brian@sdcc3.UUCP (Brian Kantor) (01/07/85)
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.
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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
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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.
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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.
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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
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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.
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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
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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.
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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.
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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.
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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,
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* 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.
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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.
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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