Network Working Group | F. Brockners, Ed. |

Internet-Draft | S. Bhandari, Ed. |

Intended status: Experimental | Cisco |

Expires: December 18, 2020 | T. Mizrahi, Ed. |

Huawei Network.IO Innovation Lab | |

S. Dara | |

Seconize | |

S. Youell | |

JPMC | |

June 16, 2020 |

Proof of Transit

draft-ietf-sfc-proof-of-transit-06

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 a 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.

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This Internet-Draft will expire on December 18, 2020.

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.

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- 1. Introduction
- 2. Conventions
- 3. Proof of Transit
- 3.1. Basic Idea
- 3.2. Solution Approach
- 3.2.1. Setup
- 3.2.2. In Transit
- 3.2.3. Verification
- 3.3. Illustrative Example
- 3.3.1. Baseline
- 3.3.1.1. Secret Shares
- 3.3.1.2. Lagrange Polynomials
- 3.3.1.3. LPC Computation
- 3.3.1.4. Reconstruction
- 3.3.1.5. Verification
- 3.3.2. Complete Solution
- 3.3.2.1. Random Polynomial
- 3.3.2.2. Reconstruction
- 3.3.2.3. Verification
- 3.3.3. Solution Deployment Considerations
- 3.4. Operational Aspects
- 3.5. Ordered POT (OPOT)
- 4. Sizing the Data for Proof of Transit
- 5. Node Configuration
- 5.1. Procedure
- 5.2. YANG Model for POT
- 5.2.1. Main Parameters
- 5.2.2. Tree Diagram
- 5.2.3. YANG Model
- 6. IANA Considerations
- 7. Security Considerations
- 7.1. Proof of Transit
- 7.2. Cryptanalysis
- 7.3. Anti-Replay
- 7.4. Anti-Preplay
- 7.5. Tampering
- 7.6. Recycling
- 7.7. Redundant Nodes and Failover
- 7.8. Controller Operation
- 7.9. Verification Scope
- 7.9.1. Node Ordering
- 7.9.2. Stealth Nodes
- 8. Acknowledgements
- 9. Contributors
- 10. References
- 10.1. Normative References
- 10.2. Informative References
- Authors' Addresses

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 nodes.

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), 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 shares of a single secret. Nodes on the path retrieve their individual shares of the secret using Shamir's Secret Sharing scheme from a central controller. The complete secret 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 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 along with data found in the packet to validate whether the packet traversed the path correctly.

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 [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
- 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 do not 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 NIST.
- 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
- SSSS:
- Shamir's Secret Sharing Scheme
- SR:
- Segment Routing

This section discusses methods and algorithms to provide for a "proof of transit" (POT) 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.)

When used in the context of IOAM, the POT information MUST be encapsulated in packets as an IOAM Proof of Transit Option-Type. The details and format of the encapsulation and the IOAM POT Option-Type format are specified in [I-D.ietf-ippm-ioam-data]. When used in conjunction with NSH [RFC8300], the POT Option-Type MUST be carried as specified in [I-D.ietf-sfc-ioam-nsh].

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.

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 (e.g., configuration mistakes). The mechanism for POT leverages "Shamir's Secret Sharing" scheme [SSS].

Shamir's secret sharing base idea: A polynomial (represented by its coefficients) of degree k is chosen as a secret by the controller. A polynomial represents a curve. A set of k+1 points on the curve define the polynomial and are thus needed to (re-)construct the polynomial. Each of these k+1 points of the polynomial is called a “share” of the secret. A single secret is associated with a particular set of k+1 nodes, which typically represent the path to be verified. k+1 shares of the single secret (i.e., k+1 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 reconstructed 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 below.

While applicable in theory, a vanilla approach based on Shamir's Secret Sharing Scheme 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 as described above which is kept constant, and a per-packet polynomial which is public and generated by the ingress node (the first node along the path). Operations are performed on the sum of those two polynomials - creating a third polynomial which is secret and per packet.

Solution approach: The overall algorithm uses two polynomials: POLY-1 and POLY-2. POLY-1 is secret and constant. A different POLY-1 is used for each path, and its value is known to the controller and to the verifier of the respective path. 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 number”, i.e. all operations are performed "modulo prime number".

Detailed algorithms are discussed next. A simple example that describes how the algorithms work is discussed in Section 3.3.

The algorithms themselves do not constrain the ranges of possible values for the different parameters and coefficients used. A deployment of the algorithms will always need to define appropriate ranges. Please refer to the YANG model in Section 5.2 for details on the units and ranges of possible values of the different parameters and coefficients.

