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Versions: (draft-fairhurst-taps-transports) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 8095

Network Working Group                                  G. Fairhurst, Ed.
Internet-Draft                                    University of Aberdeen
Intended status: Informational                          B. Trammell, Ed.
Expires: January 7, 2016                              M. Kuehlewind, Ed.
                                                              ETH Zurich
                                                           July 06, 2015


  Services provided by IETF transport protocols and congestion control
                               mechanisms
                     draft-ietf-taps-transports-06

Abstract

   This document describes services provided by existing IETF protocols
   and congestion control mechanisms.  It is designed to help
   application and network stack programmers and to inform the work of
   the IETF TAPS Working Group.

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 http://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 December 14, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Existing Transport Protocols  . . . . . . . . . . . . . . . .   4
     3.1.  Transport Control Protocol (TCP)  . . . . . . . . . . . .   4
       3.1.1.  Protocol Description  . . . . . . . . . . . . . . . .   5
       3.1.2.  Interface description . . . . . . . . . . . . . . . .   6
       3.1.3.  Transport Protocol Components . . . . . . . . . . . .   6
     3.2.  Multipath TCP (MPTCP) . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Protocol Description  . . . . . . . . . . . . . . . .   8
       3.2.2.  Interface Description . . . . . . . . . . . . . . . .   8
       3.2.3.  Transport Protocol Components . . . . . . . . . . . .   8
     3.3.  Stream Control Transmission Protocol (SCTP) . . . . . . .   9
       3.3.1.  Protocol Description  . . . . . . . . . . . . . . . .   9
       3.3.2.  Interface Description . . . . . . . . . . . . . . . .  11
       3.3.3.  Transport Protocol Components . . . . . . . . . . . .  13
     3.4.  User Datagram Protocol (UDP)  . . . . . . . . . . . . . .  13
       3.4.1.  Protocol Description  . . . . . . . . . . . . . . . .  14
       3.4.2.  Interface Description . . . . . . . . . . . . . . . .  14
       3.4.3.  Transport Protocol Components . . . . . . . . . . . .  15
     3.5.  Lightweight User Datagram Protocol (UDP-Lite) . . . . . .  15
       3.5.1.  Protocol Description  . . . . . . . . . . . . . . . .  15
       3.5.2.  Interface Description . . . . . . . . . . . . . . . .  16
       3.5.3.  Transport Protocol Components . . . . . . . . . . . .  16
     3.6.  Datagram Congestion Control Protocol (DCCP) . . . . . . .  17
       3.6.1.  Protocol Description  . . . . . . . . . . . . . . . .  17
       3.6.2.  Interface Description . . . . . . . . . . . . . . . .  19
       3.6.3.  Transport Protocol Components . . . . . . . . . . . .  19
     3.7.  Realtime Transport Protocol (RTP) . . . . . . . . . . . .  19
     3.8.  NACK-Oriented Reliable Multicast (NORM) . . . . . . . . .  20
       3.8.1.  Protocol Description  . . . . . . . . . . . . . . . .  20
       3.8.2.  Interface Description . . . . . . . . . . . . . . . .  21
       3.8.3.  Transport Protocol Components . . . . . . . . . . . .  21
     3.9.  Transport Layer Security (TLS) and Datagram TLS (DTLS) as
           a pseudotransport . . . . . . . . . . . . . . . . . . . .  22
       3.9.1.  Protocol Description  . . . . . . . . . . . . . . . .  23
       3.9.2.  Interface Description . . . . . . . . . . . . . . . .  24
       3.9.3.  Transport Protocol Components . . . . . . . . . . . .  24
     3.10. Hypertext Transport Protocol (HTTP) over TCP as a
           pseudotransport . . . . . . . . . . . . . . . . . . . . .  25
       3.10.1.  Protocol Description . . . . . . . . . . . . . . . .  25
       3.10.2.  Interface Description  . . . . . . . . . . . . . . .  26
       3.10.3.  Transport Protocol Components  . . . . . . . . . . .  27
     3.11. WebSockets  . . . . . . . . . . . . . . . . . . . . . . .  27



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       3.11.1.  Protocol Description . . . . . . . . . . . . . . . .  27
       3.11.2.  Interface Description  . . . . . . . . . . . . . . .  27
       3.11.3.  Transport Protocol Components  . . . . . . . . . . .  28
   4.  Transport Service Features  . . . . . . . . . . . . . . . . .  28
     4.1.  Complete Protocol Feature Matrix  . . . . . . . . . . . .  30
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  32
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  33
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   Most Internet applications make use of the Transport Services
   provided by TCP (a reliable, in-order stream protocol) or UDP (an
   unreliable datagram protocol).  We use the term "Transport Service"
   to mean the end-to-end service provided to an application by the
   transport layer.  That service can only be provided correctly if
   information about the intended usage is supplied from the
   application.  The application may determine this information at
   design time, compile time, or run time, and may include guidance on
   whether a feature is required, a preference by the application, or
   something in between.  Examples of features of Transport Services are
   reliable delivery, ordered delivery, content privacy to in-path
   devices, integrity protection, and minimal latency.

   The IETF has defined a wide variety of transport protocols beyond TCP
   and UDP, including SCTP, DCCP, MP-TCP, and UDP-Lite.  Transport
   services may be provided directly by these transport protocols, or
   layered on top of them using protocols such as WebSockets (which runs
   over TCP), RTP (over TCP or UDP) or WebRTC data channels (which run
   over SCTP over DTLS over UDP or TCP).  Services built on top of UDP
   or UDP-Lite typically also need to specify additional mechanisms,
   including a congestion control mechanism (such as a windowed
   congestion control, TFRC or LEDBAT congestion control mechanism).
   This extends the set of available Transport Services beyond those
   provided to applications by TCP and UDP.

   Transport protocols can also be differentiated by the features of the
   services they provide: for instance, SCTP offers a message-based
   service providing full or partial reliability and allowing to
   minimize the head of line blocking due to the support of unordered
   and unordered message delivery within multiple streams, UDP-Lite
   provides partial integrity protection, and LEDBAT can provide low-
   priority "scavenger" communication.



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2.  Terminology

   The following terms are defined throughout this document, and in
   subsequent documents produced by TAPS describing the composition and
   decomposition of transport services.

   [EDITOR'S NOTE: we may want to add definitions for the different
   kinds of interfaces that are important here.]

   Transport Service Feature:  a specific end-to-end feature that a
      transport service provides to its clients.  Examples include
      confidentiality, reliable delivery, ordered delivery, message-
      versus-stream orientation, etc.

   Transport Service:  a set of transport service features, without an
      association to any given framing protocol, which provides a
      complete service to an application.

   Transport Protocol:  an implementation that provides one or more
      different transport services using a specific framing and header
      format on the wire.

   Transport Protocol Component:  an implementation of a transport
      service feature within a protocol.

   Transport Service Instance:  an arrangement of transport protocols
      with a selected set of features and configuration parameters that
      implements a single transport service, e.g. a protocol stack (RTP
      over UDP).

   Application:  an entity that uses the transport layer for end-to-end
      delivery data across the network (this may also be an upper layer
      protocol or tunnel encapsulation).

3.  Existing Transport Protocols

   This section provides a list of known IETF transport protocol and
   transport protocol frameworks.

   [EDITOR'S NOTE: Contributions to the subsections below are welcome]

3.1.  Transport Control Protocol (TCP)

   TCP is an IETF standards track transport protocol.  [RFC0793]
   introduces TCP as follows: "The Transmission Control Protocol (TCP)
   is intended for use as a highly reliable host-to-host protocol
   between hosts in packet-switched computer communication networks, and
   in interconnected systems of such networks."  Since its introduction,



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   TCP has become the default connection-oriented, stream-based
   transport protocol in the Internet.  It is widely implemented by
   endpoints and widely used by common applications.

3.1.1.  Protocol Description

   TCP is a connection-oriented protocol, providing a three way
   handshake to allow a client and server to set up a connection, and
   mechanisms for orderly completion and immediate teardown of a
   connection.  TCP is defined by a family of RFCs [RFC4614].

