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INFORMATIONAL
Errata Exist
Network Working Group                                     H. Cruickshank
Request for Comments: 5458                          University of Surrey
Category: Informational                                        P. Pillai
                                                  University of Bradford
                                                           M. Noisternig
                                                  University of Salzburg
                                                              S. Iyengar
                                                                  Logica
                                                              March 2009


                       Security Requirements for
      the Unidirectional Lightweight Encapsulation (ULE) Protocol

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The MPEG-2 standard defined by ISO 13818-1 supports a range of
   transmission methods for a variety of services.  This document
   provides a threat analysis and derives the security requirements when
   using the Transport Stream, TS, to support an Internet network-layer
   using Unidirectional Lightweight Encapsulation (ULE) defined in RFC
   4326.  The document also provides the motivation for link-layer
   security for a ULE Stream.  A ULE Stream may be used to send IPv4
   packets, IPv6 packets, and other Protocol Data Units (PDUs) to an
   arbitrarily large number of Receivers supporting unicast and/or
   multicast transmission.

   The analysis also describes applicability to the Generic Stream
   Encapsulation (GSE) defined by the Digital Video Broadcasting (DVB)
   Project.




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Table of Contents

   1. Introduction ....................................................3
   2. Requirements Notation ...........................................4
   3. Threat Analysis .................................................7
      3.1. System Components ..........................................7
      3.2. Threats ....................................................9
      3.3. Threat Cases ..............................................10
   4. Security Requirements for IP over MPEG-2 TS ....................11
   5. Design Recommendations for ULE Security Extension Header .......14
   6. Compatibility with Generic Stream Encapsulation ................15
   7. Summary ........................................................15
   8. Security Considerations ........................................15
   9. Acknowledgments ................................................16
   10. References ....................................................16
      10.1. Normative References .....................................16
      10.2. Informative References ...................................17
   Appendix A. ULE Security Framework ................................19
      A.1. Building Block ............................................19
      A.2. Interface Definition ......................................22
   Appendix B. Motivation for ULE Link-Layer Security ................23
      B.1. Security at the IP Layer (Using IPsec) ....................23
      B.2. Link Security below the Encapsulation Layer ...............24
      B.3. Link Security as a Part of the Encapsulation Layer ........25



























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1.  Introduction

   The MPEG-2 Transport Stream (TS) has been widely accepted not only
   for providing digital TV services, but also as a subnetwork
   technology for building IP networks.  RFC 4326 [RFC4326] describes
   the Unidirectional Lightweight Encapsulation (ULE) mechanism for the
   transport of IPv4 and IPv6 Datagrams and other network protocol
   packets directly over the ISO MPEG-2 Transport Stream as TS Private
   Data.  ULE specifies a base encapsulation format and supports an
   Extension Header format that allows it to carry additional header
   information to assist in network/Receiver processing.  The
   encapsulation satisfies the design and architectural requirement for
   a lightweight encapsulation defined in RFC 4259 [RFC4259].

   Section 3.1 of RFC 4259 presents several topological scenarios for
   MPEG-2 Transmission Networks.  A summary of these scenarios is
   presented below:

   A. Broadcast TV and Radio Delivery.  This is not within the scope of
      this document.

   B. Broadcast Networks used as an ISP.  This resembles scenario A, but
      includes IP services to access the public Internet.

   C. Unidirectional Star IP Scenario.  This provides a data network
      delivering a common bit stream to typically medium-sized groups of
      Receivers.

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

   E. Point-to-Point Links.  This connectivity may be provided using a
      pair of transmit and receive interfaces.

   F. Two-Way IP Networks.

   RFC 4259 states that ULE must be robust to errors and security
   threats.  Security must also consider both unidirectional (A, B, C,
   and D) as well as bidirectional (E and F) links for the scenarios
   mentioned above.

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



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

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

   A security framework for deployment of secure ULE networks describing
   the different building blocks and the interface definitions is
   presented in Appendix A.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Other terms used in this document are defined below:

   ATSC: Advanced Television Systems Committee.  A framework and a set
   of associated standards for the transmission of video, audio, and
   data using the ISO MPEG-2 Standard.

   DVB: Digital Video Broadcast.  A framework and set of associated
   standards published by the European Telecommunications Standards
   Institute (ETSI) for the transmission of video, audio, and data using
   the ISO MPEG-2 Standard [ISO-MPEG2].

