Internet DRAFT - draft-oran-icnrg-reflexive-forwarding
draft-oran-icnrg-reflexive-forwarding
ICNRG D. Oran
Internet-Draft Network Systems Research and Design
Updates: 8569, 8609 (if approved) D. Kutscher
Intended status: Experimental HKUST(GZ)
Expires: 29 March 2024 26 September 2023
Reflexive Forwarding for CCNx and NDN Protocols
draft-oran-icnrg-reflexive-forwarding-06
Abstract
Current Information-Centric Networking protocols such as CCNx and NDN
have a wide range of useful applications in content retrieval and
other scenarios that depend only on a robust two-way exchange in the
form of a request and response (represented by an _Interest-Data
exchange_ in the case of the two protocols noted above). A number of
important applications however, require placing large amounts of data
in the Interest message, and/or more than one two-way handshake.
While these can be accomplished using independent Interest-Data
exchanges by reversing the roles of consumer and producer, such
approaches can be both clumsy for applications and problematic from a
state management, congestion control, or security standpoint. This
specification proposes a _Reflexive Forwarding_ extension to the CCNx
and NDN protocol architectures that eliminates the problems inherent
in using independent Interest-Data exchanges for such applications.
It updates RFC8569 and RFC8609.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 29 March 2024.
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Copyright Notice
Copyright (c) 2023 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Problems with pushing data . . . . . . . . . . . . . . . 5
1.2. Problems with utilizing independent exchanges . . . . . . 6
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 7
3. Overview of the Reflexive Forwarding design . . . . . . . . . 7
4. Detailed Specification of Reflexive forwarding . . . . . . . 10
4.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Reflexive Forwarding message flow . . . . . . . . . . . . 11
4.3. Consumer Operation . . . . . . . . . . . . . . . . . . . 13
4.4. Naming of Reflexive Interests . . . . . . . . . . . . . . 13
4.5. Producer Operation . . . . . . . . . . . . . . . . . . . 14
4.6. Forwarder Operation . . . . . . . . . . . . . . . . . . . 15
4.6.1. Forwarder algorithms in pseudocode . . . . . . . . . 16
4.6.1.1. Processing of a normal Interest containing a
Reflexive Name Prefix TLV . . . . . . . . . . . . . 16
4.6.1.2. Processing of a Reflexive Interest . . . . . . . 17
4.7. State coupling between producer and consumer . . . . . . 17
5. Use cases for Reflexive Interests . . . . . . . . . . . . . . 17
5.1. Achieving Remote Method Invocation with Reflexive
Interests . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. RESTful Web Interactions . . . . . . . . . . . . . . . . 20
5.3. Achieving simple data pull from consumers with reflexive
Interests . . . . . . . . . . . . . . . . . . . . . . . . 20
6. Implementation Considerations . . . . . . . . . . . . . . . . 24
6.1. Forwarder implementation considerations . . . . . . . . . 24
6.1.1. Interactions with Input Processing of Interest and Data
packets . . . . . . . . . . . . . . . . . . . . . . . 24
6.1.2. Interactions with Interest Lifetime . . . . . . . . . 25
6.1.3. Interactions with Interest aggregation and multi-path/
multi-destination forwarding . . . . . . . . . . . . 26
6.2. Consumer Implementation Considerations . . . . . . . . . 27
6.2.1. Data objects returned by the consumer to reflexive name
Interests arriving from a producer . . . . . . . . . 27
6.2.2. Terminating unwanted reflexive Interest exchanges . . 27
6.2.3. Interactions with caching . . . . . . . . . . . . . . 28
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6.3. Producer Implementation Considerations . . . . . . . . . 28
7. Operational Considerations . . . . . . . . . . . . . . . . . 28
8. Mapping to CCNx and NDN packet encodings . . . . . . . . . . 29
8.1. Packet encoding for CCNx . . . . . . . . . . . . . . . . 29
8.2. Packet encoding for NDN . . . . . . . . . . . . . . . . . 30
8.2.1. Reflexive Name Component Type . . . . . . . . . . . . 30
8.2.2. Reflexive Name Prefix TLV . . . . . . . . . . . . . . 30
8.2.3. PIT Tokens for NDNLPv2 . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
9.1. Reflexive Name Prefix TLV . . . . . . . . . . . . . . . . 31
9.2. Forward and Reverse PIT-Token hop-by-hop option TLVs . . 32
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
10.1. Collisions of reflexive Interest names . . . . . . . . . 33
10.2. Additional resource pressure on PIT and FIB . . . . . . 34
10.3. Potential Vulnerabilities from the use of PIT Tokens . . 34
10.4. Privacy Considerations . . . . . . . . . . . . . . . . . 34
11. Normative References . . . . . . . . . . . . . . . . . . . . 35
12. Informative References . . . . . . . . . . . . . . . . . . . 35
Appendix A. Alternative Designs Considered . . . . . . . . . . . 39
A.1. Handling reflexive interests using dynamic FIB entries . 40
A.1.1. Design complexities and performance issues with
FIB-based design . . . . . . . . . . . . . . . . . . 40
A.1.2. Interactions between FIB-based design and Interest
Lifetime . . . . . . . . . . . . . . . . . . . . . . 42
A.2. Reflexive forwarding using Path Steering . . . . . . . . 42
Appendix B. Implementations . . . . . . . . . . . . . . . . . . 44
B.1. Reflexive Name Segment . . . . . . . . . . . . . . . . . 45
B.2. Processing Reflexive Interests> . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
Current ICN protocols such as CCNx [RFC8569] and NDN [NDN] have a
wide range of useful applications in content retrieval and other
scenarios that depend only on a robust two-way exchange in the form
of a request and response. These ICN architectures use the terms
"consumer" and "producer" for the respective roles of the requester
and the responder, and the protocols directly capture the mechanics
of the two-way exchange through the "Interest message" carrying the
request, and the "Data message" carrying the response. Through these
constructs, the protocols are heavily biased toward a pure _pull-
based_ interaction model where requests are small (carrying little or
no user-supplied data other than the name of the requested data
object), and responses are relatively large - up to an architecture-
defined maximum transmission unit (MTU) on the order of kilobytes or
tens of kilobytes.
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A number of important applications however require interaction models
more complex than individual request/response interactions in the
same direction (i.e. between the same consumer and one or more
producers). Among these we identify three important classes which
are the target of the proposed enhancements defined in this
specification. These are described in the following paragraphs.
*Remote Method Invocation (RMI, aka RPC):* When invoking a remote
method, it is common for the method to require arguments supplied
by the caller. In conventional TCP/IP style protocols like CORBA
or HTTP "Post", these are pushed to the server as part of the
message or messages that comprise the request. In ICN-style
protocols there is an unattractive choice between inflating the
request initiation with pushed arguments, or arranging to have one
or more independent request/response pairs in the opposite
direction for the server to fetch the arguments. Both of these
approaches have substantial disadvantages. Recently, a viable
alternative emerged through the work on RICE [Krol2018] which
pioneered the main design elements proposed in this specification.
*Phone-Home scenario:* Applications in sensing, Internet-of-things
(IoT) and other types where data is produced unpredictably and
needs to be _pushed_ somewhere create a conundrum for the pure
pull-based architectures considered here. If instead one eschews
relaxing the size asymmetry between requests and responses, some
additional protocol machinery is needed. Earlier efforts in the
ICN community have recognized this issue and designed methods to
provoke a cooperating element to issue a request to return the
data the originator desires to push, essentially "phoning home" to
get the responder to fetch the data. One that has been explored
to some extent is the _Interest-Interest-Data_ exchange
[Carzaniga2011], where an Interest is sent containing the desired
request as encapsulated data. CCNx-1.0 Bidirectional Streams
[Mosko2017] are also based on a scheme where an Interest is used
to signal a name prefix that a consumer has registered for
receiving Interests from a peer in a bidirectional streaming
session.
*Peer state synchronization:* A large class of applications,
typified by those built on top of reliable order-preserving
transport protocols, require initial state synchronization between
the peers. This is accomplished with a three-way (or longer)
handshake, since employing a two-way handshake as provided in the
existing NDN and CCNx protocols exposes a number of well-know
hazards, such as _half-open connections_. When attempted for
security-related operations such as key exchange, additional
hazards such as _man-in-the-middle_ attacks become trivial to
mount. Existing alternatives, similar to those used in the two
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examples above, instead utilize either overlapping Interest-Data
exchanges in opposite directions (resulting in a four-way
handshake) or by adding initialization data to the initial request
and employing an Interest-Interest-Data protocol extension as
noted in the Phone-home scenarios above.
All of the above application interaction models present interesting
challenges, as neither relaxing the architecture to support pushing
large amounts of data, nor introducing substantial complexities
through multiple independent Interest-Data exchanges is an attractive
approach. The following subsections provide further background and
justification for why push and/or independent exchanges are
problematical.
1.1. Problems with pushing data
There are two substantial problems with the simple approach of just
allowing arbitrary amounts of data to be included with requests.
These are:
1. In ICN protocols such as NDN and CCNx, Interest messages are
intended to be small, on the order the size of a TCP ACK, as
opposed to the size of a TCP data segment. This is because the
hop-by-hop congestion control and forwarder state management
requires Interest messages to be buffered in expectation of
returning data, and possibly retransmitted hop-by-hop as opposed
to end-to-end. In addition, the need to create and manage state
on a per-Interest basis is substantially complicated if requests
in Interest messages are larger than a Path MTU (PMTU) and need
to be fragmented hop-by-hop.
2. If the payload data of a request is used for invoking a
computation (as in the RMI case described above) then substantial
bandwidth can be wasted if the computation is either refused or
abandoned for any number of reasons, including the requestor
failing an authorization check, or the responder not having
sufficient resources to execute the associated computation.
