Internet DRAFT - draft-ietf-dhc-implementation
draft-ietf-dhc-implementation
Network Working Group R. Hibbs
INTERNET-DRAFT Richard Barr Hibbs, P.E.
Category: Informational R. Stevens
Expires: December 17, 2006 (no affiliation)
June 15, 2006
Implementation Issues with RFC 2131, "Dynamic Host Configuration
Protocol (DHCPv4)"
<draft-ietf-dhc-implementation-02.txt>
Saved: Tuesday, June 15, 2006, 13:27:17
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo identifies implementation issues with RFC 2131, "Dynamic
Host Configuration Protocol," reported by a number of implementers,
assesses the severity of the problem, then proposes changes to RFC
2131 intended to overcome the issues. This is intended for use as
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the basis for discussion of RFC 2131 before it is proposed for
Internet Standard status.
Table of Contents
1 Introduction...............................................4
2 Terminology................................................4
3 Applicability..............................................4
4 Issues with RFC 2131.......................................5
4.1 Outdated RFC Boilerplate...............................5
4.2 Organization and Typography............................5
4.2.1 Outdated References.............................5
4.2.2 Typographical Errors............................5
4.2.3 Omissions.......................................6
4.2.4 Tables..........................................6
4.2.5 Inconsistencies.................................7
4.3 Policy Issues..........................................7
4.4 The Client Hardware Address, "chaddr"..................8
4.5 The DHCP Client Identifier.............................8
4.5.1 Uniqueness......................................8
4.5.2 Prohibition in DHCPOFFER and DHCPACK............9
4.6 Duplicate Address Detection............................9
4.6.1 Client-side ARP................................10
4.6.2 Server side PING...............................10
4.6.3 Other Mechanisms...............................10
4.7 DHCP Relay Agents.....................................11
4.7.1 Relay Agent Source Addresses...................11
4.7.2 Relay Agent Port Usage.........................11
4.8 Host Name, Domain Name, and FQDNs.....................11
4.9 Overloading of DHCPREQUEST............................11
4.10 DHCPINFORM...........................................12
4.11 Unicast of DHCPDISCOVER..............................12
4.12 DHCPRELEASE..........................................13
4.13 Client State Diagram.................................13
4.14 Options..............................................14
4.14.1 Which Options to Return?......................14
4.14.2 Multiple Instances of Options.................16
4.14.3 Option Ordering...............................16
4.14.4 Options 66 and 67.............................16
4.15 Vendor Classes.......................................16
4.15.1 Character Set.................................17
4.15.2 Form of the Name Space........................17
4.15.3 Relationship to Vendor Options................17
4.15.4 Multiplicity..................................17
4.16 Client/Server Retransmission.........................18
4.17 Transmission of DHCPNAKs.............................18
4.18 Use of ciaddr........................................19
4.19 Size of a BOOTP/DHCP Frame...........................19
4.19.1 Minimum Packet Size...........................19
4.19.2 Maximum Size, MTU, and Message Size Option....19
4.20 Use of giaddr........................................20
4.21 Address Selection....................................20
4.22 Use of "secs" Field..................................21
4.23 Use of "htype" and "hlen" Fields.....................21
4.24 Use of "xid" Field...................................22
4.25 Options in DHCPOFFER and DHCPACK.....................22
4.26 Lease Times..........................................23
4.27 Miscellaneous........................................23
5 Proposed Replacements for RFC 2131 Figures and Tables.....25
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5.1 Figures...............................................25
5.1.1 Figure 1: Format of a DHCP message..............25
5.1.2 Figure 2: Format of the 'flags' Field...........25
5.1.3 Figure 3: Timeline Diagram--Allocating a New
Address...............................26
5.1.4 Figure 4: Timeline Diagram--Reusing an Address..27
5.1.4 Figure 5: State-Transition Diagram for DHCP
Clients...............................28
5.2 Tables................................................29
5.2.1 Table 1: Description of fields in a DHCP
Message................................29
5.2.2 Table 2: DHCP Messages..........................30
5.2.3 Table 3: Fields Used by DHCP Servers............31
5.2.4 Table 4: Options Used by DHCP servers...........32
5.2.5 Table 5: Client Messages from Different States..32
5.2.6 Table 6: Fields Used by DHCP Clients............33
5.2.7 Table 7: Options Used by DHCP Clients...........34
5.2.8 Table 8: Host Configuration Parameters--IP
Layer..................................35
5.2.9 Table 9: Host Configuration Parameters--Link
Layer..................................36
5.2.10 Table 10: Host Configuration Parameters--TCP...36
6 Contributors..............................................37
7 IANA Considerations.......................................37
8 Security Considerations...................................37
9 References................................................37
9.1 Normative References..................................37
9.2 Informative References................................37
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1 Introduction
This memo was produced by the DHC Working Group and attempts to
identify all known implementation issues with RFC 2131 as a basis
for discussion of RFC 2131 before it is published as an Internet
Standard.
This memo grew from a discussion item during the DHC Working Group
meeting at IETF-55 in Atlanta during November 2002.
The editors have solicited input through a general call for
participation and by direct request to all implementers that they
could identify.
There are four possible outcomes of this work:
1. The RFC Editor could publish the agreed clarifications as a note
("Errata") linked to RFC 2131 in the RFC Editor's document
database. This almost insures that only the most conscientious
reviewer would ever read and apply the changes.
2. The proposed changes could be reworded and reformatted into a
new standards-track document, "RFC 2131 Errata." This has the
advantage of not disturbing RFC 2131 as it advances to Internet
Standard, but leaves the reader with multiple documents to read
for a full understanding of DHCP.
3. Proposed changes could be applied to RFC 2131 without assigning
a new document number, in effect becoming "RFC 2131-bis." This
course may not be open as the changes would still require IESG
approval, and it is unlikely that any changes other than
editorial or clarification would be permitted.
4. A new standards-track document would be created, obsoleting RFC
2131 (and possibly other documents as well.) This would
probably not require any more effort than outcome (2), but
conceivably take years to advance the document to full Standard.
The editors have not specifically addressed RFC 2132, although we
believe that it ought to be updated in conjunction with any updates
to RFC 2131. We propose that an update of the DHCP Options Document
should be separately considered in a second memo.
2 Terminology
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 [RFC2119].
3 Applicability
The intent of the work item that resulted in this memo was to
identify and clarify issues with RFC 2131 that made implementation
difficult, behavior ambiguous, or conflicted with other RFCs. The
authors imagined that RFC 2132 could also be updated as a result,
without expecting that additional RFCs might be affected.
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As our investigation proceeded, it became evident that
clarifications might very well extend to other RFCs. The exact
scope of this effort will require additional discussion by the DHC
Working Group.
4 Issues with RFC 2131
This list may not include every implementation issue for RFC 2131 as
it is based on reported problems and those known to the editors.
4.1 Outdated RFC Boilerplate
RECOMMENDATIONS:
1. "Status of This Memo" should be replaced with standardized
language for RFCs as described in "Guidelines to Authors of
Internet-Drafts," dated March 25, 2005.
2. Section 1.4, "Requirements," should be replaced with
standardized language referring to RFC 2119 regarding the
definition and interpretation of specific key words.
3. References should be separated into normative and non-normative
sections.
4.2 Organization and Typography
4.2.1 Outdated References
RECOMMENDATIONS:
O References to the "Assigned Numbers" RFC [STD 2, RFC 1700]
should be changed to the "Assigned Numbers" database maintained
by IANA. References are found in Tables 3 and 5, and in the
"References" section.
O References to the "Interaction between DHCP and BOOTP" RFC 1534
should be integrated with the text of the memo, and RFC 1534
deprecated.
4.2.2 Typographical Errors
RECOMMENDATIONS:
O Page 23, third paragraph of section 4.1 -- "received" should be
"received."
O Page 23, sixth paragraph of section 4.1 -- refers to RFC 1533,
not RFC 2132.
O Page 15, Figure 3. Table is misformatted.
O Page 18, Figure 4. Table is misformatted.
O Page 25, ninth paragraph of section 4.1 -- "uicast" should be
"unicast."
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O Page 38, first paragraph after Table 5. Orphaned sentence:
"The DHCPREQUEST message contains the same 'xid' as the
DHCPOFFER message." No, it does not. Not only that, but this
sentence makes no sense in its current location. It should be
removed.
O Page 39, Last paragraph of 4.4.3 should be moved up as the last
paragraph of 4.4.2. When the text for DHCPINFORM was added, the
text describing what a client should do if no DHCPACK is
received was mistakenly pushed below it.
O Apostrophes (') are used as single quotation marks, but outside
of an enclosing quotation (") throughout the document.
O Page 30, section 4.3.1, second from last bullet: "...client’s
vendor class identifier and client's classes identified in the
server". This text makes no sense and should be deleted.
