# Patent

## Title
System and method for multi-source multicasting in content-centric networks

## Abstract

One embodiment of the present invention provides a system for updating routing information associated with a multicast group in an information-centric network (ICN). During operation, a first node in the ICN receives an update message for the multicast group from a neighbor node. The multicast group includes a root anchor node and a number of anchor nodes with the root anchor node having a smaller name than the anchor nodes. The update message specifies a prefix associated with the multicast group and the root anchor node of the multicast group. The system selects, based on topology information stored on the first node, from neighbors of the first node one or more next-hop neighbors that meet a notification condition, and forwards the update message to the one or more next-hop neighbors.

## Background
COLOR DRAWINGS
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
BACKGROUND
Field
The present disclosure relates generally to a content-centric network (CCN). More specifically, the present disclosure relates to a system and method for supporting multicast groups with multiple sources in content-centric networks (CCNs).
Related Art
The proliferation of the Internet and e-commerce continues to fuel revolutionary changes in the network industry. Today, a significant number of information exchanges, from online movie viewing to daily news delivery, retail sales, and instant messaging, are conducted online. An increasing number of Internet applications are also becoming mobile. However, the current Internet operates on a largely location-based addressing scheme. The two most ubiquitous protocols, the Internet Protocol (IP) and Ethernet protocol, are both based on end-host addresses. That is, a consumer of content can only receive the content by explicitly requesting the content from an address (e.g., IP address or Ethernet media access control (MAC) address) that is typically associated with a physical object or location. This restrictive addressing scheme is becoming progressively more inadequate for meeting the ever-changing network demands.
Recently, information-centric network (ICN) architectures have been proposed in the industry where content is directly named and addressed. Content-Centric Networking (CCN), an exemplary ICN architecture brings a new approach to content transport. Instead of having network traffic viewed at the application level as end-to-end conversations over which content travels, content is requested or returned based on its unique name, and the network is responsible for routing content from the provider to the consumer. Note that content includes data that can be transported in the communication system, including any form of data such as text, images, video, and/or audio. A consumer and a provider can be a person at a computer or an automated process inside or outside the ICN. A piece of content can refer to the entire content or a respective portion of the content. For example, a newspaper article might be represented by multiple pieces of content embodied as data packets. A piece of content can also be associated with metadata describing or augmenting the piece of content with information such as authentication data, creation date, content owner, etc.
At the core of all ICN architectures are name resolution and routing of content, and several approaches have been proposed. In some ICN architectures, the names of data objects are mapped into addresses by means of directory servers, and then address-based routing is used for content delivery. By contrast, a number of ICN architectures use name-based routing of content, which integrates name resolution and content routing. With name-based routing, some of the routers (producers or caching sites) advertise the existence of local copies of named data objects (NDO) or name prefixes denoting a set of objects with names sharing a common prefix, and routes to them are established; the consumers of content issue content requests that are forwarded along the routes to the routers that issued the NDO or name prefix advertisements.
Among the various ICN architectures, CCN uses distributed routing protocols to establish routes over which content requests are forwarded. In CCN, a content request (called an “Interest”) may be sent over one or multiple paths to a name prefix. It has been argued that Interest-based ICN architectures, such as CCN, provide “native support” for multicasting. However, this is the case only for single-source multicasting if the names in Interests denote the source of a multicast group.
SUMMARY
One embodiment of the present invention provides a system for updating routing information associated with a multicast group in an information-centric network (ICN). During operation, a first node in the ICN receives an update message for the multicast group from a neighbor node. The multicast group includes a root anchor node and a number of anchor nodes with the root anchor node having a smaller name than the anchor nodes. The update message specifies a prefix associated with the multicast group and the root anchor node of the multicast group. The system selects, based on topology information stored on the first node, from neighbors of the first node one or more next-hop neighbors that meet a notification condition, and forwards the update message to the one or more next-hop neighbors.
In a variation on this embodiment, a next-hop neighbor that meets the notification condition is an anchor node of the multicast group, and all neighbors of the anchor node report the anchor node as a preferred anchor node.
In a variation on this embodiment, a next-hop neighbor that meets the notification condition provides a shortest path to at least one of the anchor nodes.
In a variation on this embodiment, a second node intended to join the multicast group generates a join request. The system selects a neighbor of the second node as a next-hop node to the root anchor of the multicast group based on an ordering condition. The next-hop node of the second node to the root anchor node provides a shortest path to the root anchor node. The system forwards the join request to the selected neighbor of the second node.
In a further variation, the system receives a response to the join request sent from the root anchor node or an anchor node of the multicast group. The response traverses a reverse path of the join request to the second node.
In a further variation, the system establishes a multipoint spanning tree that includes the root anchor node, the anchor nodes, and one or more routers that receive the join request.
In a variation on this embodiment, the root anchor node and the anchor nodes are one of: sources of the multicast group and receivers of the multicast group.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 illustrates an exemplary architecture of a network, in accordance with an embodiment of the present invention.
FIG. 2 presents a diagram illustrating an exemplary neighbor table maintained at a router, in accordance with an embodiment of the present invention.
FIG. 3 presents a diagram illustrating an exemplary routing table maintained at a router, in accordance with an embodiment of the present invention.
FIG. 4 presents a diagram illustrating an exemplary multipoint routing table maintained at a router, in accordance with an embodiment of the present invention.
FIG. 5 presents a diagram presenting an exemplary architecture of a router that implements ICM, in accordance with an embodiment of the present invention.
FIG. 6 illustrates the propagation of root anchor updates in an exemplary ICN, in accordance with an embodiment of the present invention.
FIG. 7 illustrates the establishment of a multi-instantiated destination spanning tree (MIDST) in an exemplary ICN, in accordance with an embodiment of the present invention.
FIG. 8 illustrates an exemplary system for information-centric multicasting, in accordance with an embodiment of the present invention.