These problems also exist in pure datagram transport protocols such
as those used for legacy RMI applications like NFS [RFC7530]. More
usual are application protocols like HTTP(s) which rely on the TCP or
QUIC 3-way handshake to establish a session and then have congestion
control and segmentation provided as part of the transport protocol,
further allowing sessions to be rejected before large amounts of data
are transmitted or significant computational resources expended.
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1.2. Problems with utilizing independent exchanges
In order to either complete a three-way handshake, or fetch data via
a pull from the original requestor, the role of consumer and producer
need to be reversed and an Interest/Data exchange initiated in the
direction opposite of the initiating exchange. When done with an
independent Interest/Data request and response, a number of
complications ensue. Among them are:
1. The originating consumer needs to have a routable name prefix
that can be used for the exchange. This means the consumer must
arrange to have its name prefix propagated in the ICN routing
system with sufficient reach that the producer issuing the
interest can be assured it is routed appropriately. While some
consumers are generally online and act as application servers,
justifying the maintenance of this routing information, many do
not. Further, in mobile environments, a pure consumer that does
not need to have a routable name prefix can benefit from the
inherent consumer mobility support in the CCNx and NDN protocols.
By requiring a routable name prefix, extra mobile routing
machinery is needed, such as that proposed in KITE [Zhang2018] or
MAPME [Auge2018].
2. The consumer name prefix in item (1) above must be communicated
to the producer as a payload, name suffix, or other field of the
initiating Interest message. Since this name in its entirety is
chosen by the consumer, it is highly problematic from a security
standpoint, as it can recruit the producer to mount a reflection
attack against the consumer's chosen victim.
3. The correlation between the exchanges in opposite directions must
be maintained by both the consumer and the producer as
independent state, as opposed to being architecturally tied
together as would be the case with a conventional 3-way handshake
finite state machine. While this can of course be accomplished
with care by both parties, experience has shown that it is error
prone (for example see the checkered history of interactions
between the SIP [RFC3261] and SDP Offer-Answer [RFC6337])
protocols. When employed as the wrapper for a key management
protocol such as with TLS [RFC8446] state management errors can
be catastrophic for security.
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2. 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 BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown
here.
3. Overview of the Reflexive Forwarding design
This specification defines a _Reflexive Forwarding_ extension to CCNx
and NDN that avoids the problems enumerated in Sections 1.1 and 1.2.
It straightforwardly exploits the hop-by-hop state and symmetric
routing properties of the current protocols.
Figure 1 below illustrates a canonical NDN/CCNx forwarder with its
conceptual data structures of the Content Store (CS), Pending
Interest Table (PIT) and Forwarding Information Base (FIB). The key
observation involves the relation between the PIT and the FIB. Upon
arrival of an Interest, a PIT entry is created which contains state
recording the incoming interface on which the Interest was received
on. If the Interest is not immediately satisfied by cached data in
the CS, the forwarder looks up the name in the FIB to ascertain the
_next-hop_ to propagate the Interest onward upstream toward the named
producer. Therefore, a chain of forwarding state is established
during Interest forwarding that couples the PIT entries of the chain
of forwarders together conceptually as _breadcrumbs_. These are used
to forward the returning Data Message over the inverse path through
the chain of forwarders until the Data message arrives at the
originating consumer. The state in the PITs is _unwound_ by
destroying it as each PIT entry is _satisfied_. This behavior is
*critical* to the feasibility of the reflexive forwarding design we
propose.
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+-----------------------------------------------------------------+
| ICN Node |
| Send data to all ======== |
| interfaces that |
| requested it |
| YES +------------------+ |
<------------------------| Pending Interest | <---------------------
| | | Table (PIT) | Data |
| | +------------------+ 1) Find (Signed) |
| | 2) Save | Name |
| V Data | NO in |
| +---------------+ | PIT? |
| | Content Store | | |
| | (CS) | | |
| +---------------+ | |
| | |
| V |
| Drop Data |
| |
+-----------------------------------------------------------------+
+-----------------------------------------------------------------+
| ICN Node |
| ======== |
| |
| +====================+|
| |Forwarding Strategy ||
| +====================+|
| |
| 1) Find name 2) Matching 3) Find matching |
| in CS? name in PIT? entry in FIB? |
| NO NO YES|
| +---------------+ +----------------+ +-------------------+ |
| | Content Store | | Pending | | Forwarding | |
--->| (CS) |-->| Interest |-->| Information Base |-->
| | | | Table (PIT) | | ( FIB) | |
| +---------------+ +----------------+ +-------------------+ |
| Return | YES YES | NO NO | |
| Data | Add | Add | Drop |
| | Incoming | new | or |
| <------| Itf. | Interest | NACK |
| V V |
| |
+-----------------------------------------------------------------+
Figure 1: ICN forwarder structure
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Given the above forwarding properties for Interests, it should be
clear that while an Interest is outstanding and ultimately arrives at
a producer who can respond to it, there is sufficient state in the
chain of forwarders to route not just a returning Data message, but
potentially another Interest directed through the inverse path to the
unique consumer who issued the original Interest. (Section 6.1.3
describes how Interest aggregation of requests to the same target
name from multiple consumers interacts with this scheme.) The key
question therefore is how to access this state in a way that it can
be used to forward Interests.
In order to achieve this _Reflexive Interest_ forwarding on the
inverse path recorded in the PIT of each forwarder, we need a few
critical design elements:
1. The Reflexive Interest needs to have a _Name_. This name is what
the originating consumer will use to match against the Data
object (or multiple Data objects — more on this later) that the
producer may request by issuing the Reflexive Interest. This
cannot be just any name, but needs to essentially name the state
already recorded in the PIT and not allow the consumer to
manufacture an arbitrary name and mount a reflection attack as
pointed out in Section 1.2, Paragraph 2, Item 2.
2. Each forwarder along the inverse path from producer to consumer
must be able to forward the reflexive Interest towards the
direction of the Consumer, without relying on global routing
information, as the Reflexive Name Prefixes are only valid while
the originating Interest/Data exchange state is present at the
forwarders. Essential to this operation is the ability to access
the PIT entry associated with the original Interest message,
since that contains the state necessary to identify the ingress
face of the original Interest, which is the unique (modulo
aggregation) output face over which the Reflexive Interest needs
to be forwarded. The Name assigned by the consumer for Reflexive
Name Prefix in theory is adequate to the task, but entails an
expensive and complicated lookup procedure. In order to make
this lookup both simple and efficient, we adopt an extended
version of the "PIT-Token" scheme pioneered by the high-speed
NIST NDN forwarder [Shi2020]. In this specification, we are
using _Forward Direction PIT Tokens_ (FPTs) that nodes attach to
forwarded Interests in the upstream direction, and _Reverse
Direction PIT Tokens_ (RPTs) that nodes attach to Reflexive
Interests (as well as regular Data messages) in the downstream
direction. We describe the specific processing requirements in
more detail below.
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3. There has to be coupling of the state between the originating
Interest-Data exchange and the enclosed Reflexive Interest-Data
exchange at both the consumer and the producer. In our design,
this is accomplished by the way reflexive interest names are
chosen.
4. Detailed Specification of Reflexive forwarding
The following sections provide the normative details on each of these
design elements.
4.1. Terminology
For general ICN-related terms, we adopt the terms defined in
[RFC8793]. For CCNx-specific terms, we use the terms defined in
[RFC8609]. This specification defines the following additional
terms:
*Reflexive Interest* (RI) An interest message sent from a producer
back towards a consumer to fetch data needed to satisfy the
original interest
*Reflexive Name Segment* A high-order typed name segment which
identifies an Interest as being a Reflexive Interest instead of a
normal Interest.
*Reflexive Name Prefix TLV* (RNP) An Interest message TLV indicating
to a producer that it may fetch data from the sending consumer by
issuing one or more corresponding Relfexive Interests. It
consists of a Name prefix whose high-order is a Reflexive Name
Segment as defined above.
*PIT Token* A hop-by-hop option in either an Interest or data
message allowing a forwarder to locate the PIT entry of the
corresponding Interest/Data exchange without the overhead of a
full exact name lookup
*Forward PIT Token* (FPT) A PIT Token added when forwarding an
Interest to refer to the PIT entry the forwarder has made for that
Interest/Data exchange
*Reverse PIT Token* (RPT) A PIT token (copied from the corresponding
forward PIT token) added to either a Data message or a Reflexive
Interest to allow a forwarder to locate the PIT entry of the
corresponding interest/Data exchange without the over head of a
full exact name lookup
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4.2. Reflexive Forwarding message flow
The overall interaction flow for reflexive forwarding is illustrated
below in Figure 2.
