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Path Steering in CCNx and NDN
draft-irtf-icnrg-pathsteering-07

The information below is for an old version of the document that is already published as an RFC.
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This is an older version of an Internet-Draft that was ultimately published as RFC 9531.
Authors Ilya Moiseenko , David R. Oran
Last updated 2024-03-11 (Latest revision 2023-09-29)
Replaces draft-oran-icnrg-pathsteering
RFC stream Internet Research Task Force (IRTF)
Intended RFC status Experimental
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Shepherd write-up Show Last changed 2023-04-09
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Send notices to ietf@dkutscher.net
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IANA expert review comments Expert confirmed that issues have been addressed.
draft-irtf-icnrg-pathsteering-07
ICNRG                                                       I. Moiseenko
Internet-Draft                                               Apple, Inc.
Intended status: Experimental                                    D. Oran
Expires: 1 April 2024                Network Systems Research and Design
                                                       29 September 2023

                     Path Steering in CCNx and NDN
                    draft-irtf-icnrg-pathsteering-07

Abstract

   Path Steering is a mechanism to discover paths to the producers of
   ICN content objects and steer subsequent Interest messages along a
   previously discovered path.  It has various uses, including the
   operation of state-of-the-art multipath congestion control algorithms
   and for network measurement and management.  This specification
   derives directly from the design published in _Path Switching in
   Content Centric and Named Data Networks_ (4th ACM Conference on
   Information-Centric Networking - ICN'17) and therefore does not
   recapitulate the design motivations, implementation details, or
   evaluation of the scheme.  Some technical details are different
   however, and where there are differences, the design documented here
   is to be considered definitive.

   This document is a product of the IRTF Information-Centric Networking
   Research Group (ICNRG).  It is not an IETF product and is not an
   Internet Standard.

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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 1 April 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/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Path Steering as an experimental extension to ICN protocol
           architectures . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Essential elements of ICN path discovery and path steering  .   5
     2.1.  Path Discovery  . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Path Steering . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Handling Path Steering errors . . . . . . . . . . . . . .   8
     2.4.  Interactions with Interest Aggregation  . . . . . . . . .   9
     2.5.  How to represent the Path Label . . . . . . . . . . . . .  10
   3.  Mapping to CCNx and NDN packet encodings  . . . . . . . . . .  11
     3.1.  Path label TLV  . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Path label encoding for CCNx  . . . . . . . . . . . . . .  12
     3.3.  Path label encoding for NDN . . . . . . . . . . . . . . .  13
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     5.1.  Cryptographic protection of a path label  . . . . . . . .  16
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Path Steering is a mechanism to discover paths to the producers of
   ICN content objects and steer subsequent Interest messages along a
   previously discovered path.  It has various uses, including the
   operation of state-of-the-art multipath congestion control algorithms
   and for network measurement and management.  This specification
   derives directly from the design published in [Moiseenko2017] and
   therefore does not recapitulate the design motivations,
   implementation details, or evaluation of the scheme.  That
   publication should be considered a normative reference as it is not
   likely a reader will be able to understand all elements of this

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   design without first having read the reference.  Some technical
   details are different however, and where there are differences, the
   design documented here is to be considered definitive.

   Path discovery and subsequent path steering in ICN networks is
   facilitated by the symmetry of forward and reverse paths in the CCNx
   and NDN architectures.  Path discovery is achieved by a consumer
   endpoint transmitting an ordinary Interest message and receiving a
   Content (Data) message containing an end-to-end path label
   constructed on the reverse path by the forwarding plane.  Path
   steering is achieved by a consumer endpoint including a path label in
   the Interest message, which is forwarded to each nexthop through the
   corresponding egress interfaces in conjunction with longest name
   prefix match (LNPM) lookup in the Forwarding Information Base (FIB).

   This document is a product of the IRTF Information-Centric Networking
   Research Group (ICNRG).  It was supported by the ICNRG participants
   during its development and through Research Group last call.  It has
   received detailed review by experts in both the CCNx and NDN
   communities.

