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Versions: (draft-cruickshank-ipdvb-sec-req) 00 01 02 03 04 05 06 07 08 09 RFC 5458

     Internet Engineering Task Force                       H.Cruickshank
     Internet Draft                                           S. Iyengar
     draft-ietf-ipdvb-sec-req-03.txt            University of Surrey, UK
                                                               P. Pillai
     Expires: December 29, 2007               University of Bradford, UK
    
    
     Category: WG Draft intended for INFORMATIONAL RFC     June 29, 2007
    
    
    
            Security requirements for the Unidirectional Lightweight
                         Encapsulation (ULE) protocol
                        draft-ietf-ipdvb-sec-req-03.txt
    
    
    Status of this Draft
    
       By submitting this Internet-Draft, each author represents that
       any applicable patent or other IPR claims of which he or she is
       aware have been or will be disclosed, and any of which he or she
       becomes aware will be disclosed, in accordance with Section 6 of
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       This Internet-Draft will expire on December 29, 2007.
    
    Abstract
    
       The MPEG-2 standard defined by ISO 13818-1 supports a range of
       transmission methods for a range of services. This document
       provides a threat analysis and derives the security requirements
       when using the Transport Stream, TS, to support an Internet
       network-layer using Unidirectional Lightweight Encapsulation
       (ULE) defined in RFC4326. The document also provides the
    

    
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       motivation for link-layer security for a ULE Stream. A ULE Stream
       may be used to send IPv4 packets, IPv6 packets, and other
       Protocol Data Units (PDUs) to an arbitrarily large number of
       Receivers supporting unicast and/or multicast transmission.
    
    
    Table of Contents
    
       1. Introduction.......................................2
       2. Requirements notation...............................4
       3. Threat Analysis....................................6
          3.1. System Components..............................6
          3.2. Threats.......................................9
          3.3. Threat Scenarios..............................10
       4. Security Requirements for IP over MPEG-2 TS...........11
       5. 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 Extension16
       7. Compatibility with Generic Stream Encapsulation........17
       8. Summary..........................................17
       9. Security Considerations............................17
       10. IANA Considerations...............................18
       11. Acknowledgments..................................18
       12. References.......................................18
          12.1. Normative References..........................18
          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
    
    
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       header information to assist in network/Receiver processing. The
       encapsulation satisfies the design and architectural requirement
       for a lightweight encapsulation defined in RFC 4259 [RFC4259].
    
       Section 3.1 of RFC 4259 presents several topological scenarios
       for MPEG-2 Transmission Networks. A summary of these scenarios
       are presented below (for full detail, please refer to RFC 4259):
    
       1. Broadcast TV and Radio Delivery.
    
       2. Broadcast Networks used as an ISP. This resembles to scenario
          1, but includes the provision of IP services providing access
          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
    
    
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       specification of the security functions from the encapsulation
       functions.
    
       This document extends the above analysis and derives a detailed
       the security requirements for ULE in MPEG-2 transmission
       networks.
    
       A security framework for deployment of secure ULE networks
       describing the different building blocks and the interface
       definitions is presented in Appendix A.
    
    2. Requirements notation
    
       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
       NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       RFC2119 [RFC2119].
    
       Other terms used in this document are defined below:
    
       ATSC: Advanced Television Systems Committee. A framework and a
       set of associated standards for the transmission of video, audio,
       and data using the ISO MPEG-2 standard.
    
       DVB: Digital Video Broadcast. A framework and set of associated
       standards published by the European Telecommunications Standards
       Institute (ETSI) for the transmission of video, audio, and data
       using the ISO MPEG-2 Standard [ISO-MPEG2].
    
       Encapsulator: A network device that receives PDUs and formats
       these into Payload Units (known here as SNDUs) for output as a
       stream of TS Packets.
    
       LLC: Logical Link Control [ISO-8802, IEEE-802].  A link-layer
       protocol defined by the IEEE 802 standard, which follows the
       Ethernet Medium Access Control Header.
    
       MAC: Message Authentication Code.
    
       MPE: Multiprotocol Encapsulation [ETSI-DAT].  A scheme that
       encapsulates PDUs, forming a DSM-CC Table Section.  Each Section
       is sent in a series of TS Packets using a single TS Logical
       Channel.
    
