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RFC 1329
Thoughts on Address Resolution for Dual MAC FDDI Networks.
P. Kuehn. May 1992.

 
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Network Working Group P. Kuehn Request for Comments: 1329 May 1992 Thoughts on Address Resolution for Dual MAC FDDI Networks Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard. Distribution of this memo is unlimited. 1. Abstract In this document an idea is submitted how IP and ARP can be used on inhomogeneous FDDI networks (FDDI networks with single MAC and dual MAC stations) by introducing a new protocol layer in the protocol suite of the dual MAC stations. Thus two dual MAC stations are able to do a load splitting across the two rings and use the double bandwidth of 200 Mbits/s as single MAC stations. The new layer is an extension of layer 3. For the user, the higher layer protocols, IP and ARP the property "dual MAC" is transparent. No modification is required in the protocol suite of single MAC stations and transparent bridges. 2. Acknowledgements This paper is a result of a diploma thesis prepared at the Technical University of Munich, Lehrstuhl fuer Kommunikationsnetze, in co- operation with the Siemens Nixdorf AG. The author would like to thank Jrg Eberspher and Bernhard Edmaier from the university, Andreas Thimmel and Jens Horstmeier from the SNI AG at Augsburg for the helpful comments and discussions. 3. Conventions Primary MAC, P-MAC MAC, placed on the primary ring Secondary MAC, S-MAC MAC, placed on the secondary ring Inhomogeneous ring configuration of a dual FDDI ring with single MAC and dual MAC stations DMARP Dual MAC Address Resolution Protocol 4. Assumptions When a dual FDDI ring wraps, both MACs in a dual MAC station are assumed to remain connected to the ring. ANSI is just investigating whether the Configuration Management in the Station Management of a Kuehn [Page 1]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 FDDI station can be modified to allow this. According to the FDDI SMT standard [1], different addresses are required for all MACs on the primary and the secondary ring. In this paper, the MAC in a single MAC station is assumed to reside on the primary ring. The application of single MAC stations which have their MAC attached to the secondary ring is not precluded, but therefor additional connectivity between the two rings is required. These configurations are beyond the scope of this document. 5. The Application of Transparent Bridges Transparent bridges can provide links to other 802 LANs or further inhomogeneous FDDI rings. The connection between two inhomogeneous FDDI rings can be realized by one or two transparent bridges. When two transparent bridges are used, one transparent bridge links the primary rings, the other the secondary rings. If two secondary rings are connected by a transparent bridge, a path of transparent bridges must exist between the two primary rings. No transparent bridges are allowed between the primary and the secondary ring. 6. Protocol Layers in Single MAC Stations The new protocol layer, named load sharing layer, is drafted to be introduced only in dual MAC stations. In single MAC stations, IP and ARP are working on top of the Subnetwork Access Protocol (SNAP) 04] and the Logical Link Control protocol (802.2 LLC) [3]. LLC type 1 is used because connectionless services are investigated only. Kuehn [Page 2]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 +--------------------------+ | IP | +--------------------------+ +--------------------------+ | ARP | +--------------------------+ | | | ARP frames | IP frames | | +--------------------------+ | SNAP | +--------------------------+ +--------------------------+ | LLC | +--------------------------+ +--------------------------++-------+ | FDDI-MAC || F | +--------------------------+| D S | +--------------------------+| D M | | FDDI PHY and PMD || I T | +--------------------------++-------+ For the ARP layer, the following model is assumed: +-------------------------------------------------------X-----------+ | - ARP entity - | | | | IP frames | | +----------------+ +----------------+ read | | | | Cache | | | entries +-------------+ | | | Administration |->-| Address Cache |------>--| Address | | | +----------------+ | | | Conversion | | | | +----------------+ | Unit | | | | ARP frames +-------------+ | | | / | | | | ___________ <- ARP requests _________________/ | IP frames | | |/ | | +-----X-------------------------------------------------X-----------+ The Address Conversion Unit handles the actual conversion of IP addresses to hardware addresses. For this purpose, it uses the information in the ARP cache. The cache administration communicates with other ARP entities by ARP and creates, deletes and renews the entries in the cache. 7. Protocol Layers in Dual MAC Stations The load sharing layer provides the same interface to ARP as SNAP does. To exchange information about addresses and reachability, the load sharing entities in dual MAC stations communicate with the Dual Kuehn [Page 3]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 MAC Address Resolution Protocol (DMARP). For the transmission of DMARP frames the SNAP SAP of LLC is used, as for IP and ARP, too. The Organizationally Unique Identifier (OUI) in the SNAP header is set to zero (24 bit), the EtherType field (16 bit) contains a new number indicating DMARP, which is not defined yet. +---------------------------------------------------------+ | IP | +---------------------------------------------------------+ +---------------------------------------------------------+ | ARP | +---------------------------------------------------------+ | ARP frames | IP frames +---------------------------------------------------------+ | Load Sharing Layer | +---------------------------------------------------------+ | | | | | | | ARP | DMARP | IP | ARP | DMARP | IP | frames | frames | frames | frames | frames | frames | | | | | | +-------------------------+ +----------------------------+ | SNAP 1 | | SNAP 2 | +-------------------------+ +----------------------------+ +-------------------------+ +----------------------------+ | LLC 1 | | LLC 2 | +-------------------------+ +----------------------------+ +-------------------------+ +----------------------------++-------+ | Primary MAC | | Secondary MAC || F | +-------------------------+ +----------------------------+| D S | +---------------------------------------------------------+| D M | | FDDI PHY and PMD || I T | +---------------------------------------------------------++-------+ 8. Running Inhomogeneous FDDI Rings 8.1. Exchange of Primary MAC Addresses between Stations IP and higher layer protocols only use the network independent IP addresses. The ARP entity takes upon the conversion of an IP address to the appropriate hardware address. To make the property dual MAC" transparent, ARP may only know the addresses of MACs on the primary ring. Therefore, the load sharing entity always delivers ARP frames to SNAP 1 for transmission. By this way, communication with ARP is done over the primary ring in normal state. A secondary MAC can receive an ARP frame when the dual ring is wrapped and the destination hardware address is a multicast or broadcast address. These frames will be discarded because they were received twice. Kuehn [Page 4]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 By this way, the associations of IP addresses to primary MAC addresses for the single MAC and dual MAC stations are stored in the ARP cache. The ARP cache contains no secondary MAC addresses. 8.2. Exchange of Secondary MAC Addresses between Dual MAC Stations The load sharing layer needs to know the secondary MAC addresses of the other dual MAC stations. The DMARP is used to get these addresses. Whenever the load sharing entity delivers an ARP frame to SNAP 1, a DMARP reply frame will be sent on the secondary ring, containing the stations primary and secondary MAC address. The destination hardware address in this DMARP frame is the broadcast MAC address, the EtherType field in the SNAP header identifies DMARP. The IP destination address is copied from the ARP frame. If the ARP frame that was transmitted parallel to the DMARP reply was a request, an ARP reply frame will be sent back to the sending station by the ARP entity in the receiving station. When the load sharing layer in the receiving station delivers this ARP reply frame to SNAP 1, it sends a DMARP reply frame on the secondary ring. By this way, DMARP exchanges the additionally required secondary MAC addresses between the dual MAC stations. This is done parallel to the exchange of the ARP frames. 8.3. Communication of Dual MAC Stations on Different Dual FDDI Rings If two inhomogeneous dual FDDI rings are connected by one transparent bridge, dual MAC stations placed on different dual FDDI rings cannot perform a load sharing. If both dual FDDI rings remain in normal state, no DMARP reply frames get from one secondary ring to the other secondary ring. A dual MAC station realizes another dual MAC station placed on the other dual ring as a single MAC station, because it only receives ARP frames from it. If one of the dual rings is wrapped, a DMARP reply frame can get on the primary ring of the other dual ring. A target station on the unwrapped ring receives this DMARP frame by the primary MAC and the load sharing entity stores the contained addresses in an entry in the address cache. This entry is marked with a control bit, named the OR-bit Other ring bit"). No load sharing will be done with a station related to an entry with the OR-bit set. If both dual FDDI rings are wrapped, the MACs of all stations reside on one ring. Now, dual MAC stations placed on different dual rings can communicate with DMARP. If a DMARP reply frame is received by the primary MAC and no entry exists for the sending station, a new entry with OR-Bit set will be created. Otherwise, the OR-bit will be set in the existing entry. If a DMARP reply frame is received by the secondary MAC and an entry with OR-bit set already exists for the Kuehn [Page 5]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 sending station, the bit will not be reset. This mechanism provides that no load sharing will be done between Dual MAC stations on different dual rings if the dual rings are linked with one transparent bridge. An additional DMARP error frame is used to provide against errors when a DMARP reply frame gets lost on the ring. 8.4. Timeout of Entries Marked with OR-Bit Set If a FDDI ring is wrapped, the DMARP reply frames are received by the primary and secondary MACs of the target dual MAC stations. In that case, the entries for dual MAC stations on the same dual ring are also marked with the OR-bit, although the load sharing is possible between these stations. When an OR-bit in an entry is set for the first time, a timer entity is started. If the timer entity runs out, a DMARP request frame is sent over SNAP 2 to the secondary MAC of the associated target) station. Then the entry will be discarded. If the request cannot be received by the target station because the network configuration has changed, there is no entry in the address cache for this station any more and no load sharing is computed. If the target station receives the DMARP request frame, it sends back a DMARP reply frame. 