Quantum Internet Research Group                             R. Van Meter
Internet-Draft                                                 T. Matsuo
Intended status: Informational                           Keio University
Expires: March 13, 2020                               September 10, 2019

                 Connection Setup in a Quantum Network


   Near-term quantum networks will grow to form a Noisy, Intermediate-
   Scale Quantum Internet (NISQI).  Connection setup will require
   adapting behavior along the path to the noise levels of individual
   elements.  In this proposal, path creation is triggered by an
   application at the Initiator, information is accumulated node-by-node
   on an outbound pass in a series of QCap (quantum capability) blocks,
   then the RuleSets are created at the Responder.  RuleSets are
   installed at the individual nodes on the return pass.  This document
   describes the architecture of connection setup in a network.  Details
   of the RuleSets and QCaps, addressing architecture, link protocols,
   routing, resource allocation (multiplexing), extension of this setup
   procedure to an internetwork, and extension to multiparty
   communications are beyond the scope of this document.

Status of This Memo

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Concepts and Glossary . . . . . . . . . . . . . . . . . . . .   3
   3.  Connection Setup Phases . . . . . . . . . . . . . . . . . . .   5
     3.1.  Short Description of Phases . . . . . . . . . . . . . . .   5
     3.2.  Rationale for this Architecture . . . . . . . . . . . . .   5
   4.  Message Contents and Elements . . . . . . . . . . . . . . . .   6
     4.1.  PathSetupRequest  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Quantum Capabilities (QCap) . . . . . . . . . . . . . . .   7
     4.3.  RuleSets  . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Processing the SetupRequest . . . . . . . . . . . . . . . . .   7
     5.1.  Initiating a Connection Setup Request . . . . . . . . . .   8
     5.2.  Outbound Processing . . . . . . . . . . . . . . . . . . .   8
     5.3.  Responder Processing  . . . . . . . . . . . . . . . . . .   9
     5.4.  Return Processing . . . . . . . . . . . . . . . . . . . .   9
   6.  Rejection and Robustness of the Setup Process . . . . . . . .   9
     6.1.  Rejection by a Repeater or Router . . . . . . . . . . . .   9
     6.2.  Rejection by a Responder  . . . . . . . . . . . . . . . .  10
     6.3.  Robustness  . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Building a connection across a quantum network [theqi] is a classical
   task.  Because of the low success probability of quantum
   communication due to photon loss and the extremely high error rates
   due to the fragile nature of quantum information, quantum
   communication between two nodes more closely resembles a coordinated
   computation distributed among the set of nodes forming the path

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   between the two nodes than a store-and-forward network session

   Use of the quantum network is driven by applications running at two
   (or more) classical nodes.  Overall behavior is similar to client-
   server computing.  The connection is initiated from a node similar to
   client and responded to by a node similar to a server.  The details
   of the sending and receiving of the classical messages are not
   specified in this document, but can be modeled as if being sent over
   a TCP socket.  Messages are assumed to be reliable and delivered in
   order.  These messages have no hard real time requirement, though the
   subsequent data phase of the operation may.

   This connection setup process must collect information about the
   hardware (channels and buffer memories) to be used, because of the
   heterogeneity of the underlying hardware.  Loss in optical channels
   naturally varies with channel length and other factors, and has a
   large impact on quantum communication performance.  Individual
   quantum buffers holding quantum bits (qubits) will vary in quality,
   as well.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Concepts and Glossary

   The following terms will be used:

   Bell pair  a common form of entangled quantum state useful in

   End node  a quantum network node with a single interface.  End nodes
           may have stationary quantum memories, or may be capable only
           of measuring photons; this distinction is beyond the scope of
           this document.

   Entanglement  the condition of a group of qubits (typically two
           qubits in this document) in a shared state that cannot be
           described using only real, non-negative, classical

   Entanglement Swapping  executed at node B splices an entangled state
           shared with node A to an entangled state shared with node C,
           creating A-C entanglement and disentangling B from both

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   Fidelity  a measure of the quality of a quantum state; roughly, the
           probability that the system holds the desired state.

   Initiator  the initiator of the classical process of establishing the
           connection by sending a message toward the Responder.

