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Network Working Group                                   B. Trammell, Ed.
Internet-Draft                                        M. Kuehlewind, Ed.
Intended status: Informational                                ETH Zurich
Expires: April 21, 2016                                 October 19, 2015


 Requirements for the design of a Substrate Protocol for User Datagrams
                                 (SPUD)
                       draft-trammell-spud-req-01

Abstract

   The Substrate Protocol for User Datagrams (SPUD) BoF session at the
   IETF 92 meeting in Dallas in March 2015 identified the potential need
   for a UDP-based encapsulation protocol to allow explicit cooperation
   with middleboxes while using new, encrypted transport protocols.
   This document proposes an initial set of requirements for such a
   protocol, and discusses tradeoffs to be made in further refining
   these requirements.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  History . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Functional Requirements . . . . . . . . . . . . . . . . . . .   5
     5.1.  Grouping of Packets (into "tubes")  . . . . . . . . . . .   5
     5.2.  Endpoint to Path Signaling  . . . . . . . . . . . . . . .   6
     5.3.  Path to Endpoint Signaling  . . . . . . . . . . . . . . .   6
     5.4.  Tube Start and End Signaling  . . . . . . . . . . . . . .   6
     5.5.  Extensibility . . . . . . . . . . . . . . . . . . . . . .   6
     5.6.  Authentication  . . . . . . . . . . . . . . . . . . . . .   7
     5.7.  Proof a device is on-path . . . . . . . . . . . . . . . .   7
     5.8.  Integrity . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.9.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Technical Requirements  . . . . . . . . . . . . . . . . . . .   7
     6.1.  Middlebox Traversal . . . . . . . . . . . . . . . . . . .   8
     6.2.  Low Overhead in Network Processing  . . . . . . . . . . .   8
     6.3.  Implementability in User-Space  . . . . . . . . . . . . .   8
     6.4.  Incremental Deployability in an Untrusted, Unreliable
           Environment . . . . . . . . . . . . . . . . . . . . . . .   8
     6.5.  Protection against trivial abuse  . . . . . . . . . . . .   8
     6.6.  No unnecessary restrictions on the superstrate  . . . . .   9
     6.7.  Minimal additional start-up latency . . . . . . . . . . .   9
     6.8.  Minimal Header Overhead . . . . . . . . . . . . . . . . .   9
     6.9.  Minimal non-productive traffic  . . . . . . . . . . . . .   9
     6.10. Preservation of Security Properties . . . . . . . . . . .  10
     6.11. Reliability, Fragmentation, and Duplication . . . . . . .  10
     6.12. Interoperability with non-encapsulated superstrates . . .  10
   7.  Open questions and discussion . . . . . . . . . . . . . . . .  10
     7.1.  Tradeoffs in tube identifiers . . . . . . . . . . . . . .  11
     7.2.  Property binding  . . . . . . . . . . . . . . . . . . . .  12
     7.3.  Tradeoffs in integrity protection . . . . . . . . . . . .  12
     7.4.  In-band, out-of-band, piggybacked, and interleaved
           signaling . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.5.  Continuum of trust among endpoints and middleboxes  . . .  13
     7.6.  Discovery and capability exposure . . . . . . . . . . . .  13
     7.7.  Hard state vs. soft state . . . . . . . . . . . . . . . .  14
     7.8.  Tube vs. superstrate association lifetime . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15



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   12. Informative References  . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Motivation

   A number of efforts to create new transport protocols or experiment
   with new network behaviors have been built on top of UDP, as it
   traverses firewalls and other middleboxes more readily than new
   protocols do.  Each such effort must, however, either manage its
   flows within common middlebox assumptions for UDP or train the
   middleboxes on the new protocol (thus losing the benefit of using
   UDP).  A common Substrate Protocol for User Datagrams (SPUD) would
   allow each effort to re-use a set of shared methods for notifying
   middleboxes of the flows' semantics, thus avoiding both the
   limitations of current flow semantics and the need to re-invent the
   mechanism for notifying the middlebox of the new semantics.

   As a concrete example, it is common for some middleboxes to tear down
   required state (such as NAT bindings) very rapidly for UDP flows.  By
   notifying the path that a particular transport using UDP maintains
   session state and explicitly signals session start and stop using the
   substrate, the using protocol may reduce or avoid the need for
   heartbeat traffic.

