Internet Engineering Task Force R. Johnson Internet Draft Jay Kumarasamy Expiration: August 2003 Kim Kinnear File: draft-ietf-dhc-subnet-alloc-00.txt Mark Stapp Cisco Systems, Inc. Subnet Allocation using DHCP <draft-ietf-dhc-subnet-alloc-00.txt> February 20, 2002 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document defines a new DHCP option which is passed between the DHCP Client to the DHCP Server to request dynamic allocation of a subnet, give specifications of subnet(s) allocated, and report usage statistics. Johnson, et. al. [Page 1]
Internet Draft Subnet Allocation using DHCP February 2003 1.0 Introduction There is a need for a DHCP Client to be able to allocate a subnet from a DHCP Server. Alternate methods of allocation, such as AAA are not appropriate for various reasons and the most straight-forward way to accomplish this allocation is via DHCP. A DHCP option to support this may be utilized by a simple Dialin Aggregation box, or even to implement a Hierarchical chain of DHCP Servers, each one in turn leasing one or more subnets to the next DHCP Server down the chain. This new DHCP option [2], the Subnet Allocation option is specified in such a way as to use one DHCP Option number, while using suboption numbers for each function. The Subnet-Request suboption tells what types of subnets are needed and how many. The "Subnet Information" suboption gives the actual subnet number(s) and allows for extra flags to convey additional information about each subnet. The "Subnet Name" suboption allows a method of passing additional information about the requested subnet(s), such as department name, user name, customer number, etc. The DHCP Server has the option of not supplying all subnets requested or even returning smaller subnets than was requested. The "Subnet Usage Statistics" suboption is used to report usage information from the DHCP Client back to the DHCP Server. 1.1 Conventions 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 [3]. This document also uses the following terms: "DHCP Client" DHCP Client or "Client" is an Internet host using DHCP to obtain configuration parameters such as a network address. "DHCP Server" A DHCP Server or "Server" is an Internet host that returns configuration parameters to DHCP Clients. Johnson, et. al. [Page 2]
Internet Draft Subnet Allocation using DHCP February 2003 2.0 Option Formats: 2.1 Subnet Allocation Option Format: 0 1 2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Len | Flags | Suboptions ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code = Subnet Allocation option code (1 octet) TBD (220 currently) Len = length of the entire option including all sub-options and excluding the "Code" and "Len" fields above (1 octet) Flags = Various flags: (None currently defined) One or more sub-options may be specified as described below. 2.2 Subnet Request suboption format: 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 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Len | Flags |i|h| Prefix | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Len = length of the suboption excluding the subcode and Len fields (1 octet) Flags = Flags field. (all unused bits MBZ) "h" = "1" = Client will be allocating addresses from this subnet. "0" = Client will be relaying DHCP requests to the Server from this subnet. "i" = "1" = Client is seeking information about previously allocated subnets. "0" = Client is seeking a new subnet allocation. Johnson, et. al. [Page 3]
Internet Draft Subnet Allocation using DHCP February 2003 Prefix = size of the subnet needed [4] (number of bits in subnet prefix) (zero (0) means no suggested size is given) (1 octet) The DHCP Server SHOULD NOT include the Subnet Request suboption in any replies to the DHCP Client. Enough information will be present in the Subnet Information suboption, such that the Subnet Request suboption qis not needed in replies to the Client. The DHCP Server SHOULD allocate a subnet with prefix size less than or equal to the size P specified in the request. In other words, a subnet at least the size requested and possibly bigger. A size P of zero (0) MAY be specified by the DHCP Client. In this case, no suggested size is given and the Server is free to return subnet(s) of whatever size is deemed appropriate by the Server. Multiple Subnet Request suboptions in a DHCP packet indicate that multiple sizes of subnets are being requested. Each Subnet Request suboption MUST result in no more than one (1) Subnet Information suboption in the DHCP OFFER message from the Server, and may result in zero (0) Subnet Information suboptions. The Client MAY also include the Subnet Information suboption (described below) in order to request a particular subnet. In this case, the Client SHOULD only include one (1) Subnet Request suboption, since it would otherwise be unclear which Subnet Information suboption refered to which Subnet Request suboption. Multiple subnet requests can be made by sending multiple DHCP DISCOVER packets. Setting