Internet Engineering Task Force                       H. Cruickshank
Internet-Draft                              University of Surrey, UK
Intended status: Informational                            S. Iyengar
Expires: March 8, 2008                      University of Surrey, UK
                                                           P. Pillai
                                          University of Bradford, UK
                                                    October 11, 2007

       Security requirements for the Unidirectional Lightweight
                     Encapsulation (ULE) protocol

Status of this Draft

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   This Internet-Draft will expire on March 8, 2007.


   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.

Table of Contents

   1. Introduction................................................2
   2. Requirements notation.......................................4
   3. Threat Analysis.............................................6
      3.1. System Components......................................6
      3.2. Threats................................................8
      3.3. Threat Scenarios......................................10
   4. Security Requirements for IP over MPEG-2 TS................11
   5. Motivation for ULE link-layer security.....................13
      5.1. Security at the IP layer (using IPSEC)................13
      5.2. Link security below the Encapsulation layer...........14
      5.3. Link security as a part of the encapsulation layer....15
   6. Design recommendations for ULE Security Header Extension...15
   7. Compatibility with Generic Stream Encapsulation............16
   8. Summary....................................................17
   9. Security Considerations....................................18
   10. IANA Considerations.......................................18
   11. Acknowledgments...........................................18
   12. References................................................18
      12.1. Normative References.................................19
      12.2. Informative References...............................19
   13. Author's Addresses........................................20
   14. IPR Notices...............................................21
      14.1. Intellectual Property Statement......................21
      14.2. Intellectual Property................................21
   15. Copyright Statement.......................................22
   Appendix A: ULE Security Framework............................22
   Document History..............................................26

1. Introduction

   The MPEG-2 Transport Stream (TS) has been widely accepted not
   only for providing digital TV services, but also as a subnetwork
   technology for building IP networks. RFC 4326 [RFC4326] describes
   the Unidirectional Lightweight Encapsulation (ULE) mechanism for
   the transport of IPv4 and IPv6 Datagrams and other network
   protocol packets directly over the ISO MPEG-2 Transport Stream as
   TS Private Data.  ULE specifies a base encapsulation format and
   supports an extension format that allows it to carry additional
   header information to assist in network/Receiver processing. The

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   encapsulation satisfies the design and architectural requirement
   for a lightweight encapsulation defined in RFC 4259 [RFC4259].

   Section 3.1 of RFC 4259 presents several topological scenarios
   for MPEG-2 Transmission Networks. A summary of these scenarios
   are presented below (for full detail, please refer to RFC 4259):

   1. Broadcast TV and Radio Delivery.

   2. Broadcast Networks used as an ISP. This resembles to scenario
      1, but includes the provision of IP services providing access
      to the public Internet.

   3. Unidirectional Star IP Scenario. It utilizes a Hub station to
      provide a data network delivering a common bit stream to
      typically medium-sized groups of Receivers.

   4. Datacast Overlay. It employs MPEG-2 physical and link layers
      to provide additional connectivity such as unidirectional
      multicast to supplement an existing IP-based Internet service.

   5. Point-to-Point Links.

   6. Two-Way IP Networks. This can be typically satellite-based and
      star-based utilising a Hub station to deliver a common bit
      stream to medium- sized groups of receivers. A bidirectional
      service is provided over a common air-interface.

   RFC 4259 states that ULE must be robust to errors and security
   threats. Security must also consider both unidirectional as well
   as bidirectional links for the scenarios mentioned above.

   An initial analysis of the security requirements in MPEG-2
   transmission networks is presented in the security considerations
   section of RFC 4259. For example, when such networks are not
   using a wireline network, the normal security issues relating to
   the use of wireless links for transport of Internet traffic
   should be considered [RFC3819].

   The security considerations of RFC 4259 recommends that any new
   encapsulation defined by the IETF should allow Transport Stream
   encryption and should also support optional link-layer
   authentication of the SNDU payload. In ULE [RFC4326], it is
   suggested that this may be provided in a flexible way using
   Extension Headers. This requires the definition of a mandatory
   header extension, but has the advantage that it decouples
   specification of the security functions from the encapsulation

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

   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]).

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   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, 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

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   with a different PID value) [RFC4259]; for example, to
   redistribute the same multicast content to two terrestrial TV
   transmission cells.

