ICNRG M. Mosko
Internet-Draft PARC, Inc.
Intended status: Experimental I. Solis
Expires: December 28, 2018 LinkedIn
C. Wood
University of California Irvine
June 26, 2018
CCNx Semantics
draft-irtf-icnrg-ccnxsemantics-09
Abstract
This document describes the core concepts of the Content Centric
Networking (CCNx) architecture and presents a network protocol based
on two messages: Interests and Content Objects. It specifies the set
of mandatory and optional fields within those messages and describes
their behavior and interpretation. This architecture and protocol
specification is independent of a specific wire encoding.
The protocol also uses a Control message called an InterestReturn,
whereby one system can return an Interest message to the previous hop
due to an error condition. This indicates to the previous hop that
the current system will not respond to the Interest.
This document is a product of the Information Centric Networking
research group (ICNRG).
Status of This Memo
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 http://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 December 28, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Architecture . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Protocol Overview . . . . . . . . . . . . . . . . . . . . 5
2. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Message Grammar . . . . . . . . . . . . . . . . . . . . . 9
2.2. Consumer Behavior . . . . . . . . . . . . . . . . . . . . 12
2.3. Publisher Behavior . . . . . . . . . . . . . . . . . . . 14
2.4. Forwarder Behavior . . . . . . . . . . . . . . . . . . . 14
2.4.1. Interest HopLimit . . . . . . . . . . . . . . . . . . 15
2.4.2. Interest Aggregation . . . . . . . . . . . . . . . . 16
2.4.3. Content Store Behavior . . . . . . . . . . . . . . . 17
2.4.4. Interest Pipeline . . . . . . . . . . . . . . . . . . 18
2.4.5. Content Object Pipeline . . . . . . . . . . . . . . . 18
3. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1. Name Examples . . . . . . . . . . . . . . . . . . . . . . 20
3.2. Interest Payload ID . . . . . . . . . . . . . . . . . . . 21
4. Cache Control . . . . . . . . . . . . . . . . . . . . . . . . 21
5. Content Object Hash . . . . . . . . . . . . . . . . . . . . . 22
6. Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7. Hashes . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. Validation . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Validation Algorithm . . . . . . . . . . . . . . . . . . 23
9. Interest to Content Object matching . . . . . . . . . . . . . 24
10. Interest Return . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Message Format . . . . . . . . . . . . . . . . . . . . . 25
10.2. ReturnCode Types . . . . . . . . . . . . . . . . . . . . 26
10.3. Interest Return Protocol . . . . . . . . . . . . . . . . 26
10.3.1. No Route . . . . . . . . . . . . . . . . . . . . . . 27
10.3.2. HopLimit Exceeded . . . . . . . . . . . . . . . . . 28
10.3.3. Interest MTU Too Large . . . . . . . . . . . . . . . 28
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10.3.4. No Resources . . . . . . . . . . . . . . . . . . . . 28
10.3.5. Path Error . . . . . . . . . . . . . . . . . . . . . 28
10.3.6. Prohibited . . . . . . . . . . . . . . . . . . . . . 28
10.3.7. Congestion . . . . . . . . . . . . . . . . . . . . . 29
10.3.8. Unsupported Content Object Hash Algorithm . . . . . 29
10.3.9. Malformed Interest . . . . . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
12. Security Considerations . . . . . . . . . . . . . . . . . . . 29
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
13.1. Normative References . . . . . . . . . . . . . . . . . . 32
13.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
This document describes the principles of the CCNx architecture. It
describes 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 concerns the semantics of the protocol and is not dependent
on a specific wire format encoding. The CCNx Messages [CCNMessages]
document describes a type-length-value (TLV) wire protocol encoding.
This section introduces the main concepts of CCNx, which are further
elaborated in the remainder of the document.
The CCNx protocol derives from the early ICN work by Jacobson et al.
[nnc]. Jacobson's version of CCNx is known as the 0.x version ("CCNx
0.x") and the present work is known as the 1.0 version ("CCNx 1.0").
