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Independent Submission J. Wu
Request for Comments: 5747 Y. Cui
Category: Experimental X. Li
ISSN: 2070-1721 M. Xu
Tsinghua University
C. Metz
Cisco Systems, Inc.
March 2010
4over6 Transit Solution Using IP Encapsulation and MP-BGP Extensions
Abstract
The emerging and growing deployment of IPv6 networks will introduce
cases where connectivity with IPv4 networks crossing IPv6 transit
backbones is desired. This document describes a mechanism for
automatic discovery and creation of IPv4-over-IPv6 tunnels via
extensions to multiprotocol BGP. It is targeted at connecting
islands of IPv4 networks across an IPv6-only backbone without the
need for a manually configured overlay of tunnels. The mechanisms
described in this document have been implemented, tested, and
deployed on the large research IPv6 network in China.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5747.
IESG Note
The mechanisms and techniques described in this document are related
to specifications developed by the IETF softwire working group and
published as Standards Track documents by the IETF, but the
relationship does not prevent publication of this document.
Wu, et al. Experimental [Page 1]
RFC 5747 4over6 March 2010 Copyright Notice Copyright (c) 2010 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. Table of Contents 1. Introduction ....................................................3 2. 4over6 Framework Overview .......................................3 3. Prototype Implementation ........................................5 3.1. 4over6 Packet Forwarding ...................................5 3.2. Encapsulation Table ........................................6 3.3. MP-BGP 4over6 Protocol Extensions ..........................7 3.3.1. Receiving Routing Information from Local CE .........8 3.3.2. Receiving 4over6 Routing Information from a Remote 4over6 PE ....................................8 4. 4over6 Deployment Experience ....................................9 4.1. CNGI-CERNET2 ...............................................9 4.2. 4over6 Testbed on the CNGI-CERNET2 IPv6 Network ............9 4.3. Deployment Experiences ....................................10 5. Ongoing Experiment .............................................11 6. Relationship to Softwire Mesh Effort ...........................12 7. IANA Considerations ............................................12 8. Security Considerations ........................................13 9. Conclusion .....................................................13 10. Acknowledgements ..............................................13 11. Normative References ..........................................14 Wu, et al. Experimental [Page 2]
RFC 5747 4over6 March 2010 1. Introduction Due to the lack of IPv4 address space, more and more IPv6 networks have been deployed not only on edge networks but also on backbone networks. However, there are still a large number of legacy IPv4 hosts and applications. As a result, IPv6 networks and IPv4 applications/hosts will have to coexist for a long period of time. The emerging and growing deployment of IPv6 networks will introduce cases where connectivity with IPv4 networks is desired. Some IPv6 backbones will need to offer transit services to attached IPv4 access networks. The method to achieve this would be to encapsulate and then transport the IPv4 payloads inside IPv6 tunnels spanning the backbone. There are some IPv6/IPv4-related tunneling protocols and mechanisms defined in the literature. But at the time that the mechanism described in this document was introduced, most of these existing techniques focused on the problem of IPv6 over IPv4, rather than the case of IPv4 over IPv6. Encapsulation methods alone, such as those defined in [RFC2473], require manual configuration in order to operate. When a large number of tunnels are necessary, manual configuration can become burdensome. To the above problem, this document describes an approach, referred to as "4over6". The 4over6 mechanism concerns two aspects: the control plane and the data plane. The control plane needs to address the problem of how to set up an IPv4-over-IPv6 tunnel in an automatic and scalable fashion between a large number of edge routers. This document describes experimental extensions to Multiprotocol Extension for BGP (MP-BGP) [RFC4271] [RFC4760] employed to communicate tunnel endpoint information and establish 4over6 tunnels between dual-stack Provider Edge (PE) routers positioned at the edge of the IPv6 backbone network. Once the 4over6 tunnel is in place, the data plane focuses on the packet forwarding processes of encapsulation and decapsulation. 