CCNx Semantics

This document describes the principles of the CCNx architecture. It describes the network protocol based on two message types: Interests and Content Objects. The description is not dependent on a specific wire format or particular encodings. This section introduces the main concepts of CCNx, which are further elaborated in the remainder of the document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

CCNx is a request and response protocol to fetch chunks of data using a name. The integrity of each chunk may be directly asserted through a digital signature or message authentication code (MAC), or, alternatively, indirectly via hash chains. Chunks may also carry weaker message integrity checks (MICs) or no integrity protection mechanism at all. Because provenance information is carried with each chunk (or larger indirectly protected block), we no longer need to rely on host identities, such as those derived from TLS certificates, to ascertain the chunk legitimacy. In this sense, data integrity is a core feature of CCNx; it does not rely on the pipe. There are several options for data confidentiality, discussed later. As a request and response protocol, CCNx may be carried over many different transports. In use today are Ethernet, TCP, UDP, 802.15.4, GTP, GRE, DTLS, TLS, and others. While the specific wire format of CCNx may vary to some extent based on transport, the core principles and behaviors of CCNx outlined in this document should remain fixed. CCNx uses hierarchical names to identify bytes of payload. The Name combines a routable prefix with an arbitrary application-dependent suffix assigned by the publisher to a piece of content. The result is a "named payload". This is different from other systems that use only self-certifying names, where the payload name is intrinsically derivable from the payload or its realization in a network object (e.g., a SHA-256 hash of the payload or network object). In human-readable form, we represent names as a "ccnx:" scheme URI , though the canonical encoding should be octet strings. In this respect, we speak of a name being made up of hierarchical path segments, which is the URI terminology. This document only defines the general properties of CCNx names. In some isolated environments, CCNx users may be able to use any name they choose and either inject that name (or prefix) into a routing protocol or use other information foraging techniques. In the Internet environment, there will be policies around the formats of names and assignments of names to publishers, though those are not specified here. The key concept of CCNx is that a subjective name is (cryptographically) bound to a fixed payload. These (publisher-generated) bindings can therefore be (cryptographically) verified. For example, a publisher could compute a cryptographic hash over the name and payload, sign the hash, and deliver the tuple {Name, Payload, Validation}. Consumers of this data can check the binding integrity by re-computing the same cryptographic hash and verifying the digital signature in Validation. Additional information would be included as needed by specific validation mechanisms. Therefore, we divide Validation in to a ValidationAlgorithm and a ValidationPayload. The ValidationAlgorithm has information about the crypto suite and parameters. In particular, the ValidationAlgorithm usually has a field called KeyId which identifies the public key used by the validation, when applicable. The ValidationPayload is the output of the validation algorithm, such as a CRC value, an HMAC output, or an RSA signature. In addition to the essential Name, Payload, and Validation sections, a CCNx user may need to include some other signaling information. This could include a hint about the type of Payload (e.g., application data, a cryptographic key, etc.) or cache control directives, etc. We will call this extra signaling information ExtraFields. A named payload is thus the tuple {{Name, ExtraFields, Payload, ValidationAlgorithm}, ValidationPayload}, where all fields in the inner tuple are covered by the validation algorithm. CCNx specifies a network protocol around Interests (request messages) and Content Objects (response messages) to move named payloads. An Interest includes the Name -- which identifies the desired response -- and two optional limiting restrictions. The first restriction on the KeyId to limit responses to those signed with a ValidationAlgorithm KeyId field equal to the restriction. The second is the ContentObjectHash restriction, which limits the response to one where the cryptographic hash of the entire named payload is equal to the restriction. The hierarchy of a CCNx Name is used for routing via the longest matching prefix in a Forwarder. The longest matching prefix is computed name segment by name segment in the hierarchical path name, where each name segment must be exactly equal to match. There is no requirement that the prefix be globally routable. Within a deployment any local routing may be used, even one that only uses a single flat (non-hierarchical) name segment. Another concept of CCNx is that there should be flow balance between Interest messages and Content Object messages. At the network level, an Interest traveling along a single path should elicit no more than one Content Object response. If some node sends the Interest along more than one path, that node should consolidate the responses such that only one Content Object flows back towards the requester. If an Interest is sent broadcast or multicast on a multiple-access media, the sender should be prepared for multiple responses unless some other media-dependent mechanism like gossip suppression or leader election is used. As an Interest travels the forward path following the Forwarding Information Base (FIB), it establishes state at each forwarder such that a Content Object response can trace its way back to the original requester(s) without the requester needing to include a routable return address. We use the notional Pending Interest Table (PIT) as a method to store state that facilitates the return of a Content Object. The PIT table is not mandated by the specification. The notional PIT table stores the last hop of an Interest plus its Name and optional restrictions. This is the data required to match a Content Object to an Interest (see ). When a Content Object arrives, it must be matched against the PIT to determine which entries it satisfies. For each such entry, at most one copy of the Content Object is sent to each listed last hop in the PIT entries. If multiple Interests with the same {Name, KeyIdRestriction, ContentObjectHashRestriction} tuple arrive at a node before a Content Object matching the first Interest comes back, they are grouped in the same PIT entry and their last hops aggregated (see ). Thus, one Content Object might satisfy multiple pending Interests in a PIT. In CCNx, higher-layer protocols often become so-called "name-based protocols" because they operate on the CCNx Name. For example, a versioning protocol might append additional name segments to convey state about the version of payload. A content discovery protocol might append certain protocol-specific name segments to a prefix to discover content under that prefix. Many such protocols may exist and apply their own rules to Names. They may be layered with each protocol encapsulating (to the left) a higher layer's Name prefix. This document also describes a control message called an InterestReturn. A network element may return an Interest message to a previous hop if there is an error processing the Interest. The returned Interest may be further processed at the previous hop or returned towards the Interest origin. When a node returns an Interest it indicates that the previous hop should not expect a response from that node for the Interest, i.e., there is no PIT entry left at the returning node for a Content Object to follow. There are multiple ways to describe larger objects in CCNx. Some options may use the namespace while others may use a structure such as a Manifest. This document does not address these options at this time. The remainder of this document describes a named payload as well as the Interest and Content Object network protocol behavior in detail.

