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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

QUIC                                                     J. Iyengar, Ed.
Internet-Draft                                                    Fastly
Intended status: Standards Track                         M. Thomson, Ed.
Expires: November 23, 2018                                       Mozilla
                                                            May 22, 2018


           QUIC: A UDP-Based Multiplexed and Secure Transport
                      draft-ietf-quic-transport-12

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 November 23, 2018.








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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
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   6
     2.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   6
   3.  Versions  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Packet Types and Formats  . . . . . . . . . . . . . . . . . .   8
     4.1.  Long Header . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Short Header  . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Version Negotiation Packet  . . . . . . . . . . . . . . .  12
     4.4.  Cryptographic Handshake Packets . . . . . . . . . . . . .  13
       4.4.1.  Initial Packet  . . . . . . . . . . . . . . . . . . .  13
       4.4.2.  Retry Packet  . . . . . . . . . . . . . . . . . . . .  14
       4.4.3.  Handshake Packet  . . . . . . . . . . . . . . . . . .  15
     4.5.  Protected Packets . . . . . . . . . . . . . . . . . . . .  16
     4.6.  Coalescing Packets  . . . . . . . . . . . . . . . . . . .  17
     4.7.  Connection ID . . . . . . . . . . . . . . . . . . . . . .  17
     4.8.  Packet Numbers  . . . . . . . . . . . . . . . . . . . . .  18
   5.  Frames and Frame Types  . . . . . . . . . . . . . . . . . . .  20
   6.  Life of a Connection  . . . . . . . . . . . . . . . . . . . .  22
     6.1.  Matching Packets to Connections . . . . . . . . . . . . .  23
       6.1.1.  Client Packet Handling  . . . . . . . . . . . . . . .  23
       6.1.2.  Server Packet Handling  . . . . . . . . . . . . . . .  23
     6.2.  Version Negotiation . . . . . . . . . . . . . . . . . . .  24
       6.2.1.  Sending Version Negotiation Packets . . . . . . . . .  25
       6.2.2.  Handling Version Negotiation Packets  . . . . . . . .  25
       6.2.3.  Using Reserved Versions . . . . . . . . . . . . . . .  26
     6.3.  Cryptographic and Transport Handshake . . . . . . . . . .  26
     6.4.  Transport Parameters  . . . . . . . . . . . . . . . . . .  27
       6.4.1.  Transport Parameter Definitions . . . . . . . . . . .  29
       6.4.2.  Values of Transport Parameters for 0-RTT  . . . . . .  31
       6.4.3.  New Transport Parameters  . . . . . . . . . . . . . .  31
       6.4.4.  Version Negotiation Validation  . . . . . . . . . . .  32



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     6.5.  Stateless Retries . . . . . . . . . . . . . . . . . . . .  33
     6.6.  Proof of Source Address Ownership . . . . . . . . . . . .  34
       6.6.1.  Client Address Validation Procedure . . . . . . . . .  34
       6.6.2.  Address Validation on Session Resumption  . . . . . .  35
       6.6.3.  Address Validation Token Integrity  . . . . . . . . .  36
     6.7.  Path Validation . . . . . . . . . . . . . . . . . . . . .  36
       6.7.1.  Initiation  . . . . . . . . . . . . . . . . . . . . .  37
       6.7.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  37
       6.7.3.  Completion  . . . . . . . . . . . . . . . . . . . . .  38
       6.7.4.  Abandonment . . . . . . . . . . . . . . . . . . . . .  38
     6.8.  Connection Migration  . . . . . . . . . . . . . . . . . .  39
       6.8.1.  Probing a New Path  . . . . . . . . . . . . . . . . .  39
       6.8.2.  Initiating Connection Migration . . . . . . . . . . .  39
       6.8.3.  Responding to Connection Migration  . . . . . . . . .  40
       6.8.4.  Loss Detection and Congestion Control . . . . . . . .  42
       6.8.5.  Privacy Implications of Connection Migration  . . . .  42
     6.9.  Server's Preferred Address  . . . . . . . . . . . . . . .  44
       6.9.1.  Communicating A Preferred Address . . . . . . . . . .  44
       6.9.2.  Responding to Connection Migration  . . . . . . . . .  44
       6.9.3.  Interaction of Client Migration and Preferred Address  45
     6.10. Connection Termination  . . . . . . . . . . . . . . . . .  45
       6.10.1.  Closing and Draining Connection States . . . . . . .  45
       6.10.2.  Idle Timeout . . . . . . . . . . . . . . . . . . . .  47
       6.10.3.  Immediate Close  . . . . . . . . . . . . . . . . . .  47
       6.10.4.  Stateless Reset  . . . . . . . . . . . . . . . . . .  48
   7.  Frame Types and Formats . . . . . . . . . . . . . . . . . . .  51
     7.1.  Variable-Length Integer Encoding  . . . . . . . . . . . .  51
     7.2.  PADDING Frame . . . . . . . . . . . . . . . . . . . . . .  52
     7.3.  RST_STREAM Frame  . . . . . . . . . . . . . . . . . . . .  52
     7.4.  CONNECTION_CLOSE frame  . . . . . . . . . . . . . . . . .  53
     7.5.  APPLICATION_CLOSE frame . . . . . . . . . . . . . . . . .  53
     7.6.  MAX_DATA Frame  . . . . . . . . . . . . . . . . . . . . .  54
     7.7.  MAX_STREAM_DATA Frame . . . . . . . . . . . . . . . . . .  54
     7.8.  MAX_STREAM_ID Frame . . . . . . . . . . . . . . . . . . .  55
     7.9.  PING Frame  . . . . . . . . . . . . . . . . . . . . . . .  56
     7.10. BLOCKED Frame . . . . . . . . . . . . . . . . . . . . . .  57
     7.11. STREAM_BLOCKED Frame  . . . . . . . . . . . . . . . . . .  57
     7.12. STREAM_ID_BLOCKED Frame . . . . . . . . . . . . . . . . .  58
     7.13. NEW_CONNECTION_ID Frame . . . . . . . . . . . . . . . . .  58
     7.14. STOP_SENDING Frame  . . . . . . . . . . . . . . . . . . .  59
     7.15. ACK Frame . . . . . . . . . . . . . . . . . . . . . . . .  60
       7.15.1.  ACK Block Section  . . . . . . . . . . . . . . . . .  61
       7.15.2.  Sending ACK Frames . . . . . . . . . . . . . . . . .  63
       7.15.3.  ACK Frames and Packet Protection . . . . . . . . . .  64
     7.16. PATH_CHALLENGE Frame  . . . . . . . . . . . . . . . . . .  65
     7.17. PATH_RESPONSE Frame . . . . . . . . . . . . . . . . . . .  65
     7.18. STREAM Frames . . . . . . . . . . . . . . . . . . . . . .  65
   8.  Packetization and Reliability . . . . . . . . . . . . . . . .  67



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     8.1.  Packet Processing and Acknowledgment  . . . . . . . . . .  67
     8.2.  Retransmission of Information . . . . . . . . . . . . . .  68
     8.3.  Packet Size . . . . . . . . . . . . . . . . . . . . . . .  70
     8.4.  Path Maximum Transmission Unit  . . . . . . . . . . . . .  70
       8.4.1.  Special Considerations for PMTU Discovery . . . . . .  71
       8.4.2.  Special Considerations for Packetization Layer PMTU
               Discovery . . . . . . . . . . . . . . . . . . . . . .  71
   9.  Streams: QUIC's Data Structuring Abstraction  . . . . . . . .  72
     9.1.  Stream Identifiers  . . . . . . . . . . . . . . . . . . .  73
     9.2.  Stream States . . . . . . . . . . . . . . . . . . . . . .  74
       9.2.1.  Send Stream States  . . . . . . . . . . . . . . . . .  74
       9.2.2.  Receive Stream States . . . . . . . . . . . . . . . .  76
       9.2.3.  Permitted Frame Types . . . . . . . . . . . . . . . .  79
       9.2.4.  Bidirectional Stream States . . . . . . . . . . . . .  79
     9.3.  Solicited State Transitions . . . . . . . . . . . . . . .  80
     9.4.  Stream Concurrency  . . . . . . . . . . . . . . . . . . .  81
     9.5.  Sending and Receiving Data  . . . . . . . . . . . . . . .  82
     9.6.  Stream Prioritization . . . . . . . . . . . . . . . . . .  82
   10. Flow Control  . . . . . . . . . . . . . . . . . . . . . . . .  83
     10.1.  Edge Cases and Other Considerations  . . . . . . . . . .  85
       10.1.1.  Response to a RST_STREAM . . . . . . . . . . . . . .  85
       10.1.2.  Data Limit Increments  . . . . . . . . . . . . . . .  85
       10.1.3.  Handshake Exemption  . . . . . . . . . . . . . . . .  86
     10.2.  Stream Limit Increment . . . . . . . . . . . . . . . . .  86
       10.2.1.  Blocking on Flow Control . . . . . . . . . . . . . .  86
     10.3.  Stream Final Offset  . . . . . . . . . . . . . . . . . .  87
   11. Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  87
     11.1.  Connection Errors  . . . . . . . . . . . . . . . . . . .  88
     11.2.  Stream Errors  . . . . . . . . . . . . . . . . . . . . .  89
     11.3.  Transport Error Codes  . . . . . . . . . . . . . . . . .  89
     11.4.  Application Protocol Error Codes . . . . . . . . . . . .  90
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  91
     12.1.  Spoofed ACK Attack . . . . . . . . . . . . . . . . . . .  91
     12.2.  Optimistic ACK Attack  . . . . . . . . . . . . . . . . .  91
     12.3.  Slowloris Attacks  . . . . . . . . . . . . . . . . . . .  92
     12.4.  Stream Fragmentation and Reassembly Attacks  . . . . . .  92
     12.5.  Stream Commitment Attack . . . . . . . . . . . . . . . .  92
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  93
     13.1.  QUIC Transport Parameter Registry  . . . . . . . . . . .  93
     13.2.  QUIC Transport Error Codes Registry  . . . . . . . . . .  94
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  96
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  96
     14.2.  Informative References . . . . . . . . . . . . . . . . .  97
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  98
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . .  98
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  99
     C.1.  Since draft-ietf-quic-transport-11  . . . . . . . . . . .  99
     C.2.  Since draft-ietf-quic-transport-10  . . . . . . . . . . .  99



