# Homa: A Receiver-Driven Low-Latency Transport Protocol Using Network Priorities

### Abstract 

* Homa: new transport protocol for DC networks 
* Provides low latency, especially for workloads with a high volume of very short messages, and it also supports large messages and high network utilization 

### Introduction 

* Existing transport protocols are ill-suited to these conditions, so the latency they provide for short messages is far higher than the hardware potential, particularly under high network loads 
* Homo 
  * Favors small messages
  * But also improves the performance of large messages in comparison to TCP-like approaches based on fair sharing 
* Two innovations 
  * Aggressive use of priority queues provided by modern network switches 
    * Assigns priorities dynamically on receivers, and it integrates the priorities with a receiver-driven flow control mechanism 
  * Controller over-commitment: a receiver allows a few senders to transmit simultaneously 
    * Utilizing the bandwidth efficiently 
* Unusual features 
  * Message-based architecture rather than a streaming approach
    * Eliminates head-of-line blocking at senders and reduces tail latency 
  * Connectionless 
  * No explicit ack 
  * At-least-once semantics 
* Metric: tail message latency (99th percentile): most important metric for datacenter application 

### Motivation and Key ideas 

Motivation 

* NIC and software stack advances 
* All prior work has focused on workloads with very large messages 
* Existing DC transport design cannot achieve the lowest possible latency for tiny messages at high network load 

Design space 

* Key design principles 
  * Transmit short messages blindly 
  * Using in-network priorities 
  * Allocate priorities dynamically at receivers in conjunction with receiver-driven rate control
  * Controlled over-commitments of receiver downlinks 
* Focus on message latency (instead of packet latency), since it reflects application performance 
* Key
  * There is no time to schedule every packet
    * Short messages are transmitted blindly, without considering potential congestion 
  * Buffering is a necessary evil 
    * Latency penalty of buffering 
  * In-network priorities are a must 
    * Assign packet priorities so that short messages bypass queued packets for longer messages 
  * Making best use of limited priorities requires receiver control 
    * In order to produce the best approximation of SRPT with only a small number of priority levels 
  * Receivers must allocate priorities dynamically
    * Two mechanisms: 
      * For messages larger than RTTbytes, the receiver communicates a priority for each packet to its sender dynamically based on the exact set of inbound messages 
      * For short messages that sent blindly, the sender cannot know about other messages inbound for the receiver 
  * Receivers must overcommit their downlink in a controlled manner 
    * Intentionally overcimmit their downlinks by granting simultaneously to a small number of senders 
  * Senders need SRPT also