The session and presentation layers (Managing NFS and NIS, 2nd Edition) - e-Reading Library

start page | rating of books | rating of authors | reviews | copyrights

Book Home

Previous

Next

1.5. The session and presentation layers

The session and presentation layers define the creation and lifetime of network connections and the format of data sent over these connections. Sessions may be built on top of any supported transport protocol -- login sessions use TCP, while services that broadcast information about the local host use UDP. The session protocol used by NFS and NIS is the Remote Procedure Call (RPC).

1.5.1. The client-server model

RPC provides a mechanism for one host to make a procedure call that appears to be part of the local process but is really executed on another machine on the network. Typically, the host on which the procedure call is executed has resources that are not available on the calling host. This distribution of computing services imposes a client/server relationship on the two hosts: the host owning the resource is a server for that resource, and the calling host becomes a client of the server when it needs access to the resource. The resource might be a centralized configuration file (NIS) or a shared filesystem (NFS).

Instead of executing the procedure on the local host, the RPC system bundles up the arguments passed to the procedure into a network datagram. The exact bundling method is determined by the presentation layer, described in the next section. The RPC client creates a session by locating the appropriate server and sending the datagram to a process on the server that can execute the RPC; see Figure 1-1. On the server, the arguments are unpacked, the server executes the result, packages the result (if any), and sends it back to the client. Back on the client side, the reply is converted into a return value for the procedure call, and the user application is re-entered as if a local procedure call had completed. This is the end of the "session," as defined in the ISO model.

Figure 1-1

Figure 1-1. Remote procedure call execution

RPC services may be built on either TCP or UDP transports, although most are UDP-oriented because they are centered around short-lived requests. Using UDP also forces the RPC call to contain enough context information for its execution independent of any other RPC requests, since UDP packets may arrive in any order, if at all.

When an RPC call is made, the client may specify a timeout period in which the call must complete. If the server is overloaded or has crashed, or if the request is lost in transit to the server, the remote call may not be executed before the timeout period expires. The action taken upon an RPC timeout varies by application; some resend the RPC call, while others may look for another server. Detailed mechanics of making an RPC call can be found in Chapter 13, "Network Diagnostic and Administrative Tools".

1.5.2. External data representation

At first look, the data presentation layer seems like overkill. Data is data, and if the client and server processes were written to the same specification, they should agree on the format of the data -- so why bother with a presentation protocol? While a presentation layer may not be needed in a purely homogeneous network, it is required in a heterogeneous network to unify differences in data representation. These differences are outlined in the following list:

Data byte ordering: Does the most significant byte of an integer go in the odd- or even-numbered byte?

Compiler behavior: Do odd-sized quantities get padded out to even-byte boundaries? How are unions handled?

Floating point numbers: What standard is used for encoding floating point numbers?

Arrays and strings: How do you transmit variable-sized objects, such as arrays and strings?

Again, a presentation protocol would not be necessary if datagrams consisted only of byte-oriented data. However, applications that use RPC expect a system call-like interface, including support for structures and data types more complex than byte streams. The presentation layer provides services for encoding and decoding argument buffers that may then be passed down to RPC for transmission to the client or server.

The External Data Representation (XDR) protocol was developed by Sun Microsystems and is used by NIS and NFS at the presentation layer. XDR is built on the notion of an immutable network byte ordering, called the canonical form. It isn't really important what the canonical form is -- your system may or may not use the same byte ordering and structure packing conventions. The canonical form simply allows network hosts to exchange structured data (as opposed to streams of bytes) independently of any peculiarities of a particular machine. All data structures are converted into the network byte ordering and padded appropriately.

The rule of XDR is "sender makes local canonical; receiver makes canonical local." Any data that goes over the network is in canonical form.[3] A host sending data on the network converts it to canonical form, and the host that receives the data converts it back into its local representation. A different way to implement the presentation layer might be "receiver makes local." In this case, the sender does nothing to the local data, and the receiver must deduce the packing and encoding technique and convert it into the local equivalent. While this scheme may send less data over the network -- since it is not subject to additional padding -- it places the burden of incorporating a new hardware architecture on the receiving side, rather than on the new machine. This doesn't seem like a major distinction, but consider having to change all existing, fielded software to handle the new machine's structure-packing conventions. It's usually worth the overhead of converting to and from canonical form to ensure that all new machines will be able to "plug in" to the network without any software changes.

[3]The canonical form matches the byte ordering of the Motorola and SPARC family of microprocessors, so these processors do not have to perform any byte swapping to translate to or from canonical form. This byte ordering is called Big Endian. Big Endian ordering is used for many Internet protocols.

The XDR and RPC layers complete the foundation necessary for a client/server distributed computing relationship. NFS and NIS are client/server applications, which means they sit at the top layer of the protocol stack and use the XDR and RPC services. To complete this introduction to network services, we'll take a look at the two mechanisms used to start and maintain servers for various network services.

