Patent Publication Number: US-9838947-B2

Title: Mobile node host route installation and withdrawal

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. patent application Ser. No. 14/543,754, filed Nov. 17, 2014, which is a continuation of U.S. patent application Ser. No. 13/536,740, filed Jun. 28, 2012, now U.S. Pat. No. 8,891,450, which claims the benefit of U.S. Provisional Application No. 61/595,587, filed Feb. 6, 2012, and of U.S. Provisional Application No. 61/598,585, filed Feb. 14, 2012, the entire content of each of which being incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to mobile networks and, more specifically, to roaming among access networks of service provider networks. 
     BACKGROUND 
     A cellular radio access network is a collection of cells that each includes at least one base station capable of transmitting and relaying signals to subscribers&#39; mobile nodes. A “cell” generally denotes a distinct area of a mobile network that utilizes a particular frequency or range of frequencies for transmission of data. A typical base station is a tower to which are affixed a number of antennas that transmit and receive the data over the particular frequency. Mobile nodes, such as cellular or mobile phones, smart phones, camera phones, personal digital assistants (PDAs) and laptop computers, may initiate or otherwise transmit a signal at the designated frequency to the base station to initiate a call or data session and begin transmitting data. 
     Mobile service provider networks convert cellular signals, e.g., Time Division Multiple Access (TDMA) signals, Orthogonal Frequency-Division Multiplexing (OFDM) signals or Code Division Multiple Access (CDMA) signals, received at a base station from mobile nodes into Internet protocol (IP) packets for transmission within packet-based networks. A number of standards have been proposed to facilitate this conversion and transmission of cellular signals to IP packets, such as a general packet radio service (GPRS) standardized by the Global System for Mobile Communications (GSM) Association, a Universal Mobile Telecommunications System (UMTS), an evolution of UMTS referred to as Long Term Evolution (LTE), as well as other standards proposed by the 3 rd  Generation Partnership Project (3GPP), 3 rd  Generation Partnership Project 2 (3GGP/2) and the Worldwide Interoperability for Microwave Access (WiMAX) forum. 
     A typical mobile service provider network, or mobile network, includes a core packet-switched network, a transport network, and one or more radio access networks (RANs). The core packet-switched network for the mobile network establishes logical connections, known as bearers, among the many service nodes on a path between a mobile node, attached to one of the radio access networks, and a packet data network (PDN). The service nodes then utilize the bearers to transport subscriber traffic exchanged between the mobile node and the PDN, which may include, for example, the Internet, an enterprise intranet, a layer 3 VPN, and a service provider&#39;s private network. Various PDNs provide a variety of packet-based data services to mobile nodes to enable the mobile nodes to exchange service data with application or other servers of the PDNs. 
     Some mobile nodes, in addition to supporting connections to a mobile network via a RAN, also support wireless local area network (WLAN) access network connections that provide connectivity to a broadband network via a WiFi “hotspot” or other wireless access point (WAP). These mobile nodes may support WLAN offload while dual-connected to a RAN of a mobile service provider network as well as to a WLAN access network that is not connected to the mobile service provider core packet-switched network. That is, such mobile nodes may route specific IP flows via the WLAN access network rather than via the RAN in order to avoid traversing the mobile service provider network. However, in such cases, a mobile node uses the local IP address allocated by the WLAN for such IP flows, and neither the WLAN nor mobile service provider network provides IP address preservation to the mobile node to allow “seamless” WLAN offload for IP flows to permit uninterrupted operation of higher-level protocols. While overlay networks that tunnel IP flows between a WLAN and a mobile service provider network or between multiple mobile service provider networks may provide inter-network connectivity, such overlay networks prevent the service providers from enforcing traffic policies. In addition, overlay networks require that the roaming mobile node participate in establishing a tunnel to connect itself to an anchor point of the mobile service provider network for the mobile node. 
     SUMMARY 
     In general, techniques are described for atomically installing and withdrawing host routes along paths connecting network routers to attenuate packet loss for mobile nodes migrating among WLAN access networks and a mobile network. The techniques may be applicable in the context of a Mobile virtual provider network (VPN) that extends a virtual routing area of a service provider network that includes a cellular access network to one or more WLAN access networks to provide network data service continuity for roaming mobile nodes. In this context, a Mobile VPN maintains an Internet Protocol (IP) VPN to whichever access network a mobile node uses to connect to the Internet and to distribute host routes in the Mobile VPN to enable the mobile node to keep its IP address while transitioning between networks, where a host route refers to a network route that is specific to a particular mobile node and uniquely identifies the network path to the mobile node within the Mobile VPN. 
     In some examples, whenever the mobile node moves from one attachment point to the next, it triggers the distribution of its host route from the new attachment point toward the service provider network hub provider edge router (“hub PE router”) that anchors the mobile node on a service provider network that provides access to packet-based services via a regional cellular access network. Routers participating in the Mobile VPN install the host route “atomically” from the attachment point to the mobile gateway so as to ensure convergence of the network forwarding plane with the host route toward the new attachment point prior to transitioning mobile node connectivity from a previous attachment point, which may include another WLAN access network, a visited service provider network, or the service provider network. Routers along the upstream propagation path cooperate to delay, if necessary, programming the forwarding plane components of the hub PE router until the remaining routers on the path have programmed their respective forwarding plane components to converge the forwarding plane along the downstream network path from the hub PE router toward the new attachment point of the mobile node. As convergence toward the mobile node is assured, the hub PE router transitions connectivity for the mobile node to the new attachment point by programming its forwarding plane components with the host route. Thereafter, the hub PE router forwards downstream network traffic destined for the mobile node along the downstream network path toward the new attachment point. 
     In some examples, whenever a mobile node detaches from a roaming attachment point (e.g., a WLAN access network or a visited service provider network) and reattaches by the cellular access network, it triggers withdrawal of a host route for the mobile node by network routers along the network path toward the roaming attachment point. Routers participating in the Mobile VPN withdraw the host route “atomically,” with the hub PE router withdrawing the host route prior to any of the downstream network routers along the network path toward the roaming attachment point withdrawing the host route. As a result, the hub PE router atomically switches over the forwarding path to begin forwarding downstream traffic to the mobile node by the service provider network, and downstream traffic may therefore be unaffected by transitional forwarding plane configurations within forwarding plane components of the downstream network routers. 
