Go to the previous, next section.
When two modules of a program are in different address spaces, or use different data representations, ILU forms messages to send across the inter-module boundary; we call a particular way of forming and interpreting these messages an RPC protocol (sometimes simply protocol). These messages may be transported between address spaces in different ways; we call a particular way of moving messages a transport. This chapter describes the various kinds of available ILU protocols and transports. ILU is extensible: additional RPC protocols and transports can be added, either at compile-time or run-time; this chapter does not describe how to do so.
When an ILU kernel server exports objects, it does so via one or more contact stacks. Each stack has an RPC protocol at the top of the stack, forming and interpreting messages, and one or more layers of transport below the protocol layer, transforming or communicating the messages in various ways. A contact stack is specified by a protocol-info string and a sequence of transport-info strings; the syntax of these strings is defined in this chapter.
Before describing any particular protocol, we will describe the abstract ILU protocol, which is layered on top of each actual protocol. It is quite simple. Two types of messages are used, one to communicate parameters to a true method, and the other to communicate results and/or exceptions from the true method to surrogate caller. Parameters and values are encoded according to a simple abstract external data representation format. This abstract protocol identifies what information is passed between modules without specifying its exact mapping to bit patterns.
The first type of message is called a request. Each request consists of a code identifying the method being requested, an authentication block identifying the principal making the call, and a list of parameter inputs to the method being called. The method is identified by passing the one-based ordinal value (that is, the index of the method in the list of methods, beginning with one) of the method, in the list of methods as specified in the ISL description of the class which actually defines the method. No more than 65278 (1-0xFEFF) methods may be directly specified for any type (though more methods may be inherited by a type). Method codes 0xFF00 to 0xFFFF are reserved for ILU internal use. The principal is identified by a block of authentication credentials information which varies depending on the specific authentication protocol used. These credentials may be either in the request header, or may appear as a parameter of the request. (Note: There should also be an ILU protocol version number somewhere here, but there isn't (yet).)
The
result message is used to convey return values and exception
values from the true method back to the caller. It consists of a Boolean
value, indicating whether the call was successful (for TRUE) or
signalled an exception (for FALSE). If successful, the return value (if any),
follows, followed by the values of any Out parameters, in the order
they are specified as parameters. If an exception was signalled, a value
between 1 and 2^16-1 follows, indicating the ordinal value specific exception in
the list specified in the definition of the method, followed by a value of the
exception type, if any was specified for the exception.
Simple numerical values, of types integer, cardinal, real, or byte,
are passed directly.
Character values are passed as integer values in the range [0,2^16-1].
Short character values are passed as integer values in the range [0,2^16-1].
Long character values are passed as integer values in the range [0,2^32-1].
Enumeration values are passed as integer values in the range [0,2^16-1], the
value being the zero-based ordinal value of the corresponding enumeration value
in the original list of enumeration values in the definition of the enumerated type.
Boolean values are passed as as integer values of either 0, for FALSE, or 1, for TRUE.
Optional values are passed by first passing a Boolean value, with TRUE indicating
that a non-NIL value is being passed, and then only in the non-NIL case
passing a value of the optional value's indicated type.
Sequence values are passed by first passing a count,
as an integer in the range [0,2^32-1] for sequences without limits, or for sequences with
limits greater than 2^16-1, or an integer in the range [0,2^16-1], for sequences
with limits less than 2^16, indicating the number
of elements in the sequence, and then that number of values of the sequence's base
type.
Array values are passed by passing a number of elements of the array's base type corresponding
to the size of the array.
Record values are passed by passing values of types corresponding to the fields of the record, following the
order in which the fields are defined in the ISL definition of the record.
Union values are passed by passing a value of the discriminant type, which indicates which branch
of the union constitutes the union's actual type, usually followed by a value of the union's actual type.
If the discriminant value indicates a branch of the union which has no associate value, only the
discriminant value is passed.
Object values are passed in several different forms,
depending on whether or not the object value is in the discriminator position,
whether or not the object's type is a singleton type, and
whether or not the object reference is NIL.
singleton type. In this case, the object is already known to both sides, and the object
is passed implicitly; that is, no actual bytes are transmitted.
singleton type. In this case,
the CRC-32 of the server ID of the object is passed as a cardinal value,
followed by the instance handle of the object, as a sequence of short character value.
