Compatible Version Negotiation for QUIC
draft-ietf-quic-version-negotiation-04
QUIC D. Schinazi
Internet-Draft Google LLC
Intended status: Standards Track E. Rescorla
Expires: 28 November 2021 Mozilla
27 May 2021
Compatible Version Negotiation for QUIC
draft-ietf-quic-version-negotiation-04
Abstract
QUIC does not provide a complete version negotiation mechanism but
instead only provides a way for the server to indicate that the
version the client offered is unacceptable. This document describes
a version negotiation mechanism that allows a client and server to
select a mutually supported version. Optionally, if the original and
negotiated version share a compatible first flight format, the
negotiation can take place without incurring an extra round trip.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the QUIC Working Group
mailing list (quic@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/quic/.
Source for this draft and an issue tracker can be found at
https://github.com/quicwg/version-negotiation.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 28 November 2021.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. Server Deployments of QUIC . . . . . . . . . . . . . . . . . 3
3. Compatible Versions . . . . . . . . . . . . . . . . . . . . . 5
4. Version Negotiation Mechanism . . . . . . . . . . . . . . . . 6
4.1. Connections and Version Negotiation . . . . . . . . . . . 6
4.2. Incompatible Version Negotiation . . . . . . . . . . . . 7
4.3. Compatible Version Negotiation . . . . . . . . . . . . . 7
5. Version Information . . . . . . . . . . . . . . . . . . . . . 8
6. Version Downgrade Prevention . . . . . . . . . . . . . . . . 9
7. Client Choice of Original Version . . . . . . . . . . . . . . 10
8. Interaction with Retry . . . . . . . . . . . . . . . . . . . 10
9. Interaction with 0-RTT . . . . . . . . . . . . . . . . . . . 10
10. Considerations for Future Versions . . . . . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
12.1. QUIC Transport Parameter . . . . . . . . . . . . . . . . 11
12.2. QUIC Transport Error Code . . . . . . . . . . . . . . . 11
13. Normative References . . . . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The version-invariant properties of QUIC [INV] define a version
negotiation (VN) packet but do not specify how an endpoint reacts
when it receives one. QUIC version 1 [QUIC] allows the server to use
a VN packet to indicate that the version the client offered is
unacceptable, but doesn't allow the client to safely make use of that
information to create a new connection with a mutually supported
version. With proper safety mechanisms in place, the VN packet can
be part of a mechanism to allow two QUIC implementations to negotiate
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between two totally disjoint versions of QUIC, at the cost of an
extra round trip. However, it is beneficial to avoid that cost
whenever possible, especially given that most incremental versions
are broadly similar to the the previous version.
This specification describes a simple version negotiation mechanism
which optionally leverages similarities between versions and can
negotiate between the set of "compatible" versions in a single round
trip.
1.1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
In this document, the Maximum Segment Lifetime (MSL) represents the
time a QUIC packet can exist in the network. Implementations can
make this configurable, and a RECOMMENDED value is one minute.
2. Server Deployments of QUIC
While this document mainly discusses a single QUIC server, it is
common for deployments of QUIC servers to include a fleet of multiple
server instances. We therefore define the following terms:
Acceptable Versions: This is the set of versions supported by a
given server instance. More specifically, these are the versions
that a given server instance will use if a client sends a first
flight using them.
Offered Versions: This is the set of versions that a given server
instance will send in a Version Negotiation packet if it receives
a first flight from an unknown version. This set will most often
be equal to the Acceptaple Versions set, except during short
transitions while versions are added or removed (see below).
Fully-Deployed Versions: This is the set of QUIC versions that is
supported and negotiated by every single QUIC server instance in
this deployment. If a deployment only contains a single server
instance, then this set is equal to the Offered Versions set,
except during short transitions while versions are added or
removed (see below).
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If a deployment contains multiple server instances, software updates
may not happen at exactly the same time on all server instances.
Because of this, a client might receive a Version Negotiation packet
from a server instance that has already been updated and the client's
resulting connection attempt might reach a different server instance
which hasn't been updated yet.
However, even when there is only a single server instance, it is
still possible to receive a stale Version Negotiation packet if the
server performs its software update while the Version Negotiation
packet is in flight.