the "h" flag to "1" indicates the Client will be allocating addresses from the allocated subnet(s) itself. This can be thought of as a "Hierarchial DHCP" design in that control of allocation for the subnet(s) will be passed to the Client. Setting the "h" flag to "0" indicates the Client wants the DHCP Server to retain control over allocation of addresses from the subnet(s). In this case, the Server should simply mark the subnet(s) as "used" by this Client and not return the subnet(s) in any Subnet Information suboptions to any other client. (Any address allocation requests on the subnet will be relayed back to the DHCP Server.) Setting the "i" flag to "1" indicates the Client is NOT seeking allocation of any subnets, but is simply seeking information from the Johnson, et. al. [Page 4]
Internet Draft Subnet Allocation using DHCP February 2003 Server as to what subnet(s) have been allocated (or reserved) for this Client. If the "i" flag is set to "1", then the "P" field SHOULD be set to "0" and MUST be ignored by the Server. Johnson, et. al. [Page 5]
Internet Draft Subnet Allocation using DHCP February 2003 2.3 Subnet Information suboption format: 0 1 2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | Len | Flags |c|s| SP1, SP2, ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Len = length of the sub-option excluding the Subcode and Len fields (1 octet) Flags = Various flags which apply to ALL Subnet Prefix Information fields specified in this suboption "c" = Client flag (explained below) "s" = Server flag (explained below) SP1,SP2 = Subnet Prefix information as specified below (variable sized) The "Client flag" ("c") is set to "1" if this Subnet Information suboption is in response to a Client request for information from the Server as to what subnet(s) have been allocated. This flag is only used in response to a Subnet Request suboption with the "i" flag set and should be zero (0) otherwise. The "Server flag" ("s") is set to "1" if the Server has additional subnet information for the Client. Johnson, et. al. [Page 6]
Internet Draft Subnet Allocation using DHCP February 2003 2.4 Subnet Prefix Information suboption Format: 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 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix | Flags |h|d| Stat-len | Optional statistics... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Addr = IPv4 address (4 octets) Prefix = specifying number of prefix bits in the subnet (1 octet) Flags = flags field (Undefined bits must be zero) (1 octet) "d" = "1" = Deprecate this subnet "h" = "1" = Client will be allocating addresses from this subnet. "0" = Client will be relaying DHCP requests to the Server from this subnet. Stat-len = length of the optional statistics information field Subnet Prefix suboption format: The "d" flag may only be returned by the Server to the Client. It's presensce means that the Client should prepare to give up the subnet. For example, if the Client is assigning addresses from this subnet to other clients, it should cease doing so immediately and should not renew any leases when client's ask for renewal. As soon as all addresses in the subnet are unallocated, the Client should send a DHCP RELEASE message to the Server, including a Subnet Prefix Information suboption for the subnet in order to release the Subnet. The format of this message is described below. The "h" flag tells the Client how the Server intends the Client to use the allocated subnet. It is interpreted in the same manner as that in the Subnet Request suboption. In response to a Subnet Request, the Server should normally specify the "h" flag in the same mannor was it was in the Subnet Request suboption from the Client. The Server MAY, however, change the "h" flag from that specified in the Subnet Request suboption if it has been configured to override the Client's request. Johnson, et. al. [Page 7]
Internet Draft Subnet Allocation using DHCP February 2003 If any usage statistics information is to be included, then the "Stat-len" field specifies the number of bytes of statistics information which is included. See below for more information. If no statistics information is included, then this byte MUST be zero. 2.4.1 Subnet Usage Statistics The Subnet Information suboption may also include usage statistics information. If this information is included, then the "Stat-len" (Statistics length) field MUST be set to the number of bytes of statistics information which is being included. The statistics information MUST be in the following form and order: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | High water | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Currently in use | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Unusable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ "High water" refers the to "high water mark" of allocated addresses within the subnet. "Currently in use" refers to the number of addresses currently allocated in the subnet. "Unusable" refers to the number of addresses which are currently unusable for any reason (such as a client returning a DHCP DECLINE, or finding the address already in use). Additional statistics may be added to this option in the future. If so, they MUST be appended to the end of the option. All statistics fields MUST remain in the same order. Use the all ones value (0xFFFF) in order to skip reporting a number for a particular field. Fewer fields may be included than what is specified in any current RFC, but all fields which are included MUST remain in order specifed here. Johnson, et. al. [Page 8]
