IPDVB Working Group H. Cruickshank
Internet-Draft S. Iyengar
Intended status: Informational University of Surrey, UK
P. Pillai
Expires: October 4, 2008 University of Bradford, UK
April 4, 2008
Security requirements for the Unidirectional Lightweight
Encapsulation (ULE) protocol
draft-ietf-ipdvb-sec-req-06.txt
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Abstract
The MPEG-2 standard defined by ISO 13818-1 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) defined in RFC4326. The document also provides the
motivation for link-layer security for a ULE Stream. A ULE Stream
may be used to send IPv4 packets, IPv6 packets, and other
Protocol Data Units (PDUs) to an arbitrarily large number of
Receivers supporting unicast and/or multicast transmission.
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Table of Contents
1. Introduction................................................2
2. Requirements notation.......................................3
3. Threat Analysis.............................................6
3.1. System Components......................................6
3.2. Threats................................................9
3.3. Threat Scenarios......................................10
4. Security Requirements for IP over MPEG-2 TS................11
5. Design recommendations for ULE Security Header Extension...13
6. Compatibility with Generic Stream Encapsulation............14
7. Summary....................................................14
8. Security Considerations....................................15
9. IANA Considerations........................................16
10. Acknowledgments...........................................16
11. References................................................16
11.1. Normative References.................................16
11.2. Informative References...............................16
12. Author's Addresses........................................18
13. IPR Notices...............................................18
13.1. Intellectual Property Statement......................19
14. Copyright Statement.......................................19
Appendix A: ULE Security Framework............................20
Appendix B: Motivation for ULE link-layer security............24
Document History..............................................27
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].
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
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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-layer
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
the security requirements for ULE in MPEG-2 transmission
networks.
A security framework for deployment of secure ULE networks
describing the different building blocks and the interface
definitions is presented in Appendix A.
2. Requirements notation
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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, IEEE-802]. 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
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
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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 layer 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
optional overhead including an Adaptation Field, encryption
details, and time stamp information to synchronise a set of
related TS Logical Channels.
ULE Stream: An MPEG-2 TS Logical Channel that carries only ULE
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encapsulated PDUs. ULE Streams may be identified by definition of
a stream_type in SI/PSI [ISO-MPEG2].
3. Threat Analysis
3.1. System Components
+------------+ +------------+
| IP | | IP |
| End Host | | End Host |
+-----+------+ +------------+
| ^
+------------>+---------------+ |
+ ULE | |
+-------------+ Encapsulator | |
SI-Data | +------+--------+ |
+-------+-------+ |MPEG-2 TS Logical Channel |
| MPEG-2 | | |
| SI Tables | | |
+-------+-------+ ->+------+--------+ |
| -->| MPEG-2 | . . .
+------------>+ Multiplexer | |
MPEG-2 TS +------+--------+ |
Logical Channel |MPEG-2 TS Mux |
| |
Other ->+------+--------+ |
MPEG-2 -->+ MPEG-2 | |
TS --->+ Multiplexer | |
---->+------+--------+ |
|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 (adapted from [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 (the ULE Encapsulator)
o SI-Table signalling generator (input to the multiplexer)
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o Receivers (the end points for ULE streams)
o TS multiplexers (including re-multiplexers)
o 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 Layer
3 (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.
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).
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 a 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). 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.)
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 Layer 2 (L2) control plane. Examples of
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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, i.e. supporting its
security objectives in the area of availability, in addition to
confidentiality and integrity. One method recently proposed [ID-
EXT] encapsulates these messages using ULE. In such cases all the
security requirements of this document apply in securing these
signalling messages.
ULE link security focuses only on the security between the ULE
Encapsulation Gateway (ULE Encapsulator) and the Receiver. In
many deployment scenarios the user of a ULE Stream has to secure
communications beyond the link since other network links are
utilised in addition to the ULE link. Therefore, if
authentication of the end-point i.e. the IP Sources is required,
or users are concerned about loss of confidentiality, integrity
or authenticity of their communication data, they will have to
employ end-to-end network security mechanisms like IPSec or
Transport Layer Security (TLS). Governmental users may be forced
by regulations to employ specific, approved implementations of
those mechanisms. Hence for such cases the confidentiality and
integrity of the user data will already be taken care of by the
end-to-end security mechanism and the ULE security measures would
focus on either providing traffic flow confidentiality for user
data that has already been encrypted or for users who choose not
to implement end-to-end security mechanisms.
ULE links may also be used for communications where the two end-
points are not under central control (e.g., when browsing a
public web site). In these cases, it may be impossible to enforce
any end-to-end security mechanisms. Yet, a common objective is
that users can rely on security assumptions as of wired links.
