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dhc-minutes-92mar.txt
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This is only a rough draft - Megan 04/15/92
Here are the minutes from the DHC WG meeting in San Diego:
The DHC WG held two meetings in San Diego, on Tuesday afternoon and Thursday
morning. In addition, Ralph Droms gave a technical presentation to the IETF
summarizing DHCP. In the Tuesday meeting, the WG discussed changes to DHCP,
as reflected in a new version of the DHCP Internet Draft, dated March, 1992:
o Modified introductory text to state that DHCP allows but does not
require the configuration of higher level protocols. Vendor extensions
for NIS have been defined. Others will be allowed.
o Extended the definitions of the 'htype', 'hlen' and 'chaddr' fields to
accommodate client identifiers that might not be hardware addresses.
o Replaced 'byte' with 'octet' throughout.
o Added text allowing servers to use the same local broadcast mechanism
as BOOTP relay agents when sending DHCP messages to clients.
o Added a new mechanism for clients that do not request dynamic address
allocation.
o Added a new 'message' vendor extension, which may be used by a server
to pass an ASCII text message to a client, e.g., an error message if
the server cannot satisfy a client request.
Several minor changes were discussed, which will be integrated into a new
version of the DHCP I-D:
o The I-D will give details of mechanisms through which hosts can check
to determine if its network address is already in use by another
host. For example, a host using ARP can issue an ARP request for
its network address to see if another host answers. As neither
RFC 826 nor RFC 1122 address this use of ARP, the DHCP I-D must
explicitly describe how a host can issue an ARP request for its own
address; specifically, the host must use some other IP address (e.g.,
0.0.0.0) as the 'sender's protocol address' in the ARP request to avoid
corrupting other hosts' ARP caches.
o Hosts must delay some short, random time at boot-up before issuing
an initial DHCP message, to avoid swamping the network with multiple
simultaneous DHCP messages in the case of synchronized host boot-up
(e.g., power fail/recover cycle).
o Figure 2 will be modified to include a second server and to delete the
final DHCPRELEASE message.
o Section 6.2.1 will be rewritten to reflect the use of the new REBOOTING
state and DHCPREQUEST message by hosts that retain a network address
across system reboots.
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o The 'message' vendor extension will specify the use of NVT ASCII text.
Use of MIME multi-media message format was considered and rejected.
o The definitions of the vendor extensions (Appendix D in the March, 1992
Internet Draft) will be extracted and submitted as a separate Internet
Draft. Definition of new vendor extensions in the future will require
only the resubmission of the vendor extension Internet Draft, rather
than the entire DHCP Internet Draft.
o The DHCP specification Internet Draft will be rewritten with careful
use of MUST/MUST NOT/SHOULD/SHOULD NOT/MAY to reduce ambiguity in the
specification. Special attention will be paid to the specification of
the fields to be used and the valid vendor extensions in each of the
DHCP messages.
There were also several topics that sparked longer discussions. These
topics, in general, remain unresolved:
o The relationship between BOOTP and DHCP should be clarified by renaming
DHCP as BOOTP.
o Various levels of compliance with the DHCP/BOOTP specification should
be indicated by ``BOOTP Level 0,'' ``BOOTP Level 1'' and ``BOOTP Level
2.''
DISCUSSION: There is the potential for confusion on the part of naive
users about the relationship between BOOTP and DHCP. For example,
BOOTP clients will continue to work with DHCP servers; will DHCP
clients work with BOOTP servers? The relay agents will still be
known as ``BOOTP relay agents''; will those work with DHCP clients and
servers?
Some client implementations of DHCP may not include all the functions
specified in the DHCP Internet Draft. In particular, members of the WG
have expressed concern that some DHCP clients may be unable to enforce
the network address lease rules, and may always require allocation of a
network address with an infinite lease.
The WG proposes renaming DHCP to BOOTP, and subsuming the specification
of the current BOOTP protocol into the DHCP/BOOTP document. The
various type of DHCP/BOOTP service would be names:
-- BOOTP Level 0 -- BOOTP as defined in RFC 951.
-- BOOTP Level 1 -- DHCP/BOOTP with automatic network address
allocation and only infinite leases.
-- BOOTP Level 2 -- DHCP/BOOTP with fully dynamic (finite lease)
network allocation.
o Does DHCP need to deliver more parameters in vendor extensions than
will fit in a single DHCP message?
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o If DHCP delivers more parameters than will fit in a single DHCP
message, how should those additional parameters be delivered?
DISCUSSION: Some members of the WG contend that DHCP must, indeed, be
prepared to deliver more parameters than will fit in a single DHCP
message. One argument in favor of that view is that specific sites
will want to deliver additional DHCP parameters, and, if the DHCP
document does not explicitly specify the use of a particular mechanism,
each site may choose a local (and potentially incompatible) mechanism.
Other members of the WG feel that DHCP can deliver sufficient
parameters in a single message to configure a host. Based on
the currently defined vendor extensions, and assuming ``reasonable''
lengths for variable length vendor extensions, DHCP could send all
parameters specified by vendor extensions in roughly 280 octets.
Counting the 'sname' and 'file' fields, a single DHCP message can
deliver 502 octets of vendor extensions. Thus, DHCP can, at present,
deliver all necessary parameters to a host in a single DHCP message.
The WG discussed a mechanism called a ``bill of lading'' to provide
reliable delivery of vendor extensions in multiple DHCP messages,
if DHCP is defined to support vendor extensions in excess of 502
octets. A bill of lading is a bit vector representing all 256 vendor
extensions. The DHCP server will deliver a bill of lading to the host,
indicating which vendor extensions the host must receive with a 1 in
the corresponding bit in the bit vector. The host will then repeatedly
ask for DHCP parameters, checking off specific vendor extensions as
they arrive, until all specified vendor extensions have been delivered.
o Should a host use DHCP at every reboot to reacquire network parameters?
o If the host cannot contact a DHCP server at reboot, should a host be
allowed to reuse previously acquired network parameters?
1. Required use of DHCP: A DHCP host should likely be required to use
DHCP whenever the local network parameters may have changed (e.g.,
system reboot, network failure and restart), as connected network
may change w/o host's or user's knowledge (consider change inside
wiring closet w/o user notification).
2. Application of lease: does the lease apply to just the network
address or does it apply to all network parameters? I.e., host
is allowed to reuse stored network address (careful here; host may
have bogus network address and not be able to contact server).
3. Authority of DHCP server: should server be able to force host to
take new network values; i.e., "remote manage" the host?
DISCUSSION: The conversation about this topic began with consideration
of whether or not to allow a DHCP host should be allowed to continue if
it cannot contact a DHCP server at reboot. THere were strong arguments
for and against -- for: allows for partial network operation even if
DHCP servers are inaccessible; against: may allow misconfigured hosts
to operate. Note that only network parameters can be misconfigured;
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DHCP guarantees that any network address will be assigned to only one
DHCP host.
The WG then discussed the relation between the 'lease' and network
parameters; should the lease apply to all parameters or just to the
network address. A proposal was floated for another bit-vector to
describe those network parameters to which the lease applied. The
idea here is to provide some degree of network management through the
guarantee that DHCP hosts would periodically reacquire new (possibly
changed) network parameters.