Internet Engineering Task Force H.Cruickshank
Internet Draft S. Iyengar
draft-ietf-ipdvb-sec-req-01.txt University of Surrey, UK
L. Duquerroy
Alcatel Alenia Space, France
Expires: September 2, 2007 P. Pillai
University of Bradford, UK
Category: WG Draft intended for PS March 2, 2007
Security requirements for the Unidirectional Lightweight
Encapsulation (ULE) protocol
draft-ietf-ipdvb-sec-req-01.txt
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Abstract
The MPEG-2 standard defined by ISO 13818-1 [ISO-MPEG2] supports a
range of transmission methods for a range of services. This
document provides a threat analysis and derives the security
requirements when using the Transport Stream, TS, to support an
Internet network-layer using Unidirectional Lightweight
Encapsulation (ULE) [RFC4326]. The document also provides the
motivation for link-level security for a ULE Stream. A ULE Stream
may be used to send IPv4 packets, IPv6 packets, and other
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Protocol Data Units (PDUs) to an arbitrarily large number of
Receivers supporting unicast and/or multicast transmission.
Table of Contents
1. Introduction..............................................2
2. Requirements notation......................................4
3. Threat Analysis...........................................6
3.1. System Components.....................................6
Figure 1: An example configuration for a unidirectional........6
Service for IP transport over MPEG-2 [RFC4259]................6
3.2. Threats..............................................8
3.3. Threat Scenarios......................................9
4. Security Requirements for IP over MPEG-2 TS...............11
5. IPsec and MPEG-2 Transmission Networks....................12
6. Motivation for ULE link-layer security....................13
6.1. Link security below the Encapsulation layer..........13
6.2. Link security as a part of the Encapsulation layer....14
7. Summary..................................................15
8. Security Considerations...................................15
9. IANA Considerations......................................16
10. Acknowledgments.........................................16
11. References..............................................16
11.1. Normative References................................16
11.2. Informative References..............................16
Author's Addresses..........................................18
Intellectual Property Statement..............................19
Disclaimer of Validity.............Error! Bookmark not defined.
Copyright Statement................Error! Bookmark not defined.
Document History............................................20
Appendix A: ULE Security Framework...........................22
1. Introduction
The MPEG-2 Transport Stream (TS) has been widely accepted not
only for providing digital TV services, but also as a subnetwork
technology for building IP networks. RFC 4326 [RFC4326] describes
the Unidirectional Lightweight Encapsulation (ULE) mechanism for
the transport of IPv4 and IPv6 Datagrams and other network
protocol packets directly over the ISO MPEG-2 Transport Stream as
TS Private Data. ULE specifies a base encapsulation format and
supports an extension format that allows it to carry additional
header information to assist in network/Receiver processing. The
encapsulation satisfies the design and architectural requirement
for a lightweight encapsulation defined in RFC 4259 [RFC4259].
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Section 3.1 of RFC 4259 presents several topological scenarios
for MPEG-2 Transmission Networks. A summary of these scenarios
are presented below (for full detail, please refer to RFC 4259).
1. Broadcast TV and Radio Delivery.
2. Broadcast Networks used as an ISP. This resembles to scenario
1, but includes the provision of IP services providing access
to the public Internet.
3. Unidirectional Star IP Scenario. It utilizes a Hub station to
provide a data network delivering a common bit stream to
typically medium-sized groups of Receivers.
4. Datacast Overlay. It employs MPEG-2 physical and link layers
to provide additional connectivity such as unidirectional
multicast to supplement an existing IP-based Internet service.
5. Point-to-Point Links.
6. Two-Way IP Networks. This can be typically satellite-based and
star-based utilising a Hub station to deliver a common bit
stream to medium- sized groups of receivers. A bidirectional
service is provided over a common air-interface.
RFC 4259 states that ULE must be robust to errors and security
threats. Security must also consider both unidirectional as well
as bidirectional links for the scenarios mentioned above.
An initial analysis of the security requirements in MPEG-2
transmission networks is presented in the security considerations
section of RFC 4259. For example, when such networks are not
using a wireline network, the normal security issues relating to
the use of wireless links for transport of Internet traffic
should be considered [RFC3819].
The security considerations of RFC 4259 recommends that any new
encapsulation defined by the IETF should allow Transport Stream
encryption and should also support optional link-level
authentication of the SNDU payload. In ULE [RFC4326], it is
suggested that this may be provided in a flexible way using
Extension Headers. This requires the definition of a mandatory
header extension, but has the advantage that it decouples
specification of the security functions from the encapsulation
functions.
This document extends the above analysis and derives a detailed
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the security requirements for ULE in MPEG-2 transmission
networks.
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC2119 [RFC2119].
Other terms used in this document are defined below:
ATSC: Advanced Television Systems Committee. A framework and a
set of associated standards for the transmission of video, audio,
and data using the ISO MPEG-2 standard.
