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RFC 1490

Network Working Group                                        T. Bradley
Request for Comments: 1490               Wellfleet Communications, Inc.
Obsoletes: 1294                                                C. Brown
                                         Wellfleet Communications, Inc.
                                                               A. Malis
                                                   Ascom Timeplex, Inc.
                                                              July 1993


              Multiprotocol Interconnect over Frame Relay





Bradley, Brown & Malis                                          [Page 1]

RFC 1490 Multiprotocol over Frame Relay July 1993 1. Conventions and Acronyms The following language conventions are used in the items of specification in this document: o Must, Shall or Mandatory -- the item is an absolute requirement of the specification. o Should or Recommended -- the item should generally be followed for all but exceptional circumstances. o May or Optional -- the item is truly optional and may be followed or ignored according to the needs of the implementor. All drawings in this document are drawn with the left-most bit as the high order bit for transmission. For example, the dawings might be labeled as: 0 1 2 3 4 5 6 7 bits +---+---+---+---+---+---+---+ +---------------------------+ | flag (7E hexadecimal) | +---------------------------+ | Q.922 Address* | +-- --+ | | +---------------------------+ : : : : +---------------------------+ Drawings that would be too large to fit onto one page if each octet were presented on a single line are drawn with two octets per line. These are also drawn with the left-most bit as the high order bit for transmission. There will be a "+" to distinguish between octets as in the following example. Bradley, Brown & Malis [Page 2]
RFC 1490 Multiprotocol over Frame Relay July 1993 |--- octet one ---|--- octet two ---| 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ +--------------------------------------------+ | Organizationally Unique | +-- +--------------------+ | Identifier | Protocol | +-----------------------+--------------------+ | Identifier | +-----------------------+ The following are common acronyms used throughout this document. BECN - Backward Explicit Congestion Notification BPDU - Bridge Protocol Data Unit C/R - Command/Response bit DCE - Data Communication Equipment DE - Discard Eligibility bit DTE - Data Terminal Equipment FECN - Forward Explicit Congestion Notification PDU - Protocol Data Unit PTT - Postal Telephone & Telegraph SNAP - Subnetwork Access Protocol 2. Introduction The following discussion applies to those devices which serve as end stations (DTEs) on a public or private Frame Relay network (for example, provided by a common carrier or PTT. It will not discuss the behavior of those stations that are considered a part of the Frame Relay network (DCEs) other than to explain situations in which the DTE must react. The Frame Relay network provides a number of virtual circuits that form the basis for connections between stations attached to the same Frame Relay network. The resulting set of interconnected devices forms a private Frame Relay group which may be either fully interconnected with a complete "mesh" of virtual circuits, or only partially interconnected. In either case, each virtual circuit is uniquely identified at each Frame Relay interface by a Data Link Connection Identifier (DLCI). In most circumstances, DLCIs have strictly local significance at each Frame Relay interface. The specifications in this document are intended to apply to both switched and permanent virtual circuits. Bradley, Brown & Malis [Page 3]
RFC 1490 Multiprotocol over Frame Relay July 1993 3. Frame Format All protocols must encapsulate their packets within a Q.922 Annex A frame [1,2]. Additionally, frames shall contain information necessary to identify the protocol carried within the protocol data unit (PDU), thus allowing the receiver to properly process the incoming packet. The format shall be as follows: +---------------------------+ | flag (7E hexadecimal) | +---------------------------+ | Q.922 Address* | +-- --+ | | +---------------------------+ | Control (UI = 0x03) | +---------------------------+ | Optional Pad (0x00) | +---------------------------+ | NLPID | +---------------------------+ | . | | . | | . | | Data | | . | | . | +---------------------------+ | Frame Check Sequence | +-- . --+ | (two octets) | +---------------------------+ | flag (7E hexadecimal) | +---------------------------+ * Q.922 addresses, as presently defined, are two octets and contain a 10-bit DLCI. In some networks Q.922 addresses may optionally be increased to three or four octets. The control field is the Q.922 control field. The UI (0x03) value is used unless it is negotiated otherwise. The use of XID (0xAF or 0xBF) is permitted and is discussed later. The pad field is used to align the remainder of the frame to a two octet boundary. There may be zero or one pad octet within the pad field and, if present, must have a value of zero. The Network Level Protocol ID (NLPID) field is administered by ISO Bradley, Brown & Malis [Page 4]
RFC 1490 Multiprotocol over Frame Relay July 1993 and CCITT. It contains values for many different protocols including IP, CLNP and IEEE Subnetwork Access Protocol (SNAP)[10]. This field tells the receiver what encapsulation or what protocol follows. Values for this field are defined in ISO/IEC TR 9577 [3]. A NLPID value of 0x00 is defined within ISO/IEC TR 9577 as the Null Network Layer or Inactive Set. Since it cannot be distinguished from a pad field, and because it has no significance within the context of this encapsulation scheme, a NLPID value of 0x00 is invalid under the Frame Relay encapsulation. The Appendix contains a list of some of the more commonly used NLPID values. There is no commonly implemented minimum maximum frame size for Frame Relay. A network must, however, support at least a 262 octet maximum. Generally, the maximum will be greater than or equal to 1600 octets, but each Frame Relay provider will specify an appropriate value for its network. A Frame Relay DTE, therefore, must allow the maximum acceptable frame size to be configurable. The minimum frame size allowed for Frame Relay is five octets between the opening and closing flags assuming a two octet Q.922 address field. This minimum increases to six octets for three octet Q.922 address and seven octets for the four octet Q.922 address format. 4. Interconnect Issues There are two basic types of data packets that travel within the Frame Relay network: routed packets and bridged packets. These packets have distinct formats and therefore, must contain an indicator that the destination may use to correctly interpret the contents of the frame. This indicator is embedded within the NLPID and SNAP header information. For those protocols that do not have a NLPID already assigned, it is necessary to provide a mechanism to allow easy protocol identification. There is a NLPID value defined indicating the presence of a SNAP header. A SNAP header is of the form: +--------------------------------------------+ | Organizationally Unique | +-- +--------------------+ | Identifier | Protocol | +-----------------------+--------------------+ | Identifier | +-----------------------+ All stations must be able to accept and properly interpret both the Bradley, Brown & Malis [Page 5]
RFC 1490 Multiprotocol over Frame Relay July 1993 NLPID encapsulation and the SNAP header encapsulation for a routed packet. The three-octet Organizationally Unique Identifier (OUI) identifies an organization which administers the meaning of the Protocol Identifier (PID) which follows. Together they identify a distinct protocol. Note that OUI 0x00-00-00 specifies that the following PID is an Ethertype. 4.1. Routed Frames Some protocols will have an assigned NLPID, but because the NLPID numbering space is so limited, not all protocols have specific NLPID values assigned to them. When packets of such protocols are routed over Frame Relay networks, they are sent using the NLPID 0x80 (which indicates a SNAP follows) followed by SNAP. If the protocol has an Ethertype assigned, the OUI is 0x00-00-00 (which indicates an Ethertype follows), and PID is the Ethertype of the protocol in use. There will be one pad octet to align the protocol data on a two octet boundary as shown below. Format of Routed Frames with Ethertypes +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+ --+ | OUI 0x00-00 | +-------------------------------+ | Ethertype | +-------------------------------+ | Protocol Data | +-------------------------------+ | FCS | +-------------------------------+ In the few cases when a protocol has an assigned NLPID (see appendix), 48 bits can be saved using the format below: Bradley, Brown & Malis [Page 6]
