IPDVB Working Group                                   H. Cruickshank
Internet-Draft                              University of Surrey, UK
Intended status: Informational                             P. Pillai
Expires: Jan 13, 2009                     University of Bradford, UK
                                                       M. Noisternig
                                     University of Salzburg, Austria
                                                          S. Iyengar
                                                          Logica, UK
                                                       14 July, 2008

        Security requirements for the Unidirectional Lightweight
                     Encapsulation (ULE) protocol

Status of this Draft

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   This Internet-Draft will expire on January 13, 2009.


   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

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   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.

Table of Contents

   1. Introduction .............................................. 2
   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. IANA Considerations ...................................... 16
   10. Acknowledgments ......................................... 16
   11. References .............................................. 16
      11.1. Normative References ............................... 16
      11.2. Informative References ............................. 17
   12. Author's Addresses ...................................... 18
   13. Intellectual Property Statement ......................... 19
   14. Full Copyright Statement ................................ 20
   Appendix A: ULE Security Framework .......................... 20
   Appendix B: Motivation for ULE link-layer security .......... 24
   Document History ............................................ 28

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].

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   Section 3.1 of RFC 4259 presents several topological scenarios
   for MPEG-2 Transmission Networks. A summary of these scenarios
   are presented below (see section 3.1 of RFC 4259):

   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].

   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 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

   This document extends the above analysis and derives in detail
   the security requirements for ULE in MPEG-2 transmission

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   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
   "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

   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

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   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, 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

   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.

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   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
   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)

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   o TS multiplexers (including re-multiplexers)

   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

   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 a L2 source [RFC4259]. In an MPEG-2
   TS, this association is made via the Packet Identifier, PID [ISO-
   MPEG2]. At the sender, each source associates a locally unique
   set of PID values with each stream it originates. However, there
   is no required relationship between the PID value used at the
   sender and that received at the Receiver. Network devices may re-
   number the PID values associated with one or more TS Logical
   Channels (e.g. ULE Streams) to prevent clashes at a multiplexer
   between input streams with the same PID carried on different
   input multiplexes (updating entries in the PMT [ISO-MPEG2], and
   other SI tables that reference the PID value). A device may also
   modify and/or insert new SI data into the control plane (also
   sent as TS Packets identified by their PID value). 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

   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.

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   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 end-points, 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, 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 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 IP
   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 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) 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

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   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 (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:

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   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 data integrity
      and authenticity and preventing replay of old messages.

   o Case 3: Globally conduct active attacks on the MPEG-TS
      multiplex. This assumes a sophisticated intruder able to over-
      ride the whole MPEG-2 transmission multiplex. The requirements
      are similar to scenario 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 outside of a
   virtual private network.

   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:

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   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.

   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

   Req 3. Integrity protection and source authentication of ULE
      Stream data are OPTIONAL. These can be used to prevent active
      attacks described in section 3.2.

   Req 4. Protection against replay attacks is OPTIONAL. This is
      used to counter 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

   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

   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

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     been evaluated. One exception is the Timestamp extension which
     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.

   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

   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 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 common DoS

   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

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                                        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.

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

   A modular design of ULE security may allow it to use and benefit
   from existing key management protocols, such as GSAKMP [RFC4535]
   and 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.

   IPsec [RFC4301] and TLS [RFC4346] 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.

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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 requirement 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).

   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

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   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 for 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. 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),
   and Yim Fun Hu (University of Bradford).

11. References

   11.1. Normative References

   [ISO-MPEG2] "Information technology -- generic coding of moving
               pictures and associated audio information systems,
               Part I", ISO 13818-1, International Standards
               Organisation (ISO), 2000.

   [RFC2119]  Bradner, S., "Key Words for Use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, 1997.

   [RFC4326]  Fairhurst, G. and B. Collini-Nocker, "Unidirectional

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               Lightweight Encapsulation (ULE) for Transmission of
               IP Datagrams over an MPEG-2 Transport Stream (TS)",
               IETF RFC 4326, December 2005.

   11.2. Informative References

   [RFC4947]   G. Fairhurst, M.-J. Montpetit, "Address Resolution
               Mechanisms for IP Datagrams over MPEG-2 Networks",
               IETF RFC 4947, July 2007.