This could cause the version downgrade prevention mechanism described
in Section 6 to falsely detect a downgrade attack. To avoid that,
server operators SHOULD perform a three-step process when they wish
to add or remove support for a version:
When adding support for a new version:
* The first step is to progressively add support for the new version
to all server instances. This step updates the Acceptable
Versions but not the Offered Versions nor the Fully-Deployed
Versions. Once all server instances have been updated, operators
wait for at least one MSL to allow any in-flight Version
Negotiation packets to arrive.
* Then, the second step is to progressively add the new version to
Offered Versions on all server instances. Once complete,
operators wait for at least another MSL.
* Finally, the third step is to progressively add the new version to
Fully-Deployed Versions on all server instances.
When removing support for a version:
* The first step is to progressively remove the version from Fully-
Deployed Versions on all server instances. Once it has been
removed on all server instances, operators wait for at least one
MSL to allow any in-flight Version Negotiation packets to arrive.
* Then, the second step is to progressively remove the version from
Offered Versions on all server instances. Once complete,
operators wait for at least another MSL.
* Finally, the third step is to progressively remove support for the
version from all server instances. That step updates the
Acceptable Versions.
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Note that this opens connections to version downgrades (but only for
partially-deployed versions) during the update window, since those
could be due to clients communicating with both updated and non-
updated server instances.
3. Compatible Versions
If A and B are two distinct versions of QUIC, A is said to be
"compatible" with B if it is possible to take a first flight of
packets from version A and convert it into a first flight of packets
from version B. As an example, if versions A and B are absolutely
equal in their wire image and behavior during the handshake but
differ after the handshake, then A is compatible with B and B is
compatible with A.
Version compatibility is not symmetric: it is possible for version A
to be compatible with version B and for B not to be compatible with
A. This could happen for example if version B is a strict superset
of version A.
Note that version compatibility does not mean that every single
possible instance of a first flight will succeed in conversion to the
other version. A first flight using version A is said to be
"compatible" with version B if two conditions are met: first that
version A is compatible with version B, and second that the
conversion of this first flight to version B is well-defined. For
example, if version B is equal to A in all aspects except it
introduced a new frame in its first flight that version A cannot
parse or even ignore, then B could still be compatible with A as
conversions would succeed for connections where that frame is not
used. In this example, first flights using version B that carry this
new frame would not be compatible with version A.
When a new version of QUIC is defined, it is assumed to not be
compatible with any other version unless otherwise specified.
Similarly, no other version is compatible with the new version unless
otherwise specified. Implementations MUST NOT assume compatibility
between versions unless explicitly specified.
Note that both endpoints might disagree on whether two versions are
compatible or not. For example, two versions could have been defined
concurrently and then specified as compatible in a third document
much later - in that scenario one endpoint might be aware of the
compatibility document while the other may not.
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4. Version Negotiation Mechanism
This document specifies two means of performing version negotiation:
one "incompatible" which requires a round trip and is applicable to
all versions, and one "compatible" that allows saving the round trip
but only applies when the versions are compatible.
The client initiates a QUIC connection by sending a first flight of
QUIC packets with a long header to the server [INV]. We'll refer to
the version of those packets as the "original version". The client's
first flight includes Version Information (see Section 5) which will
be used to optionally enable compatible version negotation (see
Section 4.3), and to prevent version downgrade attacks (see
Section 6).
Upon receiving this first flight, the server verifies whether it
knows how to parse first flights from the original version. If it
does not, then it starts incompatible version negotiation, see
Section 4.2. If the server can parse the first flight, it can either
establish the connection using the original version, or it MAY
attempt compatible version negotiation, see Section 4.3.
Note that it is possible for a server to have the ability to parse
the first flight of a given version without fully supporting it, in
the sense that it implements enough of the version's specification to
parse first flight packets but not enough to fully establish a
connection using that version.
4.1. Connections and Version Negotiation
QUIC connections are shared state between a client and a server
[INV]. The compatible version negotiation mechanism defined in this
document (see Section 4.3) is performed as part of a single QUIC
connection; that is, the packets with the original version are part
of the same connection as the packets with the negotiated version.
In comparison, the incompatible version negotiation mechanism, which
leverages QUIC Version Negotiation packets (see Section 4.2)
conceptually operates across two QUIC connections: the connection
attempt prior to receiving the Version Negotiation packet is distinct
from the connection with the incompatible version that follows.
