| ICNRG | M. Mosko |
| Internet-Draft | PARC, Inc. |
| Intended status: Experimental | I. Solis |
| Expires: July 28, 2019 | |
| C. Wood | |
| University of California Irvine | |
| January 24, 2019 |
CCNx Messages in TLV Format
draft-irtf-icnrg-ccnxmessages-latest
This document specifies the encoding of CCNx messages in a TLV packet format, including the TLV types used by each message element and the encoding of each value. The semantics of CCNx messages follow the encoding-independent CCNx Semantics specification.
This document is a product of the Information Centric Networking research group (ICNRG).
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 28, 2019.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document specifies a Type-Length-Value (TLV) packet format and the TLV type and value encodings for CCNx messages. A full description of the CCNx network protocol, providing an encoding-free description of CCNx messages and message elements, may be found in [CCNSemantics]. CCNx is a network protocol that uses a hierarchical name to forward requests and to match responses to requests. It does not use endpoint addresses, such as Internet Protocol. Restrictions in a request can limit the response by the public key of the response's signer or the cryptographic hash of the response. Every CCNx forwarder along the path does the name matching and restriction checking. The CCNx protocol fits within the broader framework of Information Centric Networking (ICN) protocols [RFC7927].
This document describes a TLV scheme using a fixed 2-byte T and a fixed 2-byte L field. The rational for this choice is described in Section 5. Briefly, this choice avoids multiple encodings of the same value (aliases) and reduces the work of a validator to ensure compliance. Unlike some uses of TLV in networking, the each network hop must evaluate the encoding, so even small validation latencies at each hop could add up to a large overall forwarding delay. For very small packets or low throughput links, where the extra bytes may become a concern, one may use a TLV compression protocol, for example [compress] and [CCNxz].
This document specifies:
This document is supplemented by this document:
The type values in Section 4 represent the values in common usage today. These values may change pending IANA assignments. All type values are relative to their parent containers. For example, each level of a nested TLV structure might define a "type = 1" with a completely different meaning. In the following, we use the symbolic names defined in that section.
Packets are represented as 32-bit wide words using ASCII art. Due to the nested levels of TLV encoding and the presence of optional fields and variable sizes, there is no concise way to represent all possibilities. We use the convention that ASCII art fields enclosed by vertical bars "|" represent exact bit widths. Fields with a forward slash "/" are variable bit widths, which we typically pad out to word alignment for picture readability.
The document represents the consensus of the ICN RG. It is the first ICN protocol from the RG, created from the early CCNx protocol [nnc] with significant revision and input from the ICN community and RG members. The draft has received critical reading by several members of the ICN community and the RG. The authors and RG chairs approve of the contents. The document is sponsored under the IRTF and is not issued by the IETF and is not an IETF standard. This is an experimental protocol and may not be suitable for any specific application and the specification may change in the future.
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.
We use 16-bit Type and 16-bit Length fields to encode TLV based packets. This provides 64K different possible types and value field lengths of up to 64KiB. With 64K possible types at each level of TLV encoding, there should be sufficient space for basic protocol types, while also allowing ample room for experimentation, application use, vendor extensions, and growth. This encoding does not allow for jumbo packets beyond 64 KiB total length. If used on a media that allows for jumbo frames, we suggest defining a media adapation envelope that allows for multiple smaller frames.
There are several global TLV definitions that we reserve at all hierarchical contexts. The TLV types in the range 0x1000 - 0x1FFF are reserved for experimental use. The TLV type T_ORG is also reserved for vendor extensions ( see Section 3.3.2). The TLV type T_PAD is used to optionally pad a field out to some desired alignment.
| Abbrev | Name | Description |
|---|---|---|
| T_ORG | Vendor Specific Information | Information specific to a vendor implementation (see below). |
| T_PAD | Padding | Adds padding to a field (see below). |
| n/a | Experimental | Experimental use. |
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+---------------+---------------+---------------+---------------+
| Type | Length |
+---------------+---------------+---------------+---------------+
The Length field contains the length of the Value field in octets. It does not include the length of the Type and Length fields. The length MAY be zero.
TLV structures are nestable, allowing the Value field of one TLV structure to contain additional TLV structures. The enclosing TLV structure is called the container of the enclosed TLV.
Type values are context-dependent. Within a TLV container, one may re-use previous type values for new context-dependent purposes.
Each packet includes the 8 byte fixed header, described below, followed by a set of TLV fields. These fields are optional hop-by-hop headers and the Packet Payload.
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+---------------+---------------+---------------+---------------+
| Version | PacketType | PacketLength |
+---------------+---------------+---------------+---------------+
| PacketType specific fields | HeaderLength |
+---------------+---------------+---------------+---------------+
/ Optional Hop-by-hop header TLVs /
+---------------+---------------+---------------+---------------+
/ PacketPayload TLVs /
+---------------+---------------+---------------+---------------+
The packet payload is a TLV encoding of the CCNx message, followed by optional Validation TLVs.
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+---------------+---------------+---------------+---------------+
| CCNx Message TLV /
+---------------+---------------+---------------+---------------+
/ Optional CCNx ValidationAlgorithm TLV /
+---------------+---------------+---------------+---------------+
/ Optional CCNx ValidationPayload TLV (ValidationAlg required) /
+---------------+---------------+---------------+---------------+
This document describes the Version "1" TLV encoding.
After discarding the fixed and hop-by-hop headers the remaining PacketPayload should be a valid protocol message. Therefore, the PacketPayload always begins with 4 bytes of type-length that specifies the protocol message (whether it is an Interest, Content Object, or other message type) and its total length. The embedding of a self-sufficient protocol data unit inside the fixed and hop-by-hop headers allows a network stack to discard the headers and operate only on the embedded message. It also de-couples the PacketType field -- which specifies how to forward the packet -- from the PacketPayload.
