What is Packet Loss and what does it represent for the network

[Wolfgang Kuehne]

Here I have some very basic questions about packet loss. I would be extremely thankful if someone would take the time to provide answers to each one of them, in a way that would make this thread useful to other beginners as well.

What is packet loss. What phenomena of a network does it represent?

How do you find out the packet loss? I assume you send a packet and wait for its acknowledgement, if the acknowledgement does not arrive the packet is considered to be lost. What if the packet has arrived but the ACK is lost on the way back. How is this handled in usual communication protocols.

Who measures the packet loss? The sending party or the receiving party?

Packet loss is a PHY or MAC layer metric, can it be used for Application Layer as well?

 

[Ogu Reginald]

www.webopedia.com/TERM/P/packet_switching.html

- - - Updated - - -

https://compnetworking.about.com/od/networkprotocols/f/packet-switch.htm

 

[D.A.(Tony)Stewart]

(1)What is packet loss.
(2)What phenomena of a network does it represent?
(3)How do you find out the packet loss?
(4) What if the packet has arrived but the ACK is lost on the way back. (5) How is this handled in usual communication protocols.
(6)Who measures the packet loss? The sending party or the receiving party?
(7) Packet loss is a PHY or MAC layer metric, can it be used for Application Layer as well?

(1) an error in 1 or more bits in a packet, detected by CRC in receiver
(2) a probability usually based on SNR of signal from some interference, or thermal noise.
(3) Receiver or transmitter can measure true error rate only it knows how the network manages error detection/correction/recovery.

Although perceived error rate can be estimated by number of retries, retransmits, selective ACKs, undeciphered messages, wrong addresses etc it is not always accurate unless expected flow , rate and message is predetermined and how error reporting is monitored and how delivery is guaranteed. e.g. TCP vs UDP
(4) same effect. unless every packet number is monitored, counted and reported, by every node, you cant tell. This requires custom monitors at expen$e of overhead.
(5) usual? There are thousands of protocols.
e.g. Streaming video ... Keep sending then restart from last packet number received OK. WHich can stutter badly if there is packet loss and sender is slow to react, but stable in bursts with adequate delay.
(6) usually both tx, rx keep track of errors for resolution by the network engineer. In simple pt-pt networks a loopback test is done. In large asymmetric networks, the Tx sets up the Rx to compare a known pattern such a pseudo-random patterns or otherwise worst-case patterns known to Network Engineer.
(7) yes, when it gets serious or more complex, usually starts by deviations in transfer rate, error-free or latency.

But if you know Windows, netstat -e in command window, it is pretty useless at finding or reporting problems, unless very specific Apps by Network guy due to errors not being reported except ones from obscure sources.