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rfc:rfc8304

Internet Engineering Task Force (IETF) G. Fairhurst Request for Comments: 8304 T. Jones Category: Informational University of Aberdeen ISSN: 2070-1721 February 2018

       Transport Features of the User Datagram Protocol (UDP)
                   and Lightweight UDP (UDP-Lite)

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.  RFC
 8303 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 offers a road map to documentation that may help
 users of the UDP and UDP-Lite protocols.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8304.

Fairhurst & Jones Informational [Page 1] RFC 8304 UDP Transport Features February 2018

Copyright Notice

 Copyright (c) 2018 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 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 . . . . . . . . . . . . .  12
 4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   6.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
   6.2.  Informative References  . . . . . . . . . . . . . . . . .  15
 Appendix A.  Multicast Primitives . . . . . . . . . . . . . . . .  17
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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, including tunnels and other upper-layer protocols
 that use UDP and UDP-Lite.
 UDP is widely implemented and deployed.  It is used for a wide range
 of applications.  A special class of applications can derive benefit
 from having partially damaged payloads delivered, rather than
 discarded, when using paths that include error-prone links.
 Applications that can tolerate payload corruption can choose to use
 UDP-Lite instead of UDP and use the application programming interface

Fairhurst & Jones Informational [Page 2] RFC 8304 UDP Transport Features February 2018

 (API) to control checksum protection.  Conversely, UDP applications
 could choose to use UDP-Lite, but this is currently less widely
 deployed, and users could encounter paths that do not support
 UDP-Lite.  These topics are discussed more in Section 3.4 of "UDP
 Usage Guidelines" [RFC8085].
 The IEEE standard 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(), sendto(), recvmsg(), and
 sendmsg() functions.  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].)  In UDP and UDP-Lite, each datagram is a
 self-contained message of a specified length, and options at the
 transport layer can be used to set properties for all subsequent
 datagrams sent using a socket or changed for each datagram.  For
 datagrams, this can require the application to use the API to set
 IP-level information (IP Time To Live (TTL), Differentiated Services
 Code Point (DSCP), IP fragmentation, etc.) for the datagrams it sends
 and receives.  In contrast, when using TCP and other connection-
 oriented transports, the IP-level information normally either remains
 the same for the duration of a connection or is controlled by the
 transport protocol rather than the application.
 Socket options are used in the sockets API to provide additional
 functions.  For example, the IP_RECVTTL socket option is used by some
 UDP multicast applications to return the IP TTL field from the 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 sockets API is a second, more cumbersome, method
 to send UDP datagrams.  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
 the 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 [RFC8303].  Specifically, this document
 provides the first pass of this process, which discusses the relevant
 RFC text describing primitives for each protocol.  [RFC8303] uses
 this input to document the usage of transport features provided by
 IETF transport protocols, describing the way UDP, UDP-Lite, and other

Fairhurst & Jones Informational [Page 3] RFC 8304 UDP Transport Features February 2018

 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 "On the Usage of Transport Features Provided
 by IETF Transport Protocols" [RFC8303].  It uses common terminology
 defined in that document and also quotes RFCs that use the
 terminology of RFC 2119 [RFC2119].

3. UDP and UDP-Lite Primitives

 UDP [RFC0768] [RFC8200] and UDP-Lite [RFC3828] are IETF Standards
 Track transport protocols.  These protocols provide unidirectional,
 datagram services, supporting transmit and receive operations 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 is used 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

 "User Datagram Protocol" [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...This
    protocol provides a procedure for application programs to send
    messages to other programs with a minimum of protocol mechanism.

Fairhurst & Jones Informational [Page 4] RFC 8304 UDP Transport Features February 2018

 The User Interface section of RFC 768 states that the user interface
 to an application should allow
    the creation of new receive ports, receive operations on the
    receive ports that return the data octets and an indication of
    source port and source address, and an operation that allows a
    datagram to be sent, specifying the data, source and destination
    ports and addresses to be sent.
 UDP has been defined for IPv6 [RFC8200], together with API extensions
 for "Basic Socket Interface Extensions for IPv6" [RFC3493].
 [RFC6935] and [RFC6936] define an update to the UDP transport
 originally specified in [RFC2460] (note that RFC 2460 has been
 obsoleted by RFC 8200).  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].
 UDP offers only a basic transport interface.  UDP datagrams may be
 directly sent and received, without exchanging messages between the
 endpoints to set up 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 address is set using the BIND call.  At the remote
 end, the remote endpoint address is set 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,
 or 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.

Fairhurst & Jones Informational [Page 5] RFC 8304 UDP Transport Features February 2018

 Guidance on the use of the services provided by UDP is provided in
 the UDP Guidelines [RFC8085].  This also states that
    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 end 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 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 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 and 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.

Fairhurst & Jones Informational [Page 6] RFC 8304 UDP Transport Features February 2018

    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:
    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 either is used after receiving a
        datagram from an endpoint that causes the creation of a
        connection or can be triggered by a third-party configuration
        or a protocol trigger (such as reception of a UDP Service
        Description Protocol (SDP) [RFC4566] record).
 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] [RFC6935] [RFC6936] [RFC8085].  When unset, this
    overrides the default UDP behavior, disabling the checksum on
    sending.  Section 4.1.3.4 of the requirements for Internet hosts
    [RFC1122] states that "An application MAY optionally be able to
    control whether a UDP checksum will be generated, but it MUST
    default to checksumming on."

