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RFC 4037

Open Pluggable Edge Services (OPES) Callout Protocol (OCP) Core

Pages: 56
Proposed Standard
Part 2 of 3 – Pages 15 to 33
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4. Transactions

An OCP transaction is a logical sequence of OCP messages processing a single original application message. The result of the processing may be zero or more application messages, adapted from the original. A typical transaction consists of two message flows: a flow from the OPES processor to the callout server (sending the original application message), and a flow from the callout server to the OPES processor (sending adapted application messages). The number of application messages produced by the callout server and whether the callout server actually modifies the original application message may depend on the requested callout service and other factors. The OPES processor or the callout server can terminate the transaction by sending a corresponding message to the other side. An OCP transaction starts with a Transaction Start (TS) message sent by the OPES processor. A transaction ends with the first Transaction End (TE) message sent or received, explicit or implied. A TE message can be sent by either side. Zero or more OCP messages associated with the transaction can be exchanged in between. The figure below illustrates a possible message sequence (prefix "P" stands for the OPES processor; prefix "S" stands for the callout server). Some message details are omitted. P: TS 10; P: AMS 10 1; ... processor sending application data to the callout server S: AMS 10 2; ... callout server sending application data to the processor ... processor sending application data to the callout server P: AME 10 1 result; S: AME 10 2 result; P: TE 10 result;
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5. Invalid Input

This specification contains many criteria for valid OCP messages and their parts, including syntax rules, semantics requirements, and relationship to agents state. In this context, "Invalid input" means messages or message parts that violate at least one of the normative rules. A message with an invalid part is, by definition, invalid. If OCP agent resources are exhausted while parsing or interpreting a message, the agent MUST treat the corresponding OCP message as invalid. Unless explicitly allowed to do otherwise, an OCP agent MUST terminate the transaction if it receives an invalid message with transaction scope and MUST terminate the connection if it receives an invalid message with a connection scope. A terminating agent MUST use the result status code of 400 and MAY specify termination cause information in the result status reason parameter (see section 10.10). If an OCP agent is unable to determine the scope of an invalid message it received, the agent MUST treat the message as having connection scope. OCP usually deals with optional but invasive application message manipulations for which correctness ought to be valued above robustness. For example, a failure to insert or remove certain optional web page content is usually far less disturbing than corrupting (making unusable) the host page while performing that insertion or removal. Most OPES adaptations are high level in nature, which makes it impossible to assess correctness of the adaptations automatically, especially if "robustness guesses" are involved.

6. Negotiation

The negotiation mechanism allows OCP agents to agree on the mutually acceptable set of features, including optional and application-specific behavior and OCP extensions. For example, transport encryption, data format, and support for a new message can be negotiated. Negotiation implies intent for a behavioral change. For a related mechanism allowing an agent to query capabilities of its counterpart without changing the counterpart's behavior, see the Ability Query (AQ) and Ability Answer (AA) message definitions. Most negotiations require at least one round trip time delay. In rare cases when the other side's response is not required immediately, negotiation delay can be eliminated, with an inherent risk of an overly optimistic assumption about the negotiation response.
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   A detected violation of negotiation rules leads to OCP connection
   termination.  This design reduces the number of negotiation scenarios
   resulting in a deadlock when one of the agents is not compliant.

   Two core negotiation primitives are supported: negotiation offer and
   negotiation response.  A Negotiation Offer (NO) message allows an
   agent to specify a set of features from which the responder has to
   select at most one feature that it prefers.  The selection is sent by
   using a Negotiation Response (NR) message.  If the response is
   positive, both sides assume that the selected feature is in effect
   immediately (see section 11.19 for details).  If the response is
   negative, no behavioral changes are assumed.  In either case, further
   offers may follow.