CCNx is a request and response protocol. A request is called an Interest and a response is called a ContentObject. CCNx also uses a 1-hop control message called InterestReturn. These are, as a group, called CCNx Messages.

The CCNx message ABNF grammar is show in . The grammar does not include any encoding delimiters, such as TLVs. Specific wire encodings are given in a separate document. If a Validation section exists, the Validation Algorithm covers from the Body (BodyName or BodyOptName) through the end of the ValidationAlg section. The InterestLifetime, CacheTime, and Return Code fields exist outside of the validation envelope and may be modified. The various fields -- in alphabetical order -- are defined as: AbsTime: Absolute times are conveyed as the 64-bit UTC time in milliseconds since the epoch (standard POSIX time). CacheTime: The absolute time after which the publisher believes there is low value in caching the content object. This is a recommendation to caches (see ). ConObjField: These are optional fields that may appear in a Content Object. ConObjHash: The value of the Content Object Hash, which is the SHA256-32 over the message from the beginning of the body to the end of the message. Note that this coverage area is different from the ValidationAlg. This value SHOULD NOT be trusted across domains (see ). ExpiryTime: An absolute time after which the content object should be considered expired (see ). HopLimit: Interest messages may loop if there are loops in the forwarding plane. To eventually terminate loops, each Interest carries a HopLimit that is decremented after each hop and no longer forwarded when it reaches zero. See . InterestField: These are optional fields that may appear in an Interest message. KeyIdRestr: The KeyId Restriction. A Content Object must have a KeyId with the same value as the restriction. ObjHashRestr: The Content Object Hash Restriction. A content object must hash to the same value as the restriction using the same HashType. The ObjHashRestr MUST use SHA256-32. KeyId: An identifier for the key used in the ValidationAlg. For public key systems, this should be the SHA-256 hash of the public key. For symmetric key systems, it should be an identifer agreed upon by the parties. KeyLink: A Link (see ) that names how to retrieve the key used to verify the ValidationPayload. A message SHOULD NOT have both a KeyLink and a PublicKey. Lifetime: The approximate time during which a requester is willing to wait for a response, usually measured in seconds. It is not strongly related to the network round trip time, though it must necessarily be larger. Name: A name is made up of a non-empty first segment followed by zero or more additional segments, which may be of 0 length. Path segments are opaque octet strings, and are thus case-sensitive if encoding UTF-8. An Interest MUST have a Name. A ContentObject MAY have a Name (see ). Payload: The message's data, as defined by PayloadType. PayloadType: The format of the Payload. If missing, assume DataType. DataType means the payload is opaque application bytes. KeyType means the payload is a DER-encoded public key. LinkType means it is a Link (see ). PublicKey: Some applications may wish to embed the public key used to verify the signature within the message itself. The PublickKey is DER encoded. A message SHOULD NOT have both a KeyLink and a PublicKey. RelTime: A relative time, measured in milli-seconds. ReturnCode: States the reason an Interest message is being returned to the previous hop (see ). SigTime: The absolute time (UTC milliseconds) when the signature was generated. Hash: Hash values carried in a Message carry a HashType to identify the algorithm used to generate the hash followed by the hash value. This form is to allow hash agility. Some fields may mandate a specific HashType.