8.5. Problems with the Application of Large FDDI Networks With an increasing number of dual FDDI rings, each one linked together by two transparent bridges, the probability increases, that one of these inhomogeneous dual FDDI rings is wrapped in the moment when two dual MAC stations exchange ARP frames and DMARP replies. If two dual MAC stations are communicating for the first time, the probability decreases that a load sharing is really computed after the exchange of DMARP replies, although this would be possible according to the network configuration. It relies upon the fact, that DMARP replies get to the primary ring over the wrapped dual ring and only entries marked with the OR-bit set are created. To solve this problem further expedients are invented: At first, entries in the address cache can be marked read-only by the setting of the R-bit. In dual MAC stations, entries can be written manually for other dual MAC stations that are frequently talked to or that have a special importance. The control bits of these entries cannot be changed by DMARP. Kuehn [Page 6]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 Next, additional control bits are introduced. One of these bits is the Hold-bit (H-bit). When two dual MAC stations exchange ARP frames and DMARP replies to create entries in their address caches, one station starts sending a DMARP reply, first. According to the network state, it sends an additional DMARP error frame, a moment later. Within a maximum period of time (see "Configuring the Timer Parameters"), all frames arrive at the neighbour station and are received by the primary and/or secondary MAC. If the OR-bit was not set for an entry within this period of time, it is clear, that no further DMARP frames will be received, which result in setting the OR-bit. For such an entry the H-bit is set. As the reception of reply and error frames is not sufficient for setting the OR-bit when the H-bit is set, the load sharing is assumed to be sure. The correctness of the H-bit will be verified in relatively long time periods by queries (query and hold frames) at the station associated. For two communicating stations there exists a possibility to get information from a third station. Always, when the OR-bit is set for an entry in a dual MAC station, a search frame is transmitted by the secondary MAC, containing the own primary MAC address and the primary MAC address of the counter station. If a third station can compute a sure load sharing with both stations (the H-bit is set for the associated entries), the stations can perform a load sharing between them, too. The third station informs these stations by sending found frames to them. 8.6. Multicast and Broadcast Addresses in IP Frames If the destination hardware address of an IP frame is a multicast or broadcast hardware address, the frame is always delivered to SNAP 1 and sent on the primary ring, because one of the addressed stations could be a single MAC station. IP frames which are delivered to the load sharing entity by SNAP 2 are discarded by the load sharing entity. Thus, the duplication of these frames can be prevented. 9. Internal Structure One load Sharing entity exists in the load sharing layer. This load sharing entity consists of the address cache, the cache administration and the multiplexer. Kuehn [Page 7]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 to ARP to ARP +----X----------------------------------------------------X--------+ | | | IP | | | ARP frames read | frames | | | entries | | | +----------------------------+ +---------+ +----------+ | | | Cache Administration |->-| Address |---->--| Multi- | | | +----------------------------|->-| Cache | | plexer | | | | | | | +---------+ | | | | | | | | +----------+ | | | ARP | DMARP | ARP | DMARP | | | | | frames | frames | frames | frames IP | IP | | | | | | | frames | frames | | | | | | | | | | +--X--------X--------X--------X-----------------------X--------X---+ to SNAP 1 to SNAP 2 to SNAP 1 to SNAP 2 9.1. The Address Cache In the address cache, the associations of primary MAC addresses to secondary MAC addresses are stored for other dual MAC stations on the network. There are no entries for single MAC stations. Because the OR- and the LS-bit (see table) always have inverted values, one of the bits is redundant. Afterwards the examination of an entry state gets easier by the introduction of both bits, they are defined together. The ARP is able to support other protocol address formats than the IP format. To support this ARP property by DMARP, the protocol type number as used in the ARP frames is stored in every entry of the address cache. So, a dual MAC station is able to communicate with another station with DMARP, even if the other station does not use IP. The numbers used in DMARP frames and the address cache for the protocol type and the address length are taken over from ARP. name length comment -------------------------------------------------------------------- P-MAC address 48 bit Address of the primary MAC in an other dual MAC station S-MAC address 48 bit Address of the secondary MAC in that station LS-bit 1 bit A load sharing can be performed with that station ("Load sharing bit") Kuehn [Page 8]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 OR-bit 1 bit No load sharing may be done with that station ("Other ring bit") H-bit 1 bit The load sharing with that station is trusty. ("Hold bit") Q-bit 1 bit A query frame was sent to that station, no hold frame was received yet ("Query bit") R-bit 1 bit This entry cannot be changed by DMARP ("Read-only bit") V-bit 1 bit The entry is valid ("Valid bit") subscript 32 bit Unique number, identifying this entry protocol type 16 bit Number of the protocol type that was last used in that station 9.2. The Multiplexer The multiplexer deals with multiplexing the IP frames upon the two FDDI rings. Broadcast and multicast frames are always sent on the primary ring. Otherwise, the contents of the address cache and a load sharing criteria are used to decide on which of the rings an IP frame has to be transmitted. If there is no entry for the primary MAC address of the destination station in the cache, the IP frame is transmitted on the primary ring. If there is an entry for the destination station and the LS-bit is set, a load sharing can be done with this station. Later on a load sharing criteria, which is beyond the scope of this document, decides, which one of the rings is used for transmission. An example for a load sharing criteria is the length of the transmit queues in the MACs. The multiplexer requires an abstract function only, which returns the appropriate ring for the transmission of an actual