   TCP provides multiplexing to multiple sockets on each host using port
   numbers.  An active TCP session is identified by its four-tuple of
   local and remote IP addresses and local port and remote port numbers.
   The destination port during connection setup has a different role as
   it is often used to indicate the requested service.

   TCP partitions a continuous stream of bytes into segments, sized to
   fit in IP packets.  ICMP-based PathMTU discovery [RFC1191][RFC1981]
   as well as Packetization Layer Path MTU Discovery (PMTUD) [RFC4821]
   are supported.

   Each byte in the stream is identified by a sequence number.  The
   sequence number is used to order segments on receipt, to identify
   segments in acknowledgments, and to detect unacknowledged segments
   for retransmission.  This is the basis of TCP's reliable, ordered
   delivery of data in a stream.  TCP Selective Acknowledgment [RFC2018]
   extends this mechanism by making it possible to identify missing
   segments more precisely, reducing spurious retransmission.

   Receiver flow control is provided by a sliding window: limiting the
   amount of unacknowledged data that can be outstanding at a given
   time.  The window scale option [RFC7323] allows a receiver to use
   windows greater than 64KB.

   All TCP senders provide Congestion Control: This uses a separate
   window, where each time congestion is detected, this congestion
   window is reduced.  A receiver detects congestion using one of three
   mechanisms: A retransmission timer, detection of loss (interpreted as
   a congestion signal), or Explicit Congestion Notification (ECN)
   [RFC3168] to provide early signaling (see
   [I-D.ietf-aqm-ecn-benefits])

   A TCP protocol instance can be extended [RFC4614] and tuned.  Some
   features are sender-side only, requiring no negotiation with the
   receiver; some are receiver-side only, some are explicitly negotiated
   during connection setup.




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   By default, TCP segment partitioning uses Nagle's algorithm [RFC0896]
   to buffer data at the sender into large segments, potentially
   incurring sender-side buffering delay; this algorithm can be disabled
   by the sender to transmit more immediately, e.g. to enable smoother
   interactive sessions.

   [EDITOR'S NOTE: add URGENT and PUSH flag (note [RFC6093] says SHOULD
   NOT use due to the range of TCP implementations that process TCP
   urgent indications differently.) ]

   A checksum provides an Integrity Check and is mandatory across the
   entire packet.  The TCP checksum does not support partial corruption
   protection as in DCCP/UDP-Lite).  This check protects from
   misdelivery of data corrupted data, but is relatively weak, and
   applications that require end to end integrity of data are
   recommended to include a stronger integrity check of their payload
   data.

   A TCP service is unicast.

3.1.2.  Interface description

   A User/TCP Interface is defined in [RFC0793] providing six user
   commands: Open, Send, Receive, Close, Status.  This interface does
   not describe configuration of TCP options or parameters beside use of
   the PUSH and URGENT flags.

   In API implementations derived from the BSD Sockets API, TCP sockets
   are created using the "SOCK_STREAM" socket type.

   The features used by a protocol instance may be set and tuned via
   this API.

   (more on the API goes here)

3.1.3.  Transport Protocol Components

   The transport protocol components provided by TCP (new version) are:

   [EDITOR'S NOTE: discussion of how to map this to features and TAPS:
   what does the higher layer need to decide? what can the transport
   layer decide based on global settings? what must the transport layer
   decide based on network characteristics?]

   o  Connection-oriented bidirectional communication using three-way
      handshake connection setup with feature negotiation and an
      explicit distinction between passive and active open: This implies
      both unicast addressing and a guarantee of return routability.



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   o  Single stream-oriented transmission: The stream abstraction atop
      the datagram service provided by IP is implemented by dividing the
      stream into segments.

   o  Limited control over segment transmission scheduling (Nagle's
      algorithm): This allows for delay minimization in interactive
      applications by preventing the transport to add additional delays
      (by deactivating Nagle's algorithm).

   o  Port multiplexing, with application-to-port mapping during
      connection setup: Note that in the presence of network address and
      port translation (NAPT), TCP ports are in effect part of the
      endpoint address for forwarding purposes.

   o  Full reliability using (S)ACK- and RTO-based loss detection and
      retransmissions: Loss is sensed using duplicated ACKs ("fast
      retransmit"), which places a lower bound on the delay inherent in
      this approach to reliability.  The retransmission timeout
      determines the upper bound on the delay (expect if also
      exponential back-off is performed).  The use of selective
      acknowlegdements further reduces the latency for retransmissions
      if multiple packets are lost during one congestion event.

   o  Error detection based on a checksum covering the network and
      transport headers as well as payload: Packets that are detected as
      corrupted are dropped, relying on the reliability mechanism to
      retransmit them.

   o  Window-based flow control, with receiver-side window management
      and signaling of available window: Scaling the flow control window
      beyond 64kB requires the use of an optional feature, which has
      performance implications in environments where this option is not
      supported; this can be the case either if the receiver does not
      implement window scaling or if a network node on the path strips
      the window scaling option.

   o  Window-based congestion control reacting to loss, delay,
      retransmission timeout, or an explicit congestion signal (ECN):
      Most commonly used is a loss signal from the reliability
      component's retransmission mechanism.  TCP reacts to a congestion
      signal by reducing the size of the congestion window;
      retransmission timeout is generally handled with a larger reaction
      than other signals.

3.2.  Multipath TCP (MPTCP)

   Multipath TCP [RFC6824] is an extension for TCP to support multi-
   homing.  It is designed to be as transparent as possible to middle-



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   boxes.  It does so by establishing regular TCP flows between a pair
   of source/destination endpoints, and multiplexing the application's
   stream over these flows.

3.2.1.  Protocol Description

   MPTCP uses TCP options for its control plane.  They are used to
   signal multipath capabilities, as well as to negotiate data sequence
   numbers, and advertise other available IP addresses and establish new
   sessions between pairs of endpoints.

3.2.2.  Interface Description

   By default, MPTCP exposes the same interface as TCP to the
   application.  [RFC6897] however describes a richer API for MPTCP-
   aware applications.

   This Basic API describes how an application can - enable or disable
   MPTCP; - bind a socket to one or more selected local endpoints; -
   query local and remote endpoint addresses; - get a unique connection
   identifier (similar to an address-port pair for TCP).

   The document also recommend the use of extensions defined for SCTP
   [RFC6458] (see next section) to deal with multihoming.

   [AUTHOR'S NOTE: research work, and some implementation, also suggest
   that the scheduling algorithm, as well as the path manager, are
   configurable options that should be exposed to higher layer.  Should
   this be discussed here?]

3.2.3.  Transport Protocol Components

   [AUTHOR'S NOTE: shouldn't it be "service feature"?]

   As an extension to TCP, MPTCP provides mostly the same components.
   By establishing multiple sessions between available endpoints, it can
   additionally provide soft failover solutions should one of the paths
   become unusable.  In addition, by multiplexing one byte stream over
   separate paths, it can achieve a higher throughput than TCP in
   certain situations (note however that coupled congestion control
   [RFC6356] might limit this benefit to maintain fairness to other
   flows at the bottleneck).  When aggregating capacity over multiple
   paths, and depending on the way packets are scheduled on each TCP
   subflow, an additional delay and higher jitter might be observed
   observed before in-order delivery of data to the applications.

   The transport protocol components provided by MPTCP in addition to
   TCP therefore are:



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   o  congestion control with load balancing over mutiple connections

   o  endpoint multiplexing of a single byte stream (higher throughput)

   o  resilience to network failure and/or handoverss

   [AUTHOR'S NOTE: it is unclear whether MPTCP has to provide data
   bundling.]  [AUTHOR'S NOTE: AF muliplexing? sub-flows can be started
   over IPv4 or IPv6 for the same session]

3.3.  Stream Control Transmission Protocol (SCTP)

   SCTP is a message oriented standards track transport protocol and the
   base protocol is specified in [RFC4960].  It supports multi-homing to
   handle path failures.  An SCTP association has multiple
   unidirectional streams in each direction and provides in-sequence
   delivery of user messages only within each stream.  This allows to
   minimize head of line blocking.  SCTP is extensible and the currently
   defined extensions include mechanisms for dynamic re-configurations
   of streams [RFC6525] and IP-addresses [RFC5061].  Furthermore, the
   extension specified in [RFC3758] introduces the concept of partial
   reliability for user messages.