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4.2. Incompatible Version Negotiation
The server starts incompatible version negotiation by sending a
Version Negotiation packet. This packet SHALL include each entry
from the server's set of Offered Versions (see Section 2) in a
Supported Version field. The server MAY add reserved versions (as
defined in the Versions section of [QUIC]) in Supported Version
fields.
Upon receiving the VN packet, the client will search for a version it
supports in the list provided by the server. If it doesn't find one,
it aborts the connection attempt. Otherwise, it selects a mutually
supported version and sends a new first flight with that version - we
refer to this version as the "negotiated version".
The new first flight will allow the endpoints to establish a
connection using the negotiated version. The handshake of the
negotiated version will exchange version information (see Section 5)
required to ensure that VN was genuine, i.e. that no attacker
injected packets in order to influence the VN process, see Section 6.
4.3. Compatible Version Negotiation
When the server can parse the client's first flight using the
original version, it can extract the client's Version Information
structure (see Section 5). This contains the list of versions that
the client thinks its first flight is compatible with.
If the server supports one of the client's compatible versions, and
the server also believes that the original version is compatible with
this version, then the server converts the client's first flight to
that version and replies to the client as if it had received the
converted first flight. The version used by the server in its reply
is refered to as the "negotiated version". The server MUST NOT reply
with a version that is not present in the client's compatible
versions, unless it is the original version.
If the server does not find a compatible version, it will use the
original version if it supports it, and if it doesn't then the server
will perform incompatible version negotiation instead, see
Section 4.2.
For the duration of the compatible version negotiation process,
clients MUST use the same 5-tuple (source and destination IP
addresses and UDP port numbers). During that time, clients MUST also
use the same Destination Connection ID, except if the server
explicitly instructs the client to use a different Destination
Connection ID (for example, a QUIC version 1 server can accomplish
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this by sending an INITIAL packet with a Source Connection ID that
differed from the client's Destination Connection ID). This allows
load balancers to ensure that packets for a given connection are
routed to the same server.
5. Version Information
During the handshake, endpoints will exchange Version Information,
which is a blob of data that is defined below. In QUIC version 1,
the Version Information is transmitted using a new transport
parameter, "version_information". The contents of Version
Information are shown below (using the notation from the "Notational
Conventions" section of [QUIC]):
Version Information {
Chosen Version (32),
Other Versions (32) ...,
}
Figure 1: Version Information Format
The content of each field is described below:
Chosen Version: The version that the sender has chosen to use for
this connection. In most cases, this field will be equal to the
value of the Version field in the long header that carries this
data.
The contents of the "Other Versions" field depends on whether it is
sent by the client or by the server.
Client-Sent Other Versions: When sent by a client, the "Other
Versions" field lists all the versions that this first flight is
compatible with, ordered by descending preference. Note that the
version in the "Chosen Version" field MUST be included in this
list to allow the client to communicate the chosen version's
preference. Note that this preference is only advisory, servers
MAY choose to use their own preference instead.
Server-Sent Other Versions: When sent by a server, the "Other
Versions" field lists all the Fully-Deployed Versions of this
server deployment, see Section 2.
Clients and servers MAY both include versions following the pattern
"0x?a?a?a?a" in their "Other Versions" list. Those versions are
reserved to exercise version negotiation (see the Versions section of
[QUIC]), and will never be selected when choosing a version to use.
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6. Version Downgrade Prevention
Clients MUST ignore any received Version Negotiation packets that
contain the version that they initially attempted. Once a client has
reacted to a Version Negotiation packet, it MUST drop all subsequent
Version Negotiation packets on that connection.
Both endpoints MUST parse their peer's Version Information during the
handshake. If parsing the Version Information failed (for example,
if it is too short or if its length is not divisible by four), then
the endpoint MUST close the connection; if the connection was using
QUIC version 1, that connection closure MUST use a transport error of
type "TRANSPORT_PARAMETER_ERROR".
If the Version Information was missing, the endpoints MAY complete
the handshake if they have reason to believe the peer might not
support this extension. However, if a client has reacted to a
Version Negotiation packet and the Version Information was missing,
the client MUST close the connection; if the connection was using
QUIC version 1, that connection closure MUST use a transport error of
type "VERSION_NEGOTIATION_ERROR".