+-----------+ +-----------+ +-----------+
| Consumer | | Forwarder | | Producer |
+-----------+ +-----------+ +-----------+
| | |
| I1 | |
|------------------------->| |
| -------------\ | |
| | Record RNP |-| |
| | in PIT(I1) | | |
| |------------| | |
| | --------------------\ |
| |-| Add FPT to I1.FPT | |
| | |-------------------| |
| | |
| | I1 |
| |---------------------------->|
| | | ------------\
| | |-| Check |
| | | | prefix, |
| | | | creating |
| | | | Reflexive |
| | | | Interest |
| | | | state |
| | | |-----------|
| | ------------------\ |
| | | I1.FPT->RI1.RPT |-|
| | |-----------------| |
| | |
| | RI1 |
| |<----------------------------|
| | ----------------------\ |
| |-| use RI1.RPT to find | |
| | | PIT(I1), | |
| | | check match | |
| | | of PIT(I1).RNP | |
| | | create PIT(RI1), | |
| | | forward RI1 | |
| | |---------------------| |
|------------------------\ | |
|| add I1.RPT and I2.FPT |-| |
|| to PIT(I2) | | |
||-----------------------| | |
| | |
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| RI1 | |
|<-------------------------| |
| | |
| RD1 obj | |
|------------------------->| |
|------------------------\ | |
|| consume PIT(I2) entry |-| |
|| and forward RD1 | | |
||-----------------------| | |
| -------------------\ | |
| | RI1.FPT->RD1.RPT |-| |
| |------------------| | |
| | -------------------\ |
| |-| satisfy PIT(RI1) | |
| | |------------------| |
| | |
| | RD1 |
| |---------------------------->|
| | | -------------\
| | |-| all |
| | | | parameters |
| | | | received, |
| | | | answer I1 |
| | | |------------|
| | |
| | D1 object |
| |<----------------------------|
| -------------------\ | |
| | satisfy PIT(I1), |-| |
| | forward D1 | | |
| |------------------| | |
| -----------------\ | |
| | I1.FPT->D1.RPT |-| |
| |----------------| | |
| | |
| D1 | |
|<-------------------------| |
| | |
Legend:
I1: Interest #1 containing the Reflexive Name Prefix TLV
RI1: Reflexive Interest with Reflexive Name Prefix Segment
RNP: Reflexive Name Prefix
FPT: Forward Direction PIT Token
RPT: Reverse Direction PIT Token
D1: Data message, answering initiating I1 Interest
RD1: Data message, answering RI
Figure 2: Message Flow Overview
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4.3. Consumer Operation
A consumer that wants to employ Reflexive Forwarding MUST create an
Interest (I1) with a Reflexive Name Prefix (RNP) TLV that is used by
the producer when issuing Reflexive Interests (RI) back to the
consumer. Upon receiving a Reflexive Interest (e.g. RI1 in
Figure 2) from a Producer in response to the Interest whose first
name segment is the RNP supplied earlier, the consumer SHOULD perform
a name match against the object specified in the Reflexive Name, and
return that object to the producer in a conventional Data message,
(e.g. RD1 in Figure 2).
4.4. Naming of Reflexive Interests
A consumer may have one or more objects for the producer to fetch,
and therefore needs to communicate enough information in its initial
Interest to allow the producer to construct properly formed Reflexive
Interest names. For some applications the set of _full names_ (see
the ICN Terminology RFC [RFC8793]) is known a priori, for example
through compile time bindings of arguments in an interface definition
or by the architectural definition of a simple sensor reading. In
other cases, the full names of the individual objects must be
communicated via the RNP in the original Interest message.
We define a new typed name segment, identified by a registered name
segment type in the IANA registry for [RFC8609]. We call this the
_Reflexive Interest Name Segment type_. It MUST be the first (i.e.
high order) name segment of any Reflexive Interest issued by a
producer. Its value is a random 128 bit quantity, assigned by the
consumer, which provides the entropy required to uniquely identify
the issuing consumer for the duration of any outstanding Interest-
Data exchange. We suggest using a UUID as specified in [RFC4122] but
any scheme that meets the randomness and entropy requirements can
suffice. The consumer SHOULD choose a different random value for
each Interest message it constructs because:
1. If there is insufficient randomness, a name collision on the
Reflexive Names could occur at any of the intermediate forwarders
which would result in the same mutability problems generated by
poor name selection in other contexts; and
2. Re-use of the same reflexive interest name over multiple
interactions might reveal linkability information that could be
used by surveillance adversaries for tracking purposes.
This initial name segment is either communicated by itself through a
_Reflexive Name Prefix TLV_ in the originating Interest, or prepended
to any object names the consumer wishes the producer to fetch
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explicitly where there is more than one object needed by the producer
for the current Interest-Data interaction. There are four cases to
consider:
1. The reflexive _fullname_ of a single object to fetch.
2. A single reflexive name prefix out of which the producer can (by
application-specific means) construct a number of _fullnames_ of
the objects it may want to fetch.
3. The reflexive _fullname_ of a FLIC Manifest [I-D.irtf-icnrg-flic]
enumerating the suffixes that may be used by the producer to
construct the necessary names. We distinguish this from the
single object fetch in case (1) above because the use of a
Manifest implies multiple reflexive Interest/Data exchanges with
the consumer.
4. Multiple Reflexive Name Prefix TLVs MAY be included in the
Interest message if none of the above 3 options covers the
desired use case.
The last of the four options above, while not explicitly outlawed,
SHOULD NOT be used. This is because it results in a longer Interest
message and requires extra PIT resources. Hence, it is more likely a
forwarder will reject the Interest for lack of resources. A
forwarder MAY optimize for the case of a single Reflexive Name TLV at
the expense of those with more than one.
A producer, upon receiving an Interest with one or more Reflexive
Name Prefix TLVs, may decide it needs to retrieve the associated data
object(s). It therefore can issue one or more Reflexive Interests by
appending the necessary name segments needed to form valid full names
of the associated objects present at the originating consumer. These
in fact comprise conventional Interest-Data exchanges, with no
alteration of the usual semantics with regard to signatures, caching,
expiration, etc. When the producer has retrieved the required
objects to complete the original Interest-Data exchange, it can issue
its Data response, which unwinds all the established state at the
producer, the consumer, and the intermediate forwarders.
4.5. Producer Operation
A producer that has received an Interest with a Reflexive Name Prefix
(RNP) MUST store the supplied RNP and the Forward PIT Token (FPT)
from the received Interest for subsequent (optional, depending on
application semantics) Reflexive Interest sending.
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When sending a Reflexive Interest back to the consumer, the producer
MUST construct a corresponding Interest name based on the RNP and
insert the received Forward PIT Token (FPT) as the Reverse PIT Token
(RPT) TLV in the reflexive Interest.
4.6. Forwarder Operation
The forwarder operation for CCNx and/or NDN is changed in the
following respects when supporting Reflexive Interests. The
requirements are slightly different for a simple forwarder meeting
the mandatory aspects of the specification, versus a forwarder
designed for high-performance, as discussed later in Section 6.1.1.
The main differences are in how PIT lookups are done, and whether the
forwarder only does the steps necessary to process the PIT Tokens
supplied by upstream and downstream forwarders, or whether it also
generates and processes its own PIT Tokens.
1. Upon receiving an Interest containing a Reflexive Name Prefix
(RNP) TLV a forwarder MAY check its CS for a matching Data
Object. if a match is found, the corresponding Data is returned
and the Reflexive Interest is considered satisfied. If no match
is found, proceed with the following steps.
*Note:* Given that reflexive interests initiate exchanges
constrained by the Interest Lifetime of the enclosing Interest/
Data exchange, the value of caching the Data responses in
forwarders is for short-term recovery from lost packets as
opposed to optimizing longer term caching gain. For this reason
we avoid mandating or strongly recommending the CS check for
reflexive interests.
2. The forwarder then MUST record the RNP as an element of the PIT
entry for that Interest. (For interactions with Interest
aggregation, also see Section 6.1.3).
3. When forwarding an Interest with a Reflexive Name Prefix (RNP)
TLV, the forwarder MAY generate a Forward PIT Token (FPT) and
append it to the forwarded Interest to be processed by the next
hop.
4. If an Interest contains a Reverse PIT Token (RPT), the forwarder
MAY use that value to access the corresponding PIT entry, or do a
direct lookup based on the Reflexive Interest Name.
5. The forwarders MUST check that the high-order Name segment of the
Interest is of type Reflexive Name Segment. If not, while this
could strictly speaking be considered an error, the forwarder
SHOULD simply process the Interest as a normal non-reflexive
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Interest and skip the steps below. A match indicates that this
is a Reflexive Interest corresponding to the original consumer to
producer Interest, so execute the following steps.
6. Create a new PIT entry for the Reflexive Interest (if resources
are sufficient). Also, see Section 6.1.1 for how PIT sharding
interacts with the location and creation of PIT entries on high-
speed forwarders.
7. Record the Forward PIT-Token (FPT), if any, in this PIT entry, as
would be done for any received Interest containing an FTP TLV.
8. Look up the ingress face from the originating Interest's PIT
entry, forward the Reflexive Interest on this face, with the
following modifications:
* Append the the RPT from the ingress face information of the
original Interest's PIT entry, if any
* If the downstream forwarder desires the upstream forwarder to
supply an RPT in any returning Data Packet for this Reflexive
interest, optionally append a FPT TLV to the Interest.
The PIT entry for the Reflexive Interest is consumed per regular
Interest/Data message forwarding requirements. The PIT entry for the
originating Interest (that communicated the Reflexive Interest Name)
is also consumed by a final Data message from the producer to the
original consumer.
4.6.1. Forwarder algorithms in pseudocode
This section provides some pseudocode examples to further explain the
details of forwarder operation. It has separate code paths for
minimal forwarder operations and those needed by high-performance
forwarders as is further discussed in Section 6.1.1.
4.6.1.1. Processing of a normal Interest containing a Reflexive Name
Prefix TLV
Create PIT entry for Interest;
IF interest contains FPT
Record FPT along with ingress face to use as RPT later;
Record RNP in PIT entry;
EITHER
Create entry in an RNP look-aside table with RNP value;
OR
Generate a FPT for this PIT entry and add to Interest;
Forward Interest upstream;
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4.6.1.2. Processing of a Reflexive Interest
IF Interest contains an RPT
use RPT to lookup up PIT entry for original interest;
ELSE
Use RNP of Interest's Name TLV to lookup original Interest PIT entry;
IF PIT entry of original Interest not is found
Issue an Interest Return with "No Route" error back to the producer;
RETURN;
ELSE
Create PIT entry for Reflexive Interest;
IF RNP of Reflexive Interest matches RNP in PIT entry
BEGIN
Extract FPT from Original Interest PIT entry (if any);
Add FPT to Reflexive interest as RPT for downstream forwarder;
Optionally, generate and add FPT for the Reflexive Interest for returning Data
END;
ELSE
Process as a normal Interest;
4.7. State coupling between producer and consumer
A consumer that wishes to use this scheme MUST utilize one of the
reflexive naming options defined in Section 4.4 and include it in the
corresponding Interest message. The Reflexive Name TLV _and_ the
full name of the requested data object (that identifies the producer)
identify the common state shared by the consumer and the producer.
