Network Working Group                                      M. Kuehlewind
Internet-Draft                                               B. Trammell
Intended status: Informational                                ETH Zurich
Expires: April 28, 2018                                 October 25, 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.

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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.  Use of Streams  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Stream versus Flow Multiplexing . . . . . . . . . . . . .   5
     4.2.  Paketization and latency  . . . . . . . . . . . . . . . .   6
     4.3.  Prioritization  . . . . . . . . . . . . . . . . . . . . .   6
   5.  Graceful connection closure . . . . . . . . . . . . . . . . .   6
   6.  Information exposure and the Connection ID  . . . . . . . . .   7
     6.1.  Server-Generated Connection ID  . . . . . . . . . . . . .   7
     6.2.  Using Server Retry for Redirection  . . . . . . . . . . .   8
   7.  Use of Versions and Cryptographic Handshake . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 (as specified in section
   3.2 in [QUIC-TLS]).  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

3.3.  Session resumption versus Keep-alive

   [EDITOR'S NOTE: see]

4.  Use of Streams

   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.  Stream data is carried within Frames, where one (UDP)
   packet on the wired can carry one of multiple stream frames.

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   Stream can be independently open and closed, gracefully or by error.
   If a critical stream for the application is closed, the application
   can generate respective error messages on the application layer to
   inform the other end or the higher layer and eventually indicate quic
   to reset the connection.  QUIC, however, does not need to know which
   streams are critical, and does not provide an interface to
   exceptional handling of any stream.  There are special streams in
   QUIC that are used for control on the QUIC connection, however, these
   streams are not exposed to the apllication.

   Mapping of application data to streams is application-specific and
   described for HTTP/s in [QUIC-HTTP].  In general data that can be
   processed independently, and therefore would suffer from head of line
   blocking, if forced to be received in order, should be transmitted
   over different streams.  If there is a logical grouping of those data
   chunks or messages, stream can be reused, or a new stream can be
   opened for each chunk/message.  However, a QUIC receiver has a
   maximum number of concurrently open streams.  If the stream limit is
   exhausted a sender is able to indicate that more streams are needed,
   however, this does not automatically lead to an increase of the
   maximum number of streams by the receiver.  Therefore it can be
   valuable to expose this maximum number to the application, or the
   number of currently still available, unused streams, and make the
   mapping of data to streams dependent on this information.

   Further, streams have a maximum number of bytes that can be sent on
   one stream.  This number is high enough (2^64) that this will usually
   not be reached with current applications.  Applications that send
   chunks of data over a very long period of time (such as days, months,
   or years), should rather utilize the 0-RTT seesion resumption ability
   provided by QUIC, than trying to maintain one connection open.

4.1.  Stream versus Flow Multiplexing

   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.  Therefore 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.

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4.2.  Paketization and latency

   Quic provides an interface that provides multiple streams to the
   application, however, the application usually doesn't have control
   how the data transmitted over one stream is mapped into frame and how
   frames are bundled into packets.  By default QUIC will try to
   maximally pack packets to minimize bandwidth consumption and
   computational costs with one or multiple same data frames.  If not
   enough data available to send QUIC may even wait for a short time,
   trading of latency and bandwidth effeciency.  This time might either
   be pre-configured or can the dynamically adjusted based on the
   observed sending pattern of the application.  If the apllication
   requires low latency, with only small chunks of data to send, it may
   be valuable to indicate to QUIC that all data should be send out
   immediately.  Or if a certain sending pattern is know by the
   application, it might also provide valuabe to QUIC how long it should
   wait to bundle frame into a packet.

4.3.  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 an interface for applications to prioritize
   streams [QUIC].  Further applications can implement their own
   prioritization scheme on top of QUIC: an an (application) protocol
   that run on top of QUIC can define explict messages for signaling
   priority, such as those defined for HTTP/2; it can define rules that
   allow an endpoint to determine priority based on context; or it can
   provide a higher level interface and leave the determination to the
   application on top.

   Priority handling of retransmissions can be implemented by the sender
   in the transport layer.  [QUIC] recommends to retransmit lost data
   before new data, unless indicated differently by the application.
   Currently QUIC only provides fully reliable stream transmission, and
   as such prioritization of retransmissionis likely beneficial in most
   cases, as gaps that get filled up and thereby free up flow control.
   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.

5.  Graceful connection closure

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

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6.  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
   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.

6.1.  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.

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6.2.  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.

7.  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.

8.  IANA Considerations

   This document has no actions for IANA.

9.  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.

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10.  Contributors

   Igor Lubashev contributed text to Section 6 on server-selected
   connection IDs.

11.  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.

12.  References

12.1.  Normative References

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-07 (work
              in progress), October 2017.

              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-07 (work in
              progress), October 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-07 (work in
              progress), October 2017.

12.2.  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

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              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,

              Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-07 (work in progress), October

   [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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