O Inconsistent style regarding placement of periods (.), commas
(,) and semi-colons (;) with respect to quotation marks
throughout the document.
O Quotation marks (single and double) are overused through the
document.
4.2.3 Omissions
In several places there is missing or incomplete information,
including:
O Table 3, pages 27 and 28. The "options" entry for "DHCPNAK" in
the "fields" portion of the table is missing. All entries in
this line should refer to the subsequent "options" table.
Suggested replacements for Table 3 are shown in sections 5.2.3
and 5.2.4.
O Page 33, Table 4, and Page 35, Figure 5, do not include the
"DHCPINFORM" message. Suggested replacement for Table 4 is
shown in section 5.2.5.
O Table 5, pages 37 and 38. The "options" entry for "DHCPDECLINE,
DHCPRELEASE" is missing. Suggested replacement for Table 5 is
shown in sections 5.2.6 and 5.2.7.
4.2.4 Tables
RECOMMENDATIONS:
O Table 3 (pages 28 and 29) and Table 5 (pages 37 and 38) should
be separated into two tables each for readability. Suggested
replacements for Table 3 are shown in sections 5.2.3 and 5.2.4,
and for Table 5 are show in sections 5.2.6 and 5.2.7.
O Table 4 should be reorganized to show all messages (except
DHCPRELEASE) that are sent from each client state. Suggested
replacement for Table 4 is shown in section 5.2.5.
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O The "Host Configuration Parameters" table now in an appendix
should be omitted and described in a revised "DHCP Options"
document (RFC 2132). Suggested replacement for that table is
shown in sections 5.2.8-5.2.10.
4.2.5 Inconsistencies
RECOMMENDATIONS:
O Page 1, Abstract, "TCPIP" should be "TCP/IP" – as it is in the
rest of the document.
O Page 10, Table 3, description of 'htype' and 'hlen' fields does
not capitalize "Ethernet."
O Lack of consistency when describing "IP broadcast." Sometimes
it is "0xffffffff IP broadcast," elsewhere "limited broadcast,"
or "broadcast." Suggest using the "255.255.255.255 IP broadcast
address" form, as that is the most specific. References
include:
a. Page 19, third paragraph of section 3.2, List item #2.
b. Page 23, fifth paragraph of section 4.1 (twice).
c. Page 25, thirteenth paragraph of section 4.1 (twice).
d. Page 32, section 4.3.2, third bullet item.
e. Page 32, section 4.3.2, fifth bullet item.
f. Page 36, second paragraph of section 4.4.1.
g. Page 39, last paragraph of section 4.4.1.
h. Page 39, second paragraph of section 4.4.3.
4. Lack of consistency when referring to the BROADCAST (B) flag:
it is also referred to as the "broadcast bit."
5. Table 3, "Fields and options used by DHCP servers," is
problematic. It indicates that we MUST fill in both the "Server
Identifier" (and siaddr) in our DHCPACK (and DHCPNAK) response.
That is a change from RFC 1541 (which specifies a "MAY" and
which is consistent with RFC 2132 section 9.7 and identical to
RFC 1533 section 9.5 wording).
4.3 Policy Issues
There has in general been a certain amount of overlap in DHCP
between protocol and policy. The matters include lease times,
whether servers are willing to extend leases, timeouts, and re-
transmission.
We SHOULD clarify what is dictated by the protocol and what is a
policy decision at a given site.
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The DHC Working Group philosophy ought to be to constrain client
behavior more closely than server behavior. DHCP interactions are
initiated (and continued) by clients: clients outnumber servers by
many tens of thousands to one; client implementers cannot be quite
certain of all the environments in which their client may ultimately
appear, whereas server implementations may be designed for very
specific environments. Policy is likely to be a matter of
centralized control, whereas clients are not likely to enjoy a
sufficient status to impose policy on servers.
The previous paragraph implies that the WORKING GROUP should tighten
the protocol with respect to such issues as retries and backoffs,
whereas servers should not be constrained on issues such as how to
uniquely identify clients, whether to offer or extend leases etc.
4.4 The Client Hardware Address, "chaddr"
The value of "chaddr" MUST NOT change from DHCPDISCOVER to
DHCPREQUEST, although the wording in Table 3 makes this point
unclear.
Further, the length of "chaddr" SHOULD be exactly specified by
"hlen," which SHOULD match the address length for "htype."
RECOMMENDATIONS:
O Update Table 3.
4.5 The DHCP Client Identifier
4.5.1 Uniqueness
DHCP servers must uniquely identify DHCP clients requesting services
in order to configure the client correctly. DHCP does not require
global uniqueness for client identifiers, only uniqueness within the
scope of [sub-] networks reachable by DHCP packets in any
installation. This is sometimes called "subnet uniqueness."
RFC 2131 provides two specific methods for identifying a client:
(1) the client identifier (DHCP Option 61) [RFC2132], and (2) the
"chaddr" field of the BOOTPREQUEST packet.
Confusion arises from the language of RFC 2131 Section 4.2. A DHCP
client "...MAY choose to explicitly provide the identifier through
the 'client identifier' option. If the client supplies a 'client
identifier,' the client MUST use that same identifier in all
subsequent messages, and the server MUST use that identifier to
identify the client. If the client does not provide a 'client
identifier' option, the server MUST use the contents of the 'chaddr'
field to identify the client."
The text of Section 4.2 goes on to state that subnet uniqueness is a
requirement for an identifier, but points out that "chaddr" may not
satisfy that requirement. Two alternatives for a unique identifier
were given: an unspecified manufacturer's serial number or a DNS
name.
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RFC 2132 adds to the confusion by stating that the client identifier
"...is expected to be unique for all clients in an administrative
domain" without specifying what an "administrative domain" is.
RFC 2132 continues by suggesting use of "...type-value pairs similar
to the 'htype'/'chaddr' fields defined in" [RFC951], and that a
"...hardware type of 0 (zero) should be used when the value field
contains an identifier other than a hardware address (e.g., a fully
qualified domain name)."
This suggestion of using type-value pairs has been widely adopted by
DHCP client implementers, but the suggestion fails to heed the
warning about uniqueness issues with "chaddr."
RECOMMENDATIONS:
1. RFC 2131 SHOULD have made required the "client identifier"
either to be globally unique or, to be unique within an
"administrative domain," and, in the latter case, defined
"administrative domain."
2. RFC 2131 SHOULD NOT have suggested the use of DNS names for the
"client identifier" without also suggesting some mechanism for
maintaining a consistent name-to-address mapping.
3. RFC 2132 SHOULD NOT have suggested using the "htype" and
"chaddr" fields as a type-value pair because of the warning in
RFC 2131 Section 4.2 about potential problems using "chaddr" for
the purpose.
4. RFC 2132 SHOULD NOT have used the word "SHOULD" when suggesting
the use of type-value pairs for "client identifier" with a type
of 0 (zero) when the value is anything other than a hardware
address.
4.5.2 Prohibition in DHCPOFFER and DHCPACK
Table 3, in the "options" section, specifies that the server MUST
NOT send the "client identifier" in the DHCPOFFER or DHCPACK
messages, but MAY send it in a DHCPNAK message. There is no good
reason why DHCPNAK should be treated differently, and there is
considerable utility in returning the client identifier, as it
allows clients further corroboration, beyond that implied by
matching "xid’s" (see 2.23), that they are the intended recipient.
RECOMMENDATION:
Change the text in Table 3, for all three message-types, to read,
"MAY -- if included, MUST BE the client identifier sent by the
client." Suggested replacement for Table 3 are shown in sections
5.2.3 and 5.2.4.
4.6 Duplicate Address Detection
RFC 2131 Page 7, section 1.6, second set of bullet items, first
bullet says that DHCP must: "Guarantee that any specific network
address will not be in use by more than one client at a time," but
the protocol as later described does not fulfill this requirement.
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Two mechanisms are presented: an ARP request generated by the
client, and an ICMP ECHO request generated by the server:
O Page 12, second paragraph of section 2.2, last sentence.
O Page 15, list item 2, section 3.1.
O Page 38, first paragraph after Table 5, section 4.4.1.
4.6.1 Client-side ARP
To meet the requirement of RFC 2131 page 7, a DHCP client MUST send
an ARP request for the IP address contained in a DHCPACK before
using it. This is presently a SHOULD:
Page 12, second paragraph of section 2.2: "... and the
client SHOULD probe the newly received address, e.g., with
ARP."
There has been confusion on this topic because many clients are
sending an ARP reply (after the DHCPACK). This often has nothing to
do with DHCP, and is triggered in many systems whenever an interface
IP address changes. (Without access to kernel code, there is
nothing to be done about it.)
RECOMMENDATION:
The Working Group should consider whether to make client ARP a MUST.
4.6.2 Server side PING
ICMP is inherently unreliable. Furthermore since success is "no
response" it is an imprecise matter to decide how long to wait
before one is certain that no response will ever occur. A possible
suggestion is a back off and retry for ping.