   TS Packet: A fixed-length 188B unit of data sent over a TS
   Multiplex [ISO-MPEG2].  Each TS Packet carries a 4B header, plus
   optional overhead including an Adaptation Field, encryption
   details, and time stamp information to synchronise a set of
   related TS Logical Channels.

3. Threat Analysis

3.1. System Components

     +------------+                                  +------------+
     |  IP        |                                  |  IP        |
     |  End Host  |                                  |  End Host  |
     +-----+------+                                  +------------+
           |                                                ^
           +------------>+---------------+                  |
                         +  IP           |                  |
           +-------------+  Encapsulator |                  |
   SI-Data |             +------+--------+                  |
   +-------+-------+            |MPEG-2 TS Logical Channel  |
   |  MPEG-2       |            |                           |
   |  SI Tables    |            |                           |
   +-------+-------+   ->+------+--------+                  |
           |          -->|  MPEG-2       |                . . .
           +------------>+  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 [RFC4259].

   As shown in Figure 1 above (from section 3.3 of [RFC4259]), there

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   are several entities within the MPEG-2 transmission network
   architecture. These include:

   o  ULE Encapsulation Gateways (the Encapsulator or ULE source)

   o  SI-Table signalling generator (input to the multiplexer)

   o  Receivers (the end points for ULE streams)

   o  TS multiplexers (including re-multiplexers)

   o  Modulators

   In a MPEG-2 TS transmission network, the originating source of TS
   Packets is either a L2 interface device (media encoder,
   encapsulation gateway, etc) or a L2 network device (TS
   multiplexer, etc). These devices may, but do not necessarily,
   have an associated IP address. In the case of an encapsulation
   gateway (e.g. ULE sender), the device may operate at L2 or Layer
   3 (L3), and is not normally the originator of an IP traffic flow,
   and usually the IP source address of the packets that it forwards
   do not correspond to an IP address associated with the device.

   The TS Packets are carried to the Receiver over a physical layer
   that usually includes Forward Error Correction (FEC) coding that
   interleaves the bytes of several consecutive, but unrelated, TS
   Packets. FEC-coding and synchronisation processing makes
   injection of single TS Packets very difficult. Replacement of a
   sequence of packets is also difficult, but possible (see section

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

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   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 [ID-AR] may
   need to be broadcast (e.g. by an Encapsulation Gateway or other
   device) to form the Layer 2 (L2) control plane. Examples of
   signalling messages include the Program Association Table (PAT),
   Program Map Table (PMT) and Network Information Table (NIT). In
   existing MPEG-2 transmission networks, these messages are
   broadcast in the clear (no encryption or integrity checks). The
   integrity as well as authenticity of these messages is important
   for correct working of the ULE network, i.e. supporting its
   security objectives in the area of availability, in addition to
   confidentiality and integrity. One method recently proposed [ID-
   EXT] encapsulates these messages using ULE. In such cases all the
   security requirements of this document apply in securing these
   signalling messages.

   ULE link security focuses only on the security between the ULE
   Encapsulation Gateway (ULE source) and the Receiver. In many
   deployment scenarios the user of a ULE Stream has to secure
   communications beyond the link since other network links are
   utilised in addition to the ULE link. Therefore, if
   authentication of the end-point i.e. the IP Sources is required,
   or users are concerned about loss of confidentiality, integrity
   or authenticity of their communication data, they will have to
   employ end-to-end network security mechanisms like IPSec or
   Transport Layer Security (TLS). Governmental users may be forced
   by regulations to employ specific, approved implementations of
   those mechanisms. Hence for such cases the confidentiality and
   integrity of the user data will already be taken care of by the
   end-to-end security mechanism and the ULE security measures would
   focus on either providing traffic flow confidentiality for user
   data that has already been encrypted or for users who choose not
   to implement end-to-end security mechanisms.

   In contrast to the above, if a ULE Stream is used to directly
   join networks which are considered physically secure, for example
   branch offices to a central office, ULE link Security could be
   the sole provider of confidentiality and integrity. In this
   scenario, governmental users could still have to employ approved
   cryptographic equipment at the network layer or above, unless a
   manufacturer of ULE Link Security equipment obtains governmental
   approval for their implementation.