   Negotiating OCP agents have to take into account prior negotiated
   (i.e., already enabled) features.  OCP agents MUST NOT make and MUST
   reject offers that would lead to a conflict with already negotiated
   features.  For example, an agent cannot offer an HTTP application
   profile for a connection that already has an SMTP application profile
   enabled, as there would be no way to resolve the conflict for a given
   transaction.  Similarly, once TLSv1 connection encryption is
   negotiated, an agent must not offer and must reject offers for SSLv2
   connection encryption (unless a negotiated feature explicitly allows
   for changing an encryption scheme on the fly).

   Negotiation Offer (NO) messages may be sent by either agent.  OCP
   extensions documenting negotiation MAY assign the initiator role to
   one of the agents, depending on the feature being negotiated.  For
   example, negotiation of transport security feature should be
   initiated by OPES processors to avoid situations where both agents
   wait for the other to make an offer.

   As either agent may make an offer, two "concurrent" offers may be
   made at the same time, by the two communicating agents.  Unmanaged
   concurrent offers may lead to a negotiation deadlock.  By giving OPES
   processor a priority, offer-handling rules (section 11.18) ensure
   that only one offer per OCP connection is honored at a time, and that
   the other concurrent offers are ignored by both agents.

6.1. Negotiation Phase

A Negotiation Phase is a mechanism ensuring that both agents have a chance to negotiate all features they require before proceeding further. Negotiation Phases have OCP connection scope and do not overlap. For each OCP agent, the Negotiation Phase starts with the first Negotiation Offer (NO) message received or the first Negotiation Response (NR) message sent, provided the message is not a part of an existing Phase. For each OCP agent, Negotiation Phase
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   ends with the first Negotiation Response (NR) message (sent or
   received), after which the agent expects no more negotiations.  Agent
   expectation rules are defined later in this section.

   During a Negotiation Phase, an OCP agent MUST NOT send messages other
   than the following "Negotiation Phase messages": Negotiation Offer
   (NO), Negotiation Response (NR), Ability Query (AQ), Ability Answer
   (AA), Progress Query (PQ), Progress Answer (PA), Progress Report
   (PR), and Connection End (CE).

   Multiple Negotiation Phases may happen during the lifespan of a
   single OCP connection.  An agent may attempt to start a new
   Negotiation Phase immediately after the old Phase is over, but it is
   possible that the other agent will send messages other than
   "Negotiation Phase messages" before receiving the new Negotiation
   Offer (NO).  The agent that starts a Phase has to be prepared to
   handle those messages while its offer is reaching the recipient.

   An OPES processor MUST make a negotiation offer immediately after
   sending a Connection Start (CS) message.  If the OPES processor has
   nothing to negotiate, the processor MUST send a Negotiation Offer
   (NO) message with an empty features list.  These two rules bootstrap
   the first Negotiation Phase.  Agents are expected to negotiate at
   least the application profile for OCP Core.  Thus, these
   bootstrapping requirements are unlikely to result in any extra work.

   Once a Negotiation Phase starts, an agent MUST expect further
   negotiations if and only if the last NO sent or the last NR received
   contained a true "Offer-Pending" parameter value.  Informally, an
   agent can keep the phase open by sending true "Offer-Pending"
   parameters with negotiation offers or responses.  Moreover, if there
   is a possibility that the agent may need to continue the Negotiation
   Phase, the agent must send a true "Offer-Pending" parameter.

6.2. Negotiation Examples

Below is an example of the simplest negotiation possible. The OPES processor is offering nothing and is predictably receiving a rejection. Note that the NR message terminates the Negotiation Phase in this case because neither of the messages contains a true "Offer-Pending" value: P: NO (); S: NR; The next example illustrates how a callout server can force negotiation of a feature that an OPES processor has not negotiated. Note that the server sets the "Offer-Pending" parameter to true when
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   responding to the processor Negotiation Offer (NO) message.  The
   processor chooses to accept the feature:

   P: NO ();
   S: NR
      Offer-Pending: true
   S: NO ({"22:ocp://feature/example/"})
      Offer-Pending: false
   P: NR {"22:ocp://feature/example/"};

   If the server seeks to stop the above negotiations after sending a
   true "Offer-Pending" value, its only option would be send an empty
   negotiation offer (see the first example above).  If the server does
   nothing instead, the OPES processor would wait for the server and
   would eventually time out the connection.

