Is there any way that we can track TCP session hop by hop?
Say we have 10 ECMP between A and Z point, what's the easiest way to track
specific session is using which path? How we can check between
servers(Linux/Unix) and between Routers(Cisco/Juniper etc)?
Somebody needs to renew their Let's Encrypt SSL cert.
A TCP connection is uniquely identified by the combination of four numbers:
The source IP address, the source port, the destination IP address and the
destination port. You used the word session, but sessions happen above TCP
in the stack and may use more than one TCP connection. Every packet in the
connection contains all four numbers and no packet from any other
connection contains the same four numbers.
If you want to track the connections, you capture the packets at each point
in the path (router products have vendor-specific ways of doing this) and
see which unique sets of the four numbers went through which router and
If you want to -test- which path a TCP connection -would- take, Ruairi's
afore-mentioned tcptraceroute is the way to go. The regular traceroute with
modern Linux servers also supports the "-T" flag which does the same thing.
It works just like regular traceroute but uses synthetic TCP SYN packets
instead of ICMP or UDP packets, allowing the packets to pass firewalls
which would otherwise block the trace.
Bear in mind that in each case you will likely only see the path taken at
the IP level. Underlying transits at the Ethernet or MPLS level are
intentionally invisible to the endpoints.
Thank you all for the reply!
I think traceroute/tcptraceroute is a good way to track tcp session as we
can use same 5 tuple as normal TCP does.
Bill brought up an interesting point about MPLS and Ethernet, I give it a
bit of think and here's what i can tell, please correct me if i'm wrong
for MPLS, everything should be the same prior enter MPLS cloud. At ingress
router, it will push MPLS label (also entropy label if enabled), but it
should be the same for traceroute traffic and actual TCP traffic(we have
same 4 tuple, or 5, including incoming interface on router), so the
label/entropy label should be same. Inside MPLS cloud, normally router will
use mpls label, src, dst ip, port number(or entropy label if enabled) as
hash seed(depends on configuration) to calculate which ECMP path it will
use. Choose member link inside lAG might be another story for non-entropy
enabled MPLS cloud, but we don't really care as they belong to same
IP(layer-3) path, but I think they should be same as well?
In the data center context, enabling sFlow continuously captures packets
from all paths and can be used to trace multi-path packet flows, whether
layer 2 (MLAG/LAG), or layer 3 (ECMP). sFlow reports physical switch ports
and captures Ethernet packet headers, so you can relate paths to MPLS
labels, Ethernet headers, IP headers, TCP/UDP headers, VxLAN tunnels, etc.
The following article provides an example: