Internet Architecture Board (IAB)                       B. Trammell, Ed.
Request for Comments: 7663                            M. Kuehlewind, Ed.
Category: Informational                                       ETH Zurich
ISSN: 2070-1721                                             October 2015


                      Report from the IAB Workshop
           on Stack Evolution in a Middlebox Internet (SEMI)

Abstract

   The Internet Architecture Board (IAB) through its IP Stack Evolution
   program, the Internet Society, and the Swiss Federal Institute of
   Technology (ETH) Zurich hosted the Stack Evolution in a Middlebox
   Internet (SEMI) workshop in Zurich on 26-27 January 2015 to explore
   the ability to evolve the transport layer in the presence of
   middlebox- and interface-related ossification of the stack.  The goal
   of the workshop was to produce architectural and engineering guidance
   on future work to break the logjam, focusing on incrementally
   deployable approaches with clear incentives to deployment both on the
   endpoints (in new transport layers and applications) as well as on
   middleboxes (run by network operators).  This document summarizes the
   contributions to the workshop and provides an overview of the
   discussion at the workshop, as well as the outcomes and next steps
   identified by the workshop.  The views and positions documented in
   this report are those of the workshop participants and do not
   necessarily reflect IAB views and positions.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7663.

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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   (http://trustee.ietf.org/license-info) in effect on the date of
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   to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Organization of This Report . . . . . . . . . . . . . . .   4
   2.  The Situation in Review . . . . . . . . . . . . . . . . . . .   4
   3.  Incentives for Stack Ossification and Evolution . . . . . . .   5
   4.  The Role and Rule of Middleboxes  . . . . . . . . . . . . . .   6
   5.  Evolving the Transport Layer  . . . . . . . . . . . . . . . .   6
   6.  Outcomes  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Minimal Signaling for Encapsulated Transports . . . . . .   7
     6.2.  Middlebox Measurement . . . . . . . . . . . . . . . . . .   8
     6.3.  Guidelines for Middlebox Design and Deployment  . . . . .   9
     6.4.  Architectural Guidelines for Transport Stack Evolution  .   9
     6.5.  Additional Activities in the IETF and IAB . . . . . . . .  10
     6.6.  Additional Activities in Other Venues . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Attendees  . . . . . . . . . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The transport layer of the Internet has become ossified, squeezed
   between narrow interfaces (from BSD sockets to pseudo-transport over
   HTTPS) and increasing in-network modification of traffic by
   middleboxes that make assumptions about the protocols running through
   them.  This ossification makes it difficult to innovate in the
   transport layer, through the deployment of new protocols or the
   extension of existing ones.  At the same time, emerging applications
   require functionality that existing protocols can provide only
   inefficiently, if at all.

   To begin to address this problem, the IAB, within the scope of its IP
   Stack Evolution Program, organized a workshop to discuss approaches
   to de-ossify transport, especially with respect to interactions with
   middleboxes and new methods for implementing transport protocols.
   Recognizing that the end-to-end principle has long been compromised,
   we start with the fundamental question of matching paths through the
   Internet with certain characteristics to application and transport
   requirements.

   We posed the following questions in the call for papers: Which paths
   through the Internet are actually available to applications?  Which
   transports can be used over these paths?  How can applications
   cooperate with network elements to improve path establishment and
   discovery?  Can common transport functionality and standardization
   help application developers to implement and deploy such approaches
   in today's Internet?  Could cooperative approaches give us a way to
   rebalance the Internet back toward its end-to-end roots?

   The call for papers encouraged a focus on approaches that are
   incrementally deployable within the present Internet.  Identified
   topics included the following:

   o  Development and deployment of transport-like features in
      application-layer protocols

   o  Methods for discovery of path characteristics and protocol
      availability along a path

   o  Methods for middlebox detection and characterization of middlebox
      behavior and functionality

   o  Methods for NAT and middlebox traversal in the establishment of
      end-to-end paths

   o  Mechanisms for cooperative path-endpoint signaling, and lessons
      learned from existing approaches

   o  Economic considerations and incentives for cooperation in
      middlebox deployment

   The Internet Architecture Board (IAB) holds occasional workshops
   designed to consider long-term issues and strategies for the
   Internet, and to suggest future directions for the Internet
   architecture.  This long-term planning function of the IAB is
   complementary to the ongoing engineering efforts performed by working
   groups of the Internet Engineering Task Force (IETF), under the
   leadership of the Internet Engineering Steering Group (IESG) and area
   directorates.

