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Versions: 00 01

MOPS                                                     M.P. Sharabayko
Internet-Draft                                           M.A. Sharabayko
Intended status: Standards Track           Haivision Network Video, GmbH
Expires: 13 March 2021                                           J. Dube
                                                               Haivision
                                                                 JS. Kim
                                                                 JW. Kim
                                                    SK Telecom Co., Ltd.
                                                        9 September 2020


                            The SRT Protocol
                      draft-sharabayko-mops-srt-01

Abstract

   This document specifies Secure Reliable Transport (SRT) protocol.
   SRT is a user-level protocol over User Datagram Protocol and provides
   reliability and security optimized for low latency live video
   streaming, as well as generic bulk data transfer.  For this, SRT
   introduces control packet extension, improved flow control, enhanced
   congestion control and a mechanism for data encryption.

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 13 March 2021.

Copyright Notice

   Copyright (c) 2020 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.



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Secure Reliable Transport Protocol  . . . . . . . . . . .   4
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   5
   3.  Packet Structure  . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Data Packets  . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Control Packets . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Handshake . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.2.  Key Material  . . . . . . . . . . . . . . . . . . . .  17
       3.2.3.  Keep-Alive  . . . . . . . . . . . . . . . . . . . . .  21
       3.2.4.  ACK (Acknowledgment)  . . . . . . . . . . . . . . . .  22
       3.2.5.  NAK (Loss Report) . . . . . . . . . . . . . . . . . .  24
       3.2.6.  Shutdown  . . . . . . . . . . . . . . . . . . . . . .  25
       3.2.7.  ACKACK  . . . . . . . . . . . . . . . . . . . . . . .  26
   4.  SRT Data Transmission and Control . . . . . . . . . . . . . .  26
     4.1.  Stream Multiplexing . . . . . . . . . . . . . . . . . . .  27
     4.2.  Data Transmission Modes . . . . . . . . . . . . . . . . .  27
       4.2.1.  Message Mode  . . . . . . . . . . . . . . . . . . . .  27
       4.2.2.  Live Mode . . . . . . . . . . . . . . . . . . . . . .  28
       4.2.3.  Buffer Mode . . . . . . . . . . . . . . . . . . . . .  28
     4.3.  Handshake Messages  . . . . . . . . . . . . . . . . . . .  28
       4.3.1.  Caller-Listener Handshake . . . . . . . . . . . . . .  31
       4.3.2.  Rendezvous Handshake  . . . . . . . . . . . . . . . .  33
     4.4.  SRT Buffer Latency  . . . . . . . . . . . . . . . . . . .  39
     4.5.  Timestamp-Based Packet Delivery . . . . . . . . . . . . .  40
       4.5.1.  Packet Delivery Time  . . . . . . . . . . . . . . . .  42
     4.6.  Too-Late Packet Drop  . . . . . . . . . . . . . . . . . .  43
     4.7.  Drift Management  . . . . . . . . . . . . . . . . . . . .  44
     4.8.  Acknowledgement and Lost Packet Handling  . . . . . . . .  46
       4.8.1.  Packet Acknowledgement (ACKs, ACKACKs)  . . . . . . .  46
       4.8.2.  Packet Retransmission (NAKs)  . . . . . . . . . . . .  47
     4.9.  Bidirectional Transmission Queues . . . . . . . . . . . .  49
     4.10. Round-Trip Time Estimation  . . . . . . . . . . . . . . .  49
     4.11. Congestion Control  . . . . . . . . . . . . . . . . . . .  50
   5.  Encryption  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  51
       5.1.1.  Encryption Scope  . . . . . . . . . . . . . . . . . .  51
       5.1.2.  AES Counter . . . . . . . . . . . . . . . . . . . . .  51
       5.1.3.  Stream Encrypting Key (SEK) . . . . . . . . . . . . .  51
       5.1.4.  Key Encrypting Key (KEK)  . . . . . . . . . . . . . .  52



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       5.1.5.  Key Material Exchange . . . . . . . . . . . . . . . .  52
       5.1.6.  KM Refresh  . . . . . . . . . . . . . . . . . . . . .  53
     5.2.  Encryption Process  . . . . . . . . . . . . . . . . . . .  54
       5.2.1.  Generating the Stream Encrypting Key  . . . . . . . .  54
       5.2.2.  Encrypting the Payload  . . . . . . . . . . . . . . .  54
     5.3.  Decryption Process  . . . . . . . . . . . . . . . . . . .  54
       5.3.1.  Restoring the Stream Encrypting Key . . . . . . . . .  55
       5.3.2.  Decrypting the Payload  . . . . . . . . . . . . . . .  55
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  56
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  56
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  56
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  56
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  56
     Normative References  . . . . . . . . . . . . . . . . . . . . .  56
     Informative References  . . . . . . . . . . . . . . . . . . . .  57
   Appendix A.  Packet Sequence List Coding  . . . . . . . . . . . .  59
   Appendix B.  SRT Access Control . . . . . . . . . . . . . . . . .  60
     B.1.  General Syntax  . . . . . . . . . . . . . . . . . . . . .  60
     B.2.  Standard Keys . . . . . . . . . . . . . . . . . . . . . .  61
     B.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  62
   Appendix C.  Changelog  . . . . . . . . . . . . . . . . . . . . .  62
     C.1.  Since Version 00  . . . . . . . . . . . . . . . . . . . .  62
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63

1.  Introduction

1.1.  Motivation

   The demand for live video streaming has been increasing steadily for
   many years.  With the emergence of cloud technologies, many video
   processing pipeline components have transitioned from on-premises
   appliances to software running on cloud instances.  While real-time
   streaming over TCP-based protocols like RTMP [RTMP] is possible at
   low bitrates and on a small scale, the exponential growth of the
   streaming market has created a need for more powerful solutions.

   To improve scalability on the delivery side, content delivery
   networks (CDNs) at one point transitioned to segmentation-based
   technologies like HLS (HTTP Live Streaming) [RFC8216] and DASH
   (Dynamic Adaptive Streaming over HTTP) [ISO23009].  This move
   increased the end-to-end latency of live streaming to over 30
   seconds, which makes it unattractive for many use cases.  Over time,
   the industry optimized these delivery methods, bringing the latency
   down to 3 seconds.







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   While the delivery side scaled up, improvements to video transcoding
   became a necessity.  Viewers watch video streams on a variety of
   different devices, connected over different types of networks.  Since
   upload bandwidth from on-premises locations is often limited, video
   transcoding moved to the cloud.

   RTMP became the de facto standard for contribution over the public
   Internet.  But there are limitations for the payload to be
   transmitted, since RTMP as a media specific protocol only supports
   two audio channels and a restricted set of audio and video codecs,
   lacking support for newer formats such as HEVC [H.265], VP9 [VP9], or
   AV1 [AV1].

   Since RTMP, HLS and DASH rely on TCP, these protocols can only
   guarantee acceptable reliability over connections with low RTTs, and
   can not use the bandwidth of network connections to their full extent
   due to limitations imposed by congestion control.  Notably, QUIC
   [I-D.ietf-quic-transport] has been designed to address these problems
   with HTTP-based delivery protocols in HTTP/3 [I-D.ietf-quic-http].
   Like QUIC, SRT [SRTSRC] uses UDP instead of the TCP transport
   protocol, but assures more reliable delivery using Automatic Repeat
   Request (ARQ), packet acknowledgments, end-to-end latency management,
   etc.

1.2.  Secure Reliable Transport Protocol

   Low latency video transmissions across reliable (usually local) IP
   based networks typically take the form of MPEG-TS [ISO13818-1]
   unicast or multicast streams using the UDP/RTP protocol, where any
   packet loss can be mitigated by enabling forward error correction
   (FEC).  Achieving the same low latency between sites in different
   cities, countries or even continents is more challenging.  While it
   is possible with satellite links or dedicated MPLS [RFC3031]
   networks, these are expensive solutions.  The use of public Internet
   connectivity, while less expensive, imposes significant bandwidth
   overhead to achieve the necessary level of packet loss recovery.
   Introducing selective packet retransmission (reliable UDP) to recover
   from packet loss removes those limitations.

   Derived from the UDP-based Data Transfer (UDT) protocol [GHG04b], SRT
   is a user-level protocol that retains most of the core concepts and
   mechanisms while introducing several refinements and enhancements,
   including control packet modifications, improved flow control for
   handling live streaming, enhanced congestion control, and a mechanism
   for encrypting packets.






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   SRT is a transport protocol that enables the secure, reliable
   transport of data across unpredictable networks, such as the
   Internet.  While any data type can be transferred via SRT, it is
   ideal for low latency (sub-second) video streaming.  SRT provides
   improved bandwidth utilization compared to RTMP, allowing much higher
   contribution bitrates over long distance connections.

   As packets are streamed from source to destination, SRT detects and
   adapts to the real-time network conditions between the two endpoints,
   and helps compensate for jitter and bandwidth fluctuations due to
   congestion over noisy networks.  Its error recovery mechanism
   minimizes the packet loss typical of Internet connections.

   To achieve low latency streaming, SRT had to address timing issues.
   The characteristics of a stream from a source network are completely
   changed by transmission over the public Internet, which introduces
   delays, jitter, and packet loss.  This, in turn, leads to problems
   with decoding, as the audio and video decoders do not receive packets
   at the expected times.  The use of large buffers helps, but latency
   is increased.  SRT includes a mechanism to keep a constant end-to-end
   latency, thus recreating the signal characteristics on the receiver
   side, and reducing the need for buffering.

   Like TCP, SRT employs a listener/caller model.  The data flow is bi-
   directional and independent of the connection initiation - either the
   sender or receiver can operate as listener or caller to initiate a
   connection.  The protocol provides an internal multiplexing
   mechanism, allowing multiple SRT connections to share the same UDP
   port, providing access control functionality to identify the caller
   on the listener side.

   Supporting forward error correction (FEC) and selective packet
   retransmission (ARQ), SRT provides the flexibility to use either of
   the two mechanisms or both combined, allowing for use cases ranging
   from the lowest possible latency to the highest possible reliability.

   SRT maintains the ability for fast file transfers introduced in UDT,
   and adds support for AES encryption.

2.  Terms 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.

   SRT:  The Secure Reliable Transport protocol described by this



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      document.

   PRNG:  Pseudo-Random Number Generator.

3.  Packet Structure

   SRT packets are transmitted as UDP payload [RFC0768].  Every UDP
   packet carrying SRT traffic contains an SRT header immediately after
   the UDP header (Figure 1).

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            SrcPort            |            DstPort            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Len              |            ChkSum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                          SRT Packet                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1: SRT packet as UDP payload

   SRT has two types of packets distinguished by the Packet Type Flag:
   data packet and control packet.

   The structure of the SRT packet is shown in Figure 2.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |F|        (Field meaning depends on the packet type)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          (Field meaning depends on the packet type)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Packet Contents                        |
   |                  (depends on the packet type)                 +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: SRT packet structure




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   F: 1 bit.  Packet Type Flag.  The control packet has this flag set to
      "1".  The data packet has this flag set to "0".

   Timestamp: 32 bits.  The timestamp of the packet, in microseconds.
      The value is relative to the time the SRT connection was
      established.  Depending on the transmission mode (Section 4.2),
      the field stores the packet send time or the packet origin time.

   Destination Socket ID: 32 bits.  A fixed-width field providing the
      SRT socket ID to which a packet should be dispatched.  The field
      may have the special value "0" when the packet is a connection
      request.

3.1.  Data Packets

   The structure of the SRT data packet is shown in Figure 3.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                    Packet Sequence Number                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |P P|O|K K|R|                   Message Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                              Data                             +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Data packet structure

   Packet Sequence Number: 31 bits.  The sequential number of the data
      packet.

   PP: 2 bits.  Packet Position Flag.  This field indicates the position
      of the data packet in the message.  The value "10b" (binary) means
      the first packet of the message. "00b" indicates a packet in the
      middle. "01b" designates the last packet.  If a single data packet
      forms the whole message, the value is "11b".

   O: 1 bit.  Order Flag.  Indicates whether the message should be
      delivered by the receiver in order (1) or not (0).  Certain
      restrictions apply depending on the data transmission mode used
      (Section 4.2).



