Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP- Lite) Transport Protocols
draft-ietf-taps-transports-usage-udp-05

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Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                                  T. Jones
Intended status: Informational                    University of Aberdeen
Expires: March 3, 2018                                   August 30, 2017

 Features of the User Datagram Protocol (UDP) and Lightweight UDP (UDP-
                       Lite) Transport Protocols
                draft-ietf-taps-transports-usage-udp-05

Abstract

   This is an informational document that describes the transport
   protocol interface primitives provided by the User Datagram Protocol
   (UDP) and the Lightweight User Datagram Protocol (UDP-Lite) transport
   protocols.  It identifies the datagram services exposed to
   applications and how an application can configure and use the
   features offered by the Internet datagram transport service.  RFCxxxx
   documents the usage of transport features provided by IETF transport
   protocols, describing the way UDP, UDP-Lite and other transport
   protocols expose their services to applications and how an
   application can configure and use the features that make up these
   services.  This document provides input to and context for that
   document, as well as offering a road map to documentation that may be
   of help to users of the UDP and UDP-Lite protocols.

   XXX RFC-Ed Note - please replace RFCxxxx with the published RFC
   number for I-D.ietf-taps-transports-usage, when these documents are
   both published XXX.

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 March 3, 2018.

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Copyright Notice

   Copyright (c) 2017 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  UDP and UDP-Lite Primitives . . . . . . . . . . . . . . . . .   4
     3.1.  Primitives Provided by UDP  . . . . . . . . . . . . . . .   4
       3.1.1.  Excluded Primitives . . . . . . . . . . . . . . . . .  11
     3.2.  Primitives Provided by UDP-Lite . . . . . . . . . . . . .  11
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Multicast Primitives . . . . . . . . . . . . . . . .  16
   Appendix B.  Revision Notes . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This document presents defined interactions between transport
   protocols and applications in the form of 'primitives' (function
   calls) for the User Datagram Protocol (UDP) [RFC0768] and the
   Lightweight User Datagram Protocol (UDP-Lite) [RFC3828].  In this
   usage, the word application refers to any program built on the
   datagram interface, and including tunnels and other upper layer
   protocols that use UDP and UDP-LIte.

   The de facto standard application programming interface (API) for
   TCP/IP applications is the "socket" interface [POSIX].  An
   application can use the recv() and send() POSIX functions as well as
   the recvfrom() and sendto() and recvmsg() and sendmsg() functions.

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   The UDP and UDP-Lite sockets API differs from that for TCP in several
   key ways.  (Examples of usage of this API are provided in [STEVENS].)

   Additional functions in the sockets API are provided by socket
   options, examples include:

   o  SO_REUSEADDR specifies the rules for validating addresses supplied
      to bind() should allow reuse of local addresses.

   o  SO_REUSEPORT specifies that the rules for validating ports
      supplied to bind() should allow reuse of a local port.

   o  IP_RECVTTL returns the IP Time To Live (TTL) field from IP header
      of a received datagram.

   Some platforms also offer applications the ability to directly
   assemble and transmit IP packets through "raw sockets" or similar
   facilities.  The raw socket API is a second, more cumbersome method
   of using UDP.  The use of this API is discussed in the RFC series in
   the UDP Guidelines [RFC8085].

   The list of transport service features and primitives in this
   document is strictly based on the parts of protocol specifications in
   RFC-series that relate to what the transport protocol provides to an
   application that uses it and how the application interacts with the
   transport protocol.  Primitives can be invoked by an application or a
   transport protocol; the latter type is called an "event".

   The description in Section 3 follows the methodology defined by the
   IETF TAPS working group in[I-D.ietf-taps-transports-usage].
   Specifically, this document provides the first pass of this process,
   which discusses the relevant RFC text describing primitives for each
   protocol.  [I-D.ietf-taps-transports-usage] uses this input to
   document the usage of transport features provided by IETF transport
   protocols, describing the way UDP, UDP-Lite and other transport
   protocols expose their services to applications and how an
   application can configure and use the features that make up these
   services.

   The presented road map to documentation of the transport interface
   may also help developers working with UDP and UDP-Lite.

2.  Terminology

   This document provides details for the Pass 1 analysis of UDP and
   UDP-Lite that is used in "Usage of Transport Features Provided by
   IETF Transport Protocols" [I-D.ietf-taps-transports-usage].  It uses
   common terminology defined in that document and also refers to the

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   terminology of RFC 2119 [RFC2119], but does not itself define new
   terms.

3.  UDP and UDP-Lite Primitives

   The User Datagram Protocol (UDP) [RFC0768][RFC8200] and UDP-Lite
   protocols [RFC3828] are IETF standards track transport protocols.
   These protocols provide unidirectional, datagram services that
   preserve message boundaries.

