TAPS Working Group B. Trammell
Internet-Draft ETH Zurich
Intended status: Informational C. Perkins
Expires: September 9, 2017 University of Glasgow
T. Pauly
Apple Inc.
M. Kuehlewind
ETH Zurich
March 08, 2017
Post Sockets, An Abstract Programming Interface for the Transport Layer
draft-trammell-taps-post-sockets-00
Abstract
This document describes Post Sockets, an asynchronous abstract
programming interface for the atomic transmission of messages in an
inherently multipath environment. Post replaces connections with
long-lived associations between endpoints, with the possibility to
cache cryptographic state in order to reduce amortized connection
latency. We present this abstract interface as an illustration of
what is possible with present developments in transport protocols
when freed from the strictures of the current sockets 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
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on September 9, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Abstractions and Terminology . . . . . . . . . . . . . . . . 5
2.1. Message Carrier . . . . . . . . . . . . . . . . . . . . . 6
2.1.1. Listener . . . . . . . . . . . . . . . . . . . . . . 7
2.1.2. Source . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.3. Sink . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.4. Responder . . . . . . . . . . . . . . . . . . . . . . 8
2.1.5. Stream . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Message . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Lifetime and Partial Reliability . . . . . . . . . . 9
2.2.2. Priority . . . . . . . . . . . . . . . . . . . . . . 10
2.2.3. Dependence . . . . . . . . . . . . . . . . . . . . . 10
2.2.4. Idempotence . . . . . . . . . . . . . . . . . . . . . 10
2.2.5. Immediacy . . . . . . . . . . . . . . . . . . . . . . 10
2.2.6. Additional Events . . . . . . . . . . . . . . . . . . 10
2.3. Association . . . . . . . . . . . . . . . . . . . . . . . 11
2.4. Remote . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.5. Local . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.6. Transient . . . . . . . . . . . . . . . . . . . . . . . . 12
2.7. Path . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.8. Policy Context . . . . . . . . . . . . . . . . . . . . . 13
3. Abstract Programming Interface . . . . . . . . . . . . . . . 14
3.1. Example Connection Patterns . . . . . . . . . . . . . . . 15
3.1.1. Client-Server . . . . . . . . . . . . . . . . . . . . 15
3.1.2. Client-Server with Happy Eyeballs and 0-RTT
establishment . . . . . . . . . . . . . . . . . . . . 16
3.1.3. Peer to Peer with Network Address Translation . . . . 17
3.1.4. Multicast Receiver . . . . . . . . . . . . . . . . . 17
3.2. Implementation Considerations . . . . . . . . . . . . . . 17
3.2.1. Message Framing and Deframing . . . . . . . . . . . . 18
3.2.2. Message Size Limitations . . . . . . . . . . . . . . 18
3.2.3. Backpressure . . . . . . . . . . . . . . . . . . . . 18
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Normative References . . . . . . . . . . . . . . . . . . 19
5.2. Informative References . . . . . . . . . . . . . . . . . 19
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Appendix A. API sketch in Golang . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The BSD Unix Sockets API's SOCK_STREAM abstraction, by bringing
network sockets into the UNIX programming model, allowing anyone who
knew how to write programs that dealt with sequential-access files to
also write network applications, was a revolution in simplicity. It
would not be an overstatement to say that this simple API is the
reason the Internet won the protocol wars of the 1980s. SOCK_STREAM
is tied to the Transmission Control Protocol (TCP), specified in 1981
[RFC0793]. TCP has scaled remarkably well over the past three and a
half decades, but its total ubiquity has hidden an uncomfortable
fact: the network is not really a file, and stream abstractions are
too simplistic for many modern application programming models.
In the meantime, the nature of Internet access, and the variety of
Internet transport protocols, is evolving. The challenges that new
protocols and access paradigms present to the sockets API and to
programming models based on them inspire the design elements of a new
approach
Many end-user devices are connected to the Internet via multiple
interfaces, which suggests it is time to promote the paths by which
two endpoints are connected to each other to a first-order object.
While implicit multipath communication is available for these
multihomed nodes in the present Internet architecture with the
Multipath TCP extension (MPTCP) [RFC6824], MPTCP was specifically
designed to hide multipath communication from the application for
purposes of compatibility. Since many multihomed nodes are connected
to the Internet through access paths with widely different properties
with respect to bandwidth, latency and cost, adding explicit path
control to MPTCP's API would be useful in many situations.
Applications also need control over cooperation with path elements
via mechanisms such as that proposed by the Path Layer UDP Substrate
(PLUS) effort (see [I-D.trammell-plus-statefulness] and
[I-D.trammell-plus-abstract-mech]).
