This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
EID 9224
Internet Engineering Task Force (IETF) M. Welzl
Request for Comments: 8303 University of Oslo
Category: Informational M. Tuexen
ISSN: 2070-1721 Muenster Univ. of Appl. Sciences
N. Khademi
University of Oslo
February 2018
On the Usage of Transport Features
Provided by IETF Transport Protocols
Abstract
This document describes how the transport protocols Transmission
Control Protocol (TCP), MultiPath TCP (MPTCP), Stream Control
Transmission Protocol (SCTP), User Datagram Protocol (UDP), and
Lightweight User Datagram Protocol (UDP-Lite) expose services to
applications and how an application can configure and use the
features that make up these services. It also discusses the service
provided by the Low Extra Delay Background Transport (LEDBAT)
congestion control mechanism. The description results in a set of
transport abstractions that can be exported in a transport services
(TAPS) API.
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/rfc8303.
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 ....................................................3
2. Terminology .....................................................5
3. Pass 1 ..........................................................6
3.1. Primitives Provided by TCP .................................6
3.1.1. Excluded Primitives or Parameters ...................9
3.2. Primitives Provided by MPTCP ..............................10
3.3. Primitives Provided by SCTP ...............................11
3.3.1. Excluded Primitives or Parameters ..................18
3.4. Primitives Provided by UDP and UDP-Lite ...................18
3.5. The Service of LEDBAT .....................................19
4. Pass 2 .........................................................20
4.1. CONNECTION-Related Primitives .............................21
4.2. DATA-Transfer-Related Primitives ..........................38
5. Pass 3 .........................................................41
5.1. CONNECTION-Related Transport Features .....................41
5.2. DATA-Transfer-Related Transport Features ..................47
5.2.1. Sending Data .......................................47
5.2.2. Receiving Data .....................................48
5.2.3. Errors .............................................49
6. IANA Considerations ............................................49
7. Security Considerations ........................................49
8. References .....................................................50
8.1. Normative References ......................................50
8.2. Informative References ....................................52
Appendix A. Overview of RFCs Used as Input for Pass 1 .............54
Appendix B. How This Document Was Developed .......................54
Acknowledgements ..................................................56
Authors' Addresses ................................................56
1. Introduction
This specification describes how transport protocols offer transport
services, such that applications using them are no longer directly
tied to a specific protocol. Breaking this strict connection can
reduce the effort for an application programmer, yet attain greater
transport flexibility by pushing complexity into an underlying
transport services (TAPS) system.
This design process has started with a survey of the services
provided by IETF transport protocols and congestion control
mechanisms [RFC8095]. The present document and [RFC8304] complement
this survey with an in-depth look at the defined interactions between
applications and the following unicast transport protocols:
Transmission Control Protocol (TCP), MultiPath TCP (MPTCP), Stream
Control Transmission Protocol (SCTP), User Datagram Protocol (UDP),
and Lightweight User Datagram Protocol (UDP-Lite). We also define a
primitive to enable/disable and configure the Low Extra Delay
Background Transport (LEDBAT) unicast congestion control mechanism.
For UDP and UDP-Lite, the first step of the protocol analysis -- a
discussion of relevant RFC text -- is documented in [RFC8304].
This snapshot in time of the IETF transport protocols is published as
an RFC to document the analysis by the authors and the TAPS Working
Group; this generates a set of transport abstractions that can be
exported in a TAPS API. It provides the basis for the minimal set of
transport services that end systems supporting TAPS should implement
[TAPS-MINSET].
The list of primitives, events, and transport features in this
document is strictly based on the parts of protocol specifications
that describe what the protocol provides to an application using it
and how the application interacts with it. Transport protocols
provide communication between processes that operate on network
endpoints, which means that they allow for multiplexing of
communication between the same IP addresses, and this multiplexing is
achieved using port numbers. Port multiplexing is therefore assumed
to be always provided and not discussed in this document.
Parts of a protocol that are explicitly stated as optional to
implement are not covered. Interactions between the application and
a transport protocol that are not directly related to the operation
of the protocol are also not covered. For example, there are various
ways for an application to use socket options to indicate its
interest in receiving certain notifications [RFC6458]. However, for
the purpose of identifying primitives, events, and transport
features, the ability to enable or disable the reception of
notifications is irrelevant. Similarly, "one-to-many style sockets"
[RFC6458] just affect the application programming style, not how the
underlying protocol operates, and they are therefore not discussed
here. The same is true for the ability to obtain the unchanged value
of a parameter that an application has previously set (e.g., via
"get" in get/set operations [RFC6458]).
The document presents a three-pass process to arrive at a list of
transport features. In the first pass (pass 1), the relevant RFC
text is discussed per protocol. In the second pass (pass 2), this
discussion is used to derive a list of primitives and events that are
uniformly categorized across protocols. Here, an attempt is made to
present or -- where text describing primitives or events does not yet
exist -- construct primitives or events in a slightly generalized
form to highlight similarities. This is, for example, achieved by
renaming primitives or events of protocols or by avoiding a strict
1:1 mapping between the primitives or events in the protocol
specification and primitives or events in the list. Finally, the
third pass (pass 3) presents transport features based on pass 2,
identifying which protocols implement them.
In the list resulting from the second pass, some transport features
are missing because they are implicit in some protocols, and they
only become explicit when we consider the superset of all transport
features offered by all protocols. For example, TCP always carries
out congestion control; we have to consider it together with a
protocol like UDP (which does not have congestion control) before we
can consider congestion control as a transport feature. The complete
list of transport features across all protocols is therefore only
available after pass 3.
Some protocols are connection oriented. Connection-oriented
protocols often use an initial call to a specific primitive to open a
connection before communication can progress and require
communication to be explicitly terminated by issuing another call to
a primitive (usually called 'Close'). A "connection" is the common
state that some transport primitives refer to, e.g., to adjust
general configuration settings. Connection establishment,
maintenance, and termination are therefore used to categorize
transport primitives of connection-oriented transport protocols in
pass 2 and pass 3. For this purpose, UDP is assumed to be used with
"connected" sockets, i.e., sockets that are bound to a specific pair
of addresses and ports [RFC8304].
2. Terminology
Transport Feature: a specific end-to-end feature that the transport
layer provides to an application. Examples include
confidentiality, reliable delivery, ordered delivery, message-
versus-stream orientation, etc.
Transport Service: a set of transport features, without an
association to any given framing protocol, which provides a
complete service to an application.
Transport Protocol: an implementation that provides one or more
transport services using a specific framing and header format on
the wire.
Transport Protocol Component: an implementation of a transport
feature within a protocol.
Transport Service Instance: an arrangement of transport protocols
with a selected set of features and configuration parameters that
implement a single transport service, e.g., a protocol stack (RTP
over UDP).
Application: an entity that uses the transport layer for end-to-end
delivery of data across the network (this may also be an upper-
layer protocol or tunnel encapsulation).
Endpoint: an entity that communicates with one or more other
endpoints using a transport protocol.
Connection: shared state of two or more endpoints that persists
across messages that are transmitted between these endpoints.
Primitive: a function call that is used to locally communicate
between an application and a transport endpoint. A primitive is
related to one or more transport features.
Event: a primitive that is invoked by a transport endpoint.
Parameter: a value passed between an application and a transport
protocol by a primitive.
Socket: the combination of a destination IP address and a
destination port number.
Transport Address: the combination of an IP address, transport
protocol, and the port number used by the transport protocol.
3. Pass 1
This first iteration summarizes the relevant text parts of the RFCs
describing the protocols, focusing on what each transport protocol
provides to the application and how it is used (abstract API
descriptions, where they are available). When presenting primitives,
events, and parameters, the use of lower- and upper-case characters
is made uniform for the sake of readability.
3.1. Primitives Provided by TCP
The initial TCP specification [RFC0793] states:
The Transmission Control Protocol (TCP) is intended for use as a
highly reliable host-to-host protocol between hosts in packet-
switched computer communication networks, and in interconnected
systems of such networks.
Section 3.8 of [RFC0793] further specifies the interaction with the
application by listing several transport primitives. It is also
assumed that an Operating System provides a means for TCP to
asynchronously signal the application; the primitives representing
such signals are called 'events' in this section. This section
describes the relevant primitives.
Open: This is either active or passive, to initiate a connection or
listen for incoming connections. All other primitives are
associated with a specific connection, which is assumed to first
have been opened. An active open call contains a socket. A
passive open call with a socket waits for a particular connection;
alternatively, a passive open call can leave the socket
unspecified to accept any incoming connection. A fully specified
passive call can later be made active by calling 'Send'.
Optionally, a timeout can be specified, after which TCP will abort
the connection if data has not been successfully delivered to the
destination (else a default timeout value is used). A procedure
for aborting the connection is used to avoid excessive
retransmissions, and an application is able to control the
threshold used to determine the condition for aborting; this
threshold may be measured in time units or as a count of
retransmission [RFC1122]. This indicates that the timeout could
also be specified as a count of retransmission.
Also optional, for multihomed hosts, the local IP address can be
provided [RFC1122]. If it is not provided, a default choice will
be made in case of active open calls. A passive open call will
await incoming connection requests to all local addresses and then
maintain usage of the local IP address where the incoming
connection request has arrived. Finally, the 'options' parameter
allows the application to specify IP options such as Source Route,
Record Route, or Timestamp [RFC1122]. It is not stated on which
segments of a connection these options should be applied, but
probably on all segments, as this is also stated in a
specification given for the usage of the Source Route IP option
(Section 4.2.3.8 of [RFC1122]). Source Route is the only non-
optional IP option in this parameter, allowing an application to
specify a source route when it actively opens a TCP connection.
Master Key Tuples (MKTs) for authentication can optionally be
configured when calling 'Open' (Section 7.1 of [RFC5925]). When
authentication is in use, complete TCP segments are authenticated,
including the TCP IPv4 pseudoheader, TCP header, and TCP data.
TCP Fast Open (TFO) [RFC7413] allows applications to immediately
hand over a message from the active open to the passive open side
of a TCP connection together with the first message establishment
packet (the SYN). This can be useful for applications that are
sensitive to TCP's connection setup delay. [RFC7413] states that
"TCP implementations MUST NOT use TFO by default, but only use TFO
if requested explicitly by the application on a per-service-port
basis." The size of the message sent with TFO cannot be more than
TCP's maximum segment size (minus options used in the SYN). For
the active open side, it is recommended to change or replace the
connect() call in order to support a user data buffer argument
[RFC7413]. For the passive open side, the application needs to
enable the reception of Fast Open requests, e.g., via a new
TCP_FASTOPEN setsockopt() socket option before listen(). The
receiving application must be prepared to accept duplicates of the
TFO message, as the first data written to a socket can be
delivered more than once to the application on the remote host.
