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ACKNOWLEDGMENTS This specification was prepared by BBN Communications Corporation under contract to the Defense Data Network Program Management Office of the Defense Communications Agency. The specification has been reviewed by the Defense Communications Engineering Center for accuracy and completeness. The draft of this specification has been disseminated to industry by the National Bureau of Standards for review and comments which have been incorporated in the final specification. This specification has been approved for use on the Defense Data Network by the DoD Protocol Standards Steering Group. Comments on this specification should be directed to the Defense Communications Agency, ATTN: Defense Data Network Program Managment Office, Code B610, Washington, D.C. 20305 Table of Contents 1 INTRODUCTION.......................................... 1 1.1 Background.......................................... 1 1.1.1 X.25 and FIPS 100/Federal Standard 1041........... 1 1.1.2 X.25-to-X.25 and X.25-to-1822 Interoperability................................ 2 1.2 Compliance.......................................... 4 1.2.1 Compliance With CCITT X.25 and FIPS 100/Fed. Std. 1041.............................. 4 1.2.2 DTE Compliance With This Specification............ 4 2 INTERFACE SPECIFICATION............................... 6 2'1 Call Establishment Conventions...................... 6 2.1.1 Addressing........................................ 6 2.1.1.1 Address Formats and Fields...................... 6 2.1.1.1.1 Reserved...................................... 7 2.1.1.1.2 Flag.......................................... 7 2.1.1.1.3 DDN host Identifier........................... 7 2.1.1.1.4 Sub-Address................................... 7 2.1.1.2 Supplying Missing Address Information........... 7 2.1.2 DDN-Specific Facilities........................... 8 2.1.2.1 Type of Service Selection....................... 8 2.1.2.2 Call Precedence................................. 9 2.1.3 Protocol Identification.......................... 10 2.1.4 Logical Channel Assignment....................... 10 2.2 Packet Level Procedures............................ 11 2.3 Link Level Procedures.............................. 12 2.3.1 Link Level Parameters and Options................ 12 2.3.2 Timer T1 and Parameter T2........................ 12 2.3.3 Maximum I Frame Size............................. 13 2.4 Physical Level Specifications...................... 14 3 BIBLIOGRAPHY......................................... 16 APPENDIX A: DDN X.25 Implementation Details............ A-1 A-1 Introduction...................................... A-1 A-2 Operational Features of DDN X.25 DCE Releases..... A-1 A-2.1 Initial Feature Support......................... A-1 A-2.2 Exception-Handling Procedures................... A-2 A-2.2.1 Non-Octet-Aligned Data........................ A-2 A-2.2.2 RESTART REQUEST Packet........................ A-2 A-2.2.3 RESET REQUEST Packet.......................... A-2 A-2.2.9 CLEAR REQUEST Packet.......................... A-3 A-2.3 Virtual Circuit Resource Availability........... A-3 A-3 Detailed Features and Facilities Specifications.................................. A-3 A-3.1 Additional Diagnostic Codes..................... A-3 A-3.2 X.25 IP Interoperability Considerations......... A-6 A-3.3 The DDN Logical Addressing Facility............. A-7 A-3.3.1 Logical Addresses............................. A-7 A-3.3.2 Enabling and Disabling Logical Addresses...... A-7 A-4 Limitations of DDN Basic X.25 Service............. A-8 A-5 Derivation of DDN X.25 Addresses.................. A-9 APPENDIX Q: DDN Synchronous Level 1 Specification...... B-1 B-1 Introduction...................................... B-1 B-2 Supported Interfaces.............................. B-1 APPENDIX C: Federal Information Processing Standard Publication 100...................................... C-1 TABLES DDN X.25 Address Fields................................... 7 "Derivation of Maximum I Frame Size".................... 14 DDN X.25 Physical Signaling Rates and Interfaces......... 15 Additional Packet Level Diagnostic Codes................ A-4 IP Precedence to X.25 Precedence Mapping................ A-6 EIA and CCITT Interchange Circuits...................... B-3 Signal Selection by CCITT Interchange Circuit Number................................................ B-4 Typical Level 1 Connection Schemes...................... B-5 Interface Type by Service Speed......................... B-7 RS-232-C Interface...................................... B-8 MIL-188-114 Interface (and equivalents)................. B-9 V.35 Interface......................................... B-10 FIGURES Typical Level 1 Connection Schemes...................... B-4 INTRODUCTION This report specifies the attachment of an X.25 host to the Defense Data Network (DDN). In particular, this report describes specific options and features of CCITT Recommendation X.25 (1980) and Federal Information Processing Standard (FIPS) 100/Federal Standard (Fed. Std.) 1041 (July 1983) required of a host X.25 implementation to enable that host to communicate with a DDN X.25 Interface Message Processor ("IMP", the DDN packet switching node). This report, in conjunction with FIPS 100/Fed. Std. 1041, should enable DDN host site managers and others planning to attach a host by means of X.25, rather than the 1822 interface, to determine, first, whether or not the X.25 implementation of the host in question is adequate for operation with DDN, and, second, what options, parameter settings, etc. must or may be selected for operation with DDN. This report assumes that the reader is familiar with CCITT Recommendation X.25 and FIPS 100/Fed. Std. 1041. A copy of FIPS 100/Fed. Std. 1041 is attached as Appendix C of this report. In this document, the term "Administration" refers to the Defense Communications Agency (DCA Code B610, Washington, D. C. 20305). 1.1 Background 1.1.1 X.25 and FIPS 100/Federal Standard 1041 The CCITT Recommendation X.25 describes the interface between host computers (data terminal equipment, or DTEs) and data circuit-terminating equipment (DCEs, which effect communication with remote hosts over computer networks) for hosts operating in the packet mode on public data networks. The X.25 interface standard is defined as three independent architectural levels, following the Open Systems Interconnection (OSI) Reference Model. The three levels are: Level 1: The PHYSICAL level of the connection. The physical, electrical, functional, and procedural characteristics to activate, _____________ * As used in this report, "1822 interface" refers to the interface specified in Bolt Beranek and Newman Inc. (BBN) Report No. 1822, "Specification for the Interconnection of a Host and an IMP," revision of December 1981. -1- maintain, and deactivate the physical link between the DTE and the DCE. Level 2: The LINK level of the connection. The link access procedure for data interchange across the link between the DTE and the DCE. Level 3: The PACKET level of the connection. The packet format and control procedures for the exchange of packets containing control information and user data between the DTE and the DCE, and between the DTE and a remote DTE. CCITT Recommendation X.25 contains many options and implementation choices. FIPS 100/Fed. Std. 1041, which specifies the general use of X.25 for the Federal Government, defines some of the choices left open in X.25. This document describes the X.25 interface to a particular network, DDN. Thus in several areas where X.25 allows a choice, a single choice appropriate for DDN is specified; in areas which X.25 leaves unspecified, addressing in particular, conventions are specified that are consistent with the overall architecture of DDN and the interoperability goals described below. The effect of this approach is to make DDN service available to hosts in a way that requires no changes to a host DTE implementation that is compliant with FIPS 100/Fed. Std. 1041 and CCITT Recommendation X.25. By implementing extensions described in this specification, a host will be able to take advantage of additional DDN features required in military networks, such as precedence and logical addressing. The reader is referred to CCITT Recommendation X.25 and to FIPS 100/Fed. Std. 1041 for detailed information not provided in the body of this document. 1.1.2 X.25-to-X.25 and X.25-to-1822 Interoperability A key goal of the DDN X.25 implementation is interoperability among all DDN subscribers. That is, effective communication should be possible, not only between subscribers attached to the DDN using identical vendor-supplied X.25 implementations, but between subscribers using different X.25 implementations, and between a subscriber using an X.25 interface to the DDN and a subscriber using an 1822 interface to the DDN. Achieving this goal of interoperability requires that all DDN -2- X.25 subscribers conform to this interface specification and implement the DoD standard higher level protocols. True interoperability among DDN hosts requires, in particular, implementation of the DoD standard protocols TCP (Transmission Control Protocol) and IP (Internet Protocol), as well as the higher-level protocols which implement DDN standard services, " when such services are provided by the host: the Telnet Protocol for character-oriented terminal support, the File Transfer Protocol (FTP) for file movement between hosts, and the Simple Mail Transfer Protocol (SMTP) for communication between electronic mail service hosts. The DDN X.25 DCE offers two types of service to X.25 DTEs: 1. DDN Standard X.25 Service, which, when used in conjunction with DoD standard protocols, provides interoperable communication between an X.25 DTE and other DDN hosts that also implement the DoD standard protocols, whether they are connected to DDN via the 1822 interface or via the X.25 interface; and 2. DDN Basic X.25 Service, which provides communication only between an X.25 DTE and other DDN X.25 DTEs implementing compatible higher-level protocols. Section 2.1.2.1 of this report describes the conventions to be used by a DTE to specify the type of service desired for each X.25 virtual call. All DDN X.25 DTEs will be required to develop and initiate a plan to use the DoD standard protocol architecture and DDN standard X.25 service. Use of DDN basic X.25 service imposes some restrictions on the nature of the network communications service that a host can obtain. These restrictions are discussed in Appendix A, Section A-4. -3- 1.2 Compliance 1.2.1 Compliance With CCITT X.25 and FIPS 100/Fed. Std. 1041 The DDN X.25 Interface Specification is compliant with CCITT Recommendation X.25 and FIPS 100/Fed. Std. 1041. The DDN X.25 DCE supports all facilities specified as E (essential) by FIPS 100/Fed. Std. 1041, and no facilities specified as A (additional). The additional facilities not supported are: (i) datagrams and associated facilities, and (ii) bilateral closed user groups. In that FIPS 100/Fed. Std. 1041 describes features for a DCE, DDN X.25 DTEs may support any or all facilities specified as either E or A by FIPS 100/Fed Std. 1041. However, DDN X.25 DTEs must not use the facilities identified above that are not supported by the DDN X.25 DCE. 1.2.2 DTE Compliance With This Specification This document specifies several areas in which the DDN X.25 DCE is capable of operating in several modes. For example, Section 2.4 lists a number of signaling rates supported by the DCE. In such cases, a DDN X.25 DTE must implement at least one of the options listed (or the set of options required of a DTE by FIPS 100/Fed. Std. 1041) but need not implement all of the options listed (unless required by FIPS 100/Fed. Std. 1041). Determining the adequacy of the options supported by a DTE vendor for meeting a DDN subscriber's requirements is the responsibility of the subscriber. In addition to the CCITT X.25 and FIPS 100/Fed. Std. 1041 requirements described in Section 1.2.1 above, DDN X.25 DTEs may wish to take advantage of additional DDN-specific features that are compatible extensions to the public standards. Implementation of a DDN-specific feature by a host is required only if the host wishes to take advantage of the service or information provided by the feature. For example, a host that wishes to establish calls only at the default precedence level assigned to it need not implement the precedence facility described in Section 2.1.2.2. However, a host that wishes to have flexibility in the precedence of the calls it establishes must implement this facility. -4- Any deficiencies with respect to this specification in a vendor-supplied X.25 DTE implementation contemplated for use with the DDN X.25 DCE should be rectified so as to attain compliance with this specification. Proper operation with DDN of an X.25 DTE that is not compliant with this specification cannot be guaranteed and should not be attempted. To this end, a test program is available through the Administration. 2 INTERFACE SPECIFICATION 2.1 Call Establishment Conventions This section specifies DDN X.25 call establishment conventions. 2.1.1 Addressing DDN addresses are assigned to subscriber DTEs by the Administration. Two basic forms of address are provided: physical addresses, which correspond to the node number and DCE port number of the node to which the DTE is connected, and logical addresses, which are mapped transparently by DCE software into a corresponding physical network address. Each DTE is assigned one physical address, and may be assigned one or more logical addresses. All DDN addresses are either twelve or fourteen BCD (binary-coded decimal) digits in length. A calling DTE need not determine whether a given address is a physical or logical address, in order to establish a call to that address. .2.1.1.1 Address Formats and Fields DDN addresses have the following format: ZZZZ F DDDDDDD (SS) The various fields of the address are presented in Table 2.1 and are explained below. Length Field Meaning (BCD digits) ZZZZ Reserved (must be zero) 4 F Flag 1 DDDDDDD DDN Host Identifier 7 (SS) Sub-address (optional) 0 or 2 TOTAL 12 or 14 Table 2.1 DDN X.25 Address Fields -6- 2.1.1.1.1 Reserved The Reserved field corresponds to the DNIC field generally used in public data networks. Pending assignment of a DDN DNIC, this field must be zero. 2.1.1.1.2 Flag The Flag field is used to differentiate physical and logical addressing. The value zero indicates physical addressing, while the value one indicates logical addressing. A value of nine is used in the setup of calls to enable and disable logical addresses; see Appendix A, Section A-3.3.1. 2.1.1.1.3 DDN Host Identifier The DDN Host Identifier is a seven-digit address, either logical or physical, assigned to a subscriber DTE by the DDN Administration. 2.1.1.1.4 Sub-Address The Sub-Address may be used by a DTE for any.purpose. It is carried across the network without modification. Its presence is optional. 2.1.1.2 Supplying Missing Address Information The DDN X.25 DCE incorporates a mechanism to supply "missing" address information in CALL REQUEST and CALL ACCEPTED packets received from an attached DTE. This mechanism is useful in DTE software testing and physical address determination. If a DTE sends a CALL REQUEST packet with no calling address field, the local DCE will insert the physical calling DDN Host Identifier with no subaddress field. If a DTE sends a CALL REQUEST or CALL ACCEPTED packet with either or both calling or called addresses that contain F = zero and DDDDDDD = zero, the local DCE will replace the DDN Host Identifier field (DDDDDDD) with the physical address of the DTE. -7- DTE implementors are cautioned that use of this mechanism in accepting calls to a DTE's logical address (See Appendix A, Section A-3.3) can result in confusion on the part of the calling DTE and is not advised. 2.1.2 DDN-Specific Facilities Two DDN-specific features are requested by means of "private" or non-CCITT facilities in CALL REQUEST and CALL ACCEPTED packets. If either or both of these facilities are requested in a CALL REQUEST or CALL ACCEPTED packet, they must follow all CCITT X.25 facilities and must be preceded by a single facility marker, two octets of zero. 2.1.2.1 Type of Service Selection The DDN X.25 provides two types of service, DDN basic X.25 service and DDN standard X.25 service. DDN standard X.25 service provides only local DTE to local DCE support of the X.25 connection. Data is carried via the network to its destination (using protocols internal to the network), where it is delivered using the access protocol of the destination host (i.e., either 1822 or DDN standard X.25 service). This access method is oriented towards DDN X.25 hosts using the DoD standard TCP/IP higher level protocols. No X.25 procedures change when using DDN standard X.25 service; however, the significance of the procedures changes (see Appendix A, Section A-3.2). There is no end-to-end X.25-level acknowledgment or guarantee of delivery of data packets with DDN standard X.25 service; reliability of DDN standard X.25 service is provided instead by the use of a reliable transport protocol. DDN basic X.25 service provides end-to-end call management with significance as described in CCITT Recommendation X.25 and FIPS 100/Fed. Std. 1041. This access method is oriented towards hosts that have existing higher level protocol implementations that require reliable packet delivery at the network level. Selection of DDN standard or DDN basic X.25 service must be made on a call-by-call basis by the DDN X.25 DTE at the time of call setup. To specify DDN standard X.25 service, a DTE must include in the CALL REQUEST packet a facility two octets long, coded as follows: 00000100 00000001 -8- If this facility is not specified, DDN basic X.25 service will be provided. 2.1.2.2 Call Precedence The precedence of a call is negotiated by an X.25 DTE by means of a facility two octets long, coded as: 00001000 000000XX where XX is the precedence, from 0 (lowest precedence) to 3 (highest precedence). If this facility is not used, the call will be established at the subscriber's default precedence. A DTE is not permitted to establish a call at a precedence level higher than that authorized for that DTE by the Administration. An attempt to do so will result in the DDN X.25 DCE returning to the DTE a CLEAR INDICATION packet with clearing cause 00001001, "Out of order," with diagnostic code 194, "Requested precedence too high". Calls of a lower precedence may be cleared by a DCE if DCE or other network resources are required, or if access to the local or remote DTE is required (for a call of higher precedence). In this event, a CLEAR INDICATION packet will be sent with the clearing cause 00000101, "Network congestion," and with a diagnostic code specifying the reason for the preemption. The diagnostic codes employed for this purpose are 192, "Cleared due to higher precedence call at local DCE," and 193, "Cleared due to higher precedence call at remote DCE". Similarly, an attempt to establish a call may be unsuccessful if network resources are engaged in calls of higher priority than that requested. In this case, a CLEAR INDICATION packet will be sent with the clearing cause 00001001, "Out of order," and with either diagnostic code 192 or 193, as appropriate. The diagnostic codes described in the preceding paragraphs are DDN-specific diagnostic codes; additional information about these codes may be found in Appendix A, Section A-3.1. -9- 2.1.3 Protocol Identification X.25 DTEs employing the DoD standard TCP/IP protocol architecture must indicate this by means of the call user data field of the CALL REQUEST packet. The first octet of this field must be set to 11001100 to identify the DoD standard protocol architecture. Indication of the use of the DoD standard protocol architecture is independent of the selection of DDN standard or DDN basic X.25 service by means of the facility specified in Section 2.1.2.1 above. Therefore, a host employing the DoD standard protocol architecture and using DDN standard X.25 service must include both the DDN standard X.25 service facility and the call user data DoD standard protocol identification in its CALL REQUEST packet. A DTE using a protocol architecture other than the standard DoD protocol architecture is free to use any call user data protocol identification recognized by the DTEs with which it wishes to communicate. Identification of protocol architectures other than the DoD standard architecture is not standardized or enforced by the Administration. Subscribers are cautioned, therefore, that conflicts among various vendor-assigned protocol identifications may arise. 2.1.4 Logical Channel Assignment The assignment of logical channels by the DDN X.25 DCE follows the requirements and guidelines of FIPS 100/Fed. Std. 1041 and Annex A of CCITT X.25. Within the guidelines of CCITT X.25 Annex A, the range of logical channel numbers assigned to permanent virtual circuits, incoming, two-way, and outgoing virtual calls for DDN DCEs is configured for each DTE attached to a DCE by the Administration. DDN X.25 DTEs must follow the logical channel selection requirements of FIPS 100/Fed. Std. 1041. The number of logical channels available to a DTE is dependent upon the configuration of the DCE to which the DTE is attached, and upon the dynamic requirements placed upon other DCEs that share the same DDN packet switching node. -10- 2.2 Packet Level Procedures DDN X.25 packet level procedures are as specified by FIPS 100/Fed. Std. 1041 and CCITT X.25. The following additional information is provided: 1. The maximum window size that may be negotiated is seven. 2. Modulo 128 packet level sequence numbering is not supported. 3. Maximum packet sizes of 16, 32, 64, 128, 256, 512, and 1024 octets may be negotiated. 4. The DDN X.25 DCE uses additional packet level diagnostic codes, specified in Appendix A, Table A-1. DDN X.25 DTEs may, but are not required to, make use of the information conveyed by these codes. 5. The Qualifier bit (Q-bit) is passed transparently by the DDN X.25 DCE in DDN basic X.25 service. DTEs using DDN basic X.25 service may use the Q- bit in any way that is consistent with FIPS 100/Fed. Std. 1041. 6. The DDN X.25 DCE implements the diagnostic packet. It is sent under conditions specified in Annex D of CCITT X.25. The DTE is not required to act on the information provided in diagnostic packets. 