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RFC 1645 - Simple Network Paging Protocol, Version 2

Abstract

   This RFC suggests a simple way for delivering wireless messages, both
   one and two-way, to appropriate receiving devices.  In its simplest
   form, SNPP provides a simple way to implement a "shim" between the
   Internet and a TAP/IXO paging terminal. In its level 3 form, it
   provides an easy-to-use (and build) method for communicating and
   receiving end-to-end acknowledgments and replies from two-way
   messaging devices (such as ReFLEX units).

   Gateways supporting this protocol, as well as SMTP, have been in use
   for well over a year at several commercial paging companies, and
   private businesses.  Client software supporting this protocol has
   become widespread, and is being integrated into many of the new
   paging and messaging products being built.  In addition to commercial
   software, email filters and SNPP client software for Unix and Windows
   (WikiPage) are available at no cost.  Please contact the author for
   more information.

   Earlier versions of this specification were reviewed by IESG members
   and the "822 Extensions" Working Group.  They preferred an alternate
   strategy, as discussed under "Relationship to Other IETF Work",
   below.

1. Introduction

   With all due apologies to the Glenayre engineers (who take offense at
   the term "nerd") beepers are as much a part of computer nerdom as X-
   terminals--perhaps, unfortunately, more. The intent of Simple Network
   Paging Protocol is to provide a standard whereby pages can be
   delivered to individual paging terminals.  The most obvious benefit
   is the elimination of the need for modems and phone lines to produce
   alphanumeric pages, and the added ease of delivery of pages to
   terminals in other cities or countries. The benefits of the Internet

   become even more realized when growing towards acknowledgment-based
   messaging such as ReFLEX paging--where it may be impossible to
   accurately predict costs associated with telco services such as 1-800
   numbers.

2. System Philosophy

   Radio paging is somewhat taken for granted, because of the wide
   availability and wide use of paging products.  However, the actual
   delivery of the page, and the process used (especially in wider area
   paging) is somewhat complicated.  When a user initiates a page, by
   dialing a number on a telephone, or entering an alphanumeric page
   through some input device, the page must ultimately be delivered to
   some paging terminal, somewhere.  In most cases, this delivery is
   made using TAP (Telocator Alphanumeric input Protocol, also known as
   IXO).  This protocol can be a somewhat convoluted, and complicated
   protocol using older style ASCII control characters and a non-
   standard checksumming routine to assist in validating the data.

   Even though TAP is widely used throughout the industry, there are
   plans on the table to move to a more flexible "standard" protocol
   referred to as TME (Telocator Message Entry Protocol).  The level two
   enhancements to SNPP (as described below) are intended for use with
   this forthcoming standard.

   The recently-added level three enhancements have been engineered for
   use, specifically, with acknowledgment-based paging.  With the recent
   advances in wireless technology, two-way paging is fast approaching
   reality--therefore creating a need for a workable end-to-end
   acknowledged protocol.  Two-way messaging, however, opens up several
   new areas of unpredictability.  The most pronounced is the subscriber
   response time.  Although deliveries from host to subscriber, and
   subsequent receipt-acknowledgments happen in a rather predictable
   manner, it is impossible to know when the subscriber will physically
   pull the unit out, read the message and respond to it.  Therefore, it
   could well be cost prohibitive to conduct such transactions online
   using a phone line as medium--especially an 800-number. This makes
   the Internet an extremely attractive alternative because of its
   (generally) usage insensitive nature.

   However, acknowledging the complexity of task, and flexibility of the
   current protocols (or the lack thereof), the final user function is
   quite simple: to deliver a page from point-of-origin to someone's
   beeper.  That is the simple, real-time function that the base
   protocol attempts to address.

3. Why not just use Email and SMTP for paging?

   Email, while quite reliable, is not always timely.  A good example of
   this is deferred messaging when a gateway is down. Suppose Mary Ghoti
   (fish@hugecompany.org) sends a message to Zaphod Beeblebrox's beeper
   (5551212@pager.pagingcompany.com). Hugecompany's gateway to the
   Internet is down causing Mary's message to be deferred.  Mary,
   however, is not notified of this delay because her message has not
   actually failed to reach its destination.  Three hours later, the
   link is restored, and (as soon as sendmail wakes up) the message is
   sent.  Obviously, if Mary's page concerned a meeting that was
   supposed to happen 2 hours ago, there will be some minor
   administrative details to work out between Mary and Zaphod!