Send: This is the primitive that an application uses to give the
local TCP transport endpoint a number of bytes that TCP should
reliably send to the other side of the connection. The 'urgent'
flag, if set, states that the data handed over by this send call
is urgent and this urgency should be indicated to the receiving
process in case the receiving application has not yet consumed all
non-urgent data preceding it. An optional timeout parameter can
be provided that updates the connection's timeout (see 'Open').
Additionally, optional parameters allow the ability to indicate
the preferred outgoing MKT (current_key) and/or the preferred
incoming MKT (rnext_key) of a connection (Section 7.1 of
[RFC5925]).
Receive: This primitive allocates a receiving buffer for a provided
number of bytes. It returns the number of received bytes provided
in the buffer when these bytes have been received and written into
the buffer by TCP. The application is informed of urgent data via
an 'urgent' flag: if it is on, there is urgent data; if it is off,
there is no urgent data or this call to 'Receive' has returned all
the urgent data. The application is also informed about the
current_key and rnext_key information carried in a recently
received segment via an optional parameter (Section 7.1 of
[RFC5925]).
Close: This primitive closes one side of a connection. It is
semantically equivalent to "I have no more data to send" but does
not mean "I will not receive any more", as the other side may
still have data to send. This call reliably delivers any data
that has already been given to TCP (and if that fails, 'Close'
becomes 'abort').
Abort: This primitive causes all pending 'Send' and 'Receive' calls
to be aborted. A TCP "RESET" message is sent to the TCP endpoint
on the other side of the connection [RFC0793].
Close Event: TCP uses this primitive to inform an application that
the application on the other side has called the 'Close'
primitive, so the local application can also issue a 'Close' and
terminate the connection gracefully. See [RFC0793], Section 3.5.
Abort Event: When TCP aborts a connection upon receiving a "RESET"
from the peer, it "advises the user and goes to the CLOSED state."
See [RFC0793], Section 3.4.
User Timeout Event: This event is executed when the user timeout
(Section 3.9 of [RFC0793]) expires (see the definition of 'Open'
in this section). All queues are flushed, and the application is
informed that the connection had to be aborted due to user
timeout.
Error_Report event: This event informs the application of "soft
errors" that can be safely ignored [RFC5461], including the
arrival of an ICMP error message or excessive retransmissions
(reaching a threshold below the threshold where the connection is
aborted). See Section 4.2.4.1 of [RFC1122].
Type-of-Service: Section 4.2.4.2 of the requirements for Internet
hosts [RFC1122] states that "The application layer MUST be able to
specify the Type-of-Service (TOS) for segments that are sent on a
connection." 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. Since then, the TOS field has been
redefined. The Differentiated Services (Diffserv) model [RFC2475]
[RFC3260] replaces this field in the IP header, assigning the six
most significant bits to carry the Differentiated Services Code
Point (DSCP) field [RFC2474].
Nagle: The Nagle algorithm delays sending data for some time to
increase the likelihood of sending a full-sized segment
(Section 4.2.3.4 of [RFC1122]). An application can disable the
Nagle algorithm for an individual connection.
User Timeout Option: The User Timeout Option (UTO) [RFC5482] allows
one end of a TCP connection to advertise its current user timeout
value so that the other end of the TCP connection can adapt its
own user timeout accordingly. In addition to the configurable
value of the user timeout (see 'Send'), there are three per-
connection state variables that an application can adjust to
control the operation of the UTO: 'adv_uto' is the value of the
UTO advertised to the remote TCP peer (default: system-wide
default user timeout); 'enabled' (default false) is a boolean-type
flag that controls whether the UTO option is enabled for a
connection. This applies to both sending and receiving.
'changeable' is a boolean-type flag (default true) that controls
whether the user timeout may be changed based on a UTO option
received from the other end of the connection. 'changeable'
becomes false when an application explicitly sets the user timeout
(see 'Send').
Set/Get Authentication Parameters: The preferred outgoing MKT
(current_key) and/or the preferred incoming MKT (rnext_key) of a
connection can be configured. Information about current_key and
rnext_key carried in a recently received segment can be retrieved
(Section 7.1 of [RFC5925]).
3.1.1. Excluded Primitives or Parameters
The 'Open' primitive can be handed optional precedence or security/
compartment information [RFC0793], but this was not included here
because it is mostly irrelevant today [RFC7414].
The 'Status' primitive was not included because the initial TCP
specification describes this primitive as "implementation dependent"
and states that it "could be excluded without adverse effect"
[RFC0793]. Moreover, while a data block containing specific
information is described, it is also stated that not all of this
information may always be available. While [RFC5925] states that
'Status' "SHOULD be augmented to allow the MKTs of a current or
pending connection to be read (for confirmation)", the same
information is also available via 'Receive', which, following
[RFC5925], "MUST be augmented" with that functionality. The 'Send'
primitive includes an optional 'push' flag which, if set, requires
data to be promptly transmitted to the receiver without delay
[RFC0793]; the 'Receive' primitive described in can (under some
conditions) yield the status of the 'push' flag. Because "push"
functionality is optional to implement for both the 'Send' and
'Receive' primitives [RFC1122], this functionality is not included
here. The requirements for Internet hosts [RFC1122] also introduce
keep-alives to TCP, but these are optional to implement and hence not
considered here. The same document also describes that "some TCP
implementations have included a FLUSH call", indicating that this
call is also optional to implement; therefore, it is not considered
here.
3.2. Primitives Provided by MPTCP
MPTCP is an extension to TCP that allows the use of multiple paths
for a single data stream. It achieves this by creating different so-
called TCP subflows for each of the interfaces and scheduling the
traffic across these TCP subflows. The service provided by MPTCP is
described as follows in [RFC6182]:
Multipath TCP MUST follow the same service model as TCP [RFC0793]:
in-order, reliable, and byte-oriented delivery. Furthermore, a
Multipath TCP connection SHOULD provide the application with no
worse throughput or resilience than it would expect from running a
single TCP connection over any one of its available paths.
Further, there are some constraints on the API exposed by MPTCP, as
stated in [RFC6182]:
A multipath-capable equivalent of TCP MUST retain some level of
backward compatibility with existing TCP APIs, so that existing
applications can use the newer transport merely by upgrading the
operating systems of the end hosts.
As such, the primitives provided by MPTCP are equivalent to the ones
provided by TCP. Nevertheless, the MPTCP RFCs [RFC6824] and
[RFC6897] clarify some parts of TCP's primitives with respect to
MPTCP and add some extensions for better control on MPTCP's subflows.
Hereafter is a list of the clarifications and extensions the above-
cited RFCs provide to TCP's primitives.
Open: "An application should be able to request to turn on or turn
off the usage of MPTCP" [RFC6897]. This functionality can be
provided through a socket option called 'tcp_multipath_enable'.
Further, if the application has not explicitly enabled MPTCP and
is binding to a specific address or interface, MPTCP must be
disabled; if the application has explicitly enabled MPTCP and is
binding to a specific address or interface, MPTCP must remain
enabled but use only the addresses explicitly provided by the
application [RFC6897].
EID 9224 (Verified) is as follows:Section: 3.2
Original Text:
Further, MPTCP must be disabled in case the application is binding
to a specific address [RFC6897].
Corrected Text:
Further, if the application has not explicitly enabled MPTCP and
is binding to a specific address or interface, MPTCP must be
disabled; if the application has explicitly enabled MPTCP and is
binding to a specific address or interface, MPTCP must remain
enabled but use only the addresses explicitly provided by the
application [RFC6897].
Notes:
[RFC6897] states two rules, not one. Section 4.2.1 says that binding to a specific address or interface "implies that the application is not aware of MPTCP and will disable the use of MPTCP on this connection". But Section 5.3.2 states: "if an application enables MPTCP but binds to a specific address or interface, MPTCP MUST be enabled, but MPTCP MUST respect the application's choice and only use addresses that are explicitly provided by the application."
The sentence in RFC 8303 omits the second rule and therefore conflicts with [RFC6897] for applications that explicitly enable MPTCP and then bind to a specific address. In addition, "a specific address" is not defined here; [RFC6897] consistently says "a specific address or interface" throughout.
Send/Receive: The sending and receiving of data does not require any
changes to the application when MPTCP is being used [RFC6824].
The MPTCP-layer will take one input data stream from an
application, and split it into one or more subflows, with
sufficient control information to allow it to be reassembled and
delivered reliably and in order to the recipient application.
The use of the Urgent Pointer is special in MPTCP [RFC6824], which
states: "a TCP subflow MUST NOT use the Urgent Pointer to
interrupt an existing mapping."
Address and Subflow Management: MPTCP uses different addresses and
allows a host to announce these addresses as part of the protocol.
The MPTCP API Considerations RFC [RFC6897] says "An application
should be able to restrict MPTCP to binding to a given set of
addresses" and thus allows applications to limit the set of
addresses that are being used by MPTCP. Further, "An application
should be able to obtain information on the pairs of addresses
used by the MPTCP subflows."
3.3. Primitives Provided by SCTP
TCP has a number of limitations that SCTP removes (Section 1.1 of
[RFC4960]). The following three removed limitations directly
translate into transport features that are visible to an application
using SCTP: 1) it allows for preservation of message delimiters; 2)
it does not provide in-order or reliable delivery unless the
application wants that; 3) multihoming is supported. In SCTP,
connections are called "associations" and they can be between not
only two (as in TCP) but multiple addresses at each endpoint.
Section 10 of the SCTP base protocol specification [RFC4960]
specifies the interaction with the application (which SCTP calls the
"Upper-Layer Protocol (ULP)"). It is assumed that the Operating
System provides a means for SCTP to asynchronously signal the
application; the primitives representing such signals are called
'events' in this section. Here, we describe the relevant primitives.
In addition to the abstract API described in Section 10 of [RFC4960],
an extension to the sockets API is described in [RFC6458]. This
covers the functionality of the base protocol [RFC4960] and some of
its extensions [RFC3758] [RFC4895] [RFC5061]. For other protocol
extensions [RFC6525] [RFC6951] [RFC7053] [RFC7496] [RFC7829]
[RFC8260], the corresponding extensions of the sockets API are
specified in these protocol specifications. The functionality
exposed to the ULP through all these APIs is considered here.