   This document defines a specific set of requirements for a SPUD
   facility, based on analysis on a target set of applications to be
   developed on SPUD developing experience with a prototype described in
   [I-D.hildebrand-spud-prototype].  It is intended as the basis for
   determining the next steps to make progress in this space, including
   possibly chartering a working group for specific protocol engineering
   work.

2.  History

   An outcome of the IAB workshop on Stack Evolution in a Middlebox
   Internet (SEMI) [I-D.iab-semi-report], held in Zurich in January
   2015, was a discussion on the creation of a substrate protocol to
   support the deployment of new transport protocols in the Internet.
   Assuming that a way forward for transport evolution in user space
   would involve encapsulation in UDP datagrams, the workshop noted that
   it may be useful to have a facility built atop UDP to provide minimal
   signaling of the semantics of a flow that would otherwise be
   available in TCP.  At the very least, indications of first and last
   packets in a flow may assist firewalls and NATs in policy decision
   and state maintenance.  This facility could also provide minimal
   application-to- path and path-to-application signaling, though there
   was less agreement about what should or could be signaled here.
   Further transport semantics would be used by the protocol running



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   atop this facility, but would only be visible to the endpoints, as
   the transport protocol headers themselves would be encrypted, along
   with the payload, to prevent inspection or modification.  This
   encryption might be accomplished by using DTLS [RFC6347] as a
   subtransport [I-D.huitema-tls-dtls-as-subtransport] or by other
   suitable methods.

   The Substrate Protocol for User Datagrams (SPUD) BoF was held at IETF
   92 in Dallas in March 2015 to develop this concept further.  It is
   clear from discussion before and during the SPUD BoF that any
   selective exposure of traffic metadata outside a relatively
   restricted trust domain must be advisory, non-negotiated, and
   declarative rather than imperative.  This conclusion matches
   experience with previous endpoint-to-middle and middle-to-endpoint
   signaling approaches.  As with other metadata systems, exposure of
   specific elements must be carefully assessed for privacy risks and
   the total of exposed elements must be so assessed.  Each exposed
   parameter should also be independently verifiable, so that each
   entity can assign its own trust to other entities.  Basic transport
   over the substrate must continue working even if signaling is ignored
   or stripped, to support incremental deployment.  These restrictions
   on vocabulary are discussed further in [stackevo-explicit-coop].
   This discussion includes privacy and trust concerns as well as the
   need for strong incentives for middlebox cooperation based on the
   information that are exposed.

3.  Terminology

   This document uses the following terms:

   o  Superstrate: : The transport protocol or protocol stack "above"
      SPUD, that uses SPUD for explicit path cooperation and path
      traversal.  The superstrate usually consists of a security layer
      (e.g.  TLS, DTLS) and a transport protocol, or a transport
      protocol with integrated security features, to protect headers and
      payload above SPUD.

   o  Endpoint: : One end of a communication session, located on a
      single node that is a source or destination of packets in that
      session.  In this document, this term may refer to either the SPUD
      implementation at the endpoint, the superstrate implementation
      running over SPUD, or the applications running over that
      superstrate.

   o  Path: : The sequence of Internet Protocol nodes and links that a
      given packet traverses from endpoint to endpoint.





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   o  Middlebox: : As defined in [RFC3234], a middlebox is any
      intermediary device performing functions other than the normal,
      standard functions of an IP router on the datagram path between a
      source host and destination host; e.g. making decisions about
      forwarding behavior based on other than addressing information,
      and/or modifying a packet before forwarding.

4.  Use Cases

   The primary use case for endpoint to path signaling, making use of
   packet grouping, is the binding of limited related semantics (start,
   ack, and stop) to a flow or a group of packets within a flow which
   are semantically related in terms of the application or superstrate.
   By explicitly signaling start and stop semantics, a flow allows
   middleboxes to use those signals for setting up and tearing down
   their relevant state (NAT bindings, firewall pinholes), rather than
   requiring the middlebox to infer this state from continued traffic.
   At best, this would allow the application to refrain from sending
   heartbeat traffic, which might result in reduced radio utilization
   and thus greater battery life on mobile platforms.

