Transcript Lecture 03 - Suraj @ LUMS

XDR, RPC, NFS
Internetworking with UNIX TCP/IP
Winter 2002-2003
Jan 28, 2003
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XDR
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Asymmetric data conversion
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Convert to what other entity understands!
Symmetric data conversion
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Convert to a standard format!
Network standard byte order
XDR has implicit types
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32 bit big-endian integer has exactly 32 bits
No explicit information about the “type” of data
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How to convert to XDR?
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XDR library routines are available
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These routines are machine dependent
Convert data from native format to XDR
and vice versa
A buffer paradigm is mostly used
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“Complete” message is converted to XDR
Code example:
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Create a buffer to hold XDR data
Append data to the buffer (or XDR stream)
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XDR: code example
#include <rpc/xdr.h>
#define BUFFER_SIZE 4000
int i;
XDR *xdr_stream;
char buf[BUFFER_SIZE];
xdrmem_create(xdr_stream, buf, BUFFER_SIZE, XDR_ENCODE);
…
i = 35;
xdr_int(xdr_stream, &i); // convert integer and append to stream
// why are we passing the address of i?
…
See figure 19.4, page 237 for other data conversion routines
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XDR: code example, receiver
#include <rpc/xdr.h>
#define BUFFER_SIZE 4000
int j;
XDR *xdr_stream;
char buf[BUFFER_SIZE];
xdrmem_create(xdr_stream, buf, BUFFER_SIZE, XDR_DECODE);
…
// someone filled in the XDR stream xdr_stream here!
…
xdr_int(xdr_stream, &j); // this is why we were passing the address of i!
…
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Using XDR with TCP and UDP
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TCP is stream oriented
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Can directly be used with XDR
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Open TCP socket and get a descriptor fd
Use FILE *fdopen(int fd, char* mode) to associate a
standard I/O stream to fd (socket fd)
Use xdrstdio_create(XDR* xdrs, FILE* stdio_stream, enum
xdr_op) to connect a new XDR stream to the I/O
descriptor
Subsequent calls to XDR conversion routines
automatically send XDR data over TCP
For use with UDP, records and datagram I/O
are described in XDR (see xdrrec_create( ))
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Remote Procedure Call
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An Extension of Procedural Call Model
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In many programming languages (not
object oriented), complete program flow is
described by procedure (or function) calls
If the procedures cross the boundary of a
computer, it becomes RPC
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A communication protocol is required to carry
out the remote call to the procedure
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Conventional Procedure Call
A single thread of execution
Control transfer from one procedure to another
Variables of the calling procedure are “frozen”
Procedures calls are usually iterative
main
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proc_A
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proc_B
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Distributed Procedure Call
Imagine a different “client-server” model
Server implements a procedure called by client
Client interacts with server as a procedure caller
Server represents an implementer of the procedure
Machine 1
Machine 2
Machine 3
main
proc_A
proc_B
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Local calls vs. RPC
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For local calls, a procedure is inactive until it is called. For RPCs,
a server process must exist waiting for a call
Local procedures generally use a few arguments. RPCs can
return arbitrary stream of data
RPCs incur higher cost due to network delays
Local procedures are generally in the same address space, so
pointers are passable
Remote procedures do not have access to caller’s I/O
descriptors
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Remote procedures do not write data to the error file of a calling
computer
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Shared Data, Remote Programs
A single remote program
Procedure 1
Procedure 2
Procedure 3
Shared global data
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RPC: Number of Arguments
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Calls with large number of arguments are
hard to read
Data aggregation
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In a struct, for example
XDR tells how to encode structs for sending these
across to another machine
Typically, all the arguments in RPC are collected in
one single struct
RPC Identification: (prog, vers, proc)
Prog: 32 bits; vers, proc: integers
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More on RPCs
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Mutual Exclusions
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At most one procedure in a remote program can
be running at a time
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Ensures integrity of shared data
Retransmissions
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Default timeout mechanism specifies a fixed and
non-adaptive timeout
A fixed number of retries is also specified
Some applications may adapt the timeout on their
own, but RPC does not adapt to network delays
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RPC Call Message Format
0
16
31
MESSAGE ID
MESSAGE TYPE (0 FOR CALL)
RPC VERSION NUMBER (2)
REMOTE PROGRAM (0x000186a3 for NFS)
REMOTE PROGRAM VERSION (2)
REMOTE PROCEDURE (1 for GETATTR)
UNIX AUTHENTICATION
ARGUMENTS FOR REMOTE PROCEDURE
(IF APPLICABLE)
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Distributed File Systems
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Network File System (generic name) or
Distributed File System
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Files are “distributed” over a network of
computers
Examples
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NFS
AFS
Coda
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Design Goals
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Uses same/similar file system interfaces as
used for local access
Facilities
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Independent of user location
Transparent to the end user
Backups
Caching
Consideration of user mobility (?)
Data sharing for a multitude of users
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NFS
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Very similar to UNIX file system
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Provides file access rather than transfer
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open( )
read( )
write( )
close( )
One copy of the file is kept
Independent of machine architecture
Uses RPC for machine-to-machine communication
Provides common login environment
It is transparent to the end user
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stat( ) call for file information
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stat(string filename, stat* stat_block);
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Provides information on a UNIX file
S_ISREG(stat_block->st_mode); //regular file
S_ISDIR(stat_block->st_mode); //directory
Symbolic links
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Created using $ ln –s realfile linkfile
Traversing a directory tree will not work using stat(
) call
Use lstat( ) to traverse directories
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File System
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Everything you can logically see, not necessarily on
the same disk
Data is organized in files; files are organized in
directories
Each file (directory is a special file) is referenced by
an i-node which contains:
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stat( ) info
Reference to the data block
i-node numbers on a physical device are unique
To differentiate between files and directories, looking
at data block is not enough; need to go to i-node
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File System
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If i-numbers associated with the two files on
a UNIX file system are same:
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Data block for the two must be the same
This is defined as hard link
To create hard link
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$ ln realfile linkfile
How to ensure that i-numbers are unique
across machines in a distributed system?
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Can’t ensure; i-number duplication is allowed
Hard links not allowed across physical devices
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Functions of NFS
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Directory mount
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Directory export
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From other computers on the local network
To other computers off the local network
Has client-server architecture
An NFS client is able to mount directories from
multiple NFS servers
Servers are stateless
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No information at the server about files open at the client
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NFS File Attributes
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Just like stat provides information for
UNIX files, “file attributes” for NFS are
described in the fattr structure
Instead of a stat( ) or lstat( ) call as in
case of UNIX file system, a remote
procedure call fills in the fattr structure
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RPC for NFS
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Runs on top of UDP
Clients specify block size, retransmit attempts, and
timeout values
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rsize, wsize, retrans, timeo
Default values are 8KB block, 5 retransmits, and 1s
timeout
timeo is for the first request; the value is doubled
with every retransmit request
Generally, processes requesting server files will block
until server responds
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