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     C.3.  Since draft-ietf-quic-transport-09  . . . . . . . . . . . 100
     C.4.  Since draft-ietf-quic-transport-08  . . . . . . . . . . . 100
     C.5.  Since draft-ietf-quic-transport-07  . . . . . . . . . . . 101
     C.6.  Since draft-ietf-quic-transport-06  . . . . . . . . . . . 102
     C.7.  Since draft-ietf-quic-transport-05  . . . . . . . . . . . 102
     C.8.  Since draft-ietf-quic-transport-04  . . . . . . . . . . . 102
     C.9.  Since draft-ietf-quic-transport-03  . . . . . . . . . . . 103
     C.10. Since draft-ietf-quic-transport-02  . . . . . . . . . . . 103
     C.11. Since draft-ietf-quic-transport-01  . . . . . . . . . . . 104
     C.12. Since draft-ietf-quic-transport-00  . . . . . . . . . . . 106
     C.13. Since draft-hamilton-quic-transport-protocol-01 . . . . . 106
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 107

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
   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 implements techniques learned from experience with TCP, SCTP and
   other transport protocols.  QUIC uses UDP as substrate so as to not
   require changes to legacy client operating systems and middleboxes to
   be deployable.  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.  This document describes the core QUIC protocol,
   including the conceptual design, wire format, and mechanisms of the
   QUIC protocol for connection establishment, stream multiplexing,
   stream and connection-level flow control, connection migration, and
   data reliability.

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




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

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.

   Stream:  A logical, bi-directional 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:  A well-formed UDP payload that can be parsed by a QUIC
      receiver.

   QUIC is a name, not an acronym.

2.1.  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 7.1




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

3.  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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4.  Packet Types and Formats

   We first describe QUIC's packet types and their formats, since some
   are referenced in subsequent mechanisms.

   All numeric values are encoded in network byte order (that is, big-
   endian) and all field sizes are in bits.  When discussing individual
   bits of fields, the least significant bit is referred to as bit 0.
   Hexadecimal notation is used for describing the value of fields.

   Any QUIC packet has either a long or a short header, as indicated by
   the Header Form bit.  Long headers are expected to be used early in
   the connection before version negotiation and establishment of 1-RTT
   keys.  Short headers are minimal version-specific headers, which are
   used after version negotiation and 1-RTT keys are established.

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

                       Figure 1: Long Header 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 4.2).  The long form allows for
   special packets - such as the Version Negotiation packet - to be
   represented in this uniform fixed-length packet format.  A long
   header contains 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 1.

   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:  Octet 1 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 4.7 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 4.7 describes the use of this
      field in more detail.

   Payload Length:  The length of the Payload field in octets, encoded
      as a variable-length integer (Section 7.1).

   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 5.6 of [QUIC-TLS].  The
      length of the packet number field is encoded in the plaintext
      packet number.  See Section 4.8 for details.

   Payload:  The payload of the packet.

   The following packet types are defined:






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                +------+-----------------+---------------+
                | Type | Name            | Section       |
                +------+-----------------+---------------+
                | 0x7F | Initial         | Section 4.4.1 |
                |      |                 |               |
                | 0x7E | Retry           | Section 4.4.2 |
                |      |                 |               |
                | 0x7D | Handshake       | Section 4.4.3 |
                |      |                 |               |
                | 0x7C | 0-RTT Protected | Section 4.5   |
                +------+-----------------+---------------+

                     Table 1: 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 1 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 Payload field (which is also the end of the long
   header packet) is determined by the value of the Payload Length
   field.  Senders can sometimes coalesce multiple packets into one UDP
   datagram.  See Section 4.6 for more details.

4.2.  Short 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
   +-+-+-+-+-+-+-+-+
   |0|K|1|1|0|R R R|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Destination Connection ID (0..144)           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Packet Number (8/16/32)                ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Protected Payload (*)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Short Header Format

   The short header can be used after the version and 1-RTT keys are
   negotiated.  This header form has the following fields:



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   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 Demultipexing 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.

   Destination Connection ID:  The Destination Connection ID is a
      connection ID that is chosen by the intended recipient of the
      packet.  See Section 4.7 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 5.6 of [QUIC-TLS].  The
      length of the packet number field is encoded in the plaintext
      packet number.  See Section 4.8 for details.

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

   The packet type in a short header currently determines only the size
   of the packet number field.  Additional types can be used to signal
   the presence of other fields.



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   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.

4.3.  Version Negotiation Packet

   A Version Negotiation packet is inherently not version-specific, and
   does not use the long packet header (see Section 4.1.  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 3: Version Negotiation Packet

   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.




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   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 6.2 for a description of the version negotiation process.

4.4.  Cryptographic Handshake Packets

   Once version negotiation is complete, the cryptographic handshake is
   used to agree on cryptographic keys.  The cryptographic handshake is
   carried in Initial (Section 4.4.1), Retry (Section 4.4.2) and
   Handshake (Section 4.4.3) packets.

   All these packets use the long header and contain the current QUIC
   version in the version field.

   In order to prevent tampering by version-unaware middleboxes,
   handshake packets are protected with a connection- and version-
   specific key, 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.

4.4.1.  Initial Packet

   The Initial packet uses long headers with a type value of 0x7F.  It
   carries the first cryptographic handshake message sent by the client.

   If the client has not previously received a Retry packet from the
   server, it populates the Destination Connection ID field with a
   randomly selected value.  This MUST be at least 8 octets in length.
   Until a packet is received from the server, the client MUST use the



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   same random value unless it also changes the Source Connection ID
   (which effectively starts a new connection attempt).  The randomized
   Destination Connection ID is used to determine packet protection
   keys.

   If the client received a Retry packet and is sending a second Initial
   packet, then it sets the Destination Connection ID 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.

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

   The first Initial packet that is sent by a client contains a packet
   number of 0.  All subsequent packets contain a packet number that is
   incremented by at least one, see (Section 4.8).

   The payload of an Initial packet conveys a STREAM frame (or frames)
   for stream 0 containing a cryptographic handshake message.  The
   stream in this packet always starts at an offset of 0 (see
   Section 6.5) and the complete cryptographic handshake message MUST
   fit in a single packet (see Section 6.3).

   The payload of a UDP datagram carrying the Initial packet MUST be
   expanded to at least 1200 octets (see Section 8), by adding PADDING
   frames to the Initial packet and/or by combining the Initial packet
   with a 0-RTT packet (see Section 4.6).

   The client uses 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, this includes the packets sent after receiving a
   Version Negotiation (Section 4.3) or Retry packet (Section 4.4.2).

4.4.2.  Retry Packet

   A Retry packet uses long headers with a type value of 0x7E.  It
   carries cryptographic handshake messages and acknowledgments.  It is
   used by a server that wishes to perform a stateless retry (see
   Section 6.5).

   The server populates the Destination Connection ID with the
   connection ID that the client included in the Source Connection ID of
   the Initial packet.  This might be a zero-length value.






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   The server includes a connection ID of its choice in the Source
   Connection ID field.  The client MUST use this connection ID in the
   Destination Connection ID of subsequent packets that it sends.

