OSPF over ATM and Proxy-PAR
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This is an older version of an Internet-Draft that was ultimately published as RFC 2844.
|Robert HAAS , Patrick Droz , Tony Przygienda
|2013-03-02 (Latest revision 1999-08-24)
|Internet Engineering Task Force (IETF)
|Intended RFC status
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|Became RFC 2844 (Experimental)
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Internet Engineering Task Force OSPF WG Internet Draft T. Przygienda/P. Droz/R. Haas draft-ietf-ospf-atm-03.txt Siara / IBM / IBM August 24, 1999 Expires: February 24, 2000 OSPF over ATM and Proxy PAR Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This draft specifes for OSPF implementors and users mechanisms describ¡ ing how the protocol operates in ATM networks over PVC and SVC meshes with the presence of Proxy PAR. These recommendations do not require any protocol changes and allow for simpler, more efficient and cost-effec¡ tive network designs. It is recommended that OSPF implementations should be able to support logical interfaces, each consisting of one or more virtual circuits and used either as numbered logical point-to-point links (one VC), logical NBMA networks (more than one VC) or point-to- multipoint networks (more than one VC), where a solution simulating broadcast interfaces is not appropriate. PAR can help to distribute across the ATM cloud configuration set-up and changes of such interfaces when OSPF capable routers are (re-)configured. Proxy-PAR can in turn be used to exchange this information between the ATM cloud and the routers connected to it. 1 Introduction Proxy-PAR and PAR have been accepted as standards by the ATM Forum in January 1999 . A more complete overview of Proxy PAR than in the T. Przygienda/P. Droz/R. Haas [Page 1] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 section below is given in . 1.1 Introduction to Proxy PAR Proxy PAR  is an extension allowing for different ATM attached devices (like routers) to interact with PAR capable switches and query information about non-ATM services without executing PAR themselves. The Proxy PAR client side in the ATM attached device is much simpler in terms of implementation complexity and memory requirements than a com¡ plete PAR protocol stack (which includes the full PNNI  protocol stack) and should allow easy implementation in e.g. existing IP routers. Additionnaly, clients can use Proxy PAR to register different non-ATM services and protocols they support. Proxy PAR has consciously not been included as part of ILMI  due to the complexity of PAR information passed in the protocol and the fact that it is intended for integration of non-ATM protocols and services only. A device executing Proxy PAR does not necessarily need to execute ILMI or UNI signaling, although this normally will be the case. The protocol in itself does not specify how the distributed service reg¡ istration and data delivered to the client is supposed to be driving other protocols so e.g. OSPF routers finding themselves through Proxy PAR could use this information in a Classical IP over ATM  fashion, forming a full mesh of point-to-point connections to interact with each other to simulate broadcast interfaces. For the same purpose LANE  or MARS  could be used. As a by-product, Proxy PAR could provide the ATM address resolution for IP attached devices but such resolution can be achieved by other protocols under specification at the IETF as well, e.g. . And last but not least, it should be mentioned here that the protocol coexists with and complements the ongoing work in IETF on server detection via ILMI extensions [9,10,11]. 1.1.1 Proxy PAR Scopes Any Proxy PAR registration is carried only within a defined scope that is set during registration and is equivalent to the PNNI routing level. Since no assumptions except scope values can be made about the informa¡ tion distributed (e.g. IP addresses bound to NSAPs are not assumed to be aligned with them in any respect such as encapsulation or functional mapping), registration information cannot be summarized. This makes a careful handling of scopes necessary to preserve the scalability. More details on the usage of scope can be found in . 