When the producer responds by sending Interests with a Reflexive Name
Prefix, the original consumer therefore has sufficient information to
map these Interests to the ongoing Interest-Data exchange.
The exchange is finished when the producer who received the original
Interest message responds with a Data message (or an Interest Return
message in the case of error) answering the original Interest. After
sending this Data message, the producer SHOULD destroy the
corresponding shared state. It MAY decide to use a timer that will
trigger a later state destruction. After receiving this Data
message, the originating consumer MUST destroy the corresponding
Interest-Data exchange state.
5. Use cases for Reflexive Interests
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5.1. Achieving Remote Method Invocation with Reflexive Interests
RICE (Remote Method Invocation in ICN) [Krol2018] used a similar
Reflexive Interest Forwarding scheme that inspired the design
specified in this document (similar to the original design captured
in Appendix A.1).
In RICE, the original Interest denotes the remote method (plus
potential parameters) to be invoked at a producer (server). Before
committing any computing resources, the server can then request
authentication credentials and (optional) parameters using reflexive
Interest-Data exchanges.
When the server has obtained the necessary credentials and input
parameters, it can decide to commit computing resources, starts the
compute process, and returns a handle ("Thunk") in the final Data
message to the original consumer (client).
The client would later request the computation results using a
regular Interest-Data exchange (outside the Reflexive-Interest
transaction), using the Thunk as a name for the computation result.
Figure 3 depicts an abstract message diagram for RICE. In addition
to the 4-way Reflexive Forwarding Handshake (see Figure 2 for the
details of the interaction), RICE adds another (standard) ICN
Interest/Data exchange for transmitting the RMI result. The Thunk
name is provided to the consumer in the D1 DATA message (answering
the initial I1 Interest).
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+-----------+ +-----------+
| Consumer | | Producer |
+-----------+ +-----------+
| |
| I1 |
|------------------------->|
| | ---------------------\
| |-| Requesting request |
| | | parameters |
| | | and credentials |
| | |--------------------|
| |
| RI1 |
|<-------------------------|
| |
| RD1 |
|------------------------->|
| | --------------------\
| |-| Commit compute |
| | | resources, |
| | | return Thunk name |
| | |-------------------|
| |
| D1 |
|<-------------------------|
| | ----------------\
| |-| Invoke Remote |
| | | Method |
| | |---------------|
| -------------------\ |
|-| After some time, | |
| | request result | |
| |------------------| |
| |
| I2 |
|------------------------->|
| |
| D2 |
|<-------------------------|
| |
Legend:
I1: Interest #1 containing the Reflexive Name Prefix TLV
D1: Data message, answering initiating I1 Interest,
returning Thunk name
RI1: Reflexive Interest issued by producer
RD1: Data message, answering RI (parameters, credentials)
I2: Regular Interest for Thunk (compute result)
D2: Data message, answering I2
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Figure 3: RICE Message Flow
5.2. RESTful Web Interactions
In todays HTTP-based web, RESTful (Representational State Transfer)
web interactions are realized by sending requests in a client/server
interaction, where the requests provides the application context (or
a reference to it). It has been noted in [Moiseenko2014] that
corresponding requests often exceed the response messages in size,
and that this raises the problems noted in Section 1.1 when
attempting to map such exchanges directly to CCNx/NDN.
Another reason not to include all request parameters in a (possibly
encrypted) Interest message is the fact that a server (that is
serving thousands of clients) would be obliged to receive, possibly
decrypt and parse the complete requests before being able to
determine whether the requestor is authorized, whether the request
can be served etc. Many non-trivial requests could thus lead to
computational overload attacks.
Using Reflexive Interest Forwarding for RESTful Web Interactions
would encode the REST request in the original request, together with
a Reflexive Interest Prefix that the server could then use to get
back to the client for authentication credentials and request
parameters, such as cookies. The request result (response message)
could either be transmitted in the Data message answering the
original request, or — in case of dynamic, longer-running
computations — in a seperate Interest/Data exchange, potentially
leveraging the Thunk scheme described in Section 5.1.
Unlike approaches where clients have to signal a globally routable
prefix to the network, this approach would not require the client
(original consumer) to expose its identity to the network (the
network only sees the temporary Reflexive Name Prefix), but it would
still be possible to authenticate the client at the server.
An initial design for achieving RESTful ICN that employs reflexive
forwarding is presented in [Kutscher2022].
5.3. Achieving simple data pull from consumers with reflexive Interests
An oft-cited use case for ICN network architectures is _Internet of
Things_ (IoT), where the sources of data are limited-resource sensor/
actuators. Many approaches have been tried (e.g. [Baccelli2014],
[Lindgren2016], [Gundogan2018]) with varying degrees of success in
addressing the issues outlined in Section 1.1. The reflexive
forwarding extension may substantially ameliorate the documented
difficulties by allowing a different model for the basic interaction
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of sensors with the ICN network.
Instead of simply acting as a producer (either directly to the
Internet or indirectly through the use of some form of application-
layer gateway), the IoT device need only act as a consumer to
initiate commuhication. When it has data to provide, it issues a
"phone-home" Interest message to a pre-configured rendezvous name
(e.g. an application-layer gateway or ICN Repo [Chen2015]) and
provides a reflexive name prefix TLV for the data it wishes to
publish. The target producer may then issue the necessary reflexive
Interest message(s) to fetch the data. Once fetched, validated, and
stored, the producer then responds to the original Interest message
with a success indication, possibly containing a Data object if
needed to allow the originating device to modify its internal state.
Alternatively, the producer might choose to not respond and allow the
original Interest to time out, although this is NOT RECOMMENDED
except in cases where the extra message transmission bandwith is at a
premium compared to the persistence of stale state in the forwarders.
We note that this interaction approach mirrors the earlier efforts
using Interest-Interest-Data designs.
Figure 4 depicts this interaction with the (optional) D1 message.
See Figure 2 for the details of the general Reflexive Forwarding
interaction.
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+-----------+ +-----------+
| Consumer | | Producer |
+-----------+ +-----------+
------------\ | |
| new IoT |-| |
| data item | | |
| produced | | |
|-----------| | |
---------------\ | |
| "phone home" |-| |
| by notifying | | |
| producer | | |
|--------------| | |
| |
| I1 |
|-------------------->|
| | --------------------\
| |-| generate Interest |
| | | for IoT data |
| | |-------------------|
| |
| RI1 |
|<--------------------|
-----------------\ | |
| send requested |-| |
| data object | | |
|----------------| | |
| |
| RD1 |
|-------------------->|
| | -----------------------\
| |-| finalize interaction |
| | | with optional |
| | | Data message |
| | |----------------------|
| |
| D1 (optional) |
|<--------------------|
| |
Legend:
I1: Interest #1 containing the Reflexive Name Prefix TLV
D1: Data message (OPTIONAL), finalizing interaction
RI1: Reflexive Interest requesting the IoT data
RD1: Data message, answering RI, returning IoT data object
D1: (optional) Data answering I1
Figure 4: "Phone Home" Message Flow
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There are two approaches that the IoT device can use for its response
to a reflexive Interest. It can simply construct a Data Message
bound through the usual ICN hash name to the reflexive Interest name.
Since the scope of any data object bound in this way is only the
duration of the enclosing Interest-Data exchange (see Section 6.2)
the producer would need to itself construct any persistent Data
object, name it, and sign it. This is sometimes the right approach,
as for some applications the identity of the originating IoT device
is not important from an operational or security point of view; in
contrast the identity of the gateway or Repo is what matters.
If alternatively, the persistent Data object should be bound from a
naming and security point of view to the originating IoT device, this
can be easily accomplished. Instead of directly placing the content
in a Data object responding to the reflexive Interest as above, the
consumer encapsulates a complete CCNx/NDN Data message (which
includes the desired name of the data) in the response to the
reflexive Interest message.
The interaction model described above brings a number potential
advantages, some obvious, some less so. We enumerate a few of them
as follows:
* By not requiring the IoT device to be actively listening for
Interests, it can sleep and only wake up if it has something to
communicate. Conversely, parties interested in obtaining data
from the device do not need to be constantly polling in order to
ascertain if there is new data available.
* No forwarder resources are tied up with state apart from the
actual reflexive forwarding interactions. All that is needed is
enough routing state in the FIB to be able to forward the "phone
home" Interest to an appropriate target producer. While this
model does not provide all the richness of a full Pub/Sub system
(like that described in [Gundogan2018]) we argue it is adequate
for a large subclass of such applications.
* The reflexive interest, through either a name suffix or Interest
payload, can give the IoT device useful context from which to
craft its Data object in response. One highly useful parameter
would be a robust clock value for the device to use as a timestamp
of the data, possibly as part of its name, to correctly place it
in a time seres of sensor readings. This substantially alleviates
the need for low-end devices to have a robust time base, as long
as they trust the producer they contact to provide it.
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6. Implementation Considerations
There are a number of important aspects to the reflexive forwarding
design which affect correctness and performance of existing
forwarder, consumer, and producer implementations desiring to support
it. This section discusses the effect of each of these elements on
the CCNx/NDN protocol architecture.