DVB: Digital Video Broadcast. A framework and set of associated
standards published by the European Telecommunications Standards
Institute (ETSI) for the transmission of video, audio, and data
using the ISO MPEG-2 Standard [ISO-MPEG2].
Encapsulator: A network device that receives PDUs and formats
these into Payload Units (known here as SNDUs) for output as a
stream of TS Packets.
LLC: Logical Link Control [ISO-8802-2, IEEE-802.2]. A link-layer
protocol defined by the IEEE 802 standard, which follows the
Ethernet Medium Access Control Header.
MAC: Message Authentication Code.
MPE: Multiprotocol Encapsulation [ETSI-DAT]. A scheme that
encapsulates PDUs, forming a DSM-CC Table Section. Each Section
is sent in a series of TS Packets using a single TS Logical
Channel.
MPEG-2: A set of standards specified by the Motion Picture
Experts Group (MPEG) and standardized by the International
Standards Organisation (ISO/IEC 13818-1) [ISO-MPEG2], and ITU-T
(in H.222 [ITU-H222]).
NPA: Network Point of Attachment. In this document, refers to a
6-byte destination address (resembling an IEEE Medium Access
Control address) within the MPEG-2 transmission network that is
used to identify individual Receivers or groups of Receivers.
PDU: Protocol Data Unit. Examples of a PDU include Ethernet
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frames, IPv4 or IPv6 datagrams, and other network packets.
PID: Packet Identifier [ISO-MPEG2]. A 13-bit field carried in
the header of TS Packets. This is used to identify the TS
Logical Channel to which a TS Packet belongs [ISO-MPEG2]. The TS
Packets forming the parts of a Table Section, PES, or other
Payload Unit must all carry the same PID value. The all-zeros
PID 0x0000 as well as other PID values is reserved for specific
PSI/SI Tables [ISO-MPEG2]. The all-ones PID value 0x1FFF
indicates a Null TS Packet introduced to maintain a constant bit
rate of a TS Multiplex. There is no required relationship
between the PID values used for TS Logical Channels transmitted
using different TS Multiplexes.
Receiver: Equipment that processes the signal from a TS Multiplex
and performs filtering and forwarding of encapsulated PDUs to the
network-layer service (or bridging module when operating at the
link layer).
SI Table: Service Information Table [ISO-MPEG2]. In this
document, this term describes a table that is defined by another
standards body to convey information about the services carried
in a TS Multiplex. A Table may consist of one or more Table
Sections; however, all sections of a particular SI Table must be
carried over a single TS Logical Channel [ISO-MPEG2].
SNDU: SubNetwork Data Unit. An encapsulated PDU sent as an MPEG-2
Payload Unit.
TS: Transport Stream [ISO-MPEG2], a method of transmission at the
MPEG-2 level using TS Packets; it represents layer 2 of the
ISO/OSI reference model. See also TS Logical Channel and TS
Multiplex.
TS Multiplex: In this document, this term defines a set of MPEG-2
TS Logical Channels sent over a single lower-layer connection.
This may be a common physical link (i.e., a transmission at a
specified symbol rate, FEC setting, and transmission frequency)
or an encapsulation provided by another protocol layer (e.g.,
Ethernet, or RTP over IP). The same TS Logical Channel may be
repeated over more than one TS Multiplex (possibly associated
with a different PID value) [RFC4259]; for example, to
redistribute the same multicast content to two terrestrial TV
transmission cells.
TS Packet: A fixed-length 188B unit of data sent over a TS
Multiplex [ISO-MPEG2]. Each TS Packet carries a 4B header, plus
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optional overhead including an Adaptation Field, encryption
details, and time stamp information to synchronise a set of
related TS Logical Channels.
3. Threat Analysis
3.1. System Components
+------------+ +------------+
| IP | | IP |
| End Host | | End Host |
+-----+------+ +------------+
| ^
+------------>+---------------+ |
+ IP | |
+-------------+ Encapsulator | |
SI-Data | +------+--------+ |
+-------+-------+ |MPEG-2 TS Logical Channel |
| MPEG-2 | | |
| SI Tables | | |
+-------+-------+ ->+------+--------+ |
| -->| MPEG-2 | . . .
+------------>+ Multiplexor | |
MPEG-2 TS +------+--------+ |
Logical Channel |MPEG-2 TS Mux |
| |
Other ->+------+--------+ |
MPEG-2 -->+ MPEG-2 | |
TS --->+ Multiplexor | |
---->+------+--------+ |
|MPEG-2 TS Mux |
| |
+------+--------+ +------+-----+
|Physical Layer | | MPEG-2 |
|Modulator +---------->+ Receiver |
+---------------+ MPEG-2 +------------+
TS Mux
Figure 1: An example configuration for a unidirectional
Service for IP transport over MPEG-2 [RFC4259].
