Internet DRAFT - draft-ietf-quic-invariants
draft-ietf-quic-invariants
QUIC M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track 15 January 2021
Expires: 19 July 2021
Version-Independent Properties of QUIC
draft-ietf-quic-invariants-13
Abstract
This document defines the properties of the QUIC transport protocol
that are common to all versions of the protocol.
Note to Readers
Discussion of this draft takes place on the QUIC working group
mailing list (quic@ietf.org (mailto:quic@ietf.org)), which is
archived at https://mailarchive.ietf.org/arch/
search/?email_list=quic.
Working Group information can be found at https://github.com/quicwg;
source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/-invariants.
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-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 July 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. An Extremely Abstract Description of QUIC . . . . . . . . . . 2
2. Fixed Properties of All QUIC Versions . . . . . . . . . . . . 2
3. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
4. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
5. QUIC Packets . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Long Header . . . . . . . . . . . . . . . . . . . . . . . 4
5.2. Short Header . . . . . . . . . . . . . . . . . . . . . . 5
5.3. Connection ID . . . . . . . . . . . . . . . . . . . . . . 6
5.4. Version . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Version Negotiation . . . . . . . . . . . . . . . . . . . . . 6
7. Security and Privacy Considerations . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Incorrect Assumptions . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. An Extremely Abstract Description of QUIC
QUIC is a connection-oriented protocol between two endpoints. Those
endpoints exchange UDP datagrams. These UDP datagrams contain QUIC
packets. QUIC endpoints use QUIC packets to establish a QUIC
connection, which is shared protocol state between those endpoints.
2. Fixed Properties of All QUIC Versions
In addition to providing secure, multiplexed transport, QUIC
[QUIC-TRANSPORT] allows for the option to negotiate a version. This
allows the protocol to change over time in response to new
requirements. Many characteristics of the protocol could change
between versions.
This document describes the subset of QUIC that is intended to remain
stable as new versions are developed and deployed. All of these
invariants are IP-version-independent.
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The primary goal of this document is to ensure that it is possible to
deploy new versions of QUIC. By documenting the properties that
cannot change, this document aims to preserve the ability for QUIC
endpoints to negotiate changes to any other aspect of the protocol.
As a consequence, this also guarantees a minimal amount of
information that is made available to entities other than endpoints.
Unless specifically prohibited in this document, any aspect of the
protocol can change between different versions.
Appendix A contains a non-exhaustive list of some incorrect
assumptions that might be made based on knowledge of QUIC version 1;
these do not apply to every version of QUIC.
3. 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.
This document defines requirements on future QUIC versions, even
where normative language is not used.
This document uses terms and notational conventions from
[QUIC-TRANSPORT].
4. Notational Conventions
The format of packets is described using the notation defined in this
section. This notation is the same as that used in [QUIC-TRANSPORT].
Complex fields are named and then followed by a list of fields
surrounded by a pair of matching braces. Each field in this list is
separated by commas.
Individual fields include length information, plus indications about
fixed value, optionality, or repetitions. Individual fields use the
following notational conventions, with all lengths in bits:
x (A): Indicates that x is A bits long
x (A..B): Indicates that x can be any length from A to B; A can be
omitted to indicate a minimum of zero bits and B can be omitted to
indicate no set upper limit; values in this format always end on
an byte boundary
x (L) = C: Indicates that x, with a length described by L, has a
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fixed value of C
x (L) ...: Indicates that x is repeated zero or more times (and that
each instance is length L)
This document uses network byte order (that is, big endian) values.
Fields are placed starting from the high-order bits of each byte.
Figure 1 shows an example structure:
Example Structure {
One-bit Field (1),
7-bit Field with Fixed Value (7) = 61,
Arbitrary-Length Field (..),
Variable-Length Field (8..24),
Repeated Field (8) ...,
}
Figure 1: Example Format
5. QUIC Packets
QUIC endpoints exchange UDP datagrams that contain one or more QUIC
packets. This section describes the invariant characteristics of a
QUIC packet. A version of QUIC could permit multiple QUIC packets in
a single UDP datagram, but the invariant properties only describe the
first packet in a datagram.