A controller generates a first polynomial (POLY-1) of degree k and k+1 points on the polynomial, corresponding to the k+1 nodes along the path. The constant coefficient of POLY-1 is considered the SECRET, which is per the definition of the SSSS algorithm [SSS]. The k+1 points are used to derive the Lagrange Basis Polynomials. The Lagrange Polynomial Constants (LPC) are retrieved from the constant coefficients of the Lagrange Basis Polynomials. Each of the k+1 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.

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, specifically each node performs

CML = CML+(((Share(POLY-1)+ Share(POLY-2)) * LPC) mod Prime, with "LPC" being the Lagrange Polynomial Constant and "Prime" being the prime number which defines the finite field arithmetic that all operations are done over. Please also refer to Section 3.3.2 below for further details how the operations are performed.

This step is performed by each node until the packet completes the path. The verifier also performs the step with its respective share.

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.

This section shows a simple example to illustrate step by step the approach described above. The example assumes a network with 3 nodes. The last node that packets traverse also serves as the verifier. A Controller communicates the required parameters to the individual nodes.

Assumption: It is to be verified whether packets passed through the 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, i.e., SECRET=10. It is important to note that all operations are done over a finite field (i.e., modulo Prime = 53).

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 by the Controller to three nodes respectively and are kept secret.

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

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 Constants (LPC) would be mod(10/3, 53), mod(-5, 53), mod(8/3, 53).LPC are computed by the Controller and communicated to the individual nodes.

- LPC(l0) = (10/3) mod 53 = 21
- LPC(l1) = (-5) mod 53 = 48
- LPC(l2) = (8/3) mod 53 = 38

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

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 LPC:

SECRET = (y0*LPC(l0) + y1*LPC(l1) + y2*LPC(l2)) mod 53 = (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, i.e. by every node. 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.

Upon completion of the path, the resulting CML is retrieved by the verifier from the packet POT data. Recall that the 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.

As observed previously, the baseline algorithm that involves a single secret polynomial is not secure. The complete solution leverages a random second polynomial, which is chosen per packet.

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 the RND value changes per packet and is public and the rest of the non-constant coefficients of POLY-2 is kept secret.

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 as

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.

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

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.

The "complete solution" described above in Section 3.3.2 could still be prone to replay or preplay attacks. An attacker could e.g. reuse the POT metadata for bypassing the verification. These threats can be mitigated by appropriate parameterization of the algorithm. Please refer to Section 7 for details.

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:

- 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.
- 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.
- 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.

POT as discussed in this document so far only verifies that a defined set of nodes have been traversed by a packet. The order in which nodes where traversed is not verified. "Ordered Proof of Transit (OPOT)" addresses the need of deployments, that require to verify the order in which nodes were traversed. OPOT extends the POT scheme with symmetric masking between the nodes.

- For each path the controller provisions all the nodes with (or asks them to agree on) two secrets per node, that we will refer to 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.
- Any two contiguous nodes in the OPOT stream share the mask for the connection between them, in the shape of symmetric keys. Masks can be refreshed as per-policy, defined at each hop or globally by the controller.
- Each mask has the same size in bits as the length assigned to CML plus RND, as described in the above sections.
- 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.
- 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.
- 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.

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

- RND: Random number (the constant coefficient of public polynomial)
- 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 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 rate | Secret/RND size | Max # of packets | Time RND lasts |
---|---|---|---|

1 Gbps | 64 | 2^64 = approx. 2*10^19 | approx. 310,000 years |

10 Gbps | 64 | 2^64 = approx. 2*10^19 | approx. 31,000 years |

100 Gbps | 64 | 2^64 = approx. 2*10^19 | approx. 3,100 years |

1 Gbps | 32 | 2^32 = approx. 4*10^9 | 2,200 seconds |

10 Gbps | 32 | 2^32 = approx. 4*10^9 | 220 seconds |

100 Gbps | 32 | 2^32 = approx. 4*10^9 | 22 seconds |

Table assumes 64 octet packets

If the symmetric masking method for ordered POT is used (Section 3.5), the masks used between nodes adjacent in the path MUST have a length equal to the sum of the ones of RND and CML.

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 the associated values (i.e. prime number, secret-share, LPC, etc.) to the nodes. The sum of all parameters for a specific node is referred to as "POT-Profile". For details see the YANG model in Section 5.2.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.

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. This means that the parameters for the algorithms are continuously refreshed. Please refer to Section 4 for choosing an appropriate refresh rate: The rate at which the POT-Profiles are communicated to the nodes is configurable and MUST be more frequent than the speed at which a POT-Profile is "used up". 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).