     The techniques of this disclosure may provide one or more advantages. For example, a Mobile VPN may cause significant churning in the forwarding plane configurations of participating routers due to mobile node transitions and concomitant host route programming and withdrawal in both the hub PE router and downstream routers. By providing for atomic host route installation and withdrawal, application of the described techniques by participating routers may in many instances avoid routing loops within the network and attendant packet loss that might otherwise occur in corresponding non-atomic host route-related operations. The techniques may consequently contribute to seamless extension of a Mobile VPN for implementing flexible roaming support among heterogeneous networks, including both cellular and WLAN access networks. 
     In one aspect, a method includes allocating an Internet Protocol (IP) address to a wireless device with a service provider network for accessing services by a cellular access network of the service provider network, wherein the service provider network includes a mobile virtual private network (VPN) that defines routes that are distinct and separate from a default routing space of the service provider network. The method also includes, when the wireless device attaches to a wireless alternate access network other than the cellular access network, dynamically extending the mobile VPN from the service provider network toward the access network by atomically installing a host route for the IP address into a VPN routing and forwarding (VRF) instance associated with the mobile VPN within a downstream router and within an upstream router to enable the wireless device to exchange network traffic via the alternate access network with the service provider network using the IP address allocated by the service provider network, wherein atomically installing the route comprises delaying the upstream router from installing the host route until the downstream router has installed the host route, and wherein, subsequent to atomically installing the route, network traffic destined for the wireless device proceeds from the upstream router toward the downstream router. 
     In another aspect, a method includes receiving notification, with a router coupled to an access network, that a wireless device is no longer attached to the access network. The method also includes setting a timer with a withdraw wait interval. The method further includes, when the timer expires without the router having received a reverse order route withdraw message for a host route to the wireless device, removing the host route from the router and sending a host route withdraw message to an upstream router that is on a path to the access network from a mobile virtual private network (VPN) to withdraw the host route from the upstream router. The method also includes, upon the router receiving the reverse order route withdrawn message for the host route to the wireless device prior to the timer expiring, removing the host route from the router and not sending the host route withdraw message to the upstream router. 
     In another aspect, a system includes a service provider network that includes a mobile virtual private network (VPN) that defines routes that are distinct and separate from a default routing space of the service provider network, wherein the service provider network allocates an Internet Protocol (IP) address to a wireless device with a service provider network for accessing services by a cellular access network of the service provider network. The system also includes a wireless alternate access network other than the cellular access network, an upstream router of the mobile VPN, a downstream router of the mobile VPN, and a wireless device that attaches to a wireless alternate access network. The upstream router and the downstream router dynamically extend the mobile VPN from the service provider network toward the access network by atomically installing a host route for the IP address into a VPN routing and forwarding (VRF) instance associated with the mobile VPN within the downstream router and within the upstream router to enable the wireless device to exchange network traffic via the alternate access network with the service provider network using the IP address allocated by the service provider network. The upstream router and the downstream router atomically install the route by delaying the upstream router from installing the host route until the downstream router has installed the host route. Subsequent to atomically installing the route, network traffic destined for the wireless device proceeds from the upstream router toward the downstream router. 
     In another aspect, a router coupled to an access network includes a routing protocol module, executing on control unit, that receives notification that a wireless device is no longer attached to the access network, wherein the routing protocol module sets a timer with a withdraw wait interval. When the timer expires without the router having received a reverse order route withdrawn message for a host route to the wireless device, the routing protocol module removes the host route from the router and sends a host route withdraw message to an upstream router that is on a path to the access network from a mobile virtual private network (VPN) to withdraw the host route from the upstream router. The routing protocol module, upon the router receiving the reverse order route withdraw message for the host route to the wireless device prior to the timer expiring, removes the host route from the router and does not send the host route withdraw message to the upstream router. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an example network system that includes network components that atomically install and withdraw host routes according to the described techniques. 
         FIG. 2  is a block diagram illustrating an example router that applies techniques described in this disclosure. 
         FIGS. 3A-3B  depict a flow chart illustrating example operations of routers to atomically install host routes according to techniques described in this disclosure. 
         FIG. 4  is a flowchart illustrating an example mode of operation to delay forwarding host route advertisements to upstream routers according to a configured delay time in accordance with techniques described herein. 
         FIG. 5  is a flowchart illustrating an example mode of operation of a router to apply reverse path host route withdrawal in accordance with techniques of this disclosure. 
         FIG. 6  is a flowchart illustrating an example operation of a router to delay conventional route withdrawal according to techniques of this disclosure. 
         FIG. 7  is a block diagram illustrating an example host route advertisement that includes, in accordance with techniques described herein, timing information to delay route installation by a hub provider edge router. 
         FIG. 8  is a block diagram illustrating an example reverse path host route withdraw message that includes, in accordance with techniques described herein, a flag directing receiving routers to perform reverse path host route withdrawal. 
     
    
    
     Like reference characters denote like elements throughout the figures and text. 
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example network system  2  that includes network components that atomically install and withdraw host routes according to the described techniques. In this example, network system  2  comprises packet data network (PDN)  12  coupled to provider network  20  to provide services to customers of the provider network  20  provider. In this way, PDN  12  represents network devices and infrastructure that provide a “service plane” of provider network  20 . Packet data network  12  supports one or more packet-based services that are available for request and use by wireless device  6 . As examples, PDN  12  may provide, for example, Internet access, bulk data delivery, voice over Internet protocol (VoIP), Internet Protocol television (IPTV), Short Messaging Service (SMS), Wireless Application Protocol (WAP) service, or customer-specific application services. Packet data network  12  may comprise, for instance, a local area network (LAN), a wide area network (WAN), the Internet, a virtual LAN (VLAN), an enterprise LAN, a layer 3 virtual private network (VPN), an Internet Protocol (IP) intranet operated by the service provider that operates provider network  20 , an enterprise IP network, or some combination thereof. Packet data network  12  executes one or more packet data protocols (PDPs), such as IP (IPv4 and/or IPv6), X.25 or Point-to-Point Protocol (PPP), to enable packet-based transport of PDN  12  services. Provider network  20  may represent a service provider (SP) network. 
     Wireless device  6  is a mobile node and represents any mobile communication device that supports both cellular radio access and wireless local area network (WLAN) access, e.g., by way of any of the IEEE 802.11 communication protocols (including so called “WiFi” communication protocols). Wireless device  6  may comprise, for example, a mobile telephone, a laptop or other mobile computer having, e.g., a 3G/4G wireless card, a smart phone, or a personal data assistant (PDA). Wireless device  6  may run one or more applications, such as VoIP clients, video games, videoconferencing, E-mail, and Internet browsers, among others. Certain applications running on wireless device  6  may require access to services offered by PDN  12 . Wireless device  6  may also be referred to, in various architectural embodiments, as User Equipment (UE) or Mobile Stations (MS). One example of a wireless device utilizing services is described in U.S. patent application Ser. No. 12/967,977 filed Dec. 14, 2010, entitled “MULTI-SERVICE VPN NETWORK CLIENT FOR MOBILE DEVICE,” incorporated herein by reference. 