   On the other hand, if Mary had used her SNPP client (or simply
   telnetted to the SNPP gateway), she would have immediately discovered
   the network problem.  She would have decided to invoke plan "B" and
   call Zaphod's pager on the telephone, ringing him that way.

   The obvious difference here is not page delivery, but the immediate
   notification of a problem that affects your message. Standard email
   and SMTP, while quite reliable in most cases, cannot be positively
   guaranteed between all nodes at all times, making it less desirable
   for emergency or urgent paging.  This inability to guarantee delivery
   could, whether rightly or wrongly, place the service provider in an
   uncomfortable position with a client who has just received his or her
   emergency page, six hours too late.

   Another advantage of using a separate protocol for paging delivery is
   that it gives the sender absolute flexibility over what is sent to
   the pager.  For instance, in the paging arena, where messages are
   sent to alphanumeric pagers, it is less desirable to send the
   recipient general header lines from a standard SMTP message.  Much of
   the information is useless, possibly redundant, and a waste of
   precious RF bandwidth.

   Therefore, when implementing an SMTP gateway, the service provider
   should elect to parse out needed information (such as the sender, and
   possibly subject) such to maximize the utility of the transmission.
   Parsing generally means less control over content and format by the
   message originator.  SNPP provides a clean, effective way to send a
   message, as written, to the recipient's pager.

   The other consideration is the relative simplicity of the SNPP
   protocol for manual telnet sessions versus someone trying to manually
   hack a mail message into a gateway.

4. The SNPP Protocol

   The SNPP protocol is a sequence of commands and replies, and is based
   on the philosophy of many other Internet protocols currently in use.
   SNPP has several input commands (the first 4 characters of each are
   significant) that solicit various server responses falling into the
   following categories:

    2xx - Successful, continue
    3xx - Begin DATA input (see "DATA" command)
    4xx - Failed with connection terminated
    5xx - Failed, but continue session

   SNPP version 3 (two-way) adds the following categories:

    7xx - UNsuccessful two-way specific transaction, but continue
          session
    8xx - Successful two-way specific transaction, continue
    9xx - Successful QUEUED two-way transaction, continue

   The first character of every server response code is a digit
   indicating the category of response.  The text portion of the
   response following the code may be altered to suit individual
   applications.

   The session interaction, especially at SNPP level one, is actually
   quite simple (hence the name).  The client initiates the connection
   with the listening server.  Upon opening the connection, the server
   issues a "220" level message (indicating the willingness of the
   server to accept SNPP commands).  The client passes pager ID
   information, and a message, then issues a "SEND" command.  The server
   then feeds the information to the paging terminal, gathers a
   response, and reports the success or failure to the client.

4.1 Examples of "simple" SNPP Transactions

   The following illustrate examples of client-server communication
   using SNPP.

4.1.1 A Typical Level One Connection

            Client                         Server

    Open Connection               -->
                                  <--  220 SNPP Gateway Ready
    PAGE 5551212                  -->
                                  <--  250 Pager ID Accepted
    MESS Your network is hosed    -->
                                  <--  250 Message OK
    SEND                          -->
                                  <--  250 Message Sent OK
    QUIT                          -->
                                  <--  221 OK, Goodbye

4.1.2 A Typical Level Two, Multiple Transaction

   The following example illustrates a single message sent to two
   pagers.  Using this level protocol, pager-specific options may be
   selected for each receiver by specifying the option prior to issuing
   the "PAGEr" command.  In this example, an alternate coverage area is
   selected for the first pager, while delayed messaging is specified
   for the second.

            Client                         Server

    Open Connection               -->
                                  <--  220 SNPP Server Ready
    COVE 2                        -->
                                  <--  250 Alternate Area Selected
    PAGE 5551212 FOOBAR           -->
                                  <--  250 Pager ID Accepted
    HOLD 9401152300 -0600         -->
                                  <--  250 Delayed Message OK
    PAGE 5552323 XYZZY            -->
                                  <--  250 Pager ID Accepted
    SUBJ Seattle Meeting          -->
                                  <--  250 Message Subject OK
    DATA                          -->
                                  <--  354 Begin Input, End With '.'
    Please meet me tomorrow at    -->
    the Seattle office            -->
                                  <--  250 DATA Accepted
    SEND                          -->
                                  <--  250 Message Sent OK
    QUIT                          -->
                                  <--  221 OK, Goodbye

4.1.3 A Typical Level Three (two-way) Transaction

   Level three transactions are inherently single-unit oriented because
   of the one-to-one issues surrounding responses.  Each transaction
   begins with the "2WAY" command and terminates with a "SEND" command.

