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1IETF CIPSO Working Group216 July, 19923 4 5 6 COMMERCIAL IP SECURITY OPTION (CIPSO 2.2)7 8 9 101. Status11 12This Internet Draft provides the high level specification for a Commercial13IP Security Option (CIPSO). This draft reflects the version as approved by14the CIPSO IETF Working Group. Distribution of this memo is unlimited.15 16This document is an Internet Draft. Internet Drafts are working documents17of the Internet Engineering Task Force (IETF), its Areas, and its Working18Groups. Note that other groups may also distribute working documents as19Internet Drafts.20 21Internet Drafts are draft documents valid for a maximum of six months.22Internet Drafts may be updated, replaced, or obsoleted by other documents23at any time. It is not appropriate to use Internet Drafts as reference24material or to cite them other than as a "working draft" or "work in25progress."26 27Please check the I-D abstract listing contained in each Internet Draft28directory to learn the current status of this or any other Internet Draft.29 30 31 32 332. Background34 35Currently the Internet Protocol includes two security options. One of36these options is the DoD Basic Security Option (BSO) (Type 130) which allows37IP datagrams to be labeled with security classifications. This option38provides sixteen security classifications and a variable number of handling39restrictions. To handle additional security information, such as security40categories or compartments, another security option (Type 133) exists and41is referred to as the DoD Extended Security Option (ESO). The values for42the fixed fields within these two options are administered by the Defense43Information Systems Agency (DISA).44 45Computer vendors are now building commercial operating systems with46mandatory access controls and multi-level security. These systems are47no longer built specifically for a particular group in the defense or48intelligence communities. They are generally available commercial systems49for use in a variety of government and civil sector environments.50 51The small number of ESO format codes can not support all the possible52applications of a commercial security option. The BSO and ESO were53designed to only support the United States DoD. CIPSO has been designed54to support multiple security policies. This Internet Draft provides the55format and procedures required to support a Mandatory Access Control56security policy. Support for additional security policies shall be57defined in future RFCs.58 59 60 61 62Internet Draft, Expires 15 Jan 93 [PAGE 1]63 64 65 66CIPSO INTERNET DRAFT 16 July, 199267 68 69 70 713. CIPSO Format72 73Option type: 134 (Class 0, Number 6, Copy on Fragmentation)74Option length: Variable75 76This option permits security related information to be passed between77systems within a single Domain of Interpretation (DOI). A DOI is a78collection of systems which agree on the meaning of particular values79in the security option. An authority that has been assigned a DOI80identifier will define a mapping between appropriate CIPSO field values81and their human readable equivalent. This authority will distribute that82mapping to hosts within the authority's domain. These mappings may be83sensitive, therefore a DOI authority is not required to make these84mappings available to anyone other than the systems that are included in85the DOI.86 87This option MUST be copied on fragmentation. This option appears at most88once in a datagram. All multi-octet fields in the option are defined to be89transmitted in network byte order. The format of this option is as follows:90 91+----------+----------+------//------+-----------//---------+92| 10000110 | LLLLLLLL | DDDDDDDDDDDD | TTTTTTTTTTTTTTTTTTTT |93+----------+----------+------//------+-----------//---------+94 95 TYPE=134 OPTION DOMAIN OF TAGS96 LENGTH INTERPRETATION97 98 99 Figure 1. CIPSO Format100 101 1023.1 Type103 104This field is 1 octet in length. Its value is 134.105 106 1073.2 Length108 109This field is 1 octet in length. It is the total length of the option110including the type and length fields. With the current IP header length111restriction of 40 octets the value of this field MUST not exceed 40.112 113 1143.3 Domain of Interpretation Identifier115 116This field is an unsigned 32 bit integer. The value 0 is reserved and MUST117not appear as the DOI identifier in any CIPSO option. Implementations118should assume that the DOI identifier field is not aligned on any particular119byte boundary.120 121To conserve space in the protocol, security levels and categories are122represented by numbers rather than their ASCII equivalent. This requires123a mapping table within CIPSO hosts to map these numbers to their124corresponding ASCII representations. Non-related