rfc1677.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   information).  Because of the limited capacity of tactical RF
   networks, resource reservation is extremely important to control
   access to these valuable resources.  Resource reservation can play a
   role in "congestion avoidance" for these limited resources as well as
   ensuring that quality-of-service data delivery requirements are met
   for multi-media communication.

   Note there is more required here than can be met by simple quality-
   of-service (QoS) based path selection and subsequent source-routing
   to get real-time data such as voice delivered.  For example, to
   support digital voice in the CSNI project, a call setup and resource
   reservation protocol was designed.  It was determined that the QoS
   mechanisms provided by the CLNP specification were not sufficient for
   our voice application path selection.  Voice calls could not be
   routed and resources reserved based on any single QoS parameter
   (e.g., delay, capacity, etc.) alone.  Some RF subnets in the CSNI
   test bed simply did not have the capability to support voice calls.
   To perform resource reservation for the voice calls, the CLNP cost
   metric was "hijacked" as essentially a Type of Service identifier to
   let the router know which datagrams were associated with a voice
   call.  The cost metric, concatenated with the source and destination
   addresses were used to form a unique identifier for voice calls in
   the router and subnet state tables.  Voice call paths were to be
   selected by the router (i.e. the "cost" metric was calculated) as a



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RFC 1677             IPng Tactical RF Requirements           August 1994


   rule-based function of each subnet's capability to support voice, its
   delay, and its capacity.  While source routing provided a possible
   means for voice datagrams to find their way from router to router,
   the network address alone was not explicit enough to direct the data
   to the correct interface, particularly in cases where there were
   multiple communication media interconnecting two routers along the
   path.  Fortunately, exclusive use of the cost QoS indicator for voice
   in CSNI was able to serve as a flag to the router for packets
   requiring special handling.

   While a simple Type of Service field as part of an IPng protocol can
   serve this purpose where there are a limited number of well known
   services (CSNI has a single special service - 2400 bps digital
   voice), a more general technique such as RSVP's Flow Specification
   can support a larger set of such services.  And a field, such as the
   one sometimes referred to as a Flow Identification (Flow ID), can
   play an important role in facilitating inter-networked data
   communication over these limited capacity networks.

   For example, the D/V ATD RF sub-network provides support for both
   connectionless datagram delivery and virtual circuit connectivity.
   To utilize this capability, an IPng could establish a virtual circuit
   connection across this RF subnetwork which meets the requirements of
   an RSVP Flow Specification. By creating an association between a
   particular Flow ID and the subnetwork header identifying the
   established virtual circuit, an IPng gateway could forward data
   across the low-capacity while removing most, if not all, of the IPng
   packet header information.  The receiving gateway could re- construct
   these fields based on the Flow Specification of the particular Flow
   ID/virtual circuit association.

   In summary, a field such as a Flow Identification can serve at least
   two important purposes:

         1)      It can be used by routers (or gateways) to identify
                 packets with special, or pre-arranged delivery
                 requirements.  It is important to realize that it may
                 not always be possible to "peek" at internet packet
                 content for this information if certain security
                 considerations are met (e.g., an encrypted transport
                 layer).

         2)      It can aid mapping datagram services to different
                 types of communication services provided by
                 specialized subnet/data link layer protocols.






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RFC 1677             IPng Tactical RF Requirements           August 1994


Multicast

   Tactical military communication has a very clear requirement for
   multicast.  Efficient dissemination of information to distributed
   warfighting participants can be the key to success in a battle.  In
   modern warfare, this information includes imagery, the "tactical
   scene" via tactical data messages, messaging information, and real-
   time interactive applications such as digital secure voice.  Many of
   the tactical RF communication media are broadcast by nature, and
   multicast routing can take advantage of this topology to distribute
   critical data to a large number of participants.  The throughput
   limitations imposed by these RF media and the physics of potential
   electronic counter measures (ECM) dictate that this information be
   distributed efficiently.  A multicast architecture is the general
   case for information flow in a tactical internetwork.