There are two active implementations of CCNx 1.0. The most complete
implementation is Community ICN (CINC) [cicn], a Linux Foundation
project hosted at fd.io. Another active implementation is CCN-lite
[ccnlite], with support for IoT systems and the RIOT operating
system. CCNx 0.x formed the basis of the Named Data Networking [ndn]
(NDN) university project.
The current CCNx 1.0 specification diverges from CCNx 0.x in a few
significant areas. The most pronounced behavioral difference between
CCNx 0.x and CCNx 1.0 is that CCNx 1.0 has a simpler response
processing behavior. In both versions, a forwarder uses a
hierarchical longest prefix match of a request name against the
forwarding information base (FIB) to send the request through the
network to a system that can issue a response. A forwarder must then
match a response's name to a request's name to determine the reverse
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path and deliver the response to the requester. In CCNx 0.x, the
Interest name may be a hierarchical prefix of the response name,
which allows a form of layer 3 content discovery. In CCNx 1.0, a
response's name must exactly equal a request's name. Content
discovery is performed by a higher-layer protocol.
CCNx Selectors [selectors] is an example of using a higher-layer
protocol on top of the CCNx 1.0 layer-3 to perform content discovery.
The selector protocol uses a method similar to the original CCNx 0.x
techniques without requiring partial name matching of a response to a
request in the forwarder.
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.
1.1. Requirements Language
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 [RFC2119].
1.2. Architecture
We describe the architecture of the network in which CCNx operates
and introduce certain terminology from [terminology]. The detailed
behavior of each component and message grammars are in Section 2.
A producer (also called a publisher) is an endpoint that encapsualtes
content in Content Objects for transport in the CCNx network. A
producer has a public/private keypair and signs (directly or
indirectly) the content objects. Usually, the producer's keyid (hash
of the public key) is well-known or may be derived from the
producer's namespace via standard means.
A producer operates within one or more namespaces. A namespace is a
name prefix that is represented in the forwarding information base
(FIB). This allows a request to reach the producer and fetch a
response (if one exists).
The forwarding information base (FIB) is a table that tells a
forwarder where to send a request. It may point to a local
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application, a local cache or content store, or to a remote system.
If there is no matching entry in the FIB, a forwarder cannot process
a request. The detailed rules on name matching to the FIB are given
in Section 2.4.4. An endpoint has a FIB, though it may be a simple
default route. An intermediate system (i.e. a router) typically has
a much larger FIB. A core CCNx forwarder, for example, would know
all the global routes.
A consumer is an endpoint that requests a name. It is beyond the
scope of this document to describe how a consumer learns of a name or
publisher keyid -- higher layer protocols build on top of CCNx handle
those tasks, such as search engines or lookup services or well known
names. The consumer constructs a request, called an Interest, and
forwards it via the endpoint's FIB. The consumer should get back
either a response, called a Content Object, that matches the Interest
or a control message, called an InterestReturn, that indicates the
network cannot handle the request.
There are three ways to detect errors in Interest handling. An
InterestReturn is a network control message that indicates a low-
level error like no route or out of resources. If an Interest
arrives at a producer, but the producer does not have the requested
content, the producer should send an application-specific error
message (e.g. a not found message). Finally, a consumer may not
receive anything, in which case it should timeout and, depending on
the application, retry the request or return an error to the
application.
1.3. Protocol Overview
The goal of CCNx is to name content and retrieve the content from the
network without binding it to a specific network endpoint. A routing
system (specified separately) populates the forwarding information
base (FIB) tables at each CCNx router with hierarchical name prefixes
that point towards the content producers under that prefix. A
request finds matching content along those paths, in which case a
response carries the data, or if no match is found a control message
indicates the failure. A request may further refine acceptable
responses with a restriction on the response's signer and the
cryptographic hash of the response. The details of these
restrictions are described below.
The CCNx name is a hierarchical series of path segments. Each path
segment has a type and zero or more bytes. Matching two names is
done as a binary comparison of the type and value, segment by
segment. The human-readable form is defined under a URI scheme
"ccnx:" [CCNxURI], though the canonical encoding of a name is a
series of (type, octet string) pairs. There is no requirement that
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any path segment be human readable or UTF-8. The first few segments
in a name will matched against the FIB and a routing protocol may put
its own restrictions on the routable name components (e.g. a maximum
length or character encoding rules). In principle, path segments and
names have unbounded length, though in practice they are limited by
the wire format encoding and practical considerations imposed by a
routing protocol. Note that in CCNx path segments use binary
comparison whereas in a URI the authority uses case-insensitive
hostname (due to DNS).