Another trend straining the traditional layering of the transport
stack associated with the SOCK_STREAM interface is the widespread
interest in ubiquitous deployment of encryption to guarantee
confidentiality, authenticity, and integrity, in the face of
pervasive surveillance [RFC7258]. Layering the most widely deployed
encryption technology, Transport Layer Security (TLS), strictly atop
TCP (i.e., via a TLS library such as OpenSSL that uses the sockets
API) requires the encryption-layer handshake to happen after the
transport-layer handshake, which increases connection setup latency
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on the order of one or two round-trip times, an unacceptable delay
for many applications. Integrating cryptographic state setup and
maintenance into the path abstraction naturally complements efforts
in new protocols (e.g. QUIC [I-D.ietf-quic-transport]) to mitigate
this strict layering.
To meet these challenges, we present the Post-Socket Application
Programming Interface (API), described in detail in this work. Post
is designed to be language, transport protocol, and architecture
independent, allowing applications to be written to a common abstract
interface, easily ported among different platforms, and used even in
environments where transport protocol selection may be done
dynamically, as proposed in the IETF's Transport Services working
group.
Post replaces the traditional SOCK_STREAM abstraction with an Message
abstraction, which can be seen as a generalization of the Stream
Control Transmission Protocol's [RFC4960] SOCK_SEQPACKET service.
Messages are sent and received on Carriers, which logically group
Messages for transmission and reception. For backward compatibility,
these Carriers can also be opened as Streams, presenting a file-like
interface to the network as with SOCK_STREAM.
Post replaces the notions of a socket address and connected socket
with an Association with a remote endpoint via set of Paths.
Implementation and wire format for transport protocol(s) implementing
the Post API are explicitly out of scope for this work; these
abstractions need not map directly to implementation-level concepts,
and indeed with various amounts of shimming and glue could be
implemented with varying success atop any sufficiently flexible
transport protocol.
The key features of Post as compared with the existing sockets API
are:
o Explicit Message orientation, with framing and atomicity
guarantees for Message transmission.
o Asynchronous reception, allowing all receiver-side interactions to
be event-driven.
o Explicit support for multistreaming and multipath transport
protocols and network architectures.
o Long-lived Associations, whose lifetimes may not be bound to
underlying transport connections. This allows associations to
cache state and cryptographic key material to enable fast
resumption of communication, and for the implementation of the API
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to explicitly take care of connection establishment mechanics such
as connection racing [RFC6555] and peer-to-peer rendezvous
[RFC5245].
o Transport protocol stack independence, allowing applications to be
written in terms of the semantics best for the application's own
design, separate from the protocol(s) used on the wire to achieve
them. This enables applications written to a single API to make
use of transport protocols in terms of the features they provide,
as in [I-D.ietf-taps-transports].
This work is the synthesis of many years of Internet transport
protocol research and development. It is inspired by concepts from
the Stream Control Transmission Protocol (SCTP) [RFC4960], TCP Minion
[I-D.iyengar-minion-protocol], and MinimaLT[MinimaLT], among other
transport protocol modernization efforts. We present Post Sockets as
an illustration of what is possible with present developments in
transport protocols when freed from the strictures of the current
sockets API. While much of the work for building parts of the
protocols needed to implement Post are already ongoing in other IETF
working groups (e.g. MPTCP, QUIC, TLS), we argue that an abstract
programming interface unifying access all these efforts is necessary
to fully exploit their potential.
2. Abstractions and Terminology
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+===============+
| Message |
+===============+
| ^ initiate() listen()
send() ready() | |
V | V V
+======================+ accept() +============+
| |<---+------| |
| Carrier | | | Listener |
| |----+ | |
+======================+ +============+
| | |
| | |
| +=======================+
| | | durable end-to-end
| | Association | state via many paths/
| | | policies and prefs
| +=======================+
| | |
| | |
| +=========+ +=========+
| | Local | | Remote |
| +=========+ +=========+
| | |
+===========+ +==========+
ephemeral | | | |
transport & | Transient |------->| Path | properties of
crypto state | | | | address pair
+===========+ +==========+
Figure 1: Abstractions and relationships in Post Sockets
Post is based on a small set of abstractions, centered around a
Message Carrier as the entry point for an application to the
networking API. The relationships among them are shown in
Figure Figure 1 and detailed in this section.
2.1. Message Carrier
A Message Carrier (or simply Carrier) is a transport protocol stack-
independent interface for sending and receiving messages between an
application and a remote endpoint; it is roughly analogous to a
socket in the present sockets API.
