4/8/2026 at 8:54:51 PM
>Many routers deprioritize or drop ICMP to save CPU.Not exactly.
Most big routers have ASICs (custom silicon) that can handle the bulk of routing decisions, like an interface card will have a chip that can directly determine where a packet needs to go and forwards it there. These are extremely fast, but limited, and are called "fast path".
Aside: Too many ACLs is a common way that packets fall off the fast path, and is why routers on the public Internet will happily forward along bogon traffic that by it's very nature is just wasting bits on the pipes.
There are some things that the fast path cannot handle, and generating ICMP TTL exceeded messages is one of them. Those go over to the router CPU, which historically has been insanely underpowered. Back when I was doing more routing it was common to have host CPUs in the multi-GHz range with multiple cores, but routers of a similar class would have a 100MHz MIPS CPU.
That's why, as the article goes on to explain, "*"s in the traceroute may not indicate a problem. It's not necessarily a literal deprioritization of ICMP.
If you ever see packet loss in a trace at one step but the steps after it aren't showing it, you can ignore that packet loss, it's likely a CPU limitation on a busy router.
by linsomniac
4/8/2026 at 11:49:13 PM
> If you ever see packet loss in a trace at one step but the steps after it aren't showing it, you can ignore that packet loss, it's likely a CPU limitation on a busy router.Trippy now includes [0] forward loss (Floss) and backward loss (Bloss) _heuristics_ to help surface such behaviour.
The idea was inspired by our previous discussion [1] on the topic on HN some time ago!
These columns are experimental and so not shown by default but can be enabled [2].
[0] https://github.com/fujiapple852/trippy/blob/master/RELEASES....
by FujiApple
4/9/2026 at 3:46:55 AM
a blog about traceroute in 2026. imagine that. hard to believe it could possibly be of any interest at all, especially written by someone that only just discovered it. but i'm oh so glad i stopped in, to learn about trippy! it looks amazing.by jiveturkey
4/9/2026 at 5:53:05 AM
Trippy looks so cool. I'm glad I wrote this article, if just to find out about Trippy.by stonecharioteer
4/9/2026 at 3:31:10 AM
So these types of routers are typically running at 100% CPU and - likely - not keeping up with packets that are not in the fast path? Ie the operators really only care about the packets in the fast path, all others can be ignored?Or are there other types of packets in the slow path that do get a delivery guarantee by the router?
I’m curious how these tradeoffs are made.
by stingraycharles
4/9/2026 at 9:47:41 AM
Why is it not on the fast path by now? Creating a single packet that doesn't need to be kept tracked of is a basic thing that shouldn't require that much ASIC space.by charcircuit
4/8/2026 at 11:33:13 PM
I worked on a board (a switch, not a router) with a chip that could saturate all 10 of its GigE ports all day long if you stayed in the fabric. But if you had to pass anything over to the built-in single-core 250 MHz CPU, you were in for a bad time. (Thankfully it also had pins for wiring up your own external CPU.)by _moof
4/9/2026 at 3:44:19 AM
2002 - Estimating Router ICMP Generation Delays[0]2015 - Characterizing ICMP Rate Limitation on Routers[1]
[0] https://pages.cs.wisc.edu/~suman/courses/640/papers/govindan...
by jiveturkey