   The following example shows a dialog with a callout server that
   insists on enabling two imaginary features: strong transport
   encryption and volatile storage for responses.  The server is
   designed not to exchange sensitive messages until both features are
   enabled.  Naturally, the volatile storage feature has to be
   negotiated securely.  The OPES processor supports one of the strong
   encryption mechanisms but prefers not to offer (to volunteer support
   for) strong encryption, perhaps for performance reasons.  The server
   has to send a true "Offer-Pending" parameter to get a chance to offer
   strong encryption (which is successfully negotiated in this case).
   Any messages sent by either agent after the (only) successful NR
   response are encrypted with "strongB" encryption scheme.  The OPES
   processor does not understand the volatile storage feature, and the
   last negotiation fails (over a strongly encrypted transport

   P: NO ({"29:ocp://example/encryption/weak"})
   S: NR
      Offer-Pending: true
   S: NO ({"32:ocp://example/encryption/strongA"},\
      Offer-Pending: true
   P: NR {"32:ocp://example/encryption/strongB"}
   ... all traffic below is encrypted using strongB ...
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   S: NO ({"31:ocp://example/storage/volatile"})
      Offer-Pending: false
   P: NR
      Unknowns: ({"31:ocp://example/storage/volatile"})
   S: CSE { 400 "33:lack of VolStore protocol support" }

   The following example from [OPES-HTTP] illustrates successful HTTP
   application profile negotiation:

   P: NO ({"54:"
      Aux-Parts: (request-header,request-body)
      SG: 5;
   S: NR {"54:"
      Aux-Parts: (request-header)
      Pause-At-Body: 30
      Wont-Send-Body: 2147483647
      Content-Encodings: (gzip)
      SG: 5;

7. 'Data Preservation' Optimization

Many adaptations do not require any data modifications (e.g., message logging or blocking). Some adaptations modify only a small portion of application message content (e.g., HTTP cookies filtering or ad insertion). Yet, in many cases, the callout service has to see complete data. By default, unmodified data would first travel from the OPES processor to the callout server and then back. The "data preservation" optimization in OCP helps eliminate the return trip if both OCP agents cooperate. Such cooperation is optional: OCP agents MAY support data preservation optimization. To avoid sending back unmodified data, a callout service has to know that the OPES processor has a copy of the data. As data sizes can be very large and the callout service may not know in advance whether it will be able to use the processor copy, it is not possible to require the processor to keep a copy of the entire original data. Instead, it is expected that a processor may keep some portion of the data, depending on processor settings and state. When an OPES processor commits to keeping a data chunk, it announces its decision and the chunk parameters via a Kept parameter of a Data Use Mine (DUM) message. The callout server MAY "use" the chunk by sending a Data Use Yours (DUY) message referring to the preserved
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   chunk.  That OCP message does not have payload and, therefore, the
   return trip is eliminated.

   As the mapping between original and adapted data is not known to the
   processor, the processor MUST keep the announced-as-preserved chunk
   until the end of the corresponding transaction, unless the callout
   server explicitly tells the processor that the chunk is not needed.
   As implied by the above requirement, the processor cannot assume that
   a data chunk is no longer needed just because the callout server sent
   a Data Use Yours (DUY) message or adapted data with, for instance,
   the same offset as the preserved chunk.

   For simplicity, preserved data is always a contiguous chunk of
   original data, described by an (offset, size) pair using a "Kept"
   parameter of a Data Use Mine (DUM) message.  An OPES processor may
   volunteer to increase the size of the kept data.  An OPES processor
   may increase the offset if the callout server indicated that the kept
   data is no longer needed.