3.2. Threats

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   The simplest type of network threat is a passive threat. This
   includes eavesdropping or monitoring of transmissions, with a
   goal to obtain information that is being transmitted. In
   broadcast networks (especially those utilising widely available
   low-cost physical layer interfaces, such as DVB) passive threats
   are considered the major threats. An example of such a threat is
   an intruder monitoring the MPEG-2 transmission broadcast and then
   extracting traffic information concerning the communication
   between IP hosts using a link. Another example is of an intruder
   trying to gain information about the communication parties by
   monitoring their ULE Receiver NPA addresses; an intruder can gain
   information by determining the layer 2 identity of the
   communicating parties and the volume of their traffic. This is a
   well-known issue in the security field; however it is more of a
   problem in the case of broadcast networks such as MPEG-2
   transmission networks because of the easy availability of
   receiver hardware and the wide geographical span of the networks.

   Active threats (or attacks) are, in general, more difficult to
   implement successfully than passive threats, and usually require
   more sophisticated resources and may require access to the
   transmitter. Within the context of MPEG-2 transmission networks,
   examples of active attacks are:

   o  Masquerading: An entity pretends to be a different entity.
      This includes masquerading other users and subnetwork control
      plane messages.

   o  Modification of messages in an unauthorised manner.

   o  Replay attacks: When an intruder sends some old (authentic)
      messages to the Receiver. In the case of a broadcast link,
      access to previous broadcast data is easy.

   o  Denial of Service attacks: When an entity fails to perform its
      proper function or acts in a way that prevents other entities
      from performing their proper functions.

   The active threats mentioned above are major security concerns
   for the Internet community [BELLOVIN]. Masquerading and
   modification of IP packets are comparatively easy in an Internet
   environment whereas such attacks are in fact much harder for
   MPEG-2 broadcast links. This could for instance motivate the
   mandatory use of sequence numbers in IPsec, but not for
   synchronous links. This is further reflected in the security
   requirements for Case 2 and 3 in section 4 below.

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   As explained in section 3.1, the PID associated with an
   Elementary Stream can be modified (e.g. in some systems by
   reception of an updated SI table, or in other systems until the
   next announcement/discovery data is received). An attacker that
   is able to modify the content of the received multiplex (e.g.
   replay data and/or control information) could inject data locally
   into the received stream with an arbitrary PID value.

3.3. Threat Scenarios

   Analysing the topological scenarios for MPEG-2 Transmission
   Networks in section 1, the security threat cases can be
   abstracted into three cases:

   o  Case 1: Monitoring (passive threat). Here the intruder
      monitors the ULE broadcasts to gain information about the ULE
      data and/or tracking the communicating parties identities (by
      monitoring the destination NPA). In this scenario, measures
      must be taken to protect the ULE data flow and the identity of
      ULE Receivers.

   o  Case 2: Locally conduct active attacks on the MPEG-TS
      multiplex. Here an intruder is assumed to be sufficiently
      sophisticated to over-ride the original transmission from the
      ULE Encapsulation Gateway and deliver a modified version of
      the MPEG-TS transmission to a single ULE Receiver or a small
      group of Receivers (e.g. in a single company site). The MPEG-2
      transmission network operator might not be aware of such
      attacks. Measures must be taken to ensure ULE source
      authentication and preventing replay of old messages.

   o  Case 3: Globally conduct active attacks on the MPEG-TS
      multiplex. Here we assume an intruder is very sophisticated
      and able to over-ride the whole MPEG-2 transmission multiplex.
      The requirements here are similar to scenario 2. The MPEG-2
      transmission network operator can usually identify such
      attacks and may resort to some means to restore the original

   For both cases 2 and 3, there can be two sub cases:

   o  Insider attacks i.e. active attacks from adversaries in the
   known of secret material.

   o  Outsider attacks i.e. active attacks from outside of a virtual
   private network.