     A service provider provisions and operates cellular access network  16  including, in some instances, mobile gateway  13  to provide network access, data transport and other services to wireless device  6 . In general, cellular access network  16  and mobile gateway  13  may implement any commonly defined cellular network architecture including those defined by standards bodies, such as the Global System for Mobile communication (GSM) Association, the 3 rd  Generation Partnership Project (3GPP), the 3 rd  Generation Partnership Project 2 (3GGP/2), the Internet Engineering Task Force (IETF), and the Worldwide Interoperability for Microwave Access (WiMAX) forum. For example, cellular access network  16  and mobile gateway  13  may together represent one or more of a GSM architecture, a General Packet Radio Service (GPRS) architecture, a Universal Mobile Telecommunications System (UMTS) architecture, and an evolution of UMTS referred to as Long Term Evolution (LTE), each of which are standardized by 3GGP. Cellular access network  16  and mobile gateway  13  may, alternatively or in conjunction with one of the above, implement a code division multiple access-2000 (“CDMA2000”) architecture. Cellular access network  16  and mobile gateway  13  may, again as an alternative or in conjunction with one or more of the above, implement a WiMAX architecture defined by the WiMAX forum. The service provider that operates provider network  20  may be different than a provider/operator of cellular access network  16  and mobile gateway  13 . That is, the relationship between the provider network  20  service provider and the cellular access network  16  service provider may be that of a provider and customer, respectively. The customer may be an Internet Service Provider (ISP), an application service provider, or another virtual private network (VPN) service provider. Alternatively, the same service provider may operate both provider network  20  and cellular access network  16 . 
     Cellular access network  16  may include a core packet-switched network and one or more radio access networks (also not shown in  FIG. 1 ) that includes one or more base stations  10  to provide regional cellular access. A core packet-switched network of cellular access network  16  may comprise, for example, a General Packet Radio Service (GPRS) core packed-switched network, or an Evolved Packet Core (EPC) or Evolved Packet System (EPS). The core packet-switched network of cellular access network  16  comprises intermediate devices required to implement the protocols of the particular architectural embodiment of cellular access network  16 , such as Serving GPRS Serving Nodes (SGSNs) and Mobility Management Entities (MMEs). Wireless device  6  communicates with cellular access network  16  using a wireless communication link to base station  10  of cellular access network  16 . Radio access networks of cellular access network  16  may include, for example, for example, a GSM Radio Access Network (GRAN), a WiMAX radio access network, a UMTS Radio Access Network (UTRAN), and/or an evolution of a UTRAN known as an E-UTRAN. Accordingly, base station  10  may represent a Node B or ENode B, for instance. Cellular access network  16  may further make use of a backhaul or transport network (not shown) that includes land-based transmission lines, frequently leased by the service provider, to transport user and control traffic between wireless device  6  and mobile gateway  13 . The backhaul network also includes network devices such as aggregation devices and routers. 
     Mobile gateway  13  is a customer edge (CE) network device that operates as a gateway and provides an interface to PDN  12  via provider network  20 . Mobile gateway  13  may represent an endpoint for reference point SGi (LTE), Gi (3GPP), and Wi(non-3GPP) as described in 3GPP TS 23.402, “Architecture Enhancements for non-3GPP Accesses,” Release 10, 3rd Generation Partnership Project, Technical Specification Group Services and System Aspects, June 2011, the entire content thereof being incorporated by reference herein. 
     Mobile gateway  13  may represent, for example, a Gateway GPRS Serving Node (GGSN), an Access Gateway (aGW), or a Packet Gateway (P-GW). Alternatively, mobile gateway  13  may represent a network element that exchanges traffic between a wireless network deployment represented by cellular access network  16  and provider network  20 . For example, mobile gateway  13  may represent may represent an interworking WLAN (I-WLAN) network element, a P-GW, a Packet Data Gateway (PDG) or enhanced PDG (ePDG), or a WLAN Access Gateway (WAG). In some embodiments, mobile gateway  13  may include functionality for a provider edge (PE) router of provider network  20 , thus incorporating aspect of hub PE router  8  functionality. Additional information regarding wireless access networks may be found in 3GPP TS 23.234, “3GPP System to Wireless Local Area Network (WLAN) Internetworking,” Release 10, 3rd Generation Partnership Project, Technical Specification Group Services and System Aspects, March 2011, the entire content thereof being incorporated by reference herein. While described herein with respect to one or more particular architectures for ease of illustration purposes, cellular access network  16  and mobile gateway  13  may implement any architecture including those set forth by any standards body and those proprietarily owned. Moreover, the techniques may apply to any mobile data protocol supported by these architectures. The techniques therefore should not be limited to cellular architectures referenced to herein and the mobile data protocols supported by these architectures. Mobile gateway  13  and elements of cellular access network  16  may, therefore, each represent an abstraction of devices found within any type of mobile network architectures. 
     Cellular access network  16  and mobile gateway  13  establish and operate an access channel for wireless device  6  to access the service-plane of provider network  20  (represented in  FIG. 1  by PDN  12 ). The access channel may represent, in accordance with different mobile access technologies, a packet data protocol (PDP) bearer or other bearer corresponding to a 3GPP or LTE architecture, a WiMAX channel, a CDMA2000 channel, or another non-3GPP access channel such as a WLAN link. To attach to cellular access network  16 , wireless device  6  initiates an attach request toward base station  10 . The attach request may include a wireless identifier in the form of a device or subscriber identifier. The wireless identifier may include, for example, an International Mobile Subscriber Identity (IMSI), a Temporary Mobile Subscriber Identity (TMSI), a P-TMSI, a System Architecture Evolution (SAE) TMSI or S-TMSI, an International Mobile Equipment Identity (IMEI), a Globally Unique Temporary Identity (GUTI), a subscriber username, an MSISDN number (e.g., a “Mobile Subscriber Integrated Services Digital Network Number”), an access point name (APN), a service provider identifier, Network Access Identifier, or other data or combination of the above-specified identifiers identifying a subscriber using wireless device  6 . In one example, the wireless identifier is of the form username@mobilevpn.apn.homeprovider, where username identifies a particular subscriber seeking access to the service identified by apn and provided by the service provider for provider network  20 , which is identified by homeprovider. 