The CCNx name, as used by the forwarder, is purposefully left as a
general octet-encoded type and value without any requirements on
human readability and character encoding. The reason for this is
that we are concerned with how a forwarder processes names. We
expect that applications, routing protocols, or other higher layers
will apply their own conventions and restrictions on the allowed path
segment types and path segment values.
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. Data integrity is
therefore a core feature of CCNx; it does not rely on the data
transmission channel. There are several options for data
confidentiality, discussed later.
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. A
named payload is thus the tuple {{Name, ExtraFields, Payload,
ValidationAlgorithm}, ValidationPayload}, where all fields in the
inner tuple are covered by the validation payload (e.g. signature).
Consumers of this data can check the binding integrity by re-
computing the same cryptographic hash and verifying the digital
signature in ValidationPayload.
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In addition to digital signatures (e.g. RSA), CCNx also supports
message authentication codes (e.g. HMAC) and message integrity codes
(e.g. SHA-256 or CRC). To maintain the cryptographic binding, there
should be at least one object with a signature or authentication
code, but not all objects require it. For example, a first object
with a signature could refer to other objects via a hash chain, a
Merkle tree, or a signed manifest. The later objects may not have
any validation and rely purely on the references. The use of an
integrity code (e.g. CRC) is intended for detecting accidental
corruption in an Interest.
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 optional matching restrictions. Restrictions limit the
possible matching Content Objects. Two restrictions exist:
KeyIdRestr and ContentObjectHashRestr. The first restriction on the
KeyId limits responses to those signed with a ValidationAlgorithm
KeyId field equal to the restriction. The second is the Content
ObjectHash 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
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a method to store state that facilitates the return of a Content
Object.
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 Section 9). 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.
An actual PIT table is not mandated by the specification. An
implementation may use any technique that gives the same external
behavior. There are, for example, research papers that use
techniques like label switching in some parts of the network to
reduce the per-node state incurred by the PIT table [dart]. Some
implementations store the PIT state in the FIB, so there is not a
second table.
If multiple Interests with the same {Name, KeyIdRestr,
ContentObjectHashRestr} 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
Section 2.4.2). Thus, one Content Object might satisfy multiple
pending Interests in a PIT.
In CCNx, higher-layer protocols are often 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.
Aggregating layer-3 content objects in to larger objects is beyond
the scope of this document. One proposed method, FLIC [flic], uses a
manifest to enumerate the pieces of a larger object. Manifests are,
themselves, Content Objects. Another option is to use a convention
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in the Content Object name, as in the CCNx Chunking [chunking]
protocol where a large object is broken in to small chunks and each
chunk receives a special name component indicating its serial order.
At the semantic level, described in this document, we do not address
fragmentation. One experimental fragmentation protocol, BeginEnd
Fragments [befrags] uses a multipoint-PPP style technique for use
over layer-2 interfaces with the CCNx Messages [CCNMessages] TLV wire
forman specification.
With these concepts, the remainder of the document specifies the
behavior of a forwarder in processing Interest, Content Object, and
InterestReturn messages.
2. Protocol
CCNx is a request and response protocol. A request is called an
Interest and a response is called a Content Object. CCNx also uses a
1-hop control message called InterestReturn. These are, as a group,
called CCNx Messages.
2.1. Message Grammar
The CCNx message ABNF [RFC5234] grammar is shown in Figure 1. 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:
o AbsTime: Absolute times are conveyed as the 64-bit UTC time in
milliseconds since the epoch (standard POSIX time).
o 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 Section 4).
o ConObjField: These are optional fields that may appear in a
Content Object.
o 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 Section 5).
