INTERNET DRAFT                                          Vivek Kashyap
<draft-ietf-ipoib-dhcp-over-infiniband-07.txt>                    IBM
Expiration Date: May 2005                               November 2004

                          DHCP over InfiniBand

Status of this memo

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    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.

1. Introduction

    The Dynamic Host Configuration Protocol(DHCP) provides a framework

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    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-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",
    document are to be interpreted as described in [RFC2119].

2. The DHCP over IPoIB mechanism

    As is noted above, the link-layer address is unavailable to the DHCP
    server because it is larger than the "chaddr" field length.
    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

    [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, [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.  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.

    According to [RFC2132] the "client-identifier" option MAY consist of
    any data, but IPoIB clients SHOULD use the format discussed below
    for the client-identifier option.

    Note: 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

2.1.1 Client-identifier values

    Every IPoIB interface is associated with an identifier referred to
    as the GID [IPoIB_ARCH]. A GID is formed by appending the port's
    EUI-64 identifier to the InfiniBand subnet prefix. An invariant GID
    is formed when the port's manufacturer assigned EUI-64 value is used
    to form the GID. A port might have additional EUI-64 values assigned
    to it by the subnet-manager(SM) [IBARCH]. Therefore a port can have
    multiple GIDs associated with it. A GID is unique in the InfiniBand

    The GID is associated with a particular hardware port. The GID and a
    QPN define an IPoIB interface at the port[IPOIB_ENCAP].  Therefore
    an implementation could associate multiple IPoIB interfaces on the
    same port by utilising a common GID but different QPNs. In such a
    case the GID is shared between multiple interfaces, and therefore,

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    the client identifier formed from just the GID is no longer unique
    in the IP subnet.

    This is not an issue if the interfaces sharing the GID are in
    different InfiniBand partitions, and thereby on different IPoIB
    links, since the client identifier need only be unique within a
    subnet. However, if the GID is shared by interfaces within the same
    partition the implementation MUST ensure a unique client identifier.
    For example, a unique client-identifier 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. 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 within a partition, then use of the GID is sufficient.

    Since a port may be associated with multiple GIDs, multiple IPoIB
    interfaces may exist on the same port while using a different GID
    from among the GIDs associated with the port. In such a case too the
    GID can form a unique client identifier.

    Therefore, one of the following formats SHOULD be used for the
    "client-identifier" option.

    1. If the QPN is used to distinguish between interfaces using the
    same GID.

    Code  Len   Type |<---------------- Client-Identifier -------------->|
   |  61 | 21  |  32 |      20 octets(link-layer address)                |

    2. A unique value, other than the QPN, may be used to distinguish
    between interfaces using the same GID. In this case a "type" of 0
    MUST be specified since the "client identifier" is not a hardware
    address [RFC2132].

    Code  Len   Type |<---------------- Client-Identifier -------------->|
   |  61 | 21  |  00 | Unique-value(4 octets)|   16 octet GID            |

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    But if the GID is not shared with another IPoIB interface then there
    is no need for another "unique-value". In such a case the GID
    suffices by itself.

    Code  Len   Type |<---------------- Client-Identifier -------------->|
   |  61 | 21  |  00 |      00 (4 octets)    |   16 octet GID            |

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.

3. Security Considerations

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

4. Acknowledgement

    This document borrows extensively from [RFC2855]. Roy Larsen pointed
    out the length discrepancy between the IPoIB link address and DHCP's
    "chaddr" field.

5. 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

    [RFC1542]       Clarifications and Extensions for the Bootstrap Protocol,

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                    W. Wimer

    [RFC951]        Bootstrap Protocol, B. Croft, J. Gilmore

    [IPoIB_ENCAP]   draft-ietf-ipoib-ip-over-infiniband-07.txt,
                    H.K. Jerry Chu, V. Kashyap


    [IBARCH]        InfiniBand Architecture Specification,

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

5.2 Informative

    [RFC2855]       DHCP for IEEE 1394, K. Fujisawa

6. Author's Address

    Vivek Kashyap

    15350, SW Koll Parkway
    Beaverton, OR 97006
    Phone: +1 503 578 3422

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