   Purification  an error detection mechanism on quantum states.
           Typically, one quantum state is used to test the condition of
           a second state; the first state is destroyed in the process.
           If the purification fails, it is unknown whether the first or
           second state was in error, and the second state is discarded
           as well.  If purification succeeds, our confidence in the
           state is improved.

   QCap    an information block describing the quantum capabilities of a
           particular node and link.

   Qubit   a quantum system with two states that can be stored in memory
           or transmitted through a channel, manipulated in a
           constrained set of operations, entangled with other qubits,
           and measured.

   Repeater  a quantum network node with two interfaces, typically
           sitting in the middle of a chain.  Repeaters do not require
           routing functionality, but otherwise have the same
           capabilities as routers.  As spacing between nodes may be
           required to be as short as ten kilometers, depending on
           technology, what would be single fiber hops in a classical
           network will be a long chain of repeaters.

   Responder  is the classical endpoint of the connection setup process,
           where the message sent by the Initiator terminates.  The
           Responder creates the RuleSets for all nodes in the path, and
           commonly will be the smarter node.

   Router  a quantum network node with a more than two interfaces,
           requiring routing capability.

   RuleSet describes the actions that a nodes should take when certain
           conditions occur.  The contents of RuleSets are beyond the
           scope of this document.

   The terms "source" and "destination" are not appropriate at the
   connection level in a quantum network, because distributed quantum
   states are not necessarily used for the unidirectional transfer of
   information.  Therefore, we use Initiator and Responder to designate
   roles in the connection setup process, but those roles do not not

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   necessarily correspond to any asymmetry during the connection
   lifetime.  Source and destination are not appropriate because:

   1.  There may not even be data transferred between nodes; the
       entanglement might be used for some shared operation that doesn't
       involve qubits moving back and forth via teleportation.  Quantum
       key distribution (QKD) is an obvious example, where both ends
       measure the entangled state and destroy it in order to get a
       classical bit.

   2.  Temporally, operations may not even happen left-to-right along
       the chain of repeaters, again violating the notion that data is

   "Source" and "destination" may be used to describe the movement of an
   individual classical message.

   Links are assumed to be point-to-point.  Multidrop physical layers
   are possible, but quantum broadcast or multicast are not directly
   possible at the physical level, and would have to be emulated.

3.  Connection Setup Phases

3.1.  Short Description of Phases

   The single-network, two-node connection setup procedure consists of
   three basic phases:

   1.  The outbound request is routed from Initiator to Responder using
       a standard NextHop-based forwarding table, accumulating
       information about the path along the way in a stack of QCaps.

   2.  When the request arrives at the Responder, the Responder uses
       that information to create a complete RuleSet for every node.
       The RuleSets are assembled into a stack with the nearest node at
       the top.

   3.  The RuleSets are sent back along the original path, with each
       node removing its RuleSet from the message (popping the stack),
       then forwarding the remaining QCaps on until it returns to the

3.2.  Rationale for this Architecture

   The outbound pass collects information about the nodes and links, to
   be used by the Responder to formulate the RuleSets.  Why is the
   information collected in this fashion rather than shared more broadly
   across the network, e.g. as part of a modified routing protocol such

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   as OSPF [RFC2328]?  Why does a single node create the RuleSets for
   all nodes, rather than allowing individual nodes to create their own
   RuleSets when they see the PathSetupRequest message?

   1.  Because Repeaters may be spaced as closely as every 10km, a full
       topology for a network listing every Repeater may be excessively
       large for routing purposes, but such information is needed for
       building RuleSets.

   2.  The information collected may be substantially larger in volume
       than simple link costs.

   3.  The information collected and used may be too dynamic for a
       routing protocol.

   4.  Sharing of this information can be unnecessary when routing is
       driven by policy decisions rather than technical capabilities.

   5.  Centralization of the RuleSet creation is necessary because all
       RuleSets must cooperate toward a single goal, and the correct
       breakdown of responsibility cannot be determined from partial

   6.  Centralization of RuleSet creation allows a Responder to upgrade
       its policies independently and to improve the process if its
       developers have found better tuning mechanisms.  A distributed
       mechanism would require that all nodes in the path upgrade at the
       same time to avoid the creation of inconsistent policies, and
       limit the ability of Responders (often service providers of some
       sort) to innovate.

4.  Message Contents and Elements

   This section outlines the principal information to be carried in the
   messages.  Detailed packet formats are beyond the scope of this
   document, and may vary from network to network.

