Network Working Group                                       F. Brockners
Internet-Draft                                               S. Bhandari
Intended status: Experimental                                    S. Dara
Expires: April 4, 2019                                      C. Pignataro
                                                                J. Leddy
                                                               S. Youell
                                                                D. Mozes

                                                              T. Mizrahi
                                                               A. Aguado
                                       Universidad Politecnica de Madrid
                                                                D. Lopez
                                                          Telefonica I+D
                                                         October 1, 2018

                            Proof of Transit


   Several technologies such as Traffic Engineering (TE), Service
   Function Chaining (SFC), and policy based routing are used to steer
   traffic through a specific, user-defined path.  This document defines
   mechanisms to securely prove that traffic transited said defined
   path.  These mechanisms allow to securely verify whether, within a
   given path, all packets traversed all the nodes that they are
   supposed to visit.

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
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on April 4, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Proof of Transit  . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Basic Idea  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Solution Approach . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Setup . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  In Transit  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.3.  Verification  . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Illustrative Example  . . . . . . . . . . . . . . . . . .   7
       3.3.1.  Basic Version . . . . . . . . . . . . . . . . . . . .   7  Secret Shares . . . . . . . . . . . . . . . . . .   8  Lagrange Polynomials  . . . . . . . . . . . . . .   8  LPC Computation . . . . . . . . . . . . . . . . .   8  Reconstruction  . . . . . . . . . . . . . . . . .   9  Verification  . . . . . . . . . . . . . . . . . .   9
       3.3.2.  Enhanced Version  . . . . . . . . . . . . . . . . . .   9  Random Polynomial . . . . . . . . . . . . . . . .   9  Reconstruction  . . . . . . . . . . . . . . . . .  10  Verification  . . . . . . . . . . . . . . . . . .  10
       3.3.3.  Final Version . . . . . . . . . . . . . . . . . . . .  11
     3.4.  Operational Aspects . . . . . . . . . . . . . . . . . . .  11
     3.5.  Ordered POT (OPOT)  . . . . . . . . . . . . . . . . . . .  12
       3.5.1.  Nested Encryption . . . . . . . . . . . . . . . . . .  12
       3.5.2.  Symmetric Masking Between Nodes . . . . . . . . . . .  12
   4.  Sizing the Data for Proof of Transit  . . . . . . . . . . . .  13
   5.  Node Configuration  . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Procedure . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  YANG Model  . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18

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   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     8.1.  Proof of Transit  . . . . . . . . . . . . . . . . . . . .  18
     8.2.  Cryptanalysis . . . . . . . . . . . . . . . . . . . . . .  19
     8.3.  Anti-Replay . . . . . . . . . . . . . . . . . . . . . . .  20
     8.4.  Anti-Preplay  . . . . . . . . . . . . . . . . . . . . . .  20
     8.5.  Anti-Tampering  . . . . . . . . . . . . . . . . . . . . .  21
     8.6.  Recycling . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.7.  Redundant Nodes and Failover  . . . . . . . . . . . . . .  21
     8.8.  Controller Operation  . . . . . . . . . . . . . . . . . .  21
     8.9.  Verification Scope  . . . . . . . . . . . . . . . . . . .  22
       8.9.1.  Node Ordering . . . . . . . . . . . . . . . . . . . .  22
       8.9.2.  Stealth Nodes . . . . . . . . . . . . . . . . . . . .  22
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Several deployments use Traffic Engineering, policy routing, Segment
   Routing (SR), and Service Function Chaining (SFC) [RFC7665] to steer
   packets through a specific set of nodes.  In certain cases,
   regulatory obligations or a compliance policy require operators to
   prove that all packets that are supposed to follow a specific path
   are indeed being forwarded across and exact set of pre-determined

   If a packet flow is supposed to go through a series of service
   functions or network nodes, it has to be proven that indeed all
   packets of the flow followed the path or service chain or collection
   of nodes specified by the policy.  In case some packets of a flow
   weren't appropriately processed, a verification device should
   determine the policy violation and take corresponding actions
   corresponding to the policy (e.g., drop or redirect the packet, send
   an alert etc.)  In today's deployments, the proof that a packet
   traversed a particular path or service chain is typically delivered
   in an indirect way: Service appliances and network forwarding are in
   different trust domains.  Physical hand-off-points are defined
   between these trust domains (i.e.  physical interfaces).  Or in other
   terms, in the "network forwarding domain" things are wired up in a
   way that traffic is delivered to the ingress interface of a service
   appliance and received back from an egress interface of a service
   appliance.  This "wiring" is verified and then trusted upon.  The
   evolution to Network Function Virtualization (NFV) and modern service
   chaining concepts (using technologies such as Locator/ID Separation
   Protocol (LISP), Network Service Header (NSH), Segment Routing (SR),

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   etc.) blurs the line between the different trust domains, because the
   hand-off-points are no longer clearly defined physical interfaces,
   but are virtual interfaces.  As a consequence, different trust layers
   should not to be mixed in the same device.  For an NFV scenario a
   different type of proof is required.  Offering a proof that a packet
   indeed traversed a specific set of service functions or nodes allows
   operators to evolve from the above described indirect methods of
   proving that packets visit a predetermined set of nodes.