   The SEMI workshop followed in part from the IAB's longer term
   interest in the evolution of the Internet and the adoption of
   Internet protocols, including the Internet Technology Adoption and
   Transition workshop [RFC7305], "What Makes for a Successful Protocol"
   [RFC5218], back to Deering's plenary talk [deering-plenary] at IETF
   51 in 2001.

1.1.  Organization of This Report

   This workshop report summarizes the contributions to, and discussions
   at the workshop, organized by topic.  We started with a summary of
   the current situation with respect to stack ossification, and
   explored the incentives that have made it that way and the role of
   incentives in evolution.  Many contributions were broadly split into
   two areas: middlebox measurement, classification, and approaches to
   defense against middlebox modification of packets; and approaches to
   support transport evolution.  All accepted position papers and
   detailed transcripts of discussion are available at
   https://www.iab.org/activities/workshops/semi/.

   The outcomes of the workshop are discussed in Section 6, including
   progress after the workshop toward each of the identified work items
   as of the time of publication of this report.

2.  The Situation in Review

   At the time of Deering's talk in 2001, network address translation
   (NAT) was identified as the key challenge to the Internet
   architecture.  Since then, the NAT traversal problem has been largely
   solved, but the boxes in the middle are getting smarter and more
   varied.

   SEMI, as the IP Stack Evolution program in general, is far from the
   first attempt to solve the problems caused by middlebox interference
   in the end-to-end model.  Just within the IETF, the MIDCOM, NSIS, and

   BEHAVE efforts have addressed this problem, and the TRAM working
   group is updating the NAT traversal outcomes of MIDCOM to reflect
   current reality.

   We believe we have an opportunity to improve the situation in the
   present, however, due to a convergence of forces.  While the tussle
   between security and middleboxes is not new, the accelerating
   deployment of cryptography for integrity and confidentiality makes
   many packet inspection and packet modification operations obsolete,
   creating pressure to improve the situation.  There is also new energy
   in the IETF around work that requires transport-layer flexibility
   we're not sure we have (e.g., WebRTC) as well as flexibility at the
   transport interface (TAPS).

3.  Incentives for Stack Ossification and Evolution

   The current situation is, of course, the result of a variety of
   processes, and the convergence of incentives for network operators,
   content providers, network equipment vendors, application developers,
   operating system developers, and end users.  Moore's Law makes it
   easier to deploy more processing on-path, network operators need to
   find ways to add value, enterprises find it more scalable to deploy
   functionality in-network than on endpoints, and middleboxes are
   something vendors can vend.  These trends increase ossification of
   the network stack.

   Any effort to reduce the resulting ossification in order to make it
   easier to evolve the transport stack, then, must consider the
   incentives to deployment of new approaches by each of these actors.

   As Christian Huitema [huitema-semi] pointed out, encryption provides
   a powerful incentive here: putting a transport protocol atop a
   cryptographic protocol atop UDP resets the transport versus middlebox
   tussle by making inspection and modification above the encryption and
   demux layer impossible.  Any transport evolution strategy using this
   approach must also deliver better performance or functionality (e.g.,
   setup latency) than existing approaches while being as deployable as
   these approaches, or moreso.

   Indeed, significant positive net value at each organization where
   change is required -- operators, application developers, equipment
   vendors, enterprise and private users -- is best to drive deployment
   of a new protocol, said Dave Thaler, pointing to [RFC5218].  All
   tussles in networking stem from conflicting incentives unavoidable in
   a free market.  For upper-layer protocols, incentives tend to favor
   protocols that work anywhere, use the most efficient mechanism that
   works, and are as simple as possible from an implementation,
   maintenance, and management standpoint.  For lower-layer protocols,

   incentives tend toward ignoring and or disabling optional features,
   as there is a positive feedback cycle between being rarely used and
   rarely implemented.

4.  The Role and Rule of Middleboxes

   Middleboxes are commonplace in the Internet and constrain the ability
   to deploy new protocols and protocol extensions.  Engineering around
   this problem requires a "bestiary" of middleboxes, a classification
   of which kinds of impairments middleboxes cause and how often,
   according to Benoit Donnet [edeline-semi].

