NAME

    pass-it-on
    
SYNOPSIS

    pass-it-on
    
DESCRIPTION

    pass-it-on listens on the for UDP packets from a peer pass-it-on 
    server and "passes the packet on" to a next peer pass-it-on server.
    
    There are three types of UDP packets. All are strictly printable 
    ASCII characters (no NULL's, new-lines, or other special characters
    are allowed):
    
    S:_signature_:_list_,_of_,_users_
    A:_signature_:
    E:_explanation_
    
    The "S" (source) packet gives a comma separated list of users. The users
    are known to the system (you can write the user names into your program,
    for simplicity). The names can only be alphanumeric. The list can be
    empty. If not empty, pass-it-on removes the first user from the list,
    and sends an "S" packet with a new recalculated signature to the hostname
    associated with the first user. The new list of users is the old list,
    with the first user removed.
    
    If the list of users is empty, no "S" packet is sent in response.
    
    The signature is exactly 13 characters, and is calculated based on the pass-it-on
    server's password. The colons are literal, and appear as the second and 16-th
    characters in the "S" packet.
    
    Every "S" packet also causes pass-it-on to send an "A" packet (acknowledgement)
    back to the sender of the "S" packet, to acknowledge it. The receiver of the
    "A" packet notes the packet, but takes no further action. Resend if an 
    acknowledgement is not received is not required. The signature of the "A" packet
    is copied from the signature of the "S" packet.
    
    If pass-it-on does not like the "S" packet, it sends an "E" packet (error)
    rather than an "A" packet back to the sender of the server. The explanation
    is arbitrary text explaining the error.
    
SIGNATURE

	The signature is exactly 13 characters.
	
	In order to prove that a packet has passed through the pass-it-on server,
	the server will sign the packet:
	
	new-signature = H(old-signature,server-secret)
	
	where H is a cryptographic hash function. Because the signature accumulates
	all server-secrets, it attests that the packet has passed through each 
	of the pass-it-on servers listed in the original "S" packet, in the order
	they were listed.
	
	Use your username concatenated with itself for your 8 letter password:
	e.g. user "burt" signs with password "burtburt".
	
OPTIONS

	There are no required options. You might want to implement a -v option
	for testing. 

THE PASS-IT-ON GAME

	OLD: Once your server is working, tweet with hashtag #csc524 your username and
	public DNS for the AWS server where your server is running.
	
	NEW: The PEER REGISTRY section of this page will be updated with the username
	and public DNS of claimed working pass-it-on servers. To run a program perennially
	in the background, even when logged out, start the program as:
	
	    nohup ./pass-it-on &
	
	To kill all such program, the most convenient way is "killall pass-it-on". If you
	don't kill running services before starting another, they will conflict over
	binding to the UDP port.
 
	
	My pass-it-on server will create a master "S" packet with a fixed starting
	signature. The list of users will end with "burt" so that the packet will
	ultimately return to me, after passing through other peers. 
	
	Since I know your passwords I can verify that the packet is traveled correctly.
	
	OLD: The "T" packet can be used to test pass-it-on servers, as you can send
	"T" packets to users named in #csc524 tweets, with your user as the last
	on the list, and expect them to be returned empty, with a known signature
	only depending on the length of the original user list.
	
NOTES
	
	You will have to make another program (a client) to create and send the
	first "S" packet, in order to test your pass-it-on server. To test it further,
	you might need to form small groups to send packets around in a 
	circle. These are also called "bake-offs", because you test whether your
	protocols can inter-operate.
	
	Receiving port is 3333. Sending port can be the receiving port, or an 
	ephemeral port. The assignment does not specify this.
	
PEER REGISTRY:

	ahae, ec2-54-242-244-20.compute-1.amazonaws.com (confirmed)
	burt, ec2-50-19-171-60.compute-1.amazonaws.com (confirmed)
	pika, ec2-54-242-231-151.compute-1.amazonaws.com (confirmed)
	nada, ec2-50-19-170-169.compute-1.amazonaws.com (confirmed)
	efie, ec2-67-202-57-111.compute-1.amazonaws.com (confirmed)
	mwei, ec2-54-242-189-57.compute-1.amazonaws.com (confirmed)
	kcbr, ec2-23-22-17-187.compute-1.amazonaws.com (confirmed)
	atve, ec2-54-243-17-171.compute-1.amazonaws.com
	neca, ec2-23-23-20-2.compute-1.amazonaws.com
	bcsa, ec2-23-22-228-54.compute-1.amazonaws.com (confirmed)
	dacl, ec2-23-20-36-19.compute-1.amazonaws.com (confirmed)
	
	
LAST UPDATED

	March 31, 2013