   SCTP was originally developed for transporting telephony signalling
   messages and is deployed in telephony signalling networks, especially
   in mobile telephony networks.  Additionally, it is used in the WebRTC
   framework for data channels and is therefore deployed in all WEB-
   browsers supporting WebRTC.

3.3.1.  Protocol Description

   SCTP is a connection oriented protocol using a four way handshake to
   establish an SCTP association and a three way message exchange to
   gracefully shut it down.  It uses the same port number concept as
   DCCP, TCP, UDP, and UDP-Lite do and only supports unicast.

   SCTP uses the 32-bit CRC32c for protecting SCTP packets against bit
   errors.  This is stronger than the 16-bit checksums used by TCP or
   UDP.  However, a partial checksum coverage as provided by DCCP or
   UDP-Lite is not supported.

   SCTP has been designed with extensibility in mind.  Each SCTP packet
   starts with a single common header containing the port numbers, a
   verification tag and the CRC32c checksum.  This common header is
   followed by a sequence of chunks.  Each chunk consists of a type
   field, flags, a length field and a value.  [RFC4960] defines how a
   receiver processes chunks with an unknown chunk type.  The support of
   extensions can be negotiated during the SCTP handshake.



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   SCTP provides a message-oriented service.  Multiple small user
   messages can be bundled into a single SCTP packet to improve the
   efficiency.  For example, this bundling may be done by delaying user
   messages at the sender side similar to the Nagle algorithm used by
   TCP.  User messages which would result in IP packets larger than the
   MTU will be fragmented at the sender side and reassembled at the
   receiver side.  There is no protocol limit on the user message size.
   ICMP-based path MTU discovery as specified for IPv4 in [RFC1191] and
   for IPv6 in [RFC1981] as well as packetization layer path MTU
   discovery as specified in [RFC4821] with probe packets using the
   padding chunks defined the [RFC4820] are supported.

   [RFC4960] specifies a TCP friendly congestion control to protect the
   network against overload.  SCTP also uses a sliding window flow
   control to protect receivers against overflow.

   Each SCTP association has between 1 and 65536 uni-directional streams
   in each direction.  The number of streams can be different in each
   direction.  Every user-message is sent on a particular stream.  User
   messages can be sent un-ordered or ordered upon request by the upper
   layer.  Un-ordered messages can be delivered as soon as they are
   completely received.  Only all ordered messages sent on the same
   stream are delivered at the receiver in the same order as sent by the
   sender.  For user messages not requiring fragmentation, this
   minimises head of line blocking.  The base protocol defined in
   [RFC4960] doesn't allow interleaving of user-messages, which results
   in sending a large message on one stream can block the sending of
   user messages on other streams.  [I-D.ietf-tsvwg-sctp-ndata]
   overcomes this limitation.  Furthermore, [I-D.ietf-tsvwg-sctp-ndata]
   specifies multiple algorithms for the sender side selection of which
   streams to send data from supporting a variety of scheduling
   algorithms including priority based ones.  The stream re-
   configuration extension defined in [RFC6525] allows to reset streams
   during the lifetime of an association and to increase the number of
   streams, if the number of streams negotiated in the SCTP handshake is
   not sufficient.

   According to [RFC4960], each user message sent is either delivered to
   the receiver or, in case of excessive retransmissions, the
   association is terminated in a non-graceful way, similar to the TCP
   behaviour.  In addition to this reliable transfer, the partial
   reliability extension defined in [RFC3758] allows the sender to
   abandon user messages.  The application can specify the policy for
   abandoning user messages.  Examples for these policies include:

   o  Limiting the time a user message is dealt with by the sender.





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   o  Limiting the number of retransmissions for each fragment of a user
      message.  If the number of retransmissions is limited to 0, one
      gets a service similar to UDP.

   o  Abandoning messages of lower priority in case of a send buffer
      shortage.

   SCTP supports multi-homing.  Each SCTP end-point uses a list of IP-
   addresses and a single port number.  These addresses can be any
   mixture of IPv4 and IPv6 addresses.  These addresses are negotiated
   during the handshake and the address re-configuration extension
   specified in [RFC5061] in combination with [RFC4895] can be used to
   change these addresses in an authenticated way during the livetime of
   an SCTP association.  This allows for transport layer mobility.
   Multiple addresses are used for improved resilience.  If a remote
   address becomes unreachable, the traffic is switched over to a
   reachable one, if one exists.  Each SCTP end-point supervises
   continuously the reachability of all peer addresses using a heartbeat
   mechanism.

   For securing user messages, the use of TLS over SCTP has been
   specified in [RFC3436].  However, this solution does not support all
   services provided by SCTP (for example un-ordered delivery or partial
   reliability), and therefore the use of DTLS over SCTP has been
   specified in [RFC6083] to overcome these limitations.  When using
   DTLS over SCTP, the application can use almost all services provided
   by SCTP.

   [I-D.ietf-tsvwg-natsupp] defines a methods for end-hosts and
   middleboxes to provide for NAT support for SCTP over IPv4.  For
   legacy NAT traversal, [RFC6951] defines the UDP encapsulation of
   SCTP-packets.  Alternatively, SCTP packets can be encapsulated in
   DTLS packets as specified in [I-D.ietf-tsvwg-sctp-dtls-encaps].  The
   latter encapsulation is used with in the WebRTC context.

   Having a well defined API is also a feature provided by SCTP as
   described in the next subsection.

3.3.2.  Interface Description

   [RFC4960] defines an abstract API for the base protocol.  An
   extension to the BSD Sockets API is defined in [RFC6458] and covers:

   o  the base protocol defined in [RFC4960].

   o  the SCTP Partial Reliability extension defined in [RFC3758].

   o  the SCTP Authentication extension defined in [RFC4895].



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   o  the SCTP Dynamic Address Reconfiguration extension defined in
      [RFC5061].

   For the following SCTP protocol extensions the BSD Sockets API
   extension is defined in the document specifying the protocol
   extensions:

   o  the SCTP SACK-IMMEDIATELY extension defined in [RFC7053].

   o  the SCTP Stream Reconfiguration extension defined in [RFC6525].

   o  the UDP Encapsulation of SCTP packets extension defined in
      [RFC6951].

   o  the additional PR-SCTP policies defined in
      [I-D.ietf-tsvwg-sctp-prpolicies].

   Future documents describing SCTP protocol extensions are expected to
   describe the corresponding BSD Sockets API extension in a "Socket API
   Considerations" section.

   The SCTP socket API supports two kinds of sockets:

   o  one-to-one style sockets (by using the socket type "SOCK_STREAM").

   o  one-to-many style socket (by using the socket type
      "SOCK_SEQPACKET").

   One-to-one style sockets are similar to TCP sockets, there is a 1:1
   relationship between the sockets and the SCTP associations (except
   for listening sockets).  One-to-many style SCTP sockets are similar
   to unconnected UDP sockets as there is a 1:n relationship between the
   sockets and the SCTP associations.

   The SCTP stack can provide information to the applications about
   state changes of the individual paths and the association whenever
   they occur.  These events are delivered similar to user messages but
   are specifically marked as notifications.

   A couple of new functions have been introduced to support the use of
   multiple local and remote addresses.  Additional SCTP-specific send
   and receive calls have been defined to allow dealing with the SCTP
   specific information without using ancillary data in the form of
   additional cmsgs, which are also defined.  These functions provide
   support for detecting partial delivery of user messages and
   notifications.





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   The SCTP socket API allows a fine-grained control of the protocol
   behaviour through an extensive set of socket options.

