Network Working Group Y. Cui Internet-Draft P. Wu Intended status: Informational J. Wu Expires: April 24, 2014 Tsinghua University T. Lemon Nominum, Inc. Q. Sun Tsinghua University October 21, 2013 DHCPv4 over IPv6 Transport draft-ietf-dhc-dhcpv4-over-ipv6-08 Abstract In IPv6 networks, there remains a need to provide IPv4 service for some residual devices. This document describes a mechanism for allocating IPv4 addresses to such devices, using DHCPv4 with an IPv6 transport. It is done by putting a special relay agent function (Client Relay Agent) on the client side, as well as extending the behavior of the server; in the case where DHCP server only supports IPv4 transport, a relay agent is extended to support IPv6 transport (IPv6-Transport Relay Agent) and relay DHCP traffic for the server, with a new Relay Agent Information sub-option added to carry the IPv6 address of the Client Relay Agent. DHCPv4 over IPv6 has been developed in the IETF, and some implementations and deployments have been carried out. But this mechanism is not recommended for future implementation or deployment. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on April 24, 2014. Copyright Notice Cui, et al. Expires April 24, 2014 [Page 1] Internet-Draft DHCPv4 over IPv6 October 2013 Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Summary . . . . . . . . . . . . . . . . . . . . . . . 4 4. Client Relay Agent IPv6 Address Sub-option . . . . . . . . . . 6 5. Client Relay Agent Behavior . . . . . . . . . . . . . . . . . 6 6. IPv6-Transport Server Behavior . . . . . . . . . . . . . . . . 7 7. IPv6-Transport Relay Agent Behavior . . . . . . . . . . . . . 8 8. Security Consideration . . . . . . . . . . . . . . . . . . . . 8 9. IANA Consideration . . . . . . . . . . . . . . . . . . . . . . 9 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 9 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11.1. Normative References . . . . . . . . . . . . . . . . . . 10 11.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Motivation for selecting this particular solution . . 11 A.1. Configuring IPv4 with DHCPv6 . . . . . . . . . . . . . . 11 A.2. Tunnel DHCPv4 over IPv6 . . . . . . . . . . . . . . . . . 11 A.3. DHCPv4 relayed over IPv6 . . . . . . . . . . . . . . . . 12 Appendix B. Discussion on One Host Retrieving Multiple Addresses through One CRA . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Cui, et al. Expires April 24, 2014 [Page 2] Internet-Draft DHCPv4 over IPv6 October 2013 1. Introduction DHCPv4 over IPv6 mechanism has been developed in the IETF. There have been implementations from ISC, Juniper, Huawei, Tsinghua University, etc. It is in active deployments in some networks, including in the China Next Generation Internet (CNGI) and China Education and Research Network 2 (CERNET2), Deutsche Telekom, and so on. Documenting this mechanism is for the benefit of vendors and operators of the existing implementations and deployments. According to [I-D.ietf-dhc-v4configuration], future usage should reference [I-D.ietf-dhc-dhcpv4-over-dhcpv6]. DHCPv4 [RFC2131] was not designed with IPv6 in mind: DHCPv4 cannot operate on an IPv6 network. However, as dual-stack networks become a reality, the need arises to allocate IPv4 addresses in an IPv6 environment. To meet this demand, this document extends the DHCPv4 protocol to allow the use of an IPv6 network for transport. A typical scenario that probably requires this feature is IPv4-over- IPv6 hub and spoke tunnel [RFC4925]. In this scenario, IPv4-over- IPv6 tunnel is used to provide IPv4 connectivity to end users (hosts or end networks) across an IPv6 network. If the IPv4 addresses of the end users are provisioned by the concentrator side, then the provisioning process should be able to cross the IPv6 network. One such tunnel mechanism is demonstrated in [I-D.ietf-softwire-public-4over6]. 