1.1.  Path Steering as an experimental extension to ICN protocol
      architectures

   There are a number of important use cases to justify extending ICN
   architectures such as CCNx [RFC8569] or NDN [NDN] to provide these
   capabilities.  These are summarized as follows:

   *  Support the discovery, monitoring and troubleshooting of multi-
      path network connectivity based on names and name prefixes.
      Analogous functions have been shown to be a crucial operational
      capability in multicast and multi-path topologies for IP.  The
      canonical tools are the well-known _traceroute_ and _ping_. For
      point-to-multipoint MPLS the more recent tree trace [RFC8029]
      protocol is used.  Equivalent diagnostic functions have been
      defined for CCNx through the ICN Ping [I-D.irtf-icnrg-icnping] and
      ICN Traceroute [I-D.irtf-icnrg-icntraceroute] specifications, both
      of which are capable of exploiting path steering if available.

   *  Perform accurate online measurement of network performance, which
      generally requires multiple consecutive packets follow the same
      path under control of an application.

   *  Improve the performance and flexibility of multi-path congestion
      control algorithms.  Congestion control schemes such as
      [Mahdian2016] and [Song2018] depend on the ability of a consumer
      to explicitly steer packets onto individual paths in a multi-path
      and/or multi-destination topology.

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   *  A consumer endpoint can mitigate content poisoning attacks by
      directing its Interests onto the network paths that bypass
      poisoned caches.

   The path discovery machinery described here may (and likely will)
   discover paths with varying properties.  [RFC9217] discusses a number
   of open questions in path aware networking, among which is how to
   assess and exploit paths having different properties.  Experimenting
   with ICN path steering may be helpful in further elucidating these
   questions and perhaps shedding light on which path properties are
   most useful for the use cases cited above.

   One nuance compared to other path aware networking approaches is that
   ICN path steering piggybacks path discovery on the base ICN data
   exchange, rather than having a separate path advertisement or
   discovery mechanism.  That means when the recorded path comes back in
   an ICN Data message response, the properties of the path are known
   only implicitly to the consumer as opposed to being explicitly
   labeled.  That makes the question of what properties a consumer uses
   to choose a path one of observation or measurement rather than
   advance selection based on an explicit advertised property (e.g SCION
   [I-D.dekater-panrg-scion-overview]).

   The utility and overall technical quality of this path steering
   capability can be assessed by how well it enables the above use cases
   and what performance and robustness effects it has on the underlying
   ICN protocols and their use in various applications.  A few of the
   open questions that should be addressed through experimentation with
   path steering include:

   *  how much more accurate and useful are measurements of RTT, packet
      loss, etc. through ping and traceroute when utilizing path
      steering?

   *  how much is the performance and robustness of multi-path
      forwarding enhanced by the use of this explicit path steering
      capability?

1.2.  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.

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1.3.  Terminology

   This document uses the general ICN terms that are defined in
   [RFC8793].  In addition we define the following terms specific to
   path steering:

   Path Discovery:  The process of sending an Interest requesting
      discovery of a path and if successful, receiving a Data containing
      a Path Label for the path the corresponding Interest traversed

   Path Steering:  The process of sending an Interest message containing
      the Path Label of a previously discovered path in order that the
      forwarders use that path when forwarding that particular Interest
      message.

   Path Label:  An optional field in the packet indicating a particular
      path from a consumer to either a producer, or a forwarder cache
      that can respond with the requested item.  In an Interest message,
      the Path Label gets built up hop by hop as the interest traverses
      a path.  In a Data message, the Path Label carries the full path
      information back to the consumer for use in one or more subsequent
      Interest messages.

   Nexthop Label:  One entry in a Path Label representing the next hop
      for the corresponding forwarder to use when a path-steered
      Interest message arrives at that forwarder.  A sequence of Nexthop
      Labels constitutes a full Path Label.

2.  Essential elements of ICN path discovery and path steering

   We elucidate the design using CCNx semantics [RFC8569] and extend its
   Packet Encoding [RFC8609] as defined in Section 3.2.  While the
   terminology is slightly different, this design can be applied also to
   NDN, by extending its bespoke packet encodings [NDNTLV] (See
   Section 3.3).

2.1.  Path Discovery

   _End-to-end Path Discovery_ for CCNx is achieved by creating a _path
   label_ and placing it as a hop-by-hop TLV in a CCNx Content (Data)
   message.  The path label is constructed hop-by-hop as the message
   traverses the reverse path of transit CCNx forwarders as shown in the
   first example in Figure 1.  The path label is updated by adding to
   the existing path label the nexthop label of the interface at which
   the Content (Data) message has arrived.  Eventually, when the
   Content(Data) message arrives at the consumer, the path label
   identifies the complete path the Content (Data) message took to reach
   the consumer.  As shown in the second example in the figure, when

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   multiple paths are available, subsequent Interests may be able to
   discover additional paths by omitting a path steering TLV and
   obtaining a new path label on the returning interest.

