QUIC [25] is the latest transport protocol standardized by the IETF. There are several characteristics that distinguish QUIC from other transport protocols:

  • QUIC supports multiple streams like SCTP

  • QUIC includes various reliability and congestion control mechanisms like TCP

  • QUIC directly integrates the security features of TLS 1.3 [26] instead of simply layering TLS above the transport layer

  • QUIC can also be used to carry datagrams [27]

  • QUIC runs above UDP in contrast with other transport protocols such as TCP, SCTP or DCCP that run above IP

From a pure layering viewpoint, QUIC can be illustrated as shown in Fig. 16.

Figure made with TikZ

Fig. 16 QUIC in the TCP/IP stack

It is unusual to layer a transport protocol above another one. QUIC opted for this solution for two main reasons. First, on most operating systems, any application can directly use UDP without requiring special privileges. This implies that QUIC can be implemented as a library which can be included directly inside applications. This contrasts with TCP, SCTP or DCCP whose implementations are either part of the operating system kernel or need special privileges to send raw packets. The second motivation is that UDP (as well as TCP and ICMP) is supported by most middleboxes while many of them block transport protocols that were not defined when they were designed [7].

It is useful to note that by using UDP, QUIC slightly increases its overhead. Each QUIC packet carries 8 bytes of headers containing the source and destination ports, the length field and a checksum. UDP also comes with a performance penalty compared to TCP. During the last decades, operating system kernels have been optimized to provide high bandwidth using TCP with techniques such as TCP Segmentation Offload (TSO) and Generic Receive Offload (GRO). In parallel, the UDP implementation has not changed significantly and most of the optimizations focused on request-response services such as DNS servers. The situation has changed recently with some effort, notably on Linux, to improve UDP’s raw throughput [28].

Frames and packets

There is an important difference between QUIC and classical protocols like TCP or UDP. TCP and UDP send segments that are composed of a header followed by a sequence of bytes that constitutes the payload. In contrast and like SCTP, a QUIC packet contains a header followed by one or more frames. The QUIC header is much simpler and shorter than the TCP header. It only carries the information which is required in all QUIC packets. We will describe later the short and long QUIC headers. Each QUIC header starts with one byte that contains some flags and a 32 bits version field. QUIC defines different types of frames that we will discuss in this chapter. Some types of QUIC frames carry user data. Other types of QUIC frames carry control information. Some of these frames are used during the handshake only while others such as acknowledgments can be sent at any time. Each QUIC frame is a sequence of byte that starts with a one byte Type field. Fig. 17 shows a QUIC packet containing two frames which is placed inside a UDP datagram. Neither the QUIC header nor the QUIC frames need to be aligned on 32 bits boundaries even if this alignment is convenient for :numref:`fig-quic-packet. An important point to note is that a QUIC packet can mix both data and control frames in any order.

Figure made with TikZ

Fig. 17 A QUIC packet is sent inside a UDP datagram and contains one or more QUIC frames

Connection establishment

QUIC uses a four-way handshake to create a QUIC connection. Fig. 18 describes this handshake. The QUIC handshake has three main purposes. First, it allows to negotiate the crypto keys required to both encrypt and authenticate the packets that will be sent later on the connection. This is mainly done using TLS 1.3 [26]. Second, it allows to negotiate different options using transport parameters. Third, the server can validate that the client can respond to the address used to send the initial packet. This validation allows to prevent denial of service attacks using spoofed addresses.

Figure made with TikZ

Fig. 18 Simplified QUIC Handshake

The client sends an Initial packet containing a CRYPTO frame. This packet carries the TLS Client Hello and the transport parameters proposed by the client for this connection. The server replies with an Initial packet containing also a CRYPTO frame. This one contains the TLS Server Hello. It is immediately followed by one or more Handshake packets containing also a CRYPTO frame with the TLS Encrypted Extensions. The contents of this frame is encrypted using the session key derived from the information contained in the TLS Client Hello and the TLS Server Hello. It mainly contains the certificate and the transport parameters of the server. This frame can be spread over several QUIC packets. The client replies with a Handshake packet that contains a CRYPTO frame with the TLS Finished message. The server later confirms the end of the TLS handshake by sending a Handshake_Done frame.

Before looking at the details of the negotiation of the cryptographic parameters, it is interesting to see how QUIC counters denial of service attacks that use spoofed addresses. During such attack, host x sends packets using the address of host y as their source. The main risk of such attacks is that the server could send a large number of packets towards address y although the host owning this address did not try to establish a QUIC connection with the server. QUIC prevents such attacks by using two distinct techniques. First, and this is unusual for transport protocols, the Initial QUIC packet sent by the client is large. The first packet sent to create a QUIC connection must contain a UDP payload of at least 1200 bytes [25]. Such a packet contains a CRYPTO frame has shown in the figure, but also padding frames to fill the packet. If an attacker wants to send spoofed packets to initiate a connection with a server, it needs to send more than one KByte for each connection attempt. This should be compared with the 40 bytes of the TCP and IPv4 headers that must be sent to initiate a TCP connection. Another advantage of sending a large initial packet, the client can also perform Path MTU discovery and detect routers that could fragment the QUIC packets.


Address spoofing

In theory, an Internet host should only send packets using its IPv4 and IPv6 source addresses. In practice, incorrectly configured hosts can use other addresses than their assigned one. Furthermore, attackers often change their source address to hide some of their activities. A frequent situation are denial of service (DoS) attacks. A simple DoS attack is when a host sends a large volume of packets to a victim. If the attacker sends these packets using another address than its official IP address, it makes it more difficult for the victim to identify the source of the attack.

But there is another category of DoS attack that is more worrisome. If an attacker can send a packets using the source address of the victim to a server, the server would return a response to the victim. These attackers use Internet servers that send a large response, possibly using multiple packets to a single request packet. They have exploited protocols such as DNS, NTP or applications such as memcached . The main problem with such attacks is that the server amplifies the volume of the attack generated by the clients. As there are very powerful servers on the Internet, this can be a huge problem and such attacks have reached volumes of hundreds of Gbps. The IETF and network operators have published recommendations to configure access networks to block spoofed packets [29]. Unfortunately, there are still portions of the Internet where attackers can send spoofed packets [30].

The second mitigation against denial-of-service attacks using spoofed packets operates on the server. When a server receives an initial packet from a client, it may respond with an initial packet as shown in Fig. 18. This could for example be the case of an enterprise server that receives a request from a host of a known enterprise subnet. The server could also want to validate the client and verify that the client can receive the packets that it sends. For this, it returns a Retry frame and a Token. This token is an opaque field that is constructed in a way that makes it easy for the server to validate the subsequent client packets and difficult for the client to predict the token that a server will create. A possible approach is to compute a secure hash of a message that contains the source IP addresses and ports used by the client, a secret value only known by the server and possibly some timing information to enable the server to ignore older tokens. Faced with the same problem, TCP syn cookies are encoded using fewer bits and thus inherently less secure. Fig. 19 shows a QUIC handshake that includes a validation of the client address.

Figure made with TikZ

Fig. 19 Simplified QUIC Handshake with address validation

Address validation improves the resilience of servers against denial of service attacks using spoofed addresses, but this comes at the expense of a longer connection establishment delay. QUIC version 1 includes several additional techniques to reduce the impact of address validation while still preventing denial of service attacks.