groups of systems may125 126 127 128Internet Draft, Expires 15 Jan 93 [PAGE 2]129 130 131 132CIPSO INTERNET DRAFT 16 July, 1992133 134 135 136have their own unique mappings. For example, one group of systems may137use the number 5 to represent Unclassified while another group may use the138number 1 to represent that same security level. The DOI identifier is used139to identify which mapping was used for the values within the option.140 141 1423.4 Tag Types143 144A common format for passing security related information is necessary145for interoperability. CIPSO uses sets of "tags" to contain the security146information relevant to the data in the IP packet. Each tag begins with147a tag type identifier followed by the length of the tag and ends with the148actual security information to be passed. All multi-octet fields in a tag149are defined to be transmitted in network byte order. Like the DOI150identifier field in the CIPSO header, implementations should assume that151all tags, as well as fields within a tag, are not aligned on any particular152octet boundary. The tag types defined in this document contain alignment153bytes to assist alignment of some information, however alignment can not154be guaranteed if CIPSO is not the first IP option.155 156CIPSO tag types 0 through 127 are reserved for defining standard tag157formats. Their definitions will be published in RFCs. Tag types whose158identifiers are greater than 127 are defined by the DOI authority and may159only be meaningful in certain Domains of Interpretation. For these tag160types, implementations will require the DOI identifier as well as the tag161number to determine the security policy and the format associated with the162tag. Use of tag types above 127 are restricted to closed networks where163interoperability with other networks will not be an issue. Implementations164that support a tag type greater than 127 MUST support at least one DOI that165requires only tag types 1 to 127.166 167Tag type 0 is reserved. Tag types 1, 2, and 5 are defined in this168Internet Draft. Types 3 and 4 are reserved for work in progress.169The standard format for all current and future CIPSO tags is shown below:170 171+----------+----------+--------//--------+172| TTTTTTTT | LLLLLLLL | IIIIIIIIIIIIIIII |173+----------+----------+--------//--------+174 TAG TAG TAG175 TYPE LENGTH INFORMATION176 177 Figure 2: Standard Tag Format178 179In the three tag types described in this document, the length and count180restrictions are based on the current IP limitation of 40 octets for all181IP options. If the IP header is later expanded, then the length and count182restrictions specified in this document may increase to use the full area183provided for IP options.184 185 1863.4.1 Tag Type Classes187 188Tag classes consist of tag types that have common processing requirements189and support the same security policy. The three tags defined in this190Internet Draft belong to the Mandatory Access Control (MAC) Sensitivity191 192 193 194Internet Draft, Expires 15 Jan 93 [PAGE 3]195 196 197 198CIPSO INTERNET DRAFT 16 July, 1992199 200 201 202class and support the MAC Sensitivity security policy.203 204 2053.4.2 Tag Type 1206 207This is referred to as the "bit-mapped" tag type. Tag type 1 is included208in the MAC Sensitivity tag type class. The format of this tag type is as209follows:210 211+----------+----------+----------+----------+--------//---------+212| 00000001 | LLLLLLLL | 00000000 | LLLLLLLL | CCCCCCCCCCCCCCCCC |213+----------+----------+----------+----------+--------//---------+214 215 TAG TAG ALIGNMENT SENSITIVITY BIT MAP OF216 TYPE LENGTH OCTET LEVEL CATEGORIES217 218 Figure 3. Tag Type 1 Format219 220 2213.4.2.1 Tag Type222 223This field is 1 octet in length and has a value of 1.224 225 2263.4.2.2 Tag Length227 228This field is 1 octet in length. It is the total length of the tag type229including the type and length fields. With the current IP header length230restriction of 40 bytes the value within this field is between 4 and 34.231 232 2333.4.2.3 Alignment Octet234 235This field is 1 octet in length and always has the value of 0. Its purpose236is to align the category bitmap field on an even octet boundary. This will237speed many implementations including router implementations.238 239 2403.4.2.4 Sensitivity Level241 242This field is 1 octet in length. Its value is from 0 to 255. The values243are ordered with 0 being the minimum value and 255 representing the maximum244value.245 246 2473.4.2.5 Bit Map of Categories248 249The length of this field is variable and ranges from 0 to 30 octets. This250provides representation of categories 0 to 239. The ordering of the bits251is left to right or MSB to LSB. For example category 0 is represented by252the most significant bit of the first byte and