Internet Engineering Task Force (IETF) M. Kühlewind

Internet Engineering Task Force (IETF) M. Kühlewind
Request for Comments: 9312 Ericsson
Category: Informational B. Trammell
ISSN: 2070-1721 Google Switzerland GmbH
September 2022

Manageability of the QUIC Transport Protocol

Abstract

This document discusses manageability of the QUIC transport protocol
and focuses on the implications of QUIC's design and wire image on
network operations involving QUIC traffic. It is intended as a
"user's manual" for the wire image to provide guidance for network
operators and equipment vendors who rely on the use of transport-
aware network functions.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9312.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
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Table of Contents

1. Introduction
2. Features of the QUIC Wire Image
2.1. QUIC Packet Header Structure
2.2. Coalesced Packets
2.3. Use of Port Numbers
2.4. The QUIC Handshake
2.5. Integrity Protection of the Wire Image
2.6. Connection ID and Rebinding
2.7. Packet Numbers
2.8. Version Negotiation and Greasing
3. Network-Visible Information about QUIC Flows
3.1. Identifying QUIC Traffic
3.1.1. Identifying Negotiated Version
3.1.2. First Packet Identification for Garbage Rejection
3.2. Connection Confirmation
3.3. Distinguishing Acknowledgment Traffic
3.4. Server Name Indication (SNI)
3.4.1. Extracting Server Name Indication (SNI) Information
3.5. Flow Association
3.6. Flow Teardown
3.7. Flow Symmetry Measurement
3.8. Round-Trip Time (RTT) Measurement
3.8.1. Measuring Initial RTT
3.8.2. Using the Spin Bit for Passive RTT Measurement
4. Specific Network Management Tasks
4.1. Passive Network Performance Measurement and Troubleshooting
4.2. Stateful Treatment of QUIC Traffic
4.3. Address Rewriting to Ensure Routing Stability
4.4. Server Cooperation with Load Balancers
4.5. Filtering Behavior
4.6. UDP Blocking, Throttling, and NAT Binding
4.7. DDoS Detection and Mitigation
4.8. Quality of Service Handling and ECMP Routing
4.9. Handling ICMP Messages
4.10. Guiding Path MTU
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

QUIC [QUIC-TRANSPORT] is a new transport protocol that is
encapsulated in UDP. QUIC integrates TLS [QUIC-TLS] to encrypt all
payload data and most control information. QUIC version 1 was
designed primarily as a transport for HTTP with the resulting
protocol being known as HTTP/3 [QUIC-HTTP].

This document provides guidance for network operations that manage
QUIC traffic. This includes guidance on how to interpret and utilize
information that is exposed by QUIC to the network, requirements and
assumptions of the QUIC design with respect to network treatment, and
a description of how common network management practices will be
impacted by QUIC.

QUIC is an end-to-end transport protocol; therefore, no information
in the protocol header is intended to be mutable by the network.
This property is enforced through integrity protection of the wire
image [WIRE-IMAGE]. Encryption of most transport-layer control
signaling means that less information is visible to the network in
comparison to TCP.

Integrity protection can also simplify troubleshooting at the end
points as none of the nodes on the network path can modify transport
layer information. However, it means in-network operations that
depend on modification of data (for examples, see [RFC9065]) are not
possible without the cooperation of a QUIC endpoint. Such
cooperation might be possible with the introduction of a proxy that
authenticates as an endpoint. Proxy operations are not in scope for
this document.

Network management is not a one-size-fits-all endeavor; for example,
practices considered necessary or even mandatory within enterprise
networks with certain compliance requirements would be impermissible
on other networks without those requirements. Therefore, presence of
a particular practice in this document should not be construed as a
recommendation to apply it. For each practice, this document
describes what is and is not possible with the QUIC transport
protocol as defined.

This document focuses solely on network management practices that
observe traffic on the wire. For example, replacement of
troubleshooting based on observation with active measurement
techniques is therefore out of scope. A more generalized treatment
of network management operations on encrypted transports is given in
[RFC9065].

QUIC-specific terminology used in this document is defined in
[QUIC-TRANSPORT].

2. Features of the QUIC Wire Image

This section discusses aspects of the QUIC transport protocol that
have an impact on the design and operation of devices that forward
QUIC packets. Therefore, this section is primarily considering the
unencrypted part of QUIC's wire image [WIRE-IMAGE], which is defined
as the information available in the packet header in each QUIC
packet, and the dynamics of that information. Since QUIC is a
versioned protocol, the wire image of the header format can also
change from version to version. However, the field that identifies
the QUIC version in some packets and the format of the Version
Negotiation packet are both inspectable and invariant
[QUIC-INVARIANTS].

This document addresses version 1 of the QUIC protocol, whose wire
image is fully defined in [QUIC-TRANSPORT] and [QUIC-TLS]. Features
of the wire image described herein may change in future versions of
the protocol except when specified as an invariant [QUIC-INVARIANTS]
and cannot be used to identify QUIC as a protocol or to infer the
behavior of future versions of QUIC.

2.1. QUIC Packet Header Structure

QUIC packets may have either a long header or a short header. The
first bit of the QUIC header is the Header Form bit and indicates
which type of header is present. The purpose of this bit is
invariant across QUIC versions.

The long header exposes more information. It contains a version
number, as well as Source and Destination Connection IDs for
associating packets with a QUIC connection. The definition and
location of these fields in the QUIC long header are invariant for
future versions of QUIC, although future versions of QUIC may provide
additional fields in the long header [QUIC-INVARIANTS].

In version 1 of QUIC, the long header is used during connection
establishment to transmit CRYPTO handshake data, perform version
negotiation, retry, and send 0-RTT data.

Short headers are used after a connection establishment in version 1
of QUIC and expose only an optional Destination Connection ID and the
initial flags byte with the spin bit for RTT measurement.

The following information is exposed in QUIC packet headers in all
versions of QUIC (as specified in [QUIC-INVARIANTS]):

version number: The version number is present in the long header and
identifies the version used for that packet. During Version
Negotiation (see Section 17.2.1 of [QUIC-TRANSPORT] and
Section 2.8), the Version field has a special value (0x00000000)
that identifies the packet as a Version Negotiation packet. QUIC
version 1 uses version 0x00000001. Operators should expect to
observe packets with other version numbers as a result of various
Internet experiments, future standards, and greasing [RFC7801].
An IANA registry contains the values of all standardized versions
of QUIC, and may contain some proprietary versions (see
Section 22.2 of [QUIC-TRANSPORT]). However, other versions of
QUIC can be expected to be seen in the Internet at any given time.

Source and Destination Connection ID: Short and long headers carry a
Destination Connection ID, which is a variable-length field. If
the Destination Connection ID is not zero length, it can be used
to identify the connection associated with a QUIC packet for load
balancing and NAT rebinding purposes; see Sections 4.4 and 2.6.
Long packet headers additionally carry a Source Connection ID.
The Source Connection ID is only present on long headers and
indicates the Destination Connection ID that the other endpoint
should use when sending packets. On long header packets, the
length of the connection IDs is also present; on short header
packets, the length of the Destination Connection ID is implicit,
as it is known from preceding long header packets.