        Client                         Server

Open Connection               -->
                              <--  220 SNPP (V3) Gateway Ready
2WAY                          -->
                              <--  250 Two-Way Mode Enabled
NOQUEUE                       -->
                              <--  250 Msg will either be Sent or Rejected
PAGER SHIRLEY                 -->
                              <--  850 Unit online; Don't call me Shirley!
ACKRead 1                     -->
                              <--  250 Read Acknowledgment Requested
DATA                          -->
                              <--  354 Begin Input, End With '.'
Little Bo Binary has lost     -->
her Sparcstation and doesn't  -->
know where to find it. Have   -->
you seen it recently?         -->
                              <--  250 DATA Accepted
RTYPE MULTICHOICE             -->
                              <--  250 Multichoice Responses Enabled
MCRESP 01 In the West Pasture -->
                              <--  250 MCR Code Accepted
MCRESP 02 GoldiFLOCKs has it  -->
                              <--  250 MCR Code Accepted
MCRESP 03 Haven't a clue      -->
                              <--  250 MCR Code Accepted
MCRESP 04 Haven't a life      -->
                              <--  250 MCR Code Accepted
MCRESP 05 Oh, GO AWAY!        -->
                              <--  250 MCR Code Accepted
SEND                          -->
                              <--  860 00321 1234 Message Delivered
QUIT                          -->
                              <--  221 OK, Goodbye

4.2 General Response Code Theory

   Before discussing specific SNPP transactions, it may be helpful to
   discuss some of the response codes.  As mentioned previously, every
   response from the SNPP server to the client contains a 3 digit code
   that categorizes the response. Several of these codes fall into the

   "general" category, and may occur more frequently throughout a given
   SNPP transaction. There are some lesser used (somewhat transaction
   specific) responses that will be discussed in conjunction with the
   format of a specific command.

4.2.1 Code 214 - Multi-line "help/info" message

   This code prefixes a line of response information (such as in
   response to the HELP command).  It should be terminated with a "250
   OK" message.  This code is used when the response will take more than
   one line to display.

4.2.2 Code 218 - Single-line "help/info" message

   This code prefixes a single line of response information (such as the
   request for a single database entry).  Unlike the 214 series, it has
   no "250" series terminator.

4.2.3 Code 250 - Successful Transaction

   This code is a general positive acknowledgment from the server
   indicating that a command was successfully processed. Additionally,
   code 250 can appear at the end of the response to a HELP command (214
   series commands--discussed below).

4.2.4 Code 421 - Fatal Error, Connection Terminated

   This code is displayed just prior to the SNPP server terminating a
   connection with a client for errors. Such a connection termination
   may occur at any time and for any reason (administrative or
   technical).

4.2.5 Code 500 - Command Not Implemented

   This code is a "fail but continue code" that appears when an illegal
   command is entered.

4.2.6 Code 503 - Duplicate Command Entry; Already Entered That

   This code indicates that the specified information has already been
   entered.  This code would appear, for instance, if the client
   attempted to enter a MESSage command after specifying a "DATA"
   sequence.

4.2.7 Codes 550 and 554 - Transaction Failed, but Continue

   These codes indicate a failed command, but the session is allowed to
   continue.  A 550 code should be used to indicate a more

   "administrative" failure (such as an invalid pager ID, or illegal
   parameter), while a 554 series indicates a more technical reason
   (such as a gateway down or equipment failure).  In addition to the
   specified failure codes, additional 550 and 554 failures may be
   specified as necessary to allow for greater flexibility.

4.2.8 Code 552 - Maximum Entries Exceeded

   This code is in response to the entry of the "n+1" item when the
   server only permits "n" items in a category.  As an example, the
   client would expect to see this message when trying to enter the 6th
   PAGEr command when the terminal only supported 5.

4.3 Level 1 Commands

   Level one commands are designed as a minimum implementation of the
   protocol.  This collection of commands may be used with either
   TAP/IXO or TME for message delivery to the paging terminal.

4.3.1 PAGEr <Pager ID>

   The PAGEr command submits a pager ID (PID) number, for inclusion in
   the next messaging transaction.  The PID used must reside in, and be
   validated by the paging terminal.  Limited validation may optionally
   be done on the server (such as all numeric, and ID length), or
   validation can be left up to the terminal at the time the page is
   sent.