In cable modem environments, PING is not helpful because it is the
cable modem termination system (CMTS) that replies from its cache:
a cache which may not be perfectly reliable and which in many cases
has been constructed by listening to the DHCP traffic in the first
place!
It is known that some network administrators try to block
propagation of ICMP ECHO messages through internal routers, which
removes one of the two address conflict detection mechanisms.
RECOMMENDATION:
Use of ICMP on the server should be a "MAY", not a "SHOULD".
4.6.3 Other Mechanisms
Both the ICMP ECHO (Ping) and Address Resolution Protocol (ARP)
mechanisms are very lightweight by design, depending on clients with
conflicting addresses to "defend" their address by responding to
queries to show that an address is in use. Is there a better
alternative to ICMP ECHO and ARP that is backward compatible with
these two protocols?
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4.7 DHCP Relay Agents
4.7.1 Relay Agent Source Addresses
There should be some text that specifies what the relay agent should
use for the IP source address of relayed packets. Because relay
agents change the payload ("giaddr" and relay agent option 82),
their operation does not amount to IP forwarding. The IP source
address they use should be their own. [Aside: for security
purposes it might have been better than they retain the source IP
address of the original packet, but it is too late to change all
that.]
RECOMMENDATION:
4.7.2 Relay Agent Port Usage
Relay agents should use port 67 as the source port number. Relay
agents always listen on port 67, but port 68 has sometimes been used
as the source port number probably because it was copied from the
source port of the incoming packet.
Cable modem vendors would like to install filters blocking outgoing
packets with source port 67.
RECOMMENDATIONS:
O Relay agents MUST use 67 as their source port number.
O Relay agents MUST NOT forward packets with non-zero giaddr
unless the source port number on the packet is 67.
4.8 Host Name, Domain Name, and FQDNs
A fully qualified domain name (FQDN) consists of two conceptual
parts: the host name portion and the domain name portion. Host
names consist of one or more non-null parts separated by the ISO
period (.) character ("separator") while domain names consist of two
or more non-null parts delimited by the separator, one of which must
be a valid top-level domain (TLD) name. DHCP options exist for
hostname (option 12) and domain name (option 15), and are proposed
for FQDN (draft-ietf-dhc-fqdn-option-05.txt) but the FQDN option is
not required to be a concatenation of hostname and domain name.
Should RFC 2131 explicitly state that the client FQDN MUST be the
host name (option 12) concatenated with the domain name (option 15)?
4.9 Overloading of DHCPREQUEST
The client sends a DHCPREQUEST message from several different
states: INIT, INIT-REBOOT, REBINDING, and RENEWING.
Differentiation among the states is done according to the context of
other message fields and option values. At this point, there
probably can be no change in this usage, but the content of other
message fields and option values should be carefully reviewed to
ensure consistency.
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4.10 DHCPINFORM
The intent of DHCPINFORM messages is to allow clients to query
servers for configuration information WHETHER OR NOT their IP
address has been assigned by DHCP.
Section 3.4 Para 2 Page 21 states: "The server SHOULD check the
network address in a DHCPINFORM message for consistency." What the
server should be checking is a mystery. Possibly the intent was
that servers should verify that the source IP address in the packet
is identical to the "ciaddr." Since the server should reply to
"ciaddr," this affords some measure of security, preventing third
parties from discovering configuration information pertaining to
other clients. Whether that is desirable, or whether instead
DHCPINFORM should be available to third parties, such as proxies,
has never been resolved.
Section 4.4.4 Para 1, "Use of broadcast and unicast," hints that
clients may be able to broadcast DHCPINFORM messages to servers:
"The DHCP client broadcasts DHCPDISCOVER, DHCPREQUEST and DHCPINFORM
messages, unless the client knows the address of a DHCP server."
This text suggests that a DHCP client may choose to broadcast a
DHCPINFORM request for whatever reason, and points out the need for
clarification of all text concerning multiple server responses and
consistency of returned options.
RECOMMENDATIONS:
O DHCPINFORM messages should be included in Table 4 to summarize
the fields and options usage with this message type. Suggested
replacement for Table 4 is shown in section 5.2.5.
O The Working Group should consider the ramifications of
permitting third party DHCPINFORMs, that is, DHCPINFORM messages
NOT sent by the DHCP client, but by other processes having
access to the ports.
O Section 4.3.1, "DHCPDISCOVER Message," and section 4.3.2,
"DHCPREQUEST Message" briefly mention consistency and uniform
responses from multiple servers: this text SHOULD be clarified
to state what consistency is expected or required of the server,
and what a client should do if a server supplies inconsistent
data.
4.11 Unicast of DHCPDISCOVER
Section 4.4.4 Paragraph 1, "Use of broadcast and unicast," hints
that clients may be able to unicast DHCPDISCOVER messages to
servers: "The DHCP client broadcasts DHCPDISCOVER, DHCPREQUEST and
DHCPINFORM messages, unless the client knows the address of a DHCP
server."
This would be pointless unless "ciaddr" were non-zero, because the
server would not know how to respond. Neither does table 4 admit
the possibility.
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We believe it is common practice for BOOTP Relay Agents to only
fill-in "giaddr" for broadcast packets. This requires
investigation: such behavior would restrict the use of unicast
DHCPDISCOVER messages to the same subnet on which the server resides
-- a very restricted condition.
One circumstance in which this might make sense is for proxies
gathering IP addresses on behalf of other clients. In that case,
the proxy could put its own IP address in "ciaddr" and perhaps use
multiple different client identifiers in multiple transmissions.
Table 5, however, asserts that ciaddr must be zero.
RECOMMENDATIONS:
O The Working Group should consider whether to allow this kind of
proxy usage, and what changes that might imply to RFC 2131.
O Tables 4 and 5 SHOULD be updated to reflect the possibility of
unicast DHCPDISCOVER messages. Suggested replacements for
Tables 4 and 5 are shown in sections 5.2.5-5.2.7.
O Figure 5 SHOULD be updated to reflect the uses of unicast and
broadcast packets. Suggested replacements for Figure 5 are
shown in sections 5.2.6 and 5.2.7.
4.12 DHCPRELEASE
There are several MUST NOT entries in the "options" portion of RFC
2131 table 5 specifying the inclusion of options in the DHCPRELEASE.
Some customers complained that a particular vendor included the
"hostname" option and that this seemed innocuous. The vendor said
that their reading of the RFC allows such an option to be included.
In the "fields" portion of table 5 there is the word "unused" for
the "sname" and "file" fields of a DHCPRELEASE message, while
"options" for the DHCPRELEASE was left blank.
A DHCPRELEASE message SHOULD be subject to some verification
criteria to reduce the chance of a bogus release. Two possible
changes to these tables are:
O In the "fields" portion of table 5, change the "xid" from
"selected by client" to "xid from server DHCPACK message."
O In the "options" portion of table 5, change the entry for
"client identifier" from "MAY" to "client identifier used in the
DHCPDISCOVER message."
4.13 Client State Diagram
Section 4.3.1 and Figure 5 do not accurately describe DHCP client
behavior: DHCP clients send messages to servers from the INIT,
INIT-REBOOT, SELECTING, REQUESTING, and BOUND states, not from
RENEWING or REBINDING.
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RECOMMENDATION:
Change the text of section 4.3.1 and its schematic representation in
Figure 5 to correctly represent the states, transitions, triggering
events, and messages sent. Suggested replacements for Figure 5 are
shown in sections 5.2.6 and 5.2.7.
4.14 Options
The language in RFC 2131 concerning whether and which options to
return to the client is convoluted and apparently contradictory.
4.14.1 Which Options to Return?
There are two opposing philosophies regarding which options servers
should return to clients: to return every option with values within
the client’s scope, or to return only those options specifically
requested by a client and within scope. The following arguments
have been cited:
O Supporting the return of every option:
a. Consistency. A network administrator wants all of the
configured options to show up on each client on the network,
regardless of client vendor.
b. A DHCP client is likely only to request the options it
supports. However, many application layer options are not
used by the DHCP client but are useful to applications.
c. A DHCP client would either need to be configured or updated
to request new options. The whole idea of DHCP is to keep
configuration on the server, not on the client, which is
pointed out in: Page 7, second and third bullets of section
1.6.
5. Supporting the return only of requested options:
a.
Some DHCP clients may reject packets containing options
that they did not request especially if they are ignorant of
their semantics; therefore a DHCP server should only return
the options requested.
b. The DHCP packet size is limited. Options are often
configured on a per-network rather than a per-client basis,
and to return unwanted options risks exhausting the space
available while options remain which the client needs.
RFC 2131 does little to resolve the matter. Two different sections
are relevant: section 3.5 describes mechanisms to limit the number
of options sent, while section 4.3.1 subsequently presents an
apparently conflicting description of how to select values for
options requested by the client.