     Cellular access network  16  passes the wireless identifier or a subset of the data included therein to mobile gateway  13 , which associates the access channel provisioned for wireless device  6  with an interface to PDN  12  via provider network  20 . Mobile gateway  13  may authenticate wireless device  6  to provider network  20  using the wireless identifier received from wireless device  6 . In addition, mobile gateway  13  allocates, or relays from provider network  20 , an IP address for wireless device  6  to use in exchanging IP packet belonging to IP traffic flows with PDN  12 . Wireless device  6  uses the access channel and its allocated IP address to engage PDN  12  services. 
     Provider network  20  supports mobile virtual private network (VPN)  18  with which customer edge (CE) devices, in this case represented by mobile gateway  13  and CE routers  11 A- 11 B (collectively, “CE routers  11 ”), served by provider network  20  may exchange routes with other CE devices also served by provider network  20 . In one example, mobile VPN  18  is an IP-based VPN established and served by provider network  20  such that mobile VPN  18  routes are distinct and separate from the default routing space of provider network  20  as well as any other VPNs supported by the provider network. One example of an IP-based VPN is described more fully in Rosen and Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs)”, Request for Comments  4364 , Network Working Group, February 2006, the entire contents of which being incorporated by reference herein. Additional details regarding Mobile VPNs are found in U.S. patent application Ser. No. 13/332,163, “VIRTUAL PRIVATE NETWORKING WITH MOBILE COMMUNICATION CONTINUITY,” filed Dec. 20, 2011, the entire content of which is incorporated as if fully set forth herein. 
     Provider network  20  uses route distribution protocols, such as Multi-Protocol Border Gateway Protocol (MP-BGP), to distribute routes specific to mobile VPN  18  that are advertised by edge routers of mobile VPN  18 , including PE routers  9 A- 9 B (collectively, “PE routers  9 ”) and hub PE router  8 , to one another. In this example, provider network  20  assigns each route within mobile VPN  18  a Multiprotocol Label Switching (MPLS) label. Accordingly, edge routers of the mobile VPN  18  may distribute an MPLS label with any advertised route. 
     In the illustrated example, an administrator configures mobile VPN  18  in a hub-and-spoke topology by configuring asymmetrical route target import and export lists in routers of mobile VPN  18 . For example, the hub and each spoke may be configured to import a different route target. When an edge router of provider network  20  receives an advertised route, the edge router compares the attached route target to its route target import list and imports routes having matching attached route targets to the virtual routing and forwarding (VRF) instance associated with mobile VPN  18 . In addition, mobile VPN  18  routes may carry a route distinguisher that ensure the address space for mobile VPN  18  remains distinct and separate from the default address space and the address space for any other VPN in which any of the edge routers of provider network  20  participate. For example, PE router  9 A may include a route distinguisher associated identifying mobile VPN  18  in MP-BGP route advertisements of mobile VPN  18  routes. 
     In accordance with the hub-and-spoke topology, the spoke sites (represented in  FIG. 1  by alternate access networks  14 A- 14 B) in mobile VPN  18  can communicate only with the hub site (represented by cellular access network  16 ); they cannot communicate with one another. A “site” refers to a network connected to mobile VPN  18  (and thus to provider network  20 ) by an attachment circuit to one of PE routers  9  or hub PE router  8  of provider network  20 . For example, CE router  11 B of alternate access network  14 B connects to PE router  9 B of mobile VPN  18  by an attachment circuit. In this respect, PE routers  9  may be considered “spoke” PE routers, and “spokes” thus radiate from hub PE router  8  through to respective spoke PE routers  9  toward respective CE routers  11 . As another example, mobile gateway  13  connects to hub PE router  8  by attachment circuit  15 . An attachment circuit may include, for example, a point-to-point protocol (PPP) connection, Asynchronous Transfer Mode (ATM) Virtual Circuit (VC), Frame Relay VC, Ethernet interface, Virtual Local Area Networks (VLAN) on an Ethernet interface, Generic Route Encapsulation (GRE) tunnel, Layer 2 Tunneling Protocol (L2TP) tunnel, or IPSec tunnel. In some instances, a single device integrates mobile gateway  13  functionality for cellular access network  16  with hub PE router  8  functionality for provider network  20 . Such a device may be referred to herein as an IP mobile gateway (IP-MGW). 
     When forwarding packets sourced by customer devices of mobile VPN  18 , including wireless device  6 , a PE router supporting mobile VPN  18  encapsulates the packets with the MPLS label that corresponds, in mobile VPN  18 , to the route that is the best match to the destination address for the packet. This MPLS packet is further encapsulated (e.g., with another MPLS label and/or with an IP header) so that it gets tunneled across the provider network  20  backbone to the proper edge router supporting mobile VPN  18 . Thus, core routers of provider network  20  do not need to know the VPN routes of mobile VPN  18 , and the routes of mobile VPN  18  may remain distinct and separate from the default address space and the address spaces of other VPNs served by provider network  20 . As mobile gateway  13  provides an interface to provider network  20  and, by extension, to mobile VPN  18  established by provider network  20 , mobile gateway  13  may alternately be referred to as a “home mobile gateway.” 
     Alternate access networks  14 A- 14 B (collectively, “alternate access networks  14 ”) connect to spokes of mobile VPN  18 . Each of alternate access networks  14  represents a non-cellular wireless access network such as a local area network (WLAN), WiMAX, or Femtocell network, for example, and includes access points, intermediate authentication, authorization, and accounting (AAA) elements and, in the illustrated examples, a respective one of CE routers  11 . In these embodiments, access points  4 A- 4 B provide local radio communication to wireless devices, e.g., by way of any of the IEEE 802.11 communication protocols, and thus operates as a WiFi or WiMAX “hotspot,” Femtocell, or other wireless access point that serves as a bridge to the wired network infrastructure represented by alternate access networks  14 . Alternate access networks  14  provide access channels between wireless devices and respective spoke PE routers  9 . Each of alternate access networks  14  may be provisioned by a service provider of provider network  20  or another service provider. 
     In some instances, any of alternate access networks  14  may couple to a service provider network other than provider network  20  that then transports data traffic between the CE router  11  of the alternate access network  14  and an autonomous system border router (ASBR) of provider network  20 . This other service provider network may be alternatively referred to as a visited service provider network for wireless device  6  when wireless device  6  attaches to any of alternate access networks  14  coupled to the visited service provider network. In some examples, any of alternate access networks  14  may represent a cellular access network of a visited service provider. 