6.1. Forwarder implementation considerations
6.1.1. Interactions with Input Processing of Interest and Data packets
Reflexive Interests are designed specifically to be no different from
any other Interest other than the use of the Reflexive Name Segment
type as their high-order name segment. This means that a forwarder
does not have to have special handling in terms of creation, and
destruction, and other Interest processing needs such as timeouts,
Interest satisfaction, and caching of returning data in the CS if
desired. However, this design does require additional processing for
Reflexive Interests not needed in the absence of reflexive
forwarding. The most significant requirements are:
* In order to locate the corresponding PIT entry for the original
Interest, the forwarder's packet input processing needs to be able
to efficiently locate the PIT entry of the original Interest that
contained the RNP TLV.
* Ensure that the high order name segment of the Reflexive Interest
matches the RNP stored in that PIT entry.
There are a few additional considerations to highlight for high-speed
forwarders however; these are discussed in the following paragraphs.
In order to achieve forwarding scalability, high speed forwarders
need to exploit available parallelism in both CPU (through multiple
cores) and memory (through multiport DRAM and limiting accesses to
both DRAM and L3 caches). One commonly-used technique is _PIT
sharding_, where the forwarder-global PIT is partitoned among cores
such that all processing of both Interest and Data for a given Name
is directed at the same core, optimizing both L1 I-cache utilization
and L2/L3/DRAM throughput and latency. This is achieved in a number
of implementations (e.g. [So2013]) by hashing the fullname in the
Interest or Data and using that hash to select the assigned
processing core (and associated memory banks). This efficiently
distributes the load and minimizes the number of memory accesses
other than to bytes of the input packet.
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Straightforward input name hashing to achieve a sharded PIT has one
potentially undesirable side effect: the original Interest containing
the Reflexive Name Prefix TLV and any resultant reflexive Interests
issued by the producer will likely hash to different PIT shards,
making any pointers that need to be traversed across shards or cross-
shard updates expensive, possibly dramatically so. One could either
optimize those accesses (as, for example, suggested in the discussion
of Interest Lifetime in Section 6.1.2) or add special input handling
of reflexive interests to steer them to the same shard as the
original interest. This latter technique is what we have specified
by making the use of PIT Tokens similar to those in [Shi2020] an
important element of the design.
6.1.2. Interactions with Interest Lifetime
If and when a producer decides to fetch data from the consumer using
one or more reflexive Interest-Data exchanges, the total latency for
the original Interest-Data exchange is inflated, potentially by
multiple RTTs. It is difficult for a consumer to predict the
inflation factor when issuing the original Interest, and hence there
can be a substantial hazard of that Interest lifetime expiring before
completion of the full multi-way exchange. This can result in
persistent failures, which is obviously highly undesirable.
There is a fairly straightforward technique that can be employed by
forwarders to avoid these "false" Interest lifetime expirations. In
the absence of a superior alternative technique, it is RECOMMENDED
that all forwarders implement the following algorithm.
If and when a reflexive Interest arrives matching the original
Interest's PIT entry, the forwarder examines the Interest lifetime of
the arriving reflexive Interest. Call this value _IL_r_. The
forwarder computes MAX(_IL_t, (IL_r * 1.5)_), and replaces _IL_t_
with this value. This in effect ensures that the remaining Interest
lifetime of the original Interest accounts for the additional 1.5
RTTs that may occur as a result of the reflexive Interest-Data
exchange.
| We note that this is not unduly expensive in this design where
| the two PIT entries are guaranteed to be in the same PIT shard
| on a high speed forwarder. The earlier design discussed in
| Appendix A.1.2 required some additional gymnastics.
While the above approach of inflating the interest lifetime of the
original Interest to accommodate the additional RTTs of reflexive
Interest-Data exchanges avoids the timeout problem, this does
introduce a new vulnerability that must be dealt with. A Producer,
either through a bug or malicious intent, could keep an originating
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Interest-Data exchange alive by continuing to send reflexive
Interests back to the consumer, while the consumer had no way to
terminate the enclosing interaction (there is no "cancel Interest"
function in either NDN nor CCNx). To eliminate this hazard, if the
consumer rejects a reflexive interest with a T_RETURN_PROHIBITED
error, the forwarder(s), in addition to satisfying the coresponding
PIT entry, MUST also delete the RNP from the original Interest's PIT
entry, thereby preventing any further reflexive Interests from
reaching the consumer. This allows the enclosing Interest-Data
exchange to either time out or be correctly ended with a Data message
or Interest Return from the Producer.
6.1.3. Interactions with Interest aggregation and multi-path/multi-
destination forwarding
As with numerous other situations where multiple Interests for the
same named object arrive containing different parameters (e.g.
Interest Lifetime, QoS, payload hash) the same phenomenon occurs for
the reflexive Name Prefix TLV. If Interests with different reflexive
name prefix TLVs collide, the forwarder MUST NOT aggregate these
Interest messages and instead MUST create a separate PIT entry for
each. This in turn means that a different Forward PIT-Token (FPT)
will be placed in the individual forwarded Interests.
Forwarders supporting multi-path forwarding may of course exploit
this capability for Interests with identical reflexive name prefix
TLVs, like any other Interests. There are two sub-cases of multi-
next hop behavior; regular multi-path (where the split traffic
reconverges further upstream) and multi-destination (where it doesn’t
and the Interest reaches multiple producers).
For multi-path, since the Interests that converge upstream carry
identical reflexive Interest name TLVs, they will get aggregated.
The forwarder might, just as for any other Interest, decide to either
do single or multi-path forwarding of that reflexive Interest. If
sent multi-path in parallel, these also will reconverge on the
inverse path and get aggregated. The inclusion of the Forward PIT-
Token (FPT) in the forwarded Interest is unaffected by multi-path
since it is only used on returning Data messages or Reflexive
Interests to access the correct PIT entry.
For multi-destination, reflexive Interests might get issued by
multiple producers, but they will carry the same reflexive name
prefix and hence be forwarded using the ingress face of the same
original Interest PIT entry until reaching the join point, at which
they will get aggregated and thus handled identically to any other
Interest(s) subject to aggregation.
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6.2. Consumer Implementation Considerations
6.2.1. Data objects returned by the consumer to reflexive name
Interests arriving from a producer
The Data objects returned to the producer in response to a reflexive
Interest are normal CCNx/NDN data objects. The object returned in
response to a reflexive Interest is named with its hash as the
trailing segment of the reflexive Interest name, and hence the scope
of the object is under most circumstances meaningful only for the
duration of the enclosing Interest-Data interaction. This property
is ideal for naming and securing data that is "part of" the enclosing
interaction — things like method arguments, authenticators, and key
exchange parameters, but not for the creation and naming of objects
intended to survive outside the current interaction's scope (c.f.
Section 5.3, which describes how to provide globally-named objects
using encapsulation). In general, the consumer should use the
following guidelines in creating Data messages in response to
reflexive Interest messages from the producer.
(a) Set the recommended cache time (T_CACHETIME) either to zero, or
a value no greater than the Interest lifetime (T_INTLIFE) of the
original Interest messsage.
(b) Set the payload type (T_PAYLDTYPE) according to the type of
object being returned (e.g. object, link, manifest)
(c) Set the expiry time (T_EXPIRY) to a value greater than _now_,
and less than or equal to the _now_ + Interest lifetime
(T_INTLIFE) of the original Interest messsage.
6.2.2. Terminating unwanted reflexive Interest exchanges
A consumer may wish to stop receiving reflxive Interests due to
possible erors or malicious behavior on the part of the producer.
Therefore, if the consumer receives an unwanted reflexive Interest,
it SHOULD reject that interest with a T_RETURN_PROHIBITED error (See
section 10.3.6 of [RFC8609] ). This will provoke the forwarders to
prevent further reflexive Interests from reaching the consumer, as
described above in Section 6.1.2, Paragraph 5.
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6.2.3. Interactions with caching
The reflexive named objects provide "local", temporary names that are
only defined for one specific interaction between a consumer and a
producer. Corresponding Data objects MUST NOT be shared among
multiple consumers (violating this would require special gyrations by
the producer since the reflexive Name utilizes per-consumer/per-
interaction random values). A producer MUST NOT issue an Interest
message for any reflexive name after it has sent the final Data
message answering the original Interest.
Forwarders MAY still cache reflexive Data objects for retransmissions
within a transactions, but they MUST invalidate or remove them from
the content store when they forward the final Data message answering
the original Interest.
6.3. Producer Implementation Considerations
Producers receiving an Interest with a Reflexive Name Segment, MAY
decide to issue Reflexive Interests for the corresponding Data
objects. All Reflexive Interest messages that a producer sends MUST
be sent over the face that the original Interest was received on.
7. Operational Considerations
This extension represents a substantial enhancement to the CCNx/NDN
protocol architecture and hence has important forward and backward
compatibility consequences. The most important of these is that
correct operation of the scheme requires an unbroken chain of
forwarders between the consumer and the desired producer that support
the Reflexive Name Prefix TLV, the Forward and Backward PIT-Token
TLVs and the corresponding forwarder capabilities specified in
Section 4.6. When this invariant is not satisfied, some means is
necessary to detect and hopefully recover from the error. We have
identified three possible approaches to handling the lack of
universal deployment of forwarders supporting the reflexive
forwarding scheme.
The first approach simply lets the producer detect the error by
getting a "no route to destination" error when trying to send an
Interest to a reflexive name. This will catch the error, but only
after forwarding resources are tied up and the producer has done some
work on the original Interest message. Further, the producer would
need a bit of smarts to determine that this is a permanent error and
not a transient to be retried. In order for the consumer to attempt
recovery, there might be a need for some explicit error returned for
the original interest to tell the consumer what the likely problem
is. This approach does not enable an obvious recovery path for the
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consumer either, since if the producer cannot easily detect the
error, the consumer has no way to know if a retry has any chance of
succeeding.