   The solution approach presented in this document is based on a small
   portion of operational data added to every packet.  This "in-situ"
   operational data is also referred to as "proof of transit data", or
   POT data.  The POT data is updated at every required node and is used
   to verify whether a packet traversed all required nodes.  A
   particular set of nodes "to be verified" is either described by a set
   of secret keys, or a set of shares of a single secret.  Nodes on the
   path retrieve their individual keys or shares of a key (using for
   e.g., Shamir's Secret Sharing scheme) from a central controller.  The
   complete key set is only known to the controller and a verifier node,
   which is typically the ultimate node on a path that performs
   verification.  Each node in the path uses its secret or share of the
   secret to update the POT data of the packets as the packets pass
   through the node.  When the verifier receives a packet, it uses its
   key(s) along with data found in the packet to validate whether the
   packet traversed the path correctly.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Abbreviations used in this document:

   HMAC:      Hash based Message Authentication Code.  For example,
              HMAC-SHA256 generates 256 bits of MAC

   IOAM:      In-situ Operations, Administration, and Maintenance

   LISP:      Locator/ID Separation Protocol

   LPC:       Lagrange Polynomial Constants

   MTU:       Maximum Transmit Unit

   NFV:       Network Function Virtualization

   NSH:       Network Service Header

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   POT:       Proof of Transit

   POT-profile:  Proof of Transit Profile that has the necessary data
              for nodes to participate in proof of transit

   RND:       Random Bits generated per packet.  Packet fields that
              donot change during the traversal are given as input to
              HMAC-256 algorithm.  A minimum of 32 bits (left most) need
              to be used from the output if RND is used to verify the
              packet integrity.  This is a standard recommendation by

   SEQ_NO:    Sequence number initialized to a predefined constant.
              This is used in concatenation with RND bits to mitigate
              different attacks discussed later.

   SFC:       Service Function Chain

   SR:        Segment Routing

3.  Proof of Transit

   This section discusses methods and algorithms to provide for a "proof
   of transit" for packets traversing a specific path.  A path which is
   to be verified consists of a set of nodes.  Transit of the data
   packets through those nodes is to be proven.  Besides the nodes, the
   setup also includes a Controller that creates secrets and secrets
   shares and configures the nodes for POT operations.

   The methods how traffic is identified and associated to a specific
   path is outside the scope of this document.  Identification could be
   done using a filter (e.g., 5-tuple classifier), or an identifier
   which is already present in the packet (e.g., path or service
   identifier, NSH Service Path Identifier (SPI), flow-label, etc.)

   The solution approach is detailed in two steps.  Initially the
   concept of the approach is explained.  This concept is then further
   refined to make it operationally feasible.

3.1.  Basic Idea

   The method relies on adding POT data to all packets that traverse a
   path.  The added POT data allows a verifying node (egress node) to
   check whether a packet traversed the identified set of nodes on a
   path correctly or not.  Security mechanisms are natively built into
   the generation of the POT data to protect against misuse (i.e.
   configuration mistakes, malicious administrators playing tricks with

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   routing, capturing, spoofing and replaying packets).  The mechanism
   for POT leverages "Shamir's Secret Sharing" scheme [SSS].

   Shamir's secret sharing base idea: A polynomial (represented by its
   coefficients) is chosen as a secret by the controller.  A polynomial
   represents a curve.  A set of well-defined points on the curve are
   needed to construct the polynomial.  Each point of the polynomial is
   called "share" of the secret.  A single secret is associated with a
   particular set of nodes, which typically represent the path, to be
   verified.  Shares of the single secret (i.e., points on the curve)
   are securely distributed from a Controller to the network nodes.
   Nodes use their respective share to update a cumulative value in the
   POT data of each packet.  Only a verifying node has access to the
   complete secret.  The verifying node validates the correctness of the
   received POT data by reconstructing the curve.

   The polynomial cannot be constructed if any of the points are missed
   or tampered.  Per Shamir's Secret Sharing Scheme, any lesser points
   means one or more nodes are missed.  Details of the precise
   configuration needed for achieving security are discussed further

   While applicable in theory, a vanilla approach based on Shamir's
   secret sharing could be easily attacked.  If the same polynomial is
   reused for every packet for a path a passive attacker could reuse the
   value.  As a consequence, one could consider creating a different
   polynomial per packet.  Such an approach would be operationally
   complex.  It would be complex to configure and recycle so many curves
   and their respective points for each node.  Rather than using a
   single polynomial, two polynomials are used for the solution
   approach: A secret polynomial which is kept constant, and a per-
   packet polynomial which is public.  Operations are performed on the
   sum of those two polynomials - creating a third polynomial which is
   secret and per packet.