     Subsequent to being provisioned with an IP address, wireless device  6  performs migration  24  and attaches to alternate access network  14 B via access point  4 B to receive network services by mobile VPN  18 . Access point  4 B may serve a particular location, such as a service provider or third-party hotspot, i.e., a commercial establishment that provides a wireless access point with which customers may establish a wireless access channel to access network services (e.g., Internet access). The location may also include a residency having a wireless access point, such as a wireless router, deployed to facilitate access to network services. 
     To ensure delivery of downstream network traffic destined for wireless device  6  to alternate access network  14 B, CE router  11 B upon attachment by wireless device  6  advertises a host route for wireless device  6  to PE router  9 B. A host route uniquely identifies wireless device  6  within mobile VPN  18 . The host route may include a /32 route (i.e., a route prefix with length  32 ) where mobile VPN  18  is an IPv4 network, where the route prefix is an IP address allocated to wireless device  6  for connectivity via cellular access network  16 . Where mobile VPN  18  is an IPv6 network, the route prefix may include a /128 route, where the route prefix is an IP address allocated to wireless device  6  for connectivity via cellular access network  16 . The host route may be advertised together with a route distinguisher identifying mobile VPN  18 . 
     Because of the hub-and-spoke configuration, hub PE router  8  is a next hop for downstream traffic for wireless devices, (including wireless device  6 ) that receive network services by cellular access network  16 , irrespective of whether the wireless device is attached to cellular access network  16  or to any of alternate access networks  14 . In accordance with the techniques of this disclosure, routers participating in the mobile VPN  18  advertise the host route for wireless device  6  upstream (i.e., toward hub PE router  8 ) together with timing information so as to ensure convergence of the network forwarding plane with the host route toward CE device  11 B prior to transitioning connectivity from the previous attachment point with cellular access network  16  to the new attachment point with one of alternate access networks  14 . 
     In the illustrated example, PE router  9 B advertises a host route for wireless device  6 , upon attachment of the wireless device to alternate access network  14 B, to hub PE router  8  using host route advertisement  25 , which conforms to a routing protocol advertisement of a route distribution protocol (e.g., Multi-Protocol Border Gateway Protocol (MP-BGP)) that is modified to facilitate atomic host route installation techniques. Host route advertisement  25  includes timing information that directs hub PE router  8  to delay installing a host route for wireless device  6  toward PE router  9 B. In addition, the term “host route advertisement,” refers to an advertisement associated with a mobile VPN, such as mobile VPN  18 . 
     In some instances, the timing information includes an absolute or global time that causes hub PE router  8  to postpone installing the included host route until the time passes. The absolute time may be defined according to a clock synchronized among at least the edge routers of mobile VPN  18 . For example, the edge routers of mobile VPN  18  may synchronize to Global Position System (GPS) timing signals that determine an expiry of the absolute time included in host route advertisement  25 . The absolute time may be expressed in host route advertisement  25  according to any of a number of temporal expressions, such as “ddMONyyyy:hh:mm:ss:[ms],” where dd refers to a calendar day of the month MON (e.g., JUL (July)) in the year yyyy, and hh, mm, ss, and ms refer to the hour, minute, second, and zero-padded millisecond of the absolute time. Alternatively, the absolute time may include ss:[ms], i.e., include only seconds and milliseconds. Absolute time may be expressed according to any of a number of different time granularities, including milliseconds, seconds, ticks, and so forth. 
     In some instances, the timing information includes a delay time interval that causes hub PE router  8  to delay, from the time of host router advertisement  25  receipt, installing the included host route until a time equal to the delay time interval expires. Like the absolute time described above, the delay time interval may be expressed according to any of a number of temporal expression. The delay time interval may include an integer that defines a number of clock ticks, milliseconds, hundreds of microseconds, and so forth. In the above-described examples, hub PE router  8  “implements” or “installs” a host route included in host route advertisement  25  by forwarding traffic directed to the host route prefix according to the host route forwarding information (e.g., a next hop included therein). 
     PE routers  9  generate timing information for host route advertisements to account for parallel processing of the host route by PE routers  9  themselves. For example, for host route advertisement  25 , PE router  9 B anticipates its own (non-zero) processing time for installing the host route and incorporates this processing time into the timing information. For example, as described in further detail below, PE router  9 B may measure the time required to program forwarding components of PE router  9 B with the host route. PE router  9 B may then include a representation of this required time in host route advertisement  25 . For example, if PE router  9 B measures the time required to program forwarding components as 10 ms, PE router  9 B may generate timing information for host route advertisement  25  to delay hub router  8  by 10 ms. In instances that use an absolute time, PE router  9 B combines the delay time (10 ms) with a current time and includes the combined time in host route advertisement  25 . In instances that use a delay time interval, PE router  9 B includes a representation of the delay time of 10 ms in host route advertisement  25 . 
     In some topologies, one or more spokes of the hub-and-spoke topology of mobile VPN  18  may include multiple routers. For example, mobile VPN  18  may include a number of different Interior Gateway Protocol (IGP) areas (e.g., Open Shortest Path First areas), with each area potentially having a separate route reflector for BGP. In such topologies, a spoke may include one or more core or provider (P) routers of provider network  20  that operate as border routers of the respective areas. The P routers are intermediate routers that forward traffic between hub PE router  8  and PE routers  9 . The P routers may, in addition to spoke PE routers  9 , incorporate timing information for host route advertisement propagation. A P router upstream to PE router  9 B, for example, combines timing information with its own (non-zero) processing time for installing the host route and incorporates this processing time into timing information to send to hub PE router  9 . In instances that use an absolute time, the P router may include the maximum of its own absolute time for installing the host route and of the absolute time received in a host route advertisement from a downstream router (e.g., PE router  9 B). In instances that use a delay interval time, the P router may include the maximum of its own delay time and the delay time interval received in a host route advertisement. Other than hub PE router  9 , upstream routers of a spoke may avoid delaying their own processing of an advertised host route due to timing information including in a host route advertisement. For example, a P router may immediately begin processing a host route advertisement to install the host route, so long as hub PE router  9  delays sufficiently such that all of the downstream routers have installed the host route prior to hub PE router  9  installing the host route. To that end, hub PE router  9  receives the timing information and delays installing the advertised host route to align with the processing time of all of the downstream routers of the spoke (on which the corresponding wireless device for the host route is attached to mobile VPN  18 ). As a result, the downstream routers of the spoke have completed transitioning to installing the host route prior, and the hub PE router  9  delayed installation of the host route is an atomic operation that may prevent routing loops within the spoke that can otherwise occur with conventional route advertisement and processing. Routers of mobile VPN  18  may apply similar techniques in instances where wireless device  6  migrates between alternate access networks  14 . 