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   In terms of priority, case 1 is considered the major threat in
   MPEG-2 transmission systems. Case 2 is likely to a lesser degree
   within certain network configurations, especially when there are
   insider attacks. Hence, protection against such active attacks
   should be used only when such a threat is a real possibility.
   Case 3 is envisaged to be less practical, because it will be very
   difficult to pass unnoticed by the MPEG-2 transmission operator.
   It will require restoration of the original transmission. The
   assumption being here is that physical access to the network
   components (multiplexers, etc) and/or connecting physical media
   is secure. Therefore case 3 is not considered further in this

4. Security Requirements for IP over MPEG-2 TS

   From the threat analysis in section 3, the following security
   requirements can be derived:

   o  Data flow confidentiality is the major requirement to mitigate
      passive threats in MPEG-2 broadcast networks.

   o  Protection of Layer 2 NPA address. In broadcast networks this
      protection can be used to prevent an intruder tracking the
      identity of ULE Receivers and the volume of their traffic.

   o  Integrity protection and authentication of the ULE source is
      required against active attacks described in section 3.2.

   o  Protection against replay attacks. This is required for the
      active attacks described in section 3.2.

   o  Layer L2 ULE Source and Receiver authentication: This is
      normally performed during the initial key exchange and
      authentication phase, before the ULE Receiver can join a
      secure session with the ULE Encapsulator (ULE source). This is
      normally receiver to hub authentication and it could be either
      unidirectional or bidirectional authentication based on the
      underlying key management protocol.

   Other general requirements are:

   o  Decoupling of ULE key management functions from ULE security
      services such as encryption and source authentication. This
      allows the independent development of both systems.

   o  Support for automated as well as manual insertion of keys and
      policy into the relevant databases.

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   o  Algorithm agility is needed. Changes in crypto algorithms,
      hashes as they become obsolete should be updated without
      affecting the overall security of the system.

   o  Traceability: To monitor transmission network using log files
      to record the activities in the network and detect any

   o  Protection against loss of service (availability) through
      malicious reconfiguration of system components (see Figure 1).

   o  Compatibility with other networking functions such as NAT
      Network Address Translation (NAT) [RFC3715] or TCP
      acceleration can be used in a wireless broadcast networks.

   o  Compatibility and operational with ULE extension headers i.e.
      allow encryption of a compressed SNDU payload.

   o  Where a ULE Stream carries a set of IP traffic flows to
      different destinations with a range of properties (multicast,
      unicast, etc), it is often not appropriate to provide IP
      confidentiality services for the entire ULE Stream. For many
      expected applications of ULE, a finer-grain control is
      therefore required, at least permitting control of data
      confidentiality/authorisation at the level of a single MAC/NPA

   Examining the threat cases in section 3.3, the security
   requirements for each case can be summarised as:

   o  Case 1: Data flow confidentiality MUST be provided to prevent
      monitoring of the ULE data (such as user information and IP
      addresses). Protection of NPA addresses MAY be provided to
      prevent tracking ULE Receivers and their communications.

   o  Case 2: In addition to case 1 requirements, new measures need
      to be implemented such as authentication schemes using Message
      Authentication Codes, digital signatures or TESLA [RFC4082] in
      order to provide integrity protection and source
      authentication, and using sequence numbers to protect against
      replay attacks. In terms of outsider attacks, group
      authentication using Message Authentication Codes should
      provide the same level of security. This will significantly
      reduce the ability of intruders to 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

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      OPTIONAL. Such measures incur additional transmission as well
      as processing overheads. Moreover, intrusion detection systems
      may also be needed by the MPEG-2 network operator. These
      should best be coupled with perimeter security policy to
      monitor most denial-of-service attacks.

   o  Case 3: As stated in section 3.3. The requirements here are
      similar to Case 2 but since the MPEG-2 transmission network
      operator can usually identify such attacks the constraints on
      intrusion detections are less than in case 2.

5. Motivation for ULE link-layer security

   Examination of the threat analysis and security requirements in
   sections 3 and 4 has shown that there is a need to provide
   security in MPEG-2 transmission networks employing ULE. This
   section compares the disadvantages when security functionalities
   are present in different layers.

5.1. 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 links. The major
   advantage of IPsec is its wide implementation in IP routers and
   hosts. IPsec in transport mode can be used for end-to-end
   security transparently over MPEG-2 transmission links with little

   In the context of MPEG-2 transmission links, if IPsec is used to
   secure a ULE link, then the ULE Encapsulator and Receivers are
   equivalent to the security gateways in IPsec terminology. A
   security gateway implementation of IPsec uses tunnel mode. Such
   usage has the following disadvantages:

   o  There is an extra transmission overhead associated with using
      IPsec in tunnel mode, i.e. the extra IP header (IPv4 or IPv6).

   o  There is a need to protect the identity (NPA) of ULE Receivers
      over the ULE broadcast medium; IPsec is not suitable for
      providing this service. In addition, the interfaces of these
      devices do not necessarily have IP addresses (they can be L2

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   o  Multicast is considered a major service over ULE links. The
      current IPsec specifications [RFC4301] only define a pairwise
      tunnel between two IPsec devices with manual keying. Work is
      in progress in defining the extra detail needed for multicast
      and to use the tunnel mode with address preservation to allow
      efficient multicasting. For further details refer to [WEIS06].

