Network Working Group K. Smith
Internet-Draft Vodafone Group
Intended status: Informational May 08, 2015
Expires: November 9, 2015
Network management of encrypted traffic
draft-smith-encrypted-traffic-management-00
Abstract
Encrypted Internet traffic may pose traffic management challenges to
network operators. This document recommends approaches to help
manage encrypted traffic, without breaching user privacy or security.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Document structure . . . . . . . . . . . . . . . . . . . 3
1.2. Security protocols . . . . . . . . . . . . . . . . . . . 3
2. Network management functions . . . . . . . . . . . . . . . . 3
2.1. Queuing . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Intrusion detection . . . . . . . . . . . . . . . . . . . 4
2.3. Policy enforcement . . . . . . . . . . . . . . . . . . . 4
2.4. SPAM and malware filtering . . . . . . . . . . . . . . . 4
3. Flow information visible to a network . . . . . . . . . . . . 4
3.1. IP 5-tuple . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. TLS Server Name Indication . . . . . . . . . . . . . . . 5
3.3. Application Layer Protocol Negotiation (ALPN) . . . . . . 5
3.4. DiffServ Code Points (DSCP) . . . . . . . . . . . . . . . 5
3.5. Explicit Congestion Notification . . . . . . . . . . . . 6
3.6. Multi Protocol Label Switching . . . . . . . . . . . . . 6
4. Inferred flow information . . . . . . . . . . . . . . . . . . 7
4.1. Heuristics . . . . . . . . . . . . . . . . . . . . . . . 7
5. Providing hints to and from the network . . . . . . . . . . . 7
5.1. Substrate Protocol for User Datagrams (SPUD) . . . . . . 7
5.2. Mobile throughput Guidance . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Networks utilise various management techniques to ensure efficient
throughput, congestion management, anti-SPAM and security measures.
Historically these functions have utilised visibility of the Internet
application layer.
This visibility is rapidly diminishing - encrypted Internet traffic
is expected to continue its upward trend, driven by increased privacy
awareness, uptake by popular services, and advocacy from the [IAB],
[RFC7258] and W3C [TAG] .
[IAB], [RFC7258] and [mm-effect-encrypt] recognise that network
management functions are impacted by encryption, and that solutions
are needed to persist them - as long as they do not threaten privacy.
These solutions would ensure the benefits of encryption do not
degrade network efficiency.
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This document lists such solutions, and points to evolving IETF work
addressing the problem.
1.1. Document structure
This document describes the network management functions that are
likely to be hindered by traffic encryption.
It then describes the technical details of existing options to fully
or partially persist these functions under encryption. 'Encryption'
in this document typically refers to HTTP over TLS [RFC2818]; other
forms of encryption are noted where applicable.
Finally, a summary is provided of ongoing IETF work which is
investigating how middleboxes along the network path can improve
encrypted traffic delivery - again without breaching user privacy or
security.
The legal, political and commercial aspects of network management are
recgnised but not covered in this technical document.
1.2. Security protocols
The following IETF protocols are considered in this document: TLS
[RFC5246] , IPsec [RFC4301] and the ongoing transport layer security
work of [TCPINC].
2. Network management functions
Editor's note: Part or all of this section may be removed where there
is duplication with any updated version of [mm-effect-encrypt]
2.1. Queuing
Traffic flowing through a network may be queued for delivery. This
is important at an access network where network conditions can change
rapidly - such as a cellular radio access network. To account for
congestion, the network will categorise content requests according to
the latency and bandwidth required to deliver that content type.
These combinations run from high-latency, low bandwidth (Email),
medium latency, medium bandwidth (Web pages), low latency high
bandwidth (video streaming), and many others including voice calls,
texts, WebRTC and VoIP. A well-managed network will triage between
these content types and deliver from each queue in bursts, to ensure
no user experiences a disrupted service.
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2.2. Intrusion detection
Networks will monitor traffic stream behaviours to identify likely
Denial of Service attacks. Tools exist at each network layer to
detect and mitigate these, including application layer detection.
2.3. Policy enforcement
Approved access to a network is a prerequisite to requests for
Internet traffic - hence network access, including any authentication
and authorisation, is not impacted by traffic encryption.