   When implementing SNPP, the user may elect to support multiple
   recipients per message sent.  However, be wary that validation-
   prior-to-sending is not possible with TAP/IXO (and is not an official
   option of the current TME specification).  What this means is that in
   order to validate a PID, one must generate a message to the pager.
   The terminal responds favorably or negatively.  When reporting
   failure of a single PID in a sequence, delineating and reporting the
   failure in a "standard format" may prove to be a challenge.

   Possible responses from the SNPP server, with suggested text, in
   response to a PAGEr command are:

    250 Pager ID Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    550 Error, Invalid Pager ID
    554 Error, failed (technical reason)

   Both level 2 and level 3 enhancements affect the PAGEr command.
   Please refer to the appropriate section(s) for details.

4.3.2 MESSage <Alpha or Numeric Message>

   The MESSage command specifies a single-line message, into the
   gateway.  Limited validation of the message may be done on the SNPP
   server (such as length), but type-of-message validation should be
   done by the paging terminal.  Duplicating the MESSage command before
   SENDing the message should produce an "503 ERROR, Message Already
   Entered" message, and allow the user to continue.

   Possible responses from the SNPP server, with suggested text, in
   response to a MESSage command are:

    250 Message OK
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    503 ERROR, Message Already Entered
    550 ERROR, Invalid Message
    554 Error, failed (technical reason)

4.3.3 RESEt

   The RESEt command clears already entered information from the server
   session, resetting it to the state of a freshly opened connection.
   This is provided, primarily, as a means to reset accidentally entered
   information during a manual session.

   Possible responses from the SNPP server, with suggested text, in
   response to a RESEt command are:

    250 RESET OK
    421 Too Many Errors, Goodbye (terminate connection
    421 Gateway Service Unavailable (terminate connection)

4.3.4 SEND

   The SEND command finalizes the current message transaction, and
   processes the page to the paging terminal.  Prior to processing, the
   PAGEr and MESSage fields (or message DATA when using the level two
   option) should be checked for the existence of information.  Should
   one of these required fields be missing, the server should respond
   "503 Error, Incomplete Information" and allow the user to continue.
   Assuming that the information is complete, the SNPP server should
   format and send the page to the paging terminal, and await a
   response.

   Possible responses from the SNPP server, with suggested text, in
   response to a SEND command are:

    250 Message Sent Successfully
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    503 Error, Pager ID or Message Incomplete
    554 Message Failed [non-administrative reason]

   Or, in the case of an illegal or non-existent pager ID, or some other
   administrative reason for rejecting the page, the server should
   respond:

    550 Failed, Illegal Pager ID (or other explanation)

   After processing a SEND command, the server should remain online to
   allow the client to submit another transaction.

   Level 3 enhancements to this command allow for other responses.
   Please refer to the appropriate section for discussion.

4.3.5 QUIT

   The QUIT command terminates the current session.  The server should
   simply respond:

    221 OK, Goodbye"

   and close the connection.

4.3.6 HELP (optional)

   The optional HELP command displays a screen of information about
   commands that are valid on the SNPP server.  This is primarily to
   assist manual users of the gateway.  Each line of the HELP screen
   (responses) are preceded by a code "214".  At the end of the HELP
   sequence, a "250" series message is issued.

   Possible responses from the SNPP server, with suggested text, in
   response to a HELP command are:

    214 [Help Text]  (repeated for each line of information)
    250 End of Help Information
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented

4.4 Level 2 - Minimum Extensions

   This section specifies minimum enhancements to the SNPP protocol for
   added functionality.

4.4.1 DATA

   The DATA command is an alternate form of the MESSage command,
   allowing for multiple line delivery of a message to the paging
   terminal.  This command's function is similar to the DATA command
   implemented in SMTP (Internet STD10, RFC821).  The SNPP server should
   only allow one DATA or MESSage command to be issued prior to a SEND.

   Possible responses from the SNPP server, with suggested text, in
   response to a DATA command are:

    354 Begin Input; End with <CRLF>'.'<CRLF>
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    503 ERROR, Message Already Entered
    500 Command Not Implemented
    550 ERROR, failed (administrative reason)
    554 ERROR, failed (technical reason)

   Upon receiving a "354" response, the client begins line input of the
   message to send to the pager.  A single period ("."), in the first
   position of the line, terminates input.  After input, the server may
   respond:

    250 Message OK
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    550 ERROR, Invalid Message (or administrative reason)
    554 ERROR, Failed (technical reason)

4.5 Level 2 - Optional Extensions

   This section discusses enhancements to the SNPP protocol for more
   control over paging functions.  These are primarily designed to
   mirror the added functionality built into the Telocator Message Entry
   (TME) protocol as specified in the TDP protocol suite. These
   functions may, optionally (as is being done by the author), be
   integrated into a paging terminal.  There is no requirement to
   implement all of these functions.  Requests for invalid functions
   should return a "500 Function Not Implemented" error.