   Encapsulator: A network device that receives Protocol Data Units
   (PDUs) and formats these into Payload Units (known here as SNDUs) for
   output as a stream of TS Packets.

   GCKS: Group Controller and Key Server.  A server that authenticates
   and provides the policy and keying material to members of a secure
   group.

   LLC: Logical Link Control [ISO-8802], [IEEE-802].  A link-layer
   protocol defined by the IEEE 802 standard, which follows the Ethernet
   Medium Access Control Header.

   MAC: Message Authentication Code.





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   MPE: Multiprotocol Encapsulation [ETSI-DAT].  A scheme that
   encapsulates PDUs, forming a Digital Storage Media Command and
   Control (DSM-CC) Table Section.  Each Section is sent in a series of
   TS Packets using a single TS Logical Channel.

   MPEG-2: A set of standards specified by the Motion Picture Experts
   Group (MPEG) and standardised by the International Standards
   Organisation (ISO/IEC 13818-1) [ISO-MPEG2], and ITU-T (in H.222
   [ITU-H222]).

   NPA: Network Point of Attachment.  In this document, refers to a
   6-byte destination address (resembling an IEEE Medium Access Control
   address) within the MPEG-2 transmission network that is used to
   identify individual Receivers or groups of Receivers.

   PDU: Protocol Data Unit.  Examples of a PDU include Ethernet frames,
   IPv4 or IPv6 Datagrams, and other network packets.

   PID: Packet Identifier [ISO-MPEG2].  A 13-bit field carried in the
   header of TS Packets.  This is used to identify the TS Logical
   Channel to which a TS Packet belongs [ISO-MPEG2].  The TS Packets
   forming the parts of a Table Section, Packetised Elementary Stream
   (PES), or other Payload Unit must all carry the same PID value.  The
   all-zeros PID 0x0000 as well as other PID values are reserved for
   specific PSI/SI Tables [ISO-MPEG2].  The all-ones PID value 0x1FFF
   indicates a Null TS Packet introduced to maintain a constant bit rate
   of a TS Multiplex.  There is no required relationship between the PID
   values used for TS Logical Channels transmitted using different TS
   Multiplexes.

   Receiver: Equipment that processes the signal from a TS Multiplex and
   performs filtering and forwarding of encapsulated PDUs to the
   network-layer service (or bridging module when operating at the link
   layer).

   SI Table: Service Information Table [ISO-MPEG2].  In this document,
   this term describes a table that is defined by another standards body
   to convey information about the services carried in a TS Multiplex.
   A Table may consist of one or more Table Sections; however, all
   sections of a particular SI Table must be carried over a single TS
   Logical Channel [ISO-MPEG2].

   SNDU: SubNetwork Data Unit.  An encapsulated PDU sent as an MPEG-2
   Payload Unit.

   TS: Transport Stream [ISO-MPEG2].  A method of transmission at the
   MPEG-2 layer using TS Packets; it represents Layer 2 of the ISO/OSI
   reference model.  See also TS Logical Channel and TS Multiplex.



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   TS Multiplex: In this document, this term defines a set of MPEG-2 TS
   Logical Channels sent over a single lower-layer connection.  This may
   be a common physical link (i.e., a transmission at a specified symbol
   rate, Forward Error Correction (FEC) setting, and transmission
   frequency) or an encapsulation provided by another protocol layer
   (e.g., Ethernet, or RTP over IP).  The same TS Logical Channel may be
   repeated over more than one TS Multiplex (possibly associated with a
   different PID value) [RFC4259]; for example, to redistribute the same
   multicast content to two terrestrial TV transmission cells.

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

   ULE Stream: An MPEG-2 TS Logical Channel that carries only ULE
   encapsulated PDUs.  ULE Streams may be identified by definition of a
   stream_type in SI/PSI [ISO-MPEG2].
