Cellular networks often sell tariffs that allow free-data access to
certain sites, known as 'zero rating'. A session to visit such a
site incurs no additional cost or data usage to the user. Such 'zero
rating
Note: this section deliberately does not go into detail on the
ramifications of encryption as regards government regulation. These
regulations include 'Lawful Intercept', adherence to Codes of
Practice on content filtering, application of court order filters.
However it is clear that these functions are impacted by encryption,
typically by allowing a less granular means of implementation. The
enforcement of any Net Neutrality regulations is unlikely to be
affected by content being encrypted.
2.4. SPAM and malware filtering
This has typically required Deep Packet Inspection to filter various
keywords, fraudulent headers and virus attachments.
3. Flow information visible to a network
3.1. IP 5-tuple
This indicates source and destination IP addresses/ports and the
transport protocol. This information is available during TLS, TCP-
layer encryption (except ports), and IP-layer encryption (IPSec);
although it may be obscured in Tunnel mode IPSec.
This allows network management at a coarse IP-source level, which
makes it of limited value where the origin server supports a blend of
service types.
Obscured from network by: IPSec Tunnel Mode
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3.2. TLS Server Name Indication
When initiating the TLS handshake, the Client may provide an
extension field (server_name) which indicates the server to which it
is attempting a secure connection. TLS SNI was standardized in 2003
to enable servers to present the "correct TLS certificate" to clients
in a deployment of multiple virtual servers hosted by the same server
infrastructure and IP-address. Although this is an optional
extension, it is today supported by all modern browsers, web servers
and developer libraries. Notable exceptions are Android 2.2 and
Internet Explorer 6 on Windows XP. It should be noted that HTTP/2
introduces the Alt-SVC method for upgrading the connection from
HTTP/1 to either unencrypted or encrypted HTTP/2. If the initial
HTTP/1 request is unencrypted, the destination alternate service name
can be identified before the communication is potentially upgraded to
encrypted HTTP/2 transport. HTTP/2 implementations MUST support the
Server Name Indication (SNI) extension.
Limitation: This information is only visible if the client is
populating the Server Name Indication extension. This need not be
done, but may be done as per TLS standard. Therefore, even if
existing network filters look out for seeing a Server Name Indication
extension, they may not find one. The per-domain nature of SNI may
not reveal the specific service or media type being accessed,
especially where the domain is of a provider offering a range of
email, video, Web pages etc. For example, certain blog or social
network feeds may be deemed 'adult content', but the Server Name
Indication will only indicate the server domain rather than a URL
path to be blocked.
Obscured from network by: not providing the SNI, IPSec
3.3. Application Layer Protocol Negotiation (ALPN)
ALPN is a TLS extenion which may be used to indicate the application
protocol within the TLS session. This is likely to be of more value
to the network where it indicates a protocol dedicated to a
particular traffic type (such as video streaming) rather than a
multi-use protocol. ALPN is used as part of HTTP/2 'h2', but will
not indicate the traffic types which may make up streams within an
HTTP/2 multiplex.
3.4. DiffServ Code Points (DSCP)
Data packets are flagged with a traffic class (class of service).
Network operators may honour a DiffServ classification entering their
network, or may choose to override it (since it is potentially open
to abuse by a service provider that classifies all its content as
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high priority). The purpose is to help manage traffic and congestion
in the network.
Limitations: This requires the content provider to flag data packets.
This is extra work for the provider, and it has potential for abuse
if a content provider simply flags all packets with high priorities.
The network would need to know which flags to trust and which to
override. The use of DiffServ within the operator network is
beneficial where the operator determines the class of service itself;
but where content is encrypted then heuristics would be needed to
predict the traffic type entering the network. HTTP/2 allows several
streams to be multiplexed over a single TCP connection. This means
that if a provider decides to send Web pages, videos, chat etc. as
individual streams over the same connection, then DiffServ would be
useless as it would apply to the TCP/IP connection as a whole.
However it may be more efficient for such Web providers to serve each
content type from separate, dedicated servers - this will become
clearer as HTTP/2 deployments are tuned for optimal delivery.
3.5. Explicit Congestion Notification
Explicit Congestion Notification (ECN) routers can exchange
congestion notification headers to ECN compliant endpoints. This is
in preference to inferring congestion from dropped packets (e.g. in
TCP). The purpose is to help manage traffic and congestion in the
network.
This solution is required to be implemented at network and service
provider. The service provider will utilise the ECN to reduce
throughput until it is notified that congestion has eased.