2. Terminology This document makes use of the following terms: o DHCPv4: IPv4 Dynamic Host Configuration Protocol [RFC2131]. o Client Relay Agent (CRA): a special DHCPv4 Relay Agent which relays between DHCPv4 client and DHCPv4 server using an IPv6 network. A CRA either sits on the same, IPv6-accessible host with the DHCPv4 client, or sits on the same link with the host running DHCPv4 client. o Host Client Relay Agent (HCRA): a CRA which sits on the same, IPv6-accessible host with the DHCPv4 client. o On-Link Client Relay Agent (LCRA): a CRA which sits on the same link with the host that runs DHCPv4 client. o IPv6-Transport Server (TSV): a DHCPv4 Server that supports IPv6 transport. The TSV listens on IPv6 for incoming DHCPv4 messages, Cui, et al. Expires April 24, 2014 [Page 3] Internet-Draft DHCPv4 over IPv6 October 2013 and sends DHCPv4 messages in IPv6 packets. o IPv6-Transport Relay Agent (TRA): a DHCPv4 Relay Agent that supports IPv6 transport. The TRA sits on a machine which has both IPv6 and IPv4 connectivity, and relays DHCP messages between a CRA and a regular DHCPv4 server. Unlike the CRA, the TRA sits on the remote end of IPv6 network, and communicates with DHCPv4 server through IPv4. o Client Relay Agent IPv6 Address Sub-option (CRA6ADDR sub-option): a new sub-option of the DHCP Relay Agent Information Option [RFC3046], defined in this document, which is used to carry the IPv6 address of the CRA. 3. Protocol Summary The scenario for DHCPv4 over IPv6 transport is shown in Figure 1. DHCPv4 clients and DHCPv4 server/relay are separated by an IPv6 network in the middle. DHCP messages between a client and the server/relay cannot naturally be forwarded to each other because they are IPv4 UDP packets, either unicast or broadcast. To bridge this gap, both the client side and the server/relay side can enable DHCPv4 over IPv6 transport. More precisely, it is necessary for them to support delivering and receiving DHCP messages in IPv6 UDP packets and thereby traverse the IPv6 network. On the client side, a special relay agent called Client Relay Agent is placed on the same host with the client, or on the link of the host. CRA is used to relay DHCP messages from the client to the server, and from the server to the client. CRA sends DHCPv4 messages to the server through unicast IPv6 UDP, and receives unicast IPv6 UDP packets with the DHCPv4 messages from the server. By using CRA, no extension is required on the DHCP client. +-------------------------+ +------+ | |DHCPv4| | |Client| +-------+ +------+ |DHCPv4 | | IPv6 Network |Server/| +------+ |Relay | |DHCPv4| +-------+ |Client| | +------+ | +-------------------------+ Cui, et al. Expires April 24, 2014 [Page 4] Internet-Draft DHCPv4 over IPv6 October 2013 Figure 1 Scenario of DHCPv4 over IPv6 Transport The IPv6-Transport DHCPv4 server is able to receive DHCP messages delivered in IPv6 UDP from the CRA, and send out DHCP messages to the CRA using IPv6 UDP (figure 2(a)). The TSV sends DHCP messages to the IPv6 address from which it receives relevant DHCP messages earlier. When CRAs communicate with an IPv6-Transport Relay Agent rather than with a server directly, the situation becomes a little more complicated. Besides the IPv6 communication with a CRA, a TRA also communicates with a regular DHCPv4 server through IPv4. Therefore, when the TRA relays DHCP messages between a CRA and the DHCPv4 server, it receives DHCP message from the CRA in IPv6 and sends it to the server in IPv4, as well as receives DHCP message from the server in IPv4 and sends it to the CRA in IPv6. The TRA sends the IPv6 address of the CRA to the DHCP server using the Client Relay Agent IPv6 Address (CRA6ADDR) suboption, defined in this document. The DHCP server returns this suboption to the TRA as required in [RFC3046]. The TRA then uses the returned CRA6ADDR suboption to determine the destination address to which to relay the response. +------+ +------+ |client| IPv6 network |TSV | |+HCRA |----------------| | +------+ +------+ +------+ +------+ +------+ |client| |LCRA | IPv6 network |TSV | | |--| |----------------| | +------+ +------+ +------+ (a)client--server case +------+ +------+ +------+ |client| IPv6 network |TRA | IPv4 network |server| |+HCRA |----------------| |--------------| | +------+ +------+ +------+ +------+ +------+ +------+ +------+ |client| |LCRA | IPv6 network |TRA | IPv4 network |server| | |--| |----------------| |--------------| | +------+ +------+ +------+ +------+ (b)client--relay--server case Cui, et al. Expires April 24, 2014 [Page 5] Internet-Draft DHCPv4 over IPv6 October 2013 Figure 2 Protocol Summary 4. Client Relay Agent IPv6 Address Sub-option The CRA6ADDR suboption is a suboption of the Relay Agent Information Option [RFC3046]. It encodes the IPv6 address of the machine from which a DHCPv4-in-IPv6 CRA-to-TRA message was received. It is used by the TRA to relay DHCPv4 replies back to the proper CRA. The TRA uses the IPv6 address encoded in this suboption as the destination IPv6 address when relaying a DHCPv4 message from the DHCP server to the CRA. The CRA6ADDR sub-option has a fixed length of 18 octets. The SubOpt code is tbd by IANA, the length field is 16, and the following 16 octets contain the CRA IPv6 address. SubOpt Len Agent Remote ID +------+------+------+------+------+- -+------+ | tbd | 16 | a1 | a2 | a3 | ... | a16 | +------+------+------+------+------+- -+------+ Figure 3 Client Relay Agent IPv6 Address Sub-option format 5. Client Relay Agent Behavior A Client Relay Agent sits on the same host with the DHCPv4 client (HCRA), or on the same link as the host (LCRA). A CRA listens for DHCP packets on IPv4 on port 67, and also listens for DHCP packets on IPv6 on port 67. A CRA is configured with one or more IPv6 addresses of TSV/TRA as the destination(s). The CRA is also configured with a global IPv6 address before the DHCPv4 client starts, so that it can forward DHCPv4 messages over IPv6. When the CRA receives any DHCP message on IPv4 with BOOTP op field set to 1, it forwards the message over UDP on IPv6 using a standard DHCP message format, with source port 67 and destination port 67. The CRA forwards the message to each TSV or TRA address with which it is configured. When the CRA receives any message on IPv6 with BOOTP op field set to 2, the CRA checks to see if the message contains option 82. If it does, the CRA silently discards the message. Otherwise, it relays Cui, et al. Expires April 24, 2014 [Page 6] Internet-Draft DHCPv4 over IPv6 October 2013 the message to the DHCP client using IPv4. When the CRA receives any message on IPv6 with BOOTP op field set to 4, it decapsulates the message as specified in DHCPv4 Relay Agent Encapsulation [I-D.ietf-dhc-dhcpv4-relay-encapsulation]. If the CRA does not support encapsulation, it silently discards the message. The LCRA or HCRA does not use the Relay Agent Information Option [RFC3046]. If either type of CRA needs to send relay agent options, it uses relay agent encapsulation as defined in [I-D.ietf-dhc-dhcpv4-relay-encapsulation]. An HCRA only serves the client inside the same host, while the LCRA serves any client on the link. When the IPv6 address of TSV/TRA is provisioned to the host running the DHCP client, it uses HCRA; else the client depends on LCRA. A HCRA serves only one link; the multiple-link case is handled by multiple HCRA instances. A LCRA does not necessarily need an IPv4 address, though it may be configured with one. In the HCRA case, the DHCPv6 client (or other IPv6 configuration processes), DHCPv4 client and CRA run on the same physical interface. In some cases, the host running the DHCPv4 client and CRA defers the operation of the DHCPv4 client until an IPv6 address of the interface has been acquired, as well as the TSV/TRA address information. If this is not done, the DHCPv4 client may send several messages that the CRA cannot relay, and this could result in long delays before the DHCPv4 client actually gets an IPv4 address. 