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COMP30023 Project 2
Remote Procedure Call
Out date: 28 April 2023
Due date: No later than 3pm Friday 19 May, 2023 AEST
Weight: 15% of the final mark
1 Project Overview
Remote Procedure Call (RPC) is a crucial technology in distributed computing that enables software applications
to communicate with each other seamlessly over a network. It provides a way for a client to call a function on a
remote server as if it were a local function call. This abstraction allows developers to build distributed systems
and applications that span multiple machines and platforms.
In this project, you will be building a custom RPC system that allows computations to be split seamlessly between
multiple computers. This system may differ from standard RPC systems, but the underlying principles of RPC
will still apply.
Your RPC system must be written in C. Submissions that do not compile and run on a Linux cloud VM, like
the one you have been provided with, may receive zero marks. You must write your own RPC code, without
using existing RPC libraries.
2 RPC System Architecture
Your task is to design and code a simple Remote Procedure Call (RPC) system using a client-server architecture.
The RPC system will be implemented in two files, called rpc.c and rpc.h. The resulting system can be linked
to either a client or a server. For marking, we will write our own clients and servers, and so you must stick to the
proposed API carefully.
For testing purposes, you may run server and client programs on the same machine (e.g., your VM).
3 Project Details
Your task is to design and code the RPC system described above. You will design the application layer protocol to
use. A skeleton is provided which uses a simple application programming interface (API). When we assess your
submission, we will link our own testing code using the same RPC system API; what you will be assessed on is
rpc.c (and any other supporting files compiled in by your Makefile).
Note that implementing the API will require you to use sockets. This uses material covered in the lectures after
the project is released. There is plenty to do on the project before you need to use sockets, so please do not say
that you cannot start because you have not yet learned about sockets.
3.1 API
The basic API implemented by rpc.c consists of the following data structures and functions.
3.1.1 Data structures
The API will send and receive data structures of the form:
typedef struct {
int data1;
size_t data2_len;
void *data2;
} rpc_data;
where data1 is an integer to be passed to the other side, and data2 is a block of bytes (of length data2_len) to
be sent. The purpose of data1 is to allow simple functions that only pass an integer to avoid memory management
issues, by setting data2_len=0 and data2=NULL. Your protocol can limit data1 to being no more than 64 bits.
Note that size_t depends on the architecture, and the sender and receiver can have different architectures. Think
how this will affect your protocol.
The handler that implements the actual remote procedure will have the signature:
rpc_data *procedure (const rpc_data *d);
That is, it takes a pointer to an rpc_data object and returns a pointer to another rpc_data object. This function
will dynamically allocate memory with malloc for both the rpc_data structure and its data2 field. It is the
responsibility of the RPC system to free those after use.
The state of the client and server will be in data structures that you define. These are declared in rpc.h as:
typedef struct rpc_client rpc_client;
typedef struct rpc_server rpc_server;
and you should provide the actual struct definitions in rpc.c. These are returned by initialization functions
(rpc_init_client and rpc_init_server, and passed to all other functions.
3.1.2 Server-side API
rpc_server *rpc_init_server (int port)
Called before rpc_register. Use this for whatever you need. It should return a pointer to a struct (that you
define) containing server state information on success and NULL on failure.
int rpc_register (rpc_server *srv, const char *name, rpc_data* (*handler)(rpc_data*))
At the server, let the subsystem know what function to call when an incoming request is received.
It should return a non-negative number on success (possibly an ID for this handler, but a constant is fine), and -1
on failure. If any of the arguments is NULL then -1 should be returned.
1. What are the valid characters in name? (We will only test printable ASCII characters between 32 and 126.)
2. How does the server know the length of name? (We will not test names longer than 1000 bytes.)
3. Should there be a minimum length for name? (We will not test for empty names.)
If there is already a function registered with name name, then the old function should be forgotten and the new
one should take its place.
To get full marks, you should be able to register at least 10 functions. You can still get most of the marks as long
as you can register one function, so implement that first.
void rpc_serve_all (rpc_server *srv)
This function will wait for incoming requests for any of the registered functions, or rpc_find, on the port specified
in rpc_init_server of any interface. If it is a function call request, it will call the requested function, send a
reply to the caller, and resume waiting for new requests. If it is rpc_find, it will reply to the caller saying whether
the name was found or not, or possibly an error code.
This function will not usually return.
It should only return if srv is NULL or you’re handling SIGINT (not a requirement).
3.1.3 Client-side API
rpc_client *rpc_init_client (const char *addr, int port)
Called before rpc_find or rpc_call. Use this for whatever you need. The string addr and integer port are the
text-based IP address and numeric port number passed in on the command line.
The function should return a non-NULL pointer to a struct (that you define) containing client state information on
success and NULL on failure.
void rpc_close_client (rpc_client *cl)
Called after the final rpc_call or rpc_find (i.e., any use of the RPC system by the client).