   SPUD may also provide some facility for SPUD-aware nodes on the path
   to signal some property of the path relative to a tube to the
   endpoints and other SPUD- aware nodes on the path.  The primary use
   case for path to application signaling is parallel to the use of ICMP
   [RFC0792], in that it describes a set of conditions (including
   errors) that applies to the datagrams as they traverse the path.
   This usage is, however, not a pure replacement for ICMP but a
   "5-tuple ICMP" for error messages which should be application-
   visible; these would traverse NATs in the same way as the traffic
   related to it, and be deliverable to the application with appropriate
   tube information.

5.  Functional Requirements

   The following requirements detail the services that SPUD must provide
   to superstrates, endpoints, and middleboxes using SPUD.

5.1.  Grouping of Packets (into "tubes")

   Transport semantics and many properties of communication that
   endpoints may want to expose to middleboxes are bound to flows or
   groups of flows (five- tuples).  SPUD must therefore provide a basic
   facility for associating packets together (into what we call a
   "tube", for lack of a better term) and associate information to these
   groups of packets.  Each packet in a SPUD "flow" (determined by
   5-tuple) belongs to exactly one tube.  Notionally, a tube consists of
   a set of packets with a set of common properties, that should



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   therefore receive equivalent treatment from the network; these tubes
   may or may not be related to separate semantic entities in the
   superstrate (e.g.  SCTP streams).

   The simplest mechanisms for association involve the addition of an
   identifier to each packet in a tube.  Current thoughts on the
   tradeoffs on requirements and constraints on this identifier space
   are given in {{tradeoffs-in-tube- identifiers}}.

5.2.  Endpoint to Path Signaling

   SPUD must be able to provide information scoped to a tube from the
   end- point(s) to all SPUD-aware nodes on the path about the packets
   in that tube.  Since it is implausible that an endpoint has pre-
   existing trust relationships to all SPUD-aware middleboxes on a
   certain path in the context of the Internet, SPUD must provide in-
   band signaling.  SPUD may in addition also offer mechanisms for out-
   of-band signaling when appropriate.  See {{in-band-out-of- band-
   piggybacked-and-interleaved-signaling}} for more discussion.

5.3.  Path to Endpoint Signaling

   SPUD must be able to provide information from a SPUD-aware middlebox
   to the endpoint.  Though this information is not scoped to a tube in
   the same way that endpoint to path signaling is, as the middleboxes
   do not originate the packets in a tube, it is still associated with a
   tube, in terms of "the properties of the path(s) this tube will
   traverse".  Path to endpoint signaling need not be in-band; see
   Section 7.4 for more discussion.

5.4.  Tube Start and End Signaling

   SPUD must provide a facility for endpoints to signal that a tube has
   started, that the start of the tube has been acknowledged and
   accepted by the remote endpoint(s), and that a tube has ended and its
   state can be forgotten by the path.  Given unreliable signaling (see
   Section 6.11) both endpoints and devices on the path must be
   resilient to the loss of any of these signals.  Specifically,
   timeouts are still necessary to clean up stale state.  See
   Section 7.7 and Section 7.8 for more discussion on tube start and end
   signaling.

5.5.  Extensibility

   SPUD must enable multiple new transport semantics and application/
   path declarations without requiring updates to SPUD implementations
   in middleboxes.




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5.6.  Authentication

   The basic SPUD protocol must not require any authentication or a
   priori trust relationship between endpoints and middleboxes to
   function.  However, SPUD should interoperate with the presentation/
   exchange of authentication information in environments where a trust
   relationship already exists, or can be easily established, either in-
   band or out-of-band, and use this information where possible and
   appropriate.

5.7.  Proof a device is on-path

   Devices may make assertions of network characteristics relevant to a
   flow.  One way these assertions can be assessed is by a demonstration
   that the device making it is on-path to the flow and so could adjust
   the characteristics to match the assertion.  SPUD must therefore
   allow endpoints to distinguish on- path devices from devices not on
   the path.  Network elements may also need to confirm that
   application-to-path assertions are made by the source indicated in
   the flow.  In both cases, return routability (as in {{protection-
   against- trivial-abuse}}) may offer one incrementally deployable
   method of testing the topology to make this confirmation.

