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)" Saved: Tuesday, June 15, 2006, 13:27:17 Intellectual Property Rights By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Status of this Memo Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Comments are solicited and should be addressed to the working group's mailing list at dhcwg@ietf.org and/or the author(s). 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 Hibbs & Stevens Expires: December 17, 2006 [Page 1] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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Issues with RFC 2131.......................................5 4.1 Outdated RFC Boilerplate...............................5 4.2 Organization and Typography............................5 4.2.1 Outdated References.............................5 4.2.2 Typographical Errors............................5 4.2.3 Omissions.......................................6 4.2.4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 Hibbs & Stevens Expires: December 17, 2006 [Page 2] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 3] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 4] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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." Hibbs & Stevens Expires: December 17, 2006 [Page 5] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 6] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 7] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 8] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 9] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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? Hibbs & Stevens Expires: December 17, 2006 [Page 10] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 11] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 12] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 13] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 14] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 15] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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? Hibbs & Stevens Expires: December 17, 2006 [Page 16] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 17] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 18] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 19] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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.'" Hibbs & Stevens Expires: December 17, 2006 [Page 20] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 21] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 22] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 23] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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). Hibbs & Stevens Expires: December 17, 2006 [Page 24] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 25] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 26] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 27] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 28] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 29] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 30] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 31] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 32] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 33] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 34] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 35] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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 Hibbs & Stevens Expires: December 17, 2006 [Page 36] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. , Stapp, M., and Rekhter, Y., "The DHCP Client FQDN Option," March 2006. Hibbs & Stevens Expires: December 17, 2006 [Page 37] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 38] Internet Draft RFC 2131 Implementation Issues June 15, 2006 Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Hibbs & Stevens Expires: December 17, 2006 [Page 39] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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? Hibbs & Stevens Expires: December 17, 2006 [Page 40] Internet Draft RFC 2131 Implementation Issues June 15, 2006 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. Hibbs & Stevens Expires: December 17, 2006 [Page 41]