Patent Publication Number: US-9407549-B2

Title: System and method for hash-based forwarding of packets with hierarchically structured variable-length identifiers

Description:
RELATED APPLICATION 
     The subject matter of this application is related to the subject matter in the following applications:
         U.S. patent application Ser. No. 12/565,005, now U.S. patent Ser. No. 12/565,005, entitled “SYSTEM FOR FORWARDING A PACKET WITH A HIERARCHICALLY STRUCTURED VARIABLE-LENGTH IDENTIFIER,” by inventors Van L. Jacobson and James D. Thornton, filed 23 Sep. 2009;   U.S. patent application Ser. No. 12/638,478, now U.S. patent Ser. No. 12/638,478, entitled “SYSTEM FOR FORWARDING PACKETS WITH HIERARCHICALLY STRUCTURED VARIABLE-LENGTH IDENTIFIERS USING AN EXACT-MATCH LOOKUP ENGINE,” by inventors Van L. Jacobson and James D. Thornton, filed 15 Dec. 2009; and   U.S. patent application Ser. No. 12/640,968, now Ser. No. 12/640,968, entitled “METHOD AND SYSTEM FOR FACILITATING FORWARDING A PACKET IN A CONTENT-CENTRIC NETWORK,” by inventors Van L. Jacobson and James D. Thornton, filed 17 Dec. 2009;
 
the disclosures of which are incorporated by reference in their entirety.
       

     BACKGROUND 
     1. Field 
     The present disclosure relates generally to facilitating communication over a data network. More specifically, the present disclosure relates to a system and method for facilitating hash-based forwarding of packets with hierarchically structured variable-length identifiers. 
     2. 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 location-based 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) closely associated with a physical object or location. This restrictive addressing scheme is becoming progressively inadequate for meeting the ever-changing network demands. 
     Recently, content centric network (CCN) architectures have been proposed in the industry. CCN 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 CCN. 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 meta-data describing or augmenting the piece of content with information such as authentication data, creation date, content owner, etc. 
     In CCN, content objects and interests are identified by their names, which is typically a hierarchically structured variable-length identifier (HSVLI). Because these names have variable lengths, it is difficult to forward packets with HSVLIs at line speed with high throughput. 
     SUMMARY 
     One embodiment of the present invention provides a system for forwarding packets with hierarchically structured variable-length identifiers (HSVLIs). During operation, the system receiving a packet with an HSVLI. The packet includes a first value and a second value. The first value uniquely represents an interest corresponding to the HSVLI. The second value is derived based on at least a subset of the HSVLI components. The system then makes a forwarding decision for the packet based on the first hash value and second hash value. 
     In a variation on this embodiment, the packet is an interest in a piece of content corresponding to the HSVLI. The first value is a hash derived based on the entire HSVLI and optionally additional information in the packet. The second value is a hash derived on the subset of the HSVLI components. The system updates the second value based on a longer prefix match for the HSVLI. 
     In a variation on this embodiment, the packet contains a content object in response to an interest in the content. 
     In a variation on this embodiment, the system maintains a pending interest table, wherein a respective entry in the pending interest table indicates a pending interest associated with the first value and optionally the second value. 
     In a variation on this embodiment, the system maintains a forwarding information base, wherein a respective entry in the forwarding information base indicates forwarding information for a packet that contains a particular second value. 
     In a further variation, the entry in the forwarding information base further indicates that a longer prefix match exists for the HSVLI. 
     In a variation on this embodiment, the packet is an interest in the piece of content. In addition, the system searches a local content store for content corresponding to the interest based on the first value and optionally the second value. 
     In a variation on this embodiment, the packet contains a content object in response to an interest in the content. The system further updates the second hash value for the packet based on a second hash value of corresponding interest packet that is previously received. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates an exemplary network where packets have hierarchically structured variable-length identifiers (HSVLIs) in accordance with an embodiment. 
         FIG. 2  illustrates an exemplary hash-forwarding header for a CCN packet, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates the process of forwarding an Interest and a corresponding Content Object, in accordance with an embodiment of the present invention. 
         FIG. 4  presents a flow chart illustrating the process of receiving and forwarding an Interest, in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates an exemplary forwarding information base (FIB), in accordance with an embodiment of the present invention. 
         FIG. 6  illustrates an exemplary pending Interest table (PIT), in accordance with an embodiment of the present invention. 
         FIG. 7  presents a flow chart illustrating an exemplary process of receiving and forwarding a Content Object, in accordance with an embodiment of the present invention. 
         FIG. 8  illustrates an exemplary system for forwarding packets with HSVLIs, in accordance with an embodiment. 
     