3.2.  Solution Approach

   Solution approach: The overall algorithm uses two polynomials: POLY-1
   and POLY-2.  POLY-1 is secret and constant.  Each node gets a point
   on POLY-1 at setup-time and keeps it secret.  POLY-2 is public,
   random and per packet.  Each node generates a point on POLY-2 each
   time a packet crosses it.  Each node then calculates (point on POLY-1
   + point on POLY-2) to get a (point on POLY-3) and passes it to
   verifier by adding it to each packet.  The verifier constructs POLY-3
   from the points given by all the nodes and cross checks whether
   POLY-3 = POLY-1 + POLY-2.  Only the verifier knows POLY-1.  The
   solution leverages finite field arithmetic in a field of size "prime

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   Detailed algorithms are discussed next.  A simple example is
   discussed in Section 3.3.

3.2.1.  Setup

   A controller generates a first polynomial (POLY-1) of degree k and
   k+1 points on the polynomial.  The constant coefficient of POLY-1 is
   considered the SECRET.  The non-constant coefficients are used to
   generate the Lagrange Polynomial Constants (LPC).  Each of the k
   nodes (including verifier) are assigned a point on the polynomial
   i.e., shares of the SECRET.  The verifier is configured with the
   SECRET.  The Controller also generates coefficients (except the
   constant coefficient, called "RND", which is changed on a per packet
   basis) of a second polynomial POLY-2 of the same degree.  Each node
   is configured with the LPC of POLY-2.  Note that POLY-2 is public.

3.2.2.  In Transit

   For each packet, the ingress node generates a random number (RND).
   It is considered as the constant coefficient for POLY-2.  A
   cumulative value (CML) is initialized to 0.  Both RND, CML are
   carried as within the packet POT data.  As the packet visits each
   node, the RND is retrieved from the packet and the respective share
   of POLY-2 is calculated.  Each node calculates (Share(POLY-1) +
   Share(POLY-2)) and CML is updated with this sum.  This step is
   performed by each node until the packet completes the path.  The
   verifier also performs the step with its respective share.

3.2.3.  Verification

   The verifier cross checks whether CML = SECRET + RND.  If this
   matches then the packet traversed the specified set of nodes in the
   path.  This is due to the additive homomorphic property of Shamir's
   Secret Sharing scheme.

3.3.  Illustrative Example

   This section shows a simple example to illustrate step by step the
   approach described above.

3.3.1.  Basic Version

   Assumption: It is to be verified whether packets passed through 3
   nodes.  A polynomial of degree 2 is chosen for verification.

   Choices: Prime = 53.  POLY-1(x) = (3x^2 + 3x + 10) mod 53.  The
   secret to be re-constructed is the constant coefficient of POLY-1,

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   i.e., SECRET=10.  It is important to note that all operations are
   done over a finite field (i.e., modulo prime).  Secret Shares

   The shares of the secret are the points on POLY-1 chosen for the 3
   nodes.  For example, let x0=2, x1=4, x2=5.

      POLY-1(2) = 28 => (x0, y0) = (2, 28)

      POLY-1(4) = 17 => (x1, y1) = (4, 17)

      POLY-1(5) = 47 => (x2, y2) = (5, 47)

   The three points above are the points on the curve which are
   considered the shares of the secret.  They are assigned to three
   nodes respectively and are kept secret.  Lagrange Polynomials

   Lagrange basis polynomials (or Lagrange polynomials) are used for
   polynomial interpolation.  For a given set of points on the curve
   Lagrange polynomials (as defined below) are used to reconstruct the
   curve and thus reconstruct the complete secret.

      l0(x) = (((x-x1) / (x0-x1)) * ((x-x2)/x0-x2))) mod 53 =
      (((x-4) / (2-4)) * ((x-5)/2-5))) mod 53 =
      (10/3 - 3x/2 + (1/6)x^2) mod 53

      l1(x) = (((x-x0) / (x1-x0)) * ((x-x2)/x1-x2))) mod 53 =
      (-5 + 7x/2 - (1/2)x^2) mod 53

      l2(x) = (((x-x0) / (x2-x0)) * ((x-x1)/x2-x1))) mod 53 =
      (8/3 - 2 + (1/3)x^2) mod 53  LPC Computation

   Since x0=2, x1=4, x2=5 are chosen points.  Given that computations
   are done over a finite arithmetic field ("modulo a prime number"),
   the Lagrange basis polynomial constants are computed modulo 53.  The
   Lagrange Polynomial Constant (LPC) would be 10/3 , -5 , 8/3.

