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Versions: (draft-hamilton-quic-transport-protocol) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                                    Fastly
Intended status: Standards Track                         M. Thomson, Ed.
Expires: April 26, 2019                                          Mozilla
                                                        October 23, 2018


           QUIC: A UDP-Based Multiplexed and Secure Transport
                      draft-ietf-quic-transport-16

Abstract

   This document defines the core of the QUIC transport protocol.  This
   document describes connection establishment, packet format,
   multiplexing, and reliability.  Accompanying documents describe the
   cryptographic handshake and loss detection.

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org), which is archived at
   <https://mailarchive.ietf.org/arch/search/?email_list=quic>.

   Working Group information can be found at <https://github.com/
   quicwg>; source code and issues list for this draft can be found at
   <https://github.com/quicwg/base-drafts/labels/-transport>.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 26, 2019.








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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Document Structure  . . . . . . . . . . . . . . . . . . .   6
     1.2.  Conventions and Definitions . . . . . . . . . . . . . . .   7
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   8
   2.  Streams . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.1.  Stream Identifiers  . . . . . . . . . . . . . . . . . . .   9
     2.2.  Stream Concurrency  . . . . . . . . . . . . . . . . . . .  10
     2.3.  Sending and Receiving Data  . . . . . . . . . . . . . . .  11
     2.4.  Stream Prioritization . . . . . . . . . . . . . . . . . .  11
   3.  Stream States: Life of a Stream . . . . . . . . . . . . . . .  12
     3.1.  Send Stream States  . . . . . . . . . . . . . . . . . . .  13
     3.2.  Receive Stream States . . . . . . . . . . . . . . . . . .  15
     3.3.  Permitted Frame Types . . . . . . . . . . . . . . . . . .  18
     3.4.  Bidirectional Stream States . . . . . . . . . . . . . . .  18
     3.5.  Solicited State Transitions . . . . . . . . . . . . . . .  19
   4.  Flow Control  . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.1.  Handling of Stream Cancellation . . . . . . . . . . . . .  21
     4.2.  Data Limit Increments . . . . . . . . . . . . . . . . . .  22
     4.3.  Stream Final Offset . . . . . . . . . . . . . . . . . . .  23
     4.4.  Flow Control for Cryptographic Handshake  . . . . . . . .  24
     4.5.  Stream Limit Increment  . . . . . . . . . . . . . . . . .  24
   5.  Connections . . . . . . . . . . . . . . . . . . . . . . . . .  24
     5.1.  Connection ID . . . . . . . . . . . . . . . . . . . . . .  24
       5.1.1.  Issuing Connection IDs  . . . . . . . . . . . . . . .  25
       5.1.2.  Consuming and Retiring Connection IDs . . . . . . . .  26
     5.2.  Matching Packets to Connections . . . . . . . . . . . . .  27
       5.2.1.  Client Packet Handling  . . . . . . . . . . . . . . .  27
       5.2.2.  Server Packet Handling  . . . . . . . . . . . . . . .  27
     5.3.  Life of a QUIC Connection . . . . . . . . . . . . . . . .  28
   6.  Version Negotiation . . . . . . . . . . . . . . . . . . . . .  28
     6.1.  Sending Version Negotiation Packets . . . . . . . . . . .  29



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     6.2.  Handling Version Negotiation Packets  . . . . . . . . . .  29
     6.3.  Using Reserved Versions . . . . . . . . . . . . . . . . .  30
   7.  Cryptographic and Transport Handshake . . . . . . . . . . . .  31
     7.1.  Example Handshake Flows . . . . . . . . . . . . . . . . .  32
     7.2.  Negotiating Connection IDs  . . . . . . . . . . . . . . .  33
     7.3.  Transport Parameters  . . . . . . . . . . . . . . . . . .  34
       7.3.1.  Values of Transport Parameters for 0-RTT  . . . . . .  35
       7.3.2.  New Transport Parameters  . . . . . . . . . . . . . .  36
       7.3.3.  Version Negotiation Validation  . . . . . . . . . . .  36
   8.  Address Validation  . . . . . . . . . . . . . . . . . . . . .  37
     8.1.  Address Validation During Connection Establishment  . . .  38
       8.1.1.  Address Validation using Retry Packets  . . . . . . .  38
       8.1.2.  Address Validation for Future Connections . . . . . .  39
       8.1.3.  Address Validation Token Integrity  . . . . . . . . .  41
     8.2.  Path Validation . . . . . . . . . . . . . . . . . . . . .  41
     8.3.  Initiating Path Validation  . . . . . . . . . . . . . . .  42
     8.4.  Path Validation Responses . . . . . . . . . . . . . . . .  42
     8.5.  Successful Path Validation  . . . . . . . . . . . . . . .  42
     8.6.  Failed Path Validation  . . . . . . . . . . . . . . . . .  43
   9.  Connection Migration  . . . . . . . . . . . . . . . . . . . .  43
     9.1.  Probing a New Path  . . . . . . . . . . . . . . . . . . .  44
     9.2.  Initiating Connection Migration . . . . . . . . . . . . .  45
     9.3.  Responding to Connection Migration  . . . . . . . . . . .  45
       9.3.1.  Handling Address Spoofing by a Peer . . . . . . . . .  46
       9.3.2.  Handling Address Spoofing by an On-path Attacker  . .  46
     9.4.  Loss Detection and Congestion Control . . . . . . . . . .  47
     9.5.  Privacy Implications of Connection Migration  . . . . . .  48
     9.6.  Server's Preferred Address  . . . . . . . . . . . . . . .  49
       9.6.1.  Communicating A Preferred Address . . . . . . . . . .  49
       9.6.2.  Responding to Connection Migration  . . . . . . . . .  49
       9.6.3.  Interaction of Client Migration and Preferred Address  50
   10. Connection Termination  . . . . . . . . . . . . . . . . . . .  50
     10.1.  Closing and Draining Connection States . . . . . . . . .  51
     10.2.  Idle Timeout . . . . . . . . . . . . . . . . . . . . . .  52
     10.3.  Immediate Close  . . . . . . . . . . . . . . . . . . . .  52
     10.4.  Stateless Reset  . . . . . . . . . . . . . . . . . . . .  53
       10.4.1.  Detecting a Stateless Reset  . . . . . . . . . . . .  56
       10.4.2.  Calculating a Stateless Reset Token  . . . . . . . .  56
       10.4.3.  Looping  . . . . . . . . . . . . . . . . . . . . . .  57
   11. Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  58
     11.1.  Connection Errors  . . . . . . . . . . . . . . . . . . .  58
     11.2.  Stream Errors  . . . . . . . . . . . . . . . . . . . . .  59
   12. Packets and Frames  . . . . . . . . . . . . . . . . . . . . .  59
     12.1.  Protected Packets  . . . . . . . . . . . . . . . . . . .  59
     12.2.  Coalescing Packets . . . . . . . . . . . . . . . . . . .  60
     12.3.  Packet Numbers . . . . . . . . . . . . . . . . . . . . .  61
     12.4.  Frames and Frame Types . . . . . . . . . . . . . . . . .  62
   13. Packetization and Reliability . . . . . . . . . . . . . . . .  65



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     13.1.  Packet Processing and Acknowledgment . . . . . . . . . .  66
       13.1.1.  Sending ACK Frames . . . . . . . . . . . . . . . . .  66
       13.1.2.  ACK Frames and Packet Protection . . . . . . . . . .  67
     13.2.  Retransmission of Information  . . . . . . . . . . . . .  67
     13.3.  Explicit Congestion Notification . . . . . . . . . . . .  69
       13.3.1.  ECN Counters . . . . . . . . . . . . . . . . . . . .  70
       13.3.2.  ECN Verification . . . . . . . . . . . . . . . . . .  70
   14. Packet Size . . . . . . . . . . . . . . . . . . . . . . . . .  71
     14.1.  Path Maximum Transmission Unit . . . . . . . . . . . . .  72
       14.1.1.  IPv4 PMTU Discovery  . . . . . . . . . . . . . . . .  73
     14.2.  Special Considerations for Packetization Layer PMTU
            Discovery  . . . . . . . . . . . . . . . . . . . . . . .  73
   15. Versions  . . . . . . . . . . . . . . . . . . . . . . . . . .  74
   16. Variable-Length Integer Encoding  . . . . . . . . . . . . . .  75
   17. Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  75
     17.1.  Packet Number Encoding and Decoding  . . . . . . . . . .  76
     17.2.  Long Header Packet . . . . . . . . . . . . . . . . . . .  77
     17.3.  Short Header Packet  . . . . . . . . . . . . . . . . . .  79
     17.4.  Version Negotiation Packet . . . . . . . . . . . . . . .  81
     17.5.  Initial Packet . . . . . . . . . . . . . . . . . . . . .  82
       17.5.1.  Starting Packet Numbers  . . . . . . . . . . . . . .  84
       17.5.2.  0-RTT Packet Numbers . . . . . . . . . . . . . . . .  84
     17.6.  Handshake Packet . . . . . . . . . . . . . . . . . . . .  85
     17.7.  Retry Packet . . . . . . . . . . . . . . . . . . . . . .  85
   18. Transport Parameter Encoding  . . . . . . . . . . . . . . . .  88
     18.1.  Transport Parameter Definitions  . . . . . . . . . . . .  90
   19. Frame Types and Formats . . . . . . . . . . . . . . . . . . .  92
     19.1.  PADDING Frame  . . . . . . . . . . . . . . . . . . . . .  93
     19.2.  RST_STREAM Frame . . . . . . . . . . . . . . . . . . . .  93
     19.3.  CONNECTION_CLOSE frame . . . . . . . . . . . . . . . . .  94
     19.4.  APPLICATION_CLOSE frame  . . . . . . . . . . . . . . . .  95
     19.5.  MAX_DATA Frame . . . . . . . . . . . . . . . . . . . . .  95
     19.6.  MAX_STREAM_DATA Frame  . . . . . . . . . . . . . . . . .  96
     19.7.  MAX_STREAM_ID Frame  . . . . . . . . . . . . . . . . . .  97
     19.8.  PING Frame . . . . . . . . . . . . . . . . . . . . . . .  98
     19.9.  BLOCKED Frame  . . . . . . . . . . . . . . . . . . . . .  98
     19.10. STREAM_BLOCKED Frame . . . . . . . . . . . . . . . . . .  99
     19.11. STREAM_ID_BLOCKED Frame  . . . . . . . . . . . . . . . .  99
     19.12. NEW_CONNECTION_ID Frame  . . . . . . . . . . . . . . . . 100
     19.13. RETIRE_CONNECTION_ID Frame . . . . . . . . . . . . . . . 101
     19.14. STOP_SENDING Frame . . . . . . . . . . . . . . . . . . . 102
     19.15. ACK Frame  . . . . . . . . . . . . . . . . . . . . . . . 102
       19.15.1.  ACK Block Section . . . . . . . . . . . . . . . . . 104
       19.15.2.  ECN section . . . . . . . . . . . . . . . . . . . . 105
     19.16. PATH_CHALLENGE Frame . . . . . . . . . . . . . . . . . . 106
     19.17. PATH_RESPONSE Frame  . . . . . . . . . . . . . . . . . . 107
     19.18. NEW_TOKEN frame  . . . . . . . . . . . . . . . . . . . . 107
     19.19. STREAM Frames  . . . . . . . . . . . . . . . . . . . . . 107



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     19.20. CRYPTO Frame . . . . . . . . . . . . . . . . . . . . . . 109
     19.21. Extension Frames . . . . . . . . . . . . . . . . . . . . 110
   20. Transport Error Codes . . . . . . . . . . . . . . . . . . . . 110
     20.1.  Application Protocol Error Codes . . . . . . . . . . . . 111
   21. Security Considerations . . . . . . . . . . . . . . . . . . . 112
     21.1.  Handshake Denial of Service  . . . . . . . . . . . . . . 112
     21.2.  Spoofed ACK Attack . . . . . . . . . . . . . . . . . . . 113
     21.3.  Optimistic ACK Attack  . . . . . . . . . . . . . . . . . 113
     21.4.  Slowloris Attacks  . . . . . . . . . . . . . . . . . . . 114
     21.5.  Stream Fragmentation and Reassembly Attacks  . . . . . . 114
     21.6.  Stream Commitment Attack . . . . . . . . . . . . . . . . 114
     21.7.  Explicit Congestion Notification Attacks . . . . . . . . 115
     21.8.  Stateless Reset Oracle . . . . . . . . . . . . . . . . . 115
   22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 116
     22.1.  QUIC Transport Parameter Registry  . . . . . . . . . . . 116
     22.2.  QUIC Frame Type Registry . . . . . . . . . . . . . . . . 117
     22.3.  QUIC Transport Error Codes Registry  . . . . . . . . . . 118
   23. References  . . . . . . . . . . . . . . . . . . . . . . . . . 121
     23.1.  Normative References . . . . . . . . . . . . . . . . . . 121
     23.2.  Informative References . . . . . . . . . . . . . . . . . 122
   Appendix A.  Sample Packet Number Decoding Algorithm  . . . . . . 123
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 124
     B.1.  Since draft-ietf-quic-transport-15  . . . . . . . . . . . 124
     B.2.  Since draft-ietf-quic-transport-14  . . . . . . . . . . . 124
     B.3.  Since draft-ietf-quic-transport-13  . . . . . . . . . . . 125
     B.4.  Since draft-ietf-quic-transport-12  . . . . . . . . . . . 126
     B.5.  Since draft-ietf-quic-transport-11  . . . . . . . . . . . 126
     B.6.  Since draft-ietf-quic-transport-10  . . . . . . . . . . . 127
     B.7.  Since draft-ietf-quic-transport-09  . . . . . . . . . . . 127
     B.8.  Since draft-ietf-quic-transport-08  . . . . . . . . . . . 128
     B.9.  Since draft-ietf-quic-transport-07  . . . . . . . . . . . 129
     B.10. Since draft-ietf-quic-transport-06  . . . . . . . . . . . 130
     B.11. Since draft-ietf-quic-transport-05  . . . . . . . . . . . 130
     B.12. Since draft-ietf-quic-transport-04  . . . . . . . . . . . 130
     B.13. Since draft-ietf-quic-transport-03  . . . . . . . . . . . 131
     B.14. Since draft-ietf-quic-transport-02  . . . . . . . . . . . 131
     B.15. Since draft-ietf-quic-transport-01  . . . . . . . . . . . 132
     B.16. Since draft-ietf-quic-transport-00  . . . . . . . . . . . 134
     B.17. Since draft-hamilton-quic-transport-protocol-01 . . . . . 134
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 134
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . . 135
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 135

1.  Introduction

   QUIC is a multiplexed and secure transport protocol that runs on top
   of UDP.  QUIC aims to provide a flexible set of features that allow




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   it to be a general-purpose secure transport for multiple
   applications.

   o  Version negotiation

   o  Low-latency connection establishment

   o  Authenticated and encrypted header and payload

   o  Stream multiplexing

   o  Stream and connection-level flow control

   o  Connection migration and resilience to NAT rebinding

   QUIC uses UDP as a substrate to avoid requiring changes in legacy
   client operating systems and middleboxes.  QUIC authenticates all of
   its headers and encrypts most of the data it exchanges, including its
   signaling.  This allows the protocol to evolve without incurring a
   dependency on upgrades to middleboxes.

1.1.  Document Structure

   This document describes the core QUIC protocol, and is structured as
   follows:

   o  Streams are the basic service abstraction that QUIC provides.

      *  Section 2 describes core concepts related to streams,

      *  Section 3 provides a reference model for stream states, and

      *  Section 4 outlines the operation of flow control.

   o  Connections are the context in which QUIC endpoints communicate.

      *  Section 5 describes core concepts related to connections,

      *  Section 6 describes version negotiation,

      *  Section 7 details the process for establishing connections,

      *  Section 8 specifies critical denial of service mitigation
         mechanisms,

      *  Section 9 describes how endpoints migrate a connection to use a
         new network paths, and




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      *  Section 10 lists the options for terminating an open
         connection.

   o  Packets and frames are the basic unit used by QUIC to communicate.

      *  Section 12 describes concepts related to packets and frames,

      *  Section 13 defines models for the transmission, retransmission,
         and acknowledgement of information, and

      *  Section 14 contains a rules for managing the size of packets.

   o  Details of encoding of QUIC protocol elements is described in:

      *  Section 15 (Versions),

      *  Section 17 (Packet Headers),

      *  Section 18 (Transport Parameters),

      *  Section 19 (Frames), and

      *  Section 20 (Errors).

   Accompanying documents describe QUIC's loss detection and congestion
   control [QUIC-RECOVERY], and the use of TLS 1.3 for key negotiation
   [QUIC-TLS].

   QUIC version 1 conforms to the protocol invariants in
   [QUIC-INVARIANTS].

1.2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Definitions of terms that are used in this document:

   Client:  The endpoint initiating a QUIC connection.

   Server:  The endpoint accepting incoming QUIC connections.

   Endpoint:  The client or server end of a connection.





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   Stream:  A logical unidirectional or bidirectional channel of ordered
      bytes within a QUIC connection.

   Connection:  A conversation between two QUIC endpoints with a single
      encryption context that multiplexes streams within it.

   Connection ID:  An opaque identifier that is used to identify a QUIC
      connection at an endpoint.  Each endpoint sets a value that its
      peer includes in packets.

   QUIC packet:  The smallest unit of data that can be exchanged by QUIC
      endpoints.

   QUIC is a name, not an acronym.

1.3.  Notational Conventions

   Packet and frame diagrams use the format described in Section 3.1 of
   [RFC2360], with the following additional conventions:

   [x]  Indicates that x is optional

   x (A)  Indicates that x is A bits long

   x (A/B/C) ...  Indicates that x is one of A, B, or C bits long

   x (i) ...  Indicates that x uses the variable-length encoding in
      Section 16

   x (*) ...  Indicates that x is variable-length

2.  Streams

   Streams in QUIC provide a lightweight, ordered byte-stream
   abstraction.

   There are two basic types of stream in QUIC.  Unidirectional streams
   carry data in one direction: from the initiator of the stream to its
   peer; bidirectional streams allow for data to be sent in both
   directions.  Different stream identifiers are used to distinguish
   between unidirectional and bidirectional streams, as well as to
   create a separation between streams that are initiated by the client
   and server (see Section 2.1).

   Either type of stream can be created by either endpoint, can
   concurrently send data interleaved with other streams, and can be
   cancelled.




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   Streams can be created by sending data.  Other processes associated
   with stream management - ending, cancelling, and managing flow
   control - are all designed to impose minimal overheads.  For
   instance, a single STREAM frame (Section 19.19) can open, carry data
   for, and close a stream.  Streams can also be long-lived and can last
   the entire duration of a connection.

   Stream offsets allow for the octets on a stream to be placed in
   order.  An endpoint MUST be capable of delivering data received on a
   stream in order.  Implementations MAY choose to offer the ability to
   deliver data out of order.  There is no means of ensuring ordering
   between octets on different streams.

   Streams are individually flow controlled, allowing an endpoint to
   limit memory commitment and to apply back pressure.  The creation of
   streams is also flow controlled, with each peer declaring the maximum
   stream ID it is willing to accept at a given time.

   An alternative view of QUIC streams is as an elastic "message"
   abstraction, similar to the way ephemeral streams are used in SST
   [SST], which may be a more appealing description for some
   applications.

2.1.  Stream Identifiers

   Streams are identified by an unsigned 62-bit integer, referred to as
   the Stream ID.  Stream IDs are encoded as a variable-length integer
   (see Section 16).  The least significant two bits of the Stream ID
   are used to identify the type of stream (unidirectional or
   bidirectional) and the initiator of the stream.

   The least significant bit (0x1) of the Stream ID identifies the
   initiator of the stream.  Clients initiate even-numbered streams
   (those with the least significant bit set to 0); servers initiate
   odd-numbered streams (with the bit set to 1).  Separation of the
   stream identifiers ensures that client and server are able to open
   streams without the latency imposed by negotiating for an identifier.

   If an endpoint receives a frame for a stream that it expects to
   initiate (i.e., odd-numbered for the client or even-numbered for the
   server), but which it has not yet opened, it MUST close the
   connection with error code STREAM_STATE_ERROR.

   The second least significant bit (0x2) of the Stream ID
   differentiates between unidirectional streams and bidirectional
   streams.  Unidirectional streams always have this bit set to 1 and
   bidirectional streams have this bit set to 0.




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   The two type bits from a Stream ID therefore identify streams as
   summarized in Table 1.

              +----------+----------------------------------+
              | Low Bits | Stream Type                      |
              +----------+----------------------------------+
              | 0x0      | Client-Initiated, Bidirectional  |
              |          |                                  |
              | 0x1      | Server-Initiated, Bidirectional  |
              |          |                                  |
              | 0x2      | Client-Initiated, Unidirectional |
              |          |                                  |
              | 0x3      | Server-Initiated, Unidirectional |
              +----------+----------------------------------+

                         Table 1: Stream ID Types

   The first bidirectional stream opened by the client is stream 0.

   A QUIC endpoint MUST NOT reuse a Stream ID.  Streams of each type are
   created in numeric order.  Streams that are used out of order result
   in opening all lower-numbered streams of the same type in the same
   direction.

2.2.  Stream Concurrency

   QUIC allows for an arbitrary number of streams to operate
   concurrently.  An endpoint limits the number of concurrently active
   incoming streams by limiting the maximum stream ID (see Section 4.5).

   The maximum stream ID is specific to each endpoint and applies only
   to the peer that receives the setting.  That is, clients specify the
   maximum stream ID the server can initiate, and servers specify the
   maximum stream ID the client can initiate.  Each endpoint may respond
   on streams initiated by the other peer, regardless of whether it is
   permitted to initiate new streams.

   Endpoints MUST NOT exceed the limit set by their peer.  An endpoint
   that receives a STREAM frame with an ID greater than the limit it has
   sent MUST treat this as a stream error of type STREAM_ID_ERROR
   (Section 11), unless this is a result of a change in the initial
   limits (see Section 7.3.1).

   A receiver cannot renege on an advertisement; that is, once a
   receiver advertises a stream ID via a MAX_STREAM_ID frame,
   advertising a smaller maximum ID has no effect.  A receiver MUST
   ignore any MAX_STREAM_ID frame that does not increase the maximum
   stream ID.



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2.3.  Sending and Receiving Data

   Endpoints uses streams to send and receive data.  Endpoints send
   STREAM frames, which encapsulate data for a stream.  STREAM frames
   carry a flag that can be used to signal the end of a stream.

   Streams are an ordered byte-stream abstraction with no other
   structure that is visible to QUIC.  STREAM frame boundaries are not
   expected to preserved when data is transmitted, when data is
   retransmitted after packet loss, or when data is delivered to the
   application at the receiver.

   When new data is to be sent on a stream, a sender MUST set the
   encapsulating STREAM frame's offset field to the stream offset of the
   first octet of this new data.  The first octet of data on a stream
   has an offset of 0.  An endpoint is expected to send every stream
   octet.  The largest offset delivered on a stream MUST be less than
   2^62.

   QUIC makes no specific allowances for partial reliability or delivery
   of stream data out of order.  Endpoints MUST be able to deliver
   stream data to an application as an ordered byte-stream.  Delivering
   an ordered byte-stream requires that an endpoint buffer any data that
   is received out of order, up to the advertised flow control limit.

   An endpoint could receive the same octets multiple times; octets that
   have already been received can be discarded.  The value for a given
   octet MUST NOT change if it is sent multiple times; an endpoint MAY
   treat receipt of a changed octet as a connection error of type
   PROTOCOL_VIOLATION.

   An endpoint MUST NOT send data on any stream without ensuring that it
   is within the data limits set by its peer.  Flow control is described
   in detail in Section 4.

2.4.  Stream Prioritization

   Stream multiplexing has a significant effect on application
   performance if resources allocated to streams are correctly
   prioritized.  Experience with other multiplexed protocols, such as
   HTTP/2 [HTTP2], shows that effective prioritization strategies have a
   significant positive impact on performance.

   QUIC does not provide frames for exchanging prioritization
   information.  Instead it relies on receiving priority information
   from the application that uses QUIC.  Protocols that use QUIC are
   able to define any prioritization scheme that suits their application
   semantics.  A protocol might define explicit messages for signaling



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   priority, such as those defined in HTTP/2; it could define rules that
   allow an endpoint to determine priority based on context; or it could
   leave the determination to the application.

   A QUIC implementation SHOULD provide ways in which an application can
   indicate the relative priority of streams.  When deciding which
   streams to dedicate resources to, QUIC SHOULD use the information
   provided by the application.  Failure to account for priority of
   streams can result in suboptimal performance.

   Stream priority is most relevant when deciding which stream data will
   be transmitted.  Often, there will be limits on what can be
   transmitted as a result of connection flow control or the current
   congestion controller state.

   Giving preference to the transmission of its own management frames
   ensures that the protocol functions efficiently.  That is,
   prioritizing frames other than STREAM frames ensures that loss
   recovery, congestion control, and flow control operate effectively.

   CRYPTO frames SHOULD be prioritized over other streams prior to the
   completion of the cryptographic handshake.  This includes the
   retransmission of the second flight of client handshake messages,
   that is, the TLS Finished and any client authentication messages.

   STREAM data in frames determined to be lost SHOULD be retransmitted
   before sending new data, unless application priorities indicate
   otherwise.  Retransmitting lost stream data can fill in gaps, which
   allows the peer to consume already received data and free up the flow
   control window.

3.  Stream States: Life of a Stream

   This section describes the two types of QUIC stream in terms of the
   states of their send or receive components.  Two state machines are
   described: one for streams on which an endpoint transmits data
   (Section 3.1); another for streams from which an endpoint receives
   data (Section 3.2).

   Unidirectional streams use the applicable state machine directly.
   Bidirectional streams use both state machines.  For the most part,
   the use of these state machines is the same whether the stream is
   unidirectional or bidirectional.  The conditions for opening a stream
   are slightly more complex for a bidirectional stream because the
   opening of either send or receive sides causes the stream to open in
   both directions.





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   An endpoint can open streams up to its maximum stream limit in any
   order, however endpoints SHOULD open the send side of streams for
   each type in order.

   Note:  These states are largely informative.  This document uses
      stream states to describe rules for when and how different types
      of frames can be sent and the reactions that are expected when
      different types of frames are received.  Though these state
      machines are intended to be useful in implementing QUIC, these
      states aren't intended to constrain implementations.  An
      implementation can define a different state machine as long as its
      behavior is consistent with an implementation that implements
      these states.

3.1.  Send Stream States

   Figure 1 shows the states for the part of a stream that sends data to
   a peer.

































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          o
          | Create Stream (Sending)
          | Create Bidirectional Stream (Receiving)
          v
      +-------+
      | Ready | Send RST_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Send STREAM /             |
          |      STREAM_BLOCKED       |
          |                           |
          | Create Bidirectional      |
          |      Stream (Receiving)   |
          v                           |
      +-------+                       |
      | Send  | Send RST_STREAM       |
      |       |---------------------->|
      +-------+                       |
          |                           |
          | Send STREAM + FIN         |
          v                           v
      +-------+                   +-------+
      | Data  | Send RST_STREAM   | Reset |
      | Sent  |------------------>| Sent  |
      +-------+                   +-------+
          |                           |
          | Recv All ACKs             | Recv ACK
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Recvd |                   | Recvd |
      +-------+                   +-------+

                     Figure 1: States for Send Streams

   The sending part of stream that the endpoint initiates (types 0 and 2
   for clients, 1 and 3 for servers) is opened by the application or
   application protocol.  The "Ready" state represents a newly created
   stream that is able to accept data from the application.  Stream data
   might be buffered in this state in preparation for sending.

   Sending the first STREAM or STREAM_BLOCKED frame causes a send stream
   to enter the "Send" state.  An implementation might choose to defer
   allocating a Stream ID to a send stream until it sends the first
   frame and enters this state, which can allow for better stream
   prioritization.




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   The sending part of a bidirectional stream initiated by a peer (type
   0 for a server, type 1 for a client) enters the "Ready" state then
   immediately transitions to the "Send" state if the receiving part
   enters the "Recv" state.

   In the "Send" state, an endpoint transmits - and retransmits as
   necessary - data in STREAM frames.  The endpoint respects the flow
   control limits of its peer, accepting MAX_STREAM_DATA frames.  An
   endpoint in the "Send" state generates STREAM_BLOCKED frames if it
   encounters flow control limits.

   After the application indicates that stream data is complete and a
   STREAM frame containing the FIN bit is sent, the send stream enters
   the "Data Sent" state.  From this state, the endpoint only
   retransmits stream data as necessary.  The endpoint no longer needs
   to track flow control limits or send STREAM_BLOCKED frames for a send
   stream in this state.  The endpoint can ignore any MAX_STREAM_DATA
   frames it receives from its peer in this state; MAX_STREAM_DATA
   frames might be received until the peer receives the final stream
   offset.

