`40549`

```
```Israel

```
```tal.mizrahi.phd@gmail.com

```
```

```
```
Seconize
BANGALORE
Bangalore, KARNATAKA
```
```INDIA

```
```
sashank@seconize.co

```
```

```
```
JP Morgan Chase
25 Bank Street
London
`E14 5JP`

United Kingdom
stephen.youell@jpmorgan.com
ops
Telemetry, Tracing, 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.

```
```
Several deployments use Traffic Engineering, policy routing, Segment
Routing (SR), and Service Function Chaining (SFC) 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 .
Abbreviations used in this document:
Hash based Message Authentication Code. For
example, HMAC-SHA256 generates 256 bits of MAC
In-situ Operations, Administration, and
Maintenance
Locator/ID Separation Protocol
Lagrange Polynomial Constants
Maximum Transmit Unit
Network Function Virtualization
Network Service Header
Proof of Transit
Proof of Transit Profile that has the
necessary data for nodes to participate in proof of transit
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.
Sequence number initialized to a predefined
constant. This is used in concatenation with RND bits to mitigate
different attacks discussed later.
Service Function Chain
Shamir's Secret Sharing Scheme
Segment Routing

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 POT information is encapsulated in packets as an IOAM Proof Of
Transit Option. The details and format of the encapsulation and the POT
Option format are specified in .
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 .
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 .
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 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 . 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 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.
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:
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 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 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 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 .
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 (), 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 .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 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: 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
for details.

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.

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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 . 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 .
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 (), 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 ).
If symmetric masking is used for OPOT (),
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 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 () 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 () 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 to this document:

```
```
&RFC2119;
&I-D.ietf-ippm-ioam-data;
&RFC7665;
Shamir's Secret Sharing
&I-D.draft-ietf-anima-autonomic-control-plane;

```
```