Identifying QUIC connections

A TCP connection is identified by a four tuple \(IP_{Client},IP_{Server},Port_{Client},Port_{Server}\). All packets belonging to a given connection carry this information in their IP and TCP headers. As QUIC runs above UDP, a simple approach could have been to use the UDP four tuple to identify each QUIC connection. Although simple, this solution would have severely restricted the flexibility of QUIC and the scalability of QUIC servers. The QUIC designers have opted for locally unique connection identifiers (CID) that are selected by the client and the server. These identifiers are placed in the QUIC packet headers during the handshake. When sending the Initial packet, the client selects a source CID that uniquely identifies the connection on the client and a random destination CID. Upon reception of this packet, the server selects its own connection identifier. It echoes the client selected CID and returns its selected CID. This is illustrated in Fig. 20.

Figure made with TikZ

Fig. 20 Connection identifiers during a simplified QUIC Handshake

The connection identifiers selected by the client and the server uniquely identify the QUIC connection. However, in contrast with TCP and UDP, the two identifiers are not present in all packets. Since a host selects a unique identifier for each connection, it only needs this identifier to identify a connection to which a packet belongs. For this reason, the QUIC packets exchanged on a connection after the handshake only contain the destination CID, i.e. the CID selected by the host that receives the packet.


Variable length CIDs

Most transport protocols rely on fixed-length fields because this simplifies the parsing of packet headers. For example, the TCP and UDP port numbers are encoded as a 16 bits field. However, using fixed-length fields also limits the extensibility of the protocol. A TCP server cannot listen to more than \(2^{16}\) different ports.

QUIC has opted for variable length CIDs to support very different use cases. On the server side, the length of the selected connection identifiers will depend on the architecture of the server. Large sites might use a load-balancer that distributes the connections to different physical servers. Such a load-balancer can leverage the CID to direct each incoming packet to the server that handles this connection. A simple CID would be composed of a server identifier chosen by the load balancer, e.g. in the high order bits of the CID, followed by a connection identifier selected by the physical server. Other designs are possible, e.g. by encrypting the CID to prevent attacks where malicious clients try to target a specific server.

One the client side, variable lengths CIDs bring another benefit. As clients typically manage a small number of QUIC connections, they can simply rely on the destination port of the packets that they receive to identify the corresponding QUIC connection. This corresponds to a zero-length connection identifier. Such a CID is never sent by the server after the handshake. This limits the byte overhead of the packets that clients receive.

A last point to note about these CIDs is their encoding inside the QUIC packets. The Initial packet contains the length and the value of both connection identifiers. The maximum length for a CID is 20 bytes. However, after the handshake, the packets that are exchanged over the QUIC connection only contain the destination CID without any field indicating its length. The host that has allocated the CID knows the length of the CIDs that it uses and can thus parse the packets that it receives without an explicit length information.

Security keys

We can now discuss how QUIC leverages TLS 1.3 to negotiate the security keys that are used to authenticate and encrypt the packets exchanged over a connection. As shown in Fig. 18, a QUIC connection starts with the exchange of four frames which can be carried in four or more packets. The first packet sent by the client contains the ClientHello TLS record. The ClientHello contains the information required to derive the session keys using Diffie-Hellman or a similar protocol. TLS 1.3 supports both finite field Diffie-Hellman and Elliptic Curve Diffie-Hellman [26]. The ClientHello message also contains TLS or QUIC parameters that the client proposes to use during the connection. The TLS Server Hello returned by the server contains the certificate that enables the client to validate the server’s identity and the information required to determine the Diffie-Hellman keys. Using these keys, the server also encrypts the TLS Encrypted Extensions message that contains the TLS and QUIC parameters that the server has selected based on the ones proposed in the ClientHello. The server also constructs the Finished message that contains a message authentication code computed over the entire TLS handshake. This message is encrypted and authenticated using the session keys derived from the Diffie-Hellman keys. The client and the server recompute the hash of the entire handshake and verify both Finished messages. If one of these messages is incorrect, this indicates that either the key has not been correctly derived or that some of the TLS messages have been tampered. In these situations, the QUIC connection is terminated with an error message. The simplified TLS handshake used by QUIC is illustrated in Fig. 21. The TLS messages shown in italics are encrypted using the session keys.

Figure made with TikZ

Fig. 21 Simplified TLS Handshake within a QUIC connection


Encrypting TLS Client Hello and TLS Server Hello

When TLS 1.3 is used above TCP, the TLS Client Hello and TLS Server Hello messages are sent in clear because the client and the server have not yet exchanged the session keys. A similar approach could have been used for QUIC, but there was a fear that middleboxes could analyze the contents of these initial QUIC messages and try to interfere with them. To add some burden on these middleboxes, QUIC encrypts the Initial packets using a secret that is derived from the destination connection ID of the client’s first Initial packet. The pseudocode below, extracted from [31], shows how the client and the server keys are derived:

initial_salt = 0x38762cf7f55934b34d179ae6a4c80cadccbb7f0a
initial_secret = HKDF-Extract(initial_salt,

client_initial_secret = HKDF-Expand-Label(initial_secret,
             "client in", "", Hash.length)
server_initial_secret = HKDF-Expand-Label(initial_secret,
                           "server in", "",Hash.length)

Since the keys used to protect the Initial packets are derived from the destination connection ID, any QUIC implementation, including those used on middleboxes, can decrypt the contents of the Initial packets.

As mentioned earlier, the client and the server can also use the TLS handshake to agree on specific QUIC parameters. These parameters are called transport parameters in QUIC [25]. 17 different transport parameters are defined in QUIC version 1 [25] and implementations can define their own transport parameters. We will discuss some of them in different sections of this document. A first example is the max_udp_payload_size parameter that indicates the largest UDP payload that an implementation is willing to receive. The minimum value for this parameter is 1200 bytes. QUIC implementations used in a datacenter supporting jumbo Ethernet frames could agree on a much larger max_udp_payload_size without risking packet fragmentation.

Another example of QUIC transport parameters are the initial_source_connection_id and the original_destination_connection_id transport parameters. As explained above, thanks to the Finished messages in the TLS handshake, the client and the servers can verify that their messages have not been tampered. Unfortunately, the authentication code included in the Finished messages is only computed based on the contents of the TLS messages (i.e. ClientHello, ServerHello, EncryptedExtensions and Finished). During the handshake, the client and the servers also propose the source and destination connection identifiers that they plan to use to identify the QUIC session. These identifiers are placed in the packet header and not inside the TLS messages. They are thus not covered by the authentication code included in the Finished message. To still authenticate these identifiers, QUIC encodes them as transport parameters that are included in the ClientHello and the EncryptedExtensions messages. The client copies the source connection identifier in the initial_source_connection_id transport parameter in its ClientHello. The server does the same when sending the ServerHello. It also copies in the original_destination_connection_id transport parameter the destination identifier used by the client to send the packet containing the ClientHello.

The QUIC packet headers

At this point, the QUIC session and the TLS security keys are known by the client and the server. They can start to exchange data. Before looking at how data is carried inside QUIC packets, it is interesting to explore in more details the packet headers that are placed inside each packet. QUIC uses variable length packet headers. Two different header formats exist. The first header format is the long header. This is the header used for the first packets of a QUIC connection.