   Both agents may benefit from data reuse.  An OPES processor has to
   allocate storage to support this optimization, but a callout server
   does not.  On the other hand, it is the callout server that is
   responsible for relieving the processor from data preservation
   commitments.  There is no simple way to resolve this conflict of
   interest on a protocol level.  Some OPES processors may allocate a
   relatively small buffer for data preservation purposes and stop
   preserving data when the buffer becomes full.  This technique would
   benefit callout services that can quickly reuse or discard kept data.
   Another processor strategy would be to size the buffer based on
   historical data reuse statistics.  To improve chances of beneficial
   cooperation, callout servers are strongly encouraged to immediately
   notify OPES processors of unwanted data.  The callout server that
   made a decision not to send Data Use Yours (DUY) messages (for a
   specific data ranges or at all) SHOULD immediately inform the OPES
   processor of that decision with the corresponding Data Preservation
   Interest (DPI) message(s) or other mechanisms.

8. 'Premature Dataflow Termination' Optimizations

Many callout services adapt small portions of large messages and would preferably not to be in the loop when that adaptation is over. Some callout services may not seek data modification and would preferably not send data back to the OPES processor, even if the OPES processor is not supporting the data preservation optimization (Section 7). By OCP design, unilateral premature dataflow termination by a callout server would lead to termination of an OCP transaction with an error. Thus, the two agents must cooperate to allow for error-free premature termination.
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   This section documents two mechanisms for premature termination of
   original or adapted dataflow.  In combination, the mechanisms allow
   the callout server to get out of the processing loop altogether.

8.1. Original Dataflow

There are scenarios where a callout server is not interested in the remaining original dataflow. For example, a simple access blocking or "this site is temporary down" callout service has to send an adapted (generated) application message but would preferably not receive original data from the OPES processor. OCP Core supports premature original dataflow termination via the Want Stop Receiving Data (DWSR) message. A callout server that does not seek to receive additional original data (beyond a certain size) sends a DWSR message. The OPES processor receiving a DWSR message terminates original dataflow by sending an Application Message End (AME) message with a 206 (partial) status code. The following figure illustrates a typical sequence of events. The downward lines connecting the two dataflows illustrate the transmission delay that allows for more OCP messages to be sent while an agent waits for the opposing agent reaction. OPES Callout Processor Server DUM> <DUM DUM> <DWSR <-- Server is ready to stop receiving ... _____/<DUM <-- Server continues as usual DUM>______/ <DUM AME> ... <-- Processor stops sending original data \_____ <DUM \______<DUM <DUM <-- Server continues to send adapted data ... <AME The mechanism described in this section has no effect on the adapted dataflow. Receiving an Application Message End (AME) message with 206 (partial) result status code from the OPES processor does not introduce any special requirements for the adapted dataflow termination. However, it is not possible to terminate adapted dataflow prematurely after the original dataflow has been prematurely terminated (see section 8.3).
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8.2. Adapted Dataflow

There are scenarios where a callout service may want to stop sending adapted data before a complete application message has been sent. For example, a logging-only callout service has to receive all application messages but would preferably not send copies back to the OPES processor. OCP Core supports premature adapted dataflow termination via a combination of Want Stop Sending Data (DWSS) and Stop Sending Data (DSS) messages. A callout service that seeks to stop sending data sends a DWSS message, soliciting an OPES processor permission to stop. While waiting for the permission, the server continues with its usual routine. An OPES processor receiving a Want Stop Sending Data message responds with a Stop Sending Data (DSS) message. The processor may then pause to wait for the callout server to terminate the adapted dataflow or may continue sending original data while making a copy of it. Once the server terminates the adapted dataflow, the processor is responsible for using original data (sent or paused after sending DSS) instead of the adapted data. The callout server receiving a DSS message terminates the adapted dataflow (see the Stop Sending Data (DSS) message definition for the exact requirements and corner cases). The following figure illustrates a typical sequence of events, including a possible pause in original dataflow when the OPES processor is waiting for the adapted dataflow to end. The downward lines connecting the two dataflows illustrate the transmission delay that allows for more OCP messages to be sent while an agent waits for the opposing agent reaction.
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   OPES                 Callout
   Processor            Server
       DUM>             <DUM
       DUM>             <DWSS    <-- Server is ready to stop sending
       ...        _____/<DUM     <-- Server continues as usual,
       DUM>______/      <DUM         waiting for DSS
       DSS>             ...
           \_____       <DUM
     possible    \______<DUM
     org-dataflow       <AME 206 <-- Server terminates adapted dataflow
     pause        _____/             upon receiving the DSS message
       DUM>                      <-- Processor resumes original dataflow
       DUM>                          to the server and starts using
       ...                           original data without adapting it