IP frame. Additionally, the multiplexer filters the received IP frames: multicast or broadcast frames received from the secondary MAC are discarded. Kuehn [Page 9]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 9.3. The Cache Administration The cache administration creates and deletes the entries in the address cache. For this purpose, it communicates with other load sharing entities in other dual MAC stations with the DMARP. The cache administration handles the delivery of ARP frames to the ARP and the SNAP entity in the station, respectively. The cache administration needs three timers for the communication with the DMARP, which have to be supported by the system environment. Each of these timers must support a timer entity for each entry in the address cache, whereby a single one is running at a time. Supported timer services: TIMER_request(time, name, subscript) TIMER_response(name, subscript) TIMER_cancel(name, subscript): A timer entity is started by the service TIMER_request and cancelled by the TIMER_cancel service request. The TIMER_response service indicates that a timer entity has run out. The parameter name is the name of a timer: OR-Entry-Timer, Hold-Timer, or Query-Timer. Each entry in the address cache is uniquely identified by a number subscript). This number is also the number of an associated timer entity. How to dispose these numbers is a question of implementation. The parameter time determines the time period when the timer runs out. This parameter has the value OR-set-timeout for the OR-Entry-Timer, Hold-time for the Hold-Timer and Query-time for the Query-Timer. 9.4. Configuring the Timer Parameters The OR-set-timeout parameter for the OR-Entry-Timer The period of time, determined by this parameter, should be essentially longer than the maximum time for a frame to travel around the entire network. The expression entire network means the network which is constituted by the subnetworks linked together with transparent bridges. When entries with OR-bit set are created continuously for a dual MAC station by the timeout mechanism, this parameter determines the periods of time between the consecutive requests that are sent to this station. If the state of the dual FDDI ring changes and an entry with LS-bit set could be created, this parameter additionally determines the maximum time until the new entry is created. (If an entry could not be created by transmission of search frames.) Therefore, the OR-set-timeout parameter should be set to some 10 seconds. Kuehn [Page 10]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 The Hold-time parameter for the Hold-Timer The period of time, determined by this parameter, should as well be essentially longer than the maximum time for a frame to travel around the entire network. When two stations communicate for the first time, they exchange ARP frames and DMARP replies. The Hold-time parameter determines the period of time until the load sharing is assumed to be accomplished after the setting of the LS-bit. In this period of time, the frames mentioned above must have reached its destination. If an entry would be marked with the H-bit incorrectly, the time until it gets corrected will be relatively long (Query time). Proposed dimension: several minutes. The Query-time parameter for the Query-Timer When an entry is marked with LS- and H-bit it is assumed, that load sharing can be performed with the associated station. To allow the correction of a wrong value of the H-bit, the correctness of the H-bit is tested in periods of time, determined by the parameter Query-time. It is tested whether a frame is received, which was sent by the secondary MAC to the secondary MAC address of the target station. (The target station acknowledges the reception of the query frame by a hold frame.) To limit the traffic caused by the query and hold frames, the parameter Query- time should be set to several minutes. 9.5. Format of DMARP Frames fieldname length comment -------------------------------------------------------------------- hardware type 16 bit 1 = "ethernet" protocol type 16 bit 2048D = "Internet Protocol" length of hardware 8 bit Value in octets, addresses 6 for 48 bit MAC addresses length of protocol 8 bit Value in octets, addresses 4 for Internet addresses operation 16 bit 1: "reply" 2: "request" 3: "error" 4: "search" 5: "found" Kuehn [Page 11]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 6: "query" 7: "hold" 1. hardware address ... octets 2. hardware address ... octets protocol address ... octets sender protocol address ... octets receiver -------------------------------------------------------------------- The value for the field "protocol type" is the same as in ARP frames. 9.6. Contents of DMARP Frames In the following tables of DMARP frames, the fields containing the length and type of protocol and hardware addresses are omitted. Format: +-------------------------------------------------------------+ | Operation | 1. hardware | 2. hardware | protocol | protocol | | | address | address | address | address | | | | | sender | receiver | +-------------------------------------------------------------+ Operation = 1 (reply), 2 (request), 3 (error): +-----------------------------------------------------------------+ | Operation | P-MAC address | S-MAC address | protocol | protocol | | | sender | sender | address | address | | | | | sender | receiver | +-----------------------------------------------------------------+ +-------------------------------------------------------------------+ | Operation=4 | P-MAC | P-MAC address | protocol | broadcast | | (search) | address | counter- | address | protocol | | | sender | station | sender | address | +-------------------------------------------------------------------+ +-------------------------------------------------------------------+ | Operation=5 | P-MAC | S-MAC address | protocol | broadcast | | (found) | address | counter- | address | protocol | | | sender | station | sender | address | +-------------------------------------------------------------------+ Kuehn [Page 12]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 +-------------------------------------------------------------------+ | Operation=6 | S-MAC | P-MAC address | protocol | broadcast | | (query) | address | counter- | address | protocol | | | sender | station | sender | address | +-------------------------------------------------------------------+ +-------------------------------------------------------------------+ | Operation=7 | P-MAC address | S-MAC address | protocol | protocol | | (hold) | sender | sender | address | address | | | | | sender | receiver | +-------------------------------------------------------------------+ Apart from the error frames all frames are sent on the secondary ring. The reply, error and search frames are addressed to the broadcast hardware address. The request, found, query and hold frames are addressed to an individual secondary MAC address. 10. Formal Description The following description is written in ESTELLE. 10.1. Global Constants, Variables and Types default individual queue; timescale ...; type PDU_type = ... ; (* format of a Protocol Data Unit: String of variable length *) HW_addr_type = ... ; (* format of a 48 bit MAC address *) PR_addr_type = ... ; (* General: format of a protocol address in an ARP or DMARP frame *) IP_addr_type = ... ; (* General: format of an IP address *) QoS_type = ... ; (* General: format of a Quality-of- -Service statement *) timer_name_type = ... ; (* Type for the name of a system timer *) flag = (reset,set); var (* The values of these variables are set in the initialization part or by external management functions. *) Kuehn [Page 13]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 My_P_MAC_addr : HW_addr_type; (* Address of the MAC, placed on the primary ring *) My_S_MAC_addr : HW_addr_type; (* Address of the MAC, placed on the secondary ring *) My_IP_address : IP_addr_type; (* IP address of this station *) Broadcast_HW_addr : HW_addr_type; (* Broadcast MAC address (48 bit) *) Broadcast_IP_addr : IP_addr_type; (* Broadcast IP address *) dmarp_QoS : QoS_type; (* Quality_of_Service-statement for DMARP frames *) ethernet : integer; (* Type statement in DMARP frames *) ip : integer; (* Number for IP as protocol type *) fddi_addr_length : integer; (* Length of a MAC address in octetts *) ip_addr_length : integer; (* Length of a IP address in octetts *) OR_set_timeout : integer; (* Parameter for the OR-Entry-Timer *) Query_time : integer; (* Parameter for the Hold-Timer *) Hold_time : integer; (* Parameter for the Query-Timer *) 10.2. Channels channel SAPchn(User,Provider); by User : UNITDATA_request ( Source_addr : HW_addr_type; Dest_addr : HW_addr_type; QoS : QoS_type; PDU : PDU_type; ) by Provider : UNITDATA_indication ( Source_addr : HW_addr_type; Dest_addr : HW_addr_type; QoS : QoS_type; PDU : PDU_type; ) channel System_Access_Point_chn(User,Provider); by User: TIMER_request(Time : integer; Timer_id : timer_name_type; subscript : integer); TIMER_cancel(Timer_id : timer_name_type; subscript : integer); Kuehn [Page 14]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 by Provider: TIMER_response(Timer_id : timer_name_type; subscript : integer); 10.3. The Module Header and Interaction Points module LS_module systemprocess; ip LS_ARPSAP : SAPchn(Provider); LS_IPSAP : SAPchn(Provider); SNAP1_ARPSAP : SAPchn(User); SNAP1_LSSAP : SAPchn(User); SNAP1_IPSAP : SAPchn(User); SNAP2_ARPSAP : SAPchn(User); SNAP2_LSSAP : SAPchn(User); SNAP2_IPSAP : SAPchn(User); LS_System_Access_Point : System_Access_Point_chn(User); end; 10.4. The Modulebody of the Load Sharing Entity body LS_body for LS_module; module multiplexer_module process; ip LS_IPSAP : SAPchn(Provider); SNAP1_IPSAP : SAPchn(User); SNAP2_IPSAP : SAPchn(User); end; module cache_administration_module process; ip LS_ARPSAP : SAPchn(Provider); SNAP1_ARPSAP : SAPchn(User); SNAP1_LSSAP : SAPchn(User); SNAP2_ARPSAP : SAPchn(User); SNAP2_LSSAP : SAPchn(User); LS_System_Access_Point : System_Access_Point_chn(User); end; body cache_administration_body for cache_administration_module; (* defined later *) end; body multiplexer_body for multiplexer_module; (* defined later *) end; modvar cache_administration : cache_administration_module; Kuehn [Page 15]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 multiplexer : multiplexer_module; initialize begin ethernet := 1; ip := 2048; fddi_addr_length := 6; ip_addr_length := 4; init cache_administration with cache_administration_body; init multiplexer with multiplexer_body; attach LS_IPSAP to multiplexer.LS_IPSAP; attach SNAP1_IPSAP to multiplexer.SNAP1_IPSAP; attach SNAP2_IPSAP to multiplexer.SNAP2_IPSAP; attach LS_ARPSAP to cache_administration.LS_ARPSAP; attach SNAP1_ARPSAP to cache_administration.SNAP1_ARPSAP; attach SNAP1_LSSAP to cache_administration.SNAP1_LSSAP; attach SNAP2_ARPSAP to cache_administration.SNAP2_ARPSAP; attach SNAP2_LSSAP to cache_administration.SNAP2_LSSAP; attach LS_System_Access_Point to cache_administration. LS_System_Access_Point; end; end; 10.5. The Modulebody for the Multiplexer body multiplexer_body for multiplexer_module; type Type_of_addr_type = (individual, multi, broad); ring_type = (primary, secondary); var act_S_MAC_addr : HW_addr_type; function determ_addrtype(HW_addr: HW_addr_type): Type_of_addr_type; primitive; (* Returns the type of a hardware address. (Individual, multicast or broadcast address) *) function get_cacheentry(prtype: integer; P_MAC_addr: HW_addr_type; var S_MAC_addr : HW_addr_type): boolean; primitive; (* Returns the associated secondary MAC address for a given primary MAC address and protocol type. If an entry exists, the value TRUE is returned. *) Kuehn [Page 16]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 function ls_criteria : ring_type; (* Returns the ring on which the actual frame should be transmitted. *) primitive; trans when LS_IPSAP.UNITDATA_request(Source_addr,Dest_addr,QoS,PDU) begin if determ_addrtype(Dest_addr) <> individual then