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Network Working Group                                         C. Huitema
Internet-Draft                                                 Microsoft
Intended status: Informational                               E. Rescorla
Expires: September 6, 2015                                       Mozilla
                                                              J. Iyengar
                                                           March 5, 2015

                     DTLS as Subtransport protocol


   The developers of "user level" transports will benefit from a
   standard implementation of authentication and encryption.  This can
   be achieved using DTLS as a sub-transport.  Using DTLS enables
   developers to benefit from the investment in TLS, and removes the
   burden and the risks of re-creating similar technology.

   There are several requirements to ensure that DTLS is a suitable sub-
   transport: zero RTT setup, low overhead, and DOS prevention.  The IAB
   SEMI workshop outlined other potential requirements: start/stop
   indication, and ability to accept indications from the network.  The
   draft presents guidelines for meeting these requirements in a new
   version of DTLS.

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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2015.

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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
   Provisions Relating to IETF Documents
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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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  DTLS as a sub transport . . . . . . . . . . . . . . . . . . .   3
   3.  Efficient retransmissions . . . . . . . . . . . . . . . . . .   4
   4.  Zero-RTT with TLS/1.3 . . . . . . . . . . . . . . . . . . . .   5
   5.  Overhead reduction  . . . . . . . . . . . . . . . . . . . . .   5
   6.  DOS resilience  . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Connection-id option  . . . . . . . . . . . . . . . . . . . .   7
   8.  Start/stop indication . . . . . . . . . . . . . . . . . . . .   7
   9.  Indication verification . . . . . . . . . . . . . . . . . . .   8
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     13.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   There is a growing demand to develop "user level" transport,
   motivated by "innovation" and "deployment."  The innovation part is
   the desire to get better performance than TCP, or especially the
   combination of TCP and TLS, addressing such issues as zero-RTT setup
   or head of queue blocking.  The deployment part is motivated by
   observation that platform upgrades are slow, and typically only reach
   a fraction of the deployed systems.  The proposed solution is to
   execute the transport functions in user space, so the transport
   innovation can be distributed with application updates.

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   Any transport innovation has to work on top of an encryption layer.
   This is good security practice, and it also prevent middleboxes from
   interfering with the transport functions.  This interference with TCP
   is widespread and effectively blocks innovation, making it hard to
   deploy even something as simple as ECN.  Encryption prevents the
   middle boxes from twiddling bits in the header.  For example,
   Google's QUIC [QUICBLOG].  protocol uses an encryption system that is
   tightly integrated with the transport layer in order to optimize
   performance and reduce the protocol's accessibility to middleboxes.

   QUIC uses a specially designed security layer, but there was a
   consensus in the IAB SEMI workshop [IABSEMI] that we don't want
   multiple applications each designing their specific key exchange and
   encryption algorithms.  The natural solution is to base the end-to-
   end transports on a standard security layer, allowing transport
   specialists can worry about efficient retransmission, congestion and
   multiplexing, while security specialists can implement the security

   The obvious candidate is DTLS [RFC6347], as the general idea of "TLS
   over UDP" allows us to reuse the TLS experience and the TLS
   implementations.  Of course, we may need to work on a new features to
   meet transport requirements.

1.1.  Requirements

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2026].

2.  DTLS as a sub transport

   Examination of DTLS to the requirements for a subtransport layer
   reveals some areas for improvement.

   Efficient retransmissions:  Part of the rationale for doing new
      transports is to explore efficient retransmission strategies, but
      this conflicts with the existing retransmission procedures that
      are embedded in standard DTLS.

   Zero-RTT setup:  DTLS/1.2's minimum connection setup requires 1-RTT.
      One of the major performance targets for new transports is low-
      latency, motivating a 0-RTT connection setup.

   Low overhead:  DTLS/1.2 record headers include elements like version
      number, epoch, sequence number or clear text length that cause a
      fair bit of overhead in a small UDP datagram.  Using some form of
      header compression would reduce that overhead.

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   DOS prevention:  TLS over UDP offers a big surface area for DOS
      attacks, as unauthenticated clients can ask a server to perform
      expensive crypto or produce large responses.  This is especially
      true if we support 0-RTT.  While DTLS has some defenses against
      DoS attacks, they may need to be strengthened.

   connection-id:  DTLS/1.2 identifies connections using the 5 tuple.
      Having an independent ID would allow for functionalities similar
      to "TCP multipath."  It would also facilitate the work of load
      balancers in front of a server farm.