   Once all stream data has been successfully acknowledged, the send
   stream enters the "Data Recvd" state, which is a terminal state.

   From any of the "Ready", "Send", or "Data Sent" states, an
   application can signal that it wishes to abandon transmission of
   stream data.  Similarly, the endpoint might receive a STOP_SENDING
   frame from its peer.  In either case, the endpoint sends a RST_STREAM
   frame, which causes the stream to enter the "Reset Sent" state.

   An endpoint MAY send a RST_STREAM as the first frame on a send
   stream; this causes the send stream to open and then immediately
   transition to the "Reset Sent" state.

   Once a packet containing a RST_STREAM has been acknowledged, the send
   stream enters the "Reset Recvd" state, which is a terminal state.

3.2.  Receive Stream States

   Figure 2 shows the states for the part of a stream that receives data
   from a peer.  The states for a receive stream mirror only some of the
   states of the send stream at the peer.  A receive stream doesn't
   track states on the send stream that cannot be observed, such as the
   "Ready" state; instead, receive streams track the delivery of data to
   the application or application protocol some of which cannot be
   observed by the sender.





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          o
          | Recv STREAM / STREAM_BLOCKED / RST_STREAM
          | Create Bidirectional Stream (Sending)
          | Recv MAX_STREAM_DATA
          | Create Higher-Numbered Stream
          v
      +-------+
      | Recv  | Recv RST_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Recv STREAM + FIN         |
          v                           |
      +-------+                       |
      | Size  | Recv RST_STREAM       |
      | Known |---------------------->|
      +-------+                       |
          |                           |
          | Recv All Data             |
          v                           v
      +-------+  Recv RST_STREAM  +-------+
      | Data  |--- (optional) --->| Reset |
      | Recvd |  Recv All Data    | Recvd |
      +-------+<-- (optional) ----+-------+
          |                           |
          | App Read All Data         | App Read RST
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Read  |                   | Read  |
      +-------+                   +-------+

                   Figure 2: States for Receive Streams

   The receiving part of a stream initiated by a peer (types 1 and 3 for
   a client, or 0 and 2 for a server) are created when the first STREAM,
   STREAM_BLOCKED, RST_STREAM, or MAX_STREAM_DATA (bidirectional only,
   see below) is received for that stream.  The initial state for a
   receive stream is "Recv".  Receiving a RST_STREAM frame causes the
   receive stream to immediately transition to the "Reset Recvd".

   The receive stream enters the "Recv" state when the sending part of a
   bidirectional stream initiated by the endpoint (type 0 for a client,
   type 1 for a server) enters the "Ready" state.

   A bidirectional stream also opens when a MAX_STREAM_DATA frame is
   received.  Receiving a MAX_STREAM_DATA frame implies that the remote
   peer has opened the stream and is providing flow control credit.  A



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   MAX_STREAM_DATA frame might arrive before a STREAM or STREAM_BLOCKED
   frame if packets are lost or reordered.

   Before creating a stream, all lower-numbered streams of the same type
   MUST be created.  That means that receipt of a frame that would open
   a stream causes all lower-numbered streams of the same type to be
   opened in numeric order.  This ensures that the creation order for
   streams is consistent on both endpoints.

   In the "Recv" state, the endpoint receives STREAM and STREAM_BLOCKED
   frames.  Incoming data is buffered and can be reassembled into the
   correct order for delivery to the application.  As data is consumed
   by the application and buffer space becomes available, the endpoint
   sends MAX_STREAM_DATA frames to allow the peer to send more data.

   When a STREAM frame with a FIN bit is received, the final offset (see
   Section 4.3) is known.  The receive stream enters the "Size Known"
   state.  In this state, the endpoint no longer needs to send
   MAX_STREAM_DATA frames, it only receives any retransmissions of
   stream data.

   Once all data for the stream has been received, the receive stream
   enters the "Data Recvd" state.  This might happen as a result of
   receiving the same STREAM frame that causes the transition to "Size
   Known".  In this state, the endpoint has all stream data.  Any STREAM
   or STREAM_BLOCKED frames it receives for the stream can be discarded.

   The "Data Recvd" state persists until stream data has been delivered
   to the application or application protocol.  Once stream data has
   been delivered, the stream enters the "Data Read" state, which is a
   terminal state.

   Receiving a RST_STREAM frame in the "Recv" or "Size Known" states
   causes the stream to enter the "Reset Recvd" state.  This might cause
   the delivery of stream data to the application to be interrupted.

   It is possible that all stream data is received when a RST_STREAM is
   received (that is, from the "Data Recvd" state).  Similarly, it is
   possible for remaining stream data to arrive after receiving a
   RST_STREAM frame (the "Reset Recvd" state).  An implementation is
   able to manage this situation as they choose.  Sending RST_STREAM
   means that an endpoint cannot guarantee delivery of stream data;
   however there is no requirement that stream data not be delivered if
   a RST_STREAM is received.  An implementation MAY interrupt delivery
   of stream data, discard any data that was not consumed, and signal
   the existence of the RST_STREAM immediately.  Alternatively, the
   RST_STREAM signal might be suppressed or withheld if stream data is




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   completely received.  In the latter case, the receive stream
   effectively transitions to "Data Recvd" from "Reset Recvd".

   Once the application has been delivered the signal indicating that
   the receive stream was reset, the receive stream transitions to the
   "Reset Read" state, which is a terminal state.

3.3.  Permitted Frame Types

   The sender of a stream sends just three frame types that affect the
   state of a stream at either sender or receiver: STREAM
   (Section 19.19), STREAM_BLOCKED (Section 19.10), and RST_STREAM
   (Section 19.2).

   A sender MUST NOT send any of these frames from a terminal state
   ("Data Recvd" or "Reset Recvd").  A sender MUST NOT send STREAM or
   STREAM_BLOCKED after sending a RST_STREAM; that is, in the "Reset
   Sent" state in addition to the terminal states.  A receiver could
   receive any of these frames in any state, but only due to the
   possibility of delayed delivery of packets carrying them.

   The receiver of a stream sends MAX_STREAM_DATA (Section 19.6) and
   STOP_SENDING frames (Section 19.14).

   The receiver only sends MAX_STREAM_DATA in the "Recv" state.  A
   receiver can send STOP_SENDING in any state where it has not received
   a RST_STREAM frame; that is states other than "Reset Recvd" or "Reset
   Read".  However there is little value in sending a STOP_SENDING frame
   after all stream data has been received in the "Data Recvd" state.  A
   sender could receive these frames in any state as a result of delayed
   delivery of packets.

3.4.  Bidirectional Stream States

   A bidirectional stream is composed of a send stream and a receive
   stream.  Implementations may represent states of the bidirectional
   stream as composites of send and receive stream states.  The simplest
   model presents the stream as "open" when either send or receive
   stream is in a non-terminal state and "closed" when both send and
   receive streams are in a terminal state.

   Table 2 shows a more complex mapping of bidirectional stream states
   that loosely correspond to the stream states in HTTP/2 [HTTP2].  This
   shows that multiple states on send or receive streams are mapped to
   the same composite state.  Note that this is just one possibility for
   such a mapping; this mapping requires that data is acknowledged
   before the transition to a "closed" or "half-closed" state.




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   +-----------------------+---------------------+---------------------+
   | Send Stream           | Receive Stream      | Composite State     |
   +-----------------------+---------------------+---------------------+
   | No Stream/Ready       | No Stream/Recv *1   | idle                |
   |                       |                     |                     |
   | Ready/Send/Data Sent  | Recv/Size Known     | open                |
   |                       |                     |                     |
   | Ready/Send/Data Sent  | Data Recvd/Data     | half-closed         |
   |                       | Read                | (remote)            |
   |                       |                     |                     |
   | Ready/Send/Data Sent  | Reset Recvd/Reset   | half-closed         |
   |                       | Read                | (remote)            |
   |                       |                     |                     |
   | Data Recvd            | Recv/Size Known     | half-closed (local) |
   |                       |                     |                     |
   | Reset Sent/Reset      | Recv/Size Known     | half-closed (local) |
   | Recvd                 |                     |                     |
   |                       |                     |                     |
   | Data Recvd            | Recv/Size Known     | half-closed (local) |
   |                       |                     |                     |
   | Reset Sent/Reset      | Data Recvd/Data     | closed              |
   | Recvd                 | Read                |                     |
   |                       |                     |                     |
   | Reset Sent/Reset      | Reset Recvd/Reset   | closed              |
   | Recvd                 | Read                |                     |
   |                       |                     |                     |
   | Data Recvd            | Data Recvd/Data     | closed              |
   |                       | Read                |                     |
   |                       |                     |                     |
   | Data Recvd            | Reset Recvd/Reset   | closed              |
   |                       | Read                |                     |
   +-----------------------+---------------------+---------------------+

           Table 2: Possible Mapping of Stream States to HTTP/2

   Note (*1):  A stream is considered "idle" if it has not yet been
      created, or if the receive stream is in the "Recv" state without
      yet having received any frames.

3.5.  Solicited State Transitions

   If an endpoint is no longer interested in the data it is receiving on
   a stream, it MAY send a STOP_SENDING frame identifying that stream to
   prompt closure of the stream in the opposite direction.  This
   typically indicates that the receiving application is no longer
   reading data it receives from the stream, but is not a guarantee that
   incoming data will be ignored.




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   STREAM frames received after sending STOP_SENDING are still counted
   toward the connection and stream flow-control windows, even though
   these frames will be discarded upon receipt.  This avoids potential
   ambiguity about which STREAM frames count toward flow control.

   A STOP_SENDING frame requests that the receiving endpoint send a
   RST_STREAM frame.  An endpoint that receives a STOP_SENDING frame
   MUST send a RST_STREAM frame for that stream, and can use an error
   code of STOPPING.  If the STOP_SENDING frame is received on a send
   stream that is already in the "Data Sent" state, a RST_STREAM frame
   MAY still be sent in order to cancel retransmission of previously-
   sent STREAM frames.

   STOP_SENDING SHOULD only be sent for a receive stream that has not
   been reset.  STOP_SENDING is most useful for streams in the "Recv" or
   "Size Known" states.

   An endpoint is expected to send another STOP_SENDING frame if a
   packet containing a previous STOP_SENDING is lost.  However, once
   either all stream data or a RST_STREAM frame has been received for
   the stream - that is, the stream is in any state other than "Recv" or
   "Size Known" - sending a STOP_SENDING frame is unnecessary.

4.  Flow Control

   It is necessary to limit the amount of data that a sender may have
   outstanding at any time, so as to prevent a fast sender from
   overwhelming a slow receiver, or to prevent a malicious sender from
   consuming significant resources at a receiver.  To this end, QUIC
   employs a credit-based flow-control scheme similar to that in HTTP/2
   [HTTP2].  A receiver advertises the number of octets it is prepared
   to receive on a given stream and for the entire connection.  This
   leads to two levels of flow control in QUIC:

   o  Stream flow control, which prevents a single stream from consuming
      the entire receive buffer for a connection.

   o  Connection flow control, which prevents senders from exceeding a
      receiver's buffer capacity for the connection, and

   A data receiver sets initial credits for all streams by sending
   transport parameters during the handshake (Section 7.3).

   A data receiver sends MAX_STREAM_DATA or MAX_DATA frames to the
   sender to advertise additional credit.  MAX_STREAM_DATA frames send
   the maximum absolute byte offset of a stream, while MAX_DATA frames
   send the maximum of the sum of the absolute byte offsets of all
   streams.



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   A receiver advertises credit for a stream by sending a
   MAX_STREAM_DATA frame with the Stream ID set appropriately.  A
   receiver could use the current offset of data consumed to determine
   the flow control offset to be advertised.  A receiver MAY send
   MAX_STREAM_DATA frames in multiple packets in order to make sure that
   the sender receives an update before running out of flow control
   credit, even if one of the packets is lost.

   Connection flow control is a limit to the total bytes of stream data
   sent in STREAM frames on all streams.  A receiver advertises credit
   for a connection by sending a MAX_DATA frame.  A receiver maintains a
   cumulative sum of bytes received on all contributing streams, which
   are used to check for flow control violations.  A receiver might use
   a sum of bytes consumed on all streams to determine the maximum data
   limit to be advertised.

   A receiver MAY advertise a larger offset at any point by sending
   MAX_STREAM_DATA or MAX_DATA frames.  A receiver cannot renege on an
   advertisement; that is, once a receiver advertises an offset,
   advertising a smaller offset has no effect.  A sender MUST therefore
   ignore any MAX_STREAM_DATA or MAX_DATA frames that do not increase
   flow control limits.

   A receiver MUST close the connection with a FLOW_CONTROL_ERROR error
   (Section 11) if the peer violates the advertised connection or stream
   data limits.

   A sender SHOULD send STREAM_BLOCKED or BLOCKED frames to indicate it
   has data to write but is blocked by flow control limits.  These
   frames are expected to be sent infrequently in common cases, but they
   are considered useful for debugging and monitoring purposes.

   A similar method is used to control the number of open streams (see
   Section 4.5 for details).

4.1.  Handling of Stream Cancellation

   There are some edge cases which must be considered when dealing with
   stream and connection level flow control.  Given enough time, both
   endpoints must agree on flow control state.  If one end believes it
   can send more than the other end is willing to receive, the
   connection will be torn down when too much data arrives.  Conversely
   if a sender believes it is blocked, while endpoint B expects more
   data can be received, then the connection can be in a deadlock, with
   the sender waiting for a MAX_STREAM_DATA or MAX_DATA frame which will
   never come.





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   On receipt of a RST_STREAM frame, an endpoint will tear down state
   for the matching stream and ignore further data arriving on that
   stream.  This could result in the endpoints getting out of sync,
   since the RST_STREAM frame may have arrived out of order and there
   may be further bytes in flight.  The data sender would have counted
   the data against its connection level flow control budget, but a
   receiver that has not received these bytes would not know to include
   them as well.  The receiver must learn the number of bytes that were
   sent on the stream to make the same adjustment in its connection flow
   controller.

   To ensure that endpoints maintain a consistent connection-level flow
   control state, the RST_STREAM frame (Section 19.2) includes the
   largest offset of data sent on the stream.  On receiving a RST_STREAM
   frame, a receiver definitively knows how many bytes were sent on that
   stream before the RST_STREAM frame, and the receiver MUST use the
   final offset to account for all bytes sent on the stream in its
   connection level flow controller.

   RST_STREAM terminates one direction of a stream abruptly.  Whether
   any action or response can or should be taken on the data already
   received is application specific.

   For a bidirectional stream, RST_STREAM has no effect on data flow in
   the opposite direction.  The RST_STREAM sender can send a
   STOP_SENDING frame to encourage prompt termination.  Both endpoints
   MUST maintain state for the stream in the unterminated direction
   until that direction enters a terminal state, or either side sends
   CONNECTION_CLOSE or APPLICATION_CLOSE.

4.2.  Data Limit Increments

   This document leaves when and how many bytes to advertise in a
   MAX_DATA or MAX_STREAM_DATA to implementations, but offers a few
   considerations.  These frames contribute to connection overhead.
   Therefore frequently sending frames with small changes is
   undesirable.  At the same time, larger increments to limits are
   necessary to avoid blocking if updates are less frequent, requiring
   larger resource commitments at the receiver.  Thus there is a trade-
   off between resource commitment and overhead when determining how
   large a limit is advertised.

   A receiver MAY use an autotuning mechanism to tune the frequency and
   amount that it increases data limits based on a round-trip time
   estimate and the rate at which the receiving application consumes
   data, similar to common TCP implementations.





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   If a sender runs out of flow control credit, it will be unable to
   send new data.  That is, the sender is blocked.  A blocked sender
   SHOULD send a STREAM_BLOCKED or BLOCKED frame.  A receiver uses these
   frames for debugging purposes.  A receiver MUST NOT wait for a
   STREAM_BLOCKED or BLOCKED frame before sending MAX_STREAM_DATA or
   MAX_DATA, since doing so will mean that a sender will be blocked for
   an entire round trip and the peer may never send a STREAM_BLOCKED or
   BLOCKED frame.

   It is generally considered best to not let the sender go into
   quiescence if avoidable.  To avoid blocking a sender, and to
   reasonably account for the possibility of loss, a receiver should
   send a MAX_DATA or MAX_STREAM_DATA frame at least two round trips
   before it expects the sender to get blocked.

   A sender sends a single BLOCKED or STREAM_BLOCKED frame only once
   when it reaches a data limit.  A sender SHOULD NOT send multiple
   BLOCKED or STREAM_BLOCKED frames for the same data limit, unless the
   original frame is determined to be lost.  Another BLOCKED or
   STREAM_BLOCKED frame can be sent after the data limit is increased.

4.3.  Stream Final Offset

   The final offset is the count of the number of octets that are
   transmitted on a stream.  For a stream that is reset, the final
   offset is carried explicitly in a RST_STREAM frame.  Otherwise, the
   final offset is the offset of the end of the data carried in a STREAM
   frame marked with a FIN flag, or 0 in the case of incoming
   unidirectional streams.

   An endpoint will know the final offset for a stream when the receive
   stream enters the "Size Known" or "Reset Recvd" state.

   An endpoint MUST NOT send data on a stream at or beyond the final
   offset.

   Once a final offset for a stream is known, it cannot change.  If a
   RST_STREAM or STREAM frame causes the final offset to change for a
   stream, an endpoint SHOULD respond with a FINAL_OFFSET_ERROR error
   (see Section 11).  A receiver SHOULD treat receipt of data at or
   beyond the final offset as a FINAL_OFFSET_ERROR error, even after a
   stream is closed.  Generating these errors is not mandatory, but only
   because requiring that an endpoint generate these errors also means
   that the endpoint needs to maintain the final offset state for closed
   streams, which could mean a significant state commitment.






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4.4.  Flow Control for Cryptographic Handshake

   Data sent in CRYPTO frames is not flow controlled in the same way as
   STREAM frames.  QUIC relies on the cryptographic protocol
   implementation to avoid excessive buffering of data, see [QUIC-TLS].
   The implementation SHOULD provide an interface to QUIC to tell it
   about its buffering limits so that there is not excessive buffering
   at multiple layers.

4.5.  Stream Limit Increment

   An endpoint limits the number of concurrently active incoming streams
   by limiting the maximum stream ID.  An initial value is set in the
   transport parameters (see Section 18.1) and is subsequently increased
   by MAX_STREAM_ID frames (see Section 19.7).

   As with stream and connection flow control, this document leaves when
   and how many streams to make available to a peer via MAX_STREAM_ID to
   implementations, but offers a few considerations.  MAX_STREAM_ID
   frames constitute minimal overhead, while withholding MAX_STREAM_ID
   frames can prevent the peer from using the available parallelism.

   The STREAM_ID_BLOCKED frame (Section 19.11) can be used to signal a
   shortage of available streams.  Implementations will likely want to
   increase the maximum stream ID as peer-initiated streams close.

5.  Connections

   A QUIC connection is a single conversation between two QUIC
   endpoints.  QUIC's connection establishment combines version
   negotiation with the cryptographic and transport handshakes to reduce
   connection establishment latency, as described in Section 7.  Once
   established, a connection may migrate to a different IP or port at
   either endpoint as described in Section 9.  Finally, a connection may
   be terminated by either endpoint, as described in Section 10.

5.1.  Connection ID

   Each connection possesses a set of connection identifiers, or
   connection IDs, each of which can be identify the connection.
   Connection IDs are independently selected by endpoints; each endpoint
   selects the connection IDs that its peer uses.

   The primary function of a connection ID is to ensure that changes in
   addressing at lower protocol layers (UDP, IP, and below) don't cause
   packets for a QUIC connection to be delivered to the wrong endpoint.
   Each endpoint selects connection IDs using an implementation-specific
   (and perhaps deployment-specific) method which will allow packets



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   with that connection ID to be routed back to the endpoint and
   identified by the endpoint upon receipt.

   Connection IDs MUST NOT contain any information that can be used to
   correlate them with other connection IDs for the same connection.  As
   a trivial example, this means the same connection ID MUST NOT be
   issued more than once on the same connection.

   Packets with long headers include Source Connection ID and
   Destination Connection ID fields.  These fields are used to set the
   connection IDs for new connections, see Section 7.2 for details.

   Packets with short headers (Section 17.3) only include the
   Destination Connection ID and omit the explicit length.  The length
   of the Destination Connection ID field is expected to be known to
   endpoints.  Endpoints using a load balancer that routes based on
   connection ID could agree with the load balancer on a fixed length
   for connection IDs, or agree on an encoding scheme.  A fixed portion
   could encode an explicit length, which allows the entire connection
   ID to vary in length and still be used by the load balancer.

   A Version Negotiation (Section 17.4) packet echoes the connection IDs
   selected by the client, both to ensure correct routing toward the
   client and to allow the client to validate that the packet is in
   response to an Initial packet.

   A zero-length connection ID MAY be used when the connection ID is not
   needed for routing and the address/port tuple of packets is
   sufficient to identify a connection.  An endpoint whose peer has
   selected a zero-length connection ID MUST continue to use a zero-
   length connection ID for the lifetime of the connection and MUST NOT
   send packets from any other local address.

   When an endpoint has requested a non-zero-length connection ID, it
   needs to ensure that the peer has a supply of connection IDs from
   which to choose for packets sent to the endpoint.  These connection
   IDs are supplied by the endpoint using the NEW_CONNECTION_ID frame
   (Section 19.12).

5.1.1.  Issuing Connection IDs

   Each Connection ID has an associated sequence number to assist in
   deduplicating messages.  The initial connection ID issued by an
   endpoint is sent in the Source Connection ID field of the long packet
   header (Section 17.2) during the handshake.  The sequence number of
   the initial connection ID is 0.  If the preferred_address transport
   parameter is sent, the sequence number of the supplied connection ID
   is 1.



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   Additional connection IDs are communicated to the peer using
   NEW_CONNECTION_ID frames (Section 19.12).  The sequence number on
   each newly-issued connection ID MUST increase by 1.  The connection
   ID randomly selected by the client in the Initial packet and any
   connection ID provided by a Reset packet are not assigned sequence
   numbers unless a server opts to retain them as its initial connection
   ID.

   When an endpoint issues a connection ID, it MUST accept packets that
   carry this connection ID for the duration of the connection or until
   its peer invalidates the connection ID via a RETIRE_CONNECTION_ID
   frame (Section 19.13).

   An endpoint SHOULD ensure that its peer has a sufficient number of
   available and unused connection IDs.  While each endpoint
   independently chooses how many connection IDs to issue, endpoints
   SHOULD provide and maintain at least eight connection IDs.  The
   endpoint can do this by always supplying a new connection ID when a
   connection ID is retired by its peer or when the endpoint receives a
   packet with a previously unused connection ID.  Endpoints that
   initiate migration and require non-zero-length connection IDs SHOULD
   provide their peers with new connection IDs before migration, or risk
   the peer closing the connection.

5.1.2.  Consuming and Retiring Connection IDs

   An endpoint can change the connection ID it uses for a peer to
   another available one at any time during the connection.  An endpoint
   consumes connection IDs in response to a migrating peer, see
   Section 9.5 for more.

   An endpoint maintains a set of connection IDs received from its peer,
   any of which it can use when sending packets.  When the endpoint
   wishes to remove a connection ID from use, it sends a
   RETIRE_CONNECTION_ID frame to its peer, indicating that the peer
   might bring a new connection ID into circulation using the
   NEW_CONNECTION_ID frame.

   An endpoint that retires a connection ID can retain knowledge of that
   connection ID for a period of time after sending the
   RETIRE_CONNECTION_ID frame, or until that frame is acknowledged.

   As discussed in Section 9.5, each connection ID MUST be used on
   packets sent from only one local address.  An endpoint that migrates
   away from a local address SHOULD retire all connection IDs used on
   that address once it no longer plans to use that address.





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5.2.  Matching Packets to Connections

   Incoming packets are classified on receipt.  Packets can either be
   associated with an existing connection, or - for servers -
   potentially create a new connection.

   Hosts try to associate a packet with an existing connection.  If the
   packet has a Destination Connection ID corresponding to an existing
   connection, QUIC processes that packet accordingly.  Note that more
   than one connection ID can be associated with a connection; see
   Section 5.1.

   If the Destination Connection ID is zero length and the packet
   matches the address/port tuple of a connection where the host did not
   require connection IDs, QUIC processes the packet as part of that
   connection.  Endpoints MUST drop packets with zero-length Destination
   Connection ID fields if they do not correspond to a single
   connection.

   Endpoints SHOULD send a Stateless Reset (Section 10.4) for any
   packets that cannot be attributed to an existing connection.

   Packets that are matched to an existing connection, but for which the
   endpoint cannot remove packet protection, are discarded.

5.2.1.  Client Packet Handling

   Valid packets sent to clients always include a Destination Connection
   ID that matches a value the client selects.  Clients that choose to
   receive zero-length connection IDs can use the address/port tuple to
   identify a connection.  Packets that don't match an existing
   connection are discarded.

   Due to packet reordering or loss, clients might receive packets for a
   connection that are encrypted with a key it has not yet computed.
   Clients MAY drop these packets, or MAY buffer them in anticipation of
   later packets that allow it to compute the key.

   If a client receives a packet that has an unsupported version, it
   MUST discard that packet.

5.2.2.  Server Packet Handling

   If a server receives a packet that has an unsupported version, but
   the packet is sufficiently large to initiate a new connection for any
   version supported by the server, it SHOULD send a Version Negotiation
   packet as described in Section 6.1.  Servers MAY rate control these
   packets to avoid storms of Version Negotiation packets.



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   The first packet for an unsupported version can use different
   semantics and encodings for any version-specific field.  In
   particular, different packet protection keys might be used for
   different versions.  Servers that do not support a particular version
   are unlikely to be able to decrypt the payload of the packet.
   Servers SHOULD NOT attempt to decode or decrypt a packet from an
   unknown version, but instead send a Version Negotiation packet,
   provided that the packet is sufficiently long.

   Servers MUST drop other packets that contain unsupported versions.

   Packets with a supported version, or no version field, are matched to
   a connection using the connection ID or - for packets with zero-
   length connection IDs - the address tuple.  If the packet doesn't
   match an existing connection, the server continues below.

   If the packet is an Initial packet fully conforming with the
   specification, the server proceeds with the handshake (Section 7).
   This commits the server to the version that the client selected.

   If a server isn't currently accepting any new connections, it SHOULD
   send an Initial packet containing a CONNECTION_CLOSE frame with error
   code SERVER_BUSY.

   If the packet is a 0-RTT packet, the server MAY buffer a limited
   number of these packets in anticipation of a late-arriving Initial
   Packet.  Clients are forbidden from sending Handshake packets prior
   to receiving a server response, so servers SHOULD ignore any such
   packets.

   Servers MUST drop incoming packets under all other circumstances.

5.3.  Life of a QUIC Connection

   TBD.

6.  Version Negotiation

   Version negotiation ensures that client and server agree to a QUIC
   version that is mutually supported.  A server sends a Version
   Negotiation packet in response to each packet that might initiate a
   new connection, see Section 5.2 for details.

   The first few messages of an exchange between a client attempting to
   create a new connection with server is shown in Figure 3.  After
   version negotiation completes, connection establishment can proceed,
   for example as shown in Section 7.1.




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   Client                                                  Server

   Packet (v=X) ->

                           <- Version Negotiation (supported=Y,Z)

   Packet (v=Y) ->

                                               <- Packet(s) (v=Y)

              Figure 3: Example Version Negotiation Exchange

   The size of the first packet sent by a client will determine whether
   a server sends a Version Negotiation packet.  Clients that support
   multiple QUIC versions SHOULD pad the first packet they send to the
   largest of the minimum packet sizes across all versions they support.
   This ensures that the server responds if there is a mutually
   supported version.

6.1.  Sending Version Negotiation Packets

   If the version selected by the client is not acceptable to the
   server, the server responds with a Version Negotiation packet (see
   Section 17.4).  This includes a list of versions that the server will
   accept.

   This system allows a server to process packets with unsupported
   versions without retaining state.  Though either the Initial packet
   or the Version Negotiation packet that is sent in response could be
   lost, the client will send new packets until it successfully receives
   a response or it abandons the connection attempt.

   A server MAY limit the number of Version Negotiation packets it
   sends.  For instance, a server that is able to recognize packets as
   0-RTT might choose not to send Version Negotiation packets in
   response to 0-RTT packets with the expectation that it will
   eventually receive an Initial packet.

6.2.  Handling Version Negotiation Packets

   When the client receives a Version Negotiation packet, it first
   checks that the Destination and Source Connection ID fields match the
   Source and Destination Connection ID fields in a packet that the
   client sent.  If this check fails, the packet MUST be discarded.

   Once the Version Negotiation packet is determined to be valid, the
   client then selects an acceptable protocol version from the list
   provided by the server.  The client then attempts to create a



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   connection using that version.  Though the content of the Initial
   packet the client sends might not change in response to version
   negotiation, a client MUST increase the packet number it uses on
   every packet it sends.  Packets MUST continue to use long headers
   (Section 17.2) and MUST include the new negotiated protocol version.