1.5.3. Internet and RPC server configuration

The XDR and RPC services are useful for applications that need to exchange data structures over the network. Each new RPC request contains all required information in its XDR-encoded arguments, just as a local procedure call gets its inputs from passed-in arguments. RPC services are usually connectionless services because RPC requests do not require the creation of a long-lived network connection between the client and server. The client communicates with the server to send its request and receive a reply, but there is no connection or environment for the communication.

There are many other network services, such as telnet and ftp, that are commonly referred to as the Internet or ARPA services. They are part of the original suite of utilities designed for use on the Internet. Internet services are generally based on the TCP protocol and are connection-oriented -- the service client establishes a connection to a server, and data is then exchanged in the form of a well-ordered byte stream. There is no need for RPC or XDR services, since the data is byte-oriented, and the service defines its own protocols for handling the data stream. The telnet service, for example, has its own protocol for querying the server about end-of-line, terminal type, and flow control conventions.

Note that RPC services are not required to be connectionless. RPC can be run over TCP, in a connection-oriented fashion. The TCP transport protocol may be used with RPC services whenever a large amount of data needs to be transferred. NIS, for example, uses UDP (in connectionless mode) for most of its operations, but switches to TCP whenever it needs to transfer an entire database from one machine to another. NFS supports either TCP or UDP for all its operations.

Most Internet services are managed by a super-daemon called inetd that accepts requests for connections to servers and starts instances of those servers on an as-needed basis. Rather than having many server processes, or daemons, running on each host, inetd starts them as requests arrive. Clients contact the inetd daemon on well-known port numbers for each service. These port numbers are published in the /etc/services file.

inetd sets up a one-to-one relationship between service clients and server-side daemons. Every rlogin shell, for example, has a client side rlogin process (that calls inetd upon invocation) and a server-side in.rlogind daemon that was started by inetd. In this regard, inetd and the services it supports are multi-threaded: they can service multiple clients at the same time, creating a new separate connection (and state information) for each client. A new server instance, or thread, is initiated by each request for that service, but a single daemon handles all incoming requests at once.

Only traffic specific to a single session moves over the connection between a client and its server. When the client is done with the service, it asks the server to terminate its connection, and the server daemon cleans up and exits. If the server prematurely ends the connection due to a crash, for example, the client drops its end of the connection as well.

Some RPC services can't afford the overhead of using inetd. The standard inetdbased services, like telnet, tend to be used for a long time, so the cost of talking to inetd and having it start a new server process is spread out over the lifetime of the connection. Many RPC calls are short in duration, lasting at most the time required to perform a disk operation.

RPC servers are generally started during the boot process and run as long as the machine is up. While the time required to start a new server process may be small compared to the time a remote login or rsh session exists, this overhead is simply too large for efficient RPC operation. As a result, RPC servers typically have one server process for the RPC service, and it executes remote requests for all clients in the same process. Some RPC servers are single-threaded: they execute requests one at a time. To achieve better performance, some RPC servers are multi-threaded: they have multiple threads of execution within the same process, sharing the same address space. There may be many clients of the RPC server, but their requests intermingle in the RPC server queue and are processed in the order in which server threads are dispatched to deal with the requests.

Instead of using pre-assigned ports and a super-server, RPC servers are designated by service number. The file /etc/rpc contains a list of RPC servers and their program numbers. Each program may contain many procedures. The NFS program, for example, contains more than a dozen procedures, one for each filesystem operation such as "read block," "write block," "create file," "make symbolic link," and so on. RPC services still must use TCP/UDP port numbers to fit the underlying protocols, so the mapping of RPC program numbers to port numbers is handled by the portmapper daemon (portmap on some systems, rpcbind on others).

When an RPC server initializes, it usually registers its service with the portmapper. The RPC server tells the portmapper which ports it will listen on for incoming requests, rather than having the portmapper listen for it, in inetd fashion. An RPC client contacts the portmapper daemon on the server to determine the port number used by the RPC server, or it may ask the portmapper to call the server indirectly on its behalf. In either case, the first RPC call from a client to a server must be made with the portmapper running. If the portmapper dies, clients will be unable to locate RPC daemons services on the server. A server without a running portmapper effectively stops serving NIS, NFS, and other RPC-based applications.

We'll come back to RPC mechanics and debugging techniques in later chapters. For now, this introduction to the configuration and use of RPC services suffices as a foundation for explaining the NFS and NIS applications built on top of them.

1.5.3.1. Socket RPC and Transport Independent RPC

RPC was originally designed to work over sockets, a programing interface for network communication introduced in the 1980s by the University of California in its 4.1c BSD version of Unix. Solaris 2.0 introduced Transport Independent RPC (TI-RPC). The motivation for TI-RPC was that it appeared that OSI networking would eventually supplant TCP/IP-based networking, and so a transport independent interface would make it easier to transition RPC applications was needed. While OSI networking did not take over, TI-RPC is still used in Solaris. TI-RPC introduces an additional configuration file, /etc/netconfig, which defines each transport that RPC services can listen for requests over. In addition to TCP and UDP, the /etc/netconfig file lists connectionless and connection-oriented loopback transports for RPC services that don't need to provide service outside the host. In Solaris 8, the /etc/netconfig file will also let you specify services over TCP and UDP on IPv6 network interfaces.