   It is important to note that, at the time of this publication, the
   TME standard is still not finalized.

4.5.1 LOGIn <loginid> [password]

   This command allows for a session login ID to be specified.  It is
   used to validate the person attempting to access the paging terminal.

   If no LOGIn command is issued, "anonymous" user status is assumed.

   Possible responses from the SNPP server, with suggested text, in
   response to a LOGIn command are:

    250 Login Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    421 Illegal Access Attempt
    550 Error, Invalid LoginID or Password
    554 Error, failed (technical reason)

4.5.2 PAGEr <PagerID> [Password/PIN]

   This PAGEr command is an enhancement to the level one specification.
   The primary difference is the ability to specify a password or PIN
   for validation or feature access.

   Before proceeding, it is important to understand the logical function
   of the PAGEr command with respect to the LEVEl, COVErage, HOLDtime,
   and ALERt commands (option parameters as described below).  Each time
   a PAGEr command is issued, it should be thought of as the last step
   in a multiple step transaction.

   When the PAGEr command is processed, the pager ID (and password) is
   submitted to the paging terminal with LEVEl, COVErage, HOLDtime, and
   ALERt.  If these parameters have not been altered, then their
   defaults are assumed for the transaction.  After the next PAGEr
   command has been processed, these option parameters are reset their
   defaults.  Using this type of "option-option-option-go" scheme, it is
   possible to specify a different priority level for "Jeff," and an
   alternate coverage area for "Kathy," while sending the same message
   to each.

   Possible responses from the SNPP server, with suggested text, in
   response to a PAGEr command are:

    250 Pager ID Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    550 Error, Invalid Pager ID or Password
    552 Max Recipients Exceeded
    554 Error, failed (technical reason)

4.5.3 LEVEl <ServiceLevel>

   The LEVEl function is used to specify an optional alternate level of
   service for the next PAGEr command.  Ideally, "ServiceLevel" should

   be an integer between 0 and 11 inclusive.  The TME protocol specifies
   ServiceLevel as follows:

    0 - Priority
    1 - Normal (default)
    2 - Five minutes
    3 - Fifteen minutes
    4 - One hour
    5 - Four hours
    6 - Twelve hours
    7 - Twenty Four hours
    8 - Carrier specific '1'
    9 - Carrier specific '2'
   10 - Carrier specific '3'
   11 - Carrier specific '4'

   The choice on how to implement this feature, or to what level it
   should be implemented, should be optional and up to the discretion of
   the carrier.

   Possible responses from the SNPP server, with suggested text, in
   response to a LEVEl command are:

    250 OK, Alternate Service Level Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    550 Error, Invalid Service Level Specified
    554 Error, failed (technical reason)

4.5.4 ALERt <AlertOverride>

   The optional ALERt command may be used to override the default
   setting and specify whether or not to alert the subscriber upon
   receipt of a message.  This option, like the previous command, alters
   the parameters submitted to the paging terminal using the PAGEr
   command.  The TME protocol specifies AlertOverride as either 0-
   DoNotAlert, or 1-Alert.

   Possible responses from the SNPP server, with suggested text, in
   response to a ALERt command are:

    250 OK, Alert Override Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    550 Error, Invalid Alert Parameter
    554 Error, failed (technical reason)

4.5.5 COVErage <AlternateArea>

   The optional COVErage command is used to override the subscriber's
   default coverage area, and allow for the selection of an alternate
   region.  This option, like the previous command, alters the
   parameters submitted to the paging terminal using the PAGEr command.
   AlternateArea is a designator for one of the following:

    - A subscriber-specific alternate coverage area
    - A carrier-defined region available to subscribers

   As an example, Mary Ghoti is a subscriber having local service in
   Chicago, Illinois (Mary's region '1').  Her account has been set up
   in such a manner as to allow Mary's pager to be paged nationwide upon
   demand (Mary's region '2').  Specifying "COVErage 2" prior to issuing
   the appropriate "PAGEr" command allows the default Chicago area to be
   overridden, and Mary's pager to be messaged nationally for that
   transaction.  It is assumed that the carrier providing Mary's service
   will keep track of how many pages have been sent to her pager in this
   manner, and will bill her accordingly.