   The SCTP kernel implementations of FreeBSD, Linux and Solaris follow
   mostly the specified extension to the BSD Sockets API for the base
   protocol and the corresponding supported protocol extensions.

3.3.3.  Transport Protocol Components

   The transport protocol components provided by SCTP are:

   o  unicast

   o  connection setup with feature negotiation and application-to-port
      mapping

   o  port multiplexing

   o  reliable or partially reliable delivery

   o  ordered and unordered delivery within a stream

   o  support for multiple concurrent streams

   o  support for stream scheduling prioritization

   o  flow control

   o  message-oriented delivery

   o  congestion control

   o  user message bundling

   o  user message fragmentation and reassembly

   o  strong error detection (CRC32C)

   o  transport layer multihoming for resilience

   o  transport layer mobility

   [EDITOR'S NOTE: update this list.]

3.4.  User Datagram Protocol (UDP)

   The User Datagram Protocol (UDP) [RFC0768] [RFC2460] is an IETF
   standards track transport protocol.  It provides a uni-directional,



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   datagram protocol which preserves message boundaries.  It provides
   none of the following transport features: error correction,
   congestion control, or flow control.  It can be used to send
   broadcast datagrams (IPv4) or multicast datagrams (IPv4 and IPv6), in
   addition to unicast (and anycast) datagrams.  IETF guidance on the
   use of UDP is provided in[RFC5405].  UDP is widely implemented and
   widely used by common applications, especially DNS.

3.4.1.  Protocol Description

   UDP is a connection-less protocol which maintains message boundaries,
   with no connection setup or feature negotiation.  The protocol uses
   independent messages, ordinarily called datagrams.  The lack of error
   control and flow control implies messages may be damaged, re-ordered,
   lost, or duplicated in transit.  A receiving application unable to
   run sufficiently fast or frequently may miss messages.  The lack of
   congestion handling implies UDP traffic may cause the loss of
   messages from other protocols (e.g., TCP) when sharing the same
   network paths.  UDP traffic can also cause the loss of other UDP
   traffic in the same or other flows for the same reasons.

   Messages with bit errors are ordinarily detected by an invalid end-
   to-end checksum and are discarded before being delivered to an
   application.  There are some exceptions to this general rule,
   however.  UDP-Lite (see [RFC3828], and below) provides the ability
   for portions of the message contents to be exempt from checksum
   coverage.  It is also possible to create UDP datagrams with no
   checksum, and while this is generally discouraged [RFC1122]
   [RFC5405], certain special cases permit its use [RFC6935].  The
   checksum support considerations for omitting the checksum are defined
   in [RFC6936].  Note that due to the relatively weak form of checksum
   used by UDP, applications that require end to end integrity of data
   are recommended to include a stronger integrity check of their
   payload data.

   On transmission, UDP encapsulates each datagram into an IP packet,
   which may in turn be fragmented by IP.  Applications concerned with
   fragmentation or that have other requirements such as receiver flow
   control, congestion control, PathMTU discovery/PLPMTUD, support for
   ECN, etc need to be provided by protocols other than UDP [RFC5405].

3.4.2.  Interface Description

   [RFC0768] describes basic requirements for an API for UDP.  Guidance
   on use of common APIs is provided in [RFC5405].

   A UDP endpoint consists of a tuple of (IP address, port number).
   Demultiplexing using multiple abstract endpoints (sockets) on the



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   same IP address are supported.  The same socket may be used by a
   single server to interact with multiple clients (note: this behavior
   differs from TCP, which uses a pair of tuples to identify a
   connection).  Multiple server instances (processes) binding the same
   socket can cooperate to service multiple clients- the socket
   implementation arranges to not duplicate the same received unicast
   message to multiple server processes.

   Many operating systems also allow a UDP socket to be "connected",
   i.e., to bind a UDP socket to a specific (remote) UDP endpoint.
   Unlike TCP's connect primitive, for UDP, this is only a local
   operation that serves to simplify the local send/receive functions
   and to filter the traffic for the specified addresses and ports
   [RFC5405].

3.4.3.  Transport Protocol Components

   The transport protocol components provided by UDP are:

   o  unidirectional

   o  port multiplexing

   o  2-tuple endpoints

   o  IPv4 broadcast, multicast and anycast

   o  IPv6 multicast and anycast

   o  IPv6 jumbograms

   o  message-oriented delivery

   o  error detection (checksum)

   o  checksum optional

3.5.  Lightweight User Datagram Protocol (UDP-Lite)

   The Lightweight User Datagram Protocol (UDP-Lite) [RFC3828] is an
   IETF standards track transport protocol.  UDP-Lite provides a
   bidirectional set of logical unicast or multicast message streams
   over a datagram protocol.  IETF guidance on the use of UDP-Lite is
   provided in [RFC5405].

3.5.1.  Protocol Description





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   UDP-Lite is a connection-less datagram protocol, with no connection
   setup or feature negotiation.  The protocol use messages, rather than
   a byte-stream.  Each stream of messages is independently managed,
   therefore retransmission does not hold back data sent using other
   logical streams.

   It provides multiplexing to multiple sockets on each host using port
   numbers.  An active UDP-Lite session is identified by its four-tuple
   of local and remote IP addresses and local port and remote port
   numbers.

   UDP-Lite fragments packets into IP packets, constrained by the
   maximum size of IP packet.

   UDP-Lite changes the semantics of the UDP "payload length" field to
   that of a "checksum coverage length" field.  Otherwise, UDP-Lite is
   semantically identical to UDP.  Applications using UDP-Lite therefore
   can not make assumptions regarding the correctness of the data
   received in the insensitive part of the UDP-Lite payload.

   As for UDP, mechanisms for receiver flow control, congestion control,
   PMTU or PLPMTU discovery, support for ECN, etc need to be provided by
   upper layer protocols [RFC5405].

   Examples of use include a class of applications that can derive
   benefit from having partially-damaged payloads delivered, rather than
   discarded.  One use is to support error tolerate payload corruption
   when used over paths that include error-prone links, another
   application is when header integrity checks are required, but payload
   integrity is provided by some other mechanism (e.g. [RFC6936].

   A UDP-Lite service may support IPv4 broadcast, multicast, anycast and
   unicast.

3.5.2.  Interface Description

   There is no current API specified in the RFC Series, but guidance on
   use of common APIs is provided in [RFC5405].

   The interface of UDP-Lite differs from that of UDP by the addition of
   a single (socket) option that communicates a checksum coverage length
   value: at the sender, this specifies the intended checksum coverage,
   with the remaining unprotected part of the payload called the "error-
   insensitive part".  The checksum coverage may also be made visible to
   the application via the UDP-Lite MIB module [RFC5097].

3.5.3.  Transport Protocol Components




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   The transport protocol components provided by UDP-Lite are:

   o  unicast

   o  IPv4 broadcast, multicast and anycast

   o  port multiplexing

   o  non-reliable, non-ordered delivery

   o  message-oriented delivery

   o  partial integrity protection

3.6.  Datagram Congestion Control Protocol (DCCP)

   Datagram Congestion Control Protocol (DCCP) [RFC4340] is an IETF
   standards track bidirectional transport protocol that provides
   unicast connections of congestion-controlled unreliable messages.

   [EDITOR'S NOTE: Gorry Fairhurst signed up as a contributor for this
   section.]

   The DCCP Problem Statement describes the goals that DCCP sought to
   address [RFC4336].  It is suitable for applications that transfer
   fairly large amounts of data and that can benefit from control over
   the trade off between timeliness and reliability [RFC4336].

   It offers low overhead, and many characteristics common to UDP, but
   can avoid "Re-inventing the wheel" each time a new multimedia
   application emerges.  Specifically it includes core functions
   (feature negotiation, path state management, RTT calculation, PMTUD,
   etc): This allows applications to use a compatible method defining
   how they send packets and where suitable to choose common algorithms
   to manage their functions.  Examples of suitable applications include
   interactive applications, streaming media or on-line games [RFC4336].