In the figures, like reference numerals refer to the same figure elements.
DETAILED DESCRIPTION
Overview
Embodiments of the present invention provide an information-centric multicasting (ICM) system for supporting multicasting groups with multiple sources in information-centric networks (ICNs). More specifically, ICM supports routing to multicast groups by establishing a multi-instantiated destination spanning tree (MIDST) for each name prefix that denotes a multicast group. All anchors of a given multicast group are connected through the MIDST for the prefix. ICM can support both source-initiated multicasting (SIM) and receiver-initiated multicasting (RIM).
Exemplary CCN Architecture
To demonstrate the operations of a link-state content routing (LCR) system, this disclosure uses CCN as an example. However, the operations of the LCR system is not limited to CCN. In general, LCR can be applied to any other type of ICN networks.
CCN uses two types of messages: Interests and Content Objects. An Interest carries the hierarchically structured variable-length identifier (HSVLI), also called the “name,” of a Content Object and serves as a request for that object. If a network element (e.g., router) receives multiple Interests for the same name, it may aggregate those Interests. A network element along the path of the Interest with a matching Content Object may cache and return that object, satisfying the Interest. The Content Object follows the reverse path of the Interest to the origin(s) of the Interest.
The terms used in the present disclosure are generally defined as follows (but their interpretation is not limited to such):

    
    
        “HSVLI:” Hierarchically structured variable-length identifier, also called a Name. It is an ordered list of Name Components, which may be variable length octet strings. In human-readable form, it can be represented in a format such as ccnx:/path/part. Also the HSVLI may not be human readable. As mentioned above, HSVLIs refer to content, and it is desirable that they be able to represent organizational structures for content and be at least partially meaningful to humans. An individual component of an HSVLI may have an arbitrary length. Furthermore, HSVLIs can have explicitly delimited components, can include any sequence of bytes, and are not limited to human-readable characters. A longest-prefix-match lookup is important in forwarding packets with HSVLIs. For example, an HSVLI indicating an Interest in “/parc/home/bob” will match both “/parc/home/bob/test.txt” and “/parc/home/bob/bar.txt.” The longest match, in terms of the number of name components, is considered the best because it is the most specific. Detailed descriptions of the HSVLIs can be found in U.S. Pat. No. 8,160,069, entitled “SYSTEM FOR FORWARIDNG A PACKET WITH A HIERARCHICHALLY STRUCTURED VARIABLE-LENGTH IDENTIFIER,” by inventors Van L. Jacobson and James D. Thornton, filed 23 Sep. 2009, the disclosure of which is incorporated herein by reference in its entirety.
        “Interest:” A request for a Content Object. The Interest specifies an HSVLI name prefix and other optional selectors that can be used to choose among multiple objects with the same name prefix. Any Content Object whose name matches the Interest name prefix (and optionally other requested parameters such as publisher key-ID match) satisfies the Interest.
        “Content Object:” A data object sent in response to an Interest. It has an HSVLI name and a Content payload that are bound together via a cryptographic signature. Optionally, all Content Objects have an implicit terminal name component made up of the SHA-256 digest of the Content Object. In one embodiment, the implicit digest is not transferred on the wire, but is computed at each hop, if needed. In this disclosure, the term “Content Object” and the term “Named Data Object (NDO)” are exchangeable.
        “Face:” In CCN, the term face is a generalization of the concept of an interface. A face may be a connection to a network or directly to an application party. A face may be configured to send and receive broadcast or multicast packets on a particular network interface, or to send and receive packets using point-to-point addressing in the underlying transport, or using a tunnel (for example a TCP tunnel). A face may also be the connection to a single application process running on the same machine, via an encapsulation like UDP or an OS-specific inter-process communication path. All messages arrive through a face and are sent out through a face. In this disclosure, the term “neighbor” is interchangeable with the term “face,” refereeing to incoming or outgoing interface of an Interest.
        “Instance:” In this disclosure, the term “instance” can be used to refer to either a receiver or a source of a multicast group.
        “Prefix:” In this disclosure, the term “prefix” can be used to refer to either a name of a specific multicast group or a name prefix for the multicast group.
        “Anchor:” In this disclosure, the term “anchor” is used to refer to a router that advertises having an instance of a multicast group locally available. More specifically, a router (or a node) that advertises for some or all the content corresponding to a prefix is referred to as an anchor of the prefix.
    
    