The range of bytes protected by the Validation includes the CCNx Message and the ValidationAlgorithm.
The ContentObjectHash begins with the CCNx Message and ends at the tail of the packet.
CCNx messages begin with an 8 byte fixed header (non-TLV format). The HeaderLength field represents the combined length of the Fixed and Hop-by-hop headers. The PacketLength field represents the entire Packet length from the first byte of Version to the last byte of the packet.
A specific PacketType may assign meaning to the "PacketType specific fields," which are otherwise reserved. For the three defined PacketTypes (Interest, ContentObject, and InterestReturn), we define those values in this document.
The PacketPayload of a CCNx packet is the protocol message itself. The Content Object Hash is computed over the PacketPayload only, excluding the fixed and hop-by-hop headers as those might change from hop to hop. Signed information or Similarity Hashes should not include any of the fixed or hop-by-hop headers. The PacketPayload should be self-sufficient in the event that the fixed and hop-by-hop headers are removed.
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+---------------+---------------+---------------+---------------+
| Version | PacketType | PacketLength |
+---------------+---------------+---------------+---------------+
| PacketType specific fields | HeaderLength |
+---------------+---------------+---------------+---------------+
The PacketType field indicates how the forwarder should process the packet. A Request Packet (Interest) has PacketType PT_INTEREST, a Response (Content Object) has PacketType PT_CONTENT, and an InterestReturn has PacketType PT_RETURN.
HeaderLength is the number of octets from the start of the packet (Version) to the end of the hop-by-hop headers. PacketLength is the number of octets from the start of the packet to the end of the packet. Both lengths have a minimum value of 8 (the fixed header itself).
The PacketType specific fields are reserved bits whose use depends on the PacketType. They are used for network-level signaling.
If the PacketType is PT_INTEREST, it indicates that the PacketPayload should be processed as an Interest message. For this type of packet, the Fixed Header includes a field for a HopLimit as well as Reserved and Flags fields. The Reserved field MUST be set to 0 in an Interest - this field will be set to a return code in the case of an Interest Return. There are currently no Flags defined, so this field MUST be set to 0.
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+---------------+---------------+---------------+---------------+
| Version | PT_INTEREST | PacketLength |
+---------------+---------------+---------------+---------------+
| HopLimit | Reserved | Flags | HeaderLength |
+---------------+---------------+---------------+---------------+
For an Interest message, the HopLimit is a counter that is decremented with each hop. It limits the distance an Interest may travel on the network. The node originating the Interest MAY put in any value - up to the maximum of 255. Each node that receives an Interest with a HopLimit decrements the value upon reception. If the value is 0 after the decrement, the Interest MUST NOT be forwarded off the node.
It is an error to receive an Interest with a 0 hop-limit from a remote node.
If the PacketType is PT_CONTENT, it indicates that the PacketPayload should be processed as a Content Object message. A Content Object defines a Flags field, however there are currently no flags defined, so the Flags field must be set to 0.
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+---------------+---------------+---------------+---------------+
| Version | PT_CONTENT | PacketLength |
+---------------+---------------+---------------+---------------+
| Reserved | Flags | HeaderLength |
+---------------+---------------+---------------+---------------+
If the PacketType is PT_RETURN, it indicates that the PacketPayload should be processed as a returned Interest message. The only difference between this InterestReturn message and the original Interest is that the PacketType is changed to PT_RETURN and a ReturnCode is is put into the ReturnCode field. All other fields are unchanged from the Interest packet. The purpose of this encoding is to prevent packet length changes so no additional bytes are needed to return an Interest to the previous hop. See [CCNSemantics] for a protocol description of this packet type.
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+---------------+---------------+---------------+---------------+
| Version | PT_RETURN | PacketLength |
+---------------+---------------+---------------+---------------+
| HopLimit | ReturnCode | Flags | HeaderLength |
+---------------+---------------+---------------+---------------+
This is the original Interest's HopLimit, as received. It is the value before being decremented at the current node (i.e. the received value).
These are the original Flags as set in the Interest.
The numeric value assigned to the return types is defined below. This value is set by the node creating the Interest Return.
A return code of "0" MUST NOT be used, as it indicates that the returning system did not modify the Return Code field.
| Type | Return Type |
|---|---|
| T_RETURN_NO_ROUTE | No Route |
| T_RETURN_LIMIT_EXCEEDED | Hop Limit Exceeded |
| T_RETURN_NO_RESOURCES | No Resources |
| T_RETURN_PATH_ERROR | Path Error |
| T_RETURN_PROHIBITED | Prohibited |
| T_RETURN_CONGESTED | Congested |
| T_RETURN_MTU_TOO_LARGE | MTU too large |
| T_RETURN_UNSUPPORTED_HASH_RESTRICTION | Unsupported ContentObjectHashRestriction |
| T_RETURN_MALFORMED_INTEREST | Malformed Interest |
This section defines global formats that may be nested within other TLVs.
The pad type may be used by protocols that prefer word-aligned data. The size of the word may be defined by the protocol. Padding 4-byte words, for example, would use a 1-byte, 2-byte, and 3-byte Length. Padding 8-byte words would use a (0, 1, 2, 3, 5, 6, 7)-byte Length.
One MUST NOT pad inside a Name. Apart from that, a pad MAY be inserted after any other TLV in the CCNx Message or in the Validation Dependent Data In the remainder of this document, we will not show optional pad TLVs.
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+---------------+---------------+---------------+---------------+
| T_PAD | Length |
+---------------+---------------+---------------+---------------+
/ variable length pad MUST be zeros /
+---------------+---------------+---------------+---------------+
Organization specific TLVs (also known as Vendor TLVs) MUST use the T_ORG type. The Length field is the length of the organization specific information plus 3. The Value begins with the 3 byte organization number derived from the last three digits of the IANA Private Enterprise Numbers, followed by the organization specific information.