   Even though the trend towards Network Function Visualization (NFV)
   allows for faster update-cycle of middleboxes and thereby more
   flexibility, the function provided by middleboxes will stay.  In
   fact, service chaining may lead to more and more add-ons to address
   and manage problems in the network, in turn further increasing the
   complexity of network management.  Ted Hardie [hardie-semi] warned
   that each instance may add a new queue and may increase the
   bufferbloat problem that is counterproductive for new emerging
   latency-sensitive applications.  However, this new flexibility also
   provides a chance to move functionality back to the end host.
   Alternately, more appropriate in-network functionality could benefit
   from additional information in application and path characteristics,
   though this in turn implies a variety of complicated trust
   relationships among nodes in the network.  In any case, an increasing
   trend of in-network functionality can be observed, especially in
   mobile networks.

   Costin Raiciu [raiciu-semi] stated that middleboxes make the Internet
   unpredictable, leading to a trade-off between efficiency and
   reachability.  While constructive cooperation with middleboxes to
   establish a clear contract between the network and the endpoint might
   be one approach to address this challenge, enforcement of contract in
   less cooperative environments might require extensive tunneling.
   Raiciu's contribution on "ninja tunneling" illustrates one such
   approach.

5.  Evolving the Transport Layer

   For evolution in the transport layer itself, various proposals have
   been discussed, reaching from the development of new protocols
   (potentially as user-level stacks) encapsulated in UDP as a transport
   identification sub-header to the use of TCP as a substrate where the
   semantics of TCP are relaxed (e.g., regarding reliability, ordering,
   flow control, etc.) and a more flexible API is provided to the
   application.

   Discussion on evolution during the workshop divided amicably along
   two lines: working to fix the deployability of TCP extensions
   (referred to in discussion as "the TCP Liberation Front") versus
   working to build new encapsulation-based mechanisms to allow wholly
   new protocols to be deployed (referred to in discussion as "the
   People's Front of UDP").  David Black [black-semi] pointed out that
   UDP encapsulation has to be adapted and separately discussed for
   every use case, which can be a long and painful process.  UDP
   encapsulation can be an approach to develop more specialized
   protocols that helps to address special needs of certain
   applications.  However, Stuart Cheshire [cheshire-semi] (as presented
   by Brian Trammell) pointed out that designing a new protocol instead
   of fixing/extending TCP might not always solve the problem.

   To address the extensibility problem of TCP, Bob Briscoe proposed
   Inner Space [briscoe-semi].  Here, the general principle is to extend
   layer X's header within layer X+1; in the case of TCP, additional TCP
   header and option space is provided within the TCP payload, such that
   it cannot presently be inspected and modified by middleboxes.

   Further, instead of only focusing on those cases where new extensions
   and protocols are not deployable, Micheal Welzl [welzl-semi] points
   out that there are also a lot of paths in the network that are not
   ossified.  To enable deployment on these paths, an end host would
   need to probe or use a happy-eyeball-like approach [RFC6555] and
   potentially fallback.  The TAPS working group implements the first
   step to decouple applications from transport protocols allowing for
   the needed flexibility in the transport layer.

6.  Outcomes

   The SEMI workshop identified several areas for further work, outlined
   below.

6.1.  Minimal Signaling for Encapsulated Transports

   Assuming that a way forward for transport evolution in user space
   would involve encapsulation in UDP datagrams, the workshop identified
   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 to 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 on exactly what should or could be signaled here.

   The workshop did note that, given the increasing deployment of
   encryption in the Internet, this facility should cooperate with
   Datagram Transport Layer Security (DTLS) [RFC6347] in order to
   selectively expose information about traffic flows where the
   transport headers and payload themselves are encrypted.

   To develop this concept further, it was decided to propose a BoF
   session that would not form a working group, SPUD (Substrate Protocol
   for User Datagrams), at the IETF 92 meeting in March in Dallas.  A
   document on use cases [SPUD-USE], a prototype specification for a
   shim protocol over UDP [SPUD-PROTO], and a separate specification of
   the use of DTLS as a subtransport layer [TLS-DTLS] were prepared
   following discussions at SEMI and presented at the BoF.

   Clear from discussion before and during the SPUD BoF, and drawing on
   experience with previous endpoint-to-middle and middle-to-endpoint
   signaling approaches, is that any selective exposure of traffic
   metadata outside a relatively restricted trust domain must be
   declarative as opposed to imperative, non-negotiated, and advisory.
   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 [EXP-COOP].

