Network Working Group Vivek Kashyap
Request for Comments: 4390 IBM
Category: Standards Track April 2006
Dynamic Host Configuration Protocol (DHCP) over InfiniBand
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
IP over Infiniband (IPoIB) link-layer address is 20 octets long.
This is larger than the 16 octets reserved for the hardware address
in a Dynamic Host Configuration Protocol/Bootstrap Protocol
(DHCP/BOOTP) message. The above inequality imposes restrictions on
the use of the DHCP message fields when used over an IPoIB network.
This document describes the use of DHCP message fields when
implementing DHCP over IPoIB.
Table of Contents
1. Introduction ....................................................2
2. The DHCP over IPoIB Mechanism ...................................2
2.1. IPoIB-specific Usage of DHCP Message Fields ................3
2.2. Use of the BROADCAST flag ..................................3
3. Security Considerations .........................................3
4. Acknowledgement .................................................4
5. References ......................................................4
5.1. Normative References .......................................4
5.2. Informative References .....................................4
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RFC 4390 DHCP Over Infiniband April 2006
1. Introduction
The Dynamic Host Configuration Protocol (DHCP) provides a framework
for passing configuration information to hosts on an IP network
[RFC 2131]. DHCP is based on the Bootstrap Protocol (BOOTP) [RFC 951]
adding the capability of automatic allocation of reusable network
addresses and additional configuration options [RFC 2131,RFC 2132].
The DHCP server receives a broadcast request from a client. The DHCP
server uses the client interface's hardware address to unicast a
reply when the client does not 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 octets in length. The IPoIB link-layer
address is 20 octets in length [RFC 4391]. Therefore, the IPoIB
link-layer address will not fit in the "chaddr" field making it
impossible for the DHCP server to unicast a reply 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 [RFC 2131] 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].
2. The DHCP over IPoIB Mechanism
As described above, the link-layer address is unavailable to the DHCP
server because the link-layer address is larger than the "chaddr"
field length. As a result, the server cannot unicast its reply to
the client. Therefore, a DHCP client MUST request that the server
send a broadcast reply by setting the BROADCAST flag when IPoIB
Address Resolution Protocol (ARP) is not possible, i.e., in
situations where the client does not know its IP address.
[RFC 1542] discourages the use of a broadcast reply. But in the case
of IPoIB, this is a necessity because the server does not receive the
link-layer address. To desynchronise broadcasts at subnet startup,
[RFC 2131] suggests that a client wait a random time (1 to 10 seconds)
before initiating server discovery. The same timeout will spread out
the DHCP server broadcast responses generated due to the use of the
BROADCAST bit.
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RFC 4390 DHCP Over Infiniband April 2006
The client hardware address, "chaddr", is unique in the subnet and
hence can be used to identify a client interface. But in the absence
of a unique "chaddr", another unique 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 [RFC 2132]. This
value must be unique within the client's subnet.
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].
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.
"chaddr" (client hardware address) field MUST be zeroed.
"client-identifier" option MUST be used in DHCP messages.
The "client identifier" used in DHCP messages MUST conform to
[RFC 4361].
2.2. Use of the BROADCAST flag
A DHCP client on IPoIB MUST set the BROADCAST flag in DHCPDISCOVER
and DHCPREQUEST messages (and set "ciaddr" to zero) to ensure that
the server (or the relay agent) broadcasts its reply to the client.
Note: As described in [RFC 2131], "ciaddr" MUST be filled in with the
client's IP address during BOUND, RENEWING or REBINDING states;
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 ARP.
3. Security Considerations
[RFC 2131] describes the security considerations relevant to DHCP.
This document does not introduce any new issues.
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RFC 4390 DHCP Over Infiniband April 2006
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.
5. References
5.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[RFC 951] Housley, R., Horting, T., and P. Yee, "TELNET
Authentication Using KEA and SKIPJACK", RFC 2951,
September 2000.
[RFC 4391] Chu, J. and V. Kashyap "Transmission of IP over
InfiniBand (IPoIB)", RFC 4391, April 2006.
[ARPPARAM] http://www.iana.org/numbers.html
[RFC 4361] Lemon, T. and B. Sommerfeld, "Node-specific Client
Identifiers for Dynamic Host Configuration Protocol
Version Four (DHCPv4)", RFC 4361, February 2006.
5.2. Informative References
[RFC 2855] Fujisawa, K., "DHCP for IEEE 1394", RFC 2855, June
2000.
[RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
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RFC 4390 DHCP Over Infiniband April 2006
Author's Address
Vivek Kashyap
15350, SW Koll Parkway
Beaverton, OR 97006
USA
Phone: +1 503 578 3422
EMail: vivk@us.ibm.com
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RFC 4390 DHCP Over Infiniband April 2006
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