Seventh lab

It is available here. And it covers IPtables command that allows you to control what packets your system listens to or ignores. It is just a very basic introduction that will enable you to create basic traffic-filtering rules.

Exam reminder

Please note next April 29th we will have a new exam, covering exclusively Chapter 6, that is, the Network Layer.

Chapter 6 test results

You can get the solved quiz here.

I have found the results a bit unfair. It seems that no one understood how fragmentation works, maybe the lab about it was not enough. So let me explain it once more:

Datragrams are fragmented so resulting fragments have a total length (header + payload) lower than the MTU of the network they are forwarded to. However, as fragmentation offset values represent blocks of eight bytes, fragmentation may only happen on certain points of the payload (those that happen to be multiples of eight).  Thus, the resulting fragment it is made of a header (typically 20 bytes for IPv4) plus the data payload (whose length is a multiple of eight). This number, the total fragment length, has to be equal or lower than the MTU.

So all the fragments (but the last) for a network of MTU=512 have to be 508 bytes long (or smaller). Though 512 is a valid size, a fragment whose length is a multiple of eight cannot be 492 bytes long, as 492 (which is 512-20) is not a multiple of eight. Therefore, the maximum fragment length would be 488 bytes (which is the closest but lower multiple of eight to 492). If data size is 488 then the total fragment size (once header is included) would be 488 + 20 = 508 bytes.

Please note that the marks, which are available in the usual place, have an average mark of 25%, an all time low. Considering we have an exam in a couple of weeks about these topics, you all know that there is a lot to improve if you want to pass that exam.