7. DTEs using DDN standard X.25 service must restrict the maximum number of data bits in a complete packet sequence to be no more than 8056. This ensures that the data from a packet sequence transmitted by an X.25 host will fit within the maximum 1822 message length limit upon delivery to an 1822 host. This restriction is necessary as existing 1822 host implementations are not re- quired to accept messages longer than 8063 bits. * ________________ * DTEs using DDN standard X.25 service will generally be transmitting Internet Protocol datagrams, the length of which, by convention, does not approach this limit. Therefore, unless a protocol other than the Internet Protocol is used with DDN standard X.25 service, this is a technical restriction that will have no practical impact upon the design of DTE software. See Appendix A, Section A-3.2. -11- DDN X.25 DTEs connecting to DDN through an X.25 Internet Private Line Interface (IPLI) must reduce the maximum complete packet sequence length by an additional 256 bits to allow for IPLI overhead. 2.3 Link Level Procedures DDN X.25 link level procedures are as specified by FIPS 100/Fed. Std. 1041 and CCITT X.25. This section presents additional information. 2.3.1 Link Level Parameters and Options 1. The default value of K, the maximum number of sequentially numbered I frames that the DCE will have outstanding (unacknowledged) at any given time, is seven. A DDN X.25 DCE may be configured on a per-DTE basis to provide optional values of K from one to six. 2. The default value of N2, the maximum number of transmissions and retransmissions of a frame following the expiration of the T1 timer, is twenty. This value can be changed to any value from one to 200 as a DCE configuration parameter on a per-DTE basis. 3. The optional 32-bit FCS is not supported. 2.3.2 Timer T1 and Parameter T2 The period of the timer T1 used by the DDN X.25 DCE reflects assumptions about the processing speed of the DTE. The DCE assumes that parameter T2, the response latency of the DTE to a frame from the DCE, is no greater than 1/2 second. Likewise, the DCE guarantees that its parameter T2, the latency in responding to frames from the DTE, is 1/2 second for signaling rates of 19.2 Kb/s or slower, and 1/4 second for faster links. A lower bound for timer T1 may be computed to be 4X + T2, based on the assumptions that: * the link propagation time is negligible, -12- * the worst-case frame transmission time is X, * timer T1 is started when a frame is scheduled for output, * each frame is scheduled just as transmission of the previous frame starts, * frames are not aborted, and * each frame and its predecessor are of maximum length Nl = 8248 bits (see Section 2.3.3 below). As an example, for a signaling rate of 9.6 Kb/s, this yields X = .86 sec. If T2 is .5 sec., the total time for the DTE to respond in the worst case should be 3.9 seconds. In fact, the DCE uses a T1 timer value of 4 seconds for a link speed of 9.6 Kb/s. In no case does the DCE use a value for T1 smaller than 3 seconds. This means that, for faster links, the DTE's T2 parameter may be lengthened because the X term in the above formula is smaller. For links of 19.2 Kb/s or faster, DTEs are expected to satisfy latency requirements that allow the DCE to use the formula 4X + T2 (DTE) < 3 seconds = T1 (DCE). The DTE may choose any value for T1 that is compatible with the DCE's T2 parameter values. The value of T1 used by the DTE may always be set longer than the formula indicates, with the result that recovery from certain types of link errors will be slower. However, the DCE's parameter T2 cannot be reduced, so the formula should be viewed as yielding a lower bound on the DTE's T1 timer. 2.3.3 Maximum I Frame Size The maximum number Nl of bits in an I Frame is 8248, accommodating a data packet with up to 1024 data octets. The derivation of this number is shown in Table 2.2. DTEs using DDN standard X.25 service must observe the restriction on the number of data bits in a complete packet sequence given in Section 2.2 above. -13- X.25 No. of Field Name Level Bits Address 2 8 Control 2 8 General Format Identifier 3 4 Logical Channel Number 3 12 Packet Type 3 8 User Data 3 8192 (max) Frame Check Sequence 2 16 TOTAL 8248 (max) Table 2.2 Derivation of Maximum I Frame Size 2.4 Physical Level Specifications The DDN X.25 physical level specification is in conformance with FIPS 100/Fed. Std. 1041 and CCITT X.25. This section presents additional information. A DDN X.25 DTE may either be collocated with its DCE or may be connected to it via an access line. In all cases the DTE presents a physical DTE interface; the DDN will supply the matching DCE interface. DDN X.25 service offers four physical level interfaces: RS-232-C (CCITT V.28), RS-449, both balanced and unbalanced (CCITT V.ll and V.10, respectively; also MIL-188- 114 balanced and unbalanced), and CCITT V.35. Appendix B of this document describes in detail the choices of physical interface available to the DDN subscriber and the specifications for each type of interface. Table 2.3, below, summarizes the physical interfaces available at each data rate supported by the DDN X.25 DCE, and indicates which interfaces are recommended at each signaling rate. A DDN X.25 DTE may implement any or all of the signaling rates shown. At each signaling rate implemented, the DTE must offer at least one of the physical interface options listed as "R" (recommended) or "A" (available) for that rate in Table 2.3. Implementors are encouraged to offer the widest variety of signaling rates and physical interfaces practical to maximize the ease of use of their equipment in DDN. -14- Physical Signaling Rate in Kb/s Interface 1.2 2.4 4.8 9.6 14.4 48 50 56 64 100 RS-232-C R R R R R - - - - - RS-449 unbal. A A A A - - - - - - (and equiv.) RS-449 balanced A A A A A A A A A R (and equiv.) CCITT V.35 - - - - - R A R R A Legend R = Recommended A = Available - = Not available (Taken from Appendix B, Table B-4 Table 2.3 DDN X.25 Physical Signaling Rates and Interfaces 3 BIBLIOGRAPHY 1. "Specification for the Interconnection of a Host and an IMP". Report No. 1822, Bolt Beranek and Newman Inc" Cambridge, MA, revision of December 1981. 2. CCITT Recommendation X.25, "Interface Between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks," International Telegraph and Telephone Consultative Committee Yellow food, Vol. VIII.2, Geneva, 1981. 3. "Defense Data Network Subscriber Interface Guide," Defense Communications Agency, Washington, DC, July 1983. 4. "Internet Protocol Transition Workbook," SRI International, Menlo Park, CA, March 1982. 5. "Internet Protocol Implementation Guide," SRI International, Menlo Park, CA, August 1982. APPENDIX A: DDN X.25 Implementation Details A-1 Introduction This Appendix serves three purposes. First, it provides information concerning the planned evolution of DDN X.25 capabilities. Second, it provides information on the use of certain DDN X.25 features and facilities at a greater level of detail than is appropriate for inclusion in the body of the DDN X.25 Interface Specification. Specifications for the use of DDN X.25 features and facilities given in this Appendix are mandatory on the part of DDN X.25 DTEs that wish to make use of these features and facilities. Finally, this Appendix presents a discussion of the limitations on the use of DDN services that will be encountered by hosts using only DDN basic X.25 service. A-2 Operational Features of DDN X.25 DCE Releases The capabilities of the DDN X.25 DCE will evolve over time from an initial set of capabilities to the full capabilities of this DDN X.25 Interface Specification. This section describes release-dependent features of the DDN X.25 DCE. Implementors should note that not all optional facilities of the specification will initially be available for use by DTEs. Releases of new DCE capabilities will be compatible with DTE hardware and software implementations that meet the full DDN X.25 Interface Specification. A-2.1 Initial Feature Support The initial release of the DDN X.25 DCE will support flow control parameter negotiation and fast select. In addition, the DDN X.25 DCE may be configured by the DDN Administration to provide non-standard default window and packet sizes as described in CCITT X.25 Sections 7.1.2 and 7.2.1. The call precedence and type of service selection facilities will be accepted, but not acted upon, by the network. Only DDN basic X.25 service will be supported. Planned future DCE releases will support all facilities specified in FIPS 100/Federal Standard 1041 with the exception of those "additional" facilities that are listed in Section 1.2.1 of this document. A-1 A detailed schedule of DDN X.25 DCE releases and the capabilities of each release will be supplied in a separate document. A-2.2 Exception-Handling Procedures Certain of the exception- or error-handling procedures of the initial release of the DDN X.25 DCE differ in detail from the procedures specified in FIPS 100/Federal Standard 1041. These differences are described below. A later release of the DDN X.25 DCE will bring these procedures into conformance. In the interim, the variances in these procedures will not preclude satisfactory operation between the DCE and a DTE, provided the DTE operates in accordance with FIPS 100/Federal Standard 1041. A-2.2.1 Non-Octet-Aligned Data Data packets received by the DDN X.25 DCE that are not aligned on an octet boundary are discarded at the link level. They are not passed to the DCE packet level, and no packet level diagnostic code is returned to the DTE. A-2.2.2 RESTART REQUEST Packet The DDN X.25 DCE will not discard, but will instead act upon, a RESTART REQUEST packet that (i) is too long (unless it exceeds the maximum frame size for the link level), or (ii) contains a non-zero cause field. A-2.2.3 RESET REQUEST Packet The DDN X.25 DCE will not discard, but will instead act upon, a RESET REQUEST packet that contains a non-zero reset cause field. A-2 A-2.2.4 CLEAR REQUEST Packet The DDN X.25 DCE will not discard, but will instead act upon, a CLEAR REQUEST packet that contains a non-zero clearing cause field. A-2.3 Virtual Circuit Resource Availability In its current implementation, the DDN X.25 packet switching node is capable of supporting a minimum of one hundred simultaneous virtual circuits. As was discussed in Section 2.1.4, resources of the node are shared dynamically among the DCEs attached to the node. Therefore, no explicit guarantees are made of the number of simultaneous virtual circuits that can be made by a single DTE. Depending upon the configuration of the node, the number of simultaneous circuits supported by the node can be significantly greater than one hundred. A-3 Detailed Features and Facilities Specifications This section provides detailed specifications and descriptions of use for certain DDN X.25 features and facilities. A-3.1 Additional Diagnostic Codes The DDN X.25 DCE is capable of providing additional information to DTEs in RESTART, RESET, CLEAR INDICATION, and DIAGNOSTIC packets by means of diagnostic codes that are extensions to the set of diagnostic codes given in Annex E of CCITT Recommendation X.25. These codes are taken from the set of codes "reserved for network specific diagnostic information," and are thus not in conflict with code assignments made in Annex E. The values of these codes, and their meanings, are given in Table A-1 below. A-3 Code Value Meaning 128 IMP is unavailable. The packet-forwarding mechanisms of the network are unavailable to the DCE. Sent in RESET, CLEAR and RESTART packets. 