The abstract API contains a 'SetProtocolParameters' primitive that
allows elements of a parameter list [RFC4960] to be adjusted; it is
stated that SCTP implementations "may allow ULP to customize some of
these protocol parameters", indicating that none of the elements of
this parameter list are mandatory to make ULP configurable. Thus, we
only consider the parameters in the abstract API that are also
covered in one of the other RFCs listed above, which leads us to
exclude the parameters 'RTO.Alpha', 'RTO.Beta', and 'HB.Max.Burst'.
For clarity, we also replace 'SetProtocolParameters' itself with
primitives that adjust parameters or groups of parameters that fit
together.
Initialize: Initialize creates a local SCTP instance that it binds
to a set of local addresses (and, if provided, a local port
number) [RFC4960]. Initialize needs to be called only once per
set of local addresses. A number of per-association
initialization parameters can be used when an association is
created, but before it is connected (via the primitive 'Associate'
below): the maximum number of inbound streams the application is
prepared to support, the maximum number of attempts to be made
when sending the INIT (the first message of association
establishment), and the maximum retransmission timeout (RTO) value
to use when attempting an INIT [RFC6458]. At this point, before
connecting, an application can also enable UDP encapsulation by
configuring the remote UDP encapsulation port number [RFC6951].
Associate: This creates an association (the SCTP equivalent of a
connection) that connects the local SCTP instance and a remote
SCTP instance. To identify the remote endpoint, it can be given
one or multiple (using "connectx") sockets (Section 9.9 of
[RFC6458]). Most primitives are associated with a specific
association, which is assumed to first have been created.
Associate can return a list of destination transport addresses so
that multiple paths can later be used. One of the returned
sockets will be selected by the local endpoint as the default
primary path for sending SCTP packets to this peer, but this
choice can be changed by the application using the list of
destination addresses. Associate is also given the number of
outgoing streams to request and optionally returns the number of
negotiated outgoing streams. An optional parameter of 32 bits,
the adaptation layer indication, can be provided [RFC5061]. If
authenticated chunks are used, the chunk types required to be sent
authenticated by the peer can be provided [RFC4895]. An
'SCTP_Cant_Str_Assoc' notification is used to inform the
application of a failure to create an association [RFC6458]. An
application could use sendto() or sendmsg() to implicitly set up
an association, thereby handing over a message that SCTP might
send during the association setup phase [RFC6458]. Note that this
mechanism is different from TCP's TFO mechanism: the message would
arrive only once, after at least one RTT, as it is sent together
with the third message exchanged during association setup, the
COOKIE-ECHO chunk).
Send: This sends a message of a certain length in bytes over an
association. A number can be provided to later refer to the
correct message when reporting an error, and a stream id is
provided to specify the stream to be used inside an association
(we consider this as a mandatory parameter here for simplicity: if
not provided, the stream id defaults to 0). A condition to
abandon the message can be specified (for example limiting the
number of retransmissions or the lifetime of the user message).
This allows control of the partial reliability extension [RFC3758]
[RFC7496]. An optional maximum lifetime can specify the time
after which the message should be discarded rather than sent. A
choice (advisory, i.e., not guaranteed) of the preferred path can
be made by providing a socket, and the message can be delivered
out-of-order if the 'unordered' flag is set. An advisory flag
indicates that the peer should not delay the acknowledgement of
the user message provided [RFC7053]. Another advisory flag
indicates whether the application prefers to avoid bundling user
data with other outbound DATA chunks (i.e., in the same packet).
A payload protocol-id can be provided to pass a value that
indicates the type of payload protocol data to the peer. If
authenticated chunks are used, the key identifier for
authenticating DATA chunks can be provided [RFC4895].
Receive: Messages are received from an association, and optionally a
stream within the association, with their size returned. The
application is notified of the availability of data via a 'Data
Arrive' notification. If the sender has included a payload
protocol-id, this value is also returned. If the received message
is only a partial delivery of a whole message, a 'partial' flag
will indicate so, in which case the stream id and a stream
sequence number are provided to the application.
Shutdown: This primitive gracefully closes an association, reliably
delivering any data that has already been handed over to SCTP. A
parameter lets the application control whether further receive or
send operations or both are disabled when the call is issued. A
return code informs about success or failure of this procedure.
Abort: This ungracefully closes an association, by discarding any
locally queued data and informing the peer that the association
was aborted. Optionally, an abort reason to be passed to the peer
may be provided by the application. A return code informs about
success or failure of this procedure.
Change Heartbeat / Request Heartbeat: This allows the application to
enable/disable heartbeats and optionally specify a heartbeat
frequency as well as requesting a single heartbeat to be carried
out upon a function call, with a notification about success or
failure of transmitting the HEARTBEAT chunk to the destination.
Configure Max. Retransmissions of an Association: The parameter
'Association.Max.Retrans' [RFC4960] (called "sasoc_maxrxt" in the
SCTP sockets API extensions [RFC6458]) allows the configuration of
the number of unsuccessful retransmissions after which an entire
association is considered as failed; this should invoke a
'Communication Lost' notification.
Set Primary: This allows the ability to set a new primary default
path for an association by providing a socket. Optionally, a
default source address to be used in IP datagrams can be provided.
Change Local Address / Set Peer Primary: This allows an endpoint to
add/remove local addresses to/from an association. In addition,
the peer can be given a hint for which address to use as the
primary address [RFC5061].
Configure Path Switchover: The abstract API contains a primitive
called 'Set Failure Threshold' [RFC4960]. This configures the
parameter 'Path.Max.Retrans', which determines after how many
retransmissions a particular transport address is considered as
unreachable. If there are more transport addresses available in
an association, reaching this limit will invoke a path switchover.
An extension called "SCTP-PF" adds a concept of "Potentially
Failed (PF)" paths to this method [RFC7829]. When a path is in PF
state, SCTP will not entirely give up sending on that path, but it
will preferably send data on other active paths if such paths are
available. Entering the PF state is done upon exceeding a
configured maximum number of retransmissions. Thus, for all paths
where this mechanism is used, there are two configurable error
thresholds: one to decide that a path is in PF state, and one to
decide that the transport address is unreachable.
Set/Get Authentication Parameters: This allows an endpoint to add/
remove key material to/from an association. In addition, the
chunk types being authenticated can be queried [RFC4895].
Add/Reset Streams, Reset Association: This allows an endpoint to add
streams to an existing association or to reset them individually.
Additionally, the association can be reset [RFC6525].
Status: The 'Status' primitive returns a data block with information
about a specified association, containing: an association
connection state; a destination transport address list;
destination transport address reachability states; current local
and peer receiver window sizes; current local congestion window
sizes; number of unacknowledged DATA chunks; number of DATA chunks
pending receipt; a primary path; the most recent Smoothed Round-
Trip Time (SRTT) on a primary path; RTO on a primary path; SRTT
and RTO on other destination addresses [RFC4960]; and an MTU per
path [RFC6458].
Enable/Disable Interleaving: This allows the negotiation of user
message interleaving support for future associations to be enabled
or disabled. For existing associations, it is possible to query
whether user message interleaving support was negotiated or not on
a particular association [RFC8260].
Set Stream Scheduler: This allows the ability to select a stream
scheduler per association, with a choice of: First-Come, First-
Served; Round-Robin; Round-Robin per Packet; Priority-Based; Fair
Bandwidth; and Weighted Fair Queuing [RFC8260].
Configure Stream Scheduler: This allows the ability to change a
parameter per stream for the schedulers: a priority value for the
Priority-Based scheduler and a weight for the Weighted Fair
Queuing scheduler.
Enable/Disable NoDelay: This turns on/off any Nagle-like algorithm
for an association [RFC6458].
Configure Send Buffer Size: This controls the amount of data SCTP
may have waiting in internal buffers to be sent or retransmitted
[RFC6458].
Configure Receive Buffer Size: This sets the receive buffer size in
octets, thereby controlling the receiver window for an association
[RFC6458].
Configure Message Fragmentation: If a user message causes an SCTP
packet to exceed the maximum fragmentation size (which can be
provided by the application and is otherwise the Path MTU (PMTU)
size), then the message will be fragmented by SCTP. Disabling
message fragmentation will produce an error instead of fragmenting
the message [RFC6458].
Configure Path MTU Discovery: Path MTU Discovery (PMTUD) can be
enabled or disabled per peer address of an association
(Section 8.1.12 of [RFC6458]). When it is enabled, the current
Path MTU value can be obtained. When it is disabled, the Path MTU
to be used can be controlled by the application.
Configure Delayed SACK Timer: The time before sending a SACK can be
adjusted; delaying SACKs can be disabled; and the number of
packets that must be received before a SACK is sent without
waiting for the delay timer to expire can be configured [RFC6458].
Set Cookie Life Value: The cookie life value can be adjusted
(Section 8.1.2 of [RFC6458]). 'Valid.Cookie.Life' is also one of
the parameters that is potentially adjustable with
'SetProtocolParameters' [RFC4960].
Set Maximum Burst: The maximum burst of packets that can be emitted
by a particular association (default 4, and values above 4 are
optional to implement) can be adjusted (Section 8.1.2 of
[RFC6458]). 'Max.Burst' is also one of the parameters that is
potentially adjustable with 'SetProtocolParameters' [RFC4960].
Configure RTO Calculation: The abstract API contains the following
adjustable parameters: 'RTO.Initial'; 'RTO.Min'; 'RTO.Max';
'RTO.Alpha'; and 'RTO.Beta'. Only the initial, minimum and
maximum RTOs are also described as configurable in the SCTP
sockets API extensions [RFC6458].
Set DSCP Value: The DSCP value can be set per peer address of an
association (Section 8.1.12 of [RFC6458]).
Set IPv6 Flow Label: The flow label field can be set per peer
address of an association (Section 8.1.12 of [RFC6458]).
Set Partial Delivery Point: This allows the ability to specify the
size of a message where partial delivery will be invoked. Setting
this to a lower value will cause partial deliveries to happen more
often [RFC6458].
Communication Up Notification: When a lost communication to an
endpoint is restored or when SCTP becomes ready to send or receive
user messages, this notification informs the application process
about the affected association, the type of event that has
occurred, the complete set of sockets of the peer, the maximum
number of allowed streams, and the inbound stream count (the
number of streams the peer endpoint has requested). If
interleaving is supported by both endpoints, this information is
also included in this notification.
Restart Notification: When SCTP has detected that the peer has
restarted, this notification is passed to the upper layer
[RFC6458].