Both the instance handle and server ID must be passed,
as the true object previously at the "known address" for the object may have been replaced
by a different object with the same instance handle, in a different kernel server.
sequence of short character value.
sequence of short character value of length zero.
This section describes the mapping of the abstract ILU protocol into the specific on-the-wire protocol used with ONC RPC One of the major goals of this mapping is to preserve compatibility with existing Sun RPC services that can be described in ISL.
A protocol info string for ONC RPC has the form
sunrpc_2_program-number_program-version where
program-number and program-version may be specified
either in decimal, or in hexidecimal with a leading string of 0x.
The program number for non-native-ONC-RPC ILU object types is
always the same (in ILU 2.0alpha1 to 2.0alpha7, 0x61A78; in ILU 2.0alpha8 and up, 0x61A79), and the program version
varies depending on the specific object type.
Use of ONC RPC requires use of a boundaried transport below it.
The request message used is that specified by the ONC RPC protocol. The
ILU method index is encoded as a 32-bit number in the "proc"
field in the ONC RPC request header. Principal identification is passed
in the "cred" field of the ONC RPC request header. By
default, ILU will pass the AUTH_UNIX authentication
information, if no authentication method is specified for the method. (This default
authentication can be disabled by defining the environment variable ILU_NO_SUNRPC_UNIX_AUTH
to any value.) For non-singleton
object types, the ONC RPC
program number passed in the "prog" slot is always the same (for ILU 2.0alpha1 - 2.0alpha7, 0x00061a78;
for ILU 2.0alpha8, 0x00061a79),
and the version
number passed in the "vers" slot is the CRC-32 hashed value of the MSTID for
the object type on which the method being invoked is defined. For singleton classes, the program number and version
specified in the singleton information is used.
The "mtype" field is set to CALL. The indicated
"rpcvers" is 2. A monotonically increasing 32-bit serial number is
used in the "xid" field. For non-singleton, non-NIL objects,
an extra argument identifying the
discriminant of the
message (the object on which the method is being invoked) is marshalled before
any of the specified arguments. This discriminant is marshalled as an XDR Unsigned Integer,
which is the CRC-32 of the server ID of the object, followed by an XDR string, which is
the plain instance handle of the object.
The reply message used is that specified by the ONC RPC protocol. The
"mtype" field is set to REPLY. The "stat" field is always set
to MSG_ACCEPTED. In the accepted_reply, the authentication
verifier is always NULL. The "stat" field may be non-zero, to signal
one of a small number of "standard" exceptions, or may be zero. This
header is then followed by one of three forms: If a "standard"
exception was raised, nothing. If the method has no exceptions, the
return values and out parameters (if any). If the method has any
exceptions defined, a 32-bit value which specifies either successful
completion (a value of 0), or an exception (a value greater than 0,
which is the ordinal value of the particular exception being signalled
in the list of exceptions specified for this method), followed by either
the return value and out parameters (if any), in the case of successful
completion, or the exception value (if any), in the case of an
exception.
The mapping of ILU types into ONC RPC types is accomplished primarily by using the appropriate XDR representation for that type.
Short integer and integer types are represented with the XDR Integer type. Long integer
types are represented as an XDR Hyper Integer.
Short cardinal, byte, and cardinal types are represented
with the XDR Unsigned Integer type. Long cardinal
types are represented as an XDR Unsigned Hyper Integer.
Short real numbers are encoded as XDR Floating-point. Real numbers are encoded
as XDR Double-precision Floating-point. Long real numbers are encoded as XDR Fixed-length
Opaque data of length 16.
Array values are encoded as XDR Fixed-length Array, except for two special cases.
If the array is multi-dimensional,
it is encoded as a flat rendering into a single-dimensional array in row-major order (the last specified
index varying most rapidly). If the array is of element-type byte or short character, it is encoded
as an array of one (in the one-dimensional case) or more (in the greater-than-one dimensional case)
values of XDR Opaque Data.
Record values are encoded as XDR Structures.
Union values are encoded as XDR Discriminated Unions, with a discriminant of type
"unsigned int" containing the ILU short cardinal discriminant.