130 Link level came up. Sent in RESTART and RESET packets. 131 Link level went down at remote DTE. Sent in CLEAR and RESET packets. 132 Remote DTE restarted. Sent in CLEAR and RESET packets. 133 Local resources not available for call establishment. The local DCE has too few resources to establish another call. Sent in CLEAR and DIAGNOSTIC packets. 134 Remote resources not available for call establishment. The remote DCE has too few resources to establish another call. Sent in CLEAR packets. 136 Remote host dead. The link to the remote DTE is down. Sent in CLEAR and RESET packets. 137 Remote IMP dead. The IMP to which the remote DTE is attached is down. Sent in CLEAR and RESET packets. 138 Logical subnetwork access barred. The remote DTE cannot be reached because of a communities-of- interest prohibition. Sent in CLEAR and RESET packets. 139 Connection lost. An internal error has occurred at either the remote or the local DCE which has made their virtual circuit data structures inconsistent. Sent in CLEAR and RESET packets. 140 Response lost. A response from the remote DCE failed to arrive within a reasonable time. Sent in CLEAR and RESET packets. A-4 141 Calling logical address not enabled or not authorized. Sent in CLEAR packets. 142 Calling logical name incorrect for this DTE. Sent in CLEAR packets. 143 Called logical name not authorized. Sent in CLEAR packets. 144 Called logical name not enabled. Sent in CLEAR packets. 145 Called logical name has no enabled DTEs. Sent in CLEAR packets. 146 Use of logical addresses invalid in this network. Sent in CLEAR packets. 147 Declared logical name now in effect. Sent in CLEAR packets. 148 Declared logical name was already in effect. Sent in CLEAR packets. 149 Declared logical name is now disabled. Sent in CLEAR packets. 150 Declared logical name was already disabled. Sent in CLEAR packets. 151 Incoming calls barred. Sent in CLEAR packets. 152 Outgoing calls barred. Sent in CLEAR packets. 192 Cleared due to higher precedence call at local DCE. Sent in CLEAR packets. 193 Cleared due to higher precedence call at remote DCE. Sent in CLEAR packets. 194 Requested precedence too high. The DTE is not authorized to establish a call at the requested precedence level. Sent in CLEAR packets. Table A-1. Additional Packet Level Diagnostic Codes A-5 A-3.2 X.25 IP Interoperability Considerations When DDN standard X.25 service is requested at call establishment (as described in Section 2.1.2.1), the call is in effect established between the DTE and a local X.25 entity. This entity subsequently extracts the IP datagrams from the X.25 data packets for transmission through the DDN Internet. This approach requires that certain conventions be followed: 1. IP datagrams are to be sent as X.25 complete packet sequences. That is, datagrams begin on packet boundaries and the M ("more data") bit is used for datagrams that are larger than one packet. Only one IP datagram is to be sent per X.25 complete packet sequence. 2. By convention, the maximum IP datagram size is 576 octets. This packet size can most efficiently be accommodated by negotiating an X.25 maximum packet size of 1024; alternatively, a DTE may use an X.25 complete packet sequence to transmit an IP datagram. 3. Because the X.25 connection is in effect terminated locally, the D and Q bits have no significance and should be set to zero. 4. The precedence bits of the IP type-of-service field are to be mapped into X.25 precedence bits (see Section 2.1.2.2) as specified in Table A-2. IP Precedence X.25 Precedence 000 00 001 01 010 10 011 - 111 11 Table A-2. IP Precedence to X.25 Precedence Mapping A-6 A-3.3 The DDN Logical Addressing Facility The DDN logical addressing facility allows references to hosts by either their physical network address or by one or more location-independent logical addresses, and allows hosts to exercise partial control over the logical address(es) by which they can be referenced. Implementation of DDN logical addressing by a host is optional. The DDN Administration will assign seven-digit logical addresses, and will maintain a logical addressing data base. The host is then responsible for notifying the network ("enabling") of the "names" (logical addresses), if any, by which it wishes to be known. It cannot receive calls addressed to a name or originate calls under that name unless it has enabled that name. It also cannot enable a name that is not authorized for that physical address. Names can also be enabled automatically by the network, under the control of the Administration. A-3.3.1 Logical Addresses Logical addressing is invoked when a called address is supplied to the IMP with the flag digit F = one. The logical address consists of seven BCD digits. This name is mapped by the logical addressing facility into a DDN physical network address. The logical name need not be unique for the physical address, nor is the physical address necessarily unique for the name. A-3.3.2 Enabling and Disabling Logical Addresses To enable and disable logical addresses, the DDN X.25 host must send declarative CALL REQUEST packets to the DCE using a called address with the format: ZZZZ F DDDDDDD (SS) where the address fields are as described in Section 2.1.1. The Flag F must be set to nine, the DDN Host Identifier field specifies the logical address under consideration, and the subaddress field, which must be present, specifies the type of transaction. Declarative calls are cleared immediately by the local DCE. A-7 If SS is zero, the logical name is enabled in normal mode,; that is, that physical port will accept incoming calls to that name, and allow outgoing calls from that name. If SS is one, the logical name is disabled. If SS is two, the logical address is enabled in reverse translation mode; in this mode, the called address field of incoming call packets will be translated into a physical address (i.e., an address containing a flag F = 0), if it was given by the calling DTE (X.25 host), as a logical address (i.e., containing a flag F = 1). Whenever a DTE comes up, or restarts, the logical names for that DTE are returned to their default state, which may be either enabled or disabled, as configured by the DDN Administration. A-4 Limitations of DDN Basic X.25 Service The Defense Data Network is an Internetwork environment. That is, DDN as a whole is made up of a number of constituent packet switching networks that are interconnected via gateways. Communication across gateways requires the use of the Internet Protocol, which, for a host accessing DDN using X.25, requires that the host implement the DoD standard protocol architecture and employ DDN standard X.25 service. In addition, a classified host is attached to a DDN constituent network of lower classification by means of an Internet Private Line Interface (IPLI). IPLIs, which themselves contain gateways, also require the use of the Internet Protocol; moreover, they do not, as currently designed, offer an X.25 host interface. These attributes of the DDN Internet have two implications for users of DDN basic X.25 service: 1. DDN hosts that do not implement IP and higher- level DDN protocols, and which use only DDN basic X.25 service, cannot communicate across gateways. Their network communication is therefore restricted to a single DDN constituent network. 2. X.25 hosts cannot be provided classified service on a constituent network of lower classification. Should X.25 host access be developed for the IPLI in the future, classified network access will be made available to hosts using DDN standard X.25 service only. A-8 A-5 Derivation of DDN X.25 Addresses All DDN hosts are assigned addresses by the Administration. The address of a DDN host may be obtained from the Network Information Center (NIC), represented as an ASCII text string in what is called "host table format". This section describes the process by which DDN X.25 addresses in the format described in Section 2.1.1 may be derived from addresses in NIC host table format. A NIC host table address consists of the ASCII text string representations of four decimal numbers separated by periods, corresponding to the four octets of a thirty-two bit Internet address. The four decimal numbers are referred to in this section as "n", "h", "l", and "i." Thus, a host table address may be represented as "n.h.l.i" Each of these four numbers will have either one, two, or three decimal digits and will never have a value greater than 255. For example, in the host table address "10.2.0.124", n=10, h=2, l=0, and i=124. To convert a host table address to a DDN X.25 address: 1. If h < 64, the host table address corresponds to the DDN X.25 physical address ZZZZ F IIIHHZZ (SS) where: ZZZZ = 0000 as required in Section 2.1.1.1.1; F = 0 because the address is a physical address; III is a three decimal digit representation of "i", right-adjusted and padded with leading zeros if required; HH is a two decimal digit representation of "h", right-adjusted and padded with leading zeros if required;, ZZ = 00 and (SS) is optional, as described in Section 2.1.1.1.4. A-9 In the example given above, the host table address 10.2.0.124 corresponds to the DDN X.25 physical address 000001240200. 2. If h > 64 or h = 64, the host table address corresponds to the DDN X.25 logical address ZZZZ F RRRRRZZ (SS) where: ZZZZ = 0000 as required in Section 2.1.1.1.1; F = 1 because the address is a logical address; RRRRR is a five decimal digit representation of the result "r" of the calculation r = h * 256 + i (note that the decimal representation of "r" will always require five digits); ZZ = 00 and (SS) is optional, as described in Section 2.1.1.1.4. Thus, the host table address 10.83.0.207 corresponds to the DDN X.25 logical address 000012145500. In both cases, the "n" and "l" fields of the host table address are not used. A-10 APPENDIX B: DDN Synchronous Level 1 Specification B-1 Introduction A host may connect to the Defense Data Network at the link level using the asynchronous bit serial protocol described in BBN Report No. 1822 as either a local host (LH) or a distant host (DH). A host may also connect to the DDN by means of a synchronous bit serial protocol at the link level, using either the method described in BBN Report No. 1822, HDH, or the DDN X.25 interface. Neither LH nor DH is recommended for new implementations. This section describes the functional, electrical, and mechanical connection (the level 1 connection) that is required when either an HDH or an X.25 host is connected to the DDN. Hosts connecting to the DDN via HDH or X.25 require a synchronous modem connection or the equivalent, which will be supplied as part of the DDN service. The host will present the DTE interface while the DDN-provided equipment will present the DCE interface. A long-term goal of the DDN is for all level 1 connections to be accomplished with the MIL-188-114 balanced interface. Its general equivalents are EIA RS-449/422, CCITT V.ll, and Fed. Std. 1031/1020. The DDN cannot implement this at present