To request a piece of content for a given {Name, [KeyIdRest], [ObjHashRestr]} tuple, a consumer creates an Interest message with those values. It MAY add a validation section, typically only a CRC32C. A consumer MAY put a Payload field in an Interest to send additional data to the producer beyond what is in the Name. The Name is used for routing and may be remembered at each hop in the notional PIT table to facilitate returning a content object; Storing large amounts of state in the Name could lead to high memory requirements. Because the Payload is not considered when forwarding an Interest or matching a Content Object to an Interest, a consumer SHOULD put an Interest Payload ID (see Section ) as part of the name to allow a forwarder to match Interests to content objects and avoid aggregating Interests with different payloads. Similarly, if a consumer uses a MAC or a signature, it SHOULD also include a unique segment as part of the name to prevent the Interest from being aggregated with other Interests or satisfied by a Content Object that has no relation to the validation. The consumer SHOULD specify an InterestLifetime, which is the length of time the consumer is willing to wait for a response. The InterestLifetime is an application-scale time, not a network round trip time (see ). If not present, the InterestLifetime will use a default value (TO_INTERESTLIFETIME). The consumer SHOULD set the Interest HopLimit to a reasonable value or use the default 255. If the consumer knows the distances to the producer via routing, it SHOULD use that value. A consumer hands off the Interest to its first forwarder, which will then forward the Interest over the network to a publisher (or replica) that may satisfy it based on the name (see ). Interest messages are unreliable. A consumer SHOULD run a transport protocol that will retry the Interest if it goes unanswered, up to the InterestLifetime. No transport protocol is specified in this document. The network MAY send to the consumer an InterestReturn message that indicates the network cannot fulfill the Interest. The ReturnCode specifies the reason for the failure, such as no route or congestion. Depending on the ReturnCode, the consumer MAY retry the Interest or MAY return an error to the requesting application. If the content was found and returned by the first forwarder, the consumer will receive a ContentObject. The consumer SHOULD: Ensure the content object is properly formatted. Verify that the returned Name matches a pending request. If the request also had KeyIdRestr and ObjHashRest, it should also validate those properties. If the content object is signed, it SHOULD cryptographically verify the signature. If it does not have the corresponding key, it SHOULD fetch the key, such as from a key resolution service or via the KeyLink. If the signature has a SigTime, the consumer MAY use that in considering if the signature is valid. For example, if the consumer is asking for dynamically generated content, it should expect the SigTime to not be before the time the Interest was generated. If the content object is signed, it should assert the trustworthiness of the signing key to the namespace. Such an assertion is beyond the scope of this document, though one may use traditional PKI methods, a trusted key resolution service, or methods like [schematized trust]. It MAY cache the content object for future use, up to the ExpiryTime if present. A consumer MAY accept a content object off the wire that is expired. It may happen that a packet expires while in flight, and there is no requirement that forwarders drop expired packets in flight. The only requirement is that content stores, caches, or producers MUST NOT respond with an expired content object.