             Discover and use first path:

                  Consumer                  Interest 1  ___  Interest 2
                     |                          |        ^       |
                     |                          |        |       |
                     |                          |        |       |
                Forwarder 1                     v        |       V
                     | (nexthop 1)          (nexthop 1)  ^   (nexthop 1)
                     |                          |        |       |
                     |                          |        |       |
                Forwarder 2                     v        |       v
        (nexthop 3) / \ (nexthop 2)         (nexthop 2)  ^   (nexthop 2)
                   /   \                        |        |       |
                  /     \                       |        |       |
                 /       \                      |        |       |
                /         \                     |        |       |
               /           \                    |        |       |
         Forwarder 4    Forwarder 3             v        |       v
   (nexthop 5)\             / (nexthop 4)   (nexthop 4)  ^   (nexthop 4)
               \           /                    |        |       |
                \         /                     |        |       |
                 \       /                      |        |       |
                  \     /                       |        |       |
                   \   /                        |        |       |
                    \ /                         v        |       v
                  Producer                     ___     Data 1   ___
                    or
               Content Store

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             Discover and use second path:

                  Consumer                  Interest 3  ___  Interest 4
                     |                          |        ^       |
                     |                          |        |       |
                     |                          |        |       |
                Forwarder 1                     v        |       V
                     | (nexthop 1)          (nexthop 1)  ^   (nexthop 1)
                     |                          |        |       |
                     |                          |        |       |
                Forwarder 2                     v        |       v
        (nexthop 3) / \ (nexthop 2)         (nexthop 3)  ^   (nexthop 3)
                   /   \                        |        |       |
                  /     \                       |        |       |
                 /       \                      |        |       |
                /         \                     |        |       |
               /           \                    |        |       |
         Forwarder 4    Forwarder 3             v        |       v
   (nexthop 5)\             / (nexthop 4)   (nexthop 5)  ^   (nexthop 5)
               \           /                    |        |       |
                \         /                     |        |       |
                 \       /                      |        |       |
                  \     /                       |        |       |
                   \   /                        |        |       |
                    \ /                         v        |       v
                  Producer                     ___     Data 2   ___
                    or
               Content Store

           Figure 1: Basic example of path discovery and steering

2.2.  Path Steering

   Due to the symmetry of forward and reverse paths in CCNx, a consumer
   application can reuse a discovered path label to fetch the same or
   similar (e.g. next chunk, or next Application Data Unit, or next
   pointer in a Manifest [I-D.irtf-icnrg-flic]) Content (Data) message
   over the discovered network path.  This _Path Steering_ is achieved
   by processing the Interest message's path label at each transit ICN
   forwarder and forwarding the Interest through the specified nexthop
   among those identified as feasible by LNPM FIB lookup (Figure 2).

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  ----------------------------------------------------------------------
                                FORWARD PATH
  ----------------------------------------------------------------------

  Interest +---------+  +-----+ (path label) +--------+ (match) Interest
  -------->| Content |->| PIT | ------------>| Label  |---------------->
           |  Store  |  +-----+              | Lookup |
           +---------+   | \ (no path label) +--------+
            |            |  \                    |\(path label mismatch)
  Data      |            |   \                   | \
  <---------+            v    \                  |  \
                    aggregate  \                 |   \
                                \                |    \
                                 \               |     +-----+  Interest
                                  +--------------|---->| FIB | -------->
                                                 |     +-----+
  Interest-Return (NACK)                         v        | (no route)
  <----------------------------------------------+<-------+

  ----------------------------------------------------------------------
                                REVERSE PATH
  ----------------------------------------------------------------------

  Interest-return(NACK) +-----+(update path label) Interest-Return(NACK)
  <---------------------|     |<----------------------------------------
                        |     |
  Data   +---------+    | PIT |  (update path label)                Data
  <------| Content |<---|     |<----------------------------------------
         |  Store  |    |     |
         +---------+    +-----+
                           |
                           | (no match)
                           v

              Figure 2: Path Steering CCNx / NDN data plane

2.3.  Handling Path Steering errors

   Over time, the state of interfaces and the FIB on forwarders may
   change such that, at any particular forwarder, a given nexthop is no
   longer valid for a given prefix.  In this case, the path label will
   point to a now-invalid nexthop.  This is detected by failure to find
   a match between the decoded nexthop ID and the nexthops of the FIB
   entry after LNPM FIB lookup.