As mentioned before, an HSVLI indicates a piece of content, is hierarchically structured, and includes contiguous components ordered from a most general level to a most specific level. The length of a respective HSVLI is not fixed. In content-centric networks, unlike a conventional IP network, a packet may be identified by an HSVLI. For example, “abcd/bob/papers/ccn/news” could be the name of the content and identifies the corresponding packet(s), i.e., the “news” article from the “ccn” collection of papers for a user named “Bob” at the organization named “ABCD.” To request a piece of content, a node expresses (e.g., broadcasts) an Interest in that content by the content's name. An Interest in a piece of content can be a query for the content according to the content's name or identifier. The content, if available in the network, is sent back from any node that stores the content to the requesting node. The routing infrastructure intelligently propagates the Interest to the prospective nodes that are likely to have the information and then carries available content back along the reverse path traversed by the Interest message. Essentially the Content Object follows the breadcrumbs left by the Interest message and thus reaches the requesting node.
FIG. 1 illustrates an exemplary architecture of a network, in accordance with an embodiment of the present invention. In this example, a network 180 comprises nodes 100-145. Each node in the network is coupled to one or more other nodes. Network connection 185 is an example of such a connection. The network connection is shown as a solid line, but each line could also represent sub-networks or super-networks, which can couple one node to another node. Network 180 can be content-centric, a local network, a super-network, or a sub-network. Each of these networks can be interconnected so that a node in one network can reach a node in other networks. The network connection can be broadband, wireless, telephonic, satellite, or any type of network connection. A node can be a computer system, an end-point representing users, and/or a device that can generate Interest or originate content.
In accordance with an embodiment of the present invention, a consumer can generate an Interest for a piece of content and forward that Interest to a node in network 180. The piece of content can be stored at a node in network 180 by a publisher or content provider, who can be located inside or outside the network. For example, in FIG. 1, the Interest in a piece of content originates at node 105. If the content is not available at the node, the Interest flows to one or more nodes coupled to the first node. For example, in FIG. 1, the Interest flows (Interest flow 150) to node 115, which does not have the content available. Next, the Interest flows (Interest flow 155) from node 115 to node 125, which again does not have the content. The Interest then flows (Interest flow 160) to node 130, which does have the content available. The flow of the Content Object then retraces its path in reverse (content flows 165, 170, and 175) until it reaches node 105, where the content is delivered. Other processes such as authentication can be involved in the flow of content.
In network 180, any number of intermediate nodes (nodes 100-145) in the path between a content holder (node 130) and the Interest generation node (node 105) can participate in caching local copies of the content as it travels across the network. Caching reduces the network load for a second subscriber located in proximity to other subscribers by implicitly sharing access to the locally cached content.
An Information-Centric Multicasting System
Various approaches have been proposed to support multicasting in ICNs, including a link-state routing approach that relies on each node flooding link-state advertisements (LSAs) stating the state of adjacent links and the existence of receivers for different multicast groups. Core Based Trees (CBTs) have also been introduced as an multicast routing approach that avoids the flooding from each multicast source or the flooding of information about those routers with attached multicast group receivers. In CBT, a pre-defined node serves as the intermediary of a multicast group and is called the core of the group. Nodes maintain routes to all network nodes and hence to all possible cores, and learn the mapping from the multicast group address to the address of the core by some external means. Each receiver of a multicast group sends join requests toward the core of the group to establish a shared multicast tree spanning all the receivers and the core. Sources simply send data packets toward the core, and data packets are sent to all receivers of the multicast group over the multicast tree. Another approach, Protocol Independent Multicast (PIM) is similar to CBT, but the multicast trees are unidirectional; hence, sources must send multicast data to the intermediary node, called the rendezvous point (RP), which then floods the data over the multicast tree.
Other multicast approaches include the pull-based approach and the push-based approach. The pull-based approach is adopted in Interest-based ICN architectures, such as CCN and named data networking (NDN). It has been argued that NDN and CCN provide native support for multicasting, given that Interests are aggregated along trees directed toward a site with the requested content. However, CCN and NDN can provide efficient support for single-source multicasting only, assuming that Interests state the name of the requested multicast source and that the presence of the multicast group source is advertised throughout the network. This approach does not work well for the case of multicasting with a large number of sources, because each multicast source must be known in the network and a tree to each such source needs to be maintained, which does not scale well.
The push-based multicast approach has been adopted by other ICN architectures that are not based on Interests. Content-Oriented Publish/Subscribe System (COPSS) is a good example of the PIM-based approach. In COPSS, users subscribe to content on a content descriptor (CD), which can be any legal content name, and each CD is associated with a Rendezvous Point (RP). The number of RPs may be as large as the number of ICN nodes. Routers maintain CD-based subscription tables to provide the same functionality as IP multicast, and COPSS supports sparse mode multicasting at the content layer. The RPs receive content from one or more publishers and send it over the multicast trees established by routers for the multicast groups. However, such push-based multicasting schemes can be inefficient, especially in supporting multi-source multicasting.
Embodiments of the present invention provide an information-centric multicasting system (ICM) that can support multi-source multicasting in Interest-base ICN architecture and that is more efficient than the PIM-based approaches. The operation of an ICM system assumes that: (a) each network node is assigned a name with a flat or hierarchical structure; (b) each multicast group can be requested by means of a unique name; (c) multicast group names (MGNs) can be denoted using either flat or hierarchical naming (such as HSVLI-based naming), and the same naming convention is used for the entire system; and (d) a routing protocol operates in the network to provide each router with at least one route to the nearest instance of each multicast group advertised in the network.
In order to implement ICM, routers maintain a number of data structures, including a link cost table (LTi), a neighbor table (NTi) or a topology table (TTi), a routing table (RTi), and a multipoint routing table (MRTi). Note that the link cost table, the neighbor/topology table, and the routing table are provided by the content routing protocol running in the ICN, and the multipoint routing table is built by the ICM.