Sending a Message over a Carrier is driven by the application, while
receipt is driven by the arrival of the last packet that allows the
Message to be assembled, decrypted, and passed to the application.
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Receipt is therefore asynchronous; given the different models for
asynchronous I/O and concurrency supported by different platforms, it
may be implemented in any number of ways. The abstract API provides
only for a way for the application to register how it wants to handle
incoming messages.
All the Messages sent to a Message Carrier will be received on the
corresponding Message Carrier at the remote endpoint, though not
necessarily reliably or in order, depending on Message properties and
the underlying transport protocol stack.
A Message Carrier that is backed by current transport protocol stack
state (such as a TCP connection; see Section 2.6) is said to be
"active": messages can be sent and received over it. A Message
Carrier can also be "dormant": there is long-term state associated
with it (via the underlying Association; see Section 2.3), and it may
be able to reactivated, but messages cannot be sent and received
immediately.
If supported by the underlying transport protocol stack, a Message
Carrier may be forked: creating a new Message Carrier associated with
a new Message Carrier at the same remote endpoint. The semantics of
the usage of multiple Message Carriers based on the same Association
are application-specific. When a Message Carrier is forked, its
corresponding Message Carrier at the remote endpoint receives a fork
request, which it must accept in order to fully establish the new
carrier. Multiple message carriers between endpoints are implemented
differently by different transport protocol stacks, either using
multiple separate transport-layer connections, or using multiple
streams of multistreaming transport protocols.
To exchange messages with a given remote endpoint, an application may
initiate a Message Carrier given its remote (see Section 2.4 and
local (see Section 2.5) identities; this is an equivalent to an
active open. There are five special cases of Message Carriers, as
well, supporting different initiation and interaction patterns,
defined in the subsections below.
2.1.1. Listener
A Listener is a special case of Message Carrier which only responds
to requests to create a new Carrier from a remote endpoint, analogous
to a server or listening socket in the present sockets API. Instead
of being bound to a specific remote endpoint, it is bound only to a
local identity; however, its interface for accepting fork requests is
identical to that for fully fledged Message Carriers.
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2.1.2. Source
A Source is a special case of Message Carrier over which messages can
only be sent, intended for unidirectional applications such as
multicast transmitters. Sources cannot be forked, and need not
accept forks.
2.1.3. Sink
A Sink is a special case of Message Carrier over which messages can
only be received, intended for unidirectional applications such as
multicast receivers. Sinks cannot be forked, and need not accept
forks.
2.1.4. Responder
A Responder is a special case of Message Carrier which may receive
messages from many remote sources, for cases in which an application
will only ever send Messages in reply back to the source from which a
Message was received. This is a common implementation pattern for
servers in client-server applications. A Responder's receiver gets a
Message, as well as a Source to send replies to. Responders cannot
be forked, and need not accept forks.
2.1.5. Stream
A Message Carrier may be irreversibly morphed into a Stream, in order
to provide a strictly ordered, reliable service as with SOCK_STREAM.
Morphing a Message Carrier into a Stream should return a "file-like
object" as appropriate for the platform implementing the API.
Typically, both ends of a communication using a stream service will
morph their respective Message Carriers independently before sending
any Messages.
Writing a byte to a Stream will cause it to be received by the
remote, in order, or will cause an error condition and termination of
the stream if the byte cannot be delivered. Due to the strong
sequential dependence on a stream, streams must always be reliable
and ordered. A Message Carrier may only be morphed to a Stream if it
uses transport protocol stack that provides reliable, ordered
service, and only before it is used to send a Message.
2.2. Message
A Message is an atomic unit of communication between applications. A
Message that cannot be delivered in its entirety within the
constraints of the network connectivity and the requirements of the
application is not delivered at all.
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Messages can represent both relatively small structures, such as
requests in a request/response protocol such as HTTP; as well as
relatively large structures, such as files of arbitrary size in a
filesystem.
In the general case, there is no mapping between a Message and
packets sent by the underlying protocol stack on the wire: the
transport protocol may freely segment messages and/or combine
messages into packets. However, a message may be marked as
immediate, which will cause it to be sent in a single packet, if it
will fit.
This implies that both the sending and receiving endpoint, whether in
the application layer or the transport layer, must guarantee storage
for the full size of an Message.
Messages are sent over and received from Message Carriers (see
Section 2.1).
On sending, Messages have properties that allow the application to
specify its requirements with respect to reliability, ordering,
priority, idempotence, and immediacy; these are described in detail
below. Messages may also have arbitrary properties which provide
additional information to the underlying transport protocol stack on
how they should be handled, in a protocol-specific way. These stacks
may also deliver or set properties on received messages, but in the
general case a received messages contains only a sequence of ordered
bytes.
2.2.1. Lifetime and Partial Reliability
A Message may have a "lifetime" - a wallclock duration before which
the Message must be available to the application layer at the remote
end. If a lifetime cannot be met, the Message is discarded as soon
as possible. Messages without lifetimes are sent reliably if
supported by the transport protocol stack. Lifetimes are also used
to prioritize Message delivery.
There is no guarantee that a Message will not be delivered after the
end of its lifetime; for example, a Message delivered over a strictly
reliable transport will be delivered regardless of its lifetime.
Depending on the transport protocol stack used to transmit the
message, these lifetimes may also be signaled to path elements by the
underlying transport, so that path elements that realize a lifetime
cannot be met can discard frames containing the Messages instead of
forwarding them.
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2.2.2. Priority
Messages have a "niceness" - a priority among other messages sent
over the same Message Carrier in an unbounded hierarchy most
naturally represented as a non-negative integer. By default,
Messages are in niceness class 0, or highest priority. Niceness
class 1 Messages will yield to niceness class 0 Messages sent over
the same Carrier, class 2 to class 1, and so on. Niceness may be
translated to a priority signal for exposure to path elements (e.g.
DSCP codepoint) to allow prioritization along the path as well as at
the sender and receiver. This inversion of normal schemes for
expressing priority has a convenient property: priority increases as
both niceness and lifetime decrease. A Message may have both a
niceness and a lifetime - Messages with higher niceness classes will
yield to lower classes if resource constraints mean only one can meet
the lifetime.
2.2.3. Dependence
A Message may have "antecedents" - other Messages on which it
depends, which must be delivered before it (the "successor") is
delivered. The sending transport uses deadlines, niceness, and
antecedents, along with information about the properties of the Paths
available, to determine when to send which Message down which Path.
2.2.4. Idempotence
A sending application may mark a Message as "idempotent" to signal to
the underlying transport protocol stack that its application
semantics make it safe to send in situations that may cause it to be
received more than once (i.e., for 0-RTT session resumption as in TCP
Fast Open, TLS 1.3, and QUIC).
2.2.5. Immediacy
A sending application may mark a Message as "immediate" to signal to
the underlying transport protocol stack that its application
semantics require it to be placed in a single packet, on its own,
instead of waiting to be combined with other messages or parts
thereof (i.e., for media transports and interactive sessions with
small messages).
2.2.6. Additional Events
Senders may also be asynchronously notified of three events on
Messages they have sent: that the Message has been transmitted, that
the Message has been acknowledged by the receiver, or that the
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Message has expired before transmission/acknowledgment. Not all
transport protocol stacks will support all of these events.
2.3. Association
An Association contains the long-term state necessary to support
communications between a Local (see Section 2.5) and a Remote (see
Section 2.4) endpoint, such as cryptographic session resumption
parameters or rendezvous information; information about the policies
constraining the selection of transport protocols and local
interfaces to create Transients (see Section 2.6) to carry Messages;
and information about the paths through the network available
available between them (see Section 2.7).
All Message Carriers are bound to an Association. New Message
Carriers will reuse an Association if they can be carried from the
same Local to the same Remote over the same Paths; this re-use of an
Association may implies the creation of a new Transient.
2.4. Remote
A Remote represents information required to establish and maintain a
connection with the far end of an Association: name(s), address(es),
and transport protocol parameters that can be used to establish a
Transient; transport protocols to use; information about public keys
or certificate authorities used to identify the remote on connection
establishment; and so on. Each Association is associated with a
single Remote, either explicitly by the application (when created by
the initiation of a Message Carrier) or a Listener (when created by
forking a Message Carrier on passive open).
A Remote may be resolved, which results in zero or more Remotes with
more specific information. For example, an application may want to
establish a connection to a website identified by a URL
https://www.example.com. This URL would be wrapped in a Remote and
passed to a call to initiate a Message Carrier. The first pass
resolution might parse the URL, decomposing it into a name, a
transport port, and a transport protocol to try connecting with. A
second pass resolution would then look up network-layer addresses
associated with that name through DNS, and store any certificates
available from DANE. Once a Remote has been resolved to the point
that a transport protocol stack can use it to create a Transient, it
is considered fully resolved.
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2.5. Local
A Local represents all the information about the local endpoint
necessary to establish an Association or a Listener: interface, port,
and transport protocol stack information, as well as certificates and
associated private keys to use to identify this endpoint.
2.6. Transient
A Transient represents a binding between a Message Carrier and the
instance of the transport protocol stack that implements it. As an
Association contains long-term state for communications between two
endpoints, a Transient contains ephemeral state for a single
transport protocol over a single Path at a given point in time.