   This section summarizes the relevant text parts of the RFCs
   describing the UDP and UDP-Lite protocols, focusing on what the
   transport protocols provide to the application and how the transport
   is used (based on abstract API descriptions, where they are
   available).  It describes how UDP is used with IPv4 or IPv6 to send
   unicast or anycast datagrams and the use to send broadcast datagrams
   for IPv4.  A set of network-layer primitives required to use UDP or
   UDP-Lite with IP multicast (for IPv4 and IPv6) have been specified in
   the RFC series.  Appendix A describes where to find documentation for
   network-layer primitives required to use UDP or UDP-Lite with IP
   multicast (for IPv4 and IPv6).

3.1.  Primitives Provided by UDP

   The User Datagram Protocol (UDP) [RFC0768] States: "This User
   Datagram Protocol (UDP) is defined to make available a datagram mode
   of packet-switched computer communication in the environment of an
   interconnected set of computer networks."  It "provides a procedure
   for application programs to send messages to other programs with a
   minimum of protocol mechanism (..)".

   The User Interface section of RFC768 states that the user interface
   to an application should be able to create receive, source, and
   destination ports and addresses (setting the source and destination
   ports and addresses), and provide operations to receive data based on
   ports (with an indication of source port and address).  Operations
   should be provided that allows datagrams be sent, specifying the
   source and destination ports and addresses to be used.

   UDP has been defined for IPv6 [RFC8200], together with API extensions
   for a Basic Socket Interface Extensions for IPv6 [RFC3493].
   [RFC6935] and [RFC6936] defines an update to the UDP transport
   originally specified in RFC2460.  This enables use of a zero UDP
   checksum mode with a tunnel protocol, providing that the method
   satisfies the requirements in the corresponding applicability
   statement [RFC6936].

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   UDP offers only a basic transport interface.  UDP datagrams may be
   directly sent and received, without exchanging messages between the
   endpoints to setup a connection (i.e., no handshake is performed by
   the transport protocol prior to communication).  Using the sockets
   API, applications can receive packets from more than one IP source
   address on a single UDP socket.  Common support allows specification
   of the local IP address, destination IP address, local port and
   destination port values.  Any or all of these can be indicated, with
   defaults supplied by the local system when these are not specified.
   The local endpoint is set using the BIND call and set on the remote
   endpoint using the CONNECT call.  The CLOSE function has local
   significance only.  It does not impact the status of the remote
   endpoint.

   Neither UDP nor UDP-Lite provide congestion control, retransmission,
   nor do they provide mechanisms for application-level packetization
   that would avoid IP fragmentation and other transport functions.
   This means that applications using UDP need to provide additional
   functions on top of the UDP transport API [RFC8085].  Some transport
   functions require parameters to be passed through the API to control
   the network layer (IPv4 or IPv6).  These additional primitives could
   be considered a part of the network layer (e.g., control of the
   setting of the Don't Fragment (DF) flag on a transmitted IPv4
   datagram), but are nonetheless essential to allow a user of the UDP
   API to implement functions that are normally associated with the
   transport layer (such as probing for the path maximum transmission
   size).  This document includes such primitives.

   Guidance on the use of the services provided by UDP is provided in
   the UDP Guidelines [RFC8085].  This also states "many operating
   systems also allow a UDP socket to be connected, i.e., to bind a UDP
   socket to a specific pair of addresses and ports.  This is similar to
   the corresponding TCP sockets API functionality.  However, 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.  Binding a UDP socket does not establish a
   connection - UDP does not notify the remote endpoint when a local UDP
   socket is bound.  Binding a socket also allows configuring options
   that affect the UDP or IP layers, for example, use of the UDP
   checksum or the IP Timestamp Option.  On some stacks, a bound socket
   also allows an application to be notified when Internet Control
   Message (ICMP) error messages are received for its transmissions
   [RFC1122]."

   The POSIX Base Specifications [POSIX] define an API that offers
   mechanisms for an application to receive asynchronous data events at
   the socket layer.  Calls such as "poll", "select" or "queue" allow an

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   application to be notified when data has arrived at a socket or when
   a socket has flushed its buffers.

   A callback-driven API to the network interface can be structured on
   top of these calls.  Implicit connection setup allows an application
   to delegate connection life management to the transport API.  The
   transport API uses protocol primitives to offer the automated service
   to the application via the sockets API.  By combining UDP primitives
   (CONNECT.UDP, SEND.UDP), a higher level API could offer a similar
   service.

