Internet DRAFT - draft-ietf-taps-transports-usage
draft-ietf-taps-transports-usage
TAPS M. Welzl
Internet-Draft University of Oslo
Intended status: Informational M. Tuexen
Expires: April 29, 2018 Muenster Univ. of Appl. Sciences
N. Khademi
University of Oslo
October 26, 2017
On the Usage of Transport Features Provided by IETF Transport Protocols
draft-ietf-taps-transports-usage-09
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 TAPS API.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 29, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
Welzl, et al. Expires April 29, 2018 [Page 1]
Internet-Draft Transport Services October 2017
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Pass 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Primitives Provided by TCP . . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . 18
4. Pass 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1. CONNECTION Related Primitives . . . . . . . . . . . . . . 20
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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 49
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
8. Security Considerations . . . . . . . . . . . . . . . . . . . 50
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
9.1. Normative References . . . . . . . . . . . . . . . . . . . 50
9.2. Informative References . . . . . . . . . . . . . . . . . . 52
Appendix A. Overview of RFCs used as input for pass 1 . . . . . . 53
Appendix B. How this document was developed . . . . . . . . . . . 54
Appendix C. Revision information . . . . . . . . . . . . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57
Welzl, et al. Expires April 29, 2018 [Page 2]
Internet-Draft Transport Services October 2017
1. 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
implements a single transport service, e.g., a protocol stack (RTP
over UDP).
Application: an entity that uses the transport layer for end-to-end
delivery 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.
2. Introduction
This specification describes an abstract interface for applications
to make use of Transport Services, such that applications 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
Welzl, et al. Expires April 29, 2018 [Page 3]
Internet-Draft Transport Services October 2017
mechanisms [RFC8095]. The present document and [FJ16] 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),
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 [FJ16].
This snapshot in time analysis of the IETF transport protocols is
published as an RFC to document the authors' and working group's
analysis, generating 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
[I-D.draft-gjessing-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, the relevant RFC text is
discussed per protocol. In the second pass, 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
Welzl, et al. Expires April 29, 2018 [Page 4]
Internet-Draft Transport Services October 2017
-- 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 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 [FJ16].
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
Welzl, et al. Expires April 29, 2018 [Page 5]
Internet-Draft Transport Services October 2017
networks". Section 3.8 in this specification [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 all segments, as this is also stated in a specification
given for the usage of source route (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
Welzl, et al. Expires April 29, 2018 [Page 6]
Internet-Draft Transport Services October 2017
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 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').
Welzl, et al. Expires April 29, 2018 [Page 7]
Internet-Draft Transport Services October 2017
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
expires (see 'open') (section 3.9 of [RFC0793]). 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 User Timeout Option (UTO): 'adv_uto'
is the value of the UTO advertised to the remote TCP peer
Welzl, et al. Expires April 29, 2018 [Page 8]
Internet-Draft Transport Services October 2017
(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. It is therefore not considered
here.
Welzl, et al. Expires April 29, 2018 [Page 9]
Internet-Draft Transport Services October 2017
3.2. Primitives Provided by MPTCP
Multipath TCP (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,
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 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, MPTCP must be disabled in case the application is binding
to a specific address [RFC6897].
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
Welzl, et al. Expires April 29, 2018 [Page 10]
Internet-Draft Transport Services October 2017
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) multi-homing 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 the section 10 of the
SCTP base protocol specification [RFC4960], an extension to the
socket 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],
[I-D.ietf-tsvwg-sctp-ndata], the corresponding extensions of the
socket 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 to adjust elements of a parameter list [RFC4960]; 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'
Welzl, et al. Expires April 29, 2018 [Page 11]
Internet-Draft Transport Services October 2017
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 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]. A
'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 setup 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 to control the partial reliability extension
[RFC3758], [RFC7496]. An optional maximum life time 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
Welzl, et al. Expires April 29, 2018 [Page 12]
Internet-Draft Transport Services October 2017
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 socket API extensions [RFC6458]), allows to configure the
number of unsuccessful retransmissions after which an entire
association is considered as failed; this should invoke a
'Communication Lost' notification.