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o ExpiryTime: An absolute time after which the content object should
be considered expired (see Section 4).
o 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.
o 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 Section 2.4.
o InterestField: These are optional fields that may appear in an
Interest message.
o KeyIdRestr: The KeyId Restriction. A Content Object must have a
KeyId with the same value as the restriction.
o ContentObjectHashRestr: The Content Object Hash Restriction. A
content object must hash to the same value as the restriction
using the same HashType. The ContentObjectHashRestr MUST use
SHA256-32.
o 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 identifier agreed
upon by the parties.
o KeyLink: A Link (see Section 6) that names how to retrieve the key
used to verify the ValidationPayload. A message SHOULD NOT have
both a KeyLink and a PublicKey.
o 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.
o 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 Content Object
MAY have a Name (see Section 9). The segments of a name are said
to be complete if its segments uniquely identify a single Content
Object. A name is exact if its segments are complete. An
Interest carrying a full name is one which specifies an exact name
and the ContentObjectHashRestr of the corresponding Content
Object.
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o Payload: The message's data, as defined by PayloadType.
o 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 one or more Links (see Section 6).
o 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.
o RelTime: A relative time, measured in milli-seconds.
o ReturnCode: States the reason an Interest message is being
returned to the previous hop (see Section 10.2).
o SigTime: The absolute time (UTC milliseconds) when the signature
was generated.
o Vendor: Vendor-specific opaque data. The Vendor data includes the
IANA Private Enterprise Numbers [EpriseNumbers], followed by
vendor-specific information. CCNx allows vendor-specific data in
most locations of the grammar.
Message := Interest / ContentObject / InterestReturn
Interest := IntHdr BodyName [Validation]
IntHdr := HopLimit [Lifetime] *Vendor
ContentObject := ConObjHdr BodyOptName [Validation]
ConObjHdr := [CacheTime / ConObjHash] *Vendor
InterestReturn:= ReturnCode Interest
BodyName := Name Common
BodyOptName := [Name] Common
Common := *Field [Payload]
Validation := ValidationAlg ValidatonPayload
Name := FirstSegment *Segment
FirstSegment := 1* OCTET / Vendor
Segment := 0* OCTET / Vendor
ValidationAlg := (RSA-SHA256 / HMAC-SHA256 / CRC32C) *Vendor
ValidatonPayload := 1* OCTET
RSA-SHA256 := KeyId [PublicKey] [SigTime] [KeyLink]
HMAC-SHA256 := KeyId [SigTime] [KeyLink]
CRC32C := [SigTime]
AbsTime := 8 OCTET ; 64-bit UTC msec since epoch
CacheTime := AbsTime
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ConObjField := ExpiryTime / PayloadType
ConObjHash := Hash ; The Content Object Hash
DataType := "1"
ExpiryTime := AbsTime
Field := InterestField / ConObjField / Vendor
Hash := HashType 1* OCTET
HashType := SHA256-32 / SHA512-64 / SHA512-32
HopLimit := OCTET
InterestField := KeyIdRestr / ContentObjectHashRestr
KeyId := 1* OCTET ; key identifier
KeyIdRestr := 1* OCTET
KeyLink := Link
KeyType := "2"
Lifetime := RelTime
Link := Name [KeyIdResr] [ContentObjectHashRestr]
LinkType := "3"
ContentObjectHashRestr := Hash
Payload := *OCTET
PayloadType := DataType / KeyType / LinkType
PublicKey := ; DER-encoded public key
RelTime := 1* OCTET ; msec
ReturnCode := ; see Section 10.2
SigTime := AbsTime
Vendor := PEN 0* OCTET
PEN := ; IANA Private Enterprise Number
Figure 1
2.2. Consumer Behavior
To request a piece of content for a given {Name, [KeyIdRest],
[ContentObjectHashRestr]} 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 3.2) 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.
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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 Section 2.4.2). 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 Section 2.4).
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 Content Object. The consumer SHOULD:
o Ensure the content object is properly formatted.
o Verify that the returned Name matches a pending request. If the
request also had KeyIdRestr or ObjHashRest, it MUST also validate
those properties.
o 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.
o 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.
o 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
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traditional PKI methods, a trusted key resolution service, or
methods like [trust].
o It MAY cache the content object for future use, up to the
ExpiryTime if present.
o 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.