5.2. Link security below the Encapsulation layer

   Link layer security can be provided at the MPEG-2 TS layer (below
   ULE). MPEG-2 TS encryption encrypts all TS Packets sent with a
   specific PID value. However, an MPEG-2 TS may typically multiplex
   several IP flows, belonging to different users, using a common
   PID. Therefore all multiplexed traffic will share the same
   security keys.

   This has the following advantages:

   o  The bit stream sent on the broadcast network does not expose
      any L2 or L3 headers, specifically all addresses, type fields,
      and length fields are encrypted prior to transmission.

   o  This method does not preclude the use of IPsec, TLS, or any
      other form of higher-layer security.

   However it has the following disadvantages:

   o  When a PID is shared between several users, each ULE Receiver
      needs to decrypt all MPEG-2 TS Packets with a matching PID,
      possibly including those that are not required to be
      forwarded. Therefore it does not have the flexibility to
      separately secure individual IP flows.

   o  When a PID is shared between several users, the ULE Receivers
      will have access to private traffic destined to other ULE
      Receivers, since they share a common PID and key.

   o  IETF-based key management is not used in existing systems.
      Existing access control mechanisms have limited flexibility in
      terms of controlling the use of key and rekeying. Therefore if
      the key is compromised, then this will impact several ULE

   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.

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   Some other systems are specified in standards such as MPE [ETSI-
   DAT], but there are currently no known implementations.

5.3. Link security as a part of the encapsulation layer

   Examining the threat analysis in section 3 has shown that
   protection of ULE link from eavesdropping and ULE Receiver
   identity are major requirements.

   There are several major advantages in using ULE link layer

   o  The protection of the complete ULE Protocol Data Unit (PDU)
      including IP addresses. The protection can be applied either
      per IP flow or per Receiver NPA address.

   o  Ability to protect the identity of the Receiver within the
      MPEG-2 transmission network at the IP layer and also at L2.

   o  Efficient protection of IP multicast over ULE links.

   o  Transparency to the use of Network Address Translation (NATs)
      [RFC3715] and TCP Performance Enhancing Proxies (PEP)
      [RFC3135], which require the ability to inspect and modify the
      packets sent over the ULE link.

   This method does not preclude the use of IPsec at L3 (or TLS
   [RFC4346] at L4). IPsec and TLS provide strong authentication of
   the end-points in the communication.

   L3 end-to-end security would partially deny the advantage listed
   just above (use of PEP, compression etc), since those techniques
   could only be applied to TCP packets bearing a TCP-encapsulated
   IPsec packet exchange, but not the TCP packets of the original
   applications, which in particular inhibits compression.

   End-to-end security (IPsec, TLS, etc.) may be used independently
   to provide strong authentication of the end-points in the
   communication. This authentication is desirable in many scenarios
   to ensure that the correct information is being exchanged between
   the trusted parties, whereas Layer 2 methods cannot provide this

6. Design recommendations for ULE Security Header Extension

   Table 1 below shows the threats that are applicable to ULE
   networks and the relevant security mechanism to mitigate those

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   threats. This would help in the design of the ULE Security
   extension header. For example this could help in the selection of
   security fields in the ULE Security extension Header design.
   Moreover the security services could also be grouped into
   profiles based on different security requirements. One example is
   to have a base profile which does payload encryption and identity
   protection. The second profile could do the above as well as
   source authentication.

                                        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.
   A modular design to ULE Security may allow it to use and benefit
   from IETF key management protocols, such as GSAKMP [RFC4535] and
   GDOI [RFC3547] protocols defined by the IETF Multicast Security
   (MSEC) working group. This does not preclude the use of other key
   management methods in scenarios where this is more appropriate.