In version 1 of QUIC, the following additional information is
exposed:

"Fixed Bit": In version 1 of QUIC, the second-most-significant bit
of the first octet is set to 1, unless the packet is a Version
Negotiation packet or an extension is used that modifies the usage
of this bit. If the bit is set to 1, it enables endpoints to
easily demultiplex with other UDP-encapsulated protocols. Even
though this bit is fixed in the version 1 specification, endpoints
might use an extension that varies the bit [QUIC-GREASE].
Therefore, observers cannot reliably use it as an identifier for
QUIC.

latency spin bit: The third-most-significant bit of the first octet
in the short header for version 1. The spin bit is set by
endpoints such that tracking edge transitions can be used to
passively observe end-to-end RTT. See Section 3.8.2 for further
details.

header type: The long header has a 2-bit packet type field following
the Header Form and Fixed Bits. Header types correspond to stages
of the handshake; see Section 17.2 of [QUIC-TRANSPORT] for
details.

length: The length of the remaining QUIC packet after the Length
field present on long headers. This field is used to implement
coalesced packets during the handshake (see Section 2.2).

token: Initial packets may contain a token, a variable-length opaque
value optionally sent from client to server, used for validating
the client's address. Retry packets also contain a token, which
can be used by the client in an Initial packet on a subsequent
connection attempt. The length of the token is explicit in both
cases.

Retry (Section 17.2.5 of [QUIC-TRANSPORT]) and Version Negotiation
(Section 17.2.1 of [QUIC-TRANSPORT]) packets are not encrypted.
Retry packets are integrity protected. Transport parameters are used
to authenticate the contents of Retry packets later in the handshake.
For other kinds of packets, version 1 of QUIC cryptographically
protects other information in the packet headers:

Packet Number: All packets except Version Negotiation and Retry
packets have an associated packet number; however, this packet
number is encrypted, and therefore not of use to on-path
observers. The offset of the packet number can be decoded in long
headers while it is implicit (depending on Destination Connection
ID length) in short headers. The length of the packet number is
cryptographically protected.

Key Phase: The Key Phase bit (present in short headers) specifies
the keys used to encrypt the packet to support key rotation. The
Key Phase bit is cryptographically protected.

2.2. Coalesced Packets

Multiple QUIC packets may be coalesced into a single UDP datagram
with a datagram carrying one or more long header packets followed by
zero or one short header packets. When packets are coalesced, the
Length fields in the long headers are used to separate QUIC packets;
see Section 12.2 of [QUIC-TRANSPORT]. The Length field is a
variable-length field, and its position in the header also varies
depending on the lengths of the Source and Destination Connection
IDs; see Section 17.2 of [QUIC-TRANSPORT].

2.3. Use of Port Numbers

Applications that have a mapping for TCP and QUIC are expected to use
the same port number for both services. However, as for all other
IETF transports [RFC7605], there is no guarantee that a specific
application will use a given registered port or that a given port
carries traffic belonging to the respective registered service,
especially when application layer information is encrypted. For
example, [QUIC-HTTP] specifies the use of the HTTP Alternative
Services mechanism [RFC7838] for discovery of HTTP/3 services on
other ports.

Further, as QUIC has a connection ID, it is also possible to maintain
multiple QUIC connections over one 5-tuple (protocol, source, and
destination IP address and source and destination port). However, if
the connection ID is zero length, all packets of the 5-tuple likely
belong to the same QUIC connection.

2.4. The QUIC Handshake

New QUIC connections are established using a handshake that is
distinguishable on the wire (see Section 3.1 for details) and
contains some information that can be passively observed.

To illustrate the information visible in the QUIC wire image during
the handshake, we first show the general communication pattern
visible in the UDP datagrams containing the QUIC handshake. Then, we
examine each of the datagrams in detail.

The QUIC handshake can normally be recognized on the wire through
four flights of datagrams labeled "Client Initial", "Server Initial",
"Client Completion", and "Server Completion" as illustrated in
Figure 1.

A handshake starts with the client sending one or more datagrams
containing Initial packets (detailed in Figure 2), which elicits the
Server Initial response (detailed in Figure 3), which typically
contains three types of packets: Initial packet(s) with the beginning
of the server's side of the TLS handshake, Handshake packet(s) with
the rest of the server's portion of the TLS handshake, and 1-RTT
packet(s), if present.

Client Server
| |
+----Client Initial----------------------->|
+----(zero or more 0-RTT)----------------->|
| |
|<-----------------------Server Initial----+
|<--------(1-RTT encrypted data starts)----+
| |
+----Client Completion-------------------->|
+----(1-RTT encrypted data starts)-------->|
| |
|<--------------------Server Completion----+
| |

Figure 1: General Communication Pattern Visible in the QUIC Handshake

As shown here, the client can send 0-RTT data as soon as it has sent
its ClientHello and the server can send 1-RTT data as soon as it has
sent its ServerHello. The Client Completion flight contains at least
one Handshake packet and could also include an Initial packet.
During the handshake, QUIC packets in separate contexts can be
coalesced (see Section 2.2) in order to reduce the number of UDP
datagrams sent during the handshake.

Handshake packets can arrive out-of-order without impacting the
handshake as long as the reordering was not accompanied by extensive
delays that trigger a spurious Probe Timeout (Section 6.2 of
[QUIC-RECOVERY]). If QUIC packets get lost or reordered, packets
belonging to the same flight might not be observed in close time
succession, though the sequence of the flights will not change
because one flight depends upon the peer's previous flight.

Datagrams that contain an Initial packet (Client Initial, Server
Initial, and some Client Completion) contain at least 1200 octets of
UDP payload. This protects against amplification attacks and
verifies that the network path meets the requirements for the minimum
QUIC IP packet size; see Section 14 of [QUIC-TRANSPORT]. This is
accomplished by either adding PADDING frames within the Initial
packet, coalescing other packets with the Initial packet, or leaving
unused payload in the UDP packet after the Initial packet. A network
path needs to be able to forward packets of at least this size for
QUIC to be used.

The content of Initial packets is encrypted using Initial Secrets,
which are derived from a per-version constant and the client's
Destination Connection ID. That content is therefore observable by
any on-path device that knows the per-version constant and is
considered visible in this illustration. The content of QUIC
Handshake packets is encrypted using keys established during the
initial handshake exchange and is therefore not visible.

Initial, Handshake, and 1-RTT packets belong to different
cryptographic and transport contexts. The Client Completion
(Figure 4) and the Server Completion (Figure 5) flights conclude the
Initial and Handshake contexts by sending final acknowledgments and
CRYPTO frames.

+----------------------------------------------------------+
| UDP header (source and destination UDP ports) |
+----------------------------------------------------------+
| QUIC long header (type = Initial, Version, DCID, SCID) (Length)
+----------------------------------------------------------+ |
| QUIC CRYPTO frame header | |
+----------------------------------------------------------+ |
| | TLS ClientHello (incl. TLS SNI) | | |
+----------------------------------------------------------+ |
| QUIC PADDING frames | |
+----------------------------------------------------------+<-+

Figure 2: Example Client Initial Datagram Without 0-RTT

A Client Initial packet exposes the Version, Source, and Destination
Connection IDs without encryption. The payload of the Initial packet
is protected using the Initial secret. The complete TLS ClientHello,
including any TLS Server Name Indication (SNI) present, is sent in
one or more CRYPTO frames across one or more QUIC Initial packets.