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

The main parameters for the information exchange between the Controller and the node used in the YANG model are as follows:

- pot-profile-index: Section 5.1 details that two POT-Profiles are used. Only one of the POT-Profiles is active at a given point in time, allowing the Controller to refresh the non-active one for future use. pot-profile-index defines which of the POT-Profiles (the "even" or "odd" POT-Profile) is currently active. pot-profile-index will be set in the first hop of the path or chain. Other nodes will not use this field.
- prime-number: Prime number used for module math computation.
- secret-share: Share of the secret of polynomial-1 used in computation for the node. If POLY-1 is defined by points (x1_i, y1_i) with i=0,..k, then for node i, the secret-share will be y1_i.
- public-polynomial: Public polynomial value for the node.. If POLY-2 is defined by points (x2_i, y2_i) with i=0,..k, then for node i, the secret-share will be y2_i.
- lpc: Lagrange Polynomial Coefficient for the node, i.e. for node i, this would be LPC(l_i), with l_i being the i-th Lagrange Basis Polynomial.
- validator?: True if the node is a verifier node.
- validator-key?: The validator-key represents the SECRET as described in the sections above. The SECRET is the constant coefficient of POLY-1(z). If POLY-1(z) = a_0 + a_1*z + a_2*z^2+..+a_k*z^k, then the SECRET would be a_0.
- bitmask?: Number of bits as mask used in controlling the size of the random value generation. 32-bits of mask is default. See Section 4 for details.

<CODE BEGINS> module: ietf-pot-profile +--rw pot-profiles +--rw pot-profile-set* [pot-profile-name] +--rw pot-profile-name string +--rw active-profile-index? profile-index-range +--rw pot-profile-list* [pot-profile-index] +--rw pot-profile-index profile-index-range +--rw prime-number uint64 +--rw secret-share uint64 +--rw public-polynomial uint64 +--rw lpc uint64 +--rw validator? boolean +--rw validator-key? uint64 +--rw bitmask? uint64 +--rw opot-masks +--rw downstream-mask* uint64 +--rw upstream-mask* uint64 <CODE ENDS>

This section shows a simplified graphical representation of the YANG data model for POT. The meaning of the symbols in these diagrams is as follows:

- Brackets "[" and "]" enclose list keys.
- Abbreviations before data node names: "rw" means configuration (read-write), and "ro" means state data (read-only).
- Symbols after data node names: "?" means an optional node, "!" means a presence container, and "*" denotes a list and leaf-list.
- Parentheses enclose choice and case nodes, and case nodes are also marked with a colon (":").
- Ellipsis ("...") stands for contents of subtrees that are not shown.

<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 SFC Working Group"; contact "WG Web: <https://tools.ietf.org/wg/sfc/> WG List: <mailto:sfc@ietf.org>"; 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 POT-Profile between a controller and nodes participating in proof of transit. Copyright (c) 2018 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX; see the RFC itself for full legal notices. Copyright (c) 2018 IETF Trust and the persons identified as authors of the code. All rights reserved. The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here."; revision 2016-06-15 { description "Initial revision."; reference ""; } typedef profile-index-range { type int32 { range "0 .. 1"; } description "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; description "Proof of transit profile index."; } leaf prime-number { type uint64; mandatory true; description "Prime number used for module math computation"; } leaf secret-share { type uint64; mandatory true; description "Share of the secret of polynomial-1 used in computation for the node. If POLY-1 is defined by points (x1_i, y1_i) with i=0,..k, then for node i, the secret-share will be y1_i."; } leaf public-polynomial { type uint64; mandatory true; description "Public polynomial value for the node. If POLY-2 is defined by points (x2_i, y2_i) with i=0,..k, then for node i, the secret-share will be y2_i."; } leaf lpc { type uint64; mandatory true; description "Lagrange Polynomial Coefficient"; } leaf validator { type boolean; default "false"; description "True if the node is a verifier node"; } leaf validator-key { type uint64; description "The validator-key represents the secret. The secret is the constant coefficient of POLY-1(z). If POLY-1(z) = a_0 + a_1*z + a_2*z^2+..+a_k*z^k, then the SECRET would be a_0."; } leaf bitmask { type uint64; default 4294967295; description "Number of bits as mask used in controlling the size of the random value generation. 32-bits of mask is default."; } uses opot-profile; } } grouping opot-profile { description "Grouping containing OPoT related data."; container opot-masks { must "count(downstream-mask) = count(upstream-mask)"; description "Masking information for OPoT support."; leaf-list downstream-mask { type uint64; max-elements 2; description "Secret stream used to demask the PoT metadata. The mask is used between nodes adjacent in the path and MUST have a length equal to the sum of the ones of RND and CML."; } leaf-list upstream-mask { type uint64; max-elements 2; description "Secret stream used to mask the PoT metadata. The mask is used between nodes adjacent in the path and MUST have a length equal to the sum of the ones of RND and CML."; } } } container pot-profiles { description "A group of proof of transit profiles."; list pot-profile-set { key "pot-profile-name"; ordered-by user; description "Set of proof of transit profiles that group parameters required to classify and compute proof of transit metadata at a node"; leaf pot-profile-name { type string; mandatory true; description "Unique identifier for each proof of transit profile"; } leaf active-profile-index { type profile-index-range; description "POT-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 ***/ } <CODE ENDS>

This document does not require any actions from IANA.