6. IPv6-Transport Server Behavior To support IPv6 transport, the behavior of DHCPv4 server is extended. The IPv6-Transport Server can listen on IPv6 port 67 for DHCPv4 messages, and send DHCPv4 messages through IPv6. A TSV listens for DHCP messages on IPv6 UDP port 67 and IPv4 UDP port 67. When it receives a DHCP message on IPv6, it retains the IPv6 source address of that message until it has sent a response. When it sends a response, it sends the response to this IPv6 address, with destination port 67. The TSV is bound to send a server identifier option [RFC2132] containing an IPv4 address which will be reachable from the client once the residual IPv4 service is set up. This follows the server id option requirement in [RFC2131]. The rest of TSV DHCP process is the same with a normal DHCPv4 server. Cui, et al. Expires April 24, 2014 [Page 7] Internet-Draft DHCPv4 over IPv6 October 2013 A TSV also listens on IPv4 UDP port 67 like a normal DHCPv4 server, and process IPv4 DHCPv4 messages normally. This requirement exists because when a DHCPv4 client renews, it sends its renewal messages directly to the server, rather than broadcasting them. Because a CRA may use relay agent encapsulation [I-D.ietf-dhc-dhcpv4-relay-encapsulation], the TSV ought to support it. A TSV that does not support it will not interoperate with a CRA that sends relay agent options. 7. IPv6-Transport Relay Agent Behavior An IPv6-Transport Relay Agent sits between an IPv6 network and an IPv4 network, and relays DHCPv4 messages between CRAs and an IPv4- only DHCPv4 server. The communication between CRAs and the TRA uses IPv6, while the communication between the TRA and the server uses IPv4. A TRA listens on IPv6 UDP port 67 for DHCP messages with BOOTP op field set to 1 or 3, as well as IPv4 UDP port 67 for DHCP messages with BOOTP op field set to 2 or 4. When relaying a DHCP message from CRA to server, the TRA adds a CRA6ADDR suboption. The TRA sets the contents of this suboption to the IPv6 source address of the message. The TRA also stores one its own IPv4 addresses in the giaddr field of the DHCP message. The TRA may include a Link Selection sub-option [RFC3527] to indicate to the DHCP server which link to use when choosing an IP address. If the received message is a RELAYFORWARD message, the TRA encapsulates the message in a new RELAYFORWARD message and stores the CRA6ADDR in the new relay segment. If it is some other message, the TRA appends a Relay Agent Information Option as described in [RFC3046], but may encapsulate it in the same way as RELAYFORWARD message instead, which depends on the implementation. When receiving a DHCP message from the DHCP server, if the message contains no CRA6ADDR suboption, the TRA discards the message. Otherwise, it processes it as required by [RFC3046] and [I-D.ietf-dhc-dhcpv4-relay-encapsulation], and forwards it to the IPv6 address recorded in the CRA6ADDR sub-option, with source port 67 and destination port 67. 8. Security Consideration This mechanism may rise a new form of DHCP protocol attack. A malicious attacker in IPv6 can interference with the DHCPv4 process by injecting fake DHCPv4-in-IPv6 messages which will be handled by TSV or TRA. However, the damage is the same with the known DHCPv4 Cui, et al. Expires April 24, 2014 [Page 8] Internet-Draft DHCPv4 over IPv6 October 2013 attack happened in IPv4. The only difference is the attacker and the victim could locate in different address families. Another impact is DHCP filtering. There are firewalls today capable of filtering DHCP traffic (DHCPv4 over IPv4 and DHCPv6 over IPv6 packets). The DHCP messages with the new, DHCPv4-in-IPv6 style may bypass these firewalls. Nevertheless it is not difficult for them to make some slight modification and adapt to the new DHCPv4 message pattern. 9. IANA Consideration IANA is requested to assign one new sub-option code from the registry of DHCP Agent Sub-Option Codes maintained in http://www.iana.org/assignments/bootp-dhcp-parameters. This sub- option code will be assigned to the Client Relay Agent IPv6 Address Sub-option. 