     Wireless device  6  performs migration  26  to detach from alternate access network  14 B and reattach to cellular access network  16  to continue to receive network services by mobile VPN  18  using a previously assigned IP address. For example, the user of wireless device  6  may leave the location served by access point  4 B, thus breaking the WLAN connection. In accordance with techniques described in this disclosure, hub PE router  8  initiates withdrawal of the host route for wireless device  6  in a reverse path toward the previous attachment spoke. That is, hub PE router  8  receives notification that wireless device  6  is now attached by mobile gateway  13  rather than by CE router  11 B. Notification may include out-of-band (e.g., not routing protocol-based) signaling or host route advertisement for wireless device  6  from mobile gateway  13  to hub PE router  8 , for example. 
     Hub PE router  8  withdraws the host route for wireless device  6  to redirect downstream network traffic for wireless device  6  toward mobile gateway  13  according to an aggregate prefix for devices attached to cellular access network  16 . In addition, subsequent to withdrawing the host route for wireless device  6 , hub PE router  8  sends reverse path host route withdraw message  27  to PE router  9 B directing PE router  9 B to withdraw the host route for wireless device  6  from its routing information base. As a result, hub PE router  8  completes withdrawing the host route for wireless device  6  prior to withdrawal of the host route for wireless device  6  by any of the downstream routers of the spoke with which wireless device  6  had a previous attachment. Atomic route withdrawal according to these techniques, proceeding from the hub toward the spoke, may prevent routing loops that can otherwise occur with conventional route withdrawal that proceeds from the spoke to the hub. Routers of mobile VPN  18  may apply similar techniques in instances where wireless device  6  migrates between alternate access networks  14 . In such instances, hub PE router  8  installs a new host route to the new attachment point in addition to initiating withdrawing the host route from the previous attachment point. 
       FIG. 2  is a block diagram illustrating an example router that applies techniques described in this disclosure. Router  100  may represent any of PE routers  9 , hub router  8 , CE routers  11 , or mobile gateway  8  of  FIG. 1 , for example. Router  100  may also represent an IP-MGW. Moreover, while described with respect to a particular network device, e.g., a router, the techniques may be implemented by any network device that may operate to perform the functionality herein described. As described in further detail with respect to  FIGS. 3-8 , components of router  100  apply atomic route installation and withdrawal techniques in accordance with techniques of this disclosure. Router  100  may apply any subset of the techniques. Moreover, the components are illustrative, for router  100  may apply the techniques using any suitable component configuration. 
     Router  100  includes a control unit  102  and interface cards  120 A- 120 B (“IFCs  120 ”) coupled to control unit  102  via internal links  122 A- 122 B. Control unit  102  may include one or more processors (not shown in  FIG. 2 ) that execute software instructions, such as those used to define a software or computer program, stored to a computer-readable storage medium (again, not shown in  FIG. 2 ), such as non-transitory computer-readable mediums including a storage device (e.g., a disk drive, or an optical drive) or a memory (such as Flash memory, random access memory or RAM) or any other type of volatile or non-volatile memory, that stores instructions to cause the one or more processors to perform the techniques described herein. Alternatively or additionally, control unit  102  may comprise dedicated hardware, such as one or more integrated circuits, one or more Application Specific Integrated Circuits (ASICs), one or more Application Specific Special Processors (ASSPs), one or more Field Programmable Gate Arrays (FPGAs), or any combination of one or more of the foregoing examples of dedicated hardware, for performing the techniques described herein. 
     In this example, control unit  102  is divided into two logical or physical “planes” to include a first control or routing plane  104 A and a second data or forwarding plane  104 B. That is, control unit  102  implements two separate functionalities, e.g., the routing and forwarding functionalities, either logically, e.g., as separate software instances executing on the same set of hardware components, or physically, e.g., as separate physical dedicated hardware components that either statically implement the functionality in hardware or dynamically execute software or a computer program to implement the functionality. 
     Control plane  104 A of control unit  102  executes the routing and signaling functionality of router  100 . In this respect, control plane  104 A represents hardware or a combination of hardware and software of control unit  102  for executing routing protocol (RP) module  106  that implements routing protocols such as MP-BGP  110  by which routing information may be received, advertised, processed, and stored in routing information base  112  (“RIB  112 ”). RIB  112  includes information defining a topology of a mobile VPN that is associated with a route target corresponding to a VRF instance. That is, the VRF instance defines participation by router  100  in a mobile VPN established by a provider network in which router  100  operates. Control plane  104 A may resolve the topology defined by routing information in RIB  112  to select or determine one or more routes through the mobile VPN. Control plane  104 A may then update data plane  104 B with these routes, where data plane  104 B maintains these routes within forwarding information  116 . Control plane  104 A may also define a default routing and forwarding instance as well as one or more additional VRF instances (not shown) for routing and forwarding in networks other than the mobile VPN. 
     Data plane  104 B represents hardware or a combination of hardware and software of control unit  102  that provides high-speed forwarding of network traffic received by interface cards  120  in accordance with forwarding information  116 . Forwarding component  117  of data plane  104 B performs lookups in forwarding information  116  based on packet key information for received packets to determine ingress and egress interfaces and corresponding encapsulations for the packets. Forwarding component  117  may include a packet forwarding engine. 
     In the illustrated example, forwarding component  117  acknowledges route updates from RP module  106  after installing the route updates. For example, RP module  106  issues route update  124  directing forwarding component  117  to program a route within forwarding information  116 . After forwarding component  117  programs the route, forwarding component  117  returns route update acknowledgement  126 . 
     Management interface  108  is a process executing on control plane  104 B that provides an interface by which administrator  107 , a network operator or network management system for instance, may modify configuration data  114  (illustrated as “config.  114 ”) of router  100 . Control unit  102  stores configuration data  100  to a computer-readable storage medium. Management interface  108  may present a shell and/or a graphical user interface (GUI) by which an administrator or other management entity may modify the configuration of router  100  using text-based commands and/or graphical interactions, respectively. In addition, or in the alterative, management interface  108  may present an agent that receives Simple Network Management Protocol (SNMP) or Netconf commands from a management entity to set and retrieve configuration and management information for router  100 . 
       FIGS. 3A-3B  depict a flowchart illustrating example operations of routers to atomically install host routes according to techniques described in this disclosure. The operations are described with respect to an instance of router  100  of  FIG. 2  operating within a mobile VPN, such as mobile VPN  18  of  FIG. 1 . 