due to the limited availability of commercial vendor hardware. In order to facilitate future DDN compatibility, all new system acquisitions should specify MIL-188-114 balanced as a required interface, in addition to an alternate interface. The selection of an alternate interface should not preclude utilization of the MIL- 188-114 balanced interface when it becomes supportable. B-2 Supported Interfaces DDN presently supports four synchronous level 1 interfaces. They are: 1. EIA RS-232-C, CCITT V.28 & V.24; 2. MIL-188-114 balanced, EIA RS-449&422, CCITT V.ll, Fed. Std. 1031/1020; 3. MIL-188-114 unbalanced, EIA RS-449&423, CCITT V.10, Fed. Std. 1031/1030; and B-1 4. CCITT V.35. Table B-1 is a dictionary of terms that relates the CCITT signal ID to the EIA signal ID and to the more common abbreviations. Table B-2 identifies signals as either required, optional, or not used. Figure B-1 and Table B-3 identify typical DTE connections to the DDN. The required subscriber services will dictate which scheme is selected for a particular DTE. Table B-4 relates required speed of service to interface type. Together, these tables and figures serve as a guide to level 1 interface selection. From these, most systems will be able to identify the most appropriate interface. However, this information is not all-inclusive. Other interface arrangements may be possible; contact your DDN representative for assistance as required. Demarcation Point (mating connectors) DTE DCE |------------] [------(1) Modem RS-232-C | | |---------] [------(2) Modem V.35 |---|--|----| | |----] [------(3) LDM RS-232-C, MIL-188-119 | | | |----] [------(4) Null Modem Cable | HOST | | |----] [------(5) SME Cable plus clock source | | | |----] [------(6) DCS MIL-188-114 |--|--|--|--| | | |-------] [------(7) DES RS-232-C, RS-449, V.35 | | | |----------] [------(8) KG MIL-188-114 balanced | |-------------] [------(9) IPLI MIL-188-114 balanced Figure B-1. Typical Level 1 Connection Schemes B-2 EIA CCITT ABBRM NAME ID ID NAME --- ----- ------ --------------------------------- AA 101 FG Frame (Chassis/Protective) Ground AB 102 SG Signal/Supply Common SC 102a -- RS-449 DTE Common RC 102b -- RS-949 DCE Common BA 103 TD Transmit Data BB 104 RD Receive Data CA 105 RTS Request to Send CB 106 CTS Clear to Send CC 107 DSR Data Set Ready CD 108.2 DTR Data Terminal Ready CF 109 DCD Data Carrier Detect CG 110 SQ Signal Quality CH 111 -- Signal Rate Selector to DCE CI 112 -- Signal Rate Selector to DTE DA 113 ETC External Transmit Clock DB 114 TC Transmit Clock DD 115 RC Receive Clock -- 116 -- Select Standby -- 117 -- Standby Indicator SBA 118 STD Secondary Transmit Data SBB 119 SRD Secondary Receive Data SCA 120 SRS Secondary Request to Send SCB 121 SCS Secondary Clear to Send SCF 122 SCD Secondary Carrier Detect SCG 123 SSQ Secondary Signal Quality -- 124 -- Select Frequency Group CE 125 RI Ringing Indicator -- 126 -- Select Transmit Frequency -- 127 -- Select Receive Frequency -- 128 -- External Receive Clock -- 129 RR Request to Receive -- 130 -- Secondary Transmit Tone -- 131 -- Receive Character Timing -- 132 -- Return to Non-Data Mode -- 133 RTR Ready to Receive . -- 134 -- Received Data Present -- 136 -- New Signal -- 140 RL Remote loopback -- 141 LL Local loopback -- 142 TM Test Status Monitor -- 191 -- Transmit Voice Answer 192 -- Receive Voice Answer Table B-1. EIA and CCITT Interchange Circuits B-3 Required: 101, 102, 103, 104, 105, 106, 107, 108.2, 109, 113, 114, and 115 Optional: 110, 125, 140, 141, and 142 (These may be required IAW future DDN developments; it is strongly recommended that these at least be available for implementation upon requirement) Not used: 111, 112, 116, 117, 118, 119, 120, 121, 122, 123, 124, 126, 127, 128, 129, 130, 131, 132, 133, 134, 136, 191, and 192 Table B-2. Signal Selection by CCITT Interchange Circuit Number B-4 Scheme (From Fig. B-1) Explanation (1) Modem RS-232 at spe eds of 1200, 2400, 4800, 9600 or 14400 b/s over long haul leased voice grade telephone facilities (2) Modem CCITT V.35 at speeds of 48, 50, 56, 64 Kb/s over leased group (37KHz) grade facilities or in CONUS the Digital Data Service facilities. (3) Limited Distance Modem LDM generally available at 9600 b/s and below in an RS-232 version. Other types are available for all speeds. (4) Null modem A Null Modem is a length of cable with the signal leads crossed so as to present a DCE interface. To be used in local connection schemes where either the DTE or the DCE has a clocking source capability. All four supported level 1 interfaces are available. If DTE clock and DCE clock are both available, DTE clock will be preferred. (5) Synchronous Modem Eliminator SME is a length of cable with a hardware device interjected. The device allows convenient crossing of signals so as to present a DCE interface. The device also provides clocking when neither the DTE nor the DCE has such capability. All four supported level 1 interfaces are available. (6) DCS Microwave DCS is generally a military microwave system which provides the MIL-188-114 balanced or unbalanced interfaces. It implies a speed of 50 Kbps and is usually found O-CONUS. Selection of this scheme requires selection of (4) or (5). (7) Data Encryption Standard DES is a commercial encryption device used by the DoD as a privacy device. DES is available with either RS-232, V.35, or RS-449/422. B-5 (8) KG KG devices are U. S. Government encryption devices under strict NSA control. The requirement for security and KG devices requires the selection of the MIL-188-114 balanced interface. (9) Internet Private Line Interface IPLI devices are security level community of interest isolation devices. The requirement for IPLI service requires the selection of the MIL- 188-114 balanced interface. Notes and Considerations 1. Interface (2), Modem, 48Kb/s is generally only available O-CONUS. 2. MIL-188-114 balanced is deemed equivalent to RS-449 with RS-422, the difference being that MIL-188-114 is more tolerant of noise on signal common and more tolerant of common mode noise. 3. MIL-188-114 unbalanced is deemed equivalent to RS-449 with RS-423. In most cases where MIL-188-114 balanced is specified, MIL-188-114 unbalanced is also available, but it is not recommended. 4. There are system enhancements under long term development for use in the DDN which may request additional control leads beyond those listed as required. The implementation of these enhancements will not limit operational capabilities but may impact the ability of the Network Monitoring Center to assist with host and host access line diagnosis. These enhancements may request signals from the optional category. Table B-3. Typical Level 1 Connection Schemes B-6 Signaling Rate in Kb/s Physical Interface 1.2 2.4 4.8 9.6 14.4 48 50 56 64 100 RS-232-C R R R R R* - - - - - MIL-188-114 A A A A - - - - - - unbal. (& equiv.) MIL-188-114 A A A A A* A A A A R** bal. (& equiv.) CCITT V.35 - - - - - R A R R A Legend R = Recommended A = Available - = Not available * = Only available using modems ** - Only available using a local cable connection Table B-4. Interface Type by Service Speed B-7 Signal Name Abbrev Pin No. EIA ID Signal Source ----------- ------ ------- ------ ------------- Frame Ground FG 1 AA DTE/DCE Transmitted Data TD 2 BA DTE Received Data RD 3 BB DCE Request to Send RTS 4 CA DTE Clear to Send CTS 5 CB DCE Data Set Ready DSR 6 CC DCE Signal Ground SG 7 AB DTE/DCE Data Carrier Detect DCD 8 CF DCE Transmit Clock TC 15 DB DCE Receive Clock RC 17 DD DCE Data Terminal Ready DTR 20 CD DTE Ext. Transmit Clock ETC 24 DA DTE Wired Spare -- 18 -- --- Wired Spare -- 22 -- --- Wired Spare -- 25 -- --- Required pins: 1, 2, 3, 4, 5, 6, 7, 8, 15, 17, 20, 24 Optional pins: 9, 10, 18, 22, 25 Notes 1. The DTE will present a CANNON DB-25P male connector with pinouts as above or equivalent hardware with identical pinouts. 2. The DCE will present a CANNON DB-2SS female connector or equivalent. Table B-5. RS-232-C Interface B-8 Signal Name Abbrev Pin Nos EIA ID Signal Source ----------- ------ ------- ------ ------------- Send D ta SD 4,22 BA DTE Send Timing ST 5,23 DB DCE Receive Data RD 6,24 BB DCE Request to Send RTS 7,25 CA DTE Receive Timing RT 8,26 DD DCE Clear to Send CTS 9,27 CB DCE Local 100pback LL 10 -- DTE Data Mode DM 11,29 CC DCE Terminal Ready TR 12,30 CD DTE Receiver Ready RR 13,31 CF DCE Remote 100pback RL 14 -- DTE Terminal Timing TT 17,35 DA DTE Test Mode TM 18 -- DCE Signal Ground SG 19 AB DTE/DCE Receive Common RC 20 RC DCE Send Common SC 37 SC DTE Wired Spare -- 1 -- --- Wired Spare -- 3,21 -- --- Required pins: 4,22; 5,23; 6,24; 7,25; 8,26; 9,27,; 11,29; 12,30; 13,31; 17,35; 19; 20; 37 Optional pins: 10; 14; 18; 1; 3,21 Notes: 1. The DTE will present a CANNON DC-37P male connector with pinouts as above or equivalent hardware with identical pinout. 2. The DCE will present a CANNON DC-37S female connector or equivalent. Table B-6. MIL-188-114 Interface (and equivalents) B-9 Signal Name Abbrev Pin Nos. EIA ID Signal Source ----------- ------ -------- ------ ------------- Frame Ground FG A AA DTE/DCE Signal Ground SG B AB DTE/DCE Transmit Data TD P/S BA DTE Receive Data RD R/T BB DCE Request to Send RTS C CA DTE Clear to Send CTS D CB DCE Data Set Ready DSR E CC DCE Data Carrier Detect DCD F CF DCE Local 100pback LL K -- DTE Ext. Transmit Clock ETC U/W DA DTE Transmit Clock TC Y/aa DB DCE Receive Clock RC V/X DD DCE Required Pins: A; B; P/S; R/T; C; D; E; F; U/W; Y/aa; V/X Optional Pins: K Notes: 1. The DTE will present a Winchester MRA(C)-34D-JTCH-H8 male connector with pinout as above or equivalent hardware with the identical pinout. 