The link cost table for a router i (LTi) lists the cost of the links from router i to each of its neighbors. In this disclosure, the link from router i to router k is denoted as (i,k) and the cost of the link is denoted as lki. In some embodiments, the cost of the link is assumed to be a positive number, which can be a function of administrative constraints and performance measurements made by router i for the link. An entry in the link cost table for link (i,k) includes the name of neighbor k and the cost of the link (lki).
The neighbor table (NTi) stores routing information for each router in a set Ni, which contains router i and its neighbor routers, to prefixes. In some embodiments, information stored in NTi for a router k regarding a prefix j is denoted NTjki, and consists of routing information for the nearest anchor and the root anchor of the prefix.
FIG. 2 presents a diagram illustrating an exemplary neighbor table maintained at a router, in accordance with an embodiment of the present invention. In FIG. 2, a neighbor table 200 maintained by a router i includes a number of entries indexed by the neighbor router's name and prefix. For example, entry 202 includes information for router k regarding prefix j. Entry 202 includes routing information for the nearest anchor of prefix j reported by k, which is a 3-tuple that states an anchor (ajki) storing prefix j, the distance from neighbor k to prefix j (djki), and the sequence number created by ajki for prefix j (snjki). Entry 202 also includes routing information for the root anchor of prefix j, which is a 3-tuple that identifies the root anchor of prefix j (rajki), the distance from neighbor k to the root anchor of j (rdjki), and the sequence number created by rajki for prefix j (rsnjki). The root anchor of a prefix is an anchor of the prefix with the smallest name, or the smallest lexicographic value. In this disclosure, |i| denotes the lexicographic value of a name i. Hence, at each router i and for any neighbor kεNi, |raji|≦|rajki|, and |raji|≦|ajki|, where raji is the root anchor for prefix j.
Note that, if prefix j is locally available at router i, then ajki=i and djki=0. In this case, router i is its own nearest anchor for prefix j, but may not be the root anchor for prefix j. The distance from router i to the root anchor raji is rdji=rdjsi+lsi, where s≠i is the next hop to root anchor raji selected by router i. If router i is the root anchor for prefix j, then rdji=0.
In some embodiments, information included in the neighbor table (NTi) can be derived from a topology table (TTi) when the network runs a content-routing protocol based on link-state information. The topology table (TTi) states the link-state information reported or forwarded by each neighbor for each router and each known prefix. In some embodiments, the information stored in TTi includes the links from i to each neighbor and to each locally available prefix, as well as the links to nodes or prefixes forwarded by neighbor routers.
The routing table (RTi) maintained at router i stores routing information for each prefix known at router i. In some embodiments, a routing table may include a plurality of entries that are indexed by prefix. FIG. 3 presents a diagram illustrating an exemplary routing table maintained at a router, in accordance with an embodiment of the present invention. In FIG. 3, a routing table 300 maintained by a router i includes a plurality of entries. Each entry includes the name of a prefix, the routing update information for the prefix, a set of valid next-hop neighbors to the prefix, a neighbor that offers the shortest distance to the prefix, and an anchor list that stores a tuple for each different valid anchor reported by any next-hop neighbor. For example, entry 302 includes a prefix (j), routing update information RUIji, a set of valid next-hop neighbors to prefix j (Sji), the shortest-path neighbor to prefix j (sjiεSji), and an anchor set Aji. Routing update information RUIji includes a 3-tuple that states the current distance from router i to prefix j (dji), the anchor of prefix j that has the smallest name among those that offer the shortest distance to j (aji) and the sequence number created by aji for prefix j (snji). Anchor set Aji stores a tuple ([m,sn(m)]εAji) for each different valid anchor reported by any next-hop neighbor, with m being the name of an anchor, and sn(m) the sequence number reported by anchor m.
The multipoint routing table (MRTi) includes a plurality of entries indexed by the prefix, and is updated by exchange of messages among routers. FIG. 4 presents a diagram illustrating an exemplary multipoint routing table maintained at a router, in accordance with an embodiment of the present invention. In FIG. 4, a multipoint routing table 400 maintained by a router i includes a plurality of entries. Each entry includes the name of a prefix, the multipoint update information for the prefix, and a list of neighbors that have joined the MIDST for the prefix. For example, entry 402 includes a prefix (j), multipoint update information for prefix j (MUIji), and a set of neighbors that have joined the MIDST for prefix j (MIDSTji). Multipoint update information MUIji includes a 3-tuple that states the root anchor for prefix j (raji), the distance to the root anchor (rdji), and the sequence number created by raji for prefix j (rsnji).
Each router updates its multipoint routing table based on update messages received from its neighbors and signaling messages exchanged among routers to join certain MIDTSs.
In some embodiments, an update message sent from a router i to its neighbor m can include the name of router i, a message sequence number (msni) used to identify the message, and a list of updates, one for each prefix that needs updating. An update for prefix j sent by router i is denoted as Uji, and states the name of the prefix j, the distance to j (udji), an anchor (uaji), and the sequence number created by anchor uaji for prefix j (usnji). On the other hand, the update message received by a router i from its neighbor k for prefix j can be denoted as Ujki, and states the name of the prefix j, the distance to j (udjki), an anchor (uajki), and the sequence number created by anchor uajki for prefix j (usnjki).
ICM supports routing to multicast groups by means of multi-instantiated destination spanning tree (MIDST). All the anchors of a given prefix corresponding to a multicast group are connected with one another through the MIDST for the prefix, which is rooted at the root anchor of the prefix. In some embodiments, the MIDST is established using routing updates exchanged only by routers located between the root anchor and other anchors of the same group.
The MIDST for a prefix can be established in a distributed manner. More specifically, a router that knows about multiple anchors for a prefix other than the anchor it considers to be the root anchor sends updates about the root anchor along the preferred path to each of the other anchors it knows. Routers that receive updates about the root anchor send their own updates to their preferred next hops to each of the other anchors they know. This way, distance updates about the root anchor propagate to all other anchors of the same prefix. Updates about the root anchor propagate only to those routers in preferred paths between the root anchor and other anchors. If router i changes its routing information for the root anchor of prefix j, it schedules an update about its root anchor to each neighbor that satisfies a root-anchor notification condition (RNC). The RNC states that router i sends a multipoint update (which includes a 3-tuple [raji,rdji,rsnji]) to a router kεNi−{i} only if the following two statements are true:

