Dynamic Host Configuration working group Baiju V. Patel, Internet Draft Intel Corp, Munil Shah Microsoft Corp. November 20, 1997 Multicast address allocation extensions to the Dynamic Host Configuration Protocol Status of this Memo This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress". To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or munnari.oz.au. A revised version of this draft document will be submitted to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested. This document will expire before February 1998. Distribution of this draft is unlimited. Abstract The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network. The multicast extensions to DHCP add additional capability of dynamic allocation of the multicast addresses and additional configuration options. 1. Introduction The multicast extensions to DHCP (MDHCP) provide configuration parameters to the multicast applications. MDHCP is built on a client-server model, where designated DHCP server allocates multicast addresses and delivers parameters associated with the address to dynamically configured hosts. Throughout the remainder Patel and Shah 1 MDHCP 11/20/97 of this document, the term "server" refers to a host providing multicast address(es) and parameters through DHCP, and the term "client" refers to a host requesting multicast address(es) and parameters from a DHCP server. MDHCP server is used at times, to indicate a DHCP server capable of handling MDHCP extensions to the DHCP protocol and the MDHCP client is used to indicate the MDHCP capable DHCP client. MDHCP is not a separate protocol, but is simply an extension to the DHCP protocol. Like DHCP, MDHCP should be a mechanism rather than a policy. MDHCP must allow local system administrators control over configuration parameters where desired; e.g., local system administrators should be able to enforce local policies concerning allocation and access to local resources where desired. The MDHCP client is not required to obtain IP address from a DHCP server in order to use MDHCP protocol. The design goals specified in the DHCP RFC also apply to MDHCP. 1.1. Requirements Throughout this document, the words that are used to define the significance of particular requirements are capitalized. These words are: o "MUST" This word or the adjective "REQUIRED" means that the item is an absolute requirement of this specification. o "MUST NOT" This phrase means that the item is an absolute prohibition of this specification. o "SHOULD" This word or the adjective "RECOMMENDED" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course. o "SHOULD NOT" This phrase means that there may exist valid reasons in particular circumstances when the listed behavior is acceptable or even useful, but the full implications should Patel and Shah 2 MDHCP 11/20/97 be understood and the case carefully weighed before implementing any behavior described with this label. o "MAY" This word or the adjective "OPTIONAL" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item. 1.2. Terminology This document uses the following terms[1]: o "DHCP client" A DHCP client is an Internet host that obtains configuration parameters (e.g., network address) using DHCP protocol o "DHCP server" A DHCP server is an Internet host that provides configuration parameters to DHCP clients. o "MDHCP client" A MDHCP client is a DHCP client that supports MDHCP extensions. o "MDHCP server" A MDHCP server is a DHCP server that supports MDHCP extensions. 1.3. Motivation and protocol requirements. For multicast applications to be ubiquitous, there is a need to standardize on a protocol to allocate multicast addresses to an application. Following are the set of requirements on such a protocol. Conflict Free Allocation: When two applications obtain a multicast address (using a common multicast address allocation protocol), both applications may be allocated identical addresses only if it can be guaranteed that no hosts will receive multicasts using same address from both the applications on the same network interface provided that the multicast scoping is implemented correctly. Patel and Shah 3 MDHCP 11/20/97 Quick response: The application should not have to wait for a long time before it able to determine if it can use a multicast address. The response time should primarily be a function of network and system delays only and should not be in the order of several minutes. Low network load: The multicast address allocation protocol is a control protocol. Therefore, it should impose minimal load on the network. Specifically, the address allocation protocol should not overload a modem line when used by a dial-in user. Work with power managed systems: System may be in on, off or low power state between the address allocation and usage period. Multicast address scopes: The protocol must be able to allocate both the administratively scoped and global addresses. Efficient use of address space: The multicast address space is smaller then IP address space. Moreover, a host or application may require multiple addresses. Therefore, efficient use of address space is a design goal of multicast address allocation protocol. 