1.2 Introduction to OSPF OSPF (Open Shortest Path First) is an Interior Gateway Protocol (IGP) and described in  from which most of the following paragraphs has been taken almost literally. OSPF distributes routing information T. Przygienda/P. Droz/R. Haas [Page 2] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 between routers belonging to a single Autonomous System. The OSPF proto¡ col is based on link-state or SPF technology. It was developed by the OSPF working group of the Internet Engineering Task Force. It has been designed expressly for the TCP/IP internet environment, including explicit support for IP subnetting, and the tagging of externally- derived routing information. OSPF also utilizes IP multicast when send¡ ing/receiving the updates. In addition, much work has been done to pro¡ duce a protocol that responds quickly to topology changes, yet involves small amounts of routing protocol traffic. To cope with the needs of NBMA and demand circuits capable networks such as Frame Relay or X.25,  has been made available that standardizes extensions to the protocol allowing for efficient operation over on- demand circuits. OSPF supports three types of networks today: ¸ Point-to-point networks: A network that joins a single pair of routers. Point- to-point networks can either be numbered or unnumbered in the latter case the interfaces do not have IP addresses nor masks. Even when numbered, both sides of the link do not have to agree on the IP subnet. ¸ Broadcast networks: Networks supporting many (more than two) attached routers, together with the capability to address a sin¡ gle physical message to all of the attached routers (broadcast). Neighboring routers are discovered dynamically on these networks using the OSPF Hello Protocol. The Hello Protocol itself takes advantage of the broadcast capability. The protocol makes further use of multicast capabilities, if they exist. An Ethernet is an example of a broadcast network. ¸ Non-broadcast networks: Networks supporting many (more than two) attached routers, but having no broadcast capability. Neighboring routers are maintained on these nets using OSPF's Hello Protocol. However, due to the lack of broadcast capability, some configura¡ tion information is necessary for the correct operation of the Hello Protocol. On these networks, OSPF protocol packets that are normally multicast need to be sent to each neighboring router, in turn. An X.25 Public Data Network (PDN) is an example of a non- broadcast network. OSPF runs in one of two modes over non-broadcast networks. The first mode, called non-broadcast multi-access (NBMA), simulates the operation of OSPF on a broadcast network. The second mode, T. Przygienda/P. Droz/R. Haas [Page 3] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 called Point-to-MultiPoint, treats the non-broadcast network as a collection of point-to-point links. Non-broadcast networks are referred to as NBMA networks or Point-to-MultiPoint networks, depending on OSPF's mode of operation over the network. 2 OSPF over ATM 2.1 Model Contrary to broadcast-simulation based solutions such as LANE  or Classical IP over ATM , this document elaborates on how to handle virtual OSPF interfaces over ATM such as NBMA, point-to-multipoint or point-to-point and allow for their auto-configuration in presence of Proxy PAR. One advantage is the circumvention of server solutions that often present single points of failure or hold large amounts of configu¡ ration information. The other main benefit is the possibility to execute OSPF on top of NBMA and point-to-multpoint ATM networks, and still benefit from the auto¡ matic discovery of OSPF neighbors. As opposed to broadcast networks, broadcast-simulation based networks (like LANE or Classical IP over ATM), and point-to-point networks, where an OSPF router dynamically dis¡ covers its neighbors by sending Hello packets to the AllSPFRouters mul¡ ticast address, this is not the case on NBMA and point-to-multipoint networks. On NBMA networks, the list of all other attached routers to the same NBMA network has to be manually configured or discovered by some other means: Proxy PAR allows to automate this configuration. Also on point-to-multipoint networks, the set of routers that are directly reachable can be either manually configured or dynamically discovered by Proxy PAR or through mechanisms like Inverse ATMARP. In an ATM network, (see 8.2 in ) Inverse ATMARP can be used to discover the IP address of the router at the remote end of a given PVC, whether or not its ATM address is known. But Inverse ATMARP does not return for instance whether the remote router is running OSPF, as opposed to Proxy PAR. Parallel to  that describes the recommended operation of OSPF over Frame Relay networks, a similar model is assumed where the underlying ATM network can be used to model single VCs as point-to-point interfaces or collections of VCs as non-broadcast interfaces, whether in NBMA or point-to-multipoint mode. Such a VC or collection of VCs is called a logical interface and specified through its type (either point-to-point, NBMA or point-to-multipoint), VPN ID (the Virtual Private Network to which interface belongs), address and mask. Layer 2 specific configura¡ tion such as address resolution method, class and quality of service of used circuits and other must be also included. As logical consequence thereof, a single, physical interface could encompass multiple IP sub¡ nets or even multiple VPNs. In contrary to layer 2 and IP addressing T. Przygienda/P. Droz/R. Haas [Page 4] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 information, when running Proxy PAR, most of the OSPF information needed to operate such a logical interface does not have to be configured into routers statically but can be provided through Proxy PAR queries. This allows for much more dynamic configuration of VC meshes in OSPF environ¡ ments than e.g. in Frame Relay solutions. Proxy PAR queries can also be issued with a subnet address set to 0.0.0.0, instead of a specific subnet address. This type of query returns information on all OSPF routers available in all subnets, within the scope specified in the query. This can be used for instance when the IP addressing information has not been configured. 2.2 Configuration of OSPF interfaces with Proxy PAR To achieve the goal of simplification of VC mesh reconfiguration, Proxy PAR allows the router to learn automatically most of the configuration that has to be provided to OSPF. Non-broadcast and point-to-point inter¡ face information can be learned across an ATM cloud as described in the ongoing sections. It is up to the implementation to possibly allow for a mixture of Proxy PAR autoconfiguration and manual configuration of neighbor information. Moreover, manual configuration could e.g. over¡ ride or complement information derived from a Proxy PAR client. Addi¡ tionally, OSPF extensions to handle on-demand circuits  can be used to allow for graceful tearing down of VCs not carrying any OSPF traffic over prolonged periods of time. The different interactions are described in sections 2.2.1, 2.2.2 and 2.2.3. Even after autoconfiguration of interfaces has been provided, the prob¡ lem of VC setups in an ATM network is unsolved since none of the nor¡ mally used mechanisms such as Classical IP  or LANE  are assumed to be present. Section 2.5 describes the behavior of OSPF routers neces¡ sary to allow for router connectivity. 2.2.1 Autoconfiguration of Non-Broadcast Multiple-Access (NMBA) Inter¡ faces Proxy PAR allows to autoconfigure the list of all routers residing on the same IP network in the same VPN by simply querying the Proxy PAR server. Each router can easily obtain the list of all OSPF routers on the same subnet with their router priorities and corresponding ATM addresses. This is the precondition for OSPF to work properly across such logical NBMA interfaces. Note that this memberlist, when learned through Proxy PAR queries, can dynamically change with PNNI (in)stabil¡ ity and general ATM network behavior. It maybe preferable for an imple¡ mentation to withdraw list membership (de-register itself as an OSPF router) e.g. much slower than detect new members (done by querying). Relying on OSPF mechanism to discover lack of reachability in the over¡ laying logical IP network could alleviate the risk of thrashing DR T. Przygienda/P. Droz/R. Haas [Page 5] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 elections and excessive information flooding. Once the DR registration is completed and the router has not been elected DR or BDR, an implemen¡ tation of  can ignore the fact that all routers on the specific NBMA subnet are available in its configuration since it only needs to main¡ tain VCs to the DR and BDR. Note that this information can serve other purposes, like for the forwarding of data packets (see section 2.4). Traditionally, router configuration for a NBMA network provides the list of all neighboring routers to allow for proper protocol operation. For stability purposes, the user may choose to provide a list of neighbors through such static means but additionally enable the operation of Proxy PAR protocol to complete the list. It is left to specific router imple¡ mentations whether the manual configuration is used in addition to the information provided by Proxy PAR, used as filter of the dynamic infor¡ mation or whether a concurrent mode of operation is prohibited. In any case it should be obvious that allowing for more flexibility