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3.  Threat Analysis

3.1.  System Components

     +------------+                                  +------------+
     |  IP        |                                  |  IP        |
     |  End Host  |                                  |  End Host  |
     +-----+------+                                  +------------+
           |                                                ^
           +------------>+---------------+                  |
                         +  ULE          |                  |
           +-------------+  Encapsulator |                  |
   SI-Data |             +------+--------+                  |
   +-------+-------+            |MPEG-2 TS Logical Channel  |
   |  MPEG-2       |            |                           |
   |  SI Tables    |            |                           |
   +-------+-------+   ->+------+--------+                  |
           |          -->|  MPEG-2       |                . . .
           +------------>+  Multiplexer  |                  |
   MPEG-2 TS             +------+--------+                  |
   Logical Channel              |MPEG-2 TS Mux              |
                                |                           |
              Other    ->+------+--------+                  |
              MPEG-2  -->+  MPEG-2       |                  |
              TS     --->+  Multiplexer  |                  |
                    ---->+------+--------+                  |
                                |MPEG-2 TS Mux              |
                                |                           |
                         +------+--------+           +------+-----+
                         |Physical Layer |           |  MPEG-2    |
                         |Modulator      +---------->+  Receiver  |
                         +---------------+  MPEG-2   +------------+
                                            TS Mux

     Figure 1: An example configuration for a unidirectional service
          for IP transport over MPEG-2 (adapted from [RFC4259])

   As shown in Figure 1 above (from Section 3.3 of [RFC4259]), there are
   several entities within the MPEG-2 transmission network architecture.
   These include:

   o ULE Encapsulation Gateways (the ULE Encapsulator)

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

   o Receivers (the endpoints for ULE Streams)

   o TS multiplexers (including re-multiplexers)



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   o Modulators

   The TS Packets are carried to the Receiver over a physical layer that
   usually includes Forward Error Correction (FEC) coding that
   interleaves the bytes of several consecutive, but unrelated, TS
   Packets.  FEC-coding and synchronisation processing makes injection
   of single TS Packets very difficult.  Replacement of a sequence of
   packets is also difficult, but possible (see Section 3.2).

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

   In an MPEG-2 network, a set of signalling messages [RFC4947] may need
   to be broadcast (e.g., by an Encapsulation Gateway or other device)
   to form the L2 control plane.  Examples of signalling messages
   include the Program Association Table (PAT), Program Map Table (PMT),
   and Network Information Table (NIT).  In existing MPEG-2 transmission
   networks, these messages are broadcast in the clear (no encryption or
   integrity checks).  The integrity as well as authenticity of these
   messages is important for correct working of the ULE network, i.e.,
   supporting its security objectives in the area of availability, in
   addition to confidentiality and integrity.  One method recently
   proposed [RFC5163] encapsulates these messages using ULE.  In such
   cases all the security requirements of this document apply in
   securing these signalling messages.

   ULE Stream security only concerns the security between the ULE
   Encapsulation Gateway (ULE Encapsulator) and the Receiver.  In many
   deployment scenarios the user of a ULE Stream has to secure
   communications beyond the link since other network links are utilised
   in addition to the ULE link.  Therefore, if authentication of the
   endpoints, i.e., the IP Sources, is required, or users are concerned



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   about loss of confidentiality, integrity, or authenticity of their
   communication data, they will have to employ end-to-end network
   security mechanisms, e.g., IPsec or Transport Layer Security (TLS).
   Governmental users may be forced by regulations to employ specific
   approved implementations of those mechanisms.  Hence, for such cases,
   the requirements for confidentiality and integrity of the user data
   will be met by the end-to-end security mechanism and the ULE security
   measures would focus on providing traffic flow confidentiality either
   for user data that has already been encrypted or for users who choose
   not to implement end-to-end security mechanisms.

   ULE links may also be used for communications where the two IP
   endpoints are not under central control (e.g., when browsing a public
   web site).  In these cases, it may be impossible to enforce any end-
   to-end security mechanisms.  Yet, a common objective is that users
   may make the same security assumptions as for wired links [RFC3819].
   ULE security could achieve this by protecting the vulnerable (in
   terms of passive attacks) ULE Stream.

   In contrast to the above, a ULE Stream can be used to link networks
   such as branch offices to a central office.  ULE link-layer security
   could be the sole provider of confidentiality and integrity.  In this
   scenario, users requiring high assurance of security (e.g.,
   government use) will need to employ approved cryptographic equipment
   (e.g., at the network layer).  An implementation of ULE Link Security
   equipment could also be certified for use by specific user
   communities.

