Network Working Group                                      M. Kuehlewind
Internet-Draft                                               B. Trammell
Intended status: Informational                                ETH Zurich
Expires: January 4, 2018                                   July 03, 2017

              Applicability of the QUIC Transport Protocol


   This document discusses the applicability of the QUIC transport
   protocol, focusing on caveats impacting application protocol
   development and deployment over QUIC.  Its intended audience is
   designers of application protocol mappings to QUIC, and implementors
   of these application protocols.

Status of This Memo

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   This Internet-Draft will expire on January 4, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Necessity of Fallback . . . . . . . . . . . . . . . . . .   3
   3.  Zero RTT  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Thinking in Zero RTT  . . . . . . . . . . . . . . . . . .   4
     3.2.  Here There Be Dragons . . . . . . . . . . . . . . . . . .   4
     3.3.  Session resumption versus Keep-alive  . . . . . . . . . .   4
   4.  Stream versus Flow Multiplexing . . . . . . . . . . . . . . .   4
   5.  Prioritization  . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Graceful connection closure . . . . . . . . . . . . . . . . .   5
   7.  Information exposure and the Connection ID  . . . . . . . . .   5
     7.1.  Server-Generated Connection ID  . . . . . . . . . . . . .   6
   8.  Using Server Retry for Redirection  . . . . . . . . . . . . .   6
   9.  Use of Versions and Cryptographic Handshake . . . . . . . . .   7
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     14.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     14.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   QUIC [QUIC] is a new transport protocol currently under development
   in the IETF quic working group, focusing on support of semantics as
   needed for HTTP/2 [QUIC-HTTP] such as stream-multiplexing to avoid
   head-of-line blocking.  Based on current deployment practices, QUIC
   is encapsulated in UDP.  The version of QUIC that is currently under
   development will integrate TLS 1.3 [TLS13] to encrypt all payload
   data and most control information.

   This document provides guidance for application developers that want
   to use the QUIC protocol without implementing it on their own.  This
   includes general guidance for application use of HTTP/2 over QUIC as
   well as the use of other application layer protocols over QUIC.  For
   specific guidance on how to integrate HTTP/2 with QUIC, see

   In the following sections we discuss specific caveats to QUIC's
   applicability, and issues that application developers must consider
   when using QUIC as a transport for their application.

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1.1.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when these words are capitalized, they
   have a special meaning as defined in [RFC2119].

2.  The Necessity of Fallback

   QUIC uses UDP as a substrate for userspace implementation and port
   numbers for NAT and middlebox traversal.  While there is no evidence
   of widespread, systematic disadvantage of UDP traffic compared to TCP
   in the Internet [Edeline16], somewhere between three [Trammell16] and
   five [Swett16] percent of networks simply block UDP traffic.  All
   applications running on top of QUIC must therefore either be prepared
   to accept connectivity failure on such networks, or be engineered to
   fall back to some other transport protocol.  This fallback SHOULD
   provide TLS 1.3 or equivalent cryptographic protection, if available,
   in order to keep fallback from being exploited as a downgrade attack.
   In the case of HTTP, this fallback is TLS 1.3 over TCP.

   These applications must operate, perhaps with impaired functionality,
   in the absence of features provided by QUIC not present in the
   fallback protocol.  For fallback to TLS over TCP, the most obvious
   difference is that TCP does not provide stream multiplexing and
   therefore stream multiplexing would need to be implemented in the
   application layer if needed.  Further, TCP without the TCP Fast Open
   extension does not support 0-RTT session resumption.  TCP Fast Open
   can be requested by the connection initiator but might no be
   supported by the far end or could be blocked on the network path.
   Note that there is some evidence of middleboxes blocking SYN data
   even if TFO was successfully negotiated (see [PaaschNanog]).

   Any fallback mechanism is likely to impose a degradation of
   performance; however, fallback MUST not silently violate the
   application's expectation of confidentiality or integrity of its
   payload data.

   Moreover, while encryption (in this case TLS) is inseparable
   integrated with QUIC, TLS negotiation over TCP can be blocked.  In
   case it is RECOMMENDED to abort the connection, allowing the
   application to present a suitable prompt to the user that secure
   communication is unavailable.

3.  Zero RTT

   QUIC provides for 0-RTT connection establishment (see section 3.2 of
   [QUIC]).  This presents opportunities and challenges for applications
   using QUIC.