|ajki|>|raji||rajki|>|raji|; and  Statement (1)

∀νεNi(ajvi=i)∀νεNi−{k}(ajki≠ajvi(djki+lki<djvi+lvi[djki+lki=djvi+livi|k|<|ν|]))  Statement (2).

Note that |i| denotes the lexicographic value of a name i.

Statement (1) indicates that router k has not reported as its anchor or root anchor the same root anchor viewed by router i. Statement (2) indicates that router i forwards the update about the root anchor to router k if either i is an anchor and all its neighbors report i as their chosen anchor, or k is the lexicographically smallest next hop to an anchor that is not the root anchor.
FIG. 5 presents a diagram presenting an exemplary architecture of a router that implements ICM, in accordance with an embodiment of the present invention. In FIG. 5, ICM router 500 includes a neighbor table module 502, a routing table module 504, a multipoint routing table module 506, an update-receiving module 508, a neighbor-selection module 510, a forwarding module 512, a request-generation module 514, and a response-receiving-and-processing module 516.
Neighbor table module 502 is responsible for maintaining and updating the neighbor table. Routing table module 504 is responsible for maintaining and updating the routing table. As discussed previously, information included in the neighbor table and the routing table and the updating of these two tables are handled by the content routing protocol running in the network. The scope of this disclosure is not limited by the specific content routing protocol used. In some embodiments, a Distance-based Content Routing (DCR) protocol is used for content routing in the ICN.
Multipoint routing table module 506 is responsible for maintaining and updating the multipoint routing table. In some embodiments, a router updates entries in the multipoint routing table based on the update messages regarding anchors and/or the root anchor of prefixes and the “join” messages exchanged among routers.
Update-receiving module 508 is responsible for receiving root anchor updates. The received update can result in multipoint routing table module 506 updating the multipoint routing table, which includes information regarding the root anchor. Neighbor-selection module 510 is responsible for selecting neighbors for forwarding the root anchor update; and forwarding module 512 is responsible for forwarding the root anchor update. In some embodiments, forwarding module 512 is also responsible for forwarding locally originated updates regarding the root anchor. Neighbor-selection module 510 selects, based on information stored in the router (such as the link cost table, the neighbor table, and the multipoint routing table), one or more routers for forwarding the root anchor update of a particular prefix. In some embodiments, neighbor-selection module 510 selects a set of neighbors that meet the RNC condition, and forwarding module 512 forwards the root anchor update to the selected neighbors. Note that, compared with other approaches that flood the network with signaling packets for multicasting, in embodiments of the present invention, the updates are only exchanged among a subset of routers in the networks. In fact, the updates are only sent to routers that are anchors in the same multicast group or relay routers located between the root anchor and other anchors.
Request-generation module 514 is responsible for generating “join” requests. More specifically, when a router wants to join a multicast group, either as a source or receiver, request-generation module 514 generates a “join” request. In some embodiments, the “join” request includes the name of the multicast group and the name of the root anchor. Upon the generation of such a “join” request, neighbor-selection module 510 selects, among the router's next-hop neighbors to the root anchor of the prefix, a neighbor that has the smallest lexicographical value. In some embodiments, a router selects a neighbor as its next-hop neighbors to the root anchor based on whether the neighbor meets the Root-Anchor Ordering Condition (ROC). More specifically, the ROC states that a router i can select neighbor kεNi as its next-hop neighbor for prefix j only if the following three statements are true:

|rajki|≦|raji|rsnjki≧rsnji;  Statement (3):

∀mεNi(rdjki+lki≦rdjmi+lmi); and  Statement (4):

rsnji<rsnjki[rsnji=rsnjkirdjki<rdji].  Statement (5):

Note that statement (3) states that root anchor reported by neighbor k has the smallest name (smallest lexicographical value) among all anchors of prefix j known by router i; statement (4) states that neighbor k must offer the shortest distance to the root anchor among all neighbors; and statement (5) orders router i with its selected next hop to the root anchor based on the distance to the root anchor and the sequence number created by the root anchor. More specifically, statement (5) states that the selected next hop k must report a larger sequence number created by the root anchor than that of i, or must provide a shorter distance to the root anchor.