Enumeration values are encoded as XDR Unsigned Integer (note that this is different
from XDR Enumerations, which are encoded as XDR Integer).
Boolean values are encoded as XDR Unsigned Integer, using the value 0 for FALSE
and the value 1 for TRUE.
Sequence values are encoded as XDR Variable-length Arrays, except for several special cases.
Sequences of short character are encoded as XDR String, sequences of byte are encoded
as XDR Variable-length Opaque Data, and sequences of character are encoded as XDR String,
where the string is the UTF-2 encoding of the Unicode characters in the sequence.
Optional values are encoded as an XDR Boolean value, followed by another encoded
value, if the Boolean value is TRUE.
Instances of an object type are encoded as either zero (in the case of a method discriminant of a singleton type),
or one, values of type XDR String.
This section describes the mapping of the abstract ILU protocol into the specific on-the-wire protocol used with Xerox Courier. One of the major goals of this mapping is to preserve compatibility with existing Xerox Courier services that can be described in ISL. Unfortunately, many if not most important Courier services use bulk data transfer, something that is still only planned for ILU.
A protocol info string for Xerox Courier has the form
courier_program-number_program-version where
program-number and program-version may be specified
either in decimal, or in hexidecimal with a leading string of 0x.
The program number for non-singleton ILU object types is
always (in ILU 2.0) 0x001yxxxx, where y is currently
1; the specific program number
and the program version varies depending on the specific object type.
Use of Xerox Courier requires use of a boundaried transport below it.
The request message used is the CallMessageBody specified in section 4.3.1 of the Courier protocol.
A monotonically increasing 16-bit serial number is passed in the transactionID field; a 32-bit program number
is passed in the programNumber field, a 16-bit number is passed in the versionNumber
field; the ILU method index
is passed as a 16-bit value in the procedureValue field. The program number
is calculated by computing the CRC-32 hash value of the MSTID of the object type on which the
method is defined, then forming a program number by using the value 0x0011 for the high-order
16 bits, and the high-order 16 bits of the CRC for the low-order 16 bits of the program number.
The version number is the low-order 16 bits of the CRC.
Successful replies are sent using the Courier ReturnMessageBody specified in section 4.3.3
of the Courier specification. The procedureResults field contains
the return value, if any, followed by the INOUT and OUT parameter values, if any.
User exceptions are signalled using the AbortMessageBody specified in section 4.3.4
of the Courier specification. The errorValue field contains a value greater than 0,
which is the ordinal value of the particular exception being signalled
in the list of exceptions specified for this method. The errorArguments field
contain the exception value, if any.
System exceptions (of exception type ilu.ProtocolError) are signalled
using the RejectMessageBody message of section 4.3.2. The rejectionDetail
field of the message contains the ProtocolError detail.
The mapping of ILU types into Courier types is accomplished primarily by using the appropriate Courier Layer 2 representation for that type.
Short integer and integer types are represented with the Courier Integer
and Long Integer types.
Long integer types are represented as an integer followed by a cardinal.
Short cardinal, byte, and cardinal types are
represented with the Courier cardinal, cardinal,
and long cardinal types, respectively. Long cardinal types
are represented as a big-endian (most significant 16 bits first) Courier array of 4 cardinals.
As the Courier protocol does not have any mapping for floating point values,
short real numbers are passed as a Courier long cardinal,
real numbers are encoded as a big-endian array of two Courier long cardinal values,
and long real numbers are encoded as big-endian array of four Courier long cardinal
values.
Array values are encoded as Courier one-dimensional
arrays. If the array is multi-dimensional, it is encoded as a
flat rendering into a single-dimensional array in row-major order (the
last specified index varying most rapidly). If the array is of type
byte or short character, the contents of the ILU
value are packed into a Courier array of unspecified two values per array
element, so that the Courier array is half the length of the actual ILU array.
Record values are encoded as Courier record values.
Union values of union types whose discriminant type can be mapped to a 16-bit value type in
the range [0,2^16-1] are passed as Courier choice values. Other
unions are passed as a Courier long cardinal, followed by the
value of the union's indicated type (if any).
Enumeration values are encoded as Courier enumeration values.
Boolean values are encoded as Courier boolean values.
Sequence values are encoded as Courier sequences, except for several special cases.