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   On detecting an invalid path label, the forwarder SHOULD respond to
   the Interest with an Interest-Return.  We therefore define a new
   _Invalid path label_ response code for the Interest Return message
   and include the current path label as a hop-by-hop header.  Each
   transit forwarder processing the Interest-Return message updates the
   path label in the same manner as Content (Data) messages, so that the
   consumer receiving the Interest-Return (NACK) can easily identify
   which path label is no longer valid.

   A consumer may alternatively request that a forwarder detecting the
   inconsistency forward the Interest by means of normal LNPM FIB lookup
   rather than returning an error.  The consumer endpoint, if it cares,
   can keep enough information about outstanding Interests to determine
   if the path label sent with the Interest fails to match the path
   label in the corresponding returned Content (Data), and use that
   information to replace stale path labels.  It does so by setting the
   FALLBACK MODE flag of the path label TLV in its Interest message.

2.4.  Interactions with Interest Aggregation

   If two or more Interests matching the same PIT entry arrive at a
   forwarder, under current behavior they will be aggregated whether or
   not they carry identical Path Labels TLVs.  This may or may not be
   appropriate.  For example, multiple Interests with different MODES
   (e.g. one with DISCOVERY MODE and one without) will get aggregated,
   and the behavior of the forwarder might therefore be dependent on the
   arrival order of those Interests.  In particular,

   *  If the DISCOVERY MODE Interest arrives first, it will be forwarded
      and potentially discover a new path, while the other Interest
      would be aggregated.  If that Interest carried no Path Label, its
      behavior is essentially unchanged, but if it carried a non
      DISCOVERY MODE Path Label, the consumer's intent for the Interest
      to traverse the specified path will be ignored and it is
      indeterminate if the chosen path will actually be used.

   *  If the two Interests arrive in the reverse order, the DISCOVERY
      MODE Interest will be aggregated and the consumer issuing it does
      not achieve its desire to discover a new path.

   Multiple Interests intended to discover paths (i.e. by carrying the
   DISCOVERY MODE flag defined in Section 2.5) might also be aggregated
   by a forwarder.  This limits the ability to discover multiple paths
   in parallel and instead must be discovered incrementally in
   subsequent exchanges.  In other words, aggregated Interests will all
   discover only one single path carried by one single Data packet.
   This has implications for management applications like Traceroute
   [I-D.irtf-icnrg-icntraceroute] which would likely perform much better

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   if they discover paths in parallel.  Hence, it is RECOMMENDED when
   employing Path Steering that such applications craft their Interests
   with unique name suffixes in order to avoid being aggregated.

      |  While path steering still operates correctly if DISCOVERY MODE
      |  Interests are aggregated, after further experimentation it may
      |  be appropriate to advise that:
      |  
      |     *  a forwarder SHOULD NOT aggregate Interests carrying
      |        different Path Labels, and
      |  
      |     *  SHOULD apply a rate limit to DISCOVERY MODE Interests in
      |        order to limit redundant traffic.

2.5.  How to represent the Path Label

   [Moiseenko2017] presents various options for how to represent a path
   label, with different tradeoffs in flexibility, performance and space
   efficiency.  For this specification, we choose the _Polynomial
   encoding_ which achieves reasonable space efficiency at the cost of
   establishing a hard limit on the length of paths that can be
   represented.

   The polynomial encoding utilizes a fixed-size bit array.  Each
   transit ICN forwarder is allocated a fixed sized portion of the bit
   array.  This design allocates 12 bits (i.e. 4095 as a _generator
   polynomial_) to each intermediate ICN forwarder.  This matches the
   scalability of today's commercial routers that support up to 4096
   physical and logical interfaces and usually do not have more than a
   few hundred active ones.

   +------------------------------------------------------------------+
   |                      Path Label bitmap                           |
   +----------+-----------------+-----------------+-------------------+
   |   index  |  nexthop label  |  nexthop label  |                   |
   +----------+-----------------+-----------------+-------------------+
   |<- 8bit ->|<---- 12bit ---->|<---- 12bit ---->|<----------------->|

                      Figure 3: Fixed size path label

   A forwarder that receives a Content (Data) message encodes the
   nexthop label in the next available slot and increments label index.
   Conversely, a forwarder that receives an Interest message reads the
   current nexthop label and decrements label index.  Therefore, the
   extra computation required at each hop to forward either an interest
   or Content Object message with a path label is minimized and
   constitutes a fairly trivial additional overhead compared to FIB
   lookup and other required operations.