Internet protocol specifications usually contain figures to represent the format of the packet headers. This graphical format is useful to get a quick grasp at a header containing fixed size fields. However, when a header contains several variable length fields, the graphical representation can become difficult to follow. The QUIC specification [25] uses the textual representation that was also used for the TLS protocol. As an example, let us consider the well-known TCP header. This header is graphically represented as shown in Fig. 22.

Figure made with TikZ

Fig. 22 Graphical representation of the TCP header

Listing 3 Textual representation of the TCP header
TCP Header Packet {
  Source Port (16),
  Destination Port (16),
  Sequence Number (32),
  Acknowledgment Number (32),
  Offset (4),
  Reserved (6),
  Urgent Flag (1),
  ACK Flag (1),
  Push Flag (1),
  RST Flag (1),
  SYN Flag (1),
  FIN Flag(1),
  Window (16),
  TCP Checksum (16),
  Urgent Pointer (16),
  TCP Options (..)

The attentive reader will easily understand the correspondence between the two formats. When explaining QUIC, we use the textual representation while we stick to the graphical one for TCP.

Listing 4 shows the QUIC long header. It starts with one byte containing the header type in the most significant bit, two bits indicating the packet type and four bits that are specific to each packet packet. Then, 32 bits carry the QUIC version number. The current version of QUIC, defined in [25], corresponds to version 0x00000001. The header then contains the destination and source connection identifiers that were described previously and then a payload that is specific to each type.

Listing 4 The QUIC long header
Long Header Packet {
  Header Form (1) = 1,                  /* high order bit of the first byte */
  Fixed Bit (1) = 1,                    /* second order bit of the first byte */
  Long Packet Type (2),                 /* third and fourth high order bits of the first byte */
  Type-Specific Bits (4),               /* low order four bits of the first byte */
  Version (32),                         /* 32 bits version number */
  Destination Connection ID Length (8), /* 8 bits */
  Destination Connection ID (0..160),   /* variable number from 0 up to 160 bits */
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Type-Specific Payload (..),           /* variable length */


Encoding packet numbers

Most transport protocols use fixed fields to encode packet numbers or byte offsets. The size of this field is always a trade-off. On one hand, a small packet number field limits the per packet overhead. On the other hand, a large packet number space is required to ensure that two packets carrying different data do not use the same packet number. TCP uses a 32 bits sequence number field that indicates the position of the first byte of the payload in the bytestream. This 32 bits field became a concern as bandwidth increased to Gbps and beyond [32].

QUIC takes a different approach to sequence numbers. Each packet contains a per-packet sequence number. This number is encoded as a variable-length integer (varint). Such a varint has a length encoded in the two most significant bits of the first byte. If these bits are set to 00, then the varint is encoded in one byte and can contain values between \(0\) and \(2^{6}-1\). If the two most significant bits are set to 01, the varint can encode values between \(0\) and \(2^{14}-1\) within two bytes. When the two high order bits are set to 11 the varint can encode values between \(0\) and \(2^{62}-1\) within four bytes.

There are other important differences between QUIC and other transport protocols when considering packet numbers. First, a QUIC sender must never reuse the same packet number for two different packets sent over a QUIC connection. If data needs to be retransmitted, it will be resent as a frame inside a new packet. Furthermore, since the largest possible packet number is \(2^{62}-1\), a QUIC sender must close the corresponding connection once it has sent a QUIC packet carrying this packet number. This puts a restriction on the duration of QUIC connections. They cannot last forever in contrast to TCP connections such as those used to support BGP sessions between routers. An application that uses QUIC must be ready to restart a connection from time to time.

This long header is used for the Initial, Handhsake and Retry packets. Some of these packet types add new flags in the first byte and additional information after the connection identifiers. Listing 5 shows the long header of the Initial packet. It contains two bits in the first byte that indicate the length of the packet number field. The packet specific part contains an option token, a length field, a packet number and a payload. The token length, length and packet number are encoded using variable length integers.

Listing 5 The QUIC long header of the Initial packet
Initial Packet {
  Header Form (1) = 1,                   /* High order bit first byte */
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 0,
  Reserved Bits (2),
  Packet Number Length (2),              /* Low order 2 bits of first byte */
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Token Length (i),
  Token (..),
  Length (i),
  Packet Number (8..32),
  Packet Payload (8..),

The QUIC short header contains fewer fields. The most significant bit of the first byte is set to 1 to indicate that the packet carries a short header. The other flags will be discussed later. The two least significant bits of this byte contain the length of the packet number field. It is interesting to note that in contrast with the long header, there is no explicit indication of the length of the destination connection identifier. This connection identifier has been assigned by the host that receives this packet and it already knows the length of the connection identifiers that it uses.

Listing 6 The QUIC short header used by 1-RTT packets
1-RTT Packet {
   Header Form (1) = 0,                /* High order bit of first byte */
   Fixed Bit (1) = 1,
   Spin Bit (1),
   Reserved Bits (2),
   Key Phase (1),
   Packet Number Length (2),           /* Low order bits of first byte */
   Destination Connection ID (0..160),
   Packet Number (8..32),
   Packet Payload (8..),

The short header format, depicted in Listing 6, is used by all QUIC packets once the session keys have been derived. This usually happens after one round-trip-time. These packets are called the 1-RTT packets in the QUIC specification [25]. This notation is used to emphasize the fact that QUIC also supports 0-RTT packets, i.e. packets that carry data and can be exchanged in parallel with the TLS handshake.


Coalescing packets

Besides the connection identifiers, another difference between the short and the long headers is the presence of the Packet Length field in the long header. This field might surprise the reader who is familiar with UDP since UDP also contains a Length field. As each QUIC packet is placed inside a UDP message, the QUIC Length field could seem redundant. This Length field was included in the QUIC long header to allow a QUIC sender to coalesce several consecutive and small packets inside a single UDP message. Some of the frames exchanged during the handshake are rather small. Sending a UDP message for each of these frames would be a waste of resources. Furthermore, a mix of Initial, Handshake and 0-RTT packets are exchanged when creating a QUIC connection. It would not be wise to require the utilization of one UDP message to send each of these packets. You might observe this optimization when analyzing packet traces collected on QUIC servers.

0-RTT data

Latency is a key concern for transport protocols. The QUIC/TLS handshake that we have described until now allows the client and the server to agree on security keys within one round-trip-time. However, one round-trip-time can be a long delay for some applications. To minimize the impact of the connection setup time, QUIC allows applications to exchange data during the QUIC/TLS handshake. Such data is called 0-RTT data. To ensure that 0-RTT data is exchanged securely, the client and the server must have previously agreed on a key which can be used to encrypt and authenticate the 0-RTT data. Such a 0-RTT key could either be a pre-shared key that the client and the server have shared without using the QUIC protocol or, and this is the most frequent solution, the key that they negotiated during a previous connection. In practice, the server enables 0-RTT by sending a TLS session ticket to the client [26]. A session ticket is an encrypted record that contains information that enables the server to recover all the state information about a session including its session keys. It is also linked to the client’s address to enable the server to verify that a given client reuses the tickets that it has received earlier. Usually, these tickets also contain an expiration date. The client places the received ticket in the TLS CLient Hello that it sends in the first Initial packet. It uses the pre-shared key corresponding to this ticket to encrypt data and place it in one or more 0-RTT packets. The server uses the information contained in the ticket to recover the key and decrypt the data of the 0-RTT packet. Listing 7 shows the format of QUIC’s 0-RTT packet.