This document does not specify the method by which names populate a Forwarding Information Base (FIB) table at forwarders (see ). A publisher is either configured with one or more name prefixes under which it may create content, or it chooses its name prefixes and informs the routing layer to advertise those prefixes. When a publisher receives an Interest, it SHOULD: Verify that the Interest is part of the publishers namespace(s). If the Interest has a Validation section, verify the ValidationPayload. Usually an Interest will only have a CRC32C unless the publisher application specifically accommodates other validations. The publisher MAY choose to drop Interests that carry a Validation section if the publisher application does not expect those signatures as this could be a form of computational denial of service. If the signature requires a key that the publisher does not have, it is NOT RECOMMENDED that the publisher fetch the key over the network, unless it is part of the application's expected behavior. Retrieve or generate the requested content object and return it to the Interest's previous hop. If the requested content cannot be returned, the publisher SHOULD reply with an InterestReturn or a content object with application payload that says the content is not available; this content object should have a short ExpiryTime in the future.

A forwarder routes Interest messages based on a Forwarding Information Base (FIB), returns Content Objects that match Interests to the Interest's previous hop, and processes InterestReturn control messages. It may also keep a cache of Content Objects in the notional Content Store table. This document does not specify the internal behavior of a forwarder -- only these and other external behaviors. In this document, we will use two processing pipelines, one for Interests and one for Content Objects. Interest processing is made up of checking for duplicate Interests in the PIT (see ), checking for a cached Content Object in the Content Store (see ), and forwarding an Interest via the FIB. Content Store processing is made up of checking for matching Interests in the PIT and forwarding to those previous hops.

Interest looping is not prevented in CCNx. An Interest traversing loops is eventually discarded using the hop-limit field of the Interest, which is decremented at each hop traversed by the Interest. Every Interest MUST carry a HopLimit. When an Interest is received from another forwarder, the HopLimit MUST be positive. A forwarder MUST decement the HopLimit of an Interest by at least 1 before it is forwarded. If the HopLimit equals 0, the Interest MUST NOT be forwarded to another forwarder; it MAY be sent to a publisher application or serviced from a local Content Store.

Interest aggregation is when a forwarder receives an Interest message that could be satisfied by another Interest message already forwarded by the node so the forwarder suppresses the new Interest; it only records the additional previous hop so a Content Object sent in response to the first Interest will satisfy both Interests. CCNx uses an interest aggregation rule that assumes the InterestLifetime is akin to a subscription time and is not a network round trip time. Some previous aggregation rules assumed the lifetime was a round trip time, but this leads to problems of expiring an Interest before a response comes if the RTT is estimated too short or interfering with an ARQ scheme that wants to re-transmit an Interest but a prior interest over-estimated the RTT. A forwarder MAY implement an Interest aggregation scheme. If it does not, then it will forward all Interest messages. This does not imply that multiple, possibly identical, Content Objects will come back. A forwarder MUST still satisfy all pending Interests, so one Content Object could satisfy multiple similar interests, even if the forwarded did not suppress duplicate Interest messages. A RECOMMENDED Interest aggregation scheme is: Two Interests are considered 'similar' if they have the same Name, KeyIdRestr, and ObjHashRestr. Let the notional value InterestExpiry (a local value at the forwarder) be equal to the receive time plus the InterestLifetime (or a platform-dependent default value if not present). An Interest record (PIT entry) is considered invalid if its InterestExpiry time is in the past. The first reception of an Interest MUST be forwarded. A second or later reception of an Interest similar to a valid pending Interest from the same previous hop MUST be forwarded. We consider these a retransmission requests. A second or later reception of an Interest similar to a valid pending Interest from a new previous hop MAY be aggregated (not forwarded). Aggregating an Interest MUST extend the InterestExpiry time of the Interest record. An implementation MAY keep a single InterestExpiry time for all previous hops or MAY keep the InterestExpiry time per previous hop. In the first case, the forwarder might send a ContentObject down a path that is no longer waiting for it, in which case the previous hop (next hop of the Content Object) would drop it.