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   KK: 2 bits.  Key-based Encryption Flag.  The flag bits indicate
      whether or not data is encrypted.  The value "00b" (binary) means
      data is not encrypted. "01b" indicates that data is encrypted with
      an even key, and "10b" is used for odd key encryption.  Refer to
      Section 5.  The value "11b" is only used in control packets.

   R: 1 bit.  Retransmitted Packet Flag.  This flag is clear when a
      packet is transmitted the first time.  The flag is set to "1" when
      a packet is retransmitted.

   Message Number: 26 bits.  The sequential number of consecutive data
      packets that form a message (see PP field).

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   Data: variable length.  The payload of the data packet.  The length
      of the data is the remaining length of the UDP packet.

3.2.  Control Packets

   An SRT control packet has the following structure.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|         Control Type        |            Subtype            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-specific Information                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+- CIF -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                   Control Information Field                   +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: Control packet structure

   Control Type: 15 bits.  Control Packet Type.  The use of these bits
      is determined by the control packet type definition.  See Table 1.

   Subtype: 16 bits.  This field specifies an additional subtype for
      specific packets.  See Table 1.




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   Type-specific Information: 32 bits.  The use of this field depends on
      the particular control packet type.  Handshake packets do not use
      this field.

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   Control Information Field (CIF): variable length.  The use of this
      field is defined by the Control Type field of the control packet.

   The types of SRT control packets are shown in Table 1.  The value
   "0x7FFF" is reserved for a user-defined type.

      +===================+==============+=========+===============+
      | Packet Type       | Control Type | Subtype | Section       |
      +===================+==============+=========+===============+
      | HANDSHAKE         |    0x0000    |   0x0   | Section 3.2.1 |
      +-------------------+--------------+---------+---------------+
      | KEEPALIVE         |    0x0001    |   0x0   | Section 3.2.3 |
      +-------------------+--------------+---------+---------------+
      | ACK               |    0x0002    |   0x0   | Section 3.2.4 |
      +-------------------+--------------+---------+---------------+
      | NAK (Loss Report) |    0x0003    |   0x0   | Section 3.2.5 |
      +-------------------+--------------+---------+---------------+
      | SHUTDOWN          |    0x0005    |   0x0   | Section 3.2.6 |
      +-------------------+--------------+---------+---------------+
      | ACKACK            |    0x0006    |   0x0   | Section 3.2.7 |
      +-------------------+--------------+---------+---------------+
      | User-Defined Type |    0x7FFF    |    -    | N/A           |
      +-------------------+--------------+---------+---------------+

                    Table 1: SRT Control Packet Types

3.2.1.  Handshake

   Handshake control packets (Control Type = 0x0000) are used to
   exchange peer configurations, to agree on connection parameters, and
   to establish a connection.

   The Control Information Field (CIF) of a handshake control packet is
   shown in Figure 5.









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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Version                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Encryption Field       |        Extension Field        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Initial Packet Sequence Number                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Maximum Transmission Unit Size                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Maximum Flow Window Size                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Handshake Type                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SRT Socket ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SYN Cookie                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                        Peer IP Address                        +
   |                                                               |
   +                                                               +
   |                                                               |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |         Extension Type        |        Extension Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                       Extension Contents                      +
   |                                                               |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

                    Figure 5: Handshake packet structure

   Version: 32 bits.  A base protocol version number.  Currently used
      values are 4 and 5.  Values greater than 5 are reserved for future
      use.

   Encryption Field: 16 bits.  Block cipher family and key size.  The
      values of this field are described in Table 2.  The default value
      is AES-128.








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                  +=======+============================+
                  | Value | Cipher family and key size |
                  +=======+============================+
                  | 0     |  No Encryption Advertised  |
                  +-------+----------------------------+
                  | 2     |          AES-128           |
                  +-------+----------------------------+
                  | 3     |          AES-192           |
                  +-------+----------------------------+
                  | 4     |          AES-256           |
                  +-------+----------------------------+

                      Table 2: Handshake Encryption
                               Field Values

   Extension Field: 16 bits.  This field is message specific extension
      related to Handshake Type field.  The value MUST be set to 0
      except for the following cases.  (1) If the handshake control
      packet is the INDUCTION message, this field is sent back by the
      Listener. (2) In the case of a CONCLUSION message, this field
      value should contain a combination of Extension Type values.  For
      more details, see Section 4.3.1.

                          +============+========+
                          | Bitmask    |  Flag  |
                          +============+========+
                          | 0x00000001 | HSREQ  |
                          +------------+--------+
                          | 0x00000002 | KMREQ  |
                          +------------+--------+
                          | 0x00000004 | CONFIG |
                          +------------+--------+

                             Table 3: Handshake
                              Extension Flags

   Initial Packet Sequence Number: 32 bits.  The sequence number of the
      very first data packet to be sent.

   Maximum Transmission Unit Size: 32 bits.  This value is typically set
      to 1500, which is the default Maximum Transmission Unit (MTU) size
      for Ethernet, but can be less.

   Maximum Flow Window Size: 32 bits.  The value of this field is the
      maximum number of data packets allowed to be "in flight"

      (i.e. the number of sent packets for which an ACK control packet
      has not yet been received).



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   Handshake Type: 32 bits.  This field indicates the handshake packet
      type.  The possible values are described in Table 4.  For more
      details refer to Section 4.3.

                      +============+================+
                      | Value      | Handshake type |
                      +============+================+
                      | 0xFFFFFFFD |      DONE      |
                      +------------+----------------+
                      | 0xFFFFFFFE |   AGREEMENT    |
                      +------------+----------------+
                      | 0xFFFFFFFF |   CONCLUSION   |
                      +------------+----------------+
                      | 0x00000000 |    WAVEHAND    |
                      +------------+----------------+
                      | 0x00000001 |   INDUCTION    |
                      +------------+----------------+

                          Table 4: Handshake Type

   SRT Socket ID: 32 bits.  This field holds the ID of the source SRT
      socket from which a handshake packet is issued.

   SYN Cookie: 32 bits.  Randomized value for processing a handshake.
      The value of this field is specified by the handshake message
      type.  See Section 4.3.

   Peer IP Address: 128 bits.  IPv4 or IPv6 address of the packet's
      sender.  The value consists of four 32-bit fields.  In the case of
      IPv4 addresses, fields 2, 3 and 4 are filled with zeroes.

   Extension Type: 16 bits.  The value of this field is used to process
      an integrated handshake.  Each extension can have a pair of
      request and response types.

















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            +=======+====================+===================+
            | Value |   Extension Type   | HS Extension Flag |
            +=======+====================+===================+
            | 1     |   SRT_CMD_HSREQ    |       HSREQ       |
            +-------+--------------------+-------------------+
            | 2     |   SRT_CMD_HSRSP    |       HSREQ       |
            +-------+--------------------+-------------------+
            | 3     |   SRT_CMD_KMREQ    |       KMREQ       |
            +-------+--------------------+-------------------+
            | 4     |   SRT_CMD_KMRSP    |       KMREQ       |
            +-------+--------------------+-------------------+
            | 5     |    SRT_CMD_SID     |       CONFIG      |
            +-------+--------------------+-------------------+
            | 6     | SRT_CMD_CONGESTION |       CONFIG      |
            +-------+--------------------+-------------------+
            | 7     |   SRT_CMD_FILTER   |       CONFIG      |
            +-------+--------------------+-------------------+
            | 8     |   SRT_CMD_GROUP    |       CONFIG      |
            +-------+--------------------+-------------------+

                 Table 5: Handshake Extension Type values

   Extension Length: 16 bits.  The length of the Extension Contents
      field in four-byte blocks.

   Extension Contents: variable length.  The payload of the extension.

3.2.1.1.  Handshake Extension Message

   In a Handshake Extension, the value of the Extension Field of the
   handshake control packet is defined as 1 for a Handshake Extension
   request (SRT_CMD_HSREQ in Table 5), and 2 for a Handshake Extension
   response (SRT_CMD_HSRSP in Table 5).

   The Extension Contents field of a Handshake Extension Message is
   structured 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          SRT Version                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SRT Flags                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Receiver TSBPD Delay     |       Sender TSBPD Delay      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 6: Handshake Extension Message structure



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   SRT Version: 32 bits.  SRT library version MUST be formed as major *
      0x10000 + minor * 0x100 + patch.

   SRT Flags: 32 bits.  SRT configuration flags (see Section 3.2.1.1.1).

   Receiver TSBPD Delay: 16 bits.  Timestamp-Based Packet Delivery
      (TSBPD) Delay of the receiver.  Refer to Section 4.5.

   Sender TSBPD Delay: 16 bits.  TSBPD of the sender.  Refer to
      Section 4.5.

3.2.1.1.1.  Handshake Extension Message Flags

                      +============+===============+
                      | Bitmask    |      Flag     |
                      +============+===============+
                      | 0x00000001 |    TSBPDSND   |
                      +------------+---------------+
                      | 0x00000002 |    TSBPDRCV   |
                      +------------+---------------+
                      | 0x00000004 |     CRYPT     |
                      +------------+---------------+
                      | 0x00000008 |   TLPKTDROP   |
                      +------------+---------------+
                      | 0x00000010 |  PERIODICNAK  |
                      +------------+---------------+
                      | 0x00000020 |   REXMITFLG   |
                      +------------+---------------+
                      | 0x00000040 |     STREAM    |
                      +------------+---------------+
                      | 0x00000080 | PACKET_FILTER |
                      +------------+---------------+

                            Table 6: Handshake
                         Extension Message Flags

   *  TSBPDSND flag defines if the TSBPD mechanism (Section 4.5) will be
      used for sending.

   *  TSBPDRCV flag defines if the TSBPD mechanism (Section 4.5) will be
      used for receiving.

   *  CRYPT flag MUST be set.  It is a legacy flag that indicates the
      party understands KK field of the SRT Packet (Figure 3).

   *  TLPKTDROP flag should be set if too-late packet drop mechanism
      will be used during transmission.  See Section 4.6.




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   *  PERIODICNAK flag set indicates the peer will send periodic NAK
      packets.  See Section 4.8.2.

   *  REXMITFLG flag MUST be set.  It is a legacy flag that indicates
      the peer understands the R field of the SRT DATA Packet
      (Figure 3).

   *  STREAM flag identifies the transmission mode (Section 4.2) to be
      used in the connection.  If the flag is set the buffer mode
      (Section 4.2.3) will be used.  Otherwise, message mode
      (Section 4.2.1) is to be used.

   *  PACKET_FILTER flag indicates if the peer supports packet filter.

3.2.1.2.  Key Material Extension Message

   If an encrypted connection is being established, the Key Material
   (KM) is first transmitted as a Handshake Extension message.  This
   extension is not supplied for unprotected connections.  The purpose
   of the extension is to let peers exchange and negotiate encryption-
   related information to be used to encrypt and decrypt the payload of
   the stream.

   The extension can be supplied with the Handshake Extension Type field
   set to either SRT_CMD_KMREQ or SRT_CMD_HSRSP (see Table 5 in
   Section 3.2.1).  For more details refer to Section 4.3.

   The KM message is placed in the Extension Contents.  See
   Section 3.2.2 for the structure of the KM message.

3.2.1.3.  Stream ID Extension Message

   The Stream ID handshake extension message can be used to identify the
   stream content.  The Stream ID value can be free-form, but there is
   also a recommended convention that can be used to achieve
   interoperability.

   The Stream ID handshake extension message has SRT_CMD_SID extension
   type (see Table 5.  The extension contents are a sequence of UTF-8
   characters.  The maximum allowed size of the StreamID extension is
   512 bytes.










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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                           |
                                  ...
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 7: Stream ID Extension Message

   The Extension Contents field holds a sequence of UTF-8 characters
   (see Figure 7).  The maximum allowed size of the StreamID extension
   is 512 bytes.  The actual size is determined by the Extension Length
   field (Figure 5), which defines the length in four byte blocks.  If
   the actual payload is less than the declared length, the remaining
   bytes are set to zeros.

   The content is stored as 32-bit little endian words.

3.2.1.4.  Group Membership Extension

   The Group Membership handshake extension is used to distinguish
   single SRT connections and bonded SRT connections (group
   connections).

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Group ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type    |     Flags     |             Weight              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 8: Group Membership Extension Message

   GroupID: 32 bits.  The identifier of a group whose members include
      the sender socket that is making a connection.  The target socket
      that should interpret it should belong to the corresponding group
      on its side (or should create one, if it doesn't exist).

