Dynamic Host Configuration Protocol (DHCP) over InfiniBand
RFC 4390

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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

Kashyap                     Standards Track                     [Page 1]
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
   [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 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 [RFC4391].  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 [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 [RFC2119].

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.

   [RFC1542] 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,
   [RFC2131] 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.

Kashyap                     Standards Track                     [Page 2]
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 [RFC2132].  This
   value must be unique within the client's subnet.

   The "client identifier" option includes a type and identifier pair.