10. Contributors The following gentlemen also contributed to the effort: Francis Dupont Internet Systems Consortium, Inc. Email: fdupont@isc.org Tomasz Mrugalski Internet Systems Consortium, Inc. Email: tomasz.mrugalski@gmail.com Dmitry Anipko Microsoft Corporation Email: danipko@microsoft.com 11. References Cui, et al. Expires April 24, 2014 [Page 9] Internet-Draft DHCPv4 over IPv6 October 2013 11.1. Normative References [I-D.ietf-dhc-dhcpv4-relay-encapsulation] Lemon, T., Deng, H., and L. Huang, "Relay Agent Encapsulation for DHCPv4", draft-ietf-dhc-dhcpv4-relay-encapsulation-01 (work in progress), July 2011. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001. [RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy, "Link Selection sub-option for the Relay Agent Information Option for DHCPv4", RFC 3527, April 2003. [RFC4361] Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4)", RFC 4361, February 2006. [RFC4925] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire Problem Statement", RFC 4925, July 2007. [RFC6842] Swamy, N., Halwasia, G., and P. Jhingran, "Client Identifier Option in DHCP Server Replies", RFC 6842, January 2013. 11.2. Informative References [I-D.ietf-dhc-dhcpv4-over-dhcpv6] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I. Farrer, "DHCPv4 over DHCPv6 Transport", draft-ietf-dhc-dhcpv4-over-dhcpv6-02 (work in progress), October 2013. [I-D.ietf-dhc-v4configuration] Rajtar, B. and I. Farrer, "Provisioning IPv4 Configuration Over IPv6 Only Networks", draft-ietf-dhc-v4configuration-02 (work in progress), September 2013. [I-D.ietf-softwire-public-4over6] Cui, Y., Wu, J., Wu, P., Vautrin, O., and Y. Lee, "Public Cui, et al. Expires April 24, 2014 [Page 10] Internet-Draft DHCPv4 over IPv6 October 2013 IPv4 over IPv6 Access Network", draft-ietf-softwire-public-4over6-10 (work in progress), July 2013. Appendix A. Motivation for selecting this particular solution We considered three possible solutions to the problem of configuring IPv4 addresses on an IPv6 network. A.1. Configuring IPv4 with DHCPv6 Use DHCPv6 instead of DHCPv4, to provision IPv4-related connectivity. In DHCPv6, the provisioned IPv4 address can be embedded into IPv6 address, or carried within a new option. Along with that, dedicated options are needed to convey IPv4-related information, such as the IPv4 address of DNS server, NTP server, etc. Therefore it will put a certain amount of IPv6-unrelated information into DHCPv6 protocol. This solution was rejected for two reasons. First, the DHCPv6 protocol does not currently provide a mechanism for recording bindings between IPv4 addresses and DHCPv6 clients. Extending DHCPv6 to provide this functionality would be a substantial change to the existing protocol. Second, a deliberate choice was made when the DHCPv6 protocol was defined to avoid simply copying existing functionality from DHCPv4. While it is possible, using DHCPv6, to deliver IPv4 addresses as IPv6-encoded IPv4 addresses, it might be necessary to add additional DHCPv6 options simply to support IPv4. These options would then remain in the protocol, long after the need for IPv4 has gone. By comparison, any extensions to DHCPv4 will naturally be forgotten when DHCPv4 is no longer needed. This means that whatever extensions we make to DHCPv4 to solve the problem, we can stop maintaining as soon as IPv4 is no longer needed. A.2. Tunnel DHCPv4 over IPv6 Use DHCPv4 for configuration, and tunnel DHCPv4-in-IPv4 messages over IPv6. Unlike the previous approach where DHCPv6 is used for both IPv4 and IPv6 connectivity, this approach preserves the separation between IPv4 and IPv6 connectivity information. It maintains the IPv4 service without major modifications to IPv6-related provisioning resources, and sustains DHCPv4 to be the IPv4-related information carrier. This approach was not chosen because it adds a requirement for DHCPv4 Cui, et al. Expires April 24, 2014 [Page 11] Internet-Draft DHCPv4 over IPv6 October 2013 to operate over an IPv4-in-IPv6 tunnel. DHCPv4 clients generally operate on broadcast networks, not on tunnels. To make DHCPv4 operate over a tunnel would require substantial