2.3. Publisher Behavior
This document does not specify the method by which names populate a
Forwarding Information Base (FIB) table at forwarders (see
Section 2.4). 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:
o Verify that the Interest is part of the publishers namespace(s).
o 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.
o 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 or not be cacheable (i.e. an expiry time of 0).
2.4. Forwarder Behavior
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
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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
Section 2.4.2), checking for a cached Content Object in the Content
Store (see Section 2.4.3), 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.
2.4.1. Interest HopLimit
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.
A loop may also terminate because the Interest is aggregated with
it's previous PIT entry along the loop. In this case, the Content
will be sent back along the loop and eventually return to a node that
already forwarded the content, so it will likely not have a PIT entry
any more. When the content reaches a node without a PIT entry, it
will be discarded. It may be that a new Interest or another looped
Interest will return to that same node, in which case the node will
either return a cached response to make a new PIT entry, as below.
The HopLimit is the last resort method to stop Interest loops where a
Content Object chases an Interest around a loop and where the
intermediate nodes, for whatever reason, no longer have a PIT entry
and do not cache the Content Object.
Every Interest MUST carry a HopLimit. An Interest received from a
local application MAY have a 0 HopLimit, which restricts the Interest
to other local sources.
When an Interest is received from another forwarder, the HopLimit
MUST be positive, otherwise the forwarder will discard the Interest.
A forwarder MUST decrement the HopLimit of an Interest by at least 1
before it is forwarded.
If the decremented HopLimit equals 0, the Interest MUST NOT be
forwarded to another forwarder; it MAY be sent to a local publisher
application or serviced from a local Content Store.
A RECOMMENDED HopLimit processing pipeline is below:
o If Interest received from a remote system:
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* If received HopLimit is 0, optionally send InterestReturn
(HopLimit Exceeded), and discard Interest.
* Otherwise, decrement the HopLimit by 1.
o Process as per Content Store and Aggregation rules.
o If the Interest will be forwarded:
* If the (potentailly decremented) HopLimit is 0, restrict
forwarding to the local system.
* Otherwise, forward as desired to local or remote systems.
2.4.2. Interest Aggregation
Interest aggregation is when a forwarder receives an Interest message
that could be satisfied by the response to another Interest message
already forwarded by the node, so the forwarder suppresses forwarding
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:
o Two Interests are considered 'similar' if they have the same Name,
KeyIdRestr, and ContentObjectHashRestr.
o 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).
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o An Interest record (PIT entry) is considered invalid if its
InterestExpiry time is in the past.
o The first reception of an Interest MUST be forwarded.
o 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.
o A second or later reception of an Interest similar to a valid
pending Interest from a new previous hop MAY be aggregated (not
forwarded). If this Interest has a larger HopLimit than the
pending Interest, it MUST be forwarded.
o 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 Content Object 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.
2.4.3. Content Store Behavior
The Content Store is a special cache that is an integral part 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 attack via cache poisoning, there are
special rules about how a Content Store behaves.
1. A forwarder MAY implement a Content Store. If it does, the
Content Store matches a Content Object to an Interest via the
normal matching rules (see Section 9).
2. If an Interest has a KeyIdRestr, then the Content Store MUST NOT
reply unless it knows the signature on the matching Content
Object is correct. It may do this by external knowledge (i.e.,
in a managed network or system with pre-populated caches) or by
having the public key and cryptographically verifying the
signature. A Content Store is NOT REQURIED to verify signatures;
if it does not, then it treats these cases like a cache miss.
3. If a Content Store chooses to verify signatures, then it MAY do
so as follows. If the public key is provided in the Content
Object itself (i.e., in the PublicKey field) or in the Interest,
the Content Store MUST verify that the public key's SHA-256 hash
is equal to the KeyId and that it verifies the signature. A
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Content Store MAY verify the digital signature of a Content
Object before it is cached, but it is not required to do so. A
Content Store 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.
4. If an Interest has an ContentObjectHashRestr, then the Content
Store MUST NOT reply unless it knows the the matching Content
Object has the correct hash. If it cannot verify the hash, then
it should treat the Interest as a cache miss.
5. It must obey the Cache Control directives (see Section 4).
2.4.4. Interest Pipeline
1. Perform the HopLimit check (see Section 2.4.1).
2. Determine if the Interest can be aggregated, as per
Section 2.4.2. If it can be, aggregate and do not forward the
Interest.