QUIC defines two types of packet header: long and short. Packets
with long headers are identified by the most significant bit of the
first byte being set; packets with a short header have that bit
cleared.
QUIC packets might be integrity protected, including the header.
However, QUIC Version Negotiation packets are not integrity
protected; see Section 6.
Aside from the values described here, the payload of QUIC packets is
version-specific and of arbitrary length.
5.1. Long Header
Long headers take the form described in Figure 2.
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Long Header Packet {
Header Form (1) = 1,
Version-Specific Bits (7),
Version (32),
Destination Connection ID Length (8),
Destination Connection ID (0..2040),
Source Connection ID Length (8),
Source Connection ID (0..2040),
Version-Specific Data (..),
}
Figure 2: QUIC Long Header
A QUIC packet with a long header has the high bit of the first byte
set to 1. All other bits in that byte are version specific.
The next four bytes include a 32-bit Version field. Versions are
described in Section 5.4.
The next byte contains the length in bytes of the Destination
Connection ID field that follows it. This length is encoded as an
8-bit unsigned integer. The Destination Connection ID field follows
the Destination Connection ID Length field and is between 0 and 255
bytes in length. Connection IDs are described in Section 5.3.
The next byte contains the length in bytes of the Source Connection
ID field that follows it. This length is encoded as an 8-bit
unsigned integer. The Source Connection ID field follows the Source
Connection ID Length field and is between 0 and 255 bytes in length.
The remainder of the packet contains version-specific content.
5.2. Short Header
Short headers take the form described in Figure 3.
Short Header Packet {
Header Form (1) = 0,
Version-Specific Bits (7),
Destination Connection ID (..),
Version-Specific Data (..),
}
Figure 3: QUIC Short Header
A QUIC packet with a short header has the high bit of the first byte
set to 0.
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A QUIC packet with a short header includes a Destination Connection
ID immediately following the first byte. The short header does not
include the Connection ID Lengths, Source Connection ID, or Version
fields. The length of the Destination Connection ID is not encoded
in packets with a short header and is not constrained by this
specification.
The remainder of the packet has version-specific semantics.
5.3. Connection ID
A connection ID is an opaque field of arbitrary length.
The primary function of a connection ID is to ensure that changes in
addressing at lower protocol layers (UDP, IP, and below) do not cause
packets for a QUIC connection to be delivered to the wrong QUIC
endpoint. The connection ID is used by endpoints and the
intermediaries that support them to ensure that each QUIC packet can
be delivered to the correct instance of an endpoint. At the
endpoint, the connection ID is used to identify the QUIC connection
for which the packet is intended.
The connection ID is chosen by each endpoint using version-specific
methods. Packets for the same QUIC connection might use different
connection ID values.
5.4. Version
The Version field contains a 4-byte identifier. This value can be
used by endpoints to identify a QUIC Version. A Version field with a
value of 0x00000000 is reserved for version negotiation; see
Section 6. All other values are potentially valid.
The properties described in this document apply to all versions of
QUIC. A protocol that does not conform to the properties described
in this document is not QUIC. Future documents might describe
additional properties that apply to a specific QUIC version, or to a
range of QUIC versions.
6. Version Negotiation
A QUIC endpoint that receives a packet with a long header and a
version it either does not understand or does not support might send
a Version Negotiation packet in response. Packets with a short
header do not trigger version negotiation.
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A Version Negotiation packet sets the high bit of the first byte, and
thus it conforms with the format of a packet with a long header as
defined in Section 5.1. A Version Negotiation packet is identifiable
as such by the Version field, which is set to 0x00000000.
Version Negotiation Packet {
Header Form (1) = 1,
Unused (7),
Version (32) = 0,
Destination Connection ID Length (8),
Destination Connection ID (0..2040),
Source Connection ID Length (8),
Source Connection ID (0..2040),
Supported Version (32) ...,
}
Figure 4: Version Negotiation Packet
Only the most significant bit of the first byte of a Version
Negotiation packet has any defined value. The remaining 7 bits,
labeled Unused, can be set to any value when sending and MUST be
ignored on receipt.