1.4. MHDCP Protocol Summary From the protocol standpoint, MDHCP is an extension of the DHCP. As in normal DHCP protocol, a MDHCP client requests multicast address(es) from the MDHCP server for a specified multicast scope. The MDHCP servers assigns multicast address(es) to the hosts to be used within the requested scope, and valid over a specific period. The DHCP server MUST provide TTL value of the address. The client, when using the assigned address should not use the TTL value larger than the one provided. The lease period is defined by the duration of the lease and the time at which the lease becomes effective. The DHCP server MUST NOT allocate the same address to different clients with overlapping lease period and scope. The protocol also allows client to request more than one address at a time. Before requesting a multicast address, a client needs to obtain the list of multicast scopes available on the MDHCP server. The multicast scope-list is one of the MDHCP configuration parameters. The scope list may be obtained through the DHCP option described in [3], or may be obtained by some other means. Similarly, the MDHCP server address (multicast) may also be obtained by the option described in [3] or can be configured on the client. The MDHCP server is not required to be co-located with a DHCP server. Therefore, in a typical deployment, there may be fewer MDHCP servers then the DHCP servers. The MDHCP protocol uses M flag and a set of options defined below. Patel and Shah 4 MDHCP 11/20/97 2. MDHCP messages and options. The following options and flags are used by MDHCP extensions. 2.1. M flag A new flag (M) is defined to differentiate the MDHCP messages from DHCP messages. All the messages (DHCPDISCOVER, DHCPOFFER etc.) use M flag defined below to indicate multicast address negotiations. The second bit of the flag field (bit 1) defines M (multicast) flag. The M bit must be set for all the message exchanges pertinent to the multicast address assignment. The client MUST obtain an IP address prior to requesting a multicast address. Therefore, B flag MUST not be set when M flag is set. 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |B|M| MBZ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ B: BROADCAST flag M: Multicast address request flag. MBZ: MUST BE ZERO (reserved for future use) 2.2. Multicast Scope Option This option is used by the client to indicate the multicast scope for the requested multicast address(es). It is also used to indicate the scope of the assigned address by the DHCP server. If this option is not specified, the DHCP server MAY allocate an address from a DEFAULT scope or reject the request. Code Len Scope Id +-----+-----+-----+-----+-----+-----+ | 101 | 4 | i1 | i2 | i3 | i4 | +-----+-----+-----+-----+-----+-----+ The client may obtain the scope list through the option described in [3] or using some other means. The scope id is the numeric representation of the scope as described in [3]. The 'code' for this option is 101 and the length is 4. Patel and Shah 5 MDHCP 11/20/97 2.3. Start time Option The start time is used in a client request (DHCPDISCOVER or DHCPREQUEST) to allow the client to request the starting time for the use of the assigned address. This option allows client to request a multicast address for use at a future time. Code Len Time +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ | 102 | 8 | t1 | t2 | t3 | t4 | t5 | t6 | t7 | t8 | +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ The time value is the decimal representation of Network Time Protocol (NTP) time values in seconds [5]. The 'code' for this option is 102 and the length is 8. If IP Address Lease Time option specifies [2] the duration of the lease beginning at Start Time option value. 2.4. Multicast TTL Option This option specifies the TTL value to be used with the multicast address. The TTL is specified as an octet with a value between 1 and 255. The implied value of this option is 255 when not included. Code Len Multicast TTL +-----+-----+-----+ | 103 | 1 | n | +-----+-----+-----+ The 'code' for this option is 103 and the length is 1. 2.5. Number of Addresses Requested Option This option specifies the number of addresses requested by the client. The client MAY obtain more than one address either by repeating the protocol for every address or by requesting all the addresses at the same time via this option. The server MAY use this option to indicate to the client the number of addresses it has allocated to the client. When the client is requesting only one address, this option need not be included. Code Len Number of Addresses +-----+-----+-----+-----+ | 104 | 2 | n | +-----+-----+-----+-----+ The 'code' for this option is 104 and the length is 2. Patel and Shah 6 MDHCP 11/20/97 2.6. Client Port Option In order to facilitate implementations outside the operating system kernel, and to allow two separate client implementations: one for DHCP and one for MDHCP, if this option is specified, the MDHCP server MUST use the source port number used in the DHCPDISCOVER, DHCPREQUEST, DHCPINFORM, and DHCPRESEASE as the destination port number in the response messages. Code Len Port +-----+-----+-----+ | 105 | 1 | n | +-----+-----+-----+ The 'code' for this option is 105 and the length is 1. 