   Discussions in the IAB SEMI workshop also pointed out that
   middleboxes interaction would be easier to manage if the UDP
   transport had some specific properties:

   Start/stop:  Many middleboxes need to assign state to UDP flows.  For
      example, NATs need to assign and maintain port mappings.  UDP
      flows do not have explicit beginning and end markers similar to
      TCP SYN/FIN/RST flags.  In the absence of such flags middleboxes
      have to resort to timer based management.  This in turn forces
      applications to use periodic "keep alive" traffic, which is

   Indication verification:  Middleboxes may wish to send informative
      messages similar to ICMP, providing for example indications about
      MTU size or congestion state.  Application that receive these
      messages need to differentiate between legitimate data coming from
      network elements "on the path" and fake signals coming from
      attackers.  This is easier if the messages coming from the network
      can copy hard to predict header elements like connection-id or
      sequence numbers.

   It is not yet clear whether these features are feasible or
   deployable, but we document them here as an outcome of the IAB SEMI

3.  Efficient retransmissions

   Protocols like QUIC implement innovative retransmission strategies,
   combining Forward Error Correction with selective acknowledgements
   and selective retransmissions.  DTLS implements a minimalist
   retransmission strategy for the messages that are part of the
   handshake protocol, as explained in section 3.2 of [RFC6347].  This
   creates a tension between adhering to the standard and efficient

   o  One could keep the QUIC retransmission for the handshake packets
      and switch to an innovative transport for the reminder of the

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      connection.  The downside is that using less efficient transport
      during the handshake risk can cause additional latency, which is
      contrary to the objective of transport innovation.

   o  One could design an innovative transmission as a layer under the
      TLS framing, effectively redesign the layering of DTLS.  This
      solves the efficiency issues, but expose the clear-text
      transmission controls to interference by middle-boxes, which may
      ultimately prevent innovation.

   o  One could consider a hybrid design that allows clear text
      innovation for the initial handshake and encrypted innovation for
      data retransmissions, but no such design is available yet.

   To put it simply, there is no consensus yet on a good solution to
   this problem.

4.  Zero-RTT with TLS/1.3

   Probably the biggest requirement is to have a 0-RTT connection setup,
   meaning that the initiator (typically the "client") can start sending
   protected upper-level data in its initial flight of datagrams.  In
   general, a 0-RTT handshake requires that both the client and server
   retain state:

   o  The client must retain the server's parameters, including a long-
      term cryptographic key.

   o  The server must retain enough state to detect replays of the
      client's initial flight.

   In DTLS 1.2 and before, the client and server are both assumed to be
   naive and so the first round-trip is used to establish this state.
   This is still necessary for situations where the client and server
   have never talked before and have no out-of-band communications
   channel, but if both sides are primed, it is possible to define a
   0-RTT handshake as well.  Such a mode will be part of (D)TLS 1.3 and
   is currently under development in the TLS WG.

5.  Overhead reduction

   DTLS is not generally very aggressive about conserving per-packet
   overhead.  The minimum DTLS record adds 13 bytes of header to the
   packet and the common AES-GCM cipher suites add another 8 bytes or a
   total of 21 bytes of header overhead (plus the authentication tag,
   which is required).  While these header bytes are not entirely
   redundant (for instance, the sequence number allows the receiver to
   deal with reordered packets) they are largely redundant in the common

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   case where the network mostly delivers packets in order essentially
   every record is application data.

   For maximum efficiency, it is desirable to have a mechanism for
   compressing this data.  [I-D.modadugu-dtls-short] describes one set
   of techniques for doing so, though research into the optimal method
   is still required.

6.  DOS resilience

   Our principal DoS concerns are:

   o  Preventing resource over-consumption on the server.

   o  Preventing the server from being used as a traffic amplifier.

