📄 rfc759.txt
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State The character string name of a state.Postel [Page 25] August 1980Internet Message ProtocolSpecification Trace Each MPM that handles the message must add its handling-stamp to this list. This will allow detection of messages being sent in a loop within the internet mail system, and aid in fault isolation. Trail When a message is sent through the internetwork environment, it acquires this trace, a list of MPMs that have handled the message. This list is then carried as the trail in a reply or acknowledgment of that message. Requests and replies always have a trace and each MPM adds its handling-stamp to this trace. Replies, in addition, have a trail which is the complete trace of the original message. Transaction Number This is a number which is uniquely associated with this transaction by the originating MPM. It identifies the transaction. (A transaction is a message and acknowledgment.) A transaction number must be unique during the time which the message (a request or reply) containing it could be active in the network. Type-of-Service A service parameter for the delivery of a message, for instance a message could be delivered (REGULAR), forwarded (FORWARD), turned over to general delivery (GENDEL) (i.e., allow a person to decide how to further attempt to deliver the message), or require priority handling (PRIORITY). User The character string name of a user. X121 Address This identifies a host in the Public Data Network environment. When used as a part of identifier, it identifies the originating host of a message. The X121 address is a sequence of up to 14 digits [29]. For use in the MPMs the X121 address is represented in decimal digits.[Page 26] Postel August 1980 Internet Message Protocol Specification Zip Code The character string representation of a postal zip code. The zip code of ISI is 90291.3.7. Data Elements The data elements defined here are similar to the data structure and encoding used in NSW [30]. Each of the diagrams which follow represent a sequence of octets. Field boundaries are denoted by the "|" character, octet boundaries by the "+" character. Each element begins with a one-octet code. The order of the information in each element is left-to-right. In fields with numeric values the high-order (or most significant) bit is the left-most bit. For transmission purposes, the leftmost octet is transmitted first. Cohen has described some of the difficulties in mapping memory order to transmission order [31]. Code Type Representation ---- ---- -------------- +------+ 0 No Operation | 0 | +------+ +------+------+------+------+------ 1 Padding | 1 | octet count | Data ... +------+------+------+------+------ +------+------+ 2 Boolean | 2 | 1/0 | +------+------+ +------+------+------+ 3 Index | 3 | Data | +------+------+------+ +------+------+------+------+------+ 4 Integer | 4 | Data | +------+------+------+------+------+Postel [Page 27] August 1980Internet Message ProtocolSpecification Extended +------+------+------+------+------ 5 Precision | 5 | octet count | Data ... Integer +------+------+------+------+------ +------+------+------+------+------ 6 Bit String | 6 | bit count | Data ... +------+------+------+------+------ +------+------+------ 7 Name String | 7 | count| Data ... +------+------+------ +------+------+------+------+------ 8 Text String | 8 | octet count | Data ... +------+------+------+------+------ +------+------+------+------+----- 9 List | 9 | octet count | Data ... +------+------+------+------+----- +------+------+------+------+------ 10 Proplist | 10 | octet count | Data ... +------+------+------+------+------ +------+ 11 End of List | 11 | +------+ Element code 0 (NOP) is an empty data element used for padding when it is necessary. It is ignored. Element code 1 (PAD) is used to transmit large amounts of data with a message for test or padding purposes. The type-octet is followed by a three-octet count of the number of octets to follow. No action is taken with this data but the count of dummy octets must be correct to indicate the next element code. Element code 2 (BOOLEAN) is a boolean data element. The octet following the type-octet has the value 1 for True and 0 for False.[Page 28] Postel August 1980 Internet Message Protocol Specification Element code 3 (INDEX) is a 16-bit unsigned integer datum. Element code 3 occupies only 3 octets. Element code 4 (INTEGER) is a signed 32-bit integer datum. This will always occupy five octets. Representation is two's complement. Element code 5 (EPI) is an extended precision integer. The type octet is followed by a three-octet count of the number of data octets to follow. Representation is two's complement. Element code 6 (BITSTR) is a bit string element for binary data. The bit string is padded on the right with zeros to fill out the last octet if the bit string does not end on an octet boundary. This data type must have the bit-count in the three-octet count field instead of the number of octets. Element code 7 (NAME) is used for the representation of character string names (or other short strings). The type octet is followed by a one-octet count of the number of characters (one per octet) to follow. Seven bit ASCII characters are used, right justified in the octet. The high order bit in the octet is zero. Element code 8 (TEXT) is used for the representation of text. The type octet is followed by a three-octet count of the number of characters (one per octet) to follow. Seven bit ASCII characters are used, right justified in the octet. The high order bit in the octet is zero. Element code 9 (LIST) can be used to create structures composed of other elements. The three-octet octet count specifies the number of octets in the whole list (i.e., the number of octets following this count field to the end of the list, not including the ENDLIST octet). The two-octet item count contains the number of elements which follow. Any element may be used including list