output SNAP1_IPSAP.UNITDATA_request(Source_addr,Dest_addr,QoS,PDU); else begin if get_cacheentry(ip,Dest_addr,act_S_MAC_addr) and (ls_criteria=secondary) then output SNAP2_IPSAP.UNITDATA_request(My_S_MAC_addr, act_S_MAC_addr,QoS,PDU); else output SNAP1_IPSAP.UNITDATA_request(Source_addr,Dest_addr,QoS,PDU); end; end; when SNAP1_IPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU) begin output LS_IPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU); end; when SNAP2_IPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU) begin if determ_addrtype(Dest_addr) = individual then begin Dest_addr := My_P_MAC_addr; output LS_IPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU); end; end; 10.6. The Modulebody for the Cache Administration body cache_administration_body for cache_administration_module; type arp_pdu_type = record hwtype : integer; prtype : integer; HW_length : integer; PR_length : integer; operation : (request,reply); HW_sender : HW_addr_type; PR_sender : PR_addr_type; HW_receiver : HW_addr_type; Kuehn [Page 17]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 PR_receiver : PR_addr_type; end; dmarp_operation_type = (request,reply,error,search,found,query,hold); dmarp_pdu_type = record hwtype : integer; prtype : integer; HW_length : integer; PR_length : integer; operation : dmarpoperation_type; HW_1 : HW_addr_type; HW_2 : HW_addr_type; PR_sender : PR_addr_type; PR_receiver : PR_addr_type; end; var arp_pdu : arp_pdu_type; dmarp_pdu : dmarp_pdu_type; send_pdu : dmarp_pdu_type; act_P_MAC_addr : HW_addr_type; function my_pr_address(prtype : integer ; praddr : PR_addr_type): boolean; (* Returns TRUE, if praddr is my station address, the protocol type is prtype. (2048d for the Internet protocol) *) primitive; function get_my_pr_addr(prtype : integer) : PR_addr_type; (* Returns my station address, the protocol has the number prtype. *) function extract_arp_pdu(PDU : PDU_type) : arp_pdu_type; (* Returns the data contained in an ARP PDU as a record. *) primitive; function extract_dmarp_pdu(PDU : PDU_type) : dmarp_pdu_type; (* Returns the data contained in an DMARP PDU as a record. *) primitive; Kuehn [Page 18]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 function assemble_dmarp_pdu(dmarp_pdu : dmarp_pdu_type): PDU; (* Returns a DMARP PDU from the data in the record. *) primitive; procedure create_entry(prtype: integer; P_MAC_addr: HW_addr_type; S_MAC_addr: HW_addr_type; LS_Bit: flag; OR_Bit: flag; H_Bit: flag; Q_Bit: flag; R_Bit: flag; V_Bit: flag); (* Creates a new entry in the address cache, if no entry with the given primary MAC address or R-bit set to one exists. The protocol type has the number prtype. The control bits are set as given in the parameters, the LS-bit is set last. *) primitive; function search_entry(prtype : integer; P_MAC_addr : HW_addr_type): boolean; (* Returns TRUE if an entry with the primary MAC address P_MAC_addr and the given protocol type was found in the address cache. *) primitive; procedure update_entry(prtype: integer; P_MAC_addr: HW_addr_type; S_MAC_addr: HW_addr_type); (* Searches an entry with the given primary MAC address P_MAC_address and updates the secondary MAC address in the entry if the R-bit is set to zero. *) primitive; procedure reset_LS_bit(prtype: integer; P_MAC_addr : HW_addr_type); (* Searches an entry with the given primary MAC address P_MAC_address and resets the LS-bit if the R-bit is reset. *) primitive; procedure set_Q_bit(prtype: integer; P_MAC_addr : HW_addr_type); (* Searches an entry with the given primary MAC address P_MAC_address and sets the Q-bit if the R-bit is reset. *) primitive; Kuehn [Page 19]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 function H_bit_set(prtype: integer; P_MAC_addr : HW_addr_type): boolean; (* Returns TRUE if an entry exists with H-bit set to one and the given P-MAC address. *) primitive; function OR_bit_set(prtype: integer; P_MAC_addr : HW_addr_type): boolean; (* Returns TRUE if an entry exists with OR-bit set to one and the given P-MAC address. *) primitive; function LS_bit_set(prtype: integer; P_MAC_addr : HW_addr_type): boolean; (* Returns TRUE if an entry exists with LS-bit set to one and the given P-MAC address. *) primitive; function Q_bit_set(prtype: integer; P_MAC_addr : HW_addr_type): boolean; (* Returns TRUE if an entry exists with Q-bit set to one and the given P-MAC address. *) primitive; function get_subscript(prtype: integer; P_MAC_addr : HW_addr_type): integer; (* Returns the subscipt number of an entry with the given primary MAC address. *) primitive; function get_broadcast_addr(prtype : integer): PR_addr_type; (* Returns the broadcast protocol address for the given protocol type. *) function get_P_MAC_addr(subscript : integer) : HW_addr_type; (* Returns the primary MAC address of the entry with the given subscript Kuehn [Page 20]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 number. *) primitive; function get_S_MAC_addr(prtype: integer; P_MAC_addr: HW_addr_type): HW_addr_type; (* Returns the secondary MAC address of the station with the given primary MAC address. *) primitive; procedure delete_entry(subscript : integer); (* Deletes the entry with the given subscript number if the R-bit is reset. *) primitive; function get_pr_type(subscript : integer) : integer; (* Returns the protocol type for the entry with the given subscript number. *) primitive; function get_pr_length(prtype : integer) : integer; (* Returns the length of a protocol address. *) primitive; trans when LS_ARPSAP.UNITDATA_request(Source_addr,Dest_addr,QoS,PDU) begin arp_pdu := extract_arp_pdu(PDU); output SNAP1_ARPSAP.UNITDATA_request(Source_addr,Dest_addr,QoS,PDU); dmarp_pdu.hwtype := ethernet; dmarp_pdu.prtype := arp_pdu.prtype; dmarp_pdu.HW_length := fddi_addr_length; dmarp_pdu.PR_length := arp_pdu.PR_length; dmarp_pdu.operation := reply; dmarp_pdu.HW_1 := My_P_MAC_addr; dmarp_pdu.HW_2 := My_S_MAC_addr; dmarp_pdu.PR_sender := arp_pdu.PR_sender; dmarp_pdu.PR_receiver := arp_pdu.PR_receiver; Kuehn [Page 