   Possible responses from the SNPP server, with suggested text, in
   response to a COVErage command are:

    250 Alternate Coverage Selected
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    550 Error, Invalid Alternate Region
    554 Error, failed (technical reason)

4.5.6 HOLDuntil <YYMMDDHHMMSS> [+/-GMTdifference]

   The HOLDuntil command allows for the delayed delivery of a message,
   to a particular subscriber, until after the time specified.  The time
   may be specified in local time (e.g. local to the paging terminal),
   or with an added parameter specifying offset from GMT (in other
   words, "-0600" specifies Eastern Standard Time).  This option, like
   the previous command, alters the parameters submitted to the paging
   terminal using the PAGEr command.

   Possible responses from the SNPP server, with suggested text, in
   response to a HOLDuntil command are:

    250 Delayed Messaging Selected
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented

    550 Error, Invalid Delivery Date/Time
    554 Error, failed (technical reason)

4.5.7 CALLerid <CallerID>

   The CALLerid function is a message-oriented function (as opposed to
   the subscriber-oriented functions just described).  This allows for
   the specification of the CallerIdentifier function as described in
   TME.  This parameter is optional, and is at the discretion of the
   carrier as to how it should be implemented or used.

   Possible responses from the SNPP server, with suggested text, in
   response to a CALLerid command are:

    250 Caller ID Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    550 Error, Invalid Caller ID
    554 Error, failed (technical reason)

4.5.8 SUBJect <MessageSubject>

   The SUBJect function allows is a message-oriented function that
   allows the sender to specify a subject for the next message to be
   sent.  This parameter is optional and is at the discretion of the
   carrier as to how it should be implemented or used.

   Possible responses from the SNPP server, with suggested text, in
   response to a SUBJect command are:

    250 Message Subject Accepted
    421 Too Many Errors, Goodbye (terminate connection)
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    550 Error, Invalid Subject Option
    554 Error, failed (technical reason)

4.6 Level 3 - Two-Way Extensions

   This section specifies enhancements to the SNPP protocol to support
   acknowledgment-based paging (2-way).  One of the more powerful
   features of ReFLEX-style paging, in addition to confirmed message
   delivery, is the ability to "seed" a message with multiple-choice
   type responses.  After the recipient views the message, she can reply
   with one of the seeded messages.  In addition to the multiple-choice
   responses (MCR's), the sender may elect to receive confirmation when
   the message is first viewed by the recipient.

4.6.1 2WAY

   The 2WAY command prefaces each two-way transaction (see previous
   example).  This places the server in the mode to receive and process
   a single 2-way transaction. The server returns to "non-2WAY" mode
   upon the completion of a SEND command or a RESEt command.  In 2WAY
   mode, it is, however, possible to do multiple MSTAtus commands (to
   check responses from field message units).  Possible responses are:

    250 OK, Beginning 2-Way Transaction
    550 Error, Standard Transaction Already Underway, use RESEt
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

   4.6.2 PING <PagerID | Alias>

   This command localizes (finds) the field message unit on the system
   and returns its location and/or status.  Because of the sensitive
   nature of location information, the subscriber may elect to have a
   generic "pager located" message (ACLU mode) rather than to return her
   actual location. Possible responses are:

    820 <Locus_Code> Unit On System, This Area
    821 Unit On System, No Location Information Available (ACLU mode)
    750 Unit Valid But Not Online At This Time
    920 Unit Not Online, But Can Queue Message for Later Delivery
    550 Can't PING; Unit NOT 2-way capable
    550 Unknown or Illegal ID
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.3 EXPTag <hours>

   Changes the default expiry time for a queued message delivery.  If
   the message is not delivered in the specified number of hours, then
   it is deleted and the MSTAtus tag is updated to reflect the inability
   to deliver (code 760).  Possible responses are:

    250 Message Expiry Time Changed to 'nnn' Hours
    550 Cannot Change Expiry Time
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.4 NOQUEUEing

   Specifies that the server should not allow message queuing for this
   2WAY transaction.  In this mode, if a pager is not online, the client
   will receive a "750" series response to a PAGEr command.  This
   command must be specified prior to a PAGEr command.  Possible
   responses are:

    250 Queuing Disabled, This Transaction
    550 Can't Disable Queueing
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.5 ACKRead <0|1>