RFC 1490 Multiprotocol over Frame Relay July 1993 Format of Routed NLPID Protocol +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | NLPID | +---------------+---------------+ | Protocol Data | +-------------------------------+ | FCS | +-------------------------------+ The NLPID encapsulation does not require a pad octet for alignment, so none is permitted. In the case of ISO protocols, the NLPID is considered to be the first octet of the protocol data. It is unnecessary to repeat the NLPID in this case. The single octet serves both as the demultiplexing value and as part of the protocol data (refer to "Other Protocols over Frame Relay for more details). Other protocols, such as IP, have a NLPID defined (0xCC), but it is not part of the protocol itself. Format of Routed IP Datagram +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | NLPID 0xCC | +---------------+---------------+ | IP Datagram | +-------------------------------+ | FCS | +-------------------------------+ 4.2. Bridged Frames The second type of Frame Relay traffic is bridged packets. These packets are encapsulated using the NLPID value of 0x80 indicating SNAP. As with other SNAP encapsulated protocols, there will be one pad octet to align the data portion of the encapsulated frame. The SNAP header which follows the NLPID identifies the format of the bridged packet. The OUI value used for this encapsulation is the 802.1 organization code 0x00-80-C2. The PID portion of the SNAP header (the two bytes immediately following the OUI) specifies the form of the MAC header, which immediately follows the SNAP header. Additionally, the PID indicates whether the original FCS is preserved within the bridged frame. The 802.1 organization has reserved the following values to be used with Frame Relay: Bradley, Brown & Malis [Page 7]
RFC 1490 Multiprotocol over Frame Relay July 1993 PID Values for OUI 0x00-80-C2 with preserved FCS w/o preserved FCS Media ------------------ ----------------- ---------------- 0x00-01 0x00-07 802.3/Ethernet 0x00-02 0x00-08 802.4 0x00-03 0x00-09 802.5 0x00-04 0x00-0A FDDI 0x00-0B 802.6 In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2, identifies bridged protocol data units (BPDUs) as defined by 802.1(d) or 802.1(g) [12]. A packet bridged over Frame Relay will, therefore, have one of the following formats: Format of Bridged Ethernet/802.3 Frame +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+ --+ | OUI 0x80-C2 | +-------------------------------+ | PID 0x00-01 or 0x00-07 | +-------------------------------+ | MAC destination address | : : | | +-------------------------------+ | (remainder of MAC frame) | +-------------------------------+ | LAN FCS (if PID is 0x00-01) | +-------------------------------+ | FCS | +-------------------------------+ Bradley, Brown & Malis [Page 8]
RFC 1490 Multiprotocol over Frame Relay July 1993 Format of Bridged 802.4 Frame +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+ --+ | OUI 0x80-C2 | +-------------------------------+ | PID 0x00-02 or 0x00-08 | +---------------+---------------+ | pad 0x00 | Frame Control | +---------------+---------------+ | MAC destination address | : : | | +-------------------------------+ | (remainder of MAC frame) | +-------------------------------+ | LAN FCS (if PID is 0x00-02) | +-------------------------------+ | FCS | +-------------------------------+ Bradley, Brown & Malis [Page 9]
RFC 1490 Multiprotocol over Frame Relay July 1993 Format of Bridged 802.5 Frame +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+ --+ | OUI 0x80-C2 | +-------------------------------+ | PID 0x00-03 or 0x00-09 | +---------------+---------------+ | pad 0x00 | Frame Control | +---------------+---------------+ | MAC destination address | : : | | +-------------------------------+ | (remainder of MAC frame) | +-------------------------------+ | LAN FCS (if PID is 0x00-03) | | | +-------------------------------+ | FCS | +-------------------------------+ Bradley, Brown & Malis [Page 10]
RFC 1490 Multiprotocol over Frame Relay July 1993 Format of Bridged FDDI Frame +-------------------------------+ | Q.922 Address | +---------------+---------------+ |Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+ --+ | OUI 0x80-C2 | +-------------------------------+ | PID 0x00-04 or 0x00-0A | +---------------+---------------+ | pad 0x00 | Frame Control | +---------------+---------------+ | MAC destination address | : : | | +-------------------------------+ | (remainder of MAC frame) | +-------------------------------+ | LAN FCS (if PID is 0x00-04) | | | +-------------------------------+ | FCS | +-------------------------------+ Bradley, Brown & Malis [Page 11]