   The client MUST use the long header format and include its selected
   version on all packets until it has 1-RTT keys and it has received a
   packet from the server which is not a Version Negotiation packet.

   A client MUST NOT change the version it uses unless it is in response
   to a Version Negotiation packet from the server.  Once a client
   receives a packet from the server which is not a Version Negotiation
   packet, it MUST discard other Version Negotiation packets on the same
   connection.  Similarly, a client MUST ignore a Version Negotiation
   packet if it has already received and acted on a Version Negotiation
   packet.

   A client MUST ignore a Version Negotiation packet that lists the
   client's chosen version.

   A client MAY attempt 0-RTT after receiving a Version Negotiation
   packet.  A client that sends additional 0-RTT packets MUST NOT reset
   the packet number to 0 as a result, see Section 17.5.2.

   Version negotiation packets have no cryptographic protection.  The
   result of the negotiation MUST be revalidated as part of the
   cryptographic handshake (see Section 7.3.3).

6.3.  Using Reserved Versions

   For a server to use a new version in the future, clients must
   correctly handle unsupported versions.  To help ensure this, a server
   SHOULD include a reserved version (see Section 15) while generating a
   Version Negotiation packet.

   The design of version negotiation permits a server to avoid
   maintaining state for packets that it rejects in this fashion.  The
   validation of version negotiation (see Section 7.3.3) only validates
   the result of version negotiation, which is the same no matter which
   reserved version was sent.  A server MAY therefore send different
   reserved version numbers in the Version Negotiation Packet and in its
   transport parameters.

   A client MAY send a packet using a reserved version number.  This can
   be used to solicit a list of supported versions from a server.





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7.  Cryptographic and Transport Handshake

   QUIC relies on a combined cryptographic and transport handshake to
   minimize connection establishment latency.  QUIC uses the CRYPTO
   frame Section 19.20 to transmit the cryptographic handshake.  Version
   0x00000001 of QUIC uses TLS 1.3 as described in [QUIC-TLS]; a
   different QUIC version number could indicate that a different
   cryptographic handshake protocol is in use.

   QUIC provides reliable, ordered delivery of the cryptographic
   handshake data.  QUIC packet protection ensures confidentiality and
   integrity protection that meets the requirements of the cryptographic
   handshake protocol:

   o  authenticated key exchange, where

      *  a server is always authenticated,

      *  a client is optionally authenticated,

      *  every connection produces distinct and unrelated keys,

      *  keying material is usable for packet protection for both 0-RTT
         and 1-RTT packets, and

      *  1-RTT keys have forward secrecy

   o  authenticated values for the transport parameters of the peer (see
      Section 7.3)

   o  authenticated confirmation of version negotiation (see
      Section 7.3.3)

   o  authenticated negotiation of an application protocol (TLS uses
      ALPN [RFC7301] for this purpose)

   The first CRYPTO frame from a client MUST be sent in a single packet.
   Any second attempt that is triggered by address validation (see
   Section 8.1) MUST also be sent within a single packet.  This avoids
   having to reassemble a message from multiple packets.

   The first client packet of the cryptographic handshake protocol MUST
   fit within a 1232 octet QUIC packet payload.  This includes overheads
   that reduce the space available to the cryptographic handshake
   protocol.

   The CRYPTO frame can be sent in different packet number spaces.  The
   sequence numbers used by CRYPTO frames to ensure ordered delivery of



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   cryptographic handshake data start from zero in each packet number
   space.

7.1.  Example Handshake Flows

   Details of how TLS is integrated with QUIC are provided in
   [QUIC-TLS], but some examples are provided here.  An extension of
   this exchange to support client address validation is shown in
   Section 8.1.1.

   Once any version negotiation and address validation exchanges are
   complete, the cryptographic handshake is used to agree on
   cryptographic keys.  The cryptographic handshake is carried in
   Initial (Section 17.5) and Handshake (Section 17.6) packets.

   Figure 4 provides an overview of the 1-RTT handshake.  Each line
   shows a QUIC packet with the packet type and packet number shown
   first, followed by the frames that are typically contained in those
   packets.  So, for instance the first packet is of type Initial, with
   packet number 0, and contains a CRYPTO frame carrying the
   ClientHello.

   Note that multiple QUIC packets - even of different encryption levels
   - may be coalesced into a single UDP datagram (see Section 12.2), and
   so this handshake may consist of as few as 4 UDP datagrams, or any
   number more.  For instance, the server's first flight contains
   packets from the Initial encryption level (obfuscation), the
   Handshake level, and "0.5-RTT data" from the server at the 1-RTT
   encryption level.

   Client                                                  Server

   Initial[0]: CRYPTO[CH] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                    <- 1-RTT[0]: STREAM[1, "..."]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[0]: STREAM[0, "..."], ACK[0] ->

                              1-RTT[1]: STREAM[55, "..."], ACK[0]
                                          <- Handshake[1]: ACK[0]

                     Figure 4: Example 1-RTT Handshake





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   Figure 5 shows an example of a connection with a 0-RTT handshake and
   a single packet of 0-RTT data.  Note that as described in
   Section 12.3, the server acknowledges 0-RTT data at the 1-RTT
   encryption level, and the client sends 1-RTT packets in the same
   packet number space.

   Client                                                  Server

   Initial[0]: CRYPTO[CH]
   0-RTT[0]: STREAM[0, "..."] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                           Handshake[0] CRYPTO[EE, CERT, CV, FIN]
                             <- 1-RTT[0]: STREAM[1, "..."] ACK[0]

   Initial[1]: ACK[0]
   Handshake[0]: CRYPTO[FIN], ACK[0]
   1-RTT[2]: STREAM[0, "..."] ACK[0] ->

                            1-RTT[1]: STREAM[55, "..."], ACK[1,2]
                                          <- Handshake[1]: ACK[0]

                     Figure 5: Example 0-RTT Handshake

7.2.  Negotiating Connection IDs

   A connection ID is used to ensure consistent routing of packets, as
   described in Section 5.1.  The long header contains two connection
   IDs: the Destination Connection ID is chosen by the recipient of the
   packet and is used to provide consistent routing; the Source
   Connection ID is used to set the Destination Connection ID used by
   the peer.

   During the handshake, packets with the long header (Section 17.2) are
   used to establish the connection ID that each endpoint uses.  Each
   endpoint uses the Source Connection ID field to specify the
   connection ID that is used in the Destination Connection ID field of
   packets being sent to them.  Upon receiving a packet, each endpoint
   sets the Destination Connection ID it sends to match the value of the
   Source Connection ID that they receive.

   When an Initial packet is sent by a client which has not previously
   received a Retry packet from the server, it populates the Destination
   Connection ID field with an unpredictable value.  This MUST be at
   least 8 octets in length.  Until a packet is received from the
   server, the client MUST use the same value unless it abandons the
   connection attempt and starts a new one.  The initial Destination




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   Connection ID is used to determine packet protection keys for Initial
   packets.

   The client populates the Source Connection ID field with a value of
   its choosing and sets the SCIL field to match.

   The Destination Connection ID field in the server's Initial packet
   contains a connection ID that is chosen by the recipient of the
   packet (i.e., the client); the Source Connection ID includes the
   connection ID that the sender of the packet wishes to use (see
   Section 5.1).  The server MUST use consistent Source Connection IDs
   during the handshake.

   On first receiving an Initial or Retry packet from the server, the
   client uses the Source Connection ID supplied by the server as the
   Destination Connection ID for subsequent packets.  That means that a
   client might change the Destination Connection ID twice during
   connection establishment.  Once a client has received an Initial
   packet from the server, it MUST discard any packet it receives with a
   different Source Connection ID.

   A client MUST only change the value it sends in the Destination
   Connection ID in response to the first packet of each type it
   receives from the server (Retry or Initial); a server MUST set its
   value based on the Initial packet.  Any additional changes are not
   permitted; if subsequent packets of those types include a different
   Source Connection ID, they MUST be discarded.  This avoids problems
   that might arise from stateless processing of multiple Initial
   packets producing different connection IDs.

   The connection ID can change over the lifetime of a connection,
   especially in response to connection migration (Section 9), see
   Section 5.1.1 for details.

7.3.  Transport Parameters

   During connection establishment, both endpoints make authenticated
   declarations of their transport parameters.  These declarations are
   made unilaterally by each endpoint.  Endpoints are required to comply
   with the restrictions implied by these parameters; the description of
   each parameter includes rules for its handling.

   The encoding of the transport parameters is detailed in Section 18.

   QUIC includes the encoded transport parameters in the cryptographic
   handshake.  Once the handshake completes, the transport parameters
   declared by the peer are available.  Each endpoint validates the
   value provided by its peer.  In particular, version negotiation MUST



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   be validated (see Section 7.3.3) before the connection establishment
   is considered properly complete.

   Definitions for each of the defined transport parameters are included
   in Section 18.1.  Any given parameter MUST appear at most once in a
   given transport parameters extension.  An endpoint MUST treat receipt
   of duplicate transport parameters as a connection error of type
   TRANSPORT_PARAMETER_ERROR.

   A server MUST include the original_connection_id transport parameter
   (Section 18.1) if it sent a Retry packet.

7.3.1.  Values of Transport Parameters for 0-RTT

   A client that attempts to send 0-RTT data MUST remember the transport
   parameters used by the server.  The transport parameters that the
   server advertises during connection establishment apply to all
   connections that are resumed using the keying material established
   during that handshake.  Remembered transport parameters apply to the
   new connection until the handshake completes and new transport
   parameters from the server can be provided.

   A server can remember the transport parameters that it advertised, or
   store an integrity-protected copy of the values in the ticket and
   recover the information when accepting 0-RTT data.  A server uses the
   transport parameters in determining whether to accept 0-RTT data.

   A server MAY accept 0-RTT and subsequently provide different values
   for transport parameters for use in the new connection.  If 0-RTT
   data is accepted by the server, the server MUST NOT reduce any limits
   or alter any values that might be violated by the client with its
   0-RTT data.  In particular, a server that accepts 0-RTT data MUST NOT
   set values for initial_max_data, initial_max_stream_data_bidi_local,
   initial_max_stream_data_bidi_remote, initial_max_stream_data_uni,
   initial_max_bidi_streams, or initial_max_uni_streams (Section 18.1)
   that are smaller than the remembered value of those parameters.

   Omitting or setting a zero value for certain transport parameters can
   result in 0-RTT data being enabled, but not usable.  The applicable
   subset of transport parameters that permit sending of application
   data SHOULD be set to non-zero values for 0-RTT.  This includes
   initial_max_data and either initial_max_bidi_streams and
   initial_max_stream_data_bidi_remote, or initial_max_uni_streams and
   initial_max_stream_data_uni.

   The value of the server's previous preferred_address MUST NOT be used
   when establishing a new connection; rather, the client should wait to
   observe the server's new preferred_address value in the handshake.



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   A server MUST reject 0-RTT data or even abort a handshake if the
   implied values for transport parameters cannot be supported.

7.3.2.  New Transport Parameters

   New transport parameters can be used to negotiate new protocol
   behavior.  An endpoint MUST ignore transport parameters that it does
   not support.  Absence of a transport parameter therefore disables any
   optional protocol feature that is negotiated using the parameter.

   New transport parameters can be registered according to the rules in
   Section 22.1.

7.3.3.  Version Negotiation Validation

   Though the cryptographic handshake has integrity protection, two
   forms of QUIC version downgrade are possible.  In the first, an
   attacker replaces the QUIC version in the Initial packet.  In the
   second, a fake Version Negotiation packet is sent by an attacker.  To
   protect against these attacks, the transport parameters include three
   fields that encode version information.  These parameters are used to
   retroactively authenticate the choice of version (see Section 6).

   The cryptographic handshake provides integrity protection for the
   negotiated version as part of the transport parameters (see
   Section 18.1).  As a result, attacks on version negotiation by an
   attacker can be detected.

   The client includes the initial_version field in its transport
   parameters.  The initial_version is the version that the client
   initially attempted to use.  If the server did not send a Version
   Negotiation packet Section 17.4, this will be identical to the
   negotiated_version field in the server transport parameters.

   A server that processes all packets in a stateful fashion can
   remember how version negotiation was performed and validate the
   initial_version value.

   A server that does not maintain state for every packet it receives
   (i.e., a stateless server) uses a different process.  If the
   initial_version matches the version of QUIC that is in use, a
   stateless server can accept the value.

   If the initial_version is different from the version of QUIC that is
   in use, a stateless server MUST check that it would have sent a
   Version Negotiation packet if it had received a packet with the
   indicated initial_version.  If a server would have accepted the
   version included in the initial_version and the value differs from



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   the QUIC version that is in use, the server MUST terminate the
   connection with a VERSION_NEGOTIATION_ERROR error.

   The server includes both the version of QUIC that is in use and a
   list of the QUIC versions that the server supports (see
   Section 18.1).

   The negotiated_version field is the version that is in use.  This
   MUST be set by the server to the value that is on the Initial packet
   that it accepts (not an Initial packet that triggers a Retry or
   Version Negotiation packet).  A client that receives a
   negotiated_version that does not match the version of QUIC that is in
   use MUST terminate the connection with a VERSION_NEGOTIATION_ERROR
   error code.

   The server includes a list of versions that it would send in any
   version negotiation packet (Section 17.4) in the supported_versions
   field.  The server populates this field even if it did not send a
   version negotiation packet.

   The client validates that the negotiated_version is included in the
   supported_versions list and - if version negotiation was performed -
   that it would have selected the negotiated version.  A client MUST
   terminate the connection with a VERSION_NEGOTIATION_ERROR error code
   if the current QUIC version is not listed in the supported_versions
   list.  A client MUST terminate with a VERSION_NEGOTIATION_ERROR error
   code if version negotiation occurred but it would have selected a
   different version based on the value of the supported_versions list.

   When an endpoint accepts multiple QUIC versions, it can potentially
   interpret transport parameters as they are defined by any of the QUIC
   versions it supports.  The version field in the QUIC packet header is
   authenticated using transport parameters.  The position and the
   format of the version fields in transport parameters MUST either be
   identical across different QUIC versions, or be unambiguously
   different to ensure no confusion about their interpretation.  One way
   that a new format could be introduced is to define a TLS extension
   with a different codepoint.

8.  Address Validation

   Address validation is used by QUIC to avoid being used for a traffic
   amplification attack.  In such an attack, a packet is sent to a
   server with spoofed source address information that identifies a
   victim.  If a server generates more or larger packets in response to
   that packet, the attacker can use the server to send more data toward
   the victim than it would be able to send on its own.




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   The primary defense against amplification attack is verifying that an
   endpoint is able to receive packets at the transport address that it
   claims.  Address validation is performed both during connection
   establishment (see Section 8.1) and during connection migration (see
   Section 8.2).

8.1.  Address Validation During Connection Establishment

   Connection establishment implicitly provides address validation for
   both endpoints.  In particular, receipt of a packet protected with
   Handshake keys confirms that the client received the Initial packet
   from the server.  Once the server has successfully processed a
   Handshake packet from the client, it can consider the client address
   to have been validated.

   Prior to validating the client address, servers MUST NOT send more
   than three times as many bytes as the number of bytes they have
   received.  This limits the magnitude of any amplification attack that
   can be mounted using spoofed source addresses.

   To ensure that the server is not overly constrained by this
   restriction, clients MUST send UDP datagrams with at least 1200
   octets of payload until the server has completed address validation,
   see Section 14.

   In order to prevent a handshake deadlock as a result of the server
   being unable to send, clients SHOULD send a packet upon a handshake
   timeout, as described in [QUIC-RECOVERY].  If the client has no data
   to retransmit and does not have Handshake keys, it SHOULD send an
   Initial packet in a UDP datagram of at least 1200 octets.  If the
   client has Handshake keys, it SHOULD send a Handshake packet.

   A server might wish to validate the client address before starting
   the cryptographic handshake.  Client addresses can be verified using
   an address validation token.  This token is delivered during
   connection establishment with a Retry packet (see Section 8.1.1) or
   in a previous connection using the NEW_TOKEN frame (see
   Section 8.1.2).

8.1.1.  Address Validation using Retry Packets

   QUIC uses token-based address validation during connection
   establishment.  Any time the server wishes to validate a client
   address, it provides the client with a token.  As long as it is not
   possible for an attacker to generate a valid token for its own
   address (see Section 8.1.3) and the client is able to return that
   token, it proves to the server that it received the token.




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   Upon receiving the client's Initial packet, the server can request
   address validation by sending a Retry packet (Section 17.7)
   containing a token.  This token is repeated by the client in an
   Initial packet after it receives the Retry packet.  In response to
   receiving a token in an Initial packet, a server can either abort the
   connection or permit it to proceed.

   A server can also use a Retry packet to defer the state and
   processing costs of connection establishment.  By giving the client a
   different connection ID to use, a server can cause the connection to
   be routed to a server instance with more resources available for new
   connections.

   A flow showing the use of a Retry packet is shown in Figure 6.

   Client                                                  Server

   Initial[0]: CRYPTO[CH] ->

                                                   <- Retry+Token

   Initial+Token[0]: CRYPTO[CH] ->

                                    Initial[0]: CRYPTO[SH] ACK[0]
                          Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                    <- 1-RTT[0]: STREAM[1, "..."]

                  Figure 6: Example Handshake with Retry

8.1.2.  Address Validation for Future Connections

   A server MAY provide clients with an address validation token during
   one connection that can be used on a subsequent connection.  Address
   validation is especially important with 0-RTT because a server
   potentially sends a significant amount of data to a client in
   response to 0-RTT data.

   The server uses the NEW_TOKEN frame Section 19.18 to provide the
   client with an address validation token that can be used to validate
   future connections.  The client may then use this token to validate
   future connections by including it in the Initial packet's header.
   The client MUST NOT use the token provided in a Retry for future
   connections.

   Unlike the token that is created for a Retry packet, there might be
   some time between when the token is created and when the token is
   subsequently used.  Thus, a resumption token SHOULD include an
   expiration time.  The server MAY include either an explicit



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   expiration time or an issued timestamp and dynamically calculate the
   expiration time.  It is also unlikely that the client port number is
   the same on two different connections; validating the port is
   therefore unlikely to be successful.

   A resumption token SHOULD be constructed to be easily distinguishable
   from tokens that are sent in Retry packets as they are carried in the
   same field.

   If the client has a token received in a NEW_TOKEN frame on a previous
   connection to what it believes to be the same server, it can include
   that value in the Token field of its Initial packet.

   A token allows a server to correlate activity between the connection
   where the token was issued and any connection where it is used.
   Clients that want to break continuity of identity with a server MAY
   discard tokens provided using the NEW_TOKEN frame.  Tokens obtained
   in Retry packets MUST NOT be discarded.

   A client SHOULD NOT reuse a token.  Reusing a token allows
   connections to be linked by entities on the network path (see
   Section 9.5).  A client MUST NOT reuse a token if it believes that
   its point of network attachment has changed since the token was last
   used; that is, if there is a change in its local IP address or
   network interface.  A client needs to start the connection process
   over if it migrates prior to completing the handshake.

   When a server receives an Initial packet with an address validation
   token, it SHOULD attempt to validate it.  If the token is invalid
   then the server SHOULD proceed as if the client did not have a
   validated address, including potentially sending a Retry.  If the
   validation succeeds, the server SHOULD then allow the handshake to
   proceed.

   Note:  The rationale for treating the client as unvalidated rather
      than discarding the packet is that the client might have received
      the token in a previous connection using the NEW_TOKEN frame, and
      if the server has lost state, it might be unable to validate the
      token at all, leading to connection failure if the packet is
      discarded.  A server MAY encode tokens provided with NEW_TOKEN
      frames and Retry packets differently, and validate the latter more
      strictly.

   In a stateless design, a server can use encrypted and authenticated
   tokens to pass information to clients that the server can later
   recover and use to validate a client address.  Tokens are not
   integrated into the cryptographic handshake and so they are not
   authenticated.  For instance, a client might be able to reuse a



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   token.  To avoid attacks that exploit this property, a server can
   limit its use of tokens to only the information needed validate
   client addresses.

8.1.3.  Address Validation Token Integrity

   An address validation token MUST be difficult to guess.  Including a
   large enough random value in the token would be sufficient, but this
   depends on the server remembering the value it sends to clients.

   A token-based scheme allows the server to offload any state
   associated with validation to the client.  For this design to work,
   the token MUST be covered by integrity protection against
   modification or falsification by clients.  Without integrity
   protection, malicious clients could generate or guess values for
   tokens that would be accepted by the server.  Only the server
   requires access to the integrity protection key for tokens.

   There is no need for a single well-defined format for the token
   because the server that generates the token also consumes it.  A
   token could include information about the claimed client address (IP
   and port), a timestamp, and any other supplementary information the
   server will need to validate the token in the future.

8.2.  Path Validation

   Path validation is used during connection migration (see Section 9
   and Section 9.6) by the migrating endpoint to verify reachability of
   a peer from a new local address.  In path validation, endpoints test
   reachability between a specific local address and a specific peer
   address, where an address is the two-tuple of IP address and port.

   Path validation tests that packets can be both sent to and received
   from a peer on the path.  Importantly, it validates that the packets
   received from the migrating endpoint do not carry a spoofed source
   address.

   Path validation can be used at any time by either endpoint.  For
   instance, an endpoint might check that a peer is still in possession
   of its address after a period of quiescence.

   Path validation is not designed as a NAT traversal mechanism.  Though
   the mechanism described here might be effective for the creation of
   NAT bindings that support NAT traversal, the expectation is that one
   or other peer is able to receive packets without first having sent a
   packet on that path.  Effective NAT traversal needs additional
   synchronization mechanisms that are not provided here.




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   An endpoint MAY bundle PATH_CHALLENGE and PATH_RESPONSE frames that
   are used for path validation with other frames.  In particular, an
   endpoint may pad a packet carrying a PATH_CHALLENGE for PMTU
   discovery, or an endpoint may bundle a PATH_RESPONSE with its own
   PATH_CHALLENGE.

   When probing a new path, an endpoint might want to ensure that its
   peer has an unused connection ID available for responses.  The
   endpoint can send NEW_CONNECTION_ID and PATH_CHALLENGE frames in the
   same packet.  This ensures that an unused connection ID will be
   available to the peer when sending a response.

8.3.  Initiating Path Validation

   To initiate path validation, an endpoint sends a PATH_CHALLENGE frame
   containing a random payload on the path to be validated.

   An endpoint MAY send multiple PATH_CHALLENGE frames to guard against
   packet loss.  An endpoint SHOULD NOT send a PATH_CHALLENGE more
   frequently than it would an Initial packet, ensuring that connection
   migration is no more load on a new path than establishing a new
   connection.

   The endpoint MUST use fresh random data in every PATH_CHALLENGE frame
   so that it can associate the peer's response with the causative
   PATH_CHALLENGE.

8.4.  Path Validation Responses

   On receiving a PATH_CHALLENGE frame, an endpoint MUST respond
   immediately by echoing the data contained in the PATH_CHALLENGE frame
   in a PATH_RESPONSE frame.  However, because a PATH_CHALLENGE might be
   sent from a spoofed address, an endpoint MUST limit the rate at which
   it sends PATH_RESPONSE frames and MAY silently discard PATH_CHALLENGE
   frames that would cause it to respond at a higher rate.

   To ensure that packets can be both sent to and received from the
   peer, the PATH_RESPONSE MUST be sent on the same path as the
   triggering PATH_CHALLENGE.  That is, from the same local address on
   which the PATH_CHALLENGE was received, to the same remote address
   from which the PATH_CHALLENGE was received.

8.5.  Successful Path Validation

   A new address is considered valid when a PATH_RESPONSE frame is
   received containing data that was sent in a previous PATH_CHALLENGE.
   Receipt of an acknowledgment for a packet containing a PATH_CHALLENGE




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   frame is not adequate validation, since the acknowledgment can be
   spoofed by a malicious peer.

   For path validation to be successful, a PATH_RESPONSE frame MUST be
   received from the same remote address to which the corresponding
   PATH_CHALLENGE was sent.  If a PATH_RESPONSE frame is received from a
   different remote address than the one to which the PATH_CHALLENGE was
   sent, path validation is considered to have failed, even if the data
   matches that sent in the PATH_CHALLENGE.

   Additionally, the PATH_RESPONSE frame MUST be received on the same
   local address from which the corresponding PATH_CHALLENGE was sent.
   If a PATH_RESPONSE frame is received on a different local address
   than the one from which the PATH_CHALLENGE was sent, path validation
   is considered to have failed, even if the data matches that sent in
   the PATH_CHALLENGE.  Thus, the endpoint considers the path to be
   valid when a PATH_RESPONSE frame is received on the same path with
   the same payload as the PATH_CHALLENGE frame.

8.6.  Failed Path Validation

   Path validation only fails when the endpoint attempting to validate
   the path abandons its attempt to validate the path.

   Endpoints SHOULD abandon path validation based on a timer.  When
   setting this timer, implementations are cautioned that the new path
   could have a longer round-trip time than the original.

   Note that the endpoint might receive packets containing other frames
   on the new path, but a PATH_RESPONSE frame with appropriate data is
   required for path validation to succeed.

   When an endpoint abandons path validation, it determines that the
   path is unusable.  This does not necessarily imply a failure of the
   connection - endpoints can continue sending packets over other paths
   as appropriate.  If no paths are available, an endpoint can wait for
   a new path to become available or close the connection.

   A path validation might be abandoned for other reasons besides
   failure.  Primarily, this happens if a connection migration to a new
   path is initiated while a path validation on the old path is in
   progress.

9.  Connection Migration

   The use of a connection ID allows connections to survive changes to
   endpoint addresses (that is, IP address and/or port), such as those




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   caused by an endpoint migrating to a new network.  This section
   describes the process by which an endpoint migrates to a new address.

   An endpoint MUST NOT initiate connection migration before the
   handshake is finished and the endpoint has 1-RTT keys.  The design of
   QUIC relies on endpoints retaining a stable address for the duration
   of the handshake.

   An endpoint also MUST NOT initiate connection migration if the peer
   sent the "disable_migration" transport parameter during the
   handshake.  An endpoint which has sent this transport parameter, but
   detects that a peer has nonetheless migrated to a different network
   MAY treat this as a connection error of type INVALID_MIGRATION.

   Not all changes of peer address are intentional migrations.  The peer
   could experience NAT rebinding: a change of address due to a
   middlebox, usually a NAT, allocating a new outgoing port or even a
   new outgoing IP address for a flow.  NAT rebinding is not connection
   migration as defined in this section, though an endpoint SHOULD
   perform path validation (Section 8.2) if it detects a change in the
   IP address of its peer.

   This document limits migration of connections to new client
   addresses, except as described in Section 9.6.  Clients are
   responsible for initiating all migrations.  Servers do not send non-
   probing packets (see Section 9.1) toward a client address until they
   see a non-probing packet from that address.  If a client receives
   packets from an unknown server address, the client MAY discard these
   packets.

9.1.  Probing a New Path

   An endpoint MAY probe for peer reachability from a new local address
   using path validation Section 8.2 prior to migrating the connection
   to the new local address.  Failure of path validation simply means
   that the new path is not usable for this connection.  Failure to
   validate a path does not cause the connection to end unless there are
   no valid alternative paths available.

   An endpoint uses a new connection ID for probes sent from a new local
   address, see Section 9.5 for further discussion.  An endpoint that
   uses a new local address needs to ensure that at least one new
   connection ID is available at the peer.  That can be achieved by
   including a NEW_CONNECTION_ID frame in the probe.

   Receiving a PATH_CHALLENGE frame from a peer indicates that the peer
   is probing for reachability on a path.  An endpoint sends a
   PATH_RESPONSE in response as per Section 8.2.



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   PATH_CHALLENGE, PATH_RESPONSE, NEW_CONNECTION_ID, and PADDING frames
   are "probing frames", and all other frames are "non-probing frames".
   A packet containing only probing frames is a "probing packet", and a
   packet containing any other frame is a "non-probing packet".

9.2.  Initiating Connection Migration

   An endpoint can migrate a connection to a new local address by
   sending packets containing non-probing frames from that address.

   Each endpoint validates its peer's address during connection
   establishment.  Therefore, a migrating endpoint can send to its peer
   knowing that the peer is willing to receive at the peer's current
   address.  Thus an endpoint can migrate to a new local address without
   first validating the peer's address.

   When migrating, the new path might not support the endpoint's current
   sending rate.  Therefore, the endpoint resets its congestion
   controller, as described in Section 9.4.

   The new path might not have the same ECN capability.  Therefore, the
   endpoint verifies ECN capability as described in Section 13.3.

   Receiving acknowledgments for data sent on the new path serves as
   proof of the peer's reachability from the new address.  Note that
   since acknowledgments may be received on any path, return
   reachability on the new path is not established.  To establish return
   reachability on the new path, an endpoint MAY concurrently initiate
   path validation Section 8.2 on the new path.