   The following datagram primitives are specified:

   CONNECT:  The CONNECT primitive allows the association of source and
      destination port sets to a socket to enable creation of a
      'connection' for UDP traffic.  This UDP connection allows an
      application to be notified of errors received from the network
      stack and provides a shorthand access to the send and receive
      primitives.  Since UDP is itself connectionless, no datagrams are
      sent because this primitive is executed.  A further connect call
      can be used to change the association.

      Two forms of usage may be identified for the CONNECT primitive:

      1.  bind(): A bind operation sets the local port, either
          implicitly, triggered by a "sendto" operation on an unbound
          unconnected socket using an ephemeral port.  Or by an explicit
          "bind" to use a configured or well-known port.

      2.  bind(); connect(): A bind operation that is followed by a
          CONNECT primitive.  The bind operation establishes the use of
          a known local port for datagrams, rather than using an
          ephemeral port.  The connect operation specifies a known
          address port combination to be used by default for future
          datagrams.  This form is used either after receiving a
          datagram from an endpoint that causes the creation of a
          connection, or can be triggered by third party configuration
          or a protocol trigger (such as reception of a UDP Service
          Description Protocol, SDP [RFC4566], record).

   LISTEN:  The roles of a client and a server are often not appropriate
      for UDP, where connections can be peer-to-peer.  The listening
      functions are performed using one of the forms of the CONNECT
      primitive described above.

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   SEND:  The SEND primitive hands over a provided number of bytes that
      UDP should send to the other side of a UDP connection in a UDP
      datagram.  The primitive can be used by an application to directly
      send datagrams to an endpoint defined by an address/port pair.  If
      a connection has been created, then the address/port pair is
      inferred from the current connection for the socket.  Connecting a
      socket allows network errors to be returned to the application as
      a notification on the send primitive.  Messages passed to the send
      primitive that cannot be sent atomically in an IP packet will not
      be sent by the network layer, generating an error.

   RECEIVE:  The RECEIVE primitive allocates a receiving buffer to
      accommodate a received datagram.  The primitive returns the number
      of bytes provided from a received UDP datagram.  Section 4.1.3.5
      of the requirements of Internet hosts [RFC1122] states "When a UDP
      datagram is received, its specific-destination address MUST be
      passed up to the application layer."

   CHECKSUM_ENABLED:  The optional CHECKSUM_ENABLED primitive controls
      whether a sender enables the UDP checksum when sending datagrams (
      [RFC0768] and [RFC6935] [RFC6936] [RFC8085]).  When unset, this
      overrides the default UDP behaviour, disabling the checksum on
      sending.  Section 4.1.3.4 of the requirements for Internet hosts
      [RFC1122] states "An application MAY optionally be able to control
      whether a UDP checksum will be generated, but it MUST default to
      checksumming on."

   REQUIRE_CHECKSUM:  The optional REQUIRE_CHECKSUM primitive determines
      whether UDP datagrams received with a zero checksum are permitted
      or discarded, UDP defaults to requiring checksums.
      Section 4.1.3.4 of the requirements for Internet hosts [RFC1122]
      states "An application MAY optionally be able to control whether
      UDP datagrams without checksums should be discarded or passed to
      the application."  Section 3.1 of the specification for UDP-Lite
      [RFC3828] requires that the checksum field is non-zero, and hence
      the UDP-Lite API must discard all datagrams received with a zero
      checksum.

   SET_IP_OPTIONS:  The SET_IP_OPTIONS primitive requests the network-
      layer to send a datagram with the specified IP options.
      Section 4.1.3.2 of the requirements for Internet hosts[RFC1122]
      states that an "application MUST be able to specify IP options to
      be sent in its UDP datagrams, and UDP MUST pass these options to
      the IP layer."

   GET_IP_OPTIONS:  The GET_IP_OPTIONS primitive retrieves the IP
      options of a datagram received at the network-layer.
      Section 4.1.3.2 of the requirements for Internet hosts[RFC1122]

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      states that a UDP receiver "MUST pass any IP option that it
      receives from the IP layer transparently to the application
      layer".