+------------------------------------------------------------+
| UDP header (source and destination UDP ports) |
+------------------------------------------------------------+
| QUIC long header (type = Initial, Version, DCID, SCID) (Length)
+------------------------------------------------------------+ |
| QUIC CRYPTO frame header | |
+------------------------------------------------------------+ |
| TLS ServerHello | |
+------------------------------------------------------------+ |
| QUIC ACK frame (acknowledging client hello) | |
+------------------------------------------------------------+<-+
| QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
+------------------------------------------------------------+ |
| encrypted payload (presumably CRYPTO frames) | |
+------------------------------------------------------------+<-+
| QUIC short header |
+------------------------------------------------------------+
| 1-RTT encrypted payload |
+------------------------------------------------------------+

Figure 3: Coalesced Server Initial Datagram Pattern

The Server Initial datagram also exposes the version number and the
Source and Destination Connection IDs in the clear; the payload of
the Initial packet is protected using the Initial secret.

+------------------------------------------------------------+
| UDP header (source and destination UDP ports) |
+------------------------------------------------------------+
| QUIC long header (type = Initial, Version, DCID, SCID) (Length)
+------------------------------------------------------------+ |
| QUIC ACK frame (acknowledging Server Initial) | |
+------------------------------------------------------------+<-+
| QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
+------------------------------------------------------------+ |
| encrypted payload (presumably CRYPTO/ACK frames) | |
+------------------------------------------------------------+<-+
| QUIC short header |
+------------------------------------------------------------+
| 1-RTT encrypted payload |
+------------------------------------------------------------+

Figure 4: Coalesced Client Completion Datagram Pattern

The Client Completion flight does not expose any additional
information; however, as the Destination Connection ID is server-
selected, it usually is not the same ID that is sent in the Client
Initial. Client Completion flights contain 1-RTT packets that
indicate the handshake has completed (see Section 3.2) on the client
and for three-way handshake RTT estimation as in Section 3.8.

+------------------------------------------------------------+
| UDP header (source and destination UDP ports) |
+------------------------------------------------------------+
| QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
+------------------------------------------------------------+ |
| encrypted payload (presumably ACK frame) | |
+------------------------------------------------------------+<-+
| QUIC short header |
+------------------------------------------------------------+
| 1-RTT encrypted payload |
+------------------------------------------------------------+

Figure 5: Coalesced Server Completion Datagram Pattern

Similar to Client Completion, Server Completion does not expose
additional information; observing it serves only to determine that
the handshake has completed.

When the client uses 0-RTT data, the Client Initial flight can also
include one or more 0-RTT packets as shown in Figure 6.

+----------------------------------------------------------+
| UDP header (source and destination UDP ports) |
+----------------------------------------------------------+
| QUIC long header (type = Initial, Version, DCID, SCID) (Length)
+----------------------------------------------------------+ |
| QUIC CRYPTO frame header | |
+----------------------------------------------------------+ |
| TLS ClientHello (incl. TLS SNI) | |
+----------------------------------------------------------+<-+
| QUIC long header (type = 0-RTT, Version, DCID, SCID) (Length)
+----------------------------------------------------------+ |
| 0-RTT encrypted payload | |
+----------------------------------------------------------+<-+

Figure 6: Coalesced 0-RTT Client Initial Datagram

When a 0-RTT packet is coalesced with an Initial packet, the datagram
will be padded to 1200 bytes. Additional datagrams containing only
0-RTT packets with long headers can be sent after the client Initial
packet, which contains more 0-RTT data. The amount of 0-RTT
protected data that can be sent in the first flight is limited by the
initial congestion window, typically to around 10 packets (see
Section 7.2 of [QUIC-RECOVERY]).

2.5. Integrity Protection of the Wire Image

As soon as the cryptographic context is established, all information
in the QUIC header, including exposed information, is integrity
protected. Further, information that was exposed in packets sent
before the cryptographic context was established is validated during
the cryptographic handshake. Therefore, devices on path cannot alter
any information or bits in QUIC packets. Such alterations would
cause the integrity check to fail, which results in the receiver
discarding the packet. Some parts of Initial packets could be
altered by removing and reapplying the authenticated encryption
without immediate discard at the receiver. However, the
cryptographic handshake validates most fields and any modifications
in those fields will result in a connection establishment failure
later.

2.6. Connection ID and Rebinding

The connection ID in the QUIC packet headers allows association of
QUIC packets using information independent of the 5-tuple. This
allows rebinding of a connection after one of the endpoints (usually
the client) has experienced an address change. Further, it can be
used by in-network devices to ensure that related 5-tuple flows are
appropriately balanced together (see Section 4.4).

Client and server each choose a connection ID during the handshake;
for example, a server might request that a client use a connection
ID, whereas the client might choose a zero-length value. Connection
IDs for either endpoint may change during the lifetime of a
connection, with the new connection ID being supplied via encrypted
frames (see Section 5.1 of [QUIC-TRANSPORT]). Therefore, observing a
new connection ID does not necessarily indicate a new connection.

[QUIC-LB] specifies algorithms for encoding the server mapping in a
connection ID in order to share this information with selected on-
path devices such as load balancers. Server mappings should only be
exposed to selected entities. Uncontrolled exposure would allow
linkage of multiple IP addresses to the same host if the server also
supports migration that opens an attack vector on specific servers or
pools. The best way to obscure an encoding is to appear random to
any other observers, which is most rigorously achieved with
encryption. As a result, any attempt to infer information from
specific parts of a connection ID is unlikely to be useful.

2.7. Packet Numbers

The Packet Number field is always present in the QUIC packet header
in version 1; however, it is always encrypted. The encryption key
for packet number protection on Initial packets (which are sent
before cryptographic context establishment) is specific to the QUIC
version while packet number protection on subsequent packets uses
secrets derived from the end-to-end cryptographic context. Packet
numbers are therefore not part of the wire image that is visible to
on-path observers.

2.8. Version Negotiation and Greasing

Version Negotiation packets are used by the server to indicate that a
requested version from the client is not supported (see Section 6 of
[QUIC-TRANSPORT]). Version Negotiation packets are not intrinsically
protected, but future QUIC versions could use later encrypted
messages to verify that they were authentic. Therefore, any
modification of this list will be detected and may cause the
endpoints to terminate the connection attempt.

Also note that the list of versions in the Version Negotiation packet
may contain reserved versions. This mechanism is used to avoid
ossification in the implementation of the selection mechanism.
Further, a client may send an Initial packet with a reserved version
number to trigger version negotiation. In the Version Negotiation
packet, the connection IDs of the client's Initial packet are
reflected to provide a proof of return-routability. Therefore,
changing this information will also cause the connection to fail.

QUIC is expected to evolve rapidly. Therefore, new versions (both
experimental and IETF standard versions) will be deployed on the
Internet more often than with other commonly deployed Internet and
transport-layer protocols. Use of the Version field for traffic
recognition will therefore behave differently than with these
protocols. Using a particular version number to recognize valid QUIC
traffic is likely to persistently miss a fraction of QUIC flows and
completely fail in the near future. Reliance on the Version field
for the purpose of admission control is also likely to lead to
unintended failure modes. Admission of QUIC traffic regardless of
version avoids these failure modes, avoids unnecessary deployment
delays, and supports continuous version-based evolution.