9.3.  Responding to Connection Migration

   Receiving a packet from a new peer address containing a non-probing
   frame indicates that the peer has migrated to that address.

   In response to such a packet, an endpoint MUST start sending
   subsequent packets to the new peer address and MUST initiate path
   validation (Section 8.2) to verify the peer's ownership of the
   unvalidated address.

   An endpoint MAY send data to an unvalidated peer address, but it MUST
   protect against potential attacks as described in Section 9.3.1 and
   Section 9.3.2.  An endpoint MAY skip validation of a peer address if
   that address has been seen recently.

   An endpoint only changes the address that it sends packets to in
   response to the highest-numbered non-probing packet.  This ensures




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   that an endpoint does not send packets to an old peer address in the
   case that it receives reordered packets.

   After changing the address to which it sends non-probing packets, an
   endpoint could abandon any path validation for other addresses.

   Receiving a packet from a new peer address might be the result of a
   NAT rebinding at the peer.

   After verifying a new client address, the server SHOULD send new
   address validation tokens (Section 8) to the client.

9.3.1.  Handling Address Spoofing by a Peer

   It is possible that a peer is spoofing its source address to cause an
   endpoint to send excessive amounts of data to an unwilling host.  If
   the endpoint sends significantly more data than the spoofing peer,
   connection migration might be used to amplify the volume of data that
   an attacker can generate toward a victim.

   As described in Section 9.3, an endpoint is required to validate a
   peer's new address to confirm the peer's possession of the new
   address.  Until a peer's address is deemed valid, an endpoint MUST
   limit the rate at which it sends data to this address.  The endpoint
   MUST NOT send more than a minimum congestion window's worth of data
   per estimated round-trip time (kMinimumWindow, as defined in
   [QUIC-RECOVERY]).  In the absence of this limit, an endpoint risks
   being used for a denial of service attack against an unsuspecting
   victim.  Note that since the endpoint will not have any round-trip
   time measurements to this address, the estimate SHOULD be the default
   initial value (see [QUIC-RECOVERY]).

   If an endpoint skips validation of a peer address as described in
   Section 9.3, it does not need to limit its sending rate.

9.3.2.  Handling Address Spoofing by an On-path Attacker

   An on-path attacker could cause a spurious connection migration by
   copying and forwarding a packet with a spoofed address such that it
   arrives before the original packet.  The packet with the spoofed
   address will be seen to come from a migrating connection, and the
   original packet will be seen as a duplicate and dropped.  After a
   spurious migration, validation of the source address will fail
   because the entity at the source address does not have the necessary
   cryptographic keys to read or respond to the PATH_CHALLENGE frame
   that is sent to it even if it wanted to.





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   To protect the connection from failing due to such a spurious
   migration, an endpoint MUST revert to using the last validated peer
   address when validation of a new peer address fails.

   If an endpoint has no state about the last validated peer address, it
   MUST close the connection silently by discarding all connection
   state.  This results in new packets on the connection being handled
   generically.  For instance, an endpoint MAY send a stateless reset in
   response to any further incoming packets.

   Note that receipt of packets with higher packet numbers from the
   legitimate peer address will trigger another connection migration.
   This will cause the validation of the address of the spurious
   migration to be abandoned.

9.4.  Loss Detection and Congestion Control

   The capacity available on the new path might not be the same as the
   old path.  Packets sent on the old path SHOULD NOT contribute to
   congestion control or RTT estimation for the new path.

   On confirming a peer's ownership of its new address, an endpoint
   SHOULD immediately reset the congestion controller and round-trip
   time estimator for the new path.

   An endpoint MUST NOT return to the send rate used for the previous
   path unless it is reasonably sure that the previous send rate is
   valid for the new path.  For instance, a change in the client's port
   number is likely indicative of a rebinding in a middlebox and not a
   complete change in path.  This determination likely depends on
   heuristics, which could be imperfect; if the new path capacity is
   significantly reduced, ultimately this relies on the congestion
   controller responding to congestion signals and reducing send rates
   appropriately.

   There may be apparent reordering at the receiver when an endpoint
   sends data and probes from/to multiple addresses during the migration
   period, since the two resulting paths may have different round-trip
   times.  A receiver of packets on multiple paths will still send ACK
   frames covering all received packets.

   While multiple paths might be used during connection migration, a
   single congestion control context and a single loss recovery context
   (as described in [QUIC-RECOVERY]) may be adequate.  A sender can make
   exceptions for probe packets so that their loss detection is
   independent and does not unduly cause the congestion controller to
   reduce its sending rate.  An endpoint might set a separate timer when
   a PATH_CHALLENGE is sent, which is cancelled when the corresponding



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   PATH_RESPONSE is received.  If the timer fires before the
   PATH_RESPONSE is received, the endpoint might send a new
   PATH_CHALLENGE, and restart the timer for a longer period of time.

9.5.  Privacy Implications of Connection Migration

   Using a stable connection ID on multiple network paths allows a
   passive observer to correlate activity between those paths.  An
   endpoint that moves between networks might not wish to have their
   activity correlated by any entity other than their peer, so different
   connection IDs are used when sending from different local addresses,
   as discussed in Section 5.1.  For this to be effective endpoints need
   to ensure that connections IDs they provide cannot be linked by any
   other entity.

   This eliminates the use of the connection ID for linking activity
   from the same connection on different networks.  Protection of packet
   numbers ensures that packet numbers cannot be used to correlate
   activity.  This does not prevent other properties of packets, such as
   timing and size, from being used to correlate activity.

   Clients MAY move to a new connection ID at any time based on
   implementation-specific concerns.  For example, after a period of
   network inactivity NAT rebinding might occur when the client begins
   sending data again.

   A client might wish to reduce linkability by employing a new
   connection ID and source UDP port when sending traffic after a period
   of inactivity.  Changing the UDP port from which it sends packets at
   the same time might cause the packet to appear as a connection
   migration.  This ensures that the mechanisms that support migration
   are exercised even for clients that don't experience NAT rebindings
   or genuine migrations.  Changing port number can cause a peer to
   reset its congestion state (see Section 9.4), so the port SHOULD only
   be changed infrequently.

   Endpoints that use connection IDs with length greater than zero could
   have their activity correlated if their peers keep using the same
   destination connection ID after migration.  Endpoints that receive
   packets with a previously unused Destination Connection ID SHOULD
   change to sending packets with a connection ID that has not been used
   on any other network path.  The goal here is to ensure that packets
   sent on different paths cannot be correlated.  To fulfill this
   privacy requirement, endpoints that initiate migration and use
   connection IDs with length greater than zero SHOULD provide their
   peers with new connection IDs before migration.





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   Caution:  If both endpoints change connection ID in response to
      seeing a change in connection ID from their peer, then this can
      trigger an infinite sequence of changes.

9.6.  Server's Preferred Address

   QUIC allows servers to accept connections on one IP address and
   attempt to transfer these connections to a more preferred address
   shortly after the handshake.  This is particularly useful when
   clients initially connect to an address shared by multiple servers
   but would prefer to use a unicast address to ensure connection
   stability.  This section describes the protocol for migrating a
   connection to a preferred server address.

   Migrating a connection to a new server address mid-connection is left
   for future work.  If a client receives packets from a new server
   address not indicated by the preferred_address transport parameter,
   the client SHOULD discard these packets.

9.6.1.  Communicating A Preferred Address

   A server conveys a preferred address by including the
   preferred_address transport parameter in the TLS handshake.

   Once the handshake is finished, the client SHOULD initiate path
   validation (see Section 8.2) of the server's preferred address using
   the connection ID provided in the preferred_address transport
   parameter.

   If path validation succeeds, the client SHOULD immediately begin
   sending all future packets to the new server address using the new
   connection ID and discontinue use of the old server address.  If path
   validation fails, the client MUST continue sending all future packets
   to the server's original IP address.

9.6.2.  Responding to Connection Migration

   A server might receive a packet addressed to its preferred IP address
   at any time after it accepts a connection.  If this packet contains a
   PATH_CHALLENGE frame, the server sends a PATH_RESPONSE frame as per
   Section 8.2.  The server MAY send other non-probing frames from its
   preferred address, but MUST continue sending all probing packets from
   its original IP address.

   The server SHOULD also initiate path validation of the client using
   its preferred address and the address from which it received the
   client probe.  This helps to guard against spurious migration
   initiated by an attacker.



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   Once the server has completed its path validation and has received a
   non-probing packet with a new largest packet number on its preferred
   address, the server begins sending non-probing packets to the client
   exclusively from its preferred IP address.  It SHOULD drop packets
   for this connection received on the old IP address, but MAY continue
   to process delayed packets.

9.6.3.  Interaction of Client Migration and Preferred Address

   A client might need to perform a connection migration before it has
   migrated to the server's preferred address.  In this case, the client
   SHOULD perform path validation to both the original and preferred
   server address from the client's new address concurrently.

   If path validation of the server's preferred address succeeds, the
   client MUST abandon validation of the original address and migrate to
   using the server's preferred address.  If path validation of the
   server's preferred address fails but validation of the server's
   original address succeeds, the client MAY migrate to its new address
   and continue sending to the server's original address.

   If the connection to the server's preferred address is not from the
   same client address, the server MUST protect against potential
   attacks as described in Section 9.3.1 and Section 9.3.2.  In addition
   to intentional simultaneous migration, this might also occur because
   the client's access network used a different NAT binding for the
   server's preferred address.

   Servers SHOULD initiate path validation to the client's new address
   upon receiving a probe packet from a different address.  Servers MUST
   NOT send more than a minimum congestion window's worth of non-probing
   packets to the new address before path validation is complete.

10.  Connection Termination

   Connections should remain open until they become idle for a pre-
   negotiated period of time.  A QUIC connection, once established, can
   be terminated in one of three ways:

   o  idle timeout (Section 10.2)

   o  immediate close (Section 10.3)

   o  stateless reset (Section 10.4)







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10.1.  Closing and Draining Connection States

   The closing and draining connection states exist to ensure that
   connections close cleanly and that delayed or reordered packets are
   properly discarded.  These states SHOULD persist for three times the
   current Retransmission Timeout (RTO) interval as defined in
   [QUIC-RECOVERY].

   An endpoint enters a closing period after initiating an immediate
   close (Section 10.3).  While closing, an endpoint MUST NOT send
   packets unless they contain a CONNECTION_CLOSE or APPLICATION_CLOSE
   frame (see Section 10.3 for details).  An endpoint retains only
   enough information to generate a packet containing a closing frame
   and to identify packets as belonging to the connection.  The
   connection ID and QUIC version is sufficient information to identify
   packets for a closing connection; an endpoint can discard all other
   connection state.  An endpoint MAY retain packet protection keys for
   incoming packets to allow it to read and process a closing frame.

   The draining state is entered once an endpoint receives a signal that
   its peer is closing or draining.  While otherwise identical to the
   closing state, an endpoint in the draining state MUST NOT send any
   packets.  Retaining packet protection keys is unnecessary once a
   connection is in the draining state.

   An endpoint MAY transition from the closing period to the draining
   period if it can confirm that its peer is also closing or draining.
   Receiving a closing frame is sufficient confirmation, as is receiving
   a stateless reset.  The draining period SHOULD end when the closing
   period would have ended.  In other words, the endpoint can use the
   same end time, but cease retransmission of the closing packet.

   Disposing of connection state prior to the end of the closing or
   draining period could cause delayed or reordered packets to be
   handled poorly.  Endpoints that have some alternative means to ensure
   that late-arriving packets on the connection do not create QUIC
   state, such as those that are able to close the UDP socket, MAY use
   an abbreviated draining period which can allow for faster resource
   recovery.  Servers that retain an open socket for accepting new
   connections SHOULD NOT exit the closing or draining period early.

   Once the closing or draining period has ended, an endpoint SHOULD
   discard all connection state.  This results in new packets on the
   connection being handled generically.  For instance, an endpoint MAY
   send a stateless reset in response to any further incoming packets.

   The draining and closing periods do not apply when a stateless reset
   (Section 10.4) is sent.



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   An endpoint is not expected to handle key updates when it is closing
   or draining.  A key update might prevent the endpoint from moving
   from the closing state to draining, but it otherwise has no impact.

   An endpoint could receive packets from a new source address,
   indicating a client connection migration (Section 9), while in the
   closing period.  An endpoint in the closing state MUST strictly limit
   the number of packets it sends to this new address until the address
   is validated (see Section 8.2).  A server in the closing state MAY
   instead choose to discard packets received from a new source address.

10.2.  Idle Timeout

   If the idle timeout is enabled, a connection that remains idle for
   longer than the advertised idle timeout (see Section 18.1) is closed.
   A connection enters the draining state when the idle timeout expires.

   Each endpoint advertises its own idle timeout to its peer.  The idle
   timeout starts from the last packet received.  In order to ensure
   that initiating new activity postpones an idle timeout, an endpoint
   restarts this timer when sending a packet.  An endpoint does not
   postpone the idle timeout if another packet has been sent containing
   frames other than ACK or PADDING, and that other packet has not been
   acknowledged or declared lost.  Packets that contain only ACK or
   PADDING frames are not acknowledged until an endpoint has other
   frames to send, so they could prevent the timeout from being
   refreshed.

   The value for an idle timeout can be asymmetric.  The value
   advertised by an endpoint is only used to determine whether the
   connection is live at that endpoint.  An endpoint that sends packets
   near the end of the idle timeout period of a peer risks having those
   packets discarded if its peer enters the draining state before the
   packets arrive.  If a peer could timeout within an RTO (see
   Section 4.3.3 of [QUIC-RECOVERY]), it is advisable to test for
   liveness before sending any data that cannot be retried safely.

10.3.  Immediate Close

   An endpoint sends a closing frame (CONNECTION_CLOSE or
   APPLICATION_CLOSE) to terminate the connection immediately.  Any
   closing frame causes all streams to immediately become closed; open
   streams can be assumed to be implicitly reset.

   After sending a closing frame, endpoints immediately enter the
   closing state.  During the closing period, an endpoint that sends a
   closing frame SHOULD respond to any packet that it receives with
   another packet containing a closing frame.  To minimize the state



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   that an endpoint maintains for a closing connection, endpoints MAY
   send the exact same packet.  However, endpoints SHOULD limit the
   number of packets they generate containing a closing frame.  For
   instance, an endpoint could progressively increase the number of
   packets that it receives before sending additional packets or
   increase the time between packets.

   Note:  Allowing retransmission of a packet contradicts other advice
      in this document that recommends the creation of new packet
      numbers for every packet.  Sending new packet numbers is primarily
      of advantage to loss recovery and congestion control, which are
      not expected to be relevant for a closed connection.
      Retransmitting the final packet requires less state.

   After receiving a closing frame, endpoints enter the draining state.
   An endpoint that receives a closing frame MAY send a single packet
   containing a closing frame before entering the draining state, using
   a CONNECTION_CLOSE frame and a NO_ERROR code if appropriate.  An
   endpoint MUST NOT send further packets, which could result in a
   constant exchange of closing frames until the closing period on
   either peer ended.

   An immediate close can be used after an application protocol has
   arranged to close a connection.  This might be after the application
   protocols negotiates a graceful shutdown.  The application protocol
   exchanges whatever messages that are needed to cause both endpoints
   to agree to close the connection, after which the application
   requests that the connection be closed.  The application protocol can
   use an APPLICATION_CLOSE message with an appropriate error code to
   signal closure.

   If the connection has been successfully established, endpoints MUST
   send any closing frames in a 1-RTT packet.  Prior to connection
   establishment a peer might not have 1-RTT keys, so endpoints SHOULD
   send closing frames in a Handshake packet.  If the endpoint does not
   have Handshake keys, or it is not certain that the peer has Handshake
   keys, it MAY send closing frames in an Initial packet.  If multiple
   packets are sent, they can be coalesced (see Section 12.2) to
   facilitate retransmission.

10.4.  Stateless Reset

   A stateless reset is provided as an option of last resort for an
   endpoint that does not have access to the state of a connection.  A
   crash or outage might result in peers continuing to send data to an
   endpoint that is unable to properly continue the connection.  An
   endpoint that wishes to communicate a fatal connection error MUST use
   a closing frame if it has sufficient state to do so.



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   To support this process, a token is sent by endpoints.  The token is
   carried in the NEW_CONNECTION_ID frame sent by either peer, and
   servers can specify the stateless_reset_token transport parameter
   during the handshake (clients cannot because their transport
   parameters don't have confidentiality protection).  This value is
   protected by encryption, so only client and server know this value.
   Tokens sent via NEW_CONNECTION_ID frames are invalidated when their
   associated connection ID is retired via a RETIRE_CONNECTION_ID frame
   (Section 19.13).

   An endpoint that receives packets that it cannot process sends a
   packet in the following layout:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|0|0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Random Octets (160..)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 7: Stateless Reset Packet

   This design ensures that a stateless reset packet is - to the extent
   possible - indistinguishable from a regular packet with a short
   header.

   The message consists of a header octet, followed by an arbitrary
   number of random octets, followed by a Stateless Reset Token.

   A stateless reset will be interpreted by a recipient as a packet with
   a short header.  For the packet to appear as valid, the Random Octets
   field needs to include at least 20 octets of random or unpredictable
   values.  This is intended to allow for a destination connection ID of
   the maximum length permitted, a packet number, and minimal payload.
   The Stateless Reset Token corresponds to the minimum expansion of the
   packet protection AEAD.  More random octets might be necessary if the
   endpoint could have negotiated a packet protection scheme with a
   larger minimum AEAD expansion.




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   An endpoint SHOULD NOT send a stateless reset that is significantly
   larger than the packet it receives.  Endpoints MUST discard packets
   that are too small to be valid QUIC packets.  With the set of AEAD
   functions defined in [QUIC-TLS], packets less than 19 octets long are
   never valid.

   An endpoint MAY send a stateless reset in response to a packet with a
   long header.  This would not be effective if the stateless reset
   token was not yet available to a peer.  In this QUIC version, packets
   with a long header are only used during connection establishment.
   Because the stateless reset token is not available until connection
   establishment is complete or near completion, ignoring an unknown
   packet with a long header might be more effective.

   An endpoint cannot determine the Source Connection ID from a packet
   with a short header, therefore it cannot set the Destination
   Connection ID in the stateless reset packet.  The Destination
   Connection ID will therefore differ from the value used in previous
   packets.  A random Destination Connection ID makes the connection ID
   appear to be the result of moving to a new connection ID that was
   provided using a NEW_CONNECTION_ID frame (Section 19.12).

   Using a randomized connection ID results in two problems:

   o  The packet might not reach the peer.  If the Destination
      Connection ID is critical for routing toward the peer, then this
      packet could be incorrectly routed.  This might also trigger
      another Stateless Reset in response, see Section 10.4.3.  A
      Stateless Reset that is not correctly routed is ineffective in
      causing errors to be quickly detected and recovered.  In this
      case, endpoints will need to rely on other methods - such as
      timers - to detect that the connection has failed.

   o  The randomly generated connection ID can be used by entities other
      than the peer to identify this as a potential stateless reset.  An
      endpoint that occasionally uses different connection IDs might
      introduce some uncertainty about this.

   Finally, the last 16 octets of the packet are set to the value of the
   Stateless Reset Token.

   A stateless reset is not appropriate for signaling error conditions.
   An endpoint that wishes to communicate a fatal connection error MUST
   use a CONNECTION_CLOSE or APPLICATION_CLOSE frame if it has
   sufficient state to do so.

   This stateless reset design is specific to QUIC version 1.  An
   endpoint that supports multiple versions of QUIC needs to generate a



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   stateless reset that will be accepted by peers that support any
   version that the endpoint might support (or might have supported
   prior to losing state).  Designers of new versions of QUIC need to be
   aware of this and either reuse this design, or use a portion of the
   packet other than the last 16 octets for carrying data.

10.4.1.  Detecting a Stateless Reset

   An endpoint detects a potential stateless reset when a packet with a
   short header either cannot be decrypted or is marked as a duplicate
   packet.  The endpoint then compares the last 16 octets of the packet
   with the Stateless Reset Token provided by its peer, either in a
   NEW_CONNECTION_ID frame or the server's transport parameters.  If
   these values are identical, the endpoint MUST enter the draining
   period and not send any further packets on this connection.  If the
   comparison fails, the packet can be discarded.

10.4.2.  Calculating a Stateless Reset Token

   The stateless reset token MUST be difficult to guess.  In order to
   create a Stateless Reset Token, an endpoint could randomly generate
   [RFC4086] a secret for every connection that it creates.  However,
   this presents a coordination problem when there are multiple
   instances in a cluster or a storage problem for an endpoint that
   might lose state.  Stateless reset specifically exists to handle the
   case where state is lost, so this approach is suboptimal.

   A single static key can be used across all connections to the same
   endpoint by generating the proof using a second iteration of a
   preimage-resistant function that takes a static key and the
   connection ID chosen by the endpoint (see Section 5.1) as input.  An
   endpoint could use HMAC [RFC2104] (for example, HMAC(static_key,
   connection_id)) or HKDF [RFC5869] (for example, using the static key
   as input keying material, with the connection ID as salt).  The
   output of this function is truncated to 16 octets to produce the
   Stateless Reset Token for that connection.

   An endpoint that loses state can use the same method to generate a
   valid Stateless Reset Token.  The connection ID comes from the packet
   that the endpoint receives.

   This design relies on the peer always sending a connection ID in its
   packets so that the endpoint can use the connection ID from a packet
   to reset the connection.  An endpoint that uses this design MUST
   either use the same connection ID length for all connections or
   encode the length of the connection ID such that it can be recovered
   without state.  In addition, it cannot provide a zero-length
   connection ID.



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   Revealing the Stateless Reset Token allows any entity to terminate
   the connection, so a value can only be used once.  This method for
   choosing the Stateless Reset Token means that the combination of
   connection ID and static key cannot occur for another connection.  A
   denial of service attack is possible if the same connection ID is
   used by instances that share a static key, or if an attacker can
   cause a packet to be routed to an instance that has no state but the
   same static key (see Section 21.8).  A connection ID from a
   connection that is reset by revealing the Stateless Reset Token
   cannot be reused for new connections at nodes that share a static
   key.

   Note that Stateless Reset packets do not have any cryptographic
   protection.

10.4.3.  Looping

   The design of a Stateless Reset is such that it is indistinguishable
   from a valid packet.  This means that a Stateless Reset might trigger
   the sending of a Stateless Reset in response, which could lead to
   infinite exchanges.

   An endpoint MUST ensure that every Stateless Reset that it sends is
   smaller than the packet which triggered it, unless it maintains state
   sufficient to prevent looping.  In the event of a loop, this results
   in packets eventually being too small to trigger a response.

   An endpoint can remember the number of Stateless Reset packets that
   it has sent and stop generating new Stateless Reset packets once a
   limit is reached.  Using separate limits for different remote
   addresses will ensure that Stateless Reset packets can be used to
   close connections when other peers or connections have exhausted
   limits.

   Reducing the size of a Stateless Reset below the recommended minimum
   size of 37 octets could mean that the packet could reveal to an
   observer that it is a Stateless Reset.  Conversely, refusing to send
   a Stateless Reset in response to a small packet might result in
   Stateless Reset not being useful in detecting cases of broken
   connections where only very small packets are sent; such failures
   might only be detected by other means, such as timers.

   An endpoint can increase the odds that a packet will trigger a
   Stateless Reset if it cannot be processed by padding it to at least
   38 octets.






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11.  Error Handling

   An endpoint that detects an error SHOULD signal the existence of that
   error to its peer.  Both transport-level and application-level errors
   can affect an entire connection (see Section 11.1), while only
   application-level errors can be isolated to a single stream (see
   Section 11.2).

   The most appropriate error code (Section 20) SHOULD be included in
   the frame that signals the error.  Where this specification
   identifies error conditions, it also identifies the error code that
   is used.

   A stateless reset (Section 10.4) is not suitable for any error that
   can be signaled with a CONNECTION_CLOSE, APPLICATION_CLOSE, or
   RST_STREAM frame.  A stateless reset MUST NOT be used by an endpoint
   that has the state necessary to send a frame on the connection.

11.1.  Connection Errors

   Errors that result in the connection being unusable, such as an
   obvious violation of protocol semantics or corruption of state that
   affects an entire connection, MUST be signaled using a
   CONNECTION_CLOSE or APPLICATION_CLOSE frame (Section 19.3,
   Section 19.4).  An endpoint MAY close the connection in this manner
   even if the error only affects a single stream.

   Application protocols can signal application-specific protocol errors
   using the APPLICATION_CLOSE frame.  Errors that are specific to the
   transport, including all those described in this document, are
   carried in a CONNECTION_CLOSE frame.  Other than the type of error
   code they carry, these frames are identical in format and semantics.

   A CONNECTION_CLOSE or APPLICATION_CLOSE frame could be sent in a
   packet that is lost.  An endpoint SHOULD be prepared to retransmit a
   packet containing either frame type if it receives more packets on a
   terminated connection.  Limiting the number of retransmissions and
   the time over which this final packet is sent limits the effort
   expended on terminated connections.

   An endpoint that chooses not to retransmit packets containing
   CONNECTION_CLOSE or APPLICATION_CLOSE risks a peer missing the first
   such packet.  The only mechanism available to an endpoint that
   continues to receive data for a terminated connection is to use the
   stateless reset process (Section 10.4).






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   An endpoint that receives an invalid CONNECTION_CLOSE or
   APPLICATION_CLOSE frame MUST NOT signal the existence of the error to
   its peer.

11.2.  Stream Errors

   If an application-level error affects a single stream, but otherwise
   leaves the connection in a recoverable state, the endpoint can send a
   RST_STREAM frame (Section 19.2) with an appropriate error code to
   terminate just the affected stream.

   Other than STOPPING (Section 3.5), RST_STREAM MUST be instigated by
   the application and MUST carry an application error code.  Resetting
   a stream without knowledge of the application protocol could cause
   the protocol to enter an unrecoverable state.  Application protocols
   might require certain streams to be reliably delivered in order to
   guarantee consistent state between endpoints.

12.  Packets and Frames

   QUIC endpoints communicate by exchanging packets.  Packets are
   carried in UDP datagrams (see Section 12.2) and have confidentiality
   and integrity protection (see Section 12.1).

   This version of QUIC uses the long packet header (see Section 17.2)
   during connection establishment and the short header (see
   Section 17.3) once 1-RTT keys have been established.

   Packets that carry the long header are Initial Section 17.5, Retry
   Section 17.7, Handshake Section 17.6, and 0-RTT Protected packets
   Section 12.1.

   Packets with the short header are designed for minimal overhead and
   are used after a connection is established.

   Version negotiation uses a packet with a special format (see
   Section 17.4).

12.1.  Protected Packets

   All QUIC packets except Version Negotiation and Retry packets use
   authenticated encryption with additional data (AEAD) [RFC5119] to
   provide confidentiality and integrity protection.  Details of packet
   protection are found in [QUIC-TLS]; this section includes an overview
   of the process.

   Initial packets are protected using keys that are statically derived.
   This packet protection is not effective confidentiality protection,



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   it only exists to ensure that the sender of the packet is on the
   network path.  Any entity that receives the Initial packet from a
   client can recover the keys necessary to remove packet protection or
   to generate packets that will be successfully authenticated.

   All other packets are protected with keys derived from the
   cryptographic handshake.  The type of the packet from the long header
   or key phase from the short header are used to identify which
   encryption level - and therefore the keys - that are used.  Packets
   protected with 0-RTT and 1-RTT keys are expected to have
   confidentiality and data origin authentication; the cryptographic
   handshake ensures that only the communicating endpoints receive the
   corresponding keys.

   The packet number field contains a packet number, which has
   additional confidentiality protection that is applied after packet
   protection is applied (see [QUIC-TLS] for details).  The underlying
   packet number increases with each packet sent, see Section 12.3 for
   details.

12.2.  Coalescing Packets

   A sender can coalesce multiple QUIC packets into one UDP datagram.
   This can reduce the number of UDP datagrams needed to complete the
   cryptographic handshake and starting sending data.  Receivers MUST be
   able to process coalesced packets.

   Coalescing packets in order of increasing encryption levels (Initial,
   0-RTT, Handshake, 1-RTT) makes it more likely the receiver will be
   able to process all the packets in a single pass.  A packet with a
   short header does not include a length, so it will always be the last
   packet included in a UDP datagram.

   Senders MUST NOT coalesce QUIC packets for different connections into
   a single UDP datagram.  Receivers SHOULD ignore any subsequent
   packets with a different Destination Connection ID than the first
   packet in the datagram.

   Every QUIC packet that is coalesced into a single UDP datagram is
   separate and complete.  Though the values of some fields in the
   packet header might be redundant, no fields are omitted.  The
   receiver of coalesced QUIC packets MUST individually process each
   QUIC packet and separately acknowledge them, as if they were received
   as the payload of different UDP datagrams.  For example, if
   decryption fails (because the keys are not available or any other
   reason) or the packet is of an unknown type, the receiver MAY either
   discard or buffer the packet for later processing and MUST attempt to
   process the remaining packets.