3.2.  Threats

   The simplest type of network threat is a passive threat.  This
   includes eavesdropping or monitoring of transmissions, with a goal to
   obtain information that is being transmitted.  In broadcast networks
   (especially those utilising widely available low-cost physical layer
   interfaces, such as DVB), the passive threats are the major threats.
   One example is an intruder monitoring the MPEG-2 transmission
   broadcast and then extracting the data carried within the link.
   Another example is an intruder trying to determine the identity of
   the communicating parties and the volume of their traffic by sniffing
   (L2) addresses.  This is a well-known issue in the security field;
   however, it is more of a problem in the case of broadcast networks
   such as MPEG-2 transmission networks because of the easy availability
   of Receiver hardware and the wide geographical span of the networks.








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

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

   o Modification of messages in an unauthorised manner.

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

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

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

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

3.3.  Threat Cases

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

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



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

   o Case 3: Globally conducting active attacks on the MPEG-TS
     multiplex.  This assumes a sophisticated intruder able to override
     the whole MPEG-2 transmission multiplex.  The requirements are
     similar to case 2.  The MPEG-2 transmission network operator can
     usually identify such attacks and provide corrective action to
     restore the original transmission.

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

   o Insider attacks, i.e., active attacks from adversaries within the
     network with knowledge of the secret material.

   o Outsider attacks, i.e., active attacks from adversaries without
     knowledge of the secret material.

   In terms of priority, Case 1 is considered the major threat in MPEG-2
   transmission systems.  Case 2 is considered a lesser threat,
   appropriate to specific network configurations, especially when
   vulnerable to insider attacks.  Case 3 is less likely to be found in
   an operational network, and is expected to be noticed by the MPEG-2
   transmission operator.  It will require restoration of the original
   transmission.  The assumption being that physical access to the
   network components (multiplexers, etc.) and/or connecting physical
   media is secure.  Therefore, Case 3 is not considered further in this
   document.

4.  Security Requirements for IP over MPEG-2 TS

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

   Req 1. Data confidentiality MUST be provided by a link that supports
      ULE Stream Security to prevent passive attacks and reduce the risk
      of active threats.







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   Req 2. Protection of L2 NPA address is OPTIONAL.  In broadcast
      networks, this protection can be used to prevent an intruder
      tracking the identity of ULE Receivers and the volume of their
      traffic.

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

   Req 4. Protection against replay attacks is OPTIONAL.  This is used
      to counter the active attacks described in Section 3.2.

   Req 5. L2 ULE Source and Receiver authentication is OPTIONAL.  This
      can be performed during the initial key exchange and
      authentication phase, before the ULE Receiver can join a secure
      session with the ULE Encapsulator (ULE source).  This could be
      either unidirectional or bidirectional authentication based on the
      underlying key management protocol.

   Other general requirements for all threat cases for link-layer
   security are:

   GReq (a) ULE key management functions MUST be decoupled from ULE
      security services such as encryption and source authentication.
      This allows the independent development of both systems.

   GReq (b) Support SHOULD be provided for automated as well as manual
      insertion of keys and policy into the relevant databases.

   GReq (c) Algorithm agility MUST be supported.  It should be possible
      to update the crypto algorithms and hashes when they become
      obsolete without affecting the overall security of the system.

   GReq (d) The security extension header MUST be compatible with other
      ULE extension headers.  The method must allow other extension
      headers (either mandatory or optional) to be used in combination
      with a security extension.  It is RECOMMENDED that these are
      placed after the security extension header.  This permits full
      protection for all headers.  It also avoids situations where the
      SNDU has to be discarded on processing the security extension
      header, while preceding headers have already been evaluated.  One
      exception is the Timestamp extension that SHOULD precede the
      security extension header [RFC5163].  In this case, the timestamp
      will be unaffected by security services such as data
      confidentiality and can be decoded without the need for key
      material.