       MPEG-2: A set of standards specified by the Motion Picture
       Experts Group (MPEG) and standardized by the International
       Standards Organisation (ISO/IEC 13818-1) [ISO-MPEG2], and ITU-T
    
    
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       (in H.222 [ITU-H222]).
    
       NPA: Network Point of Attachment.  In this document, refers to a
       6-byte destination address (resembling an IEEE Medium Access
       Control address) within the MPEG-2 transmission network that is
       used to identify individual Receivers or groups of Receivers.
    
       PDU: Protocol Data Unit.  Examples of a PDU include Ethernet
       frames, IPv4 or IPv6 datagrams, and other network packets.
    
       PID: Packet Identifier [ISO-MPEG2].  A 13-bit field carried in
       the header of TS Packets.  This is used to identify the TS
       Logical Channel to which a TS Packet belongs [ISO-MPEG2].  The TS
       Packets forming the parts of a Table Section, PES, or other
       Payload Unit must all carry the same PID value.  The all-zeros
       PID 0x0000 as well as other PID values is reserved for specific
       PSI/SI Tables [ISO-MPEG2]. The all-ones PID value 0x1FFF
       indicates a Null TS Packet introduced to maintain a constant bit
       rate of a TS Multiplex.  There is no required relationship
       between the PID values used for TS Logical Channels transmitted
       using different TS Multiplexes.
    
       Receiver: Equipment that processes the signal from a TS Multiplex
       and performs filtering and forwarding of encapsulated PDUs to the
       network-layer service (or bridging module when operating at the
       link layer).
    
       SI Table: Service Information Table [ISO-MPEG2].  In this
       document, this term describes a table that is defined by another
       standards body to convey information about the services carried
       in a TS Multiplex. A Table may consist of one or more Table
       Sections; however, all sections of a particular SI Table must be
       carried over a single TS Logical Channel [ISO-MPEG2].
    
       SNDU: SubNetwork Data Unit. An encapsulated PDU sent as an MPEG-2
       Payload Unit.
    
       TS: Transport Stream [ISO-MPEG2], a method of transmission at the
       MPEG-2 layer using TS Packets; it represents layer 2 of the
       ISO/OSI reference model.  See also TS Logical Channel and TS
       Multiplex.
    
       TS Multiplex: In this document, this term defines a set of MPEG-2
       TS Logical Channels sent over a single lower-layer connection.
       This may be a common physical link (i.e., a transmission at a
       specified symbol rate, FEC setting, and transmission frequency)
       or an encapsulation provided by another protocol layer (e.g.,
    
    
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       Ethernet, or RTP over IP). The same TS Logical Channel may be
       repeated over more than one TS Multiplex (possibly associated
       with a different PID value) [RFC4259]; for example, to
       redistribute the same multicast content to two terrestrial TV
       transmission cells.
    
       TS Packet: A fixed-length 188B unit of data sent over a TS
       Multiplex [ISO-MPEG2].  Each TS Packet carries a 4B header, plus
       optional overhead including an Adaptation Field, encryption
       details, and time stamp information to synchronise a set of
       related TS Logical Channels.
    
    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].
    
    
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       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 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
       3.2).
    
       A Receiver in a MPEG-2 TS transmission network needs to identify
       a TS Logical Channel (or MPEG-2 Elementary Stream) to reassemble
       the fragments of PDUs sent by a L2 source [RFC4259]. In a MPEG-2
       TS, this association is made via the Packet Identifier, PID [ISO-
       MPEG2]. At the sender, each source associates a locally unique
       set of PID values with each stream it originates. However, there
       is no required relationship between the PID value used at the
       sender and that received at the Receiver. Network devices may re-
       number the PID values associated with one or more TS Logical
       Channels (e.g. ULE Streams) to prevent clashes at a multiplexer
       between input streams with the same PID carried on different
       input multiplexes (updating entries in the PMT [ISO-MPEG2], and
       other SI tables that reference the PID value). A device may also
       modify and/or insert new SI data into the control plane (also
    
    
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       sent as TS Packets identified by their PID value). However there
       is only one valid source of data for each MPEG-2 Elementary
       Stream, bound to a PID value. (This observation could simplify
       the requirement for authentication of the source of a ULE
       Stream.)
    
       In an MPEG-2 network a set of signalling messages [ID-AR] may
       need to be broadcast (e.g. by an Encapsulation Gateway or other
       device) to form the Layer 2 (L2) control plane. Examples of
       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-
       EF] 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.
    