Internet Draft Subnet Allocation using DHCP February 2003 2.5 Subnet Name suboption format: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3 | Len | Name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Len = length of the sub-option excluding the Subcode and Len fields (1 octet) The Subnet Name suboption may be used in order to pass a subnet name to the server for use during allocation. This name may be used for any purpose but is intended to tell the server something extra about the needed subnet; for example, "sales department", "customer 1002", "address pool FOO", or some such. The "name" should NOT be NULL terminated since the "len" field already specifies the length of the name. Johnson, et. al. [Page 9]
Internet Draft Subnet Allocation using DHCP February 2003 2.6 Subnet Suggested Lease time suboption format: 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 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4 | Len (4) | t1 | t2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | t3 | t4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Len = length of the sub-option excluding the Subcode and Len fields (always 4 for this suboption) (1 octet) The Subnet Suggested Lease Time suboption MAY be included by the Server in order to suggest the lease time to be used by the Client when allocating addresses from the subnet allocated. The four (4) octet value of the lease time is in the same format as that of the "IP Address Lease Time" option (option 51), as described in [2]. If included, this suboption may appear only once. (Multiple such suboptions would make things ambiguous as to which applied to which subnet.) If different suggested lease times are needed, the Server SHOULD, instead, reply with only one offered subnet and should set the "Server flag" in the Subnet Information suboption to indicate to the Client that it should send another subnet request to gather the others. If this suboption is not included, the Client is free to use whatever lease time it wants for addresses allocated from this subnet. In all cases, the Client should, at each address allocation time, use a lease time which is the lesser of the remaining lease time of the subnet itself and the Server Suggested Lease Time suboption. 3.0 Requesting allocation of a subnet 3.1 Client DISCOVER message The DHCP Client creates a DHCP DISCOVER message including the Subnet Allocation option, and its set of suboptions, to request allocation of a subnet. The DHCP Client should include the Subnet Johnson, et. al. [Page 10]
Internet Draft Subnet Allocation using DHCP February 2003 Request suboption, specifying the prefix size of the subnet requested. The "h" bit should be set to "1" if the Client intends to control allocation of addresses within the subnet itself, or "0" if the Server should retain control of addresses within the subnet. More than one Subnet Allocation option may appear in a DHCP DISCOVER message, however the client SHOULD limit the number of requests, noting that the DHCP replies will need to include the Subnet Information suboption, which takes up more space. If more than one subnet size is being requested, multiple Subnet Request suboptions MAY be included or multiple DHCP DISCOVER messages MAY be sent instead. The prefix size field of each Subnet Request suboption MUST be either zero (0), or in the range of 1 to 30 inclusive. The DHCP "IP address lease time" option (code 51) MAY be included in the DHCP DISCOVER message to specify the lease time the Client is requesting. If not present, no suggested lease time is given. The DHCP "Client ID" option (code 61) MAY be included in the DHCP DISCOVER message as it may be used by the Server in performing the subnet allocation. 3.2 Server OFFER message Upon receiving a DHCP DISCOVER containing the Subnet Allocation option, the DHCP Server should respond with a DHCP OFFER message including the Subnet Information suboption in order to specify the subnet(s) which it is willing to allocate to the Client in order to fill the request(s). The Server need not reserve the subnets which are being OFFERed, but SHOULD strive to not OFFER the same subnets to another DHCP Client until a reasonable time period (implementation independent) has passed. The Server MUST NOT include the Subnet Request suboption in the OFFER. The same information is already present in the Subnet Information suboption(s) which SHOULD be included in the OFFER. The Server SHOULD also include the IP address lease time option (option 51) in the DHCP OFFER message. This gives the lease time for all subnets given in all Subnet Request suboptions contained in the DHCP OFFER message. The Server MAY also include the Renewal and/or Rebinding options in order to further control the "T1" and "T2" lease timers of the client. There MUST be only one IP address lease time, rebind, and/or renew option in the DHCP Johnson, et. al. [Page 11]
