undefined | Better HN

0 pointsstingraycharles5y ago0 comments

Aren’t MTU issues typically only up to a router? As in, even if the parent had a different MTU than Netflix uses, it wouldn’t matter since their router or the ISP’s router will transform packets between their appropriate MTUs?

And if this is true, then how could it be that Amazon works without problem and Netflix doesn’t?

0 comments

tyingq5y ago

"how could it be that Amazon works without problem and Netflix doesn’t"

Supporting Path MTU discovery (PMTUD), or perhaps just capping their outbound packets to 1450 or similar. Cloudflare found and fixed a problem in this space: https://blog.cloudflare.com/path-mtu-discovery-in-practice/

stingraycharlesOP5y ago

Oh wow, TIL about the “don’t fragment” bit and all the stuff that comes with it.

Thanks for sharing, I learned a lot from that blog post.

alaties5y ago

It's not unusual for a server to also be a router in a layer 3 link aggregation setup. It's extremely common for IPs to be load-shared amongst servers using ECMP. If each server is connected to 2 Top-of-rack (TOR) switches and advertises the route to the shared IP through both TORs, you can very easily have ICMP probes used for PMTU take the wrong route and be dropped. The result is a TCP session with a default MTU that may not work along all traversed paths and will suffer from fragmentation.

j / k navigate · click thread line to collapse