changes to the DHCPv4 client, as well as maintaining a tunnel over which to deliver DHCPv4 traffic. This also creates a chicken-and-egg problem: how do we set up an IPv4 tunnel when we do not know our IPv4 address? Solutions to these problems were proposed, but they require significant changes to the DHCP client and significant additional work to make a tunnel that could carry the DHCP packets. A.3. DHCPv4 relayed over IPv6 Use DHCPv4 for configuration, and extend it to use an IPv6 transport for relayed messages. Essentially this involves a single change to the protocol, to allow DHCPv4 servers or relay agents to send and receive packets using an IPv6 transport. No changes are required on the client. The working group chose this third solution because, of the three, it required the fewest changes to the DHCP protocol, so that it was easiest to specify and easiest to implement. Appendix B. Discussion on One Host Retrieving Multiple Addresses through One CRA This document is written with the intention of supporting a use case where a single DHCP client is configuring a single tunnel endpoint per physical link. The technique described in this document could be used by a host needing to configure more than one tunnel endpoint on the same physical link, i.e., to retrieve multiple addresses through the same CRA. DHCP server implementing this specification implements Client Identifier Option in DHCP server replies [RFC6842]. In general this specification is intended not to require modification of DHCP clients. However, DHCP clients being used to configure multiple tunnel endpoints have to be modified; otherwise there is no way for such DHCP clients to differentiate between DHCP responses. Therefore, in such case, the DHCP client using this specification uses a different client identifier for each tunnel endpoint being configured. Such DHCP clients examine the response from the DHCP server and use the client identifier to differentiate between the DHCP client state machines for each tunnel endpoint. Cui, et al. Expires April 24, 2014 [Page 12] Internet-Draft DHCPv4 over IPv6 October 2013 In order to satisfy the requirement that client identifiers be unique, DHCP clients configuring multiple tunnel endpoints implement Node-specific Client Identifiers for DHCPv4 [RFC4361]. Such clients use a different IAID for each tunnel endpoint. It is assumed here that every client state machine on a multiple- tunnel-endpoint link can hear all the DHCP messages (and subsequently accept the messages intended for it). How this is accomplished is left to the implementor. However, implementations must follow this requirement; otherwise, it will be impossible for multiple tunnel endpoints to be successfully configured. The easiest way to accomplish this is to have a single DHCP client process with multiple DHCP state machines, and to dispatch each DHCP message to the correct DHCP client state machine using the client identifier. However, this is not required; any mechanism that results in client state machines receiving the messages that are intended for them will suffice. Authors' Addresses Yong Cui Tsinghua University Department of Computer Science, Tsinghua University Beijing 100084 P.R.China Phone: +86-10-6260-3059 Email: yong@csnet1.cs.tsinghua.edu.cn Peng Wu Tsinghua University Department of Computer Science, Tsinghua University Beijing 100084 P.R.China Phone: +86-10-6278-5822 Email: pengwu.thu@gmail.com Cui, et al. Expires April 24, 2014 [Page 13] Internet-Draft DHCPv4 over IPv6 October 2013 Jianping Wu Tsinghua University Department of Computer Science, Tsinghua University Beijing 100084 P.R.China Phone: +86-10-6278-5983 Email: jianping@cernet.edu.cn Ted Lemon Nominum, Inc. 2000 Seaport Blvd Redwood City, CA 94063 USA Phone: +1-650-381-6000 Email: mellon@nominum.com Qi Sun Tsinghua University Department of Computer Science, Tsinghua University Beijing 100084 P.R.China Phone: +86-10-6278-5822 Email: sunqi@csnet1.cs.tsinghua.edu.cn Cui, et al. Expires April 24, 2014 [Page 14]