Network Working Group M. Kuehlewind
Internet-Draft B. Trammell
Intended status: Informational ETH Zurich
Expires: April 25, 2019 October 22, 2018

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

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 [QUIC-HTTP].

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.

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 inseparably 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. This presents opportunities and challenges for applications using QUIC.

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 trade-offs 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 explicitly 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 2.1 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 application.

3.3. Session resumption versus Keep-alive

Because QUIC is encapsulated in UDP, applications using QUIC must deal with short idle timeouts. Deployed stateful middleboxes will generally establish state for UDP flows on the first packet state, and keep state for much shorter idle periods than for TCP. According to a 2010 study ([Hatonen10]), UDP applications can assume that any NAT binding or other state entry will be expired after just thirty seconds of inactivity.

A QUIC application has three strategies to deal with this issue:

The first strategy is the easiest, but it only applies to certain applications.

Either the server or the client in a QUIC application can send PING frames as keep-alives, to prevent the connection and any on-path state from timing out. Recommendations for the use of keep-alives are application specific, mainly depending on the latency requirements and message frequency of the application. In this case, the application mapping must specify whether the client or server is responsible for keeping the application alive. Note that sending PING frames more frequently than every 30 seconds over long idle periods may result in a too much unproductive traffic and power usage for some situations.

Alternatively, the client (but not the server) can use session resumption instead of sending keepalive traffic. In this case, a client that wants to send data to a server over a connection idle longer than the server’s idle timeout (available from the idle_timeout transport parameter) can simply reconnect. When possible, this reconnection can use 0-RTT session resumption, reducing the latency involved with restarting the connection. This of course only applies in cases in which 0-RTT data is safe, when the client is the restarting peer, and when the data to be sent is idempotent.

The tradeoffs between resumption and keepalive need to be evaluated on a per-application basis. However, in general applications should use keepalives only in circumstances where continued communication is highly likely; [QUIC-HTTP], for instance, recommends using PING frames for keepalive only when a request is outstanding.

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 wire can carry one of multiple stream frames.

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 application.

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. If a QUIC receiver has maximum allowed concurrent streams open and the sender on the other end indicates that more streams are needed, it doesn’t automatically lead to an increase of the maximum number of streams by the receiver. Therefore it can be valuable to expose maximum number of allowed, currently open and currently used streams to the application to 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 session 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 of establishing another connection is extremely low.

4.2. Packetization and latency

QUIC provides an interface that provides multiple streams to the application; however, the application usually cannot control how data transmitted over one stream is mapped into frames or how those frames are bundled into packets. By default, QUIC will try to maximally pack packets with one or more stream data frames to minimize bandwidth consumption and computational costs (see section 8 of [QUIC]). If there is not enough data available to fill a packet, QUIC may even wait for a short time, to optimize bandwidth efficiency instead of latency. This delay can either be pre-configured or dynamically adjusted based on the observed sending pattern of the application. If the application 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. Alternatively, if the application expects to use a specific sending pattern, it can also provide a suggested delay to QUIC for how long to wait before bundle frames into a packet.

4.3. Prioritization

Stream prioritization is not exposed to either the network or the receiver. Prioritization is managed 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 application protocol that runs on top of QUIC can define explicit 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, which means that prioritization of retransmissions will be beneficial in most cases, by filling in gaps and freeing up the flow control window. For partially reliable or unreliable streams, priority scheduling of retransmissions over data of higher-priority streams might not be desirable. For such streams, QUIC could either provide an explicit interface to control prioritization, or derive the prioritization decision from the reliability level of the stream.

5. Port Selection

As QUIC is a general purpose transport protocol, there are no requirements that servers use a particular UDP port for QUIC in general. Instead, the same port number is used as would be used for the same application over TCP. In the case of HTTP the expectation is that port 443 is used, which has already been registered for “http protocol over TLS/SSL”. However, [QUIC-HTTP] also specifies the use of Alt-Svc for HTTP/QUIC discovery which allows the server to use and announce a different port number.

In general, port numbers serves two purposes: “first, they provide a demultiplexing identifier to differentiate transport sessions between the same pair of endpoints, and second, they may also identify the application protocol and associated service to which processes connect” [RFC6335]. Note that the assumption that an application can be identified in the network based on the port number is less true today, due to encapsulation, mechanisms for dynamic port assignments as well as NATs.