A Message Carrier may be served by multiple Transients at once, e.g.
when implementing multipath communication such that the separate
paths are exposed to the API by the underlying transport protocol
stack. Each Transient serves only one Message Carrier, although
multiple Transients may share the same underlying protocol stack;
e.g. when multiplexing Carriers over streams in a multistreaming
protocol.
Transients are generally not exposed by the API to the application,
though they may be accessible for debugging and logging purposes.
2.7. Path
A Path represents information about a single path through the network
used by an Association, in terms of source and destination network
and transport layer addresses within an addressing context, and the
provisioning domain [RFC7556] of the local interface. This
information may be learned through a resolution, discovery, or
rendezvous process (e.g. DNS, ICE), by measurements taken by the
transport protocol stack, or by some other path information discovery
mechanism. It is used by the transport protocol stack to maintain
and/or (re-)establish communications for the Association.
The set of available properties is a function of the transport
protocol stacks in use by an association. However, the following
core properties are generally useful for applications and transport
layer protocols to choose among paths for specific Messages:
o Maximum Transmission Unit (MTU): the maximum size of an Message's
payload (subtracting transport, network, and link layer overhead)
which will likely fit into a single frame. Derived from signals
sent by path elements, where available, and/or path MTU discovery
processes run by the transport layer.
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o Latency Expectation: expected one-way delay along the Path.
Generally provided by inline measurements performed by the
transport layer, as opposed to signaled by path elements.
o Loss Probability Expectation: expected probability of a loss of
any given single frame along the Path. Generally provided by
inline measurements performed by the transport layer, as opposed
to signaled by path elements.
o Available Data Rate Expectation: expected maximum data rate along
the Path. May be derived from passive measurements by the
transport layer, or from signals from path elements.
o Reserved Data Rate: Committed, reserved data rate for the given
Association along the Path. Requires a bandwidth reservation
service in the underlying transport protocol stack.
o Path Element Membership: Identifiers for some or all nodes along
the path, depending on the capabilities of the underlying network
layer protocol to provide this.
Path properties are generally read-only. MTU is a property of the
underlying link-layer technology on each link in the path; latency,
loss, and rate expectations are dynamic properties of the network
configuration and network traffic conditions; path element membership
is a function of network topology. In an explicitly multipath
architecture, application and transport layer requirements can be met
by having multiple paths with different properties to select from.
Transport protocol stacks can also provide signaling to devices along
the path, but this signaling is derived from information provided to
the Message abstraction.
2.8. Policy Context
A Local and a Remote is not necessarily enough to establish a Message
Carrier between two endpoints. For instance, an application may
require or prefer certain transport features (see
[I-D.ietf-taps-transports]) in the transport protocol stacks used by
the Transients underlying the Carrier; it may also prefer Paths over
one interface to those over another (e.g. WiFi access over LTE when
roaming on a foreign LTE network, due to cost). These policies are
expressed in a Policy Context bound to an Association. Multiple
policy contexts may be active at once; e.g. a system Policy Context
expressing administrative preferences about interface and protocol
selection, an application Policy Context expressing transport feature
information. The expression of policy contexts and the resolution of
conflicts among Policy Contexts is currently implementation-specific;
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note that these are equivalent to the Policy API in the NEAT
architeture [NEAT].
3. Abstract Programming Interface
We now turn to the design of an abstract programming interface to
provide a simple interface to Post's abstractions, constrained by the
following design principles:
o Flexibility is paramount. So is simplicity. Applications must be
given as many controls and as much information as they may need,
but they must be able to ignore controls and information
irrelevant to their operation. This implies that the "default"
interface must be no more complicated than BSD sockets, and must
do something reasonable.
o Reception is an inherently asynchronous activity. While the API
is designed to be as platform-independent as possible, one key
insight it is based on is that an Message receiver's behavior in a
packet-switched network is inherently asynchronous, driven by the
receipt of packets, and that this asynchronicity must be reflected
in the API. The actual implementation of receive and event
handling will need to be aligned to the method a given platform
provides for asynchronous I/O.
o A new API cannot be bound to a single transport protocol and
expect wide deployment. As the API is transport-independent and
may support runtime transport selection, it must impose the
minimum possible set of constraints on its underlying transports,
though some API features may require underlying transport features
to work optimally. It must be possible to implement Post over
vanilla TCP in the present Internet architecture.
The API we design from these principles is centered around a Carrier,
which can be created actively via initiate() or passively via a
listen(); the latter creates a Listener from which new Carriers can
be accept()ed. Messages may be created explicitly and passed to this
Carrier, or implicitly through a simplified interface which uses
default message properties (reliable transport without priority or
deadline, which guarantees ordered delivery over a single Carrier
when the underlying transport protocol stack supports it).