category 15 is represented253by the least significant bit of the second byte. Figure 4 graphically254shows this ordering. Bit N is binary 1 if category N is part of the label255for the datagram, and bit N is binary 0 if category N is not part of the256label. Except for the optimized tag 1 format described in the next section,257 258 259 260Internet Draft, Expires 15 Jan 93 [PAGE 4]261 262 263 264CIPSO INTERNET DRAFT 16 July, 1992265 266 267 268minimal encoding SHOULD be used resulting in no trailing zero octets in the269category bitmap.270 271 octet 0 octet 1 octet 2 octet 3 octet 4 octet 5272 XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX . . .273bit 01234567 89111111 11112222 22222233 33333333 44444444274number 012345 67890123 45678901 23456789 01234567275 276 Figure 4. Ordering of Bits in Tag 1 Bit Map277 278 2793.4.2.6 Optimized Tag 1 Format280 281Routers work most efficiently when processing fixed length fields. To282support these routers there is an optimized form of tag type 1. The format283does not change. The only change is to the category bitmap which is set to284a constant length of 10 octets. Trailing octets required to fill out the 10285octets are zero filled. Ten octets, allowing for 80 categories, was chosen286because it makes the total length of the CIPSO option 20 octets. If CIPSO287is the only option then the option will be full word aligned and additional288filler octets will not be required.289 290 2913.4.3 Tag Type 2292 293This is referred to as the "enumerated" tag type. It is used to describe294large but sparsely populated sets of categories. Tag type 2 is in the MAC295Sensitivity tag type class. The format of this tag type is as follows:296 297+----------+----------+----------+----------+-------------//-------------+298| 00000010 | LLLLLLLL | 00000000 | LLLLLLLL | CCCCCCCCCCCCCCCCCCCCCCCCCC |299+----------+----------+----------+----------+-------------//-------------+300 301 TAG TAG ALIGNMENT SENSITIVITY ENUMERATED302 TYPE LENGTH OCTET LEVEL CATEGORIES303 304 Figure 5. Tag Type 2 Format305 306 3073.4.3.1 Tag Type308 309This field is one octet in length and has a value of 2.310 311 3123.4.3.2 Tag Length313 314This field is 1 octet in length. It is the total length of the tag type315including the type and length fields. With the current IP header length316restriction of 40 bytes the value within this field is between 4 and 34.317 318 3193.4.3.3 Alignment Octet320 321This field is 1 octet in length and always has the value of 0. Its purpose322is to align the category field on an even octet boundary. This will323 324 325 326Internet Draft, Expires 15 Jan 93 [PAGE 5]327 328 329 330CIPSO INTERNET DRAFT 16 July, 1992331 332 333 334speed many implementations including router implementations.335 336 3373.4.3.4 Sensitivity Level338 339This field is 1 octet in length. Its value is from 0 to 255. The values340are ordered with 0 being the minimum value and 255 representing the341maximum value.342 343 3443.4.3.5 Enumerated Categories345 346In this tag, categories are represented by their actual value rather than347by their position within a bit field. The length of each category is 2348octets. Up to 15 categories may be represented by this tag. Valid values349for categories are 0 to 65534. Category 65535 is not a valid category350value. The categories MUST be listed in ascending order within the tag.351 352 3533.4.4 Tag Type 5354 355This is referred to as the "range" tag type. It is used to represent356labels where all categories in a range, or set of ranges, are included357in the sensitivity label. Tag type 5 is in the MAC Sensitivity tag type358class. The format of this tag type is as follows:359 360+----------+----------+----------+----------+------------//-------------+361| 00000101 | LLLLLLLL | 00000000 | LLLLLLLL | Top/Bottom | Top/Bottom |362+----------+----------+----------+----------+------------//-------------+363 364 TAG TAG ALIGNMENT SENSITIVITY CATEGORY RANGES365 TYPE LENGTH OCTET LEVEL366 367 Figure 6. Tag Type 5 Format368 369 3703.4.4.1 Tag Type371 372This field is one octet in length and has a value of 5.373 374 3753.4.4.2 Tag Length376 377This field is 1 octet in length. It is the total length of the tag type378including the type and length fields. With the current IP header length379restriction of 40 bytes the value within this field is between 4 and 34.380 381 3823.4.4.3 Alignment Octet383 384This field is 1 octet in length and always has the value of 0. Its purpose385is to align the category range field on an even octet boundary. This will386speed many implementations including router implementations.387 388 389 390 391 392Internet Draft, Expires 15 Jan 93 [PAGE 6]393 394 395 396CIPSO INTERNET DRAFT 16 July, 1992397 398 399 4003.4.4.4 Sensitivity Level401 402This field is 1 octet in