   SET_DF:  The SET_DF primitive allows the network-layer to fragment
      packets using the Fragment Offset in IPv4 [RFC6864] and a host to
      use Fragment Headers in IPv6 [RFC8200].  The SET_DF primitive sets
      the Don't Fragment (DF) flag in the IPv4 packet header that
      carries a UDP datagram, which allows routers to fragment IPv4
      packets.  Although some specific applications rely on
      fragmentation support, in general, a UDP application should
      implement a method that avoids IP fragmentation (section 4 of
      [RFC8085]).  NOTE: In many other IETF transports (e.g., TCP, SCTP)
      the transport provides the support needed to use DF.  However,
      when using UDP, the application is responsible for the techniques
      needed to discover the effective Path Maximum Transmission Unit
      (PMTU) allowed on the network path, coordinating with the network
      layer.  Classical PMTU Discovery (PMTUD) [RFC1191] relies upon the
      network path returning ICMP Fragmentation Needed or ICMPv6 Packet
      Too Big messages to the sender.  When these ICMP messages are not
      delivered (or filtered) a sender is unable to learn the actual
      path MTU, and UDP Datagrams larger than the PMTU will be "black
      holed".  To avoid this, an application can instead implement
      Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821] that
      does not rely upon network support for ICMPv6 messages and is
      therefore considered more robust than standard PMTUD, as
      recommended in [RFC8085] and [RFC8201].

   GET_MMS_S:  The GET_MMS_S primitive retrieves a network-layer value
      that indicates the maximum message size (MMS) that may be sent at
      the transport layer using a non-fragmented IP packet from the
      configured interface.  This value is specified in section 6.1 of
      [RFC1191] and section 5.1 of [RFC8201].  It is calculated from
      Effective Maximum Transmit Unit for Sending (EMTU_S), and the link
      MTU for the given source IP address.  This takes into account the
      size of the IP header plus space reserved by the IP layer for
      additional headers (if any).  UDP applications should use this
      value as part of a method to avoid sending UDP datagrams that
      would result in IP packets that exceed the effective PMTU allowed
      across the network path.  The effective PMTU (specified in
      Section 1 of [RFC1191]) is equivalent to the EMTU_S (specified in
      [RFC1122]).  The specification of PLPMTUD [RFC4821] states: "If
      PLPMTUD updates the MTU for a particular path, all Packetization
      Layer sessions that share the path representation (as described in
      Section 5.2) SHOULD be notified to make use of the new MTU and
      make the required congestion control adjustments".

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   GET_MMS_R:  The GET_MMS_R primitive retrieves a network-layer value
      that indicates the maximum message size (MMS) that may be received
      at the transport layer from the configured interface.  This value
      is specified in section 3.1 of [RFC1191].  It is calculated from
      Effective Maximum Transmit Unit for Receiving (EMTU_R), and the
      link MTU for the given source IP address, and takes into account
      the size of the IP header plus space reserved by the IP layer for
      additional headers (if any).

   SET_TTL:  The SET_TTL primitive sets the hop limit (TTL field) in the
      network-layer that is used in the IPv4 header of a packet that
      carries an UDP datagram.  This is used to limit the scope of
      unicast datagrams.  Section 3.2.2.4 of the requirements for
      Internet hosts [RFC1122] states an "incoming Time Exceeded message
      MUST be passed to the transport layer".

   GET_TTL:  The GET_TTL primitive retrieves the value of the TTL field
      in a network packet received at the network-layer.
      Section 3.2.2.4 of the requirements for Internet hosts [RFC1122]
      states that a UDP receiver "MAY pass the received ToS up to the
      application layer" When used for applications such as the
      Generalized TTL Security Mechanism (GTSM) [RFC5082], this needs
      the UDP receiver API to pass the received value of this field to
      the application.

   SET_IPV6_UNICAST_HOPS:  The SET_IPV6_UNICAST_HOPS primitive sets the
      network-layer hop limit field in an IPv6 packet header [RFC8200]
      carrying a UDP datagram.  For IPv6 unicast datagrams, this is
      functionally equivalent to the SET_TTL IPv4 function.

   GET_IPV6_UNICAST_HOPS:  The GET_IPV6_UNICAST_HOPS primitive is a
      network-layer function that reads the hop count in the IPv6 header
      [RFC8200] information of a received UDP datagram.  For IPv6
      unicast datagrams, this is functionally equivalent to the GET_TTL
      IPv4 function.

   SET_DSCP:  The SET_DSCP primitive is a network-layer function that
      sets the Differentiated Services (DiffServ) Code Point, DSCP, (or
      the legacy Type of Service, ToS) value [RFC2474] to be used in the
      field of an IP header of a packet that carries a UDP datagram.
      Section 2.4 of the requirements for Internet hosts[RFC1123] states
      that "Applications MUST select appropriate ToS values when they
      invoke transport layer services, and these values MUST be
      configurable.".  The application should be able to change the ToS
      during the connection lifetime, and the ToS value should be passed
      to the IP layer unchanged.  Section 4.1.4 of [RFC1122] also states
      that on reception the "UDP MAY pass the received ToS value up to
      the application layer".  The DiffServ model [RFC2475] [RFC3260]

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      replaces this field in the IP Header assigning the six most
      significant bits to carry the DSCP field [RFC2474].  Preserving
      the intention of the host requirements [RFC1122] to allow the
      application to specify the "Type of Service", this should be
      interpreted to mean that an API should allow the application to
      set the DSCP.  Section 3.1.6 of the UDP Guidelines [RFC8085]
      describes the way UDP applications should use this field.
      Normally a UDP socket will assign a single DSCP value to all
      datagrams in a flow, but a sender is allowed to use different DSCP
      values for datagrams within the same flow in certain
      cases[RFC8085].  There are guidelines for WebRTC that illustrate
      this use [RFC7657].

