rfc1688.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

3.5.  Away From Home

   When a router is present, and the correspondent does not implement
   mobility functions, the router must be capable of redirecting the
   correspondent to communicate directly with the Mobile Node.

   When no router is present, Mobile Nodes must be capable of
   communicating directly with other nodes on the same link.

   Mobility must not create an environment which is less secure than the
   current Internet.

   Changes in topology must not affect internode security mechanisms.

4.  Security

4.1.  Authentication

   Mobility registration messages must be authenticated between the home
   topological repository and Mobile Node.

   When the correspondent implements mobility functions, redirection or
   path optimization must be authenticated between the correspondent and
   Mobile Node.



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RFC 1688                     IPng Mobility                   August 1994


4.2.  Anonymity

   The capability to attach to a foreign administrative domain without
   the awareness of the foreign administration is not prohibited.
   However, any mobility mechanism must provide the ability to prevent
   such attachment.

4.3.  Location Privacy

   The capability to attach to a foreign administrative domain without
   the awareness of correspondents is not prohibited.  However, any
   mobility mechanism must provide the ability for the home
   administration to trace the current path to the point of attachment.

4.4.  Content Privacy

   Security mechanisms which provide content privacy must not obscure or
   have a dependency on the topological location of Mobile Nodes.

5.  Bandwidth

   Mobility must operate in the current link environment, and must not
   be dependent on bandwidth improvements.  The Mobile Node's directly
   attached link is likely to be bandwidth limited.

   In particular, radio frequency spectrum is already a scarce
   commodity.  Higher bandwidth links are likely to continue to be
   scarce in the mobile environment.

   Current applications of mobility using radio links include HF links
   which are subject to serious fading and noise constraints, VHF and
   UHF line of sight radio between ships or field sites, and UHF
   Satellite Communications links.

   The HF radio bandwidth is fixed at 1200 or 2400 bps by international
   treaty, statute, and custom, and is not likely to change.

   The European standard for cellular radio is 2400 bps GSM.

   The most prevalent deployed analog cellular and land-line modulation
   used by mobile nodes is 2400 bps.

   Current digital cellular deployment is 19,200 bps CDPD shared among
   many users.  At early installations, under light loads, effective FTP
   throughput has been observed as low as 200 bps.

   Future digital cellular deployment is 9,600 and 14,400 bps CDMA,
   which is shared between voice and data on a per user basis.



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RFC 1688                     IPng Mobility                   August 1994


   Effective FTP throughput has been measured as low as 7,200 bps.

   Future Personal Communications Services (PCS) will also have
   relatively little bandwidth.  In industrialized nations, the
   bandwidth available to each user is constrained by the density of
   deployment, and is commensurate with planned digital cellular
   deployment.

   It appears likely that satellite-based PCS will be widely deployed
   for basic telephony communications in many newly-industrialized and
   lesser-developed countries.  There is already significant PCS
   interest in East and SouthEast Asia, India, and South America.

   Van Jacobson header prediction is widely used, and essential to
   making the use of such links viable.

5.1.  Administrative Messages

   The number of administrative mobility messages sent or received by
   the Mobile Node must be limited to as few as possible.  In order to
   meet the frequency requirement of changing point of attachment once
   per second, registration of changes must not require more than a
   single request and reply.

   The size of administrative mobility messages must be kept as short as
   possible.  In order to meet the frequency requirement of changing
   point of attachment once per second, the registration messages must
   not total more than 120 bytes for a complete transaction, including
   link and internet headers.

5.2.  Response Time

   For most mobile links in current use, the typical TCP/IPv4 datagram
   overhead of 40 bytes is too large to maintain an acceptable typing
   response of 200 milliseconds round trip time.

   Therefore, the criteria for IPng mobility is that the response time
   not be perceptably worse than IPv4.

   This allows no more than 6 bytes of additional overhead per datagram
   to be added by IPng.

      This was a primary concern in the design of mobility forwarding
      headers.  Larger headers were rejected outright, and negotiation
      is provided for smaller headers than the default method.
      Topological headers are removed by the Foreign Agent prior to
      datagram transmission over the slower link to the Mobile Node,
      which also aids header prediction, as described below.



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RFC 1688                     IPng Mobility                   August 1994


5.3.  Header Prediction

   Header prediction can be useful in reducing bandwidth usage on
   multiple related datagrams.  It requires a point-to-point peer
   relationship between nodes, so that a header history can be
   maintained between the peers.

   Header prediction is less effective in mobile environments, as the
   header history is lost each time a Mobile Node changes its point of
   attachment.  The new Foreign Agent will not have the same history as
   the previous Agent.

   In order for header prediction to operate successfully, changing
   topological information must be removed from datagram overhead prior
   to transmission of the datagram on any final hop's directly attached
   link.  This applies to both the Mobile Node peering with a Foreign
   Agent, and also the final link to a Correspondent.  Otherwise, header
   prediction cannot be relied upon to improve bandwidth utilization on
   low-speed Mobile and Correspondent links.

   Since the changing topological information cannot be removed in the
   forwarding path of the datagram, header prediction will also be
   affected at any other pair of routers in the datagram path.  Each
   time that a Mobile Node moves, the topological portion of the header
   will change, and header history used at those routers will be
   updated.  Unless topological information is limited to as few headers
   as possible, this may render header prediction ineffective as more
   Mobile Nodes are deployed.

6.  Processing

   Mobility must operate in the current processor environment, and must
   not be dependent on hardware improvements.

   Common hardware implementations of Mobile Nodes include lower speed
   processors, and highly integrated components.  These are not readily
   upgradable.

   The most prevalent mobile platform is a low speed i86, i286 or i386.

   The most common ASIC processor is a low speed i186.

6.1.  Fixed Location

   The processing limitations require that datagram header fields which
   are frequently examined by Mobile Nodes, or used for datagram
   forwarding to or from Mobile Nodes, are in a fixed location and do
   not require lengths and offsets.



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RFC 1688                     IPng Mobility                   August 1994


      Varied number of fields was explicitly rejected in the design of
      mobility registration and forwarding headers.

6.2.  Simple Fields

   The processing limitations require that datagram header fields which
   are frequently examined by Mobile Nodes, or used for datagram
   forwarding to or from Mobile Nodes, are simple and fixed size.

      Varied length of fields was explicitly rejected in the design of
      mobility forwarding headers.

6.3.  Simple Tests

   Because the most prevalent processors are "little-endian", while
   network protocols are in practice "big-endian", the field processing
   must primarily use simple equality tests, rather than variable shifts
   and prefix matches.

6.4.  Type, Length, Value

   Fields which are not frequently examined, whether due to infrequent
   transmission or content that is not relevant in every message, must
   be of the Type, Length, Value format.

Acknowledgements

   This compilation is primarily based on the work in progress of the
   IETF Mobile IP Working Group.

Security Considerations

   Security issues are discussed in section 4.

Author's Address

   Questions about this memo can also be directed to:

   William Allen Simpson
   Daydreamer
   Computer Systems Consulting Services
   1384 Fontaine
   Madison Heights, Michigan  48071

   EMail: [e-mail] or
          [e-mail]





Simpson                                                         [Page 9]