      LPC(x0) = (10/3) mod 53 = 21

      LPC(x1) = (-5) mod 53 = 48

      LPC(x2) = (8/3) mod 53 = 38

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   For a general way to compute the modular multiplicative inverse, see
   e.g., the Euclidean algorithm.  Reconstruction

   Reconstruction of the polynomial is well-defined as

   POLY1(x) = l0(x) * y0 + l1(x) * y1 + l2(x) * y2

   Subsequently, the SECRET, which is the constant coefficient of
   POLY1(x) can be computed as below

   SECRET = (y0*LPC(l0)+y1*LPC(l1)+y2*LPC(l2)) mod 53

   The secret can be easily reconstructed using the y-values and the

   SECRET = (y0*LPC(l0) + y1*LPC(l1) + y2*LPC(l2)) mod 53 = mod (28 * 21
   + 17 * 48 + 47 * 38) mod 53 = 3190 mod 53 = 10

   One observes that the secret reconstruction can easily be performed
   cumulatively hop by hop.  CML represents the cumulative value.  It is
   the POT data in the packet that is updated at each hop with the
   node's respective (yi*LPC(i)), where i is their respective value.  Verification

   Upon completion of the path, the resulting CML is retrieved by the
   verifier from the packet POT data.  Recall that verifier is
   preconfigured with the original SECRET.  It is cross checked with the
   CML by the verifier.  Subsequent actions based on the verification
   failing or succeeding could be taken as per the configured policies.

3.3.2.  Enhanced Version

   As observed previously, the vanilla algorithm that involves a single
   secret polynomial is not secure.  Therefore, the solution is further
   enhanced with usage of a random second polynomial chosen per packet.  Random Polynomial

   Let the second polynomial POLY-2 be (RND + 7x + 10 x^2).  RND is a
   random number and is generated for each packet.  Note that POLY-2 is
   public and need not be kept secret.  The nodes can be pre-configured
   with the non-constant coefficients (for example, 7 and 10 in this
   case could be configured through the Controller on each node).  So
   precisely only RND value changes per packet and is public and the
   rest of the non-constant coefficients of POLY-2 kept secret.

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   Recall that each node is preconfigured with their respective
   Share(POLY-1).  Each node calculates its respective Share(POLY-2)
   using the RND value retrieved from the packet.  The CML
   reconstruction is enhanced as below.  At every node, CML is updated

   CML = CML+(((Share(POLY-1)+ Share(POLY-2)) * LPC) mod Prime

   Let us observe the packet level transformations in detail.  For the
   example packet here, let the value RND be 45.  Thus POLY-2 would be
   (45 + 7x + 10x^2).

   The shares that could be generated are (2, 46), (4, 21), (5, 12).

      At ingress: The fields RND = 45.  CML = 0.

      At node-1 (x0): Respective share of POLY-2 is generated i.e., (2,
      46) because share index of node-1 is 2.

      CML = 0 + ((28 + 46)* 21) mod 53 = 17

      At node-2 (x1): Respective share of POLY-2 is generated i.e., (4,
      21) because share index of node-2 is 4.

      CML = 17 + ((17 + 21)*48) mod 53 = 17 + 22 = 39

      At node-3 (x2), which is also the verifier: The respective share
      of POLY-2 is generated i.e., (5, 12) because the share index of
      the verifier is 12.

      CML = 39 + ((47 + 12)*38) mod 53 = 39 + 16 = 55 mod 53 = 2

   The verification using CML is discussed in next section.  Verification

   As shown in the above example, for final verification, the verifier

   VERIFY = (SECRET + RND) mod Prime, with Prime = 53 here

   VERIFY = (RND-1 + RND-2) mod Prime = ( 10 + 45 ) mod 53 = 2

   Since VERIFY = CML the packet is proven to have gone through nodes 1,
   2, and 3.

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3.3.3.  Final Version

   The enhanced version of the protocol is still prone to replay and
   preplay attacks.  An attacker could reuse the POT metadata for
   bypassing the verification.  So additional measures using packet
   integrity checks (HMAC) and sequence numbers (SEQ_NO) are discussed
   later "Security Considerations" section.

3.4.  Operational Aspects

   To operationalize this scheme, a central controller is used to
   generate the necessary polynomials, the secret share per node, the
   prime number, etc. and distributing the data to the nodes
   participating in proof of transit.  The identified node that performs
   the verification is provided with the verification key.  The
   information provided from the Controller to each of the nodes
   participating in proof of transit is referred to as a proof of
   transit profile (POT-profile).  Also note that the set of nodes for
   which the transit has to be proven are typically associated to a
   different trust domain than the verifier.  Note that building the
   trust relationship between the Controller and the nodes is outside
   the scope of this document.  Techniques such as those described in
   [I-D.ietf-anima-autonomic-control-plane] might be applied.

   To optimize the overall data amount of exchanged and the processing
   at the nodes the following optimizations are performed:

   1.  The points (x, y) for each of the nodes on the public and private
       polynomials are picked such that the x component of the points
       match.  This lends to the LPC values which are used to calculate
       the cumulative value CML to be constant.  Note that the LPC are
       only depending on the x components.  They can be computed at the
       controller and communicated to the nodes.  Otherwise, one would
       need to distributed the x components to all the nodes.

   2.  A pre-evaluated portion of the public polynomial for each of the
       nodes is calculated and added to the POT-profile.  Without this
       all the coefficients of the public polynomial had to be added to
       the POT profile and each node had to evaluate them.  As stated
       before, the public portion is only the constant coefficient RND
       value, the pre-evaluated portion for each node should be kept
       secret as well.