3. If forwarding the Interest, check for a hit in the Content Store,
as per Section 2.4.3. If a matching Content Object is found,
return it to the Interest's previous hop. This injects the
Content Store as per Section 2.4.5.
4. 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
Section 10.
2.4.5. Content Object Pipeline
1. It is RECOMMENDED that a forwarder that receives a content object
check that the Content Object 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.
2. A Content Object MUST be matched to all pending Interests that
satisfy the matching rules (see Section 9). Each satisfied
pending Interest MUST then be removed from the set of pending
Interests.
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3. 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.
4. 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.
3. Names
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.
There are three different types of names in CCNx: prefix, exact, and
full names. A prefix name is simply a name that does not uniquely
identify a single Content Object, but rather a namespace or prefix of
an existing Content Object name. An exact name is one which uniquely
identifies the name of a Content Object. A full name is one which is
exact and is accompanied by an explicit or implicit ConObjHash. The
ConObjHash is explicit in an Interest and implicit in a Content
Object.
Note that a forwarder does not need to know any semantics about a
name. It only needs to be able to match a prefix to forward
Interests and match an exact or full name to forward Content Objects.
It is not sensitive to the name segment types.
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.
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+-------------+-----------------------------------------------------+
| Name | Description |
+-------------+-----------------------------------------------------+
| Name | A generic name segment that includes arbitrary |
| Segment | octets. |
| | |
| Interest | An octet string that identifies the payload carried |
| Payload ID | 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 | An application-specific payload in a name segment. |
| Components | 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. |
+-------------+-----------------------------------------------------+
Table 1: CCNx Name Segment Types
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.
3.1. Name Examples
This section uses the CCNx URI [CCNxURI] representation of CCNx
names. Note that as per the message grammar, an Interest must have a
Name with at least one name segment and that name segment must have
at least 1 octet of value. A Content Object must have a similar name
or no name at all. The FIB, on the other hand, could have 0-length
names (a default route), or a first name segment with no value, or a
regular name.
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+--------------------------+----------------------------------------+
| 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. |
+--------------------------+----------------------------------------+
Table 2: CCNx Name Examples
3.2. Interest Payload ID
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.
4. Cache Control
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 Content Object 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 Content Object 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 MAY discard the content object
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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.
5. Content Object Hash
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 Section 9), 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.
6. Link
A Link is the tuple {Name, [KeyIdRestr], [ContentObjectHashRestr]}.
The information in a Link comprises the fields of an Interest which
would retrieve the Link target. A Content Object with PayloadType =
"Link" is an object whose payload is one or more Links. This tuple
may be used as a KeyLink to identify a specific object with the
certificate wrapped key. It is RECOMMENDED to include at least one
of KeyIdRestr or Content ObjectHashRestr. If neither restriction is
present, then any Content Object with a matching name from any
publisher could be returned.
7. Hashes
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 supports hash agility.
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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 [CCNMessages] specification should be taken as the
definition of acceptable hash functions and uses.
8. Validation
8.1. Validation Algorithm
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
KeyIdRestr, then that KeyIdRestr MUST exactly match the Content
Object's KeyId.
Validation Algorithms fall into three categories: MICs, MACs, and
Signatures. Validators using Message Integrity Code (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 KeyLink. MAC and
Signature validators may also include a SignatureTime, as described
above.
A PublicKeyLocator KeyLink 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.
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9. Interest to Content Object matching
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 KeyIdRestr, if present, and (c) the computed Content
ObjectHash exactly equals the Interest ContentObjectHashRestr, if
present.
The matching rules are given by this predicate, which if it evaluates
true means the Content Object matches the Interest. Ni = Name in
Interest (may not be empty), Ki = KeyIdRestr in the interest (may be
empty), Hi = ContentObjectHashRestr in Interest (may be empty).
Likewise, No, Ko, Ho are those properties in the Content Object,
where No and Ko may be empty; Ho always exists (it is an intrinsic
property of the Content Object). 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 ContentObjectHashRestr in the Interest
specifies an unsupported hash algorithm, then no Content Object 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.