3. Network-Visible Information about QUIC Flows

This section addresses the different kinds of observations and
inferences that can be made about QUIC flows by a passive observer in
the network based on the wire image in Section 2. Here, we assume a
bidirectional observer (one that can see packets in both directions
in the sequence in which they are carried on the wire) unless noted,
but typically without access to any keying information.

3.1. Identifying QUIC Traffic

The QUIC wire image is not specifically designed to be
distinguishable from other UDP traffic by a passive observer in the
network. While certain QUIC applications may be heuristically
identifiable on a per-application basis, there is no general method
for distinguishing QUIC traffic from otherwise unclassifiable UDP
traffic on a given link. Therefore, any unrecognized UDP traffic may
be QUIC traffic.

At the time of writing, two application bindings for QUIC have been
published or adopted by the IETF: HTTP/3 [QUIC-HTTP] and DNS over
Dedicated QUIC Connections [RFC9250]. These are both known to have
active Internet deployments, so an assumption that all QUIC traffic
is HTTP/3 is not valid. HTTP/3 uses UDP port 443 by convention but
various methods can be used to specify alternate port numbers. Other
applications (e.g., Microsoft's SMB over QUIC) also use UDP port 443
by default. Therefore, simple assumptions about whether a given flow
is using QUIC (or indeed which application might be using QUIC) based
solely upon a UDP port number may not hold; see Section 5 of
[RFC7605].

While the second-most-significant bit (0x40) of the first octet is
set to 1 in most QUIC packets of the current version (see Section 2.1
and Section 17 of [QUIC-TRANSPORT]), this method of recognizing QUIC
traffic is not reliable. First, it only provides one bit of
information and is prone to collision with UDP-based protocols other
than those considered in [RFC7983]. Second, this feature of the wire
image is not invariant [QUIC-INVARIANTS] and may change in future
versions of the protocol or even be negotiated during the handshake
via the use of an extension [QUIC-GREASE].

Even though transport parameters transmitted in the client's Initial
packet are observable by the network, they cannot be modified by the
network without causing a connection failure. Further, the reply
from the server cannot be observed, so observers on the network
cannot know which parameters are actually in use.

3.1.1. Identifying Negotiated Version

An in-network observer assuming that a set of packets belongs to a
QUIC flow might infer the version number in use by observing the
handshake. If the version number in an Initial packet of the server
response is subsequently seen in a packet from the client, that
version has been accepted by both endpoints to be used for the rest
of the connection (see Section 2 of [QUIC-VERSION-NEGOTIATION]).

The negotiated version cannot be identified for flows in which a
handshake is not observed, such as in the case of connection
migration. However, it might be possible to associate a flow with a
flow for which a version has been identified; see Section 3.5.

3.1.2. First Packet Identification for Garbage Rejection

A related question is whether the first packet of a given flow on a
port known to be associated with QUIC is a valid QUIC packet. This
determination supports in-network filtering of garbage UDP packets
(reflection attacks, random backscatter, etc.). While heuristics
based on the first byte of the packet (packet type) could be used to
separate valid from invalid first packet types, the deployment of
such heuristics is not recommended as bits in the first byte may have
different meanings in future versions of the protocol.

3.2. Connection Confirmation

This document focuses on QUIC version 1, and this Connection
Confirmation section applies only to packets belonging to QUIC
version 1 flows; for purposes of on-path observation, it assumes that
these packets have been identified as such through the observation of
a version number exchange as described above.

Connection establishment uses Initial and Handshake packets
containing a TLS handshake and Retry packets that do not contain
parts of the handshake. Connection establishment can therefore be
detected using heuristics similar to those used to detect TLS over
TCP. A client initiating a connection may also send data in 0-RTT
packets directly after the Initial packet containing the TLS
ClientHello. Since packets may be reordered or lost in the network,
0-RTT packets could be seen before the Initial packet.

Note that in this version of QUIC, clients send Initial packets
before servers do, servers send Handshake packets before clients do,
and only clients send Initial packets with tokens. Therefore, an
endpoint can be identified as a client or server by an on-path
observer. An attempted connection after Retry can be detected by
correlating the contents of the Retry packet with the Token and the
Destination Connection ID fields of the new Initial packet.

3.3. Distinguishing Acknowledgment Traffic

Some deployed in-network functions distinguish packets that carry
only acknowledgment (ACK-only) information from packets carrying
upper-layer data in order to attempt to enhance performance (for
example, by queuing ACKs differently or manipulating ACK signaling
[RFC3449]). Distinguishing ACK packets is possible in TCP, but is
not supported by QUIC since acknowledgment signaling is carried
inside QUIC's encrypted payload and ACK manipulation is impossible.
Specifically, heuristics attempting to distinguish ACK-only packets
from payload-carrying packets based on packet size are likely to fail
and are not recommended to use as a way to construe internals of
QUIC's operation as those mechanisms can change, e.g., due to the use
of extensions.

3.4. Server Name Indication (SNI)

The client's TLS ClientHello may contain a Server Name Indication
(SNI) extension [RFC6066] by which the client reveals the name of the
server it intends to connect to in order to allow the server to
present a certificate based on that name. If present, SNI
information is available to unidirectional observers on the client-
to-server path if it.

The TLS ClientHello may also contain an Application-Layer Protocol
Negotiation (ALPN) extension [RFC7301], by which the client exposes
the names of application-layer protocols it supports; an observer can
deduce that one of those protocols will be used if the connection
continues.

Work is currently underway in the TLS working group to encrypt the
contents of the ClientHello in TLS 1.3 [TLS-ECH]. This would make
SNI-based application identification impossible by on-path
observation for QUIC and other protocols that use TLS.

3.4.1. Extracting Server Name Indication (SNI) Information

If the ClientHello is not encrypted, SNI can be derived from the
client's Initial packets by calculating the Initial secret to decrypt
the packet payload and parsing the QUIC CRYPTO frames containing the
TLS ClientHello.

As both the derivation of the Initial secret and the structure of the
Initial packet itself are version specific, the first step is always
to parse the version number (the second through fifth bytes of the
long header). Note that only long header packets carry the version
number, so it is necessary to also check if the first bit of the QUIC
packet is set to 1, which indicates a long header.

Note that proprietary QUIC versions that have been deployed before
standardization might not set the first bit in a QUIC long header
packet to 1. However, it is expected that these versions will
gradually disappear over time and therefore do not require any
special consideration or treatment.

When the version has been identified as QUIC version 1, the packet
type needs to be verified as an Initial packet by checking that the
third and fourth bits of the header are both set to 0. Then, the
Destination Connection ID needs to be extracted from the packet. The
Initial secret is calculated using the version-specific Initial salt
as described in Section 5.2 of [QUIC-TLS]. The length of the
connection ID is indicated in the 6th byte of the header followed by
the connection ID itself.

Note that subsequent Initial packets might contain a Destination
Connection ID other than the one used to generate the Initial secret.
Therefore, attempts to decrypt these packets using the procedure
above might fail unless the Initial secret is retained by the
observer.

To determine the end of the packet header and find the start of the
payload, the Packet Number Length, the Source Connection ID Length,
and the Token Length need to be extracted. The Packet Number Length
is defined by the seventh and eighth bits of the header as described
in Section 17.2 of [QUIC-TRANSPORT], but is protected as described in
Section 5.4 of [QUIC-TLS]. The Source Connection ID Length is
specified in the byte after the Destination Connection ID. The Token
Length, which follows the Source Connection ID, is a variable-length
integer as specified in Section 16 of [QUIC-TRANSPORT].