Upon receiving such a “join” request, a router forwards such a request to its lexicographically smallest next hop to the root anchor according to the ROC. In this way, the “join” request traverses the path toward the root anchor of the prefix, until its reaches the root anchor or a router x that is already part of the MIDST of the multicast group. In some embodiments, each router that receives and forwards a “join” request stores an entry for the request for a finite period of time. In further embodiments, the entry for the request is maintained in a data structure that is similar to a pending interest table in CCN. Such an entry indicates the neighbor from which the “join” request was received, thus enabling the response to the “join” request to traverse the reverse path of the “join” request. Response-receiving-and-processing module 516 is responsible for receiving and processing the response to the “join” request. Once such a response is processed, the router becomes part of the MIDST for the prefix. As one can see, because routers only forward the “join” request to their neighbors that meet the ROC, much fewer routers and links are used in the signaling needed to establish the MIDST of the multicast group. More specifically, other than the links that are part of the preferred paths to the root anchor, only those routers along the shortest paths between the root anchor and another anchor of the prefix may participate in the propagation of the “join” request. Such a process is more efficient than the traditional approach of building shared multicast trees or Rendezvous Point (RP) based multicast trees, in which all routers must have routes to the pre-defined root anchor.
An ICM Operation Example
FIG. 6 illustrates the propagation of root anchor updates in an exemplary ICN, in accordance with an embodiment of the present invention. In FIG. 6, a network 600 includes twenty routers, each identified by its name, such as routers a, c, r, . . . , x, and y. In the example shown in FIG. 6, four routers (routers c, d, u, and o, which are shaded in the figure) serve as anchors for a particular multicast group, mg. These routers are anchors of mg because they have attached sources or attached receivers of mg, depending on the multicasting modality of mg. Among the four anchors, anchor c has the smallest lexicographic value, and is the root anchor. Note that in FIG. 6 it is assumed that each link has a unit cost.
In FIG. 6, one or more tuples are listed in lexicographic order next to each router, with each tuple stating a distance to an anchor of mg and the identifier of that anchor. The first tuple in the list states the smallest distance to mg and the anchor with the smallest name among all anchors at that same distance. For example, three tuples are listed next to router p, stating that the distance from router p to anchor o is 1 (1 hop), the distance to anchor u is 1 (1 hop), and the distance to anchor d is 2 (2 hops). In other words, the smallest distance from router p to an anchor of the prefix is 1, and such a smallest distance is to anchor o. Note that although the distances from router p to anchors o and u are both 1, anchor o is listed as the preferred anchor because it has a smaller lexicographical value than u. There are two tuples listed next to router h, stating that the distance from router h to anchor c is 2 (2 hops), and the distance to anchor u is 2 (2 hops). Similarly, because anchor c has a smaller lexicographic value than router u, even though the distance is the same, anchor c is listed in the first tuple, indicating that anchor c is the preferred anchor to router h.
In FIG. 6, the arrows indicate the direction of interfaces between two routers according to entries in their information forwarding base (FIB). For example, an arrow pointing from a router p to a router u indicates that in the FIB maintained by router p, router u is the outgoing face for Interest with the name prefix mg. On the other hand, in the FIB maintained by router u, router p is the incoming face for Interest with the name prefix mg. For illustration purposes, FIG. 6 is color coded with a particular anchor and the preferred next hops to that particular anchor having the same color. In other words, an arrow with the color of an anchor indicates that the corresponding link is the preferred next hop (i.e., the next hop that is lexicographically smallest) for the anchor. For example, link (w,b) is blue, indicating that router b is the preferred next hop to anchor c (also blue) for router w; and link (m,r) is green, indicating that router r is the preferred next hop to anchor d (also green) for router m. Arrows that are black indicate the non-preferred next hops to anchors. For example, link (m,i) is black, which can indicate that router i is the next hop to anchor o, but because o has a larger lexicographic value, this link is not the preferred link.
In FIG. 6, the root anchor (router c) is blue, and dashed arrowheads and blue arrowheads indicate those links over which updates about c being the root anchor of mg propagate in the direction away from c. Note that the propagations of the updates need to abide to the RNC. For example, router b propagates a root anchor update to e because that neighbor is the best choice for b toward anchor o, and router g propagates a root anchor update to u because it is the best choice for anchor u. Router o propagates an update stating that c is the root anchor of mg to all its neighbors, because it is an anchor with all its neighbors reporting o as their preferred anchor. Note that these are specified by statement (2) of RNC. As we can see, because of RNC, updates about the root anchor of a prefix can reach all the other anchors of the prefix. However, as we can see from FIG. 6, some routers (such as routers r and m, as they are not connected to blue or dashed arrows) need not participate in the propagation of the updates about root anchor c, and may not receive updates about mg with c as an anchor. This is advantageous compared with the traditional approaches where updates have to flood the entire network.
FIG. 7 illustrates the establishment of a multi-instantiated destination spanning tree (MIDST) in an exemplary ICN, in accordance with an embodiment of the present invention. The network shown in FIG. 7 is the same network 600 shown in FIG. 6, and includes twenty routers, each identified by its name, such as routers a, c, r, . . . , and y. As discussed previously, when an anchor, such as anchor u, wants to join the multicast group mg, whose root anchor is c, it needs to send a “join” request to its lexicographically smallest next hop to c. As shown in FIG. 7, router u can send the “join” request to router g, which forwards the request to root anchor c via routers e and b. Similarly, other anchors (d and o) also send their “join” request toward root anchor c in order to join the MIDST of mg. For example, router o may send a “join” request toward c, and when such a request reaches u, which is already in the MIDST, a response will be sent back to o, traversing the reverse path of the “join” request. The routers on the path become part of the MIDST. In FIG. 7, the links that constitute the MIDST for the prefix mg are indicated by solid orange lines, and multipoint data traffic for the prefix mg can flow in both directions of those links.
Depending on the multicast modality (SIM or RIM) adopted for a multicast group, the MIDST may be used differently in the implementation of ICM.
In source-initiated multicast (SIM), the sources of a multicast group advertise their presence in the network and receivers attempt to join the group of sources. The anchors of a multicast group are the sources of the group. Each source of a multicast group advertises its presence to an attached router using a Multicast Group Management Protocol (MGMP), which can be implemented using Interest-based signaling or push-based signaling, depending on the ICN architecture in which it is used. The MGMP message sent from a multicast source to its attached router states the name of the multicast group, the request to carry out SIM with the requester acting as a source of the group, and optional attributes. In some embodiments, the attributes may include a delete timer informing the router the length of time that the source is to remain active. The name of the multicast group indicates the fact that the multicast group operates in the SIM modality. Given that the sources of a multicast group make attached routers advertise the presence of the group by name, the routers attached to multicast sources are the anchors of the multicast group.
Routers use a content routing protocol, such as DCR, to provide routing to the nearest sources of the multicast group, and ICM builds and maintains the MIDST of the multicast group. The resulting MIDST connects all sources of the multicast group. To receive content from all the multicast sources of the group, a receiver sends simply an Interest to its attached router stating the name of the multicast group. In turn, the router sends an Interest toward the nearest known anchor of the multicast group. Content from the multicast group is delivered over the reverse path traversed by the Interest. A router attached to a source of the multicast group forwards content to each neighbor that submitted an Interest for the group, as well as each neighbor that is a member of the MIDST created for the multicast group. As a result, receivers are able to obtain content from all sources of a multicast group for which they stated an Interest. The SIM modality of ICM can be used to implement a simple extension of the single-source multicast support available in NDN and CCN. Instead of each receiver having to send Interests for each multicast source, a receiver simply sends Interests requesting content from all the sources of the multicast group, which is denoted by name in such Interests. Note that requiring receivers to submit Interest for multicast content is not as efficient as a push-based approach. ICM could be used more efficiently in CCN and NDN by defining long-term Interests that elicit multiple data objects.
In receiver-initiated multicast (RIM), the receivers of a multicast group advertise their presence in the network and the source sends content toward the nearest receivers of the group. The anchors of a multicast group in RIM are the receivers of the group. Each receiver of a multicast group advertises its presence to an attached router using a Multicast Group Management Protocol (MGMP). The MGMP message sent from a multicast receiver to its attached router states the name of the multicast group, the request to carry out the RIM modality with the requester acting as a receiver of the group, and optional attributes. In some embodiments, the attributes may include a delete timer informing the router the length of time that the source is to remain active. The name of the multicast group indicates the fact that the multicast group operates using the RIM modality. The routers attached to multicast receivers are the anchors of the multicast group. Routers use a content routing protocol, such as DCR, to provide routing to the nearest sources of the multicast group, and ICM builds and maintains the MIDST of the multicast group using the nearest-instance routing information provided by the routing protocol. The resulting MIDST connects all receivers of the multicast group. To send content to the multicast receivers of the group, a source simply sends the content to its attached router, who in turn sends the content to the nearest anchor of the group based on the nearest-instance routing information. The first anchor of the group or router in the MIDST of the multicast group that receives the content broadcasts it over the MIDST. As a result, all receivers of the multicast group obtain the content from any one source.
Computer and Communication System
FIG. 8 illustrates an exemplary system for information-centric multicasting, in accordance with an embodiment of the present invention. A system 800 for information-centric multicasting comprises a processor 810, a memory 820, and a storage 830. Storage 830 typically stores instructions that can be loaded into memory 820 and executed by processor 810 to perform the methods mentioned above. In one embodiment, the instructions in storage 830 can implement a neighbor table module 832, a routing table module 834, a multipoint routing table module 836, an update-receiving module 838, a neighbor-selection module 840, a forwarding module 842, a request-generation module 844, and a response-receiving-and-processing module 846, all of which can be in communication with each other through various means.
In some embodiments, modules 832-846 can be partially or entirely implemented in hardware and can be part of processor 810. Further, in some embodiments, the system may not include a separate processor and memory. Instead, in addition to performing their specific tasks, modules 832-846, either separately or in concert, may be part of general- or special-purpose computation engines.
Storage 830 stores programs to be executed by processor 810. Specifically, storage 830 stores a program that implements a system (application) for information-centric multicasting. During operation, the application program can be loaded from storage 830 into memory 820 and executed by processor 810. As a result, system 800 can perform the functions described above. System 800 can be coupled to an optional display 880 (which can be a touch screen display), keyboard 860, and pointing device 870, and can also be coupled via one or more network interfaces to network 882.
The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing computer-readable media now known or later developed.
The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium.
Furthermore, methods and processes described herein can be included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them.
The above description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