3.6.1.  Protocol Description

   DCCP is a connection-oriented datagram protocol, providing a three
   way handshake to allow a client and server to set up a connection,
   and mechanisms for orderly completion and immediate teardown of a
   connection.  The protocol is defined by a family of RFCs.

   It provides multiplexing to multiple sockets on each host using port
   numbers.  An active DCCP session is identified by its four-tuple of
   local and remote IP addresses and local port and remote port numbers.
   At connection setup, DCCP also exchanges the the service code



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   [RFC5595] mechanism to allow transport instantiations to indicate the
   service treatment that is expected from the network.

   The protocol segments data into messages, typically sized to fit in
   IP packets, but which may be fragmented providing they are less than
   the A DCCP interface MAY allow applications to request fragmentation
   for packets larger than PMTU, but not larger than the maximum packet
   size allowed by the current congestion control mechanism (CCMPS)
   [RFC4340].

   Each message is identified by a sequence number.  The sequence number
   is used to identify segments in acknowledgments, to detect
   unacknowledged segments, to measure RTT, etc.  The protocol may
   support ordered or unordered delivery of data, and does not itself
   provide retransmission.  There is a Data Checksum option, which
   contains a strong CRC, lets endpoints detect application data
   corruption.  It also supports reduced checksum coverage, a partial
   integrity mechanisms similar to UDP-lIte.

   Receiver flow control is supported: limiting the amount of
   unacknowledged data that can be outstanding at a given time.

   A DCCP protocol instance can be extended [RFC4340] and tuned.  Some
   features are sender-side only, requiring no negotiation with the
   receiver; some are receiver-side only, some are explicitly negotiated
   during connection setup.

   DCCP supports negotiation of the congestion control profile, to
   provide Plug and Play congestion control mechanisms.  examples of
   specified profiles include [RFC4341] [RFC4342] [RFC5662].  All IETF-
   defined methods provide Congestion Control.

   DCCP use a Connect packet to start a session, and permits half-
   connections that allow each client to choose features it wishes to
   support.  Simultaneous open [RFC5596], as in TCP, can enable
   interoperability in the presence of middleboxes.  The Connect packet
   includes a Service Code field [RFC5595] designed to allow middle
   boxes and endpoints to identify the characteristics required by a
   session.  A lightweight UDP-based encapsulation (DCCP-UDP) has been
   defined [RFC6773] that permits DCCP to be used over paths where it is
   not natively supported.  Support in NAPT/NATs is defined in [RFC4340]
   and [RFC5595].

   Upper layer protocols specified on top of DCCP include: DTLS
   [RFC5595], RTP [RFC5672], ICE/SDP [RFC6773].

   A DCCP service is unicast.




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   A common packet format has allowed tools to evolve that can read and
   interpret DCCP packets (e.g. Wireshark).

3.6.2.  Interface Description

   API characteristics include: - Datagram transmission.  - Notification
   of the current maximum packet size.  - Send and reception of zero-
   length payloads.  - Set the Slow Receiver flow control at a receiver.
   - Detect a Slow receiver at the sender.

   There is no current API specified in the RFC Series.

3.6.3.  Transport Protocol Components

   The transport protocol components provided by DCCP are:

   o  unicast

   o  connection setup with feature negotiation and application-to-port
      mapping

   o  Service Codes

   o  port multiplexing

   o  non-reliable, ordered delivery

   o  flow control (slow receiver function)

   o  drop notification

   o  timestamps

   o  message-oriented delivery

   o  partial integrity protection

3.7.  Realtime Transport Protocol (RTP)

   RTP provides an end-to-end network transport service, suitable for
   applications transmitting real-time data, such as audio, video or
   data, over multicast or unicast network services, including TCP, UDP,
   UDP-Lite, DCCP.

   [EDITOR'S NOTE: Varun Singh signed up as contributor for this
   section.  Given the complexity of RTP, suggest to have an abbreviated
   section here contrasting RTP with other transports, and focusing on
   those features that are RTP-unique.]



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3.8.  NACK-Oriented Reliable Multicast (NORM)

   NORM is an IETF standards track protocol specified in [RFC5740].  The
   protocol was designed to support reliable bulk data dissemination to
   receiver groups using IP Multicast but also provides for point-to-
   point unicast operation.  Its support for bulk data dissemination
   includes discrete file or computer memory-based "objects" as well as
   byte- and message-streaming.  NORM is designed to incorporate packet
   erasure coding as an inherent part of its selective ARQ in response
   to receiver negative acknowledgements.  The packet erasure coding can
   also be proactively applied for forward protection from packet loss.
   NORM transmissions are governed by TCP-friendly congestion control.
   NORM's reliability, congestion control, and flow control mechanism
   are distinct components and can be separately controlled to meet
   different application needs.

3.8.1.  Protocol Description

   [EDITOR'S NOTE: needs to be more clear about the application of FEC
   and packet erasure coding; expand ARQ.]

   The NORM protocol is encapsulated in UDP datagrams and thus provides
   multiplexing for multiple sockets on hosts using port numbers.  For
   purposes of loosely coordinated IP Multicast, NORM is not strictly
   connection-oriented although per-sender state is maintained by
   receivers for protocol operation.  [RFC5740] does not specify a
   handshake protocol for connection establishment and separate session
   initiation can be used to coordinate port numbers.  However, in-band
   "client-server" style connection establishment can be accomplished
   with the NORM congestion control signaling messages using port
   binding techniques like those for TCP client-server connections.

   NORM supports bulk "objects" such as file or in-memory content but
   also can treat a stream of data as a logical bulk object for purposes
   of packet erasure coding.  In the case of stream transport, NORM can
   support either byte streams or message streams where application-
   defined message boundary information is carried in the NORM protocol
   messages.  This allows the receiver(s) to join/re-join and recover
   message boundaries mid-stream as needed.  Application content is
   carried and identified by the NORM protocol with encoding symbol
   identifiers depending upon the Forward Error Correction (FEC) Scheme
   [RFC3452] configured.  NORM uses NACK-based selective ARQ to reliably
   deliver the application content to the receiver(s).  NORM proactively
   measures round-trip timing information to scale ARQ timers
   appropriately and to support congestion control.  For multicast
   operation, timer-based feedback suppression is uses to achieve group
   size scaling with low feedback traffic levels.  The feedback
   suppression is not applied for unicast operation.



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   NORM uses rate-based congestion control based upon the TCP-Friendly
   Rate Control (TFRC) [RFC4324] principles that are also used in DCCP
   [RFC4340].  NORM uses control messages to measure RTT and collect
   congestion event (e..g, loss event, ECN event, etc) information from
   the receiver(s) to support dynamic rate control adjustment.  The TCP-
   Friendly Multicast Congestion Control (TFMCC) [RFC4654] used provides
   some extra features to support multicast but is functionally
   equivalent to TFRC in the unicast case.

   NORM's reliability mechanism is decoupled from congestion control.
   This allows alternative arrangements of transport services to be
   invoked.  For example, fixed-rate reliable delivery can be supported
   or unreliable (but optionally "better than best effort" via packet
   erasure coding) delivery with rate-control per TFRC can be achieved.
   Additionally, alternative congestion control techniques may be
   applied.  For example, TFRC rate control with congestion event
   detection based on ECN for links with high packet loss (e.g.,
   wireless) has been implemented and demonstrated with NORM.

   While NORM is NACK-based for reliability transfer, it also supports a
   positive acknowledgment (ACK) mechanism that can be used for receiver
   flow control.  Again, since this mechanism is decoupled from the
   reliability and congestion control, applications that have different
   needs in this aspect can use the protocol differently.  One example
   is the use of NORM for quasi-reliable delivery where timely delivery
   of newer content may be favored over completely reliable delivery of
   older content within buffering and RTT constraints.