   Type: 8 bits.  Group type, as per SRT_GTYPE_ enumeration.

   *  0: undefined group type,

   *  1: broadcast group type,

   *  2: main/backup group type



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   *  3: balancing group type (reserved for future use)

   *  4: multicast group type (reserved for future use)

   Flags: 8 bits.  Special flags mostly reserved for the future.  See
      Figure 9.

   Weight: 16 bits.  Special value with interpretation depending on the
      Type field value.

   *  Not used with broadcast groups.

   *  Defines the link priority in backup groups.

   *  Not yet defined (reserved for future) for any other cases.

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+
   |   (zero)  |M|
   +-+-+-+-+-+-+-+

                 Figure 9: Group Membership Extension Flags

   M: 1 bit.  When set, defines synchronization on message numbers,
      otherwise transmission is synchronized on sequence numbers.

3.2.2.  Key Material

   The purpose of the Key Material Message is to let peers exchange
   encryption-related information to be used to encrypt and decrypt the
   payload of the stream.

   This message can be supplied in two possible ways:

   *  as a Handshake Extension, see Section 3.2.1.2,

   *  in the Content Information Field of the User-Defined control
      packet (described below).

   When the Key Material is transmitted as a control packet, the Control
   Type field of the SRT packet header is set to User-Defined Type (see
   Table 1), the Subtype field of the header is set to SRT_CMD_KMREQ for
   key-refresh request and SRT_CMD_KMRSP for key-refresh response
   (Table 5).  The KM Refresh mechanism is described in Section 5.1.6.

   The structure of the Key Material message is illustrated in
   Figure 10.




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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|  V  |   PT  |              Sign             |   Resv1   | KK|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              KEKI                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cipher    |      Auth     |       SE      |     Resv2     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Resv3             |     SLen/4    |     KLen/4    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Salt                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                          Wrapped Key                          +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 10: Key Material Message structure

   S: 1 bit, value = {0}.  This is a fixed-width field that is reserved
      for future usage.

   Version (V): 3 bits, value = {1}.  This is a fixed-width field that
      indicates the SRT version:

      *  1: initial version

   Packet Type (PT): 4 bits, value = {2}.  This is a fixed-width field
      that indicates the Packet Type:

      *  0: Reserved

      *  1: Media Stream Message (MSmsg)

      *  2: Keying Material Message (KMmsg)

      *  7: Reserved to discriminate MPEG-TS packet (0x47=sync byte)

   Sign: 16 bits, value = {0x2029}.  This is a fixed-width field that
      contains the signature 'HAI' encoded as a PnP Vendor ID ([PNPID])
      (in big-endian order)

   Resv1: 6 bits, value = {0}.  This is a fixed-width field reserved for
      flag extension or other usage.

   Key-based Encryption (KK): 2 bits.  This is a fixed-width field that




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      indicates which SEKs (odd and/or even) are provided in the
      extension:

      *  00b: no SEK is provided (invalid extension format)

      *  01b: even key is provided

      *  10b: odd key is provided

      *  11b: both even and odd keys are provided

   Key Encryption Key Index (KEKI): 32 bits, value = {0}.  This is a
      fixed-width field for specifying the KEK index (big-endian order)
      was used to wrap (and optionally authenticate) the SEK(s).  The
      value 0 is used to indicate the default key of the current stream.
      Other values are reserved for the possible use of a key management
      system in the future to retrieve a cryptographic context.

      *  0: Default stream associated key (stream/system default)

      *  1..255: Reserved for manually indexed keys

   Cipher: 8 bits, value = {0..2}.  This is a fixed-width field for
      specifying encryption cipher and mode:

      *  0: None or KEKI indexed crypto context

      *  2: AES-CTR [SP800-38A]

   Authentication (Auth): 8 bits, value = {0}.  This is a fixed-width
      field for specifying a message authentication code algorithm:

      *  0: None or KEKI indexed crypto context

   Stream Encapsulation (SE): 8 bits, value = {2}.  This is a fixed-
      width field for describing the stream encapsulation:

      *  0: Unspecified or KEKI indexed crypto context

      *  1: MPEG-TS/UDP

      *  2: MPEG-TS/SRT

   Resv2: 8 bits, value = {0}.  This is a fixed-width field reserved for
      future use.

   Resv3: 16 bits, value = {0}.  This is a fixed-width field reserved
      for future use.



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   SLen/4: 8 bits, value = {4}.  This is a fixed-width field for
      specifying salt length SLen in bytes divided by 4.  Can be zero if
      no salt/IV present.  The only valid length of salt defined is 128
      bits.

   KLen/4: 8 bits, value = {4,6,8}.  This is a fixed-width field for
      specifying SEK length in bytes divided by 4.  Size of one key even
      if two keys present.  MUST match the key size specified in the
      Encryption Field of the handshake packet Table 2.

   Salt (SLen): SLen * 8 bits, value = { }.  This is a variable-width
      field that complements the keying material by specifying a salt
      key.

   Wrap: (64 + n * KLen * 8) bits, value = { }.  This is a variable-
      width field for specifying Wrapped key(s), where n = (KK + 1)/2
      and the size of the wrap field is ((n * KLen) + 8) bytes.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                  Integrity Check Vector (ICV)                 +
   |                                                               |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                              xSEK                             |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                              oSEK                             |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

                     Figure 11: Unwrapped key structure

   ICV: 64 bits.  64-bit Integrity Check Vector(AES key wrap integrity).
      This field is used to detect if the keys were unwrapped properly.
      If the KEK in hand is invalid, validation fails and unwrapped keys
      are discarded.

   xSEK: variable width.  This field identifies an odd or even SEK.  If
      only one key is present, the bit set in the KK field tells which
      SEK is provided.  If both keys are present, then this field is
      eSEK (even key) and it is followed by odd key oSEK.  The length of
      this field is calculated as KLen * 8.

   oSEK: variable width.  This field with the odd key is present only
      when the message carries the two SEKs (identified by he KK field).






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3.2.3.  Keep-Alive

   Keep-alive control packets are sent after a certain timeout from the
   last time any packet (Control or Data) was sent.  The purpose of this
   control packet is to notify the peer to keep the connection open when
   no data exchange is taking place.

   The default timeout for a keep-alive packet to be sent is 1 second.

   An SRT keep-alive packet is formatted 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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|         Control Type        |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-specific Information                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 12: Keep-Alive control packet

   Packet Type: 1 bit, value = 1.  The packet type value of a keep-alive
      control packet is "1".

   Control Type: 15 bits, value = KEEPALIVE{0x0001}.  The control type
      value of a keep-alive control packet is "1".

   Reserved: 16 bits, value = 0.  This is a fixed-width field reserved
      for future use.

   Type-specific Information.  This field is reserved for future
      definition.

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   Keep-alive controls packet do not contain Control Information Field
   (CIF).








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3.2.4.  ACK (Acknowledgment)

   Acknowledgment control packets are used to provide delivery status of
   data packets.  By acknowled reception of data packets up to the
   acknowledged packet sequence number the receiver notifies the sender
   that all prior packets were received or, in case of live transmission
   mode (Section 4.2.2), preceeeding missing packets if any were dropped
   as too late to be delivered.

   ACK packets may also carry some additional information from the
   receiver like RTT, bandwidth, receiving speed, etc.  The CIF portion
   of the ACK control packet is expanded 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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|        Control Type         |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgement Number                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+- CIF -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Last Acknowledged Packet Sequence Number           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              RTT                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          RTT Variance                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Available Buffer Size                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Packets Receiving Rate                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Estimated Link Capacity                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Receiving Rate                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 13: ACK control packet

   Packet Type: 1 bit, value = 1.  The packet type value of an ACK
      control packet is "1".

   Control Type: 15 bits, value = ACK{0x0002}.  The control type value
      of an ACK control packet is "2".

   Reserved: 16 bits, value = 0.  This is a fixed-width field reserved



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      for future use.

   Acknowledgement Number: 32 bits.  This field contains the sequential
      number of the full acknowledgment packet starting from 1.

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   Last Acknowledged Packet Sequence Number: 32 bits.  This field
      contains the sequence number of the last data packet being
      acknowledged plus one.  In other words, if it the sequence number
      of the first unacknowledged packet.

   RTT: 32 bits.  RTT value, in microseconds, estimated by the receiver
      based on the previous ACK-ACKACK packet exchange.

   RTT Variance: 32 bits.  The variance of the RTT estimation, in
      microseconds.

   Available Buffer Size: 32 bits.  Available size of the receiver's
      buffer, in packets.

   Packets Receiving Rate: 32 bits.  The rate at which packets are being
      received, in packets per second.

   Estimated Link Capacity: 32 bits.  Estimated bandwidth of the link,
      in packets per second.

   Receiving Rate: 32 bits.  Estimated receiving rate, in bytes per
      second.

   There are several types of ACK packets:

   *  A Full ACK control packet is sent every 10 ms and has all the
      fields of Figure 13.

   *  A Lite ACK control packet includes only the Last Acknowledged
      Packet Sequence Number field.  The Type-specific Information field
      should be set to 0.

   *  A Small ACK includes the fields up to and including the Available
      Buffer Size field.  The Type-specific Information field should be
      set to 0.

   The sender only acknowledges the receipt of Full ACK packets (see
   ACKACK Section Section 3.2.7).




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   The Lite ACK and Small ACK packets are used in cases when the
   receiver should acknowledge received data packets more often than
   every 10 ms.  This is usually needed at high data rates.  It is up to
   the receiver to decide the condition and the type of ACK packet to
   send (Lite or Small).  The recommendation is to send a Lite ACK for
   every 64 packets received.

3.2.5.  NAK (Loss Report)

   Negative acknowledgment (NAK) control packets are used to signal
   failed data packet deliveries.  The receiver notifies the sender
   about lost data packets by sending a NAK packet that contains a list
   of sequence numbers for those lost packets.

   An SRT NAK packet is formatted 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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|        Control Type         |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-specific Information                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+- CIF (Loss List) -+-+-+-+-+-+-+-+-+-+-+-+
   |0|                 Lost packet sequence number                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|         Range of lost packets from sequence number          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                    Up to sequence number                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                 Lost packet sequence number                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 14: NAK control packet

   Packet Type: 1 bit, value = 1.  The packet type value of a NAK
      control packet is "1".

   Control Type: 15 bits, value = NAK{0x0003}.  The control type value
      of a NAK control packet is "3".

   Reserved: 16 bits, value = 0.  This is a fixed-width field reserved
      for future use.

   Type-specific Information: 32 bits.  This field is reserved for



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      future definition.

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   Control Information Field (CIF).  A single value or a range of lost
      packets sequence numbers.  See packet sequence number coding in
      Appendix A.

3.2.6.  Shutdown

   Shutdown control packets are used to initiate the closing of an SRT
   connection.

   An SRT shutdown control packet is formatted 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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|        Control Type         |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Type-specific Information                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 15: Shutdown control packet

   Packet Type: 1 bit, value = 1.  The packet type value of a shutdown
      control packet is "1".

   Control Type: 15 bits, value = SHUTDOWN{0x0005}.  The control type
      value of a shutdown control packet is "5".

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   Type-specific Information.  This field is reserved for future
      definition.

   Shutdown control packets do not contain Control Information Field
   (CIF).





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3.2.7.  ACKACK

   ACKACK control packets are sent to acknowledge the reception of a
   Full ACK, and are used in the calculation of RTT by the receiver.

   An SRT ACKACK Control packet is formatted 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
   +-+-+-+-+-+-+-+-+-+-+-+-+- SRT Header +-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|        Control Type         |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Acknowledgement Number                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination Socket ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 16: ACKACK control packet

   Packet Type: 1 bit, value = 1.  The packet type value of an ACKACK
      control packet is "1".

   Control Type: 15 bits, value = ACKACK{0x0006}.  The control type
      value of an ACKACK control packet is "6".

   Acknowledgement Number.  This field contains the Acknowledgement
      Number of the full ACK packet the reception of which is being
      acknowledged by this ACKACK packet.

   Timestamp: 32 bits.  See Section 3.

   Destination Socket ID: 32 bits.  See Section 3.

   ACKACK control packets do not contain Control Information Field
   (CIF).

4.  SRT Data Transmission and Control

   This section describes key concepts related to the handling of
   control and data packets during the transmission process.