The ContentStore is a special cache that sits on the fast path of a CCNx forwarder. It is an optional component. It serves to repair lost packets and handle flash requests for popular content. It could be pre-populated or use opportunistic caching. Because the Content Store could serve to amplify an attach via cache poisoning, there are special rules about how a Content Store behaves. A forwarder MAY implement a ContentStore. If it does, the Content Store matches a Content Object to an Interest via the normal matching rules (see ). If an Interest has a KeyIdRestr, then the ContentStore MUST NOT reply unless it knows the signature on the matching ContentObject is correct. It may do this by external knowledge (i.e., in a managed system pre-populating the cachine) or by having the public key and cryptographically verifying the signature. If the public key is provided in the ContentObject itself (i.e., in the PublicKey field) or in the Interest, the ContentStore MUST verify that the public key's SHA-256 hash is equal to the KeyId and that it verifies the signature. A ContentStore SHOULD NOT fetch keys over the network. If it cannot or has not yet verified the signature, it should treat the Interest as a cache miss. If an Interest has an ObjHashRestr, then the ContentStore MUST NOT reply unless it knows the the matching ContentObject has the correct hash. If it cannot verify the hash, then it should treat the Interest as a cache miss. It must object the Cache Control directives (see ).

Perform the HopLimit check (see ). Determine if the Interest can be aggregated, as per . If it can be, aggregate and do not forward the Interest. If forwarding the Interest, check for a hit in the Content Store, as per . If a matching Content Object is found, return it to the Interest's previous hop. This injects the ContentStore as per . Lookup the Interest in the FIB. Longest prefix match (LPM) is performed name segment by name segment (not byte or bit). It SHOULD exclude the Interest's previous hop. If a match is found, forward the Interest. If no match is found or the forwarder choses to not forward due to a local condition (e.g., congestion), it SHOULD send an InterestReturn message, as per .

It is RECOMMENDED that a forwarder that receives a content object check that the ContentObject came from an expected previous hop. An expected previous hop is one pointed to by the FIB or one recorded in the PIT as having had a matching Interest sent that way. A Content Object MUST be matched to all pending Interests that satisfy the matching rules (see ). Each satisfied pending Interest MUST then be removed from the set of pending Interests. A forwarder SHOULD NOT send more then one copy of the received Content Object to the same Interest previous hop. It may happen, for example, that two Interest ask for the same Content Object in different ways (e.g., by name and by name an KeyId) and that they both come from the same previous hop. It is normal to send the same content object multiple times on the same interface, such as Ethernet, if it is going to different previous hops. A Content Object SHOULD only be put in the Content Store if it satisfied an Interest (and passed rule #1 above). This is to reduce the chances of cache poisoning.

A CCNx name is a composition of name segments. Each name segment carries a label identifying the purpose of the name segment, and a value. For example, some name segments are general names and some serve specific purposes, such as carrying version information or the sequencing of many chunks of a large object into smaller, signed Content Objects. The name segment labels specified in this document are given in the table below. Name Segment is a general name segment, typically occurring in the routable prefix and user-specified content name. Other segment types are for functional name components that imply a specific purpose. A forwarding table entry may contain name segments of any type. Routing protocol policy and local system policy may limit what goes into forwarding entries, but there is no restriction at the core level. An Interest routing protocol, for example, may only allow binary name segments. A load balancer or compute cluster may route through additional component types, depending on their services. Name Description Name Segment A generic name segment that includes arbitrary octets. Interest Payload ID An octet string that identifies the payload carried in an Interest. As an example, the Payload ID might be a hash of the Interest Payload. This provides a way to differentiate between Interests based on the Payload solely through a Name Segment without having to include all the extra bytes of the payload itself. Application Components An application-specific payload in a name segment. An application may apply its own semantics to these components. A good practice is to identify the application in a Name segment prior to the application component segments. At the lowest level, a Forwarder does not need to understand the semantics of name segments; it need only identify name segment boundaries and be able to compare two name segments (both label and value) for equality. The Forwarder matches paths segment-by-segment against its forwarding table to determine a next hop.

This section uses the CCNx URI representation of CCNx names . Name Description ccnx:/ A 0-length name, corresponds to a default route. ccnx:/NAME= A name with 1 segment of 0 length, distinct from ccnx:/. ccnx:/NAME=foo/APP:0=bar A 2-segment name, where the first segment is of type NAME and the second segment is of type APP:0.