   Premature adapted dataflow preservation is not trivial, as the OPES
   processor relies on the callout server to provide adapted data
   (modified or not) to construct the adapted application message.  If
   the callout server seeks to quit its job, special care must be taken
   to ensure that the OPES processor can construct the complete
   application message.  On a logical level, this mechanism is
   equivalent to switching from one callout server to another
   (non-modifying) callout server in the middle of an OCP transaction.

   Other than a possible pause in the original dataflow, the mechanism
   described in this section has no effect on the original dataflow.
   Receiving an Application Message End (AME) message with 206 (partial)
   result status code from the callout server does not introduce any
   special requirements for the original dataflow termination.

8.3. Getting Out of the Loop

Some adaptation services work on application message prefixes and do not seek to be in the adaptation loop once their work is done. For example, an ad insertion service that did its job by modifying the first fragment of a web "page" would not seek to receive more original data or to perform further adaptations. The 'Getting Out of the Loop' optimization allows a callout server to get completely out of the application message processing loop. The "Getting Out of the Loop" optimization is made possible by terminating the adapted dataflow (section 8.2) and then by terminating the original dataflow (section 8.1). The order of termination is very important.
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   If the original dataflow is terminated first, the OPES processor
   would not allow the adapted dataflow to be terminated prematurely, as
   the processor would not be able to reconstruct the remaining portion
   of the adapted application message.  The processor would not know
   which suffix of the remaining original data has to follow the adapted
   parts.  The mapping between original and adapted octets is known only
   to the callout service.

   An OPES processor that received a DWSS message followed by a DWSR
   message MUST NOT send an AME message with a 206 (partial) status code
   before sending a DSS message.  Informally, this rule means that a
   callout server that wants to get out of the loop fast should send a
   DWSS message immediately followed by a DWSR message; the server does
   not have to wait for the OPES processor's permission to terminate
   adapted dataflow before requesting that the OPES processor terminates
   original dataflow.

9. Protocol Element Type Declaration Mnemonic (PETDM)

A protocol element type is a named set of syntax and semantics rules. This section defines a simple, formal declaration mnemonic for protocol element types, labeled PETDM. PETDM simplicity is meant to ease type declarations in this specification. PETDM formality is meant to improve interoperability among implementations. Two protocol elements are supported by PETDM: message parameter values and messages. All OCP Core parameter and message types are declared by using PETDM. OCP extensions SHOULD use PETDM when declaring new types. Atom, list, structure, and message constructs are four available base types. Their syntax and semantics rules are defined in section 3.1. New types can be declared in PETDM to extend base types semantics by using the following declaration templates. The new semantics rules are meant to be attached to each declaration using prose text. Text in angle brackets "<>" are template placeholders, to be substituted with actual type names or parameter name tokens. Square brackets "[]" surround optional elements such as structure members or message payload. o Declaring a new atomic type: <new-type-name>: extends atom; o Declaring a new list with old-type-name items: <new-type-name>: extends list of <old-type-name>; Unless it is explicitly noted otherwise, empty lists are valid and have the semantics of an absent parameter value.
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   o  Declaring a new structure with members:
   <new-type-name>: extends structure with {
           <old-type-nameA> <old-type-nameB> [<old-type-nameC>] ...;
           <member-name1>: <old-type-name1>;
           <member-name2>: <old-type-name2>;
           [<member-name3>: <old-type-name3>];