Sequences of N short characters or bytes are encoded as
either a Courier cardinal, for sequences with limits less than 2^16, or long cardinal,
for sequences with no limits or limits greater than 2^16-1, value of N, followed by
(N+1)/2 values of Courier unspecified, each such value
containing two short character or byte values, packed in big-endian order.
Optional values are encoded as an Courier boolean value, followed by another encoded
value, if the Boolean value is TRUE.
Instances of an object type are encoded as either zero (in the case of a method discriminant of a singleton type),
or one values of ISL short sequence of short character.
CORBA Nil object references are represented as a
zero-length short sequence of short character.
This section describes the mapping of the abstract ILU protocol
into the specific on-the-wire protocol prescribed by the OMG's
CORBA Internet Inter-ORB Protocol (IIOP),
version 1.0.
A protocol info string for the IIOP version 1.0, with the
ILU-to-IIOP mapping version 1, has the form iiop_1_0_1.
The IIOP may be used on top of either a reliable, boundaried or non-boundaried,
transport stack.
ILU request and reply messages are mapped to GIOP Request and Reply messages fairly directly. The byte order used is that native to the machine on which the message is being formed. A zero-length service context is always sent.
In a Request message, the operation name is the ISL operation name for the method, with all hyphen characters in the operation name changed to underscore characters. The Principal field is always sent as a zero-length field.
The GIOP CancelRequest, LocateRequest, MessageError, and CloseConnection messages are never sent by ILU, though one or more of them may be used in the future. ILU will send GIOP LocateReply messages in response to LocateRequest messages.
The mapping of ILU types into IIOP types is accomplished primarily by using the mapping for the corresponding CORBA type.
Short integer and integer types are marshaled as CORBA short and long types.
Long integer types are represented as an integer followed by a cardinal.
Short cardinal, byte, and cardinal types are
marshaled as the CORBA unsigned short, octet,
and unsigned long types, respectively. Long cardinal types
are marshalled as two CORBA unsigned long values, and the
byte order of the message determines which is marshalled first.
short real numbers are passed as CORBA float values.
real numbers are passed as CORBA double values,
and long real numbers are encoded as big-endian array of 16 bytes.
Array values are encoded as CORBA array values.
Record values are encoded as CORBA struct values.
Union values are encoded as CORBA union values.
Enumeration values are encoded as CORBA enum values.
Boolean values are encoded as CORBA boolean values.
Sequence values are encoded as CORBA sequence values.
Optional values are encoded as a CORBA sequence of the base type,
with an upper limit of one value.
Object values are passed as an IIOP Interoperable Object Reference (IOR), containing
at least an Internet Profile. The IOR may also contain an ILU Profile. In the case of the Internet
Profile, the object key contains four strings, separated by NUL (zero octet) characters. The first string
is always "ilu". The second string is the most specific type ID of the object (in case some intervening
ORB decides to re-write the IOR's type_id field). The third string is the server ID of the object's server.
The fourth string is the instance handle of the object.
HTTP)
HTTP in ILU allows an ILU application to interact with an existing Web
resource. That is, Web Browser to ILU, ILU to Web Server, and general ILU
to ILU over HTTP is possible.
For HTTP interaction with existing web services, an ILU application must be
able to not only get an object (a surrogate actually) representing the
resource. It must also have some means by which to specify the HTTP
headers and entity body that should be sent with the request. Similarly, an
ILU server functioning as a HTTP accessible Web resource must be able to
set status, header and entity body content.
Arbitrary programmers interpretations of these HTTP components cannot be
generally mapped into HTTP. A specific signature is needed for the GET
HEAD and POST methods so that the ILU implementation of the HTTP protocol
can know how to map arguments into actual HTTP format. In addition, a way
is needed to distinguish these methods intended for use with existing Web
services from other methods that may happen to have the same name but
different signatures.
This need is addressed by defining a specific type of object that has
declarations for how an application should structure the arguments / return
values for the GET HEAD and POST operations. Any GET HEAD or POST
operation invoked on an object that is an instance of this base type (or an
instance of a type derived directly or indirectly from that base type) has
a particular signature that the ILU protocol implementation knows how to
map to HTTP. This type, called Resource in the http interface, is defined in the http.isl file,
and any application wishing to supply Web compatible objects should make
the objects a direct or derived instance of it. A server for objects
acessible via HTTP should be created with the protocol info string
http_1_0, and should use the tcp transport. See the httest example for
a sample use of HTTP in ILU.