Listing 7 The QUIC 0-RTT packet
0-RTT Packet {
  Header Form (1) = 1,                  /* High order bit of the first byte */
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 1,
  Reserved Bits (2),
  Packet Number Length (2),            /* Low order bits of the first byte */
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Length (i),
  Packet Number (8..32),
  Packet Payload (8..),

The main benefit of these 0-RTT packets is that the client can immediately send encrypted data while sending its Initial packet. This is illustrated in Fig. 23 where the frames encrypted with the 0-RTT keys are shown in italics. Note that some of these frames can span several packets. 0-RTT packets are only sent by the QUIC client since the server can send encrypted data immediately after having sent its Handshake frames. As explained earlier, the Initial packets are also encrypted but using keys derived from the connection identifiers.

Figure made with TikZ

Fig. 23 Simplified QUIC Handshake with 0-RTT data


Replay attacks and 0-RTT packets

Thanks to the 0-RTT packets, a client can send encrypted data to the server before waiting for the secure handshake. This reduces the latency of the data transfer, but with one important caveat. QUIC does not provide any guarantee that 0-RTT data will not be replayed. QUIC’s 0-RTT data exchanged is intended for idempotent operations. A detailed discussion of the impact of replay attacks may be found in [33].

To understand the importance of these replay attacks, let us consider a simple HTTP GET request. Such a request could easily fit inside the 0-RTT packet and thus have lower latency. If a web browser uses it to request a static index.html file, there is no harm if the request is received twice by the server. However, if the GET request is part of a REST API and has side effects, then problems could occur depending on the type of side effect. Consider a REST API that allows a user to switch off the lights using his or her smartphone. Replaying this request two or three times will always result in the light being switched off. However, if the user requests to increase the room temperature by one °C, then multiple replays will obviously have different consequences.

Closing a QUIC connection

Before exploring how data can be exchanged over a QUIC connection, let us now analyze how a QUIC connection can terminate. QUIC supports three different methods to close a QUIC connection. QUIC’s approach to terminating connection is very different from the approaches used by traditional transport protocol. Before looking at these techniques, it is important to understand how QUIC interacts with Network Address Translation.


QUIC and Network Address Translation

QUIC runs above UDP and the design of QUIC was heavily influenced by the presence of NATs. NATs, like other middleboxes, maintain per-flow state. For TCP connections, many NATs rely on the SYN, FIN and RST flags to determine when state must be created or removed for a TCP connection. For UDP, this stateful approach is not possible and NATs create a new mapping when they observe the first packet of a flow and remove the mapping once the flow has been idle for sometime. The IETF recommends to maintain NAT mappings during at least two minutes [34], but measurements show that some deployed NATs use shorter timeouts [15, 35]. In practice, UDP flows should probably send a packet every 30 seconds to ensure that the on-path NATs preserve their state.

To prevent NATs from changing the mapping associated to the IP addresses and ports used for a QUIC connection, QUIC hosts will need to regularly send data over each established QUIC connection. QUIC defines a PING frame that allows any QUIC endpoint to send a frame that will trigger a response from the other peer. The PING frame does not carry data, but the receiver of a PING frame needs to acknowledge the packet containing this frame. This creates a bidirectional communication and can prevent NATs from discarding the mapping associated to the QUIC connection.

Implicit termination of QUIC connections

Each QUIC implementation keeps in its connection state the timestamp of the last QUIC packet received over this connection. During the connection establishment, the QUIC hosts can also exchange the max_idle_timeout parameter that indicates how long (in seconds) a QUIC connection can remain idle before being automatically closed. The first way to close a QUIC connection is to keep it idle for this period of time.

Explicit termination of a QUIC connection

The second technique to terminate a QUIC connection is to use the CONNECTION_CLOSE frame. This frame indicates that this connection has been closed abruptly. The host that receives the CONNECTION_CLOSE may respond with one CONNECTION_CLOSE frame. After that, it must stop sending any additional frame. It keeps the connection state for some time, but does not accept any new packet nor retransmit others. The host that sends a CONNECTION_CLOSE frame indicates that it will neither send nor accept more data. It keeps a subset of the QUIC connection state to be able to retransmit the CONNECTION_CLOSE frame if needed.

A host also sends a CONNECTION_CLOSE frame to abruptly terminate a connection if it receives an invalid frame or detects a protocol error. In this case, the CONNECTION_CLOSE frame contains a variable length integer that indicates the reason for the termination, the type of the frame that triggered the error and additional information encoded as a text string.

Figure made with TikZ

Fig. 24 A server that refuses a connection

The QUIC specification also defines a third technique called stateless reset to cope with hosts that have lost connection state after a crash or outage. It relies on a 16 bytes stateless token announced together with the connection identifier. See [25] for all the details.

Exchanging data over a QUIC connection

The data exchanged over is QUIC connection is organized in different streams. A stream is a sequence of bytes. QUIC supports both unidirectional and bidirectional streams. Both the client and the server can create new streams over a QUIC connection. Each stream is identified by a stream identifier. To prevent problems when the client and the server try to create a stream using the same identifier, QUIC restricts the utilization of the stream identifiers based its two low-order bits. A QUIC client can only create streams whose two low order bits are set to 00 (bidirectional stream) or 10 (unidirectional stream). Similarly, the low order bits of the server-initiated streams must be set to 01 (bidirectional stream) or 11 (unidirectional streams). The QUIC streams are created implicitly by sending data over the stream with the chosen identifier. The stream identifiers start at the minimum value, i.e. 0x00 to 0x03 for the respective types. If a host sends stream data for stream x before having sent data over the lower-numbered streams of that type, then those streams are implicitly created. The stream identifier is encoded using a variable length integer. The largest possible stream identifier is thus \(2^{62}-1\).

QUIC places all data inside STREAM frames that are then placed inside QUIC packets. The structure of a STREAM frame is shown in Listing 8. This frame contains the following information :

  • the Type of the Stream frame 1

  • the identifier of the stream

  • the offset, i.e. the position of the first byte of the Stream data in the bytestream

  • the length of the data

  • the Stream Data

Listing 8 The QUIC STREAM frame
STREAM Frame {
   Type (i) = 0x08..0x0f,
   Stream ID (i),
   Offset (i),
   Length (i),
   Stream Data (..),

The STREAM frame carries data, but it can also terminate the corresponding stream. The lowest order bit of the Type field acts as a FIN bit. When set to zero, it indicates that subsequent data will be sent over this stream. When set to one, it indicates that the STREAM frame contains the last bytes sent over that stream. The stream is closed once the last byte of the stream has been delivered to the user application. Once a QUIC stream has been closed, it cannot be reused again over this connection.

Using this information, the receiver can easily reassemble the data received over the different streams. As an illustration, let us consider a server that has created two streams (stream 1 and 5). The server sends ABCD.. over stream 1 and 123 over stream 5 and closes it after having sent the third digit. The data from these streams could be encoded as shown in Listing 9.