    
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    3.2. Threats
    
       The simplest type of network threat is a passive threat. This
       includes eavesdropping or monitoring of transmissions, with a
       goal to obtain information that is being transmitted. In
       broadcast networks (especially those utilising widely available
       low-cost physical layer interfaces, such as DVB) passive threats
       are considered the major threats. An example of such a threat is
       an intruder monitoring the MPEG-2 transmission broadcast and then
       extracting traffic information concerning the communication
       between IP hosts using a link. Another example is of an intruder
       trying to gain information about the communication parties by
       monitoring their ULE Receiver NPA addresses; an intruder can gain
       information by determining the layer 2 identity of the
       communicating parties and the volume of their traffic. This is a
       well-known issue in the security field; however it is more of a
       problem in the case of broadcast networks such as MPEG-2
       transmission networks because of the easy availability of
       receiver hardware and the wide geographical span of the networks.
    
       Active threats (or attacks) are, in general, more difficult to
       implement successfully than passive threats, and usually require
       more sophisticated resources and may require access to the
       transmitter. Within the context of MPEG-2 transmission networks,
       examples of active attacks are:
    
       o Masquerading: An entity pretends to be a different entity.
          This includes masquerading other users and subnetwork control
          plane messages.
    
       o Modification of messages in an unauthorised manner.
    
       o Replay attacks: When an intruder sends some old (authentic)
          messages to the Receiver. In the case of a broadcast link,
          access to previous broadcast data is easy.
    
       o Denial of Service attacks: When an entity fails to perform its
          proper function or acts in a way that prevents other entities
          from performing their proper functions.
    
       The active threats mentioned above are major security concerns
       for the Internet community [BELLOVIN]. Masquerading and
       modification of IP packets are comparatively easy in an Internet
       environment whereas such attacks are in fact much harder for
       MPEG-2 broadcast links. This could for instance motivate the
       mandatory use of sequence numbers in IPsec, but not for
       synchronous links. This is further reflected in the security
    
    
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       requirements for Case 2 and 3 in section 4 below.
    
       As explained in section 3.1, the PID associated with an
       Elementary Stream can be modified (e.g. in some systems by
       reception of an updated SI table, or in other systems until the
       next announcement/discovery data is received). An attacker that
       is able to modify the content of the received multiplex (e.g.
       replay data and/or control information) could inject data locally
       into the received stream with an arbitrary PID value.
    
    3.3. Threat Scenarios
    
       Analysing the topological scenarios for MPEG-2 Transmission
       Networks in section 1, the security threat cases can be
       abstracted into three cases:
    
       o Case 1: Monitoring (passive threat). Here the intruder
          monitors the ULE broadcasts to gain information about the ULE
          data and/or tracking the communicating parties identities (by
          monitoring the destination NPA). In this scenario, measures
          must be taken to protect the ULE 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
          transmission.
    
       For both cases 2 and 3, there can be two sub cases:
    
       o Insider attacks i.e. active attacks from adversaries in the
       known of secret material.
    
       o Outsider attacks i.e. active attacks from outside of a virtual
    
    
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       private network.
    
       In terms of priority, case 1 is considered the major threat in
       MPEG-2 transmission systems. Case 2 is likely to a lesser degree
       within certain network configurations, especially when there are
       insider attacks. Hence, protection against such active attacks
       should be used only when such a threat is a real possibility.
       Case 3 is envisaged to be less practical, because it will be very
       difficult to pass unnoticed by the MPEG-2 transmission operator.
       It will require restoration of the original transmission. The
       assumption being here is that physical access to the network
       components (multiplexers, etc) and/or connecting physical media
       is secure. Therefore case 3 is not considered further in this
       document.
    
    4. Security Requirements for IP over MPEG-2 TS
    
       From the threat analysis in section 3, the following security
       requirements can be derived:
    
       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.
    
    
    
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       o Support for automated as well as manual insertion of keys and
          policy into the relevant databases.
    
       o Algorithm agility is needed. Changes in crypto algorithms,
          hashes as they become obsolete should be updated without
          affecting the overall security of the system.
    
       o Traceability: To monitor transmission network using log files
          to record the activities in the network and detect any
          intrusion.
    
       o Protection against loss of service (availability) through
          malicious reconfiguration of system components (see Figure 1).
    
       o Secure Policy management
    
       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
          address.
    