A T_ORG MAY be used as a path segment in a Name, in which case it is a regular path segment and is part of the regular name matching.
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+---------------+---------------+---------------+---------------+
| T_ORG | Length (3+value length) |
+---------------+---------------+---------------+---------------+
| PEN[0] | PEN[1] | PEN[2] | /
+---------------+---------------+---------------+ +
/ Vendor Specific Value /
+---------------+---------------+---------------+---------------+
Hash values are used in several fields throughout a packet. This TLV encoding is commonly embedded inside those fields to specify the specific hash function used and it's value. Note that the reserved TLV types are also reserved here for user-defined experimental functions.
The LENGTH field of the hash value MUST be less than or equal to the hash function length. If the LENGTH is less than the full length, it is taken as the left LENGTH bytes of the hash function output. Only specified truncations are allowed, not arbitrary truncations.
This nested format is used because it allows binary comparison of hash values for certain fields without a router needing to understand a new hash function. For example, the KeyIdRestriction is bit-wise compared between an Interest's KeyIdRestriction field and a ContentObject's KeyId field. This format means the outer field values do not change with differing hash functions so a router can still identify those fields and do a binary comparison of the hash TLV without need to understand the specific hash used. An alternative approach, such as using T_KEYID_SHA512-256, would require each router keep an up-to-date parser and supporting user-defined hash functions here would explode the parsing state-space.
A CCNx entity MUST support the hash type T_SHA-256. An entity MAY support the remaining hash types.
| Abbrev | Lengths (octets) |
|---|---|
| T_SHA-256 | 32 |
| T_SHA-512 | 64, 32 |
| n/a | Experimental TLV types |
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+---------------+---------------+---------------+---------------+
| T_FOO | 36 |
+---------------+---------------+---------------+---------------+
| T_SHA512 | 32 |
+---------------+---------------+---------------+---------------+
/ 32-byte hash value /
+---------------+---------------+---------------+---------------+
Example nesting inside type T_FOO
A Link is the tuple: {Name, [KeyIdRestr], [ContentObjectHashRestr]}. It is a general encoding that is used in both the payload of a Content Object with PayloadType = "Link" and in the KeyLink field in a KeyLocator. A Link is essentially the body of an Interest.
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+---------------+---------------+-------------------------------+
/ Mandatory CCNx Name /
+---------------+---------------+-------------------------------+
/ Optional KeyIdRestriction /
+---------------------------------------------------------------+
/ Optional ContentObjectHashRestriction /
+---------------------------------------------------------------+
Hop-by-hop TLV headers are unordered and meaning MUST NOT be attached to their ordering. Three hop-by-hop headers are described in this document:
| Abbrev | Name | Description |
|---|---|---|
| T_INTLIFE | Interest Lifetime | The time an Interest should stay pending at an intermediate node. |
| T_CACHETIME | Recommended Cache Time | The Recommended Cache Time for Content Objects. |
| T_MSGHASH | Message Hash | The hash of the CCNx Message to end of packet using Section 3.3.3 format. |
Additional hop-by-hop headers are defined in higher level specifications such as the fragmentation specification.
The Interest Lifetime is the time that an Interest should stay pending at an intermediate node. It is expressed in milliseconds as an unsigned, network byte order integer.
A value of 0 (encoded as 1 byte %x00) indicates the Interest does not elicit a Content Object response. It should still be forwarded, but no reply is expected and a forwarder could skip creating a PIT entry.
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+---------------+---------------+---------------+---------------+
| T_INTLIFE | Length |
+---------------+---------------+---------------+---------------+
/ /
/ Lifetime (length octets) /
/ /
+---------------+---------------+---------------+---------------+
The Recommended Cache Time (RCT) is a measure of the useful lifetime of a Content Object as assigned by a content producer or upstream node. It serves as a guideline to the Content Store cache in determining how long to keep the Content Object. It is a recommendation only and may be ignored by the cache. This is in contrast to the ExpiryTime (described in Section 3.6.2.2.2) which takes precedence over the RCT and must be obeyed.
Because the Recommended Cache Time is an optional hop-by-hop header and not a part of the signed message, a content producer may re-issue a previously signed Content Object with an updated RCT without needing to re-sign the message. There is little ill effect from an attacker changing the RCT as the RCT serves as a guideline only.
The Recommended Cache Time (a millisecond timestamp) is a network byte ordered unsigned integer of the number of milliseconds since the epoch in UTC of when the payload expires. It is a 64-bit field.
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+---------------+---------------+---------------+---------------+
| T_CACHETIME | 8 |
+---------------+---------------+---------------+---------------+
/ /
/ Recommended Cache Time /
/ /
+---------------+---------------+---------------+---------------+
Within a trusted domain, an operator may calculate the message hash at a border device and insert that value into the hop-by-hop headers of a message. An egress device should remove the value. This permits intermediate devices within that trusted domain to match against a ContentObjectHashRestriction without calculating it at every hop.
The message hash is a cryptographic hash from the start of the CCNx Message to the end of the packet. It is used to match against the ContentObjectHashRestriction. The Message Hash may be of longer length than an Interest's restriction, in which case the device should use the left bytes of the Message Hash to check against the Interest's value.
The Message Hash may only carry one hash type and there may only be one Message Hash header.
The Message Hash header is unprotected, so this header is only of practical use within a trusted domain, such as an operator's autonomous system.