   After the handshake and exchange of capabilities is completed, packet
   data can be sent and received over the established connection.  To
   fully utilize the features of low latency and error recovery provided
   by SRT, the sender and receiver must handle control packets, timers,
   and buffers for the connection as specified in this section.



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4.1.  Stream Multiplexing

   Multiple SRT sockets may share the same UDP socket so that the
   packets received to this UDP socket will be correctly dispatched to
   those SRT sockets they are currently destined.

   During the handshake, the parties exchange their SRT Socket IDs.
   These IDs are then used in the Destination Socket ID field of every
   control and data packet (see Section 3).

4.2.  Data Transmission Modes

   SRT has been mainly created for Live Streaming and therefore its main
   and default transmission mode is "live".  SRT supports, however, the
   modes that the original UDT library supported, that is, buffer and
   message transmission.

4.2.1.  Message Mode

   When the STREAM flag of the handshake Extension Message
   Section 3.2.1.1 is set to 0, the protocol operates in Message mode,
   characterized as follows:

   *  Every packet has its own Packet Sequence Number.

   *  One or several consecutive SRT Data packets can form a message.

   *  All the packets belonging to the same message have a similar
      message number set in the Message Number field.

   The first packet of a message has the first bit of the Packet
   Position Flags (Section 3.1) set to 1.  The last packet of the
   message has the second bit of the Packet Position Flags set to 1.
   Thus, a PP equal to "11b" indicates a packet that forms the whole
   message.  A PP equal to "00b" indicates a packet that belongs to the
   inner part of the message.

   The concept of the message in SRT comes from UDT ([GHG04b]).  In this
   mode a single sending instruction passes exactly one piece of data
   that has boundaries (a message).  This message may span across
   multiple UDP packets (and multiple SRT data packets).  The only size
   limitation is that it shall fit as a whole in the buffers of the
   sender and the receiver.  Although internally all operations (e.g.
   ACK, NAK) on data packets are performed independently, an application
   must send and receive the whole message.  Until the message is
   complete (all packets are received) the application will not be
   allowed to read it.




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   When the Order Flag of a Data packet is set to 1, this imposes a
   sequential reading order on messages.  An Order Flag set to 0 allows
   an application to read messages that are already fully available,
   before any preceding messages that may have some packets missing.

4.2.2.  Live Mode

   Live mode is a special type of message mode where only data packets
   with their PP field set to "11b" are allowed.

   Additionally Timestamp-Based Packet Delivery (TSBPD) (Section 4.5)
   and Too-Late Packet Drop (Section 4.6) mechanisms are used in this
   mode.

4.2.3.  Buffer Mode

   Buffer mode is negotiated during the Handshake by setting the STREAM
   flag of the handshake Extension Message Flags to 1.

   In this mode consecutive packets form one continuous stream that can
   be read, with portions of any size.

4.3.  Handshake Messages

   SRT is a connection-oriented protocol.  It embraces the concepts of
   "connection" and "session".  The UDP system protocol is used by SRT
   for sending data and control packets.

   An SRT connection is characterized by the fact that it is:

   *  first engaged by a handshake process;

   *  maintained as long as any packets are being exchanged in a timely
      manner;

   *  considered closed when a party receives the appropriate close
      command from its peer (connection closed by the foreign host), or
      when it receives no packets at all for some predefined time
      (connection broken on timeout).

   SRT supports two connection configurations:

   1.  Caller-Listener, where one side waits for the other to initiate a
       connection

   2.  Rendezvous, where both sides attempt to initiate a connection





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   The handshake is performed between two parties: "Initiator" and
   "Responder":

   *  Initiator starts the extended SRT handshake process and sends
      appropriate SRT extended handshake requests.

   *  Responder expects the SRT extended handshake requests to be sent
      by the Initiator and sends SRT extended handshake responses back.

   There are two basic types of SRT handshake extensions that are
   exchanged in the handshake:

   *  Handshake Extension Message exchanges the basic SRT information;

   *  Key Material Exchange exchanges the wrapped stream encryption key
      (used only if encryption is requested).

   *  Stream ID extension exchanges some stream-specific information
      that can be used by the application to identify the incoming
      stream connection.

   The Initiator and Responder roles are assigned depending on the
   connection mode.

   For Caller-Listener connections: the Caller is the Initiator, the
   Listener is the Responder.  For Rendezvous connections: the Initiator
   and Responder roles are assigned based on the initial data
   interchange during the handshake.

   The Handshake Type field in the Handshake Structure (see Figure 5)
   indicates the handshake message type.

   Caller-Listener handshake exchange has the following order of
   Handshake Types:

   1.  Caller to Listener: INDUCTION

   2.  Listener to Caller: INDUCTION (reports cookie)

   3.  Caller to Listener: CONCLUSION (uses previously returned cookie)

   4.  Listener to Caller: CONCLUSION (confirms connection established)

   Rendezvous handshake exchange has the following order of Handshake
   Types:

   1.  After starting the connection: WAVEAHAND.




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   2.  After receiving the above message from the peer: CONCLUSION.

   3.  After receiving the above message from the peer: AGREEMENT.

   When a connection process has failed before either party can send the
   CONCLUSION handshake, the Handshake Type field will contain the
   appropriate error value for the rejected connection.  See the list of
   error codes in Table 7.

    +======+================+=========================================+
    | Code | Error          | Description                             |
    +======+================+=========================================+
    | 1000 | REJ_UNKNOWN    | Unknown reason                          |
    +------+----------------+-----------------------------------------+
    | 1001 | REJ_SYSTEM     | System function error                   |
    +------+----------------+-----------------------------------------+
    | 1002 | REJ_PEER       | Rejected by peer                        |
    +------+----------------+-----------------------------------------+
    | 1003 | REJ_RESOURCE   | Resource allocation problem             |
    +------+----------------+-----------------------------------------+
    | 1004 | REJ_ROGUE      | incorrect data in handshake             |
    +------+----------------+-----------------------------------------+
    | 1005 | REJ_BACKLOG    | listener's backlog exceeded             |
    +------+----------------+-----------------------------------------+
    | 1006 | REJ_IPE        | internal program error                  |
    +------+----------------+-----------------------------------------+
    | 1007 | REJ_CLOSE      | socket is closing                       |
    +------+----------------+-----------------------------------------+
    | 1008 | REJ_VERSION    | peer is older version than agent's min  |
    +------+----------------+-----------------------------------------+
    | 1009 | REJ_RDVCOOKIE  | rendezvous cookie collision             |
    +------+----------------+-----------------------------------------+
    | 1010 | REJ_BADSECRET  | wrong password                          |
    +------+----------------+-----------------------------------------+
    | 1011 | REJ_UNSECURE   | password required or unexpected         |
    +------+----------------+-----------------------------------------+
    | 1012 | REJ_MESSAGEAPI | Stream flag collision                   |
    +------+----------------+-----------------------------------------+
    | 1013 | REJ_CONGESTION | incompatible congestion-controller type |
    +------+----------------+-----------------------------------------+
    | 1014 | REJ_FILTER     | incompatible packet filter              |
    +------+----------------+-----------------------------------------+
    | 1015 | REJ_GROUP      | incompatible group                      |
    +------+----------------+-----------------------------------------+

                 Table 7: Handshake Rejection Reason Codes





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   The specification of the cipher family and block size is decided by
   the data Sender.  When the transmission is bidirectional, this value
   MUST be agreed upon at the outset because when both are set the
   Responder wins.  For Caller-Listener connections it is reasonable to
   set this value on the Listener only.  In the case of Rendezvous the
   only reasonable approach is to decide upon the correct value from the
   different sources and to set it on both parties (note that *AES-128*
   is the default).

4.3.1.  Caller-Listener Handshake

   This section describes the handshaking process where a Listener is
   waiting for an incoming Handshake request on a bound UDP port from a
   Caller.  The process has two phases: induction and conclusion.

4.3.1.1.  The Induction Phase

   The INDUCTION phase serves only to set a cookie on the Listener so
   that it doesn't allocate resources, thus mitigating a potential DoS
   attack that might be perpetrated by flooding the Listener with
   handshake commands.

   The Caller begins by sending the INDUCTION handshake, which contains
   the following (significant) fields:

   *  Version: MUST always be 4

   *  Encryption Field: 0

   *  Extension Field: 2

   *  Handshake Type: INDUCTION

   *  SRT Socket ID: SRT Socket ID of the Caller

   *  SYN Cookie: 0

   The Destination Socket ID of the SRT packet header in this message is
   0, which is interpreted as a connection request.

   The handshake version number is set to 4 in this initial handshake.
   This is due to the initial design of SRT that was to be compliant
   with the UDT protocol ([GHG04b]) on which it is based.

   The Listener responds with the following:

   *  Version: 5




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   *  Encryption Field: Advertised cipher family and block size.

   *  Extension Field: SRT magic code 0x4A17

   *  Handshake Type: INDUCTION

   *  SRT Socket ID: Socket ID of the Listener

   *  SYN Cookie: a cookie that is crafted based on host, port and
      current time with 1 minute accuracy to avoid SYN flooding attack
      [RFC4987]

   At this point the Listener still does not know if the Caller is SRT
   or UDT, and it responds with the same set of values regardless of
   whether the Caller is SRT or UDT.

   If the party is SRT, it does interpret the values in Version and
   Extension Field.  If it receives the value 5 in Version, it
   understands that it comes from an SRT party, so it knows that it
   should prepare the proper handshake messages phase.  It also checks
   the following:

   *  whether the Extension Flags contains the magic value 0x4A17;
      otherwise the connection is rejected.  This is a contingency for
      the case where someone who, in an attempt to extend UDT
      independently, increases the Version value to 5 and tries to test
      it against SRT.

   *  whether the Encryption Flags contain a non-zero value, which is
      interpreted as an advertised cipher family and block size.

   A legacy UDT party completely ignores the values reported in Version
   and Handshake Type.  It is, however, interested in the SYN Cookie
   value, as this must be passed to the next phase.  It does interpret
   these fields, but only in the "conclusion" message.

4.3.1.2.  The Conclusion Phase

   Once the Caller gets the SYN cookie from the Listener, it sends the
   CONCLUSION handshake to the Listener.

   The following values are set by the compliant caller:

   *  Version: 5

   *  Handshake Type: CONCLUSION

   *  SRT Socket ID: Socket ID of the Caller



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   *  SYN Cookie: the cookie previously received in the induction phase

   The Destination Socket ID in this message is the socket ID that was
   previously received in the induction phase in the SRT Socket ID field
   of the handshake structure.

   *  Encryption Flags: advertised cipher family and block size.

   *  Extension Flags: A set of flags that define the extensions
      provided in the handshake.

   The Listener responds with the same values shown above, without the
   cookie (which is not needed here), as well as the extensions for HS
   Version 5 (which will probably be exactly the same).

   There is not any "negotiation" here.  If the values passed in the
   handshake are in any way not acceptable by the other side, the
   connection will be rejected.  The only case when the Listener can
   have precedence over the Caller is the advertised Cipher Family and
   Block Size (Table 2) in the Encryption Field of the Handshake.

   The value for latency is always agreed to be the greater of those
   reported by each party.

4.3.2.  Rendezvous Handshake

   The Rendezvous process uses a state machine.  It is slightly
   different from UDT Rendezvous handshake [GHG04b], although it is
   still based on the same message request types.

   Both parties start with WAVEAHAND and use the Version value of 5.
   Legacy Version 4 clients do not look at the Version value, whereas
   Version 5 clients can detect version 5.  The parties only continue
   with the Version 5 Rendezvous process when Version is set to 5 for
   both.  Otherwise the process continues exclusively according to
   Version 4 rules [GHG04b].

   With Version 5 Rendezvous, both parties create a cookie for a process
   called the "cookie contest".  This is necessary for the assignment of
   Initiator and Responder roles.  Each party generates a cookie value
   (a 32-bit number) based on the host, port, and current time with 1
   minute accuracy.  This value is scrambled using an MD5 sum
   calculation.  The cookie values are then compared with one another.