   Because TLS runs over TCP, it inherits TCP's DoS resistance
   properties: an attacker must first establish a TCP connection before
   he can talk to the TLS implementation.  This generally means
   demonstrating that he can receive traffic at the IP address he is
   sending from.  This significantly reduces the risk of amplification
   and allows the server to differentially throttle traffic from clients
   which appear to be sending bogus handshakes.  The result is partial
   protection against resource consumption attacks, but an attacker can
   still mount such attacks if they control a large number of IP

   Any protocol which runs directly over UDP -- as DTLS does -- not
   inherit these properties.  DTLS already has anti-DoS measures in the
   form of a cookie exchange which allows the server to force the client
   to prove reachability at a given address.  This is the standard
   technique for addressing resource consumption attacks with such
   protocols and can be deployed differentially (i.e., only when under
   attack) to reduce the latency impact at normal times.  Other
   techniques which have been proposed for (D)TLS include computational

   The DTLS cookie exchange also prevents amplification attacks but
   because the server does not generally know when it is being used in
   this fashion, it is harder to know where to set the protection/
   latency tradeoff.  It is currently unclear how important
   amplification protection is (DNS already has significant
   amplification vectors) but if so, possible techniques include longer-
   term cookies and forcing the client to pad its initial flight, thus
   reducing the impact of amplification.

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7.  Connection-id option

   Many UDP applications identify the application connection implicitly
   from the "five tuple" of source and destination addresses and ports,
   and payload type.  There are however several potential advantages to
   having an explicit "connection-id:"

   o  Enabling applications to use several ports and path in parallel,
      for performance or resiliency,

   o  Enabling seamless continuation of an application over a new port
      if the preceding port becomes unusable.

   The latter problem, ports becoming unusable, is often caused by NAT
   traversal.  NAT are known to forget UDP mappings if they don't see
   traffic for some period, or for some other reason such as for example
   hash table collision.  Applications must be ready to quickly
   reestablish their connectivity.  Using an explicit connection-id
   makes this reestablishment straighforward.

   The connection-id could be encoded as a header parameter, and its use
   negotiated during the initial handshake, using techniques similar to
   the parameters negotiation proposed in [I-D.modadugu-dtls-short].

8.  Start/stop indication

   Middleboxes like NAT or firewall assign some state to the UDP flows,
   such as for example a port mapping in a NAT or an explicit port
   opening in a firewall.  For TCP flows, middleboxes can examine TCP
   flags and deduce when they see FIN or RST flags that the connection
   is getting closed.  They can then free the state associated with the
   TCP flow.  There are no such flags in UDP packets.  The start of a
   flow can be deduced implicitly from the arrival of a first packet for
   that flow, but the end cannot.  Middleboxes have to resort to timer
   based management.  The timers have to be short, and this drives
   applications to send frequent keep-alive packets to make sure that
   port mappings and port opening persists.  An explicit indication
   would enable more efficient management of resource.

   TLS and DTLS include an explicit close mechanism, in which the
   parties use the TLS Alert protocol and exchange "close notify"
   messages.  Sending such an alert message indicates that the sending
   party is done, and will not send any more messages in the TLS
   session.  When both parties have sent a "close notify" message, the
   session is effectively terminated.

   If a middlebox could monitor the transmission of "close notify"
   messages, it could effectively decide when resource can be disposed.

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   However, the alert protocol is part of the encrypted payload, and the
   only visible indication in the clear text header is a "Content type"
   indication set to "Alert", indicating that the encrypted payload
   contains an Alert message.  Closure indication is only one of the
   usages of the Alert protocol, the other usages being error indication
   and warning indication.  A middlebox that monitors Alert messages
   will only get an imperfect indication of the connection state:

   o  A closure message indicates that one party has finished sending,
      and waits until a similar closure message from the other end to
      terminate the session,

   o  An error message indicates that one party detected an error, will
      not send any more data, and will not accept any more data from the
      other party,

   o  A warning message indicates that one party detected an anomaly,
      but that the session can continue.

   The middlebox can gain information about the state of a DTLS
   connection by monitoring the Alert messages, even if that information
   is imperfect.  Alternatively, we could consider adding an explicit
   FIN bit in a revised clear-text header.