Listing 9 Data sent over two different streams
STREAM Frame {
   Type (i) = 0x0e,
   Stream ID = 1,
   Offset = 0
   Length = 2
   Stream Data = AB
STREAM Frame {
   Type (i) = 0x0e,
   Stream ID = 5,
   Offset = 0
   Length = 1
   Stream Data = 1
STREAM Frame {
   Type (i) = 0x0e,
   Stream ID = 1,
   Offset = 2
   Length = 1
   Stream Data = C
STREAM Frame {
   Type (i) = 0x0f,   /* FIN bit is set, end of stream */
   Stream ID = 5,
   Offset = 1
   Length = 2
   Stream Data = 23
STREAM Frame {
   Type (i) = 0x0e,
   Stream ID = 1,
   Offset = 3
   Length = 4
   Stream Data = DEFG

The penultimate frame shown in Listing 9 has the FIN flag set. It marks the end of stream 1 that has transported three bytes in total. The FIN flag is the normal way to gracefully close a QUIC stream.

There are however cases where an application might need to cancel a stream abruptly without closing the corresponding connection. First, the sending side of a stream can decide to reset the stream. For this, it sends a RESET_STREAM frame that carries the identifier of the stream that is canceled. The receiving side of a stream might also need to stop receiving data over a given stream. Consider for example a web browser that has started to download the different images that compose a web while the user has already clicked on another page from the same server. The streams that correspond to these images become useless. In this case, our browser can send a STOP_SENDING frame to indicate that it discards the incoming data over the corresponding streams. Upon reception of this frame, the server sends a RESET_STREAM frame to indicate that the corresponding stream has been closed.

Flow control in QUIC

Transport protocols usually allocate some resources to each established connection. Each QUIC connection requires memory to store its state, but also buffers to store the packets arrived out-of-order. In practice, the memory available for QUIC implementations is not unlimited and a QUIC receiver must control the amount of packets that the remote host can send at any time. QUIC supports flow control techniques that operate at different levels.

The first level is the connection level. During the handshake, each host can announce the maximum number of bytes that it agrees to receive initially on the connection using the initial_max_data transport parameter. This parameter contains the number of bytes that the sending host agrees to receive without further notice. If the connection uses more bytes than initially agreed, the receiver can update this limit by sending a MAX_DATA frame at any time. This frame contains a variable length integer that encodes the maximum amount of stream data that can be sent over the connection.

The utilization of different streams also consumes resources on a QUIC host. A receiver can also restrict the number of streams that the remote host can create. During the handshake, the initial_max_streams_bidi and initial_max_streams_uni transport parameters announce the maximum number of bidirectional and unidirectional streams that the receiving host can accept. This limit can be modified during the connection by sending a MAX_STREAMS frame that updates the limit.

Flow control can also take place at the stream level. During the handshake, several transport parameters allow the hosts to advertise the maximum number of bytes that they agree to receive on each stream. Different transport parameters are used to specify the limits that apply to the local/remote and unidirectional/bidirectional streams. These limits can be updated during the connection by sending MAX_STREAM_DATA frames. Each of these frames indicates the maximum amount of stream data that can be accepted on a given stream.

These limits restrict the number of streams that a host can create and the amount of bytes that it can send. If a host is blocked by any of these limits, it may sent a control frame to request the remote host to extend the limit. For each type of flow control, there is an associated control frame which can be used to request an extension of the limit.

A host should send a DATA_BLOCKED frame when it reaches the limit on the maximum amount of data set by the initial_max_data transport parameter or a previously received MAX_DATA frame. The DATA_BLOCKED frame contains the connection limit that caused the transmission to be blocked. In practice, a receiving host should increase the connection-level limit by sending MAX_DATA frames before reaching the limit. However, since this limit is function of the available memory, a host might not always be able to send a MAX_DATA frame. Fig. 25 provides an example packet flow with the utilization of these frames. We assume that the initial_max_data transport parameter was set to 100 bytes by the client during the handshake and the the server needs to send 900 bytes. The server creates a stream and sends 100 bytes in a 1-RTT packet carrying a STREAM frame. At this point, the server is blocked.

Figure made with TikZ

Fig. 25 QUIC uses DATA_BLOCKED frames when a connection’s flow control is blocked

The same applies with the STREAM_DATA_BLOCKED frame that is sent when a host reaching the per-stream limit. The STREAMS_BLOCKED frame is used when a host has reached the maximum number of established streams.


Connections blocked by flow control are not totally idle

If a QUIC host detects that a connection is blocked by flow control, it should regularly send DATA_BLOCKED or STREAM_DATA_BLOCKED frames to request the remote host to extend the current limit. The QUIC specification [25] does not recommend precisely how often these frames should be sent when a connection is blocked by flow control. Experience with QUIC deployments will enable the QUIC developers to more precisely define how often these frames should be sent.

A measurement study [36] revealed that QUIC implementations used different strategies for flow control. They identified three main types of behaviors :

  • Static Flow Control: the receive buffer size stays unchanged and the receiver simply increases the maximum allowance linearly

  • Growing Flow Control: the size of the receive buffer increases over time as the connection progresses

  • Auto-tuning: the size of the receive buffer is adjusted dynamically based on the estimated bandwidth and round-trip times

In 2020, three quarter of the studied implementations used a Static Flow Control and only one used auto-tuning. Static Flow Control can work, but this is a waste of memory. For example, Google’s Chrome starts with a 15 MBytes receive buffer and updates it linearly [36]. This should be contrasted with TCP implementations that usually start with a window of a few tens of KBytes and adjust it with auto-tuning. Auto-tuning is a key feature of modern TCP implementations [37]. It is expected that QUIC implementations will include more advanced heuristics to tune their flow control mechanism.

QUIC Loss Detection

As other transport protocols, QUIC includes mechanisms to detect transmission errors and packet losses. The transmission errors are usually detected by using the UDP checksum or at the datalink layer with the Wi-Fi or Ethernet CRCs. When a transmission error occurs, the corresponding packet is discarded and QUIC considers this error as a packet loss. Researchers have analyzed the performance of checksums and CRCs on real data [38].

Second, since QUIC used AEAD encryption schemes, all QUIC packets are authenticated and a receiver can leverage this AEAD to detect transmission errors that were undetected by the UDP checksum of the CRC of the lower layers. However, these undetected transmission errors are assumed to be rare and if QUIC a detects an invalid AEAD, it will consider that this error was caused by an attack and will stop the connection using a TLS alert [26].

There are several important differences between the loss detection and retransmission mechanisms used by QUIC and other transport protocols. First, QUIC packet numbers always increase monotonically over a QUIC connection. A QUIC sender never sends twice a packet with the same packet number over a given connection. QUIC encodes the packet numbers as variable length integers and it does not support wrap around in contrast with other transport protocols. The QUIC frames contain the valuable information that needs to be delivered reliably. If a QUIC packet is lost, the frames that it contained will be retransmitted in another QUIC packet that uses a different packet number. Thus, the QUIC packet number serves as a unique identifier of a packet. This simplifies some operations such as measuring the round-trip-time which is more difficult in protocols such as TCP when packets are transmitted [39].

Second, QUIC’s acknowledgments carry more information than the cumulative or selective acknowledgments used by TCP and related protocols. This enables the receiver to provide a more detailed view of the packets that it received. In contrast with TCP [11], once a receiver has reported that one packet was correctly received in an acknowledgment, the sender of that packet can discard the corresponding frames.