   Since it is impossible to have two sockets on the same machine bound
   to the same NIC and port and operating independently, it is virtually
   impossible that the parties will generate identical cookies.
   However, this situation may occur if an application tries to "connect



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   to itself" - that is, either connects to a local IP address, when the
   socket is bound to INADDR_ANY, or to the same IP address to which the
   socket was bound.  If the cookies are identical (for any reason), the
   connection will not be made until new, unique cookies are generated
   (after a delay of up to one minute).  In the case of an application
   "connecting to itself", the cookies will always be identical, and so
   the connection will never be established.

   When one party's cookie value is greater than its peer's, it wins the
   cookie contest and becomes Initiator (the other party becomes the
   Responder).

   At this point there are two possible "handshake flows": serial and
   parallel.

4.3.2.1.  Serial Handshake Flow

   In the serial handshake flow, one party is always first, and the
   other follows.  That is, while both parties are repeatedly sending
   WAVEAHAND messages, at some point one party - let's say Alice - will
   find she has received a WAVEAHAND message before she can send her
   next one, so she sends a CONCLUSION message in response.  Meantime,
   Bob (Alice's peer) has missed Alice's WAVEAHAND messages, so that
   Alice's CONCLUSION is the first message Bob has received from her.

   This process can be described easily as a series of exchanges between
   the first and following parties (Alice and Bob, respectively):

   1.  Initially, both parties are in the waving state.  Alice sends a
       handshake message to Bob:

       *  Version: 5

       *  Type: Extension field: 0, Encryption field: advertised
          "PBKEYLEN".

       *  Handshake Type: WAVEAHAND

       *  SRT Socket ID: Alice's socket ID

       *  SYN Cookie: Created based on host/port and current time.

   While Alice does not yet know if she is sending this message to a
   Version 4 or Version 5 peer, the values from these fields would not
   be interpreted by the Version 4 peer when the Handshake Type is
   WAVEAHAND.





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   1.  Bob receives Alice's WAVEAHAND message, switches to the
       "attention" state.  Since Bob now knows Alice's cookie, he
       performs a "cookie contest" (compares both cookie values).  If
       Bob's cookie is greater than Alice's, he will become the
       Initiator.  Otherwise, he will become the Responder.

   The resolution of the Handshake Role (Initiator or Responder) is
   essential for further processing.

   Then Bob responds:

   *  Version: 5

   *  Extension field: appropriate flags if Initiator, otherwise 0

   *  Encryption field: advertised PBKEYLEN

   *  Handshake Type: CONCLUSION

   If Bob is the Initiator and encryption is on, he will use either his
   own cipher family and block size or the one received from Alice (if
   she has advertised those values).

   1.  Alice receives Bob's CONCLUSION message.  While at this point she
       also performs the "cookie contest", the outcome will be the same.
       She switches to the "fine" state, and sends:

       *  Version: 5

       *  Appropriate extension flags and encryption flags

       *  Handshake Type: CONCLUSION

   Both parties always send extension flags at this point, which will
   contain HSREQ if the message comes from an Initiator, or HSRSP if it
   comes from a Responder.  If the Initiator has received a previous
   message from the Responder containing an advertised cipher family and
   block size in the encryption flags field, it will be used as the key
   length for key generation sent next in the KMREQ extension.

   1.  Bob receives Alice's CONCLUSION message, and then does one of the
       following (depending on Bob's role):

       *  If Bob is the Initiator (Alice's message contains HSRSP), he:

          -  switches to the "connected" state





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          -  sends Alice a message with Handshake Type AGREEMENT, but
             containing no SRT extensions (Extension Flags field should
             be 0)

       *  If Bob is the Responder (Alice's message contains HSREQ), he:

          -  switches to "initiated" state

          -  sends Alice a message with Handshake Type CONCLUSION that
             also contains extensions with HSRSP

             o  awaits a confirmation from Alice that she is also
                connected (preferably by AGREEMENT message)

   2.  Alice receives the above message, enters into the "connected"
       state, and then does one of the following (depending on Alice's
       role):

       *  If Alice is the Initiator (received CONCLUSION with HSRSP),
          she sends Bob a message with Handshake Type = AGREEMENT.

       *  If Alice is the Responder, the received message has Handshake
          Type AGREEMENT and in response she does nothing.

   3.  At this point, if Bob was Initiator, he is connected already.  If
       he was a Responder, he should receive the above AGREEMENT
       message, after which he switches to the "connected" state.  In
       the case where the UDP packet with the agreement message gets
       lost, Bob will still enter the "connected" state once he receives
       anything else from Alice.  If Bob is going to send, however, he
       has to continue sending the same CONCLUSION until he gets the
       confirmation from Alice.

4.3.2.2.  Parallel Handshake Flow

   The chances of the parallel handshake flow are very low, but still it
   may occur if the handshake messages with WAVEAHAND are sent and
   received by both peers at precisely the same time.

   The resulting flow is very much like Bob's behaviour in the serial
   handshake flow, but for both parties.  Alice and Bob will go through
   the same state transitions:

   Waving -> Attention -> Initiated -> Connected

   In the Attention state they know each other's cookies, so they can
   assign roles.  In contrast to serial flows, which are mostly based on
   request-response cycles, here everything happens completely



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   asynchronously: the state switches upon reception of a particular
   handshake message with appropriate contents (the Initiator MUST
   attach the HSREQ extension, and Responder MUST attach the "HSRSP"
   extension).

   Here's how the parallel handshake flow works, based on roles:

   Initiator:

   1.  Waving

       *  Receives WAVEAHAND message

       *  Switches to Attention

       *  Sends CONCLUSION + HSREQ

   2.  Attention

       *  Receives CONCLUSION message, which:

          -  contains no extensions:

             o  switches to Initiated, still sends CONCLUSION + HSREQ

          -  contains "HSRSP" extension:

             o  switches to Connected, sends AGREEMENT

   3.  Initiated

       *  Receives CONCLUSION message, which:

          -  Contains no extensions:

             o  REMAINS IN THIS STATE, still sends CONCLUSION + HSREQ

          -  contains "HSRSP" extension:

             o  switches to Connected, sends AGREEMENT

   4.  Connected

       *  May receive CONCLUSION and respond with AGREEMENT, but
          normally by now it should already have received payload
          packets.

   Responder:



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   1.  Waving

       *  Receives WAVEAHAND message

       *  Switches to Attention

       *  Sends CONCLUSION message (with no extensions)

   2.  Attention

       *  Receives CONCLUSION message with HSREQ.  This message might
          contain no extensions, in which case the party shall simply
          send the empty CONCLUSION message, as before, and remain in
          this state.

       *  Switches to Initiated and sends CONCLUSION message with HSRSP

   3.  Initiated

       *  Receives:

          -  CONCLUSION message with HSREQ

             o  responds with CONCLUSION with HSRSP and remains in this
                state

          -  AGREEMENT message

             o  responds with AGREEMENT and switches to Connected

          -  Payload packet

             o  responds with AGREEMENT and switches to Connected

   4.  Connected

       *  Is not expecting to receive any handshake messages anymore.
          The AGREEMENT message is always sent only once or per every
          final CONCLUSION message.

   Note that any of these packets may be missing, and the sending party
   will never become aware.  The missing packet problem is resolved this
   way:

   1.  If the Responder misses the CONCLUSION + HSREQ message, it simply
       continues sending empty CONCLUSION messages.  Only upon reception
       of CONCLUSION + HSREQ does it respond with CONCLUSION + HSRSP.




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   2.  If the Initiator misses the CONCLUSION + HSRSP response from the
       Responder, it continues sending CONCLUSION + HSREQ.  The
       Responder MUST always respond with CONCLUSION + HSRSP when the
       Initiator sends CONCLUSION + HSREQ, even if it has already
       received and interpreted it.

   3.  When the Initiator switches to the Connected state it responds
       with a AGREEMENT message, which may be missed by the Responder.
       Nonetheless, the Initiator may start sending data packets because
       it considers itself connected - it does not know that the
       Responder has not yet switched to the Connected state.  Therefore
       it is exceptionally allowed that when the Responder is in the
       Initiated state and receives a data packet (or any control packet
       that is normally sent only between connected parties) over this
       connection, it may switch to the Connected state just as if it
       had received a AGREEMENT message.

   4.  If the the Initiator has already switched to the Connected state
       it will not bother the Responder with any more handshake
       messages.  But the Responder may be completely unaware of that
       (having missed the AGREEMENT message from the Initiator).
       Therefore it does not exit the connecting state, which means that
       it continues sending CONCLUSION + HSRSP messages until it
       receives any packet that will make it switch to the Connected
       state (normally AGREEMENT).  Only then does it exit the
       connecting state and the application can start transmission.

4.4.  SRT Buffer Latency

   The SRT sender and receiver have buffers to store packets.

   On the sender, latency is the time that SRT holds a packet to give it
   a chance to be delivered successfully while maintaining the rate of
   the sender at the receiver.  If an acknowledgment (ACK) is missing or
   late for more than the configured latency, the packet is dropped from
   the sender buffer.  A packet can be retransmitted as long as it
   remains in the buffer for the duration of the latency window.  On the
   receiver, packets are delivered to an application from a buffer after
   the latency interval has passed.  This helps to recover from
   potential packet losses.  See Section 4.5, Section 4.6 for details.

   Latency is a value, in milliseconds, that can cover the time to
   transmit hundreds or even thousands of packets at high bitrate.
   Latency can be thought of as a window that slides over time, during
   which a number of activities take place, such as the reporting of
   acknowledged packets (ACKs) (Section 4.8.1) and unacknowledged
   packets (NAKs)(Section 4.8.2).




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   Latency is configured through the exchange of capabilities during the
   extended handshake process between initiator and responder.  The
   Handshake Extension Message (Section 3.2.1.1) has TSBPD delay
   information, in milliseconds, from the SRT receiver and sender.  The
   latency for a connection will be established as the maximum value of
   latencies proposed by the initiator and responder.

4.5.  Timestamp-Based Packet Delivery

   The goal of the SRT Timestamp-Based Packet Delivery (TSBPD) mechanism
   is to reproduce the output of the sending application (e.g., encoder)
   at the input of the receiving application (e.g., decoder) in live
   data transmission mode (see Section 4.2).  It attempts to reproduce
   the timing of packets committed by the sending application to the SRT
   sender.  This allows packets to be scheduled for delivery by the SRT
   receiver, making them ready to be read by the receiving application
   (see Figure 17).

   The SRT receiver, using the timestamp of the SRT data packet header,
   delivers packets to a receiving application with a fixed minimum
   delay from the time the packet was scheduled for sending on the SRT
   sender side.  Basically, the sender timestamp in the received packet
   is adjusted to the receiver's local time (compensating for the time
   drift or different time zones) before releasing the packet to the
   application.  Packets can be withheld by the SRT receiver for a
   configured receiver delay.  A higher delay can accommodate a larger
   uniform packet drop rate, or a larger packet burst drop.  Packets
   received after their "play time" are dropped if the Too-Late Packet
   Drop feature is enabled (see Section 4.6).

   The packet timestamp, in microseconds, is relative to the SRT
   connection creation time.  Packets are inserted based on the sequence
   number in the header field.  The origin time, in microseconds, of the
   packet is already sampled when a packet is first submitted by the
   application to the SRT sender unless explicitly provided.  The TSBPD
   feature uses this time to stamp the packet for first transmission and
   any subsequent retransmission.  This timestamp and the configured SRT
   latency (Section 4.4) control the recovery buffer size and the
   instant that packets are delivered at the destination (the
   aforementioned "play time" which is decided by adding the timestamp
   to the configured latency).

   It is worth mentioning that the use of the packet sending time to
   stamp the packets is inappropriate for the TSBPD feature, since a new
   time (current sending time) is used for retransmitted packets,
   putting them out of order when inserted at their proper place in the
   stream.




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   Figure 17 illustrates the key latency points during the packet
   transmission with the TSBPD feature enabled.

                 |  Sending  |              |                   |
                 |   Delay   |    ~RTT/2    |    SRT Latency    |
                 |<--------->|<------------>|<----------------->|
                 |           |              |                   |
                 |           |              |                   |
                 |           |              |                   |
       ___ Scheduled       Sent         Received           Scheduled
      /    for sending       |              |              for delivery
   Packet        |           |              |                   |
   State         |           |              |                   |
                 |           |              |                   |
                 |           |              |                   |
                 ----------------------------------------------------->
                                                                   Time

        Figure 17: Key Latency Points during the Packet Transmission

   The main packet states shown in Figure 17 are the following:

   *  "Scheduled for sending": the packet is committed by the sending
      application, stamped and ready to be sent;

   *  "Sent": the packet is passed to the UDP socket and sent;

   *  "Received": the packet is received and read from the UDP socket;

   *  "Scheduled for delivery": the packet is scheduled for the delivery
      and ready to be read by the receiving application.

