[c-nsp] Cisco IOS ping utility reports lower RTT than possible

[Martin T]

Hi Octavio,

instead of a two-card laptop I used the available ports in server
named "svr", but in principle I built the setup you described:

CISCO1921[Gi0/0] <-> [eno1]test-br[eno2] <-> [eno3]svr

"test-br" is a two-port Linux bridge in "svr" server. Now when I
execute "ping 10.66.66.1 source 10.66.66.2" in Cisco router, then the
results are following:

CISCO1921#ping 10.66.66.1 source 10.66.66.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.66.66.1, timeout is 2 seconds:
Packet sent with a source address of 10.66.66.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/9/12 ms
CISCO1921#

As seen above, minimum measurement was 8ms and average was 9ms. On the
other hand, packet capture done on eno2 clearly shows that each ICMP
"echo request" message receives a ICMP "echo reply" with >10ms delay
as it should:

svr$ sudo tcpdump -ttt -nei eno2 icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eno2, link-type EN10MB (Ethernet), capture size 262144 bytes
 00:00:00.000000 50:0f:80:a5:03:a0 > 3c:a8:2a:1e:f3:da, ethertype IPv4
(0x0800), length 114: 10.66.66.2 > 10.66.66.1: ICMP echo request, id
60632, seq 0, length 80
 00:00:00.010201 3c:a8:2a:1e:f3:da > 50:0f:80:a5:03:a0, ethertype IPv4
(0x0800), length 114: 10.66.66.1 > 10.66.66.2: ICMP echo reply, id
60632, seq 0, length 80
 00:00:00.000431 50:0f:80:a5:03:a0 > 3c:a8:2a:1e:f3:da, ethertype IPv4
(0x0800), length 114: 10.66.66.2 > 10.66.66.1: ICMP echo request, id
60632, seq 1, length 80
 00:00:00.010165 3c:a8:2a:1e:f3:da > 50:0f:80:a5:03:a0, ethertype IPv4
(0x0800), length 114: 10.66.66.1 > 10.66.66.2: ICMP echo reply, id
60632, seq 1, length 80
 00:00:00.000382 50:0f:80:a5:03:a0 > 3c:a8:2a:1e:f3:da, ethertype IPv4
(0x0800), length 114: 10.66.66.2 > 10.66.66.1: ICMP echo request, id
60632, seq 2, length 80
 00:00:00.010199 3c:a8:2a:1e:f3:da > 50:0f:80:a5:03:a0, ethertype IPv4
(0x0800), length 114: 10.66.66.1 > 10.66.66.2: ICMP echo reply, id
60632, seq 2, length 80
 00:00:00.000328 50:0f:80:a5:03:a0 > 3c:a8:2a:1e:f3:da, ethertype IPv4
(0x0800), length 114: 10.66.66.2 > 10.66.66.1: ICMP echo request, id
60632, seq 3, length 80
 00:00:00.010184 3c:a8:2a:1e:f3:da > 50:0f:80:a5:03:a0, ethertype IPv4
(0x0800), length 114: 10.66.66.1 > 10.66.66.2: ICMP echo reply, id
60632, seq 3, length 80
 00:00:00.000332 50:0f:80:a5:03:a0 > 3c:a8:2a:1e:f3:da, ethertype IPv4
(0x0800), length 114: 10.66.66.2 > 10.66.66.1: ICMP echo request, id
60632, seq 4, length 80
 00:00:00.010136 3c:a8:2a:1e:f3:da > 50:0f:80:a5:03:a0, ethertype IPv4
(0x0800), length 114: 10.66.66.1 > 10.66.66.2: ICMP echo reply, id
60632, seq 4, length 80

Cisco IOS ping command inserts the timestamp into the payload of the
ICMP "echo request" message and at least it seems to increment it, i.e
that part seems to be fine.


Martin