(!No | (Ni=No)) & (!Ki | (Ki=Ko)) & (!Hi | (Hi=Ho)) & (E No | E Hi)
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 Content
Object 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 Content ObjectHash as an
expressed field, it must be calculated in network to match against.
It is sufficient within an autonomous system to calculate a Content
ObjectHash 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 KeyIdRestr.
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10. Interest Return
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:
o The PacketType is set to InterestReturn to indicate a Feedback
message.
o 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
[CCNMessages].
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.
10.1. Message Format
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
[CCNMessages].
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10.2. ReturnCode Types
This section defines the InterestReturn ReturnCode introduced in this
RFC. The numeric values used in the packet are defined in
[CCNMessages].
+----------------------+--------------------------------------------+
| Name | Description |
+----------------------+--------------------------------------------+
| No Route (Section | The returning Forwarder has no route to |
| 10.3.1) | the Interest name. |
| | |
| HopLimit Exceeded | The HopLimit has decremented to 0 and need |
| (Section 10.3.2) | to forward the packet. |
| | |
| Interest MTU too | The Interest's MTU does not conform to the |
| large (Section | required minimum and would require |
| 10.3.3) | fragmentation. |
| | |
| No Resources | The node does not have the resources to |
| (Section 10.3.4) | process the Interest. |
| | |
| Path error (Section | There was a transmission error when |
| 10.3.5) | forwarding the Interest along a route (a |
| | transient error). |
| | |
| Prohibited (Section | An administrative setting prohibits |
| 10.3.6) | processing this Interest. |
| | |
| Congestion (Section | The Interest was dropped due to congestion |
| 10.3.7) | (a transient error). |
| | |
| Unsupported Content | The Interest was dropped because it |
| Object Hash | requested a Content Object Hash |
| Algorithm (Section | Restriction using a hash algorithm that |
| 10.3.8) | cannot be computed. |
| | |
| Malformed Interest | The Interest was dropped because it did |
| (Section 10.3.9) | not correctly parse. |
+----------------------+--------------------------------------------+
Table 3: Interest Return Reason Codes
10.3. Interest Return Protocol
This section describes the Forwarder behavior for the various Reason
codes for Interest Return. A Forwarder is not required to generate
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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.
o Verify that the Interest Return came from a next-hop to which it
actually sent the Interest.
o If a PIT entry for the corresponding Interest does not exist, the
Forwarder should ignore the Interest Return.
o 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, or
* Clear 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.
10.3.1. No Route
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.
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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.
10.3.2. HopLimit Exceeded
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.
10.3.3. Interest MTU Too Large
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.
10.3.4. No Resources
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.
10.3.5. Path Error
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.
10.3.6. Prohibited
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.
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10.3.7. Congestion
If a forwarder discards an Interest due to congestion, it MAY send a
Congestion InterestReturn to the previous hop.
10.3.8. Unsupported Content Object Hash Algorithm
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.
10.3.9. Malformed Interest
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.
11. IANA Considerations
This memo includes no request to IANA.
TO_INTERESTLIFETIME = 2 seconds.
12. Security Considerations
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
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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 [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 [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.
[CCNMessages] 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.
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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
Section 2.4.3 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. [CCNMessages] is the authoritative source for
per-field allowed hash types in that encoding.
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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]
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
13.2. Informative References
[ace] Shang, W., Yu, Y., Liang, T., Zhang, B., and L. Zhang,
"NDN-ACE: Access control for constrained environments over
named data networking", NDN Technical Report NDN-0036,
2015, <http://new.named-data.net/wp-
content/uploads/2015/12/ndn-0036-1-ndn-ace.pdf>.
[befrags] Mosko, M. and C. Tschudin, "ICN "Begin-End" Hop by Hop
Fragmentation", 2017, <https://www.ietf.org/archive/id/
draft-mosko-icnrg-beginendfragment-02.txt>.
[ccnlite] Tschudin, C., et al., University of Basel, "CCN-Lite V2",
2011-2018, <http://www.ccn-lite.net/>.
[CCNMessages]
Mosko, M., Solis, I., and C. Wood, "CCNx Messages in TLV
Format (Internet draft)", 2018, <https://www.ietf.org/id/
draft-irtf-icnrg-ccnxmessages-07.txt>.