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3.1.  Thinking in Zero RTT

   A transport protocol that provides 0-RTT connection establishment to
   recently contacted servers is qualitatively different than one that
   does not from the point of view of the application using it.
   Relative tradeoffs between the cost of closing and reopening a
   connection and trying to keep it open are different; see Section 3.3.

   Applications must be slightly rethought in order to make best use of
   0-RTT resumption.  Most importantly, application operations must be
   divided into idempotent and non-idempotent operations, as only
   idempotent operations may appear in 0-RTT packets.  This implies that
   the interface between the application and transport layer exposes
   idempotence either ecplicitly or implicitly.

3.2.  Here There Be Dragons

   Retransmission or (malicious) replay of data contained in 0-RTT
   resumption packets could cause the server side to receive two copies
   of the same data.  This is further described in [HTTP-RETRY].  Data
   sent during 0-RTT resumption also cannot benefit from perfect forward
   secrecy (PFS).

   Data in the first flight sent by the client in a connection
   established with 0-RTT MUST be idempotent.  Applications MUST be
   designed, and their data MUST be framed, such that multiple reception
   of idempotent data is recognized as such by the receiverApplications
   that cannot treat data that may appear in a 0-RTT connection
   establishment as idempotent MUST NOT use 0-RTT establishment.  For
   this reason the QUIC transport SHOULD provide an interface for the
   application to indicate if 0-RTT support is in general desired or a
   way to indicate whether data is idempotent, and/or whether PFS is a
   hard requirement for the application.

3.3.  Session resumption versus Keep-alive

   [EDITOR'S NOTE: see]

4.  Stream versus Flow Multiplexing

   QUIC's stream multiplexing feature allows applications to run
   multiple streams over a single connection, without head-of-line
   blocking between streams, associated at a point in time with a single
   five-tuple.  Streams are meaningful only to the application; since
   stream information is carried inside QUIC's encryption boundary, no
   information about the stream(s) whose frames are carried by a given
   packet is visible to the network.

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   Stream multiplexing is not intended to be used for differentiating
   streams in terms of network treatment.  Application traffic requiring
   different network treatment SHOULD therefore be carried over
   different five-tuples (i.e.  multiple QUIC connections).  Given
   QUIC's ability to send application data in the first RTT of a
   connection (if a previous connection to the same host has been
   successfully established to provide the respective credentials), the
   cost for establishing another connection are extremely low.

5.  Prioritization

   Stream prioritization is not exposed to the network, nor to the
   receiver.  Prioritization can be realized by the sender and the QUIC
   transport should provide and interface for applications to prioritize
   streams [QUIC].

   Priority handling of retransmissions may be implemented by the sender
   in the transport layer and [QUIC] does not specify a specific way how
   this must be handled.  Currently QUIC only provides fully reliable
   stream transmission, and as such prioritization of retransmission is
   likely beneficial.  For not fully reliable streams priority
   scheduling of retransmissions over data of higher-priority streams
   might not be desired.  In this case QUIC could also provide an
   interface or derive the prioritization decision from the reliability
   level of the stream.

6.  Graceful connection closure

   [EDITOR'S NOTE: give some guidance here about the steps an
   application should take; however this is still work in progress]

7.  Information exposure and the Connection ID

   QUIC exposes some information to the network in the unencrypted part
   of the header, either before the encryption context is established,
   because the information is intended to be used by the network.  QUIC
   has a long header that is used during connection establishment and
   for other control processes, and a short header that may be used for
   data transmission in an established connection.  While the long
   header is fixed and exposes some information, the short header only
   exposes the packet number by default and may optionally expose a
   connection ID.

   Given that exposing this information may make it possible to
   associate multiple addresses with a single client during rebinding,
   which has privacy implications, an application may indicate to not
   support exposure of certain information after the handshake.
   Specificially, an application that has additional information that

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   the client is not behind a NAT and the server is not behind a load
   balancer, and therefore it is unlikely that the addresses will be re-
   bound, may indicate to the transport that is wishes to not expose a
   connection ID.