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   This approach results in individual path label TLV instances being of
   fixed pre-computed size.  While this places a hard upper bound on the
   maximum number of network hops that can be represented, this is not a
   significant a practical problem in NDN and CCNx, since the size can
   be pre-set during Content(Data) message encoding based on the exact
   number of network hops traversed by the Interest message.  Even long
   paths of 24 hops will fit in a path label bitmap of 36 bytes if
   nexthop label is encoded in 12 bits.

3.  Mapping to CCNx and NDN packet encodings

3.1.  Path label TLV

   A Path label TLV is the tuple: {[Flags], [Path Label Hop Count],
   [Nexthop Label], [Path label bitmap]}.

   +================+=============+
   |      Flag      | Value (hex) |
   +================+=============+
   | DISCOVERY_MODE |     0x00    |
   +----------------+-------------+
   | FALLBACK_MODE  |     0x01    |
   +----------------+-------------+
   |  STRICT_MODE   |     0x02    |
   +----------------+-------------+
   |   Unassigned   |  0x03-0xFF  |
   +----------------+-------------+

      Table 1: Path label flags

   The Path Label Hop Count (PLHC) MUST be incremented by NDN and CCNx
   forwarders if the Interest packet carries a path label and DISCOVERY
   mode flag is set.  A producer node or a forwarder with cached data
   packet MUST use PLHC in calculation of a path label bitmap size
   suitable for encoding the entire path to the consumer.  The Path
   Label Hop Count (PLHC) MUST be set to zero in newly created Data or
   Interest-Return (NACK) packets.  A consumer node MUST reuse Path
   Label Hop Count (PLHC) together with the Path label bitmap (PLB) in
   order to correctly forward the Interest(s) along the corresponding
   network path.

   If an NDN or CCNx forwarder supports path labeling, the Nexthop label
   MUST be used to determine the correct egress interface for an
   Interest packet carrying either the FALLBACK MODE or STRICT MODE
   flag.  If any particular NDN or CCNx forwarder is configured to
   decrypt path labels of Interest packets (Section Security
   Considerations (Section 5)), then the forwarder MUST

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   1.  decrypt the path label with its own symmetric key,

   2.  update the nexthop label with outermost label in the path label,

   3.  decrement Path Label Hop Count (PLHC), and

   4.  remove the outermost label from the path label.

   If any particular NDN or CCNx forwarder is NOT configured to decrypt
   path labels of Interest packets, then path label decryption SHOULD
   NOT be performed.

   The Nexthop label MUST be ignored by NDN and CCNx forwarders if
   present in Data or Interest-Return (NACK) packets.  If any particular
   NDN or CCNx forwarder is configured to encrypt path labels of Data
   and Interest-Return (NACK) packets (Section Security Considerations
   (Section 5)), then the forwarder MUST encrypt existing path label
   with its own symmetric key, append the nexthop label of the ingress
   interface to the path label, and increment Path Label Hop Count
   (PLHC).  If any particular NDN or CCNx forwarder is NOT configured to
   encrypt path labels of Interest packets, then path label encryption
   SHOULD NOT be performed.

   NDN and CCNx forwarders MUST fallback to longest name prefix match
   (LNPM) FIB lookup if an Interest packet carries an invalid nexthop
   label and the FALLBACK MODE flag is set.

   CCNx forwarders MUST respond with an Interest Return packet
   specifying a T_RETURN_INVALID_PATH_LABEL code if Interest packet
   carries an invalid path label and the STRICT MODE flag is set.  This
   is a new Interrest return code defined herein (see Section 4 for the
   value allocation).

   CCNx forwarders MUST respond with an Interest Return packet
   specifying the existing T_RETURN_MALFORMED_INTEREST code if the
   Interest packet carries a path label TLV with both FALLBACK MODE and
   STRICT MODE flags set.

3.2.  Path label encoding for CCNx

   Path Label is an optional Hop-by-Hop header TLV that can be present
   in CCNx Interest, InterestReturn and Content Object packets.