may facili¡ tate operation but provides more possibilities for misconfiguration as well. 2.2.2 Autoconfiguration of Point-to-Multipoint Interfaces Point-to-Multipoint interfaces in ATM networks only make sense if no VCs can be dynamically set up since an SVC-capable ATM network normally pre¡ sents a NBMA cloud to OSPF. This is e.g. the case if OSPF executes over a network composed of a partial PVC or SPVC mesh or pre-determined SVC meshes. Such a network could be modeled using the point-to-multipoint OSPF interface and the neighbor detection could be provided by Proxy PAR or other means. In the Proxy PAR case the router queries for all OSPF routers on the same network in the same VPN but it installs in the interface configuration only routers that are already reachable through existing PVCs. The underlying assumption is that a router knows the remote ATM address of a PVC and can compare it with appropriate Proxy PAR registrations. If the remote ATM address of the PVC is unknown, it can be discovered by mechanisms like Inverse ARP . Proxy PAR provides a true OSPF neighbor detection mechanism, whereas a mechanism like Inverse ARP only returns addresses of directly reachable routers (which are not necessarily running OSPF), in the point-to-multi¡ point environment. 2.2.3 Autoconfiguration of Numbered Point-to-Point Interfaces OSPF point-to-point links do not necessarily have an IP address assigned and even when having one, the mask is undefined. As a precondition to successfully register a service with Proxy PAR, IP address and mask is required. Therefore, if a router desires to use Proxy PAR to advertise the local end of a point-to-point link to the router it intends to form an adjacency with, an IP address has to be provided and a netmask set or T. Przygienda/P. Droz/R. Haas [Page 6] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 a default of 255.255.255.252 (this gives as the default case a subnet with 2 routers on it) assumed. To allow the discovery of the remote end of the interface, IP address of the remote side has to be provided and a netmask set or a default of 255.255.255.252 assumed. Obviously the dis¡ covery can only be successfull when both sides of the interface are con¡ figured with the same network mask and are within the same IP network. The situation where more than two possible neighbors are discovered through queries and the interface type is set to point-to-point presents a configuration error. Sending multicast Hello packets on the point-to-point links allows to automatically discover OSPF neighbors. On the other hand, using Proxy PAR instead avoids sending Hello messages to routers which are not nec¡ essarily running OSPF. 2.2.4 Autoconfiguration of Unnumbered Point-to-Point Interfaces For reasons given already in  using unnumbered point-to-point inter¡ faces with Proxy PAR is not a very attractive alternative since the lack of an IP address prevents efficient registration and retrieval of con¡ figuration information. Relying on the numbering method based on MIB entries generates conflicts with the dynamic nature of creation of such entries and is beyond the scope of this work. 2.3 Registration of OSPF interfaces with Proxy PAR To allow other routers to discover an OSPF interface automatically, the IP address, mask, Area ID, interface type and router priority informa¡ tion given must be registered with the Proxy PAR server at an appropri¡ ate scope. A change in any of these parameters has to force a reregis¡ tration with Proxy PAR. It should be emphasized here that since the registration information can be used by other routers to resolve IP addresses against NSAPs as explained in section 2.4, whole IP address of the router must be regis¡ tered. It is not enough to just indicate the subnet up to the mask length but all address bits must be provided. 2.3.1 Registration of Non-Broadcast Multiple-Access Interfaces For an NBMA interface the appropriate parameters are available and can be registered through Proxy PAR without further complications. 2.3.2 Registration of Point-to-Multipoint Interfaces In case of a point-to-multipoint interface the router registers its information in the same fashion as in the NBMA case except that the interface type is modified accordingly. T. Przygienda/P. Droz/R. Haas [Page 7] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 2.3.3 Registration of Numbered Point-to-Point Interfaces In case of point-to-point numbered interfaces the address mask is not specified in the OSPF configuration. If the router has to use Proxy PAR to advertise its capability, a mask must be defined or a default value of 255.255.255.252 used. 2.3.4 Registration of Unnumbered Point-to-Point Interfaces Due to the lack of a configured IP address and difficulties generated by this fact as described earlier, registration of unnumbered point-to- point interfaces is not covered in this document. 