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Network Working Group                                         Y. RekhterRequest for Comments: 1937                                 Cisco SystemsCategory: Informational                                       D. Kandlur                                  T.J. Watson Research Center, IBM Corp.                                                                May 1996  "Local/Remote" Forwarding Decision in Switched Data Link SubnetworksStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.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 inRFC1620, these assumptions may be too   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 the IP address to a   Link Layer address (e.g. by using ARP), and then sends the packetRekhter & Kandlur            Informational                      [Page 1]

RFC 1937        Forwarding in Switched Data Link Subnets        May 1996   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      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 LinkRekhter & Kandlur            Informational                      [Page 2]

RFC 1937        Forwarding in Switched Data Link Subnets        May 1996   layer.   The unique capabilities provided by SVC-based Data Link subnetworks   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, and consequently the SVC   management, 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.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:Rekhter & Kandlur            Informational                      [Page 3]

RFC 1937        Forwarding in Switched Data Link Subnets        May 1996      Use a best-effort SVC to the first hop router.      Use an 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 an 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 an 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).   In the above we observe that the forwarding decision at the host is   more flexible than the "local/remote" decision of the IP subnet   model. We also observe that the host's forwarding decision may 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   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 an 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 create and use the SVC (either   dedicated or shared) is completely decoupled from the source and   destination IP addresses, but is instead 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.Rekhter & Kandlur            Informational                      [Page 4]

RFC 1937        Forwarding in Switched Data Link Subnets        May 19963.2 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 may 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.   In contrast with the IP subnet model (or the LIS model) the "remote"   outcome, and its particular option (first-hop router versus router   closer to the destination than the first-hop router), becomes   decoupled 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.Rekhter & Kandlur            Informational                      [Page 5]

RFC 1937        Forwarding in Switched Data Link Subnets        May 19963.5 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, called Local   Addressing Groups (LAGs). Each LAG would have hosts and routers. The   routers within a LAG would act as the first-hop routers for the hosts   in the LAG. If the total number of hosts and routers is not large,   then all these hosts and routers could form a single LAG. Criteria   for determining LAG sizes are outside the scope of this document.   To provide scalable routing each LAG should be given an IP address   prefix, and elements within the LAG should be assigned addresses out   of this prefix. The routers in a LAG would then advertise (via   appropriate routing protocols) routes to the prefix associated with   the LAG. These routes would be advertised as "directly reachable"   (with metric 0). Thus, routers within a LAG would act as the last-hop   routers for the hosts within the LAG.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 these 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.Rekhter & Kandlur            Informational                      [Page 6]

RFC 1937        Forwarding in Switched Data Link Subnets        May 1996   The proposed model utilizes the SVC-based infrastructure for the   applications that could benefit from the capabilities supported   within such an infrastructure, and takes advantage of a router-based   overlay for all other applications.  As such it provides a balanced   mix of router-based and switch-based infrastructures, where the   balance could be determined by the applications requirements.5. Security Considerations   Security issues are not discussed in this memo.6. Acknowledgements   The authors would like to thank Joel Halpern (NewBridge), Allison   Mankin (ISI), Tony Li (cisco Systems), Andrew Smith (BayNetworks),   and Curtis Villamizar (ANS) 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.   [RFC1577] Laubach, M., "Classical IP and ARP over ATM", January 1994.   [RFC1620] Braden, R., 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.Rekhter & Kandlur            Informational                      [Page 7]

RFC 1937        Forwarding in Switched Data Link Subnets        May 199614.  Authors' Addresses   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.comRekhter & Kandlur            Informational                      [Page 8]