RFC 1490 Multiprotocol over Frame Relay July 1993 Format of Bridged 802.6 Frame +-------------------------------+ | Q.922 Address | +---------------+---------------+ | Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+ --+ | OUI 0x80-C2 | +-------------------------------+ | PID 0x00-0B | +---------------+---------------+ ------- | Reserved | BEtag | Common +---------------+---------------+ PDU | BAsize | Header +-------------------------------+ ------- | MAC destination address | : : | | +-------------------------------+ | (remainder of MAC frame) | +-------------------------------+ | | +- Common PDU Trailer -+ | | +-------------------------------+ | FCS | +-------------------------------+ Note that in bridge 802.6 PDUs, there is only one choice for the PID value, since the presence of a CRC-32 is indicated by the CIB bit in the header of the MAC frame. The Common Protocol Data Unit (CPDU) Header and Trailer are conveyed to allow pipelining at the egress bridge to an 802.6 subnetwork. Specifically, the CPDU Header contains the BAsize field, which contains the length of the PDU. If this field is not available to the egress 802.6 bridge, then that bridge cannot begin to transmit the segmented PDU until it has received the entire PDU, calculated the length, and inserted the length into the BAsize field. If the field is available, the egress 802.6 bridge can extract the length from the BAsize field of the Common PDU Header, insert it into the corresponding field of the first segment, and immediately transmit the segment onto the 802.6 subnetwork. Thus, the bridge can begin transmitting the 802.6 PDU before it has received the complete PDU. One should note that the Common PDU Header and Trailer of the encapsulated frame should not be simply copied to the outgoing 802.6 Bradley, Brown & Malis [Page 12]
RFC 1490 Multiprotocol over Frame Relay July 1993 subnetwork because the encapsulated BEtag value may conflict with the previous BEtag value transmitted by that bridge. Format of BPDU Frame +-------------------------------+ | Q.922 Address | +-------------------------------+ | Control 0x03 | +-------------------------------+ | PAD 0x00 | +-------------------------------+ | NLPID 0x80 | +-------------------------------+ | OUI 0x00-80-C2 | +-------------------------------+ | PID 0x00-0E | +-------------------------------+ | | | BPDU as defined by | | 802.1(d) or 802.1(g)[12] | | | +-------------------------------+ 4. Data Link Layer Parameter Negotiation Frame Relay stations may choose to support the Exchange Identification (XID) specified in Appendix III of Q.922 [1]. This XID exchange allows the following parameters to be negotiated at the initialization of a Frame Relay circuit: maximum frame size N201, retransmission timer T200, and the maximum number of outstanding Information (I) frames K. A station may indicate its unwillingness to support acknowledged mode multiple frame operation by specifying a value of zero for the maximum window size, K. If this exchange is not used, these values must be statically configured by mutual agreement of Data Link Connection (DLC) endpoints, or must be defaulted to the values specified in Section 5.9 of Q.922: Bradley, Brown & Malis [Page 13]
RFC 1490 Multiprotocol over Frame Relay July 1993 N201: 260 octets K: 3 for a 16 Kbps link, 7 for a 64 Kbps link, 32 for a 384 Kbps link, 40 for a 1.536 Mbps or above link T200: 1.5 seconds [see Q.922 for further details] If a station supporting XID receives an XID frame, it shall respond with an XID response. In processing an XID, if the remote maximum frame size is smaller than the local maximum, the local system shall reduce the maximum size it uses over this DLC to the remotely specified value. Note that this shall be done before generating a response XID. The following diagram describes the use of XID to specify non-use of acknowledged mode multiple frame operation. Bradley, Brown & Malis [Page 14]