     Routing protocol module  106  measures a time required to program forwarding component  117  with new routes in forwarding information  116  ( 200 ). For example, RP module  106  may start a timer upon sending route update  124  to forwarding component  117  and read the timer value upon receiving route update acknowledgement  126 . The read timer value indicates a time required to program forwarding component  117  and correlates to the time required by router  100  to install a host route. Routing protocol module  106  may generate, based on previously read timer values under various levels of programming load, a function or histogram (for instance) that maps programming load to anticipated route programming time (i.e., the time required to install the route such that the route is operative for forwarding traffic). Routing protocol module  106  may then use this association when generating timing information. 
     Routing protocol module  106  receives a host route advertisement from a downstream router of a spoke of the hub-and-spoke topology of mobile VPN  18  ( 202 ). The host route advertisement may conform to MP-BGP  110 . If router  100  is hub PE router  8  (YES branch of  202 ), RP module  106  delays initiating host route programming based on timing information included in the host route advertisement ( 220 ). Once the delay based on the timing information has occurred, RP module  106  sends route update  124  including the advertised host route to program the advertised host route in forwarding information  116  ( 222 ). In some instances, RP module  106  delays a difference between the delay based on the timing information and the time required to program forwarding component  117 . For example, if T d  represents the delay based on the timing information and T p  represents the programming time, RP module  106  may delay T d -T p  or may avoid delaying programming the host route if T d -T p  is less than zero. 
     If router  100  is not hub PE router  8  (NO branch of  202 ), RP module  106  sends route update  124  including the advertised host route to immediately initiate programming the advertised host route ( 204 ). For example, RP module  106  may initiate programming the advertised host route by sending route update  124  including the advertised host route to forwarding component  117 . If the host route advertisement does not include timing information (NO branch of  206 ), RP module  106  generates new timing information based on the measured route programming time ( 210 ). For example, if the measured route programming time is 10 ms, RP module  106  may generate new timing information directing an upstream hub router to delay programming by 10 ms. Alternatively, RP module  106  may generate new timing information indicate an absolute time at which router  100  installs the advertised host route by summing the current time and the route programming time. If the host route advertisement includes timing information (YES branch of  206 ), RP module  106  combines the included timing information with the measured route programming time to generate new timing information ( 208 ). RP module  106  may determine the maximum delay (i.e., for the slowest router as respects programming the host route) as between the included timing information and the measured route programming time and generate the new timing information based on the maximum delay. For example, if T p  is the route programming time, T c  is the current time, and T a  is an absolute time received in the included timing information, RP module  106  may determine a maximum delay as the maximum of (T p +T c , T a ). As another example, if T d  is a delay time interval received in the included timing information, RP module  106  may determine a maximum delay as the maximum of (T d , T p ). Routing protocol module  106  encapsulates the generated new timing information in a new host route advertisement that included the received host route and sends the new host route advertisement to an upstream router of the spoke of mobile VPN  18  that includes router  100  ( 212 ). Routing protocol module  106  may include a network address of router  100  as a next hop address within the new host route advertisement. In this way, RP module  106  of routers in a spoke of a hub-and-spoke topology of a mobile VPN may implement a decentralized delay by the hub router for implementing a host route. 
       FIG. 4  is a flowchart illustrating an example mode of operation to delay forwarding host route advertisements to upstream routers according to a configured delay time in accordance with techniques described herein. Initially, management interface  108  receives a delay time in a message from administrator  107  and stores the delay time to configuration data  114  ( 300 ). Subsequently, RP module  106  receives a host route advertisement from a downstream router on a spoke of mobile VPN  18  ( 302 ). RP module  106  initiates programming the host route included within the host route advertisement to data plane  104 B ( 304 ). For example, RP module  106  may initiate programming the advertised host route by sending route update  124  including the advertised host route to forwarding component  117 . 
     Rather than immediately forwarding the host route advertising upstream along the spoke, RP module  106  reads the delay time from configuration data  114  and delays accordingly ( 306 ). For example, RP module  106  may set a timer to the delay time and wait to forward the host route advertisement upstream until the timer expires. After the delay time passes, RP module  106  forwards the host route advertisement upstream ( 308 ). Routing protocol module  106  may include a network address of router  100  as a next hop address within the forwarded host route advertisement. As a result, administrator  107  may prevent routing loops within spokes by setting a delay time for forwarding host routes upstream within RP module  106 . 
       FIG. 5  is a flow chart illustrating an example mode of operation of a router to apply reverse path host route withdrawal in accordance with techniques of this disclosure. For purposes of description, the operation is described with respect to an instance of router  100  of  FIG. 2  operating as a hub PE router  8  of  FIG. 1 . RP module  106  receives a notification that a host wireless device has attached to a new attachment point ( 400 ), where the wireless device previously attached to one of alternate access networks  14  and a corresponding host route was installed to forwarding information  116 . The new attachment point may be a different one of alternate access networks  14  or cellular access network  16 . RP module  106  atomically switches over to forwarding downstream network traffic to the wireless device toward the new attachment point by removing the corresponding host route from forwarding information  116  ( 402 ). 
     Upon removing the host route from data plane  104 B, RP module  106  subsequently facilitates graceful withdrawal of the host route among routers along the spoke toward the previous attachment point. RP module  106  queries RIB  112  to identify one or more next hops (multiple next hops may be present for load balancing) to the prefix according to the stale host route ( 404 ). RP module  106  then generates and sends, to each next hop, a reverse path route withdraw message that conforms to MP-BGP  110  and includes an attribute indicating that the route withdraw message should be forwarded downstream to next hops of the route being withdrawn (i.e., the stale host route) ( 406 ). As a result, routers downstream of hub PE router  8  applying the operation will apply a similar operation to remove the host route from their respective data planes and forward the route withdraw message in a reverse ordering (i.e. toward the spoke). RP module  106  deletes the stale host route from RIB  112  upon sending the one or more reverse path host route withdraw messages to remove the record of the prior attachment from control plane  104 A ( 408 ). Alternatively, RP module  106  may store a copy of the stale route at step  402  of the illustrated operation and delete the stale host route from RIB  112  in conjunction with removing the host route from data plane  104 B. 
       FIG. 6  is a flowchart illustrating an example operation of a router to delay conventional route withdrawal according to techniques of this disclosure. For purposes of description, the operation is described with respect to an instance of router  100  of  FIG. 2  operating as one of CE routers  11  of  FIG. 1 . RP module  106  receives notification that a host route for a wireless device attached to one of alternate access networks  14  on a spoke of mobile VPN  18  is no longer valid ( 500 ). In other words, the wireless device is no longer attached or is otherwise unreachable by the host route, and the host route must be withdrawn. The notification may represent a failed Address Resolution Protocol request or Internet Configuration Management Protocol echo request. 