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

   o Case 1: Data confidentiality (Req 1) MUST be provided to prevent
     monitoring of the ULE data (such as user information and IP
     addresses).  Protection of NPA addresses (Req 2) MAY be provided to
     prevent tracking ULE Receivers and their communications.

   o Case 2: In addition to Case 1 requirements, new measures MAY be
     implemented such as authentication schemes using Message
     Authentication Codes, digital signatures, or Timed Efficient Stream
     Loss-Tolerant Authentication (TESLA) [RFC4082] in order to provide
     integrity protection and source authentication (Reqs 3 and 5).  In
     addition, sequence numbers (Req 4) MAY be used to protect against
     replay attacks.  In terms of outsider attacks, group authentication
     using Message Authentication Codes can provide the required level
     of security (Reqs 3 and 5).  This will significantly reduce the
     ability of intruders to successfully inject their own data into the
     MPEG-TS stream.  However, scenario 2 threats apply only in specific
     service cases, and therefore authentication and protection against
     replay attacks are OPTIONAL.  Such measures incur additional
     transmission as well as processing overheads.  Moreover, intrusion
     detection systems may also be needed by the MPEG-2 network
     operator.  These should best be coupled with perimeter security
     policy to monitor common DoS attacks.

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

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


















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                                   Security Mechanism
                    -----------------------------------------------
                   |Data    |Data   |Source |Data   |Intru  |Iden  |
                   |Privacy |fresh  |Authent|Integ  |sion   |tity  |
                   |        |ness   |ication|rity   |Dete   |Prote |
                   |        |       |       |       |ction  |ction |
     Threat        |        |       |       |       |       |      |
    ---------------|--------|-------|-------|-------|-------|------|
   | Monitoring    |   X    |   -   |   -   |   -   |   -   |  X   |
   |---------------------------------------------------------------|
   | Masquerading  |   X    |   -   |   X   |   X   |   -   |  X   |
   |---------------------------------------------------------------|
   | Replay Attacks|   -    |   X   |   X   |   X   |   X   |  -   |
   |---------------------------------------------------------------|
   | DoS Attacks   |   -    |   X   |   X   |   X   |   X   |  -   |
   |---------------------------------------------------------------|
   | Modification  |   -    |   -   |   X   |   X   |   X   |  -   |
   | of Messages   |        |       |       |       |       |      |
    ---------------------------------------------------------------

         Table 1: Security techniques to mitigate network threats
                             in ULE Networks

5.  Design Recommendations for ULE Security Extension Header

   Table 1 may assist in selecting fields within a ULE Security
   Extension Header framework.

   Security services may be grouped into profiles based on security
   requirements, e.g., a base profile (with payload encryption and
   identity protection) and a second profile that extends this to also
   provide source authentication and protection against replay attacks.
   Although the use of specific security techniques is optional, it is
   RECOMMENDED that receiver devices should implement all the techniques
   in Reqs 2-5 of Section 4 to ensure interoperability of all profiles.

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








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   IPsec [RFC4301] and TLS [RFC5246] also provide a proven security
   architecture defining key exchange mechanisms and the ability to use
   a range of cryptographic algorithms.  ULE security can make use of
   these established mechanisms and algorithms.  See Appendix A for more
   details.

6.  Compatibility with Generic Stream Encapsulation

   RFC 5163 [RFC5163] describes three new Extension Headers that may be
   used with Unidirectional Link Encapsulation, ULE, [RFC4326] and the
   Generic Stream Encapsulation (GSE) that has been designed for the
   Generic Mode (also known as the Generic Stream (GS)), offered by
   second-generation DVB physical layers [GSE].

   The security threats and requirements presented in this document are
   applicable to ULE and GSE encapsulations.

7.  Summary

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

   ULE security must provide link-layer encryption and ULE Receiver
   identity protection.  The framework must support the optional ability
   to provide for link-layer authentication and integrity assurance, as
   well as protection against insertion of old (duplicated) data into
   the ULE Stream (i.e., replay protection).  This set of features is
   optional to reduce encapsulation overhead when not required.

   ULE Stream security between a ULE Encapsulation Gateway and the
   corresponding Receiver(s) is considered an additional security
   mechanism to IPsec, TLS, and application layer end-to-end security,
   and not as a replacement.  It allows a network operator to provide
   similar functions to that of IPsec, but in addition provides MPEG-2
   transmission link confidentiality and protection of ULE Receiver
   identity (NPA address).

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

8.  Security Considerations

   Link-layer (L2) encryption of IP traffic is commonly used in
   broadcast/radio links to supplement end-to-end security (e.g.,
   provided by TLS [RFC5246], SSH [RFC4251], IPsec [RFC4301]).





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   A common objective is to provide the same level of privacy as wired
   links.  It is recommended that an ISP or user provide end-to-end
   security services based on well-known mechanisms such as IPsec or
   TLS.