Data Arrive Notification: When a message is ready to be retrieved
via the 'Receive' primitive, the application is informed by this
notification.
Send Failure Notification / Receive Unsent Message / Receive
Unacknowledged Message: When a message cannot be delivered via an
association, the sender can be informed about it and learn whether
the message has just not been acknowledged or (e.g., in case of
lifetime expiry) if it has not even been sent. This can also
inform the sender that a part of the message has been successfully
delivered.
Network Status Change Notification: This informs the application
about a socket becoming active/inactive [RFC4960] or "Potentially
Failed" [RFC7829].
Communication Lost Notification: When SCTP loses communication to an
endpoint (e.g., via heartbeats or excessive retransmission) or
detects an abort, this notification informs the application
process of the affected association and the type of event (failure
OR termination in response to a shutdown or abort request).
Shutdown Complete Notification: When SCTP completes the shutdown
procedures, this notification is passed to the upper layer,
informing it about the affected association.
Authentication Notification: When SCTP wants to notify the upper
layer regarding the key management related to authenticated chunks
[RFC4895], this notification is passed to the upper layer.
Adaptation Layer Indication Notification: When SCTP completes the
association setup and the peer provided an adaptation layer
indication, this is passed to the upper layer [RFC5061] [RFC6458].
Stream Reset Notification: When SCTP completes the procedure for
resetting streams [RFC6525], this notification is passed to the
upper layer, informing it about the result.
Association Reset Notification: When SCTP completes the association
reset procedure [RFC6525], this notification is passed to the
upper layer, informing it about the result.
Stream Change Notification: When SCTP completes the procedure used
to increase the number of streams [RFC6525], this notification is
passed to the upper layer, informing it about the result.
Sender Dry Notification: When SCTP has no more user data to send or
retransmit on a particular association, this notification is
passed to the upper layer [RFC6458].
Partial Delivery Aborted Notification: When a receiver has begun to
receive parts of a user message but the delivery of this message
is then aborted, this notification is passed to the upper layer
(Section 6.1.7 of [RFC6458]).
3.3.1. Excluded Primitives or Parameters
The 'Receive' primitive can return certain additional information,
but this is optional to implement and therefore not considered. With
a 'Communication Lost' notification, some more information may
optionally be passed to the application (e.g., identification to
retrieve unsent and unacknowledged data). SCTP "can invoke" a
'Communication Error' notification and "may send" a 'Restart'
notification, making these two notifications optional to implement.
The list provided under 'Status' includes "etc.", indicating that
more information could be provided. The primitive 'Get SRTT Report'
returns information that is included in the information that 'Status'
provides and is therefore not discussed. The 'Destroy SCTP Instance'
API function was excluded: it erases the SCTP instance that was
created by 'Initialize' but is not a primitive as defined in this
document because it does not relate to a transport feature. The
'Shutdown' event informs an application that the peer has sent a
SHUTDOWN, and hence no further data should be sent on this socket
(Section 6.1 of [RFC6458]). However, if an application would try to
send data on the socket, it would get an error message anyway; thus,
this event is classified as "just affecting the application
programming style, not how the underlying protocol operates" and is
not included here.
3.4. Primitives Provided by UDP and UDP-Lite
The set of pass 1 primitives for UDP and UDP-Lite is documented in
[RFC8304].
3.5. The Service of LEDBAT
The service of the LEDBAT congestion control mechanism is described
as follows:
LEDBAT is designed for use by background bulk-transfer
applications to be no more aggressive than standard TCP congestion
control (as specified in RFC 5681) and to yield in the presence of
competing flows, thus limiting interference with the network
performance of competing flows [RFC6817].
LEDBAT does not have any primitives, as LEDBAT is not a transport
protocol. According to its specification [RFC6817]:
LEDBAT can be used as part of a transport protocol or as part of
an application, as long as the data transmission mechanisms are
capable of carrying timestamps and acknowledging data frequently.
LEDBAT can be used with TCP, Stream Control Transmission Protocol
(SCTP), and Datagram Congestion Control Protocol (DCCP), with
appropriate extensions where necessary; and it can be used with
proprietary application protocols, such as those built on top of
UDP for peer-to-peer (P2P) applications.
At the time of writing, the appropriate extensions for TCP, SCTP, or
DCCP do not exist.
A number of configurable parameters exist in the LEDBAT
specification: TARGET, which is the queuing delay target at which
LEDBAT tries to operate, must be set to 100 ms or less.
'allowed_increase' (should be 1, must be greater than 0) limits the
speed at which LEDBAT increases its rate. 'gain', which according to
[RFC6817] "MUST be set to 1 or less" to avoid a faster ramp-up than
TCP Reno, determines how quickly the sender responds to changes in
queueing delay. Implementations may divide 'gain' into two
parameters: one for increase and a possibly larger one for decrease.
We call these parameters 'Gain_Inc' and 'Gain_Dec' here.
'Base_History' is the size of the list of measured base delays, and,
according to [RFC6817], "SHOULD be 10". This list can be filtered
using a 'Filter' function, which is not prescribed [RFC6817], that
yields a list of size 'Current_Filter'. The initial and minimum
congestion windows, 'Init_CWND' and 'Min_CWND', should both be 2.
Regarding which of these parameters should be under control of an
application, the possible range goes from exposing nothing on the one
hand to considering everything that is not prescribed with a "MUST"
in the specification as a parameter on the other hand. Function
implementations are not provided as a parameter to any of the
transport protocols discussed here; hence, we do not regard the
'Filter' function as a parameter. However, to avoid unnecessarily
limiting future implementations, we consider all other parameters
above as tunable parameters that should be exposed.
4. Pass 2
This pass categorizes the primitives from pass 1 based on whether
they relate to a connection or to data transmission. Primitives are
presented following the nomenclature
"CATEGORY.[SUBCATEGORY].PRIMITIVENAME.PROTOCOL". The CATEGORY can be
CONNECTION or DATA. Within the CONNECTION category, ESTABLISHMENT,
AVAILABILITY, MAINTENANCE, and TERMINATION subcategories can be
considered. The DATA category does not have any SUBCATEGORY. The
PROTOCOL name "UDP(-Lite)" is used when primitives are equivalent for
UDP and UDP-Lite; the PROTOCOL name "TCP" refers to both TCP and
MPTCP. We present "connection" as a general protocol-independent
concept and use it to refer to, e.g., TCP connections (identifiable
by a unique pair of IP addresses and TCP port numbers), SCTP
associations (identifiable by multiple IP address and port number
pairs), as well UDP and UDP-Lite connections (identifiable by a
unique socket pair).
Some minor details are omitted for the sake of generalization --
e.g., SCTP's 'Close' [RFC4960] returns success or failure and lets
the application control whether further receive or send operations,
or both, are disabled [RFC6458]. This is not described in the same
way for TCP [RFC0793], but these details play no significant role for
the primitives provided by either TCP or SCTP (for the sake of being
generic, it could be assumed that both receive and send operations
are disabled in both cases).
The TCP 'Send' and 'Receive' primitives include usage of an 'urgent'
parameter. This parameter controls a mechanism that is required to
implement the "synch signal" used by telnet [RFC0854], but [RFC6093]
states that "new applications SHOULD NOT employ the TCP urgent
mechanism." Because pass 2 is meant as a basis for the creation of
future systems, the "urgent" mechanism is excluded. This also
concerns the notification 'Urgent Pointer Advance' in the
'Error_Report' (Section 4.2.4.1 of [RFC1122]).
Since LEDBAT is a congestion control mechanism and not a protocol, it
is not currently defined when to enable/disable or configure the
mechanism. For instance, it could be a one-time choice upon
connection establishment or when listening for incoming connections,
in which case it should be categorized under CONNECTION.ESTABLISHMENT
or CONNECTION.AVAILABILITY, respectively. To avoid unnecessarily
limiting future implementations, it was decided to place it under
CONNECTION.MAINTENANCE, with all parameters that are described in the
specification [RFC6817] made configurable.
4.1. CONNECTION-Related Primitives
ESTABLISHMENT:
Active creation of a connection from one transport endpoint to one or
more transport endpoints. Interfaces to UDP and UDP-Lite allow both
connection-oriented and connection-less usage of the API [RFC8085].
o CONNECT.TCP:
Pass 1 primitive/event: 'Open' (active) or 'Open' (passive) with
socket, followed by 'Send'
Parameters: 1 local IP address (optional); 1 destination transport
address (for active open; else the socket and the local IP address
of the succeeding incoming connection request will be maintained);
timeout (optional); options (optional); MKT configuration
(optional); and user message (optional)
Comments: if the local IP address is not provided, a default
choice will automatically be made. The timeout can also be a
retransmission count. The options are IP options to be used on
all segments of the connection. At least the Source Route option
is mandatory for TCP to provide. 'MKT configuration' refers to
the ability to configure MKTs for authentication. The user
message may be transmitted to the peer application immediately
upon reception of the TCP SYN packet. To benefit from the lower
latency this provides as part of the experimental TFO mechanism,
its length must be at most the TCP's maximum segment size (minus
TCP options used in the SYN). The message may also be delivered
more than once to the application on the remote host.
o CONNECT.SCTP:
Pass 1 primitive/event: 'Initialize', followed by 'Enable/Disable
Interleaving' (optional), followed by 'Associate'
Parameters: list of local SCTP port number / IP address pairs
('Initialize'); one or several sockets (identifying the peer);
outbound stream count; maximum allowed inbound stream count;
adaptation layer indication (optional); chunk types required to be
authenticated (optional); request interleaving on/off; maximum
number of INIT attempts (optional); maximum init. RTO for INIT
(optional); user message (optional); and remote UDP port number
(optional)
Returns: socket list or failure
Comments: 'Initialize' needs to be called only once per list of
local SCTP port number / IP address pairs. One socket will
automatically be chosen; it can later be changed in MAINTENANCE.
The user message may be transmitted to the peer application
immediately upon reception of the packet containing the
COOKIE-ECHO chunk. To benefit from the lower latency this
provides, its length must be limited such that it fits into the
packet containing the COOKIE-ECHO chunk. If a remote UDP port
number is provided, SCTP packets will be encapsulated in UDP.
o CONNECT.MPTCP:
This is similar to CONNECT.TCP except for one additional boolean
parameter that allows the ability to enable or disable MPTCP for a
particular connection or socket (default: enabled).
o CONNECT.UDP(-Lite):
Pass 1 primitive/event: 'Connect' followed by 'Send'
Parameters: 1 local IP address (default (ANY) or specified); 1
destination transport address; 1 local port (default (OS chooses)
or specified); and 1 destination port (default (OS chooses) or
specified).
