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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: August 31, 2015                              M. Kuehlewind, Ed.
                                                              ETH Zurich
                                                       February 27, 2015


  Services provided by IETF transport protocols and congestion control
                               mechanisms
                     draft-ietf-taps-transports-03

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 August 31, 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.

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 TCP, 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) or RTP (over TCP or UDP).  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 that does not suffer head-of-line blocking when used with
   multiple stream, because it can accept blocks of data out of order,
   UDP-Lite provides partial integrity protection, and LEDBAT can
   provide low-priority "scavenger" communication.

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.

   [NOTE: The terminology below was presented at the TAPS WG meeting in
   Honolulu.  While the factoring of the terminology seems
   uncontroversial, there may be some entities which still require names
   (e.g. information about the interface between the transport and lower
   layers which could lead to the availability or unavailability of




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   certain transport protocol features).  Comments are welcome via the
   TAPS mailing list.]

   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,
   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.






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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.

   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




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   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 are:

   o  unicast

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

   o  port multiplexing

   o  reliable delivery

   o  ordered delivery for each byte stream

   o  error detection (checksum)



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   o  segmentation

   o  stream-oriented delivery in a single stream

   o  data bundling (Nagle's algorithm)

   o  flow control

   o  congestion control

   [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?]

3.2.  Multipath TCP (MP-TCP)

   [EDITOR'S NOTE: a few sentences describing Multipath TCP [RFC6824] go
   here.  Note that this adds transport-layer multihoming to the
   components TCP provides]

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.

   [EDITOR'S NOTE: Michael Tuexen and Karen Nielsen signed up as
   contributors for these sections.]

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




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   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.

   SCTP provides a message-oriented service.  Multiple small user
   messages can be bundled into a single SCTP packet to improve the
   efficiency.  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.  [RFC4821] defines a method to perform packetization layer path
   MTU discovery with probe packets using the padding chunks defined the
   [RFC4820].

   [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 ordered or un-ordered upon request by the upper
   layer.  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 and
   also allows to specify a scheduler for the sender side streams
   selection.  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



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   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.

   o  Limiting the number of retransmissions for each fragment of a user
      message.

   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] can be used to change these addresses 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.

   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].



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

   o  the SCTP Authentication extension defined in [RFC4895].

   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



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   support for detecting partial delivery of user messages and
   notifications.

   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 prioritised streams

   o  flow control (slow receiver function)

   o  message-oriented delivery

   o  congestion control

   o  application PDU bundling

   o  application PDU fragmentation and reassembly

   o  integrity check

   o  transport layer multihoming for resilience

   o  transport layer mobility

   [EDITOR'S NOTE: update this list.]







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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,
   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.

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

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].




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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
   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








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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].

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

3.5.1.  Protocol Description

   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.





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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

   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



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   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
   [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



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   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.

   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



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   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.]

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

   pseudo transport

   [NOTE: A few words on TLS [RFC5246] and DTLS [RFC6347] here, and how
   they get used by other protocols to meet security goals as an add-on
   interlayer above transport.]

3.8.1.  Protocol Description

3.8.2.  Interface Description

3.8.3.  Transport Protocol Components

3.9.  Hypertext Transport Protocol (HTTP) as a pseudotransport

   [RFC3205]

   [EDITOR'S NOTE: No identified contributor for this section yet.]

3.9.1.  Protocol Description

3.9.2.  Interface Description

3.9.3.  Transport Protocol Components

3.10.  WebSockets

   [RFC6455]

   [EDITOR'S NOTE: No identified contributor for this section yet.]








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3.10.1.  Protocol Description

3.10.2.  Interface Description

3.10.3.  Transport Protocol Components

4.  Transport Service Features

   [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.



























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




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   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

   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 3.4 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)

8.  Acknowledgments

   This work is partially supported by the European Commission under
   grant agreement FP7-ICT-318627 mPlane; support does not imply
   endorsement.

9.  References

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.




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   [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.

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

   [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.

   [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.

   [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.

   [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.





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   [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.

   [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.



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   [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.

   [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.

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

   [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.

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





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

   [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.

   [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.

   [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.

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

   [I-D.ietf-aqm-ecn-benefits]
              Welzl, M. and G. Fairhurst, "The Benefits and Pitfalls of
              using Explicit Congestion Notification (ECN)", draft-ietf-
              aqm-ecn-benefits-00 (work in progress), October 2014.

   [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 a New Data Chunk for the Stream
              Control Transmission Protocol", draft-ietf-tsvwg-sctp-
              ndata-02 (work in progress), January 2015.



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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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