An Interest may also have a Payload which carries state about the Interest but is not used to match a Content Object. If an Interest contains a payload, the Interest name should contain an Interest Payload ID (IPID). The IPID allows a PIT table entry to correctly multiplex Content Objects in response to a specific Interest with a specific payload ID. The IPID could be derived from a hash of the payload or could be a GUID or a nonce. An optional Metadata field defines the IPID field so other systems could verify the IPID, such as when it is derived from a hash of the payload. No system is required to verify the IPID.

CCNx supports two fields that affect cache control. These determine how a cache or Content Store handles a Content Object. They are not used in the fast path, but only to determine if a ContentObject can be injected on to the fast path in response to an Interest. The ExpiryTime is a field that exists within the signature envelope of a Validation Algorithm. It is the UTC time in milliseconds after which the ContentObject is considered expired and MUST no longer be used to respond to an Interest from a cache. Stale content MAY be flushed from the cache. The Recommended Cache Time (RCT) is a field that exists outside the signature envelope. It is the UTC time in milliseconds after which the publisher considers the Content Object to be of low value to cache. A cache SHOULD discard it after the RCT, though it MAY keep it and still respond with it. A cache is MAY discard the content object before the RCT time too; there is no contractual obligation to remember anything. This formulation allows a producer to create a Content Object with a long ExpiryTime but short RCT and keep re-publishing the same, signed, Content Object over and over again by extending the RCT. This allows a form of "phone home" where the publisher wants to periodically see that the content is being used.

CCNx allows an Interest to restrict a response to a specific hash. The hash covers the Content Object message body and the validation sections, if present. Thus, if a Content Object is signed, its hash includes that signature value. The hash does not include the fixed or hop-by-hop headers of a Content Object. Because it is part of the matching rules (see ), the hash is used at every hop. There are two options for matching the content object hash restriction in an Interest. First, a forwarder could compute for itself the hash value and compare it to the restriction. This is an expensive operation. The second option is for a border device to compute the hash once and place the value in a header (ConObjHash) that is carried through the network. The second option, of course, removes any security properties from matching the hash, so SHOULD only be used within a trusted domain. The header SHOULD be removed when crossing a trust boundary.

A Link is the tuple {Name, KeyId, ContentObjectHashRestriction}. The information in a Link comprises the fields the fields of an Interest which would retrieve the Link target. A Content Object with PayloadType = "Link" is an object whose payload is a Link. This tuple may be used as a KeyName to identify a specific object with the certificate wrapped key.

Several protocol fields use cryptographic hash functions, which must be secure against attack and collisions. Because these hash functions change over time, with better ones appearing and old ones falling victim to attacks, it is important that a CCNx protocol implementation support hash agility. In this document, we suggest certain hashes (e.g., SHA-256), but a specific implementation may use what it deems best. The normative CCNx Messages specification should be taken as the definition of acceptable hash functions and uses.

The Validator consists of a ValidationAlgorithm that specifies how to verify the message and a ValidationPayload containing the validation output, e.g., the digital signature or MAC. The ValidationAlgorithm section defines the type of algorithm to use and includes any necessary additional information. The validation is calculated from the beginning of the CCNx Message through the end of the ValidationAlgorithm section. The ValidationPayload is the integrity value bytes, such as a MAC or signature. Some Validators contain a KeyId, identifying the publisher authenticating the Content Object. If an Interest carries a KeyIdRestriction, then that KeyIdRestriction MUST exactly match the Content Object's KeyId. Validation Algorithms fall into three categories: MICs, MACs, and Signatures. Validators using MIC algorithms do not need to provide any additional information; they may be computed and verified based only on the algorithm (e.g., CRC32C). MAC validators require the use of a KeyId identifying the secret key used by the authenticator. Because MACs are usually used between two parties that have already exchanged secret keys via a key exchange protocol, the KeyId may be any agreed-upon value to identify which key is used. Signature validators use public key cryptographic algorithms such as RSA, DSA, ECDSA. The KeyId field in the ValidationAlgorithm identifies the public key used to verify the signature. A signature may optionally include a KeyLocator, as described above, to bundle a Key or Certificate or KeyName. MAC and Signature validators may also include a SignatureTime, as described above. A PublicKeyLocator KeyName points to a Content Object with a DER-encoded X509 certificate in the payload. In this case, the target KeyId must equal the first object's KeyId. The target KeyLocator must include the public key corresponding to the KeyId. That key must validate the target Signature. The payload is an X.509 certificate whose public key must match the target KeyLocator's key. It must be issued by a trusted authority, preferably specifying the valid namespace of the key in the distinguished name.