2.7. List of Address Ranges Allocated This option is used by the server to provide the list of all the address ranges allocated to the client when client requests more than one addresses. When a client requests only one address, the server uses the ‘yiaddr’ field specify the allocated address. When a client requests more than one address, additional address ranges are listed via this option. Code Len Address Range List +-----+-----+-----+-----+-...-+-----+ | 105 | n | L1 | L2 | | Ln | +-----+-----+-----+-----+-----+-----+ Where the Address Range List is of the follwing format. StartAddress1 BlockSize1 StartAddress2 BlockSize2 ... +---+---+---+---+---+---+---+---+---+---+---+---+--...--+ |S11|S12|S13|S14|B11|B12|S21|S22|S23|S24|B21|B22| | +---+---+---+---+---+---+---+---+---+---+---+---+-------+ The 'code' for this option is TBD and the minimum length is 6. 3. MDHP protocol The client first needs to know the group multicast address of the MDHCP servers, and the multicast scope list. This address and the scope list may be obtained by requesting the options specified in [3] from DHCP servers via DHCPINFORM or from other repository of network configurations. At this point, client has two ways of obtaining the multicast address(es) from the server. Patel and Shah 7 MDHCP 11/20/97 In the first method [see Figure 1], the client is requesting address(es) from any of the MDHCP servers configured to hand out addresses from a given scope. The client selects a scope and sends out a DHCPDISCOVER message to the multicast address of the MDHCP servers. The server responds by sending DHCPOFFER message directly to the client. The client then sends DHCPREQUEST messages to the multicast address. of the MDHCP servers. The selected server responds by sending DHCPACK message directly to the client as in normal DHCP protocol. The multicast DHCPDICOVER and DHCPREQUEST messages are received by only the multicast capable DHCP servers, and therefore, there is no conflict between the MDHCP and DHCP messages. Further, the messages for renewing and releasing lease are sent directly to the MDHCP servers only, and therefore, there is conflict between DHCP and MDHCP message interpretation by a non-MDHCP capable server. The fields and options that are different from the normal DHCP message exchange are summarized in Table 1 to 3. For details on rest of the fields, please refer DHCP RFC [1]. The client can later renew or release the multicast address by using DHCPREQUEST and DHCPRELEASE message exchanges as defined in the DHCP RFC [1]. At any time, if the MDHCP server is unable to satisfy the DHCPREQUEST message (e.g., the requested address has been allocated), the server MUST respond with a DHCPNAK message. Note that all the messages in this exchange have their M flag set and B flag not set. In the second method [see Figure 2] the client is requesting address(es) directly from a specific MDHCP server. When a client knows the IP address of the MDHCP server from which it can obtain a multicast address(es) from a give scope, it MAY skip the discover phase ( i.e. DHCPDISCOVER and DHCPOFFER message exchange ) and directly start with unicasting DHCPREQUEST message to the server. If this fails, the client SHOULD revert back to the first method. The MDHCP client may need to be deployed on the client machines where DHCP client implementation is not capable of filtering out the MDHCP messages. In that case, the MDHCP client MUST use a port number different from ‘DHCP client port(68)’. The MDHCP client MUST specify this port in the DHCPDISCOVER and DHCPREQUEST messages via ‘Client Port Option’. The MDHCP Client MUST provide client identifier option when sending messages for multicast address assignment. The client generates a unique key and uses that as a client identifier in the DHCPDISCOVER message. The client identifier is the key to distinguish the client request and to avoid duplicate address allocation. Each client may be running several different multicast enabled applications, and each application may require separate multicast address(es). Client MUST use separate unique client identifier when Patel and Shah 8 MDHCP 11/20/97 requesting separate multicast address(es) for each request. A client implementation may choose to use hardware address, application instance and time of request to generate unique client identifiers. The following tables [Table 1, Table 2] describes the fields and options that are relavent to MDHCP protocol but are different from the normal DHCP protocol [1] Field DHCPOFFER DHCPACK DHCPNAK ----- --------- ------- ------- 'flags' Set 'M' Bit. set 'M' Bit set 'M' bit BROADCAST bit 0 BROADCAST bit 0 BROADCAST bit 0 'ciaddr' 'ciaddr' from 'ciaddr' from 0 DHCPDISCOVER or 0 DHCPREQUEST or 0 'yiaddr' Multicast address Multicast address assigned to client assigned to client 'siaddr' Server's IP address Server's IP address 0 reachable from the reachable from the client. client. 