After the Source Connection ID field, the Version Negotiation packet
contains a list of Supported Version fields, each identifying a
version that the endpoint sending the packet supports. A Version
Negotiation packet contains no other fields. An endpoint MUST ignore
a packet that contains no Supported Version fields, or a truncated
Supported Version.
Version Negotiation packets do not use integrity or confidentiality
protection. Specific QUIC versions might include protocol elements
that allow endpoints to detect modification or corruption in the set
of supported versions.
An endpoint MUST include the value from the Source Connection ID
field of the packet it receives in the Destination Connection ID
field. The value for Source Connection ID MUST be copied from the
Destination Connection ID of the received packet, which is initially
randomly selected by a client. Echoing both connection IDs gives
clients some assurance that the server received the packet and that
the Version Negotiation packet was not generated by an attacker that
is unable to observe packets.
An endpoint that receives a Version Negotiation packet might change
the version that it decides to use for subsequent packets. The
conditions under which an endpoint changes QUIC version will depend
on the version of QUIC that it chooses.
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See [QUIC-TRANSPORT] for a more thorough description of how an
endpoint that supports QUIC version 1 generates and consumes a
Version Negotiation packet.
7. Security and Privacy Considerations
It is possible that middleboxes could observe traits of a specific
version of QUIC and assume that when other versions of QUIC exhibit
similar traits the same underlying semantic is being expressed.
There are potentially many such traits; see Appendix A. Some effort
has been made to either eliminate or obscure some observable traits
in QUIC version 1, but many of these remain. Other QUIC versions
might make different design decisions and so exhibit different
traits.
The QUIC version number does not appear in all QUIC packets, which
means that reliably extracting information from a flow based on
version-specific traits requires that middleboxes retain state for
every connection ID they see.
The Version Negotiation packet described in this document is not
integrity-protected; it only has modest protection against insertion
by attackers. An endpoint MUST authenticate the semantic content of
a Version Negotiation packet if it attempts a different QUIC version
as a result.
8. IANA Considerations
This document makes no request of IANA.
9. References
9.1. Normative References
[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>.
[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/info/rfc8174>.
9.2. Informative References
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[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using Transport
Layer Security (TLS) to Secure QUIC", Work in Progress,
Internet-Draft, draft-ietf-quic-tls-33, 15 January 2021,
<https://tools.ietf.org/html/draft-ietf-quic-tls-33>.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", Work in Progress,
Internet-Draft, draft-ietf-quic-transport-34, 15 January
2021, <https://tools.ietf.org/html/draft-ietf-quic-
transport-34>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
Appendix A. Incorrect Assumptions
There are several traits of QUIC version 1 [QUIC-TRANSPORT] that are
not protected from observation, but are nonetheless considered to be
changeable when a new version is deployed.
This section lists a sampling of incorrect assumptions that might be
made about QUIC based on knowledge of QUIC version 1. Some of these
statements are not even true for QUIC version 1. This is not an
exhaustive list; it is intended to be illustrative only.
*Any and all of the following statements can be false for a given
QUIC version:*
* QUIC uses TLS [QUIC-TLS] and some TLS messages are visible on the
wire
* QUIC long headers are only exchanged during connection
establishment
* Every flow on a given 5-tuple will include a connection
establishment phase
* The first packets exchanged on a flow use the long header
* The last packet before a long period of quiescence might be
assumed to contain only an acknowledgment
* QUIC uses an AEAD (AEAD_AES_128_GCM [RFC5116]) to protect the
packets it exchanges during connection establishment
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* QUIC packet numbers are encrypted and appear as the first
encrypted bytes
* QUIC packet numbers increase by one for every packet sent
* QUIC has a minimum size for the first handshake packet sent by a
client
* QUIC stipulates that a client speaks first
* QUIC packets always have the second bit of the first byte (0x40)
set
* A QUIC Version Negotiation packet is only sent by a server
* A QUIC connection ID changes infrequently
* QUIC endpoints change the version they speak if they are sent a
Version Negotiation packet
* The Version field in a QUIC long header is the same in both
directions
* A QUIC packet with a particular value in the Version field means
that the corresponding version of QUIC is in use
* Only one connection at a time is established between any pair of
QUIC endpoints
Author's Address
Martin Thomson
Mozilla
Email: mt@lowentropy.net
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