After decryption, the client's Initial packets can be parsed to
detect the CRYPTO frames that contain the TLS ClientHello, which then
can be parsed similarly to TLS over TCP connections. Note that there
can be multiple CRYPTO frames spread out over one or more Initial
packets and they might not be in order, so reassembling the CRYPTO
stream by parsing offsets and lengths is required. Further, the
client's Initial packets may contain other frames, so the first bytes
of each frame need to be checked to identify the frame type and
determine whether the frame can be skipped over. Note that the
length of the frames is dependent on the frame type; see Section 18
of [QUIC-TRANSPORT]. For example, PADDING frames (each consisting of
a single zero byte) may occur before, after, or between CRYPTO
frames. However, extensions might define additional frame types. If
an unknown frame type is encountered, it is impossible to know the
length of that frame, which prevents skipping over it; therefore,
parsing fails.

3.5. Flow Association

The QUIC connection ID (see Section 2.6) is designed to allow a
coordinating on-path device, such as a load balancer, to associate
two flows when one of the endpoints changes address. This change can
be due to NAT rebinding or address migration.

The connection ID must change upon intentional address change by an
endpoint and connection ID negotiation is encrypted; therefore, it is
not possible for a passive observer to link intended changes of
address using the connection ID.

When one endpoint's address unintentionally changes, as is the case
with NAT rebinding, an on-path observer may be able to use the
connection ID to associate the flow on the new address with the flow
on the old address.

A network function that attempts to use the connection ID to
associate flows must be robust to the failure of this technique.
Since the connection ID may change multiple times during the lifetime
of a connection, packets with the same 5-tuple but different
connection IDs might or might not belong to the same connection.
Likewise, packets with the same connection ID but different 5-tuples
might not belong to the same connection either.

Connection IDs should be treated as opaque; see Section 4.4 for
caveats regarding connection ID selection at servers.

3.6. Flow Teardown

QUIC does not expose the end of a connection; the only indication to
on-path devices that a flow has ended is that packets are no longer
observed. Therefore, stateful devices on path such as NATs and
firewalls must use idle timeouts to determine when to drop state for
QUIC flows; see Section 4.2.

3.7. Flow Symmetry Measurement

QUIC explicitly exposes which side of a connection is a client and
which side is a server during the handshake. In addition, the
symmetry of a flow (whether it is primarily client-to-server,
primarily server-to-client, or roughly bidirectional, as input to
basic traffic classification techniques) can be inferred through the
measurement of data rate in each direction. Note that QUIC packets
containing only control frames (such as ACK-only packets) may be
padded. Padding, though optional, may conceal connection roles or
flow symmetry information.

3.8. Round-Trip Time (RTT) Measurement

The round-trip time (RTT) of QUIC flows can be inferred by
observation once per flow during the handshake in passive TCP
measurement; this requires parsing of the QUIC packet header and
recognition of the handshake, as illustrated in Section 2.4. It can
also be inferred during the flow's lifetime if the endpoints use the
spin bit facility described below and in Section 17.3.1 of
[QUIC-TRANSPORT]. RTT measurement is available to unidirectional
observers when the spin bit is enabled.

3.8.1. Measuring Initial RTT

In the common case, the delay between the client's Initial packet
(containing the TLS ClientHello) and the server's Initial packet
(containing the TLS ServerHello) represents the RTT component on the
path between the observer and the server. The delay between the
server's first Handshake packet and the Handshake packet sent by the
client represents the RTT component on the path between the observer
and the client. While the client may send 0-RTT packets after the
Initial packet during connection re-establishment, these can be
ignored for RTT measurement purposes.

Handshake RTT can be measured by adding the client-to-observer and
observer-to-server RTT components together. This measurement
necessarily includes all transport- and application-layer delay at
both endpoints.

3.8.2. Using the Spin Bit for Passive RTT Measurement

The spin bit provides a version-specific method to measure per-flow
RTT from observation points on the network path throughout the
duration of a connection. See Section 17.4 of [QUIC-TRANSPORT] for
the definition of the spin bit in Version 1 of QUIC. Endpoint
participation in spin bit signaling is optional. While its location
is fixed in this version of QUIC, an endpoint can unilaterally choose
to not support "spinning" the bit.

Use of the spin bit for RTT measurement by devices on path is only
possible when both endpoints enable it. Some endpoints may disable
use of the spin bit by default, others only in specific deployment
scenarios, e.g., for servers and clients where the RTT would reveal
the presence of a VPN or proxy. To avoid making these connections
identifiable based on the usage of the spin bit, all endpoints
randomly disable "spinning" for at least one eighth of connections,
even if otherwise enabled by default. An endpoint not participating
in spin bit signaling for a given connection can use a fixed spin
value for the duration of the connection or can set the bit randomly
on each packet sent.

When in use, the latency spin bit in each direction changes value
once per RTT any time that both endpoints are sending packets
continuously. An on-path observer can observe the time difference
between edges (changes from 1 to 0 or 0 to 1) in the spin bit signal
in a single direction to measure one sample of end-to-end RTT. This
mechanism follows the principles of protocol measurability laid out
in [IPIM].

Note that this measurement, as with passive RTT measurement for TCP,
includes all transport protocol delay (e.g., delayed sending of
acknowledgments) and/or application layer delay (e.g., waiting for a
response to be generated). It therefore provides devices on path a
good instantaneous estimate of the RTT as experienced by the
application.

However, application-limited and flow-control-limited senders can
have application- and transport-layer delay, respectively, that are
much greater than network RTT. For example, if the sender only sends
small amounts of application traffic periodically, where the
periodicity is longer than the RTT, spin bit measurements provide
information about the application period rather than network RTT.

Since the spin bit logic at each endpoint considers only samples from
packets that advance the largest packet number, signal generation
itself is resistant to reordering. However, reordering can cause
problems at an observer by causing spurious edge detection and
therefore inaccurate (i.e., lower) RTT estimates, if reordering
occurs across a spin bit flip in the stream.

Simple heuristics based on the observed data rate per flow or changes
in the RTT series can be used to reject bad RTT samples due to lost
or reordered edges in the spin signal, as well as application or flow
control limitation; for example, QoF [TMA-QOF] rejects component RTTs
significantly higher than RTTs over the history of the flow. These
heuristics may use the handshake RTT as an initial RTT estimate for a
given flow. Usually such heuristics would also detect if the spin is
either constant or randomly set for a connection.

An on-path observer that can see traffic in both directions (from
client to server and from server to client) can also use the spin bit
to measure "upstream" and "downstream" component RTT; i.e, the
component of the end-to-end RTT attributable to the paths between the
observer and the server and between the observer and the client,
respectively. It does this by measuring the delay between a spin
edge observed in the upstream direction and that observed in the
downstream direction, and vice versa.

Raw RTT samples generated using these techniques can be processed in
various ways to generate useful network performance metrics. A
simple linear smoothing or moving minimum filter can be applied to
the stream of RTT samples to get a more stable estimate of
application-experienced RTT. RTT samples measured from the spin bit
can also be used to generate RTT distribution information, including
minimum RTT (which approximates network RTT over longer time windows)
and RTT variance (which approximates one-way packet delay variance as
seen by an application end-point).

