Network Working Group B. Trammell
Internet-Draft ETH Zurich
Intended status: Informational May 07, 2015
Expires: November 8, 2015
Thoughts a New Transport Encapsulation Architecture
draft-trammell-stackevo-newtea-01
Abstract
This document explores architectural considerations for using
encapsulation in support of stack evolution and new transport
protocol deployment in an increasingly encrypted Internet. These
architectural considerations are based on an idealized architecture
where all interactions among applications, endpoints, and the path
occur explicitly, with this cooperation enforced cryptographically.
This idealized architecture is then lensed through the state of
devices in the present Internet and how they would impair the
deployability of such an architecture, in order to support an
incremental deployment of this approach.
Status of This Memo
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1. Introduction and Background
The current work of the IAB IP Stack Evolution Program is to support
the evolution of the Internet's transport layer and its interfaces to
other layers in the Internet Protocol stack. The need for this work
is driven by two trends. First is the development and increased
deployment of cryptography in Internet protocols to protect against
pervasive monitoring [RFC7258], which will break many middleboxes
used in the operation and management of Internet-connected networks
and which assume access to plaintext content. An additional
encapsulation layer to allow selective, explicit metadata exchange
between the endpoints and devices on path to replace ad-hoc packet
inspection is one approach to retain network manageability in an
encrypted Internet.
Second is the increased deployment of new applications (e.g.
interactive media as in RTCWEB [I-D.ietf-rtcweb-overview]) for which
the abstractions provided by today's transport APIs (i.e., either a
single reliable stream as in SOCK_STREAM over TCP, or an unreliable,
unordered packet sequence as in SOCK_DGRAM over UDP) are inadequate.
This evolution is constrained by the presence of middleboxes which
interfere with connectivity or packet invariability in the presence
of new transport protocols or transport protocol extensions.
Parts of this problem are presently being addressed in various ways
by the IETF. The Transport Services (TAPS) Working Group is defining
a new abstract interface for specifying transport requirements to the
transport layer, with a vocabulary based upon existing transport
protocol service features. This will allow future transport layers
(implemented in userspace libraries, in operating system kernels, or
some combination of the two) to select a wire protocol based upon
these requirements and the properties of the path between the
endpoints, including the impairments of middleboxes along that path.
The Substrate Protocol for User Datagrams (SPUD) Birds of a Feather
(BoF) session at IETF 92 in Dallas in March 2015 discussed use cases
and a prototype protocol [I-D.hildebrand-spud-prototype] for
encapsulating opaque content in UDP, with a facility for signaling
limited transport semantics and binding metadata to packets and flows
in a flexible way. This encapsulation is designed to provide
explicit cooperation between endpoints and middleboxes where this
makes sense, while allowing new transport protocol development to
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happen both in the kernel - to which it has largely been restricted
due to the history of the development of TCP/IP - as well as in
userspace. The outcome of the BoF session was to continue the
discussion about the architecture, transport semantics and metadata
vocabulary, and experimental implementation of this approach on the
mailing list established for the BoF (spud@ietf.org)
SPUD is not the only protocol-level work to address explicit
communication between endpoints and devices along the path: work in
the Transport Layer Security (TLS) working group
[I-D.huitema-tls-dtls-as-subtransport] discusses the possibility and
provides a gap analysis for running a "minimal common subtransport"
exposing common transport semantics as in SPUD directly over the
Datagram Transport Layer Security (DTLS) protocol [RFC6347].
These efforts aim at building flexible mechanisms to solve the
problem of expanding the interface between the transport layer and
the applications above it as well as the problem of making explicit
the contract between the transport layer and devices on path which
should, in an end-to-end Internet, limit themselves to lower-layer
interactions, but practically speaking have not done so for the past
two decades.
This document aims to provide an architectural basis for these
efforts, enumerating a set of architectural assumptions for transport
evolution based upon new encapsulations, and discussing limitations
on the vocabulary used in each of these new interfaces necessary to
achieve deployment.