'chaddr' 'chaddr' from 'chaddr' from 'chaddr' from client DHCPDISCOVER client DHCPREQUEST client DHCPREQUEST message message message 'file' may contain options may contain options (unused) 'options' options options Option DHCPOFFER DHCPACK DHCPNAK ------ --------- ------- ------- IP address lease time MUST MUST MUST NOT Lease Start Time MUST MUST MUST NOT Server identifier MUST MUST MUST Multicast Scope MUST MUST MUST NOT Table 1: Fields and options that are different in multicast DHCP server messages. Patel and Shah 9 MDHCP 11/20/97 Field DHCPDISCOVER DHCPREQUEST DHCPRELEASE ----- ------------ ----------- ----------- 'flags' Set 'M' Bit. set 'M' Bit set 'M' bit BROADCAST bit 0 BROADCAST bit 0 BROADCAST bit 0 'ciaddr' 0 or client's 0 or client's 0 network addr network addr 'chaddr' ignored ignored ignored 'options' options options (unused) Option DHCPDISCOVER DHCPREQUEST DHCPRELEASE ------ ------------ ----------- ----------- Start time MAY MAY MUST NOT Client identifier MUST MUST MAY Multicast Scope SHOULD SHOULD MAY Client Port MUST(if using MUST (if using MUST NOT a different a different port) port) Table 2: Fields and options that are different in multicast DHCP client messages Patel and Shah 10 MDHCP 11/20/97 Server Client Server (not selected) (selected) v v v | | | | Obtain IP address | | | | |Begin multicast address request| | | | | _____________/|\_____________ | |/ DHCPDISCOVER | DHCPDISCOVER \| | | | Determines | Determines address(es) | address(es) | | | |\ | ____________/| | \_________ | /DHCPOFFER | | DHCPOFFER\ |/ | | \ | | | Collects replies | | \| | | Selects Address(es) | | | | | _____________/|\_____________ | |/ DHCPREQUEST | DHCPREQUEST \| | | | | | Commits address(es) | | | | | _____________/| | |/ DHCPACK | | | | | assignment complete | | | | . . . | | | | Graceful release | | | | | |\_____________ | | | DHCPRELEASE \| | | | | | Discards lease | | | v v v Figure 1: Timeline diagram of messages exchanged between MDHCP client and servers using group multicast address for MDHCP capable servers. Patel and Shah 11 MDHCP 11/20/97 Client Server v v | | |\_____________ | | DHCPREQUEST \| | | | Commits address(es) | | | _____________/| |/ DHCPACK | | | | assignment complete | | . . | | Graceful release | | | |\_____________ | | DHCPRELEASE \| | | | Discards lease | | v v v Figure 2: Timeline diagram of messages exchanged between MDHCP client and servers when MDHCP server has already been selected 4. MDHCP Protocol properties Conflict free address allocation: In the intranet case, each MDHCP server MAY be allocated part of the administratively scoped address space. As long as the address space managed by MDHCP servers is non-overlapping for a given administrative scope, the protocol will allocate conflict free addresses. MDHCP protocol does not directly address the mechanisms for determining address allocation outside Intranet. However, we propose to use MDHCP as a front end to any future address allocation protocol for the Internet. The DHCP protocol will preserve conflict free address allocation property of the internet multicast address allocation protocol. Small response time: The response time for MDHCP protocol is strictly based on the network propagation delay and the load on the MDHCP server. The MDHCP protocol does not require a client system to be on all the time. Thus, it poses no additional requirements on power managed systems. Patel and Shah 12 MDHCP 11/20/97 Multicast address scopes: The administratively scoped multicast address may be directly allocated by MDHCP server. However, it is envisioned that the MDHCP protocol will be indirectly used for Internet wide Multicast addresses allocation. In such deployment, the MDHCP server will act as a front-end to future Internet multicast address allocation protocols. Efficient use of address space: The multicast address space may be statically partitioned between MDHCP servers to provide sufficient reliability and load management on servers. However, the multicast based address request will be able to obtain addresses from any of the available servers. 5. Security Considerations This document does not explicitly address security considerations to avoid redundant effort with the work in progress in DHC working group of IETF on securing DHCP. 6. Acknowledgements The authors would like to thank Rajeev Byrisetty, Steve Deering, Peter Ford, Mark Handley, Van Jacobson, David Oran, Thomas Pfenning, Dave Thayler, Ramesh Vyaghrapuri and the participants of IETF for their assistance with this protocol. 7. References [1] Droms, R., "Dynamic Host Configuration Protocol", RFC1541, October 1993. [2] Alexander, S., and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 1533, Lachman Technology, Inc., Bucknell University, October 1993. [3] Patel, B., and Shah, M., ``Multicast address allocation extensions options'' [4] Meyer, D., ``Administratively scoped IP Multicast'', Internet draft, [5] D. Mills, ``Network Time Protocol version 2 specification and implementation'', 8. Author's Address Baiju V. Patel Intel Corp. 2111 NE 25th Ave. Hillsboro, OR 97124 Patel and Shah 13 MDHCP 11/20/97 Phone: 503 264 2422 EMail: baiju@mailbox.jf.intel.com Munil Shah Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone:425 703 3924 Email:munils@microsoft.com This document will expire on April, 1998 Patel and Shah 14