length. Its value is from 0 to 255. The values403are ordered with 0 being the minimum value and 255 representing the maximum404value.405 406 4073.4.4.5 Category Ranges408 409A category range is a 4 octet field comprised of the 2 octet index of the410highest numbered category followed by the 2 octet index of the lowest411numbered category. These range endpoints are inclusive within the range of412categories. All categories within a range are included in the sensitivity413label. This tag may contain a maximum of 7 category pairs. The bottom414category endpoint for the last pair in the tag MAY be omitted and SHOULD be415assumed to be 0. The ranges MUST be non-overlapping and be listed in416descending order. Valid values for categories are 0 to 65534. Category41765535 is not a valid category value.418 419 4203.4.5 Minimum Requirements421 422A CIPSO implementation MUST be capable of generating at least tag type 1 in423the non-optimized form. In addition, a CIPSO implementation MUST be able424to receive any valid tag type 1 even those using the optimized tag type 1425format.426 427 4284. Configuration Parameters429 430The configuration parameters defined below are required for all CIPSO hosts,431gateways, and routers that support multiple sensitivity labels. A CIPSO432host is defined to be the origination or destination system for an IP433datagram. A CIPSO gateway provides IP routing services between two or more434IP networks and may be required to perform label translations between435networks. A CIPSO gateway may be an enhanced CIPSO host or it may just436provide gateway services with no end system CIPSO capabilities. A CIPSO437router is a dedicated IP router that routes IP datagrams between two or more438IP networks.439 440An implementation of CIPSO on a host MUST have the capability to reject a441datagram for reasons that the information contained can not be adequately442protected by the receiving host or if acceptance may result in violation of443the host or network security policy. In addition, a CIPSO gateway or router444MUST be able to reject datagrams going to networks that can not provide445adequate protection or may violate the network's security policy. To446provide this capability the following minimal set of configuration447parameters are required for CIPSO implementations:448 449HOST_LABEL_MAX - This parameter contains the maximum sensitivity label that450a CIPSO host is authorized to handle. All datagrams that have a label451greater than this maximum MUST be rejected by the CIPSO host. This452parameter does not apply to CIPSO gateways or routers. This parameter need453not be defined explicitly as it can be implicitly derived from the454PORT_LABEL_MAX parameters for the associated interfaces.455 456 457 458Internet Draft, Expires 15 Jan 93 [PAGE 7]459 460 461 462CIPSO INTERNET DRAFT 16 July, 1992463 464 465 466 467HOST_LABEL_MIN - This parameter contains the minimum sensitivity label that468a CIPSO host is authorized to handle. All datagrams that have a label less469than this minimum MUST be rejected by the CIPSO host. This parameter does470not apply to CIPSO gateways or routers. This parameter need not be defined471explicitly as it can be implicitly derived from the PORT_LABEL_MIN472parameters for the associated interfaces.473 474PORT_LABEL_MAX - This parameter contains the maximum sensitivity label for475all datagrams that may exit a particular network interface port. All476outgoing datagrams that have a label greater than this maximum MUST be477rejected by the CIPSO system. The label within this parameter MUST be478less than or equal to the label within the HOST_LABEL_MAX parameter. This479parameter does not apply to CIPSO hosts that support only one network port.480 481PORT_LABEL_MIN - This parameter contains the minimum sensitivity label for482all datagrams that may exit a particular network interface port. All483outgoing datagrams that have a label less than this minimum MUST be484rejected by the CIPSO system. The label within this parameter MUST be485greater than or equal to the label within the HOST_LABEL_MIN parameter.486This parameter does not apply to CIPSO hosts that support only one network487port.488 489PORT_DOI - This parameter is used to assign a DOI identifier value to a490particular network interface port. All CIPSO labels within datagrams491going out this port MUST use the specified DOI identifier. All CIPSO492hosts and gateways MUST support either this parameter, the NET_DOI493parameter, or the HOST_DOI parameter.494 495NET_DOI - This parameter is used to assign a DOI identifier value to a496particular IP network address. All CIPSO labels within datagrams destined497for the particular IP network MUST use the specified DOI identifier. All498CIPSO hosts and gateways MUST support either this parameter, the