Subject: RTTs and cellular network optimizations

From: Marcus Ihlar <[email-masked]>
Date: Tue, 14 Mar 2017 16:30:37 +0000

Hi,

In issue 269 a case for packet number echoes is made.

By having packet number echoes visible in the QUIC public header a passive observer can get estimates of RTTs and in-flight packets.

This also allows for active network optimizations that are less intrusive / ossifying than what we typically see today.

Cellular networks have typically been plagued by severe buffer bloat issues due to lack of proper AQM functionality in the RAN.

Many operators tend to deploy Performance Enhancing Proxies (PEP) as a way to mitigate this problem. However, this solution is quite crude and requires transparent termination of connections or manipulation of TCP segments.

One possible optimization technique is a "centralized" AQM function that reacts to increasing RTT:s. Such an AQM is a simplified form of a PEP, the main difference being that the AQM does not terminate or manipulate connections in any way.

We have tested this concept using TCP in a cellular network lab comparing it to Google-QUIC and TCP without AQM.

We also ran the same tests with a TCP PEP. The PEP and AQM were deployed at the Gi (interface point between Internet and cellular core network).

The tests consisted of 10MB file downloads from Google drive using a dedicated server.

The access consisted of a single LTE cell where the signal strength could be attenuated.

These tests showed a significant decrease in buffer induced latency while having small visible impact on throughput.

We also saw that QUIC would greatly benefit from AQM when using a loss-based CCA over LTE.

The centralized AQM had similar effects on buffer-induced latency as the TCP Proxy

See test result details below:

Minimum RTT between server and device was ~50ms, RTT between server and PEP / AQM is ~30ms.

The measurements were performed at the server side except for the PEP case where measurements were made at the proxy. Since the PEP terminates TCP traffic we have added the server-proxy RTT to the PEP results.

Full signal strength - Single user in cell

TCP              - Median RTT: 161ms, p95 RTT: 294ms

QUIC            - Median RTT: 60ms,   p95 RTT: 72ms

TCP + AQM - Median RTT: 88ms,   p95 RTT: 112ms

TCP + PEP   - Median RTT: 65ms,   p95 RTT: 170ms

Full signal strength - Multiple users in cell

TCP              - Median RTT: 429ms, p95 RTT: 900ms

QUIC            - Median RTT: 342ms, p95 RTT: 594ms

TCP + AQM - Median RTT: 91ms,   p95 RTT: 127ms

TCP + PEP   - Median RTT: 75ms,   p95 RTT: 200ms

Weak signal strength - Single user in cell

TCP              - Median RTT: 279ms, p95 RTT: 405ms

QUIC            - Median RTT: 362ms, p95 RTT: 690ms

TCP + AQM - Median RTT: 88ms,   p95 RTT: 117ms

TCP + PEP   - Median RTT: 79ms,   p95 RTT: 207ms

Weak signal strength - Multiple users in cell

TCP              - Median RTT: 413ms, p95 RTT: 990ms

QUIC            - Median RTT: 717ms, p95 RTT: 1051ms

TCP + AQM - Median RTT: 93ms,   p95 RTT: 125ms

TCP + PEP   - Median RTT: 85ms,   p95 RTT: 220ms

//Marcus Ihlar