2. The DCE will present a mating female connector. Table B-7. V.35 Interface B-10 APPENDIX C FEDERAL INFORMATION PROCESSING STANDARDS PUBLICATION 100 FEDERAL STANDARD 1041 1983 JULY 6 ANNOUNCING THE JOINT STANDARD FOR INTERFACE BETWEEN DATA TERMINAL EQUIPMENT (DTE) AND DATA CIRCUIT-TERMINAL EQUIPMENT (DCE) FOR OPERATION WITH PACKET-SWITCHED DATA COMMUNICATIONS NETWORKS Federal Information Processing Standards Publication are developed and issued by the National Bureau of Standards pursuant to section 111(f)(2) of the Federal Property and Administrative Services Act of 1949, as amended, Public Law 89-306 (79 Stat.1127), Executive order 11717 (38 FR 12315 dated May 11, 1973), and Part 6 of Title 15 Code of Federal Regulations (CFR). Federal Standards in the "telecommunication" series are developed by the Office of the Manager, National Communication System. These Federal Standards are issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended. Name of Standard: Interface Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Operation with Packet-Switched Data Communications Networks. Category of Standard: Hardware, Data Transmission. Explanation: Federal automated data processing equipment, services, and telecommunication equipment using public packet-switched data communications networks (PSDCN) based on the family of CCITT Recommendations derived from X.l and X.2 shall employ the interface and protocols specified in this joint standard. In addition, designers of these internally operated and maintained Federal networks employing packet-switched technology should consider the use of this interface as appropriate. The joint standard provides: - A family of physical layer interfaces, from which a particular interface may be selected; and - A single data link layer control procedure; and - Packet level procedures for virtual calls and permanent virtual circuits, and an optional datagram operation. The mandatory interface attributes of this joint standard are summarized as follows: PHYSICAL LEVEL Transmission rates: 2.4, 4.8, 9.6 Kbits/s Interface: one or more of the following: RS-232-C, X.2l, RS-449 LINK LEVEL: Procedure: LAPB Parameter K: 7 Smallest N l: l64 Octets PACKET LEVEL: Services: Virtual call and permanent virtual circuit Packet types: All basic plus Diagnostic packets. Packet Reject shall not be used. User data field Octet-aligned length: Packet sequence Modulo 8 numbering: D bit procedure: Supported by all DCEs; DTE need not employ the D bit when sending to the DCE, but no DTE shall reject incoming packet with the D bit set to l or 0 as having this bit in error unless it is known by receiver that the sender has no D bit capability. X.25 diagnostic Use standard codes whenever they apply; non-std codes: codes may be used for events not listed in X.25 within a period of 24 months after the effective date of this standard. Fast Select: DCEs shall implement fast select; DTE need not employ fast select when sending to DCE, but all DTEs with higher level functionality which allows response to fast select must be able to accept incoming fast select packet. Interrupt packet: Receipt of a DTE interrupt packet before a previous DTE interrupt packet has been confirmed is an error condition. Duplicated facility The last appearing facility code should be codes: treated by the DTE as if it were the only appearance of that code. Non-zero cause field Discarded of restart request packet: Restart request too Discarded long in state r1: This joint standard is intended to enhance interoperability by specifying certain subsets and other constraints on Federal use of CCITT Recommendation X.25. The Government's intent in employing this joint standard is to reduce the cost of acquiring and using Federal automated data processing equipment, services, and telecommunication equipment with PSDCN. The joint standard is also intended to reduce the cost of acquiring and using Government-owned or leased PSDCN. These goals will be achieved by: - increasing the available alternative sources of supply; - Increasing the reutilization of Government resources; and, - Assuring the required interoperability. Approving Authority: Secretary of Commerce (Federal Information Processing Standards). Administrator, General Services Administration (Federal Standards). Maintenance Agency: The National Bureau of Standards and the Office of the Manager, National Communications System will jointly maintain this standard coordinating as necessary with the General Services Administration (GSA). Cross Index: The following are related standards upon which this FIPS PUB is based. The inclusion of a particular standard on this list does not necessarily mean that the standard is applicable in all cases to which this FIPS PUB applies. (a) International Standard 2110-1980: Data Communication-25 pin DTE/DCE Interface Connector and Pin Assignments. (b) International Telegraph and Telephone Consultative Committee (CCITT) recommendations V.24 (1980): List of Definitions for Interchange Circuits Between Data Terminal Equipment and Data Circuit Terminating Equipment. (c) CCITT Recommendation V.28 (1980) Electrical Characteristics for Unbalanced Double-Current Interchange Circuits. (d) Electronics Industries Association (EIA) RS-232-C (1969 August): Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange. (e) International Standard 4902-1980: Data Communication-37-Pin and 9-Pin DTE/DCE Interface Connectors and Pin Assignments. (f) CCITT recommendation V.11(X.27) (1980): electrical Characteristics for Balanced Double-Current Interchange Circuits for General Use with Integrated Circuit Equipment in the Field of Data Communications. (g) EIA RS-422-A (1978 June): Electrical Characteristics of Balanced Voltage Digital Interface Circuits. (h) Federal Standard 1020A (1980 January): Telecommunications: Electrical Characteristics of Balanced Voltage Digital Interface Circuits. (i) CCITT Recommendation V.10 (X26) (1980): Electrical Characteristics for Unbalanced Double-Current Interchange Circuits for General Use with Integrated Circuit Equipment in the Field of Data Communications. (k) Federal Standard 1030A (1980 January): Telecommunications: Electrical characteristics of Unbalanced Voltage Digital Interface Circuits. (l) CCITT Recommendation X.21bis (1980): Use on Public Data Networks of Data Terminal Equipment which are Designed for Interfacing to Synchronous V-series Modems. (m) CCITT Recommendation V.54 (1980): Loop Test Devices for Modems. (n) EIA RS-449 (1977 November): general Purpose 37-Position Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment. (o) Federal Standard 1031 (1980 June): Telecommunications General Purpose 37-position and 9-position Interface Between Data Terminal Equipment and Data Circuit Terminating Equipment (implementing instructions in the form of a Federal Property Management Regulation have not yet been issued. the General Services Administration is considering canceling FED-STD 1031. Furthermore, a Federal Information Processing Standard for ADP applications corresponding to Federal Standard 1031 has not been adopted by the National Bureau of Standards.) (p) International Standard 4903-1980: Data Communication-15-pin DTE/DCE Interface Connector and Pin Assignments. (q) EIA Industrial Electronics Bulletin No. 12 (1977 November): Application Notes on Interconnection Between Interface Circuits Using RS-449 and RS-232-C. (r) Draft International Standard 2593 (1980): Data Communication-34-pin DTE/DCE Interface Connector and Pin Assignments. (s) CCITT Recommendation V.35 (1980): Data Transmission at 48 Kilobits per second Using 60-108 kHz Group Band Circuits. (t) CCITT Recommendation X.21 (1980): general Purpose Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment for Synchronous Operation on Public Data Networks. (u) CCITT recommendation V.5 (1980): Standardization of Data-Signalling Rates for Synchronous Data Transmission in the General Switched Telephone networks. (v) CCITT Recommendation V.6 (1980): Standardization of Data-Signalling Rates for Synchronous Data Transmission on Leased Telephone-Type Circuits. (w) American National Standard X3.1-1976: Synchronous Signalling Rates for Data Transmission. (x) Federal Information Processing Standard Publication 22-1 (1977 September): Synchronous Signaling Rates Between Data Terminal and Data Communication Equipment. (FIPS PUB 22-1 is identified also as FED-STD 1013.) (y) Federal Standard 1013 (1977 August): Telecommunications: Synchronous Signaling Rates Between Data Terminal Equipment and Data Circuit-Terminating Equipment utilizing 4 kHz Circuits (FED-STD 1013) is identified also as FIPS PUB 22-1.) (z) American National Standard X3.36-1975: Synchronous High-Speed Data Signaling Rates Between Data Terminal Equipment and Data Communication Equipment. (aa) Federal Information Processing Standards Publication 37 (1975 June): Synchronous High Speed Data Signaling Rates Between Data Terminal Equipment and Data Communication Equipment. (FIPS PUB 37 is identified also as FED-STD 1001.) (ab) Federal Standard 1001 (1975 June): Telecommunications: Synchronous High-Speed Data Signaling Rates Between Data Terminal Equipment and Data Communications Equipment. (FED-STD 1001 is identified also as FIPS PUB 37.) (ac) EIA RS-269-B (1976 January): Synchronous Signaling Rates for Data transmission. (ad) International Standard 3309-1979: Data Communication-High Level Data Link control Procedures-Frame Structure. (ae) International Standard 4335-1979: Data Communication-High Level Data Link control Procedures-Elements of Procedures. (af) Addendum 1 to International Standard 4335-1979: Data Communication-High Level Data Link control Procedures-Elements of Procedures. (ag) Addendum 2 to International Standard 4335-1979: Data Communication-High Level Data Link Control Procedures-Elements of procedures. (ah) International Standard 6256-1980: Data Communication-High -Level Data Link Control Procedures-Balanced Class of Procedures. (ai) American National Standard X3.66-1979: Advanced Data Communication Control procedures (ADCCP). (aj) Federal Information Processing Standards Publication 71 (1980 May) as revised by the Federal Register notice 47 FR 23798, dated June 1, 1982 and corrected by the notice 47 FR 25397 dated June 11, 1982: Advanced Data Communication Control Procedures (ADCCP). (FIPS PUB 71 is technically consistent with FED-STD 1003A.) (ak) Federal Information Processing Standards Publication 78 (1980 September): Guideline for Implementing Advanced Data Communication Control Procedures (ADCCP). (al) Federal Standard 1003A (1981 August): Telecommunications: Synchronous bit-Oriented Data Link Control Procedures (FED-STD 1003A is technically consistent with FIPS PUB 71.) (am) CCITT Recommendation X.25 (1980): Interface Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks. (an) Draft Proposed International Standard 7498: Data Processing-Open Systems Interconnection-Basic Reference Model. (ao) CCITT Recommendation X.1 (1980): International User Classes of Service in Public Data Networks. (ap) CCITT Recommendation X.2 (1980): International User Facilities in Public Data Networks. (aq) CCITT Recommendation X.96 (1980): Call Progress Signals in Public Data Networks. Applicability: The technical specifications of this joint standard shall be employed in the acquisition, design, and development of all federal automated data processing equipment, services, and telecommunication equipment and PSDCN whenever an interface based on CCITT Recommendation X.25 (1980), Interface Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equiment (DCE) for Terminals Operating in the Packet Mode on Public Networks>1, is required. Referred to below as CCITT Recommendation X.25, Recommendation X.25, or X.25. Implementation: The provisions of this joint standard are effective July 6, 1983. Any applicable equipment or service ordered on or after the effective date, or procurement action for which solicitation documents have not been issued by that date, must conform to the provisions of this standard unless a waiver has been granted in accordance with the procedures described below. This joint standard shall be reviewed by the Institute for Computer Sciences and Technology, National Bureau of Standards and the Office of the Manager, National Communications System, within five years after its effective date. This review shall take into account technological trends and other factors to determine if the joint standard should be affirmed, revised, or withdrawn. Specifications: This joint standard adopts a subset, identified below, of the International Telegraph and Telephone Consultative Committee's Recommendation X.25. (a) At the physical level, the provisions of Section 1 of CCITT Recommendation X.25 shall be used. As a minimum, networks shall support dedicated circuit access; other types of access (e.g., through the general switched telephone network) may also be