PORT_DOI499parameter, or the HOST_DOI parameter.500 501HOST_DOI - This parameter is used to assign a DOI identifier value to a502particular IP host address. All CIPSO labels within datagrams destined for503the particular IP host will use the specified DOI identifier. All CIPSO504hosts and gateways MUST support either this parameter, the PORT_DOI505parameter, or the NET_DOI parameter.506 507This list represents the minimal set of configuration parameters required508to be compliant. Implementors are encouraged to add to this list to509provide enhanced functionality and control. For example, many security510policies may require both incoming and outgoing datagrams be checked against511the port and host label ranges.512 513 5144.1 Port Range Parameters515 516The labels represented by the PORT_LABEL_MAX and PORT_LABEL_MIN parameters517MAY be in CIPSO or local format. Some CIPSO systems, such as routers, may518want to have the range parameters expressed in CIPSO format so that incoming519labels do not have to be converted to a local format before being compared520against the range. If multiple DOIs are supported by one of these CIPSO521 522 523 524Internet Draft, Expires 15 Jan 93 [PAGE 8]525 526 527 528CIPSO INTERNET DRAFT 16 July, 1992529 530 531 532systems then multiple port range parameters would be needed, one set for533each DOI supported on a particular port.534 535The port range will usually represent the total set of labels that may536exist on the logical network accessed through the corresponding network537interface. It may, however, represent a subset of these labels that are538allowed to enter the CIPSO system.539 540 5414.2 Single Label CIPSO Hosts542 543CIPSO implementations that support only one label are not required to544support the parameters described above. These limited implementations are545only required to support a NET_LABEL parameter. This parameter contains546the CIPSO label that may be inserted in datagrams that exit the host. In547addition, the host MUST reject any incoming datagram that has a label which548is not equivalent to the NET_LABEL parameter.549 550 5515. Handling Procedures552 553This section describes the processing requirements for incoming and554outgoing IP datagrams. Just providing the correct CIPSO label format555is not enough. Assumptions will be made by one system on how a556receiving system will handle the CIPSO label. Wrong assumptions may557lead to non-interoperability or even a security incident. The558requirements described below represent the minimal set needed for559interoperability and that provide users some level of confidence.560Many other requirements could be added to increase user confidence,561however at the risk of restricting creativity and limiting vendor562participation.563 564 5655.1 Input Procedures566 567All datagrams received through a network port MUST have a security label568associated with them, either contained in the datagram or assigned to the569receiving port. Without this label the host, gateway, or router will not570have the information it needs to make security decisions. This security571label will be obtained from the CIPSO if the option is present in the572datagram. See section 4.1.2 for handling procedures for unlabeled573datagrams. This label will be compared against the PORT (if appropriate)574and HOST configuration parameters defined in section 3.575 576If any field within the CIPSO option, such as the DOI identifier, is not577recognized the IP datagram is discarded and an ICMP "parameter problem"578(type 12) is generated and returned. The ICMP code field is set to "bad579parameter" (code 0) and the pointer is set to the start of the CIPSO field580that is unrecognized.581 582If the contents of the CIPSO are valid but the security label is583outside of the configured host or port label range, the datagram is584discarded and an ICMP "destination unreachable" (type 3) is generated585and returned. The code field of the ICMP is set to "communication with586destination network administratively prohibited" (code 9) or to587 588 589 590Internet Draft, Expires 15 Jan 93 [PAGE 9]591 592 593 594CIPSO INTERNET DRAFT 16 July, 1992595 596 597 598"communication with destination host administratively prohibited"599(code 10). The value of the code field used is dependent upon whether600the originator of the ICMP message is acting as a CIPSO host or a CIPSO601gateway. The recipient of the ICMP message MUST be able to handle either602value. The same procedure is performed if a CIPSO can not be added to an603IP packet because it is too large to fit in the IP options area.604 605If the error is triggered by receipt of an ICMP message, the message606is discarded and no response is permitted (consistent with general ICMP607processing rules).608 609 6105.1.1 Unrecognized