Use this for whatever you need; it should at least free(cl).
If it is (mistakenly) called on a client that has already been closed, or cl == NULL, it should return without error.
(Think: How can you tell if it has already been closed?)
rpc_handle *rpc_find (rpc_client *cl, const char *name)
At the client, tell the subsystem what details are required to place a call. The return value is a handle (not handler)
for the remote procedure, which is passed to the following function.
If name is not registered, it should return NULL. If any of the arguments are NULL then NULL should be returned. If
the find operation fails, it returns NULL.
rpc_data *rpc_call (rpc_client *cl, rpc_handle *h, const rpc_data *data)
This function causes the subsystem to run the remote procedure, and returns the value.
If the call fails, it returns NULL. NULL should be returned if any of the arguments are NULL. If this returns a
non-NULL value, then it should dynamically allocate (by malloc) both the rpc_data structure and its data2
field. The client will free these by rpc_data_free (defined below).
The skeleton gives an example of how these functions can be used.
3.1.4 Shared API
void *rpc_data_free (rpc_data* data)
Frees the memory allocated for a dynamically allocated rpc_data struct.
Note that there is a reference implementation of this function in the skeleton code.
3.2 Planning task
When you are designing the protocol, ask yourself the following questions. Think of the answer for this particular
project, and separately for the case of a “real world” RPC server.
Put the answers in a plain text file answers.txt, beginning with your name, login ID and student ID. These,
together with protocol description below, are worth 1 mark.
1. Should the server accept calls from everyone, or just a subset of users?
2. Should authentication etc. be provided by the RPC framework, or by the functions that use the RPC frame-
3. What transport layer protocol should be used? What are the trade-offs?
4. In which function(s) should the socket(s) be created?
5. Should rpc_client and rpc_server be allocated dynamically or statically? What are the implications
for the client and server code?
6. What happens if one host uses big-endian byte order and the other uses little-endian? How does that relate
to “network byte order”?
3.3 Protocol
You should design and document a simple application layer protocol for this RPC system. (It can be very simple.)
Describe it in the file answers.txt, in enough detail that someone else could implement the protocol. Together
with the questions above, this is worth 1 mark.
Note that size_t is system-dependent. You will need to choose a system-independent way to encode the size of
the data block in the packet. You can use a universal encoding, like Elias gamma coding (see Wikipedia) which
can specify arbitrarily long strings, or you can use a fixed sized field, which is simpler to decode but limits the size
of the string. Explain your choice in your protocol description.
In all test cases, len < 100 000.
If data2_len is too large to be encoded in your packet format, the relevant function in rpc.c should print
"Overlength error" to stderr and return an error.
The protocol should specify error responses for routine failures, such as a request for a procedure that does not
The protocol should work correctly even if there are IP layer packet loss and duplication.
The protocol should handle the fact that IP packets have a maximum allowed size.
Decide what transport layer protocol to use. (You will almost certainly choose TCP, but briefly mention the pros
and cons of alternatives.)
The transport layer protocol should run on top of IPv6.
3.4 Test harness
Code isn’t complete until it has been thoroughly tested.
If your Makefile produces executables rpc-server and rpc-client, then they will be executed by the CI with
the command line arguments shown below, with the results of stdout and stderr included in the test transcript.
No marks are allocated to rpc-server and rpc-client, either in execution or in code quality.
To run your server program on your VM prompt, type:
./rpc-server -p &
./rpc-client -i -p
The & tells the operating system to run the server in the background.
ip-address is the IPv6 address of the VM on which the server is running.
port is the TCP (or other transport layer) port number of the server.
The server is expected to listen for incoming connections on the port passed via command line arguments, on any
of the hosts IPv6 network addresses.
4 Stretch goal: non-blocking performance
Many RPC operations, such as database look-ups, take a substantial time to complete. Instead of making each
request wait for all those before it to complete, it is possible to continue to accept and start processing new requests
while previous requests are executing, with each call returning as soon as it is finished. The simplest way to do
this is with multiple threads: each time a request (or connection) is received, a new thread is spawned to execute
the procedure, and is destroyed once the result has been sent back to the caller. Alternatives include creating new
processes with fork(2), or using select(2).
If you implement non-blocking, include the line “#define NONBLOCKING" in your code. Otherwise, this func-
tionality will not be tested/marked.
5 Marking Criteria
The marks are broken down as follows.
The column “In CI” specifies how many of the marks allocated for this are tested by the continuous integration
system available before submission. If you pass these in CI, you can be confident of getting those marks. There
are also tests that will only be run on your final submission, which give the remaining marks.