However, whenever a non-standard port is used which does not enable easy mapping to a registered service name, this can lead to blocking by network elements such as firewalls that rely on the port number as a first order of filtering.

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 always exposes some information (such as the version and Connection IDs), the short header exposes at most only a single Connection ID.

7.1. Server-Generated Connection ID

QUIC supports a server-generated Connection ID, transmitted to the client during connection establishment (see Section 6.1 of [QUIC]). Servers behind load balancers may need to 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 retaining connection state.

Server-generated connection IDs should seek to obscure any encoding, of routing identities or any other information. Exposing the server mapping would allow linkage of multiple IP addresses to the same host if the server also supports migration. Furthermore, this opens an attack vector on specific servers or pools.

The best way to obscure an encoding is to appear random to observers, which is most rigorously achieved with encryption.

7.2. Mitigating Timing Linkability with Connection ID Migration

While sufficiently robust connection ID generation schemes will mitigate linkability issues, they do not provide full protection. Analysis of the lifetimes of six-tuples (source and destination addresses as well as the migrated CID) may expose these links anyway.

In the limit where connection migration in a server pool is rare, it is trivial for an observer to associate two connection IDs. Conversely, in the opposite limit where every server handles multiple simultaneous migrations, even an exposed server mapping may be insufficient information.

The most efficient mitigation for these attacks is operational, either by using a load balancing architecture that loads more flows onto a single server-side address, by coordinating the timing of migrations to attempt to increase the number of simultaneous migrations at a given time, or through other means.

7.3. Using Server Retry for Redirection

QUIC provides a Server Retry packet that can be sent 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 choose 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.

8. Use of Versions and Cryptographic Handshake

Versioning in QUIC may change the protocol’s behavior completely, except for the meaning of a few header fields that have been declared to be invariant [QUIC-INVARIANTS]. A version of QUIC with a higher version number will not necessarily provide a better service, but might simply provide a different feature set. As such, 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.

9. IANA Considerations

This document has no actions for IANA.

10. 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.

11. Contributors

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

12. 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.

13. References

13.1. Normative References

[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed and Secure Transport", Internet-Draft draft-ietf-quic-transport-15, October 2018.
[QUIC-INVARIANTS] Thomson, M., "Version-Independent Properties of QUIC", Internet-Draft draft-ietf-quic-invariants-03, October 2018.
[QUIC-TLS] Thomson, M. and S. Turner, "Using Transport Layer Security (TLS) to Secure QUIC", Internet-Draft draft-ietf-quic-tls-15, October 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M. and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011.
[TLS13] Thomson, M. and S. Turner, "Using Transport Layer Security (TLS) to Secure QUIC", Internet-Draft draft-ietf-quic-tls-15, October 2018.

13.2. Informative References

[Edeline16] Edeline, K., Kuehlewind, M., Trammell, B., Aben, E. and B. Donnet, "Using UDP for Internet Transport Evolution (arXiv preprint 1612.07816)", December 2016.
[Hatonen10] Hatonen, S., Nyrhinen, A., Eggert, L., Strowes, S., Sarolahti, P. and M. Kojo, "An experimental study of home gateway characteristics (Proc. ACM IMC 2010)", October 2010.
[HTTP-RETRY] Nottingham, M., "Retrying HTTP Requests", Internet-Draft draft-nottingham-httpbis-retry-01, February 2017.
[I-D.nottingham-httpbis-retry] Nottingham, M., "Retrying HTTP Requests", Internet-Draft draft-nottingham-httpbis-retry-01, February 2017.
[PaaschNanog] Paasch, C., "Network Support for TCP Fast Open (NANOG 67 presentation)", June 2016.
[QUIC-HTTP] Bishop, M., "Hypertext Transfer Protocol (HTTP) over QUIC", Internet-Draft draft-ietf-quic-http-15, October 2018.
[Swett16] Swett, I., "QUIC Deployment Experience at Google (IETF96 QUIC BoF presentation)", July 2016.
[Trammell16] 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, Switzerland EMail:
Brian Trammell ETH Zurich Gloriastrasse 35 8092 Zurich, Switzerland EMail: