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Versions: 00 01 02 03 RFC 1937

                                                          Yakov Rekhter
                                                          Cisco Systems
                                                          Dilip Kandlur
                                 T.J. Watson Research Center, IBM Corp.
                                                      Curtis Villamizar
                                                                    ANS
                                                          November 1995


  "Local/Remote" Forwarding Decision in Switched Data Link Subnetworks
                      <draft-ietf-rolc-apr-03.txt>



Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
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Abstract


   The IP architecture assumes that each Data Link subnetwork is labeled
   with a single IP subnet number. A pair of hosts with the same subnet
   number communicate directly  (with no routers); a pair of hosts with
   different subnet numbers always communicate through one or more
   routers. As indicated in RFC1620, these assumptions may be too



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   restrictive for large data networks, and specifically for networks
   based on switched virtual circuit (SVC) based technologies (e.g. ATM,
   Frame Relay, X.25), as these assumptions impose constraints on
   communication among hosts and routers through a network.  The
   restrictions may preclude full utilization of the capabilities
   provided by the underlying SVC-based Data Link subnetwork.  This
   document describes extensions to the IP architecture that relaxes
   these constraints, thus enabling the full utilization of the services
   provided by SVC-based Data Link subnetworks.


1  Background


   The following briefly recaptures the concept of the IP Subnet.  The
   topology is assumed to be composed of hosts and routers
   interconnected via links (Data Link subnetworks).  An IP address of a
   host with an interface attached to a particular link is a tuple
   <prefix length, address prefix, host number>, where host number is
   unique within the subnet address prefix.  When a host needs to send
   an IP packet to a destination, the host needs to determine whether
   the destination address identifies an interface that is connected to
   one of the links the host is attached to, or not.  This referred to
   as the "local/remote" decision. The outcome of the "local/remote"
   decision is based on (a) the destination address, and (b) the address
   and the prefix length associated with the the local interfaces.  If
   the outcome is "local", then the host resolves IP address to Link
   Layer address (e.g. by using ARP), and then sends the packet directly
   to that destination (using the Link layer services).  If the outcome
   is "remote", then the host uses one of its first-hop routers (thus
   relying on the services provided by IP routing).

   To summarize, two of the important attributes of the IP subnet model
   are:

      hosts with a common subnet address prefix are assumed to be
      attached to a common link (subnetwork), and thus communicate with
      each other directly, without any routers - "local";

      hosts with different subnet address prefixes are assumed to be
      attached to different links (subnetworks), and thus communicate



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      with each other only through routers - "remote".


   A typical example of applying the IP subnet architecture to an SVC-
   based Data Link subnetwork is "Classical IP and ARP over ATM"
   (RFC1577).  RFC1577 provides support for ATM deployment that follows
   the traditional IP subnet model and introduces the notion of a
   Logical IP Subnetwork (LIS).  The consequence of this model is that a
   host is required to setup an ATM SVC to any host within its LIS; for
   destinations outside its LIS the host must forward packets through a
   router.  It is important to stress that this "local/remote" decision
   is based solely on the information carried by the destination address
   and the address and prefix lengths associated with the local
   interfaces.


2  Motivations


   The diversity of TCP/IP applications results in a wide range of
   traffic characteristics.  Some applications last for a very short
   time and generate only a small number of packets between a pair of
   communicating hosts (e.g. ping, DNS). Other applications have a short
   lifetime, but generate a relatively large volume of packets (e.g.
   FTP). There are also applications that have a relatively long
   lifetime, but generate relatively few packets (e.g.  Telnet).
   Finally, we anticipate the emergence of applications that have a
   relatively long lifetime and generate a large volume of packets (e.g.
   video-conferencing).

   SVC-based Data Link subnetworks offer certain unique capabilities
   that are not present in other (non-SVC) subnetworks (e.g. Ethernet,
   Token Ring).  The ability to dynamically establish and tear-down SVCs
   between communicating entities attached to an SVC-based Data Link
   subnetwork enables the dynamic dedication and redistribution of
   certain communication resources (e.g. bandwidth) among the entities.
   This dedication and redistribution of resources could be accomplished
   by relying solely on the mechanism(s) provided by the Data Link
   layer.