5.8.  Integrity

   SPUD must provide integrity protection of SPUD-encapsulated packets,
   though the details of this integrity protection are still open; see
   {{tradeoffs-in- integrity-protection}}. Endpoints should be able to
   detect changes to headers SPUD uses for its own signaling (whether
   due to error, accidental modification, or malicious modification), as
   well as the injection of packets into a SPUD flow (defined by
   5-tuple) or tube by nodes other than the remote endpoint.  Integrity
   protection of the superstrate is left up to the superstrate.

5.9.  Privacy

   SPUD must allow endpoints to control the amount of information
   exposed to middleboxes, with the default being the minimum necessary
   for correct functioning.

6.  Technical Requirements

   The following requirements detail the constraints on how the SPUD
   facility must meet its functional requirements.







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6.1.  Middlebox Traversal

   SPUD must be able to traverse middleboxes that are not SPUD-aware.
   Therefore SPUD must be encapsulated in a transport protocol that is
   known to be accepted on a large fraction of paths in the Internet, or
   implement some form of probing to determine in advance which
   transport protocols will be accepted on a certain path.  This
   encapsulation will require port numbers to support NAPT- connected
   endpoints.  UDP encapsulation is the only mechanism that meets these
   requirements.

6.2.  Low Overhead in Network Processing

   SPUD must be low-overhead, specifically requiring very little effort
   to recognize that a packet is a SPUD packet and to determine the tube
   it is associated with.

6.3.  Implementability in User-Space

   To enable fast deployment SPUD and superstrates must be implementable
   without requiring kernel replacements or modules on the endpoints,
   and without having special privilege (root or "jailbreak") on the
   endpoints.  Usually all operating systems will allow a user to open a
   UDP socket.  This indicates UDP- based encapsulation, either
   exclusively or as a mandatory-to-implement feature.

6.4.  Incremental Deployability in an Untrusted, Unreliable Environment

   SPUD must operate in the present Internet.  In order to maximize
   deployment, it should also be useful between endpoints even before
   the deployment of middleboxes that understand it.  The information
   exposed over SPUD must provide incentives for adoption by both
   endpoints and middleboxes, and must maximize privacy (by minimizing
   information exposed).  Further, SPUD must be robust to packet loss,
   duplication and reordering by the underlying network service.  SPUD
   must work in multipath, multicast, and endpoint multi- homing
   environments.

   Incremental deployability likely requires limitations of the
   vocabulary used in signaling, to ensure that each actor in a
   nontrusted environment has incentives to participate in the signaling
   protocol honestly; see {{stackevo- explicit-coop}} for more.

6.5.  Protection against trivial abuse

   Malicious background traffic is a serious problem for UDP- based
   protocols due to the ease of forging source addresses in UDP together
   with the only limited deployment of network egress filtering



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   [RFC2827].  Trivial abuse includes flooding and state exhaustion
   attacks, as well as reflection and amplification attacks.  SPUD must
   provide minimal protection against this trivial abuse.  This probably
   implies that SPUD should provide:

   o  a proof of return routability,

   o  a feedback channel between endpoints,

   o  a method to probabilistically discriminiate legitimate SPUD
      traffic from reflected malicious traffic, and

   o  mechanisms to protect against state exhaustion and other denial-
      of-service attacks.

   We note that return routability excludes use of a UDP source port
   that does not accept traffic (i.e., for one-way communication, as is
   commonly done for unidirectional UDP tunnels, e.g., MPLS in UDP
   [RFC7510] as an entropy input.)

6.6.  No unnecessary restrictions on the superstrate

   Beyond those restrictions deemed necessary as common features of any
   secure, responsible transport protocol (see Section 6.5), SPUD must
   impose only minimal restrictions on the transport protocols it
   encapsulates.  However, to serve as a substrate, it is necessary to
   factor out the information that middleboxes commonly rely on and
   endpoints are commonly willing to expose.  This information should be
   included in SPUD, and might itself impose additional restrictions to
   the superstrate.

6.7.  Minimal additional start-up latency

   SPUD should not introduce additional start-up latency for
   superstrates.

6.8.  Minimal Header Overhead

   To avoid reducing network performance, the information and coding
   used in SPUD should be designed to use the minimum necessary amount
   of additional space in encapsulation headers.

6.9.  Minimal non-productive traffic

   SPUD should minimize additional non-productive traffic (e.g.
   keepalives), and should provide mechanisms to allow its superstrates
   to minimize their reliance on non-productive traffic.




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6.10.  Preservation of Security Properties

   The use of SPUD must not weaken the security properties of the
   superstrate.  If the superstrate includes payload encryption for
   confidentiality, for example, the use of SPUD must not allow deep
   packet inspection systems to have access to the plaintext.  While a
   box along the path may indicate a particular flow is adminstratively
   prohibited or why it is prohibited, SPUD itself must not be used to
   negotiate the means to lift the prohibition.

6.11.  Reliability, Fragmentation, and Duplication

   As any information provided by SPUD is anyway opportunistic, SPUD
   need not provide reliable signaling for the information associated
   with a tube.  Signals must be idempotent; all middleboxes and
   endpoints must gracefully handle receiving duplicate signal
   information.  To avoid issues with fragment reassembly, all in-band
   SPUD signaling information must fit within a single packet.  Any
   facilities requiring more than an MTU's worth of data in a single
   signal should use an out-of-band method which does provide
   reliability - this method may be an existing transport or
   superstrate/SPUD combination, or a "minimal transport" defined by
   SPUD for its own use.

6.12.  Interoperability with non-encapsulated superstrates

   It is presumed that "superstrate X with SPUD" is a distinct entity on
   the wire from "superstrate X".  The APIs the superstrate presents to
   the application should be equivalent, and the two wire protocols
   should be freely transcodeable between each other, with the caveat
   that the variant without SPUD would not necessarily support features
   enabling communication with the path.  However, there is no
   requirement that the headers the superstrate uses be the same in the
   SPUD and non-SPUD variants.  Headers that the superstrate chooses
   always to expose to the path can therefore be encoded in the SPUD
   layer but not appear in an upper-layer header.

7.  Open questions and discussion

   The preceding requirements reflect the present best understanding of
   the authors of the functional and technical requirements on an
   encapsulation-based protocol for common middlebox-endpoint
   cooperation for superstrates.  There remain a few large open
   questions and points for discussion, detailed in the subsections
   below.






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7.1.  Tradeoffs in tube identifiers

   Grouping packets into tubes requires some sort of notional tube
   identifier; for purposes of this discussion we will assume this
   identifier to be a simple vector of N bits.  The properties of the
   tube identifier are subject to tradeoffs on the requirements for
   privacy, security, ease of implementation, and header overhead
   efficiency.

   We first assume that the 5-tuple of source and destination IP
   address, UDP (or other transport protocol) port, and IP protocol
   identifier (17 for UDP) is used in the Internet as an existing flow
   identifier, due to the widespread deployment of network address and
   port translation.  The question then arises whether tube identifiers
   should be scoped to 5-tuples (i.e., a tube is identified by a 6-tuple
   including the tube identifier) or should be separate, and presumed to
   be globally unique.

   If globally unique, N must be sufficiently large to minimize the
   probability of collision among multiple tubes having the same
   identifier along the same path during some period of time.  A 128-bit
   UUID [RFC4122] or an identifier generated using an equivalent
   algorithm would be useful as such a globally- unique tube identifier.
   An advantage of globally unique tube identifiers would be migration
   of per-tube state across multiple five-tuples for mobility support in
   multipath protocols.  However, globally unique tube identifiers would
   also introduce new possibilities for user and node tracking, with a
   serious negative impact on privacy.  This alone probably speaks
   against using globally unique identifiers for SPUD.

   In the case of 5-tuple-scoped identifiers, mobility must be supported
   separately from the tube identification mechanism.  This could be
   specific to each superstrate (i.e., hidden from the path), or SPUD
   could provide a general endpoint-to-path tube grouping signal to
   allow an endpoint to explicitly expose the fact that one tube is
   related to another to the path.  Even in this case, N must still be
   sufficiently large, and the bits in the identifier sufficiently
   random, that possession of a valid tube ID implies that a node can
   observe packets belonging to the tube.  This reduces the chances of
   success of blind packet injection attacks of packets with guessed
   valid tube IDs.