21]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 PDU := assemble_dmarp_pdu(dmarp_pdu); output SNAP2_LSSAP.UNITDATA_request(My_S_MAC_addr,Broadcast_HW_addr, dmarp_QoS,PDU); end; when SNAP1_ARPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU) begin output LS_ARPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU); end; when SNAP2_ARPSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU) begin end; when SNAP1_LSSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU) begin dmarp_pdu := extract_dmarp_pdu(PDU); if ((dmarp_pdu.operation = error) or (dmarp_pdu.operation = reply)) then begin if my_pr_address(dmarp_pdu.prtype,dmarp_pdu.PR_receiver) then begin if not H_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then begin if not OR_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then begin if LS_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then begin output LS_System_Access_point.TIMER_cancel( "Hold_Timer",get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); create_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2, reset,set,reset,reset,reset,set); end; output LS_System_Access_point.TIMER_request( OR_set_timeout,"OR_Entry_Timer", get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); send_pdu.hwtype := ethernet; send_pdu.prtype := dmarp_pdu.prtype; send_pdu.HW_length := fddi_addr_length; send_pdu.PR_length := dmarp_pdu.PR_length; send_pdu.operation := search; send_pdu.HW_1 := My_P_MAC_addr; send_pdu.HW_2 := dmarp_pdu.HW_1; send_pdu.PR_sender := get_my_pr_addr(dmarp_pdu.prtype); send_pdu.PR_receiver := get_broadcast_addr(dmarp_pdu.prtype); PDU := assemble_dmarp_pdu(dmarp_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,Broadcast_HW_addr,dmarp_QoS,PDU); end else begin if dmarp_pdu.operation=error then update_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2); end; end else begin Kuehn [Page 22]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 if dmarp_pdu.operation = error then update_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2); end; end else begin if my_pr_address(dmarp_pdu.prtype,dmarp_pdu.PR_sender) and (dmarp_pdu.operation = reply) then begin dmarp_pdu.operation := error; PDU := assemble_dmarp_pdu(dmarp_pdu); output SNAP1_LSSAP.UNITDATA_request( My_P_MAC_addr,Broadcast_HW_addr,dmarp_QoS,PDU); end else begin if dmarp_pdu.operation=error and search_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1) then update_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2); end; end; end; end; when SNAP2_LSSAP.UNITDATA_indication(Source_addr,Dest_addr,QoS,PDU) begin dmarp_pdu := extract_dmarp_pdu(PDU); if (dmarp_pdu.operation = found) and my_pr_address(dmarp_pdu.prtype,dmarp_pdu.PR_receiver) then begin if not H_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then begin if OR_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then begin output LS_System_Access_Point. TIMER_cancel("OR_Entry_Timer", get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); end; if LS_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then begin output LS_System_Access_Point. TIMER_cancel("Hold_Timer", get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); end; create_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2, set,reset,set,reset,reset,set); output LS_System_Access_Point.TIMER_request(Query_time,"Query_Timer", get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); end; end else begin if (dmarp_pdu.operation = reply) or (dmarp_pdu.operation = request) then begin if search_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1) then update_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2); end; if (dmarp_pdu.operation=request) and my_pr_address(dmarp_pdu.prtype,dmarp_pdu.PR_receiver) then begin send_pdu.hwtype := dmarp_pdu.hwtype; send_pdu.prtype := dmarp_pdu.prtype; Kuehn [Page 23]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 send_pdu.HW_length := fddi_addr_length; send_pdu.PR_length := dmarp_pdu.PR_length; send_pdu.operation := reply; send_pdu.HW_1 := My_P_MAC_addr; send_pdu.HW_2 := My_S_MAC_addr; send_pdu.PR_sender := get_my_pr_addr(dmarp_pdu.prtype); send_pdu.PR_receiver := dmarp_pdu.PR_sender; PDU := assemble_dmarp_pdu(dmarp_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,Broadcast_HW_addr,dmarp_QoS,PDU); end else begin if my_pr_address(dmarp_pdu.prtype,dmarp_pdu.pr_receiver) then begin case dmarp_pdu.operation of reply: begin if not ( OR_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) or LS_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) )then begin create_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2, set,reset,reset,reset,reset,set); output LS_System_Access_Point.TIMER_request(Hold_time, "Hold_Timer",get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); end; end; error: begin if not ( OR_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) or H_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) ) then begin if LS_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then output LS_System_access_point.TIMER_cancel( "Hold_Timer",get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); create_entry(dmarp_pdu.prtype,dmarp_pdu.HW_1,dmarp_pdu.HW_2, reset,set,reset,reset,reset,set); output LS_System_access_point.TIMER_request( OR_set_timeout,"OR_Entry_Timer", get_subscript(dmarp_pdu.prtype,dmarp_pdu.HW_1)); send_pdu.hwtype := ethernet; send_pdu.prtype := dmarp_pdu.prtype; send_pdu.HW_length := fddi_addr_length; send_pdu.PR_length := dmarp_pdu.PR_length; send_pdu.operation := search; send_pdu.HW_1 := My_P_MAC_addr; send_pdu.HW_2 := dmarp_pdu.HW_1; send_pdu.PR_sender := get_my_pr_addr(dmarp_pdu.prtype); send_pdu.PR_receiver := get_broadcast_addr(dmarp_pdu.prtype); PDU := assemble_dmarp_pdu(dmarp_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,Broadcast_HW_addr,dmarp_QoS,PDU); end; end; Kuehn [Page 24]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 search: begin if not (dmarp_pdu.HW_1=My_P_MAC_addr or dmarp_pdu.HW_2=My_P_MAC_addr) then begin if H_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) and