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

   There are some security issues that were raised in RFC 4326 [RFC4326]
   that are not addressed in this document (i.e., are out of scope),
   e.g.:

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

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

   o There is a potential security issue when a Receiver receives a PDU
     with two Length fields.  The Receiver would need to validate the
     actual length and the Length field and ensure that inconsistent
     values are not propagated by the network.

9.  Acknowledgments

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

10.  References

10.1.  Normative References

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

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







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

10.2.  Informative References

   [BELLOVIN]  S. Bellovin, "Security Problems in the TCP/IP Protocol
               Suite", Computer Communications Review 2:19, pp. 32-48,
               April 1989.  http://www.cs.columbia.edu/~smb/

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

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

   [IEEE-802]  "Local and metropolitan area networks-Specific
               requirements Part 2: Logical Link Control", IEEE 802.2,
               IEEE Computer Society, (also ISO/IEC 8802-2), 1998.

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

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

   [RFC3135]   Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
               Shelby, "Performance Enhancing Proxies Intended to
               Mitigate Link-Related Degradations", RFC 3135, June 2001.

   [RFC3547]   Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
               Group Domain of Interpretation", RFC 3547, July 2003.

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

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





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   [RFC4082]   Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
               Briscoe, "Timed Efficient Stream Loss-Tolerant
               Authentication (TESLA): Multicast Source Authentication
               Transform Introduction", RFC 4082, June 2005.

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

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

   [RFC4301]   Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.

   [RFC4535]   Harney, H., Meth, U., Colegrove, A., and G. Gross,
               "GSAKMP: Group Secure Association Key Management
               Protocol", RFC 4535, June 2006.

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

   [RFC5163]   Fairhurst, G. and B. Collini-Nocker, "Extension Formats
               for Unidirectional Lightweight Encapsulation (ULE) and
               the Generic Stream Encapsulation (GSE)", RFC 5163, April
               2008.

   [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5374]   Weis, B., Gross, G., and D. Ignjatic, "Multicast
               Extensions to the Security Architecture for the Internet
               Protocol", RFC 5374, November 2008.
















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Appendix A.  ULE Security Framework

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

A.1.  Building Blocks

   This ULE Security framework describes the following building blocks
   as shown in Figure 2 below:

   o The Key Management Block

   o The ULE Security Extension Header Block

   o The ULE Databases Block

   Within the Key Management Block, the communication between the Group
   Member entity and the Group Server entity happens in the control
   plane.  The ULE Security Header Block applies security to the ULE
   SNDU and this happens in the ULE data plane.  The ULE Security
   Databases Block acts as the interface between the Key Management
   Block (control plane) and the ULE Security Header Block (ULE data
   plane) as shown in Figure 2.  The Security Databases Block exists in
   both the group member and server sides.  However, it has been omitted
   from Figure 2 just for clarity.


























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

              Figure 2: Secure ULE Framework Building Blocks

A.1.1.  Key Management Block

   A key management framework is required to provide security at the ULE
   level using extension headers.  This key management framework is
   responsible for user authentication, access control, and Security
   Association negotiation (which include the negotiations of the
   security algorithms to be used and the generation of the different
   session keys as well as policy material).  The key management
   framework can be either automated or manual.  Hence, this key
   management client entity (shown as the Key Management Group Member
   Block in Figure 2) will be present in all ULE Receivers as well as at
   the ULE Encapsulators.  The ULE Encapsulator could also be the Key
   Management Group Server Entity (shown as the Key Management Group
   Server Block in Figure 2).




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   This happens when the ULE Encapsulator also acts as the Key
   Management Group Server.  Deployment may use either automated key
   management protocols (e.g., GSAKMP [RFC4535]) or manual insertion of
   keying material.

A.1.2.  ULE Security Databases Block

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

   o ULE-SAD: ULE Security Association Database contains all the
     Security Associations that are currently established with different
     ULE peers.

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

   While traditionally link-layer security has operated using simple
   policy mechanisms, it is envisaged that ULE security should provide
   flexibility comparable to IPsec.  The above design is based on the
   two databases defined for IPsec [RFC4301].  These databases could be
   used to implement either simple policies (as in traditional link
   security services) or more complex policies (as in IPsec).