6. RFC 2131, Section 3.5:
"First, most parameters have defaults defined in the Host
Requirements RFCs; if the client receives no parameters from
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the server that override the defaults, a client uses those
default values."
The list of parameters with a cross-reference to the defining RFC is
given in Appendix A of RFC 2131.
Several sources contend that virtually none of the parameters in the
list have a meaningful default value, which raises the issue of
viability of the technique described in this section for reducing
total server response message size.
Even if the option has a default value defined in [RFC1122], RFC
2131 is silent on the question of whether or not the server MUST,
SHOULD, or MAY choose not to send that option when its value is the
same as the default.
7. RFC 2131, Section 4.3:
"IF the server has been explicitly configured with a default
value for the parameter, the server MUST include that value
in an appropriate option in the 'option' field, ELSE IF the
server recognizes the parameter as a parameter defined in
the Host Requirements Document, the server MUST include the
default value for that parameter as given in the Host
Requirements Document in an appropriate option in the
'option' field, ELSE the server MUST NOT return a value for
that parameter."
The word "default" in the first statement seems misplaced. The
second statement seems contrary to the intent of minimizing the
amount of data sent by the server: if the scope of the Host
Requirements RFCs applies to all Internet-connected hosts, then a
DHCP server SHOULD NOT have to supply these values -- they should
already be assumed by the client as the default for the requested
option.
There is no mention of a minimum set of parameters to be sent to a
requesting client, nor any mention of which parameters to send if
the client does not request any not any guidance for what to do when
there is more data than will fit in a response packet. Can the
options be somehow prioritized? Could additional options be
obtained using the DHCPINFORM mechanism? Should an additional bit
in the "flags" field be defined as a "packet overflow" bit?
RECOMMENDATIONS:
O Clients MUST include the same parameter request list on all
messages.
O Clients MUST be prepared to receive responses containing options
they did not request and/or whose semantics are unknown. They
MAY choose silently to ignore such options.
O Language implying that parameters in "Requirements for Internet
Hosts" have defaults should be removed.
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4.14.2 Multiple Instances of Options
Page 24, seventh paragraph, section 4.1: "Options may appear only
once, unless otherwise specified in the options document. The
client concatenates the values of multiple instances of the same
option into a single parameter list for configuration."
RECOMMENDATION:
The first sentence SHOULD begin "Options MUST appear only once,
unless...." The second sentence belongs in the options memo
[RFC2132] for options where there can be multiple instances.
Together, these two sentences are confusing.
4.14.3 Option Ordering
A number of clients require that the DHCP message type be the first
option (after the magic cookie).
RECOMMENDATION:
With the exception of option 82, which must be last (save option 255
which acts as a terminator), the clients MUST NOT make any
assumption about the ordering of options.
4.14.4 Options 66 and 67
Options 66 (TFTP server name) and 67 (bootfile name) were introduced
as an alternative to the fixed fields "sname" and "file" in BOOTP.
As discussed elsewhere, space is at a premium in DHCP, and reserving
64 octets ("sname") and 128 octets ("file") to contain values are
that potentially, and commonly, much shorter is wasteful.
Furthermore, the existence of these options allows the client to
either request those values from the server or not, according to
need.
At present, servers are at liberty to return values for these
options in either the fixed fields, or encapsulated like any other
DHCP option. Clients have sometimes assumed only the former. RFC
2131 should address this issue.
RECOMMENDATIONS:
1. Using "sname" and "file" for these options SHOULD be deprecated.
2. Clients MUST be prepared, at least for the time being, for
either method of delivery.
4.15 Vendor Classes
Page 3, section 1.1, first paragraph - includes the following
sentence: "The classing mechanism for identifying DHCP clients to
DHCP servers has been extended to include "vendor" classes as
defined in sections 4.2 and 4.3." Vendor classing has been there
since RFC 1541, thus there is nothing new about it. Should this
section be referring to User classing?
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4.15.1 Character Set
Some new clients have spaces in their identifier, which broke some
implementations with configuration file records delimited by
whitespace.
4.15.2 Form of the Name Space
An early suggestion (RFC 1541 time-frame), expressed symbolically,
was the form "Stock symbol/Organization...." e.g., "SUNW.class-
1.class-2" or "CMU.edu.class-1.class-2". This would have had the
advantage of preventing collisions between vendors. This was not
adopted, and it is probably too late to resurrect it.
4.15.3 Relationship to Vendor Options
Text is needed describing how each unique vendor class identifier
implies a 254 unique encapsulated option name space. There are 254,
because even within the vendor space options 0 and 255 retain their
meaning as the pad value and terminator, respectively. An
occasional misconception is that there is only a single unique
encapsulated 254 option name space shared by all vendors, with the
effect that the same values being returned to *any* client
regardless of vendor class identifier. Obviously, we should include
text to clarify the relationship between Vendor Class identifier and
the encapsulated Vendor option.
4.15.4 Multiplicity
How many vendor class identifiers can a client have? Only one class
identifier, because the client is unique to a specific vendor. If
the client were to send more than one vendor class option it would
be impossible for the server to decide which set of encapsulated
vendor options to select.
Here is some more text regarding vendor options from a note by Mike
Carney regarding the use of vendor class / encapsulated options:
Vendor class support requires the ability to configure a
DHCP server to support a new vendor class by associating
that vendor class identifier with 254 options whose types
can then be defined by following the DHCP client's
documentation. Each group of 254 options has the "scope" of
that vendor. For example, suppose we have the following two
clients:
Vendor Class "SunBeam.Toaster.2slots"
Options for this class:
Code Len Data
1 2 Darkness setting ( a 2 octet integer)
Vendor Class "Voxpopulis.Answering.Machine"
Options for this class:
Code Len Data
1 4 Outgoing message index (pointer to messages)
Both clients are on the same network ("Kitchen"), and are clients of
the same DHCP server. Note that both use encapsulated option code
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1. Looks like a conflict, but it is not: in the syntax of the DHCP
server's configuration table, one configures two new options, each
which has the "scope" of the vendor class.
What this means is that when the toaster boots, the DHCP server only
returns vendor class options associated with the
"SunBeam.Toaster.2slots" class. When the answering machine boots,
it only sees vendor class options associated with the
"GE.Answering.Machine" class. Clients of vendor classes not
currently configured on the server do not see any encapsulated
vendor options.
4.16 Client/Server Retransmission
Because DHCP servers are the passive participants and DHCP clients
are the active participants, the DHCP protocol is susceptible to
poorly behaved clients (retransmitting too fast, for example).
However, there is no text describing this susceptibility.
Furthermore, the use of the power-of-2 retransmission algorithm is a
SHOULD/MAY. This probably should be MUST. If we need different
retransmission algorithms for different media, then we should
develop/document them in table form. The specification as it stands
is too loose and does cause inter-operability problems:
O Page 16, section 3.1, last sentence of list item 3.1.
O Page 17, third paragraph of list item 5, section 3.1
O Page 24, eighth paragraph of section 4.1
O Page 36, first paragraph of section 4.4.1
4.17 Transmission of DHCPNAKs
DHCPNAKs MUST be broadcast or unicast to at the link level because
the client has no valid IP address. The same comment applies to
DHCPOFFERs but with one significant difference: a DHCPOFFER has a
valid "yiaddr" which a relay agent can use as the destination IP
address. It is not clear that whatever mechanism relay agents are
using to transmit offers will work when "yiaddr" is 0.0.0.0.
Therefore, for safety’s sake, the servers MUST set the broadcast bit
in DHCPNAK packets. The text describing a server's behavior when
the client is accessible through a BOOTP relay agent does not do
this:
O Page 19, last paragraph of list item 2, section 3.2.
O Page 23, fifth paragraph of section 4.1.
O Page 32, Last paragraph of "DHCPREQUEST generated during INIT-
REBOOT state," bullet, section 4.3.2.
This last describes the behavior that is required -- a server MUST
set the broadcast bit in order for the relay agent to properly
broadcast the DHCPNAK.
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RECOMMENDATION:
Items (1) and (2) above should either duplicate the text of (3), or
should reference section 4.3.2.
4.18 Use of ciaddr
According to RFC 951 and RFC 1542, clients use "ciaddr" when they
have received an IP address from a source outside of BOOTP/DHCP, and
can respond to ARPs.
The text in RFC 2131 is mostly supportive of this point with the
following exception:
Page 32, "DHCPREQUEST generated during REBINDING state:"
section 4.3.2: "The DHCP server SHOULD check 'ciaddr' for
correctness before replying to the DHCPREQUEST."
RECOMMENDATION:
This line should be struck from the document. Servers trust
"ciaddr," period.
4.19 Size of a BOOTP/DHCP Frame
The description in RFC 2131 relating to the size constraints of DHCP
packets (Page 10, first paragraph after Table 1, section 2) is
inadequate.