Comments: associates a transport address creating a UDP(-Lite)
socket connection. This can be called again with a new transport
address to create a new connection. The CONNECT function allows
an application to receive errors from messages sent to a transport
address.
AVAILABILITY:
Preparing to receive incoming connection requests.
o LISTEN.TCP:
Pass 1 primitive/event: 'Open' (passive)
Parameters: 1 local IP address (optional); 1 socket (optional);
timeout (optional); buffer to receive a user message (optional);
and MKT configuration (optional)
Comments: if the socket and/or local IP address is provided, this
waits for incoming connections from only and/or to only the
provided address. Else this waits for incoming connections
without this/these constraint(s). ESTABLISHMENT can later be
performed with 'Send'. If a buffer is provided to receive a user
message, a user message can be received from a TFO-enabled sender
before the TCP's connection handshake is completed. This message
may arrive multiple times. 'MKT configuration' refers to the
ability to configure MKTs for authentication.
o LISTEN.SCTP:
Pass 1 primitive/event: 'Initialize', followed by the
'Communication Up' or 'Restart' notification and possibly the
'Adaptation Layer' notification
Parameters: list of local SCTP port number / IP address pairs
(initialize)
Returns: socket list; outbound stream count; inbound stream count;
adaptation layer indication; chunks required to be authenticated;
and interleaving supported on both sides yes/no
Comments: 'Initialize' needs to be called only once per list of
local SCTP port number / IP address pairs. 'Communication Up' can
also follow a 'Communication Lost' notification, indicating that
the lost communication is restored. If the peer has provided an
adaptation layer indication, an 'Adaptation Layer' notification is
issued.
o LISTEN.MPTCP:
This is similar to LISTEN.TCP except for one additional boolean
parameter that allows the ability to enable or disable MPTCP for a
particular connection or socket (default: enabled).
o LISTEN.UDP(-Lite):
Pass 1 primitive/event: 'Receive'
Parameters: 1 local IP address (default (ANY) or specified); 1
destination transport address; local port (default (OS chooses) or
specified); and destination port (default (OS chooses) or
specified)
Comments: the 'Receive' function registers the application to
listen for incoming UDP(-Lite) datagrams at an endpoint.
MAINTENANCE:
Adjustments made to an open connection, or notifications about it.
These are out-of-band messages to the protocol that can be issued at
any time, at least after a connection has been established and before
it has been terminated (with one exception: CHANGE_TIMEOUT.TCP can
only be issued for an open connection when DATA.SEND.TCP is called).
In some cases, these primitives can also be immediately issued during
ESTABLISHMENT or AVAILABILITY, without waiting for the connection to
be opened (e.g., CHANGE_TIMEOUT.TCP can be done using TCP's 'Open'
primitive). For UDP and UDP-Lite, these functions may establish a
setting per connection but may also be changed per datagram message.
o CHANGE_TIMEOUT.TCP:
Pass 1 primitive/event: 'Open' or 'Send' combined with unspecified
control of per-connection state variables
Parameters: timeout value (optional); adv_uto (optional); boolean
uto_enabled (optional, default false); and boolean changeable
(optional, default true)
Comments: when sending data, an application can adjust the
connection's timeout value (the time after which the connection
will be aborted if data could not be delivered). If 'uto_enabled'
is true, the 'timeout value' (or, if provided, the value
'adv_uto') will be advertised for the TCP on the other side of the
connection to adapt its own user timeout accordingly.
'uto_enabled' controls whether the UTO option is enabled for a
connection. This applies to both sending and receiving.
'changeable' controls whether the user timeout may be changed
based on a UTO option received from the other end of the
connection; it becomes false when the 'timeout value' is used.
o CHANGE_TIMEOUT.SCTP:
Pass 1 primitive/event: 'Change Heartbeat' combined with
'Configure Max. Retransmissions of an Association'
Parameters: 'Change Heartbeat': heartbeat frequency and 'Configure
Max. Retransmissions of an Association': Association.Max.Retrans
Comments: 'Change Heartbeat' can enable/disable heartbeats in SCTP
as well as change their frequency. The parameter
'Association.Max.Retrans' defines after how many unsuccessful
transmissions of any packets (including heartbeats) the
association will be terminated; thus, these two primitives/
parameters together can yield a similar behavior for SCTP
associations as CHANGE_TIMEOUT.TCP does for TCP connections.
o DISABLE_NAGLE.TCP:
Pass 1 primitive/event: not specified
Parameters: one boolean value
Comments: the Nagle algorithm delays data transmission to increase
the chance of sending a full-sized segment. An application must
be able to disable this algorithm for a connection.
o DISABLE_NAGLE.SCTP:
Pass 1 primitive/event: 'Enable/Disable NoDelay'
Parameters: one boolean value
Comments: Nagle-like algorithms delay data transmission to
increase the chance of sending a full-sized packet.
o REQUEST_HEARTBEAT.SCTP:
Pass 1 primitive/event: 'Request Heartbeat'
Parameters: socket
Returns: success or failure
Comments: requests an immediate heartbeat on a path, returning
success or failure.
o ADD_PATH.MPTCP:
Pass 1 primitive/event: not specified
Parameters: local IP address and optionally the local port number
Comments: the application specifies the local IP address and port
number that must be used for a new subflow.
o ADD_PATH.SCTP:
Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'
Parameters: local IP address
o REM_PATH.MPTCP:
Pass 1 primitive/event: not specified
Parameters: local IP address; local port number; remote IP
address; and remote port number
Comments: the application removes the subflow specified by the IP/
port-pair. The MPTCP implementation must trigger a removal of the
subflow that belongs to this IP/port-pair.
o REM_PATH.SCTP:
Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'
Parameters: local IP address
o SET_PRIMARY.SCTP:
Pass 1 primitive/event: 'Set Primary'
Parameters: socket
Returns: result of attempting this operation
Comments: update the current primary address to be used, based on
the set of available sockets of the association.
o SET_PEER_PRIMARY.SCTP:
Pass 1 primitive/event: 'Change Local Address / Set Peer Primary'
Parameters: local IP address
Comments: this is only advisory for the peer.
o CONFIG_SWITCHOVER.SCTP:
Pass 1 primitive/event: 'Configure Path Switchover'
Parameters: primary max retrans (number of retransmissions after
which a path is considered inactive) and PF max retrans (number of
retransmissions after which a path is considered to be
"Potentially Failed", and others will be preferably used)
(optional)
o STATUS.SCTP:
Pass 1 primitive/event: 'Status', 'Enable/Disable Interleaving',
and 'Network Status Change' notification
Returns: data block with information about a specified
association, containing: association connection state; destination
transport address list; destination transport address reachability
states; current local and peer receiver window sizes; current
local congestion window sizes; number of unacknowledged DATA
chunks; number of DATA chunks pending receipt; primary path; most
recent SRTT on primary path; RTO on primary path; SRTT and RTO on
other destination addresses; MTU per path; and interleaving
supported yes/no
Comments: the 'Network Status Change' notification informs the
application about a socket becoming active/inactive; this only
affects the programming style, as the same information is also
available via 'Status'.
o STATUS.MPTCP:
Pass 1 primitive/event: not specified
Returns: list of pairs of tuples of IP address and TCP port number
of each subflow. The first of the pair is the local IP and port
number, while the second is the remote IP and port number.
o SET_DSCP.TCP:
Pass 1 primitive/event: not specified
Parameters: DSCP value
Comments: this allows an application to change the DSCP value for
outgoing segments.
o SET_DSCP.SCTP:
Pass 1 primitive/event: 'Set DSCP value'
Parameters: DSCP value
Comments: this allows an application to change the DSCP value for
outgoing packets on a path.
o SET_DSCP.UDP(-Lite):
Pass 1 primitive/event: 'Set_DSCP'
Parameter: DSCP value
Comments: this allows an application to change the DSCP value for
outgoing UDP(-Lite) datagrams. [RFC7657] and [RFC8085] provide
current guidance on using this value with UDP.
o ERROR.TCP:
Pass 1 primitive/event: 'Error_Report'
Returns: reason (encoding not specified) and subreason (encoding
not specified)
Comments: soft errors that can be ignored without harm by many
applications; an application should be able to disable these
notifications. The reported conditions include at least: ICMP
error message arrived and excessive retransmissions.
o ERROR.UDP(-Lite):
Pass 1 primitive/event: 'Error_Report'
Returns: Error report
Comments: this returns soft errors that may be ignored without
harm by many applications; an application must connect to be able
receive these notifications.
o SET_AUTH.TCP:
Pass 1 primitive/event: not specified
Parameters: current_key and rnext_key
Comments: current_key and rnext_key are the preferred outgoing MKT
and the preferred incoming MKT, respectively, for a segment that
is sent on the connection.
o SET_AUTH.SCTP:
Pass 1 primitive/event: 'Set/Get Authentication Parameters'
Parameters: key_id; key; and hmac_id
o GET_AUTH.TCP:
Pass 1 primitive/event: not specified
Parameters: current_key and rnext_key
Comments: current_key and rnext_key are the preferred outgoing MKT
and the preferred incoming MKT, respectively, that were carried on
a recently received segment.
o GET_AUTH.SCTP:
Pass 1 primitive/event: 'Set/Get Authentication Parameters'
Parameters: key_id and chunk_list
o RESET_STREAM.SCTP:
Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'
Parameters: sid and direction
o RESET_STREAM-EVENT.SCTP:
Pass 1 primitive/event: 'Stream Reset' notification
Parameters: information about the result of RESET_STREAM.SCTP
Comments: this is issued when the procedure for resetting streams
has completed.
o RESET_ASSOC.SCTP:
Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'
Parameters: information related to the extension, as defined in
[RFC3260]
o RESET_ASSOC-EVENT.SCTP:
Pass 1 primitive/event: 'Association Reset' notification
Parameters: information about the result of RESET_ASSOC.SCTP
Comments: this is issued when the procedure for resetting an
association has completed.
o ADD_STREAM.SCTP:
Pass 1 primitive/event: 'Add/Reset Streams, Reset Association'
Parameters: number of outgoing and incoming streams to be added
o ADD_STREAM-EVENT.SCTP:
Pass 1 primitive/event: 'Stream Change' notification
Parameters: information about the result of ADD_STREAM.SCTP
Comments: this is issued when the procedure for adding a stream
has completed.