The current state of API development is illustrated as a set of
interfaces and function prototypes in the Go programming language in
Appendix A; future revisions of this document will give more a more
abstract specification of the API as development completes.
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3.1. Example Connection Patterns
Here, we illustrate the usage of the API outlined in Appendix A for
common connection patterns. Note that error handling is ignored in
these illustrations for ease of reading.
3.1.1. Client-Server
Here's an example client-server application. The server echoes
messages. The client sends a message and prints what it receives.
The client in Figure 2 connects, sends a message, and sets up a
receiver to print messages received in response. The carrier is
inactive after the Initiate() call; the Send() call blocks until the
carrier can be activated.
// connect to a server given a remote
func sayHello() {
carrier := Initiate(local, remote)
carrier.Send([]byte("Hello!"))
carrier.Ready(func (msg InMessage) {
fmt.Println(string([]byte(msg))
return false
})
carrier.Close()
}
Figure 2: Example client
The server in Figure 3 creates a Listener, which accepts Carriers and
passes them to a server. The server echos the content of each
message it receives.
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// run a server for a specific carrier, echo all its messages
func runMyServerOn(carrier Carrier) {
carrier.Ready(func (msg InMessage) {
carrier.Send(msg)
})
}
// accept connections forever, spawn servers for them
func acceptConnections() {
listener := Listen(local)
listener.Accept(func(carrier Carrier) bool {
go runMyServerOn(carrier)
return true
})
}
Figure 3: Example server
The Responder allows the server to be significantly simplified, as
shown in Figure 4.
func echo(msg InMessage, reply Sink) {
reply.Send(msg)
}
Respond(local, echo)
Figure 4: Example responder
3.1.2. Client-Server with Happy Eyeballs and 0-RTT establishment
The fundamental design of a client need not change at all for happy
eyeballs [RFC6555] (selection of multiple potential protocol stacks
through connection racing); this is handled by the Post Sockets
implementation automatically. If this connection racing is to use
0-RTT data (i.e., as provided by TCP Fast Open [RFC7413], the client
must mark the outgoing message as idempotent.
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// connect to a server given a remote
func sayHelloQuickly() {
carrier := Initiate(local, remote)
carrier.SendMsg(OutMessage{Content: []byte("Hello!"), Idempotent: true}, nil, nil, nil)
carrier.Ready(func (msg InMessage) {
fmt.Println(string([]byte(msg)))
return false
})
carrier.Close()
}
3.1.3. Peer to Peer with Network Address Translation
In the client-server examples shown above, the Remote given to the
Initiate call refers to the name and port of the server to connect
to. This need not be the case, however; a Remote may also refer to
an identity and a rendezvous point for rendezvous as in ICE
[RFC5245]. Here, each peer does its own Initiate call
simultaneously, and the result on each side is a Carrier attached to
an appropriate Association.
3.1.4. Multicast Receiver
A multicast receiver is implemented using a Sink attached to a Local
encapsulating a multicast address on which to receive multicast
datagrams. The following example prints messages received on the
multicast address forever.
func receiveMulticast() {
sink = NewSink(local)
sink.Ready(func (msg InMessage) {
fmt.Println(string([]byte(msg)))
return true
})
}
3.2. Implementation Considerations
Here we discuss an incomplete list of API implementation
considerations that have arisen with experimentation with the
prototype in Appendix A.
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3.2.1. Message Framing and Deframing
An obvious goal of Post Sockets is interoperability with non-Post
Sockets endpoints: a Post Sockets endpoint using a given protocol
stack must be able to communicate with another endpoint using the
same protocol stack, but not using Post Sockets. This implies that
the underlying transport protocol stack must support object framing,
in order to delimit Messages carried by protocol stacks that are not
themselves message-oriented.
Another goal of Post Sockets is to work over unmodified TCP. We
could simply define a Message Carrier over TCP to support only stream
morphing, but this would fall far short of our goal to transport
independence. Another approach is to recognize that almost every
protocol using TCP already has its own message delimiters, and to
allow the receiver of a Message to provide a deframing primitive to
the API. Experimentation with the best way to achieve this within
Post Sockets is underway.
3.2.2. Message Size Limitations
Ideally, Messages can be of infinite size. However, protocol stacks
and protocol stack implementations may impose their own limits on
message sizing; For example, SCTP [RFC4960] and TLS
[I-D.ietf-tls-tls13] impose record size limitations of 64kB and 16kB,
respectively. Message sizes may also be limited by the available
buffer at the receiver, since a Message must be fully assembled by
the transport layer before it can be passed on to the application
layer. Since not every transport protocol stack implements the
signaling necessary to negotiate or expose message size limitations,
these are currently configured out of band, and are probably best
exposed through the policy context.