Internet Draft Subnet Allocation using DHCP February 2003 OFFER message. If different lease times are required for some of the allocated subnets, then the server should only return Subnet Information suboption(s) for those subnets with the same lease time. If the Client requires more subnets, another DHCP DISCOVER message will need to be issued to collect the other needed subnets. Also see the "s" flag described below. The Server MAY set the "Server flag" ("s") to "1" to indicate that it would like to allocate one or more additional subnet(s) to the Client. This indicates that the Client should send another DISCOVER message specifying a zero prefix size field, P, in order to request the additional subnet allocation(s) information. This may be necessary if the subnets are to be allocated with different lease times, for example. The "Client flag" ("c") MUST be set to zero (0) to indicate this is a response to a client request for a new subnet allocation and not a request for information about already allocated subnets. The Server SHOULD set the DHCP yiaddr value to all zeros (0.0.0.0) and the Client MUST ignore fields having to do with address assignment if the packet contains a Subnet Allocation option. In other words, a DHCP packet exchange can not provide subnet allocation and address assignment simultaneously. 3.3 Client DHCP REQUEST message When sending a DHCP REQUEST, the Client MUST NOT modify any fields of all Subnet Information suboptions received from the Server. However, the Client MAY choose to not include some Subnet Information suboptions when issuing the DHCP REQUEST. Subnet Request suboptions MUST NOT be included in the DHCP REQUEST message, only the Subnet Information suboption(s) should be included. 3.4 Server DHCP ACK message The DHCP Server, upon receipt of the Client's REQUEST message, MAY refuse allocation of any subnets (for example, if they have been allocated elsewhere in the meantime), however since the Server should have set aside the subnets offered for a short period of time, and since the Client should have requested the subnets within a short period of time after receiving the offer(s) from the server(s), this last minute rejection should be rare. The DHCP Server MAY NOT change the subnet address(es) or prefix size(s), however it MAY remove some Subnet Information suboptions from the list. Johnson, et. al. [Page 12]
Internet Draft Subnet Allocation using DHCP February 2003 The Server SHOULD include the IP address lease time option specifying the lease period for all subnet(s) in the ACK. All subnets allocated in one DHCP message will have the same lease time and only one IP address lease time option must appear in the DHCP message. If the Server has internal information which states that the Client should be allocated more subnets than were requested, the Server MAY set the "s" bit in the Subnet Information suboption to indicate that the Client needs to request more subnets (as described above). The allocatable unit is the tuple (subnet address, prefix size). Multiple subnets may be allocated in one DHCP ACK, however since there can be only one Lease-time option, each subnet allocated is assigned the same lease time. Each subnet lease MAY be RENEWed or RELEASEd individually. 4.0 Client renewal of subnet lease 4.1 Client RENEW REQUEST message The Client MUST renew all subnets allocated with a lease time in much the same manner as renewing an allocated IP address. Renewal timers need not be set in exactly the same manner, however. If Renewal and/or Rebinding options were included in the ACK of the subnet allocation, then these "T1" and "T2" timers should be used just as they would be in the case of address allocation timers. The REQUEST message MUST include a Subnet Information suboption for which the Client is seeking renewal of the lease. This Subnet Information suboption may optionally include subnet usage statistics, as described above w.r.t. the Subnet Information suboption format. The subnet IP address field (Address) and subnet prefix field (Prefix) MUST agree with the values as they were originally allocated to the Client by the Server. In any of the statistics fields (High, Current, Ususable), a value of all ones (0xffff) SHOULD be used if the Client has no information to report for a statistic. 4.2 Server RENEW response Johnson, et. al. [Page 13]