A method named GET HEAD or POST, invoked on an object that is a direct or
indirect instance of the Resource type, automatically has its Request
and Response mapped to/from HTTP in a manner compatible with existing Web
services. The fairly straightforward mapping from the ILU http Interface to
HTTP Protocol is outlined described below:
ILU Method Name Method name in Request's Request line (if using a Proxy server, scheme + location of object +) ILU Object ID + any params/queries present in the Request.URI field Request-URI in Request's Request line Request.headers Headers in Request Request.body Entity-Body in Request Response.status Status-Code and Reason-Phrase in Response's Status-Line Response.headers Headers in Response Response.body Entity-Body in Response
The HTTP implementation will automatically insert a Content-Length header
when necessary and possible, and takes care of the colon separators between
header names and values. It will also deal with older servers that
sometimes omits the CR from the required CRLF line termination.
Note that existing Web tools (e.g. browsers) will always send the 'path'
of the resource. On the ILU HTTP end, this means that
the object identifier will always begin with a forward slash. For
example, asking a browser to retrieve http://www.foo.bar.com/hello.html
where www.foo.bar.com is serviced by an ILU HTTP server,
will result in that server trying to invoke a GET operation on the
object whose object identifier is /hello.html. Omitting any
path info, i.e. asking the browser to retrieve http://www.foo.bar.com
would result in a GET on an object whose object identifier is simply /.
Regarding the Request-URI field on the client side, it is really only necessary to put in any 'param's and/or 'queries'. Any path information in this field is just ignored, since the path info needed is to form the request is based on the object's instance handle. So for example, a client my simply put the string ";param1;param2?query" into the Request-URI field instead of "http://www.foo.bar.com/hello.html;param1;param2?query".
If operations need to occur through a proxy server, the environment variable ILU_HTTP_PROXY_INFO should be set to the proxy server name, colon, and port number e.g. ourproxyserver.foo.bar.com:8000.
For other situations, i.e. general ILU to ILU communication that just
happens to be occurring over HTTP, the mapping is still consistent with
HTTP protocol, but a more general format is used. ILU specific information
such as the ilu_Server ID is placed in a header, and the marshaling of
arguments is done entirely within the entity body. In keeping with some
idea of human readability, marshaled arguments, with the exception of
potentially huge byte-vectors, are encoded as readable ASCII strings -
e.g. 3.1416 encodes as "3.1416". Readers concerned about utmost efficiency should
note that for general ILU-ILU communication, another protocol such as ONC RPC
is a much better choice than the current HTTP implementation. The HTTP
protocol implementation could however be easily changed to use a more
efficient encoding, similar to what's used in ONC RPC for example.
A transport stack consists of a sequence of transport layers. The last, or "bottom", layer does some kind of low-level I/O; the other layers are "filters" or "modifiers" on the transport services provided by the lower part of the stack. That is, every tail of a transport stack implements an abstraction called simply "a transport" (in English; in C, it is ilu_Transport); each transport layer (except the bottom) implements a transport in terms of another transport (the one implemented by the rest of the stack).
A transport stack is specified by a sequence of strings, each one of which specifies a transport layer. This section is a catalog of the built-in kinds of transport layers and their specification strings.
Some transports convey delimited messages (where each message is a byte sequence), others simply convey a byte sequence (that must be parsed into messages by something else). The former are called boundaried, the latter are not. Some RPC protocols require a boundaried transport, others require non-boundaried transports.
Some transports are reliable, and some aren't. Unreliable transports are deprecated in ILU, but included for interoperability with existing software that uses only unreliable transports. With these transport, messages may be delivered more than once. The ILU implementation of UDP on the server side filters out multiple receipts of the same request. Asynchronous methods may not be called over this transport mechanism, as reliable delivery of the request packet cannot be recognized by the client side. Non-asynchronous methods use the reply message as an acknowledgement that the request was received.
A TCP transport layer is reliable, not boundaried, and goes on the bottom.
A TCP transport is specified by a transport-info of the form tcp_host_port.