   The Packet Number field of a Retry packet MUST be set to 0.  This
   value is subsequently protected as normal.  [[Editor's Note: This
   isn't ideal, because it creates a "cheat" where the client assumes a
   value.  That's a problem, so I'm tempted to suggest that this include
   any value less than 2^30 so that normal processing works - and can be
   properly exercised.]]

   A Retry packet is never explicitly acknowledged in an ACK frame by a
   client.  Receiving another Initial packet implicitly acknowledges a
   Retry packet.

   After receiving a Retry packet, the client uses a new Initial packet
   containing the next cryptographic handshake message.  The client
   retains the state of its cryptographic handshake, but discards all
   transport state.  The Initial packet that is generated in response to
   a Retry packet includes STREAM frames on stream 0 that start again at
   an offset of 0.

   Continuing the cryptographic handshake is necessary to ensure that an
   attacker cannot force a downgrade of any cryptographic parameters.
   In addition to continuing the cryptographic handshake, the client
   MUST remember the results of any version negotiation that occurred
   (see Section 6.2).  The client MAY also retain any observed RTT or
   congestion state that it has accumulated for the flow, but other
   transport state MUST be discarded.

   The payload of the Retry packet contains at least two frames.  It
   MUST include a STREAM frame on stream 0 with offset 0 containing the
   server's cryptographic stateless retry material.  It MUST also
   include an ACK frame to acknowledge the client's Initial packet.  It
   MAY additionally include PADDING frames.  The next STREAM frame sent
   by the server will also start at stream offset 0.

4.4.3.  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.

   A server sends its cryptographic handshake in one or more Handshake
   packets in response to an Initial packet if it does not send a Retry
   packet.  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.



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   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 4.7).

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

   Servers MUST NOT send more than three Handshake packets without
   receiving a packet from a verified source address.  Source addresses
   can be verified through an address validation token, receipt of the
   final cryptographic message from the client, or by receiving a valid
   PATH_RESPONSE frame from the client.

   If the server expects to generate more than three Handshake packets
   in response to an Initial packet, it SHOULD include a PATH_CHALLENGE
   frame in each Handshake packet that it sends.  After receiving at
   least one valid PATH_RESPONSE frame, the server can send its
   remaining Handshake packets.  Servers can instead perform address
   validation using a Retry packet; this requires less state on the
   server, but could involve additional computational effort depending
   on implementation choices.

   The payload of this packet contains STREAM frames and could contain
   PADDING, ACK, PATH_CHALLENGE, or PATH_RESPONSE frames.  Handshake
   packets MAY contain CONNECTION_CLOSE frames if the handshake is
   unsuccessful.

4.5.  Protected Packets

   All QUIC packets are protected.  Packets that are protected with the
   static handshake keys or the 0-RTT keys are sent with long headers;
   all packets protected with 1-RTT keys are sent with short headers.
   The different packet types explicitly indicate the encryption level
   and therefore the keys that are used to remove packet protection.

   Packets protected with 0-RTT keys use a type value of 0x7C.  The
   connection ID fields for a 0-RTT packet MUST match the values used in
   the Initial packet (Section 4.4.1).

   The client can send 0-RTT packets after receiving a Handshake packet
   (Section 4.4.3), if that packet does not complete the handshake.
   Even if the client receives a different connection ID in the
   Handshake packet, it MUST continue to use the same Destination
   Connection ID for 0-RTT packets, see Section 4.7.

   The version field for protected packets is the current QUIC version.



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   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 4.8 for
   details.

   The payload is protected using authenticated encryption.  [QUIC-TLS]
   describes packet protection in detail.  After decryption, the
   plaintext consists of a sequence of frames, as described in
   Section 5.

4.6.  Coalescing Packets

   A sender can coalesce multiple QUIC packets (typically a
   Cryptographic Handshake packet and a Protected packet) into one UDP
   datagram.  This can reduce the number of UDP datagrams needed to send
   application data during the handshake and immediately afterwards.  A
   packet with a short header does not include a length, so it has to be
   the last packet included in a UDP datagram.

   The sender MUST NOT coalesce QUIC packets belonging to different QUIC
   connections into a single UDP 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.

4.7.  Connection ID

   A connection ID is used to ensure consistent routing of packets.  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 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.

   During the handshake, an endpoint might receive multiple packets with
   the long header, and thus be given multiple opportunities to update



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   the Destination Connection ID it sends.  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
   Handshake); 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.

   Short headers 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 connection-ID based load balancer could agree with
   the load balancer on a fixed or minimum length and on an encoding for
   connection IDs.  This 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.

   The very first packet sent by a client includes a random value for
   Destination Connection ID.  The same value MUST be used for all 0-RTT
   packets sent on that connection (Section 4.5).  This randomized value
   is used to determine the handshake packet protection keys (see
   Section 5.3.2 of [QUIC-TLS]).

   A Version Negotiation (Section 4.3) packet MUST use both connection
   IDs selected by the client, swapped to ensure correct routing toward
   the client.

   The connection ID can change over the lifetime of a connection,
   especially in response to connection migration (Section 6.8).
   NEW_CONNECTION_ID frames (Section 7.13) are used to provide new
   connection ID values.

4.8.  Packet Numbers

   The packet number is an integer in the range 0 to 2^62-1.  The value
   is used in determining the cryptographic nonce for packet encryption.
   Each endpoint maintains a separate packet number for sending and
   receiving.  The packet number for sending MUST start at zero for the
   first packet sent and MUST increase by at least one after sending a
   packet.

   A QUIC endpoint MUST NOT reuse a packet number within the same
   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 any further



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   packets; a server MAY send a Stateless Reset (Section 6.10.4) in
   response to further packets that it receives.

   In the QUIC long and short packet headers, the number of bits
   required to represent the packet number are 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 determine
   how many bits of the packet number are provided, as shown in Table 2.

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

            Table 2: Packet Number Encodings for Packet Headers

   Note that these encodings are similar to those in Section 7.1, but
   use different values.

   The encoded packet number is protected as described in Section 5.6
   [QUIC-TLS].  Protection of the packet number is removed prior to
   recovering the full packet number.  The full packet number is
   reconstructed at the receiver based on the number of significant bits
   present, the content 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 0x1f94 will be decoded as
   0xaa831f94.

   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




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   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.

   A Version Negotiation packet (Section 4.3) does not include a packet
   number.  The Retry packet (Section 4.4.2) has special rules for
   populating the packet number field.

5.  Frames and Frame Types

   The payload of all packets, after removing packet protection,
   consists of a sequence of frames, as shown in Figure 4.  Version
   Negotiation and Stateless Reset 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 4: Contents of Protected Payload

   Protected 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 byte,
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type (8)    |           Type-Dependent Fields (*)         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 5: Generic Frame Layout

   Frame types are listed in Table 3.  Note that the Frame Type byte in
   STREAM frames is used to carry other frame-specific flags.  For all
   other frames, the Frame Type byte simply identifies the frame.  These
   frames are explained in more detail as they are referenced later in
   the document.






































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            +-------------+-------------------+--------------+
            | Type Value  | Frame Type Name   | Definition   |
            +-------------+-------------------+--------------+
            | 0x00        | PADDING           | Section 7.2  |
            |             |                   |              |
            | 0x01        | RST_STREAM        | Section 7.3  |
            |             |                   |              |
            | 0x02        | CONNECTION_CLOSE  | Section 7.4  |
            |             |                   |              |
            | 0x03        | APPLICATION_CLOSE | Section 7.5  |
            |             |                   |              |
            | 0x04        | MAX_DATA          | Section 7.6  |
            |             |                   |              |
            | 0x05        | MAX_STREAM_DATA   | Section 7.7  |
            |             |                   |              |
            | 0x06        | MAX_STREAM_ID     | Section 7.8  |
            |             |                   |              |
            | 0x07        | PING              | Section 7.9  |
            |             |                   |              |
            | 0x08        | BLOCKED           | Section 7.10 |
            |             |                   |              |
            | 0x09        | STREAM_BLOCKED    | Section 7.11 |
            |             |                   |              |
            | 0x0a        | STREAM_ID_BLOCKED | Section 7.12 |
            |             |                   |              |
            | 0x0b        | NEW_CONNECTION_ID | Section 7.13 |
            |             |                   |              |
            | 0x0c        | STOP_SENDING      | Section 7.14 |
            |             |                   |              |
            | 0x0d        | ACK               | Section 7.15 |
            |             |                   |              |
            | 0x0e        | PATH_CHALLENGE    | Section 7.16 |
            |             |                   |              |
            | 0x0f        | PATH_RESPONSE     | Section 7.17 |
            |             |                   |              |
            | 0x10 - 0x17 | STREAM            | Section 7.18 |
            +-------------+-------------------+--------------+

                           Table 3: Frame Types

6.  Life of a Connection

   A QUIC connection is a single conversation between two QUIC
   endpoints.  QUIC's connection establishment intertwines version
   negotiation with the cryptographic and transport handshakes to reduce
   connection establishment latency, as described in Section 6.3.  Once
   established, a connection may migrate to a different IP or port at
   either endpoint, due to NAT rebinding or mobility, as described in



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   Section 6.8.  Finally a connection may be terminated by either
   endpoint, as described in Section 6.10.