3.8.2.  Interface Description

   The NORM specification does not describe a specific application
   programming interface (API) to control protocol operation.  A freely-
   available, open source reference implementation of NORM is available
   at https://www.nrl.navy.mil/itd/ncs/products/norm, and a documented
   API is provided for this implementation.  While a sockets-like API is
   not currently documented, the existing API supports the necessary
   functions for that to be implemented.

3.8.3.  Transport Protocol Components

   The transport protocol components provided by NORM are:

   o  unicast

   o  multicast

   o  port multiplexing (UDP ports)




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   o  reliable delivery

   o  unordered delivery of in-memory data or file bulk content objects

   o  error detection (UDP checksum)

   o  segmentation

   o  stream-oriented delivery in a single stream

   o  object-oriented delivery of discrete data or file items

   o  data bundling (Nagle's algorithm)

   o  flow control (timer-based and/or ack-based)

   o  congestion control

   o  packet erasure coding (both proactively and as part of ARQ)

3.9.  Transport Layer Security (TLS) and Datagram TLS (DTLS) as a
      pseudotransport

   Transport Layer Security (TLS) and Datagram TLS (DTLS) are IETF
   protocols that provide several security-related features to
   applications.  TLS is designed to run on top of a reliable streaming
   transport protocol (usually TCP), while DTLS is designed to run on
   top of a best-effort datagram protocol (usually UDP).  At the time of
   writing, the current version of TLS is 1.2; it is defined in
   [RFC5246].  DTLS provides nearly identical functionality to
   applications; it is defined in [RFC6347] and its current version is
   also 1.2.  The TLS protocol evolved from the Secure Sockets Layer
   (SSL) protocols developed in the mid 90s to support protection of
   HTTP traffic.

   While older versions of TLS and DTLS are still in use, they provide
   weaker security guarantees.  [RFC7457] outlines important attacks on
   TLS and DTLS.  [RFC7525] is a Best Current Practices (BCP) document
   that describes secure configurations for TLS and DTLS to counter
   these attacks.  The recommendations are applicable for the vast
   majority of use cases.

   [NOTE: The Logjam authors (weakdh.org) give (inconclusive) evidence
   that one of the recommendations of [RFC7525], namely the use of
   DHE-1024 as a fallback, may not be sufficient in all cases to counter
   an attacker with the resources of a nation-state.  It is unclear at
   this time if the RFC is going to be updated as a result, or whether
   there will be an RFC7525bis.]



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3.9.1.  Protocol Description

   Both TLS and DTLS provide the same security features and can thus be
   discussed together.  The features they provide are:

   o  Confidentiality

   o  Data integrity

   o  Peer authentication (optional)

   o  Perfect forward secrecy (optional)

   The authentication of the peer entity can be omitted; a common web
   use case is where the server is authenticated and the client is not.
   TLS also provides a completely anonymous operation mode in which
   neither peer's identity is authenticated.  It is important to note
   that TLS itself does not specify how a peering entity's identity
   should be interpreted.  For example, in the common use case of
   authentication by means of an X.509 certificate, it is the
   application's decision whether the certificate of the peering entity
   is acceptable for authorization decisions.  Perfect forward secrecy,
   if enabled and supported by the selected algorithms, ensures that
   traffic encrypted and captured during a session at time t0 cannot be
   later decrypted at time t1 (t1 > t0), even if the long-term secrets
   of the communicating peers are later compromised.

   As DTLS is generally used over an unreliable datagram transport such
   as TCP, applications will need to tolerate loss, re-ordered, or
   duplicated datagrams.  Like TLS, DTLS conveys application data in a
   sequence of independent records.  However, because records are mapped
   to unreliable datagrams, there are several features unique to DTLS
   that are not applicable to TLS:

   o  Record replay detection (optional)

   o  Record size negotiation (estimates of PMTU and record size
      expansion factor)

   o  Coveyance of IP don't fragment (DF) bit settings by application

   o  An anti-DoS stateless cookie mechanism (optional)

   Generally, DTLS follows the TLS design as closely as possible.  To
   operate over datagrams, DTLS includes a sequence number and limited
   forms of retransmission and fragmentation for its internal
   operations.  The sequence number may be used for detecting replayed
   information, according to the windowing procedure described in



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   Section 4.1.2.6 of [RFC6347].  Note also that DTLS bans the use of
   stream ciphers, which are essentially incompatible when operating on
   independent encrypted records.

3.9.2.  Interface Description

   TLS is commonly invoked using an API provided by packages such as
   OpenSSL, wolfSSL, or GnuTLS.  Using such APIs entails the
   manipulation of several important abstractions, which fall into the
   following categories: long-term keys and algorithms, session state,
   and communications/connections.  There may also be special APIs
   required to deal with time and/or random numbers, both of which are
   needed by a variety of encryption algorithms and protocols.

   Considerable care is required in the use of TLS APIs in order to
   create a secure application.  The programmer should have at least a
   basic understanding of encryption and digital signature algorithms
   and their strengths, public key infrastructure (including X.509
   certificates and certificate revocation), and the sockets API.  See
   [RFC7525] and [RFC7457], as mentioned above.

   As an example, in the case of OpenSSL, the primary abstractions are
   the library itself and method (protocol), session, context, cipher
   and connection.  After initializing the library and setting the
   method, a cipher suite is chosen and used to configure a context
   object.  Session objects may then be minted according to the
   parameters present in a context object and associated with individual
   connections.  Depending on how precisely the programmer wishes to
   select different algorithmic or protocol options, various levels of
   details may be required.

3.9.3.  Transport Protocol Components

   Both TLS and DTLS employ a layered architecture.  The lower layer is
   commonly called the record protocol.  It is responsible for
   fragmenting messages, applying message authentication codes (MACs),
   encrypting data, and invoking transmission from the underlying
   transport protocol.  DTLS augments the TLS record protocol with
   sequence numbers used for ordering and replay detection.

   Several protocols are layered on top of the record protocol.  These
   include the handshake, alert, and change cipher spec protocols.
   There is also the data protocol, used to carry application traffic.
   The handshake protocol is used to establish cryptographic and
   compression parameters when a connection is first set up.  In DTLS,
   this protocol also has a basic fragmentation and retransmission
   capability and a cookie-like mechanism to resist DoS attacks.  (TLS
   compression is not recommended at present).  The alert protocol is



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   used to inform the peer of various conditions, most of which are
   terminal for the connection.  The change cipher spec protocol is used
   to synchronize changes in cryptographic parameters for each peer.

3.10.  Hypertext Transport Protocol (HTTP) over TCP as a pseudotransport

   Hypertext Transfer Protocol (HTTP) is an application-level protocol
   widely used on the Internet.  Version 1.1 of the protocol is
   specified in [RFC7230] [RFC7231] [RFC7232] [RFC7233] [RFC7234]
   [RFC7235], and version 2 in [RFC7540].  Furthermore, HTTP is used as
   a substrate for other application-layer protocols.  There are various
   reasons for this practice listed in [RFC3205]; these include being a
   well-known and well-understood protocol, reusability of existing
   servers and client libraries, easy use of existing security
   mechanisms such as HTTP digest authentication [RFC2617] and TLS
   [RFC5246], the ability of HTTP to traverse firewalls which makes it
   work with a lot of infrastructure, and cases where a application
   server often needs to support HTTP anyway.

   Depending on application's needs, the use of HTTP as a substrate
   protocol may add complexity and overhead in comparison to a special-
   purpose protocol (e.g. HTTP headers, suitability of the HTTP security
   model etc.).  [RFC3205] address this issues and provides some
   guidelines and concerns about the use of HTTP standard port 80 and
   443, the use of HTTP URL scheme and interaction with existing
   firewalls, proxies and NATs.

   Though not strictly bound to TCP, HTTP is almost exclusively run over
   TCP, and therefore inherits its properties when used in this way.