    
    
     In the figures, like reference numerals refer to the same figure elements. 
     DETAILED DESCRIPTION 
     Overview 
     Embodiments of the present invention provide a system and method for using fixed size, flat byte strings to forward CCN packets with Hierarchically Structured Variable Length Identifiers (HSVLIs), thus simplifying the work done at a packet forwarder. A first byte string, referred to as the Similarity Hash (SH), represents the query in an Interest. The Similarity Hash remains invariant as a packet moves through the network. A second byte string, called the Forwarding Hash (FH), represents the longest matching prefix in the routing tables in various forwarding devices (e.g., routers, switches, etc.) along a data path that matches the Interest name. The Forwarding Hash may change hop-by-hop if the underlying routing tables change, such that it always represents the best match at the previous hop. A Content Object, sent in response to an SH/FH Interest, carries the SH/FH header along the return path so the Content Object may be forwarded along the proper path. 
     In general, 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. A Content Object contains, among other information, the same HSVLI, the object&#39;s payload, and cryptographic information used to bind the HSVLI to the payload. 
     The terms used in the present disclosure are generally defined as follows (but their interpretation are 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 represented in a format such as ccnx:/path/part. There is not a host or query string. As mentioned above, HSVLIs refer to content, and it is desirable that they be able to represent organizational structures for content and at least partially meaningful to humans. An individual component of a 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.   “Interest”: A request for a Content Object that specifies a HSVLI name prefix and other optional selectors to choose among multiple objects with the same name prefix. Any Content Object whose name matches the Interest name prefix and selectors satisfies the Interest.   “Content Object”: A data object sent in response to an Interest. It has a HSVLI name and a Contents payload that is 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 transfered on the wire, but is computed at each hop, if needed.   “Similarity Hash”: In an Interest, the Name and several fields called Selectors limit the possible content objects that match the interest. Taken together, they uniquely identify the query in the Interest. The Similarity Hash is a hash over those fields. Two interests with the same SH are considered identical queries.   “Flatname”: a CCN name organized as an ordered set of a varint (see below) length and name component bytes.   “Varint”: A variable sized unsigned integer encoded, for example, as a series of 7-bit values in big-endian ordered octets. Each high-order octet bit is a continuation bit; if it is set, then the next octet is part of the value.       