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+---------------+---------------+---------------+---------------+
| T_MSGHASH | (length + 4) |
+---------------+---------------+---------------+---------------+
| (hash type) | length |
+---------------+---------------+---------------+---------------+
/ hash value /
+---------------+---------------+---------------+---------------+
Message Hash Header
The top-level TLV types listed below exist at the outermost level of a CCNx protocol message.
| Abbrev | Name | Description |
|---|---|---|
| T_INTEREST | Interest | An Interest MessageType. |
| T_OBJECT | Content Object | A Content Object MessageType |
| T_VALIDATION_ALG | Validation Algorithm | The method of message verification such as Message Integrity Check (MIC), a Message Authentication Code (MAC), or a cryptographic signature. |
| T_VALIDATION_PAYLOAD | Validation Payload | The validation output, such as the CRC32C code or the RSA signature. |
This is the format for the CCNx protocol message itself. The CCNx message is the portion of the packet between the hop-by-hop headers and the Validation TLVs. The figure below is an expansion of the "CCNx Message TLV" depicted in the beginning of Section 3. The CCNx message begins with MessageType and runs through the optional Payload. The same general format is used for both Interest and Content Object messages which are differentiated by the MessageType field. The first enclosed TLV of a CCNx Message is always the Name TLV. This is followed by an optional Message TLVs and an optional Payload TLV.
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+---------------+---------------+---------------+---------------+
| MessageType | MessageLength |
+---------------+---------------+---------------+---------------+
| Name TLV (Type = T_NAME) |
+---------------+---------------+---------------+---------------+
/ Optional Message TLVs (Various Types) /
+---------------+---------------+---------------+---------------+
/ Optional Payload TLV (Type = T_PAYLOAD) /
+---------------+---------------+---------------+---------------+
| Abbrev | Name | Description |
|---|---|---|
| T_NAME | Name | The CCNx Name requested in an Interest or published in a Content Object. |
| T_PAYLOAD | Payload | The message payload. |
A Name is a TLV encoded sequence of segments. The table below lists the type values appropriate for these Name segments. A Name MUST NOT include PAD TLVs.
As described in CCNx Semantics, using the CCNx URI notation, a T_NAME with 0 length corresponds to ccnx:/ (the default route) and is distinct from a name with one zero length segment, such as ccnx:/NAME=. In the TLV encoding, ccnx:/ corresponds to T_NAME with 0 length, while ccnx:/NAME= corresponds to T_NAME with 4 length and T_NAMESEGMENT with 0 length.
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+---------------+---------------+---------------+---------------+
| T_NAME | Length |
+---------------+---------------+---------------+---------------+
/ Name segment TLVs /
+---------------+---------------+---------------+---------------+
| Symbolic Name | Name | Description |
|---|---|---|
| T_NAMESEGMENT | Name segment | A generic name Segment. |
| T_IPID | Interest Payload ID | An identifier that represents the Interest Payload field. As an example, the Payload ID might be a hash of the Interest Payload. This provides a way to differentiate between Interests based on their payloads without having to parse all the bytes of the payload itself; instead using only this Payload ID Name segment. |
| T_APP:00 - T_APP:4096 | Application Components | Application-specific payload in a name segment. An application may apply its own semantics to the 4096 reserved types. |
4096 special application payload name segments are allocated. These have application semantics applied to them. A good convention is to put the application's identity in the name prior to using these name segments.
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+---------------+---------------+---------------+---------------+
| (T_NAME) | %x14 (20) |
+---------------+---------------+---------------+---------------+
| (T_NAME_SEGMENT) | %x03 (3) |
+---------------+---------------+---------------+---------------+
| f o o |(T_NAME_SEGMENT)
+---------------+---------------+---------------+---------------+
| | %x03 (3) | b |
+---------------+---------------+---------------+---------------+
| a r | (T_NAME_SEGMENT) |
+---------------+---------------+---------------+---------------+
| %x02 (2) | h | i |
+---------------+---------------+---------------+---------------+
For example, a name like "ccnx:/foo/bar/hi" would be encoded as:
The InterestPayloadID is a name segment created by the origin of an Interest to represent the Interest Payload. This allows the proper multiplexing of Interests based on their name if they have different payloads. A common representation is to use a hash of the Interest Payload as the InterestPayloadID.
As part of the TLV 'value', the InterestPayloadID contains a one identifier of method used to create the InterestPayloadID followed by a variable length octet string. An implementation is not required to implement any of the methods to receive an Interest; the InterestPayloadID may be treated only as an opaque octet string for purposes of multiplexing Interests with different payloads. Only a device creating an InterestPayloadID name segment or a device verifying such a segment need to implement the algorithms.
It uses the Section 3.3.3 encoding of hash values.
In normal operations, we recommend displaying the InterestPayloadID as an opaque octet string in a CCNx URI, as this is the common denominator for implementation parsing.
The InterestPayloadID, even if it is a hash, should not convey any security context. If a system requires confirmation that a specific entity created the InterestPayload, it should use a cryptographic signature on the Interest via the ValidationAlgorithm and ValidationPayload or use its own methods inside the Interest Payload.
Each message type (Interest or Content Object) is associated with a set of optional Message TLVs. Additional specification documents may extend the types associated with each.
There are two Message TLVs currently associated with an Interest message: the KeyIdRestriction selector and the ContentObjectHashRestr selector are used to narrow the universe of acceptable Content Objects that would satisfy the Interest.
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+---------------+---------------+---------------+---------------+
| MessageType | MessageLength |
+---------------+---------------+---------------+---------------+
| Name TLV |
+---------------+---------------+---------------+---------------+
/ Optional KeyIdRestriction TLV /
+---------------------------------------------------------------+
/ Optional ContentObjectHashRestriction TLV /
+---------------------------------------------------------------+
| Abbrev | Name | Description |
|---|---|---|
| T_KEYIDRESTR | KeyIdRestriction | A Section 3.3.3 representation of the KeyId |
| T_OBJHASHRESTR | ContentObjectHashRestriction | A Section 3.3.3 representation of the hash of the specific Content Object that would satisfy the Interest. |
An Interest MAY include a KeyIdRestriction selector. This ensures that only Content Objects with matching KeyIds will satisfy the Interest. See Section 3.6.4.1.4.1 for the format of a KeyId.