3.10.1.  Protocol Description

   Hypertext Transfer Protocol (HTTP) is a request/response protocol.  A
   client sends a request containing a request method, URI and protocol
   version followed by a MIME-like message (see [RFC7231] for the
   differences between an HTTP object and a MIME message), containing
   information about the client and request modifiers.  The message can
   contain a message body carrying application data as well.  The server
   responds with a status or error code followed by a MIME-like message
   containing information about the server and information about carried
   data and it can include a message body.  It is possible to specify a
   data format for the message body using MIME media types [RFC2045].
   Furthermore, the protocol has numerous additional features; features
   relevant to pseudotransport are described below.

   Content negotiation, specified in [RFC7231], is a mechanism provided
   by HTTP for selecting a representation on a requested resource.  The
   client and server negotiate acceptable data formats, charsets, data



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   encoding (e.g. data can be transferred compressed, gzip), etc.  HTTP
   can accommodate exchange of messages as well as data streaming (using
   chunked transfer encoding [RFC7230]).  It is also possible to request
   a part of a resource using range requests specified in [RFC7233].
   The protocol provides powerful cache control signalling defined in
   [RFC7234].

   HTTP 1.1's and HTTP 2.0's persistent connections can be use to
   perform multiple request-response transactions during the life-time
   of a single HTTP connection.  Moreover, HTTP 2.0 connections can
   multiplex many request/response pairs in parallel on a single
   connection.  This reduces connection establishment overhead and the
   effect of TCP slow-start on each transaction, important for HTTP's
   primary use case.

   It is possible to combine HTTP with security mechanisms, like TLS
   (denoted by HTTPS), which adds protocol properties provided by such a
   mechanism (e.g. authentication, encryption, etc.).  TLS's
   Application-Layer Protocol Negotiation (ALPN) extension [RFC7301] can
   be used for HTTP version negotiation within TLS handshake which
   eliminates addition round-trip.  Arbitrary cookie strings, included
   as part of the MIME headers, are often used as bearer tokens in HTTP.

   Application layer protocols using HTTP as substrate may use existing
   method and data formats, or specify new methods and data formats.
   Furthermore some protocols may not fit a request/response paradigm
   and instead rely on HTTP to send messages (e.g. [RFC6546]).  Because
   HTTP is working in many restricted infrastructures, it is also used
   to tunnel other application-layer protocols.

3.10.2.  Interface Description

   There are many HTTP libraries available exposing different APIs.  The
   APIs provide a way to specify a request by providing a URI, a method,
   request modifiers and optionally a request body.  For the response,
   callbacks can be registered that will be invoked when the response is
   received.  If TLS is used, API expose a registration of callbacks in
   case a server requests client authentication and when certificate
   verification is needed.

   World Wide Web Consortium (W3C) standardized the XMLHttpRequest API
   [XHR], an API that can be use for sending HTTP/HTTPS requests and
   receiving server responses.  Besides XML data format, request and
   response data format can also be JSON, HTML and plain text.
   Specifically JavaScript and XMLHttpRequest are a ubiquitous
   programming model for websites, and more general applications, where
   native code is less attractive.




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   Representational State Transfer (REST) [REST] is another example how
   applications can use HTTP as transport protocol.  REST is an
   architecture style for building application on the Internet.  It uses
   HTTP as a communication protocol.

3.10.3.  Transport Protocol Components

   The transport protocol components provided by HTTP, when used as a
   pseudotransport, are:

   o  unicast

   o  reliable delivery

   o  ordered delivery

   o  message and stream-oriented

   o  object range request

   o  message content type negotiation

   o  congestion control

   HTTPS (HTTP over TLS) additionally provides the following components:

   o  authentication (of one or both ends of a connection)

   o  confidentiality

   o  integrity protection

3.11.  WebSockets

   [RFC6455]

   [EDITOR'S NOTE: Salvatore Loreto will contribute text for this
   section.]

3.11.1.  Protocol Description

3.11.2.  Interface Description









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3.11.3.  Transport Protocol Components

4.  Transport Service Features

   [EDITOR'S NOTE: This section is still work-in-progress.  This list is
   probably not complete and/or too detailed.]

   The transport protocol components analyzed in this document which can
   be used as a basis for defining common transport service features,
   normalized and separated into categories, are as follows:

   o  Control Functions

      *  Addressing

         +  unicast

         +  broadcast (IPv4 only)

         +  multicast

         +  anycast

         +  something on ports and NAT

      *  Multihoming support

         +  multihoming for resilience

         +  multihoming for mobility

            -  specify handover latency?

         +  multihoming for load-balancing

            -  specify interleaving delay?

      *  Multiplexing

         +  application to port mapping

         +  single vs. multiple streaming

   o  Delivery

      *  reliability

         +  reliable delivery



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         +  partially reliable delivery

            -  packet erasure coding

         +  unreliable delivery

            -  drop notification

            -  Integrity protection

               o  checksum for error detection

               o  partial checksum protection

               o  checksum optional

      *  ordering

         +  ordered delivery

         +  unordered delivery

            -  unordered delivery of in-memory data

      *  type/framing

         +  stream-oriented delivery

         +  message-oriented delivery

         +  object-oriented delivery of discrete data or file items

            -  object content type negotiation

         +  range-based partical object transmission

         +  file bulk content objects

   o  Transmission control

      *  rate control

         +  timer-based

         +  ACK-based

      *  congestion control




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      *  flow control

      *  segmentation

      *  data/message bundling (Nagle's algorithm)

      *  stream scheduling prioritization

   o  Security

      *  authentication of one end of a connection

      *  authentication of both ends of a connection

      *  confidentiality

      *  cryptographic integrity protection

   The next revision of this document will define transport service
   features based upon this list.

   [EDITOR'S NOTE: this section will drawn from the candidate features
   provided by protocol components in the previous section - please
   discuss on taps@ietf.org list]

4.1.  Complete Protocol Feature Matrix

   [EDITOR'S NOTE: Dave Thaler has signed up as a contributor for this
   section.  Michael Welzl also has a beginning of a matrix which could
   be useful here.]

   [EDITOR'S NOTE: The below is a strawman proposal below by Gorry
   Fairhurst for initial discussion]

   The table below summarises protocol mechanisms that have been
   standardised.  It does not make an assessment on whether specific
   implementations are fully compliant to these specifications.

   +-----------------+---------+---------+---------+---------+---------+
   | Mechanism       | UDP     | UDP-L   | DCCP    | SCTP    | TCP     |
   +-----------------+---------+---------+---------+---------+---------+
   | Unicast         | Yes     | Yes     | Yes     | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Mcast/IPv4Bcast | Yes(2)  | Yes     | No      | No      | No      |
   |                 |         |         |         |         |         |
   | Port Mux        | Yes     | Yes     | Yes     | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Mode            | Dgram   | Dgram   | Dgram   | Dgram   | Stream  |



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   |                 |         |         |         |         |         |
   | Connected       | No      | No      | Yes     | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Data bundling   | No      | No      | No      | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Feature Nego    | No      | No      | Yes     | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Options         | No      | No      | Support | Support | Support |
   |                 |         |         |         |         |         |
   | Data priority   | *       | *       | *       | Yes     | No      |
   |                 |         |         |         |         |         |
   | Data bundling   | No      | No      | No      | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Reliability     | None    | None    | None    | Select  | Full    |
   |                 |         |         |         |         |         |
   | Ordered deliv   | No      | No      | No      | Stream  | Yes     |
   |                 |         |         |         |         |         |
   | Corruption Tol. | No      | Support | Support | No      | No      |
   |                 |         |         |         |         |         |
   | Flow Control    | No      | No      | Support | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | PMTU/PLPMTU     | (1)     | (1)     | Yes     | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | Cong Control    | (1)     | (1)     | Yes     | Yes     | Yes     |
   |                 |         |         |         |         |         |
   | ECN Support     | (1)     | (1)     | Yes     | TBD     | Yes     |
   |                 |         |         |         |         |         |
   | NAT support     | Limited | Limited | Support | TBD     | Support |
   |                 |         |         |         |         |         |
   | Security        | DTLS    | DTLS    | DTLS    | DTLS    | TLS, AO |
   |                 |         |         |         |         |         |
   | UDP encaps      | N/A     | None    | Yes     | Yes     | None    |
   |                 |         |         |         |         |         |
   | RTP support     | Support | Support | Support | ?       | Support |
   +-----------------+---------+---------+---------+---------+---------+

   Note (1): this feature requires support in an upper layer protocol.