4. Specific Network Management Tasks

In this section, we review specific network management and
measurement techniques and how QUIC's design impacts them.

4.1. Passive Network Performance Measurement and Troubleshooting

Limited RTT measurement is possible by passive observation of QUIC
traffic; see Section 3.8. No passive measurement of loss is possible
with the present wire image. Limited observation of upstream
congestion may be possible via the observation of Congestion
Experienced (CE) markings in the IP header [RFC3168] on ECN-enabled
QUIC traffic.

On-path devices can also make measurements of RTT, loss, and other
performance metrics when information is carried in an additional
network-layer packet header (Section 6 of [RFC9065] describes the use
of Operations, Administration, and Management (OAM) information).
Using network-layer approaches also has the advantage that common
observation and analysis tools can be consistently used for multiple
transport protocols; however, these techniques are often limited to
measurements within one or multiple cooperating domains.

4.2. Stateful Treatment of QUIC Traffic

Stateful treatment of QUIC traffic (e.g., at a firewall or NAT
middlebox) is possible through QUIC traffic and version
identification (Section 3.1) and observation of the handshake for
connection confirmation (Section 3.2). The lack of any visible end-
of-flow signal (Section 3.6) means that this state must be purged
either through timers or least-recently-used eviction depending on
application requirements.

While QUIC has no clear network-visible end-of-flow signal and
therefore does require timer-based state removal, the QUIC handshake
indicates confirmation by both ends of a valid bidirectional
transmission. As soon as the handshake completed, timers should be
set long enough to also allow for short idle time during a valid
transmission.

[RFC4787] requires a network state timeout that is not less than 2
minutes for most UDP traffic. However, in practice, a QUIC endpoint
can experience lower timeouts in the range of 30 to 60 seconds
[QUIC-TIMEOUT].

In contrast, [RFC5382] recommends a state timeout of more than 2
hours for TCP given that TCP is a connection-oriented protocol with
well-defined closure semantics. Even though QUIC has explicitly been
designed to tolerate NAT rebindings, decreasing the NAT timeout is
not recommended as it may negatively impact application performance
or incentivize endpoints to send very frequent keep-alive packets.

Therefore, a state timeout of at least two minutes is recommended for
QUIC traffic, even when lower state timeouts are used for other UDP
traffic.

If state is removed too early, this could lead to black-holing of
incoming packets after a short idle period. To detect this
situation, a timer at the client needs to expire before a re-
establishment can happen (if at all), which would lead to
unnecessarily long delays in an otherwise working connection.

Furthermore, not all endpoints use routing architectures where
connections will survive a port or address change. Even when the
client revives the connection, a NAT rebinding can cause a routing
mismatch where a packet is not even delivered to the server that
might support address migration. For these reasons, the limits in
[RFC4787] are important to avoid black-holing of packets (and hence
avoid interrupting the flow of data to the client), especially where
devices are able to distinguish QUIC traffic from other UDP payloads.

The QUIC header optionally contains a connection ID, which could
provide additional entropy beyond the 5-tuple. The QUIC handshake
needs to be observed in order to understand whether the connection ID
is present and what length it has. However, connection IDs may be
renegotiated after the handshake, and this renegotiation is not
visible to the path. Therefore, using the connection ID as a flow
key field for stateful treatment of flows is not recommended as
connection ID changes will cause undetectable and unrecoverable loss
of state in the middle of a connection. In particular, the use of
the connection ID for functions that require state to make a
forwarding decision is not viable as it will break connectivity, or
at minimum, cause long timeout-based delays before this problem is
detected by the endpoints and the connection can potentially be re-
established.

Use of connection IDs is specifically discouraged for NAT
applications. If a NAT hits an operational limit, it is recommended
to rather drop the initial packets of a flow (see also Section 4.5),
which potentially triggers TCP fallback. Use of the connection ID to
multiplex multiple connections on the same IP address/port pair is
not a viable solution as it risks connectivity breakage in case the
connection ID changes.

4.3. Address Rewriting to Ensure Routing Stability

While QUIC's migration capability makes it possible for a connection
to survive client address changes, this does not work if the routers
or switches in the server infrastructure route using the address-port
4-tuple. If infrastructure routes on addresses only, NAT rebinding
or address migration will cause packets to be delivered to the wrong
server. [QUIC-LB] describes a way to addresses this problem by
coordinating the selection and use of connection IDs between load
balancers and servers.

Applying address translation at a middlebox to maintain a stable
address-port mapping for flows based on connection ID might seem like
a solution to this problem. However, hiding information about the
change of the IP address or port conceals important and security-
relevant information from QUIC endpoints, and as such, would
facilitate amplification attacks (see Section 8 of [QUIC-TRANSPORT]).
A NAT function that hides peer address changes prevents the other end
from detecting and mitigating attacks as the endpoint cannot verify
connectivity to the new address using QUIC PATH_CHALLENGE and
PATH_RESPONSE frames.

In addition, a change of IP address or port is also an input signal
to other internal mechanisms in QUIC. When a path change is
detected, path-dependent variables like congestion control parameters
will be reset, which protects the new path from overload.

4.4. Server Cooperation with Load Balancers

In the case of networking architectures that include load balancers,
the connection ID can be used as a way for the server to signal
information about the desired treatment of a flow to the load
balancers. Guidance on assigning connection IDs is given in
[QUIC-APPLICABILITY]. [QUIC-LB] describes a system for coordinating
selection and use of connection IDs between load balancers and
servers.

4.5. Filtering Behavior

[RFC4787] describes possible packet-filtering behaviors that relate
to NATs but are often also used in other scenarios where packet
filtering is desired. Though the guidance there holds, a
particularly unwise behavior admits a handful of UDP packets and then
makes a decision to whether or not filter later packets in the same
connection. QUIC applications are encouraged to fall back to TCP if
early packets do not arrive at their destination
[QUIC-APPLICABILITY], as QUIC is based on UDP and there are known
blocks of UDP traffic (see Section 4.6). Admitting a few packets
allows the QUIC endpoint to determine that the path accepts QUIC.
Sudden drops afterwards will result in slow and costly timeouts
before abandoning the connection.

4.6. UDP Blocking, Throttling, and NAT Binding

Today, UDP is the most prevalent DDoS vector, since it is easy for
compromised non-admin applications to send a flood of large UDP
packets (while with TCP the attacker gets throttled by the congestion
controller) or to craft reflection and amplification attacks;
therefore, some networks block UDP traffic. With increased
deployment of QUIC, there is also an increased need to allow UDP
traffic on ports used for QUIC. However, if UDP is generally enabled
on these ports, UDP flood attacks may also use the same ports. One
possible response to this threat is to throttle UDP traffic on the
network, allocating a fixed portion of the network capacity to UDP
and blocking UDP datagrams over that cap. As the portion of QUIC
traffic compared to TCP is also expected to increase over time, using
such a limit is not recommended; if this is done, limits might need
to be adapted dynamically.