       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
    
    
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          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
          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
       packets.
    
       It is possible to use IPsec to secure ULE links. The major
       advantage of IPsec is its wide implementation in IP routers and
       hosts. IPsec in transport mode can be used for end-to-end
       security transparently over MPEG-2 transmission links with little
       impact.
    
       In the context of MPEG-2 transmission links, if IPsec is used to
       secure a ULE link, then the ULE Encapsulator and Receivers are
       equivalent to the security gateways in IPsec terminology. A
       security gateway implementation of IPsec uses tunnel mode. Such
       usage has the following disadvantages:
    
       o There is an extra transmission overhead associated with using
          IPsec in tunnel mode, i.e. the extra IP header (IPv4 or IPv6).
    
    
    
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       o There is a need to protect the identity (NPA) of ULE Receivers
          over the ULE broadcast medium; IPsec is not suitable for
          providing this service. In addition, the interfaces of these
          devices do not necessarily have IP addresses (they can be L2
          devices).
    
       o Multicast is considered a major service over ULE links. The
          current IPsec specifications [RFC4301] only define a pairwise
          tunnel between two IPsec devices with manual keying. Work is
          in progress in defining the extra detail needed for multicast
          and to use the tunnel mode with address preservation to allow
          efficient multicasting. For further details refer to [WEIS06].
    
    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 Each ULE Receiver needs to decrypt all MPEG-2 TS Packets with
          a matching PID, possibly including those that are not required
          to be forwarded. Therefore it does not have the flexibility to
          separately secure individual IP flows.
    
       o ULE Receivers will have access to private traffic destined to
          other ULE Receivers, since they share a common PID and key.
    
       o IETF-based key management are not used in existing systems.
          Existing access control mechanisms have limited flexibility in
          terms of controlling the use of key and rekeying. Therefore if
          the key is compromised, then this will impact several ULE
          Receivers.
    
    
    
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       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.
    
    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
       security:
    
       o The protection of the complete ULE Protocol Data Unit (PDU)
          including IP addresses. The protection can be applied either
          per IP flow or per Receiver NPA address.
    
       o Ability to protect the identity of the Receiver within the
          MPEG-2 transmission network at the IP layer and also at L2.
    
       o Efficient protection of IP multicast over ULE links.
    
       o Transparency to the use of Network Address Translation (NATs)
          [RFC3715] and TCP Performance Enhancing Proxies (PEP)
          [RFC3135], which require the ability to inspect and modify the
          packets sent over the ULE link.
    
       This method does not preclude the use of IPsec at L3 (or TLS
       [RFC4346] at L4). IPsec and TLS provide strong authentication of
       the end-points in the communication. This authentication is
       desirable in many scenarios to ensure that the correct
       information is being exchanged between the trusted entities,
       whereas Layer 2 methods cannot provide this guarantee.
    
       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
    
    
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       the trusted parties, whereas Layer 2 methods cannot provide this
       guarantee.
    
    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
       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
                        -----------------------------------------------
                       |Payload |Counter|Source |Data   |Intru  |Iden  |
                       |Encry   |/Nonce |Authent|Integ  |sion   |tity  |
                       |ption   |       |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 Mesages    |        |       |       |       |       |      |
        ---------------------------------------------------------------
                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.
    
    
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    7. Compatibility with Generic Stream Encapsulation
    
       The [ID-EF] document describes two new Header Extensions that may
       be used with Unidirectional Link Encapsulation, ULE, [RFC4326]
       and the Generic Stream Encapsulation (GSE) that has been designed
       for the Generic Mode (also known as the Generic Stream (GS),
       offered by second-generation DVB physical layers, and
       specifically for DVB-S2 [ID-EF].
    
       The security threats and requirement presented in this document
       are applicable to ULE and GSE encapsulations. It might be
       desirable to authenticate some/all of the headers; such decision
       can be part of the security policy for the MPEG-2 transmission
       network.
    
    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.
    
    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.
    
    
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       provided by TLS [RFC4346], SSH [RFC4251], IPsec [RFC4301). A
       common objective is to provide the same level of privacy as wired
       links. It is recommended that an ISP or user provide end-to-end
       security services based on well known mechanisms such as IPsec or
       TLS.
    
