1. No way to detect whole RPC loss. Since there is no outer connection state, if every packet in the send-side of a RPC is lost then there is no way for a server to detect that it should issue a resend. The RPC is just lost to the ether. This affects small messages, like messages that fit in a single packet, more since there are fewer packets in the send-side.
2. Related to the above, there is no builtin encryption support. So, if you want encryption then you need to layer it either above or below.
3. Benchmarked performance is awful. The 60 kB average message case in [2] Table 4 takes 5(!) hyperthreads to average 20 Gbit/s. That is just 4 Gbit/s per hyperthread. Even a totally naive one-packet per system call network protocol design and implementation should get to ~8 Gbit/s per hyperthread. 30 Gbit/s per hyperthread is easy with just a little focus on performance.
4. Despite all the performance design problems in QUIC (though still faster than Homa) it already solves basically every problem Homa is trying to solve in a much cleaner way. Stream IDs correspond to RPC IDs. Stream Max corresponds to Grants. Multiple streams under single Client allows prioritization.
Except you do not randomly lose entire messages. You can compact small messages into packets. You get more precise RTT time allowing more accurate pacing/congestion calculations. You get builtin encryption. It survives ossified middleboxs. It has multiple ack frames/packets reducing ack overhead.
The only real difference is that Homa uses explicit receiver Resend instead of implicit sender Resend. Except that actually consumes significantly more receiver resources in non-trivial loss scenarios especially due to Resend packets only supporting a single Resend span. It also incurs higher latency and has higher requirements on the entire lossy network path due to all the extra transiting data that you do not want to lose.
And those are just serious problems off the top of my head after reloading the RFC into my head. I can come up with some more if needed.
Compared to TCP or Homa at a protocol design level? Not really.
However, software QUIC implementations are generally much slower than software TCP implementations. This is not due to any fundamental protocol design limitations, it is just that most/every QUIC implementation is poorly implemented for maximizing performance.
You can, of course, do multiple times more throughput than QUIC or TCP with a protocol better designed for performance, but that is likely orthogonal to your question.
Veserv · · focus · HN ↗
1. No way to detect whole RPC loss. Since there is no outer connection state, if every packet in the send-side of a RPC is lost then there is no way for a server to detect that it should issue a resend. The RPC is just lost to the ether. This affects small messages, like messages that fit in a single packet, more since there are fewer packets in the send-side.
2. Related to the above, there is no builtin encryption support. So, if you want encryption then you need to layer it either above or below.
3. Benchmarked performance is awful. The 60 kB average message case in [2] Table 4 takes 5(!) hyperthreads to average 20 Gbit/s. That is just 4 Gbit/s per hyperthread. Even a totally naive one-packet per system call network protocol design and implementation should get to ~8 Gbit/s per hyperthread. 30 Gbit/s per hyperthread is easy with just a little focus on performance.
4. Despite all the performance design problems in QUIC (though still faster than Homa) it already solves basically every problem Homa is trying to solve in a much cleaner way. Stream IDs correspond to RPC IDs. Stream Max corresponds to Grants. Multiple streams under single Client allows prioritization.
Except you do not randomly lose entire messages. You can compact small messages into packets. You get more precise RTT time allowing more accurate pacing/congestion calculations. You get builtin encryption. It survives ossified middleboxs. It has multiple ack frames/packets reducing ack overhead.
The only real difference is that Homa uses explicit receiver Resend instead of implicit sender Resend. Except that actually consumes significantly more receiver resources in non-trivial loss scenarios especially due to Resend packets only supporting a single Resend span. It also incurs higher latency and has higher requirements on the entire lossy network path due to all the extra transiting data that you do not want to lose.
And those are just serious problems off the top of my head after reloading the RFC into my head. I can come up with some more if needed.
[1] <a href="https://github.com/johnousterhout/homa-rfc/blob/main/draft-ousterhout-tsvwg-homa-00.md" rel="nofollow">https://github.com/johnousterhout/homa-rfc/blob/main/draft-o...
[2] <a href="https://www.usenix.org/system/files/atc21-ousterhout.pdf" rel="nofollow">https://www.usenix.org/system/files/atc21-ousterhout.pdf
KerrAvon · · focus · HN ↗
Veserv · · focus · HN ↗
However, software QUIC implementations are generally much slower than software TCP implementations. This is not due to any fundamental protocol design limitations, it is just that most/every QUIC implementation is poorly implemented for maximizing performance.
You can, of course, do multiple times more throughput than QUIC or TCP with a protocol better designed for performance, but that is likely orthogonal to your question.