A Content Object satisfies an Interest if and only if (a) the Content Object name, if present, exactly matches the Interest name, and (b) the ValidationAlgorithm KeyId of the Content Object exactly equals the Interest KeyIdRestriction, if present, and (c) the computed ContentObjectHash exactly equals the Interest ContentObjectHashRestriction, if present. The matching rules are given by this predicate, which if it evaluates true means the ContentObject matches the Interest. Ni = Name in Interest (may not be empty), Ki = KeyIdRestriction in the interest (may be empty), Hi = ContentObjectHashRestriction in Interest (may be empty). Likewise, No, Ko, Ho are those properties in the ContentObject, where No and Ko may be empty; Ho always exists. For binary relations, we use & for AND and | for OR. We use E for the EXISTS (not empty) operator and ! for the NOT EXISTS operator. As a special case, if the ContentObjectHashRestriction in the Interest specifies an unsupported hash algorithm, then no ContentObject can match the Interest so the system should drop the Interest and MAY send an InterestReturn to the previous hop. In this case, the predicate below will never get executed because the Interest is never forwarded. If the system is using the optional behavior of having a different system calculate the hash for it, then the system may assume all hash functions are supported and leave it to the other system to accept or reject the Interest.

As one can see, there are two types of attributes one can match. The first term depends on the existence of the attribute in the ContentObject while the next two terms depend on the existence of the attribute in the Interest. The last term is the "Nameless Object" restriction which states that if a Content Object does not have a Name, then it must match the Interest on at least the Hash restriction. If a Content Object does not carry the ContentObjectHash as an expressed field, it must be calculated in network to match against. It is sufficient within an autonomous system to calculate a ContentObjectHash at a border router and carry it via trusted means within the autonomous system. If a Content Object ValidationAlgorithm does not have a KeyId then the Content Object cannot match an Interest with a KeyIdRestriction.

This section describes the process whereby a network element may return an Interest message to a previous hop if there is an error processing the Interest. The returned Interest may be further processed at the previous hop or returned towards the Interest origin. When a node returns an Interest it indicates that the previous hop should not expect a response from that node for the Interest -- i.e., there is no PIT entry left at the returning node. The returned message maintains compatibility with the existing TLV packet format (a fixed header, optional hop-by-hop headers, and the CCNx message body). The returned Interest packet is modified in only two ways: The PacketType is set to InterestReturn to indicate a Feedback message. The ReturnCode is set to the appropriate value to signal the reason for the return The specific encodings of the Interest Return are specified in . A Forwarder is not required to send any Interest Return messages. A Forwarder is not required to process any received Interest Return message. If a Forwarder does not process Interest Return messages, it SHOULD silently drop them. The Interest Return message does not apply to a Content Object or any other message type. An Interest Return message is a 1-hop message between peers. It is not propagated multiple hops via the FIB. An intermediate node that receives an InterestReturn may take corrective actions or may propagate its own InterestReturn to previous hops as indicated in the reverse path of a PIT entry.

The Interest Return message looks exactly like the original Interest message with the exception of the two modifications mentioned above. The PacketType is set to indicate the message is an InterestReturn and the reserved byte in the Interest header is used as a Return Code. The numeric values for the PacketType and ReturnCodes are in .

This section defines the InterestReturn ReturnCode introduced in this RFC. The numeric values used in the packet are defined in . Name Description No Route The returning Forwarder has no route to the Interest name. HopLimit Exceeded The HopLimit has decremented to 0 and need to forward the packet. Interest MTU too large The Interest's MTU does not conform to the required minimum and would require fragmentation. No Resources The node does not have the resources to process the Interest. Path error There was a transmission error when forwarding the Interest along a route (a transient error). Prohibited An administrative setting prohibits processing this Interest. Congestion The Interest was dropped due to congestion (a transient error). Unsupported Content Object Hash Algorithm The Interest was dropped because it requested a Content Object Hash Restriction using a hash algorithm that cannot be computed. Malformed Interest The Interest was dropped because it did not correctly parse.