tag types611 612The default condition for any CIPSO implementation is that an613unrecognized tag type MUST be treated as a "parameter problem" and614handled as described in section 4.1. A CIPSO implementation MAY allow615the system administrator to identify tag types that may safely be616ignored. This capability is an allowable enhancement, not a617requirement.618 619 6205.1.2 Unlabeled Packets621 622A network port may be configured to not require a CIPSO label for all623incoming datagrams. For this configuration a CIPSO label must be624assigned to that network port and associated with all unlabeled IP625datagrams. This capability might be used for single level networks or626networks that have CIPSO and non-CIPSO hosts and the non-CIPSO hosts627all operate at the same label.628 629If a CIPSO option is required and none is found, the datagram is630discarded and an ICMP "parameter problem" (type 12) is generated and631returned to the originator of the datagram. The code field of the ICMP632is set to "option missing" (code 1) and the ICMP pointer is set to 134633(the value of the option type for the missing CIPSO option).634 635 6365.2 Output Procedures637 638A CIPSO option MUST appear only once in a datagram. Only one tag type639from the MAC Sensitivity class MAY be included in a CIPSO option. Given640the current set of defined tag types, this means that CIPSO labels at641first will contain only one tag.642 643All datagrams leaving a CIPSO system MUST meet the following condition:644 645 PORT_LABEL_MIN <= CIPSO label <= PORT_LABEL_MAX646 647If this condition is not satisfied the datagram MUST be discarded.648If the CIPSO system only supports one port, the HOST_LABEL_MIN and the649HOST_LABEL_MAX parameters MAY be substituted for the PORT parameters in650the above condition.651 652The DOI identifier to be used for all outgoing datagrams is configured by653 654 655 656Internet Draft, Expires 15 Jan 93 [PAGE 10]657 658 659 660CIPSO INTERNET DRAFT 16 July, 1992661 662 663 664the administrator. If port level DOI identifier assignment is used, then665the PORT_DOI configuration parameter MUST contain the DOI identifier to666use. If network level DOI assignment is used, then the NET_DOI parameter667MUST contain the DOI identifier to use. And if host level DOI assignment668is employed, then the HOST_DOI parameter MUST contain the DOI identifier669to use. A CIPSO implementation need only support one level of DOI670assignment.671 672 6735.3 DOI Processing Requirements674 675A CIPSO implementation MUST support at least one DOI and SHOULD support676multiple DOIs. System and network administrators are cautioned to677ensure that at least one DOI is common within an IP network to allow for678broadcasting of IP datagrams.679 680CIPSO gateways MUST be capable of translating a CIPSO option from one681DOI to another when forwarding datagrams between networks. For682efficiency purposes this capability is only a desired feature for CIPSO683routers.684 685 6865.4 Label of ICMP Messages687 688The CIPSO label to be used on all outgoing ICMP messages MUST be equivalent689to the label of the datagram that caused the ICMP message. If the ICMP was690generated due to a problem associated with the original CIPSO label then the691following responses are allowed:692 693 a. Use the CIPSO label of the original IP datagram694 b. Drop the original datagram with no return message generated695 696In most cases these options will have the same effect. If you can not697interpret the label or if it is outside the label range of your host or698interface then an ICMP message with the same label will probably not be699able to exit the system.700 701 7026. Assignment of DOI Identifier Numbers =703 704Requests for assignment of a DOI identifier number should be addressed to705the Internet Assigned Numbers Authority (IANA).706 707 7087. Acknowledgements709 710Much of the material in this RFC is based on (and copied from) work711done by Gary Winiger of Sun Microsystems and published as Commercial712IP Security Option at the INTEROP 89, Commercial IPSO Workshop.713 714 7158. Author's Address716 717To submit mail for distribution to members of the IETF CIPSO Working718Group, send mail to: cipso@wdl1.wdl.loral.com.719 720 721 722Internet Draft, Expires 15 Jan 93 [PAGE 11]723 724 725 726CIPSO INTERNET DRAFT 16 July, 1992727 728 729 730 731To be added to or deleted from this distribution, send mail to:732cipso-request@wdl1.wdl.loral.com.733 734 7359. References736 737RFC 1038, "Draft Revised IP Security Option", M. St. Johns, IETF, January7381988.739 740RFC 1108, "U.S. Department of Defense Security Options741for the Internet Protocol", Stephen Kent, IAB, 1 March, 1991.742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788Internet Draft, Expires 15 Jan 93 [PAGE 12]789 790 791 792