     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&#39;s name. An interest in a piece of content can be a query for the content according to the content&#39;s name or identifier. The content, if available in the network, is routed back to it from any node that stores the content. 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 path which the interest traversed. 
       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 interests or originate content. 
     In accordance with an embodiment of the present invention, a consumer can generate an Interest in a piece of content and then send 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  105  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 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 
     Hash Forwarding 
     Hash forwarding relies on each node using the same hash function to encode name prefixes and compute similarity hashes. The hash function and its usage for Hash Forwarding is described below. 
     In general, a CCN packet, either for an interest or content object, has a header that includes a Similarity Hash (SH) and a Forwarding Hash (FH). SH is used to uniquely identify a piece of content, and can be a hash of the name and one or more fields in the packet. In one embodiment, SH is only computed by the source node that initiates an Interest, and optionally verified by an authoritative source node generating content or responding from a long-term repository. Any two Interests containing the same SH are considered to contain a request for the same piece of content. Any Content Object packet that contains the same SH is considered to be a correct response to the corresponding Interest. In essence, SH can be used in place of the name for purposes of identifying a piece of content. 
     FH is computed based on one or more components of an Interest packet&#39;s name. In general, the source node of an Interest packet may compute FH based on the highest-level hierarchy of the name components (wherein the highest hierarchy is “/”). As the Interest packet travels through the network at each forwarder, the FH may or may not be updated based on the longest match conducted at each forwarder. Every time the FH is updated, it is updated to a hash that corresponds to a more specific subset of the name components. For example, for an Interest packet with a name “/apple/pie/is/good,” at the source node a packet&#39;s FH might be H{/}. As the packet is forwarded through the network, this FH can be updated to H{/apple/pie/is} and later to H{/apple/pie/is/good}. In general, the FH of a packet could become more or less specific with respect to the name components (which means the match to the name becomes “longer” or “shorter”) along the data path toward the destination. 
     The high-level of CCN hash forwarding operates as follows. A node issues an Interest for a Content Object and receives back at most one Content Object per Interest it sends. The Content Object&#39;s name is expected to be equal to or at least match a suffix of the Interest name, and to satisfy the various selectors in the Interest. In embodiments of the present invention, the system speeds up this processing by pre-computing the SH and longest-matching prefix (LMP) FH. The assumption is that the LMP FH does not change frequently in-route, and that intermediate nodes do not need to do much expensive longest match for CCN flatnames. In particular, a forwarder does not necessarily evaluate the name or selectors when matching content in its Content Store (which serves as a cache for previously seen Content Objects). It may use exact match on the SH. 
     A forwarder typically maintains several data structures: The Pending Interest Table (PIT) tracks outstanding Interests the forwarder has seen, for which the forwarder is awaiting a response. It also aggregates similar Interests (Interest with the same Similarity Hash), so one Content Object may be replicated and forwarded to multiple reverse paths corresponding to multiple pending Interests. The PIT tracks the interfaces out of which an Interest has been sent and ensures that similar Interests are not sent multiple times out the same interfaces. The PIT also ensures that similar Interests can flow in all directions. A forwarder, for example, with three interfaces  1 ,  2  and  3 , may forward an interest received from interface  1  toward interfaces  2  and  3 . At a later time, it receives a similar Interest from Interface  2 . It may forward that Interest out of interface  1 , but not  3 . 
     The Content Store (CS) is an optional component. It stores recently seen or high-value Content Objects so later requests for the same object can be answered without forwarding an Interest. Cache policy and retention policy can be applied. 
     The Forwarding Information Base (FIB) contains information indicating the Interest forwarding routes. Typically, a routing protocol is used to populate the FIB. In one embodiment, the entries in the FIB are indexed based on the Forwarding Hashes. 
     In general, a forwarder matches both the SH and FH of an Interest on the return path of a Content Object. This is because a malicious user could put in an SH for /popular/content and an FH for a /colluding/site, for example. The content object form /colluding/site would have malicious content, but an SH for /popular/content would be benign. If forwarders do not validate that the Content Object matches the full pending Interest with both SH and FH, and only reverse-path forwards with the SH, the malicious content could pollute the network. 
     To summarize the behavior of forwarding, an Interest is forwarded based on its FH. If an intermediate node has a more specific route (i.e., a forwarding entry that matches a longer portion of the name), it may update the FH to the more specific hash. When a Content Object is returned, an intermediate node will re-swap the FH label. When an intermediate node receives a Content Object, it verifies that it came from the expected direction, based on the PIT entry and SH/FH headers. An exception to this is if an Interest was routed along the default route (an empty FH), then the FH header in the Content Object is not swapped. 
     A PIT entry stores the SH, which is invariant in forwarding, the ingress FH, and the egress FH. The egress FH matches a Content Object&#39;s FH when it is received, and the ingress FH is label swapped to the Content Object when it is reverse-path forwarded toward the owner of the Interest. It is possible that the PIT stores multiple ingress FH&#39;s. 