As shown in Figure 1 above (from section 3.3 of [RFC4259]), there
are several entities within the MPEG-2 transmission network
architecture. These include:
o ULE Encapsulation Gateways (or Encapsulator or ULE source)
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o SI-Table signalling generator (input to the multiplexor)
o Receivers
o TS multiplexers (including re-multiplexers)
o Modulators
In an MPEG-2 network a set of signalling messages [ID-AR] may
need to be broadcast (e.g. by an Encapsulation Gateway or other
device) to form the L2 control plane. Examples of signalling
messages include the Program Association Table (PAT), Program Map
Table (PMT) and Network Information Table (NIT). In existing
MPEG-2 transmission networks, these messages are broadcast in the
clear (no encryption or integrity checks). The integrity as well
as authenticity of these messages is important for correct
working of the ULE network. Even though securing these messages
is an orthogonal issue, one method recently proposed [ID-EF]
encapsulates these messages using ULE. In such cases all the
security requirements of this draft apply in securing these
signalling messages.
ULE link security focuses only on the security between the ULE
Encapsulation Gateway (ULE source) and the Receiver. Securing the
ULE source and receivers eliminates the need to consider security
issues regarding the remaining system components, such as
multiplexers, re-multiplexers and modulators.
In a MPEG-2 TS transmission network, the originating source of TS
Packets is either a L2 interface device (media encoder,
encapsulation gateway, etc) or a L2 network device (TS
multiplexer, etc). These devices may, but do not necessarily,
have an associated IP address. In the case of an encapsulation
gateway (e.g. ULE sender), the device may operate at L2 or L3,
and is not normally the originator of an IP traffic flow, and
usually the IP source address of the packets that it forwards do
not correspond to an IP address associated with the device. When
authentication of the IP source is required this must be provided
by IPsec, TLS, etc. operating at a higher layer.
The TS Packets are carried to the Receiver over a physical layer
that usually includes Forward Error Correction (FEC) coding that
interleaves the bytes of several consecutive, but unrelated, TS
Packets. FEC coding and synchronisation processing makes
injection of single TS Packets very difficult. Replacement of a
sequence of packets is also difficult, but possible (see section
3.2 below).
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A Receiver in a MPEG-2 TS transmission network needs to identify
a TS Logical Channel (or MPEG-2 Elementary Stream) to reassemble
the fragments of PDUs sent by a L2 source [RFC4259]. In an MPEG-2
TS, this association is made via the Packet Identifier, PID [ISO-
MPEG2]. At the sender, each source associates a locally unique
set of PID values with each stream it originates. However, there
is no required relationship between the PID value used at the
sender and that received at the Receiver. Network devices may re-
number the PID values associated with one or more TS Logical
Channels (e.g. ULE Streams) to prevent clashes at a multiplexer
between input streams with the same PID carried on different
input multiplexes (updating entries in the PMT [ISO-MPEG2], and
other SI tables that reference the PID value). A device may also
modify and/or insert new SI data into the control plane (also
sent as TS Packets identified by their PID value).
The PID associated with an Elementary Stream can therefore be
modified (e.g. in some systems by reception of an updated SI
table, or in other systems until the next announcement/discovery
data is received). An attacker that is able to modify the content
of the received multiplex (e.g. replay data and/or control
information) could inject data locally into the received stream
with an arbitrary PID value.
One method to provide security is to secure the entire Stream at
the MPEG-2 TS level. This stream of TS Packets carried in a
multiplex is usually received by many Receivers. The approach is
well-suited to TV-transmission, data-push, etc, where the PID
carries one or a set of flows (e.g. video/audio Packetized
Elementary Stream (PES) Packets) with similar security
requirements.
Where a ULE Stream carries a set of IP traffic flows to different
destinations with a range of properties (multicast, unicast,
etc), it is often not appropriate to provide IP confidentiality
services for the entire ULE Stream. For many expected
applications of ULE, a finer-grain control is therefore required,
at least permitting control of data confidentiality/authorisation
at the level of a single MAC/NPA address. However there is only
one valid source of data for each MPEG-2 Elementary Stream, bound
to a PID value. This observation could simplify the requirement
for authentication of the source of a ULE Stream.
3.2. Threats
The simplest type of network threat is a passive threat. This
includes eavesdropping or monitoring of transmissions, with a
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goal to obtain information that is being transmitted. In
broadcast networks (especially those utilising widely available
low-cost physical layer interfaces, such as DVB) passive threats
are considered the major threats. An example of such a threat is
an intruder monitoring the MPEG-2 transmission broadcast and then
extracting traffic information concerning the communication
between IP hosts using a link. Another example is of an intruder
trying to gain information about the communication parties by
monitoring their ULE Receiver NPA addresses; an intruder can gain
information by determining the layer 2 identity of the
communicating parties and the volume of their traffic. This is a
well-known issue in the security field; however it is more of a
problem in the case of broadcast networks such as MPEG-2
transmission networks because of the easy availability of
receiver hardware and the wide geographical span of the networks.