The host needs to convey an IP address. The host can either be a dotted decimal notation of an IP address (e.g., 13.2.116.14), or be a hostname that can be mapped into an IP address.
The host in a TCP transport-info used to create a mooring (i.e., one used to tell a server how to export itself) can use special notations to mean "any IP address of this host". Those notations are 0, 0.0.0.0, and localhost. These special notations are replaced with either a name or address of the host when an SBH is produced; an SBH cannot contain such special notations. 0 and 0.0.0.0 are replaced by an address; localhost is replaced by a name, if possible, and perhaps an address otherwise. The first replacement name considered is "the hostname" of the machine. Exactly what this is (and how it is set) is system specific, but you should beware that it may or may not be a Fully Qualified Domain Name. If it can be converted to an IP address by the usual means, and a socket bound to that address, that name is used. Otherwise, the replacement is "127.0.0.1" (the loopback address -- an address that means "this machine" everywhere) or "localhost" (the canonical name for the loopback address) and the mooring's socket is bound to 0.0.0.0 (the "any" address). We bind to 0.0.0.0 rather than 127.0.0.1 because some systems (e.g., Linux) won't let us bind to 127.0.0.1.
If you use a host name instead of an address, think about how widely it can be interpreted. If it's a Fully Qualified Domain Name, the client has to be able to use the DNS to resolve it -- but not all systems include DNS support. If it's not a FQDN, the resulting SBH can only be distributed within the organization that manages the name's mapping.
The port can either be a decimal string identifying a specific port (which, of course, must not be used for anything else), or, for a mooring, be 0, which constitutes a request for a new, unused port. In the latter case, a decimal string for that port will be substituted in the transport-info.
Examples are:
tcp_augustus_0tcp_13.2.116.14_12321tcp_localhost_12321tcp_0_0
A UDP transport is boundaried, not reliable, and on the bottom.
A UDP transport supports only messages that are no longer than the maximum size of a UDP packet; an attempt to send a longer message will cause an I/O error to be raised in the sender.
One could imagine creating a reliable boundaried transport layer, either using RDP (an IP protocol similar to UDP; see RFC 1151) or building on UDP. Of course, such a transport layer could not be used to communicate with ONC RPC/UDP peers.
A UDP transport is specified by a transport-info of the form udp_host_port. The host and port convey IP host address and port just as for TCP.
Examples are:
udp_augustus_0udp_13.2.116.14_12321udp_localhost_12321udp_0_0
A SunRPC Record Marking transport is reliable and boundaried, and is built on top of some other transport that is reliable and not boundaried. A SunRPC Record Marking transport layer is specified by a transport-info of the form sunrpcrm.
The canonical form is:
sunrpcrm
In-memory transport layers are not normally used or seen by users or applications; they are automatically created and used by ILU for cross-representation calls within an address space.
An In-Memory transport layer is reliable, boundaried, and on the bottom. An In-Memory transport layer is specified by the string inmem.
The canonical form is:
inmem
Security transport layers may be added to a transport stack
to provide some form of authenticated connection. It uses the IETF Common Authentication Technology Working Group's
Generic Security Service (GSS) API to add various flavors of security to
the messages that flow back and forth over the transport. Generally speaking, each outgoing message
will be "wrapped" by the standard GSS routine gss_wrap, and each incoming message will
be "unwrapped" by the standard GSS routine gss_unwrap. This transport also includes a mechanism
for identifying callers that is integrated with the specific security scheme being used.
Use of this transport requires linking against a GSS library, implemented according to the ANSI C mapping for the GSS spec, and against an implementation of the specific GSS scheme being used.
The security transport layer is reliable and
unboundaried, and requires a reliable, boundaried, transport stack below
it. It is specified, on the server side, by a string of the form security_1_scheme-name, where
scheme-name identifies some specific GSS security scheme. Scheme names are typically dotted-decimal
strings, representing OIDs for specific schemes. Two special names are also understood, "Xerox.ILU.GSS.NIL" and
"Xerox.ILU.GSS.SSL". Examples are:
security_1_Xerox.ILU.GSS.NIL-- use security with the ILU GSS NIL schemesecurity_1_1.2.840.113550.9.1.3-- another way of saying the same thing
Go to the previous, next section.