2.4 IP address to NSAP Resolution Using Proxy PAR As a byproduct of Proxy PAR presence, an OSPF implementation could use the information in registrations for the resolution of IP addresses to ATM NSAPs on a subnet without having to use static data or mechanisms such as ATMARP . This again should allow for drastic simplification of number of mechanisms involved in operation of OSPF over ATM to pro¡ vide an IP overlay. In a system perspective, the OSPF component, the Proxy PAR client, the IP to NSAP address resolution table, and the ATM circuit manager can be depicted as in Figure 1. Figure 1 shows an example of components inter¡ actions triggered by the result of a Proxy PAR query from the Proxy PAR client. 2.5 Connection Setup Mechanisms This sections describes OSPF behavior in an ATM network under different assumptions in terms of signaling capabilities and preset connectivity. 2.5.1 OSPF in PVC Environments In environments where only partial PVCs (or SPVCs) meshes are available and modeled as point-to-multipoint interfaces, the routers see reachable routers through autodiscovery provided by Proxy PAR. This leads to expected OSPF behavior. In cases where a full mesh of PVCs is present, such a network should preferably be modeled as NBMA. Note that in such a case, PVCs failures will translate into not so obvious routing failures. 2.5.2 OSPF in SVC Environments In SVC-capable environments the routers can initiate VCs after having discovered the appropriate neighbors, preferably driven by the need to T. Przygienda/P. Droz/R. Haas [Page 8] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 __________ _________ | | | | | OSPF |<-------------------|Proxy PAR|<---(Proxy PAR query) |__________| notify | client | ^ neighbor changes |_________| | | send and | | maintain Proxy PAR receive | | entries in table OSPF msg | | | | | | ____V____ ____V_____ | ATM | | | | circuit |-------------------->|IP to NSAP| | manager | check | table | |_________| IP to NSAP bindings |__________| Figure 1: System perspective of typical components interactions + + + | +---+ | | +--+ |---|RTA|---| +-------+ | +--+ |H1|---| +---+ | | ATM | |---|H2| +--+ | | +---+ | Cloud | +---+ | +--+ |LAN Y |---|RTB|-------------|RTC|---| + | +---+ | PPAR | +---+ | + +-------+ + Figure 2: Simple Topology with Router B and Router C operating across NBMA ATM interfaces with Proxy PAR send data such as Hello-packets. This can lead to race conditions where both sides can open a VC simultaneously. It is generally desirable to avoid wasting this valuable resource: if the router with lower Router ID detects that the VC initiated by the other side is bidirectional, it is free to close its own VC and use the detected one. Note that this either T. Przygienda/P. Droz/R. Haas [Page 9] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 requires the OSPF implementation to be aware of the VCs used to send and receive Hello messages, or the component responsible of managing VCs to be aware of the usage of particular VCs. Observe that this behavior operates correctly in case OSPF over Demand Circuits extensions are used  over SVC capable interfaces. It is possible to avoid most of the time the set-up of redundant VCs by delaying the sending of the first OSPF Hello from the router with the lower Router ID, by an amount of time larger than the interval between the queries from the Proxy PAR client to the server. Chances are that the router with the higher Router ID opens the VC (or use an already existing VC) and sends the OSPF Hello first, if its interval between queries is smaller than the Hello delay of the router with the lower Router ID. Since this interval can vary depending on particular needs and implementations, the race conditions described above can still be expected to happen, albeit presumably less often. The existence of VCs used for OSPF exchanges is orthogonal to the number and type of VCs the router chooses to use within the logical interface to forward data to other routers. OSPF implementations are free to use any of these VCs (in case they are aware of their existence) to send packets if their endpoints are adequate and must accept hello packets arriving on any of the VCs belonging to the logical interface even if OSPF operating on such an interface is