   IPsec or TLS also provide a proven security architecture defining
   key exchange mechanisms and the ability to use a range of
   cryptographic algorithms. ULE security can make use of these
   established mechanisms and algorithms.

7. Compatibility with Generic Stream Encapsulation

   The [ID-EXT] document describes two new Header Extensions that
   may be used with Unidirectional Link Encapsulation, ULE,
   [RFC4326] and the Generic Stream Encapsulation (GSE) that has
   been designed for the Generic Mode (also known as the Generic

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   Stream (GS)), offered by second-generation DVB physical layers,
   and specifically for DVB-S2 [ID-EXT].

   The security threats and requirement presented in this document
   are applicable to ULE and GSE encapsulations. It might be
   desirable to authenticate some/all of the headers; such decision
   can be part of the security policy for the MPEG-2 transmission

8. Summary

   This document analyses a set of threats and security
   requirements. It also defines the requirements for ULE security
   and states the motivation for link security as a part of the
   Encapsulation layer.

   ULE security includes a need to provide link-layer encryption and
   ULE Receiver identity protection. There is an optional
   requirement for link-layer authentication and integrity assurance
   as well as protection against insertion of old (duplicated) data
   into the ULE stream (i.e. replay protection). This is optional
   because of the associated overheads for the extra features and
   they are only required for specific service cases.

   ULE link security (between a ULE Encapsulation Gateway to
   Receivers) is considered as an additional security mechanism to
   IPsec, TLS, and application layer end-to-end security, and not as
   a replacement. It allows a network operator to provide similar
   functions to that of IPsec, but in addition provides MPEG-2
   transmission link confidentiality and protection of ULE Receiver
   identity (NPA). End-to-end security mechanism may then be used
   additionally and independently for providing strong
   authentication of the end-points in the communication.

   Annexe 1 describes a set of building blocks that may be used to
   realise a framework that provides ULE security functions.

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9. Security Considerations

   Link-layer (L2) encryption of IP traffic is commonly used in
   broadcast/radio links to supplement End-to-End security (e.g.
   provided by TLS [RFC4346], SSH [RFC4251], IPsec [RFC4301). A
   common objective is to provide the same level of privacy as wired
   links. It is recommended that an ISP or user provide end-to-end
   security services based on well known mechanisms such as IPsec or

   This document provides a threat analysis and derives the security
   requirements to provide link encryption and optional link-layer
   integrity / authentication of the SNDU payload.

   There are some security issues that were raised in RFC 4326
   [RFC4326] that are not addressed in this document (out of scope)
   such as:

   o  The security issue with un-initialised stuffing bytes.  In
      ULE, these bytes are set to 0xFF (normal practice in MPEG-2).

   o  Integrity issues related to the removal of the LAN FCS in a
      bridged networking environment.  The removal for bridged
      frames exposes the traffic to potentially undetected
      corruption while being processed by the Encapsulator and/or

   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.

10. IANA Considerations

   This document does not define any protocol and does not require
   any IANA assignments but a subsequent document that defines a
   layer 2 security extension to ULE will require IANA involvement.

11. Acknowledgments

   The authors acknowledge the help and advice from Gorry Fairhurst
   (University of Aberdeen). The authors also acknowledge
   contributions from Laurence Duquerroy and Stephane Coombes (ESA),
   Yim Fun Hu (University of Bradford) and Michael Noisternig from
   University of Salzburg.

12. References

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12.1. 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.

12.2. Informative References

   [ID-AR]     G. Fairhurst, M-J Montpetit "Address Resolution
               Mechanisms for IP Datagrams over MPEG-2 Networks",
               Work in Progress <draft-ietf-ipdvb-ar-05.txt.

   [ID-EXT]    G. Fairhurst and B. Collini-Nocker, "Extension
               Formats for Unidirectional Lightweight Encapsulation
               (ULE) and the Generic Stream Encapsulation (GSE)",
               Work in Progress, draft-ietf-ipdvb-ule-ext-04.txt,
               August 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),

   [ISO-8802]  ISO/IEC 8802.2, "Logical Link Control", International
               Standards Organisation (ISO), 1998.

   [ITU-H222]  H.222.0, "Information technology, Generic coding of
               moving pictures and associated audio information
               Systems", International Telecommunication Union,
               (ITU-T), 1995.

