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Does it make a difference to specify packet size during ping tests (RedHat)?

Engineers in my company asked for ping checks with packet size of 2000 bytes (-s 2000). On certain systems, this fails, but if using default settings (no "-s" option), it yielded good results. Those who specified that ping size would not say why this was requested; after a while, the ping size issue was dropped, and ping testing with default settings was used.

My questions:
- Does it make a difference to specify packet size?
- If it does make a difference or helps find network issues, what size is good to specify?
- Where may I find some official references regarding this?

  • Default ping packet size is 56b+28b for headers=84b. According to man ping -c option is for ping count. Did you try -s option instead? – Deeh Mar 21 at 18:26
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    man ping, ss64.com/bash/ping.html and linux.die.net/man/8/ping are authoritative references for ping, – K7AAY Mar 21 at 20:10
  • Are the engineers looking for confirmation on Jumbo frames support? – Atul Mar 22 at 15:41
  • regarding ping option "-c or -s", i made a mistake and typed -c instead of -s, i corrected post. – Dmitry Osinovsky Mar 23 at 12:40
  • regarding "Jumbo frames"; our network is supposed to support 1G traffic but i dont think this request had anything to do with this but i will attempt to find out. i am under impression that when trying to test for Jumbo frames packet size jumps to 9000bytes, correct me if i'm wrong..., also i understand that packet loss chances increase in these cases. is 2000bytes would be considered as "jumbo frame" test... (did not think so). as i understand for Jumbo frame MTU should be increased, on our network it is set to 1500 last time i checked. – Dmitry Osinovsky Mar 23 at 12:48
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Packet size is helpful to troubleshoot the network by saying this I mean that some network packets does not transit if there is MTU mismatch:

Example:

If router1 has mtu configured 1500 (default) and router2 is configured `2000' then router1 will be able to send packets to router2 but router2 will not be able to transit packet to router2

reason:

router2 will send packet size of 2000 which will be dropped as router1 can only accept 1500 of less size of packet.

  • Great reply! Also make sure to set the DF bit. Otherwise the network may appear to be OK, when it's actually having to chop up the ICMP payload into fragments and reassemble them later on. – troglobit Mar 23 at 13:01
  • what is the best way to check for MTU size, is there a way to run a check on whole network and to see if there is a mismatch. "ifconfig" does not show it, i used command "cat /sys/class/net/eth0/mtu" and it does output "1500" as MTU size.... not sure if thats the best way of checking it. – Dmitry Osinovsky Mar 23 at 13:06
  • I think ifconfig should have MTU as per my linux box. [nsp@nsp-1 bin]$ ifconfig | grep mtu eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 – Vivek Kanadiya Mar 25 at 12:25

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