POT is a mechanism that is used for verifying the path through which a packet was forwarded. The security considerations of IOAM in general are discussed in [I-D.ietf-ippm-ioam-data]. Specifically, it is assumed that POT is used in a confined network domain, and therefore the potential threats that POT is intended to mitigate should be viewed accordingly. POT prevents spoofing and tampering; an attacker cannot maliciously create a bogus POT or modify a legitimate one. Furthermore, a legitimate node that takes part in the POT protocol cannot masquerade as another node along the path. These considerations are discussed in detail in the rest of this section.

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.

- 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.
- 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 chains.

If symmetric masking is used to assure OPOT (Section 3.5), the nodes need to keep two additional secrets: the downstream 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.

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 done.

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

A passive attacker could reuse a set of older RND and the intermediate CML values. Thus, an attacker can attack an old (replayed) RND and CML with a new packet in order to bypass some of the nodes along the path.

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.

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:

- Ingress node and Verifier are configured with common pre shared key
- Ingress node generates a Message Authentication Code (MAC) from packet fields using standard HMAC algorithm.
- 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.
- The verifier regenerates the HMAC from the packet fields and compares with RND. To ensure the POT data is in fact that of the packet.

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 preplay attacks. A mitigation mechanism may be included in future versions of the solution.

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.

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), mask values must be periodically updated as well, at least as frequently as the other secrets are.

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.

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.

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 POT-Profiles.

If OPOT (Section 3.5) 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 interface.

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.

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 interface.

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

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 advice.

In addition to editors and authors listed on the title page, the following people have contributed substantially to this document and should be considered coauthors:

Carlos Pignataro Cisco Systems, Inc. 7200-11 Kit Creek Road Research Triangle Park, NC 27709 United States Email: cpignata@cisco.com

John Leddy Email: john@leddy.net

David Mozes Email: mosesster@gmail.com

Alejandro Aguado Universidad Politecnica de Madrid Campus Montegancedo, Boadilla del Monte Madrid 28660 Spain Phone: +34 910 673 086 Email: a.aguadom@fi.upm.es

Diego R. Lopez Telefonica I+D Editor Jose Manuel Lara, 9 (1-B) Seville 41013 Spain Phone: +34 913 129 041 Email: diego.r.lopez@telefonica.com

[I-D.ietf-ippm-ioam-data] |
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H., Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov, P., remy@barefootnetworks.com, r., daniel.bernier@bell.ca, d. and J. Lemon, "Data Fields for In-situ OAM", Internet-Draft draft-ietf-ippm-ioam-data-09, March 2020. |

[I-D.ietf-sfc-ioam-nsh] |
Brockners, F. and S. Bhandari, "Network Service Header (NSH) Encapsulation for In-situ OAM (IOAM) Data", Internet-Draft draft-ietf-sfc-ioam-nsh-03, March 2020. |

[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. and C. Pignataro, "Service Function Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/RFC7665, October 2015. |

[RFC8300] |
Quinn, P., Elzur, U. and C. Pignataro, "Network Service Header (NSH)", RFC 8300, DOI 10.17487/RFC8300, January 2018. |

[SSS] |
Wikipedia, "Shamir's Secret Sharing" |

[I-D.ietf-anima-autonomic-control-plane] |
Eckert, T., Behringer, M. and S. Bjarnason, "An Autonomic Control Plane (ACP)", Internet-Draft draft-ietf-anima-autonomic-control-plane-18, August 2018. |

Frank Brockners (editor)
Cisco Systems, Inc.
Hansaallee 249, 3rd Floor
DUESSELDORF,
NORDRHEIN-WESTFALEN
40549
Germany
EMail: fbrockne@cisco.com

Shwetha Bhandari (editor)
Cisco Systems, Inc.
Cessna Business Park, Sarjapura Marathalli Outer Ring Road
Bangalore, KARNATAKA 560 087,
India
EMail: shwethab@cisco.com

Stephen Youell
JP Morgan Chase
25 Bank Street
London,
E14 5JP
United Kingdom
EMail: stephen.youell@jpmorgan.com