2. Terminology
This document borrows terminology from [I-D.ietf-taps-transports],
specifically Transport Service, Transport Service Feature, Transport
Protocol, and Transport Protocol Component, for discussing the
composition of transport services.
[EDITOR'S NOTE: define Application Endpoint, Endhost, and Routable
Endpoint, as well as Midpoint, Middlebox, etc., using existing
terminology where applicable. A defined terminology here will help
avoid imprecision in this conversation.]
3. An Architecture for Explicit Path-Endpoint Cooperation
The present Internet architecture is rife with implicit cooperation
between endpoints and devices on the path between them. For example,
network address translators (NATs) rewrite addresses and ports in
order to increase the size of the Internet at the expense of
universal reachability, but this translation is not explicitly
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invoked by either endpoint. Traffic classification is often required
for network management purposes, and often uses deep packet
inspection to determine the traffic class of a particular packet or
flow.
It is this implicit cooperation which has led to the ossification of
the transport layer in the Internet. Implicit cooperation requires
devices along the path to make assumptions about the format of the
packets and the nature of the traffic they are forwarding, which in
turn leads to problems using protocols which don't meet these
assumptions. It also forces application and transport protocol
developers to build protocols that operate in this presumed, least-
common-denominator network environment.
We take the position that this situation can be improved by replacing
implicit cooperation with explicit cooperation. We first explore the
properties of an ideal architecture for explicit cooperation, then
consider the constraints imposed by the present configuration of the
Internet which would make transition to this ideal architecture
infeasible. From this we derive a set of architectural principles
and protocol design requirements which will support an incrementally
deployable approach to explicit cooperation between applications on
endpoints and middleboxes in the Internet.
3.1. Principles: What does good look like?
We can take some guidance for the future from the original Internet
architecture.
The original Internet architecture defined the split between TCP and
IP by defining IP to contain those functions the gateways need to
handle (and possibly de- and re-encapsulate, including
fragmentation), while defining TCP to contain functions that can be
handled by hosts end-to-end [RFC0791]. Gateways were essentially
trusted not to meddle in TCP.
As a first principle, a strict division between hop-to-hop and end-
to-end functions is desirable to restore and maintain end-to-end
service in the Internet.
In the original architecture, there was no provision for "in-network
functionality" beyond forwarding, fragmentation, and basic
diagnostics. This was as much a function of adherence to the end-to-
end-principle [Saltzer84] as a desire to limit computational
complexity and state requirements on the gateways.
In-network functions in the Internet Protocol as presently defined
are explicit. Forwarding is inherently explicit: placing an address
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in the destination address field, the endpoint (and by extension, the
application) indicates that a packet should be sent to a given
address. The contract for fragmentation was implicit in IPv4, but
in-network fragmentation was removed in IPv6 [RFC2460].
We note that layer boundaries can be enforced using sufficiently
strong cryptography.
As a second principle, the presence of in-network functionality along
a path which results in the modification of packet streams received
at the other end of a connection should be explicit.
For optional functionality, the applications at either end of a
connection should have appropriate explicit control over the presence
of those functions on path, even if they are present by default. For
those functions which are necessary, without which there is no end-
to-end connectivity (e.g. NATs in many environments), or which are
otherwise non-optional for operational reasons (e.g. firewalls), the
functionality should be explicitly discoverable by the applications
on either end.
This explicitness extends into the transport stack in the endpoint.
When applications can clearly define transport requirements, instead
of implicitly lensing them through known implementations of each
socket type, these transport requirements can be exposed to and/or
matched with properties of devices along the path, where that is
useful.
[EDITOR'S NOTE: this is perhaps a bit further than we want to
actually go, but this would seem to be the logical conclusion of
"make path interaction explicit"]
3.2. Impairments: What keeps us from getting there?
The clear separation of network and transport layer has been steadily
eroded over the past twenty years or so. Network address and port
translation (NAPT) have effectively made the first four bytes of the
transport header a de-facto part of the network layer, and have made
it difficult to deploy protocols where NAPT devices don't know that
the ports are safe to touch: anything other than UDP and TCP.
