tpcdump
tcpdump can be used to verify ToS, DSCP, PCP, or VLAN ID values.
RDMA traffic requires switch port mirroring or an inline protocol analyzer to capture RDMA traffic.
To find each value in tcpdump:
| Value | tcpdump Option | tcpdump Output |
|---|
| ToS | -v | Run ping with -Q 24 to set ToS=24 (0x18): # ping -I enp175s0f0 -Q 24 192.168.0.3 Use tcpdump with -v and look for tos 0x18 in the IP header: # tcpdump -nXX -v -i enp175s0f0
15:05:53.197406 IP (tos 0x18, ttl 64, id 20320, offset 0, flags [DF], proto ICMP (1), length 84)
192.168.0.1 > 192.168.0.3: ICMP echo request, id 14718, seq 3, length 64
0x0000: 6805 caa3 8778 6805 caa3 8978 0800 4518 h....xh....x..E.
0x0010: 0054 4f60 4000 4001 69dc c0a8 0001 c0a8 .TO`@.@.i.......
0x0020: 0003 0800 d918 397e 0003 312f 585e 0000 ......9~..1/X^..
0x0030: 0000 9a05 0300 0000 0000 1011 1213 1415 ................
0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425 ...........!"#$%
0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435 &'()*+,-./012345
0x0060: 3637 |
| DSCP | -v | DSCP is not shown explicitly in tcpdump. DSCP is based on ToS value. Example: ToS 0x18 maps to DSCP 0x6. - Start with ToS 0x18.
- Convert 0x18 from hex to decimal.
0x18 = 0b0001_1000 - Look at the upper 6 bits of the ToS field to find DSCP.
0x18 = 0b00011000 0b000110 = 0x06 - DSCP = 0x6
|
| VLAN ID and VLAN priority (PCP) | -e vlan | Run ping on VLAN 200 with -Q 24 to set ToS=24 (0x18): # ping -I enp175s0f0.200 -Q 24 192.168.200.3 Run tcpdump with -e vlan and look for vlan 200 and p 4 in the VLAN header: # tcpdump -nXX -v -i enp175s0f0 -e vlan
15:23:37.554911 68:05:ca:a3:89:78 > 68:05:ca:a3:87:78, ethertype 802.1Q (0x8100), length 102: vlan 200, p 4, ethertype IPv4, (tos 0x18, ttl 64, id 53320, offset 0, flags [DF], proto ICMP (1), length 84)
192.168.200.1 > 192.168.200.3: ICMP echo request, id 15739, seq 3, length 64
0x0000: 6805 caa3 8778 6805 caa3 8978 8100 80c8 h....xh....x.... # PCP is 3 bits: 0x8 → 0b1000 → 4
0x0010: 0800 4518 0054 d048 4000 4001 58ea c0a8 ..E..T.H@.@.X...
0x0020: c801 c0a8 c803 0800 82a4 3d7b 0003 5933 ..........={..Y3
0x0030: 585e 0000 0000 bf78 0800 0000 0000 1011 X^.....x........
0x0040: 1213 1415 1617 1819 1a1b 1c1d 1e1f 2021 ...............!
0x0050: 2223 2425 2627 2829 2a2b 2c2d 2e2f 3031 "#$%&'()*+,-./01
0x0060: 3233 3435 3637 |