Active threats (or attacks) are, in general, more difficult to
implement successfully than passive threats, and usually require
more sophisticated resources and may require access to the
transmitter. Within the context of MPEG-2 transmission networks,
examples of active attacks are:
o Masquerading: An entity pretends to be a different entity.
This includes masquerading other users and subnetwork control
plane messages.
o Modification of messages in an unauthorised manner.
o Replay attacks: When an intruder sends some old (authentic)
messages to the Receiver. In the case of a broadcast link,
access to previous broadcast data is easy.
o Denial of Service attacks: When an entity fails to perform its
proper function or acts in a way that prevents other entities
from performing their proper functions.
The active threats mentioned above are major security concerns
for the Internet community [BELLOVIN]. Masquerading and
modification of IP packets are comparatively easy in an Internet
environment whereas such attacks are in fact much harder for
broadcast links. This could for instance motivate the use of
sequence numbers in IPsec, but not the mandatory use of them on
synchronous links and this is further reflected in the security
requirements for Case 2 and 3 in section 4 below.
3.3. Threat Scenarios
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Analysing the topological scenarios for MPEG-2 Transmission
Networks in section 1, the security threat cases can be
abstracted into three cases:
o Case 1: Monitoring (passive threat). Here the intruder
monitors the ULE broadcasts to gain information about the ULE
data and/or tracking the communicating parties identities (by
monitoring the destination NPA). In this scenario, measures
must be taken to protect the ULE data and the identity of ULE
Receivers.
o Case 2: Local hijacking of the MPEG-TS multiplex (active
threat). Here an intruder is assumed to be sufficiently
sophisticated to over-ride the original transmission from the
ULE Encapsulation Gateway and deliver a modified version of
the MPEG-TS transmission to a single ULE Receiver or a small
group of Receivers (e.g. in a single company site). The MPEG
transmission network operator might not be aware of such
attacks. Measures must be taken to ensure ULE source
authentication and preventing replay of old messages.
o Case 3: Global hijacking of the MPEG-TS multiplex (active
threat). Here we assume an intruder is very sophisticated and
able to hijack the whole MPEG transmission multiplex. The
requirements here are similar to scenario 2. The MPEG
transmission network operator can usually identify such
attacks and may resort to some means to restore the original
transmission.
For both cases 2 and 3, there can be two sub cases:
o Insider attacks i.e. active attacks from adversaries in the
known of secret material.
o Outsider attacks i.e. active attacks from outside of a virtual
private network.
In terms of priority, case 1 is considered the major threat in
MPEG transmission systems. Case 2 is likely to a lesser degree
within certain network configurations, especially when there are
insider attacks. Hence, protection against such active actives
should be used only when such a threat is a real possibility.
Case 3 is envisaged to be less practical, because it will be very
difficult to pass unnoticed by the MPEG transmission operator. It
will require restoration of the original transmission. The
assumption being here is that physical access to the network
components (multiplexors, etc) and/or connecting physical media
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is secure. Therefore case 3 is not considered further in this
document.
4. Security Requirements for IP over MPEG-2 TS
From the threat analysis in section 3, the following security
requirements can be derived:
o Data confidentiality is the major requirement to mitigate
passive threats in MPEG-2 broadcast networks.
o Protection of Layer 2 NPA address. In broadcast networks this
protection can be used to prevent an intruder tracking the
identity of ULE Receivers and the volume of their traffic.
o Integrity protection and authentication of the ULE source is
required against active attacks described in section 3.2.
o Protection against replay attacks. This is required for the
active attacks described in section 3.2.
o Layer L2 ULE Source and Receiver authentication: This is
normally performed during the initial key exchange and
authentication phase, before the ULE Receiver can join a
secure session with the ULE Encapsulator (ULE source).
Other general requirements are:
o Decoupling of ULE key management functions from ULE security
services such as encryption and source authentication. This
allows the independent development of both systems.
o Support for automated as well as manual insertion of keys and
policy into the relevant databases.
o Algorithm agility is needed. Changes in crypto algorithms,
hashes as they become obsolete should be updated without
affecting the overall security of the system.
o Traceability: To monitor transmission network using log files
to record the activities in the network and detect any
intrusion.
o Authentication of control and management messages in MPEG-2
transmission networks such as the SI tables (see Figure 1).
o Secure Policy management
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o Compatibility with other networking functions such as NAT
Network Address Translation (NAT) [RFC3715] or TCP
acceleration can be used in a wireless broadcast networks.
o Compatibility and operational with ULE extension headers i.e.
allow encryption of a compressed SNDU payload.