Quality of Service and Policy-Based Routing

   Quality of service and policy based routing are of particular
   importance in a tactical environment with limited communication
   resources, limited bandwidth, and possible degradation and/or denial
   of service.  Priority is a very important criteria in the tactical
   setting.  In the tactical RF world of limited resources (limited
   bandwidth, radio assets, etc.) there will be instances when there is
   not sufficient capacity to provide all users with their perception of
   required communication capability.  It is extremely important for a
   shared, automated communication system to delegate capacity higher
   priority users.  Unlike the commercial world, where everyone has a
   more equal footing, it is possible in the military environment to
   assign priority to users or even individual datagrams.  An example of
   this is the tactical data exchange.  Tactical data messages are
   generally single-datagram messages containing information on the
   location, bearing, identification, etc., of entities detected by
   sensors.  In CSNI, tactical data messages were assigned 15 different
   levels of CLNP priority.  This ensured that important messages, such
   as a rapidly approaching enemy missile's trajectory, were given
   priority over less important messages, such as a friendly, slow-
   moving tanker's heading.

Applicability

   There will be a significant amount of applicability to tactical RF
   networks.  The current IP and CLNP protocols are being given
   considerable attention in the tactical RF community as a means to
   provide communication interoperability across a large set of
   heterogeneous RF networks in use by different services and countries.
   The applicability of IPng can only improve with the inclusion of
   features critical to supporting QoS and Policy based routing,



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RFC 1677             IPng Tactical RF Requirements           August 1994


   security, real-time multi-media data delivery, and extended
   addressing.  It must be noted that it is very important that the IPng
   protocol headers not grow overly large.  There is a sharp tradeoff
   between the value added by these headers (interoperability, global
   addressing, etc.) and the degree of communication performance
   attainable on limited capacity RF networks.  Regardless of the data
   rate that future RF networks will be capable of supporting, there is
   always a tactical advantage in utilizing your resources more
   efficiently.

Datagram Service

   The datagram service paradigm provides many useful features for
   tactical communication networks.  The "memory" provided by datagram
   headers, provides an inherent amount of survivability essential to
   the dynamics of the tactical communication environment.  The
   availability of platforms for routing and relaying is never 100%
   certain in a tactical scenario.  The efficiency with which multi-cast
   can be implemented in a connectionless network is highly critical in
   the tactical environment where rapid, efficient information
   dissemination can be a deciding factor.  And, as has been proven,
   with several different Internet applications and experiments, a
   datagram service is capable of providing useful connection-oriented
   and real-time communication services.

   Consideration should be given in IPng to how it can co-exist with
   other architectures such as switching fabrics which offer demand-
   based control over topology and connectivity.  The military owns many
   of its own communication resources and one of the large problems in
   managing the military communication infrastructure is directing those
   underlying resources to where they are needed.  Traditional
   management (SNMP, etc.) is of course useful here, but RF
   communication media can be somewhat dynamically allocated.  Circuit
   switching designs offer some advantages here.  Dial-up IP routing is
   an example of an integrated solution.  The IPng should be capable of
   supporting a similar type of operation.

Support of Communication Media

   The tactical communication environment includes a very broad spectrum
   of communication media from shipboard fiber-optic LANs to very low
   data rate (<2400 bps) RF links.  Many of the RF links, even higher
   speed ones, can exhibit error statistics not necessarily well-
   serviced by higher layer reliable protocols (i.e., TCP).  In these
   cases, efficient lower layer protocols can be implemented to provide
   reliable datagram delivery at the link layer, but at the cost of
   highly variable delay performance.




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RFC 1677             IPng Tactical RF Requirements           August 1994


   It is also important to recognize that RF communication cannot be
   viewed from the IPng designer as simple point-to-point  links.
   Often, highly complex, unique subnetwork protocols are utilized to
   meet requirements of survivability, communications performance with
   limited bandwidth, anti- jam and/or low probability of detection
   requirements.  In some of these cases IPng will be one of several
   Layer 3 protocols sharing the subnetwork.

   It is understood that IPng cannot be the panacea of Layer 3
   protocols, particularly when it comes to providing special mechanisms
   to support the endangered-specie low data rate user.  However, note
   that there are many valuable low data rate applications useful to the
   tactical user.  And low user data rates, coupled with efficient
   networking protocols can allow many more users share limited RF
   bandwidth.  As a result, any mechanisms which facilitate compression
   of network headers can be considered highly valuable in an IPng
   candidate.

Security Considerations

   Security issues are discussed throughout this memo.

Author's Address

   R. Brian Adamson
   Communication Systems Branch
   Information Technology Division
   Naval Research Laboratory
   NRL Code 5523
   Washington, DC 20375

   EMail: [e-mail]



















Adamson                                                         [Page 9]