       This document provides a threat analysis and derives the security
       requirements to provide link encryption and optional link-layer
       integrity / authentication of the SNDU payload.
    
       There are some security issues that were raised in RFC 4326
       [RFC4326] that are not addressed in this document (out of scope)
       such as:
    
       o The security issue with un-initialised stuffing bytes.  In
          ULE, these bytes are set to 0xFF (normal practice in MPEG-2).
    
       o Integrity issues related to the removal of the LAN FCS in a
          bridged networking environment.  The removal for bridged
          frames exposes the traffic to potentially undetected
          corruption while being processed by the Encapsulator and/or
          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.
    
    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
    
    12.1. Normative References
    
       [ISO-MPEG2] "Information technology -- generic coding of moving
    
    
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                   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.
    
       [IEEE-802]  "Local and metropolitan area networks-Specific
                   requirements Part 2: Logical Link Control", IEEE
                   802.2, IEEE Computer Society, (also ISO/IEC 8802-2),
                   1998.
    
       [ISO-8802]  ISO/IEC 8802.2, "Logical Link Control", International
                   Standards Organisation (ISO), 1998.
    
       [ITU-H222] H.222.0, "Information technology, Generic coding of
                   moving pictures and associated audio information
                   Systems", International Telecommunication Union,
                   (ITU-T), 1995.
    
       [RFC4259]  Montpetit, M.-J., Fairhurst, G., Clausen, H.,
                   Collini-Nocker, B., and H. Linder, "A Framework for
                   Transmission of IP Datagrams over MPEG-2 Networks",
                   IETF RFC 4259, November 2005.
    
       [RFC4326]  Fairhurst, G. and B. Collini-Nocker, "Unidirectional
                   Lightweight Encapsulation (ULE) for Transmission of
                   IP Datagrams over an MPEG-2 Transport Stream (TS)",
                   IETF RFC 4326, December 2005.
    
       [ETSI-DAT] EN 301 192, "Digital Video Broadcasting (DVB); DVB
                   Specifications for Data Broadcasting", European
                   Telecommunications Standards Institute (ETSI).
    
       [BELLOVIN]  S.Bellovin, "Problem Area for the IP Security
                   protocols", Computer Communications Review 2:19, pp.
                   32-48, April 989. http://www.cs.columbia.edu/~smb/
    
       [RFC4082]  A. Perrig, D. Song, " Timed Efficient Stream Loss-
                   Tolerant Authentication (TESLA): Multicast Source
                   Authentication Transform Introduction", IETF RFC
    
    
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                   4082, June 2005.
    
       [RFC4535]  H Harney, et al, "GSAKMP: Group Secure Association
                   Group Management Protocol", IETF RFc 4535, June 2006.
    
       [RFC3547]  M. Baugher, et al, "GDOI: The Group Domain of
                   Interpretation", IETF RFC 3547.
    
       [WEIS06]   Weis B., et al, "Multicast Extensions to the Security
                   Architecture for the Internet", <draft-ietf-msec-
                   ipsec-extensions-02.txt>, June 2006, IETF Work in
                   Progress.
    
       [RFC3715]  B. Aboba and W Dixson, "IPsec-Network Address
                   Translation (NAT) Compatibility Requirements" IETF
                   RFC 3715, March 2004.
    
       [RFC4346]  T. Dierks, E. Rescorla, "The Transport Layer Security
                   (TLS) Protocol Version 1.1", IETF RFC 4346, April
                   2006.
    
       [RFC3135]  J. Border, M. Kojo, eyt. al., "Performance Enhancing
                   Proxies Intended to Mitigate Link-Related
                   Degradations", IETF RFC 3135, June 2001.
    
       [RFC4301]  Kent, S. and Seo K., "Security Architecture for the
                   Internet Protocol", IETF RFC 4301, December 2006.
    
       [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
                   Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J.,
                   and L. Wood, "Advice for Internet Subnetwork
                   Designers", BCP 89, IETF RFC 3819, July 2004.
    
       [RFC4251]  T. Ylonen, C. Lonvick, Ed., "The Secure Shell (SSH)
                   Protocol Architecture", IETF RFC 4251, January 2006.
    
       [ID-EF]    G. Fairhurst, "Extension Formats for the ULE
                   Encapsulation to support the Generic Stream
                   Encapsulation (GSE)", Work in Progress < draft-ietf-
                   ipdvb-ule-ext-03.txt>.
    