   We should note here that there is a potential tension between the
   management of resource and the identification of sessions discussed
   in Section 7.  The use of the context identifier allows sessions to
   spread over multiple addresses and ports, and also allows multiple
   sessions to share the same addresses and ports.  If such multiplexing
   is in place, the middleboxes would have to allocate resources per
   context rather than per address and port tuples, but would have no
   guarantee to see all the alert messages for a specific session.

9.  Indication verification

   Middleboxes could send messages to applications, using ICMP or
   perhaps simply sending UDP messages using the same five-tuple as the
   application.  Assuming that such messages can be delivered, the
   application will have to verify that they come from a legitimate
   source, for example a middlebox on the path providing an indication
   about acceptable MTU.

   There is always a risk that such indications will be misused, and
   that malevolent third parties would try to provide false indications
   in order to disrupt the application.  The application must thus be
   able to distinguish between legitimate and spurious indication.

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   The middlebox could echo some parameters of the clear text header in
   order to "prove" that they are on path.  Typical parameters would be
   the context ID or the sequence numbers.  For this to be effective,
   these parameters should be "hard to guess," which implies for example
   unpredictable assignment of context ID or initial sequence numbers.
   Of course, such desire for unpredictability conflicts with the desire
   for low overhead, as compressed headers are inherently easier to
   predict than long numbers.

   One question for any indication verification scheme is how much of
   the connection the middlebox needs to be able to see.  For instance,
   if initial sequence numbers or DTLS handshake nonces are used to
   demonstrate that the middlebox is on-path, then the middlebox needs
   to be on-path for the entire connection and maintain connection

10.  Security Considerations

   This document proposes that user level transports use DTLS as a
   component, instead of inventing their own transport.  We believe that
   this componentized approach will avoid many of the pitfalls of
   inventing or implementing special purpose security designs.  Instead,
   applications will benefit from the experience accured in the design
   and evolution of TLS [RFC5246] and will be able to reuse already
   developed TLS and DTLS implementations.

   We note that there is a definitive DOS exposure when running a
   cryptographic protocol over UDP, and that this exposure is increased
   when we attempt to enable zero RTT setup.  The risk and the
   corresponding mitigations are described in Section 6.  Here again, we
   believe that it is beneficial to reuse the DOS mitigations developed
   for DTLS and designed for the zero RTT setup options of TLS/1.3

   Any start/stop mechanism solving the requirement presented in
   Section 8 opens the door to an attack is similar to but distinct from
   TCP RST attacks, where injected RST packets terminate connections.
   An on path attacker could inject bogus packets with a "Stop"
   indication, to cause connection state to be torn down at middleboxes,
   potentially causing the connection to be abruptly terminated.  The
   middleboxes will not be able to separate these injected packets from
   legitimate "Stop" packets sent by the endpoints, because they cannot
   verify the end-to-end authentication of packets.

   Participants to the SEMI workshop [IABSEMI] envisage a "path to
   application" messaging system in which intermediate relays would
   provide information to the application, such as for example MTU size
   or congestion notification.  Such messages would not benefit from the

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   end to end authentication and encryption provided by DTLS.  Allowing
   such messages exposes the application to denial of service attacks.
   Some potential mitigations are described in Section 9

11.  IANA Considerations

   This draft references [I-D.modadugu-dtls-short], which proposed four
   new extensions for DTLS.  A future version of this draft will very
   likely propose the registration of similar extensions, using the
   mechanisms defined in [RFC5246] and [RFC6066].

12.  Acknowledgments

   The inspiration for this draft came from discussions in the IAB SEMI
   workshop, and from studies of the QUIC protocol.

13.  References

13.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

13.2.  Informative References

              Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-04 (work
              in progress), January 2015.

              Modadugu, N. and E. Rescorla, "Extensions for Datagram
              Transport Layer Security (TLS) in Low Bandwidth
              Environments", draft-modadugu-dtls-short-00 (work in
              progress), March 2006.

   [IABSEMI]  Kuehlewind, M. and B. Trammell, "IAB Workshop on Stack
              Evolution in a Middlebox Internet (SEMI)", Jan 2015,

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              Roskind, J., "Experimenting with QUIC", June 2013,

Authors' Addresses

   Christian Huitema

   Email: huitema@microsoft.com

   Eric Rescorla

   Email: ekr@rtfm.com

   Jana Iyengar

   Email: jri@google.com

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