:-[] :-|| Vivek Khokhar rambles here :-O :-[]

schtasks /create /tn “My Scheduled Jobs” /tr “php-win.exe D:\Program Files\xampp\htdocs\somefile.php” /sc minute

make sure that php-win.exe is in path while giving this command

Assumption:

- eth0 is the external interface (Connected to internet)
- eth1 is the internal interface (local network) (Fixed IP)

Edit /etc/sysctl.conf to enable ip forwarding permanently.

net.ipv4.ip_forward = 1

Edit /etc/sysconfig/iptables-config and make following changes:

IPTABLES_MODULES=”ip_conntrack_netbios_ns ip_conntrack ip_conntrack_ftp ip_conntrack_irc iptable_nat ip_nat_ftp ip_nat_irc”
IPTABLES_SAVE_ON_STOP=”yes”
IPTABLES_SAVE_ON_RESTART=”yes”

# To clear out any existing rules and set default policy,
# run following commands on command prompt
iptables -P INPUT ACCEPT
iptables -F INPUT
iptables -P OUTPUT ACCEPT
iptables -F OUTPUT
iptables -P FORWARD DROP
iptables -F FORWARD
iptables -t nat -F

# FWD: Allow all connections OUT and only existing and related ones IN

iptables -A FORWARD -i eth0 -o eth1 -m state –state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

# Enabling MASQUERADE functionality on eth0
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Pure ALOHA protocol

Pure ALOHA protocol is a random access protocol used for data transfer.

User accesses a channel as soon as a message is ready to be transmitted. After a transmission the user waits for an acknowledgement on either the same channel or separate feedback channel. In case of collision (i.e. when a NACK is received), the terminal waits for a random period of time and retransmits the message. As the number of users increase, a greater delay occurs because the probability of collision increases.

Slotted ALOHA protocol

In Slotted ALOHA protocol, time is divided into equal time slots of length greater than the packet duration (time taken to transmit a fixed length packet).

The subscribers (users), each have synchronized clocks and transmit a message only at the beginning of a new time slot, thus resulting in a discrete distribution of packets (each subscriber is assigned a time slot in which he can transmit.). (Note here: two users may get same time slot at peak times)

This prevents partial collisions (i.e. packets collide with portion of another). This implies that either a packet will collide completely or not at all.

As the number of users increase a greater delay will occur due to duplicate time slot assignments and hence greater number of complete collisions. Repeated transmission of lost packets would make things even slower.

But, the vulnerable period for slotted aloha is only one packet duration, since partial collisions are prevented through synchronization.

Overall, slotted ALOHA provides a maximum channel utilization of .368 Erlangs, double that of pure ALOHA.

Limitation of Aloha in general

ALOHA protocols do not listen to the channel before transmission, and therefore do not exploit information about other users. By listening to the channel before starting transmission, greater efficiencies may be achieved. This is where CSMA protocols come into the picture which I will cover in a separate post.

Transmission control Protocol (TCP) was developed before OSI model. Therefore, the layers in the TCP/IP protocol do not match exactly with those in the OSI model. The TCP/IP protocol is made of 5 layers: physical, data link, network, transport, and application. OSI has 7 layers.

Here follows the comparison:

Application layer of TCP/IP = Session, presentation, and application layers of the OSI model.

TCP & UDP protocol of TCP/IP = Transport layer of OSI model

(IGMP, ICMP, ARP, RARP)IP protocol of TCP/IP = Network layer of OSI model

At Data Link & Physical layers of OSI, TCP/IP does not define any specific protocol of its own. IP layer of TCP/IP is capable of interfacing with many available standards.  for example Ethernet, Token Ring, FDDI, HSSI, and ATM.

I got this error today while deploying rails application on running lighttpd server.
Errno::ENOENT (No such file or directory - getcwd)
I replaced application directory without stopping lighty and got error while trying accessing app via browser.
I went off by merely restarting lighthttpd

Seems like server creates/uses some file “within” rails app folder which got erased while replacing the folder.