6.1.  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 a
   NEW_CONNECTION_ID frame (Section 7.13) would associate more than one
   connection ID with a connection.

   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.

6.1.1.  Client Packet Handling

   Valid packets sent to clients always include a Destination Connection
   ID that matches the 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 MAY be 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.

6.1.2.  Server Packet Handling

   If a server receives a packet that has an unsupported version and
   sufficient length to be an Initial packet for some version supported
   by the server, it SHOULD send a Version Negotiation packet as
   described in Section 6.2.1.  Servers MAY rate control these packets
   to avoid storms of Version Negotiation packets.

   The first packet for an unsupported version can use different
   semantics and encodings for any version-specific field.  In



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   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 content 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 as described in Section 6.1.  If not matched, the server
   continues below.

   If the packet is an Initial packet fully conforming with the
   specification, the server proceeds with the handshake (Section 6.3).
   This commits the server to the version that the client selected.

   If a server isn't currently accepting any new connections, it SHOULD
   send a Handshake 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.
   They SHOULD send a Stateless Reset (Section 6.10.4) if a connection
   ID is present in the header.

6.2.  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 6.1 for details.

   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 their Initial packets to reflect
   the largest minimum Initial packet size of all their versions.  This
   ensures that that the server responds if there are any mutually
   supported versions.







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6.2.1.  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 4.3).  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.

6.2.2.  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
   connection using that version.  Though the contents 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 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.

   Version negotiation packets have no cryptographic protection.  The
   result of the negotiation MUST be revalidated as part of the
   cryptographic handshake (see Section 6.4.4).




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6.2.3.  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 3) 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 6.4.4) 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.

6.3.  Cryptographic and Transport Handshake

   QUIC relies on a combined cryptographic and transport handshake to
   minimize connection establishment latency.  QUIC allocates stream 0
   for 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 this stream with reliable, ordered delivery of data.
   In return, the cryptographic handshake provides QUIC with:

   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 6.4)

   o  authenticated confirmation of version negotiation (see
      Section 6.4.4)




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

   o  for the server, the ability to carry data that provides assurance
      that the client can receive packets that are addressed with the
      transport address that is claimed by the client (see Section 6.6)

   The initial cryptographic handshake message MUST be sent in a single
   packet.  Any second attempt that is triggered by address validation
   MUST also be sent within a single packet.  This avoids having to
   reassemble a message from multiple packets.  Reassembling messages
   requires that a server maintain state prior to establishing a
   connection, exposing the server to a denial of service risk.

   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.

   Details of how TLS is integrated with QUIC is provided in more detail
   in [QUIC-TLS].

6.4.  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 format of the transport parameters is the TransportParameters
   struct from Figure 6.  This is described using the presentation
   language from Section 3 of [I-D.ietf-tls-tls13].


















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      uint32 QuicVersion;

      enum {
         initial_max_stream_data(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),
         (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<22..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 6: Definition of TransportParameters

   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 encode the transport parameters.

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



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   completes, the transport parameters declared by the peer are
   available.  Each endpoint validates the value provided by its peer.
   In particular, version negotiation MUST be validated (see
   Section 6.4.4) before the connection establishment is considered
   properly complete.

   Definitions for each of the defined transport parameters are included
   in Section 6.4.1.  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.

6.4.1.  Transport Parameter Definitions

   An endpoint MUST include the following parameters in its encoded
   TransportParameters:

   initial_max_stream_data (0x0000):  The initial stream maximum data
      parameter contains the initial value for the maximum data that can
      be sent on any newly created stream.  This parameter is encoded as
      an unsigned 32-bit integer in units of octets.  This is equivalent
      to an implicit MAX_STREAM_DATA frame (Section 7.7) being sent on
      all streams immediately after opening.

   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 7.6) for the connection immediately
      after completing the handshake.

   idle_timeout (0x0003):  The idle timeout is a value in seconds that
      is encoded as an unsigned 16-bit integer.  The maximum value is
      600 seconds (10 minutes).

   An endpoint MAY use the following transport parameters:

   initial_max_bidi_streams (0x0002):  The initial maximum bidirectional
      streams parameter contains the initial maximum number of
      application-owned bidirectional streams the peer may initiate,
      encoded as an unsigned 16-bit integer.  If this parameter is
      absent or zero, application-owned bidirectional streams cannot be
      created until a MAX_STREAM_ID frame is sent.  Note that a value of
      0 does not prevent the cryptographic handshake stream (that is,
      stream 0) from being used.  Setting this parameter is equivalent
      to sending a MAX_STREAM_ID (Section 7.8) immediately after
      completing the handshake containing the corresponding Stream ID.
      For example, a value of 0x05 would be equivalent to receiving a



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      MAX_STREAM_ID containing 20 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
      application-owned 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 7.8) 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.

   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 4.5).

   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 7.15.  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,
      Handshake, and Retry packets.  Values above 20 are invalid.

   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 6.10.4.  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 6.9.

   A client MUST NOT include a stateless reset token or a preferred
   address.  A server MUST treat receipt of either transport parameter
   as a connection error of type TRANSPORT_PARAMETER_ERROR.








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6.4.2.  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 or initial_max_stream_data that are
   smaller than the remembered value of those parameters.  Similarly, a
   server MUST NOT reduce the value of initial_max_bidi_streams or
   initial_max_uni_streams.

   Omitting or setting a zero value for certain transport parameters can
   result in 0-RTT data being enabled, but not usable.  The following
   transport parameters SHOULD be set to non-zero values for 0-RTT:
   initial_max_bidi_streams, initial_max_uni_streams, initial_max_data,
   initial_max_stream_data.

   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.

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

6.4.3.  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 13.1.




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6.4.4.  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 6.2).

   The cryptographic handshake provides integrity protection for the
   negotiated version as part of the transport parameters (see
   Section 6.4).  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 4.3, 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
   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.

   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.



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   The server includes a list of versions that it would send in any
   version negotiation packet (Section 4.3) 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.

6.5.  Stateless Retries

   A server can process an initial cryptographic handshake messages from
   a client without committing any state.  This allows a server to
   perform address validation (Section 6.6), or to defer connection
   establishment costs.

   A server that generates a response to an initial packet without
   retaining connection state MUST use the Retry packet (Section 4.4.2).
   This packet causes a client to reset its transport state and to
   continue the connection attempt with new connection state while
   maintaining the state of the cryptographic handshake.

   A server MUST NOT send multiple Retry packets in response to a client
   handshake packet.  Thus, any cryptographic handshake message that is
   sent MUST fit within a single packet.

   In TLS, the Retry packet type is used to carry the HelloRetryRequest
   message.








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6.6.  Proof of Source Address Ownership

   Transport protocols commonly spend a round trip checking that a
   client owns the transport address (IP and port) that it claims.
   Verifying that a client can receive packets sent to its claimed
   transport address protects against spoofing of this information by
   malicious clients.

   This technique is used primarily to avoid QUIC from being used for
   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.

   Several methods are used in QUIC to mitigate this attack.  Firstly,
   the initial handshake packet is padded to at least 1200 octets.  This
   allows a server to send a similar amount of data without risking
   causing an amplification attack toward an unproven remote address.

   A server eventually confirms that a client has received its messages
   when the cryptographic handshake successfully completes.  This might
   be insufficient, either because the server wishes to avoid the
   computational cost of completing the handshake, or it might be that
   the size of the packets that are sent during the handshake is too
   large.  This is especially important for 0-RTT, where the server
   might wish to provide application data traffic - such as a response
   to a request - in response to the data carried in the early data from
   the client.

   To send additional data prior to completing the cryptographic
   handshake, the server then needs to validate that the client owns the
   address that it claims.

   Source address validation is therefore performed during the
   establishment of a connection.  TLS provides the tools that support
   the feature, but basic validation is performed by the core transport
   protocol.

   A different type of source address validation is performed after a
   connection migration, see Section 6.7.

6.6.1.  Client Address Validation Procedure

   QUIC uses token-based address validation.  Any time the server wishes
   to validate a client address, it provides the client with a token.
   As long as the token cannot be easily guessed (see Section 6.6.3), if




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   the client is able to return that token, it proves to the server that
   it received the token.

   During the processing of the cryptographic handshake messages from a
   client, TLS will request that QUIC make a decision about whether to
   proceed based on the information it has.  TLS will provide QUIC with
   any token that was provided by the client.  For an initial packet,
   QUIC can decide to abort the connection, allow it to proceed, or
   request address validation.