Welzl, et al. Expires April 29, 2018 [Page 13]
Internet-Draft Transport Services October 2017
Set Primary: This allows 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 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 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: 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 [RFC4960] and
MTU per path [RFC6458].
Welzl, et al. Expires April 29, 2018 [Page 14]
Internet-Draft Transport Services October 2017
Enable / Disable Interleaving: This allows to enable or disable the
negotiation of user message interleaving support for future
associations. For existing associations it is possible to query
whether user message interleaving support was negotiated or not on
a particular association [I-D.ietf-tsvwg-sctp-ndata].
Set Stream Scheduler: This allows to select a stream scheduler per
association, with a choice of: First Come First Serve, Round
Robin, Round Robin per Packet, Priority Based, Fair Bandwidth,
Weighted Fair Queuing [I-D.ietf-tsvwg-sctp-ndata].
Configure Stream Scheduler: This allows 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 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 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; 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].
Welzl, et al. Expires April 29, 2018 [Page 15]
Internet-Draft Transport Services October 2017
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;
RTO.Beta. Only the initial, minimum and maximum RTO 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 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.
Welzl, et al. Expires April 29, 2018 [Page 16]
Internet-Draft Transport Services October 2017
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 assocation.
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.
Assocation 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].
Welzl, et al. Expires April 29, 2018 [Page 17]
Internet-Draft Transport Services October 2017
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 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
[FJ16].
3.5. The service of LEDBAT
The service of the Low Extra Delay Background Transport (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
Welzl, et al. Expires April 29, 2018 [Page 18]
Internet-Draft Transport Services October 2017
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 numer 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 100ms 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], yielding 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, and 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
Welzl, et al. Expires April 29, 2018 [Page 19]
Internet-Draft Transport Services October 2017
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'
Welzl, et al. Expires April 29, 2018 [Page 20]
Internet-Draft Transport Services October 2017
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); 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 Master Key Tuples (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 attemps (optional); maximum init. RTO for INIT
(optional); user message (optional); 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.
Welzl, et al. Expires April 29, 2018 [Page 21]
Internet-Draft Transport Services October 2017
o CONNECT.MPTCP:
This is similar to CONNECT.TCP except for one additional boolean
parameter that allows 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); 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);
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 TCP's connection handshake is completed. This message may
arrive multiple times. 'MKT configuration' refers to the ability
to configure Master Key Tuples (MKTs) for authentication.
Welzl, et al. Expires April 29, 2018 [Page 22]
Internet-Draft Transport Services October 2017
o LISTEN.SCTP:
Pass 1 primitive / event: 'Initialize', followed by 'Communication
Up' or 'Restart' notification and possibly '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;
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 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); 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
Welzl, et al. Expires April 29, 2018 [Page 23]
Internet-Draft Transport Services October 2017
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); boolean changeable
(optional, default true)
Comments: when sending data, an application can adjust the
connection's timeout value (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 '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; '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.
Welzl, et al. Expires April 29, 2018 [Page 24]
Internet-Draft Transport Services October 2017
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 to send 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 to send 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'
Welzl, et al. Expires April 29, 2018 [Page 25]
Internet-Draft Transport Services October 2017
Parameters: local IP address
o REM_PATH.MPTCP:
Pass 1 primitive / event: not specified
Parameters: local IP address; local port number; remote IP
address; 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'
Welzl, et al. Expires April 29, 2018 [Page 26]
Internet-Draft Transport Services October 2017
Parameters: primary max retrans (no. of retransmissions after
which a path is considered inactive); PF max retrans (no. 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; 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.
Welzl, et al. Expires April 29, 2018 [Page 27]
Internet-Draft Transport Services October 2017
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); 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; 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
Welzl, et al. Expires April 29, 2018 [Page 28]
Internet-Draft Transport Services October 2017
Parameters: current_key; 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; hmac_id
o GET_AUTH.TCP:
Pass 1 primitive / event: not specified
Parameters: current_key; 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; chunk_list
o RESET_STREAM.SCTP:
Pass 1 primitive / event: 'Add / Reset Streams, Reset Association'
Parameters: sid; 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.