Third, a QUIC sender autonomously decides which frames it sends inside each packet. A QUIC packet may contain both data and control frames, or only data or only control information. If a QUIC packet is lost, the frames that it contained could be retransmitted in different packets. A QUIC implementation thus needs to buffer the frames and mark the in-flight ones to be able to retransmit them if the corresponding packet was lost.

Fourth, most QUIC packets are explicitly acknowledged. The only exception are the packets that only contain ACK, PADDING or CONNECTION_CLOSE frames. A packet that contains any other QUIC frame is called an ack-eliciting packet because its delivery will be confirmed by the transmission of an acknowledgment. A QUIC packet that carries both an ACK and a STREAM frame will thus be acknowledged.

With this in mind, it is interesting to look at the format of the QUIC acknowledgments and then analyze how they can be used. Listing 10 provides the format of an ACK frame. It can be sent at any time in a QUIC packet. Two types are used to distinguish between the acknowledgments that contain information about the received ECN flags (type 0x03) or only regular acknowledgments (type 0x02). The first information contained in the ACK frame is the largest packet number that is acknowledged by this ACK frame. This is usually the highest packet number received. The second information is the ACK delay. This is the delay in microseconds between the reception of the packet having the largest acknowledged number by the receiver and the transmission of the acknowledgment. This information is important to ensure that round-trip-times are accurately measured, even if a receiver delays acknowledgments. This is illustrated in Fig. 26. The ACK Range Count field contains the number of ACK ranges that are included in the QUIC ACK frame. This number can be set to zero if all packets were received in sequence without any gap. In this case, the First ACK Range field contains the number of the packet that arrived before the Largest Acknowledged packet number.

Listing 10 The QUIC ACK Frame
ACK Frame {
   Type (i) = 0x02..0x03,
   Largest Acknowledged (i),
   ACK Delay (i),
   ACK Range Count (i),
   First ACK Range (i),
   ACK Range (..) ...,
   [ECN Counts (..)],

Figure made with TikZ

Fig. 26 Utilization of the QUIC ACK delay

An ACK frame contains 0 or more ACK Ranges. The format of an ACK range is shown in Listing 11. Each range indicates first the number of unacknowledged packets since the smallest acknowledged packet in the preceding range (or the first ACK range). The next field indicates the number of consecutive acknowledged packets.

Listing 11 A QUIC ACK range
ACK Range {
   Gap (i),
   ACK Range Length (i),

As an example, consider a host that received the following QUIC packets: 3,4,6,7,8,9,11,14,16,18. To report all the received packets, it will generate the ACK frame shown in Listing 12.

Listing 12 Sample QUIC ACK Frame
ACK Frame {
   Type (i) = 0x02,
   Largest Acknowledged=18,
   ACK Delay=x,
   ACK Range Count=5,
   First ACK Range=0,
   ACK Range #0 [Gap=2, ACK Range Length=1],
   ACK Range #1 [Gap=2, ACK Range Length=1],
   ACK Range #2 [Gap=3, ACK Range Length=1],
   ACK Range #3 [Gap=2, ACK Range Length=4],
   ACK Range #4 [Gap=2, ACK Range Length=2]

The QUIC specification recommends to send one ACK frame after having received two ack-eliciting packets. This corresponds roughly to TCP’s delayed acknowledgments strategy. However, there is ongoing work to allow the sender to provide more guidelines on when and how ACK frames should be sent [40].


When should QUIC hosts send acknowledgments

A measurement study [36] analyzed how QUIC implementations generate acknowledgments. Two of the studied implementations sent acknowledgments every N packets (2 for one implementation and 10 for the other). Other implementations used ack frequencies that varied during the data transfer.


Fig. 27 Acknowledgment frequencies for different QUIC servers

The acknowledgment frequencies should be compared with TCP that usually acknowledges every second packet. It is likely that QUIC implementations will tune the generation of their acknowledgments in the coming years based on feedback from deployment.

It is interesting to observe that since the ACK frames are sent inside QUIC packets, they can also be acknowledged. Sending an ACK in response to another ACK could result in an infinite exchange of ACK frames. To prevent this problem, a QUIC sender cannot send an ACK frame in response to a non-eliciting QUIC packet and the ACK frames are one of the non-eliciting frame types. Note that if a receiver that receives many STREAM frames and thus sends many ACK frames wants to obtain information about the reception of its ACK frame, it can simply send one ACK frame inside a packet that contains an eliciting frame, e.g. a PING frame. This frame will trigger the receiver to acknowledge it and the previously sent ACK frames.

In contrast with other reliable transport protocols, QUIC does not use cumulative acknowledgments. As explained earlier, QUIC never retransmits a packet with the same packet number. When a packet is lost, this creates a gap that the receiver reports using an ACK Range. Such a gap will never be filled by retransmissions and obviously should not be reported by the receiver forever. In practice, a receiver will send the acknowledgment that corresponds to a given packet number several times and then will assume that the acknowledgment has been received. A receiver can also rely on other heuristics to determine that a given ACK Range should not be reported anymore. This is the case if the ACK frame was included in a packet that has been acknowledged by the other peer, but also when the gap was noticed several round-trip times ago.

QUIC also allows a receiver to send information about the ECN flags in the received packets. Two flags of the IP header [41] are reserved to indicate support for Explicit Congestion Notification. The QUIC ECN count field shown in Listing 13 contains three counters for the different values of the ECN flags. These counters are incremented upon the reception of each QUIC packet based on the values of the ECN flag of the received packet. Unfortunately, there are still many operational problems when using ECN in the global Internet [42]. Time will tell whether it is easier to deploy ECN with QUIC than with TCP.

Listing 13 A QUIC ECN Count
ECN Counts {
   ECT0 Count (i),
   ECT1 Count (i),
   ECN-CE Count (i),


QUIC also acknowledges control frames

Besides the STREAM frames that carry user data, QUIC uses several different frame types to exchange control information. These control frames, like the data frames, are ack-eliciting frames. This implies a host that receives such a frame needs to acknowledge it using an ACK frame.

Fig. 29 illustrates the beginning of a QUIC connection with the exchange of the Initial packets and the corresponding acknowledgments. The client sends its TLS Client Hello inside a CRYPTO frame in an Initial packet. This is the first packet sent by the client and thus its packet number is 0. The server replies with a TLS Server Hello inside a CRYPTO frame in an Initial packet. Since this is the first packet sent by the server, its packet number is also 0. The packet also contains an ACK frame that acknowledges the reception of the packet containing the TLS Client Hello.

The Handshake, 0-RTT and 1-RTT packets are acknowledged similarly using ACK frames. Handshake packets are acknowledged in other Handshake packets while 0-RTT and 1-RTT packets are acknowledged inside 1-RTT packets.

Figure made with TikZ

Fig. 29 QUIC also acknowledges Initial frames


Not all QUIC servers use 0 as the packet number of their first Initial packet

The example shows a QUIC connection where the client sent its Initial packet with packet number 0 and the server also replied with a packet number set to 0. This is what most QUIC implementations do. However, the QUIC specification does not strictly requires this. In fact, facebook servers in October 2022 appear to use random packet numbers for the Initial packet that they sent in response to a client. This is probably use to detect or mitigate some forms of attacks since the client must receive the server’s Initial packet to be able to produce a valid acknowledgment.