An Interest MAY contain a ContentObjectHashRestriction selector. This is the hash of the Content Object - the self-certifying name restriction that must be verified in the network, if an Interest carried this restriction. It is calculated from the beginning of the CCNx Message to the end of the packet. The LENGTH MUST be from one of the allowed values for that hash (see Section 3.3.3).
The ContentObjectHashRestriction SHOULD be of type T_SHA-256 and of length 32 bytes.
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+---------------+---------------+---------------+---------------+
| T_OBJHASHRESTR | LENGTH+4 |
+---------------+---------------+---------------+---------------+
| <hash type> | LENGTH |
+---------------+---------------+---------------+---------------+
/ LENGTH octets of hash /
+---------------+---------------+---------------+---------------+
The following message TLVs are currently defined for Content Objects: PayloadType (optional) and ExpiryTime (optional).
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+---------------+---------------+---------------+---------------+
| MessageType | MessageLength |
+---------------+---------------+---------------+---------------+
| Name TLV |
+---------------+---------------+---------------+---------------+
/ Optional PayloadType TLV /
+---------------------------------------------------------------+
/ Optional ExpiryTime TLV /
+---------------------------------------------------------------+
| Abbrev | Name | Description |
|---|---|---|
| T_PAYLDTYPE | PayloadType | Indicates the type of Payload contents. |
| T_EXPIRY | ExpiryTime | The time at which the Payload expires, as expressed in the number of milliseconds since the epoch in UTC. If missing, Content Object may be used as long as desired. |
The PayloadType is a network byte order integer representing the general type of the Payload TLV. Link. If this field is missing, a "Data" type is assumed.
The Data type indicate that the Payload of the ContentObject is opaque application bytes. The Key type indicates that the Payload is a DER encoded public key. The Link type indicates that the Payload is one or more
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+---------------+---------------+---------------+---------------+
| T_PAYLDTYPE | Length |
+---------------+---------------+---------------+---------------+
| PayloadType /
+---------------+
The ExpiryTime is the time at which the Payload expires, as expressed by a timestamp containing the number of milliseconds since the epoch in UTC. It is a network byte order unsigned integer in a 64-bit field. A cache or end system should not respond with a Content Object past its ExpiryTime. Routers forwarding a Content Object do not need to check the ExpiryTime. If the ExpiryTime field is missing, the Content Object has no expressed expiration and a cache or end system may use the Content Object for as long as desired.
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_EXPIRY | 8 |
+---------------+---------------+---------------+---------------+
/ ExpiryTime /
/ /
+---------------+---------------+---------------+---------------+
The Payload TLV contains the content of the packet. It MAY be of zero length. If a packet does not have any payload, this field MAY be omitted, rather than carrying a zero length.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_PAYLOAD | Length |
+---------------+---------------+---------------+---------------+
/ Payload Contents /
+---------------+---------------+---------------+---------------+
Both Interests and Content Objects have the option to include information about how to validate the CCNx message. This information is contained in two TLVs: the ValidationAlgorithm TLV and the ValidationPayload TLV. The ValidationAlgorithm TLV specifies the mechanism to be used to verify the CCNx message. Examples include verification with a Message Integrity Check (MIC), a Message Authentication Code (MAC), or a cryptographic signature. The ValidationPayload TLV contains the validation output, such as the CRC32C code or the RSA signature.
An Interest would most likely only use a MIC type of validation - a crc, checksum, or digest.
The ValidationAlgorithm is a set of nested TLVs containing all of the information needed to verify the message. The outermost container has type = T_VALIDATION_ALG. The first nested TLV defines the specific type of validation to be performed on the message. The type is identified with the "ValidationType" as shown in the figure below and elaborated in the table below. Nested within that container are the TLVs for any ValidationType dependent data, for example a Key Id, Key Locator etc.
Complete examples of several types may be found in Section 3.6.4.1.5
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_VALIDATION_ALG | ValidationAlgLength |
+---------------+---------------+---------------+---------------+
| ValidationType | Length |
+---------------+---------------+---------------+---------------+
/ ValidationType dependent data /
+---------------+---------------+---------------+---------------+
| Abbrev | Name | Description |
|---|---|---|
| T_CRC32C | CRC32C | Castagnoli CRC32 (iSCSI, ext4, etc.), with normal form polynomial 0x1EDC6F41. |
| T_HMAC-SHA256 | HMAC-SHA256 | HMAC (RFC 2104) using SHA256 hash. |
| T_RSA-SHA256 | RSA-SHA256 | RSA public key signature using SHA256 digest. |
| EC-SECP-256K1 | SECP-256K1 | Elliptic Curve signature with SECP-256K1 parameters (see [ECC]). |
| EC-SECP-384R1 | SECP-384R1 | Elliptic Curve signature with SECP-384R1 parameters (see [ECC]). |
MICs do not require additional data in order to perform the verification. An example is CRC32C that has a "0" length value.
MACs are useful for communication between two trusting parties who have already shared private keys. Examples include an RSA signature of a SHA256 digest or others. They rely on a KeyId. Some MACs might use more than a KeyId, but those would be defined in the future.
Signature type Validators specify a digest mechanism and a signing algorithm to verify the message. Examples include RSA signature og a SHA256 digest, an Elliptic Curve signature with SECP-256K1 parameters, etc. These Validators require a KeyId and a mechanism for locating the publishers public key (a KeyLocator) - optionally a PublicKey or Certificate or KeyLink.