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   Retry packets (Section 17.7), Version Negotiation packets
   (Section 17.4), and packets with a short header cannot be followed by
   other packets in the same UDP datagram.

12.3.  Packet Numbers

   The packet number is an integer in the range 0 to 2^62-1.  Where
   present, packet numbers are encoded as a variable-length integer (see
   Section 16).  This number is used in determining the cryptographic
   nonce for packet protection.  Each endpoint maintains a separate
   packet number for sending and receiving.

   Version Negotiation (Section 17.4) and Retry Section 17.7 packets do
   not include a packet number.

   Packet numbers are divided into 3 spaces in QUIC:

   o  Initial space: All Initial packets Section 17.5 are in this space.

   o  Handshake space: All Handshake packets Section 17.6 are in this
      space.

   o  Application data space: All 0-RTT and 1-RTT encrypted packets
      Section 12.1 are in this space.

   As described in [QUIC-TLS], each packet type uses different
   protection keys.

   Conceptually, a packet number space is the context in which a packet
   can be processed and acknowledged.  Initial packets can only be sent
   with Initial packet protection keys and acknowledged in packets which
   are also Initial packets.  Similarly, Handshake packets are sent at
   the Handshake encryption level and can only be acknowledged in
   Handshake packets.

   This enforces cryptographic separation between the data sent in the
   different packet sequence number spaces.  Each packet number space
   starts at packet number 0.  Subsequent packets sent in the same
   packet number space MUST increase the packet number by at least one.

   0-RTT and 1-RTT data exist in the same packet number space to make
   loss recovery algorithms easier to implement between the two packet
   types.

   A QUIC endpoint MUST NOT reuse a packet number within the same packet
   number space in one connection (that is, under the same cryptographic
   keys).  If the packet number for sending reaches 2^62 - 1, the sender
   MUST close the connection without sending a CONNECTION_CLOSE frame or



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   any further packets; an endpoint MAY send a Stateless Reset
   (Section 10.4) in response to further packets that it receives.

   A receiver MUST discard a newly unprotected packet unless it is
   certain that it has not processed another packet with the same packet
   number from the same packet number space.  Duplicate suppression MUST
   happen after removing packet protection for the reasons described in
   Section 9.3 of [QUIC-TLS].  An efficient algorithm for duplicate
   suppression can be found in Section 3.4.3 of [RFC2406].

   Packet number encoding at a sender and decoding at a receiver are
   described in Section 17.1.

12.4.  Frames and Frame Types

   The payload of QUIC packets, after removing packet protection,
   commonly consists of a sequence of frames, as shown in Figure 8.
   Version Negotiation, Stateless Reset, and Retry packets do not
   contain frames.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame 1 (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame 2 (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Frame N (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 8: QUIC Payload

   QUIC payloads MUST contain at least one frame, and MAY contain
   multiple frames and multiple frame types.

   Frames MUST fit within a single QUIC packet and MUST NOT span a QUIC
   packet boundary.  Each frame begins with a Frame Type, indicating its
   type, followed by additional type-dependent fields:











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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Frame Type (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-Dependent Fields (*)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 9: Generic Frame Layout

   The frame types defined in this specification are listed in Table 3.
   The Frame Type in STREAM frames is used to carry other frame-specific
   flags.  For all other frames, the Frame Type field simply identifies
   the frame.  These frames are explained in more detail in Section 19.





































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          +-------------+----------------------+---------------+
          | Type Value  | Frame Type Name      | Definition    |
          +-------------+----------------------+---------------+
          | 0x00        | PADDING              | Section 19.1  |
          |             |                      |               |
          | 0x01        | RST_STREAM           | Section 19.2  |
          |             |                      |               |
          | 0x02        | CONNECTION_CLOSE     | Section 19.3  |
          |             |                      |               |
          | 0x03        | APPLICATION_CLOSE    | Section 19.4  |
          |             |                      |               |
          | 0x04        | MAX_DATA             | Section 19.5  |
          |             |                      |               |
          | 0x05        | MAX_STREAM_DATA      | Section 19.6  |
          |             |                      |               |
          | 0x06        | MAX_STREAM_ID        | Section 19.7  |
          |             |                      |               |
          | 0x07        | PING                 | Section 19.8  |
          |             |                      |               |
          | 0x08        | BLOCKED              | Section 19.9  |
          |             |                      |               |
          | 0x09        | STREAM_BLOCKED       | Section 19.10 |
          |             |                      |               |
          | 0x0a        | STREAM_ID_BLOCKED    | Section 19.11 |
          |             |                      |               |
          | 0x0b        | NEW_CONNECTION_ID    | Section 19.12 |
          |             |                      |               |
          | 0x0c        | STOP_SENDING         | Section 19.14 |
          |             |                      |               |
          | 0x0d        | RETIRE_CONNECTION_ID | Section 19.13 |
          |             |                      |               |
          | 0x0e        | PATH_CHALLENGE       | Section 19.16 |
          |             |                      |               |
          | 0x0f        | PATH_RESPONSE        | Section 19.17 |
          |             |                      |               |
          | 0x10 - 0x17 | STREAM               | Section 19.19 |
          |             |                      |               |
          | 0x18        | CRYPTO               | Section 19.20 |
          |             |                      |               |
          | 0x19        | NEW_TOKEN            | Section 19.18 |
          |             |                      |               |
          | 0x1a - 0x1b | ACK                  | Section 19.15 |
          +-------------+----------------------+---------------+

                           Table 3: Frame Types






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   All QUIC frames are idempotent.  That is, a valid frame does not
   cause undesirable side effects or errors when received more than
   once.

   The Frame Type field uses a variable length integer encoding (see
   Section 16) with one exception.  To ensure simple and efficient
   implementations of frame parsing, a frame type MUST use the shortest
   possible encoding.  Though a two-, four- or eight-octet encoding of
   the frame types defined in this document is possible, the Frame Type
   field for these frames is encoded on a single octet.  For instance,
   though 0x4007 is a legitimate two-octet encoding for a variable-
   length integer with a value of 7, PING frames are always encoded as a
   single octet with the value 0x07.  An endpoint MUST treat the receipt
   of a frame type that uses a longer encoding than necessary as a
   connection error of type PROTOCOL_VIOLATION.

13.  Packetization and Reliability

   A sender bundles one or more frames in a QUIC packet (see
   Section 12.4).

   A sender can minimize per-packet bandwidth and computational costs by
   bundling as many frames as possible within a QUIC packet.  A sender
   MAY wait for a short period of time to bundle multiple frames before
   sending a packet that is not maximally packed, to avoid sending out
   large numbers of small packets.  An implementation may use knowledge
   about application sending behavior or heuristics to determine whether
   and for how long to wait.  This waiting period is an implementation
   decision, and an implementation should be careful to delay
   conservatively, since any delay is likely to increase application-
   visible latency.

   Stream multiplexing is achieved by interleaving STREAM frames from
   multiple streams into one or more QUIC packets.  A single QUIC packet
   can include multiple STREAM frames from one or more streams.

   One of the benefits of QUIC is avoidance of head-of-line blocking
   across multiple streams.  When a packet loss occurs, only streams
   with data in that packet are blocked waiting for a retransmission to
   be received, while other streams can continue making progress.  Note
   that when data from multiple streams is bundled into a single QUIC
   packet, loss of that packet blocks all those streams from making
   progress.  Implementations are advised to bundle as few streams as
   necessary in outgoing packets without losing transmission efficiency
   to underfilled packets.






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13.1.  Packet Processing and Acknowledgment

   A packet MUST NOT be acknowledged until packet protection has been
   successfully removed and all frames contained in the packet have been
   processed.  For STREAM frames, this means the data has been enqueued
   in preparation to be received by the application protocol, but it
   does not require that data is delivered and consumed.

   Once the packet has been fully processed, a receiver acknowledges
   receipt by sending one or more ACK frames containing the packet
   number of the received packet.

13.1.1.  Sending ACK Frames

   To avoid creating an indefinite feedback loop, an endpoint MUST NOT
   send an ACK frame in response to a packet containing only ACK or
   PADDING frames, even if there are packet gaps which precede the
   received packet.  The endpoint MUST however acknowledge packets
   containing only ACK or PADDING frames when sending ACK frames in
   response to other packets.

   While PADDING frames do not elicit an ACK frame from a receiver, they
   are considered to be in flight for congestion control purposes
   [QUIC-RECOVERY].  Sending only PADDING frames might cause the sender
   to become limited by the congestion controller (as described in
   [QUIC-RECOVERY]) with no acknowledgments forthcoming from the
   receiver.  Therefore, a sender should ensure that other frames are
   sent in addition to PADDING frames to elicit acknowledgments from the
   receiver.

   An endpoint MUST NOT send more than one packet containing only an ACK
   frame per received packet that contains frames other than ACK and
   PADDING frames.

   The receiver's delayed acknowledgment timer SHOULD NOT exceed the
   current RTT estimate or the value it indicates in the "max_ack_delay"
   transport parameter.  This ensures an acknowledgment is sent at least
   once per RTT when packets needing acknowledgement are received.  The
   sender can use the receiver's "max_ack_delay" value in determining
   timeouts for timer-based retransmission.

   Strategies and implications of the frequency of generating
   acknowledgments are discussed in more detail in [QUIC-RECOVERY].

   To limit ACK Blocks to those that have not yet been received by the
   sender, the receiver SHOULD track which ACK frames have been
   acknowledged by its peer.  Once an ACK frame has been acknowledged,
   the packets it acknowledges SHOULD NOT be acknowledged again.



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   Because ACK frames are not sent in response to ACK-only packets, a
   receiver that is only sending ACK frames will only receive
   acknowledgements for its packets if the sender includes them in
   packets with non-ACK frames.  A sender SHOULD bundle ACK frames with
   other frames when possible.

   To limit receiver state or the size of ACK frames, a receiver MAY
   limit the number of ACK Blocks it sends.  A receiver can do this even
   without receiving acknowledgment of its ACK frames, with the
   knowledge this could cause the sender to unnecessarily retransmit
   some data.  Standard QUIC [QUIC-RECOVERY] algorithms declare packets
   lost after sufficiently newer packets are acknowledged.  Therefore,
   the receiver SHOULD repeatedly acknowledge newly received packets in
   preference to packets received in the past.

13.1.2.  ACK Frames and Packet Protection

   ACK frames MUST only be carried in a packet that has the same packet
   number space as the packet being ACKed (see Section 12.1).  For
   instance, packets that are protected with 1-RTT keys MUST be
   acknowledged in packets that are also protected with 1-RTT keys.

   Packets that a client sends with 0-RTT packet protection MUST be
   acknowledged by the server in packets protected by 1-RTT keys.  This
   can mean that the client is unable to use these acknowledgments if
   the server cryptographic handshake messages are delayed or lost.
   Note that the same limitation applies to other data sent by the
   server protected by the 1-RTT keys.

   Endpoints SHOULD send acknowledgments for packets containing CRYPTO
   frames with a reduced delay; see Section 4.3.1 of [QUIC-RECOVERY].

13.2.  Retransmission of Information

   QUIC packets that are determined to be lost are not retransmitted
   whole.  The same applies to the frames that are contained within lost
   packets.  Instead, the information that might be carried in frames is
   sent again in new frames as needed.

   New frames and packets are used to carry information that is
   determined to have been lost.  In general, information is sent again
   when a packet containing that information is determined to be lost
   and sending ceases when a packet containing that information is
   acknowledged.

   o  Data sent in CRYPTO frames is retransmitted according to the rules
      in [QUIC-RECOVERY], until all data has been acknowledged.




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   o  Application data sent in STREAM frames is retransmitted in new
      STREAM frames unless the endpoint has sent a RST_STREAM for that
      stream.  Once an endpoint sends a RST_STREAM frame, no further
      STREAM frames are needed.

   o  The most recent set of acknowledgments are sent in ACK frames.  An
      ACK frame SHOULD contain all unacknowledged acknowledgments, as
      described in Section 13.1.1.

   o  Cancellation of stream transmission, as carried in a RST_STREAM
      frame, is sent until acknowledged or until all stream data is
      acknowledged by the peer (that is, either the "Reset Recvd" or
      "Data Recvd" state is reached on the send stream).  The content of
      a RST_STREAM frame MUST NOT change when it is sent again.

   o  Similarly, a request to cancel stream transmission, as encoded in
      a STOP_SENDING frame, is sent until the receive stream enters
      either a "Data Recvd" or "Reset Recvd" state, see Section 3.5.

   o  Connection close signals, including those that use
      CONNECTION_CLOSE and APPLICATION_CLOSE frames, are not sent again
      when packet loss is detected, but as described in Section 10.

   o  The current connection maximum data is sent in MAX_DATA frames.
      An updated value is sent in a MAX_DATA frame if the packet
      containing the most recently sent MAX_DATA frame is declared lost,
      or when the endpoint decides to update the limit.  Care is
      necessary to avoid sending this frame too often as the limit can
      increase frequently and cause an unnecessarily large number of
      MAX_DATA frames to be sent.

   o  The current maximum stream data offset is sent in MAX_STREAM_DATA
      frames.  Like MAX_DATA, an updated value is sent when the packet
      containing the most recent MAX_STREAM_DATA frame for a stream is
      lost or when the limit is updated, with care taken to prevent the
      frame from being sent too often.  An endpoint SHOULD stop sending
      MAX_STREAM_DATA frames when the receive stream enters a "Size
      Known" state.

   o  The maximum stream ID for a stream of a given type is sent in
      MAX_STREAM_ID frames.  Like MAX_DATA, an updated value is sent
      when a packet containing the most recent MAX_STREAM_ID for a
      stream type frame is declared lost or when the limit is updated,
      with care taken to prevent the frame from being sent too often.

   o  Blocked signals are carried in BLOCKED, STREAM_BLOCKED, and
      STREAM_ID_BLOCKED frames.  BLOCKED streams have connection scope,
      STREAM_BLOCKED frames have stream scope, and STREAM_ID_BLOCKED



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      frames are scoped to a specific stream type.  New frames are sent
      if packets containing the most recent frame for a scope is lost,
      but only while the endpoint is blocked on the corresponding limit.
      These frames always include the limit that is causing blocking at
      the time that they are transmitted.

   o  A liveness or path validation check using PATH_CHALLENGE frames is
      sent periodically until a matching PATH_RESPONSE frame is received
      or until there is no remaining need for liveness or path
      validation checking.  PATH_CHALLENGE frames include a different
      payload each time they are sent.

   o  Responses to path validation using PATH_RESPONSE frames are sent
      just once.  A new PATH_CHALLENGE frame will be sent if another
      PATH_RESPONSE frame is needed.

   o  New connection IDs are sent in NEW_CONNECTION_ID frames and
      retransmitted if the packet containing them is lost.
      Retransmissions of this frame carry the same sequence number
      value.  Likewise, retired connection IDs are sent in
      RETIRE_CONNECTION_ID frames and retransmitted if the packet
      containing them is lost.

   o  PADDING frames contain no information, so lost PADDING frames do
      not require repair.

   Upon detecting losses, a sender MUST take appropriate congestion
   control action.  The details of loss detection and congestion control
   are described in [QUIC-RECOVERY].

13.3.  Explicit Congestion Notification

   QUIC endpoints use Explicit Congestion Notification (ECN) [RFC3168]
   to detect and respond to network congestion.  ECN allows a network
   node to indicate congestion in the network by setting a codepoint in
   the IP header of a packet instead of dropping it.  Endpoints react to
   congestion by reducing their sending rate in response, as described
   in [QUIC-RECOVERY].

   To use ECN, QUIC endpoints first determine whether a path supports
   ECN marking and the peer is able to access the ECN codepoint in the
   IP header.  A network path does not support ECN if ECN marked packets
   get dropped or ECN markings are rewritten on the path.  An endpoint
   verifies the path, both during connection establishment and when
   migrating to a new path (see Section 9).






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13.3.1.  ECN Counters

   On receiving a packet with an ECT or CE codepoint, an endpoint that
   can access the IP ECN codepoints increases the corresponding ECT(0),
   ECT(1), or CE count, and includes these counters in subsequent (see
   Section 13.1) ACK frames (see Section 19.15).

   A packet detected by a receiver as a duplicate does not affect the
   receiver's local ECN codepoint counts; see (Section 21.7) for
   relevant security concerns.

   If an endpoint receives a packet without an ECT or CE codepoint, it
   responds per Section 13.1 with an ACK frame.  If an endpoint does not
   have access to received ECN codepoints, it acknowledges received
   packets per Section 13.1 with an ACK frame.

13.3.2.  ECN Verification

   Each endpoint independently verifies and enables use of ECN by
   setting the IP header ECN codepoint to ECN Capable Transport (ECT)
   for the path from it to the other peer.  Even if ECN is not used on
   the path to the peer, the endpoint MUST provide feedback about ECN
   markings received (if accessible).

   To verify both that a path supports ECN and the peer can provide ECN
   feedback, an endpoint MUST set the ECT(0) codepoint in the IP header
   of all outgoing packets [RFC8311].

   If an ECT codepoint set in the IP header is not corrupted by a
   network device, then a received packet contains either the codepoint
   sent by the peer or the Congestion Experienced (CE) codepoint set by
   a network device that is experiencing congestion.

   If a packet sent with an ECT codepoint is newly acknowledged by the
   peer in an ACK frame without ECN feedback, the endpoint stops setting
   ECT codepoints in subsequent packets, with the expectation that
   either the network or the peer no longer supports ECN.

   To protect the connection from arbitrary corruption of ECN codepoints
   by the network, an endpoint verifies the following when an ACK frame
   is received:

   o  The increase in ECT(0) and ECT(1) counters MUST be at least the
      number of packets newly acknowledged that were sent with the
      corresponding codepoint.






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   o  The total increase in ECT(0), ECT(1), and CE counters reported in
      the ACK frame MUST be at least the total number of packets newly
      acknowledged in this ACK frame.

   An endpoint could miss acknowledgements for a packet when ACK frames
   are lost.  It is therefore possible for the total increase in ECT(0),
   ECT(1), and CE counters to be greater than the number of packets
   acknowledged in an ACK frame.  When this happens, the local reference
   counts MUST be increased to match the counters in the ACK frame.

   Upon successful verification, an endpoint continues to set ECT
   codepoints in subsequent packets with the expectation that the path
   is ECN-capable.

   If verification fails, then the endpoint ceases setting ECT
   codepoints in subsequent packets with the expectation that either the
   network or the peer does not support ECN.

   If an endpoint sets ECT codepoints on outgoing packets and encounters
   a retransmission timeout due to the absence of acknowledgments from
   the peer (see [QUIC-RECOVERY]), or if an endpoint has reason to
   believe that a network element might be corrupting ECN codepoints,
   the endpoint MAY cease setting ECT codepoints in subsequent packets.
   Doing so allows the connection to traverse network elements that drop
   or corrupt ECN codepoints in the IP header.

14.  Packet Size

   The QUIC packet size includes the QUIC header and integrity check,
   but not the UDP or IP header.

   Clients MUST ensure that the first Initial packet they send is sent
   in a UDP datagram that is at least 1200 octets.  Padding the Initial
   packet or including a 0-RTT packet in the same datagram are ways to
   meet this requirement.  Sending a UDP datagram of this size ensures
   that the network path supports a reasonable Maximum Transmission Unit
   (MTU), and helps reduce the amplitude of amplification attacks caused
   by server responses toward an unverified client address, see
   Section 8.

   The payload of a UDP datagram carrying the Initial packet MUST be
   expanded to at least 1200 octets, by adding PADDING frames to the
   Initial packet and/or by combining the Initial packet with a 0-RTT
   packet (see Section 12.2).

   The datagram containing the first Initial packet from a client MAY
   exceed 1200 octets if the client believes that the Path Maximum
   Transmission Unit (PMTU) supports the size that it chooses.



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   A server MAY send a CONNECTION_CLOSE frame with error code
   PROTOCOL_VIOLATION in response to the first Initial packet it
   receives from a client if the UDP datagram is smaller than 1200
   octets.  It MUST NOT send any other frame type in response, or
   otherwise behave as if any part of the offending packet was processed
   as valid.

   The server MUST also limit the number of bytes it sends before
   validating the address of the client, see Section 8.

14.1.  Path Maximum Transmission Unit

   The Path Maximum Transmission Unit (PMTU) is the maximum size of the
   entire IP header, UDP header, and UDP payload.  The UDP payload
   includes the QUIC packet header, protected payload, and any
   authentication fields.

   All QUIC packets SHOULD be sized to fit within the estimated PMTU to
   avoid IP fragmentation or packet drops.  To optimize bandwidth
   efficiency, endpoints SHOULD use Packetization Layer PMTU Discovery
   ([PLPMTUD]).  Endpoints MAY use PMTU Discovery ([PMTUDv4], [PMTUDv6])
   for detecting the PMTU, setting the PMTU appropriately, and storing
   the result of previous PMTU determinations.

   In the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP
   packets larger than 1280 octets.  Assuming the minimum IP header
   size, this results in a QUIC packet size of 1232 octets for IPv6 and
   1252 octets for IPv4.  Some QUIC implementations MAY be more
   conservative in computing allowed QUIC packet size given unknown
   tunneling overheads or IP header options.

   QUIC endpoints that implement any kind of PMTU discovery SHOULD
   maintain an estimate for each combination of local and remote IP
   addresses.  Each pairing of local and remote addresses could have a
   different maximum MTU in the path.

   QUIC depends on the network path supporting an MTU of at least 1280
   octets.  This is the IPv6 minimum MTU and therefore also supported by
   most modern IPv4 networks.  An endpoint MUST NOT reduce its MTU below
   this number, even if it receives signals that indicate a smaller
   limit might exist.

   If a QUIC endpoint determines that the PMTU between any pair of local
   and remote IP addresses has fallen below 1280 octets, it MUST
   immediately cease sending QUIC packets on the affected path.  This
   could result in termination of the connection if an alternative path
   cannot be found.




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14.1.1.  IPv4 PMTU Discovery

   Traditional ICMP-based path MTU discovery in IPv4 [PMTUDv4] is
   potentially vulnerable to off-path attacks that successfully guess
   the IP/port 4-tuple and reduce the MTU to a bandwidth-inefficient
   value.  TCP connections mitigate this risk by using the (at minimum)
   8 bytes of transport header echoed in the ICMP message to validate
   the TCP sequence number as valid for the current connection.
   However, as QUIC operates over UDP, in IPv4 the echoed information
   could consist only of the IP and UDP headers, which usually has
   insufficient entropy to mitigate off-path attacks.

   As a result, endpoints that implement PMTUD in IPv4 SHOULD take steps
   to mitigate this risk.  For instance, an application could:

   o  Set the IPv4 Don't Fragment (DF) bit on a small proportion of
      packets, so that most invalid ICMP messages arrive when there are
      no DF packets outstanding, and can therefore be identified as
      spurious.

   o  Store additional information from the IP or UDP headers from DF
      packets (for example, the IP ID or UDP checksum) to further
      authenticate incoming Datagram Too Big messages.

   o  Any reduction in PMTU due to a report contained in an ICMP packet
      is provisional until QUIC's loss detection algorithm determines
      that the packet is actually lost.

14.2.  Special Considerations for Packetization Layer PMTU Discovery

   The PADDING frame provides a useful option for PMTU probe packets.
   PADDING frames generate acknowledgements, but they need not be
   delivered reliably.  As a result, the loss of PADDING frames in probe
   packets does not require delay-inducing retransmission.  However,
   PADDING frames do consume congestion window, which may delay the
   transmission of subsequent application data.

   When implementing the algorithm in Section 7.2 of [PLPMTUD], the
   initial value of search_low SHOULD be consistent with the IPv6
   minimum packet size.  Paths that do not support this size cannot
   deliver Initial packets, and therefore are not QUIC-compliant.

   Section 7.3 of [PLPMTUD] discusses trade-offs between small and large
   increases in the size of probe packets.  As QUIC probe packets need
   not contain application data, aggressive increases in probe size
   carry fewer consequences.





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15.  Versions

   QUIC versions are identified using a 32-bit unsigned number.

   The version 0x00000000 is reserved to represent version negotiation.
   This version of the specification is identified by the number
   0x00000001.

   Other versions of QUIC might have different properties to this
   version.  The properties of QUIC that are guaranteed to be consistent
   across all versions of the protocol are described in
   [QUIC-INVARIANTS].

   Version 0x00000001 of QUIC uses TLS as a cryptographic handshake
   protocol, as described in [QUIC-TLS].

   Versions with the most significant 16 bits of the version number
   cleared are reserved for use in future IETF consensus documents.

   Versions that follow the pattern 0x?a?a?a?a are reserved for use in
   forcing version negotiation to be exercised.  That is, any version
   number where the low four bits of all octets is 1010 (in binary).  A
   client or server MAY advertise support for any of these reserved
   versions.

   Reserved version numbers will probably never represent a real
   protocol; a client MAY use one of these version numbers with the
   expectation that the server will initiate version negotiation; a
   server MAY advertise support for one of these versions and can expect
   that clients ignore the value.

   [[RFC editor: please remove the remainder of this section before
   publication.]]

   The version number for the final version of this specification
   (0x00000001), is reserved for the version of the protocol that is
   published as an RFC.

   Version numbers used to identify IETF drafts are created by adding
   the draft number to 0xff000000.  For example, draft-ietf-quic-
   transport-13 would be identified as 0xff00000D.

   Implementors are encouraged to register version numbers of QUIC that
   they are using for private experimentation on the GitHub wiki at
   <https://github.com/quicwg/base-drafts/wiki/QUIC-Versions>.






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16.  Variable-Length Integer Encoding

   QUIC packets and frames commonly use a variable-length encoding for
   non-negative integer values.  This encoding ensures that smaller
   integer values need fewer octets to encode.

   The QUIC variable-length integer encoding reserves the two most
   significant bits of the first octet to encode the base 2 logarithm of
   the integer encoding length in octets.  The integer value is encoded
   on the remaining bits, in network byte order.

   This means that integers are encoded on 1, 2, 4, or 8 octets and can
   encode 6, 14, 30, or 62 bit values respectively.  Table 4 summarizes
   the encoding properties.

          +------+--------+-------------+-----------------------+
          | 2Bit | Length | Usable Bits | Range                 |
          +------+--------+-------------+-----------------------+
          | 00   | 1      | 6           | 0-63                  |
          |      |        |             |                       |
          | 01   | 2      | 14          | 0-16383               |
          |      |        |             |                       |
          | 10   | 4      | 30          | 0-1073741823          |
          |      |        |             |                       |
          | 11   | 8      | 62          | 0-4611686018427387903 |
          +------+--------+-------------+-----------------------+

                   Table 4: Summary of Integer Encodings

   For example, the eight octet sequence c2 19 7c 5e ff 14 e8 8c (in
   hexadecimal) decodes to the decimal value 151288809941952652; the
   four octet sequence 9d 7f 3e 7d decodes to 494878333; the two octet
   sequence 7b bd decodes to 15293; and the single octet 25 decodes to
   37 (as does the two octet sequence 40 25).

   Error codes (Section 20) and versions Section 15 are described using
   integers, but do not use this encoding.

17.  Packet Formats

   All numeric values are encoded in network byte order (that is, big-
   endian) and all field sizes are in bits.  Hexadecimal notation is
   used for describing the value of fields.








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17.1.  Packet Number Encoding and Decoding

   Packet numbers in long and short packet headers are encoded as
   follows.  The number of bits required to represent the packet number
   is first reduced by including only a variable number of the least
   significant bits of the packet number.  One or two of the most
   significant bits of the first octet are then used to represent how
   many bits of the packet number are provided, as shown in Table 5.

          +---------------------+----------------+--------------+
          | First octet pattern | Encoded Length | Bits Present |
          +---------------------+----------------+--------------+
          | 0b0xxxxxxx          | 1 octet        | 7            |
          |                     |                |              |
          | 0b10xxxxxx          | 2              | 14           |
          |                     |                |              |
          | 0b11xxxxxx          | 4              | 30           |
          +---------------------+----------------+--------------+

            Table 5: Packet Number Encodings for Packet Headers

   Note that these encodings are similar to those in Section 16, but use
   different values.

   Finally, the encoded packet number is protected as described in
   Section 5.3 of [QUIC-TLS].

   The sender MUST use a packet number size able to represent more than
   twice as large a range than the difference between the largest
   acknowledged packet and packet number being sent.  A peer receiving
   the packet will then correctly decode the packet number, unless the
   packet is delayed in transit such that it arrives after many higher-
   numbered packets have been received.  An endpoint SHOULD use a large
   enough packet number encoding to allow the packet number to be
   recovered even if the packet arrives after packets that are sent
   afterwards.

   As a result, the size of the packet number encoding is at least one
   more than the base 2 logarithm of the number of contiguous
   unacknowledged packet numbers, including the new packet.

