Patent Publication Number: US-9420057-B2

Title: Federated cache network

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present Application claims priority to U.S. Provisional Patent Application No. 61/828,685 filed on May 30, 2013, which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates in general, to method of cache optimization, and in particular, to cache placement and optimization in a backhaul or access network of a network service provider or a multiple-system operator. 
     BACKGROUND OF THE INVENTION 
     Today, caching is an integral part of the Internet infrastructure—it is extensively deployed for many different reasons. For example, caching is used to reduce bandwidth costs, share server costs, or increase response speed. There exist numerous caching technologies: browser caching, content distribution network, push caching, adaptive caching, active caching, streaming caches, etc. 
     Caching is important to network service providers (NSPs) and multiple-system operators (MSOs), which provide fixed or mobile broadband bandwidth to retail customers. For these providers, reducing customer churn rate is a major business driver. One way to reduce churn rate is to enhance user experience through caching. 
     Most NSPs and MSOs own a bandwidth infrastructure, which is both an asset and a liability—operating the infrastructure consumes working capital. As they have sunk significant capital in building up the infrastructure—it is to their advantage to leverage their existing infrastructure. 
     From the cost perspective, server and storage costs have dropped relative to bandwidth costs. The reason is that bandwidth costs are tied to the physical distance between a transmitter and a receiver. In contrast, the costs of computing and storage have decreased exponentially as the relevant technologies have made continuous improvements. However, the costs of laying cables and maintaining the ground lease for cell towers have actually risen over the years along with the general inflation. 
     Therefore, caching is a way to substitute the more expensive bandwidth resources with the less expensive computing and storage resources, while still improving the performance of content access in an average sense. 
     For NSPs, 2 user-experience metrics are important: (1) response speed of interactive applications, and (2) throughput of large-file applications. For interactive applications such as web browsing, the user is most concerned with the speed that a web page is rendered. For large-file applications such as video streaming, the user is most concerned with the speed that the video buffer is filled, which directly translates into the download throughput. Caching will help both types of application. 
     In the current solutions for web caching, cache nodes (devices that implement caching) are not placed in access or backhaul networks. Instead, cache nodes are placed in data centers or hub locations. The reason for doing this is that cache nodes are servers that need a protected environment. To expose a server in an outdoor environment, the server has to be ruggedized to withstand high temperature, rain, and other weather-related impacts. 
     However, as the server technologies have improved, it is now economical to place ruggedized cache nodes even in an outdoor setting such as the cell transmission tower of a mobile carrier, or the street cabinet (or local multiplexer) of a cable system operator. These outdoor locations are in the access or backhaul network of an NSP or MSO. 
     Placing cache nodes at such locations brings new technical challenges. For example, in a mobile carrier network, a user can move from one cell to the next. Once a user moves, the user device changes its network attachment point—the mobile carrier has to perform handoff by rerouting the packets to a different location. The handoff process breaks content caching at the cell towers as caching is tied to TCP (transmission control protocol) termination. 
     In TCP termination, packets destined for either the mobile device or the server must traverse the termination point. But once the mobile device changes its network attachment point, packets destined for either the mobile device or the server may not pass through the termination point anymore. The termination point has to be relocated. Preferably, the cache content associated with the mobile device is also relocated to the same place. 
     For MSOs and fixed-line NSPs, there are no mobility (handoff) issues. However, for all NSPs and MSOs, caches installed in access or backhaul network nodes are serving a highly restricted group of users; the cache content should be optimized with this in mind. 
     Therefore, there is a need to improve caching by moving cache nodes to the access or backhaul networks of NSPs and MSOs. The new type of cache nodes has to be optimized for handoff and serving a highly restricted group of users. 
     BRIEF SUMMARY OF THE INVENTION 
     A system and method, called FCN (federated cache network), places and optimizes cache nodes in an access or backhaul network of an NSP or MSO. 
     FCN caches in a backhaul or access network form a federated network—if a content object is not found in a local cache, the original content request is forwarded to an upstream cache, which is a cache on a path toward the core network of an MSO or NSP from the local cache. If the content object is not found an FCN, the content request is forwarded to an origin server. 
     In an FCN, there are 3 types of cache: trunk cache, branch cache, and leaf cache. The trunk cache has a capacity roughly equal to the sum of capacities of all directly connected (there is no 3 rd  FCN cache between the path connecting the 2 caches) leaf and branch caches in the same FCN. A branch cache has a capacity roughly equal to the sum of capacities of all the directly connected downstream (in the direction toward the end user equipment) leaf and branch caches. 