   There was much interest in the room in continuing work on an approach
   like the one under discussion.  It was relatively clear that the
   state of the discussion and prototyping activity now is not yet
   mature enough for standardization within an IETF working group.  An
   appropriate venue for continuing the work remains unclear.

   Discussion continues on the spud mailing list (spud@ietf.org).  The
   UDP shim layer prototype is described by [SPUD-PROTO].

6.2.  Middlebox Measurement

   Discussion about the impairments caused by middleboxes quickly
   identified the need to get more and better data about how prevalent
   certain types of impairments are in the network.  It doesn't make
   much sense, for instance, to engineer complex workarounds for certain
   types of impairments into transport protocols if those impairments
   are relatively rare.  There are dedicated measurement studies for
   certain types of impairment, but the workshop noted that prevalence
   data might be available from error logs from TCP stacks and
   applications on both clients and servers: these entities are in a
   position to know when attempts to use particular transport features
   failed, providing an opportunity to measure the network as a side
   effect of using it.  Many clients already have a feature for sending

   these bug reports back to their developers.  These present
   opportunities to bring data to bear on discussion and decisions about
   protocol engineering in an Internet full of middleboxes.

   The HOPS (How Ossified is the Protocol Stack) informal birds of a
   feather session ("Bar BoF") was held at the IETF 92 meeting in
   Dallas, to discuss approaches to get aggregated data from these logs
   about potential middlebox impairment, focusing on common data formats
   and issues of preserving end-user privacy.  While some discussion
   focused on aggregating impairment observations at the network level,
   initial work will focus on making relative prevalence information
   available on an Internet-wide scope.  The first activity identified
   has been to match the types of data required to answer questions
   relevant to protocol engineering to the data that currently is or can
   easily be collected.

   A mailing list (hops@ietf.org) has been established to continue
   discussion.

6.3.  Guidelines for Middlebox Design and Deployment

   The workshop identified the potential to update [RFC3234] to provide
   guidelines on middlebox design, implementation, and deployment in
   order to reduce inadvertent or accidental impact on stack
   ossification in existing and new middlebox designs.  The IAB Stack
   Evolution Program will follow up on this with the participants in the
   now-closed BEHAVE working group, as it most closely follows the work
   of that group.  It will draw in part on the work of the BEHAVE
   working group, and on experience with STUN, TURN, and ICE, all of
   which focus more specifically on network address translation.

6.4.  Architectural Guidelines for Transport Stack Evolution

   The workshop identified the need for architectural guidance in
   general for transport stack evolution: tradeoffs between user- and
   kernel-space implementations, tradeoffs in and considerations for
   encapsulations (especially UDP), tradeoffs in implicit versus
   explicit interaction with devices along the path, and so on.  This
   document will be produced by the IAB IP Stack Evolution Program; the
   new transport encapsulations document [EXP-COOP] may evolve into the
   basis for this work.

   Further, due to the underlying discuss on trust and a needed "balance
   of power" between the end hosts and the network, the workshop
   participants concluded that it is necessary to define approaches
   based on the cryptographic protocol to enable transport protocol
   extensibility.

6.5.  Additional Activities in the IETF and IAB

   The workshop identified the need to socialize ideas connected to
   transport stack evolution within the IETF community, including
   presentations in the transport and applications open area meetings on
   protocol extensibility, UDP encapsulation considerations, and the
   application of TLS/DTLS in order to prevent middlebox meddling.  Much
   of the energy coming out of the workshop went into the SPUD BoF (see
   Section 6.1), so these presentations will be given at future
   meetings.

   There are also clear interactions between the future work following
   the SEMI workshop and the IAB's Privacy and Security Program; Privacy
   and Security program members will be encouraged to follow
   developments in transport stack evolution to help especially with
   privacy implications of the outcomes of the workshop.

6.6.  Additional Activities in Other Venues

   Bob Briscoe informally liaised the SEMI workshop discussions to the
   ETSI Network Function Virtualization (NFV) Industry Specification
   Group (ISG) following the workshop, focusing as well on the
   implications of end-to-end encryption on the present and future of
   in-network functionality.  In the ISG's Security Working Group, he
   proposed text for best practices on middlebox access to data in the
   presence of end-to-end encryption.

7.  Security Considerations

   This document presents no security considerations.

8.  Informative References

   [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>.

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes For a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
              <http://www.rfc-editor.org/info/rfc5218>.

   [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>.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <http://www.rfc-editor.org/info/rfc6555>.

