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
draft-ietf-quic-applicability-03
Abstract
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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The Necessity of Fallback . . . . . . . . . . . . . . . . . . 3
3. Zero RTT . . . . . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Stream versus Flow Multiplexing . . . . . . . . . . . . . 6
4.2. Packetization and latency . . . . . . . . . . . . . . . . 7
4.3. Prioritization . . . . . . . . . . . . . . . . . . . . . 7
5. Port Selection . . . . . . . . . . . . . . . . . . . . . . . 8
6. Graceful connection closure . . . . . . . . . . . . . . . . . 8
7. Information exposure and the Connection ID . . . . . . . . . 8
7.1. Server-Generated Connection ID . . . . . . . . . . . . . 9
7.2. Mitigating Timing Linkability with Connection ID
Migration . . . . . . . . . . . . . . . . . . . . . . . . 9
7.3. Using Server Retry for Redirection . . . . . . . . . . . 9
8. Use of Versions and Cryptographic Handshake . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1. Normative References . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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].
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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.
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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
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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:
o Ignore it, if the application-layer protocol consists only of
interactions with no or very short idle periods.
o Ensure there are no long idle periods.
o Resume the session after a long idle period, using 0-RTT
resumption when appropriate.
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.
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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
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(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
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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.
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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
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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.
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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", draft-ietf-quic-transport-15 (work
in progress), October 2018.
[QUIC-INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
draft-ietf-quic-invariants-03 (work in progress), October
2018.
[QUIC-TLS]
Thomson, M. and S. Turner, "Using Transport Layer Security
(TLS) to Secure QUIC", draft-ietf-quic-tls-15 (work in
progress), 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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc6335>.
[TLS13] Thomson, M. and S. Turner, "Using Transport Layer Security
(TLS) to Secure QUIC", draft-ietf-quic-tls-15 (work in
progress), October 2018.
13.2. Informative References
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[Edeline16]
Edeline, K., Kuehlewind, M., Trammell, B., Aben, E., and
B. Donnet, "Using UDP for Internet Transport Evolution
(arXiv preprint 1612.07816)", December 2016,
<https://arxiv.org/abs/1612.07816>.
[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", draft-
nottingham-httpbis-retry-01 (work in progress), February
2017.
[I-D.nottingham-httpbis-retry]
Nottingham, M., "Retrying HTTP Requests", draft-
nottingham-httpbis-retry-01 (work in progress), February
2017.
[PaaschNanog]
Paasch, C., "Network Support for TCP Fast Open (NANOG 67
presentation)", June 2016,
<https://www.nanog.org/sites/default/files/
Paasch_Network_Support.pdf>.
[QUIC-HTTP]
Bishop, M., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-15 (work in progress), October
2018.
[Swett16] Swett, I., "QUIC Deployment Experience at Google (IETF96
QUIC BoF presentation)", July 2016,
<https://www.ietf.org/proceedings/96/slides/
slides-96-quic-3.pdf>.
[Trammell16]
Trammell, B. and M. Kuehlewind, "Internet Path
Transparency Measurements using RIPE Atlas (RIPE72 MAT
presentation)", May 2016, <https://ripe72.ripe.net/wp-
content/uploads/presentations/86-atlas-udpdiff.pdf>.
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Authors' Addresses
Mirja Kuehlewind
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: mirja.kuehlewind@tik.ee.ethz.ch
Brian Trammell
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: ietf@trammell.ch
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