Fairhurst & Jones Informational [Page 7] RFC 8304 UDP Transport Features February 2018

 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 that "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 be non-zero;
    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]
    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 and
    the Stream Control Transmission Protocol (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 MTU (PMTU) allowed on the network
    path, coordinating with the network layer.  Classical Path MTU
    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 PMTU, 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

Fairhurst & Jones Informational [Page 8] RFC 8304 UDP Transport Features February 2018

    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 MTU 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.
 GET_MMS_R:  The GET_MMS_R primitive retrieves a network-layer value
    that indicates the 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 MTU for
    Receiving (EMTU_R) and the link MTU for the given source IP
    address, and it 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 a 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 that "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 an IP packet received at the network layer.  An application
    using the Generalized TTL Security Mechanism (GTSM) [RFC5082] can
    use this information to trust datagrams with a TTL value within
    the expected range, as described in Section 3 of RFC 5082.

Fairhurst & Jones Informational [Page 9] RFC 8304 UDP Transport Features February 2018

 SET_MIN_TTL:  The SET_MIN_TTL primitive restricts datagrams delivered
    to the application to those received with an IP TTL value greater
    than or equal to the passed parameter.  This primitive can be used
    to implement applications such as GTSM [RFC5082] too, as described
    in Section 3 of RFC 5082, but this RFC does not specify this
    method.
 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.  This is
    specified in Section 6.3 of RFC 3542.  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 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 up to the application layer."  The Diffserv model
    [RFC2475] [RFC3260] 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" should be
    interpreted to mean that an API should allow the application to
    set the DSCP.  Section 3.1.8 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

Fairhurst & Jones Informational [Page 10] RFC 8304 UDP Transport Features February 2018

    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 [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 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 requirements for Internet hosts [RFC1122]
    states that "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.

3.1.1. Excluded Primitives

 In the requirements for Internet hosts [RFC1122], Section 3.4
 describes GET_MAXSIZES and ADVISE_DELIVPROB, and Section 3.3.4.4
 describes GET_SRCADDR.  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, as 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.

Fairhurst & Jones Informational [Page 11] RFC 8304 UDP Transport Features February 2018

3.2. Primitives Provided by UDP-Lite

 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; 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.
    Section 3.3 of the UDP-Lite specification [RFC3828] 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 [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

Fairhurst & Jones Informational [Page 12] RFC 8304 UDP Transport Features February 2018

    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. IANA Considerations

 This document does not require any IANA actions.

5. 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].

6. References

6.1. Normative References

 [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>.

Fairhurst & Jones Informational [Page 13] RFC 8304 UDP Transport Features February 2018

 [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>.
 [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>.
 [RFC8303]  Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
            Transport Features Provided by IETF Transport Protocols",
            RFC 8303, DOI 10.17487/RFC8303, February 2018,
            <https://www.rfc-editor.org/info/rfc8303>.

Fairhurst & Jones Informational [Page 14] RFC 8304 UDP Transport Features February 2018

6.2. Informative References

 [POSIX]    IEEE, "Standard for Information Technology - Portable
            Operating System Interface (POSIX(R)) Base
            Specifications", Issue 7, IEEE 1003.1,
            <http://ieeexplore.ieee.org/document/7582338/>.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
            December 1998, <https://www.rfc-editor.org/info/rfc2460>.
 [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>.
 [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>.

Fairhurst & Jones Informational [Page 15] RFC 8304 UDP Transport Features February 2018

 [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>.
 [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", Volume 1,
            ISBN-13: 9780131411555, October 2003.

Fairhurst & Jones Informational [Page 16] RFC 8304 UDP Transport Features February 2018

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 by using the Any Source Multicast (ASM)
 model or 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 signaled 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:
 JoinHostGroup:  Section 7.1 of "Host Extensions for IP Multicasting"
    [RFC1112] provides a function that allows receiving traffic from
    an IP multicast group.
 JoinLocalGroup:  Section 7.3 of "Host Extensions for IP Multicasting"
    [RFC1112] provides a function that allows receiving traffic from a
    local IP multicast group.
 LeaveHostGroup:  Section 7.1 of "Host Extensions for IP Multicasting"
    [RFC1112] provides a function that allows leaving an IP multicast
    group.
 LeaveLocalGroup:  Section 7.3 of "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.

Fairhurst & Jones Informational [Page 17] RFC 8304 UDP Transport Features February 2018

 IP_MULTICAST_TTL:  This sets the time-to-live field t to use for
    outgoing IPv4 multicast packets.  This is used to limit the 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 this 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.
 The socket interface extensions for MSF [RFC3678] updates the
 multicast interface to add support for MSF for IPv4 and IPv6 required
 by IGMPv3.  Section 3 defines both basic and advanced APIs, and
 Section 5 describes protocol-independent versions of these APIs.
 Four sets of API functionality are therefore defined:
 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."

Fairhurst & Jones Informational [Page 18] RFC 8304 UDP Transport Features February 2018

 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 4.2 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:
 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.

Fairhurst & Jones Informational [Page 19] RFC 8304 UDP Transport Features February 2018

 MCAST_UNBLOCK_SOURCE:  This removes a previous MSF set by
       MCAST_BLOCK_SOURCE.
 MCAST_LEAVE_GROUP:  This leaves an ASM or SSM group.
 MCAST_LEAVE_SOURCE_GROUP:  This leaves an 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 RFC 5790).

Acknowledgements

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

Authors' Addresses

 Godred Fairhurst
 University of Aberdeen
 School of Engineering
 Fraser Noble Building
 Fraser Noble Building Aberdeen  AB24 3UE
 United Kingdom
 Email: gorry@erg.abdn.ac.uk
 Tom Jones
 University of Aberdeen
 School of Engineering
 Fraser Noble Building
 Aberdeen  AB24 3UE
 United Kingdom
 Email: tom@erg.abdn.ac.uk

Fairhurst & Jones Informational [Page 20]

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