## Claims

1. A computer-executable method for updating routing information associated with a multicast group in an information-centric network (ICN), the method comprising:
receiving, by a first node in the ICN, an update message for the multicast group from a neighbor node:
wherein the multicast group is associated with a prefix of a Hierarchically Structured Variable Length Identifier (HSVLI), which comprises a variable number of name components ordered from a most general name component to a most specific name component;
wherein the multicast group includes a number of anchor nodes associated with the HSVLI prefix, including a root anchor node having a name with a smallest lexicographical value among the anchor nodes; and
wherein the update message specifies the HSVLI prefix;

selecting, based on topology information stored on the first node, from neighbors of the first node one or more next-hop neighbors that meet a notification condition, wherein the one or more neighbors meeting the notification condition provide a minimal sum of distance and link cost to the first node; and
forwarding the update message to the one or more next-hop neighbors.

2. The method of claim 1, wherein a next-hop neighbor that meets the notification condition is an anchor node of the multicast group, and all neighbors of the anchor node report the anchor node as a preferred anchor node.

3. The method of claim 1, wherein the one or more neighbors meeting, the notification condition provide a smallest lexicographical value among neighbors possessing the minimal sum of distance and link cost to the first node.

4. The method of claim 1, further comprising:
generating, by a second node intended to join the multicast group, a join request;
selecting a neighbor of the second node as a next-hop node to the root anchor of the multicast group based on an ordering condition, wherein the next-hop node of the second node to the root anchor node provides a shortest path to the root anchor node; and
forwarding the join request to the selected neighbor of the second node.