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   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |         T_PATH_LABEL          |          Length + 4           |
   +---------------+---------------+---------------+---------------+
   |     Flags     |  Path Label   |        Nexthop Label          |
   |               |  Hop Count    |                               |
   +---------------+---------------+---------------+---------------+
   /                                                               /
   /               Path label bitmap (Length octets)               /
   /                                                               /
   +---------------+---------------+---------------+---------------+

            Figure 4: Path label Hop-by-Hop header TLV for CCNx

3.3.  Path label encoding for NDN

   Path Label is an optional TLV for NDN Interest and Data packets which
   is carried in the NDN Link Adaptation Protocol [NDNLPv2] used to wrap
   NDN packets for carriage over various link layer protocols.  NDNLPv2
   was chosen over the NDN packet itself since it can carry hop-by-hop
   information that potentially mutates at each hop and therefore cannot
   be included in the secured hash computation or the signature of NDN
   packets.  Further, it can be used instead of the existing
   NextHopFaceId TLV since it not only can specify the single outgoing
   face for a consumer, but manages the selection and forwarding over an
   entire path.  The Path Label TLV in NDNLPv2 is defined below:

   PathLabel         = PATH-LABEL-TYPE TLV-LENGTH
                       PathLabelFlags
                       PathLabelBitmap

   PathLabelFlags    = PATH-LABEL-FLAGS-TYPE
                       TLV-LENGTH ; == 1
                       OCTET

   NexthopLabel      = PATH-LABEL-NEXTHOP-LABEL-TYPE
                       TLV-LENGTH ; == 2
                       2 OCTET

   PathLabelHopCount = PATH-LABEL-HOP-COUNT-TYPE
                       TLV-LENGTH ; == 1
                       OCTET

   PathLabelBitmap   = PATH-LABEL-BITMAP-TYPE
                       TLV-LENGTH ; == 64
                       64 OCTET

                      Figure 5: Path label TLV for NDN

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   +============================+=========================+
   |            Flag            | (Suggested) Value (hex) |
   +============================+=========================+
   | T_PATH_LABEL               |           0x0A          |
   +----------------------------+-------------------------+
   | T_PATH_LABEL_FLAGS         |           0x0B          |
   +----------------------------+-------------------------+
   | T_PATH_LABEL_BITMAP        |           0x0D          |
   +----------------------------+-------------------------+
   | T_PATH_LABEL_NEXTHOP_LABEL |           0x0E          |
   +----------------------------+-------------------------+
   | T_PATH_LABEL_HOP_COUNT     |           0x0F          |
   +----------------------------+-------------------------+

         Table 2: TLV-TYPE number assignments for NDN

4.  IANA Considerations

   IANA is requested to make the following assignments:

   1.  Please assign the value 0x000A (if still available) for
       T_PATH_LABEL in the *CCNx Hop-by-Hop Types* registry established
       by [RFC8609].

   2.  Please assign the value 0x0A (if still available) for the
       T_RETURN_INVALID_PATH_LABEL in the *CCNx Interest Return Code
       Types"* registry established by [RFC8609].

5.  Security Considerations

   A path is invalidated by renumbering nexthop label(s).  A malicious
   consumer can attempt to mount an attack by transmitting Interests
   with path labels which differ only in a single now-invalid nexthop
   label in order to _brute force_ a valid nexthop label.  If such an
   attack succeeds, a malicious consumer would be capable of steering
   Interests over a network path that may not match the paths computed
   by the routing algorithm or learned adaptively by the forwarders.

   When a label lookup fails, by default an _Invalid path label_
   Interest-Return (NACK) message is returned to the consumer.  This
   contains a path label identical to the one included in the
   corresponding Interest message.  A malicious consumer can therefore
   analyze the message's Hop Count field to infer which specific nexthop
   label had failed and direct an attack to influence path steering at
   that hop.  This threat can be mitigated by the following
   countermeasures:

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   *  A nexthop label of larger size is harder to crack.  If nexthop
      labels are not allocated in a predictable fashion by the routers,
      brute forcing a 32-bit nexthop label requires on average O(2^31)
      Interests.  However, this specification uses nexthop labels with
      much less entropy (12 bits), so depending on computational
      hardness is not workable.

   *  An ICN forwarder can periodically update nexthop labels to limit
      the maximum lifetime of paths.  It is RECOMMENDED that forwarders
      update path labels at least every few minutes.