   If QUIC decides to request address validation, it provides the
   cryptographic handshake with a token.  The contents of this token are
   consumed by the server that generates the token, so there is no need
   for a single well-defined format.  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.

   The cryptographic handshake is responsible for enacting validation by
   sending the address validation token to the client.  A legitimate
   client will include a copy of the token when it attempts to continue
   the handshake.  The cryptographic handshake extracts the token then
   asks QUIC a second time whether the token is acceptable.  In
   response, QUIC can either abort the connection or permit it to
   proceed.

   A connection MAY be accepted without address validation - or with
   only limited validation - but a server SHOULD limit the data it sends
   toward an unvalidated address.  Successful completion of the
   cryptographic handshake implicitly provides proof that the client has
   received packets from the server.

6.6.2.  Address Validation on Session Resumption

   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.

   A different type of token is needed when resuming.  Unlike the token
   that is created during a handshake, 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.  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.




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   This token can be provided to the cryptographic handshake immediately
   after establishing a connection.  QUIC might also generate an updated
   token if significant time passes or the client address changes for
   any reason (see Section 6.8).  The cryptographic handshake is
   responsible for providing the client with the token.  In TLS the
   token is included in the ticket that is used for resumption and
   0-RTT, which is carried in a NewSessionTicket message.

6.6.3.  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.

   In TLS the address validation token is often bundled with the
   information that TLS requires, such as the resumption secret.  In
   this case, adding integrity protection can be delegated to the
   cryptographic handshake protocol, avoiding redundant protection.  If
   integrity protection is delegated to the cryptographic handshake, an
   integrity failure will result in immediate cryptographic handshake
   failure.  If integrity protection is performed by QUIC, QUIC MUST
   abort the connection if the integrity check fails with a
   PROTOCOL_VIOLATION error code.

6.7.  Path Validation

   Path validation is used by an endpoint to verify reachability of a
   peer over a specific path.  That is, it tests 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.

   Path validation is used during connection migration (see Section 6.8
   and Section 6.9) by the migrating endpoint to verify reachability of
   a peer from a new local address.  Path validation is also used by the
   peer to verify that the migrating endpoint is able to receive packets
   sent to the its new address.  That is, that the packets received from
   the migrating endpoint do not carry a spoofed source address.





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   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.

   An endpoint MAY bundle PATH_CHALLENGE and PATH_RESPONSE frames that
   are used for path validation with other frames.  For instance, 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.

6.7.1.  Initiation

   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 additional PATH_CHALLENGE frames to handle
   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.

6.7.2.  Response

   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, with the following stipulation.  Since a
   PATH_CHALLENGE might be sent from a spoofed address, an endpoint MAY
   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: from the same local address on which the
   PATH_CHALLENGE was received, to the same remote address from which
   the PATH_CHALLENGE was received.




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6.7.3.  Completion

   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
   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.

6.7.4.  Abandonment

   An endpoint SHOULD abandon path validation after sending some number
   of PATH_CHALLENGE frames or after some time has passed.  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.

   If path validation fails, the path is deemed 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.






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6.8.  Connection Migration

   QUIC allows connections to survive changes to endpoint addresses
   (that is, IP address and/or port), such as those caused by a 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.

   This document limits migration of connections to new client
   addresses, except as described in Section 6.9.  Clients are
   responsible for initiating all migrations.  Servers do not send non-
   probing packets (see Section 6.8.1) toward a client address until it
   sees a non-probing packet from that address.  If a client receives
   packets from an unknown server address, the client MAY discard these
   packets.

6.8.1.  Probing a New Path

   An endpoint MAY probe for peer reachability from a new local address
   using path validation Section 6.7 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 6.8.5 for further discussion.

   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 6.7.

   PATH_CHALLENGE, PATH_RESPONSE, 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".

6.8.2.  Initiating Connection Migration

   A endpoint can migrate a connection to a new local address by sending
   packets containing frames other than 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



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   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 6.8.4.

   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 6.7 on the new path.

6.8.3.  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 6.7) 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 6.8.3.1 and
   Section 6.8.3.2.  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
   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 6.6) to the client.

6.8.3.1.  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,



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   connection migration might be used to amplify the volume of data that
   an attacker can generate toward a victim.

   As described in Section 6.8.3, 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 6.8.3, it does not need to limit its sending rate.

6.8.3.2.  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.

   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.







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6.8.4.  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 arm a separate alarm when
   a PATH_CHALLENGE is sent, which is disarmed when the corresponding
   PATH_RESPONSE is received.  If the alarm fires before the
   PATH_RESPONSE is received, the endpoint might send a new
   PATH_CHALLENGE, and restart the alarm for a longer period of time.

6.8.5.  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.  The
   NEW_CONNECTION_ID message can be sent to provide an unlinkable
   connection ID for use in case a peer wishes to explicitly break
   linkability between two points of network attachment.





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   An endpoint that does not require the use of a connection ID should
   not request that its peer use a connection ID.  Such an endpoint does
   not need to provide new connection IDs using the NEW_CONNECTION_ID
   frame.

   An endpoint might need to send packets on multiple networks without
   receiving any response from its peer.  To ensure that the endpoint is
   not linkable across each of these changes, a new connection ID is
   needed for each network.  To support this, multiple NEW_CONNECTION_ID
   messages are needed.  Each NEW_CONNECTION_ID is marked with a
   sequence number.  Connection IDs MUST be used in the order in which
   they are numbered.

   An endpoint that to break linkability upon changing networks MUST use
   a previously unused connection ID provided by its peer.  Protection
   of packet numbers ensures that packet numbers cannot be used to
   correlate connections.  Other properties of packets, such as timing
   and size, might be used to correlate activity, but no explicit
   correlation can be used to link activity across paths.

   Clients MAY change 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 6.8.4), so the port SHOULD
   only be changed infrequently.

   An endpoint that receives a successfully authenticated packet with a
   previously unused connection ID MUST use the next available
   connection ID for any packets it sends to that address.  To avoid
   changing connection IDs multiple times when packets arrive out of
   order, endpoints MUST change only in response to a packet that
   increases the largest received packet number.  Failing to do this
   could allow for use of that connection ID to link activity on new
   paths.  There is no need to move to a new connection ID if the
   address of a peer changes without also changing the connection ID.








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6.9.  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.

6.9.1.  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 6.7) 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.

6.9.2.  Responding to Connection Migration

   A server might receive a packet addressed to its preferred IP address
   at any time after the handshake is completed.  If this packet
   contains a PATH_CHALLENGE frame, the server sends a PATH_RESPONSE
   frame as per Section 6.7, but the server MUST continue sending all
   other 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.

   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 to the client exclusively from its
   preferred IP address.  It SHOULD drop packets for this connection



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   received on the old IP address, but MAY continue to process delayed
   packets.

6.9.3.  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 using the
   original address from the client's new 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 6.8.3.1 and Section 6.8.3.2.  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.

6.10.  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 6.10.2)

   o  immediate close (Section 6.10.3)

   o  stateless reset (Section 6.10.4)

6.10.1.  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].



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   An endpoint enters a closing period after initiating an immediate
   close (Section 6.10.3).  While closing, an endpoint MUST NOT send
   packets unless they contain a CONNECTION_CLOSE or APPLICATION_CLOSE
   frame (see Section 6.10.3 for details).

   In the closing state, only a packet containing a closing frame can be
   sent.  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 6.10.4) is sent.

   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.




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   An endpoint could receive packets from a new source address,
   indicating a client connection migration (Section 6.8), 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 6.7).  A server in the closing state MAY
   instead choose to discard packets received from a new source address.

6.10.2.  Idle Timeout

   A connection that remains idle for longer than the idle timeout (see
   Section 6.4.1) is closed.  A connection enters the draining state
   when the idle timeout expires.

   The time at which an idle timeout takes effect won't be perfectly
   synchronized on both endpoints.  An endpoint that sends packets near
   the end of an idle period could have those packets discarded if its
   peer enters the draining state before the packet is received.

6.10.3.  Immediate Close

   An endpoint sends a closing frame, either CONNECTION_CLOSE or
   APPLICATION_CLOSE, to terminate the connection immediately.  Either
   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
   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



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   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.

6.10.4.  Stateless Reset

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

   To support this process, the server sends a stateless_reset_token
   value during the handshake in the transport parameters.  This value
   is protected by encryption, so only client and server know this
   value.