o SET_STREAM_SCHEDULER.SCTP:
Pass 1 primitive/event: 'Set Stream Scheduler'
Parameters: scheduler identifier
Comments: choice of First-Come, First-Served; Round-Robin; Round-
Robin per Packet; Priority-Based; Fair Bandwidth; and Weighted
Fair Queuing.
o CONFIGURE_STREAM_SCHEDULER.SCTP:
Pass 1 primitive/event: 'Configure Stream Scheduler'
Parameters: priority
Comments: the priority value only applies when Priority-Based or
Weighted Fair Queuing scheduling is chosen with
SET_STREAM_SCHEDULER.SCTP. The meaning of the parameter differs
between these two schedulers, but in both cases, it realizes some
form of prioritization regarding how bandwidth is divided among
streams.
o SET_FLOWLABEL.SCTP:
Pass 1 primitive/event: 'Set IPv6 Flow Label'
Parameters: flow label
Comments: this allows an application to change the IPv6 header's
flow label field for outgoing packets on a path.
o AUTHENTICATION_NOTIFICATION-EVENT.SCTP:
Pass 1 primitive/event: 'Authentication' notification
Returns: information regarding key management
o CONFIG_SEND_BUFFER.SCTP:
Pass 1 primitive/event: 'Configure Send Buffer Size'
Parameters: size value in octets
o CONFIG_RECEIVE_BUFFER.SCTP:
Pass 1 primitive/event: 'Configure Receive Buffer Size'
Parameters: size value in octets
Comments: this controls the receiver window.
o CONFIG_FRAGMENTATION.SCTP:
Pass 1 primitive/event: 'Configure Message Fragmentation'
Parameters: one boolean value (enable/disable) and maximum
fragmentation size (optional; default: PMTU)
Comments: if fragmentation is enabled, messages exceeding the
maximum fragmentation size will be fragmented. If fragmentation
is disabled, trying to send a message that exceeds the maximum
fragmentation size will produce an error.
o CONFIG_PMTUD.SCTP:
Pass 1 primitive/event: 'Configure Path MTU Discovery'
Parameters: one boolean value (PMTUD on/off) and PMTU value
(optional)
Returns: PMTU value
Comments: this returns a meaningful PMTU value when PMTUD is
enabled (the boolean is true), and the PMTU value can be set if
PMTUD is disabled (the boolean is false).
o CONFIG_DELAYED_SACK.SCTP:
Pass 1 primitive/event: 'Configure Delayed SACK Timer'
Parameters: one boolean value (delayed SACK on/off); timer value
(optional); and number of packets to wait for (default 2)
Comments: if delayed SACK is enabled, SCTP will send a SACK either
upon receiving the provided number of packets or when the timer
expires, whatever occurs first.
o CONFIG_RTO.SCTP:
Pass 1 primitive/event: 'Configure RTO Calculation'
Parameters: init (optional); min (optional); and max (optional)
Comments: this adjusts the initial, minimum, and maximum RTO
values.
o SET_COOKIE_LIFE.SCTP:
Pass 1 primitive/event: 'Set Cookie Life Value'
Parameters: cookie life value
o SET_MAX_BURST.SCTP:
Pass 1 primitive/event: 'Set Maximum Burst'
Parameters: max burst value
Comments: not all implementations allow values above the default
of 4.
o SET_PARTIAL_DELIVERY_POINT.SCTP:
Pass 1 primitive/event: 'Set Partial Delivery Point'
Parameters: partial delivery point (integer)
Comments: this parameter must be smaller or equal to the socket
receive buffer size.
o SET_CHECKSUM_ENABLED.UDP:
Pass 1 primitive/event: 'Checksum_Enabled'
Parameters: 0 when zero checksum is used at sender, 1 for checksum
at sender (default)
o SET_CHECKSUM_REQUIRED.UDP:
Pass 1 primitive/event: 'Require_Checksum'
Parameter: 0 to allow zero checksum, 1 when a non-zero checksum is
required (default) at the receiver
o SET_CHECKSUM_COVERAGE.UDP-Lite:
Pass 1 primitive/event: 'Set_Checksum_Coverage'
Parameters: coverage length at sender (default maximum coverage)
o SET_MIN_CHECKSUM_COVERAGE.UDP-Lite:
Pass 1 primitive/event: 'Set_Min_Coverage'
Parameter: coverage length at receiver (default minimum coverage)
o SET_DF.UDP(-Lite):
Pass 1 primitive event: 'Set_DF'
Parameter: 0 when DF is not set (default) in the IPv4 header, 1
when DF is set
o GET_MMS_S.UDP(-Lite):
Pass 1 primitive event: 'Get_MM_S'
Comments: this retrieves the maximum transport-message size that
may be sent using a non-fragmented IP packet from the configured
interface.
o GET_MMS_R.UDP(-Lite):
Pass 1 primitive event: 'Get_MMS_R'
Comments: this retrieves the maximum transport-message size that
may be received from the configured interface.
o SET_TTL.UDP(-Lite) (IPV6_UNICAST_HOPS):
Pass 1 primitive/event: 'Set_TTL' and 'Set_IPV6_Unicast_Hops'
Parameters: IPv4 TTL value or IPv6 Hop Count value
Comments: this allows an application to change the IPv4 TTL of
IPv6 Hop Count value for outgoing UDP(-Lite) datagrams.
o GET_TTL.UDP(-Lite) (IPV6_UNICAST_HOPS):
Pass 1 primitive/event: 'Get_TTL' and 'Get_IPV6_Unicast_Hops'
Returns: IPv4 TTL value or IPv6 Hop Count value
Comments: this allows an application to read the IPv4 TTL of the
IPv6 Hop Count value from a received UDP(-Lite) datagram.
o SET_ECN.UDP(-Lite):
Pass 1 primitive/event: 'Set_ECN'
Parameters: ECN value
Comments: this allows a UDP(-Lite) application to set the Explicit
Congestion Notification (ECN) code point field for outgoing
UDP(-Lite) datagrams. It defaults to sending '00'.
o GET_ECN.UDP(-Lite):
Pass 1 primitive/event: 'Get_ECN'
Parameters: ECN value
Comments: this allows a UDP(-Lite) application to read the ECN
code point field from a received UDP(-Lite) datagram.
o SET_IP_OPTIONS.UDP(-Lite):
Pass 1 primitive/event: 'Set_IP_Options'
Parameters: options
Comments: this allows a UDP(-Lite) application to set IP options
for outgoing UDP(-Lite) datagrams. These options can at least be
the Source Route, Record Route, and Timestamp option.
o GET_IP_OPTIONS.UDP(-Lite):
Pass 1 primitive/event: 'Get_IP_Options'
Returns: options
Comments: this allows a UDP(-Lite) application to receive any IP
options that are contained in a received UDP(-Lite) datagram.
o CONFIGURE.LEDBAT:
Pass 1 primitive/event: N/A
Parameters: enable (boolean); target; allowed_increase; gain_inc;
gain_dec; base_history; current_filter; init_cwnd; and min_cwnd
Comments: 'enable' is a newly invented parameter that enables or
disables the whole LEDBAT service.
TERMINATION:
Gracefully or forcefully closing a connection or being informed about
this event happening.
o CLOSE.TCP:
Pass 1 primitive/event: 'Close'
Comments: this terminates the sending side of a connection after
reliably delivering all remaining data.
o CLOSE.SCTP:
Pass 1 primitive/event: 'Shutdown'
Comments: this terminates a connection after reliably delivering
all remaining data.
o ABORT.TCP:
Pass 1 primitive/event: 'Abort'
Comments: this terminates a connection without delivering
remaining data and sends an error message to the other side.
o ABORT.SCTP:
Pass 1 primitive/event: 'Abort'
Parameters: abort reason to be given to the peer (optional)
Comments: this terminates a connection without delivering
remaining data and sends an error message to the other side.
o ABORT.UDP(-Lite):
Pass 1 primitive event: 'Close'
Comments: this terminates a connection without delivering
remaining data. No further UDP(-Lite) datagrams are sent/received
for this transport service instance.
o TIMEOUT.TCP:
Pass 1 primitive/event: 'User Timeout' event
Comments: the application is informed that the connection is
aborted. This event is executed on expiration of the timeout set
in CONNECTION.ESTABLISHMENT.CONNECT.TCP (possibly adjusted in
CONNECTION.MAINTENANCE.CHANGE_TIMEOUT.TCP).
o TIMEOUT.SCTP:
Pass 1 primitive/event: 'Communication Lost' event
Comments: the application is informed that the connection is
aborted. This event is executed on expiration of the timeout that
should be enabled by default (see the beginning of Section 8.3 in
[RFC4960]) and was possibly adjusted in
CONNECTION.MAINTENANCE.CHANGE_TIMEOOUT.SCTP.
o ABORT-EVENT.TCP:
Pass 1 primitive/event: not specified
o ABORT-EVENT.SCTP:
Pass 1 primitive/event: 'Communication Lost' event
Returns: abort reason from the peer (if available)
Comments: the application is informed that the other side has
aborted the connection using CONNECTION.TERMINATION.ABORT.SCTP.
o CLOSE-EVENT.TCP:
Pass 1 primitive/event: not specified
o CLOSE-EVENT.SCTP:
Pass 1 primitive/event: 'Shutdown Complete' event
Comments: the application is informed that
CONNECTION.TERMINATION.CLOSE.SCTP was successfully completed.