[ccnxke] Mosko, M., Uzun, E., and C. Wood, "CCNx Key Exchange
Protocol Version 1.0", 2017,
<https://www.ietf.org/archive/id/draft-wood-icnrg-
ccnxkeyexchange-02.txt>.
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[CCNxURI] Mosko, M. and C. Wood, "The CCNx URI Scheme (Internet
draft)", 2017,
<http://tools.ietf.org/html/draft-mosko-icnrg-ccnxuri-02>.
[chunking]
Mosko, M., "CCNx Content Object Chunking", 2016,
<https://www.ietf.org/archive/id/draft-mosko-icnrg-
ccnxchunking-02.txt>.
[cicn] Muscariello, L., et al., Cisco Systems, "Community ICN
(CICN)", 2017-2018, <https://wiki.fd.io/view/Cicn>.
[dart] Garcia-Luna-Aceves, J. and M. Mirzazad-Barijough, "A
Light-Weight Forwarding Plane for Content-Centric
Networks", 2016, <https://arxiv.org/pdf/1603.06044.pdf>.
[EpriseNumbers]
IANA, "IANA Private Enterprise Numbers", 2015,
<http://www.iana.org/assignments/enterprise-numbers/
enterprise-numbers>.
[esic] Mosko, M. and C. Wood, "Encrypted Sessions In CCNx
(ESIC)", 2017, <https://www.ietf.org/id/draft-wood-icnrg-
esic-01.txt>.
[flic] Tschudin, C. and C. Wood, "File-Like ICN Collection
(FLIC)", 2017, <https://www.ietf.org/archive/id/draft-
tschudin-icnrg-flic-03.txt>.
[mobile] Mosko, M., Uzun, E., and C. Wood, "Mobile Sessions in
Content-Centric Networks", IFIP Networking, 2017,
<http://dl.ifip.org/db/conf/networking/
networking2017/1570334964.pdf>.
[ndn] UCLA, "Named Data Networking", 2007,
<http://www.named-data.net>.
[nnc] Jacobson, V., Smetters, D., Thornton, J., Plass, M.,
Briggs, N., and R. Braynard, "Networking Named Content",
2009, <http://dx.doi.org/10.1145/1658939.1658941>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, <https://www.rfc-
editor.org/info/rfc5234>.
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[RFC7927] Kutscher, D., Eum, S., Pentikousis, K., Psaras, I.,
Corujo, D., Saucez, D., Schmidt, T., and M. Waehlisch,
"Information-Centric Networking (ICN) Research
Challenges", 2016, <https://trac.tools.ietf.org/html/
rfc7927>.
[RFC7945] Pentikousis, K., Ohlman, B., Davies, E., Spirou, S., and
G. Boggia, "Information-Centric Networking: Evaluation and
Security Considerations", 2016,
<https://trac.tools.ietf.org/html/rfc7945>.
[selectors]
Mosko, M., "CCNx Selector Based Discovery", 2017,
<https://raw.githubusercontent.com/mmosko/ccnx-protocol-
rfc/master/docs/build/draft-mosko-icnrg-selectors-01.txt>.
[terminology]
Wissingh, B., Wood, C., Afanasyev, A., Zhang, L., Oran,
D., and C. Tschudin, "Information-Centric Networking
(ICN): CCN and NDN Terminology", 2017,
<https://www.ietf.org/id/draft-irtf-icnrg-terminology-
00.txt>.
[trust] Tschudin, C., Uzun, E., and C. Wood, "Trust in
Information-Centric Networking: From Theory to Practice",
2016, <https://doi.org/10.1109/ICCCN.2016.7568589>.
Authors' Addresses
Marc Mosko
PARC, Inc.
Palo Alto, California 94304
USA
Phone: +01 650-812-4405
Email: marc.mosko@parc.com
Ignacio Solis
LinkedIn
Mountain View, California 94043
USA
Email: nsolis@linkedin.com
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Christopher A. Wood
University of California Irvine
Irvine, California 92697
USA
Phone: +01 315-806-5939
Email: woodc1@uci.edu
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