   The unique capabilities provided by SVC-based Data Link subnetworks



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   do not come "for free".  The mechanisms that provide dedication and
   redistribution of resources have certain overhead (e.g. the time
   needed to establish an SVC, resources associated with maintaining a
   state for an SVC). There may also be a monetary cost associated with
   establishing and maintaining an SVC. Therefore, it is very important
   to be cognizant of such an overhead and to carefully balance the
   benefits provided by the mechanisms against the overhead introduced
   by such mechanisms.

   One of the key issues for using SVC-based Data Link subnetworks in
   the TCP/IP environment is the issue of switched virtual circuit (SVC)
   management.  This includes SVC establishment and tear-down, class of
   service specification, and SVC sharing.  At one end of the spectrum
   one could require SVC establishment between communicating entities
   (on a common Data Link subnetwork) for any application. At the other
   end of the spectrum, one could require communicating entities to
   always go through a router, regardless of the application.  Given the
   diversity of TCP/IP applications, either extreme is likely to yield a
   suboptimal solution with respect to the ability to efficiently
   exploit capabilities provided by the underlying Data Link layer.

   The traditional IP subnet model is too restrictive for flexible and
   adaptive use of SVC-based Data Link subnetworks  - the use of a
   subnetwork is driven by information completely unrelated to the
   characteristics of individual applications.  To illustrate the
   problem consider "Classical IP and ARP over ATM" (RFC1577).  RFC1577
   provides support for ATM deployment that follows the traditional IP
   subnet model, and introduces the notion of a Logical IP Subnetwork
   (LIS).  The consequence of this model is that a host is required to
   setup an SVC to any host within its LIS, and it must forward packets
   to destinations outside its LIS through a router.  This
   "local/remote" forwarding decision is based solely on the information
   carried in the source and destination addresses and the subnet mask
   associated with the source address, and has no relation to the nature
   of the applications that generated these packets. This leads to a
   situation where SVC management is controlled by totally irrelevant
   factors.







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3  QoS/Traffic Driven "Local/Remote" Decision


   Consider a host attached to an SVC-based Data Link subnetwork, and
   assume that the "local/remote" decision the host could make is not
   constrained by the IP subnet model. When such a host needs to send a
   packet to a destination, the host might consider any of the following
   options:

      Use a best-effort SVC to the first hop router.

      Use a SVC to the first hop router dedicated to a particular type
      of service (ie: predictive real time).

      Use a dedicated SVC to the first hop router.

      Use a best-effort SVC to a router closer to the destination than
      the first hop router.

      Use a SVC to a router closer to the destination than the first hop
      router dedicated to a particular type of service.

      Use a dedicated SVC to a router closer to the destination than the
      first hop router.

      Use a best-effort SVC directly to the destination (if the
      destination is on the same Data Link subnetwork as the host).

      Use a SVC directly to the destination dedicated to a particular
      type of service (if the destination is on the same Data Link
      subnetwork as the host).

      Use a dedicated SVC directly to the destination (if the
      destination is on the same Data Link subnetwork as the host).

   We may observe that the forwarding decision at the host is more
   flexible than the "local/remote" decision of the IP subnet model. We
   may also observe that the host's forwarding decision allows to take
   into account QoS and/or traffic requirements of the applications
   and/or cost factors associated with establishing and maintaining a
   VC, and thus improve the overall SVC management. Therefore, removing



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   constraints imposed by the IP subnet model is an important step
   towards better SVC management.


3.1 Extending the scope of possible "local" outcomes


   A source may have a SVC (either dedicated or shared) to a destination
   if both the source and the destination are on a common Data Link
   subnetwork. The ability to have the SVC (either dedicated or shared)
   is completely decoupled from the source and destination IP addresses,
   but is coupled to the QoS and/or traffic characteristics of the
   application. In other words, the ability to establish a direct VC
   (either dedicated or shared) between a pair of hosts on a common Data
   Link subnetwork has nothing to do with the IP addresses of the hosts.
   In contrast with the IP subnet model (or the LIS mode), the "local"
   outcome becomes divorced from the addressing information.


3.2 Allowing the "remote" outcome where applicable


   A source may go through one or more routers to reach a destination if
   either (a) the destination is not on the same Data Link subnetwork as
   the source, or (b) the destination is on the same Data Link
   subnetwork as the source, but the QoS and/or traffic requirements of
   the application on the source do not justify a direct (either
   dedicated or shared) VC.

   When the destination is not on the same Data Link subnetwork as the
   source, the source could select between either (a) using its first-
   hop (default) router, or (b) establishing a "shortcut" to a router
   closer to the destination than the first-hop router.  The source
   should be able to select between these two choices irrespective of
   the source and destination IP addresses.

   When the destination is on the same Data Link subnetwork as the
   source, but the QoS and/or traffic requirements do not justify a
   direct VC, the source should be able to go through a router
   irrespective of the source and destination IP addresses.




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   In contrast with the IP subnet model (or the LIS model) the "remote"
   outcome, and its particular option (first-hop router vs router closer
   to the destination than the first-hop router), becomes divorced from
   the addressing information.


3.3 Sufficient conditions for direct connectivity


   The ability of a host to establish an SVC to a peer  on a common
   switched Data Link subnetwork is predicated on its knowledge  of the
   Link Layer address of the peer or an intermediate point closer to the
   destination.  This document assumes the existence of mechanism(s)
   that can provide the host with this information. Some of the possible
   alternatives are NHRP, ARP, or static configuration; other
   alternatives are not precluded.  The ability to acquire the Link
   Layer address of the peer should not be viewed as an indication that
   the host and the peer can establish an SVC - the two may be on
   different Data Link subnetworks, or may be on a common Data Link
   subnetwork that is partitioned.


3.4 Some of the implications


   Since the "local/remote" decision would depend on factors other than
   the addresses of the source and the destination, a pair of hosts may
   simultaneously be using two different means to reach each other,
   forwarding traffic for applications with different QoS/and or traffic
   characteristics differently.


3.5 Address assignment


   It is expected that if the total number of hosts and routers on a
   common SVC-based Data Link subnetwork is sufficiently large, then the
   hosts and routers could be partitioned into groups, where each group
   would have hosts and routers. The routers within a group would act as
   the first-hop routers for the hosts in the group. If the total number
   of hosts and routers is not large, then all these hosts and routers



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   could form a single group. Criteria for determining group sizes are
   outside the scope of this document.

   To provide scalable routing each group should be given an IP address
   prefix, and elements within the group should be assigned addresses
   out of this prefix. The routers in a group would then advertise (via
   appropriate routing protocols) routes to the prefix associated with
   the group. These routes would be advertised as "directly reachable"
   (with metric 0). Thus, routers within a group would act as the last-
   hop routers for the hosts within the group.



3.6  Using Point to Point Links


   If RFC1577 is used as an underlying model, the router based overlay
   is assumed to be comprised of LIS.  The LIS model may not be
   appropriate for some situation.  A large group may be served by a
   single router or a small number of routers to provide some
   redundancy.

   The desired behavior can be accomplished using RFC1577 LIS by
   allowing the LIS to become very small, containing two hosts each and
   relaxing the restriction prohibiting shortcut VC.  If the all zeros
   network address is to be reserved, a four address LIS is needed for
   each host in the group.  ATM ARP service is also needed.  The LIS
   model clearly has some inefficiencies for such a case.

   An alternate way to model the relation among hosts and routers within
   each group is by modelling VC as point to point links.  Using
   numbered point to point links, the two addreses on the link need not
   be adjacent.  If each end of the link has a distinct address, the
   model is a numbered point to point link.  A router may allow the use
   of a single address for the entire router, reusing the same address
   as the near end of all of the point to point connections.  Similarly,
   a host can use a single address for point to point links to more than
   one router, if a backup router is used.  This model is an unnumbered
   point to point link, unnumbered since the link endpoints are not
   uniquely numbered, just the nodes.  For SNMP manageability, on
   unnumbered point to point links, the far end address can be used for



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   identification of a link.