A truly infinite message service - e.g. large file transfer where
both endpoints have committed persistent storage to the message - is
probably best realized as a layer above Post Sockets, and may be
added as a new type of Message Carrier to a future revision of this
document.
3.2.3. Backpressure
Regardless of how asynchronous reception is implemented, it is
important for an application to be able to apply receiver
backpressure, to allow the protocol stack to perform receiver flow
control. Depending on how asynchronous I/O works in the platform,
this could be implemented by having a maximum number of concurrent
receive callbacks, for example.
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4. Acknowledgments
Many thanks to Laurent Chuat and Jason Lee at the Network Security
Group at ETH Zurich for contributions to the initial design of Post
Sockets. Thanks to Joe Hildebrand, Martin Thomson, and Michael Welzl
for their feedback, as well as the attendees of the Post Sockets
workshop in February 2017 in Zurich for the discussions, which have
improved the design described herein.
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
This support does not imply endorsement.
5. References
5.1. Normative References
[I-D.ietf-taps-transports]
Fairhurst, G., Trammell, B., and M. Kuehlewind, "Services
provided by IETF transport protocols and congestion
control mechanisms", draft-ietf-taps-transports-14 (work
in progress), December 2016.
5.2. Informative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-01 (work
in progress), January 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-18 (work in progress),
October 2016.
[I-D.iyengar-minion-protocol]
Jana, J., Cheshire, S., and J. Graessley, "Minion - Wire
Protocol", draft-iyengar-minion-protocol-02 (work in
progress), October 2013.
[I-D.trammell-plus-abstract-mech]
Trammell, B., "Abstract Mechanisms for a Cooperative Path
Layer under Endpoint Control", draft-trammell-plus-
abstract-mech-00 (work in progress), September 2016.
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[I-D.trammell-plus-statefulness]
Kuehlewind, M., Trammell, B., and J. Hildebrand,
"Transport-Independent Path Layer State Management",
draft-trammell-plus-statefulness-02 (work in progress),
December 2016.
[MinimaLT]
Petullo, W., Zhang, X., Solworth, J., Bernstein, D., and
T. Lange, "MinimaLT, Minimal-latency Networking Through
Better Security", May 2013.
[NEAT] Grinnemo, K-J., Tom Jones, ., Gorry Fairhurst, ., David
Ros, ., Anna Brunstrom, ., and . Per Hurtig, "Towards a
Flexible Internet Transport Layer Architecture", June
2016.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
2012, <http://www.rfc-editor.org/info/rfc6555>.
[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,
<http://www.rfc-editor.org/info/rfc6824>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
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[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<http://www.rfc-editor.org/info/rfc7556>.
Appendix A. API sketch in Golang
The following sketch is a snapshot of an API currently under
development in Go, available at https://github.com/mami-project/
postsocket. The details of the API are still under development; once
the API definition stabilizes, this will be expanded into prose in a
future revision of this draft.
// The interface to path information is TBD
type Path interface{}
// An association encapsulates an endpoint pair and the set of paths between them.
type Association interface {
Local() Local
Remote() Remote
Paths() []Path
}
// A message together with with metadata needed to send it
type OutMessage struct {
// The content of this message, as a byte array
Content []byte
// The niceness of this message. 0 is highest priority.
Niceness uint
// The lifetime of this message. After this duration, the message may expire.
Lifetime time.Duration
// Pointers to messages that must be sent before this one.
Antecedent []*OutMessage
// True if the message is safe to send such that it may be received multiple times (i.e. for 0-RTT).
Idempotent bool
}
// A message received from a stream
type InMessage []byte
// A Carrier is a transport protocol stack-independent interface for sending and
// receiving messages between an application and a remote endpoint; it is roughly
// analogous to a socket in the present sockets API.
type Carrier interface {
// Send a byte array on this Carrier as a message with default metadata
// and no notifications.
Send(buf []byte) error
// Send a message on this Carrier. The optional onSent function will be
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// called when the protocol stack instance has sent the message. The
// optional onAcked function will be called when the receiver has
// acknowledged the message. The optional onExpired function will be
// called if the message's lifetime expired before the message coult be
// sent. If the Carrier is not active, attempt to activate the Carrier
// before sending.
Sendmsg(msg *OutMessage, onSent func(), onAcked func(), onExpired func()) error
// Signal that an application is ready to receive messages via a given callback.