Task # and description Marks In CI
1. Client correctly finds module on server 2 1
2. Remote procedure is called correctly 2 1
3. Results are correctly returned to client 2 1
4. Supports multiple procedures 2 1
5. Portability and safety 2 1
6. Build quality 1 1
7. Quality of software practices 2 0
8. Planning task and protocol description, in answers.txt 1 0
9. Stretch goal. Non-blocking operation works 1 0.5
The Continuous Integration (CI) tests will cover at least half of the marks for code execution (tasks 1–6 and the
stretch goal). If you pass all CI tests, and pass tasks 7–8 then you will pass.
Code that does not compile and run on your cloud VM will usually be awarded zero marks for parts 1–6. Use the
Git continuous integration (CI) infrastructure to ensure your submission is valid. This is very important. Nearly
every year, someone gets code working on their own computer but it fails on the cloud. Please push your code to
Git regularly, and check the CI output.
Your submission will be tested and marked with the following criteria:
Task 1. Client correctly finds module on server Your protocol causes the client to ask the server for informa-
tion on a remote function. If the function exists on the server, the server replies, and results in a valid data structure
at the client being created, ready for Task 2. If an unregistered function is requested, NULL should be returned.
Task 2. Remote procedure is called correctly The remote procedure is called. These marks are awarded even
if an error occurs causing the result not to be received by the client.
Note that a single remote procedure may be called multiple times.
Task 3. Results are correctly returned to the client The client receives the correct result and continues execu-
tion. This should not result in any memory overflow, even if a large block of data is returned.
Task 4. Supports multiple procedures Allows multiple calls to rpc_register with different function names.
The protocol must indicate which function is being called by rpc_call, and NULL if an unregistered function is
passed to rpc_find or an invalid handle is passed to rpc_call.
Task 5. Portability and safety The server component of the RPC system return errors on failure and does not
crash after listen(2), e.g., due to malformed input, unexpected termination of connection etc.
The client component of the RPC system return errors on failure, e.g. cannot connect to server, if the server shuts
down in the middle of an operation etc.
If you implement timeouts (not a requirement), the timeout duration must be longer than 30 seconds.
Data is sent in network byte order, regardless of whether the client and server are big-endian or little-endian.
(Note that the testing system will check this by snooping on packets. Encryption or compression will break this.
If you use either of those, please comment it clearly in your code so that it can be tested by hand.)
Task 6. Build quality Running make clean && make -B should produce an object file rpc.o or static library
rpc.a, which contains everything needed for the RPC system, and will be linked to our test client and server
code. Include in your Makefile (in 1 line) a LDFLAGS variable describing any linker flag(s) that your submission
If this fails for any reason, you will be told the reason, and be allowed to resubmit (with the usual late penalty). If
it still fails, you will get 0 for Tasks 1–5 and the stretch goal. Test this by committing regularly, and checking the
CI feedback. (If you need help, ask on the forum.)
A 0.5 mark penalty will be applied if compiling using “-Wall” yields a warning or if your final commit contains
any executable, .o, or .a files.
Task 7. Quality of software practices Factors considered include quality of code, based on the choice of
variable names, comments, formatting (e.g. consistent indentation and spacing), structure (e.g. abstraction, mod-
ularity), use of global variables, proper management of memory including not leaking memory, and proper use
of version control, based on the regularity of commit and push events, their content and associated commit mes-
sages. Profanity or abuse in the commit messages will also result in mark deductions; everyone gets frustrated,
but commits must remain professional.
Task 8. Answers to questions The answers in answers.c should be short (perhaps one or two lines each) and
clear. The justifications given are more important than the choices made.
Stretch goal. Non-blocking operation works The server starts remote procedures as soon as a request is
received, and return in the order in which they complete, which may be different from the order in which requests
Since RPC calls are blocking at the client, this will only occur in the case of multiple clients or a multithreaded
client. You may assume that the client is single threaded, or at least has locks around the RPC interface, so that
within a transport layer connection – responses are in the order of the requests.
Your RPC system should be able to support at least 10 concurrent clients.
6 Submission
All code must be written in C (e.g., it should not be a C wrapper over non C-code) and cannot use any external
libraries, except standard libraries as noted below.
You can reuse the code that you wrote for your other individual projects if you clearly specify when and for what
purpose you have written it (e.g., the code and name of the subject, project description and date, that can be verified
if needed). You may use code which we have provided for practicals. You may use libc and POSIX functions
(e.g., to print, create sockets, manipulate threads etc.). Your code must compile and run on the provided VMs.
The repository must contain a Makefile which produces an object file rpc.o or static library rpc.a. The repos-
itory must contain all source files required for compilation.
Place the Makefile at the root of your repository, and ensure that running make places the requested files there too.
If the GitLab CI does not find these files, the marking system will not either.
Ensure that your RPC system does not write to stdout. However, the client and server can.