Different Validation Algorithms require access to different pieces of data contained in the ValidationAlgorithm TLV. As described above, Key Ids, Key Locators, Public Keys, Certificates, Links and Key Names all play a role in different Validation Algorithms. Any number of Validation Dependent Data containers can be present in a Validation Algorithm TLV.
Following is a table of CCNx ValidationType dependent data types:
| Abbrev | Name | Description |
|---|---|---|
| T_KEYID | SignerKeyId | An identifier of the shared secret or public key associated with a MAC or Signature. |
| T_PUBLICKEY | Public Key | DER encoded public key. |
| T_CERT | Certificate | DER encoded X509 certificate. |
| T_KEYLINK | KeyLink | A CCNx Link object. |
| T_SIGTIME | SignatureTime | A millsecond timestamp indicating the time when the signature was created. |
The KeyId is the publisher key identifier. It is similar to a Subject Key Identifier from X509 [RFC 5280, Section 4.2.1.2]. It should be derived from the key used to sign, such as from the SHA-256 hash of the key. It applies to both public/private key systems and to symmetric key systems.
The KeyId is represented using the Section 3.3.3. If a protocol uses a non-hash identifier, it should use one of the reserved values.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_KEYID | LENGTH+4 |
+---------------+---------------+---------------+---------------+
| <hash type> | LENGTH |
+---------------+---------------+---------------+---------------+
/ LENGTH octets of hash /
+---------------+---------------+---------------+---------------+
A Public Key is a DER encoded Subject Public Key Info block, as in an X509 certificate.
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+---------------+---------------+
| T_PUBLICKEY | Length |
+---------------+---------------+---------------+---------------+
/ Public Key (DER encoded SPKI) /
+---------------+---------------+---------------+---------------+
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_CERT | Length |
+---------------+---------------+---------------+---------------+
/ Certificate (DER encoded X509) /
+---------------+---------------+---------------+---------------+
A KeyLink type KeyLocator is a Link.
The KeyLink ContentObjectHashRestr, if included, is the digest of the Content Object identified by KeyLink, not the digest of the public key. Likewise, the KeyIdRestr of the KeyLink is the KeyId of the ContentObject, not necessarily of the wrapped key.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| T_KEYKINK | Length |
+---------------+---------------+-------------------------------+
/ Link /
+---------------------------------------------------------------+
The SignatureTime is a millisecond timestamp indicating the time at which a signature was created. The signer sets this field to the current time when creating a signature. A verifier may use this time to determine whether or not the signature was created during the validity period of a key, or if it occurred in a reasonable sequence with other associated signatures. The SignatureTime is unrelated to any time associated with the actual CCNx Message, which could have been created long before the signature. The default behavior is to always include a SignatureTime when creating an authenticated message (e.g. HMAC or RSA).
SignatureTime is a network byte ordered unsigned integer of the number of milliseconds since the epoch in UTC of when the signature was created. It is a fixed 64-bit field.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+-------------------------------+
| T_SIGTIME | 8 |
+---------------+---------------+-------------------------------+
/ SignatureTime /
+---------------------------------------------------------------+
As an example of a MIC type validation, the encoding for CRC32C validation would be:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_VALIDATION_ALG | 4 |
+---------------+---------------+---------------+---------------+
| T_CRC32C | 0 |
+---------------+---------------+---------------+---------------+
As an example of a MAC type validation, the encoding for an HMAC using a SHA256 hash would be:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_VALIDATION_ALG | 40 |
+---------------+---------------+---------------+---------------+
| T_HMAC-SHA256 | 36 |
+---------------+---------------+---------------+---------------+
| T_KEYID | 32 |
+---------------+---------------+---------------+---------------+
/ KeyId /
/---------------+---------------+-------------------------------+
As an example of a Signature type validation, the encoding for an RSA public key signing using a SHA256 digest and Public Key would be:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_VALIDATION_ALG | 44 + Variable Length |
+---------------+---------------+---------------+---------------+
| T_RSA-SHA256 | 40 + Variable Length |
+---------------+---------------+---------------+---------------+
| T_KEYID | 32 |
+---------------+---------------+---------------+---------------+
/ KeyId /
/---------------+---------------+-------------------------------+
| T_PUBLICKEY | Variable Length (~ 160) |
+---------------+---------------+---------------+---------------+
/ Public Key (DER encoded SPKI) /
+---------------+---------------+---------------+---------------+
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| T_VALIDATION_PAYLOAD | ValidationPayloadLength |
+---------------+---------------+---------------+---------------+
/ Type-dependent data /
+---------------+---------------+---------------+---------------+
The ValidationPayload contains the validation output, such as the CRC32C code or the RSA signature.
This section details each kind of protocol value that can be registered. Each type registry can be updated by incrementally expanding the type space, i.e., by allocating and reserving new types. As per [RFC5226] this section details the creation of the "CCNx Registry" and several sub-registries.
| Property | Value |
|---|---|
| Name | CCNx Registry |
| Abbrev | CCNx |
The following packet types should be allocated. A PacketType MUST be 1 byte. New packet types are allocated via "RFC Required" action.
| Property | Value |
|---|---|
| Name | Packet Type Registry |
| Parent | CCNx Registry |
| Review process | RFC Required |
| Syntax | 1 octet |
| Type | Name | Reference |
|---|---|---|
| %x00 | PT_INTEREST | Fixed Header Types |
| %x01 | PT_CONTENT | Fixed Header Types |
| %x02 | PT_RETURN | Fixed Header Types |
The following InterestReturn code types should be allocated.
