IBM
Expiration Date: October 2002 April 2002
DHCP over InfiniBand
Status of this memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC 2026.
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
An InfiniBand network uses a link-layer addressing scheme that
is 20-bytes long. This is larger than the 16-bytes reserved for
the hardware address in DHCP/BOOTP message. The above
inequality imposes restrictions on the use of the DHCP message
fields when used over an IP over InfiniBand(IPoIB) network.
This document describes the use of DHCP message fields when
implementing DHCP over IPoIB.
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1. Introduction
The Dynamic Host Configuration Protocol(DHCP) provides a
framework for passing configuration information to hosts on a
TCP/IP network [RFC2131]. DHCP is based on the Bootstrap
Protocol (BOOTP) [RFC951] adding the capability of automatic
allocation of reusable network addresses and additional
configuration options [RFC2131,RFC2132].
The DHCP server receives a broadcast request from the DHCP
client. The DHCP server uses the client interface's
hardware-address to unicast a reply back when the client
doesn't yet have an IP address assigned to it. The 'chaddr'
field in the DHCP message carries the client's hardware
address.
The 'chaddr' field is 16-bytes in length. The IPoIB link-layer
address is 20-bytes in length. Therefore the IPoIB link-layer
address will not fit in the 'chaddr' field making it
impossible for the DHCP server to unicast a reply back to the
client.
To ensure interoperability the usage of the fields and the
method for DHCP interaction must be clarified. This document
describes the IPoIB specific usage of some fields of DHCP. See
[RFC2131] for the mechanism of DHCP and the explanations of
each field.
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 [RFC2119].
2. The DHCP over IPoIB mechanism
As noted above, because of the link-layer address length being
larger than the 'chaddr' field length the link-layer address
is unavailable to the DHCP server. Therefore, a DHCP client
MUST request that the server sends a broadcast reply by
setting the BROADCAST flag when IPoIB ARP is not possible i.e.
in situations where the client does not know its IP address.
RFC1542 notes that the use of a broadcast reply is discouraged
but in the case of IPoIB this is a necessity. There is no
option but to broadcast back to the client since it is not
possible to reply the client's unicast address. To
desynchronise broadcasts at subnet startup, the RFC2131
suggests that a client wait a random time (1 to 10 seconds)
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before initiating server discovery. The same timeout will
equally spread out the DHCP server broadcast responses
generated due to the use of the use of the BROADCAST bit.
The client hardware address, 'chaddr', is unique in the subnet
and hence can be used to identify the client interface. But in
the absence of a unique chaddr the client-identifier must be
used.
The DHCP protocol states that the 'client identifier' option
may be used as the unique identifying value for the client.
This value must be unique within the subnet the client is a
member of.
The client identifier option includes a type and identifier
pair. The identifier included in the client-identifier option
may consist of a hardware address or any other unique value
such as the DNS name of the client. When a hardware address is
used, the type field should be one of the ARP hardware types
listed in [ARPPARAM]. A type of 0 (zero) should be used when
the included identifier is other than a hardware address.
[RFC2132]
The client-identifier itself SHOULD not be interpreted by the
server. [RFC2132]
2.1 IPoIB specific usage of DHCP message fields
A DHCP client, when working over an IPoIB interface, MUST
follow the following rules:
'htype' (hardware address type) MUST be 32 [ARPPARAM]
'hlen' (hardware address length) MUST be 0.
The 'chaddr' (client hardware address) field MUST be zeroed.
The server and the relay agent MUST ignore 'chaddr' on
receipt.
The 'client identifier' option MUST be used in DHCP messages.
'client identifier' option MAY consist of any data.
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IPoIB clients SHOULD use the following format for the
client-identifier option:
Code Len Type Client-Identifier
+-----+-----+-----+-----+-----+-----+-----+-------------------....----+
| 61 | 21 | 32 |Interface-id (4 bytes) | GID (16 bytes) |
+-----+-----+-----+-----+-----+-----+-----+-------------------....----+
Every IPoIB interface is associated with an identifier
referred to as the GID [IPoIB_ARCH]. The GID is unique in the
InfiniBand fabric. An invariant GID is formed by appending the
port's EUI-64 identifier to the InfiniBand subnet prefix.
The GID is associated with a particular hardware port. The GID
and a QPN define an IPoIB interface at the port. Therefore an
implementation may associate multiple IPoIB interfaces on the
same port. It is up to the implementation to ensure a unique
client-identifier when multiple IPoIB interfaces are defined
over the same port and same GID. A unique, invariant
'interface-id' value be included in addition to the GID to
achieve this.
Note: a port may be associated with multiple GIDs. Therefore,
multiple IPoIB interfaces may exist on the same port while
using a different GID from among the GIDs associated with the
port.
A unique interface-id may be formed by including the QPN
associated with the relevant IPoIB interface if the
implementation is designed to keep this association constant
across boots. A timestamp or some other value unique to the
implementation may also be used for the same purpose.
If there is only one IPoIB interface associated with a
particular GID, then use of the GID with the 'interface-id'
zeroed is sufficient. By default, an implementation zeroes out
the interface-id field in the client identifier described
above.
This document does not preclude the use of other 'client
identifier' type, such as fully qualified domain name(FQDN) or
the EUI-64 value associated with the interface.
2.2 Use of the BROADCAST flag
A DHCP client on IPoIB SHOULD set a BROADCAST flag in
DHCPDISCOVER and DHCPREQUEST messages (and set 'ciaddr' to
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zero) to ensure that the server (or the relay agent)
broadcasts its reply to the client.
Note: As described in [RFC2131], 'ciaddr' MUST be filled in
with client's IP address during BOUND, RENEWING or REBINDING
state, therefore, the BROADCAST flag MUST NOT be set. In these
cases, the DHCP server unicasts DHCPACK message to the address
in 'ciaddr'. The link address will be resolved by IPoIB ARP.
3. Security Considerations
DHCP currently provides no authentication or security
mechanisms. Potential exposures to attack are discussed
in section 7 of the DHCP protocol specification [RFC2131].
A malicious client can falsify the client-identifier,
thus masquerading as another client.
4. Acknowledgement
This document borrows extensively from [RFC 2855]. Roy Larsen
pointed out the length discrepancy between the IPoIB link
address and DHCP's chaddr field.
References
[RFC2119] Key words for use in RFCs to Indicate Requirement Levels
S, Bradner
[RFC2131] Dynamic Host Configuration Protocol, R. Droms
[RFC2132] DHCP Options and BOOTP Vendor Extensions,
S. Alexander, R. Droms
[RFC951] Bootstrap Protocol, B. Croft, J. Gilmore
[RFC1542] Clarifications and Extensions for the Bootstrap Protocol
W. Wimer
[ARPPARAM] http://www.iana.org/numbers.html
[RFC2855] DHCP for IEEE 1394, K. Fujisawa
[IPoIB_ARCH] draft-ietf-ipoib-architecture-01.txt, V. Kashyap
[IPoIB_ENCAP] draft-ietf-ipoib-ip-over-infiniband-00.txt,
V. Kashyap, H.K. Jerry Chu
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Author's Address
Vivek Kashyap
IBM
15450, SW Koll Parkway
Beaverton
OR 97006
Phone: +1 503 578 3422
EMail: vivk@us.ibm.com
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