   It is worth noting that the round-trip time (RTT) of an SRT link may
   vary in time.  However the actual end-to-end latency on the link
   becomes fixed and is approximately equal to (RTT_0/2 + SRT Latency)
   once the SRT handshake exchange happens, where RTT_0 is the actual
   value of the round-trip time during the SRT handshake exchange (the
   value of the round-trip time once the SRT connection has been
   established).

   The value of sending delay depends on the hardware performance.
   Usually it is relatively small (several microseconds) in contrast to
   RTT_0/2 and SRT latency which are measured in milliseconds.








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4.5.1.  Packet Delivery Time

   Packet delivery time is the moment, estimated by the receiver, when a
   packet should be delivered to the upstream application.  The
   calculation of packet delivery time (PktTsbpdTime) is performed upon
   receiving a data packet according to the following formula:

   PktTsbpdTime = TsbpdTimeBase + PKT_TIMESTAMP + TsbpdDelay + Drift

   where

   *  TsbpdTimeBase is the time base that reflects the time difference
      between local clock of the receiver and the clock used by the
      sender to timestamp packets being sent (see Section 4.5.1.1);

   *  PKT_TIMESTAMP is the data packet timestamp, in microseconds;

   *  TsbpdDelay is the receiver's buffer delay (or receiver's buffer
      latency, or SRT Latency).  This is the time, in milliseconds, that
      SRT holds a packet from the moment it has been received till the
      time it should be delivered to the upstream application;

   *  Drift is the time drift used to adjust the fluctuations between
      sender and receiver clock, in microseconds.

   SRT Latency (TsbpdDelay) should be a buffer time large enough to
   cover the unexpectedly extended RTT time, and the time needed to
   retransmit the lost packet.  The value of minimum TsbpdDelay is
   negotiated during the SRT handshake exchange and is equal to 120
   milliseconds.  The recommended value of TsbpdDelay is 3-4 times RTT.

   It is worth noting that TsbpdDelay limits the number of packet
   retransmissions to a certain extent making impossible to retransmit
   packets endlessly.  This is important for live data transmission.

4.5.1.1.  TSBPD Time Base Calculation

   The initial value of TSBPD time base (TsbpdTimeBase) is calculated at
   the moment of the second handshake request is received as follows:

   TsbpdTimeBase = T_NOW - HSREQ_TIMESTAMP

   where T_NOW is the current time according to the receiver clock;
   HSREQ_TIMESTAMP is the handshake packet timestamp, in microseconds.

   The value of TsbpdTimeBase is approximately equal to the initial one-
   way delay of the link RTT_0/2, where RTT_0 is the actual value of the
   round-trip time during the SRT handshake exchange.



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   During the transmission process, the value of TSBPD time base may be
   adjusted in two cases:

   1.  During the TSBPD wrapping period.  The TSBPD wrapping period
       happens every 01:11:35 hours.  This time corresponds to the
       maximum timestamp value of a packet (MAX_TIMESTAMP).
       MAX_TIMESTAMP is equal to 0xFFFFFFFF, or the maximum value of
       32-bit unsigned integer, in microseconds (Section 3).  The TSBPD
       wrapping period starts 30 seconds before reaching the maximum
       timestamp value of a packet and ends once the packet with
       timestamp within (30, 60) seconds interval is delivered (read
       from the buffer).  The updated value of TsbpdTimeBase will be
       recalculated as follows:

      TsbpdTimeBase = TsbpdTimeBase + MAX_TIMESTAMP + 1

   2.  By drift tracer.  See Section 4.7 for details.

4.6.  Too-Late Packet Drop

   The Too-Late Packet Drop (TLPKTDROP) mechanism allows the sender to
   drop packets that have no chance to be delivered in time, and allows
   the receiver to skip missing packets that have not been delivered in
   time.  The timeout of dropping a packet is based on the TSBPD
   mechanism (see Section 4.5).

   In the SRT, when Too-Late Packet Drop is enabled, and a packet
   timestamp is older than 125% of the SRT latency, it is considered too
   late to be delivered and may be dropped by the sender.  However, the
   sender keeps packets for at least 1 second in case the SRT latency is
   not enough for a large RTT (that is, if 125% of the SRT latency is
   less than 1 second).

   When enabled on the receiver, the receiver drops packets that have
   not been delivered or retransmitted in time, and delivers the
   subsequent packets to the application when it is their time to play.

   In pseudo-code, the algorithm of reading from the receiver buffer is
   the following:












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   <CODE BEGINS>
   pos = 0;  /* Current receiver buffer position */
   i = 0;    /* Position of the next available in the receiver buffer
                packet relatively to the current buffer position pos */

   while(True) {
       // Get the position i of the next available packet
       // in the receiver buffer
       i = next_avail();
       // Calculate packet delivery time PktTsbpdTime
       // for the next available packet
       PktTsbpdTime = delivery_time(i);

       if T_NOW < PktTsbpdTime:
           continue;

       Drop packets which buffer position number is less than i;

       Deliver packet with the buffer position i;

       pos = i + 1;
   }
   <CODE ENDS>

   where T_NOW is the current time according to the receiver clock.

   The TLPKTDROP mechanism can be turned off to always ensure a clean
   delivery.  However, a lost packet can simply pause a delivery for
   some longer, potentially undefined time, and cause even worse tearing
   for the player.  Setting higher SRT latency will help much more in
   the case when TLPKTDROP causes packet drops too often.

4.7.  Drift Management

   When the sender enters "connected" status it tells the application
   there is a socket interface that is transmitter-ready.  At this point
   the application can start sending data packets.  It adds packets to
   the SRT sender's buffer at a certain input rate, from which they are
   transmitted to the receiver at scheduled times.












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   A synchronized time is required to keep proper sender/receiver buffer
   levels, taking into account the time zone and round-trip time (up to
   2 seconds for satellite links).  Considering addition/subtraction
   round-off, and possibly unsynchronized system times, an agreed-upon
   time base drifts by a few microseconds every minute.  The drift may
   accumulate over many days to a point where the sender or receiver
   buffers will overflow or deplete, seriously affecting the quality of
   the video.  SRT has a time management mechanism to compensate for
   this drift.

   When a packet is received, SRT determines the difference between the
   time it was expected and its timestamp.  The timestamp is calculated
   on the receiver side.  The RTT tells the receiver how much time it
   was supposed to take.  SRT maintains a reference between the time at
   the leading edge of the send buffer's latency window and the
   corresponding time on the receiver (the present time).  This allows
   to convert packet timestamp to the local receiver time.  Based on
   this time, various events (packet delivery, etc.) can be scheduled.

   The receiver samples time drift data and periodically calculates a
   packet timestamp correction factor, which is applied to each data
   packet received by adjusting the inter-packet interval.  When a
   packet is received it is not given right away to the application.  As
   time advances, the receiver knows the expected time for any missing
   or dropped packet, and can use this information to fill any "holes"
   in the receive queue with another packet (see Section 4.5).

   It is worth noting that the period of sampling time drift data is
   based on a number of packets rather than time duration to ensure
   enough samples, independently of the media stream packet rate.  The
   effect of network jitter on the estimated time drift is attenuated by
   using a large number of samples.  The actual time drift being very
   slow (affecting a stream only after many hours) does not require a
   fast reaction.

   The receiver uses local time to be able to schedule events -- to
   determine, for example, if it is time to deliver a certain packet
   right away.  The timestamps in the packets themselves are just
   references to the beginning of the session.  When a packet is
   received (with a timestamp from the sender), the receiver makes a
   reference to the beginning of the session to recalculate its
   timestamp.  The start time is derived from the local time at the
   moment that the session is connected.  A packet timestamp equals
   "now" minus "StartTime", where the latter is the point in time when
   the socket was created.






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4.8.  Acknowledgement and Lost Packet Handling

   To enable the Automatic Repeat reQuest of data packet
   retransmissions, a sender stores all sent data packets in its buffer.

   The SRT receiver periodically sends acknowledgments (ACKs) for the
   received data packets so that the SRT sender can remove the
   acknowledged packets from its buffer (Section 4.8.1).  Once the
   acknowledged packets are removed, their retransmission is no longer
   possible and presumably not needed.

   Upon receiving the full acknowledgment (ACK) control packet, the SRT
   sender should acknowledge its reception to the receiver by sending an
   ACKACK control packet with the sequence number of the full ACK packet
   being acknowledged.

   The SRT receiver also sends NAK control packets to notify the sender
   about the missing packets (Section 4.8.2).  The sending of a NAK
   packet can be triggered immediately after a gap in sequence numbers
   of data packets is detected.  In addition, a Periodic NAK report
   mechanism can be used to send NAK reports periodically.  The NAK
   packet in that case will list all the packets that the receiver
   considers being lost up to the moment the Periodic NAK report is
   sent.

   Upon reception of the NAK packet, the SRT sender prioritizes
   retransmissions of lost packets over the regular data packets to be
   transmitted for the first time.

   The retransmission of the missing packet is repeated until the
   receiver acknowledges its receipt, or if both peers agree to drop
   this packet (see Section 4.6).

4.8.1.  Packet Acknowledgement (ACKs, ACKACKs)

   At certain intervals (see below), the SRT receiver sends an
   acknowledgment (ACK) that causes the acknowledged packets to be
   removed from the SRT sender's buffer.

   An ACK control packet contains the sequence number of the packet
   immediately following the latest in the list of received packets.
   Where no packet loss has occurred up to the packet with sequence
   number n, an ACK would include the sequence number (n + 1).

   An ACK (from a receiver) will trigger the transmission of an ACKACK
   (by the sender), with almost no delay.  The time it takes for an ACK
   to be sent and an ACKACK to be received is the RTT.  The ACKACK tells
   the receiver to stop sending the ACK position because the sender



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   already knows it.  Otherwise, ACKs (with outdated information) would
   continue to be sent regularly.  Similarly, if the sender does not
   receive an ACK, it does not stop transmitting.

   There are two conditions for sending an acknowledgment.  A full ACK
   is based on a timer of 10 milliseconds (the ACK period or
   synchronization time interval SYN).  For high bitrate transmissions,
   a "light ACK" can be sent, which is an ACK for a sequence of packets.
   In a 10 milliseconds interval, there are often so many packets being
   sent and received that the ACK position on the sender does not
   advance quickly enough.  To mitigate this, after 64 packets (even if
   the ACK period has not fully elapsed) the receiver sends a light ACK.
   A light ACK is a shorter ACK (SRT header and one 32-bit field).  It
   does not trigger an ACKACK.

   When a receiver encounters the situation where the next packet to be
   played was not successfully received from the sender, it will "skip"
   this packet (see Section 4.6) and send a fake ACK.  To the sender,
   this fake ACK is a real ACK, and so it just behaves as if the packet
   had been received.  This facilitates the synchronization between SRT
   sender and receiver.  The fact that a packet was skipped remains
   unknown by the sender.  Skipped packets are recorded in the
   statistics on the SRT receiver.

4.8.2.  Packet Retransmission (NAKs)

   The SRT receiver sends NAK control packets to notify the sender about
   the missing packets.  The NAK packet sending can be triggered
   immediately after a gap in sequence numbers of data packets is
   detected.

   Upon reception of the NAK packet, the SRT sender prioritizes
   retransmissions of lost packets over the regular data packets to be
   transmitted for the first time.

   The SRT sender maintains a list of lost packets (loss list) that is
   built from NAK reports.  When scheduling packet transmission, it
   looks to see if a packet in the loss list has priority and sends it
   if so.  Otherwise, it sends the next packet scheduled for the first
   transmission list.  Note that when a packet is transmitted, it stays
   in the buffer in case it is not received by the SRT receiver.

   NAK packets are processed to fill in the loss list.  As the latency
   window advances and packets are dropped from the sending queue, a
   check is performed to see if any of the dropped or resent packets are
   in the loss list, to determine if they can be removed from there as
   well so that they are not retransmitted unnecessarily.