   *  A void Hop Count field in an _Invalid path label_ Interest-Return
      (NACK) message would not give out the information on which
      specific nexthop label had failed.  An attacker might need to
      brute force all nexthop labels in all combinations.  However, some
      useful diagnostic capability is lost by obscuring the hop count.
      For example the locus of routing churn is harder to pin down
      through analysis of path-steered pings or traceroutes.  A
      forwarder MAY choose to invalidate the hop count in addition to
      changing nexthop labels periodically as above.

   Because ICN forwarders maintain per-face state and forwarding state
   for Interest messages, state inflation attacks are a general concern.
   The addition of path steering capabilities in Interest and Data
   messages does not, however, constitute a meaningful increase in
   susceptibility to such attacks.  This is because:

   *  The labels that identify each forwarding face is state O(number of
      faces) and constitutes a small increase to the existing state
      needed to represent a face.

   *  Interest message data is placed in the PIT.  The path steering
      header does in fact inflate the size of the Interest message and
      hence the PIT state, but not by an amount that is a concern.  The
      forwarder needs to protect against state inflation attacks on the
      PIT in general, and an attacker can mount one as or more easily
      just by issuing interests with long names and/or by including
      Interest payload data.

   ICN protocols can be susceptible to a variety of cache poisoning
   attacks, where a colluding consumer and producer arrange for bogus
   content (with either invalid or inappropriate signatures) to populate
   forwarder caches.  These are generally confined to on-path attacks.
   It is also theoretically possible to launch a similar attack without
   a cooperating producer such that the caches of on-path routers become
   poisoned with the content from off-path routers (i.e. physical
   connectivity, but no route in a FIB for a given prefix).  We estimate
   that without any prior knowledge of the network topology, the

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   complexity of this type of attack is in the ballpark of Breadth-
   First-Search and Depth-First-Search algorithms with the additional
   burden of transmitting 2^31 Interests in order to crack a nexthop
   label on each hop.  Relatively short periodic update of nexthop
   labels and anti- _label scan_ heuristics implemented in the ICN
   forwarder may successfully mitigate this type of attack.

5.1.  Cryptographic protection of a path label

   If the countermeasures listed above do not provide sufficient
   protection against malicious mis-steering of Interests, the path
   label can be made opaque to the consumer endpoint via hop-by-hop
   symmetric cryptography applied to the path labels (Figure 6).  This
   method is viable due to the symmetry of forward and reverse paths in
   CCNx and NDN architectures combined with ICN path steering requiring
   only reads/writes of the topmost nexthop label (i.e. active nexthop
   label) in the path label.  This way a path steering capable ICN
   forwarder receiving a Data (Content) message encrypts the current
   path label with its own non-shared symmetric key prior to adding a
   new nexthop label to the path label.  The Data (Content) message is
   forwarded downstream with unencrypted topmost (i.e active) nexthop
   label and encrypted remaining content of the path label.  As a
   result, a consumer endpoint receives a Data (Content) message with a
   unique path label exposing only the topmost nexthop label as
   cleartext.  A path steering forwarder receiving an Interest message
   performs label lookup using the topmost nexthop label, decrypts the
   path label with its own non-shared symmetric key, and forwards the
   message upstream.

   Cryptographic protection of a path label does not require any key
   negotiation among ICN forwarders, and is no more expensive than
   MACsec or IPsec.  It is also quite possible that strict hop-by-hop
   path label encryption is not necessary and path label encryption only
   on the border routers of the trusted administrative or routing
   domains may suffice.

                               Producer
                               |      ^
                               |      |
        Path Label TLV         |      |           Path Label TLV
   +-----------------------+   |      |     +-----------------------+
   |nexthop label=456      |   v      |     |nexthop label=456      |
   |encrypted path label={}|  Forwarder 3   |encrypted path label={}|
   +-----------------------+   |      ^     +-----------------------+
                               |      |
   path label is encrypted     |      |     path label is decrypted
   with Forwarder 3            |      |     with Forwarder 3
   symmetric key               |      |     symmetric key