   A server 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| Type (6)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Connection ID (144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packet Number (8/16/32)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Random Octets (*)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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

   A server generates a random 18-octet Destination Connection ID field.
   For a client that depends on the server including a connection ID,
   this will mean that this value differs from previous packets.  Ths
   results in two problems:

   o  The packet might not reach the client.  If the Destination
      Connection ID is critical for routing toward the client, then this
      packet could be incorrectly routed.  This causes the stateless
      reset to be ineffective in causing errors to be quickly detected
      and recovered.  In this case, clients 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 client to identify this as a potential stateless reset.
      A server that occasionally uses different connection IDs might
      introduce some uncertainty about this.

   The Packet Number field is set to a randomized value.  The server
   SHOULD send a packet with a short header and a packet number length
   of 1 octet.  Using the shortest possible packet number encoding
   minimizes the perceived gap between the last packet that the server
   sent and this packet.  A server MAY indicate a different packet
   number length, but a longer packet number encoding might allow this
   message to be identified as a stateless reset more easily using
   heuristics.

   After the Packet Number, the server pads the message with an
   arbitrary number of octets containing random values.

   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.  A server
   that supports multiple versions of QUIC needs to generate a stateless
   reset that will be accepted by clients that support any version that
   the server might support (or might have supported prior to losing
   state).  Designers of new versions of QUIC need to be aware of this




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   and either reuse this design, or use a portion of the packet other
   than the last 16 octets for carrying data.

6.10.4.1.  Detecting a Stateless Reset

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

6.10.4.2.  Calculating a Stateless Reset Token

   The stateless reset token MUST be difficult to guess.  In order to
   create a Stateless Reset Token, a server could randomly generate
   [RFC4086] a secret for every connection that it creates.  However,
   this presents a coordination problem when there are multiple servers
   in a cluster or a storage problem for a server 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 three inputs: the static key,
   the server's connection ID (see Section 4.7), and an identifier for
   the server instance.  A server could use HMAC [RFC2104] (for example,
   HMAC(static_key, server_id || connection_id)) or HKDF [RFC5869] (for
   example, using the static key as input keying material, with server
   and connection identifiers as salt).  The output of this function is
   truncated to 16 octets to produce the Stateless Reset Token for that
   connection.

   A server that loses state can use the same method to generate a valid
   Stateless Reset Secret.  The connection ID comes from the packet that
   the server receives.

   This design relies on the client always sending a connection ID in
   its packets so that the server can use the connection ID from a
   packet to reset the connection.  A server that uses this design
   cannot allow clients to use a zero-length connection ID.

   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
   server instance, connection ID, and static key cannot occur for
   another connection.  A connection ID from a connection that is reset



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   by revealing the Stateless Reset Token cannot be reused for new
   connections at the same server without first changing to use a
   different static key or server identifier.

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

7.  Frame Types and Formats

   As described in Section 5, packets contain one or more frames.  This
   section describes the format and semantics of the core QUIC frame
   types.

7.1.  Variable-Length Integer Encoding

   QUIC 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).




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   Error codes (Section 11.3) are described using integers, but do not
   use this encoding.

7.2.  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.

7.3.  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 11.4) which indicates why the stream is being
      closed.





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   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.

7.4.  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)     |   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 11.3 (APPLICATION_CLOSE uses codes from
      the application protocol error code space, see Section 11.4).

   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].

7.5.  APPLICATION_CLOSE frame

   An APPLICATION_CLOSE frame (type=0x03) uses the same format as the
   CONNECTION_CLOSE frame (Section 7.4), except that it uses error codes
   from the application protocol error code space (Section 11.4) instead
   of the transport error code space.



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   Other than the error code space, the format and semantics of the
   APPLICATION_CLOSE frame are identical to the CONNECTION_CLOSE frame.

7.6.  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)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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, with the
   exception of data on stream 0.  The sum of the largest received
   offsets on all streams - including streams in terminal states, but
   excluding stream 0 - MUST NOT exceed the value advertised by a
   receiver.  An endpoint MUST terminate a connection with a
   QUIC_FLOW_CONTROL_RECEIVED_TOO_MUCH_DATA 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 6.4.2).

7.7.  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.




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   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.

   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 6.4.2).

7.8.  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:




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   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 6.4.2).

7.9.  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 6.10).  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.








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7.10.  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 10.2.1).  BLOCKED frames can be used as input to tuning of
   flow control algorithms (see Section 10.1.2).

   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.

7.11.  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 7.10).

   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.





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7.12.  STREAM_ID_BLOCKED Frame

   A sender MAY 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 7.8).  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.

7.13.  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 6.8.5).

   The NEW_CONNECTION_ID 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 (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Length (8)  |          Connection ID (32..144)            ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:





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   Sequence:  A variable-length integer.  This value starts at 0 and
      increases by 1 for each connection ID that is provided by the
      server.  The connection ID that is assigned during the handshake
      is assumed to have a sequence of -1.  That is, the value selected
      during the handshake comes immediately before the first value that
      a server can send.

   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.

   Connection ID:  A connection ID of the specified length.

   Stateless Reset Token:  A 128-bit value that will be used to for a
      stateless reset when the associated connection ID is used (see
      Section 6.10.4).

   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.

7.14.  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 9.2.1).
   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:









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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        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 11.4).

7.15.  ACK Frame

   Receivers send ACK frames (type=0x0d) to inform senders which packets
   they have received and processed.  The ACK frame contains any number
   of ACK blocks.  ACK blocks are ranges of acknowledged packets.

   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]).

   A client MUST NOT acknowledge Retry packets.  Retry packets include
   the packet number from the Initial packet it responds to.  Version
   Negotiation 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 (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 7: ACK Frame Format



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   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 the 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 6.4.1).  Scaling in this fashion
      allows for a larger range of values with a shorter encoding at the
      cost of lower resolution.

   ACK Block Count:  The number of Additional ACK Block (and Gap) fields
      after the First ACK Block.

   ACK Blocks:  Contains one or more blocks of packet numbers which have
      been successfully received, see Section 7.15.1.

7.15.1.  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.












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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First ACK Block (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Gap (i)                         ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Additional ACK Block (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 8: 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
   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:



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     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_ERROR indicating an error in an ACK frame (that is,
   0x10d).

   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.

   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.

7.15.2.  Sending ACK Frames

   Implementations MUST NOT generate packets that only contain ACK
   frames in response to packets which only contain ACK frames.
   However, they MUST acknowledge packets containing only ACK frames
   when sending ACK frames in response to other packets.
   Implementations MUST NOT send more than one packet containing only
   ACK frames per received packet that contains frames other than ACK
   frames.  Packets containing non-ACK frames MUST be acknowledged
   immediately or when a delayed ack timer expires.

   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.

   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



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   lost after sufficiently newer packets are acknowledged.  Therefore,
   the receiver SHOULD repeatedly acknowledge newly received packets in
   preference to packets received in the past.

7.15.3.  ACK Frames and Packet Protection

   ACK frames that acknowledge protected packets MUST be carried in a
   packet that has an equivalent or greater level of packet protection.

   Packets that are protected with 1-RTT keys MUST be acknowledged in
   packets that are also protected with 1-RTT keys.

   A packet that is not protected and claims to acknowledge a packet
   number that was sent with packet protection is not valid.  An
   unprotected packet that carries acknowledgments for protected packets
   MUST be discarded in its entirety.

   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.

   Unprotected packets, such as those that carry the initial
   cryptographic handshake messages, MAY be acknowledged in unprotected
   packets.  Unprotected packets are vulnerable to falsification or
   modification.  Unprotected packets can be acknowledged along with
   protected packets in a protected packet.

   An endpoint SHOULD acknowledge packets containing cryptographic
   handshake messages in the next unprotected packet that it sends,
   unless it is able to acknowledge those packets in later packets
   protected by 1-RTT keys.  At the completion of the cryptographic
   handshake, both peers send unprotected packets containing
   cryptographic handshake messages followed by packets protected by
   1-RTT keys.  An endpoint SHOULD acknowledge the unprotected packets
   that complete the cryptographic handshake in a protected packet,
   because its peer is guaranteed to have access to 1-RTT packet
   protection keys.

   For instance, a server acknowledges a TLS ClientHello in the packet
   that carries the TLS ServerHello; similarly, a client can acknowledge
   a TLS HelloRetryRequest in the packet containing a second TLS
   ClientHello.  The complete set of server handshake messages (TLS
   ServerHello through to Finished) might be acknowledged by a client in
   protected packets, because it is certain that the server is able to
   decipher the packet.



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7.16.  PATH_CHALLENGE Frame

   Endpoints can use PATH_CHALLENGE frames (type=0x0e) to check
   reachability to the peer and for path validation during connection
   establishment and 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 7.17) containing the same Data.

7.17.  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 7.16).

   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 UNSOLICITED_PATH_RESPONSE.

7.18.  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




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      first octets of the stream, or the end of a stream that includes
      no data).

   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 9: 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 9.1).

   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.




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   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.

   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.

   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.