4 Conclusions


   Different approaches to SVC-based Data Link subnetworks used by
   TCP/IP yield substantially different results with respect to the
   ability of TCP/IP applications to efficiently exploit the
   functionality provided by such subnetworks.  For example, in the case
   of ATM both LAN Emulation [LANE] and "classical" IP over ATM
   [RFC1577] localize host changes below the IP layer, and therefore may
   be good first steps in the ATM deployment.  However, these approaches
   alone are likely to be inadequate for the full utilization of ATM.

   It appears that any model that does not allow SVC management based on
   QoS and/or traffic requirements will preempt the full use of SVC-
   based Data Link subnetworks.  Enabling more direct connectivity for
   applications that could benefit from the functionality provided by
   SVC-based Data Link subnetworks, while relying on strict hop by hop
   paths for other applications, could facilitate exploration of the
   capabilities provided by the subnetworks.

   While this document does not define any specific coupling between
   various QoS, traffic characteristics and other parameters, and SVC
   management, it is important to stress that efforts towards
   standardization of various QoS, traffic characteristics, and other
   parameters than an application could use (through an appropriate API)
   to influence SVC management are essential for flexible and adaptive
   use of SVC-based Data Link subnetworks.

   The proposed model utilizes the SVC-based infrastructure for the
   applications that could benefit from the capabilities supported
   within such an infrastructure, and take advantage of a based router-
   based overlay for all other applications.  The router based overlay
   could be based on RFC1577 if the restriction prohibiting subnet
   shortcut connections is eliminated from RFC1577, replacing it with a
   statement that any shortcut requires mechanisms beyond what is
   described in RFC1577.  Use of a point to point model is suggested as
   an alternate to the LIS model for situations where the NBMA LIS model
   simply does not fit.  As such it provides a balanced mix of router-



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   based and switch-based infrastructures, where the balance could be
   determined by the applications requirements.

   The approach proposed in this document combines switch-based
   infrastructure with router-based overlay and uses each for that which
   it is best suited: switch-based infrastructure for applications that
   can justify an SVC establishment; router-based overlay for all other
   applications.



6 Security Considerations


   Security issues are not discussed in this document.


7 Acknowledgements


   The authors would like to thank Joel Halpern (NewBridge), Allison
   Mankin (ISI), Tony Li (cisco Systems), Andrew Smith (BayNetworks) for
   their review and comments.


References


   [LANE] "LAN Emulation over ATM specification- version 1", ATM Forum,
   Feb.95.

   [Postel 81] Postel, J., Sunshine, C., Cohen, D., "The ARPA Internet
   Protocol", Computer Networks, 5, pp. 261-271, 1983.

   [RFC792]  Postel, J., "Internet Control Message Protocol- DARPA
   Internet Program Protocol Specification", STD 5, RFC 792, ISI,
   September 1981.

   [RFC1122]  Braden, R., Editor, "Requirements for Internet Hosts -
   Communication Layers", STD 3, RFC 1122, USC/ISI, October 1989.




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   [RFC1577] Laubach, M., "Classical IP and ARP over ATM", January 1994.

   [RFC1620] Braden, B., Postel, J., Rekhter, Y., Internet Architecture
   Extensions for Shared Media", May 1994.

   [RFC1755] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E.,
   Malis, A., "ATM Signalling Support for IP over ATM", January 1995.


14  Authors' Address


   Yakov Rekhter
   Cisco Systems
   170 West Tasman Drive,
   San Jose, CA 95134-1706
   Phone:  (914) 528-0090
   email:  yakov@cisco.com

   Dilip Kandlur
   T.J. Watson Research Center IBM Corporation
   P.O. Box 704
   Yorktown Heights, NY 10598
   Phone:  (914) 784-7722
   email:  kandlur@watson.ibm.com

   Curtis Villamizar
   ANS
   100 Clearbrook Road
   Elmsford, N.Y. 10523
   email: curtis@ans.net













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