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                               |      |
                               |      |
                               |      |
                               |      |
                               |      |
        Path Label TLV         |      |           Path Label TLV
   +-----------------------+   |      |     +-----------------------+
   |nexthop label=634      |   v      |     |nexthop label=634      |
   |encrypted path label=  |  Forwarder 2   |encrypted path label=  |
   | {456}                 |   |      ^     | {456}                 |
   +-----------------------+   |      |     +-----------------------+
                               |      |
   path label is encrypted     |      |     path label is decrypted
   with Forwarder 2            |      |     with Forwarder 2
   symmetric key               |      |     symmetric key
                               |      |
                               |      |
                               |      |
                               |      |
                               |      |
        Path Label TLV         |      |           Path Label TLV
   +-----------------------+   |      |     +-----------------------+
   |nexthop label=912      |   v      |     |nexthop label=912      |
   |encrypted path label=  |  Forwarder 1   |encrypted path label=  |
   | {634, encrypted path  |   |      ^     | {634, encrypted path  |
   | label {456}}          |   |      |     | label {456}}          |
   +-----------------------+   |      |     +-----------------------+
                               |      |
   path label is encrypted     |      |     path label is decrypted
   with Forwarder 1            |      |     with Forwarder 1
   symmetric key               |      |     symmetric key
                               |      |
                               |      |
                               |      |
                               |      |
                               v      |
                               Consumer

         Figure 6: Path label protection with hop-by-hop symmetric
                                cryptography

6.  References

6.1.  Normative References

   [Moiseenko2017]
              Moiseenko, I. and D. Oran, "Path Switching in Content
              Centric and Named Data Networks, in 4th ACM Conference on

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              Information-Centric Networking (ICN 2017)",
              DOI 10.1145/3125719.3125721, September 2017,
              <https://conferences.sigcomm.org/acm-icn/2017/proceedings/
              icn17-2.pdf>.

   [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>.

6.2.  Informative References

   [I-D.dekater-panrg-scion-overview]
              de Kater, C., Rustignoli, N., and A. Perrig, "SCION
              Overview", Work in Progress, Internet-Draft, draft-
              dekater-panrg-scion-overview-04, 7 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-dekater-
              panrg-scion-overview-04>.

   [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-icnping]
              Mastorakis, S., Oran, D. R., Gibson, J., Moiseenko, I.,
              and R. Droms, "ICN Ping Protocol Specification", Work in
              Progress, Internet-Draft, draft-irtf-icnrg-icnping-12, 28
              August 2023, <https://datatracker.ietf.org/doc/html/draft-
              irtf-icnrg-icnping-12>.

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   [I-D.irtf-icnrg-icntraceroute]
              Mastorakis, S., Oran, D. R., Moiseenko, I., Gibson, J.,
              and R. Droms, "ICN Traceroute Protocol Specification",
              Work in Progress, Internet-Draft, draft-irtf-icnrg-
              icntraceroute-11, 17 August 2023,
              <https://datatracker.ietf.org/doc/html/draft-irtf-icnrg-
              icntraceroute-11>.

   [Mahdian2016]
              Mahdian, M., Arianfar, S., Gibson, J., and D. Oran,
              "MIRCC: Multipath-aware ICN Rate-based Congestion Control,
              in Proceedings of the 3rd ACM Conference on Information-
              Centric Networking", DOI 10.1145/2984356.2984365, 2022,
              <http://conferences2.sigcomm.org/acm-icn/2016/proceedings/
              p1-mahdian.pdf>.

   [NDN]      "Named Data Networking", various,
              <https://named-data.net/project/execsummary/>.

   [NDNLPv2]  "Named Data Networking Link Adaptation Protocol v2",
              various, <https://redmine.named-
              data.net/projects/nfd/wiki/NDNLPv2>.

   [NDNTLV]   "NDN Packet Format Specification 0.3.", 2022,
              <https://named-data.net/doc/NDN-packet-spec/current/>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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>.

   [RFC9217]  Trammell, B., "Current Open Questions in Path-Aware
              Networking", RFC 9217, DOI 10.17487/RFC9217, March 2022,
              <https://www.rfc-editor.org/info/rfc9217>.

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   [Song2018] Song, J., Lee, M., and T. Kwon, "SMIC: Subflow-level
              Multi-path Interest Control for Information Centric
              Networking, in 5th ACM Conference on Information-Centric
              Networking", DOI 10.1145/3267955.3267971, 2018,
              <https://conferences.sigcomm.org/acm-icn/2018/proceedings/
              icn18-final62.pdf>.

Authors' Addresses

   Ilya Moiseenko
   Apple, Inc.
   Cupertino, CA
   United States of America
   Email: iliamo@mailbox.org

   Dave Oran
   Network Systems Research and Design
   4 Shady Hill Square
   Cambridge, MA 02138
   United States of America
   Email: daveoran@orandom.net

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