Examining the threat cases in section 3.3, the security
requirements for each case can be summarised as:
o Case 1: Data confidentiality MUST be provided to prevent
monitoring of the ULE data (such as user information and IP
addresses). Protection of NPA addresses MUST be provided to
prevent tracking ULE Receivers and their communications.
o Case 2: In addition to case 1 requirements, new measures need
to be implemented such as authentication schemes using Message
Authentication Codes, digital signatures or TESLA [RFC4082]
and using sequence numbers to prevent replay attacks in terms
of insider attacks. In terms of outsider attacks group
authentication using Message Authentication Codes should
provide the same level of security. This will significantly
reduce the ability of intruders to inject their own data into
the MPEG-TS stream. However, scenario 2 threats apply only in
specific service cases and therefore source authentication and
protection against replay attacks are OPTIONAL. Such measures
incur extra link transmission and processing overheads.
o Case 3: The requirements here are similar to Case 2. In
addition, intrusion detection is also desirable by the MPEG-2
network operator.
5. IPsec and MPEG-2 Transmission Networks
The security architecture for the Internet Protocol [RFC4301]
describes security services for traffic at the IP layer. This
architecture primarily defines services for the Internet Protocol
(IP) unicast packets, as well as manually configured IP multicast
packets.
It is possible to use IPsec to secure ULE links. The major
advantage of IPsec is its wide implementation in IP routers and
hosts. IPsec in transport mode can be used for end-to-end
security transparently over MPEG-2 transmission links with little
impact.
In the context of MPEG-2 transmission links, if IPsec is used to
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secure a ULE link, then the ULE Encapsulator and Receivers are
equivalent to the security gateways in IPsec terminology. A
security gateway implementation of IPsec uses tunnel mode. Such
usage has the following disadvantages:
o There is an extra transmission overhead associated with using
IPsec in tunnel mode, i.e. the extra IP header (IPv4 or IPv6).
o There is a need to protect the identity (NPA) of ULE Receivers
over the ULE broadcast medium; IPsec is not suitable for
providing this service. In addition, the interfaces of these
devices do not necessarily have IP addresses (they can be L2
devices).
o Multicast is considered a major service over ULE links. The
current IPsec specifications [RFC4301] only define a pairwise
tunnel between two IPsec devices with manual keying. Work is
in progress in defining the extra detail needed for multicast
and to use the tunnel mode with address preservation to allow
efficient multicasting. For further details refer to [WEIS06].
6. Motivation for ULE link-layer security
Examination of the threat analysis and security requirements in
sections 3 and 4 has shown that there is a need to provide link-
layer (L2) security in MPEG-2 transmission networks employing
ULE.
ULE link security (between a ULE Encapsulation Gateway to
Receivers) is therefore considered an additional security
mechanism to IPsec, TLS, and application layer security, not a
replacement. It allows a network operator to provide similar
functions to that of IPsec [RFC4301], but in addition provides
MPEG-2 transmission link confidentiality and protection of ULE
Receiver identity (NPA).
A modular design to ULE Security may allow it to use and benefit
from IETF key management protocols, such as the Multicast
Security group (MSEC) GSAKMP [RFC4535] and GDOI [RFC3547]
protocols. This does not preclude the use of other key management
methods in scenarios where this is more appropriate.
6.1. Link security below the Encapsulation layer
Link layer security can be provided at the MPEG-TS level (below
ULE. MPEG-TS encryption encrypts all TS Packets sent with a
specific PID value. However, MPEG-TS may typically multiplex
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several IP flows, belonging to different users, using a common
PID. Therefore all multiplexed traffic will share the same
security keys.
This has the following advantages:
o The bit stream sent on the broadcast network does not expose
any L2 or L3 headers, specifically all addresses, type fields,
and length fields are encrypted prior to transmission.
o This method does not preclude the use of IPsec, or any other
form of higher-layer security.
However it has the following disadvantages:
o Each ULE Receiver needs to decrypt all MPEG-2 TS Packets with
a matching PID, possibly including those that are not required
to be forwarded. Therefore it does not have the flexibility to
separately secure individual IP flows.
o ULE Receivers will have access to private traffic destined to
other ULE Receivers, since they share a common PID and key.
o Encryption of the MPE NPA address is not permitted in such
systems.
o IETF-based key management are not used in existing systems.
Existing access control mechanisms have limited flexibility in
terms of controlling the use of key and rekeying. Therefore if
the key is compromised, then this will impact several ULE
Receivers.
In practice there are few L2 security systems for MPEG
transmission networks. Conditional access for digital TV
broadcasting is one example. However, this approach is optimised
for TV services and is not well-suited to IP packet transmission.
Some other systems are specified in standards such as MPE [ETSI-
DAT], but there are currently no known implementations.
