DHC Working Group B. Joshi Internet-Draft P. Kurapati Expires: December 16, 2007 Infosys Technologies Ltd. June 14, 2007 Relay Chaining in DHCPv4 draft-kurapati-dhc-relay-chaining-dhcpv4-01.txt Status of this Memo 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. 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. This Internet-Draft will expire on December 16, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract DHCP Relay Agents eliminate the necessity of having a DHCP server on each physical network. In certain network configurations, a DHCP server may be multiple subnets away from the DHCP client and multiple Relay Agents may be configured to relay DHCP messages to and from DHCP client. Such configuration can be supported only when each Relay Agent adds certain Information to DHCP messages before relaying them. This additional information helps in relaying the DHCP reply back to the DHCP client through the same path. This mechanism is Joshi & Kurapati Expires December 16, 2007 [Page 1] Internet-Draft Relay Chaining in DHCPv4 June 2007 referred as Relay Chaining. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. New sub-options . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. peer-relay-agent-information Sub Option . . . . . . . . . 6 3.2. peer-address Sub Option . . . . . . . . . . . . . . . . . 6 4. Relay Chaining . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Handling DHCP messages in Relay Agent . . . . . . . . . . 7 4.1.1. Handling Broadcast DHCP messages . . . . . . . . . . . 7 4.1.2. Handling Unicast DHCP messages . . . . . . . . . . . . 8 5. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 10 6. Special Scenarios . . . . . . . . . . . . . . . . . . . . . . 11 6.1. Multiple Layer 2 Relay Agents between DHCP client and Layer 3 Relay Agent . . . . . . . . . . . . . . . . . . . 11 6.2. One or multiple Layer 2 Relay Agents between two Layer 3 Relay Agents . . . . . . . . . . . . . . . . . . . . . . 11 7. Security Consideration . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Normative Reference . . . . . . . . . . . . . . . . . . . 14 9.2. Informative Reference . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 Intellectual Property and Copyright Statements . . . . . . . . . . 16 Joshi & Kurapati Expires December 16, 2007 [Page 2] Internet-Draft Relay Chaining in DHCPv4 June 2007 1. Introduction In some network configurations, DHCP server and clients are separated by multiple Relay Agents. One such case is the network configuration where Access Concentrators operate in Transparent Bridging mode as described in document [5]. In such configurations, there are situations where each of those relay agents need to add relay-agent- information to the DHCP messages received on its downstream interface. Relay chaining support in DHCPv4 will help in solving following problems: o In some deployments, Layer 3 Relay Agent uses unnumbered interfaces. When these Layer 3 Relay Agents are used along with Layer 2 Relay agents as described in [5] , they need to maintain internal states to identify the outgoing interface. Maintaining state information for each packet will not scale as number of DHCP clients increases. With Relay Chaining, Layer 3 Relay Agent can add its own Relay Agent Information option that can be used to identify the outgoing interface for DHCP reply messages. o When a network configuration supports multiple Layer 3 Relay Agents between DHCP Clients and DHCP server, a DHCP message need to be relayed through these Relay Agents to reach DHCP server or DHCP Clients. This can not be supported with present mechanism. This document describes the enhancements to Relay Agent functionality when multiple Relay Agents are present between DHCP clients and servers. Joshi & Kurapati Expires December 16, 2007 [Page 3] Internet-Draft Relay Chaining in DHCPv4 June 2007 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 [1]. This document uses the following terms: o "Access Concentrator" An Access Concentrator is a router or switch at the broadband access provider's edge of a public broadband access network. This document assumes that the Access Concentrator acts as a Transparent Bridge and includes the DHCP relay agent functionality. For example: In DSL environment, this is typically known as DSLAM.(Digital Subscriber Line Access Multiplexer) o "DHCP client" A DHCP client is an Internet host using DHCP to obtain configuration parameters such as a network address. o "Layer 3 Relay Agent" A Layer 3 Relay Agent is a third-party agent that transfers Bootstrap Protocol (BOOTP) and DHCP messages between clients and servers residing on different subnets, per RFC951 [6] and RFC1542 [7]. o "DHCP server" A DHCP server is an Internet host that returns configuration parameters to DHCP clients. o "downstream" Downstream is the direction from the edge network towards the DHCP Clients. o "Transparent Bridge" A device which does bridging based on MAC learning principles. Bridge learns the Source MAC of the incoming frames and updates a table with MAC/Interface information. While forwarding data packets, bridge looks at this table to find the outgoing interface. o "upstream" Upstream is the direction from the DHCP Clients towards the edge Joshi & Kurapati Expires December 16, 2007 [Page 4] Internet-Draft Relay Chaining in DHCPv4 June 2007 network. o "Unnumbered Interfaces" An interface with no IP address associated with it. IP packets received on this interface will be processed like any other numbered IP interface. It may use a local IP address of another interface while forwarding packets. Joshi & Kurapati Expires December 16, 2007 [Page 5] Internet-Draft Relay Chaining in DHCPv4 June 2007 3. New sub-options The 'Relay Chaining in DHCPv4' requires the definition of following new sub-options in Relay Agent Information [Option 82] option for DHCP packet beyond those defined by RFC2131 [2] and RFC2132 [8]. See also Section 7, IANA Considerations. 