   Note (2): this feature requires support in an upper layer protocol
   when used with IPv6.

5.  IANA Considerations

   This document has no considerations for IANA.

6.  Security Considerations





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   This document surveys existing transport protocols and protocols
   providing transport-like services.  Confidentiality, integrity, and
   authenticity are among the features provided by those services.  This
   document does not specify any new components or mechanisms for
   providing these features.  Each RFC listed in this document discusses
   the security considerations of the specification it contains.

7.  Contributors

   [Editor's Note: turn this into a real contributors section with
   addresses once we figure out how to trick the toolchain into doing
   so]

   o  Section 3.2 on MPTCP was contributed by Simone Ferlin-Oliviera
      (ferlin@simula.no) and Olivier Mehani
      (olivier.mehani@nicta.com.au)

   o  Section 3.4 on UDP was contributed by Kevin Fall (kfall@kfall.com)

   o  Section 3.3 on SCTP was contributed by Michael Tuexen (tuexen@fh-
      muenster.de)

   o  Section 3.8 on NORM was contributed by Brian Adamson
      (brian.adamson@nrl.navy.mil)

   o  Section 3.9 on MPTCP was contributed by Ralph Holz
      (ralph.holz@nicta.com.au) and Olivier Mehani
      (olivier.mehani@nicta.com.au)

   o  Section 3.10 on HTTP was contributed by Dragana Damjanovic
      (ddamjanovic@mozilla.com)

8.  Acknowledgments

   Thanks to Karen Nielsen, Joe Touch, and Michael Welzl for the
   comments, feedback, and discussion.  This work is partially supported
   by the European Commission under grant agreement FP7-ICT-318627
   mPlane; support does not imply endorsement.

   [EDITOR'S NOTE: add H2020-NEAT ack].

9.  References









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9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

9.2.  Informative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

   [RFC3205]  Moore, K., "On the use of HTTP as a Substrate", BCP 56,
              RFC 3205, February 2002.





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   [RFC3390]  Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
              Initial Window", RFC 3390, October 2002.

   [RFC3436]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
              Layer Security over Stream Control Transmission Protocol",
              RFC 3436, December 2002.

   [RFC3452]  Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
              M., and J. Crowcroft, "Forward Error Correction (FEC)
              Building Block", RFC 3452, December 2002.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758, May 2004.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

   [RFC4324]  Royer, D., Babics, G., and S. Mansour, "Calendar Access
              Protocol (CAP)", RFC 4324, December 2005.

   [RFC4336]  Floyd, S., Handley, M., and E. Kohler, "Problem Statement
              for the Datagram Congestion Control Protocol (DCCP)", RFC
              4336, March 2006.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4341]  Floyd, S. and E. Kohler, "Profile for Datagram Congestion
              Control Protocol (DCCP) Congestion Control ID 2: TCP-like
              Congestion Control", RFC 4341, March 2006.

   [RFC4342]  Floyd, S., Kohler, E., and J. Padhye, "Profile for
              Datagram Congestion Control Protocol (DCCP) Congestion
              Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
              March 2006.

   [RFC4614]  Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap
              for Transmission Control Protocol (TCP) Specification
              Documents", RFC 4614, September 2006.

   [RFC4654]  Widmer, J. and M. Handley, "TCP-Friendly Multicast
              Congestion Control (TFMCC): Protocol Specification", RFC
              4654, August 2006.






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   [RFC4820]  Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
              Parameter for the Stream Control Transmission Protocol
              (SCTP)", RFC 4820, March 2007.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061, September
              2007.

   [RFC5097]  Renker, G. and G. Fairhurst, "MIB for the UDP-Lite
              protocol", RFC 5097, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification", RFC
              5348, September 2008.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, November
              2008.

   [RFC5595]  Fairhurst, G., "The Datagram Congestion Control Protocol
              (DCCP) Service Codes", RFC 5595, September 2009.

   [RFC5596]  Fairhurst, G., "Datagram Congestion Control Protocol
              (DCCP) Simultaneous-Open Technique to Facilitate NAT/
              Middlebox Traversal", RFC 5596, September 2009.

   [RFC5662]  Shepler, S., Eisler, M., and D. Noveck, "Network File
              System (NFS) Version 4 Minor Version 1 External Data
              Representation Standard (XDR) Description", RFC 5662,
              January 2010.

   [RFC5672]  Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM)
              Signatures -- Update", RFC 5672, August 2009.




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   [RFC5740]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "NACK-Oriented Reliable Multicast (NORM) Transport
              Protocol", RFC 5740, November 2009.

   [RFC6773]  Phelan, T., Fairhurst, G., and C. Perkins, "DCCP-UDP: A
              Datagram Congestion Control Protocol UDP Encapsulation for
              NAT Traversal", RFC 6773, November 2012.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083, January 2011.

   [RFC6093]  Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, January 2011.

   [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration", RFC
              6525, February 2012.

   [RFC6546]  Trammell, B., "Transport of Real-time Inter-network
              Defense (RID) Messages over HTTP/TLS", RFC 6546, April
              2012.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298, June
              2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
              Congestion Control for Multipath Transport Protocols", RFC
              6356, October 2011.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
              6455, December 2011.

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458, December 2011.





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   [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, July 2012.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

   [RFC6897]  Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
              Interface Considerations", RFC 6897, March 2013.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935, April 2013.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, April 2013.

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951, May 2013.

   [RFC7053]  Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
              IMMEDIATELY Extension for the Stream Control Transmission
              Protocol", RFC 7053, November 2013.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

   [RFC7232]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Conditional Requests", RFC 7232, June 2014.

   [RFC7233]  Fielding, R., Lafon, Y., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Range Requests", RFC 7233,
              June 2014.

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
              2014.

   [RFC7235]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", RFC 7235, June 2014.






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   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, July 2014.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, "TCP Extensions for High Performance", RFC
              7323, September 2014.

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, February 2015.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, May 2015.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol Version 2 (HTTP/2)", RFC 7540, May 2015.

   [I-D.ietf-aqm-ecn-benefits]
              Fairhurst, G. and M. Welzl, "The Benefits of using
              Explicit Congestion Notification (ECN)", draft-ietf-aqm-
              ecn-benefits-05 (work in progress), June 2015.

   [I-D.ietf-tsvwg-sctp-dtls-encaps]
              Tuexen, M., Stewart, R., Jesup, R., and S. Loreto, "DTLS
              Encapsulation of SCTP Packets", draft-ietf-tsvwg-sctp-
              dtls-encaps-09 (work in progress), January 2015.

   [I-D.ietf-tsvwg-sctp-prpolicies]
              Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
              "Additional Policies for the Partial Reliability Extension
              of the Stream Control Transmission Protocol", draft-ietf-
              tsvwg-sctp-prpolicies-07 (work in progress), February
              2015.

   [I-D.ietf-tsvwg-sctp-ndata]
              Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol", draft-ietf-tsvwg-
              sctp-ndata-03 (work in progress), March 2015.

   [I-D.ietf-tsvwg-natsupp]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation
              Support", draft-ietf-tsvwg-natsupp-07 (work in progress),
              February 2015.



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   [XHR]      van Kesteren, A., Aubourg, J., Song, J., and H. Steen,
              "XMLHttpRequest working draft
              (http://www.w3.org/TR/XMLHttpRequest/)", 2000.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures, Ph. D. (UC Irvune),
              Chapter 5: Representational State Transfer", 2000.

Authors' Addresses

   Godred Fairhurst (editor)
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen AB24 3UE

   Email: gorry@erg.abdn.ac.uk


   Brian Trammell (editor)
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: ietf@trammell.ch


   Mirja Kuehlewind (editor)
   ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Email: mirja.kuehlewind@tik.ee.ethz.ch

















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