ULE security could achieve this by protecting the vulnerable (in
terms of passive attacks) ULE link.
In contrast to the above, if a ULE Stream is used to directly
join networks which are considered physically secure, for example
branch offices to a central office, ULE link Security could be
the sole provider of confidentiality and integrity. In this
scenario, governmental users could still have to employ approved
cryptographic equipment at the network layer or above, unless a
manufacturer of ULE Link Security equipment obtains governmental
approval for their implementation.
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3.2. Threats
The simplest type of network threat is a passive threat. This
includes eavesdropping or monitoring of transmissions, with a
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
MPEG-2 broadcast links. This could for instance motivate the
mandatory use of sequence numbers in IPsec, but not for
synchronous links. This is further reflected in the security
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requirements for Case 2 and 3 in section 4 below.
As explained in section 3.1, the PID associated with an
Elementary Stream can 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.
3.3. Threat Scenarios
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 payload data and the identity
of ULE Receivers.
o Case 2: Locally conduct active attacks on the MPEG-TS
multiplex. 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-2
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: Globally conduct active attacks on the MPEG-TS
multiplex. Here we assume an intruder is very sophisticated
and able to over-ride the whole MPEG-2 transmission multiplex.
The requirements here are similar to scenario 2. The MPEG-2
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 within
the network with knowledge of the secret material.
o Outsider attacks i.e. active attacks from outside of a virtual
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private network.
In terms of priority, case 1 is considered the major threat in
MPEG-2 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 attacks
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-2 transmission operator.
It will require restoration of the original transmission. The
assumption being here is that physical access to the network
components (multiplexers, etc) and/or connecting physical media
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:
Req 1. Data confidentiality MUST be considered in order to
mitigate passive and active threats in MPEG-2 broadcast
networks.
Req 2. Protection of Layer 2 NPA address MAY be provided. In
broadcast networks this protection can be used to prevent an
intruder tracking the identity of ULE Receivers and the volume
of their traffic.
Req 3. Integrity protection and authentication of the ULE source
MAY be provided to prevent active attacks described in section
3.2.
Req 4. Protection against replay attacks MAY be provided. This is
required for the active attacks described in section 3.2.
Req 5. Layer L2 ULE Source and Receiver authentication MAY be
provided. 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).
This is normally receiver to hub authentication and it could
be either unidirectional or bidirectional authentication based
on the underlying key management protocol.
Other general requirements are:
GReq (a) ULE key management functions SHALL be decoupled from ULE
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security services such as encryption and source authentication.
This allows the independent development of both systems.
GReq (b) Support SHOULD be provided for automated as well as
manual insertion of keys and policy into the relevant
databases.
GReq (c) Algorithm agility MUST be supported. Changes in crypto
algorithms, hashes as they become obsolete should be updated
without affecting the overall security of the system.
GReq (d) Traceability SHOULD be supported to monitor the
transmission network using log files to record the activities
in the network and detect any intrusion.
GReq (e) Protection against loss of service (availability)
through malicious reconfiguration of system components (see
Figure 1) MUST be present.
GReq (f) The security system MUST be compatible with other
networking functions such as NAT Network Address Translation
(NAT) [RFC3715] or TCP acceleration can be used in a wireless
broadcast networks.
GReq (g) The security extension header MUST be compatible with
other ULE extension headers
GReq (h) 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 MAY
therefore be required, at least permitting control of data
confidentiality/authorisation at the level of a single MAC/NPA
address.
Examining the threat cases in section 3.3, the security
requirements for each case can be summarised as:
o Case 1: Data confidentiality (Req 1) MUST be provided to
prevent monitoring of the ULE data (such as user information
and IP addresses). Protection of NPA addresses (Req 2) MAY be
provided to prevent tracking ULE Receivers and their
communications.
o Case 2: In addition to case 1 requirements, new measures MAY
be implemented such as authentication schemes using Message
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Authentication Codes, digital signatures or TESLA [RFC4082] in
order to provide integrity protection and source
authentication (Req 2, Req 3 and Req 5). In addition sequence
numbers (Req 4) MAY be used to protect against replay attacks.
In terms of outsider attacks, group authentication using
Message Authentication Codes should provide the same level of
security (Req 3 and 5). This will significantly reduce the
ability of intruders to successfully inject their own data
into the MPEG-TS stream. However, scenario 2 threats apply
only in specific service cases, and therefore authentication
and protection against replay attacks are OPTIONAL. Such
measures incur additional transmission as well as processing
overheads. Moreover, intrusion detection systems may also be
needed by the MPEG-2 network operator. These should best be
coupled with perimeter security policy to monitor most denial-
of-service attacks.
o Case 3: As stated in section 3.3, the requirements here are
similar to Case 2 but since the MPEG-2 transmission network
operator can usually identify such attacks the constraints on
intrusion detections are less than in case 2.
The general requirements GReq(a) to GReq(h) are good security
practices and apply to all the scenarios above, where
appropriate.
5. Design recommendations for ULE Security Header Extension
Table 1 below shows the threats that are applicable to ULE
networks and the relevant security mechanism to mitigate those
threats. This would help in the design of the ULE Security
extension header. For example this could help in the selection of
security fields in the ULE Security extension Header design.
Moreover the security services could also be grouped into
profiles based on different security requirements. One example is
to have a base profile which does payload encryption and identity
protection. The second profile could do the above as well as
source authentication.
A modular design to ULE Security may allow it to use and benefit
from IETF key management protocols, such as GSAKMP [RFC4535] and
GDOI [RFC3547] protocols defined by the IETF Multicast Security
(MSEC) working group. This does not preclude the use of other key
management methods in scenarios where this is more appropriate.
IPsec or TLS also provide a proven security architecture defining
key exchange mechanisms and the ability to use a range of
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cryptographic algorithms. ULE security can make use of these
established mechanisms and algorithms.
Mitigation of Threat
-----------------------------------------------
| Data | Data |Source |Data |Intru |Iden |
|Privacy | fresh |Authent|Integ |sion |tity |
| | ness |ication|rity |Dete |Prote |
| | | | |ction |ction |
Attack | | | | | | |
---------------|--------|-------|-------|-------|-------|------|
| Monitoring | X | - | - | - | - | X |
|---------------------------------------------------------------|
| Masquerading | X | - | X | X | - | X |
|---------------------------------------------------------------|
| Replay Attacks| - | X | X | X | X | - |
|---------------------------------------------------------------|
| Dos Attacks | - | X | X | X | X | - |
|---------------------------------------------------------------|
| Modification | - | - | X | X | X | - |
| of Messages | | | | | | |
---------------------------------------------------------------
Table 1: Security techniques to mitigate network threats
in ULE Networks.
6. Compatibility with Generic Stream Encapsulation
The [ID-EXT] document describes two new Header Extensions that
may be used with Unidirectional Link Encapsulation, ULE,
[RFC4326] and the Generic Stream Encapsulation (GSE) that has
been designed for the Generic Mode (also known as the Generic
Stream (GS)), offered by second-generation DVB physical layers,
and specifically for DVB-S2 [ID-EXT].
The security threats and requirement presented in this document
are applicable to ULE and GSE encapsulations. It might be
desirable to authenticate some/all of the headers; such decision
can be part of the security policy for the MPEG-2 transmission
network.
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.
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ULE security includes a need to provide link-layer encryption and
ULE Receiver identity protection. There is an optional
requirement for link-layer 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.
ULE link security (between a ULE Encapsulation Gateway to
Receivers) is considered as an additional security mechanism to
IPsec, TLS, and application layer end-to-end security, and not as
a replacement. It allows a network operator to provide similar
functions to that of IPsec, but in addition provides MPEG-2
transmission link confidentiality and protection of ULE Receiver
identity (NPA). End-to-end security mechanism may then be used
additionally and independently for providing strong
authentication of the end-points in the communication.
Annexe 1 describes a set of building blocks that may be used to
realise a framework that provides ULE security functions.
8. Security Considerations
Link-layer (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 common objective is to provide the same level of privacy as
wired links. It is recommended that an ISP or user provide end-
to-end security services based on well known mechanisms such as
IPsec or TLS.
This document provides a threat analysis and derives the security
requirements to provide link encryption and optional link-layer
integrity / authentication of the SNDU payload.
There are some security issues that were raised in RFC 4326
[RFC4326] that are not addressed in this document (out of scope)
such as:
o The security issue with un-initialised stuffing bytes. In
ULE, these bytes are set to 0xFF (normal practice in MPEG-2).
o Integrity issues related to the removal of the LAN FCS in a
bridged networking environment. The removal for bridged
frames exposes the traffic to potentially undetected
corruption while being processed by the Encapsulator and/or
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Receiver.
o There is a potential security issue when a Receiver receives a
PDU with two Length fields: The Receiver would need to
validate the actual length and the Length field and ensure
that inconsistent values are not propagated by the network.
9. IANA Considerations
There are no IANA actions defined in this document.
10. Acknowledgments
The authors acknowledge the help and advice from Gorry Fairhurst
(University of Aberdeen). The authors also acknowledge
contributions from Laurence Duquerroy and Stephane Coombes (ESA),
Yim Fun Hu (University of Bradford) and Michael Noisternig from
University of Salzburg.
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.
[ID-EXT] G. Fairhurst and B. Collini-Nocker, "Extension
Formats for Unidirectional Lightweight Encapsulation
(ULE) and the Generic Stream Encapsulation (GSE)",
Work in Progress, draft-ietf-ipdvb-ule-ext-07.txt,
January 2008.
[IEEE-802] "Local and metropolitan area networks-Specific
requirements Part 2: Logical Link Control", IEEE
802.2, IEEE Computer Society, (also ISO/IEC 8802-2),
1998.
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[ISO-8802] ISO/IEC 8802.2, "Logical Link Control", International
Standards Organisation (ISO), 1998.
[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
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(TLS) Protocol Version 1.1", IETF RFC 4346, April
2006.
[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.
12. 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
Prashant Pillai
Mobile and Satellite Communications Research Centre (MSCRC)
School of Engineering, Design and Technology
University of Bradford
Richmond Road, Bradford BD7 1DP
UK
Email: p.pillai@bradford.ac.uk
13. IPR Notices
Copyright (c) The IETF Trust (2007).
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13.1. 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.
Intellectual Property Statement
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
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.
14. Copyright Statement
Copyright (C) The IETF Trust (2008).
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Appendix A: ULE Security Framework
This section defines a security framework for the deployment of
secure ULE networks.
A.1 Building Blocks
This ULE Security framework defines the following building blocks
as shown in figure 2 below:
o The Key Management Block
o The ULE Security Extension Header Block
o The ULE Databases Block
Within the Key Management block the communication between the
Group Member entity and the Group Server entity happens in the
control plane. The ULE Security header block applies security to
the ULE SNDU and this happens in the ULE data plane. The ULE
Security databases block acts as the interface between the Key
management block (control plane) and the ULE Security Header
block (ULE data plane) as shown in figure 2.
------
+------+----------+ +----------------+ / \
| Key Management |/---------\| Key Management | |
| Block |\---------/| Block | |
| Group Member | | Group Server | Control
+------+----------+ +----------------+ Plane
| | |
| | |
| | \ /
----------- Key management <-> ULE Security databases -----
| |
\ /
+------+----------+
| ULE |
| SAD / SPD |
| Databases |
| Block |
+------+-+--------+
/ \
| |
----------- ULE Security databases <-> ULE Security Header ----
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| | / \
| | |
| | |
+------+-+--------+ ULE Data
| ULE Security | Plane
| Extension Header| |
| Block | |
+-----------------+ \ /
-----
Figure 2: Secure ULE Framework Building Blocks
A.1.1 Key Management Block
A key management framework is required to provide security at the
ULE level using extension headers. 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). The Key
management framework can be either automated or manual. Hence
this key management client entity (shown as the Key Management
Group Member block in figure 2) will be present in all ULE
receivers as well as at the ULE Encapsulators. The ULE
Encapsulator could also be the Key Management Group Server Entity
(shown as the Key Management Group Server block in figure 2. This
happens when the ULE Encapsulator also acts as the Key Management
Group Server. Deployment may use either automated key management
protocols (e.g. GSAKMP [RFC4535]) or manual insertion of keying
material.
A.1.2 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 document.
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
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could be other extension headers (either mandatory or optional).
It is RECOMMENDED that these are placed after the security
extension header. This permits full protection for all headers.
It avoids situations where the SNDU has to be discarded on
processing the security extension header, while preceding headers
have already have been evaluated. One exception is the Timestamp
extension which SHOULD precede the security extension header [ID-
EXT]. When applying the security services for example
confidentiality, input to the cipher algorithm will cover the
fields from the end of the security extension header to the end
of the PDU.
+-------+------+-------------------------------+------+
| ULE |SEC | Protocol Data Unit | |
|Header |Header| |CRC-32|
+-------+------+-------------------------------+------+
Figure 3: ULE Security Header Extension Placement
A.1.3 ULE Security 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. These 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].
The exact details of the header patterns that the SPD and SAD
will have to support for all use cases will be defined in a
separate document. This document only highlights the need for
such interfaces between the ULE data plane and the Key Management
control plane.
A.2 Interface definition
Two new interfaces have to be defined between the blocks as shown
in Figure 2 above. These interfaces are:
o Key Management block <-> ULE Security databases block
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o ULE Security databases block <-> ULE Security Header block
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 Security
Extension Header Block to get the keys and policy material for
generation of the security extension header.
A.2.1 Key Management <-> ULE Security databases
This interface is between the Key Management group member block
(GM client) and the ULE Security Database block (shown in figure
2). The Key Management GM entity 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 Key Management could be either
automated (e.g. GSAKMP [RFC4535] or GDOI [RFC3547]) 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);
The definitions of the variables are as follows:
. Database - This is a pointer to the ULE Security databases
. record - This is the rows of security attributes to be
entered or modified in the above databases
. Unique_ID - This is the primary key to lookup records (rows
of security attributes) in the above databases
A.2.2 ULE Security Databases <-> ULE Security Header
This interface is between the ULE Security Database and the ULE
Security Extension Header block as shown in figure 2. To send
traffic, firstly the ULE encapsulator using the ULE_Security_ID,
Destination Address and possibly the PID, searches the ULE
Security Database for the relevant security record. It then uses
the data in the record to create the ULE security extension
header. For received traffic, the ULE decapsulator on receiving
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the ULE SNDU will first get the record from the Security Database
using the ULE_Security_ID, the Destination Address and possibly
the PID. It then uses this information to decrypt the ULE
extension header. For both cases (either send or receive traffic)
only one interface is needed since the only difference between
the sender and receiver is the direction of the flow of traffic:
. Get_record_database (char * Database, char * record, char *
Unique_ID);
Appendix B: 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
security in MPEG-2 transmission networks employing ULE. This
section compares the disadvantages when security functionalities
are present in different layers.
B.1 Security at the IP layer (using IPsec)
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
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
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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].
B.2 Link security below the encapsulation layer
Link layer security can be provided at the MPEG-2 TS layer (below
ULE). MPEG-2 TS encryption encrypts all TS Packets sent with a
specific PID value. However, an MPEG-2 TS may typically multiplex
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, TLS, or any
other form of higher-layer security.
However it has the following disadvantages:
o When a PID is shared between several users, 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 When a PID is shared between several users, the ULE Receivers
will have access to private traffic destined to other ULE
Receivers, since they share a common PID and key.
o IETF-based key management that is very flexible and secure is
not used in existing MPEG-2 based systems. Existing access
control mechanisms in such systems 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.
Currently there are few deployed L2 security systems for MPEG-2
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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.
B.3 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 layer
security:
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 at the IP layer and also at L2.
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.
L3 end-to-end security would partially deny the advantage listed
just above (use of PEP, compression etc), since those techniques
could only be applied to TCP packets bearing a TCP-encapsulated
IPsec packet exchange, but not the TCP packets of the original
applications, which in particular inhibits compression.
End-to-end security (IPsec, TLS, etc.) may be used independently
to 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 parties, whereas Layer 2 methods cannot provide this
guarantee.
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>>> NOTE to RFC Editor: Please remove this appendix prior to
publication]
Document History
Working Group Draft 00
o Fixed editorial mistakes and ID style for WG adoption.
Working Group Draft 01
o Fixed editorial mistakes and added an appendix which shows the
preliminary framework for securing the ULE network.
Working Group Draft 02
o Fixed editorial mistakes and added some important changes as
pointed out by Knut Eckstein (ESA), Gorry Fairhurst and
UNISAL.
o Added section 4.1 on GSE. Extended the security considerations
section.
o Extended the appendix to show the extension header placement.
o The definition of the header patterns for the ULE Security
databases will be defined in a separate draft.
o Need to include some words on key management transport over
air interfaces, actually key management bootstrapping.
Working Group Draft 03
o Fixed editorial mistakes and added some important changes as
pointed out by Gorry Fairhurst.
o Table 1 added in Section 6.2 to list the different security
techniques to mitigate the various possible network threats.
o Figure 2 modified to clearly explain the different interfaces
present in the framework.
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o New Section 7 has been added.
o New Section 6 has been added.
o The previous sections 5 and 6 have been combined to section 5.
o Sections 3, 8 and 9 have been rearranged and updated with
comments and suggestions from Michael Noisternig from
University of Salzburg.
o The Authors and the Acknowledgments section have been updated.
Working Group Draft 04
o Fixed editorial mistakes and added some important changes as
pointed out by DVB-GBS group, Gorry Fairhurst and Laurence
Duquerroy.
o Table 1 modified to have consistent use of Security Services.
o Text modified to be consistent with the draft-ietf-ipdvb-ule-
ext-04.txt
Working Group Draft 06
o Fixed editorial mistakes and added some important changes as
pointed out by Pat Cain and Gorry Fairhurst.
o Figure 1 modified to have consistent use of Security Services.
o Text modified in Section 4 to clearly state the requirements.
o Moved Section 5 to the Appendix B
o Updated IANA consideration section
o Numbered the different requirements and cross referenced them
within the text.
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