   The new structure may have anonymous members and named members.
   Neither group has to exist.  Note that it is always possible for
   extensions to add more members to old structures without affecting
   type semantics because unrecognized members are ignored by compliant

   o  Declaring a new message with parameters:
   <new-type-name>: extends message with {
           <old-type-nameA> <old-type-nameB> [<old-type-nameC>] ...;
           <parameter-name1>: <old-type-name1>;
           <parameter-name2>: <old-type-name2>;
           [<parameter-name3>: <old-type-name3>];

   The new type name becomes the message name.  Just as when a structure
   is extended, the new message may have anonymous parameters and named
   parameters.  Neither group has to exist.  Note that it is always
   possible for extensions to add more parameters to old messages
   without affecting type semantics because unrecognized parameters are
   ignored by compliant agents.

   o  Extending a type with more semantics details:

   <new-type-name>: extends <old-type-name>;

   o  Extending a structure- or message-base type:
   <new-type-name>: extends <old-type-name> with {
           <old-type-nameA> <old-type-nameB> [<old-type-nameC>] ...;
           <member-name1>: <old-type-name1>;
           <member-name2>: <old-type-name2>;
           [<member-name3>: <old-type-name3>];
   New anonymous members are appended to the anonymous members of the
   old type, and new named members are merged with named members of the
   old type.
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   o  Extending a message-base type with payload semantics:
   <new-type-name>: extends <old-type-name> with {
   } and payload;
   Any any OCP message can have payload, but only some message types
   have known payload semantics.  Like any parameter, payload may be
   required or optional.

   o  Extending type semantics without renaming the type:
   <old-type-name>: extends <namespace>::<old-type-name>;
   The above template can be used by OCP Core extensions that seek to
   change the semantics of OCP Core types without renaming them.  This
   technique is essential for extending OCP messages because the message
   name is the same as the message type name.  For example, an SMTP
   profile for OCP might use the following declaration to extend an
   Application Message Start (AMS) message with Am-Id, a parameter
   defined in that profile:

   AMS: extends Core::AMS with {
           Am-Id: am-id;

   All extended types may be used as replacements of the types they
   extend.  For example, a Negotiation Offer (NO) message uses a
   parameter of type Features.  Features (section 10.12) is a list of
   feature (section 10.11) items.  A Feature is a structure-based type.
   An OCP extension (e.g., an HTTP application profile) may extend the
   feature type and use a value of that extended type in a negotiation
   offer.  Recipients that are aware of the extension will recognize
   added members in feature items and negotiate accordingly.  Other
   recipients will ignore them.

   The OCP Core namespace tag is "Core".  OCP extensions that declare
   types MUST define their namespace tags (so that other extensions and
   documentation can use them in their PETDM declarations).

9.1. Optional Parameters

Anonymous parameters are positional: The parameter's position (i.e., the number of anonymous parameters to the left) is its "name". Thus, when a structure or message has multiple optional anonymous parameters, parameters to the right can be used only if all parameters to the left are present. The following notation [name1] [name2] [name3] ... [nameN] is interpreted as
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   [name1 [ name2 [ name3 ... [nameN] ... ]]]

   When an anonymous parameter is added to a structure or message that
   has optional anonymous parameters, the new parameter has to be
   optional and can only be used if all old optional parameters are in
   use.  Named parameters do not have these limitations, as they are not
   positional, but associative; they are identified by their explicit
   and unique names.