Welzl, et al. Expires April 29, 2018 [Page 29]
Internet-Draft Transport Services October 2017
o RESET_ASSOC.SCTP:
Pass 1 primitive / event: 'Add / Reset Streams, Reset Association'
Parameters: information related to the extension, 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 if 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 Serve, Round Robin, Round
Robin per Packet, Priority Based, Fair Bandwidth, Weighted Fair
Queuing.
Welzl, et al. Expires April 29, 2018 [Page 30]
Internet-Draft Transport Services October 2017
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.
Welzl, et al. Expires April 29, 2018 [Page 31]
Internet-Draft Transport Services October 2017
o CONFIG_FRAGMENTATION.SCTP:
Pass 1 primitive / event: 'Configure Message Fragmentation'
Parameters: one boolean value (enable/disable); 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); 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); number of packets to wait for (default 2)
Comments: if delayed SACK is enabled, SCTP will send a SACK upon
either 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); max (optional)
Comments: this adjusts the initial, minimum and maximum RTO
values.
Welzl, et al. Expires April 29, 2018 [Page 32]
Internet-Draft Transport Services October 2017
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 receiver
o SET_CHECKSUM_COVERAGE.UDP-Lite:
Pass 1 primitive / event: 'Set_Checksum_Coverage'
Parameters: coverage length at sender (default maximum coverage)
Welzl, et al. Expires April 29, 2018 [Page 33]
Internet-Draft Transport Services October 2017
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
Welzl, et al. Expires April 29, 2018 [Page 34]
Internet-Draft Transport Services October 2017
Comments: this allows an application to read the the IPv4 TTL of
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 ECN
codepoint field for outgoing UDP(-Lite) datagrams. 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
codepoint 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 Time Stamp 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.
Welzl, et al. Expires April 29, 2018 [Page 35]
Internet-Draft Transport Services October 2017
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; 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.
Welzl, et al. Expires April 29, 2018 [Page 36]
Internet-Draft Transport Services October 2017
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.
Welzl, et al. Expires April 29, 2018 [Page 37]
Internet-Draft Transport Services October 2017
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
category 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); 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); 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'
Welzl, et al. Expires April 29, 2018 [Page 38]
Internet-Draft Transport Services October 2017
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. IP address
and Port 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); 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); partial flag
Welzl, et al. Expires April 29, 2018 [Page 39]
Internet-Draft Transport Services October 2017
(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; 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
Welzl, et al. Expires April 29, 2018 [Page 40]
Internet-Draft Transport Services October 2017
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, UDP(-Lite)
o Specify which IP Options must always be used
Protocols: TCP, UDP(-Lite)
o Request multiple streams
Protocols: SCTP
o Limit the number of inbound streams
Protocols: SCTP
Welzl, et al. Expires April 29, 2018 [Page 41]
Internet-Draft Transport Services October 2017
o Specify number of attempts and/or timeout for the first
establishment message
Protocols: TCP, SCTP
o Obtain multiple sockets
Protocols: SCTP
o Disable MPTCP
Protocols: MPTCP
o Configure authentication
Protocols: TCP, SCTP
Comments: with TCP, this allows to configure Master Key Tuples
(MKTs). In SCTP, this allows to specify 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, UDP(-Lite)
o Listen, N specified local interfaces
Protocols: SCTP
o Listen, all local interfaces
Protocols: TCP, SCTP, UDP(-Lite)
Welzl, et al. Expires April 29, 2018 [Page 42]
Internet-Draft Transport Services October 2017
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, UDP(-Lite)
o Disable MPTCP
Protocols: MPTCP
o Configure authentication
Protocols: TCP, SCTP
Comments: with TCP, this allows to configure Master Key Tuples
(MKTs). In SCTP, this allows to specify 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, SCTP
o Suggest timeout to the peer
Protocols: TCP
o Disable Nagle algorithm
Protocols: TCP, 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, SCTP
MPTCP Parameters: source-IP; source-Port; destination-IP;
destination-Port
SCTP Parameters: local IP address
Welzl, et al. Expires April 29, 2018 [Page 43]
Internet-Draft Transport Services October 2017
o Remove path
Protocols: MPTCP, SCTP
MPTCP Parameters: source-IP; source-Port; destination-IP;
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, 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; interleaving supported
yes/no
MPTCP parameters: subflow-list (identified by source-IP; source-
Port; destination-IP; destination-Port)
o Specify DSCP field
Protocols: TCP, SCTP, UDP(-Lite)
o Notification of ICMP error message arrival
Protocols: TCP, UDP(-Lite)
o Change authentication parameters
Protocols: TCP, SCTP
o Obtain authentication information
Protocols: TCP, SCTP
o Reset Stream
Protocols: SCTP
o Notification of Stream Reset
Protocols: STCP
Welzl, et al. Expires April 29, 2018 [Page 44]
Internet-Draft Transport Services October 2017
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
Welzl, et al. Expires April 29, 2018 [Page 45]
Internet-Draft Transport Services October 2017
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.