   [RFC5163]  G. Fairhurst and B. Collini-Nocker, "Extension Header
               formats for Unidirectional Lightweight Encapsulation
               (ULE) and the Generic Stream Encapsulation (GSE)",
               IETF RFC 5163, April 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),

   [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]  M.-J. Montpetit, G. Fairhurst, H. Clausen, B.
               Collini-Nocker, and H. Linder, "A Framework for
               Transmission of IP Datagrams over MPEG-2 Networks",
               IETF RFC 4259, November 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/

   [GSE]       TS 102 606, "Digital Video Broadcasting (DVB);
               Generic Stream Encapsulation (GSE) Protocol,
               "European  Telecommunication Standards, Institute
               (ETSI), 2007.

   [RFC4082]  A. Perrig, D. Song, "Timed Efficient Stream Loss-

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               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.

   [WEIS08]   B. Weis, et al, "Multicast Extensions to the Security
               Architecture for the Internet", <draft-ietf-msec-
               ipsec-extensions-09.txt>, June 2008, IETF Work in

   [RFC3715]  B. Aboba, W. Dixson, "IPsec-Network Address
               Translation (NAT) Compatibility Requirements" IETF
               RFC 3715, March 2004.

   [RFC4346]  T. Dierks, E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.1", IETF RFC 4346, April

   [RFC3135]  J. Border, M. Kojo, eyt. al., "Performance Enhancing
               Proxies Intended to Mitigate Link-Related
               Degradations", IETF RFC 3135, June 2001.

   [RFC4301]  S. Kent, K. Seo, "Security Architecture for the
               Internet Protocol", IETF RFC 4301, December 2006.

   [RFC3819]  P. Karn, C. Bormann, G. Fairhurst, D. Grossman, R.
               Ludwig, J. Mahdavi, G. Montenegro, J. Touch, 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
   Email: h.cruickshank@surrey.ac.uk

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   Prashant Pillai
   Mobile and Satellite Communications Research Centre (MSCRC)
   School of Engineering, Design and Technology
   University of Bradford
   Richmond Road, Bradford BD7 1DP
   Email: p.pillai@bradford.ac.uk

   Michael Noisternig
   Multimedia Comm. Group, Dpt. of Computer Sciences
   University of Salzburg
   Jakob-Haringer-Str. 2
   5020 Salzburg
   Email: mnoist@cosy.sbg.ac.at

   Sunil Iyengar
   Space & Defence
   Springfield Drive
   Surrey KT22 7LP
   Email: sunil.iyengar@logica.com

13. 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

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   the IETF at ietf-ipr@ietf.org.

14. Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   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

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.

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     +------+----------+           +----------------+           / \
     | 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 ----
            | |                                                 / \
            | |                                                  |
            | |                                                  |
     +------+-+--------+                                    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

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   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

   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.

        | ULE   |SEC   |     Protocol Data Unit        |      |
        |Header |Header|                               |CRC-32|
               Figure 3: ULE Security Extension Header Placement

   A.1.3 ULE Security Databases Block

   There needs to be two databases, i.e. similar to the IPsec

   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.

   The design of these two databases may 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 described in a
   separate document. This document only highlights the need for

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   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

   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 *

   o Update_record_database (char * Database, char * record, char *

   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

   o record - This is the rows of security attributes to be entered
      or modified in the above databases

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   o 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. 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
   interface is as follows:

   o Get_record_database (char * Database, char * record, char *

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

   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

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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) 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

   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 [WEIS08].

   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 key and rekeying. Therefore if
      the key is compromised, then this will impact several ULE

   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 major advantages in using ULE link layer

   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

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   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

   >>> NOTE to RFC Editor: Please remove this appendix prior to

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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

   o Added section 4.1 on GSE. Extended the security considerations

   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.

   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.

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   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

   o Table 1 modified to have consistent use of Security Services.

   o Text modified to be consistent with the draft-ietf-ipdvb-ule-

   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.

   Working Group Draft 07

   o Rephrased some sentences throughout the document to add more
      clarity, mainly due to suggestions by Gorry Fairhurst.

   o Updated section 4 to more clearly specify requirements,
      choosing more appropriate RFC 2119 keywords, and removed some
      overly general requirements.

   o Moved security header placement recommendation from appendix
      to list of general requirements in section 4, as suggested by
      Gorry Fairhurst.

   o Modified text in appendix section A.2.2 to correctly specify

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      which information sender and receiver use to look up security
      information within the database.

   o Fixed some editorial mistakes and updated the reference list.

   Working Group Draft 08

   o Rephrased some sentences to add more clarity.

   o Fixed some editorial mistakes pointed out by Gorry Fairhurst.

   o Described the interface definitions in section A.2 as examples
      rather than requirements.

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