   [RFC4259]   Montpetit, M.-J., Fairhurst, G., Clausen, H.,
               Collini-Nocker, B., and H. Linder, "A Framework for
               Transmission of IP Datagrams over MPEG-2 Networks",
               IETF RFC 4259, November 2005.

   [RFC4326]   Fairhurst, G. and B. Collini-Nocker, "Unidirectional
               Lightweight Encapsulation (ULE) for Transmission of
               IP Datagrams over an MPEG-2 Transport Stream (TS)",
               IETF RFC 4326, December 2005.

   [ETSI-DAT]  EN 301 192, "Digital Video Broadcasting (DVB); DVB
               Specifications for Data Broadcasting", European

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               Telecommunications Standards Institute (ETSI).

   [BELLOVIN]  S.Bellovin, "Problem Area for the IP Security
               protocols", Computer Communications Review 2:19, pp.
               32-48, April 989.

   [RFC4082]   A. Perrig, D. Song, " Timed Efficient Stream Loss-
               Tolerant Authentication (TESLA): Multicast Source
               Authentication Transform Introduction", IETF RFC
               4082, June 2005.

   [RFC4535]   H Harney, et al, "GSAKMP: Group Secure Association
               Group Management Protocol", IETF RFc 4535, June 2006.

   [RFC3547]   M. Baugher, et al, "GDOI: The Group Domain of
               Interpretation", IETF RFC 3547.

   [WEIS06]    Weis B., et al, "Multicast Extensions to the Security
               Architecture for the Internet", <draft-ietf-msec-
               ipsec-extensions-02.txt>, June 2006, IETF Work in

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

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

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

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

   [RFC3819]   Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
               Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J.,
               and L. Wood, "Advice for Internet Subnetwork
               Designers", BCP 89, IETF RFC 3819, July 2004.

   [RFC4251]   T. Ylonen, C. Lonvick, Ed., "The Secure Shell (SSH)
               Protocol Architecture", IETF RFC 4251, January 2006.

13. Author's Addresses

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   Haitham Cruickshank,
   Centre for Communications System Research (CCSR),
   University of Surrey,
   Guildford, Surrey, GU2 7XH

   Sunil Iyengar,
   Centre for Communications System Research (CCSR),
   University of Surrey,
   Guildford, Surrey, GU2 7XH

   Prashant Pillai,
   Mobile and Satellite Communications Research Centre (MSCRC),
   School of Engineering, Design and Technology,
   University of Bradford,
   Richmond Road, Bradford BD7 1DP

14. IPR Notices

   Copyright (c) The IETF Trust (2007).

14.1. Intellectual Property Statement

   Full Copyright Statement

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided

14.2. Intellectual Property

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   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

   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights that may cover technology that may be required
   to implement this standard.  Please address the information to
   the IETF at

15. Copyright Statement

   Copyright (C) The IETF Trust (2007).

Appendix A: ULE Security Framework

   This section defines a security framework for the deployment of
   secure ULE networks.

A.1 Building Blocks

   This ULE Security framework defines the following building blocks
   as shown in figure 2 below:

   o  The Key Management Block

   o  The ULE Security Extension Header Block

   o  The ULE Databases Block

   Within the Key Management block the communication between the
   Group Member entity and the Group Server entity happens in the

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   control plane. The ULE Security header block applies security to
   the ULE SNDU and this happens in the ULE data plane. The ULE
   Security databases block acts as the interface between the Key
   management block (control plane) and the ULE Security Header
   block (ULE data plane) as shown in figure 2.

     +------+----------+           +----------------+           / \
     | Key Management  |/---------\| Key Management |            |
     |     Block       |\---------/|     Block      |            |
     |  Group Member   |           |  Group Server  |        Control
     +------+----------+           +----------------+          Plane
            | |                                                  |
            | |                                                  |
            | |                                                 \ /
     ----------- Key management <-> ULE Security databases     -----
            | |
            \ /
     |      ULE        |
     |   SAD / SPD     |
     |    Databases    |
     |      Block      |
            / \
            | |
    ----------- ULE Security databases <-> ULE Security Header ----
            | |                                                 / \
            | |                                                  |
            | |                                                  |
     +------+-+--------+                                    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

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   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 sources (encapsulation gateways).
   The ULE source could also be the Key Management Group Server
   Entity (shown as the Key Management Group Server block in figure
   2. This happens when the ULE source also acts as the Key
   Management Group Server. Deployment may use either automated key
   management protocols (e.g. GSAKMP [RFC4535]) or manual insertion
   of keying material.