4.19.1 Minimum Packet Size
RFC 951 states that a minimum BOOTP frame is 300 octets in length.
Some BOOTP relay agents have been known to drop frames of less than
300 octets. RFC 951 is explicit on this point, but RFC 2131 just
refers to RFC 951. Since DHCP is intended to be backward compatible
with BOOTP, the protocol should continue to observe this lower
bound.
RECOMMENDATION:
Text should be added stating explicitly that the minimum size of a
DHCP frame is 300 octets.
4.19.2 Maximum Size, MTU, and Message Size Option
It has been thought necessary to avoid fragmentation of the IP
packets in DHCP/BOOTP due to concerns that some clients would be
unable to reassemble fragments before the IP stack is properly
configured. RFC 951 states, "For simplicity it is assumed that the
BOOTP packet is never fragmented." Regardless of theoretical
limitations in IP stack implementations, it is certain that there
are several DHCP/BOOTP implementations, at both ends of the
protocol, which will not reassemble.
Various comments in the WORKING GROUP imply that fragmentation could
be avoided were the client consistently to include the MTU of the
link layer interface. However, clients cannot be expected to be
omniscient about other media over which packets travel en route to
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servers. Servers must be endowed with this knowledge, which they
MUST use to avoid packet fragmentation.
Once the IP stack is configured, and the IP stack is fully
configured, the aforementioned limitation ceases to exist, and later
stages of the protocol could allow larger packets (up to the UDP
limit). DHCPINFORMs, especially, could benefit from this
relaxation. There probably should be explicit text to allow larger
packets (presumably up to the maximum PDU size) for later stages of
the protocol.
A number of clients send small packets with the assumption that
servers will not return a packet that is any larger than the one
received from the client. Clients MUST NOT assume this. If the
client cannot process a response larger than a certain size, the
client MUST use the message size option to inform servers of this
size. Note that this is NOT the same option as the MTU.
RECOMMENDATIONS:
O Servers and relay agents MUST ensure that IP datagram
fragmentation does not occur at any stage in the protocol before
the client IP stack is fully configured.
O Clients SHOULD communicate their link-layer frame size to the
DHCP server via the DHCP MTU option.
O Clients MUST NOT assume that servers will return a packet no
larger than the one they send. If the client has a limit on the
size of the packet that it can process it MUST convey that limit
to the server in the "maximum message size" option (57).
O Page 21, second paragraph, section 3.5, the first sentence
SHOULD be changed to "The client SHOULD include the 'maximum
DHCP message size' option to let the server know how large the
server may make its DHCP messages, and the value of this option
SHOULD be the MTU of the [client] network interface being
configured."
O The WORKING GROUP SHOULD consider whether to allow fragmentation
of packets after the client is fully configured, and how servers
can divine this fact (e.g. a non-zero "ciaddr.")
4.20 Use of giaddr
Page 23, fifth paragraph, section 4.1: "If the 'giaddr’ field is
zero and the 'ciaddr' field is nonzero, then the server unicasts
DHCPOFFER and DHCPACK messages to the address in 'ciaddr.'" True
for DHCPACK, false for DHCPOFFER (a DHCPDISCOVER will never have
anything but 0 as "ciaddr.")
4.21 Address Selection
Page 27, third paragraph, section 4.3.1: "Note that, in some
network architectures (e.g., internets with more than on IP subnet
assigned to a physical network segment), it may be the case that the
DHCP client should be assigned an address from a different subnet
than the address recording in 'giaddr.'"
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There are two differing view of this sentence:
There is considerable detail in the rest of RFC 2131 trying
to get the use of "giaddr" clear as it relates to BOOTP
relay agents (RFC 951 and RFC 1542), then this sentence
"undoes" this work. Serving multiple IP networks on the
same wire should be either described in detail in its own
section (with caveats) or as a separate informational RFC.
Otherwise, the use of "giaddr" is unclear.
Alternatively:
Additional supporting text should be added to RFC 2131 to
the effect that servers having knowledge of network topology
MAY choose to offer an address inconsistent with "giaddr"
but consistent with that topology. Furthermore, the address
offered may differ depending upon the contents of the vendor
class, user class, and even the client identifier. All of
these things are policy matters for the server.
4.22 Use of "secs" Field
The "secs" field has not been discussed much: many clients simply
leave its value as zero, and few if any servers have used its value
to modify their behavior. These practices seem acceptable. The
value of "secs" SHOULD be the elapsed time (in seconds) since the
client began trying to acquire or extend a lease on an IP address.
A sixteen bit field, its maximum value is 65536. It is conceivable
that due to server or network failure that a client may have been
waiting longer than this.
RECOMMENDATION:
A client MAY choose to leave to ignore the secs field. If so, its
value MUST be set to zero. If the client chooses to insert a value,
the value SHOULD be time elapsed since the client began negotiating
for an IP address. If the client has been waiting longer than 65536
seconds its value SHOULD be 65536. The value SHOULD NOT wrap around
to zero.
4.23 Use of "htype" and "hlen" Fields
At least one vendor has used chaddr as a place holder for a value
that was not in fact a link-layer (hardware) address, while at the
same type using an htype of 1 (meant to be Ethernet) but an hlen of
16 (instead of 6). Many servers will reject a packet with this kind
of inconsistency between the htype and hlen fields.
Values of htype not equal to zero MUST correspond to the link-level
medium to which the DHCP client is attached according to IANA’s
Assigned Numbers database.
RECOMMENDATIONS:
1. The value of hlen MUST be consistent with the length of a link-
level address implied by htype.
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2. An htype of zero SHOULD be used to mean that chaddr is an
identifier unrelated to a specific link-level medium.
4.24 Use of "xid" Field
This field exists to allow clients to match replies to requests. In
two places RFC 2131 erroneously states that the client should use
the "xid" in the server’s DHCPOFFER as the value in its follow up
request.
1. Table 5, DHCPREQUEST column.
2. Section 4.4.1, Paragraph 5.
In principle, the 32 bits of "xid" should be sufficient to make the
chance of collisions almost nil, provided it is randomly generated
as 2131 suggests in section 4.4.1 paragraph 3. However, some
vendors have admitted to generating "xid" which may not be
sufficiently uniformly distributed.
The randomness requirement on "xid" is not as stringent as would be
required, say, in selecting a cryptographic key. It is quite
permissible that the initial key be predictable given sufficient
knowledge of the client, but clients MUST ensure that these
identifiers are generated in such a way that the chance of collision
with other clients in the DHCP administrative domain is what one
should expect from a truly random number.
Permitting the use of the same "xid" on a re-transmission might
marginally improve the efficiency of the protocol. Server responses
to the first transmission, which arrived after the timeout and
retransmission would be accepted, and might avoid yet another client
timeout.
4.25 Options in DHCPOFFER and DHCPACK
RFC 2131 says that the options delivered in these two cases should
not be in conflict. It does not say what "conflict" means in that
case. This SHOULD be clarified.
RECOMMENDATIONS:
1. Servers MAY deliver a full configuration in a DHCPOFFER, but are
NOT required to do so. The DHCPOFFER MUST contain an IP address
and a lease time, and MAY contain other information. As a
client will presumably choose among multiple offers based on
some criteria, perhaps completeness of response, the server
SHOULD be permitted to return the 'parameter request list'
including the option code for each option it is prepared to
deliver in a DHCPACK message.
2. In a DHCPACK message, the lease time offered MUST be at least as
long as that in the DHCPOFFER. The IP address MUST be the same.
3. The options delivered in a DHCPACK message MAY differ from those
in a DHCPOFFER. Some DHCP servers attempt to balance the load
for other services by permuting lists. For instance, a server
configured with three DNS server addresses may rotate that list
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each time a client is serviced. It is a problem for some
servers to deliver an identical OFFER and ACK (it implies
keeping state.)
4. If a particularly long option list must be delivered to the
client, it might not be possible to fit all options in the DHCP
payload of a UDP packet. RFC 2131 appears to permit a long list
of options to be sent partly in the DHCPOFFER message and partly
in the DHCPACK message.
4.26 Lease Times
RFC 2131 has some language (section 4.3.1) that might be interpreted
as constraining the duration of a lease that can be offered based on
history or what the client wants. This SHOULD be rewritten to make
clear that in a DHCPOFFER a server can offer whatever lease time
that local policy finds acceptable, without regard to what the
client requests, or what was offered last time around. If a server
offers a longer lease than the client requested, the client can
simply enter the RENEWING or REBINDING states, or send a DHCPRELEASE
message according to its desired [earlier] times.
For RENEWALS, the text should be made clear that servers are not
obligated to extend leases merely because the client wishes an
extension [see also general comment below about policy.]
There has sometimes been an issue with T1 and T2 times as follows.