   SET_ECN:  The SET_ECN primitive is a network-layer function that sets
      the Explicit Congestion Notification (ECN) field in the IP Header
      of a UDP datagram.  The ECN field defaults to a value of 00.  When
      the use of the ToS field was redefined by DiffServ [RFC3260], 2
      bits of the field were assigned to support ECN [RFC3168].
      Section 3.1.5 of the UDP Guidelines [RFC8085] describes the way
      UDP applications should use this field.  NOTE: In many other IETF
      transports (e.g., TCP) the transport provides the support needed
      to use ECN, when using UDP, the application or higher layer
      protocol is itself responsible for the techniques needed to use
      ECN.

   GET_ECN:  The GET_ECN primitive is a network-layer function that
      returns the value of the ECN field in the IP Header of a received
      UDP datagram.  Section 3.1.5 of the UDP Guidelines [RFC8085]
      states that a UDP receiver "MUST check the ECN field at the
      receiver for each UDP datagram that it receives on this port",
      requiring the UDP receiver API to pass to pass the received ECN
      field up to the application layer to enable appropriate congestion
      feedback.

   ERROR_REPORT  The ERROR_REPORT event informs an application of "soft
      errors", including the arrival of an ICMP or ICMPv6 error message.
      Section 4.1.4 of the host requirements [RFC1122] states "UDP MUST
      pass to the application layer all ICMP error messages that it
      receives from the IP layer."  For example, this event is required
      to implement ICMP-based Path MTU Discovery [RFC1191] [RFC8201].
      UDP applications must perform a CONNECT to receive ICMP errors.

   CLOSE:  The close primitive closes a connection.  No further
      datagrams can be sent or received.  Since UDP is itself
      connectionless, no datagrams are sent when this primitive is
      executed.

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3.1.1.  Excluded Primitives

   Section 3.4 of the host requirements [RFC1122] also describes
   "GET_MAXSIZES, GET_SRCADDR (Section 3.3.4.3) and ADVISE_DELIVPROB:".
   These mechanisms are no longer used.  It also specifies use of the
   Source Quench ICMP message, which has since been deprecated
   [RFC6633].

   The IPV6_V6ONLY function is a network-layer primitive that applies to
   all transport services, defined in Section 5.3 of the basic socket
   interface for IPv6 [RFC3493].  This restricts the use of information
   from the name resolver to only allow communication of AF_INET6
   sockets to use IPv6 only.  This is not considered part of the
   transport service.

3.2.  Primitives Provided by UDP-Lite

   The Lightweight User Datagram Protocol (UDP-Lite) [RFC3828] provides
   similar services to UDP.  It changed the semantics of the UDP
   "payload length" field to that of a "checksum coverage length" field.
   UDP-Lite requires the pseudo-header checksum to be computed at the
   sender and checked at a receiver.  Apart from the length and coverage
   changes, UDP-Lite is semantically identical to UDP.

   The sending interface of UDP-Lite differs from that of UDP by the
   addition of a single (socket) option that communicates the checksum
   coverage length.  This specifies the intended checksum coverage, with
   the remaining unprotected part of the payload called the "error-
   insensitive part".

   The receiving interface of UDP-Lite differs from that of UDP by the
   addition of a single (socket) option that specifies the minimum
   acceptable checksum coverage.  The UDP-Lite Management Information
   Base (MIB) [RFC5097] further defines the checksum coverage method.
   Guidance on the use of services provided by UDP-Lite is provided in
   the UDP Guidelines [RFC8085].

   UDP-Lite requires use of the UDP or UDP-Lite checksum, and hence it
   is not permitted to use the "DISABLE_CHECKSUM:" function to disable
   use of a checksum, nor is it possible to disable receiver checksum
   processing using the "REQUIRE_CHECKSUM:" function . All other
   primitives and functions for UDP are permitted.

   In addition, the following are defined:

   SET_CHECKSUM_COVERAGE:  The SET_CHECKSUM_COVERAGE primitive sets the
      coverage area for a sent datagram.  UDP-Lite traffic uses this
      primitive to set the coverage length provided by the UDP checksum.