   3.  To provide flexibility on the size of the cumulative and random
       numbers carried in the POT data a field to indicate this is
       shared and interpreted at the nodes.

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3.5.  Ordered POT (OPOT)

   In certain scenarios preserving the order of the nodes traversed by
   the packet may be needed.  Two alternatives, one based on "nested
   encryption", and another based on "symmetric masking between nodes"
   are described here for preserving the order

3.5.1.  Nested Encryption

   1.  The controller provisions all the nodes with their respective
       secret keys.

   2.  The controller provisions the verifier with all the secret keys
       of the nodes.

   3.  For each packet, the ingress node generates a random number RND
       and encrypts it with its secret key to generate CML value

   4.  Each subsequent node on the path encrypts CML with their
       respective secret key and passes it along

   5.  The verifier is also provisioned with the expected sequence of
       nodes in order to verify the order

   6.  The verifier receives the CML, RND values, re-encrypts the RND
       with keys in the same order as expected sequence to verify.

   With this nested encryption approach it is possible to retain the
   order in which the nodes are traversed, at the cost of:

   1.  Standard AES encryption would need 128 bits of RND, CML.  This
       results in a 256 bits of additional overhead is added per packet

   2.  In hardware platforms that do not support native encryption
       capabilities like (AES-NI).  This approach would have
       considerable impact on the computational latency

3.5.2.  Symmetric Masking Between Nodes

   1.  The controller provisions all the nodes with (or asks them to
       agree on) a couple of secrets, that we will refer as masks, one
       for the connection from the upstream node(s), another for the
       connection to the downstream node(s).  For obvious reasons, the
       ingress and egress (verifier) nodes only receive one, for
       downstream and upstream, respectively.

   2.  Any two contiguous nodes in the OPOT stream share the mask for
       the connection between them, in the shape of symmetric keys.

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       Masks can be refreshed as per-policy, defined at each hop or
       globally by the controller.

   3.  Each mask has the same size in bits as the length assigned to CML
       plus RND, as described in the above sections.

   4.  Whenever a packet is received at an intermediate node, the
       CML+RND sequence is deciphered (by XORing, though other ciphering
       schemas MAY be possible) with the upstream mask before applying
       the procedures described in Section 3.3.2.

   5.  Once the new values of CML+RND are produced, they are ciphered
       (by XORing, though other ciphering schemas MAY be possible) with
       the downstream mask before transmitting the packet to the next
       node downstream.

   6.  The ingress node only applies step 5 above, while the verifier
       only applies step 4 before running the verification procedure.

   The described process allows the verifier to check if the packet has
   followed the correct order while traversing the path.  In particular,
   the reconstruction process will fail if the order is not respected,
   as the deciphering process will produce invalid CML and RND values,
   and the interpolation (secret reconstruction) will finally generate a
   wrong verification value.

   This procedure does not impose a high computational burden, does not
   require additional packet overhead, can be deployed on chains of any
   length, does not require any node to be aware of any additional
   information than the upstream and downstream masks, and can be
   integrated with the other operational mechanisms applied by the
   controller to distribute shares and other secret material.

4.  Sizing the Data for Proof of Transit

   Proof of transit requires transport of two data fields in every
   packet that should be verified:

   1.  RND: Random number (the constant coefficient of public

   2.  CML: Cumulative

   The size of the data fields determines how often a new set of
   polynomials would need to be created.  At maximum, the largest RND
   number that can be represented with a given number of bits determines
   the number of unique polynomials POLY-2 that can be created.  The
   table below shows the maximum interval for how long a single set of

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   polynomials could last for a variety of bit rates and RND sizes: When
   choosing 64 bits for RND and CML data fields, the time between a
   renewal of secrets could be as long as 3,100 years, even when running
   at 100 Gbps.

   |   Transfer  |  Secret/RND  | Max # of packets |   Time RND lasts  |
   |     rate    |     size     |                  |                   |
   |    1 Gbps   |      64      |  2^64 = approx.  |  approx. 310,000  |
   |             |              |     2*10^19      |       years       |
   |   10 Gbps   |      64      |  2^64 = approx.  |   approx. 31,000  |
   |             |              |     2*10^19      |       years       |
   |   100 Gbps  |      64      |  2^64 = approx.  |   approx. 3,100   |
   |             |              |     2*10^19      |       years       |
   |    1 Gbps   |      32      |  2^32 = approx.  |   2,200 seconds   |
   |             |              |      4*10^9      |                   |
   |   10 Gbps   |      32      |  2^32 = approx.  |    220 seconds    |
   |             |              |      4*10^9      |                   |
   |   100 Gbps  |      32      |  2^32 = approx.  |     22 seconds    |
   |             |              |      4*10^9      |                   |

                      Table assumes 64 octet packets

                   Table 1: Proof of transit data sizing

   If the symmetric masking method for ordered POT is used
   (Section 3.5.2), the masks used between nodes adjacent in the path
   MUST have a length equal to the sum of the ones of RND and CML.