A second approach is to bump the CCNx/NDN protocol version to
explicitly indicate the lack of compatability. Such Interests would
be rejected by forwarders not supporting these protocol extensions.
A consumer wishing to use the Reflexive Name Prefix TLV together with
Reverse PIT-Tokens, would use the higher protocol version on those
Interest messages (but could of course continue to use the current
version number on other Interest messages). This is a big hammer,
but may be called for in this situation because:
(a) it detects the problem immediately and deterministically, and
(b) one could assume an ICN routing protocol that would only forward
to a next hop that supports the updated protocol version number.
The supported forwarder protocol versions would have been
communicated in the routing protocol ahead of time.
A third option is to, as a precondition to utilizing the protocol in
a deployment, create and deploy a neighbor capability exchange
protocol which will tell a downstream forwarder if the upstream can
handle the new TLV. This might avoid the large hammer of updating
the protocol version, but of course this puts a pretty strong
dependency on somebody actually designing and publishing such a
protocol! On the other hand, a neighbor capability exchange protocol
for CCNx/NDN would have a number of other substantial benefits, which
makes it worth seriously considering anyway.
8. Mapping to CCNx and NDN packet encodings
8.1. Packet encoding for CCNx
For CCNx[RFC8609] this specification defines one new Name Segment TLV
type, and two hop-by-hop option TLVs.
+==================+==============+==========================+
| Abbrev | Name | Description |
+==================+==============+==========================+
| T_REFLEXIVE_NAME | Reflexive | Name segment to use as |
| | Name Segment | name prefix in Reflexive |
| | | Interest Messages |
+------------------+--------------+--------------------------+
Table 1: Reflexive Name TLV
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+========+=========+===============================================+
| Abbrev | Name | Description |
+========+=========+===============================================+
| T_FPT | Forward | 1-32 byte value chosen by the forwarder for a |
| | PIT | PIT entry communicated upstream toward a |
| | TOKEN | producer |
+--------+---------+-----------------------------------------------+
| T_RPT | Reverse | 1-32 byte value placed in either a Data |
| | PIT | packet or a Reflexive Interest packet by a |
| | TOKEN | producer or forwarder to allow the downstream |
| | | forwarder to access the PIT entry identified |
| | | by a received forward PIT Token (FPT) |
+--------+---------+-----------------------------------------------+
Table 2: Hop-by-hop PIT Token TLVs
8.2. Packet encoding for NDN
*These are proposed assignments based on [NDNTLV]. Suggestions from
the NDN team would be greatly appreciated.*
8.2.1. Reflexive Name Component Type
The NDN Name component TLVs needs to have a new component type added
with type RNP (for reflexive name prefix). We suggest something
like: *TBD*
| *Note:*It seems like the current 0.2.1 packet format only has
| allocated two name component types — a _GenericNameComponent_
| and a _ImplicitSha256DigestComponent_. Shouldn't there be more
| types by now or is this spec out of date?
8.2.2. Reflexive Name Prefix TLV
The Reflexive Name Prefix TLV needs to be added to the NDN Interest
packet format. We suggest using [RFC4122], hence something like:
+---------+----------+------------------------+
| RNP ::= | RNP-TYPE | TLV-LENGTH(=16) BYTE8) |
+---------+----------+------------------------+
Table 3: Proposed Reflexive Name Prefix TLV
for NDN Interest Packet
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8.2.3. PIT Tokens for NDNLPv2
The current NDN Link Protocol current has an assignment for the PIT
Token mechanism pioneered in [Shi2020]. That approach only needed a
single field, since PIT Tokens are only used to express one
"direction" — for consumer-to-producer Interests and producer-to-
consumer Data messages. This specification employs PIT Tokens not
only on enclosing Interest-Data exchanges, but also on Reflexive
Interests to locate the PIT entry of an enclosing Interest on
reception by a forwarder. Therefore we suggest that the existing
NDNLPv2 assignment of
+---------------+--------------------------------------+
| LpHeaderField | PitToken |
+---------------+--------------------------------------+
| PitToken | PIT-TOKEN-TYPE TLV-LENGTH 1*32OCTET> |
+---------------+--------------------------------------+
Table 4: Current NDNLPv2 PIT Token assignment
be renamed to indicate its use in the forward direction of consumer
to producer Interests and returning Data, and a second allocation be
done for a _Reverse PIT Token_ specifically for inclusion in
Reflexive Interests as follows:
+-----------------+--------------------------------------+
| LpHeaderField | ReversePitToken |
+-----------------+--------------------------------------+
| ReversePitToken | PIT-TOKEN-TYPE TLV-LENGTH 1*32OCTET> |
+-----------------+--------------------------------------+
Table 5: Proposed NDNLPv2 Reverse PIT Token assignment
9. IANA Considerations
9.1. Reflexive Name Prefix TLV
Please add the T_REFLEXIVE_NAME segment TLV type to the CCNx Name
Segment Types TLV types registry of [RFC8609] (see
https://www.iana.org/assignments/ccnx/ccnx.xhtml#ccnx-name-segment-
types), with Length 16 bytes and type of 128 bit random value.
| If possible, please assign the value 0x0003 for this TLV type
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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_REFLEXIVE_NAME | 16 |
+---------------+---------------+---------------+---------------+
| |
| 128 bit value randomly assigned by consumer |
+-------------------------------+-------------------------------+
Figure 5: Reflexive Name segment type
9.2. Forward and Reverse PIT-Token hop-by-hop option TLVs
Please add the T_FPT and T_RPT TLVs to the CCNx Hop-by-Hop Type
Registry of [RFC8609] (see https://www.iana.org/assignments/ccnx/
ccnx.xhtml#ccnx-hop-by-hop-types), with Length 1-32 bytes and type of
random value.
| If possible, please assign the value 0x000A for T_FPT and
| 0x000B for T_RPT.
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_FPT | 1-32 |
+---------------+---------------+---------------+---------------+
| |
| 1-32 byte value randomly assigned by forwarder |
+-------------------------------+-------------------------------+
Figure 6: Forward PIT-Token hop-by-hop TLV
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_RPT | 1-32 |
+---------------+---------------+---------------+---------------+
| |
| 1-32 byte value randomly assigned by forwarder |
+-------------------------------+-------------------------------+
Figure 7: Reverse PIT-Token hop-by-hop TLV
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10. Security Considerations
One of the major motivations for the reflexive forwarding extension
specified in this document is in fact to enable better security and
privacy characteristics for ICN networks. The main considerations
are presented in Section 1, but we briefly recapitulate them here:
* Current approaches to authentication and data transfer often use
payloads in Interest messages, which are clumsy to secure
(Interest messages must be signed) and as a consequence make it
very difficult to ensure consumer privacy. Reflexive forwarding
moves all sensitive data to the Data messages sent in response to
reflexive Interests, which are secured in the same manner as all
other Data messages in the CCNx and NDN protocol designs.
* In many scenarios, consumers are forced to also act as producers
so that data may be fetched by either a particular, or arbitrary
other party. The means the consumer must arrange to have a
routable name prefix and that prefix be disseminated by the
routing protocol or other means. This represents both a privacy
hazard (by revealing possible important information about the
consumer) and a security concern as it opens up the consumer to
the full panoply of flooding and crafted Interest Denial of
Service attacks.
* In order to achieve multi-way handshakes, in current designs a
consumer wishing a producer to communicate back must inform the
producer of what (globally routable) name to use. This gives the
consumer a convenient means to mount a variety of reflection
attacks by enlisting the producer to send Interests to desired
victims.
As a major protocol extension however, this design brings its own
potential security issues, which are discussed in the following
subsections.
10.1. Collisions of reflexive Interest names
Reflexive Interest names are constructed using 128-bit random
numbers. This is intended to ensure an off-path attacker cannot
easily manufacture a matching reflexive Interest and either
masquerade as the producer, or mount a denial of service attack on
the consumer. It also limits tracking through the linkability of
Interests containing a re-used random value.
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Therefore consumers MUST utilize a robust means of generating these
random values, and it is RECOMMENDED that the [RFC4122] format be
used, with a pseudo-random number generator (PRNG) approved for use
with cryptographic protocols.
10.2. Additional resource pressure on PIT and FIB
Normal Interest message processing in CCNx and NDN needs to consider
effect of various resource depletion attacks on the PIT, particularly
in the form of Interest flooding attacks (see [Gasti2012] for a good
overview of DoS and DDoS mitigation on ICN networks). Interest
messages utilizing this reflexive forwarding extension can place
additional resource pressure on the PIT.
While this does not represent a new DoS/DDoS attack vector, the
ability of a malicious consumer to utilize this extension in an
attack does represent an increased risk of resource depletion,
especially if such Interests are given unfair access to PIT and FIB
resources. Implementers SHOULD therefore protect PIT and FIB
resources by weighing requests for reflexive forwarding resources
appropriately relative to other Interests.
10.3. Potential Vulnerabilities from the use of PIT Tokens
By including PIT Tokens in the CCNx or NDN protocol, an attacker has
the opportunity to manipulate these values by either replacement or
elision. So far we do not have enough experimental data nor formal
security analysis to assess whether useful attacks against the
protocol via the PIT Tokens can be mounted. The fields are carried
differently in CCNx and NDN, but in both cases they are outside the
cryptographic integrity envelope and not encrypted for
confidentiality as part of the base protocols.
For both cases however, the potential vulnerabilities can be foiled,
at least for point-to-point communication over an L2 hop, by
employing either link-layer encryption (in the case of CCNx), or by
encrypting the NDNLPv2 protocol, which carries these fields for NDN.