offered. CClTT Recommendation X.1 standardizes data signalling rates of 2.4, 4.8, 9.6, and 48 kbits/s for packet mode interfaces. At a minimum, networks shall support the synchronous data signalling rates of 2.4, 4.8, and 9.6 kbits/s full duplex; other speeds (e.g., 19.2 kbits/s) may also be offered. The 48 kbits/s rate need not be supported in those locations where it is not available; 56 kbits/s is recommended in its place (see American National Standard X3.36-1975 and related documents referenced above). The term "user class of service" used in X.25 refers to the data signalling rate of DTE/DCE interface. In accordance with CCITT Recommendation X.25, networks shall provide one or more of the following interface options: i. CCITT Recommendation X.21; ii. EIA RS-232-C, which is essentially equivalent to one of the options in CCITT Recommendation X.21bis; iii. CCITT Recommendation X.21bis option that is equivalent to RS-449 using only the EIA RS-423A unbalanced electrical characteristics. Interworking between EIA RS-232-C on one side of the interface and RS-449 on the other side is permitted in accordance with EIA Industrial Electronics Bulletin Number 12. Where interworking with RS-232-C equipment is not required, the provisions described below employing RS-449 with the RS-422A electrical characteristics may optionally be employed at signalling rates below 48 kbit/s. Networks which support 48 or 56 kbits/s data signalling rates shall provide one or more of the following interface options: i. CCITT Recommendation X.21; ii. CCITT Recommendation X.21bis option that specifies CCITT Recommendation V.35; or iii. CCITT Recommendation X.21bis option that specifies CCITT Recommendation V.36 which is equivalent to EIA RS-449. NOTE: Current study in national and international standards groups may result in the development of additional physical interfaces. Each such physical interface will be evaluated for inclusion in this joint standard. If there are significant savings, one physical interface may be selected as the future mandatory physical interface. NOTE: DTE purchasers and designers should determine which physical interface(s) is provided by the associated DCE(s). (b) Only the LAPB link level procedures shall be used. NOTE: These procedures are a subset of those described in FIPS PUB 71 and Federal Standard 1003A and correspond to FIPS PUB 78 recommended class B. This subset is identified as follows: i. Link configuration: two combined stations on a point-to-point link. ii. Class of procedures: balanced asynchronous (BA) with options two and eight. The RSET command shall not be used. (RSET is found in option 11 of the Fips PUB 71. RSET is part of the basic repertoire in Federal Standard 1003A; option 11 of federal Standard 1003A deletes the RSET command. Note that RSET is not part of CCITT Recommendation X.25.) iii. Two-way simultaneous operation shall be employed. iv. The smallest N1, (the maximum number of bits in an information frame excluding flags and zero bit insertion for transparency), which shall be supported shall be 164 octets (the maximum length of) fast select caIl setup packet). If a DTE neither transmits, nor receives for processing by higher level functionality fast select packets, an N1 as small as 135 octets may be supported by the DTE. v. The address of the combined station provided by the network shall be 10000000; the address of the other combined station shall be 11000000; where the left-hand bit is the least significant bit (bit number 1) and shall be transmitted first. This convention is consistent with the provisions of FIPS 71 and Federal Standard 1003A. vi. The FCS shall be a 16-bit sequence as indicated in Section 2.2.7. DTE/DCE may also employ the 32-bit FCS as indicated in FIPS PUB 71 (revised) and FED-STD 1003A. DTE/DCE equipment using the 32-bit FCS shall be able to also operate with the 16-bit FCS. The smallest N1 shall be 166 octets when the 32-bit FCS is used. If a DTE neither transmits, nor receives for processing by higher level functionality fast select packets, an Nl as small as 137 octets may be supported by the DTE when the 32-bit FCS is used. NOTE: FIPS PUB 78 provides a detailed discussion of the relative merits of the 16-bit and 32-bit FCS. vii. The frame reject information field shall be padded with 4 zero bits in bit positions 21 through 24 of the information field to provide a length of three octets. viii. It is required that all implementations be capable of operating with K=7; optionally, values of 1 to 6 are permissible with modulo 8 operation and values 1 to 127 are permissible with modulo 128 operation. NOTE: DTE purchasers and designers should determine that values of k other than 7 are supported by the associated DCE(s). (c) The user data field of packets shall be an integral number of octets. If a packet is received which shows a user data field not equal to an integral number of octets, the receiving DTE/DCE shall follow the packet level procedures for processing a packet type which is too long. A new diagnostic code "non-octet aligned packet," consistent with the Data Communications-X.25 Packet Layer Specification for Terminal Equiment, ISO DP 8208, November 8, 1982, is recommended as #82. (d) The reject packet shall not be used. (e) All DCE restart confirmation, DCE reset confirmation, and DCE clear confirmation packets shall be interpreted by the DTE as having local significance only. (f) The D-bit shall be implemented by all networks. DTE's need not employ the D-bit procedures when transmitting to the network, but no DTE shall reject incoming packets with the D-bit set to 1 or 0 as having this bit in error unless the receiving DTE knows the remote DTE has not implemented the D-bit procedure; in this case, the receipt of a D-bit set to 1 may be treated by the receiving DTE as an error condition. (g) The selection of logical channel number for new virtual calls shall follow the procedures suggested in Section 4.1.2 Note 2, Annex A Note 5, and Annex A Note 6, of the CCITT Recommendation X.25. (h) It is required that all implementations be capable of operating with packet sequence numbering modulo 8; optionally, implementations of packet sequence numbering modulo 128 are also permitted. NOTE: DTE purchasers and designers should determine if the associated DCE(s) support packet sequence numbering modulo 128. (i) All DTE's and DCE's shall follow the flow control principles outlined in the first two sentences of the first paragraph of Section 4.4. 1.3 of CCITT Recommendation X.25. (j) The alternative procedure for passing packets containing a P(S) that is out of sequence but within the window as described in the third paragraph of Section 4.4.1.3 of CCITT Recommendation X.25 shall not be used. (k) The second sentence of Section 4.4.1.4 Note 2 shall not apply. This sentence permits networks to defer updating the window for data packets with D =0, and sent within the window but before a data packet with D= 1, until the network receives a corresponding P(R) for the packet with D= 1. (1) The resetting cause field of a reset request packet shall be set to zero. If a reset request is received with a non-zero resetting cause field, the packet shall be discarded. The network shall then initiate the resetting procedure with the resetting cause field indicating local/remote procedure error. (m) The clearing cause field of a clear request packet shall be set to zero. If a clear request packet is received with a non-zero clearing cause field, the packet shall be discarded. The network shall then initiate the clearing procedure with the clearing cause field indicating local/remote procedure error. (n) The restarting cause field of a restart request packet shall be set to zero. If a restart request packet is received with a non-zero restart cause field, the restart request packet shall be discarded without further action. Optionally, the DCE may generate a diagnostic packet with a recommended diagnostic code #81 (improper cause code from DTE), which is consistent with the <1Data Communication-X.25 Packet Layer>1 <1Specification for Data Terminal Equiment,>1 ISO DP 8208, November 8, 1982. (o) A diagnostic code shall be provided in all clear request, reset request, and restart request packets in accordance with the codes listed in Annex E of CCITT Recommendation X.25 whenever they apply; non-assigned codings in X.25 may be used for events not listed in X.25 within the period of 24 months after the effective date of this standard. Prior to the end of this 24 month period, this standard will be reviewed by NBS to determine whether the standard should be revised to incorporate a different table. After this revision, codes not specifically listed shall not be used. (p) A generic diagnostic code shall not be used when a more specific diagnostic code is known to be applicable. (q) The network diagnostic codes shall be used in accordance with the codes listed in Annex E of CCITT Recommendation X.25 whenever they apply; non-assigned codings in X.25 may be used for events not listed in X.25 within the period of 24 months after the effective date of this standard. Prior to the end of this 24 month period, this standard will be reviewed by NBS to determine whether the standard should be revised to incorporate a different table. After this revision, network diagnostic codes not specifically listed shall not be used. (r) The network shall consider the receipt of a DTE interrupt packet before a previous DTE interrupt packet has been confirmed as an error, and shall execute the error procedure described in Annex C, Table C-4/X.25 and the corresponding note 2. (s) The timeouts and time limits specified in Annex D shall be observed by all DTE and DCE equipment. T21 shall not be less than the value given in table D-2/X.25. The preferred actions listed in table D-2/X.25 shall be followed. (t) When the link level procedures enter the logically disconnected state, the associated packet level procedures shall clear all virtual calls and reset all permanent virtual circuits and datagram logical channels. When the link level procedures reenter the information transfer state, the associated packet level procedures shall execute the restart procedure. The terms "logically disconnected state" and "information transfer state" are used as defined in American National Standard X3.66-1979 (referenced above). Link level procedures enter the logically disconnected state when a DISC command is sent and a UA response is received, for example. The link level procedure shall also be considered to be in the logically disconnected state after N2 (re)transmissions of SABM or DISC, where N2 is as defined in CClTT Recommendation X.25. The logically disconnected state is not assumed after N2 (re)transmissions of other types of frames. (u) lf a restart request packet is received in state rl which exceeds the maximum permitted length, the DCE shall discard the restart request packet without further action. Optionally, the DCE may generate a diagnostic packet with diagnostic code #39 (packet too long). (v) In the event that a facility code appears more than once in a facility field, the receiving DTE detecting this condition should treat the last appearance of the particular code as if it were the only appearance of that code. (w) All networks shall supply diagnostic packets when their use is suggested in CClTT Recommendation X.25. No DTE shall rejcct diagnostic packets as errors. (x) ln Section 6.1.1, the second paragraph, the last phrase, "and is set to 0 in all other packets", shall be interpreted that the Qualifier bit is set to 0 in all other packets except data packets. For the case of data packets, the Qualifier bit is set to 0 or 1 as indicated in Section 4.3.6 of CClTT Recommendation X.25. (y) The list of user facilities for packet-switched data networks, extracted from CCITT Recommendation X.2, is given below. These facilities are described in Section 7 of CCITT Recommendation X.25. The following further constraints apply: i. Networks shall provide the facilities designated as essential "E" below. ii. Networks shall also implement the Fast Select and Fast Select Acceptance facilities to facilitate more efficient operation in conveying higher layer protocol information or user data during call establishment. DTE's need not employ fast select packets when transmitting to the network, but all DTE's associated with the higher level functionality which allows response to a fast select packet must be able to accept incoming fast select packets. iii. The packet retransmission facility shall not be used. iv. All DTE's which employ any of the facilities labelled as additional "A'' below (except Fast Select and Fast Select Acceptance) shall also be capable of operating without employing any A facilities (except Fast Select and Fast Select Acceptance). v. The throughput class value of 48,000 bits/s may be interpreted as 56,000 bits/s in those locations where 56,000 bits/s access is used. Facilities of packet-switched data networks: User Facility VC PVC DG* Optional user facilities assigned for an agreed contractual period: Extended packet sequence numbering (modulo) A A A* Non-standard default window sizes A A A* Non-standard default packet sizes 16, 32, 64, 256, 512, 1024 A A - Default throughput class assignment A A A* Flow control parameter negotiation E - - Throughput class negotation E - - Packet retransmission A*** A*** A*** Incoming calls barred E - E* Outgoing calls barred E - E* One-way logical channel outgoing E - A* One-way logical channel incoming A - A* Closed user group E - E* Closed user group with outgoing access A - A* Closed user group with incoming access A - A* Incoming calls barred within a closed user group A - A* Outgoing calls barred within a closed user group A - A* Bilateral closed user group A - A* Bilateral closed user group with outgoing access A - A* Reverse charging acceptance A - A* Fast select acceptance A** - - Datagram queue length selection* - - A* Datagram service signal logical channel* - - A* Datagram non-delivery indication* - - E* Datagram delivery confirmation* - - E* D-bit modification A A - Optional user facilities requested by the DTE on a per call basis Closed user group selection E - E* Bilateral closed user group selection A - A* Reverse charging A - A* RPOA selection A - A* Flow control parameter negotiation E - - Fast select A** - - Throughput class negotiation E - - Abbreviated address calling FS - A* Datagram non-delivery indication - - E* Datagram delivery confirmation - - E* NOTE: Detailed explanations of these facilities are provided in CCITT Recommendation X.25. LEGEND: E = An essential user facility to be offered by all networks. A = An additional user facility which may be offered by certain networks. FS = Further study is required. This standard will be modified when this study is complete. - = Not applicable. DG = Applicable when the datagram service is being used.* VC = Applicable when the virtual call service is being used. PVC = Applicable when the permanent virtual circuit service is being used. * - The datagram service and its related facilities may be used only when: - there is to be a one-way transfer of information which does not require recovery at the network layer; and, - a response to this transfer of information is not required at the network layer. NOTES: 1. At the present time, the transfer of datagram packets across international borders through public packet-switching networks is not permitted 2. DCE's are not required to provide datagram service. DTE's are not required to generate or accept datagrams and datagram-related packets. ** - Fast select shall be provided by all DCE's. All DTE's associated with the higher level functionality which allows response to a fast select packet must be capable of accepting incoming fast select packets, but need not generate fast select packets. *** The packet retransmission facilities shall not be used. (z) The list of the applicable call progress signals, extracted from CCITT Recommendation X.96, is given below. These signal definitions apply to the cause codes specified in CCITT Recommendation X.25. The related circumstances giving rise to each call progress signal is also defined in table 1 below. The significance of categories indicates broadly the type of action expected of the DTE receiving the signal: Category Significance A Requested action confirmed by network. B Call cleared because the procedure is complete. C1 and C2 Call cleared. The calling DTE should call again soon: the next attempt may be successful. However, after a number of unsuccessful call attempts with the same response, the cause could be assumed to be in Category D1 or D2. The interval between successive attempts and the number of maximum attempts will depend on a number of circumstances including: - nature of the call progress signal - users' traffic pattern - tariffs - possible regulations by the network provider. OR Reset. The DTE may continue to transmit data recognizing that data loss may have occurred. D1 and D2 Call cleared. The calling DTE should take other action to clarify when the call attempt might be successful. OR Reset (for permanent virtual circuit only). The DTE should cease data transmission and take other action as appropriate. C1 and D1 Due to subscriber condition. C2 and D2 Due to network condition. The sequence of call progress signals in table 1 implies, for Categories C and D, the order of call set-up processing by the network. ln general, the DTE can assume, on receiving a call progress signal, that no condition higher up in the table is present. Network congestion is an exception to this general rule. The actual coding of call progress signals does not necessarily reflect this sequence. Users and DTE manufacturers are warned to make due allowance to possible later extensions to this table by providing appropriate fallback routines for unexpected signals. ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- Call Progress Definition Category Signal ------------------------------------------------------------------------------- Delivery The datagram has been A confirmation accepted by the destination DTE. Local procedure A procedure error caused by the DTE C1 error is detected by the DCE at the local DTE/DCE interface. Network A condition exists in the network C2 congestion such as: 1) temporary network congestion 2) temporary fault condition within the network, including procedure error within a network or an international link. Invalid A facility requested by the calling D1 or D2 facility DTE is detected as invalid by the DCE request at the local DTE/DCE interface. Possible reasons include: - request for a facility which has not been subscribed to by the DTE; - request for a facility which is not available in the local network: - request for a facility which has not been recognized as valid by the local DCE. RPOA out The RPOA nominated by the calling DTE is D2 of order unable to forward the call. Not The called DTE address is D1 obtainable out of the numbering plan or not assigned to any DTE. Access barred The calling DTE is not permitted D1 the connection to the called DTE. Possible reasons include: - unauthorized access between the calling DTE and thc called DTE. - incompatible closed user group. Reverse charging The called DTE has not subscribed D1 acceptance not to the reverse charging acceptance subscribed facility. Fast select The called DTE has not subscribed D1 acceptance not to the fast select acceptance subscribed facility. Incompatible The remote DTE/DCE interface or the D1 destination or the transit network does not support a function or facility requested (eg.the datagram service). Out of Order The remote number is out of order. D1 or D2 Possible reasons include: - DTE is Uncontrolled Not Ready: - DCE Power off: - Network fault in the local loop: - X.25 Level 1 not functioning: - X.25 Level 2 not in operation. Number busy The called DTE is detected by the DCE C1 as engaged on other call(s), and therefore as not being able to accept the incoming call. (In the case of the datagram service..the queue at the destination DCE is full.) Remote A procedure error caused by the D1 procedure remote DTE is detected by the DCE error at the remote DTE/DCE interface. Network Network is ready to resume normal C1 operational operation after a temporary failure or congestion. Remote DTE Remote DTE/DCE interface is ready C1 or D1 operational to resume normal operation after a temporary failure or out of order condition (e.g., restart at the remote DTE/DCE interface. Loss of data may have occurred. DTE originated The remote DTE has intiated B or D1 a clear, reset, or restart procedure. Waivers: Waiver of this standard is required when an interface based on CCITT Recommendation X.25 (1980) is to be employed and has either one of the following conditions: 1) The interface has options that are not permitted by this standard; 2) The interface does not implement all options mandated by this standard. Heads of agencies desiring a waiver from the requirements stated in this standard, so as to acquire applicable equipment or service not conforming to this standard, shall submit a request for waiver to the Administrator, General Services Administration, for review and approval. Approval will be granted if, in the judgment of the Administrator after consultation with the Assistant Secretary of Commerce for Productivity, Technology and Innovation, based on all available information including that provided in the waiver requests, a major adverse economic or operational impact would occur through conformance with this standard. A request for waiver shall include a justification for the waiver, including a description and discussion of the adverse economic or operational impact that would result from conforming to this standard as compared to the alternative for which the waiver is requested. ICST and NCS will provide technical assistance, as required, to GSA. Where to Obtain Copies: Copies of this publication are for sale by the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161. When ordering, refer to Federal Information Processing Standards Publication 100 (FIPS-PUB- l00)/Federal Standard 1041 (FED-STD 1041), and title. When microfiche is desired, this should be specified. Payment may be made by check, money order, purchase order, credit card, or deposit account. The CCITT X.25 specifications upon which this publication is based may also be obtained from NTIS. Specify PB82-187766; the cost is $50; telephone (703) 487-4650.