5.  Node Configuration

   A POT system consists of a number of nodes that participate in POT
   and a Controller, which serves as a control and configuration entity.
   The Controller is to create the required parameters (polynomials,
   prime number, etc.) and communicate those to the nodes.  The sum of
   all parameters for a specific node is referred to as "POT-profile".
   This document does not define a specific protocol to be used between
   Controller and nodes.  It only defines the procedures and the
   associated YANG data model.

5.1.  Procedure

   The Controller creates new POT-profiles at a constant rate and
   communicates the POT-profile to the nodes.  The controller labels a
   POT-profile "even" or "odd" and the Controller cycles between "even"
   and "odd" labeled profiles.  The rate at which the POT-profiles are

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   communicated to the nodes is configurable and is more frequent than
   the speed at which a POT-profile is "used up" (see table above).
   Once the POT-profile has been successfully communicated to all nodes
   (e.g., all NETCONF transactions completed, in case NETCONF is used as
   a protocol), the controller sends an "enable POT-profile" request to
   the ingress node.

   All nodes maintain two POT-profiles (an even and an odd POT-profile):
   One POT-profile is currently active and in use; one profile is
   standby and about to get used.  A flag in the packet is indicating
   whether the odd or even POT-profile is to be used by a node.  This is
   to ensure that during profile change the service is not disrupted.
   If the "odd" profile is active, the Controller can communicate the
   "even" profile to all nodes.  Only if all the nodes have received the
   POT-profile, the Controller will tell the ingress node to switch to
   the "even" profile.  Given that the indicator travels within the
   packet, all nodes will switch to the "even" profile.  The "even"
   profile gets active on all nodes and nodes are ready to receive a new
   "odd" profile.

   Unless the ingress node receives a request to switch profiles, it'll
   continue to use the active profile.  If a profile is "used up" the
   ingress node will recycle the active profile and start over (this
   could give rise to replay attacks in theory - but with 2^32 or 2^64
   packets this isn't really likely in reality).

5.2.  YANG Model

   This section defines that YANG data model for the information
   exchange between the Controller and the nodes.

   <CODE BEGINS> file "ietf-pot-profile@2016-06-15.yang"
   module ietf-pot-profile {

     yang-version 1;

     namespace "urn:ietf:params:xml:ns:yang:ietf-pot-profile";

     prefix ietf-pot-profile;

     organization "IETF xxx Working Group";

     contact "";

     description "This module contains a collection of YANG
                  definitions for proof of transit configuration
                  parameters. The model is meant for proof of
                  transit and is targeted for communicating the

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                  POT-profile between a controller and nodes
                  participating in proof of transit.";

     revision 2016-06-15 {
         "Initial revision.";

     typedef profile-index-range {
       type int32 {
         range "0 .. 1";
         "Range used for the profile index. Currently restricted to
          0 or 1 to identify the odd or even profiles.";

     grouping pot-profile {
       description "A grouping for proof of transit profiles.";
       list pot-profile-list {
         key "pot-profile-index";
         ordered-by user;
         description "A set of pot profiles.";

         leaf pot-profile-index {
           type profile-index-range;
           mandatory true;
             "Proof of transit profile index.";

         leaf prime-number {
           type uint64;
           mandatory true;
             "Prime number used for module math computation";

         leaf secret-share {
           type uint64;
           mandatory true;
             "Share of the secret of polynomial 1 used in computation";

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         leaf public-polynomial {
           type uint64;
           mandatory true;
             "Pre evaluated Public polynomial";

         leaf lpc {
           type uint64;
           mandatory true;
             "Lagrange Polynomial Coefficient";

         leaf validator {
           type boolean;
           default "false";
             "True if the node is a verifier node";

         leaf validator-key {
           type uint64;
             "Secret key for validating the path, constant of poly 1";

         leaf bitmask {
           type uint64;
           default 4294967295;
             "Number of bits as mask used in controlling the size of the
              random value generation. 32-bits of mask is default.";

     container pot-profiles {
       description "A group of proof of transit profiles.";

       list pot-profile-set {
         key "pot-profile-name";
         ordered-by user;
           "Set of proof of transit profiles that group parameters
            required to classify and compute proof of transit
            metadata at a node";

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         leaf pot-profile-name {
           type string;
           mandatory true;
             "Unique identifier for each proof of transit profile";

         leaf active-profile-index {
           type profile-index-range;
             "Proof of transit profile index that is currently active.
              Will be set in the first hop of the path or chain.
              Other nodes will not use this field.";

         uses pot-profile;
     /*** Container: end ***/
   /*** module: end ***/

6.  IANA Considerations

   IANA considerations will be added in a future version of this

7.  Manageability Considerations

   Manageability considerations will be addressed in a later version of
   this document.

8.  Security Considerations

   Different security requirements achieved by the solution approach are
   discussed here.