   When scoped to 5-tuples, the forward and backward directions of a
   bidirectional flow probably have different tube IDs, since these will
   necessarily take different paths and may interact with a different
   set of middleboxes due to asymmetric routing.  SPUD will therefore
   require some facility to note that one tube is the "reverse"




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   direction of another, a general case of the tube grouping signal
   above.

7.2.  Property binding

   Related to identifier scope is the scope of properties bound to SPUD
   packets by endpoints.  SPUD may support both per-tube properties as
   well as per-packet properties.  Properties signaled per packet reduce
   state requirements at middleboxes, but also increase per-packet
   overhead.  It is likely that both types of property binding are
   necessary, but the selection of which properties to bind how must be
   undertaken carefully.  It is also possible that SPUD will provide a
   very limited set of per-packet signals (such as ECN) using flags in
   the SPUD header, and require all more complicated properties to be
   bound per- tube.

7.3.  Tradeoffs in integrity protection

   In order to protect the integrity of information carried by SPUD
   against forging by malicious devices along the path, it would be
   necessary to be able to authenticate the originator of that
   information.  We presume that the authentication of endpoints is a
   generally desirable property, and to be handled by the superstrate;
   in this case, SPUD may be able borrow that authentication to protect
   the integrity of endpoint-originated information.

   However, in the Internet, it is not in the general case possible for
   the endpoint to authenticate every middlebox that might see packets
   it sends and receives.  In this case information produced by
   middleboxes may enjoy less integrity protection than that produced by
   endpoints.  In addition, endpoint authentication of middleboxes and
   vice-versa may be better conducted out-of- band (treating the
   middlebox as an endpoint for the authentication protocol) than in-
   band (treating the middlebox as a participant in a 3+ party
   communication).

7.4.  In-band, out-of-band, piggybacked, and interleaved signaling

   Discussions about SPUD to date have focused on the possibility of in-
   band signaling from endpoints to middleboxes and back - the signaling
   channel happens on the same 5-tuple as the data carried by the
   superstrate.  However, there are a wide variety of potential
   signaling arrangements: in-band signaling can be piggybacked (where
   signaling happens on packets sent by the superstrate) and/or
   interleaved (where SPUD and the superstrate each have their own
   packets).  Signaling can also be out-of-band (on a different five
   tuple, or even over a completely different protocol).  Out of band
   signaling for path-to-endpoint information can use direct return,



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   allowing a device on the path to communicate directly with an
   endpoint (i.e., as with ICMP).  More discussion on the tradeoffs here
   is given in [stackevo-explicit-coop].

   The tradeoffs here must be carefully weighed, and the final approach
   may use a mix of all these communication patterns where SPUD provides
   different signaling patterns for different situations.  E.g., a
   middlebox might need to generate out-of-band signals for error
   messages or can provide requested information in-band and feedback
   over the receiver if a minimum or maximum value from all SPUD-aware
   middleboxes on path should be discovered.

7.5.  Continuum of trust among endpoints and middleboxes

   There are different security considerations for different security
   contexts.  The end-to-end context is one; anything that only needs to
   be seen by the path shouldn't be exposed in SPUD, but rather by the
   superstrate.  There are multiple different types of end-to-middle
   context based on levels of trust between end and middle - is the
   middlebox on the same network as the endpoint, under control of the
   same owner?  Is there some contract between the application user and
   the middlebox operator?  SPUD should support different levels of
   trust than the default ("untrusted, but presumed honest due to
   limitations on the signaling vocabulary") and fully-authenticated;
   how these points along the continuum are to be implemented and how
   they relate to each other needs to be explored further.

7.6.  Discovery and capability exposure

   There are three open issues in discovery and capability exposure.
   First, an endpoint needs to discover if the other communication
   endpoint understands SPUD.  Second, endpoints need to test whether
   SPUD is potentially not usable along a path because of middleboxes
   that block SPUD packets or strip the SPUD header.  If such
   impairments exist in the path, a SPUD sender needs to fall back to
   some other approach to achieve the goals of the superstrate.  Third,
   endpoints might want to be able to discover SPUD-aware middleboxes
   along the path, and to discover which parts of the vocabulary that
   can be spoken by the endpoints are supported by those middleboxes as
   well as the other communication endpoint, and vice versa.