Let us say that a new lease is offered with a certain T0 (T0 is
lease duration) and T1 = T0/2. Then, when T1 expired, the client
attempts a renewal. The server in question, for whatever reason,
does not want to extend the lease, but is willing to confirm the
residual time (T0/2). If it also returns T1 in the options, it
should ensure that T1 is adjusted from the original value else the
new ACK will have T0 and T1 identical.
4.27 Miscellaneous
There are many SHOULDs and SHOULD NOTs that should perhaps be
converted into MUSTs or MUST NOTs. Here is a summary:
1. Page 16, item 4, section 3.1 "The server SHOULD NOT check the
offered network address at this point." (MUST NOT)
2. Page 16, item 5, section 3.1 "The client SHOULD perform a final
check on the parameters..." (MUST)
3. Page 17, item 5, section 3.1 "The client SHOULD wait a minimum
of ten seconds..." (MUST)
4. Page 18, item 2, section 3.2 "Servers SHOULD NOT check that the
client's network address is already in use..." (MUST NOT)
5. Page 19, item 2, second paragraph, section 3.2 "...servers
SHOULD respond with a DHCPNAK message to the client" (MUST).
The following sentences are rather dubious in this paragraph as
well.
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6. Page 21, first paragraph, section 3.4 "The servers SHOULD
unicast the DHCPACK replay to the address given in the 'ciaddr'
field of the DHCPINFORM message" (MUST)
7. Page 22, last paragraph, section 3.5 "If a server receives a
DHCP request message with an invalid 'requested IP address', the
server SHOULD respond to the client with a DHCPNAK message...."
(MUST)
RECOMMENDATION:
The WORKING GROUP should review the use of emphasis words (e.g.,
MAY) in RFC 2131. Those SHOULDs that remain should list the valid
exceptions (some do; most don't).
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5 Proposed Replacements for RFC 2131 Figures and Tables
5.1 Figures
5.1.1 Figure 1: Format of a DHCP message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| op (1) | htype (1) | hlen (1) | hops (1) |
+---------------+---------------+---------------+---------------+
| xid (4) |
+-------------------------------+-------------------------------+
| secs (2) | flags (2) |
+-------------------------------+-------------------------------+
| ciaddr (4) |
+---------------------------------------------------------------+
| yiaddr (4) |
+---------------------------------------------------------------+
| siaddr (4) |
+---------------------------------------------------------------+
| giaddr (4) |
+---------------------------------------------------------------+
| |
| chaddr (16) |
| |
| |
+---------------------------------------------------------------+
| |
| sname (64) |
+---------------------------------------------------------------+
| |
| file (128) |
+---------------------------------------------------------------+
| |
| options (variable) |
+---------------------------------------------------------------+
Figure 1: Format of a DHCP Message
5.1.2 Figure 2: Format of the 'flags' Field
. . . . . . . . . . 1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B| MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-----------------------------------------------
| Key: |
| B: BROADCAST flag |
| MBZ: MUST BE ZERO (reserved for future use) |
-----------------------------------------------
Figure 2: Format of the 'flags' Field
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5.1.3 Figure 3: Timeline Diagram--Allocating a New Address
Server Server
(not selected) Client (selected)
v v v
| | |
| Begins initialization |
| | |
| ______________/|\______________ |
|/ DHCPDISCOVER | DHCPDISCOVER \|
| | |
Determines | Determines
configuration | configuration
| | /|
|\ | ____________/ |
| \___________ | / DHCPOFFER |
| DHCPOFFER \ |/ |
| \ | |
| \| |
| Collects replies |
| Selects configuration |
| | |
| ______________/|\______________ |
|/ DHCPREQUEST | DHCPREQUEST \|
| | |
| | Commits|
| | configuration|
| | |
| | ______________/|
| |/ DHCPACK |
| | |
| Initialization complete |
| | |
. . .
. . .
| | |
| Graceful shutdown |
| | |
| |\______________ |
| | DHCPRELEASE \|
| | |
| | Discards|
| | lease|
| | |
v v v
Figure 3: Timeline Diagram of Messages Exchanged between DHCP Client
and Servers When Allocating a New Network Address
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5.1.4 Figure 4: Timeline Diagram--Reusing an Address
Server Client Server
v v v
| | |
| Begins |
| initialization |
| | |
| /|\ |
| ____________/ | \___________ |
| /DHCPREQUEST | DHCPREQUEST\ |
|/ | \|
| | |
Locates | Locates
configuration | configuration
| | |
|\ | /|
| \ | ___________/ |
| \ | / DHCPACK |
| \ _______ |/ |
| DHCPACK\ | |
| Initialization |
| complete |
| \| |
| | |
| (Subsequent |
| DHCPACKS |
| ignored) |
| | |
| | |
v v v
Figure 4: Timeline Diagram of Messages Exchanged between DHCP
Client and Servers When Reusing a Previously Allocated Address
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5.1.4 Figure 5: State-Transition Diagram for DHCP Clients
+-------+
Have Unexpired | | Do Not Have
+---- Lease ------| START |--- Lease ----+
V | | V
+--------+ +-------+ +-------+
| | +-------------------------->| |<-------------------+
| INIT- | | +------------------>| INIT | |
| REBOOT | DHCPNAK/ | +-------->| |<---------------+ |
| | Restart | | +---+---+ | |
+----+---+ | | | | | |
| | DHCPNAK/ | Broadcast | |
Broadcast | Discard offer | DHCPDISCOVER | |
DHCPREQUEST | | DHCPACK | | |
| | | (not accepted)/ v | |
V | | Send +-----------+ | |
+---------+-+ | DHCPDECLINE | |<-----+ | |
| | | | | SELECTING | | | |
| REBOOTING | | +----+ | | DHCPOFFER/ | |
| | | | +-+-------+-+ Collect | |
+-+---------+ | | | | replies | |
| | | Select offer/ | | | |
DHCPACK/ | | send +--------+ | |
Record lease, set | | DHCPREQUEST | |
timers T1, T2 +-+----+-----+ | DHCPNAK, |
| | |<-------+ Lease expires/ |
| +-------->| REQUESTING | Halt network |
| | | | | |
| DHCPOFFER/ +-+--------+-+ +-----------+ | |
| Discard | | | | | |
| | | DHCPACK (accepted)/ +----+ REBINDING +---+ |
| +-----------+ Record lease, set | | | |
| timers T1, T2 | +-----------+ |
| | DHCPACK/ ^ |
| | Record lease, set | |
| | timers T1, T2 | |
| | | T2 expires/ |
| v | Broadcast |
| +-----+-+ | DHCPREQUEST |
+----------------->| +<-----------+ | |
+-------------------+ BOUND +-------------------------+ |
| DHCPOFFER,+--->| |<-------------+ |
| DHCPACK, or| +-+---+-+ | |
| DHCPNAK/ | | | DHCPACK/ |
| Discard +------+ | Record lease, set |
| T1 expires/ timers T1, T2 |
| Send DHCPREQUEST | |
| to leasing server +-----+----+ |
| | | | |
Lease expires/ | | RENEWING +--DHCPNAK/ ----->|
halt network +--------->| | halt network |
| +----------+ |
+-----------------------------------------------------------------+
Figure 5: State-transition diagram for DHCP clients
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5.2 Tables
5.2.1 Table 1: Description of fields in a DHCP Message
---------------------------------------------------------------------
|FIELD |OCTETS|DESCRIPTION |
-----------------------------------------------------------------------
|'op' | 1 |Message op code / message type. |
| | |1 = BOOTREQUEST, 2 = BOOTREPLY |
|'htype' | 1 |Hardware address type, see ARP section in "Assigned |
| | |Numbers" RFC; e.g., '1' = 10mb Ethernet. |
|'hlen' | 1 |Hardware address length (e.g., '6' for 10mb |
| | |Ethernet). |
|'hops' | 1 |Client sets to zero, optionally used by relay agents|
| | |when booting via a relay agent. |
|'xid' | 4 |Transaction ID, a random number chosen by the |
| | |client, used by the client and server to associate |
| | |messages and responses between a client and a |
| | |server. |
|'secs' | 2 |Filled in by client, seconds elapsed since client |
| | |began address acquisition or renewal process. |
|'flags' | 2 |Flags (see Figure 2). |
|'ciaddr' | 4 |Client IP address; only filled in if client is in |
| | |BOUND, RENEW or REBINDING state and can respond |
| | |to ARP requests. |
|'yiaddr' | 4 |"your" (client) IP address. |
|'siaddr' | 4 |IP address of next server to use in bootstrap; |
| | |returned in DHCPOFFER, DHCPACK by server. |
|'giaddr' | 4 |Relay agent IP address, used in booting via a |
| | |relay agent. |
|'chaddr' | 16 |Client hardware address. |
|'sname' | 64 |Optional server host name, null terminated string. |
|'file' | 128 |Boot file name, null terminated string; "generic" |
| | |name or null in DHCPDISCOVER, fully qualified |
| | |directory-path name in DHCPOFFER. |
|'options'|vari- |Optional parameters field. See the options |
| | able|documents for a list of defined options. |