8.1.  Proof of Transit

   Proof of correctness and security of the solution approach is per
   Shamir's Secret Sharing Scheme [SSS].  Cryptographically speaking it
   achieves information-theoretic security i.e., it cannot be broken by
   an attacker even with unlimited computing power.  As long as the
   below conditions are met it is impossible for an attacker to bypass
   one or multiple nodes without getting caught.

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   o  If there are k+1 nodes in the path, the polynomials (POLY-1, POLY-
      2) should be of degree k.  Also k+1 points of POLY-1 are chosen
      and assigned to each node respectively.  The verifier can re-
      construct the k degree polynomial (POLY-3) only when all the
      points are correctly retrieved.

   o  Precisely three values are kept secret by individual nodes.  Share
      of SECRET (i.e. points on POLY-1), Share of POLY-2, LPC, P.  Note
      that only constant coefficient, RND, of POLY-2 is public. x values
      and non-constant coefficient of POLY-2 are secret

   An attacker bypassing a few nodes will miss adding a respective point
   on POLY-1 to corresponding point on POLY-2 , thus the verifier cannot
   construct POLY-3 for cross verification.

   Also it is highly recommended that different polynomials should be
   used as POLY-1 across different paths, traffic profiles or service

   If symmetric masking is used to assure OPOT (Section 3.5.2), the
   nodes need to keep two additional secrets: the upstream and upstream
   masks, that have to be managed under the same conditions as the
   secrets mentioned above.  And it is equally recommended to employ a
   different set of mask pairs across different paths, traffic profiles
   or service chains.

8.2.  Cryptanalysis

   A passive attacker could try to harvest the POT data (i.e., CML, RND
   values) in order to determine the configured secrets.  Subsequently
   two types of differential analysis for guessing the secrets could be

   o  Inter-Node: A passive attacker observing CML values across nodes
      (i.e., as the packets entering and leaving), cannot perform
      differential analysis to construct the points on POLY-1.  This is
      because at each point there are four unknowns (i.e.  Share(POLY-
      1), Share(Poly-2) LPC and prime number P) and three known values
      (i.e.  RND, CML-before, CML-after).  The application of symmetric
      masking for OPOT makes inter-node analysis less feasible.

   o  Inter-Packets: A passive attacker could observe CML values across
      packets (i.e., values of PKT-1 and subsequent PKT-2), in order to
      predict the secrets.  Differential analysis across packets could
      be mitigated using a good PRNG for generating RND.  Note that if
      constant coefficient is a sequence number than CML values become
      quite predictable and the scheme would be broken.  If symmetric
      masking is used for OPOT, inter-packet analysis could be applied

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      to guess mask values, what requires a proper refresh rate for
      masks, at least as high as the one used for LPCs.

8.3.  Anti-Replay

   A passive attacker could reuse a set of older RND and the
   intermediate CML values to bypass certain nodes in later packets.
   Such attacks could be avoided by carefully choosing POLY-2 as a
   (SEQ_NO + RND).  For example, if 64 bits are being used for POLY-2
   then first 16 bits could be a sequence number SEQ_NO and next 48 bits
   could be a random number.

   Subsequently, the verifier could use the SEQ_NO bits to run classic
   anti-replay techniques like sliding window used in IPSEC.  The
   verifier could buffer up to 2^16 packets as a sliding window.
   Packets arriving with a higher SEQ_NO than current buffer could be
   flagged legitimate.  Packets arriving with a lower SEQ_NO than
   current buffer could be flagged as suspicious.

   For all practical purposes in the rest of the document RND means
   SEQ_NO + RND to keep it simple.

   The solution discussed in this memo does not currently mitigate
   replay attacks.  An anti-replay mechanism may be included in future
   versions of the solution.

8.4.  Anti-Preplay

   An active attacker could try to perform a man-in-the-middle (MITM)
   attack by extracting the POT of PKT-1 and using it in PKT-2.
   Subsequently attacker drops the PKT-1 in order to avoid duplicate POT
   values reaching the verifier.  If the PKT-1 reaches the verifier,
   then this attack is same as Replay attacks discussed before.

   Preplay attacks are possible since the POT metadata is not dependent
   on the packet fields.  Below steps are recommended for remediation:

   o  Ingress node and Verifier are configured with common pre shared

   o  Ingress node generates a Message Authentication Code (MAC) from
      packet fields using standard HMAC algorithm.

   o  The left most bits of the output are truncated to desired length
      to generate RND.  It is recommended to use a minimum of 32 bits.

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   o  The verifier regenerates the HMAC from the packet fields and
      compares with RND.  To ensure the POT data is in fact that of the

   If an HMAC is used, an active attacker lacks the knowledge of the
   pre-shared key, and thus cannot launch preplay attacks.

   The solution discussed in this memo does not currently mitigate
   prereplay attacks.  A mitigation mechanism may be included in future
   versions of the solution.

8.5.  Anti-Tampering

   An active attacker could not insert any arbitrary value for CML.
   This would subsequently fail the reconstruction of the POLY-3.  Also
   an attacker could not update the CML with a previously observed
   value.  This could subsequently be detected by using timestamps
   within the RND value as discussed above.