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   There is a counter for the packets that are resent.  If there is no
   ACK for a packet, it will stay in the loss list and can be resent
   more than once.  Packets in the loss list are prioritized.

   If packets in the loss list continue to block the send queue, at some
   point this will cause the send queue to fill.  When the send queue is
   full, the sender will begin to drop packets without even sending them
   the first time.  An encoder (or other application) may continue to
   provide packets, but there's no place for them, so they will end up
   being thrown away.

   This condition where packets are unsent does not happen often.  There
   is a maximum number of packets held in the send buffer based on the
   configured latency.  Older packets that have no chance to be
   retransmitted and played in time are dropped, making room for newer
   real-time packets produced by the sending application.  See
   Section 4.5, Section 4.6 for details.

   In addition to the regular NAKs, the Periodic NAK report mechanism
   can be used to send NAK reports periodically.  The NAK packet in that
   case will have all the packets that the receiver considers being lost
   at the time of sending the Periodic NAK report.

   SRT Periodic NAK reports are sent with a period of (RTT + 4 * RTTVar)
   / 2 (so called NAKInterval), with a 20 milliseconds floor, where RTT
   and RTTVar are defined in section Section 4.10.  A NAK control packet
   contains a compressed list of the lost packets.  Therefore, only lost
   packets are retransmitted.  By using NAKInterval for the NAK reports
   period, it may happen that lost packets are retransmitted more than
   once, but it helps maintain low latency in the case where NAK packets
   are lost.

   An ACKACK tells the receiver to stop sending the ACK position because
   the sender already knows it.  Otherwise, ACKs (with outdated
   information) would continue to be sent regularly.

   An ACK serves as a ping, with a corresponding ACKACK pong, to measure
   RTT.  The time it takes for an ACK to be sent and an ACKACK to be
   received is the RTT.  Each ACK has a number.  A corresponding ACKACK
   has that same number.  The receiver keeps a list of all ACKs in a
   queue to match them.  Unlike a full ACK, which contains the current
   RTT and several other values in the Control Information Field (CIF)
   (Section 3.2.4), a light ACK just contains the sequence number.  All
   control messages are sent directly and processed upon reception, but
   ACKACK processing time is negligible (the time this takes is included
   in the round-trip time).





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4.9.  Bidirectional Transmission Queues

   Once an SRT connection is established, both peers can send data
   packets simultaneously.

4.10.  Round-Trip Time Estimation

   Round-trip time (RTT) in SRT is estimated during the transmission of
   data packets based on a difference in time between an ACK packet is
   sent out and a corresponding ACKACK packet is received back by the
   SRT receiver.

   An ACK sent by the receiver triggers an ACKACK from the sender with
   minimal processing delay.  The ACKACK response is expected to arrive
   at the receiver roughly one RTT after the corresponding ACK was sent.

   The SRT receiver records the time when an ACK is sent out.  The ACK
   carries a unique sequence number (independent of the data packet
   sequence number).  The corresponding ACKACK also carries the same
   sequence number.  Upon receiving the ACKACK, SRT calculates the RTT
   by comparing the difference between the ACKACK arrival time and the
   ACK departure time.  In the following formula, RTT is the current
   value that the receiver maintains and rtt is the recent value that
   was just calculated from an ACK/ACKACK pair:

   RTT = RTT * 0.875 + rtt * 0.125

   RTT variance RTTVar is obtained as follows:

   RTTVar = RTTVar * 0.75 + abs(RTT - rtt) * 0.25

   where abs() means an absolute value.

   Both RTT and RTTVar are measured in microseconds.  The initial value
   of RTT is 100 milliseconds, RTTVar is 50 milliseconds.

   The smoothed RTT calculated by the receiver as well as the RTT
   variance RTTVar are sent with the next full acknowledgement packet
   (see Section 3.2.4).  Note that the first ACK in an SRT session might
   contain an initial RTT value of 100 milliseconds, because the early
   calculations may not be precise.

   The sender always gets the RTT from the receiver.  It does not have
   an analog to the ACK/ACKACK mechanism, i.e. it can not send a message
   that guarantees an immediate return without processing.  Upon an ACK
   reception, the SRT sender updates its own RTT and RTTVar values using
   the same formulas as above, in which case rtt is the most recent
   value it receives, i.e., carried by an incoming ACK.



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   Note that an SRT socket can both send and receive data packets.  RTT
   and RTTVar are updated by the socket based on algorithms for the
   sender (using ACK packets) and for the receiver (using ACK-ACKACK
   pairs).  When an SRT socket receives data, it updates its local RTT
   and RTTVar, which can be used for its own sender as well.

4.11.  Congestion Control

   SRT provides certain mechanisms for the sender to get some feedback
   from the receiving side through the ACK packets (Section 3.2.4).
   Every 10 ms the sender receives the latest values of RTT and RTT
   variance, Available Buffer Size, Packets Receiving Rate and Estimated
   Link Capacity.  Upon reception of the NAK packet (Section 3.2.5) the
   sender can detect packet losses during the transmission.  These
   mechanisms provide a solid background for various congestion control
   algorithms.

   Given that SRT can operate in live and file transfer modes, there are
   two groups of congestion control algorithms possible.

   For live transmission mode (Section 4.2.2) the congestion control
   algorithm does not need to control the sending pace of the data
   packets, as the sending timing is provided by the live input.
   Although certain limitations on the minimal inter-sending time of
   consecutive packets can be applied in order to avoid congestion
   during fluctuations of the source bitrate.  Also it is allowed to
   drop those packets that can not be delivered in time.

   For file transfer, any known File Congestion Control algorithms like
   CUBIC [RFC8312] and BBR [BBR] can apply, including the congestion
   control mechanism proposed in UDT [GHG04b], [GuAnAO].  The UDT
   congestion control relies on the available link capacity, packet loss
   reports (NAK) and packet acknowledgements (ACKs).  It then slows down
   the output of packets as needed by adjusting the packet sending pace.
   In periods of congestion, it can block the main stream and focus on
   the lost packets.

5.  Encryption

   This section describes the encryption mechanism that protects the
   payload of SRT streams.  Based on standard cryptographic algorithms,
   the mechanism allows an efficient stream cipher with a key
   establishment method.








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5.1.  Overview

   SRT implements encryption using AES [AES] in counter mode (AES-CTR)
   [SP800-38A] with a short-lived key to encrypt and decrypt the media
   stream.  The AES-CTR cipher is suitable for continuous stream
   encryption that permits decryption from any point, without access to
   start of the stream (random access), and for the same reason
   tolerates packet loss.  It also offers strong confidentiality when
   the counter is managed properly.

5.1.1.  Encryption Scope

   SRT encrypts only the payload of SRT data packets (Section 3.1),
   while the header is left unencrypted.  The unencrypted header
   contains the Packet Sequence Number field used to keep the
   synchronization of the cipher counter between the encrypting sender
   and the decrypting receiver.  No constraints apply to the payload of
   SRT data packets as no padding of the payload is required by counter
   mode ciphers.

5.1.2.  AES Counter

   The counter for AES-CTR is the size of the cipher's block, i.e. 128
   bits.  It is derived from a 128-bit sequence consisting of

   *  a block counter in the least significant 16 bits, which counts the
      blocks in a packet,

   *  a packet index - based on the packet sequence number in the SRT
      header - in the next 32 bits,

   *  eighty zeroed bits.

   The upper 112 bits of this sequence are XORed with an Initialization
   Vector (IV) to produce a unique counter for each crypto block.  The
   IV is derived from the Salt provided in the Keying Material
   (Section 3.2.2):

   IV = MSB(112, Salt): Most significant 112 bits of the salt.

5.1.3.  Stream Encrypting Key (SEK)

   The key used for AES-CTR encryption is called the "Stream Encrypting
   Key" (SEK).  It is used for up to 2^25 packets with further rekeying.
   The short-lived SEK is generated by the sender using a pseudo-random
   number generator (PRNG), and transmitted within the stream, wrapped
   with another longer-term key, the Key Encrypting Key (KEK), using a
   known AES key wrap protocol.



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   For connection-oriented transport such as SRT, there is no need to
   periodically transmit the short-lived key since no additional party
   can join a stream in progress.  The keying material is transmitted
   within the connection handshake packets, and for a short period when
   rekeying occurs.

5.1.4.  Key Encrypting Key (KEK)

   The Key Encrypting Key (KEK) is derived from a secret (passphrase)
   shared between the sender and the receiver.  The KEK provides access
   to the Stream Encrypting Key, which in turn provides access to the
   protected payload of SRT data packets.  The KEK has to be at least as
   long as the SEK.

   The KEK is generated by a password-based key generation function
   (PBKDF2) [RFC2898], using the passphrase, a number of iterations
   (2048), a keyed-hash (HMAC-SHA1) [RFC2104], and a key length value
   (KLen).  The PBKDF2 function hashes the passphrase to make a long
   string, by repetition or padding.  The number of iterations is based
   on how much time can be given to the process without it becoming
   disruptive.

5.1.5.  Key Material Exchange

   The KEK is used to generate a wrap [RFC3394] that is put in a key
   material (KM) message by the initiator of a connection (i.e. caller
   in caller-listener handshake and initiator in the rendezvous
   handshake, see Section 4.3) to send to the responder (listener).  The
   KM message contains the key length, the salt (one of the arguments
   provided to the PBKDF2 function), the protocol being used (e.g.  AES-
   256) and the AES counter (which will eventually change, see
   Section 5.1.6).

   On the other side, the responder attempts to decode the wrap to
   obtain the Stream Encrypting Key. In the protocol for the wrap there
   is a padding, which is a known template, so the responder knows from
   the KM that it has the right KEK to decode the SEK.  The SEK
   (generated and transmitted by the initiator) is random, and cannot be
   known in advance.  The KEK formula is calculated on both sides, with
   the difference that the responder gets the key length (KLen) from the
   initiator via the key material (KM).  It is the initiator who decides
   on the configured length.  The responder obtains it from the material
   sent by the initiator.

   The responder returns the same KM message to show that it has the
   same information as the initiator, and that the encoded material will
   be decrypted.  If the responder does not return this status, this
   means that it does not have the SEK.  All incoming encrypted packets



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   received by the responder will be lost (undecrypted).  Even if they
   are transmitted successfully, the receiver will be unable to decrypt
   them, and so packets will be dropped.  All data packets coming from
   responder will be unencrypted.

5.1.6.  KM Refresh

   The short lived SEK is regenerated for cryptographic reasons when a
   pre-determined number of packets has been encrypted.  The KM refresh
   period is determined by the implementation.  The receiver knows which
   SEK (odd or even) was used to encrypt the packet by means of the KK
   field of the SRT Data Packet (Section 3.1).

   There are two variables used to determine the KM Refresh timing:

   *  KM Refresh Period specifies the number of packets to be sent
      before switching to the new SEK,

   *  KM Pre-Announcement Period specifies when a new key is announced
      in a number of packets before key switchover.  The same value is
      used to determine when to decommission the old key after
      switchover.

   The recommended KM Refresh Period is after 2^25 packets encrypted
   with the same SEK are sent.  The recommended KM Pre-Announcement
   Period is 4000 packets (i.e. a new key is generated, wrapped, and
   sent at 2^25 minus 4000 packets; the old key is decommissioned at
   2^25 plus 4000 packets).

   Even and odd keys are alternated during transmission the following
   way.  The packets with the earlier key #1 (let it be the odd key)
   will continue to be sent.  The receiver will receive the new key #2
   (even), then decrypt and unwrap it.  The receiver will reply to the
   sender if it is able to understand.  Once the sender gets to the
   2^25th packet using the odd key (key #1), it will then start to send
   packets with the even key (key #2), knowing that the receiver has
   what it needs to decrypt them.  This happens transparently, from one
   packet to the next.  At 2^25 plus 4000 packets the first key will be
   decommissioned automatically.

   Both keys live in parallel for two times the Pre-Announcement Period
   (e.g. 4000 packets before the key switch, and 4000 packets after).
   This is to allow for packet retransmission.  It is possible for
   packets with the older key to arrive at the receiver a bit late.
   Each packet contains a description of which key it requires, so the
   receiver will still have the ability to decrypt it.