// Messages will be given to the callback until it returns false, or until the
// Carrier is closed.
Ready(receive func(InMessage) bool) error
// Retrieve the Association over which this Carrier is running.
Association() *Association
// Retrieve the active Transients over which this carrier is running, if active.
Transients() []Transient
// Determine whether the Carrier is currently active
IsActive() bool
// Ensure that the Carrier is active and ready to send and receive messages.
// Attempts to bring up at least one Transient.
Activate(isActive func()) error
// Terminate the Carrier
Close()
// Mutate to a file-like object
AsStream() io.ReadWriteCloser
// Attempt to fork a new Carrier for communicating with the same Remote
Fork() (Carrier, error)
// Signal that an application is ready to accept forks via a given callback.
// Forked carriers will be given to the callback until it returns false or
// until the Carrier is closed.
Accept(accept func(Carrier) bool) error
}
// Initiate a Carrier from a given Local to a given Remote. Returns a new
// Carrier, which may be bound to an existing or a new Association. The
// initiated Carrier is not yet active.
func Initiate(local Local, remote Remote) (Carrier, error)
type Listener interface {
// Signal that an application is ready to accept forks via a given callback.
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// Accept will terminate when the callback returns false, or until the
// Listener is closed.
Accept(accept func(Carrier) bool) error
// Terminate this Listener
Close()
}
// Create a Listener on a given Local which will pass new Carriers to the
// given channel until that channel is closed.
func Listen(local Local) (Listener, error)
// A Source is a unidirectional, send-only Carrier.
type Source interface {
// Send a byte array on this Source as a message with default metadata
// and no notifications.
Send(buf []byte) error
// Send a message on this Source. The optional onSent function will be
// called when the protocol stack instance has sent the message. The
// optional onAcked function will be called when the receiver has
// acknowledged the message. The optional onExpired function will be
// called if the message's lifetime expired before the message coult be
// sent. If the Source is not active, attempt to activate the Source
// before sending.
Sendmsg(msg *OutMessage, onSent func(), onAcked func(), onExpired func()) error
// Retrieve the Association over which this Source is running.
Association() *Association
// Determine whether the Source is currently active
IsActive() bool
// Ensure that the Source is active and ready to send messages.
// Attempts to bring up at least one Transient.
Activate() error
// Terminate the Source
Close()
}
// Initiate a Source from a given Local to a given Remote. Returns a new
// Source, which may be bound to an existing or a new Association. The
// initiated Source is not yet active.
func NewSource(local Local, remote Remote) (Source, error)
// A Sink is a unidirectional, receive-only Carrier, bound only to a local.
type Sink interface {
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// Signal that an application is ready to receive messages via a given callback.
// Messages will be given to the callback until it returns false, or until the
// Sink is closed.
Ready(receive func(InMessage) bool) error
// Retrieve the Association over which this Sink is running.
Association() *Association
// Terminate the Sink
Close()
}
// Initiate a Sink on a given Local. Returns a new
// Sink, which may be bound to an existing or a new Association.
func NewSink(local Local) (Sink, error)
// Initiate a Responder on a given Local. For each incoming Message, calls the
// respond function with the Message and a Sink to send replies to. Calls the
// Responder until it returns False, then terminates
func Respond(local Local, respond func(msg InMessage, reply Sink) bool) error
// A local identity
type Local struct {
// A string identifying an interface or set of interfaces to accept messages and new carriers on.
Interface string
// A transport layer port
Port int
// A set of zero or more end entity certificates, together with private
// keys, to identify this application with.
Certificates []tls.Certificate
}
// Encapsulate a remote identity. Since the contents of a Remote are highly
// dependent on its level of resolution; some examples are below.
type Remote interface {
// Resolve this Remote Identity to a
Resolve() ([]RemoteIdentity, error)
// Returns True if the Remote is completely resolved; i.e., cannot be resol
Complete() bool
}
// Remote consisting of a URL
type URLRemote struct {
URL string
}
// Remote encapsulating a name and port number
type NamedEndpointRemote struct {
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Hostname string
Port int
}
// Remote encapsulating an IP address and port number
type IPEndpointRemote struct {
Address net.IP
Port int
}
// Remote encapsulating an IP address and port number, and a set of presented certificates
type IPEndpointCertRemote struct {
Address net.IP
Port int
Certificates []tls.Certificate
}
Authors' Addresses
Brian Trammell
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: ietf@trammell.ch
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
Tommy Pauly
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
United States of America
Email: tpauly@apple.com
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Mirja Kuehlewind
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: mirja.kuehlewind@tik.ee.ethz.ch
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