not aware of their existence. An OSPF implementation may ignore connections being initiated by another router that has not been discovered by Proxy PAR. The OSPF implementa¡ tion will anyway ignore a neighbor whose Proxy PAR registration indi¡ cates that it is not adjacent. As an example consider the topology in Figure 2 where router RTB and RTC are connected to a common ATM cloud offering Proxy PAR services. Assum¡ ing that RTB's OSPF implementation is aware of SVCs initiated on the interface and RTC only makes minimal use of Proxy PAR information the following sequence could develop illustrating some of the cases described above: 1. RTC and RTB register with ATM cloud as Proxy PAR capable and discover each other as adjacent OSPF routers. 2. RTB sends a hello which forces it to establish a SVC connec¡ tion to RTC. 3. RTC sends a hello to RTB but disregards the already existing VC and establishes a new VC to RTB to deliver the packet. 4. RTB sees a new bi-directional VC and assuming here that RTC's OSPF Id is higher, closes the VC originated in step 2. T. Przygienda/P. Droz/R. Haas [Page 10] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 5. Host H1 sends data to H2 and RTB establishes a new data SVC between itself and RTC. 6. RTB sends a Hello to RTC and decides to do it using the newly establish data SVC. RTC must accept the hello despite the min¡ imal implementation. 3 Acknowledgments Comments and contributions from several sources, especially Rob Coltun, Doug Dykeman, John Moy and Alex Zinin are included in this work. 4 Security Consideration Several aspects are to be considered when talking about security of operating OSPF over ATM and/or Proxy PAR. The security of registered information handed to the ATM cloud must be guaranteed by the underlying PNNI protocol. Extensions to PNNI are available and given their imple¡ mentation spoofing of registrations and/or denial-of-service issues can be addressed . The registration itself through proxy PAR is not secured and appropriate mechanisms are for further study. However, even if the security at the ATM layer is not guaranteed, OSPF security mecha¡ nisms can be used to verify that detected neighbors are authorized to interact with the entity discovering them. 5 Bibliography  P. Droz, "PNNI Augmented Routing (PAR) Version 1.0." ATM Forum af- ra-0104.000, January 1999.  T. P. P. Droz, "Proxy PAR." Internet Draft draft-ietf-ion-proxypar- arch-01, February 1999.  ATM-Forum, "Private Network-Network Interface Specification Version 1.0." ATM Forum af-pnni-0055.000, March 1996.  ATM-Forum, "Interim Local Management Interface (ILMI) Specification 4.0." ATM Forum 95-0417R8, June 1996.  J. H. M. Laubach, "Classical IP and ARP over ATM, RFC 2225." Inter¡ net Engineering Task Force, April 1998.  ATM-Forum, "LAN Emulation over ATM 1.0." ATM Forum af-lane-0021.000, January 1995.  G. Armitage, "Support for Multicast over UNI 3.0/3.1 based ATM net¡ works, RFC 2022." Internet Engineering Task Force, November 1996. T. Przygienda/P. Droz/R. Haas [Page 11] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999  R. Coltun, "The OSPF Opaque LSA Option, RFC 2370." Internet Engi¡ neering Task Force, July 1998.  M. Davison, "ILMI-Based Server Discovery for ATMARP, RFC 2601." Internet Engineering Task Force, June 1999.  M. Davison, "ILMI-Based Server Discovery for MARS, RFC 2602." Internet Engineering Task Force, June 1999.  M. Davison, "ILMI-Based Server Discovery for NHRP, RFC 2603." Internet Engineering Task Force, June 1999.  J. Moy, "OSPF Version 2 - RFC 2328." Internet Engineering Task Force, April 1998.  J. Moy, "Extending OSPF to Support Demand Circuits, RFC 1793." Internet Engineering Task Force, April 1995.  O. deSouza and M. Rodrigues, "Guidelines for Running OSPF Over Frame Relay Networks, RFC 1586." Internet Engineering Task Force, March 1994.  A. M. T. Bradley, C. Brown, "Inverse Address Resolution Protocol, RFC 2390." Internet Engineering Task Force, September 1999.  T. Przygienda and C. Bullard, "Baseline Text for PNNI Peer Authen¡ tication and Cryptographic Data Integrity." ATM Forum 97-0472, July 1997. Authors' Addresses Tony Przygienda Siara Systems 300 Ferguson Drive Mountain View California 94043 firstname.lastname@example.org Patrick Droz IBM Research Division Saumerstrasse 4 8803 Ruschlikon Switzerland email@example.com T. Przygienda/P. Droz/R. Haas [Page 12] Internet Draft OSPF over ATM and Proxy PAR August 24, 1999 Robert Haas IBM Research Division Saumerstrasse 4 8803 Ruschlikon Switzerland firstname.lastname@example.org T. Przygienda/P. Droz/R. Haas [Page 13]