   For example, if an endpoint has received an acknowledgment for packet
   0x6afa2f, sending a packet with a number of 0x6b2d79 requires a
   packet number encoding with 14 bits or more; whereas the 30-bit
   packet number encoding is needed to send a packet with a number of
   0x6bc107.





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   At a receiver, protection of the packet number is removed prior to
   recovering the full packet number.  The full packet number is then
   reconstructed based on the number of significant bits present, the
   value of those bits, and the largest packet number received on a
   successfully authenticated packet.  Recovering the full packet number
   is necessary to successfully remove packet protection.

   Once packet number protection is removed, the packet number is
   decoded by finding the packet number value that is closest to the
   next expected packet.  The next expected packet is the highest
   received packet number plus one.  For example, if the highest
   successfully authenticated packet had a packet number of 0xaa82f30e,
   then a packet containing a 14-bit value of 0x9b3 will be decoded as
   0xaa8309b3.  Example pseudo-code for packet number decoding can be
   found in Appendix A.

17.2.  Long Header Packet

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|   Type (7)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Length (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Payload (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 10: Long Header Packet Format

   Long headers are used for packets that are sent prior to the
   completion of version negotiation and establishment of 1-RTT keys.
   Once both conditions are met, a sender switches to sending packets
   using the short header (Section 17.3).  The long form allows for
   special packets - such as the Version Negotiation packet - to be
   represented in this uniform fixed-length packet format.  Packets that
   use the long header contain the following fields:




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   Header Form:  The most significant bit (0x80) of octet 0 (the first
      octet) is set to 1 for long headers.

   Long Packet Type:  The remaining seven bits of octet 0 contain the
      packet type.  This field can indicate one of 128 packet types.
      The types specified for this version are listed in Table 6.

   Version:  The QUIC Version is a 32-bit field that follows the Type.
      This field indicates which version of QUIC is in use and
      determines how the rest of the protocol fields are interpreted.

   DCIL and SCIL:  The octet following the version contains the lengths
      of the two connection ID fields that follow it.  These lengths are
      encoded as two 4-bit unsigned integers.  The Destination
      Connection ID Length (DCIL) field occupies the 4 high bits of the
      octet and the Source Connection ID Length (SCIL) field occupies
      the 4 low bits of the octet.  An encoded length of 0 indicates
      that the connection ID is also 0 octets in length.  Non-zero
      encoded lengths are increased by 3 to get the full length of the
      connection ID, producing a length between 4 and 18 octets
      inclusive.  For example, an octet with the value 0x50 describes an
      8-octet Destination Connection ID and a zero-length Source
      Connection ID.

   Destination Connection ID:  The Destination Connection ID field
      follows the connection ID lengths and is either 0 octets in length
      or between 4 and 18 octets.  Section 7.2 describes the use of this
      field in more detail.

   Source Connection ID:  The Source Connection ID field follows the
      Destination Connection ID and is either 0 octets in length or
      between 4 and 18 octets.  Section 7.2 describes the use of this
      field in more detail.

   Length:  The length of the remainder of the packet (that is, the
      Packet Number and Payload fields) in octets, encoded as a
      variable-length integer (Section 16).

   Packet Number:  The packet number field is 1, 2, or 4 octets long.
      The packet number has confidentiality protection separate from
      packet protection, as described in Section 5.3 of [QUIC-TLS].  The
      length of the packet number field is encoded in the plaintext
      packet number.  See Section 17.1 for details.

   Payload:  The payload of the packet.

   The following packet types are defined:




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                 +------+-----------------+--------------+
                 | Type | Name            | Section      |
                 +------+-----------------+--------------+
                 | 0x7F | Initial         | Section 17.5 |
                 |      |                 |              |
                 | 0x7E | Retry           | Section 17.7 |
                 |      |                 |              |
                 | 0x7D | Handshake       | Section 17.6 |
                 |      |                 |              |
                 | 0x7C | 0-RTT Protected | Section 12.1 |
                 +------+-----------------+--------------+

                     Table 6: Long Header Packet Types

   The header form, type, connection ID lengths octet, destination and
   source connection IDs, and version fields of a long header packet are
   version-independent.  The packet number and values for packet types
   defined in Table 6 are version-specific.  See [QUIC-INVARIANTS] for
   details on how packets from different versions of QUIC are
   interpreted.

   The interpretation of the fields and the payload are specific to a
   version and packet type.  Type-specific semantics for this version
   are described in the following sections.

   The end of the packet is determined by the Length field.  The Length
   field covers both the Packet Number and Payload fields, both of which
   are confidentiality protected and initially of unknown length.  The
   size of the Payload field is learned once the packet number
   protection is removed.  The Length field enables packet coalescing
   (Section 12.2).

17.3.  Short Header Packet

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|R R R|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Destination Connection ID (0..144)           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Packet Number (8/16/32)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Protected Payload (*)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 11: Short Header Packet Format




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   The short header can be used after the version and 1-RTT keys are
   negotiated.  Packets that use the short header contain the following
   fields:

   Header Form:  The most significant bit (0x80) of octet 0 is set to 0
      for the short header.

   Key Phase Bit:  The second bit (0x40) of octet 0 indicates the key
      phase, which allows a recipient of a packet to identify the packet
      protection keys that are used to protect the packet.  See
      [QUIC-TLS] for details.

   [[Editor's Note: this section should be removed and the bit
   definitions changed before this draft goes to the IESG.]]

   Third Bit:  The third bit (0x20) of octet 0 is set to 1.

   [[Editor's Note: this section should be removed and the bit
   definitions changed before this draft goes to the IESG.]]

   Fourth Bit:  The fourth bit (0x10) of octet 0 is set to 1.

   [[Editor's Note: this section should be removed and the bit
   definitions changed before this draft goes to the IESG.]]

   Google QUIC Demultiplexing Bit:  The fifth bit (0x8) of octet 0 is
      set to 0.  This allows implementations of Google QUIC to
      distinguish Google QUIC packets from short header packets sent by
      a client because Google QUIC servers expect the connection ID to
      always be present.  The special interpretation of this bit SHOULD
      be removed from this specification when Google QUIC has finished
      transitioning to the new header format.

   Reserved:  The sixth, seventh, and eighth bits (0x7) of octet 0 are
      reserved for experimentation.  Endpoints MUST ignore these bits on
      packets they receive unless they are participating in an
      experiment that uses these bits.  An endpoint not actively using
      these bits SHOULD set the value randomly on packets they send to
      protect against unwanted inference about particular values.

   Destination Connection ID:  The Destination Connection ID is a
      connection ID that is chosen by the intended recipient of the
      packet.  See Section 5.1 for more details.

   Packet Number:  The packet number field is 1, 2, or 4 octets long.
      The packet number has confidentiality protection separate from
      packet protection, as described in Section 5.3 of [QUIC-TLS].  The




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      length of the packet number field is encoded in the plaintext
      packet number.  See Section 17.1 for details.

   Protected Payload:  Packets with a short header always include a
      1-RTT protected payload.

   The header form and connection ID field of a short header packet are
   version-independent.  The remaining fields are specific to the
   selected QUIC version.  See [QUIC-INVARIANTS] for details on how
   packets from different versions of QUIC are interpreted.

17.4.  Version Negotiation Packet

   A Version Negotiation packet is inherently not version-specific, and
   does not use the long packet header (see Section 17.2).  Upon receipt
   by a client, it will appear to be a packet using the long header, but
   will be identified as a Version Negotiation packet based on the
   Version field having a value of 0.

   The Version Negotiation packet is a response to a client packet that
   contains a version that is not supported by the server, and is only
   sent by servers.

   The layout of a Version Negotiation packet is:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|  Unused (7) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Version (32)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Supported Version 1 (32)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   [Supported Version 2 (32)]                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   [Supported Version N (32)]                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 12: Version Negotiation Packet



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   The value in the Unused field is selected randomly by the server.

   The Version field of a Version Negotiation packet MUST be set to
   0x00000000.

   The server MUST include the value from the Source Connection ID field
   of the packet it receives in the Destination Connection ID field.
   The value for Source Connection ID MUST be copied from the
   Destination Connection ID of the received packet, which is initially
   randomly selected by a client.  Echoing both connection IDs gives
   clients some assurance that the server received the packet and that
   the Version Negotiation packet was not generated by an off-path
   attacker.

   The remainder of the Version Negotiation packet is a list of 32-bit
   versions which the server supports.

   A Version Negotiation packet cannot be explicitly acknowledged in an
   ACK frame by a client.  Receiving another Initial packet implicitly
   acknowledges a Version Negotiation packet.

   The Version Negotiation packet does not include the Packet Number and
   Length fields present in other packets that use the long header form.
   Consequently, a Version Negotiation packet consumes an entire UDP
   datagram.

   See Section 6 for a description of the version negotiation process.

17.5.  Initial Packet

   An Initial packet uses long headers with a type value of 0x7F.  It
   carries the first CRYPTO frames sent by the client and server to
   perform key exchange, and carries ACKs in either direction.

   In order to prevent tampering by version-unaware middleboxes, Initial
   packets are protected with connection- and version-specific keys
   (Initial keys) as described in [QUIC-TLS].  This protection does not
   provide confidentiality or integrity against on-path attackers, but
   provides some level of protection against off-path attackers.

   An Initial packet (shown in Figure 13) has two additional header
   fields that are added to the Long Header before the Length field.









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   +-+-+-+-+-+-+-+-+
   |1|    0x7f     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Token Length (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Token (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Length (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Payload (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 13: Initial Packet

   These fields include the token that was previously provided in a
   Retry packet or NEW_TOKEN frame:

   Token Length:  A variable-length integer specifying the length of the
      Token field, in bytes.  This value is zero if no token is present.
      Initial packets sent by the server MUST set the Token Length field
      to zero; clients that receive an Initial packet with a non-zero
      Token Length field MUST either discard the packet or generate a
      connection error of type PROTOCOL_VIOLATION.

   Token:  The value of the token.

   The client and server use the Initial packet type for any packet that
   contains an initial cryptographic handshake message.  This includes
   all cases where a new packet containing the initial cryptographic
   message needs to be created, such as the packets sent after receiving
   a Version Negotiation (Section 17.4) or Retry packet (Section 17.7).

   A server sends its first Initial packet in response to a client
   Initial.  A server may send multiple Initial packets.  The
   cryptographic key exchange could require multiple round trips or
   retransmissions of this data.





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   The payload of an Initial packet includes a CRYPTO frame (or frames)
   containing a cryptographic handshake message, ACK frames, or both.
   PADDING and CONNECTION_CLOSE frames are also permitted.  An endpoint
   that receives an Initial packet containing other frames can either
   discard the packet as spurious or treat it as a connection error.

   The first packet sent by a client always includes a CRYPTO frame that
   contains the entirety of the first cryptographic handshake message.
   This packet, and the cryptographic handshake message, MUST fit in a
   single UDP datagram (see Section 7).  The first CRYPTO frame sent
   always begins at an offset of 0 (see Section 7).

   Note that if the server sends a HelloRetryRequest, the client will
   send a second Initial packet.  This Initial packet will continue the
   cryptographic handshake and will contain a CRYPTO frame with an
   offset matching the size of the CRYPTO frame sent in the first
   Initial packet.  Cryptographic handshake messages subsequent to the
   first do not need to fit within a single UDP datagram.

17.5.1.  Starting Packet Numbers

   The first Initial packet sent by either endpoint contains a packet
   number of 0.  The packet number MUST increase monotonically
   thereafter.  Initial packets are in a different packet number space
   to other packets (see Section 12.3).

17.5.2.  0-RTT Packet Numbers

   Packet numbers for 0-RTT protected packets use the same space as
   1-RTT protected packets.

   After a client receives a Retry or Version Negotiation packet, 0-RTT
   packets are likely to have been lost or discarded by the server.  A
   client MAY attempt to resend data in 0-RTT packets after it sends a
   new Initial packet.

   A client MUST NOT reset the packet number it uses for 0-RTT packets.
   The keys used to protect 0-RTT packets will not change as a result of
   responding to a Retry or Version Negotiation packet unless the client
   also regenerates the cryptographic handshake message.  Sending
   packets with the same packet number in that case is likely to
   compromise the packet protection for all 0-RTT packets because the
   same key and nonce could be used to protect different content.

   Receiving a Retry or Version Negotiation packet, especially a Retry
   that changes the connection ID used for subsequent packets, indicates
   a strong possibility that 0-RTT packets could be lost.  A client only
   receives acknowledgments for its 0-RTT packets once the handshake is



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   complete.  Consequently, a server might expect 0-RTT packets to start
   with a packet number of 0.  Therefore, in determining the length of
   the packet number encoding for 0-RTT packets, a client MUST assume
   that all packets up to the current packet number are in flight,
   starting from a packet number of 0.  Thus, 0-RTT packets could need
   to use a longer packet number encoding.

   A client SHOULD instead generate a fresh cryptographic handshake
   message and start packet numbers from 0.  This ensures that new 0-RTT
   packets will not use the same keys, avoiding any risk of key and
   nonce reuse; this also prevents 0-RTT packets from previous handshake
   attempts from being accepted as part of the connection.

17.6.  Handshake Packet

   A Handshake packet uses long headers with a type value of 0x7D.  It
   is used to carry acknowledgments and cryptographic handshake messages
   from the server and client.

   Once a client has received a Handshake packet from a server, it uses
   Handshake packets to send subsequent cryptographic handshake messages
   and acknowledgments to the server.

   The Destination Connection ID field in a Handshake packet contains a
   connection ID that is chosen by the recipient of the packet; the
   Source Connection ID includes the connection ID that the sender of
   the packet wishes to use (see Section 7.2).

   The first Handshake packet sent by a server contains a packet number
   of 0.  Handshake packets are their own packet number space.  Packet
   numbers are incremented normally for other Handshake packets.

   The payload of this packet contains CRYPTO frames and could contain
   PADDING, or ACK frames.  Handshake packets MAY contain
   CONNECTION_CLOSE or APPLICATION_CLOSE frames.  Endpoints MUST treat
   receipt of Handshake packets with other frames as a connection error.

17.7.  Retry Packet

   A Retry packet uses a long packet header with a type value of 0x7E.
   It carries an address validation token created by the server.  It is
   used by a server that wishes to perform a stateless retry (see
   Section 8.1).








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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |1|    0x7e     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Version (32)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |DCIL(4)|SCIL(4)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Destination Connection ID (0/32..144)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Source Connection ID (0/32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    ODCIL(8)   |      Original Destination Connection ID (*)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Retry Token (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 14: Retry Packet

   A Retry packet (shown in Figure 14) only uses the invariant portion
   of the long packet header [QUIC-INVARIANTS]; that is, the fields up
   to and including the Destination and Source Connection ID fields.  A
   Retry packet does not contain any protected fields.  Like Version
   Negotiation, a Retry packet contains the long header including the
   connection IDs, but omits the Length, Packet Number, and Payload
   fields.  These are replaced with:

   ODCIL:  The length of the Original Destination Connection ID field.
      The length is encoded in the least significant 4 bits of the
      octet, using the same encoding as the DCIL and SCIL fields.  The
      most significant 4 bits of this octet are reserved.  Unless a use
      for these bits has been negotiated, endpoints SHOULD send
      randomized values and MUST ignore any value that it receives.

   Original Destination Connection ID:  The Original Destination
      Connection ID contains the value of the Destination Connection ID
      from the Initial packet that this Retry is in response to.  The
      length of this field is given in ODCIL.

   Retry Token:  An opaque token that the server can use to validate the
      client's address.

   The server populates the Destination Connection ID with the
   connection ID that the client included in the Source Connection ID of
   the Initial packet.





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   The server includes a connection ID of its choice in the Source
   Connection ID field.  This value MUST not be equal to the Destination
   Connection ID field of the packet sent by the client.  The client
   MUST use this connection ID in the Destination Connection ID of
   subsequent packets that it sends.

   A server MAY send Retry packets in response to Initial and 0-RTT
   packets.  A server can either discard or buffer 0-RTT packets that it
   receives.  A server can send multiple Retry packets as it receives
   Initial or 0-RTT packets.

   A client MUST accept and process at most one Retry packet for each
   connection attempt.  After the client has received and processed an
   Initial or Retry packet from the server, it MUST discard any
   subsequent Retry packets that it receives.

   Clients MUST discard Retry packets that contain an Original
   Destination Connection ID field that does not match the Destination
   Connection ID from its Initial packet.  This prevents an off-path
   attacker from injecting a Retry packet.

   The client responds to a Retry packet with an Initial packet that
   includes the provided Retry Token to continue connection
   establishment.

   A client sets the Destination Connection ID field of this Initial
   packet to the value from the Source Connection ID in the Retry
   packet.  Changing Destination Connection ID also results in a change
   to the keys used to protect the Initial packet.  It also sets the
   Token field to the token provided in the Retry.  The client MUST NOT
   change the Source Connection ID because the server could include the
   connection ID as part of its token validation logic (see
   Section 8.1.2).

   All subsequent Initial packets from the client MUST use the
   connection ID and token values from the Retry packet.  Aside from
   this, the Initial packet sent by the client is subject to the same
   restrictions as the first Initial packet.  A client can either reuse
   the cryptographic handshake message or construct a new one at its
   discretion.

   A client MAY attempt 0-RTT after receiving a Retry packet by sending
   0-RTT packets to the connection ID provided by the server.  A client
   that sends additional 0-RTT packets without constructing a new
   cryptographic handshake message MUST NOT reset the packet number to 0
   after a Retry packet, see Section 17.5.2.





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   A server acknowledges the use of a Retry packet for a connection
   using the original_connection_id transport parameter (see
   Section 18.1).  If the server sends a Retry packet, it MUST include
   the value of the Original Destination Connection ID field of the
   Retry packet (that is, the Destination Connection ID field from the
   client's first Initial packet) in the transport parameter.

   If the client received and processed a Retry packet, it validates
   that the original_connection_id transport parameter is present and
   correct; otherwise, it validates that the transport parameter is
   absent.  A client MUST treat a failed validation as a connection
   error of type TRANSPORT_PARAMETER_ERROR.

   A Retry packet does not include a packet number and cannot be
   explicitly acknowledged by a client.

18.  Transport Parameter Encoding

   The format of the transport parameters is the TransportParameters
   struct from Figure 15.  This is described using the presentation
   language from Section 3 of [TLS13].






























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      uint32 QuicVersion;

      enum {
         initial_max_stream_data_bidi_local(0),
         initial_max_data(1),
         initial_max_bidi_streams(2),
         idle_timeout(3),
         preferred_address(4),
         max_packet_size(5),
         stateless_reset_token(6),
         ack_delay_exponent(7),
         initial_max_uni_streams(8),
         disable_migration(9),
         initial_max_stream_data_bidi_remote(10),
         initial_max_stream_data_uni(11),
         max_ack_delay(12),
         original_connection_id(13),
         (65535)
      } TransportParameterId;

      struct {
         TransportParameterId parameter;
         opaque value<0..2^16-1>;
      } TransportParameter;

      struct {
         select (Handshake.msg_type) {
            case client_hello:
               QuicVersion initial_version;

            case encrypted_extensions:
               QuicVersion negotiated_version;
               QuicVersion supported_versions<4..2^8-4>;
         };
         TransportParameter parameters<0..2^16-1>;
      } TransportParameters;

      struct {
        enum { IPv4(4), IPv6(6), (15) } ipVersion;
        opaque ipAddress<4..2^8-1>;
        uint16 port;
        opaque connectionId<0..18>;
        opaque statelessResetToken[16];
      } PreferredAddress;

               Figure 15: Definition of TransportParameters





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   The "extension_data" field of the quic_transport_parameters extension
   defined in [QUIC-TLS] contains a TransportParameters value.  TLS
   encoding rules are therefore used to describe the encoding of
   transport parameters.

   QUIC encodes transport parameters into a sequence of octets, which
   are then included in the cryptographic handshake.

18.1.  Transport Parameter Definitions

   An endpoint MAY use the following transport parameters:

   idle_timeout (0x0003):  The idle timeout is a value in seconds that
      is encoded as an unsigned 16-bit integer.  If this parameter is
      absent or zero then the idle timeout is disabled.

   max_packet_size (0x0005):  The maximum packet size parameter places a
      limit on the size of packets that the endpoint is willing to
      receive, encoded as an unsigned 16-bit integer.  This indicates
      that packets larger than this limit will be dropped.  The default
      for this parameter is the maximum permitted UDP payload of 65527.
      Values below 1200 are invalid.  This limit only applies to
      protected packets (Section 12.1).

   ack_delay_exponent (0x0007):  An 8-bit unsigned integer value
      indicating an exponent used to decode the ACK Delay field in the
      ACK frame, see Section 19.15.  If this value is absent, a default
      value of 3 is assumed (indicating a multiplier of 8).  The default
      value is also used for ACK frames that are sent in Initial and
      Handshake packets.  Values above 20 are invalid.

   disable_migration (0x0009):  The endpoint does not support connection
      migration (Section 9).  Peers MUST NOT send any packets, including
      probing packets (Section 9.1), from a local address other than
      that used to perform the handshake.  This parameter is a zero-
      length value.

   max_ack_delay (0x000c):  An 8 bit unsigned integer value indicating
      the maximum amount of time in milliseconds by which the endpoint
      will delay sending acknowledgments.  If this value is absent, a
      default of 25 milliseconds is assumed.

   Either peer MAY advertise an initial value for flow control of each
   type of stream on which they might receive data.  Each of the
   following transport parameters is encoded as an unsigned 32-bit
   integer in units of octets:





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   initial_max_stream_data_bidi_local (0x0000):  The initial stream
      maximum data for bidirectional, locally-initiated streams
      parameter contains the initial flow control limit for newly
      created bidirectional streams opened by the endpoint that sets the
      transport parameter.  In client transport parameters, this applies
      to streams with an identifier ending in 0x0; in server transport
      parameters, this applies to streams ending in 0x1.

   initial_max_stream_data_bidi_remote (0x000a):  The initial stream
      maximum data for bidirectional, peer-initiated streams parameter
      contains the initial flow control limit for newly created
      bidirectional streams opened by the endpoint that receives the
      transport parameter.  In client transport parameters, this applies
      to streams with an identifier ending in 0x1; in server transport
      parameters, this applies to streams ending in 0x0.

   initial_max_stream_data_uni (0x000b):  The initial stream maximum
      data for unidirectional streams parameter contains the initial
      flow control limit for newly created unidirectional streams opened
      by the endpoint that receives the transport parameter.  In client
      transport parameters, this applies to streams with an identifier
      ending in 0x3; in server transport parameters, this applies to
      streams ending in 0x2.

   If present, transport parameters that set initial flow control limits
   (initial_max_stream_data_bidi_local,
   initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni)
   are equivalent to sending a MAX_STREAM_DATA frame (Section 19.6) on
   every stream of the corresponding type immediately after opening.  If
   the transport parameter is absent, streams of that type start with a
   flow control limit of 0.

   initial_max_data (0x0001):  The initial maximum data parameter
      contains the initial value for the maximum amount of data that can
      be sent on the connection.  This parameter is encoded as an
      unsigned 32-bit integer in units of octets.  This is equivalent to
      sending a MAX_DATA (Section 19.5) for the connection immediately
      after completing the handshake.  If the transport parameter is
      absent, the connection starts with a flow control limit of 0.

   initial_max_bidi_streams (0x0002):  The initial maximum bidirectional
      streams parameter contains the initial maximum number of
      bidirectional streams the peer may initiate, encoded as an
      unsigned 16-bit integer.  If this parameter is absent or zero,
      bidirectional streams cannot be created until a MAX_STREAM_ID
      frame is sent.  Setting this parameter is equivalent to sending a
      MAX_STREAM_ID (Section 19.7) immediately after completing the
      handshake containing the corresponding Stream ID.  For example, a



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      value of 0x05 would be equivalent to receiving a MAX_STREAM_ID
      containing 16 when received by a client or 17 when received by a
      server.

   initial_max_uni_streams (0x0008):  The initial maximum unidirectional
      streams parameter contains the initial maximum number of
      unidirectional streams the peer may initiate, encoded as an
      unsigned 16-bit integer.  If this parameter is absent or zero,
      unidirectional streams cannot be created until a MAX_STREAM_ID
      frame is sent.  Setting this parameter is equivalent to sending a
      MAX_STREAM_ID (Section 19.7) immediately after completing the
      handshake containing the corresponding Stream ID.  For example, a
      value of 0x05 would be equivalent to receiving a MAX_STREAM_ID
      containing 18 when received by a client or 19 when received by a
      server.

   A server MUST include the following transport parameter if it sent a
   Retry packet:

   original_connection_id (0x000d):  The value of the Destination
      Connection ID field from the first Initial packet sent by the
      client.  This transport parameter is only sent by the server.

   A server MAY include the following transport parameters:

   stateless_reset_token (0x0006):  The Stateless Reset Token is used in
      verifying a stateless reset, see Section 10.4.  This parameter is
      a sequence of 16 octets.

   preferred_address (0x0004):  The server's Preferred Address is used
      to effect a change in server address at the end of the handshake,
      as described in Section 9.6.

   A client MUST NOT include an original connection ID, a stateless
   reset token, or a preferred address.  A server MUST treat receipt of
   any of these transport parameters as a connection error of type
   TRANSPORT_PARAMETER_ERROR.

19.  Frame Types and Formats

   As described in Section 12.4, packets contain one or more frames.
   This section describes the format and semantics of the core QUIC
   frame types.








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19.1.  PADDING Frame

   The PADDING frame (type=0x00) has no semantic value.  PADDING frames
   can be used to increase the size of a packet.  Padding can be used to
   increase an initial client packet to the minimum required size, or to
   provide protection against traffic analysis for protected packets.

   A PADDING frame has no content.  That is, a PADDING frame consists of
   the single octet that identifies the frame as a PADDING frame.

19.2.  RST_STREAM Frame

   An endpoint may use a RST_STREAM frame (type=0x01) to abruptly
   terminate a stream.

   After sending a RST_STREAM, an endpoint ceases transmission and
   retransmission of STREAM frames on the identified stream.  A receiver
   of RST_STREAM can discard any data that it already received on that
   stream.

   An endpoint that receives a RST_STREAM frame for a send-only stream
   MUST terminate the connection with error PROTOCOL_VIOLATION.

   The RST_STREAM frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Application Error Code (16)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Final Offset (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Stream ID:  A variable-length integer encoding of the Stream ID of
      the stream being terminated.

   Application Protocol Error Code:  A 16-bit application protocol error
      code (see Section 20.1) which indicates why the stream is being
      closed.

   Final Offset:  A variable-length integer indicating the absolute byte
      offset of the end of data written on this stream by the RST_STREAM
      sender.




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19.3.  CONNECTION_CLOSE frame

   An endpoint sends a CONNECTION_CLOSE frame (type=0x02) to notify its
   peer that the connection is being closed.  CONNECTION_CLOSE is used
   to signal errors at the QUIC layer, or the absence of errors (with
   the NO_ERROR code).

   If there are open streams that haven't been explicitly closed, they
   are implicitly closed when the connection is closed.

   The CONNECTION_CLOSE frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Code (16)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Frame Type (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Reason Phrase Length (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reason Phrase (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of a CONNECTION_CLOSE frame are as follows:

   Error Code:  A 16-bit error code which indicates the reason for
      closing this connection.  CONNECTION_CLOSE uses codes from the
      space defined in Section 20.

   Frame Type:  A variable-length integer encoding the type of frame
      that triggered the error.  A value of 0 (equivalent to the mention
      of the PADDING frame) is used when the frame type is unknown.

   Reason Phrase Length:  A variable-length integer specifying the
      length of the reason phrase in bytes.  Note that a
      CONNECTION_CLOSE frame cannot be split between packets, so in
      practice any limits on packet size will also limit the space
      available for a reason phrase.

   Reason Phrase:  A human-readable explanation for why the connection
      was closed.  This can be zero length if the sender chooses to not
      give details beyond the Error Code.  This SHOULD be a UTF-8
      encoded string [RFC3629].







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19.4.  APPLICATION_CLOSE frame

   An APPLICATION_CLOSE frame (type=0x03) is used to signal an error
   with the protocol that uses QUIC.

   The APPLICATION_CLOSE frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Error Code (16)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Reason Phrase Length (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Reason Phrase (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of an APPLICATION_CLOSE frame are as follows:

   Error Code:  A 16-bit error code which indicates the reason for
      closing this connection.  APPLICATION_CLOSE uses codes from the
      application protocol error code space, see Section 20.1.

   Reason Phrase Length:  This field is identical in format and
      semantics to the Reason Phrase Length field from CONNECTION_CLOSE.

   Reason Phrase:  This field is identical in format and semantics to
      the Reason Phrase field from CONNECTION_CLOSE.

   APPLICATION_CLOSE has similar format and semantics to the
   CONNECTION_CLOSE frame (Section 19.3).  Aside from the semantics of
   the Error Code field and the omission of the Frame Type field, both
   frames are used to close the connection.