The server component of the RPC system should not shut down by itself. SIGINT (like CTRL-C) will be used to
terminate the server between test cases. You may notice that a port and interface which has been bound to a socket
sometimes cannot be reused until after a timeout. To make your testing and our marking easier, please override
this behaviour by placing the following lines before the bind() call:
int enable = 1;
if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(int)) < 0) {
Make sure that all source code is committed and pushed to git. Executable files (that is, all files with the executable
bit which are in your repository) will be removed before marking. Hence, ensure that none of your source files
have the executable flag set. (You can verify this by cloning your repo onto your VM, and using ls -l; only
directories should have “x” flags.)
For your own protection, it is advisable to commit your code to git at least once per day. Be sure to push after
you commit. The git history may be considered for matters such as special consideration, extensions and potential
plagiarism. Your commit messages should be a short-hand chronicle of your implementation progress and will be
used for evaluation in the Quality of Software Practices criterion.
You must submit the full 40-digit SHA1 hash of your chosen commit to the Project 2 Assignment on LMS. You
must also push your submission to the repository named comp30023-2023-project-2 in the subgroup with
your username of the group comp30023-2023-projects on gitlab.eng.unimelb.edu.au.
You will be allowed to update your chosen commit. However, only the last commit hash submitted to LMS before
the deadline will be marked without late penalty.
You should ensure that the commit which you submitted is accessible from a fresh clone of your repository. For
example (below, the ... are added for clarity to break the line):
git clone https://gitlab.eng.unimelb.edu.au/comp30023-2023-projects...
cd comp30023-2023-project-2
git checkout
Late submissions will incur a deduction of 2 mark per day (or part thereof).
Extension policy: If you believe you have a valid reason to require an extension, please fill in the form accessible
on Project 2 Assignment on LMS. Extensions will not be considered otherwise. Requests for extensions are not
automatic and are considered on a case by case basis.
7 Testing
The skeleton available from https://gitlab.eng.unimelb.edu.au/comp30023-2023-projects/project2
gives a simple client and server. You can modify them to test your system more thoroughly.
Continuous Integration Testing: To provide you with feedback on your progress before the deadline, we will
set up a Continuous Integration (CI) pipeline on GitLab.
Though you are strongly encouraged to use this service, the usage of CI is not assessed, i.e., we do not require CI
tasks to complete for a submission to be considered for marking.
Note that test cases which are available on GitLab are not exhaustive. Hence, you are encouraged to write unit
and integration tests to further test your own implementation.
The requisite .gitlab-ci.yml file will be provided and placed in your repository, but is also available from the
project2 repository linked above.
Please, please use this CI feature. Almost all failed projects come from not fixing bugs that are reported by CI.
8 Getting help
Please see Project 2 Module on LMS.
9 Collaboration and Plagiarism
You may discuss this project abstractly with your classmates but what gets typed into your program must be
individual work, not copied from anyone else. Do not share your code and do not ask others to give you their
programs. The best way to help your friends in this regard is to say a very firm “no” if they ask to see your program,
point out that your “no”, and their acceptance of that decision, are the only way to preserve your friendship. See
https://academicintegrity.unimelb.edu.au for more information.
Note also that solicitation of solutions via posts to online forums, whether or not there is payment involved, is also
Academic Misconduct. You should not post your code to any public location (e.g., GitHub) until final subject
marks are released.
If you use a small amount of code not written by you, you must attribute that code to the source you got it from
(e.g., a book or Stack Exchange).
Do not post your code on the subject’s discussion board Ed.
Plagiarism policy: You are reminded that all submitted project work in this subject is to be your own individual
work. Automated similarity checking software will be used to compare submissions. It is University policy that
cheating by students in any form is not permitted, and that work submitted for assessment purposes must be the
independent work of the student concerned.
Using git properly is an important step in the verification of authorship. We should see the stages of your code
being written, not just the finished product.
AI software such as ChatGPT can generate code, but it will not earn you marks. You are allowed to use AI tools,
but if you do then you must strictly adhere to the following rules.
1. Have a file called AI.txt.
2. That file must state the query you gave to the AI, and the response it gave.
3. You will only be marked on the differences between your final submission and the AI output.
If the AI has built you something that gains you points for Task 1, then you will not get points for Task 1;
the AI will get all those points.
If the AI has built you something that gains no marks by itself, but you only need to modify five lines to get
something that works, then you will get credit for identifying and modifying those five lines.
4. If you ask a generic question like “How do I convert an integer to network byte order?” or “What does the
error ‘implicit declaration of function rpc_close_server’ mean?” then you will not lose any
marks for using its answer, but please report it in your AI.txt file.
If these rules seem too strict, then do not use the AI tools.
These issues are new, and this may not be the best policy, but it is this year’s policy. If you have suggestions for
better rules for future years, please mention them on the forum.
Good luck!

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