4.1.  PathSetupRequest

   At minimum, the PathSetupRequest message must contain:

   1.  node addresses for the Initiator and Responder

   2.  the class of service requested [qiroadmap]

   3.  minimum performance parameters (fidelity and throughput)

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4.2.  Quantum Capabilities (QCap)

   A QCap (quantum capabilities) block to be added to the stack in the
   PathSetupRequest message describes the functions, performance and
   quality of the node and link.  This may include:

   1.  the fidelity of Bell pairs created by the quantum channel

   2.  the fidelity of local operations performed by the node for
       purification or entanglement swapping

   3.  the rate at which entanglement can be created (Bell pairs per

   The details of the required information may differ between networks.
   A standardized form of this information for sharing between networks
   will be used for internetworking operation.

4.3.  RuleSets

   A RuleSet block in the stack in the PathSetupResponse message
   describes the rules to be executed at each node.  A rule consists of
   a Condition clause and an Action clause.  A Condition clause lists
   the existence of particular entangled states, or the reception of
   particular messages.  The Action clause describes the actions of
   purification, entanglement swapping, or even discarding an entangled
   state, as appropriate.  The details are beyond the scope of this

   In order to implement multiplexing schemes (e.g. buffer-space
   multiplexing, time-division multiplexing, or statistical
   multiplexing) based on the RuleSet-based network architecture, a
   RuleSet may include descriptors that define the usable resources for
   each link involved in that specific connection.

   If a link carries only a single connection, all resources available
   may be fully assigned to that single connection to maximize the
   throughput.  However, a link may receive a second RuleSet generated
   for a new connection.  In that case, the nodes must be able to
   correctly update and reassign the available resources.  Further
   details of the resource reservation and reclamation process are
   beyond the scope of this document.

5.  Processing the SetupRequest

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5.1.  Initiating a Connection Setup Request

   An Initiator, driven by an application request for quantum network
   services between itself and the Responder, builds the
   PathSetupRequest, populates the first QCap block, selects the next
   hop, and sends the request.  Note that there is no need for either
   the Initiator or the Responder to know the entire network topology,
   only be able to select a next hop appropriately.  The details of the
   routing are beyond the scope of this document.

5.2.  Outbound Processing

   Creation of the RuleSets requires knowledge of the number of nodes
   involved.  A quantum node adds its own address when receiving the
   request packet, before sending to the next node.  The stack size
   indicates how many nodes are involved.  Additionally, the RuleSet
   creator may require information regarding links between nodes along
   the path - e.g. to be used when optimizing the order of entanglement

   The pseudocode below outlines the processing on receipt of the
   PathSetupRequest message.

      procedure ProcessFlatPathSetupRequest(Msg)
          if (MyAddr != Msg.ConnSpec.Responder)
              // Process and forward
              NextQuantumHop = GetNextQuantumHop(Msg.ConnSpec.Responder)
              LinkInfo = GetLinkInfo(NextQuantumHop)
              // have reached the far end, need to build RuleSets
              // for everybody, then return
              ReturnMsg = ProcessFlatPath(Msg)
              MyRuleSet = ReturnMsg.RuleSetStack.Pop()
              NextQuantumHop = ReturnMsg.RuleSetStack.Top.Addr

   Note that although we use the term "NextQuantumHop" here, that refers
   to a neighboring quantum node, and does not imply that the classical
   node's neighbor is necessarily the same; it could, in theory, pass
   through multiple nodes to get there.

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5.3.  Responder Processing

   The Responder accepts the final PathSetupRequest message with the
   complete stack of information about node capabilities and links, and
   builds a corresponding stack of RuleSets, one per node in the path.
   The Responder's processing is outlined in the "then" clause of the
   pseudocode above.  The details of this creation process are beyond
   the scope of this document, and may be kept secret from other nodes
   in the path.

5.4.  Return Processing

   The pseudocode below outlines the processing on receipt of the
   PathSetupReturn message.

       procedure ProcessFlatPathSetupReturn(Msg)
           MyRuleSet = ReturnMsg.RuleSetStack.Pop()
           If (ReturnMsg.RuleSetStack.Size != 0)
               NextQuantumHop = ReturnMsg.RuleSetStack.Top.Addr

   The RuleSetStack should only be empty after the Initiator node of the
   original request removes its RuleSet, so this should be followed by
   activating the connection.