     The cache nodes in an FCN form a tree topology with the trunk cache at the root. Each FCN has only one trunk cache. The trunk cache node is directly connected to a group of downstream branch or leaf cache nodes. A branch cache node is directly connected to a group of branch or leaf cache nodes, which are further downstream in the backhaul or access network. A leaf cache node is not connected to any downstream cache nodes. 
     The content of the trunk cache is roughly the aggregate of contents in all the directly connected downstream branch and leaf caches; the content of a branch cache is roughly the aggregate of contents in all the directly connected downstream leaf and branch caches. 
     The trunk cache node may be placed at the edge of the core network, or inside the core network of an MSO or NSP. A branch cache node may be located inside a backhaul or access network, at a concentrator or hub location. A leaf cache node may be located at a location close to end customer equipment. 
     In an FCN, TCP (transmission control protocol) termination must be used in conjunction with caching. TCP termination points are placed at a location with a cache node. 
     If an FCN is embedded in a mobile carrier network, the mobility management entity (MME) communicates with the FCN cache nodes and their associated TCP termination devices. 
     If a mobile device is moving away from a current base transmission station toward a new base transmission station, the MME of the mobile network informs the TCP termination device at the current base transmission station of the new base transmission station. All live and terminated TCP sessions associated with the mobile device are copied and re-terminated at the new base transmission station. A block of cache content that is associated with the mobile device is pushed to the cache node at the new base transmission station. 
     In an FCN, the technique of TCP flow-aggregation may be used to improve average throughput between 2 cache nodes. A “permanent” TCP session is used to aggregate all or a large portion of the TCP traffic between 2 cache nodes. 
     Optionally, jumbo frames are used for the TCP traffic between cache nodes in an FCN. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features in accordance with the present invention will become apparent from the following descriptions of embodiments in conjunction with the accompanying drawings, and in which: 
         FIG. 1  depicts the topology of a federated cache network in a mobile carrier network. 
         FIG. 2  depicts the topology of a federated cache network in an MSO network. 
         FIG. 3  illustrates the concepts of downstream, upstream, and direct connectedness. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A system and method, called FCN, places and optimizes cache nodes in an access or backhaul network of a NSP or an MSO, which provides fixed or mobile bandwidth to the Internet to retail customers. 
     The cache nodes in an FCN are semi-autonomous—each cache has an independent name-address space. The caches in an FCN are federated in the sense that, whenever a content object is not found in a local cache, the content request is forwarded to an upstream cache, which is a cache on a path toward the core network of an MSO or NSP from the local cache. 
     In an FCN, there are 3 types of cache nodes: trunk cache node, branch cache node, and leaf cache node. In an FCN, there is only one trunk cache, while there are a plurality of branch caches and leaf caches. 
     A difference between the 3 types of FCN cache is in their cache capacity. The trunk cache node has a capacity roughly equal to the sum of capacities of all directly connected (the path between the 2 caches does not go through a 3 rd  FCN cache) leaf and branch caches in the same FCN. A branch cache node has a capacity roughly equal to the sum of capacities of all the directly connected downstream leaf and branch cache nodes in the same FCN. 
     The cache nodes in an FCN form a tree topology with the trunk cache at the root. The trunk cache node is directly connected to a group of downstream branch and leaf cache nodes. A branch cache node is directly connected to a group of downstream branch and leaf caches. A leaf cache node is not connected with any downstream cache nodes. 
     Further, the cache content of the trunk node is roughly the aggregate of cache contents of all directly connected downstream branch and leaf caches; the cache content of a branch node is roughly the aggregate of cache contents of all directly connected downstream leaf and branch caches. 
     If a content object is not found at a cache node, the original content request is forwarded to a directly connected upstream (toward the core network) cache node—the upstream cache node may be a branch cache node or the trunk cache node. If the content object is not found at the trunk cache, the content request is forwarded to an origin server. 
     The trunk cache node may be placed at the edge of, or inside, the core network of an NSP or MSO. A branch cache node is located inside a backhaul or access network of an NSP or MSO, at a concentrator or hub location. A branch cache node may sit at a location that joins a number of branches in an access or backhaul network toward end user equipment. A leaf cache node may be located in a backhaul or access network that is close to the last piece of bandwidth to end customer equipment. 
     In a mobile carrier network, leaf cache nodes may be located at base transmission stations or access points; in a DSL (digital subscriber line) network, leaf cache nodes may be located at DSLAM (digital subscriber line access multiplexer) nodes; in an MSO network, leaf nodes may be located at street cabinets or local multiplexers. 