RFC 1490 Multiprotocol over Frame Relay July 1993 Non-use of Acknowledged Mode Multiple Frame Operation +---------------+ | Address | (2,3 or 4 octets) | | +---------------+ | Control 0xAF | +---------------+ | format 0x82 | +---------------+ | Group ID 0x80 | +---------------+ | Group Length | (2 octets) | 0x00-0E | +---------------+ | 0x05 | PI = Frame Size (transmit) +---------------+ | 0x02 | PL = 2 +---------------+ | Maximum | (2 octets) | Frame Size | +---------------+ | 0x06 | PI = Frame Size (receive) +---------------+ | 0x02 | PL = 2 +---------------+ | Maximum | (2 octets) | Frame Size | +---------------+ | 0x07 | PI = Window Size +---------------+ | 0x01 | PL = 1 +---------------+ | 0x00 | +---------------+ | 0x09 | PI = Retransmission Timer +---------------+ | 0x01 | PL = 1 +---------------+ | 0x00 | +---------------+ | FCS | (2 octets) | | +---------------+ 6. Fragmentation Issues Fragmentation allows the exchange of packets that are greater than the maximum frame size supported by the underlying network. In the Bradley, Brown & Malis [Page 15]
RFC 1490 Multiprotocol over Frame Relay July 1993 case of Frame Relay, the network may support a maximum frame size as small as 262 octets. Because of this small maximum size, it is recommended, but not required, to support fragmentation and reassembly. Unlike IP fragmentation procedures, the scope of Frame Relay fragmentation procedure is limited to the boundary (or DTEs) of the Frame Relay network. The general format of fragmented packets is the same as any other encapsulated protocol. The most significant difference being that the fragmented packet will contain the encapsulation header. That is, a packet is first encapsulated (with the exception of the address and control fields) as defined above. Large packets are then broken up into frames appropriate for the given Frame Relay network and are encapsulated using the Frame Relay fragmentation format. In this way, a station receiving fragments may reassemble them and then put the reassembled packet through the same processing path as a packet that had not been fragmented. Within Frame Relay fragments are encapsulated using the SNAP format with an OUI of 0x00-80-C2 and a PID of 0x00-0D. Individual fragments will, therefore, have the following format: +---------------+---------------+ | Q.922 Address | +---------------+---------------+ | Control 0x03 | pad 0x00 | +---------------+---------------+ | NLPID 0x80 | OUI 0x00 | +---------------+---------------+ | OUI 0x80-C2 | +---------------+---------------+ | PID 0x00-0D | +---------------+---------------+ | sequence number | +-+-------+-----+---------------+ |F| RSVD |offset | +-+-------+-----+---------------+ | fragment data | | . | | . | | . | +---------------+---------------+ | FCS | +---------------+---------------+ The sequence field is a two octet identifier that is incremented Bradley, Brown & Malis [Page 16]
RFC 1490 Multiprotocol over Frame Relay July 1993 every time a new complete message is fragmented. It allows detection of lost frames and is set to a random value at initialization. The reserved field is 4 bits long and is not currently defined. It must be set to 0. The final bit is a one bit field set to 1 on the last fragment and set to 0 for all other fragments. The offset field is an 11 bit value representing the logical offset of this fragment in bytes divided by 32. The first fragment must have an offset of zero. The following figure shows how a large IP datagram is fragmented over Frame Relay. In this example, the complete datagram is fragmented into two Frame Relay frames. Bradley, Brown & Malis [Page 17]
RFC 1490 Multiprotocol over Frame Relay July 1993 Frame Relay Fragmentation Example +-----------+-----------+ | Q.922 Address | +-----------+-----------+ | Ctrl 0x03 | pad 0x00 | +-----------+-----------+ |NLPID 0x80 | OUI 0x00 | +-----------+-----------+ | OUI 0x80-C2 | +-----------+-----------+ +-----------+-----------+ |ctrl 0x03 |NLPID 0xCC | | PID 0x00-0D | +-----------+-----------+ +-----------+-----------+ | | | sequence number n | | | +-+------+--+-----------+ | | |0| RSVD |offset (0) | | | +-+------+--+-----------+ | | | ctrl 0x03 |NLPID 0xCC | | | +-----------+-----------+ | | | first m bytes of | | large IP datagram | ... | IP datagram | | | | | | | +-----------+-----------+ | | | FCS | | | +-----------+-----------+ | | | | +-----------+-----------+ | | | Q.922 Address | | | +-----------+-----------+ | | | Ctrl 0x03 | pad 0x00 | +-----------+-----------+ +-----------+-----------+ |NLPID 0x80 | OUI 0x00 | +-----------+-----------+ | OUI 0x80-C2 | +-----------+-----------+ | PID 0x00-0D | +-----------+-----------+ | sequence number n | +-+------+--+-----------+ |1| RSVD |offset (m/32) | +-+------+--+-----------+ | remainder of IP | | datagram | +-----------+-----------+ | FCS | +-----------+-----------+ Fragments must be sent in order starting with a zero offset and ending with the final fragment. These fragments must not be Bradley, Brown & Malis [Page 18]