Limitation: As with DiffServ, operators may not trust an external
entity to mark packets in a fair/consistent manner.
3.6. Multi Protocol Label Switching
Description: on entering an MPLS-compliant network, IP packets are
flagged with a 'Forward Equivalence Class' (FEC). This allows the
network to make packet-forwarding decisions according to their
latency requirements. MPLS routers within the network parse and act
upon the FEC value. The FEC is set according to the source IP
address and port. The purpose is to help managing traffic and
congestion in the network. This requires deployment of an MPLS
'backbone' with label-aware switches/ routers.
Limitations: an up-to-date correspondence table between Websites and
server IP address must be created. Then, the category(s) of traffic
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have to be consistently mapped to a set of MPLS labels ,which entails
a significant effort to setup and maintain.
Note: MPLS can specify how OSI Layer 3 (IP layer) traffic can be
routed over Layer 2 (Data Link); DiffServ only operates over Layer 3.
DiffServ is potentially a less complex integration as it is applied
at the network edge servers only.
4. Inferred flow information
4.1. Heuristics
Heuristics can be used to map given input data to particular
conclusions via some heuristic reasoning. Examples of input data to
this reasoning include IP destination address, TCP destination port,
server name from SNI, typical traffic pattern (e.g. occurrence of IP
packets and TCP segments over time). The accuracy of heuristics
depends on whether the observed traffic originates from a source
delivering a single service, or a blend of services. In many
scenarios, this makes it possible to directly classify the traffic
related to a specific server/service even when the traffic is fully
encrypted.
If the server/service is co-located on an infrastructure with other
services that shares the same IP-address, the encrypted traffic
cannot be directly classified. However, commercial traffic
classifiers today typically apply heuristic methods, using traffic
pattern matching algorithms to be able to identify the traffic. As
an example, classifier products are able to identify popular VoIP
services using heuristic methods although the traffic is encrypted
and mostly peer-to-peer.
5. Providing hints to and from the network
The following draft protocols aim to support a secure and privacy-
aware dialogue between client, server and a network middlebox. This
follows the cooperative path to endpoint signalling as discussed at
the IAB SEMI workshop [SEMI], with the network following a more
clearly-defined role in encrypted traffic delivery. These hints can
allow information item exchange between the endpoints and the
network, to assist queuing mechanisms and traffic pacing that
accounts for network congestion and variable connection strength.
5.1. Substrate Protocol for User Datagrams (SPUD)
SPUD [SPUD] allows network devices on the path between endpoints to
participate explicitly in a 'tube' of grouped UDP packets. The
network involvement is outside of the end-to-end context, to minimise
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any privacy or security breach. The initial prototype is based on
UDP packets but will investigate the support of additional transport
layers (such as TCP).
5.2. Mobile throughput Guidance
Mobile Throughput Guidance In-band Signalling [MTG] allows the
network to inform the server endpoint as to what bandwidth the TCP
connection can reasonably expect. This allows the server to adapt
their throughput pacing based on dynamic network conditions, which
can assist mechanisms such as Adaptive Bitrate Streaming and TCP
congestion control.
6. Acknowledgements
The editor would like to thank the GSMA Web Working Group for their
contributions, in particular to the technical solutions and network
management functions.
7. IANA Considerations
There are no IANA consideraions.
8. Security Considerations
The intention of this document is to consider how to persist network
management of encrypted traffic, without breaching user privacy or
end-to-end security. In particular this document does not recommend
any approach that intercepts or modifies client-server Transport
Layer Security.
9. References
9.1. Normative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
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9.2. Informative References
[IAB] IAB, "IAB statement on Internet confidentiality", n.d.,
.
[MTG] IETF, "Mobile Throughput Guidance Inband Signaling
Protocol", n.d., .
[SEMI] IAB, "IAB workshop, 'Stack Evolution in a Middlebox
Internet'", n.d.,
.
[SPUD] IETF, "Substrate Protocol for User Datagrams", n.d.,
.
[TAG] W3C, "Securing the Web", n.d., .
[TCPINC] IETF, "TCP Increased Security", n.d.,
.
[mm-effect-encrypt]
IETF, "Effect of Ubiquitous Encryption", n.d.,
.
Author's Address
Kevin Smith
Vodafone Group
Email: kevin.smith@vodafone.com
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