10.4. Privacy Considerations
ICN architectures like CCNx and NDN provide a rich tapestry of
interesting privacy issues, which have been extensively explored in
the research literature. The fundamental tradeoffs for privacy
concern the risk of exposing the names of information objects to the
forwarding elements of the network, which is a necessary property of
any name-based routing and forwarding design. Numerous approaches
have been explored with varying degrees of success, such as onion
routing ([DiBenedettoGTU12]), name encryption ([Ghali2017]), and name
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obfuscation ([Arianfar2011]) among others.
Reflexive forwarding does not change the overall landscape of privacy
tradeoffs, nor seem to introduce additional hazards. In fact, the
privacy exposures are confined to the inverse path of forwarders from
the producer to the consumer, through which the original Interest
forwarding may have already exposed names on path. Similar name
privacy techniques to those cited above may be equally applied to the
names in reflexive Interests.
While the individual reflexive Interest-Data exchanges have similar
properties to those in any NDN or CCNx exchange, the target usages by
applications may have interaction patterns that are subject to
relatively straightforward fingerprinting by adversaries. For
example, a particular remote method invocation may fingerprint simply
through the count of arguments fetched by the producer and their
sizes. The attacker must however be on path, which somewhat
ameliorates the exposure hazards.
11. 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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8569] Mosko, M., Solis, I., and C. Wood, "Content-Centric
Networking (CCNx) Semantics", RFC 8569,
DOI 10.17487/RFC8569, July 2019,
<https://www.rfc-editor.org/info/rfc8569>.
[RFC8609] Mosko, M., Solis, I., and C. Wood, "Content-Centric
Networking (CCNx) Messages in TLV Format", RFC 8609,
DOI 10.17487/RFC8609, July 2019,
<https://www.rfc-editor.org/info/rfc8609>.
12. Informative References
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[Arianfar2011]
Arianfar, S., Koponen, T., Raghavan, B., and S. Shenker,
"On preserving privacy in content-oriented networks, in
ICN '11: Proceedings of the ACM SIGCOMM workshop on
Information-centric networking",
DOI https://doi.org/10.1145/2018584.2018589, August 2011,
<https://dl.acm.org/doi/10.1145/2018584.2018589>.
[Auge2018] Augé, J., Carofiglio, G., Grassi, G., Muscariello, L.,
Pau, G., and X. Zeng, "MAP-Me: Managing Anchor-Less
Producer Mobility in Content-Centric Networks, in IEEE
Transactions on Network, Volume 15, Issue 2",
DOI 10.1109/TNSM.2018.2796720, June 2018,
<https://ieeexplore.ieee.org/document/8267132>.
[Baccelli2014]
Baccelli, E., Mehlis, C., Hahm, O., Schmidt, T., and M.
Wählisch, "Information centric networking in the IoT:
experiments with NDN in the wild, in ACM-ICN '14:
Proceedings of the 1st ACM Conference on Information-
Centric Networking", DOI 10.1145/2660129.2660144,
September 2014,
<https://dl.acm.org/doi/abs/10.1145/2660129.2660144>.
[Carzaniga2011]
Carzaniga, A., Papalini, M., and A.L. Wolf, "Content-Based
Publish/Subscribe Networking and Information-Centric
Networking", DOI 10.1145/2018584.2018599, September 2011,
<https://conferences.sigcomm.org/sigcomm/2011/papers/icn/
p56.pdf>.
[Chen2015] Chen, S., Cao, J., and L. Zhu, "NDSS: A Named Data Storage
System, in International Conference on Cloud and Autonomic
Computing", DOI 10.1109/ICCAC.2015.12, September 2014,
<https://ieeexplore.ieee.org/document/7312154>.
[DiBenedettoGTU12]
DiBenedetto, S., Gasti, P., Tsudik, G., and E. Uzun,
"ANDaNA: Anonymous Named Data Networking Application, in
NDSS 2012", DOI https://arxiv.org/abs/1112.2205v2, 2102,
<https://www.ndss-symposium.org/ndss2012/andana-anonymous-
named-data-networking-application>.
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[Gasti2012]
Gasti, P., Tsudik, G., Uzun, Ersin., and L. Zhang, "DoS
and DDoS in Named Data Networking, in 22nd International
Conference on Computer Communication and Networks
(ICCCN)", DOI 10.1109/ICCCN.2013.6614127, August 2013,
<https://ieeexplore.ieee.org/document/6614127>.
[Ghali2017]
Tsudik, G., Ghali, C., and C. Wood, "When encryption is
not enough: privacy attacks in content-centric networking,
in ICN '17: Proceedings of the 4th ACM Conference on
Information-Centric Networking",
DOI https://doi.org/10.1145/3125719.3125723, September
2017,
<https://dl.acm.org/doi/abs/10.1145/3125719.3125723>.
[Gundogan2018]
Gündoğan, C., Kietzmann, P., Schmidt, T., and M. Wählisch,
"HoPP: publish-subscribe for the constrained IoT, in ICN
'18: Proceedings of the 5th ACM Conference on Information-
Centric Networking", DOI 10.1145/3267955.3269020,
September 2018,
<https://dl.acm.org/doi/abs/10.1145/3267955.3269020>.
[I-D.irtf-icnrg-flic]
Tschudin, C., Wood, C. A., Mosko, M., and D. R. Oran,
"File-Like ICN Collections (FLIC)", Work in Progress,
Internet-Draft, draft-irtf-icnrg-flic-04, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-irtf-icnrg-
flic-04>.
[I-D.irtf-icnrg-pathsteering]
Moiseenko, I. and D. R. Oran, "Path Steering in CCNx and
NDN", Work in Progress, Internet-Draft, draft-irtf-icnrg-
pathsteering-04, 2 September 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-icnrg-
pathsteering-04>.
[Krol2018] Krol, M., Habak, K., Oran, D., Kutscher, D., and I.
Psaras, "RICE: Remote Method Invocation in ICN, in
Proceedings of the 5th ACM Conference on Information-
Centric Networking — ICN '18",
DOI 10.1145/3267955.3267956, September 2018,
<https://conferences.sigcomm.org/acm-icn/2018/proceedings/
icn18-final9.pdf>.
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[Kutscher2022]
Kutscher, D. and D. Oran, "RESTful information-centric
networking: statement, in ICN '22: Proceedings of the 9th
ACM Conference on Information-Centric Networking",
DOI https://doi.org/10.1145/3517212.3558089, September
2022, <https://dl.acm.org/doi/10.1145/3517212.3558089>.
[Lindgren2016]
Lindgren, A., Ben Abdessiem, F., Ahlgren, B., Schlelén,
O., and A.M. Malik, "Design choices for the IoT in
Information-Centric Networks, in 13th IEEE Annual Consumer
Communications and Networking Conference (CCNC)",
DOI 10.1109/CCNC.2016.7444905, January 2016,
<https://ieeexplore.ieee.org/abstract/document/7444905>.
[Moiseenko2014]
Moiseenko, I., Stapp, M., and D. Oran, "Communication
patterns for web interaction in named data networking",
DOI 10.1145/2660129.2660152, September 2014,
<https://dl.acm.org/doi/10.1145/2660129.2660152>.
[Mosko2017]
Mosko, M., "CCNx 1.0 Bidirectional Streams",
arXiv 1707.04738, July 2017,
<https://arxiv.org/abs/1707.04738>.
[NDN] "Named Data Networking", 2020,
<https://named-data.net/project/execsummary/>.
[NDNTLV] "NDN Packet Format Specification", 2016,
<http://named-data.net/doc/ndn-tlv/>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC6337] Okumura, S., Sawada, T., and P. Kyzivat, "Session
Initiation Protocol (SIP) Usage of the Offer/Answer
Model", RFC 6337, DOI 10.17487/RFC6337, August 2011,
<https://www.rfc-editor.org/info/rfc6337>.
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[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <https://www.rfc-editor.org/info/rfc7530>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8793] Wissingh, B., Wood, C., Afanasyev, A., Zhang, L., Oran,
D., and C. Tschudin, "Information-Centric Networking
(ICN): Content-Centric Networking (CCNx) and Named Data
Networking (NDN) Terminology", RFC 8793,
DOI 10.17487/RFC8793, June 2020,
<https://www.rfc-editor.org/info/rfc8793>.
[Shi2020] Shi, J., Pesavento, D., and L. Benmohamed, "NDN-DPDK: NDN
Forwarding at 100 Gbps on Commodity Hardware, in
Proceedings of the 7th ACM Conference on Information-
Centric Networking — ICN '20",
DOI 10.1145/3405656.3418715, September 2020,
<https://dl.acm.org/doi/10.1145/3405656.3418715>.
[So2013] So, W., Narayanan, A., and D. Oran, "Named data networking
on a router: Fast and DoS-resistant forwarding with hash
tables, in proceedings of Architectures for Networking and
Communications Systems", DOI 10.1109/ANCS.2013.6665203,
October 2013,
<https://ieeexplore.ieee.org/document/6665203>.
[Zhang2018]
Zhang, Y., Xia, Z., Mastorakis, S., and L. Zhang, "KITE:
Producer Mobility Support in Named Data Networking, in
Proceedings of the 5th ACM Conference on Information-
Centric Networking — ICN '18",
DOI 10.1145/3267955.3267959, September 2018,
<https://conferences.sigcomm.org/acm-icn/2018/proceedings/
icn18-final23.pdf>.
Appendix A. Alternative Designs Considered
During development of this specification, a number of alternative
designs were considered and at least partially documented. This
appendix explains them for historical purposes, and explains why
these were considered inferior to the design we settled on to carry
forward.