Welzl, et al. Expires April 29, 2018 [Page 46]
Internet-Draft Transport Services October 2017
o Close after reliably delivering all remaining data, causing an
event informing the application on the other side
Protocols: TCP, 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
informing the application on the other side
Protocols: TCP, 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
informing the application on the other side
Protocols: UDP(-Lite)
o Timeout event when data could not be delivered for too long
Protocols: TCP, 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 to transfer 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
we here 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
Welzl, et al. Expires April 29, 2018 [Page 47]
Internet-Draft Transport Services October 2017
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, 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 id 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 we here 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
Welzl, et al. Expires April 29, 2018 [Page 48]
Internet-Draft Transport Services October 2017
o Receive a message
Protocols: SCTP, 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, 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. 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).
7. IANA Considerations
This memo includes no request to IANA.
Welzl, et al. Expires April 29, 2018 [Page 49]
Internet-Draft Transport Services October 2017
8. 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].
9. References
9.1. Normative References
[FJ16] Fairhurst, G. and T. Jones, "Features of the User Datagram
Protocol (UDP) and Lightweight UDP (UDP-Lite) Transport
Protocols", draft-ietf-taps-transports-usage-udp-04 (work
in progress), July 2017.
[I-D.ietf-tsvwg-sctp-ndata]
Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol",
draft-ietf-tsvwg-sctp-ndata-08 (work in progress),
October 2016.
[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>.
Welzl, et al. Expires April 29, 2018 [Page 50]
Internet-Draft Transport Services October 2017
[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>.
Welzl, et al. Expires April 29, 2018 [Page 51]
Internet-Draft Transport Services October 2017
[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>.
9.2. Informative References
[I-D.draft-gjessing-taps-minset]
Gjessing, S. and M. Welzl, "A Minimal Set of Transport
Services for TAPS Systems", draft-gjessing-taps-minset-05
(work in progress), June 2017.
[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/
Welzl, et al. Expires April 29, 2018 [Page 52]
Internet-Draft Transport Services October 2017
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>.
Appendix A. Overview of RFCs used as input for pass 1
Welzl, et al. Expires April 29, 2018 [Page 53]
Internet-Draft Transport Services October 2017
TCP: [RFC0793], [RFC1122], [RFC5482], [RFC5925], [RFC7413]
MPTCP: [RFC6182], [RFC6824], [RFC6897]
SCTP: RFCs without a socket API specification: [RFC3758], [RFC4895],
[RFC4960], [RFC5061].
RFCs that include a socket API specification: [RFC6458],
[RFC6525], [RFC6951], [RFC7053], [RFC7496] [RFC7829].
UDP(-Lite): See [FJ16]
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
[RFC2119]-style requirements levels 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 pass 1 subsections was developed copy+pasting all the
relevant text parts from the relevant RFCs, then adjusting
terminology to match the terminology in Section 1 and adjusting
(shortening!) phrasing to match the general style of the document.