2. 4over6 Framework Overview In the topology shown in Figure 1, a number of IPv6-only P routers compose a native IPv6 backbone. The PE routers are dual stack and referred to as 4over6 PE routers. The IPv6 backbone acts as a transit core to transport IPv4 packets across the IPv6 backbone. This enables each of the IPv4 access islands to communicate with one another via 4over6 tunnels spanning the IPv6 transit core. Wu, et al. Experimental [Page 3]
RFC 5747 4over6 March 2010 _._._._._ _._._._._ | IPv4 | | IPv4 | | access | | access | | island | | island | _._._._._ _._._._._ | | Dual-Stack Dual-Stack "4over6 PE" "4over6 PE" | | | | __+____________________+__ 4over6 / : : : : \ IPv6 only Tunnels | : : : : | transit core between | : [P] : | with multiple PEs | : : : : | [P routers] | : : : : | \_._._._._._._._._._._._._./ | / \ | | | Dual-Stack Dual-Stack "4over6 PE" "4over6 PE" | | | _._._._._ _._._._._ | IPv4 | | IPv4 | | access | | access | | island | | island | _._._._._ _._._._._ Figure 1: IPv4 over IPv6 Network Topology As shown in Figure 1, there are multiple dual-stack PE routers connected to the IPv6 transit core. In order for the ingress 4over6 PE router to forward an IPv4 packet across the IPv6 backbone to the correct egress 4over6 PE router, the ingress 4over6 PE router must learn which IPv4 destination prefixes are reachable through each egress 4over6 PE router. MP-BGP will be extended to distribute the destination IPv4 prefix information between peering dual-stack PE routers. Section 4 of this document presents the definition of the 4over6 protocol field in MP-BGP, and Section 5 describes MP-BGP's extended behavior in support of this capability. After the ingress 4over6 PE router learns the correct egress 4over6 PE router via MP-BGP, it will forward the packet across the IPv6 backbone using IP encapsulation. The egress 4over6 PE router will receive the encapsulated packet, remove the IPv6 header, and then forward the original IPv4 packet to its final IPv4 destination. Section 6 describes the procedure of packet forwarding. Wu, et al. Experimental [Page 4]
RFC 5747 4over6 March 2010 3. Prototype Implementation An implementation of the 4over6 mechanisms described in this document was developed, tested, and deployed on Linux with kernel version 2.4. The prototype system is composed of three components: packet forwarding, the encapsulation table, and MP-BGP extensions. The packet forwarding and encapsulation table are Linux kernel modules, and the MP-BGP extension was developed by extending Zebra routing software. The following sections will discuss these parts in detail. 3.1. 4over6 Packet Forwarding Forwarding an IPv4 packet through the IPv6 transit core includes three parts: encapsulation of the incoming IPv4 packet with the IPv6 tunnel header, transmission of the encapsulated packet over the IPv6 transit backbone, and decapsulation of the IPv6 header and forwarding of the original IPv4 packet. Native IPv6 routing and forwarding are employed in the backbone network since the P routers take the 4over6 tunneled packets as just native IPv6 packets. Therefore, 4over6 packet forwarding involves only the encapsulation process and the decapsulation process, both of which are performed on the 4over6 PE routers. Tunnel from Ingress PE to Egress PE ----------------------------> Tunnel Tunnel Entry-Point Exit-Point Node Node +-+ IPv4 +--+ IPv6 Transit Core +--+ IPv4 +-+ |S|-->--//-->--|PE|=====>=====//=====>=====|PE|-->--//-->--|D| +-+ +--+ +--+ +-+ Original Ingress PE Egress PE Original Packet (Encapsulation) (Decapsulation) Packet Source Destination Node Node Figure 2: Packet Forwarding along 4over6 Tunnel As shown in Figure 2, packet encapsulation and decapsulation are both on the dual-stack 4over6 PE routers. Figure 3 shows the format of packet encapsulation and decapsulation. Wu, et al. Experimental [Page 5]