Further, if UDP traffic is desired to be throttled, it is recommended
to block individual QUIC flows entirely rather than dropping packets
indiscriminately. When the handshake is blocked, QUIC-capable
applications may fall back to TCP. However, blocking a random
fraction of QUIC packets across 4-tuples will allow many QUIC
handshakes to complete, preventing TCP fallback, but these
connections will suffer from severe packet loss (see also
Section 4.5). Therefore, UDP throttling should be realized by per-
flow policing as opposed to per-packet policing. Note that this per-
flow policing should be stateless to avoid problems with stateful
treatment of QUIC flows (see Section 4.2), for example, blocking a
portion of the space of values of a hash function over the addresses
and ports in the UDP datagram. While QUIC endpoints are often able
to survive address changes, e.g., by NAT rebindings, blocking a
portion of the traffic based on 5-tuple hashing increases the risk of
black-holing an active connection when the address changes.

Note that some source ports are assumed to be reflection attack
vectors by some servers; see Section 8.1 of [QUIC-APPLICABILITY]. As
a result, NAT binding to these source ports can result in that
traffic being blocked.

4.7. DDoS Detection and Mitigation

On-path observation of the transport headers of packets can be used
for various security functions. For example, Denial of Service (DoS)
and Distributed DoS (DDoS) attacks against the infrastructure or
against an endpoint can be detected and mitigated by characterizing
anomalous traffic. Other uses include support for security audits
(e.g., verifying the compliance with cipher suites), client and
application fingerprinting for inventory, and providing alerts for
network intrusion detection and other next-generation firewall
functions.

Current practices in detection and mitigation of DDoS attacks
generally involve classification of incoming traffic (as packets,
flows, or some other aggregate) into "good" (productive) and "bad"
(DDoS) traffic, and then differential treatment of this traffic to
forward only good traffic. This operation is often done in a
separate specialized mitigation environment through which all traffic
is filtered; a generalized architecture for separation of concerns in
mitigation is given in [DOTS-ARCH].

Efficient classification of this DDoS traffic in the mitigation
environment is key to the success of this approach. Limited first
packet garbage detection as in Section 3.1.2 and stateful tracking of
QUIC traffic as mentioned in Section 4.2 above may be useful during
classification.

Note that using a connection ID to support connection migration
renders 5-tuple-based filtering insufficient to detect active flows
and requires more state to be maintained by DDoS defense systems if
support of migration of QUIC flows is desired. For the common case
of NAT rebinding, where the client's address changes without the
client's intent or knowledge, DDoS defense systems can detect a
change in the client's endpoint address by linking flows based on the
server's connection IDs. However, QUIC's linkability resistance
ensures that a deliberate connection migration is accompanied by a
change in the connection ID. In this case, the connection ID cannot
be used to distinguish valid, active traffic from new attack traffic.

It is also possible for endpoints to directly support security
functions such as DoS classification and mitigation. Endpoints can
cooperate with an in-network device directly by e.g., sharing
information about connection IDs.

Another potential method could use an on-path network device that
relies on pattern inferences in the traffic and heuristics or machine
learning instead of processing observed header information.

However, it is questionable whether connection migrations must be
supported during a DDoS attack. While unintended migration without a
connection ID change can be supported much easier, it might be
acceptable to not support migrations of active QUIC connections that
are not visible to the network functions performing the DDoS
detection. As soon as the connection blocking is detected by the
client, the client may be able to rely on the 0-RTT data mechanism
provided by QUIC. When clients migrate to a new path, they should be
prepared for the migration to fail and attempt to reconnect quickly.

Beyond in-network DDoS protection mechanisms, TCP SYN cookies
[RFC4987] are a well-established method of mitigating some kinds of
TCP DDoS attacks. QUIC Retry packets are the functional analogue to
SYN cookies, forcing clients to prove possession of their IP address
before committing server state. However, there are safeguards in
QUIC against unsolicited injection of these packets by intermediaries
who do not have consent of the end server. See [QUIC-RETRY] for
standard ways for intermediaries to send Retry packets on behalf of
consenting servers.

4.8. Quality of Service Handling and ECMP Routing

It is expected that any QoS handling in the network, e.g., based on
use of Diffserv Code Points (DSCPs) [RFC2475] as well as Equal-Cost
Multi-Path (ECMP) routing, is applied on a per-flow basis (and not
per-packet) and as such that all packets belonging to the same active
QUIC connection get uniform treatment.

Using ECMP to distribute packets from a single flow across multiple
network paths or any other nonuniform treatment of packets belong to
the same connection could result in variations in order, delivery
rate, and drop rate. As feedback about loss or delay of each packet
is used as input to the congestion controller, these variations could
adversely affect performance. Depending on the loss recovery
mechanism that is implemented, QUIC may be more tolerant of packet
reordering than typical TCP traffic (see Section 2.7). However, the
recovery mechanism used by a flow cannot be known by the network and
therefore reordering tolerance should be considered as unknown.

Note that the 5-tuple of a QUIC connection can change due to
migration. In this case different flows are observed by the path and
may be treated differently, as congestion control is usually reset on
migration (see also Section 3.5).

4.9. Handling ICMP Messages

Datagram Packetization Layer PMTU Discovery (DPLPMTUD) can be used by
QUIC to probe for the supported PMTU. DPLPMTUD optionally uses ICMP
messages (e.g., IPv6 Packet Too Big (PTB) messages). Given known
attacks with the use of ICMP messages, the use of DPLPMTUD in QUIC
has been designed to safely use but not rely on receiving ICMP
feedback (see Section 14.2.1 of [QUIC-TRANSPORT]).

Networks are recommended to forward these ICMP messages and retain as
much of the original packet as possible without exceeding the minimum
MTU for the IP version when generating ICMP messages as recommended
in [RFC1812] and [RFC4443].

4.10. Guiding Path MTU

Some network segments support 1500-byte packets, but can only do so
by fragmenting at a lower layer before traversing a network segment
with a smaller MTU, and then reassembling within the network segment.
This is permissible even when the IP layer is IPv6 or IPv4 with the
Don't Fragment (DF) bit set, because fragmentation occurs below the
IP layer. However, this process can add to compute and memory costs,
leading to a bottleneck that limits network capacity. In such
networks, this generates a desire to influence a majority of senders
to use smaller packets to avoid exceeding limited reassembly
capacity.

For TCP, Maximum Segment Size (MSS) clamping (Section 3.2 of
[RFC4459]) is often used to change the sender's TCP maximum segment
size, but QUIC requires a different approach. Section 14 of
[QUIC-TRANSPORT] advises senders to probe larger sizes using DPLPMTUD
[DPLPMTUD] or Path Maximum Transmission Unit Discovery (PMTUD)
[RFC1191] [RFC8201]. This mechanism encourages senders to approach
the maximum packet size, which could then cause fragmentation within
a network segment of which they may not be aware.

If path performance is limited when forwarding larger packets, an on-
path device should support a maximum packet size for a specific
transport flow and then consistently drop all packets that exceed the
configured size when the inner IPv4 packet has DF set or IPv6 is
used.

Networks with configurations that would lead to fragmentation of
large packets within a network segment should drop such packets
rather than fragmenting them. Network operators who plan to
implement a more selective policy may start by focusing on QUIC.

QUIC flows cannot always be easily distinguished from other UDP
traffic, but we assume at least some portion of QUIC traffic can be
identified (see Section 3.1). For networks supporting QUIC, it is
recommended that a path drops any packet larger than the
fragmentation size. When a QUIC endpoint uses DPLPMTUD, it will use
a QUIC probe packet to discover the PMTU. If this probe is lost, it
will not impact the flow of QUIC data.