6.  Rejection and Robustness of the Setup Process

6.1.  Rejection by a Repeater or Router

   A repeater or router that receives a PathSetupRequest may reject the
   request if it has no quantum communication resources available.  It
   should not reject the request simply because it believes the
   requirements of the request (fidelity or rate) to be difficult to
   fulfill; that responsibility lies with the Responder.

   When a node rejects the PathSetupRequest, it shall inform the other
   nodes along the portion of the path that have already received the
   PathSetupRequest by creating a PathSetupResponse message with an
   error code that indicates failure and sending that message to the
   node on the top of the stack.  As with a successful
   PathSetupResponse, the list of nodes to which the message must be
   sent is created as a stack.  Other than the addresses and the error
   code, the message may be empty; no RuleSets are required.  The

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   message is then iteratively returned, with each node popping its own
   address and forwarding to the next.

6.2.  Rejection by a Responder

   A Responder may reject a PathSetupRequest for any reason:

   1.  As with any classical system, it may simply choose to reject the
       request for any service-related reason, such as security,
       licensing, etc.

   2.  It may determine that the request cannot be fulfilled with the
       resources offered by nodes in the path.

   When a node rejects the PathSetupRequest, it shall inform the other
   nodes along the path by creating a PathSetupReturn message with an
   error code that indicates failure and sending that message to the
   node on the top of the stack.  As with a successful
   PathSetupResponse, the list of nodes to which the message must be
   sent is created as a stack.  Other than the addresses and the error
   code, the message may be empty; no RuleSets are required.  The
   message is then iteratively returned, with each node popping its own
   address and forwarding to the next.

6.3.  Robustness

   As the rate of connection initiation increases, competition for
   resources will also increase.  A soft reservation mechanism that
   temporarily allocates resources in the anticipation of reception of a
   RuleSet may be used, with the reservation timing out and resources
   being released if no RuleSet arrives within a certain period.
   Specification of this mechanism is beyond the scope of this document.

   Deeper integration of routing with real-time availability of
   resources is beyond the scope of this document.

7.  Contributors

   Besides the authors, Luciano Aparicio, Clement Durand, Dominic
   Horsman, Shota Nagayama, Takahiko Satoh, Shigeya Suzuki, Amin
   Taherkhani, and Joe Touch have made substantial contributions to the
   network architecture and the concepts described here.

   We also thank Chia-Hung Chien, Kaori Ishizaki, Bill Munro, Kae
   Nemoto, Takafumi Oka, Shinnosuke Ozawa, and Thaddeus Ladd.

   Comments by Wojciech Kozlowski, Gyananjay Rai and Patrick Gelard are
   reflected in this draft.

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8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Security implications of this entire process are extensive.

   To minimize the probability of tampering, each information block
   added to the request on the outbound leg should be signed by the node
   adding the block.

   Each information block describes hardware configuration, and
   therefore inherently leaks information about the network topology and
   condition.  This document addresses only connection setup within a
   single network.  Internetwork connection setup will require
   mechanisms to limit the leaking of sensitive network information
   across organizational boundaries.

   Likewise, each RuleSet should be signed to prevent tampering during
   the PathSetupResponse phase.

   Both the Request and Response phase may be encrypted using
   appropriate public key mechanisms.

   It is also known that quantum networks may be vulnerable to attacks
   not possible in classical networks.  These concerns are beyond the
   scope of this document.

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,

10.2.  Informative References

              Wehner, S., Elkouss, D., and R. Hanson, "Quantum internet:
              A vision for the road ahead", Science 362, 2018.

              Van Meter, R., "Quantum Networking", Wiley-iSTE , 2014.

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   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,

   [theqi]    Kimble, J., "The Quantum Internet", Nature 453, 1023-1030,

Authors' Addresses

   Rodney Van Meter
   Keio University
   5322 Endo
   Fujisawa, Kanagawa  252-0882

   Phone: +81-46-649-3529
   Email: rdv@sfc.wide.ad.jp

   Takaaki Matsuo
   Keio University
   5322 Endo
   Fujisawa, Kanagawa  252-0882

   Phone: +81-46-649-3529
   Email: kaaki@sfc.wide.ad.jp

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