RFC 5747 4over6 March 2010 +----------------------------------//-----+ | IPv4 Header | Packet Payload | +----------------------------------//-----+ < Original IPv4 Packet > | |(Encapsulation on ingress PE) | v < Tunnel IPv6 Headers > < Original IPv4 Packet > +-----------+ - - - - - +-------------+-----------//--------------+ | IPv6 | IPv6 | IPv4 | | | | Extension | | Packet Payload | | Header | Headers | Header | | +-----------+ - - - - - +-------------+-----------//--------------+ < Tunnel IPv6 Packet > | |(Decapsulation on egress PE) | v +----------------------------------//-----+ | IPv4 Header | Packet Payload | +----------------------------------//-----+ < Original IPv4 Packet > Figure 3: Packet Encapsulation and Decapsulation on Dual-Stack 4over6 PE Router The encapsulation format to apply is IPv4 encapsulated in IPv6, as outlined in [RFC2473]. 3.2. Encapsulation Table Each 4over6 PE router maintains an encapsulation table as depicted in Figure 4. Each entry in the encapsulation table consists of an IPv4 prefix and its corresponding IPv6 address. The IPv4 prefix is a particular network located in an IPv4 access island network. The IPv6 address is the 4over6 virtual interface (VIF) address of the 4over6 PE router that the IPv4 prefix is reachable through. The encapsulation table is built and maintained using local configuration information and MP-BGP advertisements received from remote 4over6 PE routers. The 4over6 VIF is an IPv6 /128 address that is locally configured on each 4over6 router. This address, as an ordinary global IPv6 address, must also be injected into the IPv6 IGP so that it is reachable across the IPv6 backbone. Wu, et al. Experimental [Page 6]
RFC 5747 4over6 March 2010 +-------------+------------------------------------------------+ | IPv4 Prefix | IPv6 Advertising Address Family Border Router | +-------------+------------------------------------------------+ Figure 4: Encapsulation Table When an IPv4 packet arrives at the ingress 4over6 PE router, a lookup in the local IPv4 routing table will result in a pointer to the local encapsulation table entry with the matching destination IPv4 prefix. There is a corresponding IPv6 address in the encapsulation table. The IPv4 packet is encapsulated in an IPv6 header. The source address in the IPv6 header is the IPv6 VIF address of the local 4over6 PE router and the destination address is the IPv6 VIF address of the remote 4over6 PE router contained in the local encapsulation table. The packet is then subjected to normal IPv6 forwarding for transport across the IPv6 backbone. When the encapsulated packet arrives at the egress 4over6 PE router, the IPv6 header is removed and the original IPv4 packet is forwarded to the destination IPv4 network based on the outcome of the lookup in the IPv4 routing table contained in the egress 4over6 PE router. 3.3. MP-BGP 4over6 Protocol Extensions Each 4over6 PE router possesses an IPv4 interface connected to an IPv4 access network(s). It can peer with other IPv4 routers using IGP or BGP routing protocols to exchange local IPv4 routing information. Routing information can also be installed on the 4over6 PE router using static configuration methods. Each 4over6 PE also possesses at least one IPv6 interface to connect it into the IPv6 transit backbone. The 4over6 PE typically uses IGP routing protocols to exchange IPv6 backbone routing information with other IPv6 P routers. The 4over6 PE router will also form an MP-iBGP (Internal BGP) peering relationship with other 4over6 PE routers connected to the IPv6 backbone network. The use of MP-iBGP suggests that the participating 4over6 PE routers that share a route reflector or form a full mesh of TCP connections are contained in the same autonomous system (AS). This implementation is in fact only deployed over a single AS. This was not an intentional design constraint but rather reflected the single AS topology of the CNGI-CERNET2 (China Next Generation Internet - China Education and Research Network) national IPv6 backbone used in the testing and deployment of this solution. Wu, et al. Experimental [Page 7]
