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Network Working Group                                       A. NicholsonRequest for Comments: 1306                                      J. Young                                                     Cray Research, Inc.                                                              March 1992Experiences Supporting By-Request Circuit-Switched T3 NetworksStatus of this Memo   This RFC provides information for the Internet community.  It does   not specify an Internet standard.  Distribution of this memo is   unlimited.Abstract   This memo describes the experiences of a project team at Cray   Research, Inc., in implementing support for circuit-switched T3   services.  While the issues discussed may not be directly relevant to   the research problems of the Internet, they may be interesting to a   number of researchers and implementers.   Developers at Cray Research, Inc. were presented with an opportunity   to use a circuit-switched T3 network for wide area networking.  They   devised an architectural model for using this new resource.  This   involves activating the circuit-switched connection when an   application program engages in a bulk data transfer, and releasing   the connection when the transfer is complete.   Three software implementations for this feature have been tested, and   the results documented here.  A variety of issues are involved, and   further research is necessary.  Network users are beginning to   recognize the value of this service, and are planning to make use of   by-request circuit-switched networks.  A standard method of access   will be needed to ensure interoperability among vendors of circuit-   switched network support products.Acknowledgements   The authors thank the T3 project team and other members of the   Networking Group at Cray Research, Inc., for their efforts: Wayne   Roiger, Gary Klesk, Joe Golio, John Renwick, Dave Borman and Craig   Alesso.Nicholson & Young                                               [Page 1]

RFC 1306          Experiences with Circuit-Switched T3        March 1992Overview   Users of wide-area networks often must make a compromise between low   cost and high speed when accessing long haul connections.  The high   money cost of dedicated high speed connections makes them   uneconomical for scientists and engineers with limited budgets.  For   many traditional applications this has not been a problem.  Datasets   can be maintained on the remote computer and results were presented   in a text-only form where a low-speed connection would suffice.   However, for visualization and other data transfer intensive   applications, this limitation can severely impact the usability of   high performance computing tools which are available only through   long-haul network connections.   Supercomputers are one such high performance tool.  Many users who   can benefit from access to supercomputers are limited by slow network   connections to a centrally located supercomputer.  A solution to this   problem is to use a circuit-switched network to provide high speed   network connectivity at a reduced cost by allocating the network only   when it is needed.   Consider how a researcher using a visualization application might   efficiently use a dedicated low speed link and a circuit switched   high speed link.  The researcher logs in to the remote supercomputer   over the low speed link.  After running whatever programs are   necessary to prepare the visualization, the high speed connection is   activated and used to transfer the graphics data to the researcher's   workstation.   We built and demonstrated this capability in September, 1990, at the   Telecommunications Association show in San Diego, using this type of   visualization application.  Further, it will be available in a   forthcoming release of our system software.Architectural Model   We developed our support for circuit switched services around a   simple model of a switched network.  At some point in the path   between two hosts, there is a switched network connection.  This   connection is likely to connect two enterprise networks operated by   the same organization.  Administrative overlap between the two   networks is useful for accounting and configuration purposes.  We   believe that with further investigation circuit switched network   support could be extended to multiple switched links in an internet   environment.   The switch which makes the network connection operates on a "by-   request" basis (also called "on-demand").  When it receives a requestNicholson & Young                                               [Page 2]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   to make a network connection it will do so (if possible), and breaks   the connection when requested.  The switch will not activate   automatically if there is an attempt to transfer data over an   incomplete connection.   We also made the assumption that the circuit would be switched on a   connection basis rather than a packet basis.  When an application   begins sending data utilizing the switched connection, it will send   all the data it has, without stopping, until it is finished.  At this   time it will release the connection.  It is assumed that the quantity   of data will be large enough that the circuit setup time is   negligible relative to the period of the transfer.  Otherwise, it is   not worth the effort to support the circuit switched network for   small data transfers.   This model requires that just before the application begins a large   bulk transfer of data, a request message is sent to the switch asking   that the switched network connection be activated.  Once the link is   up, the application begins sending data, and the network routes all   the data from the application through the switched network.  As soon   as all the data has been sent, a message is sent to the switch to   turn off the switched link, and the network returns to routing data   through the slower link.   The prototype system we built for the TCA show was designed around   this model of circuit switched services.  We connected a FDDI   backbone at Cray Research in Eagan, Minnesota to the TCA show's FDDI   network through 2 NSC 703 FDDI/T1/T3 routers.  MCI provided a   dedicated T1 line and a switched T3 line, using a DSC DS3 T3 switch   located in Dallas, Texas.  These networks provided connectivity   between a Cray Research computer in Eagan to a Sun workstation on the   show floor in San Diego.Alternative Solution Strategies   The first aspect of using the switched services involved the circuit   switch.  The DS3 switch available to us was accessed via a dial up   modem, and it communicated using a subset of the CCITT Q.295   protocol.  Activating the switch required a 4 message exchange and   deactivation required a 3 message exchange.  We felt the protocol was   awkward and might be different for other switch hardware.   Furthermore, we believed that the dial up aspect of communicating   with the switch suffered from the same drawbacks.  A good solution   would require a cleaner method of controlling the switch from the   source host requesting the switched line.   The next aspect of using switched services involves the source host   software which requests and releases the switched network.  Ideally,Nicholson & Young                                               [Page 3]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   the switched network is activated just before data transfer takes   place and it is released as soon as all data has been sent.  We   considered using special utility programs which a user could execute   to control the link, special system libraries which application   programs could call, or building the capability into the kernel.  We   also considered the possibility that these methods could send   messages to a daemon running on the source host which would then   communicate with another entity actually controlling the switch.   The last aspect of using switched services we considered is selection   of the switch controlled network.  This involves both policy issues   and routing issues.  Policy issues include which users running which   applications will be able to use higher cost switched links.  And   packets must be routed amongst multiple connections offering varying   levels of service after they leave their source.Implementations   We have developed a model for switch control through the internetwork   which we believe to be reasonable.  However, we have experimented   with three different source host implementations.  These different   implementations are detailed here.Switch control   Our simplest design decisions involved the switch itself.  We decided   that the complex protocol and dial up line must be hidden from the   source host requesting the switched link.  We decided that the source   host would use a simple request/release protocol with messages sent   through the regular network (as opposed to dial up lines or other   connections).  Some host accessible through the local network would   run a program translating the simple request and release messages   into the more complicated switch protocol and also have the modem to   handle the dial up connection.   This has a variety of advantages.  First, it isolates differences in   switch hardware.  Second, multiple hosts may access the switch   without requiring multiple modems for the dial up line.  And it   provides a central point of control for switch access.  We did not   consider any alternatives to this model of switch control.   Our initial implementation used a simple translator daemon running on   a Sun workstation.  Listening on a raw IP port, this program would   wait for switch control messages.  Upon receipt of such a message, it   would dial up the switch and attempt to handle the request.  It would   then send back a success or failure response.  This host, in   conjunction with the translator daemon software, is referred to as   the switch controller.  The switch controller we used was local toNicholson & Young                                               [Page 4]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   our enterprise network; however, it could reside anywhere in the   Internet.   Later we designed a simple protocol for switch control, which was   implemented in the translator daemon.  This protocol is documented inRFC 1307, "Dynamically Switched Link Control Protocol".Source Control of the Switched Link   This problem involves a decision regarding what entity on the source   host will issue the switch request and release messages to the switch   controller, and when those messages will be issued.  Because we do   not have very much field experience with this service, we do not feel   that it is appropriate to recommend one method over the others.  They   all have advantages and disadvantages.   What we did do is make 3 different implementations of the request   software and can report our experiences with each.  These are one set   of special utility programs which communicate with the switch   controller, and 2 kernel implementations.  We did not experiment with   special libraries, nor did we implement a daemon for switch control   messages on the source host.Switch control user programs   This implementation of source host control of the switch is the   simplest.  Two programs were written which would communicate requests   to the switch controller; one for activating the connection, and   another to deactivate the connection.  The applications using this   feature were then put into shell scripts with the switch control   programs for simple execution.   This approach has the significant advantage of not requiring any   kernel modifications to any machine.  Furthermore, application   programs do not need to be modified to access this feature.  And   access to the circuit-switched links can be controlled using the   access permissions for the switch-control programs.   However, there are disadvantages as well.  First, there is   significant potential for the switch to be active (and billing the   user) for the dead time while the application program is doing tasks   other than transferring bulk data.  The granularity of turning the   switch on and off is limited to a per-application basis.   Another disadvantage is that most applications use only the   destination host's address for transfer, and this is the only   information available to the transport and network layers for routing   data packets.  Some other method must be used to distinguish betweenNicholson & Young                                               [Page 5]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   traffic which should use the circuit-switched connection and lower-   priority traffic.  This problem can be addressed using route aliases,   described below.Kernel switch control   We have made two different implementations of switch control   facilities within the operating system kernel.  Both rely upon the   routing lookup code in the kernel to send switch connect and tear   down messages.  The difference is in how the time delay between   request of the switch and a response is handled.   For starters, routing table entries were expanded to include the   internet address of the switch controller and state information for   the switched connection.  If there is a switch controller address   specified, then the connection must be set up before packets may be   sent on this route.  We also added a separate module to handle the   sending and receiving of the switch control messages.   When a routing lookup is satisfied, the routing code would check   whether the routing table entry specified a switch controller.  If   so, then the routine requesting switch setup would be called.  This   would send a message on the Internet to the switch controller to   setup the connection.   In our first implementation, the routing lookup call would return   immediately after sending the switch connection request message.  It   would be the responsibility of the transport protocol to deal with   the time delay while the connection is setup, and to tear down if the   switched connection could not be made.  This has significant   ramifications.  In the case of UDP and IP, packets must be buffered   for later transmission or face almost certain extermination as they   will probably start arriving at the switched connection before it is   ready to carry traffic.  Because of this problem, we decided that   this feature would not be available for UDP or IP traffic.   We did make this work for TCP.  Since TCP is already designed to work   so that it buffers all data for possible later retransmission, this   was not a problem.  Our first cut was to change TCP to check that the   route it was using was up if it is a switch controlled route.  TCP   would not send any data until the route was complete, and it would   close the connection if the switch did not come up.   This did not work well at first because every time TCP tried to send   data before the switch came up, the retransmit time would be reset   and backed off.  The rtt estimate, retransmit timeouts and the   congestion control mechanism were seriously skewed before any data   was ever sent.  The retransmit timer would expire as many as 3 timesNicholson & Young                                               [Page 6]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   before data could be transmitted.  We solved this problem by adding   another timer for handling the delay while the route came up, and not   allowing the delay to affect any of the normal rtt timers.   Our experiences with this approach were not particularly positive,   and we decided to try another.  We also felt that unreliable datagram   protocols should be able to use the service without excessive   reworking.  Our alternative still sends the switch control message   when a routing lookup finds a controlled route.  However, we now   suspend execution of the thread of control until a response comes   back from the switch controller.   This proved to be easier to implement in many ways.  However, there   were two major areas requiring changes outside the routing code.   First, we decided that if the switch refused to activate the   connection, it was pointless to try again.  So we changed the routing   lookup interface so that it could return an error specifying a   permanent error condition.  The transport layer could then return an   appropriate error such as a host unreachable condition.   The other, more complex issue deals with the suspension of the thread   of execution.  Our operating system, UNICOS, is an ATT System V   derivative, and our networking subsystem is based on the BSD tahoe   and reno releases.  The only way to suspend execution is to sleep.   This is fine, as long as there is a user context to put to sleep.   However, it is not a good idea to go to sleep when processing network   interrupts, as when forwarding a packet.   We solved this problem by using a global flag regarding whether it   was ok for the switch control message code to sleep.  If it is   necessary to send a message and sleep, then the flag must be set and   an error is returned if sleeping is not allowed.  User system calls   which might cause a switch control message to be sent set and clear   the flag upon entrance and exit.  We also made it impossible to   forward packets on a switch controlled route.  We feel that this is   reasonable since the overhead of switch control should be incurred   only when an application program has made an explicit request to   begin transfer of data.   The one other change we made was to make sure that TCP freed the   route it is using upon entering TIME_WAIT state.  There is no point   in holding the circuit open for two minutes in case we need to   retransmit the final ack.  Of course, this assumes that an alternate   path exists for the the peer to retransmit its fin.   The advantage of building this facility into the kernel is that it   allows a fine degree of control over when the switch will and will   not need to be activated.  Many applications which open a dataNicholson & Young                                               [Page 7]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   connection, transmit their bulk data, and then close the connection   will not require modifications and will make efficient use of the   resource.  It also opens the possibility that applications written to   use type-of-service can use the same network connection for low-   bandwidth interactive traffic, change the type-of-service (thus   activating the switched connection) for bulk transfers, and then   release the switch upon returning to interactive traffic.   Putting this feature into the kernel also allows strong control over   when and how the switched link can be used, keeping accounting   information, and limiting multiple use access to the switched link.   The disadvantage is that significant kernel modifications are   required, and some implementation details can be very difficult to   handle.Switch control libraries   The switch control programs we used were built on a library of simple   switch control routines; however, we did not alter any standard   applications to use this library.  We did consider some advantages   and disadvantages.  On the plus side, it is possible to achieve a   satisfactory degree of switch control without requiring any kernel   modifications.   The primary disadvantage of this approach is that all applications   must be altered and recompiled.  This is particularly inconvenient   when source is not available.Link Selection   When an application wishes to send data over a circuit-switched   connection, it will be necessary to select the switched link over   other links.  This selection process may need to take place many   times, depending on the local network between the source host and the   bridge to the circuit switched connection.   For example, if the kernel routing code is controlling the link, then   there must be a way to choose a controlled route over another route.   Further downstream, there must be a way to route packets to the   switched link rather than other links.   This issue has the potential for great complexity, and we avoided as   much of the complexity as possible.  Policy routing and local routing   across multiple connections are fertile areas for work and it is   outside the scope of this work to address those issues.  Instead we   opted for simple answers to difficult questions.Nicholson & Young                                               [Page 8]

RFC 1306          Experiences with Circuit-Switched T3        March 1992   First of all, we added no special policies to link accessibility   beyond that already found in UNICOS.  And we handled local routing   issues to the NSC FDDI/T1/T3 routers with routing table manipulation   and IP Type-of-Service.   We came up with three solutions for selecting a routing table entry.   The first possibility is to use the type-of-service bits, which   seemed natural to us.  We changed the routing table to include type-   of-service values associated with routing entries, and the routing   lookups would select using the type-of-service.  UNICOS already   supports a facility to mark connections with a type-of-service value.   A controlled route could be marked with high throughput type-of-   service and an application wishing to transfer bulk data could set   the socket for high throughput before making the connection.  It   could also be possible to change the type-of-service on an existing   connection and start using the switched link if one is available.   Using the type-of-service bits have the advantage that downstream   routers can also use this information.  In our demonstration system,   the NSC FDDI/T1/T3 routers were configured to transfer packets with   high throughput type-of-service over the T3 connection and all others   over the T1 connection.   Another possibility is to take advantage of the multiple addresses of   a multi-homed host.  Routing tables could be set up so that packets   for one of the addresses get special treatment by traveling over the   switched link.  The routing table in the source host would have an   entry for accessing the switch controller when sending to the high   throughput destination address.   We also derived a method we call route aliasing.  Route aliasing   involves associating extra addresses to a single host.  However,   rather than the destination being an actual multi-homed host, the   alias is known only to the source host and is used as an alternative   lookup key.  When an application tries to connect to the alias   address the routing lookup returns an aliased route.  The route alias   contains the actual address of the host, but because of looking up   the special address, the switch is activated.  The alias could also   specify a type-of-service value to send in the packets so that   downstream routers could properly route the packets to the switched   link.  We realize that some may bemoan the waste of the limited   Internet address space for aliases; however, only the source host is   aware of the alias, and the primary shortage is with Internet network   addresses rather than host addresses.  In fact, we argue that this is   a more efficient use of the already sparse allocation of host   addresses available with each network address.Nicholson & Young                                               [Page 9]

RFC 1306          Experiences with Circuit-Switched T3        March 1992Future considerations   We believe that by-request services will become increasingly   important to certain classes of users.  Many data centers make high   performance resources available over a wide area, and these will be   the first users to take advantage of wide-area circuit-switched   networks.  Some users, such as CICNet ([2]), are already interested   in deploying this capability and telecom vendors are working to   satisfy this need.  However, there are a lot of issues involved in   providing this functionality.  We are working to involve others in   this process.References   [1]  Nicholson, et. al., "High Speed Networking at Cray Research",        Computer Communications Review, January 1991.   [2]  CICNet DS3 Working Group, "High Performance Applications on        CICNet: Impact on Design and Capacity", public report, CICNet,        Inc., June 1991.   [3]  Young, J., and A. Nicholson, "Dynamically Switched Link Control        Protocol",RFC 1307, Cray Research, Inc., March 1992.Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Andy Nicholson   Cray Research, Inc.   655F Lone Oak Drive   Eagan, MN 55123   Phone: (612) 452-6650   EMail: droid@cray.com   Jeff Young   Cray Research, Inc.   655F Lone Oak Drive   Eagan, MN 55123   Phone: (612) 452-6650   EMail: jsy@cray.comNicholson & Young                                              [Page 10]