3.1. peer-relay-agent-information Sub Option This sub-option is defined under Relay Agent Information option [Option 82]. A Relay Agent can use 'peer-relay-agent-information' sub-option to store the Relay Agent Information option added by the previous Relay Agent. The value field of this sub-option contains the Relay Agent Information option present in the recevied DHCP message. The new sub-option is as shown in the figure below: SubOpt Len Value +------+------+--------------------------+ | X | Len | Option 82 of Previous RA | +------+------+--------------------------+ / \ / \ +------+-----+--------------------+ | 82 | N | i1 | i2 | ... | iN | +------+-----+--------------------+ Code Len Relay Agent Information Figure 1 3.2. peer-address Sub Option 'peer-address' sub-option is used to store the previous Relay Agent's reachable IP address. It SHOULD be typically populated with the same address as mentioned in the 'giaddr' field of the received message. If there are multiple Relay Agents involved, this sub-option helps in forwarding the reply back to the DHCP client through the same path. The 'peer-address' sub-option is as follows: SubOpt Len Sub-option Value +--------+--------+--------------------------------+ | X | 4 | Previous RA's Address (giaddr) | +--------+--------+--------------------------------+ Figure 2 Joshi & Kurapati Expires December 16, 2007 [Page 6] Internet-Draft Relay Chaining in DHCPv4 June 2007 4. Relay Chaining Relay Chaining is defined as the mechanism where multiple Relay Agents are involved in relaying a DHCP message between DHCP clients and server. Each Relay Agent adds Relay Agent Information option to the DHCP message as described below. The option of using relay chaining mechanism MUST be configurable on Relay Agent in order to provide compatibility to the previous solutions. | +-----+ | |Host1|------+ +-----+ | | +-----+ | |Host2|------+ +--------+ +-----+ | | | +--------+ +--------+ +------| | | | | | | | Relay | | Relay | | DHCP | | | Agent |---..---| Agent |--..--| Server | | | #1 | | #2 | | | +-----+ +------| | | | +--------+ |Host3|------+ | | +--------+ +-----+ | +--------+ | +-----+ | |Host4|------+ +-----+ | | | Figure 3 A typical network configuration where Relay Chaining is required is depicted in Figure 3. In the above case, two Relay Agents are involved. Relay Agent #1 does not know the DHCP server and so is configured to reach Relay Agent #2 for all DHCP messages. Relay Agent #2 knows how to reach DHCP server and so relays a DHCP message directly to DHCP server. DHCP server generates the reply messages and sends it to Relay Agent #2 which relays the same to Relay Agent #1. 4.1. Handling DHCP messages in Relay Agent 4.1.1. Handling Broadcast DHCP messages Joshi & Kurapati Expires December 16, 2007 [Page 7] Internet-Draft Relay Chaining in DHCPv4 June 2007 o When a Relay Agent receives a DHCP request message which does not contain the Relay Agent Information option, it SHOULD add the Relay Agent Information option (Option 82 as described in RFC 3046 [3]) and 'giaddr' field as it deems appropriate. It should relay the DHCP message to the DHCP server or next Relay Agent. If a Relay Agent is a Layer 2 relay agent, it MUST NOT populate the 'giaddr' field in the DHCP message and should process it as per [5] o When a Relay Agent receives a DHCP request message which contains a Relay Agent Information option, it SHOULD add the Relay Agent Information Option as it deems appropriate and also populate the 'peer-relay-agent-information' sub-option as described in section 3.1. o If a DHCP message received by a Relay Agent has a non-zero 'giaddr' field, it MUST add 'peer-address' sub-option in the Relay Agent Information option with the same IP address as described in section 3.2. o When a Relay Agent receives the reply message from the server, it MAY verify the Relay Agent Information option and SHALL silently discard the packet if it had not added the Relay Agent Information option. A Relay Agent MUST remove the Relay Agent Information option it had added and process the reply message as follows: * A Relay Agent MUST look for 'peer-relay-agent-information' sub- option in the Relay Agent Information option it had added and if it finds this sub-option, it MUST extract the contents which is the Option 82 of the previous Relay Agent. Before relaying this message, it MUST append the Relay Agent Information option of the previous Relay Agent as it is to the DHCP message. Relay Agent MAY use the Relay Agent Information it had added, to identify the outgoing interface. * A Relay Agent MUST also look for 'peer-address' sub-option and if it finds this sub-option, it must extract the IP address from the sub-option. It MUST set this IP address as 'giaddr' and relay the DHCP reply to this IP address. If this sub- option is not available, it SHOULD broadcast the reply to the outgoing interface. 