   Activates or deactivates message "read" acknowledgment.  When
   activated, instructs the field message unit to return a message when
   the subscriber actually views the received message.  This feature is
   independent of the actual reply.  Possible responses are:

    250 Read Acknowledgment <Enabled|Disabled>
    550 Cannot modify Read Acknowledgment
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.6 RTYPe <Reply_Type_Code>

   Changes the type of reply expected from the field message unit that
   is acceptable to the client program.  Initial appropriate reply type
   codes are:

    NONE        - (default) No Reply Permitted
    YESNO       - Seeds a simple "Yes" or "No reply
    SIMREPLY    - Only pre-coded replies from providers's reply base
    MULTICHOICE - Allows full multiple choice replies
    TEXT        - Allows full text replies (generated by field unit)

   Possible responses to an RTYPe command are:

    250 Reply Type Accepted
    550 Illegal Reply Type
    503 Already Entered That
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.7 MCREsponse <2-byte_Code> Response_Text

   This command is issued prior the the SEND command, and "seeds" the
   transaction with an acceptable multiple choice response. Each
   response is specific to the current message.  The number of
   acceptable responses may be limited by the SNPP server as desired by
   the provider.  Examples of MCREsponse(s) are:

    MCREsponse 1E2C Here is one response
    MCREsponse 0002 This is another response

   Responses from the SNPP server to the client are:

    250 Response Added to Transaction
    502 Error! Would Duplicate Previously Entered MCResponse
    550 Invalid MCResponse Code
    550 MCResponses Not Enabled
    552 Too Many MCResponses Entered
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.8 PAGEr

   In 2WAY mode, the following enhanced responses are available:

    850 Two-Way Unit Online and Available; Transaction Accepted
    950 Unit NOT Online; Message Will be Queued for Later Delivery
    750 Two-Way Unit NOT Online; Transaction Denied
    550 Error, Pager Not 2WAY Capable
    550 Error, RTYPe Mode Invalid for This Unit
    503 Already Selected PAGEr
    421 Gateway Service Unavailable (terminate connection)
    554 Error, failed (technical reason)

4.6.9 SEND

   Instructs the SNPP server to "launch" the message (plus attached
   response codes) to the field message unit.  A successful SEND command
   will return, to the client, a "Message_Tag" number and a "Pass_Code"
   for periodic status checking.  The client then uses the MSTAtus
   command to check the progression of the transaction. The
   "Message_Tag" functions as a "record locator," while the "Pass_Code"
   should be a randomly generated "PIN" code to authorize checking of
   the "Message_Tag."

   Response codes to a SEND command, as well as the MSTAtus command,
   indicate the degree of "finality" to the transaction.  Based on the

   delivery process, there are four categories.  Together with their
   response code prefixes, these are:

    86x - Initial message delivered, awaiting requested action(s)
    87x - Intermediate processing completed, awaiting closure
    88x - Transaction concluded (final)
    96x - Queued transaction
   These prefixes make a multi-tiered transaction relatively simple to
   follow to closure.  When an 88x series response code is received from
   the server, all requested portions of the transaction have been
   processed, and no further status changes will take place.

   The SEND command should reply with the first tier of message
   processing. Following this, the status of the message in the system
   is checked, periodically, using the MSTAtus command.

   Possible responses to a SEND command are:

    860 <Message_Tag> <Pass_Code> Delivered, Awaiting Read Ack
    861 <Message_Tag> <Pass_Code> Delivered, Awaiting Reply (MCR)

    880 <Message_Tag> <Pass_Code> Message Delivered

    960 <Message_Tag> <Pass_Code> OK, Message QUEUED for Delivery

    550 Delivery Failed!  Message destroyed.
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

4.6.10 MSTAtus <Message_Tag> <Pass_Code>

   This is used by a client program to periodically check the status of
   delivery and response of a given message.  The SEND command returns
   the "Message_Tag" and "Pass_Code" required to check the status. A
   "Message_Tag" may be (should be) expired by the SNPP server after an
   appropriate amount of time has passed.  Expiration of these tags is
   vendor dependent, and may accelerate after the first check after
   final disposition of the message (such as after a client program has
   successfully received the field unit's response code).