| Property | Value |
|---|---|
| Name | Interest Return Code |
| Parent | CCNx Registry |
| Review process | Specification Required |
| Syntax | 1 octet |
| Type | Name | Reference |
|---|---|---|
| %x00 | Reserved | |
| %x01 | T_RETURN_NO_ROUTE | Fixed Header Types |
| %x02 | T_RETURN_LIMIT_EXCEEDED | Fixed Header Types |
| %x03 | T_RETURN_NO_RESOURCES | Fixed Header Types |
| %x04 | T_RETURN_PATH_ERROR | Fixed Header Types |
| %x05 | T_RETURN_PROHIBITED | Fixed Header Types |
| %x06 | T_RETURN_CONGESTED | Fixed Header Types |
| %x07 | T_RETURN_MTU_TOO_LARGE | Fixed Header Types |
| %x08 | T_RETURN_UNSUPPORTED_HASH_RESTRICTION | Fixed Header Types |
| %x09 | T_RETURN_MALFORMED_INTEREST | Fixed Header Types |
The following hop-by-hop types should be allocated.
| Property | Value |
|---|---|
| Name | Hop-by-Hop Type Registry |
| Parent | CCNx Registry |
| Review process | RFC Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | Reserved | |
| %x0001 | T_INTLIFE | Hop-by-hop TLV headers |
| %x0002 | T_CACHETIME | Hop-by-hop TLV headers |
| %x0003 | T_MSGHASH | Hop-by-hop TLV headers |
| %x0004 - %x0007 | Reserved | |
| %x0FFE | T_PAD | Pad |
| %x0FFF | T_ORG | Organization-Specific TLVs |
| %x1000-%x1FFF | Reserved | Experimental Use |
The following top-level types should be allocated.
| Property | Value |
|---|---|
| Name | Top-Level Type Registry |
| Parent | CCNx Registry |
| Review process | RFC Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | Reserved | |
| %x0001 | T_INTEREST | Top-Level Types |
| %x0002 | T_OBJECT | Top-Level Types |
| %x0003 | T_VALIDATION_ALG | Top-Level Types |
| %x0004 | T_VALIDATION_PAYLOAD | Top-Level Types |
The following name segment types should be allocated.
| Property | Value |
|---|---|
| Name | Name Segment Type Registry |
| Parent | CCNx Registry |
| Review process | Specification Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | Reserved | |
| %x0001 | T_NAMESEGMENT | Name |
| %x0002 | T_IPID | Name |
| %x0010 - %x0013 | Reserved | Used in other drafts |
| %x0FFF | T_ORG | Organization-Specific TLVs |
| %x1000 - %x1FFF | T_APP:00 - T_APP:4096 | Application Components |
The following CCNx message segment types should be allocated.
| Property | Value |
|---|---|
| Name | Message Type Registry |
| Parent | CCNx Registry |
| Review process | RFC Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | T_NAME | Message Types |
| %x0001 | T_PAYLOAD | Message Types |
| %x0002 | T_KEYIDRESTR | Message Types |
| %x0003 | T_OBJHASHRESTR | Message Types |
| %x0005 | T_PAYLDTYPE | Content Object Message Types |
| %x0006 | T_EXPIRY | Content Object Message Types |
| %x0007 - %x000C | Reserved | Used in other RFC drafts |
| %x0FFE | T_PAD | Pad |
| %x0FFF | T_ORG | Organization-Specific TLVs |
| %x1000-%x1FFF | Reserved | Experimental Use |
The following payload types should be allocated.
| Property | Value |
|---|---|
| Name | PayloadType Registry |
| Parent | CCNx Registry |
| Review process | Specification Required |
| Syntax | Variable length unsigned integer |
| Type | Name | Reference |
|---|---|---|
| %x00 | T_PAYLOADTYPE_DATA | Payload Types |
| %x01 | T_PAYLOADTYPE_KEY | Payload Types |
| %x02 | T_PAYLOADTYPE_LINK | Payload Types |
The following validation algorithm types should be allocated. Note: registration requires public specification of the algorithm.
| Property | Value |
|---|---|
| Name | Validation Algorithm Type Registry |
| Parent | CCNx Registry |
| Review process | Specification Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | Reserved | |
| %x0001 | Unassigned | |
| %x0002 | T_CRC32C | Validation Algorithm |
| %x0003 | Unassigned | |
| %x0004 | T_HMAC-SHA256 | Validation Algorithm |
| %x0005 | T_RSA-SHA256 | Validation Algorithm |
| %x0006 | EC-SECP-256K1 | Validation Algorithm |
| %x0007 | EC-SECP-384R1 | Validation Algorithm |
| %x0FFE | T_PAD | Pad |
| %x0FFF | T_ORG | Organization-Specific TLVs |
| %x1000-%x1FFF | Reserved | Experimental Use |
The following validation dependent data types should be allocated.
| Property | Value |
|---|---|
| Name | Validation Dependent Data Type Registry |
| Parent | CCNx Registry |
| Review process | RFC Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | Reserved | |
| %x0001 - %x0008 | Unassigned | |
| %x0009 | T_KEYID | Validation Dependent Data |
| %x000A | T_PUBLICKEYLOC | Validation Dependent Data |
| %x000B | T_PUBLICKEY | Validation Dependent Data |
| %x000C | T_CERT | Validation Dependent Data |
| %x000D | T_LINK | Validation Dependent Data |
| %x000E | T_KEYLINK | Validation Dependent Data |
| %x000F | T_SIGTIME | Validation Dependent Data |
| %x0FFF | T_ORG | Organization-Specific TLVs |
| %x1000-%x1FFF | Reserved | Experimental Use |
The following CCNx hash function types should be allocated. Note: registration requires public specification of the algorithm.
| Property | Value |
|---|---|
| Name | Hash Function Type Registry |
| Parent | CCNx Registry |
| Review process | Specification Required |
| Syntax | 2 octet TLV type |
| Type | Name | Reference |
|---|---|---|
| %x0000 | Reserved | |
| %x0001 | T_SHA-256 | Hash Format |
| %x0002 | T_SHA-512 | Hash Format |
| %x0FFF | T_ORG | Organization-Specific TLVs |
| %x1000-%x1FFF | Reserved | Experimental Use |
The CCNx protocol is a layer 3 network protocol, which may also operate as an overlay using other transports, such as UDP or other tunnels. It includes intrinsic support for message authentication via a signature (e.g. RSA or elliptic curve) or message authentication code (e.g. HMAC). In lieu of an authenticator, it may instead use a message integrity check (e.g. SHA or CRC). CCNx does not specify an encryption envelope, that function is left to a high-layer protocol (e.g. [esic]).