RFC 1490 Multiprotocol over Frame Relay July 1993 interrupted with other packets or information intended for the same DLC. An end station must be able to re-assemble up to 2K octets and is suggested to support up to 8K octet re-assembly. If at any time during this re-assembly process, a fragment is corrupted or a fragment is missing, the entire message is dropped. The upper layer protocol is responsible for any retransmission in this case. Note that there is no reassembly timer, nor is one needed. This is because the Frame Relay service is required to deliver frames in order. This fragmentation algorithm is not intended to reliably handle all possible failure conditions. As with IP fragmentation, there is a small possibility of reassembly error and delivery of an erroneous packet. Inclusion of a higher layer checksum greatly reduces this risk. 7. Address Resolution There are situations in which a Frame Relay station may wish to dynamically resolve a protocol address. Address resolution may be accomplished using the standard Address Resolution Protocol (ARP) [6] encapsulated within a SNAP encoded Frame Relay packet as follows: +-----------------------+-----------------------+ | Q.922 Address | +-----------------------+-----------------------+ | Control (UI) 0x03 | pad 0x00 | +-----------------------+-----------------------+ | NLPID = 0x80 | | SNAP Header +-----------------------+ OUI = 0x00-00-00 + Indicating | | ARP +-----------------------+-----------------------+ | PID = 0x0806 | +-----------------------+-----------------------+ | ARP packet | | . | | . | | . | +-----------------------+-----------------------+ Where the ARP packet has the following format and values: Data: ar$hrd 16 bits Hardware type ar$pro 16 bits Protocol type ar$hln 8 bits Octet length of hardware address (n) Bradley, Brown & Malis [Page 19]
RFC 1490 Multiprotocol over Frame Relay July 1993 ar$pln 8 bits Octet length of protocol address (m) ar$op 16 bits Operation code (request or reply) ar$sha noctets source hardware address ar$spa moctets source protocol address ar$tha noctets target hardware address ar$tpa moctets target protocol address ar$hrd - assigned to Frame Relay is 15 decimal (0x000F) [7]. ar$pro - see assigned numbers for protocol ID number for the protocol using ARP. (IP is 0x0800). ar$hln - length in bytes of the address field (2, 3, or 4) ar$pln - protocol address length is dependent on the protocol (ar$pro) (for IP ar$pln is 4). ar$op - 1 for request and 2 for reply. ar$sha - Q.922 source hardware address, with C/R, FECN, BECN, and DE set to zero. ar$tha - Q.922 target hardware address, with C/R, FECN, BECN, and DE set to zero. Because DLCIs within most Frame Relay networks have only local significance, an end station will not have a specific DLCI assigned to itself. Therefore, such a station does not have an address to put into the ARP request or reply. Fortunately, the Frame Relay network does provide a method for obtaining the correct DLCIs. The solution proposed for the locally addressed Frame Relay network below will work equally well for a network where DLCIs have global significance. The DLCI carried within the Frame Relay header is modified as it traverses the network. When the packet arrives at its destination, the DLCI has been set to the value that, from the standpoint of the receiving station, corresponds to the sending station. For example, in figure 1 below, if station A were to send a message to station B, it would place DLCI 50 in the Frame Relay header. When station B received this message, however, the DLCI would have been modified by the network and would appear to B as DLCI 70. Bradley, Brown & Malis [Page 20]