8.6.  Recycling

   The solution approach is flexible for recycling long term secrets
   like POLY-1.  All the nodes could be periodically updated with shares
   of new SECRET as best practice.  The table above could be consulted
   for refresh cycles (see Section 4).

   If symmetric masking is used for OPOT (Section 3.5.2), mask values
   must be periodically updated as well, at least as frequently as the
   other secrets are.

8.7.  Redundant Nodes and Failover

   A "node" or "service" in terms of POT can be implemented by one or
   multiple physical entities.  In case of multiple physical entities
   (e.g., for load-balancing, or business continuity situations -
   consider for example a set of firewalls), all physical entities which
   are implementing the same POT node are given that same share of the
   secret.  This makes multiple physical entities represent the same POT
   node from an algorithm perspective.

8.8.  Controller Operation

   The Controller needs to be secured given that it creates and holds
   the secrets, as need to be the nodes.  The communication between
   Controller and the nodes also needs to be secured.  As secure
   communication protocol such as for example NETCONF over SSH should be
   chosen for Controller to node communication.

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   The Controller only interacts with the nodes during the initial
   configuration and thereafter at regular intervals at which the
   operator chooses to switch to a new set of secrets.  In case 64 bits
   are used for the data fields "CML" and "RND" which are carried within
   the data packet, the regular intervals are expected to be quite long
   (e.g., at 100 Gbps, a profile would only be used up after 3100 years)
   - see Section 4 above, thus even a "headless" operation without a
   Controller can be considered feasible.  In such a case, the
   Controller would only be used for the initial configuration of the

   If OPOT (Section 3.5.2) is applied using symmetric masking, the
   Controller will be required to perform a a periodic refresh of the
   mask pairs.  The use of OPOT SHOULD be configurable as part of the
   required level of assurance through the Controller management

8.9.  Verification Scope

   The POT solution defined in this document verifies that a data-packet
   traversed or transited a specific set of nodes.  From an algorithm
   perspective, a "node" is an abstract entity.  It could be represented
   by one or multiple physical or virtual network devices, or is could
   be a component within a networking device or system.  The latter
   would be the case if a forwarding path within a device would need to
   be securely verified.

8.9.1.  Node Ordering

   POT using Shamir's secret sharing scheme as discussed in this
   document provides for a means to verify that a set of nodes has been
   visited by a data packet.  It does not verify the order in which the
   data packet visited the nodes.

   In case the order in which a data packet traversed a particular set
   of nodes needs to be verified as well, the alternate schemes related
   to OPOT (Section 3.5) have to be considered.  Since these schemes
   introduce at least additional control requirements, the selection of
   order verification SHOULD be configurable the Controller management

8.9.2.  Stealth Nodes

   The POT approach discussed in this document is to prove that a data
   packet traversed a specific set of "nodes".  This set could be all
   nodes within a path, but could also be a subset of nodes in a path.
   Consequently, the POT approach isn't suited to detect whether

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   "stealth" nodes which do not participate in proof-of-transit have
   been inserted into a path.

9.  Acknowledgements

   The authors would like to thank Eric Vyncke, Nalini Elkins, Srihari
   Raghavan, Ranganathan T S, Karthik Babu Harichandra Babu, Akshaya
   Nadahalli, Erik Nordmark, and Andrew Yourtchenko for the comments and

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997, <

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/
              RFC7665, October 2015, <

   [SSS]      "Shamir's Secret Sharing", <

10.2.  Informative References

              Eckert, T., Behringer, M., and S. Bjarnason, "An Autonomic
              Control Plane (ACP)", draft-ietf-anima-autonomic-control-
              plane-18 (work in progress), August 2018.

Authors' Addresses

   Frank Brockners
   Cisco Systems, Inc.
   Hansaallee 249, 3rd Floor


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   Shwetha Bhandari
   Cisco Systems, Inc.
   Cessna Business Park, Sarjapura Marathalli Outer Ring Road
   Bangalore, KARNATAKA 560 087


   Sashank Dara
   Cisco Systems, Inc.
   Cessna Business Park, Sarjapura Marathalli Outer Ring Road
   BANGALORE, Bangalore, KARNATAKA 560 087


   Carlos Pignataro
   Cisco Systems, Inc.
   7200-11 Kit Creek Road
   Research Triangle Park, NC  27709
   United States


   John Leddy


   Stephen Youell
   JP Morgan Chase
   25 Bank Street
   London  E14 5JP
   United Kingdom


   David Mozes


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   Tal Mizrahi
   6 Hamada St.
   Yokneam  20692


   Alejandro Aguado
   Universidad Politecnica de Madrid
   Campus Montegancedo, Boadilla del Monte
   Madrid  28660

   Phone: +34 910 673 086

   Diego R. Lopez
   Telefonica I+D
   Editor Jose Manuel Lara, 9 (1-B)
   Seville  41013

   Phone: +34 913 129 041

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