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      Section 3.3 of the UDP-Lite MIB [RFC5097] states that
      "Applications that wish to define the payload as partially
      insensitive to bit errors ...  Should do this by an explicit
      system call on the sender side."  The default is to provide the
      same coverage as for UDP.

   SET_MIN_COVERAGE  The SET_MIN_COVERAGE primitive sets the minimum
      acceptable coverage protection for received datagrams.  UDP-Lite
      traffic uses this primitive to set the coverage length that is
      checked on receive.  (Section 1.1 of the UDP-Lite MIB [RFC5097]
      describes the corresponding MIB entry as
      udpliteEndpointMinCoverage.)  Section 3.3 of the UDP-Lite
      specification [RFC3828] states that "applications that wish to
      receive payloads that were only partially covered by a checksum
      should inform the receiving system by an explicit system call".
      The default is to require only minimal coverage of the datagram
      payload.

4.  Acknowledgements

   This work was partially funded by the European Union's Horizon 2020
   research and innovation programme under grant agreement No. 644334
   (NEAT).  The views expressed are solely those of the author(s).
   Thanks to all who have commented or contributed, including Joe Touch,
   Ted Hardie and Aaron Falk, Tommy Pauly.

5.  IANA Considerations

   This memo includes no request to IANA.

   The authors request the section to be removed during conversion into
   an RFC by the RFC Editor.

6.  Security Considerations

   Security considerations for the use of UDP and UDP-Lite are provided
   in the referenced RFCs.  Security guidance for application usage is
   provided in the UDP-Guidelines [RFC8085].

7.  References

7.1.  Normative References

   [I-D.ietf-taps-transports-usage]
              Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
              Transport Features Provided by IETF Transport Protocols",
              draft-ietf-taps-transports-usage-08 (work in progress),
              August 2017.

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

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,
              <https://www.rfc-editor.org/info/rfc1112>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989, <https://www.rfc-
              editor.org/info/rfc1122>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989, <https://www.rfc-
              editor.org/info/rfc1123>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990, <https://www.rfc-
              editor.org/info/rfc1191>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, DOI 10.17487/RFC3493, February 2003,
              <https://www.rfc-editor.org/info/rfc3493>.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
              and G. Fairhurst, Ed., "The Lightweight User Datagram
              Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
              2004, <https://www.rfc-editor.org/info/rfc3828>.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,
              <https://www.rfc-editor.org/info/rfc6864>.

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   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013, <https://www.rfc-
              editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017, <https://www.rfc-
              editor.org/info/rfc8200>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017, <https://www.rfc-
              editor.org/info/rfc8201>.

7.2.  Informative References

   [POSIX]    "IEEE Std. 1003.1-2001, , "Standard for Information
              Technology - Portable Operating System Interface (POSIX)",
              Open Group Technical Standard: Base Specifications Issue
              6, ISO/IEC 9945:2002", December 2001.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998, <https://www.rfc-
              editor.org/info/rfc2474>.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
              <https://www.rfc-editor.org/info/rfc3260>.

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   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
              <https://www.rfc-editor.org/info/rfc3376>.

   [RFC3678]  Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
              Extensions for Multicast Source Filters", RFC 3678,
              DOI 10.17487/RFC3678, January 2004, <https://www.rfc-
              editor.org/info/rfc3678>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004, <https://www.rfc-
              editor.org/info/rfc3810>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4604]  Holbrook, H., Cain, B., and B. Haberman, "Using Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Protocol Version 2 (MLDv2) for Source-
              Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
              August 2006, <https://www.rfc-editor.org/info/rfc4604>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <https://www.rfc-editor.org/info/rfc5082>.

   [RFC5097]  Renker, G. and G. Fairhurst, "MIB for the UDP-Lite
              protocol", RFC 5097, DOI 10.17487/RFC5097, January 2008,
              <https://www.rfc-editor.org/info/rfc5097>.

   [RFC5790]  Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
              DOI 10.17487/RFC5790, February 2010, <https://www.rfc-
              editor.org/info/rfc5790>.

   [RFC6633]  Gont, F., "Deprecation of ICMP Source Quench Messages",
              RFC 6633, DOI 10.17487/RFC6633, May 2012,
              <https://www.rfc-editor.org/info/rfc6633>.

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   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015, <https://www.rfc-
              editor.org/info/rfc7657>.

   [STEVENS]  "Stevens, W., Fenner, B., and A. Rudoff, "UNIX Network
              Programming, The sockets Networking API", Addison-
              Wesley.", 2004.