RFC 5747 4over6 March 2010 3.3.1. Receiving Routing Information from Local CE When a 4over6 PE router learns routing information from the locally attached IPv4 access networks, the 4over6 MP-iBGP entity should process the information as follows: 1. Install and maintain local IPv4 routing information in the IPv4 routing database. 2. Install and maintain new entries in the encapsulation table. Each entry should consist of the IPv4 prefix and the local IPv6 VIF address. 3. Advertise the new contents of the local encapsulation table in the form of MP_REACH_NLRI update information to remote 4over6 PE routers. The format of these updates is as follows: * AFI = 1 (IPv4) * SAFI = 67 (4over6) * NLRI = IPv4 network prefix * Network Address of Next Hop = IPv6 address of its 4over6 VIF 4. A new Subsequent Address Family Identifier (SAFI) BGP 4over6 (67) has been assigned by IANA. We call a BGP update with a SAFI of 67 as 4over6 routing information. 3.3.2. Receiving 4over6 Routing Information from a Remote 4over6 PE A local 4over6 PE router will receive MP_REACH_NLRI updates from remote 4over6 routers and use that information to populate the local encapsulation table and the BGP routing database. After validating the correctness of the received attribute, the following procedures are used to update the local encapsulation table and redistribute new information to the local IPv4 routing table: 1. Validate the received BGP update packet as 4over6 routing information by AFI = 1 (IPv4) and SAFI = 67 (4over6). 2. Extract the IPv4 network address from the NLRI field and install as the IPv4 network prefix. 3. Extract the IPv6 address from the Network Address of the Next Hop field and place that as an associated entry next to the IPv4 network index. (Note, this describes the update of the local encapsulation table.) Wu, et al. Experimental [Page 8]
RFC 5747 4over6 March 2010 4. Install and maintain a new entry in the encapsulation table with the extracted IPv4 prefix and its corresponding IPv6 address. 5. Redistribute the new 4over6 routing information to the local IPv4 routing table. Set the destination network prefix as the extracted IPv4 prefix, set the Next Hop as Null, and Set the OUTPUT Interface as the 4over6 VIF on the local 4over6 PE router. Therefore, when an ingress 4over6 PE router receives an IPv4 packet, the lookup in its IPv4 routing table will have a result of the output interface as the local 4over6 VIF, where the incoming IPv4 packet will be encapsulated with a new IPv6 header, as indicated in the encapsulation table. 4. 4over6 Deployment Experience 4.1. CNGI-CERNET2 A prototype of the 4over6 solution is implemented and deployed on CNGI-CERNET2. CNGI-CERNET2 is one of the China Next Generation Internet (CNGI) backbones, operated by the China Education and Research Network (CERNET). CNGI-CERNET2 connects approximately 25 core nodes distributed in 20 cities in China at speeds of 2.5-10 Gb/s. The CNGI-CERNET2 backbone is IPv6-only with some attached customer premise networks (CPNs) being dual stack. The CNGI-CERNET2 backbone, attached CNGI-CERNET2 CPNs, and CNGI-6IX Exchange all have globally unique AS numbers. This IPv6 backbone is used to provide transit IPv4 services for customer IPv4 networks connected via 4over6 PE routers to the backbone. 4.2. 4over6 Testbed on the CNGI-CERNET2 IPv6 Network Figure 5 shows 4over6 deployment network topology. Wu, et al. Experimental [Page 9]
RFC 5747 4over6 March 2010 +-----------------------------------------------------+ | IPv6 (CERNET2) | | | +-----------------------------------------------------+ | | | | Tsinghua|Univ. Peking|Univ. SJTU| Southeast|Univ. +------+ +------+ +------+ +------+ |4over6| ... |4over6| |4over6| ... |4over6| |router| |router| |router| |router| +------+ +------+ +------+ +------+ | | | | | | | | | | | | +-----------+ +-----------+ +-----------+ +-----------+ |IPv4 access| ... |IPv4 access| |IPv4 access| ... |IPv4 access| | network | | network | | network | | network | +-----------+ +-----------+ +-----------+ +-----------+ | +----------------------+ | IPv4 (Internet) | | | +----------------------+ Figure 5: 4over6 Deployment Network Topology The IPv4-only access networks are equipped with servers and clients running different applications. The 4over6 PE routers are deployed at 8 x IPv6 nodes of CNGI-CERNET2, located in seven universities and five cities across China. As suggested in Figure 5, some of the IPv4 access networks are connected to both IPv6 and IPv4 networks, and others are only connected to the IPv6 backbone. In the deployment, users in different IPv4 networks can communicate with each other through 4over6 tunnels. 