If a client has reacted to a Version Negotiation packet, it MUST
validate that the server's "Other Versions" field does not contain
the client's original version, and that the client would have
selected the same negotiated version if it had received a Version
Negotiation packet whose Supported Versions field had the same
contents as the server's "Other Versions" field. If any of these
checks fail, the client MUST close the connection; if the connection
was using QUIC version 1, that connection closure MUST use a
transport error of type "VERSION_NEGOTIATION_ERROR". This connection
closure prevents an attacker from being able to use forged Version
Negotiation packets to force a version downgrade.
After the process of version negotiation in this document completes,
the version in use for the connection is the version that the server
sent in the "Chosen Version" field of its Version Information. That
remains true even if other versions were used in the Version field of
long headers at any point in the lifetime of the connection;
endpoints MUST NOT change the version that they consider to be in use
based on the Version field of long headers as that field could be
forged by attackers.
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7. Client Choice of Original Version
The client's first flight SHOULD be sent using the version that the
server is most likely to support (in the absence of other
information, this will often be the oldest version the client
supports).
8. Interaction with Retry
QUIC version 1 features retry packets, which the server can send to
validate the client's IP address before parsing the client's first
flight. This impacts compatible version negotiation because a server
who wishes to send a retry packet before parsing the client's first
flight won't have parsed the client's Version Information yet. If a
future document wishes to define compatibility between two versions
that support retry, that document MUST specify how version
negotiation (both compatible and incompatible) interacts with retry
during a handshake that requires both. For example, that could be
accomplished by having the server send a retry packet first and
validating the client's IP address before starting version
negotiation and deciding whether to use compatible version
negotiation on that connection (in that scenario the retry packet
would be sent using the original version).
9. Interaction with 0-RTT
QUIC version 1 allows sending data from the client to the server
during the handshake, by using 0-RTT packets. If a future document
wishes to define compatibility between two versions that support
0-RTT, that document MUST address the scenario where there are 0-RTT
packets in the client's first flight. For example, this could be
accomplished by defining which transformations are applied to 0-RTT
packets. Alternatively, that document could specify that compatible
version negotiation causes 0-RTT data to be rejected by the server.
10. Considerations for Future Versions
In order to facilitate the deployment of future versions of QUIC,
designers of future versions SHOULD attempt to design their new
version such that commonly deployed versions are compatible with it.
For example, a successor to QUIC version 1 may wish to design its
transport parameters in a way that does not preclude compatibility.
Additionally, frames in QUIC version 1 do not use a self-describing
encoding, so unrecognized frame types cannot be parsed or ignored
(see the Extension Frames section of [QUIC]); this means that new
versions that wish to be very similar to QUIC version 1 and
compatible with it should avoid introducing new frames in initial
packets.
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11. Security Considerations
The security of this version negotiation mechanism relies on the
authenticity of the Version Information exchanged during the
handshake. In QUIC version 1, transport parameters are authenticated
ensuring the security of this mechanism. Negotiation between
compatible versions will have the security of the weakest common
version.
The requirement that versions not be assumed compatible mitigates the
possibility of cross-protocol attacks, but more analysis is still
needed here.
12. IANA Considerations
12.1. QUIC Transport Parameter
If this document is approved, IANA shall assign the following entry
in the QUIC Transport Parameter Registry:
+----------+---------------------+---------------+
| Value | Parameter Name | Reference |
+----------+---------------------+---------------+
| 0xFF73DB | version_information | This document |
+----------+---------------------+---------------+
12.2. QUIC Transport Error Code
If this document is approved, IANA shall assign the following entry
in the QUIC Transport Error Codes Registry:
+--------+---------------------------+---------------+
| Value | Parameter Name | Reference |
+--------+---------------------------+---------------+
| 0x53F8 | VERSION_NEGOTIATION_ERROR | This document |
+--------+---------------------------+---------------+
13. Normative References
[INV] Thomson, M., "Version-Independent Properties of QUIC",
Work in Progress, Internet-Draft, draft-ietf-quic-
invariants-13, 14 January 2021,
<https://tools.ietf.org/html/draft-ietf-quic-invariants-
13>.
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[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-34, 14 January 2021,
<https://tools.ietf.org/html/draft-ietf-quic-transport-
34>.
[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/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
Acknowledgments
The authors would like to thank Martin Thomson, Mike Bishop, Nick
Banks, Ryan Hamilton, and Roberto Peon for their input and
contributions.
Authors' Addresses
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043,
United States of America
Email: dschinazi.ietf@gmail.com
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
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