Traceroute – Inside Per Packet Load Balancing Infrastructure

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Could you confirm that in an infrastructure configured with "per packet" load balancing on the Routers (same route same metric), the usage of traceroute become unusable ? In this case the infrastucture should delivered traceroute results like random results for the same destination host ?

Best Answer

There is no way to trace the route of a packet in typical IP infrastructure. There was an experimental extension for it but it was never widely implemented.

So what traceroute actually does it send out probes with increasing TTL and look for the time-exceeded messages.

So in a per-packet load balanced infrstructure traceroute will be able to tell you how far along the path(s) each router it finds is but it cannot tell whether router(s) it finds at hop "n" and router(s) it finds at hop "n+1" are on the same path or different paths.

per-packet load balancing is rarely used through, most load balancing is done on a basis of "flows" because re-ordering packets within a flow often leads to poor performance. The flow is characterised by some combination of header fields, often source/destination IPs and source/destination ports.

In a per flow load balanced environment it is possible to get useful traceroutes but you have to be careful what traceroute implementation you use. To get a self-consistent trace the traceroute implementation must ensure that the relavent header fields are kept consistent such that all packets are seen as part of the same flow.

I recall a presentation (think it was a uknof video) which mentioned a traceroute monitoring tool that had the ability to trace with consistent source ports across multiple traces and report which paths had problems. It looks like the video was https://www.youtube.com/watch?v=jqaiXtBF4ug and the tool was "fbtracert"

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