19.5.  MAX_DATA Frame

   The MAX_DATA frame (type=0x04) is used in flow control to inform the
   peer of the maximum amount of data that can be sent on the connection
   as a whole.

   The frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Maximum Data (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The fields in the MAX_DATA frame are as follows:

   Maximum Data:  A variable-length integer indicating the maximum
      amount of data that can be sent on the entire connection, in units
      of octets.

   All data sent in STREAM frames counts toward this limit.  The sum of
   the largest received offsets on all streams - including streams in
   terminal states - MUST NOT exceed the value advertised by a receiver.
   An endpoint MUST terminate a connection with a FLOW_CONTROL_ERROR
   error if it receives more data than the maximum data value that it
   has sent, unless this is a result of a change in the initial limits
   (see Section 7.3.1).

19.6.  MAX_STREAM_DATA Frame

   The MAX_STREAM_DATA frame (type=0x05) is used in flow control to
   inform a peer of the maximum amount of data that can be sent on a
   stream.

   An endpoint that receives a MAX_STREAM_DATA frame for a receive-only
   stream MUST terminate the connection with error PROTOCOL_VIOLATION.

   An endpoint that receives a MAX_STREAM_DATA frame for a send-only
   stream it has not opened MUST terminate the connection with error
   PROTOCOL_VIOLATION.

   Note that an endpoint may legally receive a MAX_STREAM_DATA frame on
   a bidirectional stream it has not opened.

   The frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Stream Data (i)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in the MAX_STREAM_DATA frame are as follows:

   Stream ID:  The stream ID of the stream that is affected encoded as a
      variable-length integer.

   Maximum Stream Data:  A variable-length integer indicating the
      maximum amount of data that can be sent on the identified stream,
      in units of octets.



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   When counting data toward this limit, an endpoint accounts for the
   largest received offset of data that is sent or received on the
   stream.  Loss or reordering can mean that the largest received offset
   on a stream can be greater than the total size of data received on
   that stream.  Receiving STREAM frames might not increase the largest
   received offset.

   The data sent on a stream MUST NOT exceed the largest maximum stream
   data value advertised by the receiver.  An endpoint MUST terminate a
   connection with a FLOW_CONTROL_ERROR error if it receives more data
   than the largest maximum stream data that it has sent for the
   affected stream, unless this is a result of a change in the initial
   limits (see Section 7.3.1).

19.7.  MAX_STREAM_ID Frame

   The MAX_STREAM_ID frame (type=0x06) informs the peer of the maximum
   stream ID that they are permitted to open.

   The frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Stream ID (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in the MAX_STREAM_ID frame are as follows:

   Maximum Stream ID:  ID of the maximum unidirectional or bidirectional
      peer-initiated stream ID for the connection encoded as a variable-
      length integer.  The limit applies to unidirectional steams if the
      second least signification bit of the stream ID is 1, and applies
      to bidirectional streams if it is 0.

   Loss or reordering can mean that a MAX_STREAM_ID frame can be
   received which states a lower stream limit than the client has
   previously received.  MAX_STREAM_ID frames which do not increase the
   maximum stream ID MUST be ignored.

   A peer MUST NOT initiate a stream with a higher stream ID than the
   greatest maximum stream ID it has received.  An endpoint MUST
   terminate a connection with a STREAM_ID_ERROR error if a peer
   initiates a stream with a higher stream ID than it has sent, unless
   this is a result of a change in the initial limits (see
   Section 7.3.1).





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19.8.  PING Frame

   Endpoints can use PING frames (type=0x07) to verify that their peers
   are still alive or to check reachability to the peer.  The PING frame
   contains no additional fields.

   The receiver of a PING frame simply needs to acknowledge the packet
   containing this frame.

   The PING frame can be used to keep a connection alive when an
   application or application protocol wishes to prevent the connection
   from timing out.  An application protocol SHOULD provide guidance
   about the conditions under which generating a PING is recommended.
   This guidance SHOULD indicate whether it is the client or the server
   that is expected to send the PING.  Having both endpoints send PING
   frames without coordination can produce an excessive number of
   packets and poor performance.

   A connection will time out if no packets are sent or received for a
   period longer than the time specified in the idle_timeout transport
   parameter (see Section 10).  However, state in middleboxes might time
   out earlier than that.  Though REQ-5 in [RFC4787] recommends a 2
   minute timeout interval, experience shows that sending packets every
   15 to 30 seconds is necessary to prevent the majority of middleboxes
   from losing state for UDP flows.

19.9.  BLOCKED Frame

   A sender SHOULD send a BLOCKED frame (type=0x08) when it wishes to
   send data, but is unable to due to connection-level flow control (see
   Section 4).  BLOCKED frames can be used as input to tuning of flow
   control algorithms (see Section 4.2).

   The BLOCKED frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Offset (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The BLOCKED frame contains a single field.

   Offset:  A variable-length integer indicating the connection-level
      offset at which the blocking occurred.






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19.10.  STREAM_BLOCKED Frame

   A sender SHOULD send a STREAM_BLOCKED frame (type=0x09) when it
   wishes to send data, but is unable to due to stream-level flow
   control.  This frame is analogous to BLOCKED (Section 19.9).

   An endpoint that receives a STREAM_BLOCKED frame for a send-only
   stream MUST terminate the connection with error PROTOCOL_VIOLATION.

   The STREAM_BLOCKED frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Offset (i)                          ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The STREAM_BLOCKED frame contains two fields:

   Stream ID:  A variable-length integer indicating the stream which is
      flow control blocked.

   Offset:  A variable-length integer indicating the offset of the
      stream at which the blocking occurred.

19.11.  STREAM_ID_BLOCKED Frame

   A sender SHOULD send a STREAM_ID_BLOCKED frame (type=0x0a) when it
   wishes to open a stream, but is unable to due to the maximum stream
   ID limit set by its peer (see Section 19.7).  This does not open the
   stream, but informs the peer that a new stream was needed, but the
   stream limit prevented the creation of the stream.

   The STREAM_ID_BLOCKED frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The STREAM_ID_BLOCKED frame contains a single field.

   Stream ID:  A variable-length integer indicating the highest stream
      ID that the sender was permitted to open.




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19.12.  NEW_CONNECTION_ID Frame

   An endpoint sends a NEW_CONNECTION_ID frame (type=0x0b) to provide
   its peer with alternative connection IDs that can be used to break
   linkability when migrating connections (see Section 9.5).

   The NEW_CONNECTION_ID frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Length (8)  |            Sequence Number (i)              ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Connection ID (32..144)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Length:  An 8-bit unsigned integer containing the length of the
      connection ID.  Values less than 4 and greater than 18 are invalid
      and MUST be treated as a connection error of type
      PROTOCOL_VIOLATION.

   Sequence Number:  The sequence number assigned to the connection ID
      by the sender.  See Section 5.1.1.

   Connection ID:  A connection ID of the specified length.

   Stateless Reset Token:  A 128-bit value that will be used for a
      stateless reset when the associated connection ID is used (see
      Section 10.4).

   An endpoint MUST NOT send this frame if it currently requires that
   its peer send packets with a zero-length Destination Connection ID.
   Changing the length of a connection ID to or from zero-length makes
   it difficult to identify when the value of the connection ID changed.
   An endpoint that is sending packets with a zero-length Destination
   Connection ID MUST treat receipt of a NEW_CONNECTION_ID frame as a
   connection error of type PROTOCOL_VIOLATION.




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   Transmission errors, timeouts and retransmissions might cause the
   same NEW_CONNECTION_ID frame to be received multiple times.  Receipt
   of the same frame multiple times MUST NOT be treated as a connection
   error.  A receiver can use the sequence number supplied in the
   NEW_CONNECTION_ID frame to identify new connection IDs from old ones.

   If an endpoint receives a NEW_CONNECTION_ID frame that repeats a
   previously issued connection ID with a different Stateless Reset
   Token or a different sequence number, the endpoint MAY treat that
   receipt as a connection error of type PROTOCOL_VIOLATION.

19.13.  RETIRE_CONNECTION_ID Frame

   An endpoint sends a RETIRE_CONNECTION_ID frame (type=0x1b) to
   indicate that it will no longer use a connection ID that was issued
   by its peer.  This may include the connection ID provided during the
   handshake.  Sending a RETIRE_CONNECTION_ID frame also serves as a
   request to the peer to send additional connection IDs for future use
   (see Section 5.1).  New connection IDs can be delivered to a peer
   using the NEW_CONNECTION_ID frame (Section 19.12).

   Retiring a connection ID invalidates the stateless reset token
   associated with that connection ID.

   The RETIRE_CONNECTION_ID frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sequence Number (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Sequence Number:  The sequence number of the connection ID being
      retired.  See Section 5.1.2.

   Receipt of a RETIRE_CONNECTION_ID frame containing a sequence number
   greater than any previously sent to the peer MAY be treated as a
   connection error of type PROTOCOL_VIOLATION.

   An endpoint cannot send this frame if it was provided with a zero-
   length connection ID by its peer.  An endpoint that provides a zero-
   length connection ID MUST treat receipt of a RETIRE_CONNECTION_ID
   frame as a connection error of type PROTOCOL_VIOLATION.






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19.14.  STOP_SENDING Frame

   An endpoint may use a STOP_SENDING frame (type=0x0c) to communicate
   that incoming data is being discarded on receipt at application
   request.  This signals a peer to abruptly terminate transmission on a
   stream.

   Receipt of a STOP_SENDING frame is only valid for a send stream that
   exists and is not in the "Ready" state (see Section 3.1).  Receiving
   a STOP_SENDING frame for a send stream that is "Ready" or non-
   existent MUST be treated as a connection error of type
   PROTOCOL_VIOLATION.  An endpoint that receives a STOP_SENDING frame
   for a receive-only stream MUST terminate the connection with error
   PROTOCOL_VIOLATION.

   The STOP_SENDING frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Application Error Code (16)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   Stream ID:  A variable-length integer carrying the Stream ID of the
      stream being ignored.

   Application Error Code:  A 16-bit, application-specified reason the
      sender is ignoring the stream (see Section 20.1).

19.15.  ACK Frame

   Receivers send ACK frames (types 0x1a and 0x1b) to inform senders of
   packets they have received and processed.  The ACK frame contains one
   or more ACK Blocks.  ACK Blocks are ranges of acknowledged packets.
   If the frame type is 0x1b, ACK frames also contain the sum of ECN
   marks received on the connection up until this point.

   QUIC acknowledgements are irrevocable.  Once acknowledged, a packet
   remains acknowledged, even if it does not appear in a future ACK
   frame.  This is unlike TCP SACKs ([RFC2018]).

   It is expected that a sender will reuse the same packet number across
   different packet number spaces.  ACK frames only acknowledge the




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   packet numbers that were transmitted by the sender in the same packet
   number space of the packet that the ACK was received in.

   Version Negotiation and Retry packets cannot be acknowledged because
   they do not contain a packet number.  Rather than relying on ACK
   frames, these packets are implicitly acknowledged by the next Initial
   packet sent by the client.

   An ACK frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Largest Acknowledged (i)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Delay (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Block Count (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Blocks (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [ECN Section]                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 16: ACK Frame Format

   The fields in the ACK frame are as follows:

   Largest Acknowledged:  A variable-length integer representing the
      largest packet number the peer is acknowledging; this is usually
      the largest packet number that the peer has received prior to
      generating the ACK frame.  Unlike the packet number in the QUIC
      long or short header, the value in an ACK frame is not truncated.

   ACK Delay:  A variable-length integer including the time in
      microseconds that the largest acknowledged packet, as indicated in
      the Largest Acknowledged field, was received by this peer to when
      this ACK was sent.  The value of the ACK Delay field is scaled by
      multiplying the encoded value by 2 to the power of the value of
      the "ack_delay_exponent" transport parameter set by the sender of
      the ACK frame.  The "ack_delay_exponent" defaults to 3, or a
      multiplier of 8 (see Section 18.1).  Scaling in this fashion
      allows for a larger range of values with a shorter encoding at the
      cost of lower resolution.

   ACK Block Count:  A variable-length integer specifying the number of
      Additional ACK Block (and Gap) fields after the First ACK Block.




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   ACK Blocks:  Contains one or more blocks of packet numbers which have
      been successfully received, see Section 19.15.1.

19.15.1.  ACK Block Section

   The ACK Block Section consists of alternating Gap and ACK Block
   fields in descending packet number order.  A First Ack Block field is
   followed by a variable number of alternating Gap and Additional ACK
   Blocks.  The number of Gap and Additional ACK Block fields is
   determined by the ACK Block Count field.

   Gap and ACK Block fields use a relative integer encoding for
   efficiency.  Though each encoded value is positive, the values are
   subtracted, so that each ACK Block describes progressively lower-
   numbered packets.  As long as contiguous ranges of packets are small,
   the variable-length integer encoding ensures that each range can be
   expressed in a small number of octets.

   The ACK frame uses the least significant bit(bit (that is, type 0x1b)
   to indicate ECN feedback and report receipt of packets with ECN
   codepoints of ECT(0), ECT(1), or CE in the packet's IP header.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First ACK Block (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 17: ACK Block Section

   Each ACK Block acknowledges a contiguous range of packets by
   indicating the number of acknowledged packets that precede the
   largest packet number in that block.  A value of zero indicates that
   only the largest packet number is acknowledged.  Larger ACK Block



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   values indicate a larger range, with corresponding lower values for
   the smallest packet number in the range.  Thus, given a largest
   packet number for the ACK, the smallest value is determined by the
   formula:

      smallest = largest - ack_block

   The range of packets that are acknowledged by the ACK Block include
   the range from the smallest packet number to the largest, inclusive.

   The largest value for the First ACK Block is determined by the
   Largest Acknowledged field; the largest for Additional ACK Blocks is
   determined by cumulatively subtracting the size of all preceding ACK
   Blocks and Gaps.

   Each Gap indicates a range of packets that are not being
   acknowledged.  The number of packets in the gap is one higher than
   the encoded value of the Gap Field.

   The value of the Gap field establishes the largest packet number
   value for the ACK Block that follows the gap using the following
   formula:

     largest = previous_smallest - gap - 2

   If the calculated value for largest or smallest packet number for any
   ACK Block is negative, an endpoint MUST generate a connection error
   of type FRAME_ENCODING_ERROR indicating an error in an ACK frame.

   The fields in the ACK Block Section are:

   First ACK Block:  A variable-length integer indicating the number of
      contiguous packets preceding the Largest Acknowledged that are
      being acknowledged.

   Gap (repeated):  A variable-length integer indicating the number of
      contiguous unacknowledged packets preceding the packet number one
      lower than the smallest in the preceding ACK Block.

   Additional ACK Block (repeated):  A variable-length integer
      indicating the number of contiguous acknowledged packets preceding
      the largest packet number, as determined by the preceding Gap.

19.15.2.  ECN section

   The ECN section should only be parsed when the ACK frame type byte is
   0x1b.  The ECN section consists of 3 ECN counters as shown below.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ECT(0) Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ECT(1) Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ECN-CE Count (i)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   ECT(0) Count:  A variable-length integer representing the total
      number packets received with the ECT(0) codepoint.

   ECT(1) Count:  A variable-length integer representing the total
      number packets received with the ECT(1) codepoint.

   CE Count:  A variable-length integer representing the total number
      packets received with the CE codepoint.

19.16.  PATH_CHALLENGE Frame

   Endpoints can use PATH_CHALLENGE frames (type=0x0e) to check
   reachability to the peer and for path validation during connection
   migration.

   PATH_CHALLENGE frames contain an 8-byte payload.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                            Data (8)                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data:  This 8-byte field contains arbitrary data.

   A PATH_CHALLENGE frame containing 8 octets that are hard to guess is
   sufficient to ensure that it is easier to receive the packet than it
   is to guess the value correctly.

   The recipient of this frame MUST generate a PATH_RESPONSE frame
   (Section 19.17) containing the same Data.








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19.17.  PATH_RESPONSE Frame

   The PATH_RESPONSE frame (type=0x0f) is sent in response to a
   PATH_CHALLENGE frame.  Its format is identical to the PATH_CHALLENGE
   frame (Section 19.16).

   If the content of a PATH_RESPONSE frame does not match the content of
   a PATH_CHALLENGE frame previously sent by the endpoint, the endpoint
   MAY generate a connection error of type PROTOCOL_VIOLATION.

19.18.  NEW_TOKEN frame

   A server sends a NEW_TOKEN frame (type=0x19) to provide the client a
   token to send in the header of an Initial packet for a future
   connection.

   The NEW_TOKEN frame is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Token Length (i)  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Token (*)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of a NEW_TOKEN frame are as follows:

   Token Length:  A variable-length integer specifying the length of the
      token in bytes.

   Token:  An opaque blob that the client may use with a future Initial
      packet.

19.19.  STREAM Frames

   STREAM frames implicitly create a stream and carry stream data.  The
   STREAM frame takes the form 0b00010XXX (or the set of values from
   0x10 to 0x17).  The value of the three low-order bits of the frame
   type determine the fields that are present in the frame.

   o  The OFF bit (0x04) in the frame type is set to indicate that there
      is an Offset field present.  When set to 1, the Offset field is
      present; when set to 0, the Offset field is absent and the Stream
      Data starts at an offset of 0 (that is, the frame contains the
      first octets of the stream, or the end of a stream that includes
      no data).




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   o  The LEN bit (0x02) in the frame type is set to indicate that there
      is a Length field present.  If this bit is set to 0, the Length
      field is absent and the Stream Data field extends to the end of
      the packet.  If this bit is set to 1, the Length field is present.

   o  The FIN bit (0x01) of the frame type is set only on frames that
      contain the final offset of the stream.  Setting this bit
      indicates that the frame marks the end of the stream.

   An endpoint that receives a STREAM frame for a send-only stream MUST
   terminate the connection with error PROTOCOL_VIOLATION.

   A STREAM frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Stream ID (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [Offset (i)]                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [Length (i)]                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Stream Data (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 18: STREAM Frame Format

   The STREAM frame contains the following fields:

   Stream ID:  A variable-length integer indicating the stream ID of the
      stream (see Section 2.1).

   Offset:  A variable-length integer specifying the byte offset in the
      stream for the data in this STREAM frame.  This field is present
      when the OFF bit is set to 1.  When the Offset field is absent,
      the offset is 0.

   Length:  A variable-length integer specifying the length of the
      Stream Data field in this STREAM frame.  This field is present
      when the LEN bit is set to 1.  When the LEN bit is set to 0, the
      Stream Data field consumes all the remaining octets in the packet.

   Stream Data:  The bytes from the designated stream to be delivered.

   When a Stream Data field has a length of 0, the offset in the STREAM
   frame is the offset of the next byte that would be sent.




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   The first byte in the stream has an offset of 0.  The largest offset
   delivered on a stream - the sum of the re-constructed offset and data
   length - MUST be less than 2^62.

19.20.  CRYPTO Frame

   The CRYPTO frame (type=0x18) is used to transmit cryptographic
   handshake messages.  It can be sent in all packet types.  The CRYPTO
   frame offers the cryptographic protocol an in-order stream of bytes.
   CRYPTO frames are functionally identical to STREAM frames, except
   that they do not bear a stream identifier; they are not flow
   controlled; and they do not carry markers for optional offset,
   optional length, and the end of the stream.

   A CRYPTO frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Offset (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Length (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Crypto Data (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 19: CRYPTO Frame Format

   The CRYPTO frame contains the following fields:

   Offset:  A variable-length integer specifying the byte offset in the
      stream for the data in this CRYPTO frame.

   Length:  A variable-length integer specifying the length of the
      Crypto Data field in this CRYPTO frame.

   Crypto Data:  The cryptographic message data.

   There is a separate flow of cryptographic handshake data in each
   encryption level, each of which starts at an offset of 0.  This
   implies that each encryption level is treated as a separate CRYPTO
   stream of data.

   Unlike STREAM frames, which include a Stream ID indicating to which
   stream the data belongs, the CRYPTO frame carries data for a single
   stream per encryption level.  The stream does not have an explicit
   end, so CRYPTO frames do not have a FIN bit.




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19.21.  Extension Frames

   QUIC frames do not use a self-describing encoding.  An endpoint
   therefore needs to understand the syntax of all frames before it can
   successfully process a packet.  This allows for efficient encoding of
   frames, but it means that an endpoint cannot send a frame of a type
   that is unknown to its peer.

   An extension to QUIC that wishes to use a new type of frame MUST
   first ensure that a peer is able to understand the frame.  An
   endpoint can use a transport parameter to signal its willingness to
   receive one or more extension frame types with the one transport
   parameter.

   Extension frames MUST be congestion controlled and MUST cause an ACK
   frame to be sent.  The exception is extension frames that replace or
   supplement the ACK frame.  Extension frames are not included in flow
   control unless specified in the extension.

   An IANA registry is used to manage the assignment of frame types, see
   Section 22.2.

20.  Transport Error Codes

   QUIC error codes are 16-bit unsigned integers.

   This section lists the defined QUIC transport error codes that may be
   used in a CONNECTION_CLOSE frame.  These errors apply to the entire
   connection.

   NO_ERROR (0x0):  An endpoint uses this with CONNECTION_CLOSE to
      signal that the connection is being closed abruptly in the absence
      of any error.

   INTERNAL_ERROR (0x1):  The endpoint encountered an internal error and
      cannot continue with the connection.

   SERVER_BUSY (0x2):  The server is currently busy and does not accept
      any new connections.

   FLOW_CONTROL_ERROR (0x3):  An endpoint received more data than it
      permitted in its advertised data limits (see Section 4).

   STREAM_ID_ERROR (0x4):  An endpoint received a frame for a stream
      identifier that exceeded its advertised maximum stream ID.

   STREAM_STATE_ERROR (0x5):  An endpoint received a frame for a stream
      that was not in a state that permitted that frame (see Section 3).



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   FINAL_OFFSET_ERROR (0x6):  An endpoint received a STREAM frame
      containing data that exceeded the previously established final
      offset.  Or an endpoint received a RST_STREAM frame containing a
      final offset that was lower than the maximum offset of data that
      was already received.  Or an endpoint received a RST_STREAM frame
      containing a different final offset to the one already
      established.

   FRAME_ENCODING_ERROR (0x7):  An endpoint received a frame that was
      badly formatted.  For instance, a frame of an unknown type, or an
      ACK frame that has more acknowledgment ranges than the remainder
      of the packet could carry.

   TRANSPORT_PARAMETER_ERROR (0x8):  An endpoint received transport
      parameters that were badly formatted, included an invalid value,
      was absent even though it is mandatory, was present though it is
      forbidden, or is otherwise in error.

   VERSION_NEGOTIATION_ERROR (0x9):  An endpoint received transport
      parameters that contained version negotiation parameters that
      disagreed with the version negotiation that it performed.  This
      error code indicates a potential version downgrade attack.

   PROTOCOL_VIOLATION (0xA):  An endpoint detected an error with
      protocol compliance that was not covered by more specific error
      codes.

   INVALID_MIGRATION (0xC):  A peer has migrated to a different network
      when the endpoint had disabled migration.

   CRYPTO_ERROR (0x1XX):  The cryptographic handshake failed.  A range
      of 256 values is reserved for carrying error codes specific to the
      cryptographic handshake that is used.  Codes for errors occurring
      when TLS is used for the crypto handshake are described in
      Section 4.8 of [QUIC-TLS].

   See Section 22.3 for details of registering new error codes.

20.1.  Application Protocol Error Codes

   Application protocol error codes are 16-bit unsigned integers, but
   the management of application error codes are left to application
   protocols.  Application protocol error codes are used for the
   RST_STREAM (Section 19.2) and APPLICATION_CLOSE (Section 19.4)
   frames.

   There is no restriction on the use of the 16-bit error code space for
   application protocols.  However, QUIC reserves the error code with a



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   value of 0 to mean STOPPING.  The application error code of STOPPING
   (0) is used by the transport to cancel a stream in response to
   receipt of a STOP_SENDING frame.

21.  Security Considerations

21.1.  Handshake Denial of Service

   As an encrypted and authenticated transport QUIC provides a range of
   protections against denial of service.  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.

   Once a connection is established QUIC endpoints might accept some
   unauthenticated ICMP packets (see Section 14.1.1), but the use of
   these packets is extremely limited.  The only other type of packet
   that an endpoint might accept is a stateless reset (Section 10.4)
   which relies on the token being kept secret until it is used.

   During the creation of a connection, QUIC only provides protection
   against attack from off the network path.  All QUIC packets contain
   proof that the recipient saw a preceding packet from its peer.

   The first mechanism used is the source and destination connection
   IDs, which are required to match those set by a peer.  Except for an
   Initial and stateless reset packets, an endpoint only accepts packets
   that include a destination connection that matches a connection ID
   the endpoint previously chose.  This is the only protection offered
   for Version Negotiation packets.

   The destination connection ID in an Initial packet is selected by a
   client to be unpredictable, which serves an additional purpose.  The
   packets that carry the cryptographic handshake are protected with a
   key that is derived from this connection ID and salt specific to the
   QUIC version.  This allows endpoints to use the same process for
   authenticating packets that they receive as they use after the
   cryptographic handshake completes.  Packets that cannot be
   authenticated are discarded.  Protecting packets in this fashion
   provides a strong assurance that the sender of the packet saw the
   Initial packet and understood it.

   These protections are not intended to be effective against an
   attacker that is able to receive QUIC packets prior to the connection
   being established.  Such an attacker can potentially send packets
   that will be accepted by QUIC endpoints.  This version of QUIC
   attempts to detect this sort of attack, but it expects that endpoints
   will fail to establish a connection rather than recovering.  For the



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   most part, the cryptographic handshake protocol [QUIC-TLS] is
   responsible for detecting tampering during the handshake, though
   additional validation is required for version negotiation (see
   Section 7.3.3).

   Endpoints are permitted to use other methods to detect and attempt to
   recover from interference with the handshake.  Invalid packets may be
   identified and discarded using other methods, but no specific method
   is mandated in this document.

21.2.  Spoofed ACK Attack

   An attacker might be able to receive an address validation token
   (Section 8) from the server and then release the IP address it used
   to acquire that token.  The attacker may, in the future, spoof this
   same address (which now presumably addresses a different endpoint),
   and initiate a 0-RTT connection with a server on the victim's behalf.
   The attacker can then spoof ACK frames to the server which cause the
   server to send excessive amounts of data toward the new owner of the
   IP address.

   There are two possible mitigations to this attack.  The simplest one
   is that a server can unilaterally create a gap in packet-number
   space.  In the non-attack scenario, the client will send an ACK frame
   with the larger value for largest acknowledged.  In the attack
   scenario, the attacker could acknowledge a packet in the gap.  If the
   server sees an acknowledgment for a packet that was never sent, the
   connection can be aborted.

   The second mitigation is that the server can require that
   acknowledgments for sent packets match the encryption level of the
   sent packet.  This mitigation is useful if the connection has an
   ephemeral forward-secure key that is generated and used for every new
   connection.  If a packet sent is protected with a forward-secure key,
   then any acknowledgments that are received for them MUST also be
   forward-secure protected.  Since the attacker will not have the
   forward-secure key, the attacker will not be able to generate
   forward-secure protected packets with ACK frames.

21.3.  Optimistic ACK Attack

   An endpoint that acknowledges packets it has not received might cause
   a congestion controller to permit sending at rates beyond what the
   network supports.  An endpoint MAY skip packet numbers when sending
   packets to detect this behavior.  An endpoint can then immediately
   close the connection with a connection error of type
   PROTOCOL_VIOLATION (see Section 10.3).




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21.4.  Slowloris Attacks

   The attacks commonly known as Slowloris [SLOWLORIS] try to keep many
   connections to the target endpoint open and hold them open as long as
   possible.  These attacks can be executed against a QUIC endpoint by
   generating the minimum amount of activity necessary to avoid being
   closed for inactivity.  This might involve sending small amounts of
   data, gradually opening flow control windows in order to control the
   sender rate, or manufacturing ACK frames that simulate a high loss
   rate.

   QUIC deployments SHOULD provide mitigations for the Slowloris
   attacks, such as increasing the maximum number of clients the server
   will allow, limiting the number of connections a single IP address is
   allowed to make, imposing restrictions on the minimum transfer speed
   a connection is allowed to have, and restricting the length of time
   an endpoint is allowed to stay connected.

21.5.  Stream Fragmentation and Reassembly Attacks

   An adversarial sender might intentionally send fragments of stream
   data in order to cause disproportionate receive buffer memory
   commitment and/or creation of a large and inefficient data structure.

   An adversarial receiver might intentionally not acknowledge packets
   containing stream data in order to force the sender to store the
   unacknowledged stream data for retransmission.

   The attack on receivers is mitigated if flow control windows
   correspond to available memory.  However, some receivers will over-
   commit memory and advertise flow control offsets in the aggregate
   that exceed actual available memory.  The over-commitment strategy
   can lead to better performance when endpoints are well behaved, but
   renders endpoints vulnerable to the stream fragmentation attack.