-----------------------------------------------------------------------
Table 1: Description of Fields in a DHCP message
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5.2.2 Table 2: DHCP Messages
---------------------------------------------------------------------
|Message |Usage |
---------------------------------------------------------------------
|DHCPDISCOVER|Client broadcast to locate available servers. |
| | |
|DHCPOFFER |Server to client in response to DHCPDISCOVER with |
| |offer of configuration parameters. |
| | |
|DHCPREQUEST |Client message to servers either (a) requesting |
| |offered parameters from one server and implicitly |
| |declining offers from all others, (b) confirming |
| |correctness of previously allocated address after, |
| |e.g., system reboot, or (c) extending the lease on a |
| |particular network address. |
| | |
|DHCPACK |Server to client with configuration parameters, |
| |including committed network address. |
| | |
|DHCPNAK |Server to client indicating client's notion of network|
| |address is incorrect (e.g., client has moved to new |
| |subnet) or client's lease as expired |
| | |
|DHCPDECLINE |Client to server indicating network address is already|
| |in use. |
| | |
|DHCPRELEASE |Client to server relinquishing network address and |
| |canceling remaining lease. |
| | |
|DHCPINFORM |Client to server, asking only for local configuration |
| |parameters; client already has externally configured |
| |network address. |
---------------------------------------------------------------------
Table 2: DHCP Messages
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5.2.3 Table 3: Fields Used by DHCP Servers
---------------------------------------------------------------------
|Field |DHCPOFFER |DHCPACK |DHCPNAK |
---------------------------------------------------------------------
|'op' |2 |2 |2 |
|'htype' | (From "Assigned Numbers" Database) |
|'hlen' | (Hardware address length in octets) |
|'hops' |0 |0 |0 |
|'xid' |'xid' from client |'xid' from client |'xid' from client |
| |DHCPDISCOVER |DHCPREQUEST |DHCPREQUEST |
| |message |message |message |
|'secs' |0 |0 |0 |
|'ciaddr' |0 |'ciaddr' from |0 |
| | |DHCPREQUEST or 0 | |
|'yiaddr' |IP address offered |IP address |0 |
| |to client |assigned to client| |
|'siaddr' |IP address of next |IP address of next|0 |
| |bootstrap server |bootstrap server | |
|'flags' |'flags' from |'flags' from |'flags' from |
| |client DHCPDISCOVER|client DHCPREQUEST|client DHCPREQUEST|
| |message |message |message |
|'giaddr' |'giaddr' from |'giaddr' from |'giaddr' from |
| |client DHCPDISCOVER|client DHCPREQUEST|client DHCPREQUEST|
| |message |message |message |
|'chaddr' |'chaddr' from |'chaddr' from |'chaddr' from |
| |client DHCPDISCOVER|client DHCPREQUEST|client DHCPREQUEST|
| |message |message |message |
|'sname' |Server host name |Server host name |(unused) |
| |or options |or options | |
|'file' |Client boot file |Client boot file |(unused) |
| |name or options |name or options | |
|'options'|(see Table 4) |(see Table 4) |(see Table 4) |
---------------------------------------------------------------------
Table 3: Fields Used by DHCP Servers
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5.2.4 Table 4: Options Used by DHCP servers
---------------------------------------------------------------------
|Option |DHCPOFFER|DHCPACK |DHCPNAK |
---------------------------------------------------------------------
|Requested IP address |MUST NOT |MUST NOT |MUST NOT|
|IP address lease time |MUST |MUST (DHCPREQUEST) |MUST NOT|
| | |MUST NOT (DHCPINFORM)| |
|Use 'file'/'sname' fields |MAY |MAY |MUST NOT|
|DHCP message type |DHCPOFFER|DHCPACK |DHCPNAK |
|Parameter request list |MUST NOT |MUST NOT |MUST NOT|
|Message |SHOULD |SHOULD |SHOULD |
|Client identifier |MUST NOT |MUST NOT |MAY |
|Vendor class identifier |MAY |MAY |MAY |
|Server identifier |MUST |MUST |MUST |
|Maximum message size |MUST NOT |MUST NOT |MUST NOT|
|All others |MAY |MAY |MUST NOT|
---------------------------------------------------------------------
Table 4: Options Used by DHCP servers
5.2.5 Table 5: Client Messages from Different States
---------------------------------------------------------------------
| |INIT-REBOOT |SELECTING |RENEWING |REBINDING |
---------------------------------------------------------------------
|broad/uni-cast|broadcast |broadcast |unicast |broadcast |
|server-ip |MUST NOT |MUST |MUST NOT |MUST NOT |
|requested-ip |MUST |MUST |MUST NOT |MUST NOT |
|ciaddr |zero |zero |IP address |IP address|
---------------------------------------------------------------------
Table 5: Client Messages from Different States
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5.2.6 Table 6: Fields Used by DHCP Clients
---------------------------------------------------------------------
|Field |DHCPDISCOVER, |DHCPREQUEST |DHCPDECLINE, |
| |DHCPINFORM | |DHCPRELEASE |
---------------------------------------------------------------------
|'op' |1 |1 |1 |
|'htype' | (From "Assigned Numbers" Database) |
|'hlen' | (Hardware address length in octets) |
|'hops' |0 |0 |0 |
|'xid' |selected by client|'xid' from server |selected by |
| | |DHCPOFFER message |client |
|'secs' |seconds since |seconds since |0 |
| |DHCP process |DHCP process | |
| |started |started | |
|'flags' |Set 'BROADCAST' |Set 'BROADCAST' |0 |
| |flag if client |flag if client | |
| |requires broadcast|requires broadcast | |
| |reply |reply | |
|'ciaddr' |0 (DHCPDISCOVER) |0 or client's |0 (DHCPDECLINE) |
| |client's |network address |client's network |
| |network address |(BOUND/RENEW/REBIND)|address |
| |(DHCPINFORM) | |(DHCPRELEASE) |
|'yiaddr' |0 |0 |0 |
|'siaddr' |0 |0 |0 |
|'giaddr' |0 |0 |0 |
|'chaddr' |client's hardware |client's hardware |client's hardware|
| |address |address |address |
|'sname' |options, if |options, if |(unused) |
| |indicated in |indicated in | |
| |'sname/file' |'sname/file' | |
| |option; otherwise |option; otherwise | |
| |unused |unused | |
|'file' |options, if |options, if |(unused) |
| |indicated in |indicated in | |
| |'sname/file' |'sname/file' | |
| |option; otherwise |option; otherwise | |
| |unused |unused | |
|'options'|(see Table 7) |(see Table 7) |(see Table 7) |
---------------------------------------------------------------------
Table 6: Fields Used by DHCP Clients
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5.2.7 Table 7: Options Used by DHCP Clients
---------------------------------------------------------------------
|Option |DHCPDISCOVER,|DHCPREQUEST |DHCPDECLINE, |
| |DHCPINFORM | |DHCPRELEASE |
---------------------------------------------------------------------
|Requested IP address |MAY |MUST (in |MUST |
| |(DISCOVER) |SELECTING or |(DHCPDECLINE)|
| |MUST NOT |INIT-REBOOT) |MUST NOT |
| |(INFORM) |MUST NOT (in |(DHCPRELEASE)|
| | |BOUND or | |
| | |RENEWING) | |
|IP address lease time |MAY |MAY |MUST NOT |
| |(DISCOVER) | | |
| |MUST NOT | | |
| |(INFORM) | | |
|Use 'file'/'sname' |MAY |MAY |MAY |
|fields | | | |
|DHCP message type |DHCPDISCOVER/|DHCPREQUEST |DHCPDECLINE/ |
| |DHCPINFORM | |DHCPRELEASE |
|Client identifier |MAY |MAY |MAY |
|Vendor class identifier|MAY |MAY |MUST NOT |
|Server identifier |MUST NOT |MUST (after |MUST |
| | |SELECTING) | |
| | |MUST NOT (after| |
| | |INIT-REBOOT, | |
| | |BOUND, RENEWING| |
| | |or REBINDING) | |
|Parameter request list |MAY |MAY |MUST NOT |
|Maximum message size |MAY |MAY |MUST NOT |
|Message |SHOULD NOT |SHOULD NOT |SHOULD |
|Site-specific |MAY |MAY |MUST NOT |
|All others |MAY |MAY |MUST NOT |
---------------------------------------------------------------------
Table 7: Options Used by DHCP Clients
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5.2.8 Table 8: Host Configuration Parameters--IP Layer
---------------------------------------------------------------------
| IP-layer parameters, per host: |
---------------------------------------------------------------------
|Parameter | Permissible Values | Reference |
---------------------------------------------------------------------
|Be a router | on/off | HR 3.1 |
|Non-local source routing | on/off | HR 3.3.5 |
|Policy filters for | | |
|non-local source routing | (list) | HR 3.3.5 |
|Maximum reassembly size | integer | HR 3.3.2 |
|Default TTL | integer | HR 3.2.1.7 |
|PMTU aging timeout | integer | MTU 6.6 |
|MTU plateau table | (list) | MTU 7 |
---------------------------------------------------------------------
| IP-layer parameters, per interface: |
---------------------------------------------------------------------
|Parameter | Permissible Values | Reference |