   [RFC7305]  Lear, E., Ed., "Report from the IAB Workshop on Internet
              Technology Adoption and Transition (ITAT)", RFC 7305,
              DOI 10.17487/RFC7305, July 2014,
              <http://www.rfc-editor.org/info/rfc7305>.

   [SPUD-USE] Hardie, T., "Use Cases for SPUD", Work in Progress,
              draft-hardie-spud-use-cases-01, February 2015.

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

   [TLS-DTLS] Huitema, C., Rescorla, E., and J. Jana, "DTLS as
              Subtransport protocol", Work in Progress,
              draft-huitema-tls-dtls-as-subtransport-00, March 2015.

   [EXP-COOP] Trammell, B., Ed., "Architectural Considerations for
              Transport Evolution with Explicit Path Cooperation", Work
              in Progress, draft-trammell-stackevo-explicit-coop-00,
              September 2015.

   [black-semi]
              Black, D., "UDP Encapsulation: Framework Considerations",
              January 2015, <https://www.iab.org/wp-content/
              IAB-uploads/2014/12/semi2015_black.pdf>.

   [briscoe-semi]
              Briscoe, B., "Tunneling Through Inner Space", October
              2014, <https://www.iab.org/wp-content/IAB-uploads/2014/12/
              semi2015_briscoe.pdf>.

   [cheshire-semi]
              Cheshire, S., "Restoring the Reputation of the
              Much-Maligned TCP", January 2015, <https://www.iab.org/
              wp-content/IAB-uploads/2015/01/semi2015-cheshire.pdf>.

   [deering-plenary]
              Deering, S., "Watching the Waist of the Protocol
              Hourglass", August 2001,
              <https://www.ietf.org/proceedings/51/slides/plenary-1>.

   [edeline-semi]
              Edeline, K. and B. Donnet, "On a Middlebox
              Classification", January 2015, <https://www.iab.org/
              wp-content/IAB-uploads/2014/12/semi2015_edeline.pdf>.

   [hardie-semi]
              Hardie, T., "Network Function Virtualization and Path
              Character", January 2015, <https://www.iab.org/wp-content/
              IAB-uploads/2014/12/semi2015_hardie.pdf>.

   [huitema-semi]
              Huitema, C., "The Secure Transport Tussle", October 2014,
              <https://www.iab.org/wp-content/IAB-uploads/2014/12/
              semi2015_huitema.pdf>.

   [raiciu-semi]
              Raiciu, C., Olteanu, V., and , "Good Cop, Bad Cop: Forcing
              Middleboxes to Cooperate", January 2015,
              <https://www.iab.org/wp-content/IAB-uploads/2015/01/
              ninja.pdf>.

   [welzl-semi]
              Welzl, M., Fairhurst, G., and D. Ros, "Ossification: a
              result of not even trying?", January 2015,
              <https://www.iab.org/wp-content/IAB-uploads/2014/12/
              semi2015_welzl.pdf>.

Appendix A.  Attendees

   The following people attended the SEMI workshop:

   Mary Barnes, Richard Barnes, David Black, Marc Blanchet, Bob Briscoe,
   Ken Calvert, Spencer Dawkins, Benoit Donnet, Lars Eggert, Gorry
   Fairhurst, Aaron Falk, Mat Ford, Ted Hardie, Joe Hildebrand, Russ
   Housley, Felipe Huici, Christian Huitema, Jana Iyengar, Mirja
   Kuehlewind, Eliot Lear, Barry Leiba, Xing Li, Szilveszter Nadas, Erik
   Nordmark, Colin Perkins, Bernhard Plattner, Miroslav Ponec, Costin
   Raiciu, Philipp Schmidt, Martin Stiemerling, Dave Thaler, Brian
   Trammell, Michael Welzl, Brandon Williams, Dan Wing, and Aaron Yi
   Ding.

   Additionally, Stuart Cheshire and Eric Rescorla contributed to the
   workshop but were unable to attend.

Acknowledgments

   The IAB thanks the SEMI Program Committee: Brian Trammell, Mirja
   Kuehlewind, Joe Hildebrand, Eliot Lear, Mat Ford, Gorry Fairhurst,
   and Martin Stiemerling.  We additionally thank Prof. Dr. Bernhard
   Plattner of the Communication Systems Group at ETH for hosting the
   workshop, and the Internet Society for its support.  Thanks to
   Suzanne Woolf and Aaron Falk for the feedback and review.

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