To illustrate how QUIC uses acknowledgments, let us consider a simple QUIC connection. The client starts a QUIC connection with a new server, sends a request, receives a response and then closes the connection. There are no losses in this connection. Fig. 30 illustrates this connection.

Figure made with TikZ

Fig. 30 Acknowledgments in a short QUIC connection

The connection starts when the client sends an Initial packet containing a CRYPTO frame with the TLS Client Hello. The server replies with an Initial packet that contains an acknowledgment and a CRYPTO frame with the TLS Server Hello. The server then sends an Initial packet containing the TLS Encrypted Extensions. Since this is the first Initial packet, its packet number is set to 0. In practice, it is likely that the server will need to send several packets to carry the certificates contained in this packet. Note that the server cannot send more than 3 packets in response to the client’s Initial packet. This limit was introduced in the QUIC specification to restrict the ability of attackers to trigger DDoS attacks by sending spoofed packets to QUIC servers [25]. If the CRYPTO frame containing the certificates is too long, the server might need to wait for acknowledgments from the client before sending its last Handshake packets. The client confirms the reception of the server’s Initial packet by sending its last Initial packet that contains an ACK frame. This is the second Initial packet sent by the client and its packet number is thus 1. Since this packet only contains an ACK frame, the server does not return an acknowledgment to confirm its reception.

The client replies to the Handshake packet with another Handshake packet that contains a CRYPTO frame and acknowledges the Handshake packets sent by the server. The client’s Handshake packet is acknowledged by the server. The server starts the data exchange by sending a 1-RTT packet carrying one or more stream frames to create the required streams. Since this is the first 1-RTT packet sent by the server, its packet number is set to 0. The client then sends its request in a STREAM frame. The server replies with a 1-RTT packet that contains a STREAM frame with its response. The client ends the connection by sending a CONNECTION_CLOSE frame. In the example, the server replies with a CONNECTION_CLOSE frame, but the QUIC specification [25] only indicates that a host may respond with a CONNECTION_CLOSE in this case.

Observing QUIC connections

We have now reviewed the main components of the QUIC specification. To illustrate it, let us consider a simple scenario where a client opens a QUIC connection with a server. We leverage Maxime Piraux’s QUIC tracker [43]. In this example, we use the packet trace collected using the nghttp2.org server on November 22, 2021 at 18:04:52 CET. This trace is available from https://quic-tracker.info.ucl.ac.be/traces/20211122/148. You can see the contents of the packets, download it as a .pcap trace or visualize it using QLog/QViz [44] from this web site.

This trace contains 16 packets. The scenario is a simple handshake with an exchange of data.


Fig. 31 Sample QUIC tracker trace with nghttp2.org containing a successful handshake

To initiate the connection, the client sends an Initial QUIC packet. It is interesting to analyze the content of this packet. It starts with a long QUIC header shown in Listing 14.

Listing 14 The QUIC header of the first packet sent by the client
Long Header Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type = 00,
  Type-Specific Bits (4) = 0000,
  Version (32) = 0xff00001d,
  Destination Connection ID Length (8) = 8,
  Destination Connection ID (0..160) = 0x6114ca6ecbe483bb,
  Source Connection ID Length (8) = 8,
  Source Connection ID (0..160) = 0xc9f54d3c298296b9,
  Token Length (i) = 0,
  Length (i) = 1226,
  Packet Number (8..32) = 0,
  Packet Payload (8..) = CRYPTO,
  Type-Specific Payload (..)

The client proposes a 64 bits connection identifier and uses a random 64 bits identifier for the destination connection identifier. There is no token in this packet since this is the first connection from this client to the server. It is useful to note that the packet number of this Initial packet is set to zero. All QUIC connections start with a packet whose packet number is set to zero in contrast with TCP that uses a random sequence number. The packet contains a CRYPTO frame shown in Listing 15.

Listing 15 The CRYPTO frame of the first QUIC packet sent by the client
CRYPTO Frame {
   Type (i) = 0x06,
   Offset (i) = 0,
   Length (i) = 245,
   Crypto Data = ClientHello

The CRYPTO frame starts at offset 0 and has a length of 245 bytes. It contains a TLS 1.3 ClientHello message whose format is specified in [26]. This ClientHello includes a 32 bytes secure random number, a set of proposed cipher suites and a series of TLS extensions. One of these extensions carries the QUIC transport parameters proposed by the client. On this connection, the QUIC tracker client proposed the following ones:

  • initial_max_stream_data_bidi_local = 0x80004000

  • initial_max_stream_data_uni = 0x80004000

  • initial_max_data = 0x80008000

  • initial_max_streams_bidi = 0x1

  • initial_max_streams_uni = 0x3

  • max_idle_timeout = 0x6710

  • active_connection_id_limit = 0x4

  • max_packet_size = 0x45c0

  • inital_source_connection_id = 0xc9f54d3c298296b9

Finally, the first QUIC packet contains a PADDING frame with 960 dummy bytes. The entire packet is 1236 bytes long.

The server responds to this Initial packet with two packets. The first one is an Initial packet. It starts with the header shown in Listing 16.

Listing 16 The QUIC header of the first packet sent by the client
Long Header Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type = 10,
  Type-Specific Bits (4) = 0000,
  Version (32) = 0xff00001d,
  Destination Connection ID Length (8) = 8,
  Destination Connection ID (0..160) = 0xc9f54d3c298296b9,
  Source Connection ID Length (8) = 18,
  Source Connection ID (0..160) = 0x8d3470255ae3b0b3fad3c40515132a813dfa,
  Token Length (i) = 0,
  Length (i) = 149,
  Packet Number (8..32) = 0,
  Packet Payload (...)

This server uses 18 bytes to encode its connection identifier and proposes the first identifier in the long header. The packet payload contains two frames: an ACK frame and a CRYPTO frame. The ACK frame (Listing 17) acknowledges the reception of the Initial packet sent by the client. The CRYPTO frame contains the TLS ServerHello.

Listing 17 The ACK Frame of the first packet sent by the server
ACK Frame {
   Type (i) = 0x02,
   Largest Acknowledged = 0,
   ACK Delay = 0,
   ACK Range Count = 0,
   First ACK Range = 0

The payload of these Initial packets is encrypted using the static key derived from the connection identifiers included in the long header.

The server then sends three Handshake packets carrying a CRYPTO frame that contains the TLSEncryptedExtensions. These extensions are encrypted using the TLS key. They mainly contain the server certificate. It is interesting to note that the packet_number field of the first Handshake packet sent by the server is also set to zero. This is the second, but not the last, packet that we observe with this packet_number. QUIC handles packet numbers differently then other protocols. QUIC considers that a QUIC connection is divided in three phases:

  1. The exchange of the Initial packets

  2. The exchange of the Handshake packets

  3. The exchange of the other packets (0-RTT, 1-RTT, … packets)

A QUIC host restarts the packet_number at zero in each phase. This explains why it is possible to observe different packets (of different types) with the same packet_number over a QUIC connection.

The three Handshake packets sent by the server contain the beginning of the TLSEncryptedExtensions sent by the server. To prevent denial of service attacks, the server cannot send more than three full-length packets in response to a packet sent by the client. The server thus needs to wait for an acknowledgment from the client before sending additional packets.