   A.1.2 ULE Extension Header Block

   A new security extension header for the ULE protocol is required
   to provide the security features of data confidentiality, data
   integrity, data authentication and mechanisms to prevent replay
   attacks. Security keying material will be used for the different
   security algorithms (for encryption/decryption, MAC generation,
   etc.), which are used to meet the security requirements,
   described in detail in Section 4 of this document.

   This block will use the keying material and policy information
   from the ULE security database block on the ULE payload to
   generate the secure ULE Extension Header or to decipher the
   secure ULE extension header to get the ULE payload. An example
   overview of the ULE Security extension header format along with
   the ULE header and payload is shown in figure 3 below. There
   could be other extension headers (either mandatory or optional)
   but these will always be placed after the security extension
   header. However, there is an exception: the timestamp extension
   may be placed before the security extension header [ID-EXT]. When
   applying the security services for example confidentiality, input
   to the cipher algorithm will cover the fields from the end of the
   security extension header to the end of the PDU.

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

   A.1.3 ULE Security Databases Block

   There needs to be two databases i.e. similar to the IPSec

   o  ULE-SAD: ULE Secure Association Database contains all the

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      Security Associations that are currently established with
      different ULE peers.

   o  ULE-SPD: ULE Secure Policy Database contains the policies as
      defined by the system manager. These policies describe the
      security services that must be enforced

   The design of these two databases will be based on IPSec
   databases as defined in RFC4301 [RFC4301].

   The exact details of the header patterns that the SPD and SAD
   will have to support for all use cases will be defined in a
   separate document. This document only highlights the need for
   such interfaces between the ULE data plane and the Key Management
   control plane.

A.2 Interface definition

   Two new interfaces have to be defined between the blocks as shown
   in Figure 2 above. These interfaces are:

   o  Key Management block <-> ULE Security databases block

   o  ULE Security databases block  <-> ULE Security Header block

   While the first interface is used by the Key Management Block to
   insert keys, security associations and policies into the ULE
   Database Block, the second interface is used by the ULE Security
   Extension Header Block to get the keys and policy material for
   generation of the security extension header.

   A.2.1 Key Management <-> ULE Security databases

   This interface is between the Key Management group member block
   (GM client) and the ULE Security Database block (shown in figure
   2). The Key Management GM entity will communicate with the GCKS
   and then get the relevant security information (keys, cipher
   mode, security service, ULE_Security_ID and other relevant keying
   material as well as policy) and insert this data into the ULE
   Security database block. The Key Management could be either
   automated (e.g. GSAKMP [RFC4535] or GDOI [RFC3547]) or manually
   inserted using this interface. The following three interface
   functions are defined:

  . Insert_record_database (char * Database, char * record, char *

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  . Update_record_database (char * Database, char * record, char *
  . Delete_record_database (char * Database, char * Unique_ID);

   The definitions of the variables are as follows:

     . Database - This is a pointer to the ULE Security databases
     . record - This is the rows of security attributes to be
        entered or modified in the above databases
     . Unique_ID - This is the primary key to lookup records (rows
        of security attributes) in the above databases

   A.2.2 ULE Security Databases <-> ULE Security Header

   This interface is between the ULE Security Database and the ULE
   Security Extension Header block as shown in figure 2. To send
   traffic, firstly the ULE encapsulator using the ULE_Security_ID,
   Destination Address and possibly the PID, searches the ULE
   Security Database for the relevant security record. It then uses
   the data in the record to create the ULE security extension
   header. For received traffic, the ULE decapsulator on receiving
   the ULE SNDU will first get the record from the Security Database
   using the ULE_Security_ID, the Destination Address and possibly
   the PID. It then uses this information to decrypt the ULE
   extension header. For both cases (either send or receive traffic)
   only one interface is needed since the only difference between
   the sender and receiver is the direction of the flow of traffic:

  . Get_record_database (char * Database, char * record, char *

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

Document History

   Working Group Draft 00

   o  Fixed editorial mistakes and ID style for WG adoption.

   Working Group Draft 01

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   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.

   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

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      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-

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