4.2. DATA-Transfer-Related Primitives
All primitives in this section refer to an existing connection, i.e.,
a connection that was either established or made available for
receiving data (although this is optional for the primitives of
UDP(-Lite)). In addition to the listed parameters, all sending
primitives contain a reference to a data block, and all receiving
primitives contain a reference to available buffer space for the
data. Note that CONNECT.TCP and LISTEN.TCP in the ESTABLISHMENT and
AVAILABILITY categories also allow to transfer data (an optional user
message) before the connection is fully established.
o SEND.TCP:
Pass 1 primitive/event: 'Send'
Parameters: timeout (optional); current_key (optional); and
rnext_key (optional)
Comments: this gives TCP a data block for reliable transmission to
the TCP on the other side of the connection. The timeout can be
configured with this call (see also
CONNECTION.MAINTENANCE.CHANGE_TIMEOUT.TCP). 'current_key' and
'rnext_key' are authentication parameters that can be configured
with this call (see also CONNECTION.MAINTENANCE.SET_AUTH.TCP).
o SEND.SCTP:
Pass 1 primitive/event: 'Send'
Parameters: stream number; context (optional); socket (optional);
unordered flag (optional); no-bundle flag (optional); payload
protocol-id (optional); pr-policy (optional) pr-value (optional);
sack-immediately flag (optional); and key-id (optional)
Comments: this gives SCTP a data block for transmission to the
SCTP on the other side of the connection (SCTP association). The
'stream number' denotes the stream to be used. The 'context'
number can later be used to refer to the correct message when an
error is reported. The 'socket' can be used to state which path
should be preferred, if there are multiple paths available (see
also CONNECTION.MAINTENANCE.SETPRIMARY.SCTP). The data block can
be delivered out of order if the 'unordered' flag is set. The
'no-bundle flag' can be set to indicate a preference to avoid
bundling. The 'payload protocol-id' is a number that will, if
provided, be handed over to the receiving application. Using
pr-policy and pr-value, the level of reliability can be
controlled. The 'sack-immediately' flag can be used to indicate
that the peer should not delay the sending of a SACK corresponding
to the provided user message. If specified, the provided key-id
is used for authenticating the user message.
o SEND.UDP(-Lite):
Pass 1 primitive/event: 'Send'
Parameters: IP address and port number of the destination endpoint
(optional if connected)
Comments: this provides a message for unreliable transmission
using UDP(-Lite) to the specified transport address. The IP
address and port number may be omitted for connected UDP(-Lite)
sockets. All CONNECTION.MAINTENANCE.SET_*.UDP(-Lite) primitives
apply per message sent.
o RECEIVE.TCP:
Pass 1 primitive/event: 'Receive'
Parameters: current_key (optional) and rnext_key (optional)
Comments: 'current_key' and 'rnext_key' are authentication
parameters that can be read with this call (see also
CONNECTION.MAINTENANCE.GET_AUTH.TCP).
o RECEIVE.SCTP:
Pass 1 primitive/event: 'Data Arrive' notification, followed by
'Receive'
Parameters: stream number (optional)
Returns: stream sequence number (optional) and partial flag
(optional)
Comments: if the 'stream number' is provided, the call to receive
only receives data on one particular stream. If a partial message
arrives, this is indicated by the 'partial flag', and then the
'stream sequence number' must be provided such that an application
can restore the correct order of data blocks that comprise an
entire message.
o RECEIVE.UDP(-Lite):
Pass 1 primitive/event: 'Receive'
Parameters: buffer for received datagram
Comments: all CONNECTION.MAINTENANCE.GET_*.UDP(-Lite) primitives
apply per message received.
o SENDFAILURE-EVENT.SCTP:
Pass 1 primitive/event: 'Send Failure' notification, optionally
followed by 'Receive Unsent Message' or 'Receive Unacknowledged
Message'
Returns: cause code; context; and unsent or unacknowledged message
(optional)
Comments: 'cause code' indicates the reason of the failure, and
'context' is the context number if such a number has been provided
in DATA.SEND.SCTP, for later use with 'Receive Unsent Message' or
'Receive Unacknowledged Message', respectively. These primitives
can be used to retrieve the unsent or unacknowledged message (or
part of the message, in case a part was delivered) if desired.
o SEND_FAILURE.UDP(-Lite):
Pass 1 primitive/event: 'Send'
Comments: this may be used to probe for the effective PMTU when
using in combination with the 'MAINTENANCE.SET_DF' primitive.
o SENDER_DRY-EVENT.SCTP:
Pass 1 primitive/event: 'Sender Dry' notification
Comments: this informs the application that the stack has no more
user data to send.
o PARTIAL_DELIVERY_ABORTED-EVENT.SCTP:
Pass 1 primitive/event: 'Partial Delivery Aborted' notification
Comments: this informs the receiver of a partial message that the
further delivery of the message has been aborted.
5. Pass 3
This section presents the superset of all transport features in all
protocols that were discussed in the preceding sections, based on the
list of primitives in pass 2 but also on text in pass 1 to include
transport features that can be configured in one protocol and are
static properties in another (congestion control, for example).
Again, some minor details are omitted for the sake of generalization
-- e.g., TCP may provide various different IP options, but only
source route is mandatory to implement, and this detail is not
visible in the pass 3 transport feature "Specify IP options". As
before, "UDP(-Lite)" represents both UDP and UDP-Lite, and "TCP"
refers to both TCP and MPTCP.
5.1. CONNECTION-Related Transport Features
ESTABLISHMENT:
Active creation of a connection from one transport endpoint to one or
more transport endpoints.
o Connect
Protocols: TCP, SCTP, and UDP(-Lite)
o Specify which IP options must always be used
Protocols: TCP and UDP(-Lite)
o Request multiple streams
Protocols: SCTP
o Limit the number of inbound streams
Protocols: SCTP
o Specify number of attempts and/or timeout for the first
establishment message
Protocols: TCP and SCTP
o Obtain multiple sockets
Protocols: SCTP
o Disable MPTCP
Protocols: MPTCP
o Configure authentication
Protocols: TCP and SCTP
Comments: with TCP, this allows the configuration of MKTs. In
SCTP, this allows the specification of which chunk types must
always be authenticated. DATA, ACK, etc., are different 'chunks'
in SCTP; one or more chunks may be included in a single packet.
o Indicate an Adaptation Layer (via an adaptation code point)
Protocols: SCTP
o Request to negotiate interleaving of user messages
Protocols: SCTP
o Hand over a message to reliably transfer (possibly multiple times)
before connection establishment
Protocols: TCP
o Hand over a message to reliably transfer during connection
establishment
Protocols: SCTP
o Enable UDP encapsulation with a specified remote UDP port number
Protocols: SCTP
AVAILABILITY:
Preparing to receive incoming connection requests.
o Listen, 1 specified local interface
Protocols: TCP, SCTP, and UDP(-Lite)
o Listen, N specified local interfaces
Protocols: SCTP
o Listen, all local interfaces
Protocols: TCP, SCTP, and UDP(-Lite)
o Obtain requested number of streams
Protocols: SCTP
o Limit the number of inbound streams
Protocols: SCTP
o Specify which IP options must always be used
Protocols: TCP and UDP(-Lite)
o Disable MPTCP
Protocols: MPTCP
o Configure authentication
Protocols: TCP and SCTP
Comments: with TCP, this allows the configuration of MKTs. In
SCTP, this allows the specification of which chunk types must
always be authenticated. DATA, ACK, etc., are different 'chunks'
in SCTP; one or more chunks may be included in a single packet.
o Indicate an Adaptation Layer (via an adaptation code point)
Protocols: SCTP
MAINTENANCE:
Adjustments made to an open connection, or notifications about it.
o Change timeout for aborting connection (using retransmit limit or
time value)
Protocols: TCP and SCTP
o Suggest timeout to the peer
Protocols: TCP
o Disable Nagle algorithm
Protocols: TCP and SCTP
o Request an immediate heartbeat, returning success/failure
Protocols: SCTP
o Notification of excessive retransmissions (early warning below
abortion threshold)
Protocols: TCP
o Add path
Protocols: MPTCP and SCTP
MPTCP Parameters: source-IP; source-Port; destination-IP; and
destination-Port
SCTP Parameters: local IP address
o Remove path
Protocols: MPTCP and SCTP
MPTCP Parameters: source-IP; source-Port; destination-IP; and
destination-Port
SCTP Parameters: local IP address
o Set primary path
Protocols: SCTP
o Suggest primary path to the peer
Protocols: SCTP
o Configure Path Switchover
Protocols: SCTP
o Obtain status (query or notification)
Protocols: SCTP and MPTCP
SCTP parameters: association connection state; destination
transport address list; destination transport address reachability
states; current local and peer receiver window sizes; current
local congestion window sizes; number of unacknowledged DATA
chunks; number of DATA chunks pending receipt; primary path; most
recent SRTT on primary path; RTO on primary path; SRTT and RTO on
other destination addresses; MTU per path; and interleaving
supported yes/no
MPTCP parameters: subflow-list (identified by source-IP;
source-Port; destination-IP; and destination-Port)
o Specify DSCP field
Protocols: TCP, SCTP, and UDP(-Lite)
o Notification of ICMP error message arrival
Protocols: TCP and UDP(-Lite)
o Change authentication parameters
Protocols: TCP and SCTP
o Obtain authentication information
Protocols: TCP and SCTP
o Reset Stream
Protocols: SCTP
o Notification of Stream Reset
Protocols: STCP
o Reset Association
Protocols: SCTP
o Notification of Association Reset
Protocols: STCP
o Add Streams
Protocols: SCTP
o Notification of Added Stream
Protocols: STCP
o Choose a scheduler to operate between streams of an association
Protocols: SCTP
o Configure priority or weight for a scheduler
Protocols: SCTP
o Specify IPv6 flow label field
Protocols: SCTP
o Configure send buffer size
Protocols: SCTP
o Configure receive buffer (and rwnd) size
Protocols: SCTP
o Configure message fragmentation
Protocols: SCTP
o Configure PMTUD
Protocols: SCTP
o Configure delayed SACK timer
Protocols: SCTP
o Set Cookie life value
Protocols: SCTP
o Set maximum burst
Protocols: SCTP
o Configure size where messages are broken up for partial delivery
Protocols: SCTP
o Disable checksum when sending
Protocols: UDP
o Disable checksum requirement when receiving
Protocols: UDP
o Specify checksum coverage used by the sender
Protocols: UDP-Lite
o Specify minimum checksum coverage required by receiver
Protocols: UDP-Lite
o Specify DF field
Protocols: UDP(-Lite)
o Get max. transport-message size that may be sent using a non-
fragmented IP packet from the configured interface
Protocols: UDP(-Lite)
o Get max. transport-message size that may be received from the
configured interface
Protocols: UDP(-Lite)
o Specify TTL/Hop Count field
Protocols: UDP(-Lite)
o Obtain TTL/Hop Count field
Protocols: UDP(-Lite)
o Specify ECN field
Protocols: UDP(-Lite)
o Obtain ECN field
Protocols: UDP(-Lite)
o Specify IP options
Protocols: UDP(-Lite)
o Obtain IP options
Protocols: UDP(-Lite)
o Enable and configure "Low Extra Delay Background Transfer"
Protocols: A protocol implementing the LEDBAT congestion control
mechanism
TERMINATION:
Gracefully or forcefully closing a connection, or being informed
about this event happening.
o Close after reliably delivering all remaining data, causing an
event informing the application on the other side
Protocols: TCP and SCTP
Comments: a TCP endpoint locally only closes the connection for
sending; it may still receive data afterwards.
o Abort without delivering remaining data, causing an event that
informs the application on the other side
Protocols: TCP and SCTP
Comments: in SCTP, a reason can optionally be given by the
application on the aborting side, which can then be received by
the application on the other side.
o Abort without delivering remaining data, not causing an event that
informs the application on the other side
Protocols: UDP(-Lite)
o Timeout event when data could not be delivered for too long
Protocols: TCP and SCTP
Comments: the timeout is configured with CONNECTION.MAINTENANCE
"Change timeout for aborting connection (using retransmit limit or
time value)".