Appendix A.  Multicast Primitives

   This appendix describes primitives that are used when UDP and UDP-
   Lite support IPv4/IPv6 Multicast.  Multicast services are not
   considered by the IETF TAPS WG, but the currently specified
   primitives are included for completeness in this appendix.  Guidance
   on the use of UDP and UDP-Lite for multicast services is provided in
   the UDP Guidelines[RFC8085].

   IP multicast may be supported using the Any Source Multicast (ASM)
   model or by the Source-Specific Multicast (SSM) model.  The latter
   requires use of a Multicast Source Filter (MSF) when specifying an IP
   multicast group destination address.

   Use of multicast requires additional primitives at the transport API
   that need to be called to coordinate operation of the IPv4 and IPv6
   network layer protocols.  For example, to receive datagrams sent to a
   group, an endpoint must first become a member of a multicast group at
   the network layer.  Local multicast reception is signalled for IPv4
   by the Internet Group Management Protocol (IGMP) [RFC3376] [RFC4604].
   IPv6 uses the equivalent Multicast Listener Discovery (MLD) protocol
   [RFC3810] [RFC5790], carried over ICMPv6.  A lightweight version of
   these protocols has also been specified [RFC5790].

   The following are defined:

   JoinGroup:  Section 7.1 of the Host Extensions for IP Multicasting
      [RFC1112] provides a function that allows receiving traffic from
      an IP multicast group.

   JoinLocalGroup:  Section 7.2 of the Host Extensions for IP
      Multicasting [RFC1112] provides a function that allows receiving
      traffic from a local IP multicast group.

   LeaveHostGroup:  Section 7.1 of the Host Extensions for IP
      Multicasting [RFC1112] provides a function that allows leaving an
      IP multicast group.

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   LeaveLocalGroup:  Section 7.2 of the Host Extensions for IP
      Multicasting [RFC1112] provides a function that allows leaving a
      local IP multicast group.

   IPV6_MULTICAST_IF:  Section 5.2 of the basic socket extensions for
      IPv6 [RFC3493] states that this sets the interface that will be
      used for outgoing multicast packets.

   IP_MULTICAST_TTL:  This sets the time to live field t to use for
      outgoing IPv4 multicast packets.  This is used to limit scope of
      multicast datagrams.  Methods such as The Generalized TTL Security
      Mechanism (GTSM) [RFC5082], set this value to ensure link-local
      transmission.  GTSM also requires the UDP receiver API to pass the
      received value of this field to the application.

   IPV6_MULTICAST_HOPS:  Section 5.2 of the basic socket extensions for
      IPv6 [RFC3493] states that sets the hop count to use for outgoing
      multicast IPv6 packets.  (This is equivalent to IP_MULTICAST_TTL
      used for IPv4 multicast).

   IPV6_MULTICAST_LOOP:  Section 5.2 of the basic socket extensions for
      IPv6 [RFC3493] states that this sets whether a copy of a datagram
      is looped back by the IP layer for local delivery when the
      datagram is sent to a group to which the sending host itself
      belongs).

   IPV6_JOIN_GROUP:  Section 5.2 of the basic socket extensions for IPv6
      [RFC3493] provides a function that allows an endpoint to join an
      IPv6 multicast group.

   SIOCGIPMSFILTER:  Section 8.1 of the socket interface for MSF
      [RFC3678] provides a function that allows reading the multicast
      source filters.

   SIOCSIPMSFILTER:  Section 8.1 of the socket interface for MSF
      [RFC3678] provides a function that allows setting/modifying the
      multicast source filters.

   IPV6_LEAVE_GROUP:  Section 5.2 of the basic socket extensions for
      IPv6 [RFC3493] provides a function that allows leaving an IPv6
      multicast group.

   Section 4.1.1 of the Socket Interface Extensions for MSF [RFC3678]
   updates the multicast interface to add support for MSF for IPv4 and
   IPv6 required by IGMPv3.  Three sets of API functionality are
   defined:

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   1.  IPv4 Basic (Delta-based) API.  "Each function call specifies a
       single source address which should be added to or removed from
       the existing filter for a given multicast group address on which
       to listen."

   2.  IPv4 Advanced (Full-state) API.  "This API allows an application
       to define a complete source-filter comprised of zero or more
       source addresses, and replace the previous filter with a new
       one."

   3.  Protocol-Independent Basic MSF (Delta-based) API.

   4.  Protocol-Independent Advanced MSF (Full-state) API.

   It specifies the following primitives:

   IP_ADD_MEMBERSHIP:  This is used to join an ASM group.

   IP_BLOCK_SOURCE:  This MSF can block data from a given multicast
      source to a given ASM or SSM group.

