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Network Working Group                                     H. CruickshankRequest for Comments: 5458                          University of SurreyCategory: Informational                                        P. Pillai                                                  University of Bradford                                                           M. Noisternig                                                  University of Salzburg                                                              S. Iyengar                                                                  Logica                                                              March 2009Security Requirements forthe Unidirectional Lightweight Encapsulation (ULE) ProtocolStatus 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 toBCP 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 inRFC4326.  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.Cruickshank, et al.          Informational                      [Page 1]

RFC 5458             Security Requirements for ULE            March 2009Table of Contents1. Introduction ....................................................32. Requirements Notation ...........................................43. Threat Analysis .................................................73.1. System Components ..........................................73.2. Threats ....................................................93.3. Threat Cases ..............................................104. Security Requirements for IP over MPEG-2 TS ....................115. Design Recommendations for ULE Security Extension Header .......146. Compatibility with Generic Stream Encapsulation ................157. Summary ........................................................158. Security Considerations ........................................159. Acknowledgments ................................................1610. References ....................................................1610.1. Normative References .....................................1610.2. Informative References ...................................17Appendix A. ULE Security Framework ................................19A.1. Building Block ............................................19A.2. Interface Definition ......................................22Appendix B. Motivation for ULE Link-Layer Security ................23B.1. Security at the IP Layer (Using IPsec) ....................23B.2. Link Security below the Encapsulation Layer ...............24B.3. Link Security as a Part of the Encapsulation Layer ........25Cruickshank, et al.          Informational                      [Page 2]

RFC 5458             Security Requirements for ULE            March 20091.  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 inRFC 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 ofRFC 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].Cruickshank, et al.          Informational                      [Page 3]

RFC 5458             Security Requirements for ULE            March 2009   The security considerations ofRFC 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 inAppendix 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 inRFC 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.Cruickshank, et al.          Informational                      [Page 4]

RFC 5458             Security Requirements for ULE            March 2009   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.Cruickshank, et al.          Informational                      [Page 5]

RFC 5458             Security Requirements for ULE            March 2009   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].Cruickshank, et al.          Informational                      [Page 6]

RFC 5458             Security Requirements for ULE            March 20093.  Threat Analysis3.1.  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 (fromSection 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)Cruickshank, et al.          Informational                      [Page 7]

RFC 5458             Security Requirements for ULE            March 2009   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 (seeSection 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 concernedCruickshank, et al.          Informational                      [Page 8]

RFC 5458             Security Requirements for ULE            March 2009   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.Cruickshank, et al.          Informational                      [Page 9]

RFC 5458             Security Requirements for ULE            March 2009   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 inSection 4   below.   As explained inSection 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   inSection 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.Cruickshank, et al.          Informational                     [Page 10]

RFC 5458             Security Requirements for ULE            March 2009   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 inSection 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.Cruickshank, et al.          Informational                     [Page 11]

RFC 5458             Security Requirements for ULE            March 2009   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 inSection 3.2.   Req 4. Protection against replay attacks is OPTIONAL.  This is used      to counter the active attacks described inSection 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.Cruickshank, et al.          Informational                     [Page 12]

RFC 5458             Security Requirements for ULE            March 2009   Examining the threat cases inSection 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 inSection 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.Cruickshank, et al.          Informational                     [Page 13]

RFC 5458             Security Requirements for ULE            March 2009                                   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 Networks5.  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 ofSection 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.Cruickshank, et al.          Informational                     [Page 14]

RFC 5458             Security Requirements for ULE            March 2009   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.  SeeAppendix A for more   details.6.  Compatibility with Generic Stream EncapsulationRFC 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]).Cruickshank, et al.          Informational                     [Page 15]

RFC 5458             Security Requirements for ULE            March 2009   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 inRFC 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.  References10.1.  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.Cruickshank, et al.          Informational                     [Page 16]

RFC 5458             Security Requirements for ULE            March 2009   [RFC4326]   Fairhurst, G. and B. Collini-Nocker, "Unidirectional               Lightweight Encapsulation (ULE) for Transmission of IP               Datagrams over an MPEG-2 Transport Stream (TS)",RFC4326, 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",BCP89,RFC 3819, July 2004.Cruickshank, et al.          Informational                     [Page 17]

RFC 5458             Security Requirements for ULE            March 2009   [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",RFC4947, 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.Cruickshank, et al.          Informational                     [Page 18]

RFC 5458             Security Requirements for ULE            March 2009Appendix 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.Cruickshank, et al.          Informational                     [Page 19]

RFC 5458             Security Requirements for ULE            March 2009                                                              -----    +------+----------+           +----------------+           / \    | 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 BlocksA.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).Cruickshank, et al.          Informational                     [Page 20]

RFC 5458             Security Requirements for ULE            March 2009   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 inSection 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.Cruickshank, et al.          Informational                     [Page 21]

RFC 5458             Security Requirements for ULE            March 2009   +-------+------+-------------------------------+------+   | ULE   |SEC   |     Protocol Data Unit        |      |   |Header |Header|                               |CRC-32|   +-------+------+-------------------------------+------+            Figure 3: ULE Security Extension Header PlacementA.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 databasesCruickshank, et al.          Informational                     [Page 22]

RFC 5458             Security Requirements for ULE            March 2009   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 databasesA.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   Sections3 and4 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.Cruickshank, et al.          Informational                     [Page 23]

RFC 5458             Security Requirements for ULE            March 2009   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.Cruickshank, et al.          Informational                     [Page 24]

RFC 5458             Security Requirements for ULE            March 2009   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 inSection 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.Cruickshank, et al.          Informational                     [Page 25]

RFC 5458             Security Requirements for ULE            March 2009   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.comCruickshank, et al.          Informational                     [Page 26]