RFC 1490 Multiprotocol over Frame Relay July 1993 ~~~~~~~~~~~~~~~ ( ) +-----+ ( ) +-----+ | |-50------(--------------------)---------70-| | | A | ( ) | B | | |-60-----(---------+ ) | | +-----+ ( | ) +-----+ ( | ) ( | ) <---Frame Relay ~~~~~~~~~~~~~~~~ network 80 | +-----+ | | | C | | | +-----+ Figure 1 Lines between stations represent data link connections (DLCs). The numbers indicate the local DLCI associated with each connection. DLCI to Q.922 Address Table for Figure 1 DLCI (decimal) Q.922 address (hex) 50 0x0C21 60 0x0CC1 70 0x1061 80 0x1401 If you know about frame relay, you should understand the correlation between DLCI and Q.922 address. For the uninitiated, the translation between DLCI and Q.922 address is based on a two byte address length using the Q.922 encoding format. The format is: 8 7 6 5 4 3 2 1 +------------------------+---+--+ | DLCI (high order) |c/r|ea| +--------------+----+----+---+--+ | DLCI (lower) |FECN|BECN|DE |EA| +--------------+----+----+---+--+ For ARP and its variants, the FECN, BECN, C/R and DE bits are assumed to be 0. When an ARP message reaches a destination, all hardware addresses Bradley, Brown & Malis [Page 21]
RFC 1490 Multiprotocol over Frame Relay July 1993 will be invalid. The address found in the frame header will, however, be correct. Though it does violate the purity of layering, Frame Relay may use the address in the header as the sender hardware address. It should also be noted that the target hardware address, in both ARP request and reply, will also be invalid. This should not cause problems since ARP does not rely on these fields and in fact, an implementation may zero fill or ignore the target hardware address field entirely. As an example of how this address replacement scheme may work, refer to figure 1. If station A (protocol address pA) wished to resolve the address of station B (protocol address pB), it would format an ARP request with the following values: ARP request from A ar$op 1 (request) ar$sha unknown ar$spa pA ar$tha undefined ar$tpa pB Because station A will not have a source address associated with it, the source hardware address field is not valid. Therefore, when the ARP packet is received, it must extract the correct address from the Frame Relay header and place it in the source hardware address field. This way, the ARP request from A will become: ARP request from A as modified by B ar$op 1 (request) ar$sha 0x1061 (DLCI 70) from Frame Relay header ar$spa pA ar$tha undefined ar$tpa pB Station B's ARP will then be able to store station A's protocol address and Q.922 address association correctly. Next, station B will form a reply message. Many implementations simply place the source addresses from the ARP request into the target addresses and then fills in the source addresses with its addresses. In this case, the ARP response would be: ARP response from B ar$op 2 (response) ar$sha unknown ar$spa pB ar$tha 0x1061 (DLCI 70) ar$tpa pA Bradley, Brown & Malis [Page 22]
RFC 1490 Multiprotocol over Frame Relay July 1993
RFC 1490 Multiprotocol over Frame Relay July 1993 [3] Information technology - Telecommunications and Information Exchange between systems - Protocol Identification in the Network Layer, ISO/IEC TR 9577: 1990 (E) 1990-10-15. [4] Baker, F., Editor, "Point to Point Protocol Extensions for Bridging", RFC 1220, ACC, April 1991. [5] International Standard, Information Processing Systems - Local Area Networks - Logical Link Control, ISO 8802-2: 1989 (E), IEEE Std 802.2-1989, 1989-12-31. [6] Plummer, D., "An Ethernet Address Resolution Protocol - or - Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT, November 1982. [7] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1340, USC/Information Sciences Institute, July 1992. [8] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse Address Resolution Protocol", STD 38, RFC 903, Stanford University, June 1984. [9] Postel, J. and Reynolds, J., "A Standard for the Transmission of IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information Sciences Institute, February 1988. [10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks: Overview and architecture", IEEE Standards 802-1990. [11] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol", RFC 1293, Wellfleet Communications, Inc., January 1992. [12] IEEE, "IEEE Standard for Local and Metropolitan Networks: Media Access Control (MAC) Bridges", IEEE Standard 802.1D-1990. [13] PROJECT 802 - LOCAL AND METROPOLITAN AREA NETWORKS, Draft Standard 802.1G: Remote MAC Bridging, Draft 6, October 12, 1992. 14. Security Considerations Security issues are not discussed in this memo. Bradley, Brown & Malis [Page 34]
RFC 1490 Multiprotocol over Frame Relay July 1993 15. Authors' Addresses Terry Bradley Wellfleet Communications, Inc. 15 Crosby Drive Bedford, MA 01730 Phone: (617) 280-2401 Email: tbradley@wellfleet.com



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