   QUIC deployments SHOULD provide mitigations against stream
   fragmentation attacks.  Mitigations could consist of avoiding over-
   committing memory, limiting the size of tracking data structures,
   delaying reassembly of STREAM frames, implementing heuristics based
   on the age and duration of reassembly holes, or some combination.

21.6.  Stream Commitment Attack

   An adversarial endpoint can open lots of streams, exhausting state on
   an endpoint.  The adversarial endpoint could repeat the process on a
   large number of connections, in a manner similar to SYN flooding
   attacks in TCP.




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   Normally, clients will open streams sequentially, as explained in
   Section 2.1.  However, when several streams are initiated at short
   intervals, transmission error may cause STREAM DATA frames opening
   streams to be received out of sequence.  A receiver is obligated to
   open intervening streams if a higher-numbered stream ID is received.
   Thus, on a new connection, opening stream 2000001 opens 1 million
   streams, as required by the specification.

   The number of active streams is limited by the concurrent stream
   limit transport parameter, as explained in Section 2.2.  If chosen
   judiciously, this limit mitigates the effect of the stream commitment
   attack.  However, setting the limit too low could affect performance
   when applications expect to open large number of streams.

21.7.  Explicit Congestion Notification Attacks

   An on-path attacker could manipulate the value of ECN codepoints in
   the IP header to influence the sender's rate.  [RFC3168] discusses
   manipulations and their effects in more detail.

   An on-the-side attacker can duplicate and send packets with modified
   ECN codepoints to affect the sender's rate.  If duplicate packets are
   discarded by a receiver, an off-path attacker will need to race the
   duplicate packet against the original to be successful in this
   attack.  Therefore, QUIC receivers ignore ECN codepoints set in
   duplicate packets (see Section 13.3).

21.8.  Stateless Reset Oracle

   Stateless resets create a possible denial of service attack analogous
   to a TCP reset injection.  This attack is possible if an attacker is
   able to cause a stateless reset token to be generated for a
   connection with a selected connection ID.  An attacker that can cause
   this token to be generated can reset an active connection with the
   same connection ID.

   If a packet can be routed to different instances that share a static
   key, for example by changing an IP address or port, then an attacker
   can cause the server to send a stateless reset.  To defend against
   this style of denial service, endpoints that share a static key for
   stateless reset (see Section 10.4.2) MUST be arranged so that packets
   with a given connection ID always arrive at an instance that has
   connection state, unless that connection is no longer active.

   In the case of a cluster that uses dynamic load balancing, it's
   possible that a change in load balancer configuration could happen
   while an active instance retains connection state; even if an
   instance retains connection state, the change in routing and



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   resulting stateless reset will result in the connection being
   terminated.  If there is no chance in the packet being routed to the
   correct instance, it is better to send a stateless reset than wait
   for connections to time out.  However, this is acceptable only if the
   routing cannot be influenced by an attacker.

22.  IANA Considerations

22.1.  QUIC Transport Parameter Registry

   IANA [SHALL add/has added] a registry for "QUIC Transport Parameters"
   under a "QUIC Protocol" heading.

   The "QUIC Transport Parameters" registry governs a 16-bit space.
   This space is split into two spaces that are governed by different
   policies.  Values with the first byte in the range 0x00 to 0xfe (in
   hexadecimal) are assigned via the Specification Required policy
   [RFC8126].  Values with the first byte 0xff are reserved for Private
   Use [RFC8126].

   Registrations MUST include the following fields:

   Value:  The numeric value of the assignment (registrations will be
      between 0x0000 and 0xfeff).

   Parameter Name:  A short mnemonic for the parameter.

   Specification:  A reference to a publicly available specification for
      the value.

   The nominated expert(s) verify that a specification exists and is
   readily accessible.  Expert(s) are encouraged to be biased towards
   approving registrations unless they are abusive, frivolous, or
   actively harmful (not merely aesthetically displeasing, or
   architecturally dubious).

   The initial contents of this registry are shown in Table 7.














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     +--------+-------------------------------------+---------------+
     | Value  | Parameter Name                      | Specification |
     +--------+-------------------------------------+---------------+
     | 0x0000 | initial_max_stream_data_bidi_local  | Section 18.1  |
     |        |                                     |               |
     | 0x0001 | initial_max_data                    | Section 18.1  |
     |        |                                     |               |
     | 0x0002 | initial_max_bidi_streams            | Section 18.1  |
     |        |                                     |               |
     | 0x0003 | idle_timeout                        | Section 18.1  |
     |        |                                     |               |
     | 0x0004 | preferred_address                   | Section 18.1  |
     |        |                                     |               |
     | 0x0005 | max_packet_size                     | Section 18.1  |
     |        |                                     |               |
     | 0x0006 | stateless_reset_token               | Section 18.1  |
     |        |                                     |               |
     | 0x0007 | ack_delay_exponent                  | Section 18.1  |
     |        |                                     |               |
     | 0x0008 | initial_max_uni_streams             | Section 18.1  |
     |        |                                     |               |
     | 0x0009 | disable_migration                   | Section 18.1  |
     |        |                                     |               |
     | 0x000a | initial_max_stream_data_bidi_remote | Section 18.1  |
     |        |                                     |               |
     | 0x000b | initial_max_stream_data_uni         | Section 18.1  |
     |        |                                     |               |
     | 0x000c | max_ack_delay                       | Section 18.1  |
     |        |                                     |               |
     | 0x000d | original_connection_id              | Section 18.1  |
     +--------+-------------------------------------+---------------+

            Table 7: Initial QUIC Transport Parameters Entries

22.2.  QUIC Frame Type Registry

   IANA [SHALL add/has added] a registry for "QUIC Frame Types" under a
   "QUIC Protocol" heading.

   The "QUIC Frame Types" registry governs a 62-bit space.  This space
   is split into three spaces that are governed by different policies.
   Values between 0x00 and 0x3f (in hexadecimal) are assigned via the
   Standards Action or IESG Review policies [RFC8126].  Values from 0x40
   to 0x3fff operate on the Specification Required policy [RFC8126].
   All other values are assigned to Private Use [RFC8126].

   Registrations MUST include the following fields:




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   Value:  The numeric value of the assignment (registrations will be
      between 0x00 and 0x3fff).  A range of values MAY be assigned.

   Frame Name:  A short mnemonic for the frame type.

   Specification:  A reference to a publicly available specification for
      the value.

   The nominated expert(s) verify that a specification exists and is
   readily accessible.  Specifications for new registrations need to
   describe the means by which an endpoint might determine that it can
   send the identified type of frame.  An accompanying transport
   parameter registration (see Section 22.1) is expected for most
   registrations.  The specification needs to describe the format and
   assigned semantics of any fields in the frame.

   Expert(s) are encouraged to be biased towards approving registrations
   unless they are abusive, frivolous, or actively harmful (not merely
   aesthetically displeasing, or architecturally dubious).

   The initial contents of this registry are tabulated in Table 3.

22.3.  QUIC Transport Error Codes Registry

   IANA [SHALL add/has added] a registry for "QUIC Transport Error
   Codes" under a "QUIC Protocol" heading.

   The "QUIC Transport Error Codes" registry governs a 16-bit space.
   This space is split into two spaces that are governed by different
   policies.  Values with the first byte in the range 0x00 to 0xfe (in
   hexadecimal) are assigned via the Specification Required policy
   [RFC8126].  Values with the first byte 0xff are reserved for Private
   Use [RFC8126].

   Registrations MUST include the following fields:

   Value:  The numeric value of the assignment (registrations will be
      between 0x0000 and 0xfeff).

   Code:  A short mnemonic for the parameter.

   Description:  A brief description of the error code semantics, which
      MAY be a summary if a specification reference is provided.

   Specification:  A reference to a publicly available specification for
      the value.





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   The initial contents of this registry are shown in Table 8.  Values
   from 0xFF00 to 0xFFFF are reserved for Private Use [RFC8126].

















































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   +------+---------------------------+----------------+---------------+
   | Valu | Error                     | Description    | Specification |
   | e    |                           |                |               |
   +------+---------------------------+----------------+---------------+
   | 0x0  | NO_ERROR                  | No error       | Section 20    |
   |      |                           |                |               |
   | 0x1  | INTERNAL_ERROR            | Implementation | Section 20    |
   |      |                           | error          |               |
   |      |                           |                |               |
   | 0x2  | SERVER_BUSY               | Server         | Section 20    |
   |      |                           | currently busy |               |
   |      |                           |                |               |
   | 0x3  | FLOW_CONTROL_ERROR        | Flow control   | Section 20    |
   |      |                           | error          |               |
   |      |                           |                |               |
   | 0x4  | STREAM_ID_ERROR           | Invalid stream | Section 20    |
   |      |                           | ID             |               |
   |      |                           |                |               |
   | 0x5  | STREAM_STATE_ERROR        | Frame received | Section 20    |
   |      |                           | in invalid     |               |
   |      |                           | stream state   |               |
   |      |                           |                |               |
   | 0x6  | FINAL_OFFSET_ERROR        | Change to      | Section 20    |
   |      |                           | final stream   |               |
   |      |                           | offset         |               |
   |      |                           |                |               |
   | 0x7  | FRAME_ENCODING_ERROR      | Frame encoding | Section 20    |
   |      |                           | error          |               |
   |      |                           |                |               |
   | 0x8  | TRANSPORT_PARAMETER_ERROR | Error in       | Section 20    |
   |      |                           | transport      |               |
   |      |                           | parameters     |               |
   |      |                           |                |               |
   | 0x9  | VERSION_NEGOTIATION_ERROR | Version        | Section 20    |
   |      |                           | negotiation    |               |
   |      |                           | failure        |               |
   |      |                           |                |               |
   | 0xA  | PROTOCOL_VIOLATION        | Generic        | Section 20    |
   |      |                           | protocol       |               |
   |      |                           | violation      |               |
   |      |                           |                |               |
   | 0xC  | INVALID_MIGRATION         | Violated       | Section 20    |
   |      |                           | disabled       |               |
   |      |                           | migration      |               |
   +------+---------------------------+----------------+---------------+

            Table 8: Initial QUIC Transport Error Codes Entries




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23.  References

23.1.  Normative References

   [PLPMTUD]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [PMTUDv4]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [PMTUDv6]  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>.

   [QUIC-RECOVERY]
              Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", draft-ietf-quic-recovery-16 (work
              in progress), October 2018.

   [QUIC-TLS]
              Thomson, M., Ed. and S. Turner, Ed., "Using Transport
              Layer Security (TLS) to Secure QUIC", draft-ietf-quic-
              tls-16 (work in progress), October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.






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   [RFC5119]  Edwards, T., "A Uniform Resource Name (URN) Namespace for
              the Society of Motion Picture and Television Engineers
              (SMPTE)", RFC 5119, DOI 10.17487/RFC5119, February 2008,
              <https://www.rfc-editor.org/info/rfc5119>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8311]  Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,
              <https://www.rfc-editor.org/info/rfc8311>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

23.2.  Informative References

   [EARLY-DESIGN]
              Roskind, J., "QUIC: Multiplexed Transport Over UDP",
              December 2013, <https://goo.gl/dMVtFi>.

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              draft-ietf-quic-invariants-03 (work in progress), October
              2018.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,
              <https://www.rfc-editor.org/info/rfc2018>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.



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   [RFC2360]  Scott, G., "Guide for Internet Standards Writers", BCP 22,
              RFC 2360, DOI 10.17487/RFC2360, June 1998,
              <https://www.rfc-editor.org/info/rfc2360>.

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, DOI 10.17487/RFC2406, November
              1998, <https://www.rfc-editor.org/info/rfc2406>.

   [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>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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>.

   [SLOWLORIS]
              RSnake Hansen, R., "Welcome to Slowloris...", June 2009,
              <https://web.archive.org/web/20150315054838/
              http://ha.ckers.org/slowloris/>.

   [SST]      Ford, B., "Structured streams", ACM SIGCOMM Computer
              Communication Review Vol. 37, pp. 361,
              DOI 10.1145/1282427.1282421, October 2007.

Appendix A.  Sample Packet Number Decoding Algorithm

   The following pseudo-code shows how an implementation can decode
   packet numbers after packet number protection has been removed.















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   DecodePacketNumber(largest_pn, truncated_pn, pn_nbits):
      expected_pn  = largest_pn + 1
      pn_win       = 1 << pn_nbits
      pn_hwin      = pn_win / 2
      pn_mask      = pn_win - 1
      // The incoming packet number should be greater than
      // expected_pn - pn_hwin and less than or equal to
      // expected_pn + pn_hwin
      //
      // This means we can't just strip the trailing bits from
      // expected_pn and add the truncated_pn because that might
      // yield a value outside the window.
      //
      // The following code calculates a candidate value and
      // makes sure it's within the packet number window.
      candidate_pn = (expected_pn & ~pn_mask) | truncated_pn
      if candidate_pn <= expected_pn - pn_hwin:
         return candidate_pn + pn_win
      // Note the extra check for underflow when candidate_pn
      // is near zero.
      if candidate_pn > expected_pn + pn_hwin and
         candidate_pn > pn_win:
         return candidate_pn - pn_win
      return candidate_pn

Appendix B.  Change Log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

   Issue and pull request numbers are listed with a leading octothorp.

B.1.  Since draft-ietf-quic-transport-15

   Substantial editorial reorganization; no technical changes.

B.2.  Since draft-ietf-quic-transport-14

   o  Merge ACK and ACK_ECN (#1778, #1801)

   o  Explicitly communicate max_ack_delay (#981, #1781)

   o  Validate original connection ID after Retry packets (#1710, #1486,
      #1793)

   o  Idle timeout is optional and has no specified maximum (#1765)





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   o  Update connection ID handling; add RETIRE_CONNECTION_ID type
      (#1464, #1468, #1483, #1484, #1486, #1495, #1729, #1742, #1799,
      #1821)

   o  Include a Token in all Initial packets (#1649, #1794)

   o  Prevent handshake deadlock (#1764, #1824)

B.3.  Since draft-ietf-quic-transport-13

   o  Streams open when higher-numbered streams of the same type open
      (#1342, #1549)

   o  Split initial stream flow control limit into 3 transport
      parameters (#1016, #1542)

   o  All flow control transport parameters are optional (#1610)

   o  Removed UNSOLICITED_PATH_RESPONSE error code (#1265, #1539)

   o  Permit stateless reset in response to any packet (#1348, #1553)

   o  Recommended defense against stateless reset spoofing (#1386,
      #1554)

   o  Prevent infinite stateless reset exchanges (#1443, #1627)

   o  Forbid processing of the same packet number twice (#1405, #1624)

   o  Added a packet number decoding example (#1493)

   o  More precisely define idle timeout (#1429, #1614, #1652)

   o  Corrected format of Retry packet and prevented looping (#1492,
      #1451, #1448, #1498)

   o  Permit 0-RTT after receiving Version Negotiation or Retry (#1507,
      #1514, #1621)

   o  Permit Retry in response to 0-RTT (#1547, #1552)

   o  Looser verification of ECN counters to account for ACK loss
      (#1555, #1481, #1565)

   o  Remove frame type field from APPLICATION_CLOSE (#1508, #1528)






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B.4.  Since draft-ietf-quic-transport-12

   o  Changes to integration of the TLS handshake (#829, #1018, #1094,
      #1165, #1190, #1233, #1242, #1252, #1450, #1458)

      *  The cryptographic handshake uses CRYPTO frames, not stream 0

      *  QUIC packet protection is used in place of TLS record
         protection

      *  Separate QUIC packet number spaces are used for the handshake

      *  Changed Retry to be independent of the cryptographic handshake

      *  Added NEW_TOKEN frame and Token fields to Initial packet

      *  Limit the use of HelloRetryRequest to address TLS needs (like
         key shares)

   o  Enable server to transition connections to a preferred address
      (#560, #1251, #1373)

   o  Added ECN feedback mechanisms and handling; new ACK_ECN frame
      (#804, #805, #1372)

   o  Changed rules and recommendations for use of new connection IDs
      (#1258, #1264, #1276, #1280, #1419, #1452, #1453, #1465)

   o  Added a transport parameter to disable intentional connection
      migration (#1271, #1447)

   o  Packets from different connection ID can't be coalesced (#1287,
      #1423)

   o  Fixed sampling method for packet number encryption; the length
      field in long headers includes the packet number field in addition
      to the packet payload (#1387, #1389)

   o  Stateless Reset is now symmetric and subject to size constraints
      (#466, #1346)

   o  Added frame type extension mechanism (#58, #1473)

B.5.  Since draft-ietf-quic-transport-11

   o  Enable server to transition connections to a preferred address
      (#560, #1251)




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   o  Packet numbers are encrypted (#1174, #1043, #1048, #1034, #850,
      #990, #734, #1317, #1267, #1079)

   o  Packet numbers use a variable-length encoding (#989, #1334)

   o  STREAM frames can now be empty (#1350)

B.6.  Since draft-ietf-quic-transport-10

   o  Swap payload length and packed number fields in long header
      (#1294)

   o  Clarified that CONNECTION_CLOSE is allowed in Handshake packet
      (#1274)

   o  Spin bit reserved (#1283)

   o  Coalescing multiple QUIC packets in a UDP datagram (#1262, #1285)

   o  A more complete connection migration (#1249)

   o  Refine opportunistic ACK defense text (#305, #1030, #1185)

   o  A Stateless Reset Token isn't mandatory (#818, #1191)

   o  Removed implicit stream opening (#896, #1193)

   o  An empty STREAM frame can be used to open a stream without sending
      data (#901, #1194)

   o  Define stream counts in transport parameters rather than a maximum
      stream ID (#1023, #1065)

   o  STOP_SENDING is now prohibited before streams are used (#1050)

   o  Recommend including ACK in Retry packets and allow PADDING (#1067,
      #882)

   o  Endpoints now become closing after an idle timeout (#1178, #1179)

   o  Remove implication that Version Negotiation is sent when a packet
      of the wrong version is received (#1197)

B.7.  Since draft-ietf-quic-transport-09

   o  Added PATH_CHALLENGE and PATH_RESPONSE frames to replace PING with
      Data and PONG frame.  Changed ACK frame type from 0x0e to 0x0d.
      (#1091, #725, #1086)



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   o  A server can now only send 3 packets without validating the client
      address (#38, #1090)

   o  Delivery order of stream data is no longer strongly specified
      (#252, #1070)

   o  Rework of packet handling and version negotiation (#1038)

   o  Stream 0 is now exempt from flow control until the handshake
      completes (#1074, #725, #825, #1082)

   o  Improved retransmission rules for all frame types: information is
      retransmitted, not packets or frames (#463, #765, #1095, #1053)

   o  Added an error code for server busy signals (#1137)

   o  Endpoints now set the connection ID that their peer uses.
      Connection IDs are variable length.  Removed the
      omit_connection_id transport parameter and the corresponding short
      header flag. (#1089, #1052, #1146, #821, #745, #821, #1166, #1151)

B.8.  Since draft-ietf-quic-transport-08

   o  Clarified requirements for BLOCKED usage (#65, #924)

   o  BLOCKED frame now includes reason for blocking (#452, #924, #927,
      #928)

   o  GAP limitation in ACK Frame (#613)

   o  Improved PMTUD description (#614, #1036)

   o  Clarified stream state machine (#634, #662, #743, #894)

   o  Reserved versions don't need to be generated deterministically
      (#831, #931)

   o  You don't always need the draining period (#871)

   o  Stateless reset clarified as version-specific (#930, #986)

   o  initial_max_stream_id_x transport parameters are optional (#970,
      #971)

   o  Ack Delay assumes a default value during the handshake (#1007,
      #1009)

   o  Removed transport parameters from NewSessionTicket (#1015)



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B.9.  Since draft-ietf-quic-transport-07

   o  The long header now has version before packet number (#926, #939)

   o  Rename and consolidate packet types (#846, #822, #847)

   o  Packet types are assigned new codepoints and the Connection ID
      Flag is inverted (#426, #956)

   o  Removed type for Version Negotiation and use Version 0 (#963,
      #968)

   o  Streams are split into unidirectional and bidirectional (#643,
      #656, #720, #872, #175, #885)

      *  Stream limits now have separate uni- and bi-directional
         transport parameters (#909, #958)

      *  Stream limit transport parameters are now optional and default
         to 0 (#970, #971)

   o  The stream state machine has been split into read and write (#634,
      #894)

   o  Employ variable-length integer encodings throughout (#595)

   o  Improvements to connection close

      *  Added distinct closing and draining states (#899, #871)

      *  Draining period can terminate early (#869, #870)

      *  Clarifications about stateless reset (#889, #890)

   o  Address validation for connection migration (#161, #732, #878)

   o  Clearly defined retransmission rules for BLOCKED (#452, #65, #924)

   o  negotiated_version is sent in server transport parameters (#710,
      #959)

   o  Increased the range over which packet numbers are randomized
      (#864, #850, #964)








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B.10.  Since draft-ietf-quic-transport-06

   o  Replaced FNV-1a with AES-GCM for all "Cleartext" packets (#554)

   o  Split error code space between application and transport (#485)

   o  Stateless reset token moved to end (#820)

   o  1-RTT-protected long header types removed (#848)

   o  No acknowledgments during draining period (#852)

   o  Remove "application close" as a separate close type (#854)

   o  Remove timestamps from the ACK frame (#841)

   o  Require transport parameters to only appear once (#792)

B.11.  Since draft-ietf-quic-transport-05

   o  Stateless token is server-only (#726)

   o  Refactor section on connection termination (#733, #748, #328,
      #177)

   o  Limit size of Version Negotiation packet (#585)

   o  Clarify when and what to ack (#736)

   o  Renamed STREAM_ID_NEEDED to STREAM_ID_BLOCKED

   o  Clarify Keep-alive requirements (#729)

B.12.  Since draft-ietf-quic-transport-04

   o  Introduce STOP_SENDING frame, RST_STREAM only resets in one
      direction (#165)

   o  Removed GOAWAY; application protocols are responsible for graceful
      shutdown (#696)

   o  Reduced the number of error codes (#96, #177, #184, #211)

   o  Version validation fields can't move or change (#121)

   o  Removed versions from the transport parameters in a
      NewSessionTicket message (#547)




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   o  Clarify the meaning of "bytes in flight" (#550)

   o  Public reset is now stateless reset and not visible to the path
      (#215)

   o  Reordered bits and fields in STREAM frame (#620)

   o  Clarifications to the stream state machine (#572, #571)

   o  Increased the maximum length of the Largest Acknowledged field in
      ACK frames to 64 bits (#629)

   o  truncate_connection_id is renamed to omit_connection_id (#659)

   o  CONNECTION_CLOSE terminates the connection like TCP RST (#330,
      #328)

   o  Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)

B.13.  Since draft-ietf-quic-transport-03

   o  Change STREAM and RST_STREAM layout

   o  Add MAX_STREAM_ID settings

B.14.  Since draft-ietf-quic-transport-02

   o  The size of the initial packet payload has a fixed minimum (#267,
      #472)

   o  Define when Version Negotiation packets are ignored (#284, #294,
      #241, #143, #474)

   o  The 64-bit FNV-1a algorithm is used for integrity protection of
      unprotected packets (#167, #480, #481, #517)

   o  Rework initial packet types to change how the connection ID is
      chosen (#482, #442, #493)

   o  No timestamps are forbidden in unprotected packets (#542, #429)

   o  Cryptographic handshake is now on stream 0 (#456)

   o  Remove congestion control exemption for cryptographic handshake
      (#248, #476)

   o  Version 1 of QUIC uses TLS; a new version is needed to use a
      different handshake protocol (#516)



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   o  STREAM frames have a reduced number of offset lengths (#543, #430)

   o  Split some frames into separate connection- and stream- level
      frames (#443)

      *  WINDOW_UPDATE split into MAX_DATA and MAX_STREAM_DATA (#450)

      *  BLOCKED split to match WINDOW_UPDATE split (#454)

      *  Define STREAM_ID_NEEDED frame (#455)

   o  A NEW_CONNECTION_ID frame supports connection migration without
      linkability (#232, #491, #496)

   o  Transport parameters for 0-RTT are retained from a previous
      connection (#405, #513, #512)

      *  A client in 0-RTT no longer required to reset excess streams
         (#425, #479)

   o  Expanded security considerations (#440, #444, #445, #448)

B.15.  Since draft-ietf-quic-transport-01

   o  Defined short and long packet headers (#40, #148, #361)

   o  Defined a versioning scheme and stable fields (#51, #361)

   o  Define reserved version values for "greasing" negotiation (#112,
      #278)

   o  The initial packet number is randomized (#35, #283)

   o  Narrow the packet number encoding range requirement (#67, #286,
      #299, #323, #356)

   o  Defined client address validation (#52, #118, #120, #275)

   o  Define transport parameters as a TLS extension (#49, #122)

   o  SCUP and COPT parameters are no longer valid (#116, #117)

   o  Transport parameters for 0-RTT are either remembered from before,
      or assume default values (#126)

   o  The server chooses connection IDs in its final flight (#119, #349,
      #361)




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   o  The server echoes the Connection ID and packet number fields when
      sending a Version Negotiation packet (#133, #295, #244)

   o  Defined a minimum packet size for the initial handshake packet
      from the client (#69, #136, #139, #164)

   o  Path MTU Discovery (#64, #106)

   o  The initial handshake packet from the client needs to fit in a
      single packet (#338)

   o  Forbid acknowledgment of packets containing only ACK and PADDING
      (#291)

   o  Require that frames are processed when packets are acknowledged
      (#381, #341)

   o  Removed the STOP_WAITING frame (#66)

   o  Don't require retransmission of old timestamps for lost ACK frames
      (#308)

   o  Clarified that frames are not retransmitted, but the information
      in them can be (#157, #298)

   o  Error handling definitions (#335)

   o  Split error codes into four sections (#74)

   o  Forbid the use of Public Reset where CONNECTION_CLOSE is possible
      (#289)

   o  Define packet protection rules (#336)

   o  Require that stream be entirely delivered or reset, including
      acknowledgment of all STREAM frames or the RST_STREAM, before it
      closes (#381)

   o  Remove stream reservation from state machine (#174, #280)

   o  Only stream 1 does not contribute to connection-level flow control
      (#204)

   o  Stream 1 counts towards the maximum concurrent stream limit (#201,
      #282)

   o  Remove connection-level flow control exclusion for some streams
      (except 1) (#246)



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   o  RST_STREAM affects connection-level flow control (#162, #163)

   o  Flow control accounting uses the maximum data offset on each
      stream, rather than bytes received (#378)

   o  Moved length-determining fields to the start of STREAM and ACK
      (#168, #277)

   o  Added the ability to pad between frames (#158, #276)

   o  Remove error code and reason phrase from GOAWAY (#352, #355)

   o  GOAWAY includes a final stream number for both directions (#347)

   o  Error codes for RST_STREAM and CONNECTION_CLOSE are now at a
      consistent offset (#249)

   o  Defined priority as the responsibility of the application protocol
      (#104, #303)

B.16.  Since draft-ietf-quic-transport-00

   o  Replaced DIVERSIFICATION_NONCE flag with KEY_PHASE flag

   o  Defined versioning

   o  Reworked description of packet and frame layout

   o  Error code space is divided into regions for each component

   o  Use big endian for all numeric values

B.17.  Since draft-hamilton-quic-transport-protocol-01

   o  Adopted as base for draft-ietf-quic-tls

   o  Updated authors/editors list

   o  Added IANA Considerations section

   o  Moved Contributors and Acknowledgments to appendices

Acknowledgments

   Special thanks are due to the following for helping shape pre-IETF
   QUIC and its deployment: Chris Bentzel, Misha Efimov, Roberto Peon,
   Alistair Riddoch, Siddharth Vijayakrishnan, and Assar Westerlund.




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   This document has benefited immensely from various private
   discussions and public ones on the quic@ietf.org and proto-
   quic@chromium.org mailing lists.  Our thanks to all.

Contributors

   The original authors of this specification were Ryan Hamilton, Jana
   Iyengar, Ian Swett, and Alyssa Wilk.

   The original design and rationale behind this protocol draw
   significantly from work by Jim Roskind [EARLY-DESIGN].  In
   alphabetical order, the contributors to the pre-IETF QUIC project at
   Google are: Britt Cyr, Jeremy Dorfman, Ryan Hamilton, Jana Iyengar,
   Fedor Kouranov, Charles Krasic, Jo Kulik, Adam Langley, Jim Roskind,
   Robbie Shade, Satyam Shekhar, Cherie Shi, Ian Swett, Raman Tenneti,
   Victor Vasiliev, Antonio Vicente, Patrik Westin, Alyssa Wilk, Dale
   Worley, Fan Yang, Dan Zhang, Daniel Ziegler.

Authors' Addresses

   Jana Iyengar (editor)
   Fastly

   Email: jri.ietf@gmail.com


   Martin Thomson (editor)
   Mozilla

   Email: martin.thomson@gmail.com





















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