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5.2.  Encryption Process

5.2.1.  Generating the Stream Encrypting Key

   On the sending side SEK, Salt and KEK are generated the following
   way:

   SEK  = PRNG(KLen)
   Salt = PRNG(128)
   KEK = PBKDF2(passphrase, LSB(64,Salt), Iter, Klen)

   where

   *  PBKDF2 is the PKCS#5 Password Based Key Derivation Function
      [RFC2898],

   *  passphrase is the pre-shared passphrase,

   *  Salt is the field of the KM message,

   *  LSB(n, v) is the function taking n least significant bits of v,

   *  Iter=2048 defines the number of iterations for PBKDF2,

   *  KLen is the field of the KM message.

   Wrap = AESkw(KEK, SEK)

   where AESkw(KEK, SEK) is the key wrapping function [RFC3394].

5.2.2.  Encrypting the Payload

   The encryption of the payload of the SRT DATA packet is done with
   AES-CTR

   EncryptedPayload = AES_CTR_Encrypt(SEK, IV, UnencryptedPayload)

   where the Initialization Vector is derived as

   IV = (MSB(112, Salt) << 2) XOR (PktSeqNo)

   *  PktSeqNo is the value of the Packet Sequence Number field of the
      SRT data packet.

5.3.  Decryption Process






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5.3.1.  Restoring the Stream Encrypting Key

   For the receiver to be able to decrypt the incoming stream it has to
   know the stream encrypting key (SEK) used by the sender.  The
   receiver must know the passphrase used by the sender.  The remaining
   information can be extracted from the Keying Material message.

   The Keying Material message contains the AES-wrapped [RFC3394] SEK
   used by the encoder.  The Key-Encryption Key (KEK) required to unwrap
   the SEK is calculated as:

   KEK = PBKDF2(passphrase, LSB(64,Salt), Iter, KLen)

   where

   *  PBKDF2 is the PKCS#5 Password Based Key Derivation Function
      [RFC2898],

   *  passphrase is the pre-shared passphrase,

   *  Salt is the field of the KM message,

   *  LSB(n, v) is the function taking n least significant bits of v,

   *  Iter=2048 defines the number of iterations for PBKDF2,

   *  KLen is the field of the KM message.

   SEK = AESkuw(KEK, Wrap)

   where AESkuw(KEK, Wrap) is the key unwrapping function.

5.3.2.  Decrypting the Payload

   The decryption of the payload of the SRT data packet is done with
   AES-CTR

   DecryptedPayload = AES_CTR_Encrypt(SEK, IV, EncryptedPayload)

   where the Initialization Vector is derived as

   IV = (MSB(112, Salt) << 2) XOR (PktSeqNo)

   *  PktSeqNo is the value of the Packet Sequence Number field of the
      SRT data packet.






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6.  Security Considerations

   SRT supports confidentiality of user data using stream ciphering
   based on AES.  Session keys for ciphering are delivered through
   control packets during handshake, with the protection by Key
   Encryption Key, which is generated by a sender and receiver with pre-
   shared secret such as passphrase.  As in UDT, careful uses of SYN
   Cookies may help to deter denial of service attacks.  Appropriate
   security policy including key size, key refresh period, as well as
   passphrase should be managed by security officers, which is out of
   scope of the present document.

7.  IANA Considerations

   This document makes no requests of the IANA.

Contributors

   This specification is heavily based on the SRT Protocol Technical
   Overview [SRTTO] written by Jean Dube and Steve Matthews.

   In alphabetical order, the contributors to the pre-IETF SRT project
   and specification at Haivision are: Marc Cymontkowski, Roman
   Diouskine, Jean Dube, Mikolaj Malecki, Steve Matthews, Maria
   Sharabayko, Maxim Sharabayko, Adam Yellen.

   The contributors to this specification at SK Telecom are Jeongseok
   Kim and Joonwoong Kim.

   We cannot list all the contributors to the open-sourced
   implementation of SRT on GitHub.  But we appreciate the help,
   contribution, integrations and feedback of the SRT and SRT Alliances
   community.

Acknowledgments

   The basis of the SRT protocol and its implementation was the UDP-
   based Data Transfer Protocol [GHG04b].  The authors thank Yunhong Gu
   and Robert Grossman, the authors of the UDP-based Data Transfer
   Protocol [GHG04b].

   TODO acknowledge.

References

Normative References





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   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [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>.

Informative References

   [AES]      National Institute of Standards and Technology, "FIPS Pub
              197: Advanced Encryption Standard (AES)", November 2001,
              <http://csrc.nist.gov/publications/fips/fips197/fips-
              197.pdf>.

   [AV1]      Rivaz, P.d. and J. Haughton, "AV1 Bitstream & Decoding
              Process Specification", September 2020,
              <https://aomediacodec.github.io/av1-spec/av1-spec.pdf>.

   [BBR]      Cardwell, N., Cheng, Y., Gunn, C.S., Yeganeh, S.H., and V.
              Jacobson, "BBR: Congestion-Based Congestion Control",
              October 2016.

   [GHG04b]   Gu, Y., Hong, X., and R.L. Grossman, "Experiences in
              Design and Implementation of a High Performance Transport
              Protocol", DOI 10.1109/SC.2004.24, December 2004,
              <https://doi.org/10.1109/SC.2004.24>.

   [GuAnAO]   Gu, Y., Hong, X., and R.L. Grossman, "An Analysis of AIMD
              Algorithm with Decreasing Increases", October 2004.

   [H.265]    International Telecommunications Union, "H.265 : High
              efficiency video coding", ITU-T Recommendation H.265,
              2019.

   [I-D.ietf-quic-http]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-29, 9 June 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-quic-http-29.txt>.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-29, 9 June 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-29.txt>.



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   [ISO13818-1]
              ISO, "Information technology -- Generic coding of moving
              pictures and associated audio information: Systems", ISO/
              IEC 13818-1, September 2020.

   [ISO23009] ISO, "Information technology -- Dynamic adaptive streaming
              over HTTP (DASH)", ISO/IEC 23009:2019, September 2020.

   [PNPID]    "PNP ID AND ACPI ID REGISTRY", September 2020,
              <https://uefi.org/PNP_ACPI_Registry>.

   [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>.

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898,
              DOI 10.17487/RFC2898, September 2000,
              <https://www.rfc-editor.org/info/rfc2898>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
              September 2002, <https://www.rfc-editor.org/info/rfc3394>.

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

   [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>.

   [RFC8216]  Pantos, R., Ed. and W. May, "HTTP Live Streaming",
              RFC 8216, DOI 10.17487/RFC8216, August 2017,
              <https://www.rfc-editor.org/info/rfc8216>.

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,
              <https://www.rfc-editor.org/info/rfc8312>.





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   [RTMP]     "Real-Time Messaging Protocol", September 2020,
              <https://www.adobe.com/devnet/rtmp.html>.

   [SP800-38A]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation", December 2001.

   [SRTSRC]   "SRT fully functional reference implementation", September
              2020, <https://github.com/Haivision/srt>.

   [SRTTO]    Dube, J. and S. Matthews, "SRT Protocol Technical
              Overview", December 2019.

   [VP9]      WebM, "VP9 Video Codec", September 2020,
              <https://www.webmproject.org/vp9>.

Appendix A.  Packet Sequence List Coding

   For any single packet sequence number, it uses the original sequence
   number in the field.  The first bit MUST start with "0".

    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|                   Sequence Number                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 18: Single sequence numbers coding

   For any consecutive packet sequence numbers that the difference
   between the last and first is more than 1, only record the first (a)
   and the the last (b) sequence numbers in the list field, and modify
   the the first bit of a to "1".

    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|                   Sequence Number a (first)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                   Sequence Number b (last)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 19: Range of sequence numbers coding








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Appendix B.  SRT Access Control

   One type of information that can be interchanged when a connection is
   being established in SRT is the Stream ID, which can be used in a
   caller-listener connection layout.  This is a string of maximum 512
   characters set on the caller side.  It can be retrieved at the
   listener side on the newly accepted connection.

   SRT listener can notify an upstream application about the connection
   attempt when a HS conclusion arrives, exposing the contents of the
   Stream ID extension message.  Based on this information, the
   application can accept or reject the connection, select the desired
   data stream, or set an appropriate passphrase for the connection.

   The Stream ID value can be used as free-form, but there is a
   recommended convention so that all SRT users speak the same language.
   The intent of the convention is to:

   *  promote readability and consistency among free-form names,

   *  interpret some typical data in the key-value style.

B.1.  General Syntax

   This recommended syntax starts with the characters known as an
   executable specification in POSIX: #!.

   The next two characters are:

   : - this marks the YAML format, the only one currently used
   The content format, which is either:
       : - the comma-separated keys with no nesting
       { - like above, but nesting is allowed and must end with }

   (Nesting means that you can have multiple level brace-enclosed parts
   inside.)

   The form of the key-value pair is:

   key1=value1,key2=value2...











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B.2.  Standard Keys

   Beside the general syntax, there are several top-level keys treated
   as standard keys.  All single letter key definitions, including those
   not listed in this section, are reserved for future use.  Users can
   additionally use custom key definitions with user_* or companyname_*
   prefixes, where user and companyname are to be replaced with an
   actual user or company name.

   The existing key values MUST not be extended, and MUST not differ
   from those described in this section.

   The following keys are standard:

   *  u: User Name, or authorization name, that is expected to control
      which password should be used for the connection.  The application
      should interpret it to distinguish which user should be used by
      the listener party to set up the password.

   *  r: Resource Name identifies the name of the resource and
      facilitates selection should the listener party be able to serve
      multiple resources.

   *  h: Host Name identifies the hostname of the resource.  For
      example, to request a stream with the URI somehost.com/videos/
      querry.php?vid=366 the hostname field should have somehost.com,
      and the resource name can have videos/querry.php?vid=366 or simply
      366.  Note that this is still a key to be specified explicitly.
      Support tools that apply simplifications and URI extraction are
      expected to insert only the host portion of the URI here.

   *  s: Session ID is a temporary resource identifier negotiated with
      the server, used just for verification.  This is a one-shot
      identifier, invalidated after the first use.  The expected usage
      is when details for the resource and authorization are negotiated
      over a separate connection first, and then the session ID is used
      here alone.

   *  t: Type specifies the purpose of the connection.  Several standard
      types are defined, but users may extend the use:

      -  stream (default, if not specified): for exchanging the user-
         specified payload for an application-defined purpose,

      -  file: for transmitting a file, where r is the filename,






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      -  auth: for exchanging sensible data.  The r value states its
         purpose.  No specific possible values for that are known so far
         (FUTURE USE).

   *  m: Mode expected for this connection:

      -  request (default): the caller wants to receive the stream,

      -  publish: the caller wants to send the stream data,

      -  bidirectional: bidirectional data exchange is expected.

   Note that "m" is not required in the case where Stream ID is not used
   to distinguish authorization or resources, and the caller is expected
   to send the data.  This is only for cases where the listener can
   handle various purposes of the connection and is therefore required
   to know what the caller is attempting to do.

B.3.  Examples

   The example content of the StreamID is:

   #!::u=admin,r=bluesbrothers1_hi

   It specifies the username and the resource name of the stream to be
   served to the caller.

   #!::u=johnny,t=file,m=publish,r=results.csv

   This specifies that the file is expected to be transmitted from the
   caller to the listener and its name is results.csv.

Appendix C.  Changelog

C.1.  Since Version 00

   *  Improved and extended the description of "Encryption" section,

   *  Improved and extended the description of "Round-Trip Time
      Estimation" section,

   *  Extended the description of "Handshake" section with "Stream ID
      Extension Message", "Group Membership Extension" subsections,

   *  Extended "Handshake Messages" section with the detailed
      description of handshake procedure,

   *  Improved "Key Material" section description,



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   *  Changed packet structure formatting for "Packet Structure"
      section,

   *  Did minor additions to the "Acknowledgement and Lost Packet
      Handling" section,

   *  Fixed broken links,

   *  Extended the list of references.

Authors' Addresses

   Maxim Sharabayko
   Haivision Network Video, GmbH

   Email: maxsharabayko@haivision.com


   Maria Sharabayko
   Haivision Network Video, GmbH

   Email: msharabayko@haivision.com


   Jean Dube
   Haivision

   Email: jdube@haivision.com


   Jeongseok Kim
   SK Telecom Co., Ltd.

   Email: jeongseok.kim@sk.com


   Joonwoong Kim
   SK Telecom Co., Ltd.

   Email: joonwoong.kim@sk.com











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