   The tag record contains a "Sequence" number which is an incremental
   counter that rises as the record's status changes (such as from a
   delivery acknowledgment to a reply).  In addition, date and time of
   the current transaction should be kept in the following format:

    YYMMDDHHMMSS+GMT   (example: 950925143501+7)

   Because of the tiered structure of replies, possible responses to an
   MSTAtus command are:

    860 <Sequence> <Date&Time> Delivered, Awaiting Read Confirmation
    861 <Sequence> <Date&Time> Delivered, Awaiting Reply (MCR)

    870 <Sequence> <Date&Time> Delivered, Read, Awaiting Reply (MCR)

    880 <Sequence> <Date&Time> Message Delivered (No Reply Pending)
    881 <Sequence> <Date&Time> Message Delivered and Read by Recipient
    888 <Sequence> <Date&Time> <Reply_Code> MCR Reply Received
    889 <Sequence> <Date&Time> <Full_Text_Response>

    960 <Sequence> <Date&Time> Message Queued; Awaiting Delivery

    780 <Sequence> <Date&Time> MESSAGE EXPIRED Before Delivery!

    550 Unknown or Illegal Message_Tag or Pass_Code
    421 Gateway Service Unavailable (terminate connection)
    500 Command Not Implemented
    554 Error, failed (technical reason)

   After a closure-series (88x) command has been returned to the client,
   acceleration of message tag deletion may be desired to maximize use
   of resources on the server.

KTAG <Message_Tag> <Pass_Code>

   Used to "kill" the message tag after final reading (or when no
   further responses are desired).  This is more of a courtesy feature
   that allows the client to "clean up" rather than wait for the SNPP
   server to expire the tag.

4.7 Illegal Commands

   Should the client issue an illegal command, the server may respond in
   one of the two following ways:

    421 Too Many Errors, Goodbye (terminate connection)
    500 Command Not Implemented, Try Again

   The number of illegal commands allowed before terminating the
   connection should be at the discretion of the operator of the SNPP
   server.  The only response that has not been discussed is:

    421 SERVER DOWN, Goodbye

   This is used to refuse or terminate connections when the gateway is
   administratively down, or when there is some other technical or
   administrative problem with the paging terminal.

4.8 Timeouts

   The SNPP server can, optionally, have an inactivity timeout
   implemented.  At the expiration of the allotted time, the server
   responds "421 Timeout, Goodbye" and closes the connection.

4.9 Rigidity of Command Structure

   The commands from client to server should remain constant. However,
   since the first character of the response indicates success or
   failure, the text of the server responses could be altered to suit
   the tastes of the operator of the SNPP server. It is suggested that
   the response codes mirror SMTP response codes as closely as possible.

5. Revision History

   Originally, when proposed, the author employed POP2 style
   result/response codes.  The Internet community suggested that this
   '+' and '-' style theory be altered to provide numeric response codes
   -- similar to those used in other services such as SMTP.  The
   protocol has been altered to this specification from the first
   proposed draft.

   Administrative errors (Illegal Pager ID, for example) have been
   separated from technical errors (out-of-space on disk, for example).
   Administrative failures are generally preceded with a 550 series
   response, while technical failures bear a 554 series code.

   Level two enhancements to the protocol have been added in preparation
   for TME deployment.

   Level three enhancements to the protocol have been added in
   preparation for acknowledgment-based messaging.

   Error code "502 Command not implemented" was changed to a general
   "500 Command not recognized" failure result to closer follow SMTP.

6. Relationship to Other IETF Work

   The strategy of this specification, and many of its details, were
   reviewed by an IETF Working Group and three IESG members.  They
   concluded that an approach using the existing email infrastructure
   was preferable, due in large measure to the very high costs of
   deploying a new protocol and the advantages of using the Internet's

   most widely-distributed applications protocol infrastructure.  Most
   reviewers felt that no new protocol was needed at all because the
   special "deliver immediately or fail" requirements of SNPP could be
   accomplished by careful configuration of clients and servers.  The
   experimental network printing protocol [4] was identified as an
   example of an existing infrastructure approach to an existing
   problem. Other reviewers believed that a case could be made for new
   protocol details to identify paging clients and servers to each other
   and negotiate details of the transactions, but that it would be
   sensible to handle those details as extensions to SMTP [1, 2] rather
   than deploying a new protocol structure.

   The author, while recognizing these positions, believes that there is
   merit in a separate protocol to isolate details of TAP/IXO and its
   evolving successors from users and, indeed, from mail-based
   approaches that might reach systems that would act as SMTP/MIME [3]
   to SNPP gateways.  Such systems and gateways are, indeed, undergoing
   design and development concurrent with this work.  See the section
   "Why not just use Email and SMTP?" for additional discussion of the
   author's view of the classical electronic email approach.