Internet Draft Subnet Allocation using DHCP February 2003 The Server MAY respond to a subnet RENEW request with either an ACK or NAK response. If a NAK response is given the Client MUST immediately stop using the subnet(s) specified and, if possible, notify all Clients with addresses allocated from this subnet that their addresses are no longer valid. The Client MAY, of course, send a DHCP DISCOVER message containing the Subnet Allocation option and the Subnet Request suboption in order to acquire another subnet for use. In general, the Server should ask the Client to "free" subnets by using the "Deprecate" bit of the Subnet Information suboption in an ACK message (see below). If an ACK response is given, the "Deprecate" ("d") bit of the flags field in the Subnet Information suboption may also be set. This indicates the DHCP Client should "prepare to stop using this subnet". If the Client is allocating IP addresses for other clients out of this subnet (probably via DHCP), the Client SHOULD immediately stop allocating such addresses. Once all allocated addresses in the subnet have been released, the Client SHOULD send a DHCP RELEASE message, including the Subnet Information suboption (with optional usage statistics) in order to release the subnet(s) back to the Server. 4.3 Client RELEASE message The DHCP Client should send a DHCP RELEASE message in order to release allocated subnet(s) back to the Server when it is finished using them. This message MUST NOT include the Subnet Request suboption, but MUST include one or more Subnet Information suboptions, and optionally including usage statistics. The Client MUST release the same subnet(s) of the same prefix size (Prefix), as was originally allocated. The Client MAY release a subset of the subnets which were allocated originally. In other words, the allocatable unit is the tuple (subnet address, prefix size). Multiple subnets may be allocated in one DHCP ACK, however each subnet MAY be released individually. 4.4 Server RECONFIGURE message The DHCP Server may issue a DHCP RECONFIGURE message containing the Subnet Allocation option and the Subnet Information suboption. This message effectively immediately times out the Client's lease(s) for the allocated subnet(s). Upon receiving this message, the DHCP Client MUST issue a DHCP REQUEST message to the DHCP Server in order to renew the lease on the subnet mentioned. Johnson, et. al. [Page 14]
Internet Draft Subnet Allocation using DHCP February 2003 No other subnets allocated to the Client are effected. As is the case with all DHCP Renewal messages, the Client may include subnet usage information in the Subnet Information suboption in order to report subnet usage statistics, or set the "Stat-len" field to zero (0) if no statistics are to be reported. If the Server responds to this REQUEST with a DHCP NAK message, then the Client MUST immediately stop using the subnet(s) and, if possible, notify all Clients with addresses allocated from this/these subnet(s) that their addresses are no longer valid. The Client MAY, of course, send a DHCP DISCOVER message containing the Subnet Allocation option and the Subnet Request suboption in order to acquire another subnet for use. 5.0 Client requesting subnet allocation information: The DHCP Client may request from the DHCP Server a list of what subnets are currently allocated to Client. This may be used to recover from a restart if the Client does not have local storage in order to retain the information itself. 5.1 Client DISCOVER message The DHCP Client DISCOVER message, in order to discover already allocated subnet information, should contain a Subnet Request suboption, with the "Prefix" field set to zero (0) and with the "i" flag set to "1" to indicate that the Client is seeking already allocated subnet information from the Server. No Subnet Information suboptions should be included in this message. This DISCOVER message MAY be unicast to a particular DHCP Server, or broadcast in the normal fashion. 5.2 Server OFFER response Any DHCP Server which has allocated subnets to the Client should respond to the DISCOVER message with a DHCP OFFER message The OFFER message should contain one or more Subnet Information suboption(s) telling the subnet address(es) and prefix(es) of the subnet(s) allocated to the Client. The Server SHOULD, internally, retain an ordered list of subnets which are allocated to each Client. The subnet(s) information Johnson, et. al. [Page 15]