The client sends two packets to carry these acknowledgments. First, it sends an Initial packet as the sixth packet of the trace. This packet belongs to the packet numbering space of the Initial packets. Its packet number is 1 since this is the second Initial packet sent by the client. The next acknowledgment is carried inside an Handshake packet. It acknowledges the Handshake packets 0-2 sent by the server. Since this is the first Handshake packet sent by the client, its packet number is also 0.

The server then sends the eighth packet that contains the last part of the TLSEncryptedExtensions in a CRYPTO frame. By combining the information contained in the Handshake packets and the Initial packets, the client can derive the session keys.

The server immediately sends its first 1-RTT packet. This packet contains a short header shown in Listing 18.

Listing 18 The QUIC short header of the first 1-RTT packet sent by the server
1-RTT Packet {
   Header Form (1) = 0,
   Fixed Bit (1) = 1,
   Spin Bit (1) = 0,
   Reserved Bits (2)= 00,
   Key Phase (1) = 0,
   Packet Number Length (2)= 0,
   Destination Connection ID = 0xc9f54d3c298296b9,
   Packet Number = 0,

This short header contains the connection identifier proposed by the client in the first Initial packet. The payload contains STREAM frames that create three streams. The client replies with two packets. The tenth packet of the trace is a Handshake packet that carries two frames. The CRYPTO frame contains the TLS Finished message that finalizes the TLS handshake. The ACK frame acknowledges the four Handshake packets sent by the server.

The first 1-RTT packet sent by the client contains an ACK frame that acknowledges the 1-RTT packet sent by the server and flow control information. The client sends a MAX_DATA frame to restrict the amount of data that the server can send and one MAX_STREAM frame for each of the three streams created by the server.

The twelfth packet of the trace is more interesting. It contains five different frames that are sent by the server. First, the server send two NEW_CONNECTION_ID frames that advertise two 18 bytes long connection identifiers which can be used by the client to migrate the connection later. The next frame is the HANDSHAKE_DONE frame that confirms the TLS handshake. The server also sends a NEW_TOKEN frame that contains a 57 bytes long token that the client will be able to use in subsequent connections with the server. The last frame is a CRYPTO frame that contains two TLS New Session Tickets.

A closer look at other QUIC handshakes

It is interesting to analyze how different servers perform the handshake using QUIC tracker. Let us first explore the trace collected with cloudflare-quic.com on the same day shown in Fig. 32. There are several differences with the nghttp2 trace that we analyzed above. First, the server sends two small packets in response to the client’s Initial. The first packet only contains an ACK frame. It advertises a 20 bytes long connection identifier. The second packet contains a CRYPTO frame with a TLS Hello Retry Request. This message indicates that the server did not agree with the key_share parameter of the TLS Client Hello sent in the first packet. The client acknowledges this packet and sends a new TLS Client Hello in the fourth packet. The server replies with a TLS Server Hello and then the TLSEncryptedExtensions in three QUIC packets. The certificate used by cloudflare-quic.com is more compact than the one used by nghttp2.org.


Fig. 32 Sample quic tracker trace from cloudflare-quic.com with a successful handshake

The 1-RTT packets are also slightly different. The first 1-RTT packet sent by the server contains the HANDSHAKE_DONE frame, a CRYPTO frame with two TLS New Session Ticket messages and a STREAM frame that creates one stream. The server then sends two short packet. Each of these packets contains a STREAM frame that creates a new stream. These two short packets could have been packed in the first 1-RTT packet sent by the server. In contrast with nghttp2.org, cloudflare-quic.com does advertise new connection identifiers.

Our third example is picoquic. The QUIC tracker trace with test.privateoctopus.com contains 13 packets.


Fig. 33 Sample QUIC tracker trace from test.privateoctopus.com with a successful handshake

picoquic uses 64 bits long connection identifiers. It manages to fit its TLS Encrypted Extensions within two Handshake packets. The first 1-RTT packet that it sends contains a PING frame. The second 1-RTT packet contains one CRYPTO frame that advertises one TLS New Session Ticket, three NEW_CONNECTION_ID frames and a NEW_TOKEN frame. This test server does not try to create new streams in contrast with the two others.


Comparing QUIC servers

It is interesting to use the traces collected by QUIC tracker to analyze how different servers have selected some of the optional features of QUIC. A first difference between the servers is the length of the server-selected connection identifiers. The graph below shows that in November 2021 many servers advertised 8 bytes CIDs, but some have opted for much longer CIDs.


Fig. 34 Length of the connection identifiers advertised by different QUIC servers (Nov 2021)

Observing 0-RTT data in QUIC

The ability to send data immediately was one of the requirements for the design of QUIC. It is interesting to observe how QUIC uses the 0-RTT packets for this purpose. We use a trace collected between QUIC tracker and picoquic as our example. This trace covers two QUIC connections shown in Fig. 36.


Fig. 36 Sample QUIC trace with test.privateoctopus.com with 0-RTT packets

During the first QUIC connection, QUIC tracker receives one TLS session ticket in the CRYPTO frame contained in the 1-RTT packet that the server sent with packet number set to 0. This ticket contains all the information required by the server to retrieve the key in a subsequent connection. QUIC tracker starts the second connection by sending an Initial packet. This packet contains a CRYPTO frame that contains the TLS Client Hello message. A comparison between this TLS Client Hello and the one sent to create the first connection shows that the latter contains the psk_key_exchange_modes TLS extension. This extension contains the information that enables the server to recover the key required to decrypt the 0-RTT packet. In this example, the client sends a 0-RTT that contains the beginning of a simple HTTP GET.

QUIC streams

As QUIC support multiple streams, it is interesting to analyze how the streams are managed over a real QUIC connection. For this example, we use a trace between QUIC tracker and quic.tech summarized in Fig. 37. In the example, the QUIC tracker creates four streams and sends one HTTP GET request over each of them.


Fig. 37 Sample QUIC trace with quic.tech using multiple streams

In this trace, the client creates four streams in its first STREAM frame sent in the first 1-RTT packet. Listing 19 shows the first of these STREAM frames. The Type of the STREAM is one octet structured as 0b00001OLF where O is set to 1 if the STREAM frame contains an Offset field. Bit L is set to 1 if the frame contains a Length field. Finally, the F is set to 1 to mark the end of the STREAM. In this test, QUIC Tracker sends 17 bytes over each stream and closes it.

Listing 19 The first QUIC STREAM frame sent by QUIC Tracker
STREAM Frame {
   Type (i) = 0b00001011,  # Offset=0, Length=1, FIN=1
   Stream ID = 8,
   Length = 17,
   Stream Data = GET /index.html\r\n

The server sends each response in a STREAM frame. Listing 20 shows the frame returned by the server. Its Offset bit is set to 1. It carries the entire HTML page and its Offset field could have been ignored since this is the first frame of the stream.

Listing 20 The QUIC STREAM frame returned by the server
STREAM Frame {
   Type (i) = 0b00001111,  # Offset=1, Length=1, FIN=1
   Stream ID = 8,
   Offset = 0,
   Length = 462,
   Stream Data = <!DOCTYPE html>...



All STREAM frames have a type that starts with 0b0001.... The three low order bits of the STREAM frame indicate the presence of the Offset and Length fields. The lowest order bit is the FIN bit.