itself. +------+------+------+------+------+------+ | 9 | octet count | item count | +------+------+------+------+------+------+ +------+------/---+ repeated | element | +------+------/---+ +-------+ |ENDLIST| +-------+ In some situations it may not be possible to know the length of a listPostel [Page 29] August 1980Internet Message ProtocolSpecification until the head of it has been transmitted. To allow for this a special ENDLIST element is defined. A list of undetermined length is transmitted with the octet count cleared to zero, and the item count cleared to zero. A null or empty List, one with no elements, has an octet count of two (2) and an item count of zero (0). The ENDLIST element always follows a LIST, even when the length is determined. Element code 10 (PROPLIST) is the Property List element. It is a special case of the list element, in which the elements are in pairs and the first element of each pair is a name. It has the following form: +------+------+------+------+------+ | 10 | octet count | pair | +------+------+------+------+------+ +------+------/---+------+------/---+ repeated | name element | value element | +------+------/---+------+------/---+ +-------+ |ENDLIST| +-------+ The Property List structure consists of a set of unordered <name,value> pairs. The pairs are composed of a name which must be a NAME element and a value which may be any kind of element. Following the type code is a three-octet octet count of the following octets. Following the octet count is a one-octet pair count of the number of <name,value> pairs in the property list. The name of a <name,value> pair is to be unique within the property list, that is, there shall be at most one occurrence of any particular name in one property list. In some situations it may not be possible to know the length of a property list until the head of it has been transmitted. To allow for this the special ENDLIST element is defined. A property list of undetermined length is transmitted with the octet count cleared to zero, and the pair count cleared to zero. A null or empty property list, one with no elements, has an octet count of one (1) and an pair count of zero (0). The ENDLIST element always follows a property list, even when the length is determined. Element code 11 (ENDLIST) is the end of list element. It marks the end of the corresponding list or property list.[Page 30] Postel August 1980 Internet Message Protocol Specification Structure Sharing When messages are batched in message-bags for transmission, it may often be the case that the same document will be sent to more than one recipient. Since the document portion can usually be expected to be the major part of the message, much repeated data would be sent if a copy of the document for each recipient were to be shipped in the message-bag. To avoid this redundancy, messages may be assembled in the message-bag so that actual data appears on its first occurrence and only references to it appear in later occurrences. When data is shared, the first occurrence of the data will be tagged, and later locations where the data should appear will only reference the earlier tagged location. All references to copied data point to earlier locations in the message-bag. The data to be retrieved is indicated by the tag. This is a very general sharing mechanism. PLEASE NOTE THAT THE MPM WILL NOT SUPPORT THE FULL USE OF THIS MECHANISM. THE MPM WILL ONLY SUPPORT SHARING OF WHOLE DOCUMENTS. No other level of sharing will be supported by the MPMs. This sharing mechanism may be used within a document as long as all references refer to tags within the same document. Sharing is implemented by placing a share-tag on the first occurrence of the data to be shared, and placing a share-reference at the locations where copies of that data should occur. +------+------+------+ 12 Share Tag | 12 | share-index | +------+------+------+ +------+------+------+ 13 Share Reference | 13 | share-index | +------+------+------+ Element code 12 (S-TAG) is a share tag element. The two octets following the type-octet specify the shared data identification code for the following data element. Note that s-tag is not a DATA element, in the sense that data elements encode higher level objects. Element code 13 (S-REF) is a share reference element. The twoPostel [Page 31] August 1980Internet Message ProtocolSpecification octets following the type-octet specify the referenced shared data identification code. An example of using this mechanism is ( ( <a>, <b> ) ( <c>, <b> ) ) could be coded as follows to share <b> ( ( <a>, <s-tag-1><b> ) ( <c>, <s-ref-1> ) ) To facilitate working with structures which may contain shared data, the two high-order bits of the list and property list element codes are reserved for indicating if the structure contains data to be shared or contains a reference to shared data. That is, if the high-order bit of the list or property list element code octet is set to one then the property list contains a share-reference to shared data. Or, if the second high-order bit is set to one the structure contains a share-tag for data to be shared. The example above is now repeated in detail showing the use of the high-order bits. +------+------+------+------+------+------+------+------+ |11 - 9|01 - 9| <a> | 12 | 0 | 1 | <b> | 11 | +------+------+------+------+------+------+------+------+ +------+------+------+------+------+------+------+ |10 - 9| <c> | 13 | 0 | 1 | 11 | 11 | +------+------+------+------+------+------+------+ It is not considered an error for an element to be tagged but not referenced. A substructure with internal sharing may be created. If such a substructure is closed with respect to sharing -- that is, all references to its tagged elements are within the substructure -- then there is no need for the knowledge of the sharing to propagate up the hierarchy of lists. For example, if the substructure is: 00-LIST ( a b c b ) which ⌨️ 快捷键说明
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