Protocols to support with NAT traversal (e.g. Interactive
Connectivity Establishment [RFC5245]) do not address this fundamental
problem.
Mechanisms that could be used to support explicit cooperation between
applications and middleboxes could be supported within the network
layer. The IPv6 Hop-by-Hop Options Header is explicitly intended for
this purpose, and a new hop-by-hop option could be defined. However,
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there are some limitations to using this header: it is only supported
by IPv6, it may itself cause packets to be dropped, it may not be
handled efficiently (or indeed at all) by currently deployed routers
and middleboxes, and it requires changes to operating system stacks
at the endpoints to allow applications to access these headers.
One of the effects of the fact that cryptography enforces layer
boundaries is that applications and transports run over HTTPS de
facto [I-D.blanchet-iab-internetoverport443], since HTTPS is the most
widely implemented, accessible, and deployable way for application
developers to get this enforcement.
However, the greatest barriers to explicit cooperation between
applications and devices along the path is the lack of explicit trust
among them. While it is possible to assign trust within the "first
hop" administrative domains, especially when the endpoint and network
operator are the same entity, building and operating an
infrastructure for assigning and maintaining these trust
relationships within an Internet context is currently impractical.
Finally, the erosion of the end-to-end principle has not occurred in
a vacuum. There are incentives to deploy in-network functions, and
services that are impaired by them have already worked around these
impairments. For example, the present trend toward service
recentralization ("cloud computing") can be seen in part as the
market's response to the end of end-to-end. Tf every application-
layer transaction is mediated by services owned by the application's
operator, two-end NAT traversal is no longer important. This new
architecture for services has additional implications for the types
of interactions supported, and for the types of business models
encouraged, which may in turn make some of the concerns about limited
deployability of new transport protocols moot.
3.3. What can we do?
First we turn to the problem of re-separation of the network layer
from the transport layer. NAPT, as noted, has effectively made the
ports part of the network layer, and this change is not easy to undo,
so we can make this explicit. In many NAPT environments only UDP and
TCP traffic will be forewarded, and a packet with a TCP header may be
assumed by middleboxes to have TCP semantics; therefore, the solution
space is constrained to putting the "new" separation between the
network and transport layers within a UDP encapsulation. This has a
further positive implication for incremental deployability: it is
possible to implement UDP-based encapsulations in userspace
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4. Encapsulation and signaling mechanisms
[EDITOR'S NOTE: frontmatter on encaps]
4.1. Encapsulations are narrow
A good deal of experience with tunnels has shown that the per-stream
overhead of a given encapsulation is generally less important than
its impact on MTU. For instance, the SPUD prototype as presently
defined needs at least 20 additional bytes in the header per packet:
2 each for source and destination UDP port, 2 for UDP length, 2 for
UDP checksum, 8 to identify tubes, 1 for control bits for SPUD
itself, and 3 for the smallest possible CBOR map containing a single
opaque higher layer datagram. For 1500-byte Ethernet frames, the
marginal cost of SPUD before is therefore 1.33% in byte terms, but it
does imply that 1450 byte application datagrams will no longer fit in
a single SPUD-over-UDP-over-IPv4-over Ethernet packet.
This fact has two implications for encapsulation design: First,
maximum payload size per packet should be communicated up to the
higher layer, as an explicit feature of the transport layer's
interface. Second, link-layer MTU is a fundamental property of each
link along a path, so any signaling protocol allowing path elements
to communicate to the endpoint should treat MTU as one of the most
important properties along the path to explicitly signal.
5. Implicit trust in endpoint-path signaling
In a perfect world, the trust relationships among endpoints and
elements on path would be precisely and explicitly defined: an
endpoint would explicitly delegate some processing to a network
element on its behalf, network elements would be able to trust any
command from any endpoint, and the integrity and authenticity of
signaling in both directions would be cryptographically protected.