   Implementation note: 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.  An implementation is therefore
   advised to bundle as few streams as necessary in outgoing packets
   without losing transmission efficiency to underfilled packets.

8.  Packetization and Reliability

   A sender bundles one or more frames in a QUIC packet (see Section 5).

   A sender SHOULD 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.

8.1.  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.  Any stream state transitions triggered by the frame MUST
   have occurred.  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.  To avoid creating an indefinite



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   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
   acknowledge packets containing only ACK or PADDING frames in the next
   ACK frame that it sends.

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

8.2.  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  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 7.15.2.

   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 9.3.

   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 6.10.

   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,



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      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
      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.

   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].




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8.3.  Packet Size

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

   Clients MUST pad any Initial packet it sends to have a QUIC packet
   size of at least 1200 octets.  Sending an Initial packet of this size
   ensures that the network path supports a reasonably sized packet, and
   helps reduce the amplitude of amplification attacks caused by server
   responses toward an unverified client address.

   An Initial packet MAY exceed 1200 octets if the client knows that the
   Path Maximum Transmission Unit (PMTU) supports the size that it
   chooses.

   A server MAY send a CONNECTION_CLOSE frame with error code
   PROTOCOL_VIOLATION in response to an Initial packet 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.

8.4.  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 wish to 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.





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   QUIC depends on the network path supporting a 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.

8.4.1.  Special Considerations for 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.

8.4.2.  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.



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   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 tradeoffs 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.

9.  Streams: QUIC's Data Structuring Abstraction

   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 only; 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 9.1).

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

   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.

   The creation and destruction of streams are expected to have minimal
   bandwidth and computational cost.  A single STREAM frame may create,
   carry data for, and terminate a stream, or a stream may last the
   entire duration of a connection.

   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.





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9.1.  Stream Identifiers

   Streams are identified by an unsigned 62-bit integer, referred to as
   the Stream ID.  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.

   The two type bits from a Stream ID therefore identify streams as
   summarized in Table 5.

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

                         Table 5: Stream ID Types

   Stream ID 0 (0x0) is a client-initiated, bidirectional stream that is
   used for the cryptographic handshake.  Stream 0 MUST NOT be used for
   application data.

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



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   Stream IDs are encoded as a variable-length integer (see
   Section 7.1).

9.2.  Stream States

   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 9.2.1); another for streams from which an endpoint receives
   data (Section 9.2.2).

   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.

   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.

9.2.1.  Send Stream States

   Figure 10 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       |
          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 10: 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.

   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 if the
   receiving part enters the "Recv" state.

   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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   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.

9.2.2.  Receive Stream States

   Figure 11 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
          v
      +-------+
      | Recv  | Recv RST_STREAM
      |       |-----------------------.
      +-------+                       |
          |                           |
          | Recv STREAM + FIN         |
          v                           |
      +-------+                       |
      | Size  | Recv RST_STREAM       |
      | Known +---------------------->|
      +-------+                       |
          |                           |
          | Recv All Data             |
          v                           v
      +-------+                   +-------+
      | Data  | Recv RST_STREAM   | Reset |
      | Recvd +<-- (optional) --->| Recvd |
      +-------+                   +-------+
          |                           |
          | App Read All Data         | App Read RST
          v                           v
      +-------+                   +-------+
      | Data  |                   | Reset |
      | Read  |                   | Read  |
      +-------+                   +-------+

                   Figure 11: 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.

   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 10.3) 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
   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.




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9.2.3.  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 7.18), STREAM_BLOCKED (Section 7.11), and RST_STREAM
   (Section 7.3).

   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 7.7) and
   STOP_SENDING frames (Section 7.14).

   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.

9.2.4.  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 6 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 6: 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.

9.3.  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.

9.4.  Stream Concurrency

   An endpoint limits the number of concurrently active incoming streams
   by adjusting the maximum stream ID.  An initial value is set in the
   transport parameters (see Section 6.4.1) and is subsequently
   increased by MAX_STREAM_ID frames (see Section 7.8).

   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 initiated 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
   offsets (see Section 6.4.2).

   A receiver MUST NOT renege on an advertisement; that is, once a
   receiver advertises a stream ID via a MAX_STREAM_ID frame, it MUST
   NOT subsequently advertise a smaller maximum ID.  A sender may
   receive MAX_STREAM_ID frames out of order; a sender MUST therefore
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9.5.  Sending and Receiving Data

   Once a stream is created, endpoints may use the stream to send and
   receive data.  Each endpoint may send a series of STREAM frames
   encapsulating data on a stream until the stream is terminated in that
   direction.  Streams are an ordered byte-stream abstraction, and they
   have no other structure within them.  STREAM frame boundaries are not
   expected to be preserved in retransmissions from the sender or during
   delivery 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 byte 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.  The cryptographic
   handshake stream, Stream 0, is exempt from the connection-level data
   limits established by MAX_DATA.  Data on stream 0 other than the
   initial cryptographic handshake message is still subject to stream-
   level data limits and MAX_STREAM_DATA.  This message is exempt from
   flow control because it needs to be sent in a single packet
   regardless of the server's flow control state.  This rule applies
   even for 0-RTT handshakes where the remembered value of
   MAX_STREAM_DATA would not permit sending a full initial cryptographic
   handshake message.

   Flow control is described in detail in Section 10, and congestion
   control is described in the companion document [QUIC-RECOVERY].

9.6.  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



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   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
   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.

   Stream 0 MUST 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 flow
   control window.

10.  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.  This section
   describes QUIC's flow-control mechanisms.





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   QUIC employs a credit-based flow-control scheme similar to HTTP/2's
   flow control [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: (i)
   Connection flow control, which prevents senders from exceeding a
   receiver's buffer capacity for the connection, and (ii) Stream flow
   control, which prevents a single stream from consuming the entire
   receive buffer for a connection.

   A data receiver sends MAX_STREAM_DATA or MAX_DATA frames to the
   sender to advertise additional credit.  MAX_STREAM_DATA frames send
   the the maximum absolute byte offset of a stream, while MAX_DATA
   sends the maximum sum of the absolute byte offsets of all streams
   other than stream 0.

   A receiver MAY advertise a larger offset at any point by sending
   MAX_DATA or MAX_STREAM_DATA frames.  A receiver MUST NOT renege on an
   advertisement; that is, once a receiver advertises an offset, it MUST
   NOT subsequently advertise a smaller offset.  A sender could receive
   MAX_DATA or MAX_STREAM_DATA frames out of order; a sender MUST
   therefore ignore any flow control offset that does not move the
   window forward.

   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 BLOCKED or STREAM_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 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 except stream 0.  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
   contributing streams to determine the maximum data limit to be
   advertised.



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10.1.  Edge Cases and Other Considerations

   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_DATA or MAX_STREAM_DATA
   frame which will never come.

   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 avoid this de-synchronization, a RST_STREAM sender MUST include
   the final byte offset sent on the stream in the RST_STREAM frame.  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.

10.1.1.  Response to a RST_STREAM

   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 an application-specific issue, but it will often be the
   case that upon receipt of a RST_STREAM an endpoint will choose to
   stop sending data in its own direction.  If the sender of a
   RST_STREAM wishes to explicitly state that no future data will be
   processed, that endpoint MAY send a STOP_SENDING frame at the same
   time.

10.1.2.  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



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   undesirable.  At the same time, infrequent updates require larger
   increments to limits if blocking is to be avoided.  Thus, larger
   updates require a receiver to commit to larger resource commitments.
   Thus there is a tradeoff 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.

10.1.3.  Handshake Exemption

   During the initial handshake, an endpoint could need to send a larger
   message on stream 0 than would ordinarily be permitted by the peer's
   initial stream flow control window.  Since MAX_STREAM_DATA frames are
   not permitted in these early packets, the peer cannot provide
   additional flow control window in order to complete the handshake.

   Endpoints MAY exceed the flow control limits on stream 0 prior to the
   completion of the cryptographic handshake.  (That is, in Initial,
   Retry, and Handshake packets.)  However, once the handshake is
   complete, endpoints MUST NOT send additional data beyond the peer's
   permitted offset.  If the amount of data sent during the handshake
   exceeds the peer's maximum offset, the endpoint cannot send
   additional data on stream 0 until the peer has sent a MAX_STREAM_DATA
   frame indicating a larger maximum offset.

10.2.  Stream Limit Increment

   As with 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.

   Implementations will likely want to increase the maximum stream ID as
   peer-initiated streams close.  A receiver MAY also advance the
   maximum stream ID based on current activity, system conditions, and
   other environmental factors.