7.1.  Server-Generated Connection ID

   QUIC supports a server-generated Connection ID, transmitted to the
   client during connection establishment: see Section 5.7 of [QUIC]
   Servers behind load balancers should propose a Connection ID during
   the handshake, encoding the identity of the server or information
   about its load balancing pool, in order to support stateless load
   balancing.  Once the server generates a Connection ID that encodes
   its identity, every CDN load balancer would be able to forward the
   packets to that server without needing information about every
   specific flow it is forwarding.

   Server-generated Connection IDs must not encode any information other
   that that needed to route packets to the appropriate backend
   server(s): typically the identity of the backend server or pool of
   servers, if the data-center's load balancing system keeps "local"
   state of all flows itself.  Care must be exercised to ensure that the
   information encoded in the Connection ID is not sufficient to
   identify unique end users.  Note that by encoding routing information
   in the Connection ID, load balancers open up a new attack vector that
   allows bad actors to direct traffic at a specific backend server or
   pool.  It is therefore recommended that Server-Generated Connection
   ID includes a cryptographic MAC that the load balancer pool server
   are able to identify and discard packets featuring an invalid MAC.

8.  Using Server Retry for Redirection

   QUIC provide a Server Retry packet that can be send by a server in
   response to the Client Initial packet.  The server may choose a new
   connection ID in that packet and the client will retry by sending
   another Client Initial packet with the server-selected connection ID.
   This mechanism can be used to redirect a connection to a different
   server, e.g. due to performance reasons or when servers in a server
   pool are upgraded gradually, and therefore may support different
   versions of QUIC.  In this case, it is assumed that all servers
   belonging to a certain pool are served in cooperation with load
   balancers that forward the traffic based on the connection ID.  A
   server can chose the connection ID in the Server Retry packet such
   that the load balancer will redirect the next Client Initial packet
   to a different server in that pool.

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9.  Use of Versions and Cryptographic Handshake

   Versioning in QUIC may change the the protocol's behavior completely,
   except for the meaning of a few header fields that have been declared
   to be fixed.  As such version of QUIC with a higher version number
   does not necessarily provide a better service, but might simply
   provide a very different service, so an application needs to be able
   to select which versions of QUIC it wants to use.

   A new version could use an encryption scheme other than TLS 1.3 or
   higher.  [QUIC] specifies requirements for the cryptographic
   handshake as currently realized by TLS 1.3 and described in a
   separate specification [QUIC-TLS].  This split is performed to enable
   light-weight versioning with different cryptographic handshakes.

10.  IANA Considerations

   This document has no actions for IANA.

11.  Security Considerations

   See the security considerations in [QUIC] and [QUIC-TLS]; the
   security considerations for the underlying transport protocol are
   relevant for applications using QUIC, as well.

   Application developers should note that any fallback they use when
   QUIC cannot be used due to network blocking of UDP SHOULD guarantee
   the same security properties as QUIC; if this is not possible, the
   connection SHOULD fail to allow the application to explicitly handle
   fallback to a less-secure alternative.  See Section 2.

12.  Contributors

   Igor Lubashev contributed text to Section 7 on server-selected
   connection IDs.

13.  Acknowledgments

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
   for Education, Research, and Innovation under contract no. 15.0268.
   This support does not imply endorsement.

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14.  References

14.1.  Normative References

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-04 (work
              in progress), June 2017.

              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-04 (work in
              progress), June 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [TLS13]    Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-04 (work in
              progress), June 2017.

14.2.  Informative References

              Edeline, K., Kuehlewind, M., Trammell, B., Aben, E., and
              B. Donnet, "Using UDP for Internet Transport Evolution
              (arXiv preprint 1612.07816)", December 2016,

              Nottingham, M., "Retrying HTTP Requests", draft-
              nottingham-httpbis-retry-01 (work in progress), February

              Nottingham, M., "Retrying HTTP Requests", draft-
              nottingham-httpbis-retry-01 (work in progress), February

              Paasch, C., "Network Support for TCP Fast Open (NANOG 67
              presentation)", June 2016,

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              Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-04 (work in progress), June

   [Swett16]  Swett, I., "QUIC Deployment Experience at Google (IETF96
              QUIC BoF presentation)", July 2016,

              Trammell, B. and M. Kuehlewind, "Internet Path
              Transparency Measurements using RIPE Atlas (RIPE72 MAT
              presentation)", May 2016, <

Authors' Addresses

   Mirja Kuehlewind
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich


   Brian Trammell
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich


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