The CCNx message format includes the ability to attach MICs (e.g. SHA-256 or CRC), MACs (e.g. HMAC), and Signatures (e.g. RSA or ECDSA) to all packet types. This does not mean that it is a good idea to use an arbitrary ValidationAlgorithm, nor to include computationally expensive algorithms in Interest packets, as that could lead to computational DoS attacks. Applications should use an explicit protocol to guide their use of packet signatures. As a general guideline, an application might use a MIC on an Interest to detect unintentionally corrupted packets. If one wishes to secure an Interest, one should consider using an encrypted wrapper and a protocol that prevents replay attacks, especially if the Interest is being used as an actuator. Simply using an authentication code or signature does not make an Interests secure. There are several examples in the literature on how to secure ICN-style messaging [mobile] [ace].
As a layer 3 protocol, this document does not describe how one arrives at keys or how one trusts keys. The CCNx content object may include a public key embedded in the object or may use the PublicKeyLocator field to point to a public key (or public key certificate) that authenticates the message. One key exchange specification is CCNxKE [mobile], which is similar to the TLS 1.3 key exchange except it is over the CCNx layer 3 messages. Trust is beyond the scope of a layer-3 protocol protocol and left to applications or application frameworks.
The combination of an ephemeral key exchange (e.g. CCNxKE) and an encapsulating encryption (e.g. [esic]) provides the equivalent of a TLS tunnel. Intermediate nodes may forward the Interests and Content Objects, but have no visibility inside. It also completely hides the internal names in those used by the encryption layer. This type of tunneling encryption is useful for content that has little or no cache-ability as it can only be used by someone with the ephemeral key. Short term caching may help with lossy links or mobility, but long term caching is usually not of interest.
Broadcast encryption or proxy re-encryption may be useful for content with multiple uses over time or many consumers. There is currently no recommendation for this form of encryption.
The specific encoding of messages will have security implications. This document uses a type-length-value (TLV) encoding. We chose to compromise between extensibility and unambiguous encodings of types and lengths. Some TLVs use variable length T and variable length L fields to accomodate a wide gamut of values while trying to be byte-efficient. Our TLV encoding uses a fixed length 2-byte T and 2-byte L. Using a fixed-length T and L field solves two problems. The first is aliases. If one is able to encode the same value, such as 0x2 and 0x02, in different byte lengths then one must decide if they mean the same thing, if they are different, or if one is illegal. If they are different, then one must always compare on the buffers not the integer equivalents. If one is illegal, then one must validate the TLV encoding -- every field of every packet at every hop. If they are the same, then one has the second problem: how to specify packet filters. For example, if a name has 6 name components, then there are 7 T's and 7 L's, each of which might have up to 4 representations of the same value. That would be 14 fields with 4 encodings each, or 1001 combinations. It also means that one cannot compare, for example, a name via a memory function as one needs to consider that any embedded T or L might have a different format.
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.
Caching nodes must take caution when processing content objects. It is essential that the Content Store obey the rules outlined in [CCNSemantics] to avoid certain types of attacks. Unlike NDN, CCNx 1.0 has no mechanism to work around an undesired result from the network (there are no "excludes"), so if a cache becomes poisoned with bad content it might cause problems retrieving content. There are three types of access to content from a content store: unrestricted, signature restricted, and hash restricted. If an Interest has no restrictions, then the requester is not particular about what they get back, so any matching cached object is OK. In the hash restricted case, the requester is very specific about what they want and the content store (and every forward hop) can easily verify that the content matches the request. In the signature verified case (often used for initial manifest discovery), the requester only knows the KeyId that signed the content. It is this case that requires the closest attention in the content store to avoid amplifying bad data. The content store must only respond with a content object if it can verify the signature -- this means either the content object carries the public key inside it or the Interest carries the public key in addition to the KeyId. If that is not the case, then the content store should treat the Interest as a cache miss and let an endpoint respond.
A user-level cache could perform full signature verification by fetching a public key according to the PublicKeyLocator. That is not, however, a burden we wish to impose on the forwarder. A user-level cache could also rely on out-of-band attestation, such as the cache operator only inserting content that it knows has the correct signature.
The CCNx grammar allows for hash algorithm agility via the HashType. It specifies a short list of acceptable hash algorithms that should be implemented at each forwarder. Some hash values only apply to end systems, so updating the hash algorithm does not affect forwarders -- they would simply match the buffer that includes the type-length-hash buffer. Some fields, such as the ConObjHash, must be verified at each hop, so a forwarder (or related system) must know the hash algorithm and it could cause backward compatibility problems if the hash type is updated.
A CCNx name uses binary matching whereas a URI uses a case insensitive hostname. Some systems may also use case insensitive matching of the URI path to a resource. An implication of this is that human-entered CCNx names will likely have case or non-ASCII symbol mismatches unless one uses a consistent URI normalization to the CCNx name. It also means that an entity that registers a CCNx routable prefix, say ccnx:/example.com, would need separate registrations for simple variations like ccnx:/Example.com. Unless this is addressed in URI normalization and routing protocol conventions, there could be phishing attacks.
For a more general introduction to ICN-related security concerns and approaches, see [RFC7927] and [RFC7945]
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |