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Network Working Group H. Cruickshank
Request for Comments: 5458 University of Surrey
Category: Informational P. Pillai
University of Bradford
M. Noisternig
University of Salzburg
S. Iyengar
Logica
March 2009
Security Requirements for
the Unidirectional Lightweight Encapsulation (ULE) Protocol
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
The MPEG-2 standard defined by ISO 13818-1 supports a range of
transmission methods for a variety 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 RFC
4326. 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.
The analysis also describes applicability to the Generic Stream
Encapsulation (GSE) defined by the Digital Video Broadcasting (DVB)
Project.
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Table of Contents
1. Introduction ....................................................3
2. Requirements Notation ...........................................4
3. Threat Analysis .................................................7
3.1. System Components ..........................................7
3.2. Threats ....................................................9
3.3. Threat Cases ..............................................10
4. Security Requirements for IP over MPEG-2 TS ....................11
5. Design Recommendations for ULE Security Extension Header .......14
6. Compatibility with Generic Stream Encapsulation ................15
7. Summary ........................................................15
8. Security Considerations ........................................15
9. Acknowledgments ................................................16
10. References ....................................................16
10.1. Normative References .....................................16
10.2. Informative References ...................................17
Appendix A. ULE Security Framework ................................19
A.1. Building Block ............................................19
A.2. Interface Definition ......................................22
Appendix B. Motivation for ULE Link-Layer Security ................23
B.1. Security at the IP Layer (Using IPsec) ....................23
B.2. Link Security below the Encapsulation Layer ...............24
B.3. Link Security as a Part of the Encapsulation Layer ........25
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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 Header 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 is
presented below:
A. Broadcast TV and Radio Delivery. This is not within the scope of
this document.
B. Broadcast Networks used as an ISP. This resembles scenario A, but
includes IP services to access the public Internet.
C. Unidirectional Star IP Scenario. This provides a data network
delivering a common bit stream to typically medium-sized groups of
Receivers.
D. Datacast Overlay. This employs MPEG-2 physical and link layers to
provide additional connectivity such as unidirectional multicast
to supplement an existing IP-based Internet service.
E. Point-to-Point Links. This connectivity may be provided using a
pair of transmit and receive interfaces.
F. Two-Way IP Networks.
RFC 4259 states that ULE must be robust to errors and security
threats. Security must also consider both unidirectional (A, B, C,
and D) as well as bidirectional (E and F) 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].
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The security considerations of RFC 4259 recommend that any new
encapsulation defined by the IETF should allow Transport Stream
encryption and should also support optional link-layer authentication
of the Subnetwork Data Unit (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 Extension
Header, but has the advantage that it decouples specification of the
security functions from the encapsulation functions.
This document extends the above analysis and derives in detail 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
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 RFC 2119 [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 Protocol Data Units
(PDUs) and formats these into Payload Units (known here as SNDUs) for
output as a stream of TS Packets.
GCKS: Group Controller and Key Server. A server that authenticates
and provides the policy and keying material to members of a secure
group.
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.
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MPE: Multiprotocol Encapsulation [ETSI-DAT]. A scheme that
encapsulates PDUs, forming a Digital Storage Media Command and
Control (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 standardised 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 Packets
forming the parts of a Table Section, Packetised Elementary Stream
(PES), or other Payload Unit must all carry the same PID value. The
all-zeros PID 0x0000 as well as other PID values are 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.
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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, Forward Error Correction (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 188-byte unit of data sent over a TS
Multiplex [ISO-MPEG2]. Each TS Packet carries a 4-byte 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
encapsulated PDUs. ULE Streams may be identified by definition of a
stream_type in SI/PSI [ISO-MPEG2].
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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)
o Receivers (the endpoints for ULE Streams)
o TS multiplexers (including re-multiplexers)
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o Modulators
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 an 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 an 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). 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 [RFC4947] 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, i.e.,
supporting its security objectives in the area of availability, in
addition to confidentiality and integrity. One method recently
proposed [RFC5163] encapsulates these messages using ULE. In such
cases all the security requirements of this document apply in
securing these signalling messages.
ULE Stream security only concerns 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
endpoints, i.e., the IP Sources, is required, or users are concerned
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about loss of confidentiality, integrity, or authenticity of their
communication data, they will have to employ end-to-end network
security mechanisms, e.g., 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 requirements for confidentiality and integrity of the user data
will be met by the end-to-end security mechanism and the ULE security
measures would focus on providing traffic flow confidentiality either
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 IP
endpoints 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
may make the same security assumptions as for wired links [RFC3819].
ULE security could achieve this by protecting the vulnerable (in
terms of passive attacks) ULE Stream.
In contrast to the above, a ULE Stream can be used to link networks
such as branch offices to a central office. ULE link-layer security
could be the sole provider of confidentiality and integrity. In this
scenario, users requiring high assurance of security (e.g.,
government use) will need to employ approved cryptographic equipment
(e.g., at the network layer). An implementation of ULE Link Security
equipment could also be certified for use by specific user
communities.
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), the passive threats are the major threats.
One example is an intruder monitoring the MPEG-2 transmission
broadcast and then extracting the data carried within the link.
Another example is an intruder trying to determine the identity of
the communicating parties and the volume of their traffic by sniffing
(L2) addresses. 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.
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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 (DoS) 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 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 Cases
Analysing the topological scenarios for MPEG-2 Transmission Networks
in Section 1, the security threats 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 address). In this scenario, measures must be taken
to protect the ULE payload data and the identity of ULE Receivers.
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o Case 2: Locally conducting active attacks on the MPEG-TS multiplex.
Here an intruder is assumed to be sufficiently sophisticated to
override 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 data integrity and authenticity and preventing replay of
old messages.
o Case 3: Globally conducting active attacks on the MPEG-TS
multiplex. This assumes a sophisticated intruder able to override
the whole MPEG-2 transmission multiplex. The requirements are
similar to case 2. The MPEG-2 transmission network operator can
usually identify such attacks and provide corrective action 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 adversaries without
knowledge of the secret material.
In terms of priority, Case 1 is considered the major threat in MPEG-2
transmission systems. Case 2 is considered a lesser threat,
appropriate to specific network configurations, especially when
vulnerable to insider attacks. Case 3 is less likely to be found in
an operational network, and is expected to be noticed by the MPEG-2
transmission operator. It will require restoration of the original
transmission. The assumption being 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 provided by a link that supports
ULE Stream Security to prevent passive attacks and reduce the risk
of active threats.
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Req 2. Protection of L2 NPA address is OPTIONAL. 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 source authentication of ULE Stream
data are OPTIONAL. These can be used to prevent the active
attacks described in Section 3.2.
Req 4. Protection against replay attacks is OPTIONAL. This is used
to counter the active attacks described in Section 3.2.
Req 5. L2 ULE Source and Receiver authentication is OPTIONAL. This
can be 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 could be
either unidirectional or bidirectional authentication based on the
underlying key management protocol.
Other general requirements for all threat cases for link-layer
security are:
GReq (a) ULE key management functions MUST be decoupled from ULE
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. It should be possible
to update the crypto algorithms and hashes when they become
obsolete without affecting the overall security of the system.
GReq (d) The security extension header MUST be compatible with other
ULE extension headers. The method must allow other extension
headers (either mandatory or optional) to be used in combination
with a security extension. It is RECOMMENDED that these are
placed after the security extension header. This permits full
protection for all headers. It also avoids situations where the
SNDU has to be discarded on processing the security extension
header, while preceding headers have already been evaluated. One
exception is the Timestamp extension that SHOULD precede the
security extension header [RFC5163]. In this case, the timestamp
will be unaffected by security services such as data
confidentiality and can be decoded without the need for key
material.
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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
Authentication Codes, digital signatures, or Timed Efficient Stream
Loss-Tolerant Authentication (TESLA) [RFC4082] in order to provide
integrity protection and source authentication (Reqs 3 and 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 can provide the required level
of security (Reqs 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 common DoS 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.
Table 1 below shows the threats that are applicable to ULE networks,
and the relevant security mechanisms to mitigate those threats.
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Security Mechanism
-----------------------------------------------
|Data |Data |Source |Data |Intru |Iden |
|Privacy |fresh |Authent|Integ |sion |tity |
| |ness |ication|rity |Dete |Prote |
| | | | |ction |ction |
Threat | | | | | | |
---------------|--------|-------|-------|-------|-------|------|
| 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
5. Design Recommendations for ULE Security Extension Header
Table 1 may assist in selecting fields within a ULE Security
Extension Header framework.
Security services may be grouped into profiles based on security
requirements, e.g., a base profile (with payload encryption and
identity protection) and a second profile that extends this to also
provide source authentication and protection against replay attacks.
Although the use of specific security techniques is optional, it is
RECOMMENDED that receiver devices should implement all the techniques
in Reqs 2-5 of Section 4 to ensure interoperability of all profiles.
A modular design of ULE security may allow it to use and benefit from
existing key management protocols, such as the Group Secure
Association Key Management Protocol (GSAKMP) [RFC4535] and the Group
Domain of Interpretation (GDOI) [RFC3547] 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.
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IPsec [RFC4301] and TLS [RFC5246] 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. See Appendix A for more
details.
6. Compatibility with Generic Stream Encapsulation
RFC 5163 [RFC5163] describes three new Extension Headers 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 [GSE].
The security threats and requirements presented in this document are
applicable to ULE and GSE encapsulations.
7. Summary
This document analyses a set of threats and security requirements.
It defines the requirements for ULE security and states the
motivation for link security as a part of the Encapsulation layer.
ULE security must provide link-layer encryption and ULE Receiver
identity protection. The framework must support the optional ability
to provide 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 set of features is
optional to reduce encapsulation overhead when not required.
ULE Stream security between a ULE Encapsulation Gateway and the
corresponding Receiver(s) is considered 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 address).
Appendix A 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 [RFC5246], SSH [RFC4251], IPsec [RFC4301]).
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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 (i.e., are out of scope),
e.g.:
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 of bridged frames exposes the
traffic to potentially undetected corruption while being processed
by the Encapsulator and/or 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. 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), and Yim Fun Hu
(University of Bradford).
10. References
10.1. 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, March 1997.
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[RFC4326] Fairhurst, G. and B. Collini-Nocker, "Unidirectional
Lightweight Encapsulation (ULE) for Transmission of IP
Datagrams over an MPEG-2 Transport Stream (TS)", RFC
4326, December 2005.
10.2. Informative References
[BELLOVIN] S. Bellovin, "Security Problems in the TCP/IP Protocol
Suite", Computer Communications Review 2:19, pp. 32-48,
April 1989. http://www.cs.columbia.edu/~smb/
[ETSI-DAT] EN 301 192, "Digital Video Broadcasting (DVB); DVB
Specifications for Data Broadcasting", European
Telecommunications Standards Institute (ETSI).
[GSE] TS 102 606, "Digital Video Broadcasting (DVB); Generic
Stream Encapsulation (GSE) Protocol, "European
Telecommunication Standards, Institute (ETSI), 2007.
[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.
[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.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC 3135, June 2001.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3715] Aboba, B. and W. Dixon, "IPsec-Network Address
Translation (NAT) Compatibility Requirements", RFC 3715,
March 2004.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and
L. Wood, "Advice for Internet Subnetwork Designers", BCP
89, RFC 3819, July 2004.
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[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[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", RFC 4259, November
2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC4947] Fairhurst, G. and M. Montpetit, "Address Resolution
Mechanisms for IP Datagrams over MPEG-2 Networks", RFC
4947, July 2007.
[RFC5163] Fairhurst, G. and B. Collini-Nocker, "Extension Formats
for Unidirectional Lightweight Encapsulation (ULE) and
the Generic Stream Encapsulation (GSE)", RFC 5163, April
2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
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Appendix A. ULE Security Framework
This section describes a security framework for the deployment of
secure ULE networks.
A.1. Building Blocks
This ULE Security framework describes 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. The Security Databases Block exists in
both the group member and server sides. However, it has been omitted
from Figure 2 just for clarity.
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-----
+------+----------+ +----------------+ / \
| Key Management |/---------\| Key Management | |
| Group Member |\---------/| Group Server | |
| Block | | Block | Control
+------+----------+ +----------------+ Plane
| | |
| | |
| | \ /
----------- Key management <-> ULE Security databases -----
| |
\ /
+------+----------+
| ULE |
| SAD / SPD |
| Databases |
| Block |
+------+-+--------+
/ \
| |
----------- ULE Security databases <-> ULE Security Header ----
| | / \
| | |
| | |
+------+-+--------+ 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).
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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 Security Databases Block
There needs to be two databases, i.e., similar to the IPsec
databases.
o ULE-SAD: ULE Security Association Database contains all the
Security Associations that are currently established with different
ULE peers.
o ULE-SPD: ULE Security Policy Database contains the policies as
described by the system manager. These policies describe the
security services that must be enforced.
While traditionally link-layer security has operated using simple
policy mechanisms, it is envisaged that ULE security should provide
flexibility comparable to IPsec. The above design is based on the
two databases defined for IPsec [RFC4301]. These databases could be
used to implement either simple policies (as in traditional link
security services) or more complex policies (as in IPsec).
The exact details of the header patterns that the SPD and SAD will
have to support for all use cases will be described 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.1.3. ULE Extension Header Block
A new security extension header for the ULE protocol is required to
provide the security features of data confidentiality, identity
protection, 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.
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+-------+------+-------------------------------+------+
| ULE |SEC | Protocol Data Unit | |
|Header |Header| |CRC-32|
+-------+------+-------------------------------+------+
Figure 3: ULE Security Extension Header Placement
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
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 Block of a group member
(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 security information could be manually
inserted using this interface.
Examples of interface functions are:
o Insert_record_database (char * Database, char * record, char *
Unique_ID);
o Update_record_database (char * Database, char * record, char *
Unique_ID);
o Delete_record_database (char * Database, char * Unique_ID);
The definitions of the variables are as follows:
o Database - This is a pointer to the ULE Security databases
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o record - This is the rows of security attributes to be entered or
modified in the above databases
o Unique_ID - This is the primary key to look up 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. When sending
traffic, the ULE encapsulator uses the Destination Address, the PID,
and possibly other information such as L3 source and destination
addresses to locate the relevant security record within the ULE
Security Database. It then uses the data in the record to create the
ULE security extension header. For received traffic, the ULE
decapsulator on receiving the ULE SNDU will use the Destination
Address, the PID, and a ULE Security ID inserted by the ULE
encapsulator into the security extension to retrieve the relevant
record from the Security Database. 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 main
difference between the sender and receiver is the direction of the
flow of traffic. An example of such an interface is as follows:
o 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 placement of security functionalities 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 Streams. 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.
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In the context of MPEG-2 transmission links, if IPsec is used to
secure a ULE Stream, 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 address) 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 [RFC5374].
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.
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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 keying and rekeying. Therefore, if the key
is compromised, this will impact several ULE Receivers.
Currently, there are few deployed L2 security systems for MPEG-2
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 and these methods are
not applicable to GSE.
B.3. Link Security as a Part of the Encapsulation Layer
Examining the threat analysis in Section 3 has shown that protection
of ULE Stream from eavesdropping and ULE Receiver identity are major
requirements.
There are several 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
[RFC5246] at L4). IPsec and TLS provide strong authentication of the
endpoints in the communication.
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L3 end-to-end security would partially deny the advantage listed
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 endpoints 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.
Authors' Addresses
Haitham Cruickshank
Centre for Communications System Research (CCSR)
University of Surrey
Guildford, Surrey, GU2 7XH
UK
EMail: h.cruickshank@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
Michael Noisternig
Multimedia Comm. Group, Dpt. of Computer Sciences
University of Salzburg
Jakob-Haringer-Str. 2
5020 Salzburg
Austria
EMail: mnoist@cosy.sbg.ac.at
Sunil Iyengar
Space & Defence
Logica
Springfield Drive
Leatherhead
Surrey KT22 7LP
UK
EMail: sunil.iyengar@logica.com
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