This section describes the Forwarder behavior for the various Reason codes for Interest Return. A Forwarder is not required to generate any of the codes, but if it does, it MUST conform to this specification. If a Forwarder receives an Interest Return, it SHOULD take these standard corrective actions. A forwarder is allowed to ignore Interest Return messages, in which case its PIT entry would go through normal timeout processes. Verify that the Interest Return came from a next-hop to which it actually sent the Interest.If a PIT entry for the corresponding Interest does not exist, the Forwarder should ignore the Interest Return.If a PIT entry for the corresponding Interest does exist, the Forwarder MAY do one of the following: Try a different forwarding path, if one exists, and discard the Interest Return, orClear the PIT state and send an Interest Return along the reverse path. If a forwarder tries alternate routes, it MUST ensure that it does not use same same path multiple times. For example, it could keep track of which next hops it has tried and not re-use them. If a forwarder tries an alternate route, it may receive a second InterestReturn, possibly of a different type than the first InterestReturn. For example, node A sends an Interest to node B, which sends a No Route return. Node A then tries node C, which sends a Prohibited. Node A should choose what it thinks is the appropriate code to send back to its previous hop If a forwarder tries an alternate route, it should decrement the Interest Lifetime to account for the time spent thus far processing the Interest.

If a Forwarder receives an Interest for which it has no route, or for which the only route is back towards the system that sent the Interest, the Forwarder SHOULD generate a "No Route" Interest Return message. How a forwarder manages the FIB table when it receives a No Route message is implementation dependent. In general, receiving a No Route Interest Return should not cause a forwarder to remove a route. The dynamic routing protocol that installed the route should correct the route or the administrator who created a static route should correct the configuration. A forwarder could suppress using that next hop for some period of time.

A Forwarder MAY choose to send HopLimit Exceeded messages when it receives an Interest that must be forwarded off system and the HopLimit is 0.

If a Forwarder receives an Interest whose MTU exceeds the prescribed minimum, it MAY send an "Interest MTU Too Large" message, or it may silently discard the Interest. If a Forwarder receives an "Interest MTU Too Large" is SHOULD NOT try alternate paths. It SHOULD propagate the Interest Return to its previous hops.

If a Forwarder receives an Interest and it cannot process the Interest due to lack of resources, it MAY send an InterestReturn. A lack of resources could be the PIT table is too large, or some other capacity limit.

If a forwarder detects an error forwarding an Interest, such as over a reliable link, it MAY send a Path Error Interest Return indicating that it was not able to send or repair a forwarding error.

A forwarder may have administrative policies, such as access control lists, that prohibit receiving or forwarding an Interest. If a forwarder discards an Interest due to a policy, it MAY send a Prohibited InterestReturn to the previous hop. For example, if there is an ACL that says /parc/private can only come from interface e0, but the Forwarder receives one from e1, the Forwarder must have a way to return the Interest with an explanation.

If a forwarder discards an Interest due to congestion, it MAY send a Congestion InterestReturn to the previous hop.

If a Content Object Hash Restriction specifies a hash algorithm the forwarder cannot verify, the Interest should not be accepted and the forwarder MAY send an InterestReturn to the previous hop.

If a forwarder detects a structural or syntactical error in an Interest, it SHOULD drop the interest and MAY send an InterestReturn to the previous hop. This does not imply that any router must validate the entire structure of an Interest.

This memo includes no request to IANA. TO_INTERESTLIFETIME = 2 seconds.

The Interest Return message has no authenticator from the previous hop. Therefore, the payload of the Interest Return should only be used locally to match an Interest. A node should never forward that Interest payload as an Interest. It should also verify that it sent the Interest in the Interest Return to that node and not allow anyone to negate Interest messages. If two peers require authenticated messaging, they must use an external mechanism such as MACSEC.