Small doubt regarding ICMP

[chandra_rc16]

What does the fouth line mean exactly ?

1. The packet's destination IP address matches the device's IP address or the broadcast address. The device de-encapsulates the packet.

2. The packet is an ICMP packet. The ICMP process processes it.

3. The ICMP process received an Echo Reply message.

4. The ICMP process has not sent an ICMP message with this identification recently. It drops the message.

Thanks in advance.

Regards,
Chandu       

[Jon Marshall]

Chandu

It's a little diffcult to say without seeing more details about the question but from what you have supplied it looks like you are referring to a device that has sent a ping to another device and now has received a reply from the device ie. echo reply.

What the last line is saying is that if a device receives an echo reply and it has not sent out an echo then it drops the packet. In more general terms -

if you receive a response to a ping but you haven't sent out a ping request in the first place then you should drop the packet.

Jon

[chandra_rc16]

Jon,

Actually i was doing simulation on PT. So the topology is same as in my previous thread of ICMP.

So i pinged WS1 from WS2. Then WS2 generated an ICMP packet ...however WS1 has received it & replied to the WS2.

What i did was before the reply from WS1 reach the WS2 i aborted the ping on WS2. Then after the packet reached WS2 and it got dropped... then i opened that frame there i saw the above 4lines.

Regards,
Chandu

[Jon Marshall]

Chandu

ICMP echo/echo replies are matched to each other by -

1) sending the same data back in the datagram as was sent

2) using sequence numbers to match the echo requests against the eho replies.

If you stopped the ICMP process on the WS2 before the echo reply was received then WS2 simply sees it as an echo reply packet but it has nothing to match it against because you stopped the ICMP process from running.

Jon

[chandra_rc16]

Ok, so WS2 thought it doen't send any packet and ignored that reply, right ?

Regards,
Chandu

[Jon Marshall · Accepted solution]

Chandu

Once you stopped the ICMP process on WS2 it then just sees the echo reply from WS1 as a single packet ie. it has not kept track of the sequence number etc. because you stopped the process.

It has no way of knowing this is a response to an actual echo request it sent out. So it drops it.

Jon

[John Blakley]

Chandra,

For me at least, this is a very difficult question since we're not seeing the context from where it was pulled from. What is this in regards to? My first thought is that if a device receives an icmp reply, but the device didn't send an icmp echo, the icmp process should drop it.

HTH,
John

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[redwhitejomonkey]

had the same issue a while back, when implemented PAT. I've double and triple check the routing table and that was correct on both wan Router, but when I implemented PAT the same Source IP public Ip and Dest IP was the same but with different source port. Same error, " The ICMP process has not sent an ICMP message with this identification recently. It drops the message".

R1 WAN IP: 200.13.20.62/26

R2 WAN IP: 200.13.20.1/26

After changing the starting IP NAT POOL, the issue was corrected and i was able to successfully ping on both ends. 

probably unrelated to issue