5.2. DATA-Transfer-Related Transport Features
All transport features in this section refer to an existing
connection, i.e., a connection that was either established or made
available for receiving data. Note that TCP allows the transfer of
data (a single optional user message, possibly arriving multiple
times) before the connection is fully established. Reliable data
transfer entails delay -- e.g., for the sender to wait until it can
transmit data or due to retransmission in case of packet loss.
5.2.1. Sending Data
All transport features in this section are provided by DATA.SEND from
pass 2. DATA.SEND is given a data block from the application, which
here we call a "message" if the beginning and end of the data block
can be identified at the receiver, and "data" otherwise.
o Reliably transfer data, with congestion control
Protocols: TCP
o Reliably transfer a message, with congestion control
Protocols: SCTP
o Unreliably transfer a message, with congestion control
Protocols: SCTP
o Unreliably transfer a message, without congestion control
Protocols: UDP(-Lite)
o Configurable Message Reliability
Protocols: SCTP
o Choice of stream
Protocols: SCTP
o Choice of path (destination address)
Protocols: SCTP
o Ordered message delivery (potentially slower than unordered)
Protocols: SCTP
o Unordered message delivery (potentially faster than ordered)
Protocols: SCTP and UDP(-Lite)
o Request not to bundle messages
Protocols: SCTP
o Specifying a 'payload protocol-id' (handed over as such by the
receiver)
Protocols: SCTP
o Specifying a key identifier to be used to authenticate a message
Protocols: SCTP
o Request not to delay the acknowledgement (SACK) of a message
Protocols: SCTP
5.2.2. Receiving Data
All transport features in this section are provided by DATA.RECEIVE
from pass 2. DATA.RECEIVE fills a buffer provided by the
application, with what here we call a "message" if the beginning and
end of the data block can be identified at the receiver, and "data"
otherwise.
o Receive data (with no message delimiting)
Protocols: TCP
o Receive a message
Protocols: SCTP and UDP(-Lite)
o Choice of stream to receive from
Protocols: SCTP
o Information about partial message arrival
Protocols: SCTP
Comments: in SCTP, partial messages are combined with a stream
sequence number so that the application can restore the correct
order of data blocks an entire message consists of.
5.2.3. Errors
This section describes sending failures that are associated with a
specific call to DATA.SEND from pass 2.
o Notification of an unsent (part of a) message
Protocols: SCTP and UDP(-Lite)
o Notification of an unacknowledged (part of a) message
Protocols: SCTP
o Notification that the stack has no more user data to send
Protocols: SCTP
o Notification to a receiver that a partial message delivery has
been aborted
Protocols: SCTP
6. IANA Considerations
This document does not require any IANA actions.
7. Security Considerations
Authentication, confidentiality protection, and integrity protection
are identified as transport features [RFC8095]. These transport
features are generally provided by a protocol or layer on top of the
transport protocol; none of the transport protocols considered in
this document provides these transport features on its own.
Therefore, these transport features are not considered in this
document, with the exception of native authentication capabilities of
TCP and SCTP for which the security considerations in [RFC5925] and
[RFC4895] apply.
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].
8. References
8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
RFC 5482, DOI 10.17487/RFC5482, March 2009,
<https://www.rfc-editor.org/info/rfc5482>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
<https://www.rfc-editor.org/info/rfc6182>.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/info/rfc6525>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<https://www.rfc-editor.org/info/rfc6817>.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.
[RFC6897] Scharf, M. and A. Ford, "Multipath TCP (MPTCP) Application
Interface Considerations", RFC 6897, DOI 10.17487/RFC6897,
March 2013, <https://www.rfc-editor.org/info/rfc6897>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.
[RFC7053] Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
IMMEDIATELY Extension for the Stream Control Transmission
Protocol", RFC 7053, DOI 10.17487/RFC7053, November 2013,
<https://www.rfc-editor.org/info/rfc7053>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC7496] Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
"Additional Policies for the Partially Reliable Stream
Control Transmission Protocol Extension", RFC 7496,
DOI 10.17487/RFC7496, April 2015,
<https://www.rfc-editor.org/info/rfc7496>.
[RFC7829] Nishida, Y., Natarajan, P., Caro, A., Amer, P., and K.
Nielsen, "SCTP-PF: A Quick Failover Algorithm for the
Stream Control Transmission Protocol", RFC 7829,
DOI 10.17487/RFC7829, April 2016,
<https://www.rfc-editor.org/info/rfc7829>.
[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>.
[RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", RFC 8260,
DOI 10.17487/RFC8260, November 2017,
<https://www.rfc-editor.org/info/rfc8260>.
[RFC8304] Fairhurst, G. and T. Jones, "Transport Features of the
User Datagram Protocol (UDP) and Lightweight UDP (UDP-
Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018,
<https://www.rfc-editor.org/info/rfc8304>.
8.2. Informative References
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, DOI 10.17487/RFC0854, May
1983, <https://www.rfc-editor.org/info/rfc854>.
[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>.
[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>.
[RFC5461] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
DOI 10.17487/RFC5461, February 2009,
<https://www.rfc-editor.org/info/rfc5461>.
[RFC6093] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
January 2011, <https://www.rfc-editor.org/info/rfc6093>.
[RFC7414] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>.
[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>.
[RFC8095] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.
[TAPS-MINSET]
Welzl, M. and S. Gjessing, "A Minimal Set of Transport
Services for TAPS Systems", Work in Progress, draft-ietf-
taps-minset-01, February 2018.
Appendix A. Overview of RFCs Used as Input for Pass 1
TCP: [RFC0793], [RFC1122], [RFC5482], [RFC5925], and
[RFC7413].
MPTCP: [RFC6182], [RFC6824], and [RFC6897].
SCTP: RFCs without a sockets API specification:
[RFC3758], [RFC4895], [RFC4960], and [RFC5061].
RFCs that include a sockets API specification:
[RFC6458], [RFC6525], [RFC6951], [RFC7053], [RFC7496],
and [RFC7829].
UDP(-Lite): See [RFC8304].
LEDBAT: [RFC6817].
Appendix B. How This Document Was Developed
This section gives an overview of the method that was used to develop
this document. It was given to contributors for guidance, and it can
be helpful for future updates or extensions.
This document is only concerned with transport features that are
explicitly exposed to applications via primitives. It also strictly
follows RFC text: if a transport feature is truly relevant for an
application, the RFCs should say so, and they should describe how to
use and configure it. Thus, the approach followed for developing
this document was to identify the right RFCs, then analyze and
process their text.
Primitives that "MAY" be implemented by a transport protocol were
excluded. To be included, the minimum requirement level for a
primitive to be implemented by a protocol was "SHOULD". Where style
requirement levels as described in [RFC2119] are not used, primitives
were excluded when they are described in conjunction with statements
like, e.g., "some implementations also provide" or "an implementation
may also". Excluded primitives or parameters were briefly described
in a dedicated subsection.
Pass 1: This began by identifying text that talks about primitives.
An API specification, abstract or not, obviously describes primitives
-- but we are not *only* interested in API specifications. The text
describing the 'Send' primitive in the API specified in [RFC0793],
for instance, does not say that data transfer is reliable. TCP's
reliability is clear, however, from this text in Section 1 of
[RFC0793]:
The Transmission Control Protocol (TCP) is intended for use as a
highly reliable host-to-host protocol between hosts in packet-
switched computer communication networks, and in interconnected
systems of such networks.
Some text for the pass 1 subsections was developed by copying and
pasting all the relevant text parts from the relevant RFCs then
adjusting the terminology to match that in Section 2 and shortening
phrasing to match the general style of the document. An effort was
made to formulate everything as a primitive description such that the
primitive descriptions became as complete as possible (e.g., the
'SEND.TCP' primitive in pass 2 is explicitly described as reliably
transferring data); text that is relevant for the primitives
presented in this pass but still does not fit directly under any
primitive was used in a subsection's introduction.
Pass 2: The main goal of this pass is unification of primitives. As
input, only text from pass 1 was used (no exterior sources). The
list in pass 2 is not arranged by protocol (i.e., "first protocol X,
here are all the primitives; then protocol Y, here are all the
primitives, ...") but by primitive (i.e., "primitive A, implemented
this way in protocol X, this way in protocol Y, ..."). It was a goal
to obtain as many similar pass 2 primitives as possible. For
instance, this was sometimes achieved by not always maintaining a 1:1
mapping between pass 1 and pass 2 primitives, renaming primitives,
etc. For every new primitive, the already-existing primitives were
considered to try to make them as coherent as possible.
For each primitive, the following style was used:
o PRIMITIVENAME.PROTOCOL:
Pass 1 primitive/event:
Parameters:
Returns:
Comments:
The entries "Parameters", "Returns", and "Comments" were skipped when
a primitive had no parameters, no described return value, or no
comments seemed necessary, respectively. Optional parameters are
followed by "(optional)". When known, default values were provided.
Pass 3: The main point of this pass is to identify transport features
that are the result of static properties of protocols, for which all
protocols have to be listed together; this is then the final list of
all available transport features. This list was primarily based on
text from pass 2, with additional input from pass 1 (but no external
sources).
Acknowledgements
The authors would like to thank (in alphabetical order) Bob Briscoe,
Spencer Dawkins, Aaron Falk, David Hayes, Karen Nielsen, Tommy Pauly,
Joe Touch, and Brian Trammell for providing valuable feedback on this
document. We especially thank Christoph Paasch for providing input
related to Multipath TCP and Gorry Fairhurst and Tom Jones for
providing input related to UDP(-Lite). This work has received
funding from the European Union's Horizon 2020 research and
innovation programme under grant agreement No. 644334 (NEAT).
Authors' Addresses
Michael Welzl
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Email: michawe@ifi.uio.no
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Steinfurt 48565
Germany
Email: tuexen@fh-muenster.de
Naeem Khademi
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Email: naeemk@ifi.uio.no