     During operation, when a node creates an Interest, the node encapsulates the Interest in a header. It computes the Similarity Hash and places it in the header&#39;s SH field. If the node has knowledge of the proper Forwarding Hash, it places the FH in the FH field. The node then sends the Interest packet to the next-hop forwarder. 
     A node may obtain the FH in several ways: hash the first name component; use a directory service; use the FH returned in a Content Object from a previous Interest for the same prefix; or encode the FH in a specific link format. 
     When a forwarder receives an Interest on an ingress interface, it performs the following actions: The forwarder looks up the SH/FH in the PIT. If no entry exists, it creates a PIT entry for the Interest, then proceeds to check the Content Store. To create a PIT entry, the forwarder records the SH and FH of the Interest and notes the ingress port on which the Interest is received. If the remaining time of the PIT entry is less than the Interest&#39;s requested holdtime, the forwarder can extend the PIT entry&#39;s remaining time. Note that the holdtime is a suggested maximum time to hold the Interest in a PIT. The forwarder then proceeds to forward the Interest. 
     If a forwarder implements a Content Store, it can lookup the FH in the FIB, and determine if there is a more specific route FH′ (which is an FH corresponding to a longer, or more specific, portion of the name). If not, set FH′=FH. The forwarder then matches the SH and FH′ in the Content Store. If there is an exact match, the forwarder returns the Content Object and consumes the PIT entry. The returned object carries SH/FH, unless FH was the default route, in which case it carries SH/FH′. If there is no exact match in the Content Store, the forwarder forwards the Interest. 
     To forward the Interest, the forward first looks up the FH in the FIB and finds the longest matching prefix in the FIB, based on the name of the Interest, then forwards the Interest out those ports. The forwarder is precluded from forwarding the Interest on the port from which it is received. Call the longest matching FIB forwarding hash FH′ and the set of egress interfaces E. As an example, if the FIB is a hash table, the forwarder looks up the FH as the key. If the entry exists and it has no children (meaning that there does not exist a longer match with the Interest&#39;s name), the forwarder uses that FIB entry. If the entry has one or more children, the forwarder examines the children to determine if a longer match is possible. The forwarder then removes the Interest&#39;s ingress interface from E. The forwarder further looks up the SH/FH′ in the PIT. If the Interest&#39;s hop limit (as decremented above) is greater than the PIT entries “maximum hop limit”, the forwarder sets the PIT entry&#39;s maximum hop limit to the Interest&#39;s hop limit, and internally marks the Interest as “hop limit extended.” If the Interest is not marked as “hop limit extended,” the forwarder removes any egress interfaces already used from E. In addition, the forwarder links SH/FH′ to SH/FH, if they are different. This may be a one to many mapping relationship. If E is not empty, the forwarder updates the FH in the interest with the longest matching FIB hash, and then forwards the Interest. 
     If an end-system content producer receives an Interest, it may create a Content Object that satisfies the body of the Interest and return it along the reverse path. The returned object carries the SH/FH received in the Interest. An end system may verify that the SH is properly calculated to match the body of the Interest. 
     An intermediate forwarder receiving a Content Object first verifies whether the SH and FH of the received Content Object are in the PIT. If they are not, the forwarder drops the Content Object. The forwarder then verifies that the Content Object arrived from a port over which a corresponding Interest was previously forwarded, or over which the corresponding Interest could have been forwarded. If this condition is not met, the forwarder drops the Content Object. 
     If the forwarder implements a Content Store, the forwarder adds the object to the store if the object&#39;s holdtime permits it. Then the forwarder forwards the object along the reverse path, label swapping the object&#39;s FH to the reverse path&#39;s FH, except if the reverse path FH was the default route (empty) in which case the forwarder does not change the FH. This is done by following the links from SH/FH′ to SH/FH, if any exists. Subsequently, the forwarder consumes the PIT entries satisfied by the Content Object. 
     An end system receiving a Content Object should verify that the Content Object actually satisfies the original Interest. It should also verify the integrity of the Content Object&#39;s hash and signature. 
       FIG. 2  illustrates an exemplary hash-forwarding header for a CCN packet, in accordance with an embodiment of the present invention. In this example, a CCN packet  200  includes a payload portion  208  and a hash forwarding header, which in turn can include a holdtime field  202 , an FH field  204 , an SH field  206 . Payload portion  208  may include the full CCN content name (i.e., HSVLI), and additional fields associated with an Interest or Content Object. 
     Holdtime field  202  indicates the holdtime which is a suggested maximum time to hold the message at a forwarder. For an Interest, the holdtime is the desired time to keep the Interest in the PIT until a response comes. For a Content Object, the holdtime is the maximum time to keep the Content Object in the fast response cache. 
     In one embodiment, the Similarity Hash is only computed by the source node, and optionally verified by an authoritative source node generating content or responding from a long-term repository. The Similarity Hash can use the SHA-256 hashing algorithm. 
     The Forwarding Hashes can be computed in a similar way. The Forwarding Hash is used and possibly computed by forwarding nodes based on entries in their FIB table. Speed of computation is important, and collision resistance only needs to be good enough to distinguish between allowed routing names. In one embodiment, the Forwarding Hash uses FNV-1a 128-bit [FNV] with the standard FNV_offset and FNV_prime: 
     