5. The method of claim 4, further comprising:
receiving a response to the join request sent from the root anchor node or an anchor node of the multicast group;
wherein the response traverses a reverse path of the join request to the second node.

6. The method of claim 4, further comprising:
establishing a multi-instantiated destination spanning tree (MIDST) associated with the HSVLI prefix, wherein the MIDST connects the root anchor node, the anchor nodes, and one or more routers that receive the join request; and
updating, in a multipoint routing table entry associated with the HSVLI prefix, a list of neighbors that have joined the MIDST associated with the HSVLI prefix.

7. The method of claim 1, wherein the root anchor node and the anchor nodes are one of:
sources of the multicast group; and
receivers of the multicast group.

8. A non-transitory computer-readable storage medium storing instructions that when executed by a computing device cause the computing device to perform a method for updating routing information associated with a multicast group in an information-centric network (ICN), the method comprising:
receiving, by a first node in the ICN, an update message for the multicast group from a neighbor node:
wherein the multicast group is associated with a prefix of a Hierarchically Structured Variable Length Identifier (HSVLI), which comprises a variable number of name components ordered from a most general name component to a most specific name component;
wherein the multicast group includes a number of anchor nodes associated with the HSVLI prefix, including a root anchor node having a name with a smallest lexicographical value among the anchor nodes; and
wherein the update message specifies the HSVLI prefix;
selecting, based on topology information stored on the first node, from neighbors of the first node one or more next-hop neighbors that meet a notification condition, wherein the one or more neighbors meeting the notification condition provide a minimal sum of distance and link cost to the first node; and
forwarding the update message to the one or more next-hop neighbors.

9. The computer-readable storage medium of claim 8, wherein a next-hop neighbor that meets the notification condition is an anchor node of the multicast group, and all neighbors of the anchor node report the anchor node as a preferred anchor node.

10. The computer-readable storage medium of claim 8, wherein the one or more neighbors meeting the notification condition provide a smallest lexicographical value among neighbors possessing the minimal sum of distance and link cost to the first node.

11. The computer-readable storage medium of claim 8, wherein the method further comprises:
generating, by a second node intended to join the multicast group, a join request;
selecting a neighbor of the second node as a next-hop node to the root anchor of the multicast group based on an ordering condition, wherein the next-hop node of the second node to the root anchor node provides a shortest path to the root anchor node; and
forwarding the join request to the selected neighbor of the second node.

12. The computer-readable storage medium of claim 11, wherein the method further comprises:
receiving a response to the join request sent from the root anchor node or an anchor node of the multicast group;
wherein the response traverses a reverse path of the join request to the second node.

13. The computer-readable storage medium of claim 11, wherein the method further comprises establishing a multi-instantiated destination spanning tree (MIDST) associated with the HSVLI prefix, wherein the MIDST connects the root anchor node, the anchor nodes, and one or more routers that receive the join request; and
updating, in a multipoint routing table entry associated with the HSVLI prefix, a list of neighbors that have joined the MIDST associated with the HSVLI prefix.

14. The computer-readable storage medium of claim 8, wherein the root anchor node and the anchor nodes are one of:
sources of the multicast group; and
receivers of the multicast group.

15. A computer system for updating routing information associated with a multicast group in an information-centric network (ICN), the system comprising:
a processor; and
a storage device coupled to the processor and storing instructions which when executed by the processor cause the processor to perform a method, the method comprising:
receiving, by a first node in the ICN, an update message for the multicast group from a neighbor node:
wherein the multicast group is associated with a prefix of a Hierarchically Structured Variable Length Identifier (HSVLI), which comprises a variable number of name components ordered from a most general name component to a most specific name component;
wherein the multicast group includes a number of anchor nodes associated with the HSVLI prefix, including a root anchor node having a name with a smallest lexicographical value among the anchor nodes: and
wherein the update message specifies the HSVLI prefix;
selecting, based on topology information stored on the first node, from neighbors of the first node one or more next-hop neighbors that meet a notification condition,

wherein the one or more neighbors meeting the notification condition provide a minimal sum of distance and link cost to the first node; and
forwarding the update message to the one or more next-hop neighbors.

16. The system of claim 15, wherein a next-hop neighbor that meets the notification condition is an anchor node of the multicast group, and all neighbors of the anchor node report the anchor node as a preferred anchor node.

17. The system of claim 15, wherein the one or more neighbors meeting, the notification condition provide a smallest lexicographical value among neighbors possessing the minimal sum of distance and link cost to the first node.

18. The system of claim 15, wherein the method further comprises:
generating, by a second node intended to join the multicast group, a join request;
selecting a neighbor of the second node as a next-hop node to the root anchor of the multicast group based on an ordering condition, wherein the next-hop node of the second node to the root anchor node provides a shortest path to the root anchor node; and
forwarding the join request to the selected neighbor of the second node.

19. The system of claim 18, wherein the method further comprises:
receiving a response to the join request sent from the root anchor node or an anchor node of the multicast group;
wherein the response traverses a reverse path of the join request to the second node.

20. The system of claim 18, wherein the method further comprises establishing a multi-instantiated destination spanning tree (MIDST) associated with the HSVLI prefix, wherein the MIDST connects the root anchor node, the anchor nodes, and one or more routers that receive the join request; and
updating, in a multipoint routing table entry associated with the HSVLI prefix, a list of neighbors that have joined the MIDST associated with the HSVLI prefix.

21. The system of claim 15, wherein the method further comprises establishing a MIDST that includes the root anchor node, the anchor nodes, and one or more routers that receive the join request.