Internet Draft Subnet Allocation using DHCP February 2003 returned in the OFFER message are the first subnet(s) from this list. If the end of the list has been reached, then the "s" bit should be set to "0". If there are more subnets in the list, the "s" bit should be set to "1". to indicate to the Client that more information is available. If this is the initial OFFER to the client, the "c" flag should be set to "1". 5.3 Client additional DISCOVER messages The Client, upon receiving any Server OFFER messages containing Subnet Information suboption information with the "c" ("Client") bit set, should gather the subnet address and prefix information from the message. If the "s" bit is set in the Subnet Information suboption, then the client MUST construct a new DHCP DISCOVER message containing the Subnet Allocation option and the Subnet Information suboption, and send this message back to the same DHCP Server originating the OFFER message. The "c" and "s" bits MUST retain the same settings they had in the Server's OFFER message and the subnet address ("A") and prefix size ("P") fields MUST be unaltered as well. If the "s" bit in the Subnet Information suboption from the Server was "0", then it indicates the Server has no more information about subnets allocated to the Client. In this case, the Client MUST NOT send a REQUEST response to the Server. 5.4 Server additional OFFER messages The Server, upon receiving a DISCOVER message from a Client containing a Subnet Information suboption with the "c" and the "s" bits set, MUST use the subnet address ("A") and prefix size ("P") fields in order to locate the position in the internal table of allocated subnets for this Client, and then return an OFFER message containing a Subnet Information suboption giving information about the next set of subnets allocated to this Client. If this finishes the list in the table for this Client, then the "s" bit MUST be set to "0" to indicate there is no more information. 6.0 DHCP Server Subnet Allocation method The actual method of allocating subnets on the DHCP Server, as well as the configuration of what networks may be subnetted and how, is left up to the implementation. Johnson, et. al. [Page 16]
Internet Draft Subnet Allocation using DHCP February 2003 7.0 Examples Only the Subnet Allocation option and accompanying suboptions are displayed in these examples. All other fields in the DHCP messages are described in [1]. For the purposes of these examples, "SAC" stands for the actual code number allocated for the "Subnet Allocation option Code". 7.1 Example 1: DHCP Client requesting a subnet with prefix size 24 from which the Client will allocate addresses to other clients. The Server responds with allocation of exactly the size requested: Client sends DHCP DISCOVER including the Subnet Allocation option with the Subnet-Request suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 5 | 0 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 |0|0| 24 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Server responds with DHCP OFFER including Subnet Allocation option with a Subnet Information suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | 0 |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Johnson, et. al. [Page 17]
Internet Draft Subnet Allocation using DHCP February 2003 Client sends DHCP REQUEST including Subnet Allocation option with Subnet Information suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | 0 |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Server responds with DHCP ACK including Subnet Allocation option with Subnet-Info suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | 0 |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Later Client sends DHCP RELEASE including Subnet Allocation option with Subnet Information suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | 0 |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Johnson, et. al. [Page 18]
Internet Draft Subnet Allocation using DHCP February 2003 7.2 Example 2: DHCP Client requesting a subnet with prefix size 24 and a subnet with prefix size 30: Client sends DHCP DISCOVER including the Subnet Allocation option with the Subnet-Request suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 9 | 0 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 |0|0| 24 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 |0|0| 30 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Server responds with DHCP OFFER including Subnet Allocation option with Subnet Information suboption: Offer includes 1 subnet of size 24 and 1 subnet of size 28. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 18 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 15 | |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 10 | 0 | 3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 28 | |0|0| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Client sends DHCP REQUEST including Subnet Allocation option with Subnet Information suboption: Johnson, et. al. [Page 19]
Internet Draft Subnet Allocation using DHCP February 2003 Client decides that the subnet of size 28 is not sufficient so doesn't include it into the request message. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Server responds with DHCP ACK including Subnet Allocation option with Subnet Information suboption: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Later Client sends DHCP REQUEST message in order to renew the lease on the one subnet, including subnet usage information. It reports that a maximum of 10 addresses were allocated from the subnet since the last report, 7 addresses are currently allocated, and 2 addresses were found to be unusable. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 17 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 14 | |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 6 | 0 | 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 7 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Johnson, et. al. [Page 20]