4.3. Deployment Experiences A number of 4over6 PE routers were deployed and configured to support the 4over6 transit solution. MP-BGP peerings were established, and successful distribution of 4over6 SAFI information occurred. Inspection of the BGP routing and encapsulation tables confirmed that the correct entries were sent and received. ICMP ping traffic indicated that IPv4 packets were successfully transiting the IPv6 backbone. In addition, other application protocols were successfully tested per the following: Wu, et al. Experimental [Page 10]
RFC 5747 4over6 March 2010 o HTTP. A client running Internet Explorer in one IPv4 client network was able to access and download multiple objects from an HTTP server located in another IPv4 client network. o P2P. BitComet software running on several PCs placed in different IPv4 client networks were able to find each other and share files. Other protocols, including FTP, SSH, IM (e.g., MSN, Google Talk), and Multimedia Streaming, all functioned correctly. 5. Ongoing Experiment Based on the above successful experiment, we are going to have further experiments in the following two aspects. 1. Inter-AS 4over6 The above experiment is only deployed over a single AS. With the growth of the network, there could be multiple ASes between the edge networks. Specifically, the Next Hop field in MP-BGP indicates the tunnel endpoint in the current 4over6 technology. However, in the Inter-AS scenario, the tunnel endpoint needs to be separated from the field of Next Hop. Moreover, since the technology of 4over6 is deployed on the router running MP-BGP, the supportability of 4over6 on each Autonomous System Border Router (ASBR) will be a main concern in the Inter-AS experiment. We may consider different situations: (1) Some ASBRs do not support 4over6; (2) ASBRs only support the 4over6 control plane (i.e., MP- BGP extension of 4over6) rather than 4over6 data plane; (3) ASBRs support both the control plane and the data plane for 4over6. 2. Multicast 4over6 The current 4over6 technology only supports unicast routing and data forwarding. With the deployment of network-layer multicast in multiple IPv4 edge networks, we need to extend the 4over6 technology to support multicast including both multicast tree manipulation on the control plane and multicast traffic forwarding on the data plane. Based on the current unicast 4over6 technology providing the unicast connectivity of edge networks over the backbone in another address family, the multicast 4over6 will focus on the mapping technologies between the multicast groups in the different address families. Wu, et al. Experimental [Page 11]
RFC 5747 4over6 March 2010 6. Relationship to Softwire Mesh Effort The 4over6 solution was presented at the IETF Softwires Working Group Interim meeting in Hong Kong in January 2006. The existence of this large-scale implementation and deployment clearly showed that MP-BGP could be employed to support tunnel setup in a scalable fashion across an IPv6 backbone. Perhaps most important was the use-case presented -- an IPv6 backbone that offers transit to attached client IPv4 networks. The 4over6 solution can be viewed as a precursor to the Softwire Mesh Framework proposed in the softwire problem statement [RFC4925]. However, there are several differences with this solution and the effort that emerged from the Softwires Working Group called "softwire Mesh Framework" [RFC5565] and the related solutions [RFC5512] [RFC5549]. o MP-BGP Extensions. 4over6 employs a new SAFI (BGP 4over6) to convey client IPv4 prefixes between 4over6 PE routers. Softwire Mesh retains the original AFI-SAFI designations, but it uses a modified MP_REACH_NLRI format to convey IPv4 Network Layer Reachability Information (NLRI) prefix information with an IPv6 next_hop address [RFC5549]. o Encapsulation. 4over6 assumes IP-in-IP or it is possible to configure Generic Routing Encapsulation (GRE). Softwires uses those two scenarios configured locally or for IP headers that require dynamic updating. As a result, the BGP encapsulation SAFI is introduced in [RFC5512]. o Multicast. The basic 4over6 solution only implemented unicast communications. The multicast communications are specified in the Softwire Mesh Framework and are also supported by the multicast extension of 4over6. o Use-Cases. The 4over6 solution in this document specifies the 4over6 use-case, which is also pretty easy to extend for the use- case of 6over4. The Softwire Mesh Framework supports both 4over6 and 6over4. 7. IANA Considerations A new SAFI value (67) has been assigned by IANA for the BGP 4over6 SAFI. Wu, et al. Experimental [Page 12]