10.2.1.  Blocking on Flow Control

   If a sender does not receive a MAX_DATA or MAX_STREAM_DATA frame when
   it has run out of flow control credit, the sender will be blocked and
   SHOULD send a BLOCKED or STREAM_BLOCKED frame.  These frames are
   expected to be useful for debugging at the receiver; they do not
   require any other action.  A receiver SHOULD NOT wait for a BLOCKED



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   or STREAM_BLOCKED frame before sending MAX_DATA or MAX_STREAM_DATA,
   since doing so will mean that a sender is unable to send for an
   entire round trip.

   For smooth operation of the congestion controller, 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 possibiity 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.

10.3.  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.

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




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   application-level errors can be isolated to a single stream (see
   Section 11.2).

   The most appropriate error code (Section 11.3) 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 6.10.4) 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 7.4,
   Section 7.5).  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 6.10.4).

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







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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 7.3) with an appropriate error code to
   terminate just the affected stream.

   Stream 0 is critical to the functioning of the entire connection.  If
   stream 0 is closed with either a RST_STREAM or STREAM frame bearing
   the FIN flag, an endpoint MUST generate a connection error of type
   PROTOCOL_VIOLATION.

   Other than STOPPING (Section 9.3), 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.

11.3.  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 10).

   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 9.2).





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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_FORMAT_ERROR (0x7):  An endpoint received a frame that was
      badly formatted.  For instance, an empty STREAM frame that omitted
      the FIN flag, or an ACK frame that has more acknowledgment ranges
      than the remainder of the packet could carry.  This is a generic
      error code; an endpoint SHOULD use the more specific frame format
      error codes (0x1XX) if possible.

   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.

   UNSOLICITED_PATH_RESPONSE (0xB):  An endpoint received a
      PATH_RESPONSE frame that did not correspond to any PATH_CHALLENGE
      frame that it previously sent.

   FRAME_ERROR (0x1XX):  An endpoint detected an error in a specific
      frame type.  The frame type is included as the last octet of the
      error code.  For example, an error in a MAX_STREAM_ID frame would
      be indicated with the code (0x106).

   Codes for errors occuring when TLS is used for the crypto handshake
   are defined in Section 11 of [QUIC-TLS].  See Section 13.2 for
   details of registering new error codes.

11.4.  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 7.3) and APPLICATION_CLOSE (Section 7.5) frames.



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   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
   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.

12.  Security Considerations

12.1.  Spoofed ACK Attack

   An attacker might be able to receive an address validation token
   (Section 6.6) 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.

12.2.  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 6.10.3).






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12.3.  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.

12.4.  Stream Fragmentation and Reassembly Attacks

   An adversarial endpoint might intentionally fragment the data on
   stream buffers in order to cause disproportionate memory commitment.
   An adversarial endpoint could open a stream and send some STREAM
   frames containing arbitrary fragments of the stream content.

   The attack 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 the stream
   fragmentation attack.  Mitigations could consist of avoiding over-
   committing memory, delaying reassembly of STREAM frames, implementing
   heuristics based on the age and duration of reassembly holes, or some
   combination.

12.5.  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.

   Normally, clients will open streams sequentially, as explained in
   Section 9.1.  However, when several streams are initiated at short
   intervals, transmission error may cause STREAM DATA frames opening



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   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 9.4.  If chosen
   judisciously, 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.

13.  IANA Considerations

13.1.  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.  The 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  | Section 6.4.1 |
           |        |                          |               |
           | 0x0001 | initial_max_data         | Section 6.4.1 |
           |        |                          |               |
           | 0x0002 | initial_max_bidi_streams | Section 6.4.1 |
           |        |                          |               |
           | 0x0003 | idle_timeout             | Section 6.4.1 |
           |        |                          |               |
           | 0x0004 | preferred_address        | Section 6.4.1 |
           |        |                          |               |
           | 0x0005 | max_packet_size          | Section 6.4.1 |
           |        |                          |               |
           | 0x0006 | stateless_reset_token    | Section 6.4.1 |
           |        |                          |               |
           | 0x0007 | ack_delay_exponent       | Section 6.4.1 |
           |        |                          |               |
           | 0x0008 | initial_max_uni_streams  | Section 6.4.1 |
           +--------+--------------------------+---------------+

            Table 7: Initial QUIC Transport Parameters Entries

13.2.  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.  Note
   that FRAME_ERROR takes the range from 0x100 to 0x1FF and private use
   occupies the range from 0xFE00 to 0xFFFF.

   +-----------+------------------------+---------------+--------------+
   | Value     | Error                  | Description   | Specificatio |
   |           |                        |               | n            |
   +-----------+------------------------+---------------+--------------+
   | 0x0       | NO_ERROR               | No error      | Section 11.3 |
   |           |                        |               |              |
   | 0x1       | INTERNAL_ERROR         | Implementatio | Section 11.3 |
   |           |                        | n error       |              |
   |           |                        |               |              |
   | 0x2       | SERVER_BUSY            | Server        | Section 11.3 |
   |           |                        | currently     |              |
   |           |                        | busy          |              |
   |           |                        |               |              |
   | 0x3       | FLOW_CONTROL_ERROR     | Flow control  | Section 11.3 |
   |           |                        | error         |              |
   |           |                        |               |              |
   | 0x4       | STREAM_ID_ERROR        | Invalid       | Section 11.3 |
   |           |                        | stream ID     |              |
   |           |                        |               |              |
   | 0x5       | STREAM_STATE_ERROR     | Frame         | Section 11.3 |
   |           |                        | received in   |              |
   |           |                        | invalid       |              |
   |           |                        | stream state  |              |
   |           |                        |               |              |
   | 0x6       | FINAL_OFFSET_ERROR     | Change to     | Section 11.3 |
   |           |                        | final stream  |              |
   |           |                        | offset        |              |
   |           |                        |               |              |
   | 0x7       | FRAME_FORMAT_ERROR     | Generic frame | Section 11.3 |
   |           |                        | format error  |              |
   |           |                        |               |              |
   | 0x8       | TRANSPORT_PARAMETER_ER | Error in      | Section 11.3 |
   |           | ROR                    | transport     |              |
   |           |                        | parameters    |              |
   |           |                        |               |              |
   | 0x9       | VERSION_NEGOTIATION_ER | Version       | Section 11.3 |
   |           | ROR                    | negotiation   |              |
   |           |                        | failure       |              |
   |           |                        |               |              |
   | 0xA       | PROTOCOL_VIOLATION     | Generic       | Section 11.3 |
   |           |                        | protocol      |              |
   |           |                        | violation     |              |
   |           |                        |               |              |
   | 0xB       | UNSOLICITED_PATH_RESPO | Unsolicited   | Section 11.3 |



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   |           | NSE                    | PATH_RESPONSE |              |
   |           |                        | frame         |              |
   |           |                        |               |              |
   | 0x100-0x1 | FRAME_ERROR            | Specific      | Section 11.3 |
   | FF        |                        | frame format  |              |
   |           |                        | error         |              |
   +-----------+------------------------+---------------+--------------+

            Table 8: Initial QUIC Transport Error Codes Entries

14.  References

14.1.  Normative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

   [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-12 (work
              in progress), May 2018.

   [QUIC-TLS]
              Thomson, M., Ed. and S. Turner, Ed., "Using Transport
              Layer Security (TLS) to Secure QUIC", draft-ietf-quic-
              tls-12 (work in progress), May 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>.





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   [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>.

   [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>.

14.2.  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-01 (work in progress), May
              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>.

   [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>.




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   [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.  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.

Appendix B.  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.

   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.




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Appendix C.  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.

C.1.  Since draft-ietf-quic-transport-11

   o  Enable server to transition connections to a preferred address
      (#560, #1251)

   o  Packet numbers are encrypted (#1174, #1043, #1048, #1034, #850,
      #990, #734, #1079)

   o  Packet numbers use a variable-length encoding (#989, #1334)

   o  STREAM frames can now be empty (#1350)

C.2.  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)





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   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)

C.3.  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)

   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)

C.4.  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)




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   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)

C.5.  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-directinal
         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)



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   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)

C.6.  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)

C.7.  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)

C.8.  Since draft-ietf-quic-transport-04

   o  Introduce STOP_SENDING frame, RST_STREAM only resets in one
      direction (#165)




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   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)

   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)

C.9.  Since draft-ietf-quic-transport-03

   o  Change STREAM and RST_STREAM layout

   o  Add MAX_STREAM_ID settings

C.10.  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)



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   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)

   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)

C.11.  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)




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   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)

   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)




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   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)

   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)

C.12.  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

C.13.  Since draft-hamilton-quic-transport-protocol-01

   o  Adopted as base for draft-ietf-quic-tls

   o  Updated authors/editors list



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   o  Added IANA Considerations section

   o  Moved Contributors and Acknowledgments to appendices

Authors' Addresses

   Jana Iyengar (editor)
   Fastly

   Email: jri.ietf@gmail.com


   Martin Thomson (editor)
   Mozilla

   Email: martin.thomson@gmail.com



































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