   In addition, endpoints may need to discover and negotiate which
   superstrates are available for a given interaction.  SPUD could
   assist here.  However, it is explicitly not a goal of SPUD to expose
   information about the details of the superstrate to middleboxes.






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7.7.  Hard state vs. soft state

   The initial thinking on signaling envisions "hard state" in
   middleboxes that is established when the middlebox observes the start
   of a SPUD tube and is torn down when the middlebox observes the end
   (stop) of a SPUD tube.  Such state can be abandoned as a result of
   network topology changes (e.g., routing update in response to link or
   node failure).  An alternative is a "soft state" approach that
   requires periodic refresh of state in middleboxes, but cleanly times
   out and discards abandoned state.  SPUD has the opportunity to use
   different timeouts than the defaults that are required for current
   NAT and firewall pinhole maintenance.  Of course, applications will
   still have to detect non-SPUD middleboxes that use shorter timers.

7.8.  Tube vs. superstrate association lifetime

   The requirements as presently defined use tube start and stop
   signaling for two things: (1) setting up and tearing down state along
   the path, and (2) signaling superstrate such as association startup,
   acceptance, and teardown, which may have security implications.
   These may require separate signaling.  Specifically, if tube start
   acknowledgement is to be used to provide explicit guarantees to the
   path about the acceptability of a tube to a remote endpoint, it
   cannot be a completely unreliable signal.  Second, the lifetime of a
   tube may be much shorter than the lifetime of a superstrate
   association, and the creation of a new tube over an existing
   association may need to be treated differently by endpoints and path
   devices than a tube creation coincident with an association creation.

8.  Security Considerations

   The security-relevant requirements for SPUD deal mainly with endpoint
   authentication and the integrity of exposed information (Section 5.6,
   Section 5.8, Section 5.9, and Section 7.3); protection against
   attacks (Section 5.7, Section 6.5, and Section 7.1 and); and the
   trust relationships among endpoints and middleboxes Section 7.5.
   These will be further addressed in protocol definition work following
   from these requirements.

9.  IANA Considerations

   This document has no actions for IANA.

10.  Contributors

   In addition to the editors, this document is the work of David Black,
   Ken Calvert, Ted Hardie, Joe Hildebrand, Jana Iyengar, and Eric
   Rescorla.



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11.  Acknowledgments

   Thanks to Roland Bless, Cameron Byrne, Toerless Eckert, Daniel Kahn
   Gillmor, Tom Herbert, and Christian Huitema for feedback and comments
   on these requirements, as well as to the participants at the SPUD BoF
   at IETF 92 meeting in Dallas inand the IAB SEMI workshop in Zurich
   for the discussions leading to this work.

12.  Informative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <http://www.rfc-editor.org/info/rfc792>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,
              <http://www.rfc-editor.org/info/rfc3234>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI
              10.17487/RFC4122, July 2005,
              <http://www.rfc-editor.org/info/rfc4122>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC7510]  Xu, X., Sheth, N., Yong, L., Callon, R., and D. Black,
              "Encapsulating MPLS in UDP", RFC 7510, DOI 10.17487/
              RFC7510, April 2015,
              <http://www.rfc-editor.org/info/rfc7510>.

   [I-D.hildebrand-spud-prototype]
              Hildebrand, J. and B. Trammell, "Substrate Protocol for
              User Datagrams (SPUD) Prototype", draft-hildebrand-spud-
              prototype-03 (work in progress), March 2015.

   [I-D.huitema-tls-dtls-as-subtransport]
              Huitema, C., Rescorla, E., and J. Jana, "DTLS as
              Subtransport protocol", draft-huitema-tls-dtls-as-
              subtransport-00 (work in progress), March 2015.





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   [stackevo-explicit-coop]
              Trammell, B., "Architectural Considerations for Transport
              Evolution with Explicit Path Cooperation", September 2015.

   [I-D.iab-semi-report]
              Trammell, B. and M. Kuehlewind, "IAB Workshop on Stack
              Evolution in a Middlebox Internet (SEMI) Report", draft-
              iab-semi-report-01 (work in progress), July 2015.

Authors' Addresses

   Brian Trammell (editor)
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: ietf@trammell.ch


   Mirja Kuehlewind (editor)
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch
























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