   IP_UNBLOCK_SOURCE:  This updates an MSF to undo a previous call to
      IP_UNBLOCK_SOURCE for an ASM or SSM group.

   IP_DROP_MEMBERSHIP:  This is used to leave an ASM or SSM group.  (In
      SSM, this drops all sources that have been joined for a particular
      group and interface.  The operations are the same as if the socket
      had been closed.)

   Section 4.1.2 of the socket interface for MSF [RFC3678] updates the
   interface to add IPv4 MSF support to IGMPv3 using ASM:

   IP_ADD_SOURCE_MEMBERSHIP:  This is used to join an SSM group.

   IP_DROP_SOURCE_MEMBERSHIP:  This is used to leave an SSM group.

   Section 4.1.2 of the socket interface for MSF [RFC3678] defines the
   Advanced (Full-state) API:

   setipv4sourcefilter  This is used to join an IPv4 multicast group, or
      to enable multicast from a specified source.

   getipv4sourcefilter:  This is used to leave an IPv4 multicast group,
      or to filter multicast from a specified source.

   Section 5.1 of the socket interface for MSF [RFC3678] specifies
   Protocol-Independent Multicast API functions:

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   MCAST_JOIN_GROUP  This is used to join an ASM group.

   MCAST_JOIN_SOURCE_GROUP  This is used to join an SSM group.

   MCAST_BLOCK_SOURCE:  This is used to block a source in an ASM group.

   MCAST_UNBLOCK_SOURCE:  This removes a previous MSF set by
      MCAST_BLOCK_SOURCE.

   MCAST_LEAVE_GROUP:  This leaves a SSM group.

   MCAST_LEAVE_GROUP:  This leaves an ASM or SSM group.

   Section 5.2 of the socket interface for MSF [RFC3678] specifies the
   Protocol-Independent Advanced MSF (Full-state) API applicable for
   both IPv4 and IPv6:

   setsourcefilter  This is used to join an IPv4 or IPv6 multicast
      group, or to enable multicast from a specified source.

   getsourcefilter:  This is used to leave an IPv4 or IPv6 multicast
      group, or to filter multicast from a specified source.

   The Lightweight IGMPv3 (LW_IGMPv3) and MLDv2 protocol [RFC5790]
   updates this interface (in Section 7.2 of RFC5790).

Appendix B.  Revision Notes

   Note to RFC-Editor: please remove this entire section prior to
   publication.

   Individual draft -00:

   o  This is the first version.  Comments and corrections are welcome
      directly to the authors or via the IETF TAPS working group mailing
      list.

   Individual draft -01:

   o  Includes ability of a UDP receiver to disallow zero checksum
      datagrams.

   o  Fixes to references and some connect on UDP usage.

   Individual draft -02:

   o  Fixes to address issues noted by WG.

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   o  Completed Multicast section to specify modern APIs.

   o  Noted comments on API usage for UDP.

   o  Feedback from various reviewers.

   Individual draft -03:

   o  Removes pass 2 and 3 of the TAPS analysis from this revision.
      These are expected to be incorporated into a combined draft of the
      TAPS WG.

   o  Fixed Typos.

   TAPS WG draft -00:

   o  Expected to progress with draft-ietf-taps-transports-usage of the
      TAPS WG.

   TAPS WG draft -01:

   o  No intentional changes were made to the specification of
      primitives, this update is editorial

   o  Reorganised text to eliminate the appendices.

   o  Editorial changes were make to complete the document for a WGLC.

   o  Rephrasing to eliminate using references as nouns, and to make
      text more consistent.

   o  One appendix was incorporated.

   o  This appendix was moved to the end (for later deletion by the RFC-
      Ed).

   TAPS WG draft -02:

   o  Updated to align with latest taps-transport-usage ID.

   o  Revised to clarify MTU usage and track work in IPv6 PMTU

   o  Usage of DF clarified.

   o

   TAPS WG draft -03

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   o  edit to MMS entries.

   TAPS WG draft -04

   o  Typos noted by Tommy Pauly 4/6/2017 and corrected here.

   o  Checked and corrected parenthesis and use of period.

   o  Document Shepherd review 7/2017.

   o  Fixed citations and abbreviations.

   TAPS WG draft -05

   o  AD review 8/2017.

   o  Updates to reflect published RFCs and refer to PMTUD for IPv6.

   o  Aligned to latest TAPS transport usage ID.

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Fraser Noble Building Aberdeen  AB24 3UE
   UK

   Email: gorry@erg.abdn.ac.uk

   Tom Jones
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   UK

   Email: tom@erg.abdn.ac.uk

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