       
         
           
             
               
                 
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     To compute a Forwarding Hash over a CCN name, the system can run the FNV-1a 128-bit over each name component using the flatname format, in cumulative order, to the desired number of components. 
       FIG. 3  illustrates the process of forwarding an Interest and a corresponding Content Object, in accordance with an embodiment of the present invention. In this example, a client end system  300  is coupled to a network  300  and wishes to obtain content named /apple/pie/is/good from a content server  314 . A number of forwarding nodes, such as forwarders  302 ,  304 ,  306 ,  308 ,  310 , and  312 , couple end system  300  with content server  314 . In one embodiment, these forwarders can be IP routers. In this example, forwarders  302  and  312  can be enterprise gateway routers for the respective enterprise networks associated with end system  300  and content server  314 . Forwarders  304  and  310  can be edge routers providing access into core network  300  to the enterprise networks. 
     During operation, end system  300  initiates an Interest for /apple/pie/is/good. Assuming that end system  300  has no knowledge about how to forward the Interest packet, end system  300  forwards the Interest to the default gateway router  302 , setting the Interest&#39;s FH to H{/}, that is, the FH is computed based on the highest hierarchy “/” in the HSVLI. Gateway router  302  also has no specific routing information on how to forward the Interest for /apple/pie/is/good, so it forwards it to edge router  304  with the same FH. Assuming that edge router  304  has routing information for /apple/pie/is, edge router then replaces the Interest&#39;s FH with H{/apple/pie/is}. Subsequently, core routers  306  and  308  can forward the Interest based on this updated FH through core network  300  without having to parse the full HSVLI, using an exact match of the FH in their respective FIB. When the Interest reaches edge router  310 , edge router  310  forwards the Interest, based on the same FH=H{/apple/pie/is/good}, to gateway router  312 , which is within the enterprise network where content server  314  resides. Since gateway router  312  has the routing information for the full HSVLI /apple/pie/is/good, gateway router  312  replaces the FH with H{/apple/pie/is/good}, and forwards the Interest to content server  314 . 
     When content server  314  returns a Content Object, it sets the Content Object&#39;s SH to be the same SH as the Interest, and the FH to be H{/apple/pie/is/good}. Subsequently, the Content Object is reverse-path forwarded back to end system  300 . At each hop, the FH of the Content Object is updated so that it matches the FH of the Interest that was previously received on the same link where the Content Object is to be forwarded. For example, at router  312 , the Content Object&#39;s FH is replaced with H{/apple/pie/is}, and at edge router  304  the FH is again replaced with H{/}. This reverse-path forwarding mechanism ensures that the Content Object travels along the same data path on which the Interest has traveled, and hence can prevent any malicious entity from tampering with or spoofing the returned Content Object. 
       FIG. 4  presents a flow chart illustrating the process of receiving and forwarding an Interest, in accordance with an embodiment of the present invention. During operation, the system receives an Interest (operation  400 ). The system then determines whether the SH of the Interest is in the PIT (operation  402 ). If so, the system adds the ingress port on which the Interest is received to the corresponding PIT entry (operation  404 ). Otherwise, the system further determines whether the content corresponding to the SH is in the local Content Store (operation  406 ). If so, the system returns the matching Content Object (operation  408 ). If not, the system then adds a corresponding entry to the PIT (operation  407 ). 
     Next, the system determines whether the Interest&#39;s FH is in the FIB (operation  410 ). If the FH is not in the FIB, the system drops the Interest packet (operation  412 ). If the FH is in the FIB, the system further determines whether the corresponding FIB entry has a child, which means that the FIB contains a longer prefix match than the current FH indicates (operation  414 ). If the FIG entry does not have a child, the system forwards the Interest packet based on the egress port indicated by the FIB entry (operation  416 ). If the FIB entry has a child, the system then updates the Interest packet&#39;s FH based on the longer prefix match indicated by the child by rehashing the matched prefix, and forwards the packet accordingly (operation  420 ). 