     In an FCN, the technique of TCP (transmission control protocol) termination must be used in conjunction with caching. TCP termination is also known as TCP splicing or TCP splitting. At a termination point, a cache node is made available. In an FCN, TCP termination points are usually placed at a location with a cache node. 
     If an FCN is implemented in a mobile carrier network, a technical hookup is set up so that the mobility management entity (MME) communicates with the FCN cache nodes and their associated TCP termination devices. 
     In a mobile network, a mobile device may move away from a current transmission station or access point to a new transmission station or access point. When and if such a movement takes place, the MME of the mobile network informs the TCP termination device at the current transmission station or access point that an associated mobile device is moving toward a new transmission station or access point. All live and terminated TCP sessions associated with the mobile device are copied and re-terminated at the new transmission station or access point, in anticipation of the actual handoff, if it does happen. In addition, a block of cache content that has been determined to be associated with the mobile device is pushed to the cache node at the new transmission station or access point. 
     In an FCN, the technique of TCP flow-aggregation may be used to improve throughput between 2 cache nodes. 2 cache nodes may communicate with each other—a cache can communicate with an upstream or downstream cache, or a cache may communicate with another cache that is neither upstream nor downstream. A “permanent” TCP session may be set up between 2 cache nodes. The “permanent” TCP session is then used to aggregate all or a large portion of the TCP traffic between the 2 cache nodes. Optionally, jumbo frames are used for the TCP traffic between cache nodes in an FCN. 
     For mobile carriers, an access network is often called a backhaul network. Sometimes, an access network for a metropolitan area is referred to as a metro network, and a core network is also known as a long-haul network. For a large operator, the core network is often connected with multiple access networks, and each access network is connected to the core through a gateway or hub. 
     In 3G (3 rd  generation) mobile networks, such a gateway is called a GGSN (gateway GPRS support node); in 4G (4 th  generation) networks, such a gateway is often called a PGW (packet gateway)—GPRS stands for general packet radio service. Both GGSN and PGW are used for a large service area—a gateway for a smaller service area is called a SGSN (serving GPRS support node) in 3G networks and a SGW (serving gateway) in 4G networks. For a large mobile carrier, the service area is further subdivided—a subdivision gateway is often called a RNC (radio network controller) in 3G networks. Finally, at the cell level, a base station is called a BTS (base transmission station) in 3G networks and an e-Node-B in 4G networks. 
     At the cell level, there are different types of cell: macro-, micro-, pico-, and femto-cell. Some base stations are wireless relays, while others are base stations, which are connected to the core network through a fixed broadband link. All the cell stations are candidates for placing an FCN cache. However, depending on the power supply and physical conditions, it may or may not be suitable to install a cache node. 
     If an NSP is a DSL (digital subscriber line) provider, cache nodes can be installed at a DSLAM (digital subscriber line access multiplexer) node, a central office node, or a core network node. If an NSP is a cable provider, cache nodes can be installed at the nation head end, a regional hub, a local multiplexer, or a street cabinet. 
       FIG. 1  depicts an embodiment of an FCN embedded in a 4G mobile carrier network. The trunk cache  201  is directly attached to PGW  101 , which is connected to the core network  300 . PGW  101  is connected to a SGW  102  and an RNC  103 . The branch caches  202  are attached to SGW  102  and RNC  103 . Both RNC  103  and SGW  102  are located in the access network  100 . SGW  102  is connected to a number of eNodeB  103 ; RNC  103  is connected to a number of BTS  104 . The leaf caches  203  are attached to either an eNodeB  103  or a BTS  104 . 
       FIG. 2  depicts an embodiment of an FCN embedded in an MSO network. The trunk cache  201  is directly attached to a distribution hub  101 , which is connected to the transport ring  300 , which is equivalent to the core network in a mobile carrier. The distribution hub  101  is connected to 2 local multiplexers  102  and  103 . The branch caches  202  are attached to the local multiplexers  102  and  103 . Both local multiplexers  103  and  102  are located in the access network  100 . The local multiplexers  102  and  103  are connected to a number of street cabinets  103  and  104 . The leaf caches  203  are attached to a street cabinet  103  or  104 . 
       FIG. 3  illustrates the concepts of upstream, downstream, and direct connectedness. The direction toward the core network  100  is upstream; the direction toward user equipment is downstream, which is opposite to the upstream. The dash cloud  500  represents the access network. The trunk cache  201  is directly connected to downstream branch caches  301  and  302  and a downstream leaf cache  404 . Branch cache  301  is directly connected to downstream leaf caches  401 ,  405  and another downstream branch cache  303 . Branch  301  is upstream of leaf cache  405 .