10. Message Parameter Types

This section defines parameter value types that are used for message definitions (section 11). Before using a parameter value, an OCP agent MUST check whether the value has the expected type (i.e., whether it complies with all rules from the type definition). A single rule violation means that the parameter is invalid. See Section 5 for rules on processing invalid input. OCP extensions MAY define their own types. If they do, OCP extensions MUST define types with exactly one base format and MUST specify the type of every new protocol element they introduce.

10.1. uri

uri: extends atom; Uri (universal resource identifier) is an atom formatted according to URI rules in [RFC2396]. Often, a uri parameter is used as a unique (within a given scope) identifier. Uni semantics is incomplete without the scope specification. Many uri parameters are URLs. Unless it is noted otherwise, URL identifiers do not imply the existence of a serviceable resource at the location they specify. For example, an HTTP request for an HTTP-based URI identifier may result in a 404 (Not Found) response.

10.2. uni

uni: extends atom; Uni (unique numeric identifier) is an atom formatted as dec-number and with a value in the [0, 2147483647] range, inclusive. A uni parameter is used as a unique (within a given scope) identifier. Uni semantics is incomplete without the scope specification. Some OCP messages create identifiers (i.e., bring them into scope). Some OCP messages destroy them (i.e, remove them
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   from scope).  An OCP agent MUST NOT create the same uni value more
   than once within the same scope.  When creating a new identifier of
   the same type and within the same scope as some old identifier, an
   OCP agent MUST use a higher numerical value for the new identifier.
   The first rule makes uni identifiers suitable for cross-referencing
   logs and other artifacts.  The second rule makes efficient checks of
   the first rule possible.

   For example, a previously used transaction identifier "xid" must not
   be used for a new Transaction Start (TS) message within the same OCP
   transaction, even if a prior Transaction End (TE) message was sent
   for the same transaction.

   An OCP agent MUST terminate the state associated with uni uniqueness
   scope if all unique values have been used up.

10.3. size

size: extends atom; Size is an atom formatted as dec-number and with a value in the [0, 2147483647] range, inclusive. Size value is the number of octets in the associated data chunk. OCP Core cannot handle application messages that exceed 2147483647 octets in size, that require larger sizes as a part of OCP marshaling process, or that use sizes with granularity other than 8 bits. This limitation can be addressed by OCP extensions, as hinted in section 15.1.

10.4. offset

offset: extends atom; Offset is an atom formatted as dec-number and with a value in the [0, 2147483647] range, inclusive. Offset is an octet position expressed in the number of octets relative to the first octet of the associated dataflow. The offset of the first data octet has a value of zero.

10.5. percent

percent: extends atom; Percent is an atom formatted as dec-number and with a value in the [0, 100] range, inclusive.
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   Percent semantics is incomplete unless its value is associated with a
   boolean statement or assertion.  The value of 0 indicates absolute
   impossibility.  The value of 100 indicates an absolute certainty.  In
   either case, the associated statement can be relied upon as if it
   were expressed in boolean rather than probabilistic terms.  Values in
   the [1,99] inclusive range indicate corresponding levels of certainty
   that the associated statement is true.

10.6. boolean

boolean: extends atom; Boolean type is an atom with two valid values: true and false. A boolean parameter expresses the truthfulness of the associated statement.

10.7. xid

xid: extends uni; Xid, an OCP transaction identifier, uniquely identifies an OCP transaction within an OCP connection.

10.8. sg-id

sg-id: extends uni; Sg-id, a service group identifier, uniquely identifies a group of services on an OCP connection.

10.9. modp

modp: extends percent; Modp extends the percent type to express the sender's confidence that application data will be modified. The boolean statement associated with the percentage value is "data will be modified". Modification is defined as adaptation that changes the numerical value of at least one data octet.