However, both the economic reality that the users at the endpoints
and the operators of the network may not always have aligned
interests, as well as the difficulty of universal key exchange and
trust distribution among widely heterogeneous devices across
administrative domain boundaries, require us to take a different
approach toward building deployable, useful metadata signaling.
We observe that imperative signaling approaches in the past have
often failed in that they give each actor incentives to lie.
Markings to ask the network to explicitly treat some packets as more
important than others will see their value inflate away - i.e., most
packets from most sources will be so marked - unless some mechanism
is built to police those markings. Reservation protocols suffer from
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the same problem: for example, if an endpoint really needs 1Mbps, but
there is no penalty for reserving 1.5Mbps "just in case", a
conservative strategy on the part of the endpoint leads to over-
reservation.
5.1. Declarative marking
An alternate approach is to treat these markings as declarative and
advisory, and to treat all markings on packets and flows as relative
to other markings on packets and flows from the same sender. In this
case, where neither endpoints nor path elements can reliably predict
the actions other elements in the network will take with respect to
the marking, and where no endpoint can ask for special treatment at
the expense of other endpoints, the incentives to marking inflation
are greatly diminished.
5.2. Verifiable marking
Second, markings and declarations should be defined in such a way
that they are verifiable, and verification built end to endpoints and
middleboxes wherever practical. Suppose for example an endpoint
declares that it will send constant-bitrate, loss-insensitive traffic
at 192kbps. The average data rate for the given flow is trivially
verifiable at any endpoint. A firewall which uses this data for
traffic classification and differential quality of service may spot-
check the data rate for such flows, and penalize those flows and
sources which are clearly mismarking their traffic.
It is probably not possible, especially in an environment with
ubiquitous opportunistic encryption [RFC7435], to define a useful
marking vocabulary such that every marking will be so easily
verifiable. However, in an environment in which markings are
variably-trusted and verified, trustworthiness can be dynamically
assigned by each device, as well as
the trustworthiness of each endpoint and path can be independently
assessed by any node involved in a communication, and reputation-
tracking approaches can be used to signal how believable a
declaration is to transport protocols which use those declarations,
as well as to provide an additional incentive to mark honestly.
6. IANA Considerations
This document has no considerations for IANA.
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7. Security Considerations
This revision of this document presents no security considerations.
A more rigorous definition of the limits of declarative and
verifiable marking would need to be evaluated against a specified
threat model, but we leave this to future work.
8. Acknowledgments
Many thanks to the attendees of the IAB Workshop on Stack Evolution
in a Middlebox Internet (SEMI) in Zurich, 26-27 January 2015; most of
the thoughts in this document follow directly from discussions at
SEMI. This work is partially supported by the European Commission
under Grant Agrement FP7-318627 mPlane; support does not imply
endorsement by the Commission of the content of this work.
9. Informative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, December 2014.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-13
(work in progress), November 2014.
[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-03 (work
in progress), February 2015.
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[I-D.hildebrand-spud-prototype]
Hildebrand, J. and B. Trammell, "Substrate Protocol for
User Datagrams (SPUD) Prototype", draft-hildebrand-spud-
prototype-03 (work in progress), March 2015.
[I-D.huitema-tls-dtls-as-subtransport]
Huitema, C., Rescorla, E., and J. Jana, "DTLS as
Subtransport protocol", draft-huitema-tls-dtls-as-
subtransport-00 (work in progress), March 2015.
[I-D.blanchet-iab-internetoverport443]
Blanchet, M., "Implications of Blocking Outgoing Ports
Except Ports 80 and 443", draft-blanchet-iab-
internetoverport443-02 (work in progress), July 2013.
[Saltzer84]
Saltzer, J., Reed, D., and D. Clark, "End-to-End Arguments
in System Design (ACM Trans. Comp. Sys.)", 1984.
Author's Address
Brian Trammell
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: ietf@trammell.ch
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