IPv4 routers generate an ICMP message when a packet is dropped
because the link MTU was exceeded. [RFC8504] specifies how an IPv6
node generates an ICMPv6 PTB in this case. PMTUD relies upon an
endpoint receiving such PTB messages [RFC8201], whereas DPLPMTUD does
not reply upon these messages, but can still optionally use these to
improve performance Section 4.6 of [DPLPMTUD].

A network cannot know in advance which discovery method is used by a
QUIC endpoint, so it should send a PTB message in addition to
dropping an oversized packet. A generated PTB message should be
compliant with the validation requirements of Section 14.2.1 of
[QUIC-TRANSPORT], otherwise it will be ignored for PMTU discovery.
This provides a signal to the endpoint to prevent the packet size
from growing too large, which can entirely avoid network segment
fragmentation for that flow.

Endpoints can cache PMTU information in the IP-layer cache. This
short-term consistency between the PMTU for flows can help avoid an
endpoint using a PMTU that is inefficient. The IP cache can also
influence the PMTU value of other IP flows that use the same path
[RFC8201] [DPLPMTUD], including IP packets carrying protocols other
than QUIC. The representation of an IP path is implementation
specific [RFC8201].

5. IANA Considerations

This document has no actions for IANA.

6. Security Considerations

QUIC is an encrypted and authenticated transport. That means once
the cryptographic handshake is complete, QUIC endpoints discard most
packets that are not authenticated, greatly limiting the ability of
an attacker to interfere with existing connections.

However, some information is still observable as supporting
manageability of QUIC traffic inherently involves trade-offs with the
confidentiality of QUIC's control information; this entire document
is therefore security-relevant.

More security considerations for QUIC are discussed in
[QUIC-TRANSPORT] and [QUIC-TLS], which generally consider active or
passive attackers in the network as well as attacks on specific QUIC
mechanism.

Version Negotiation packets do not contain any mechanism to prevent
version downgrade attacks. However, future versions of QUIC that use
Version Negotiation packets are required to define a mechanism that
is robust against version downgrade attacks. Therefore, a network
node should not attempt to impact version selection, as version
downgrade may result in connection failure.

7. References

7.1. Normative References

[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.

[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

7.2. Informative References

[DOTS-ARCH]
Mortensen, A., Ed., Reddy.K, T., Ed., Andreasen, F.,
Teague, N., and R. Compton, "DDoS Open Threat Signaling
(DOTS) Architecture", RFC 8811, DOI 10.17487/RFC8811,
August 2020, <https://www.rfc-editor.org/info/rfc8811>.

[DPLPMTUD] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.

[IPIM] Allman, M., Beverly, R., and B. Trammell, "Principles for
Measurability in Protocol Design", 9 December 2016,
<https://arxiv.org/abs/1612.02902>.

[QUIC-APPLICABILITY]
Kühlewind, M. and B. Trammell, "Applicability of the QUIC
Transport Protocol", RFC 9308, DOI 10.17487/RFC9308,
September 2022, <https://www.rfc-editor.org/info/rfc9308>.

[QUIC-GREASE]
Thomson, M., "Greasing the QUIC Bit", RFC 9287,
DOI 10.17487/RFC9287, August 2022,
<https://www.rfc-editor.org/info/rfc9287>.

[QUIC-HTTP]
Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/info/rfc9114>.

[QUIC-INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
RFC 8999, DOI 10.17487/RFC8999, May 2021,
<https://www.rfc-editor.org/info/rfc8999>.

[QUIC-LB] Duke, M., Banks, N., and C. Huitema, "QUIC-LB: Generating
Routable QUIC Connection IDs", Work in Progress, Internet-
Draft, draft-ietf-quic-load-balancers-14, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
load-balancers-14>.

[QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/info/rfc9002>.

[QUIC-RETRY]
Duke, M. and N. Banks, "QUIC Retry Offload", Work in
Progress, Internet-Draft, draft-ietf-quic-retry-offload-
00, 25 May 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-quic-retry-offload-00>.

[QUIC-TIMEOUT]
Roskind, J., "QUIC", IETF-88 TSV Area Presentation, 7
November 2013,
<https://www.ietf.org/proceedings/88/slides/slides-88-
tsvarea-10.pdf>.

[QUIC-VERSION-NEGOTIATION]
Schinazi, D. and E. Rescorla, "Compatible Version
Negotiation for QUIC", Work in Progress, Internet-Draft,
draft-ietf-quic-version-negotiation-10, 27 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
version-negotiation-10>.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.

[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC3449] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.

[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
2006, <https://www.rfc-editor.org/info/rfc4459>.

[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <https://www.rfc-editor.org/info/rfc4787>.

[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.

[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<https://www.rfc-editor.org/info/rfc5382>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[RFC7605] Touch, J., "Recommendations on Using Assigned Transport
Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
August 2015, <https://www.rfc-editor.org/info/rfc7605>.

[RFC7801] Dolmatov, V., Ed., "GOST R 34.12-2015: Block Cipher
"Kuznyechik"", RFC 7801, DOI 10.17487/RFC7801, March 2016,
<https://www.rfc-editor.org/info/rfc7801>.

[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.

[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
RFC 7983, DOI 10.17487/RFC7983, September 2016,
<https://www.rfc-editor.org/info/rfc7983>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.

[RFC8504] Chown, T., Loughney, J., and T. Winters, "IPv6 Node
Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
January 2019, <https://www.rfc-editor.org/info/rfc8504>.

[RFC9065] Fairhurst, G. and C. Perkins, "Considerations around
Transport Header Confidentiality, Network Operations, and
the Evolution of Internet Transport Protocols", RFC 9065,
DOI 10.17487/RFC9065, July 2021,
<https://www.rfc-editor.org/info/rfc9065>.

[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/info/rfc9250>.

[TLS-ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-14, 13 February 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-14>.

[TMA-QOF] Trammell, B., Gugelmann, D., and N. Brownlee, "Inline Data
Integrity Signals for Passive Measurement", Traffic
Measurement and Analysis, TMA 2014, Lecture Notes in
Computer Science, vol. 8406, pp. 15-25,
DOI 10.1007/978-3-642-54999-1_2, April 2014,
<https://link.springer.com/
chapter/10.1007/978-3-642-54999-1_2>.

[WIRE-IMAGE]
Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.

Acknowledgments

Special thanks to last call reviewers Elwyn Davies, Barry Leiba, Al
Morton, and Peter Saint-Andre.

This work was 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.

Contributors

The following people have contributed significant text to and/or
feedback on this document:

Chris Box

Dan Druta

David Schinazi

Gorry Fairhurst

Ian Swett

Igor Lubashev

Jana Iyengar

Jared Mauch

Lars Eggert

Lucas Purdue

Marcus Ihlar

Mark Nottingham

Martin Duke

Martin Thomson

Matt Joras

Mike Bishop

Nick Banks

Thomas Fossati

Sean Turner

Authors' Addresses

Mirja Kühlewind
Ericsson
Email: [email protected]

Brian Trammell
Google Switzerland GmbH
Gustav-Gull-Platz 1
CH-8004 Zurich
Switzerland
Email: [email protected]