     RP module  106  starts a timer to delay withdrawing the host route according to a withdraw wait interval set in configuration data  114  ( 502 ). If RP module  106  receives a reverse path host route withdraw message from an upstream router of the spoke within the withdraw wait interval (YES branch of  504 ), RP module  106  clears the stale host route from RIB  112  and forwarding information  116  ( 506 ). If, on the other hand, the timer expires without RP module  106  having received a reverse path host route withdraw message (YES branch of  508 ), RP module  106  executes a conventional host withdrawal by sending a host route withdraw message to the corresponding one of PE routers  9 , that is, an upstream router on the spoke ( 510 ) and removing the host route from RIB  112  and forwarding information  116  ( 506 ). The host route withdraw message and reverse path host route withdraw message may conform to MP-BGP  110  and the techniques of this disclosure. By delaying the withdraw wait interval, CE routers  11  may allow reverse path host route withdrawal initiated by hub PE router  8  to proceed downstream toward PE routers  9  to further atomicity of the switchover to the new attachment point. In other words, conventional host route withdrawal avoids causing host route withdrawal until the reverse path host route withdrawal has had a chance to run its course or, at the very least, that the stale host route be first withdrawn from hub PE router  8 . If the timer expires and no reverse path host route withdraw message is forthcoming, CE routers  11  resume conventional route withdrawal responsibilities. 
       FIG. 7  is a block diagram illustrating an example host route advertisement that includes, in accordance with techniques described herein, timing information to delay route installation by a hub provider edge router. Extended BGP UPDATE message  600  conforms to MP-BGP and includes MP-REACH-NLRI  602  advertising a host route for a wireless device newly attached to a spoke of a hub-and-spoke mobile VPN topology. Extended BGP UPDATE message  600  may represent an example instance of host route advertisement  25  illustrated in  FIG. 1 . For purposes of illustration, extended BGP UPDATE message  600  is illustrated using glyphs, rather than with packet fields. 
     MP-REACH-NLRI  602  of extended BGP UPDATE message  600  specifies an Address Family Identifier (AFI)  604  of 1 in this example to indicate IPv4 network addresses, along with a value for the Subsequent AFI (SAFI)  606  of 241 to identify delay NLRI  608  as carrying timing information. AFI  604  and SAFI  606  may in some instances have different values, as assigned by a private party or by the Internet Assigned Numbers Authority (IANA). 
     Delay Network Layer Reachability Information (NLRI)  608  specifies a host route for the wireless device having IP address 10.1.0.65, identified in prefix  610 C and assigned to facilitate access to network services by a cellular access network. Route distinguisher  610 B is associated with a mobile VPN, such as mobile VPN  18  of  FIG. 1 , and label  610 A specifies an MPLS label for an LSP from a hub PE router of the mobile VPN toward a spoke PE router to which the wireless device is attached. A value of delay  610 D specifies a delay interval time added by an router of the spoke, advertising extended BGP UPDATE message  600 , to direct a hub PE router of the mobile VPN to delay installing the route included in delay NLRI  608 . In this example, the delay  610 D value of 100 indicates that the hub PE router is to delay installing the route 100 ms. Other example timing information formats and values may be used, as described in further detail above. 
       FIG. 8  is a block diagram illustrating an example reverse path host route withdraw message that includes, in accordance with techniques described herein, a flag directing receiving routers to perform reverse path host route withdrawal. Extended BGP UPDATE message  700  conforms to MP-BGP and includes MP-UNREACH-NLRI  702  advertising a no longer reachable host route for a wireless device previously attached to a spoke of a hub-and-spoke mobile VPN topology. Extended BGP UPDATE message  700  may represent an example instance of reverse path host route withdraw message  27  illustrated in  FIG. 1 . For purposes of illustration, extended BGP UPDATE message  700  is illustrated using glyphs, rather than with packet fields. 
     MP-UNREACH-NLRI  702  of extended BGP UPDATE message  700  specifies an Address Family Identifier (AFI)  704  of 1 in this example to indicate IPv4 network addresses, along with a value for the Subsequent AFI (SAFI)  706  of 241 to identify delay NLRI  608  as carrying timing information. AFI  704  and SAFI  706  may in some instances have different values, as assigned by a private party or by the Internet Assigned Numbers Authority (IANA). 
     Reverse Path Network Layer Reachability Information (NLRI)  708  specifies a host route for the wireless device having IP address 10.1.0.65, identified in prefix  710 C and assigned to facilitate access to network services by a cellular access network. Route distinguisher  710 B is associated with a mobile VPN, such as mobile VPN  18  of  FIG. 1 , and label  710 A specifies an MPLS label for an LSP from a hub PE router of the mobile VPN toward a spoke PE router to which the wireless device was previously attached. Reverse Path NLRI  708  includes reverse field  710 D with a set flag to indicate that extended BGP UPDATE message  700  is a reverse path host route withdraw message. A router that receives extended BGP UPDATE message  700  removes the host route prefix  710 C advertised as unreachable and forwards a representation of extended BGP UPDATE message  700  to one or more next hops (now stale) for host route prefix  710 C as indicated in a routing information base of the router. 
     The techniques described herein may be implemented in hardware, software, firmware, or any combination thereof. Various features described as modules, units or components may be implemented together in an integrated logic device or separately as discrete but interoperable logic devices or other hardware devices. In some cases, various features of electronic circuitry may be implemented as one or more integrated circuit devices, such as an integrated circuit chip or chipset. 
     If implemented in hardware, this disclosure may be directed to an apparatus such a processor or an integrated circuit device, such as an integrated circuit chip or chipset. Alternatively or additionally, if implemented in software or firmware, the techniques may be realized at least in part by a computer-readable data storage medium comprising instructions that, when executed, cause a processor to perform one or more of the methods described above. For example, the computer-readable data storage medium may store such instructions for execution by a processor. 
     A computer-readable medium may form part of a computer program product, which may include packaging materials. A computer-readable medium may comprise a computer data storage medium such as random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), Flash memory, magnetic or optical data storage media, and the like. In some examples, an article of manufacture may comprise one or more computer-readable storage media. 
     In some examples, the computer-readable storage media may comprise non-transitory media. The term “non-transitory” may indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium may store data that can, over time, change (e.g., in RAM or cache). 
     The code or instructions may be software and/or firmware executed by processing circuitry including one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, functionality described in this disclosure may be provided within software modules or hardware modules. 
     Various embodiments have been described. These and other embodiments are within the scope of the following examples.