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

A.1.3.  ULE Extension Header Block

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

   This block will use the keying material and policy information from
   the ULE Security Database Block on the ULE payload to generate the
   secure ULE Extension Header or to decipher the secure ULE extension
   header to get the ULE payload.  An example overview of the ULE
   Security extension header format along with the ULE header and
   payload is shown in Figure 3 below.





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   +-------+------+-------------------------------+------+
   | ULE   |SEC   |     Protocol Data Unit        |      |
   |Header |Header|                               |CRC-32|
   +-------+------+-------------------------------+------+

            Figure 3: ULE Security Extension Header Placement

A.2.  Interface Definition

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

   o Key Management Block <-> ULE Security Databases Block

   o ULE Security Databases Block <-> ULE Security Header Block

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

A.2.1.  Key Management <-> ULE Security Databases

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

   Examples of interface functions are:

   o Insert_record_database (char * Database, char * record, char *
     Unique_ID);

   o Update_record_database (char * Database, char * record, char *
     Unique_ID);

   o Delete_record_database (char * Database, char * Unique_ID);

   The definitions of the variables are as follows:

   o Database - This is a pointer to the ULE Security databases




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

   o Unique_ID - This is the primary key to look up records (rows of
     security attributes) in the above databases

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

   This interface is between the ULE Security Database and the ULE
   Security Extension Header Block as shown in Figure 2.  When sending
   traffic, the ULE encapsulator uses the Destination Address, the PID,
   and possibly other information such as L3 source and destination
   addresses to locate the relevant security record within the ULE
   Security Database.  It then uses the data in the record to create the
   ULE security extension header.  For received traffic, the ULE
   decapsulator on receiving the ULE SNDU will use the Destination
   Address, the PID, and a ULE Security ID inserted by the ULE
   encapsulator into the security extension to retrieve the relevant
   record from the Security Database.  It then uses this information to
   decrypt the ULE extension header.  For both cases (either send or
   receive traffic) only one interface is needed since the main
   difference between the sender and receiver is the direction of the
   flow of traffic.  An example of such an interface is as follows:

   o Get_record_database (char * Database, char * record, char *
     Unique_ID);

Appendix B.  Motivation for ULE Link-Layer Security

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

B.1.  Security at the IP Layer (Using IPsec)

   The security architecture for the Internet Protocol [RFC4301]
   describes security services for traffic at the IP layer.  This
   architecture primarily defines services for the Internet Protocol
   (IP) unicast packets, as well as manually configured IP multicast
   packets.

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





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

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

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

   o Multicast is considered a major service over ULE links.  The
     current IPsec specifications [RFC4301] only define a pairwise
     tunnel between two IPsec devices with manual keying.  Work is in
     progress in defining the extra detail needed for multicast and to
     use the tunnel mode with address preservation to allow efficient
     multicasting.  For further details refer to [RFC5374].

B.2.  Link Security below the Encapsulation Layer

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

   This has the following advantages:

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

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

   However it has the following disadvantages:

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





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

   o IETF-based key management that is very flexible and secure is not
     used in existing MPEG-2 based systems.  Existing access control
     mechanisms in such systems have limited flexibility in terms of
     controlling the use of keying and rekeying.  Therefore, if the key
     is compromised, this will impact several ULE Receivers.

   Currently, there are few deployed L2 security systems for MPEG-2
   transmission networks.  Conditional access for digital TV
   broadcasting is one example.  However, this approach is optimised for
   TV services and is not well-suited to IP packet transmission.  Some
   other systems are specified in standards such as MPE [ETSI-DAT], but
   there are currently no known implementations and these methods are
   not applicable to GSE.

B.3.  Link Security as a Part of the Encapsulation Layer

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

   There are several advantages in using ULE link-layer security:

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

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

   o Efficient protection of IP multicast over ULE links.

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

   This method does not preclude the use of IPsec at L3 (or TLS
   [RFC5246] at L4).  IPsec and TLS provide strong authentication of the
   endpoints in the communication.








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

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

Authors' Addresses

   Haitham Cruickshank
   Centre for Communications System Research (CCSR)
   University of Surrey
   Guildford, Surrey, GU2 7XH
   UK
   EMail: h.cruickshank@surrey.ac.uk

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

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

   Sunil Iyengar
   Space & Defence
   Logica
   Springfield Drive
   Leatherhead
   Surrey KT22 7LP
   UK
   EMail: sunil.iyengar@logica.com






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