02 - Packet’s load tolerance

Keep up packets

• bandwidth: measures capacity (a bigger pipe would mean higher bandwidth)
• latency: measures data speed (how quickly does the water in the pipe reaches its destination)
• throughput: measures data transmitted and received during a specific time (the water running through the time)

Example: you can have a 100Gb/s NIC (bandwidth), but can transmit only at 40Gb/s (throughput) because of overheads

Recap: model

Level	OSI	TCP/IP	Examples
1	physical	same	ethernet
2	data link	same	ethernet
3	network	same	ip
4	transport	same	tcp/udp
5	session	-
6	presentation	-
7	application	same

Socket

• first in 1983
• level of abstraction very useful because nowadays are still used
• unix file with basic function that can send and receive “data”
• it is between transport and application layer

The other things are managed by the OS!

Questions

1. transfer data between network controller and the end host

2. Notify the end host about network packet reception

   • we can use interrupts

     • letteralmente il SO che dice “ao, *ferma tutto* … che te serve? … Ok, mo vedi d’annattenne” e si rimette in ascolto

   • if CPU gets an interrupt and keeps to be interrupted, this leads to a livelock: a state where the system is perfectly able to work but it cannot do any work

   • deadlock: process waits for some resource to do the work

Interrupt would occur each 6ns. It is not sustainable for the cpu to work in this way, so, some possible alternatives:

1. interrupt coalescing (mix): generate an interrupt every n packets
2. polling: disable interrupts all together, use CPU polling to check for new arrivals
3. hybrid: a mix of these two
   • interrupts → polling → interrupts
   • there is a threshold when the rate exceed then switch to polling, then to interrupts

Advantages

• lower cpu utilization
• higher throughput

Disadvantages

• higher latency

3. build a packet with multiple protocols and headers