10.10. result

result: extends structure with { atom [atom]; }; The OCP processing result is expressed as a structure with two documented members: a required Uni status code and an optional
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   reason.  The reason member contains informative textual information
   not intended for automated processing.  For example:

   { 200 OK }
   { 200 "6:got it" }
   { 200 "27:27 octets in UTF-8 encoding" }

   This specification defines the following status codes:

   Result Status Codes

   |   code |     text     | semantics                                 |
   |    200 |      OK      | Overall success.  This specification does |
   |        |              | not contain any general actions for a 200 |
   |        |              | status code recipient.                    |
   |    206 |    partial   | Partial success.  This status code is     |
   |        |              | documented for Application Message End    |
   |        |              | (AME) messages only.  The code indicates  |
   |        |              | that the agent terminated the             |
   |        |              | corresponding dataflow prematurely (i.e., |
   |        |              | more data would be needed to reconstruct  |
   |        |              | a complete application message).          |
   |        |              | Premature termination of one dataflow     |
   |        |              | does not introduce any special            |
   |        |              | requirements for the other dataflow       |
   |        |              | termination.  See dataflow termination    |
   |        |              | optimizations (section 8) for use cases.  |
   |    400 |    failure   | An error, exception, or trouble.  A       |
   |        |              | recipient of a 400 (failure) result of an |
   |        |              | AME, TE, or CE message MUST destroy any   |
   |        |              | state or data associated with the         |
   |        |              | corresponding dataflow, transaction, or   |
   |        |              | connection.  For example, an adapted      |
   |        |              | version of the application message data   |
   |        |              | must be purged from the processor         |
   |        |              | cache if the OPES processor receives an   |
   |        |              | Application Message End (AME) message     |
   |        |              | with result code of 400.                  |

   Specific OCP messages may require code-specific actions.

   Extending result semantics is made possible by adding new "result"
   structure members or by negotiating additional result codes (e.g., as
   a part of a negotiated profile).  A recipient of an unknown (in
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   then-current context) result code MUST act as if code 400 (failure)
   were received.

   The recipient of a message without the actual result parameter, but
   with an optional formal result parameter, MUST act as if code 200
   (OK) were received.

   Textual information (the second anonymous parameter of the result
   structure) is often referred to as "reason" or "reason phrase".  To
   assist manual troubleshooting efforts, OCP agents are encouraged to
   include descriptive reasons with all results indicating a failure.

   In this specification, an OCP message with result status code of 400
   (failure) is called "a message indicating a failure".

10.11. feature

feature: extends structure with { uri; }; The feature type extends structure to relay an OCP feature identifier and to reserve a "place" for optional feature-specific parameters (sometimes called feature attributes). Feature values are used to declare support for and to negotiate use of OCP features. This specification does not define any features.

10.12. features

features: extends list of feature; Features is a list of feature values. Unless it is noted otherwise, the list can be empty, and features are listed in decreasing preference order.

10.13. service

service: extends structure with { uri; }; Service structure has one anonymous member, an OPES service identifier of type uri. Services may have service-dependent parameters. An OCP extension defining a service for use with OCP MUST define service identifier and service-dependent parameters, if there are any, as additional "service" structure members. For example, a service value may look like this:
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   {"41:" "8:blowfish"}

10.14. services

services: extends list of service; Services is a list of service values. Unless it is noted otherwise, the list can be empty, and the order of the values is the requested or actual service application order.

10.15. Dataflow Specializations

Several parameter types, such as offset apply to both original and adapted dataflow. It is relatively easy to misidentify a type's dataflow affiliation, especially when parameters with different affiliations are mixed together in one message declaration. The following statements declare new dataflow-specific types by using their dataflow-agnostic versions (denoted by a <type> placeholder). The following new types refer to original data only: org-<type>: extends <type>; The following new types refer to adapted data only: adp-<type>: extends <type>; The following new types refer to the sender's dataflow only: my-<type>: extends <type>; The following new types refer to the recipient's dataflow only: your-<type>: extends <type>; OCP Core uses the above type-naming scheme to implement dataflow specialization for the following types: offset, size, and sg-id. OCP extensions SHOULD use the same scheme.

(page 33 continued on part 3)

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