Welzl, et al. Expires April 29, 2018 [Page 54]
Internet-Draft Transport Services October 2017
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 ("first protocol X, here
are all the primitives; then protocol Y, here are all the primitives,
..") but by primitive ("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 a default value is known, this was
also 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).
Appendix C. Revision information
XXX RFC-Ed please remove this section prior to publication.
-00 (from draft-welzl-taps-transports): this now covers TCP based on
all TCP RFCs (this means: if you know of something in any TCP RFC
that you think should be addressed, please speak up!) as well as
Welzl, et al. Expires April 29, 2018 [Page 55]
Internet-Draft Transport Services October 2017
SCTP, exclusively based on [RFC4960]. We decided to also incorporate
[RFC6458] for SCTP, but this hasn't happened yet. Terminology made
in line with [RFC8095]. Addressed comments by Karen Nielsen and
Gorry Fairhurst; various other fixes. Appendices (TCP overview and
how-to-contribute) added.
-01: this now also covers MPTCP based on [RFC6182], [RFC6824] and
[RFC6897].
-02: included UDP, UDP-Lite, and all extensions of SCTPs. This
includes fixing the [RFC6458] omission from -00.
-03: wrote security considerations. The "how to contribute" section
was updated to reflect how the document *was* created, not how it
*should be* created; it also no longer wrongly says that Experimental
RFCs are excluded. Included LEDBAT. Changed abstract and intro to
reflect which protocols/mechanisms are covered (TCP, MPTCP, SCTP,
UDP, UDP-Lite, LEDBAT) instead of talking about "transport
protocols". Interleaving and stream scheduling added
(draft-ietf-tsvwg-sctp-ndata). TFO added. "Set protocol parameters"
in SCTP replaced with per-parameter (or parameter group) primitives.
More primitives added, mostly previously overlooked ones from
[RFC6458]. Updated terminology (s/transport service feature/
transport feature) in line with an update of [RFC8095]. Made
sequence of transport features / primitives more logical. Combined
MPTCP's add/rem subflow with SCTP's add/remove local address.
-04: changed UDP's close into an ABORT (to better fit with the
primitives of TCP and SCTP), and incorporated the corresponding
transport feature in step 3 (this addresses a comment from Gorry
Fairhurst). Added TCP Authentication (RFC 5925, section 7.1).
Changed TFO from looking like a primitive in pass 1 to be a part of
'open'. Changed description of SCTP authentication in pass 3 to
encompass both TCP and SCTP. Added citations of [RFC8095] and minset
[I-D.draft-gjessing-taps-minset] to the intro, to give the context of
this document.
-05: minor fix to TCP authentication (comment from Joe Touch),
several fixes from Gorry Fairhurst and Tom Jones. Language fixes;
updated to align with latest taps-transport-usage-udp ID.
-06: addressed WGLC comments from Aaron Falk and Tommy Pauly.
-07: addressed AD review comments from Spencer Dawkins.
-08: removed "delivery number" which was based on an error in RFC
4960: https://tools.ietf.org/html/
draft-ietf-tsvwg-rfc4960-errata-02#section-3.34.
Welzl, et al. Expires April 29, 2018 [Page 56]
Internet-Draft Transport Services October 2017
-09: for consistency with the draft-ietf-taps-minset-00, adjusted the
following transport features in "pass 3": "Choice between unordered
(potentially faster) or ordered delivery of messages" divided into
two transport features (one for unordered, one for ordered); the word
"reliably" was added to the transport features "Hand over a message
to reliably transfer (possibly multiple times) before connection
establishment" and "Hand over a message to reliably transfer during
connection establishment". Fixed RFC2119-style language into
explicit citations (comment by Eric Rescorla and others). Addressed
editorial comments by Mirja Kuehlewind, Ben Campbell, Benoit Claise
and the Gen-ART reviewer Roni Even, except for moving terminology
section after the intro because the terminology is already used in
the intro text.
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
Welzl, et al. Expires April 29, 2018 [Page 57]