H_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_2) then begin send_pdu.hwtype := ethernet; send_pdu.prtype := dmarp_pdu.prtype; send_pdu.HW_length := fddi_addr_length; send_pdu.PR_length := dmarp_pdu.PR_length; send_pdu.operation := found; send_pdu.HW_1 := dmarp_pdu.HW_2; send_pdu.HW_2 := get_S_MAC_addr(dmarp_pdu.prtype, dmarp_pdu.HW_2); send_pdu.PR_sender := get_my_pr_addr(dmarp_pdu.prtype); send_pdu.PR_receiver := get_broadcast_addr(dmarp_pdu.prtype); PDU := assemble_dmarp_pdu(send_pdu); output SNAP2_LSSAP.UNITDATA_request(My_S_MAC_addr, get_S_MAC_addr(dmarp_pdu.prtype,dmarp_pdu.HW_1),dmarp_QoS,PDU); send_pdu.HW_1 := dmarp_pdu.HW_1; send_pdu.HW_2 := get_S_MAC_addr(dmarp_pdu.prtype, dmarp_pdu.HW_1); PDU := assemble_dmarp_pdu(send_pdu); output SNAP2_LSSAP.UNITDATA_request(My_S_MAC_addr, get_S_MAC_addr(dmarp_pdu.prtype,dmarp_pdu.HW_2),dmarp_QoS,PDU); end; end; end; Query: begin if dmarp_pdu.HW_2 = My_P_MAC_addr then begin send_pdu.hwtype := ethernet; send_pdu.prtype := dmarp_pdu.prtype; send_pdu.HW_length := dmarp_pdu.HW_length; send_pdu.PR_length := dmarp_pdu.PR_length; send_pdu.operation := hold; send_pdu.HW_1 := My_P_MAC_addr; send_pdu.HW_2 := My_S_MAC_addr; send_pdu.PR_sender := get_my_pr_addr(dmarp_pdu.prtype); send_pdu.PR_receiver := dmarp_pdu.PR_sender; PDU := assemble_dmarp_pdu(send_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,dmarp_pdu.HW_1,dmarp_QoS,PDU); end; end; Hold: begin if H_bit_set(dmarp_pdu.prtype,dmarp_pdu.HW_1) then reset_Q_bit(dmarp_pdu.prtype,dmarp_pdu.HW_1); Kuehn [Page 25]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 end; end; end; end; end; end; when LS_System_Access_Point.TIMER_response(Timer_name,subscript) begin case Timer_name of "OR_Entry_Timer": begin act_P_MAC_addr := get_P_MAC_addr(subscript); if OR_bit_set(get_pr_type(subscript),act_P_MAC_addr) then begin send_pdu.hwtype := ethernet; send_pdu.prtype := get_pr_type(subscript); send_pdu.HW_length := fddi_addr_length; send_pdu.PR_length := get_pr_length(send_pdu.prtype); send_pdu.operation := request; send_pdu.HW_1 := My_P_MAC_addr; send_pdu.HW_2 := My_S_MAC_addr; send_pdu.PR_sender := get_my_pr_addr(send_pdu.prtype); send_pdu.PR_receiver := get_broadcast_addr(send_pdu.prtype); PDU := assemble_dmarp_pdu(send_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,get_S_MAC_addr(send_pdu.prtype,act_P_MAC_addr), dmarp_QoS,PDU); delete_entry(subscript); end; end; "Hold_Timer": begin act_P_MAC_addr := get_P_MAC_addr(subscript); if (not H_bit_set(get_pr_type(subscript),act_P_MAC_addr)) and LS_bit_set(get_pr_type(subscript),act_P_MAC_addr) then begin set_H_bit(get_pr_type(subscript),act_P_MAC_addr); output LS_System_Access_point.TIMER_request( Query_time,"Query_Timer",subscript); end; end; "Query_Timer": begin act_P_MAC_addr := get_P_MAC_addr(subscript); send_pdu.hwtype := ethernet; send_pdu.prtype := get_pr_type(subscript); send_pdu.HW_length := fddi_addr_length; send_pdu.PR_length := get_pr_length(send_pdu.prtype); send_pdu.PR_sender := get_my_pr_addr(send_pdu.prtype); send_pdu.PR_receiver := get_broadcast_addr(send_pdu.prtype); if Q_bit_set(get_pr_type(subscript),act_P_MAC_addr) then begin send_pdu.HW_1 := My_P_MAC_addr; Kuehn [Page 26]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 send_pdu.HW_2 := My_S_MAC_addr; send_pdu.operation := request; PDU := assemble_dmarp_pdu(send_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,get_S_MAC_addr(send_pdu.prtype,act_P_MAC_addr), dmarp_QoS,PDU); delete_entry(subscript); end else begin send_pdu.HW_1 := My_S_MAC_addr; send_pdu.HW_2 := get_P_MAC_addr(subscript); send_pdu.operation := query; PDU := assemble_dmarp_pdu(send_pdu); output SNAP2_LSSAP.UNITDATA_request( My_S_MAC_addr,get_S_MAC_addr(send_pdu.prtype,send_pdu.HW_2), dmarp_QoS,PDU); set_Q_bit(send_pdu.prtype,send_pdu.HW_2); end; end; end; end; end; (* body *) 11. Summary The introduction of the load sharing layer in the protocol layering of the dual MAC stations allows the application of IP and ARP on inhomogeneous FDDI rings. The protocol suite of single MAC stations needs no modification. By the load sharing layer, the property "dual MAC" is transparent for ARP, IP and the higher layer protocols. In dual MAC stations, any load sharing criteria may be implemented in the multiplexer of the load sharing entity. The conversion of addresses, the exchange of address and reachability information between dual MAC stations and the proper transmission of multicast and broadcast frames is taken upon by the load sharing entity. 12. References [1] ANSI, "FDDI Station Management (SMT)", ANSI X3T9/90-X3T9.5/84-49 Rev 6.2, May 1990. [2] ANSI, "FDDI Media Access Control (MAC-2)", X3T9/90-X3T9.5/88-139 Rev 3.2, June 1990. [3] ISO, "Information processing systems- Local area networks- Part 2: Logical link control", ISO 8802-2:1989, August 1989. [4] IEEE, "Draft Standard P802.1A Overview and Architecture", P802.1A/D9-89/74, September 1989. Kuehn [Page 27]
RFC 1329 Address Resolution for Dual MAC FDDI Networks May 1992 [5] Plummer, C., "An Ethernet Address Resolution Protocol --or-- Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", RFC 826, MIT, November 1982. [6] Reynolds, J., and Postel, J., "Assigned Numbers", RFC 1060, USC/Information Sciences Institute, March 1990. [7] Postel, J., "Internet Protocol", RFC 791, USC/Information Sciences Institute, September 1981. [8] Katz, D., "A Proposed Standard for the Transmission of IP Datagrams over FDDI Networks", RFC 1188, Merit/NSFNET, October 1990. [9] Internet Engineering Task Force, Braden, R., Editor, "Requirements for Internet Hosts -- Communication Layers", RFC 1122, IETF, October 1989. [10] Katz, D., "The Use of Connectionless Network Layer Protocols over FDDI Networks", Merit/NSFNET, 1990. 13. Security Considerations Security issues are not discussed in this memo. 14. Author's Address Peter Kuehn Raiffeisenstrasse 9b 8933 Untermeitingen Germany Phone: .. 82 32 / 7 46 02 EMail: thimmela@sniabg.wa.sni.de Kuehn [Page 28]

   

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