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A.1. Handling reflexive interests using dynamic FIB entries
The original draft specification employed the use of dynamically-
created FIB entries for forwarding Reflexive Interests. In this
approach, at each forwarder along the inverse path from producer to
consumer, a FIB entry must be present that matches this name via
Longest Name Prefix Match (LNPM), so that when the reflexive interest
arrives, the forwarder can forward it downstream toward the
originating consumer. This FIB entry would point directly to the
incoming interface on which the corresponding original Interest
arrived. The FIB entry needs to be created as part of the forwarding
of the original Interest so that it is available in time to catch any
reflexive Interest issued by the producer. It would usually make
sense to destroy this FIB entry when the Data message satisfying the
original Interest arrives since this avoids any dangling stale state.
Given the design details discussed below, stale FIB state would not
represent a correctness hazard and hence could be done lazily if
desired in an implementation.
In this scheme, the forwarder operates as follows:
1. The forwarder creates short-lifetime FIB entries for any
Reflexive Interest Name prefixes communicated in an Interest
message. If the forwarder does not have sufficient resources to
do so, it rejects the Interest with the T_RETURN_NO_RESOURCES
error — the same error used if the forwarder were lacking
sufficient PIT resources to process the Interest message.
2. Those FIB entries are queried whenever an Interest message
arrives whose first name segment is of the type _Reflexive
Interest Name Segment (RNP)_
3. The FIB entry gets removed eventually, after the corresponding
Data message has been forwarded. One option would be to remove
the FIB directly after the Data message has been forwarded.
However, the forwarder might choose do lazy cleanup.
There are a number of additional considerations with this design that
need to be dealt with.
A.1.1. Design complexities and performance issues with FIB-based design
When processing an Interest containing the reflexive name TLV and
creating the necessary FIB entry, the forwarder also creates a _back
pointer_ from that FIB entry to the PIT entry for the Interest
message that created it. This PIT entry contains the current value
of the remaining Interest lifetime or alternatively a value from
which the remaining Interest lifetime can be easily computed. Call
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this value _IL_t_.
The forwarder input thread could key off the high-order name segment
type (one byte) and if reflexive, do a reflexive FIB lookup instead
of a full name hash. The reflexive FIB entry would contain the shard
identity of the matching Interest (concretely, the core id servicing
the shard) and steer the reflexive interest there. The reflexive
name prefix FIB lookup would have to be competitive performance-wise
with a full-name hash for this to win, however. Experimentation is
needed to further evaluate such implementation tradeoffs for input
packet load balancing in high-speed forwarders.
The FIB is a performance-critical data structure in any forwarder, as
it needs to support relatively expensive longest name prefix match
(LNPM) lookup algorithms. A number of well-known FIB data structures
are heavily optimized for read access, since for normal Interest
message processing the FIB changes slowly — only after topological
changes or routing protocol updates. Support for reflexive names
using dynamic FIB entries changes this, as FIB entries would be
created and destroyed rapidly as Interest messages containing
reflexive name TLVs are processed and the corresponding Data messages
come back.
While it may be feasible, especially in low-end forwarders handling a
low packet forwarding rate to ignore this problem, for high-speed
forwarders there are a number of hazards, including:
1. If the entire FIB needs to be locked in order to insert or remove
entries, this could cause inflated forwarding delays or in
extreme cases, forwarding performance collapse.
2. A number of high-speed forwarder implementations employ a sharded
PIT scheme (see Section 6.1.1) to better parallelize forwarding
across processing cores. The FIB, however, is still a shared
data structure which is either read without read locks across
cores, or explicitly copied such that there is a separate copy of
the FIB for each PIT shard. Clearly, a high update rate without
read locks and/or updating many copies of the FIB are
unattractive implementation options. (Note: unlike the adopted
scheme in the main specification, by just depending on a dynamic
FIB it is not feasible to force reflexive interests to be hashed
or be otherwise directed to the PIT shard holding the original
Interest state).
There are any number of alternative FIB implementations that can work
adequately. The most straightforward would be to simply implement a
"special" FIB for just reflexive name lookups. This is feasible
because reflexive names deterministically contain the distinguished
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high-order name segment type of T_REFLEXIVE_NAME, whose content is a
64-bit value that can be easily hashed to a FIB entry directly,
avoiding the more expensive LNPM lookup. Inserts and deletes then
devolve to the well-understood problem of hash table maintenance.
A.1.2. Interactions between FIB-based design and Interest Lifetime
If Interest lifetime handling is implemented naively, it may run
afoul of a sharded PIT forwarder implementation, since the PIT entry
for the reflexive Interest and the PIT entry for the original
Interest may be in different shards. Therefore, if the update is
done cross-shard on each reflexive Interest arrival, performance may
suffer, perhaps dramatically. Instead, the following approach to
updating the Interest lifetime after computing the new value is would
be needed by this FIB-based design for sharded-PIT forwarders.
When creating the reflexive FIB entry as above in Appendix A.1.1,
copy the remaining Interest lifetime from the PIT entry. Do the PIT
update if and only if this value is about to expire, thus paying the
cross-shard update cost only if the original Interest is about to
expire. A further optimization at the cost of modest extra
complexity is to instead _queue_ the update to the core holding the
shard of the original PIT entry rather than doing the update
directly. If the PIT entry expires or is satisfied, instead of
removing it the associated core checks the update queue and does the
necessary update.
A.2. Reflexive forwarding using Path Steering
We also considered leveraging Path Steering
[I-D.irtf-icnrg-pathsteering] Path Labels that inform the forwarder
at each hop which outgoing face to use for for forwarding the
Reflexive Interest. In this approach, the producer, when creating
and issuing the Reflexive Interest with the Reflexive Name Prefix
includes a Path Label to strictly steer the forwarding at all hops
from the producer to the consumer (strict mode Path Steering). This
means, the Reflexive Interest carries the Reflexive Name Prefix, but
forwarders do not apply LNPM or any other outgoing face selection
based on the name. It also eliminates the need for dynamic FIB
entries as discussed above in Appendix A.1. Instead the forwarding
is strictly steered by the Path Label using regular Path Steering
semantics.
The message flow using Path Steering would look like the following:
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+-----------+ +-----------+ +-----------+
| Consumer | | Forwarder | | Producer |
+-----------+ +-----------+ +-----------+
| -----------------------------\ | |
|-| Create I1 with additional, | | |
| | emptyPath Label | | |
| | data structure | | |
| | for reverse discovery | | |
| |----------------------------| | |
| | |
| I1 with Path Label | |
| and RNP TLV | |
|----------------------------------->| |
| | -----------------\ |
| |-| Add path label | |
| | | for adjacency | |
| | | to Consumer | |
| | |----------------| |
| | |
| | I1 |
| |-------------------------->|
| | ------------------\ |
| | | Create RI state |-|
| | |-----------------| |
| | |
| | RI with RNP |
| | and path label |
| | (strict mode) |
| |<--------------------------|
| --------------------------\ | |
| | perform path label |-| |
| | switching (no FIB info) | | |
| |-------------------------| | |
| | |
| RI with RNP | |
|<-----------------------------------| |
| | |
| D2 (RNP) | |
|----------------------------------->| |
| | --------------------\ |
| |-| regular PIT-based | |
| | | forwarding | |
| | |-------------------| |
| | |
| | D2 (RNP) |
| |-------------------------->|
| | -----------------\ |
| | | all parameters |-|
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| | | received, | |
| | | answer orig. | |
| | | I1 Interest | |
| | |----------------| |
| | |
| | D1 |
| |<--------------------------|
| --------------------\ | |
| | regular PIT-based |-| |
| | forwarding | | |
| |-------------------| | |
| | |
| D1 | |
|<-----------------------------------| |
| | |
Legend:
I1: Interest #1 containing the Reflexive Name Prefix TLV
RI: Reflexive Interest with Reflexive Name Prefix Segment
RNP: Reflexive Name Prefix
D1: Data message, answering initiating I1 Interest
D2: Data message, answering RI
Figure 8: Message Flow Overview using Path Steering
Path Steering uses Path Label data structures on the downstream path
(from producer to consumer) to discover and collect hop-by-hop
forwarding information so that consumers can then specify selected
paths for subsequent Interests. Reflexive Forwarding would use the
same data structure, but for "reverse discovery", i.e., in the
upstream direction from consumer to producer.
From an operational perspective the path-steering approach does not
exhibit good properties with respect to backward compatibility.
Without a complete path of forwarders between consumer and producer
that support path steering, reflexive interests cannot reach the
intended consumer. While we might envision a way to steer a
subsequent Interest onto a working path as proposed in
[I-D.irtf-icnrg-pathsteering], there is no capability to force
Interest routing away from an otherwise working path not supporting
the reflexive name TLV.
Appendix B. Implementations
An implementation of Reflexive Forwarind in NDN is available at
https://github.com/nflsjxc/Reflexive-Forwarding-NDN-demo/tree/master.
It differs from this specification in the following ways:
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B.1. Reflexive Name Segment
Currently Reflexive Name Component is the suffix instead of the
prefix for the Reflexive Names. This is easier for prefix matching
in NFD.
B.2. Processing Reflexive Interests>
Currently we omit the RNP checking of Reflexive Interest with RNP in
PIT entry because NFD now uses name-based PITs.
Generally speaking there are 2 types of interests in the message
flow:
1. Reflexive Interest (Consumer -> Producer)
2. Reflexive Interest (Producer -> Consumer)
Currently the two types of Interests are identified by the Reflexive
Name value, if RN=9999, then it is the Reflexive Interest from
Producer to Consumer.
Authors' Addresses
Dave Oran
Network Systems Research and Design
4 Shady Hill Square
Cambridge, MA 02138
United States of America
Email: daveoran@orandom.net
Dirk Kutscher
HKUST(GZ)
Guangzhou
China
Email: ietf@dkutscher.net
URI: https://dirk-kutscher.info
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