    13. Author's Addresses
    
       Haitham Cruickshank
       Centre for Communications System Research (CCSR)
       University of Surrey
       Guildford, Surrey, GU2 7XH
    
    
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       UK
       Email: h.cruickshank@surrey.ac.uk
    
       Sunil Iyengar
       Centre for Communications System Research (CCSR)
       University of Surrey
       Guildford, Surrey, GU2 7XH
       UK
       Email: S.Iyengar@surrey.ac.uk
    
       Prashant Pillai
       Mobile and Satellite Communications Research Centre (MSCRC)
       School of Engineering, Design and Technology
       University of Bradford
       Richmond Road, Bradford BD7 1DP
       UK
       Email: P.Pillai@bradford.ac.uk
    
    
    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
       on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
       REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
       IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
       WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
       WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
       ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR
       FITNESS FOR A PARTICULAR PURPOSE.
    
    14.2. Intellectual Property
    
       The IETF takes no position regarding the validity or scope of any
       Intellectual Property Rights or other rights that might be
       claimed to pertain to the implementation or use of the technology
    
    
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       described in this document or the extent to which any license
       under such rights might or might not be available; nor does it
       represent that it has made any independent effort to identify any
       such rights.  Information on the procedures with respect to
       rights in RFC documents can be found in BCP 78 and BCP 79.
    
       Copies of IPR disclosures made to the IETF Secretariat and any
       assurances of licenses to be made available, or the result of an
       attempt made to obtain a general license or permission for the
       use of such proprietary rights by implementers or users of this
       specification can be obtained from the IETF on-line IPR
       repository at http://www.ietf.org/ipr.
    
       The IETF invites any interested party to bring to its attention
       any copyrights, patents or patent applications, or other
       proprietary rights that may cover technology that may be required
       to implement this standard.  Please address the information to
       the IETF at ietf-ipr@ietf.org.
    
    
    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
       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
    
    
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       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
       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
    
    
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       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. In this way all extension headers (if any) follow the
       security extension header. When applying the security services
       for example confidentiality, input to the cipher algorithm will
       cover the fields from the end of the security extension header to
       the end of the PDU.
    
            +-------+------+-------------------------------+------+
            | ULE   |SEC   |     Protocol Data Unit        |      |
            |Header |Header|                               |CRC-32|
            +-------+------+-------------------------------+------+
                   Figure 3: ULE Security Header Extension Placement
    
       A.1.3 ULE Security Databases Block
    
       There needs to be two databases i.e. similar to the IPSec
       databases.
    
       o ULE-SAD: ULE Secure Association Database contains all the
          Security Associations that are currently established with
          different ULE peers.
    
    
    
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       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 *
         Unique_ID);
       . Update_record_database (char * Database, char * record, char *
         Unique_ID);
       . Delete_record_database (char * Database, char * Unique_ID);
    
    
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       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 *
         Unique_ID);
    
    
       >>> NOTE to RFC Editor: Please remove this appendix prior to
       publication]
    
    Document History
    
    
       Working Group Draft 00
    
       o Fixed editorial mistakes and ID style for WG adoption.
    
       Working Group Draft 01
    
       o Fixed editorial mistakes and added an appendix which shows the
    
    
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          preliminary framework for securing the ULE network.
    
       Working Group Draft 02
    
       o Fixed editorial mistakes and added some important changes as
          pointed out by Knut Eckstein (ESA), Gorry Fairhurst and
          UNISAL.
    
       o Added section 4.1 on GSE. Extended the security considerations
          section.
    
       o Extended the appendix to show the extension header placement.
    
       o The definition of the header patterns for the ULE Security
          databases will be defined in a separate draft.
    
       o Need to include some words on key management transport over
          air interfaces, actually key management bootstrapping.
    
    
       Working Group Draft 03
    
       o Fixed editorial mistakes and added some important changes as
          pointed out by Gorry Fairhurst.
    
       o Table 1 added in Section 6.2 to list the different security
          techniques to mitigate the various possible network threats.
    
       o Figure 2 modified to clearly explain the different interfaces
          present in the framework.
    
       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.
    
    
    
    
    
    
    
    Cruickshank           Expires December 29, 2007           [Page 27]
    

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