---------------------------------------------------------------------
|IP address | (address) | HR 3.3.1.6 |
|Subnet mask | (address mask) | HR 3.3.1.6 |
|MTU | integer | HR 3.3.3 |
|All-subnets-MTU | on/off | HR 3.3.3 |
|Broadcast address flavor | 0.0.0.0 or | HR 3.3.6 |
| | 255.255.255.255 | |
|Perform mask discovery | on/off | HR 3.2.2.9 |
|Be a mask supplier | on/off | HR 3.2.2.9 |
|Perform router discovery | on/off | RD 5.1 |
|Router solicitation address | (address) | RD 5.1 |
|Default routers, list of: | | |
| router address | (address) | HR 3.3.1.6 |
| preference level | integer | HR 3.3.1.6 |
|Static routes, list of: | | |
| destination | (host/subnet/net) | HR 3.3.1.2 |
| destination mask | (address mask) | HR 3.3.1.2 |
| type-of-service | integer | HR 3.3.1.2 |
| first-hop router | (address) | HR 3.3.1.2 |
| ignore redirects | on/off | HR 3.3.1.2 |
| PMTU | integer | MTU 6.6 |
| perform PMTU discovery | on/off | MTU 6.6 |
---------------------------------------------------------------------
| Key: |
| HR -- Host Requirements Communications Layers (RFC 1122, |
| Internet Standard) |
| MTU -- Path MTU Discovery (RFC 1191, Proposed Standard) |
| RD -- Router Discovery (RFC 1256, Proposed Standard) |
---------------------------------------------------------------------
Table 8: Host Configuration Parameters--IP Layer
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5.2.9 Table 9: Host Configuration Parameters--Link Layer
---------------------------------------------------------------------
| Link-layer parameters, per interface: |
---------------------------------------------------------------------
|Parameter | Permissible Values | Reference |
---------------------------------------------------------------------
|Trailers | on/off | HR 2.3.1 |
|ARP cache timeout | integer | HR 2.3.2.1 |
|Ethernet encapsulation | (RFC 894/RFC 1042) | HR 2.3.3 |
---------------------------------------------------------------------
| Key: |
| HR -- Host Requirements Communications Layers (RFC 1122, |
| Internet Standard) |
---------------------------------------------------------------------
Table 9: Host Configuration Parameters--Link Layer
5.2.10 Table 10: Host Configuration Parameters--TCP
---------------------------------------------------------------------
| TCP parameters, per host: |
---------------------------------------------------------------------
|Parameter | Permissible Values | Reference |
---------------------------------------------------------------------
|TTL | integer | HR 4.2.2.19 |
|Keep-alive interval | integer | HR 4.2.3.6 |
|Keep-alive data size | 0/1 | HR 4.2.3.6 |
---------------------------------------------------------------------
| Key: |
| HR -- Host Requirements Communications Layers (RFC 1122, |
| Internet Standard) |
---------------------------------------------------------------------
Table 10: Host Configuration Parameters--TCP
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6 Contributors
This document is the result of work undertaken the by DHCP working
group. The editors would like to include a number of contributors
to this effort including Mike Carney of Sun Microsystems, Steve
Tulloh of Shadow Support, Bernie Volz, Ted Lemon of Nominum, Simon
Vogl, Edward Mascarenhas of SGI, Andre Kostur of Incognito, Bud
Millwood of Weird Solutions, Patrick Guélat of ImproWare Network
Services, and Swamy Narasimha of Nokia.
7 IANA Considerations
This memo contains no values requiring IANA attention.
8 Security Considerations
(To be defined when suggested text changes for RFC 2131 are
completed.)
A separate Internet-Draft is being created to provide a threat
analysis of RFCs 2131 and 3118.
9 References
9.1 Normative References
[RFC951] Croft, W., and Gilmore, J., "Bootstrap Protocol," RFC 951,
September 1985.
[RFC1123] R. Braden, "Requirements for Internet Hosts -- Application
and Support," October 1989.
[RFC1542] W. Wimer, "Clarifications and Extensions for the Bootstrap
Protocol" RFC 1542, October 1993
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol," March
1997.
[RFC2132] Alexander, S. and Droms, R., "DHCP Options and BOOTP
Vendor Extensions," March 1997.
9.2 Informative References
[RFC3203] T'Joens, Y., Hublet, C., and De Schrijver, P., "The DHCP
Reconfigure Extension," July 2001.
[RFC4388] Woundy, R., and Kinnear, K., "Dynamic Host Configuration
Protocol (DHCP) Leasequery," February 2006.
<draft-ietf-dhc-fqdn-option-13.txt>, Stapp, M., and Rekhter, Y.,
"The DHCP Client FQDN Option," March 2006.
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Authors' Addresses
Barr Hibbs
Richard Barr Hibbs, P.E.
952 Sanchez Street
San Francisco, California 94114-3362
USA
Phone: +1-(415)-648-3920
E-mail: rbhibbs@pacbell.net
Rob Stevens
308 Arthur Avenue
Aptos, California 95003-5202
USA
Phone: +1-(831)-688-9722
E-mail: robs@cruzio.com
Full Copyright Statement
Copyright (C) The Internet Society (2006). All rights reserved.
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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APPENDIX: NOTES
This appendix will be removed when this memo goes to Working Group
Last Call.
Issues List
Open, unresolved issues about RFC 2131 include:
1. What is the correct use of client identifier in DHCPOFFER and
DHCPACK messages?
2. Are there any effective alternatives to ICMP ECHO and ARP for
address-in-use detection?
3. What is the definition of a Fully Qualified Domain Name?
4. Should DHCPINFORM messages be allowed from client proxies?
5. Should client proxies, in general, be allowed, and how does a
client proxy know the IP address of a DHCP server?
6. Should a DHCP server send only options requested by a client, or
should a server send all options for which it is configured with
a value?
7. Should required usage of "xid" and "client identifier" be
changed to support server verification of DHCPRELEASE messages?
8. What is the correct statement about selecting an IP address to
offer a client when the offered address is on a different subnet
than the client's "giaddr?"
9. Should a new flags bit to signify "more options data available"
be added?
10. Do we need a new "Maximum Relay MTU Size" option to ensure that
all reply packets sent by a server will pass without fragmenting
or dropping packets?
11. Would it help to set a sort of "more to come" option, indicating
that more options will follow in a consecutive DHCPACK, where
the subsequent DHCPACKs would have a "additional information"
option indicating that the message contains only new options
data similar to a DHCPACK in response to a DHCPINFORM message?
12. Are unicast DHCPDISCOVER messages permitted? What are the
requirements for specific message fields and options in this
case?
13. What level of consistency is required among responses from
multiple servers?
14. Can the REBINDING state be entered from the BOUND state on
expiration of T2, or only if there is a timeout in RENEWING
state?
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15. Should the text of RFC 4361, Node-Specific Client Identifiers,
be folded into a revised RFC 2131 and RFC 2132?
16. Should the text of RFC 4388, DHCP Leasequery, be folded into a
revised RFC 2131 and RFC 2132?
Changes from Prior Drafts
"-00" Draft
The "-00" revision was the initial version of this memo, submitted
to the Internet-Drafts editor on 23 February 2003.
"-01" Draft
The "-01" revision contains substantial changes following a detailed
review of DHC Working Group mailing list discussions on RFC 2131
clarification issues, consideration of several directed questions,
and comments received by the authors. Changes include:
O Reorganization of the document to group all typographical errors
together, separate from protocol or policy issues.
O Elimination of "Interaction with DNS" and "Client and Server
Administration" sections because the authors saw no clear
resolution to the topics.
O Creation of an issues list in section 4.1.
"-02" Draft
The "-02" revision consists of numerous minor typographical,
spelling, and grammatical updates, plus:
O Replaced all Internet-Draft Boilerplate with current versions.
O Removed all of Section 5, "Notes" to this appendix.
O Replaced Section 5 with the proposed figures and tables for use
with revised text of RFC 2131-bis.
O Removed Appendix A.
O Separation of Section 9, "References," into Normative and
Informative subsections.
O Updated Authors' Addresses.
O Added text to Section 4.5.1 covering RFC 4361, "Node-Specific
Client Identifiers for DHCPv4."
O Recent mailing list discussion about the correct use of the
"secs" field was incorporated into Section 4.22.
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