       FIG. 5  illustrates an exemplary forwarding information base (FIB), in accordance with an embodiment of the present invention. In this example, a FIB  500  includes an FH column  502 , an egress port(s) column  504 , and a child indication column  506 . FH column  502  stores the FHs for which the FIB maintains the proper forwarding (i.e., egress port(s)) information. Egress port(s) column  504  indicates one or more egress ports via which an Interest packet can be forwarded. 
     Child indication column  506  stores an indicator which indicates whether the forwarder has a longer prefix match for the HSVLI associated with the current FH. In one embodiment, child indication column  506  stores a pointer to the longer prefix match, based on which the system can re-compute the FH. 
       FIG. 6  illustrates an exemplary pending Interest table (PIT), in accordance with an embodiment of the present invention. In this example, a PIT  600  includes an SH column  602 , an ingress port(s) column  604 , an egress FH column  606 , and an ingress FH column  608 . SH column  602  stores the SH for a pending Interest and is used to look up an pending Interest in PIT  600 . Ingress port(s) column  604  indicates one or more ingress ports on which an Interest is received. These ports will be used to send back the Content Objects corresponding to the pending Interest. Egress FH column  606  indicates the FH a corresponding received Content Object should have, which is used to confirm that the Content Object is received via the correct reverse path. Ingress FH column  608  indicates the new FH that should be used to update the old FH of a received Content Object. Note that the terms “egress” and “ingress” are used here with reference to the corresponding Interest, not the Content Object. 
       FIG. 7  presents a flow chart illustrating an exemplary process of receiving and forwarding a Content Object, in accordance with an embodiment of the present invention. During operation, the system first receives a Content Object packet (operation  702 ). The system then determines whether the SH of the Content Object is in the PIT (operation  704 ). If it is not in the PIT, the system discards the packet (operation  706 ). Otherwise, the system further determines whether the FH in the Content Object matches the egress FH (corresponding to egress FH column  606  in  FIG. 6 ) in the corresponding PIT entry (operation  708 ). If not, the system discards the packet (operation  706 ). Otherwise, the system updates the Content Object&#39;s FH, if the corresponding PIT entry indicates that a different FH should be used for the Content Object before it is sent out (corresponding to ingress FH column  608  in  FIG. 6 ) (operation  710 ). 
     Subsequently, the system determines whether the ingress port on which the Content Object is received matches the FIB entry corresponding to the updated FH (operation  712 ). If not, the system discards the packet (operation  706 ). Otherwise, the system forwards the Content Object packet to the ports indicated by the PIT entry (corresponding to ingress port(s) column  604  in  FIG. 6 ) (operation  714 ). 
       FIG. 8  illustrates an exemplary system for forwarding packets with HSVLIs, in accordance with an embodiment. A system  800  for forwarding packets with HSVLIs comprises a processor  810 , a memory  820 , and a storage  830 . Storage  830  typically stores instructions which can be loaded into memory  820  and executed by processor  810  to perform the hash-forwarding methods mentioned above. In one embodiment, the instructions in storage  830  can implement a hash module  832 , a PIT module  834 , and a FIB module  836 , all of which can be in communication with each other through various means. 
     In some embodiments, modules  832 ,  834 , and  836  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 ,  834 , and  836 , 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 performing hash-based forwarding of packets with HSVLIs. 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 , keyboard  860 , and pointing device  870 , and 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.