RFC 5747 4over6 March 2010 8. Security Considerations Tunneling mechanisms, especially automatic ones, often have potential problems of Distributed Denial of Service (DDoS) attacks on the tunnel entry-point or tunnel exit-point. As the advantage, the BGP 4over6 extension doesn't allocate resources to a single flow or maintain the state for a flow. However, since the IPv6 tunnel endpoints are globally reachable IPv6 addresses, it would be trivial to spoof IPv4 packets by encapsulating and sending them over IPv6 to the tunnel interface. This could bypass IPv4 Reverse Path Forwarding (RPF) or other antispoofing techniques. Also, any IPv4 filters may be bypassed. An iBGP peering relationship may be maintained over IPsec or other secure communications. 9. Conclusion The emerging and growing deployment of IPv6 networks, in particular, IPv6 backbone networks, will introduce cases where connectivity with IPv4 networks is desired. Some IPv6 backbones will need to offer transit services to attached IPv4 access networks. The 4over6 solution outlined in this document supports such a capability through an extension to MP-BGP to convey IPv4 routing information along with an associated IPv6 address. Basic IP encapsulation is used in the data plane as IPv4 packets are tunneled through the IPv6 backbone. An actual implementation has been developed and deployed on the CNGI- CERNET2 IPv6 backbone. 10. Acknowledgements During the design procedure of the 4over6 framework and definition of BGP-MP 4over6 extension, Professor Ke Xu gave the authors many valuable comments. The support of the IETF Softwires WG is also gratefully acknowledged with special thanks to David Ward, Alain Durand, and Mark Townsley for their rich experience and knowledge in this field. Yakov Rekhter provided helpful comments and advice. Mark Townsley reviewed this document carefully and gave the authors a lot of valuable comments, which were very important for improving this document. The deployment and test for the prototype system was conducted among seven universities -- namely, Tsinghua University, Peking University, Beijing University of Post and Telecommunications, Shanghai Jiaotong University, Huazhong University of Science and Technology, Southeast Wu, et al. Experimental [Page 13]
RFC 5747 4over6 March 2010 University, and South China University of Technology. The authors would like to thank everyone involved in this effort at these universities. 11. Normative References [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, December 1998. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007. [RFC4925] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire Problem Statement", RFC 4925, July 2007. [RFC5512] Mohapatra, P. and E. Rosen, "The BGP Encapsulation Subsequent Address Family Identifier (SAFI) and the BGP Tunnel Encapsulation Attribute", RFC 5512, April 2009. [RFC5549] Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network Layer Reachability Information with an IPv6 Next Hop", RFC 5549, May 2009. [RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh Framework", RFC 5565, June 2009. Wu, et al. Experimental [Page 14]
RFC 5747 4over6 March 2010 Authors' Addresses 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 Yong Cui Tsinghua University Department of Computer Science, Tsinghua University Beijing 100084 P.R. China Phone: +86-10-6278-5822 EMail: cy@csnet1.cs.tsinghua.edu.cn Xing Li Tsinghua University Department of Electronic Engineering, Tsinghua University Beijing 100084 P.R. China Phone: +86-10-6278-5983 EMail: xing@cernet.edu.cn Mingwei Xu Tsinghua University Department of Computer Science, Tsinghua University Beijing 100084 P.R. China Phone: +86-10-6278-5822 EMail: xmw@csnet1.cs.tsinghua.edu.cn Chris Metz Cisco Systems, Inc. 3700 Cisco Way San Jose, CA 95134 USA EMail: chmetz@cisco.com Wu, et al. Experimental [Page 15]