4.1.2. Handling Unicast DHCP messages As DHCP Clients unicast RENEW, RELEASE and INFORM messages directly to the DHCP server, these messages are not intercepted by Relay Agents and so these messages does not have any Relay Agent Information options added to them. Joshi & Kurapati Expires December 16, 2007 [Page 8] Internet-Draft Relay Chaining in DHCPv4 June 2007 Some existing Relay Agent implementations maintain lease/location informations for each DHCP client. These implementations intercepts unicast DHCP messages to keep the lease/location information updated. So these Relay Agent adds Relay Agent Information option to unicast DHCP messages as well. Relay Agents and DHCP server process them similar to broadcast messages as described above in section 5.1.1. Joshi & Kurapati Expires December 16, 2007 [Page 9] Internet-Draft Relay Chaining in DHCPv4 June 2007 5. DHCP Server Behavior DHCP server would still find only one single Relay Agent Information option [ Option 82 ] in the DHCP message which has been relayed by multiple relay agents. Some existing DHCP servers may use the Relay Agent Information option to apply the IP Address and other parameter assignment policies. These DHCP servers will have to employ recursive lookup algorithm to find the relevant Option 82 [the Option 82 which was added by the first Relay Agent]. Server MUST echo back the entire Option 82 as it is. Joshi & Kurapati Expires December 16, 2007 [Page 10] Internet-Draft Relay Chaining in DHCPv4 June 2007 6. Special Scenarios 6.1. Multiple Layer 2 Relay Agents between DHCP client and Layer 3 Relay Agent There may be a network scenario when there are multiple Layer 2 Relay Agents configured between DHCP clients and Layer 3 Relay Agent. In this case, as described above, each Layer 2 Relay Agent MUST add 'Relay Agent Information' option and MUST not add 'peer-address' sub- option in it. A Layer 2 Relay Agent can use this Relay Agent Information option while relaying the DHCP Reply message back to the DHCP client. 6.2. One or multiple Layer 2 Relay Agents between two Layer 3 Relay Agents There is a very rare possibility of one or multiple Layer 2 Relay Agents being present between two Layer 3 Relay Agents. In this case, all the DHCP messages exchanged between these two Layer 3 Relay Agents would be unicast messages. Typically a Layer 2 Relay Agent should snoop these DHCP messages only if it maintains Lease/Location information as described in [5]. In such cases, Layer 2 Relay Agents SHOULD add Relay Agent Information option as described in section 4.1.1. Joshi & Kurapati Expires December 16, 2007 [Page 11] Internet-Draft Relay Chaining in DHCPv4 June 2007 7. Security Consideration o A Relay Agents relaying DHCP messages to another Relay Agent are essentially DHCP clients for the DHCP messages. Thus, RFC3118 [4] is an appropriate mechanism for these DHCP messages. o To restrict the number of Relay Agents in Relay Chaining, as defined in RFC 1542 [7], a Relay Agent must silently discard the DHCP message whose 'hops' field exceeds the value 16. A network manager can use configuration option to set this threshold to a smaller value. Joshi & Kurapati Expires December 16, 2007 [Page 12] Internet-Draft Relay Chaining in DHCPv4 June 2007 8. IANA Considerations This document needs IANA to provide a unique number for the following new suboptions in Relay Agent Information option [Option 82]: o To carry the peer relay agent information option. Please refer to section 3.1 for more details. o To carry the peer address. Please refer to section 3.2 for more details. Joshi & Kurapati Expires December 16, 2007 [Page 13] Internet-Draft Relay Chaining in DHCPv4 June 2007 9. References 9.1. Normative Reference [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001. [4] Droms, R. and B. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [5] Joshi, B., Kurapati, P., Kamath, M., and S. De Cnodder, "Layer 2 Relay Agent", draft draft-joshi-dhc-layer2-relay-agent-01.txt, February 2007. 9.2. Informative Reference [6] Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951, September 1985. [7] Wimer, W., "Clarifications and Extensions for the Bootstrap Protocol", RFC 1542, October 1993. [8] Droms, R. and S. Alexander, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. Joshi & Kurapati Expires December 16, 2007 [Page 14] Internet-Draft Relay Chaining in DHCPv4 June 2007 Authors' Addresses Bharat Joshi Infosys Technologies Ltd. 44 Electronics City, Hosur Road Bangalore 560 100 India Email: bharat_joshi@infosys.com URI: http://www.infosys.com/ Pavan Kurapati Infosys Technologies Ltd. 44 Electronics City, Hosur Road Bangalore 560 100 India Email: pavan_kurapati@infosys.com URI: http://www.infosys.com/ Joshi & Kurapati Expires December 16, 2007 [Page 15] Internet-Draft Relay Chaining in DHCPv4 June 2007 Intellectual Property Statement 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. 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