Internet Draft Subnet Allocation using DHCP February 2003 Server responds with ACK, however signals Client that the subnet should be deprecated. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Client crashes at this point and upon recovery sends a DISCOVER asking for information about all subnets which were allocated to it. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 5 | 0 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | |1|0| 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Server responds with an OFFER, giving the subnet information of the one subnet which is allocated to the Client. Also the Server specifies that the one allocated subnet should be immediately deprecated. Note that the "s" ("Server") bit is zero (0) thus indicating that there is no more information available for this Client. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | |1|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ Johnson, et. al. [Page 21]
Internet Draft Subnet Allocation using DHCP February 2003 Client responds with RELEASE message after having deprecated the subnet: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SAC | 11 | 0 | SIS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 8 | |0|0| 10 | 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | 0 | 24 | |0|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | +-+-+-+-+-+-+-+-+ 8.0 Differences with DHCPv6 Prefix Delegation The following differences may be noticed between Subnet Allocation as described in this document and DHCPv6 Prefix Delegation as described in [6]: * This option does not use anything like an "IA_PD" as is used in DHCPv6. * If the Server can not allocate a subnet, it remains silent, instead of returning a special saying nothing is available. * DHCPv6 Prefix Delegation has no mechanism for returning subnet/prefix usage statistics. * DHCPv6 has no equivalent to the "subnet deprecation" flag as described here. * DHCPv6 Prefix Delegation makes no mention of what Client actions should result from receiving a NAK during a RENEW of a delegation. * DHCPv6 has no equivalent of the subnet allocation "Network name" suboption, which may be used by the Server for various purposes, such as to specify a pool to use when allocating a subnet. * DHCPv6 Prefix Delegation corresponds to "Hierarchical Subnet Allocation" (setting the "h" flag in the Prefix Information suboption). There is no V6 equivalent of clearing the "h" flag, in which the Server retains authority over allocation of addresses from the subnet. * DHCPv6 Prefix Delegation has nothing to correspond to the Suggested Lease Time suboption. Johnson, et. al. [Page 22]
Internet Draft Subnet Allocation using DHCP February 2003 9.0 Security Considerations Potential exposures to attack are discussed in section 7 of the DHCP protocol specification [1]. The Subnet Allocation option can be used to hoard all allocatable subnets on a network. It is suggested that DHCP Authentication be used with this option. Message authentication in DHCP for intradomain use where the out-of- band exchange of a shared secret is feasible is defined in RFC 3118 [5]. Potential exposures to attack are discussed in section 7 of the DHCP protocol specification in RFC 2131 [1]. 10.0 IANA Considerations IANA has assigned a value of TBD for the DHCP option code described in this document. No assignment of values for the suboption codes need be made at this time. New values may only be defined by IETF Consensus, as described in [6]. Basically, this means that they are defined by RFCs approved by the IESG. 11.0 Intellectual Property Rights The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights. References [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, Bucknell University, March 1997. [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell University, March 1997. [3] Bradner, S., "Key words for use in RFCs to indicate requirement levels", RFC 2119, Harvard University, March 1997. [4] Pummill, T., Manning, B., "Variable Length Subnet Table For IPv4", RFC1878, Alantec, USC/Information Sciences Institute, December, 1995 [5] Droms, R., "Authentication for DHCP Messages", RFC 3118, June 2001 Johnson, et. al. [Page 23]
Internet Draft Subnet Allocation using DHCP February 2003 [6] Troan, O., Droms, R., "IPv6 Prefix Options for DHCPv6", draft-ietf-dhc-dhcpv6-opt-prefix-delegation-02.txt, February, 2003 Author Information: Richard Johnson Cisco Systems, Inc. San Jose, Ca. email: raj@cisco.com Jay Kumarasamy Cisco Systems, Inc. San Jose, Ca. email: jayk@cisco.com Kim Kinnear Cisco Systems, Inc. San Jose, Ca. email: kkinnear@cisco.com Mark Stapp Cisco Systems, Inc. San Jose, Ca. email: mjs@cisco.com Johnson, et. al. [Page 24]
