INTERNET DRAFT                                          Vivek Kashyap
<draft-ietf-ipoib-dhcp-over-infiniband-10.txt>                    IBM
Expiration Date: September 2005                            March 2005



                          DHCP over InfiniBand


Status of this memo

    By submitting this Internet-Draft, I certify that any applicable
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    Copyright (C) The Internet Society (2001).  All Rights Reserved.


Abstract

    An InfiniBand network uses a link-layer addressing scheme that is
    20-octets long. This is larger than the 16-octets 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.

. Introduction

    The Dynamic Host Configuration Protocol (DHCP) provides a framework



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    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 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-octets in length. The IPoIB link-layer
    address is 20-octets 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 [RFC2119].

. 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 back to the client.  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 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 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", another unique client identifier must
    be used.




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    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 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].

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
    [DHC_3315id].

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 [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 ARP.

. Security Considerations

    [RFC2131] describes the security considerations relevant to DHCP.
    This document does not introduce any new issues.

. Acknowledgement

    This document borrows extensively from [RFC2855]. Roy Larsen




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    pointed out the length discrepancy between the IPoIB link address
    and DHCP's "chaddr" field.

. References

5.1 Normative

    [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

    [IPoIB_ENCAP]   draft-ietf-ipoib-ip-over-infiniband-09.txt,
                    H.K. Jerry Chu, V. Kashyap

    [ARPPARAM]      http://www.iana.org/numbers.html

    [IBARCH]        InfiniBand Architecture Specification,
                    www.infinibandta.org/specs

    [IPoIB_ARCH]    draft-ietf-ipoib-architecture-04.txt, V. Kashyap

    [DHC_3315id]    draft-ietf-dhc-3315id-for-v4-04.txt,
                    T. Lemon, B. Sommerfeld

5.2 Informative

    [RFC2855]       DHCP for IEEE 1394, K. Fujisawa

    [RFC1542]       Clarifications and Extensions for the Bootstrap Protocol,
                    W. Wimer

. 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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Acknowledgment

    Funding for the RFC Editor function is currently provided by the
    Internet Society.









Kashyap                                                         [Page 5]