6.2. Link security as a part of the Encapsulation layer
Examining the threat analysis in section 3 has shown that
protection of ULE link from eavesdropping and ULE Receiver
identity are major requirements.
There are several major advantages in using ULE link level
security:
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o The protection of the complete ULE Protocol Data Unit (PDU)
including IP addresses. The protection can be applied either
per IP flow or per Receiver NPA address.
o Ability to protect the identity of the Receiver within the
MPEG-2 transmission network.
o Efficient protection of IP multicast over ULE links.
o Transparency to the use of Network Address Translation (NATs)
[RFC3715] and TCP Performance Enhancing Proxies (PEP)
[RFC3135], which require the ability to inspect and modify the
packets sent over the ULE link.
This method does not preclude the use of IPsec at L3 (or TLS
[RFC4346] at L4). IPsec and TLS provide strong authentication of
the end-points in the communication. This authentication is
desirable in many scenarios to ensure that the correct
information is being exchanged between the trusted entities,
whereas Layer 2 methods cannot provide this guarantee.
IPsec /TLS also provide a proven security architecture defining
key exchange mechanisms and the ability to use a range of
cryptographic algorithms. ULE security can make use of these
established mechanisms and algorithms but the advantages are
distinct from those when using IPsec or TLS.
7. Summary
This document analyses a set of threats and security
requirements. It also defines the requirements for ULE security
and states the motivation for link security as a part of the
Encapsulation layer.
This includes a need to provide L2 encryption and ULE Receiver
identity protection. There is an optional requirement for L2
authentication and integrity assurance as well as protection
against insertion of old (duplicated) data into the ULE stream
(i.e. replay protection). This is optional because of the
associated overheads for the extra features and they are only
required for specific service cases
8. Security Considerations
Link-level (L2) encryption of IP traffic is commonly used in
broadcast/radio links to supplement End-to-End security (e.g.
provided by TLS [RFC4346], SSH [RFC4251], IPsec [RFC4301). A
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common objective is to provide the same level of privacy as wired
links. An ISP or User may also wish to provide end-to-end
security services to the end-users (based on well known
mechanisms such as IPsec or TLS).
This document provides a threat analysis and derives the security
requirements to provide optional link encryption and link-level
integrity / authentication of the SNDU payload.
9. IANA Considerations
This document does not define any protocol and does not require
any IANA assignments.
10. Acknowledgments
The authors acknowledge the help and advice from Gorry Fairhurst
(University of Aberdeen). The authors also acknowledge
contributions from Stephane Coombes (ESA) and Yim Fun Hu
(University of Bradford).
11. References
11.1. Normative References
[ISO-MPEG2] "Information technology -- generic coding of moving
pictures and associated audio information systems,
Part I", ISO 13818-1, International Standards
Organisation (ISO), 2000.
[RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, 1997.
11.2. Informative References
[ID-AR] G. Fairhurst, M-J Montpetit "Address Resolution
Mechanisms for IP Datagrams over MPEG-2 Networks",
Work in Progress <draft-ietf-ipdvb-ar-05.txt.
[IEEE-802.2]"Local and metropolitan area networks-Specific
requirements Part 2: Logical Link Control", IEEE
802.2, IEEE Computer Society, (also ISO/IEC 8802-2),
1998.
[ISO-8802-2] ISO/IEC 8802.2, "Logical Link Control", International
Standards Organisation (ISO), 1998.
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[ITU-H222] H.222.0, "Information technology, Generic coding of
moving pictures and associated audio information
Systems", International Telecommunication Union,
(ITU-T), 1995.
[RFC4259] Montpetit, M.-J., Fairhurst, G., Clausen, H.,
Collini-Nocker, B., and H. Linder, "A Framework for
Transmission of IP Datagrams over MPEG-2 Networks",
IETF RFC 4259, November 2005.
[RFC4326] Fairhurst, G. and B. Collini-Nocker, "Unidirectional
Lightweight Encapsulation (ULE) for Transmission of
IP Datagrams over an MPEG-2 Transport Stream (TS)",
IETF RFC 4326, December 2005.
[ETSI-DAT] EN 301 192, "Digital Video Broadcasting (DVB); DVB
Specifications for Data Broadcasting", European
Telecommunications Standards Institute (ETSI).
[BELLOVIN] S.Bellovin, "Problem Area for the IP Security
protocols", Computer Communications Review 2:19, pp.
32-48, April 989. http://www.cs.columbia.edu/~smb/
[RFC4082] A. Perrig, D. Song, " Timed Efficient Stream Loss-
Tolerant Authentication (TESLA): Multicast Source
Authentication Transform Introduction", IETF RFC
4082, June 2005.
[RFC4535] H Harney, et al, "GSAKMP: Group Secure Association
Group Management Protocol", IETF RFc 4535, June 2006.
[RFC3547] M. Baugher, et al, "GDOI: The Group Domain of
Interpretation", IETF RFC 3547.
[WEIS06] Weis B., et al, "Multicast Extensions to the Security
Architecture for the Internet", <draft-ietf-msec-
ipsec-extensions-02.txt>, June 2006, IETF Work in
Progress.
[RFC3715] B. Aboba and W Dixson, "IPsec-Network Address
Translation (NAT) Compatibility Requirements" IETF
RFC 3715, March 2004.
[RFC4346] T. Dierks, E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", IETF RFC 4346, April
2006.
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[RFC3135] J. Border, M. Kojo, eyt. al., "Performance Enhancing
Proxies Intended to Mitigate Link-Related
Degradations", IETF RFC 3135, June 2001.
[RFC4301] Kent, S. and Seo K., "Security Architecture for the
Internet Protocol", IETF RFC 4301, December 2006.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J.,
and L. Wood, "Advice for Internet Subnetwork
Designers", BCP 89, IETF RFC 3819, July 2004.
[RFC4251] T. Ylonen, C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", IETF RFC 4251, January 2006.
[ID-EF] G. Fairhurst, "Extension Formats for the ULE
Encapsulation to support the Generic Stream
Encapsulation (GSE)", Work in Progress < draft-
fairhurst-ipdvb-ule-ext-xx.txt>.
Author's Addresses
Haitham Cruickshank
Centre for Communications System Research (CCSR)
University of Surrey
Guildford, Surrey, GU2 7XH
UK
Email: h.cruickshank@surrey.ac.uk
Sunil Iyengar
Centre for Communications System Research (CCSR)
University of Surrey
Guildford, Surrey, GU2 7XH
UK
Email: S.Iyengar@surrey.ac.uk
Laurence Duquerroy
Research Department/Advanced Telecom Satellite Systems
Alcatel Space, Toulouse
France
E-Mail: Laurence.Duquerroy@space.alcatel.fr
Prashant Pillai
Mobile and Satellite Communications Research Centre
School of Engineering, Design and Technology
University of Bradford
Richmond Road, Bradford BD7 1DP
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UK
Email: P.Pillai@bradford.ac.uk
12. IPR Notices
Copyright (c) The IETF Trust (2007).
12.1. Intellectual Property Statement
Full Copyright Statement
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided
on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE.
12.2. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be
claimed to pertain to the implementation or use of the technology
described in this document or the extent to which any license
under such rights might or might not be available; nor does it
represent that it has made any independent effort to identify any
such rights. Information on the procedures with respect to
rights in RFC documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the
use of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR
repository at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
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proprietary rights that may cover technology that may be required
to implement this standard. Please address the information to
the IETF at ietf-ipr@ietf.org.
13. Copyright Statement
Copyright (C) The IETF Trust (2007).
>>> NOTE to RFC Editor: Please remove this appendix prior to
publication]
Document History
Individual Draft-ID-00
o This draft is intended as a study item for proposed future
work by the IETF in this area.
o Major security concerns from Steve Bellovin who also agreed
that Layer 2 Address hiding is a necessary security service.
o Motivation for security over ULE was not clear (Joerg Ott)
o Not sure where this was leading for Non -IP traffic and key
management (Margaret Wasserman).
o It was recommended that a separate requirements draft was
first needed before suggesting any solutions (Bellovin)
Issues to be resolved in next revision (01):
o Title change (inserted "security requirements " rather than
"security extension")
o Separate security requirements draft
o Threat Analysis
Individual Draft -ID-01
o Load of discussion on the mailing lists regarding signalling
traffic security and the threats involved (Gorry, Montpetit
and Art)
o Benefits of IPSec over ULE as well as the fact the document
was not easy to read (Gerrard Gessler and Wolfgang Fritche)
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o What were the benefits of NPA protection?
o Threats in broadcast networks was raised by Gorry
Issues to be resolved in next revision (02):
o Add section 3 on threats
o Address Signalling threats
o Make the document easy to read
Individual Draft -ID-02
o Merged draft with the one proposed by Prashant and included
him as an author.
o Define the threat scenarios and the security requirement for
these scenarios.
o English fixed
Issues to be resolved in next revision (03):
o Include subsection 3.1 Threat Scenarios and the requirements
for these scenarios
o English Fixed
Individual Draft -ID-03
o Major comments and suggestions (Michael Noistering) regarding
authentication and integrity assurance. He also suggested that
the threat scenarios (section 3.2) should be expanded.
o Elaborate the impact of threats for IP as opposed to Layer 2
(Gorry Fairhurst)
Issues to be resolved in next revision (04):
o Expanded the threat scenarios.
o Algorithm Agility added as a requirement (gorry)
o Nits have been taken care of and addressed.
o English Fixed
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Individual Draft -ID-04
o Minor comment from Michael regarding replay protection
o Minor comments form Gorry.
Issues to be resolved in next revision (05):
o Nits have been taken care of and addressed.
o English Fixed
Working Group Draft 00
o Fixed editorial mistakes and ID style for WG adoption.
Working Group Draft 01
o Fixed editorial mistakes and added some changes as pointed out
by Knut (ESA) and added an appendix which shows the framework
for securing the ULE network.
Appendix A: ULE Security Framework
This section aims to define a preliminary security framework for
widespread deployment of secure ULE networks.
Building Blocks
This ULE Security framework defines the following building blocks
as shown in the figure 2 below:
1. The Key Management Block
2. The ULE Extension Header Block
3. The ULE Databases Block
+------+----------+ +----------------
| Key Management |/------------\| Key Management |
| Block |\------------/| Block |
| Group Member | | Group Server |
+------+----------+ +----------------
| |
| |
| |
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| |
| |
\ /
+------+----------+
| ULE |
| SAD / SPD |
| Interface Block |
+------+-+--------+
/ \
| |
| |
| |
| |
| |
| |
+------+-+--------+
| ULE Security |
| Extension Header|
| Block |
+-----------------+
Figure 2: Secure ULE framework Building Blocks
1. Key Management Block
In order to provide security at the ULE level using extension
headers, a key management framework is required. This key
management framework is responsible for user authentication,
access control, and Security Association negotiation (which
include the negotiations of the security algorithms to be used
and the generation of the different session keys as well as
policy material). This Key management framework can be either
automated or manual. Hence Key management client entity will be
present in all ULE receivers as well as ULE sources. In some
cases the ULE source could also be the Key Server Entity. Key
management protocols like GSAKMP may be used or manual insertion
of keying material can also be deployed.
2.ULE Extension header Block
A new security extension header for the ULE protocol is required
to provide the security features of data confidentiality, data
integrity, data authentication and mechanisms to prevent replay
attacks. Security keying material will be used for the different
security algorithms (for encryption/decryption, MAC generation,
etc.), which are used to meet the security requirements,
described in detail in Section 4 of this draft.
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This block will use the keying material and policy information from
the ULE security database block on the ULE payload to generate the
secure ULE extension Header or to decipher the secure ULE extension
header to get the ULE payload. An example overview of the ULE
Security extension header format along with the ULE header and
payload is shown in figure 3 below. There could be other extension
headers placed before or after the ULE Security Header extension
+-------+------+-------------------------------+------+
| ULE |SEC | Protocol Data Unit | |
|Header |Header| |CRC-32|
+-------+------+-------------------------------+------+
Figure 3: ULE Sec Header extension Placement
3. ULE Databases Block
There needs to be two databases i.e. similar to the IPSec
databases.
o ULE-SAD: ULE Secure Association Database contains all the
Security Associations that are currently established with
different ULE peers.
o ULE-SPD: ULE Secure Policy Database contains the policies as
defined by the system manager. Those policies describe the
security services that must be enforced
The design of these two databases will be based on IPSec
databases as defined in RFC4301 [RFC4301].
Interface definition
Two new interfaces have to be defined between the three blocks as
shown in figure 2 above. These interfaces are:
o Key management <-> ULE Security databases
o ULE Security databases <-> ULE interfaces
While the first interface is used by the Key Management Block to
insert keys, security associations and policies into the ULE
Database Block, the second interface is used by the ULE Extension
Header Block to get the keys and policy material for the ULE
Payloads.
1. Key management <-> ULE Security databases
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This interface is between the Key Management client block (GM
client) and the ULE Security Database block. The Key management
client will communicate with the GCKS and then get the relevant
security information (keys, cipher mode, security service,
ULE_Security_ID and other relevant keying material as well as
policy) and insert this data into the ULE Security database
block. The ULE Security database block holds the records of all
security associations currently used as well as information for
security policy control. The Key management could be either
automated (GSAKMP) or manually inserted using this interface. The
following three interface functions are defined:
. Insert_record_database (char * Database, char * record, char *
Unique_ID);
. Update_record_database (char * Database, char * record, char *
Unique_ID);
. Delete_record_database (char * Database, char * Unique_ID);
2. ULE Security databases <-> ULE interfaces
This interface is between the ULE Security Database and the ULE
Engine. For Outbound Traffic, firstly the ULE Engine using the
Destination Address and the ULE_Security_ID searches the ULE
Security Database for the relevant security record. It then uses
the record data to create the ULE security extension header
[ref]. For inbound traffic, the ULE engine on receiving the ULE
packet will first get the record from the Security Database using
the Destination Address and the ULE_Security_ID. It then uses
this information to decrypt the ULE extension header.
In both cases only one interface is needed:
. Get_record_database (char * Database, char * record, char *
Unique_ID);
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