Is Explicit Congestion Notification usable with UDP? Stephen McQuistin and Colin Perkins

Transcription

1 Is Explicit Congestion Notification usable with UDP? Stephen McQuistin and Colin Perkins

2 Introduction Explicit congestion notification (ECN) ECN with TCP/IP long established: RFC 3168 Extends IP to mark packets, TCP to return the congestion feedback to sender widely implemented Numerous measurement studies; starting to be enabled by default Use of ECN with UDP/IP uncommon: ECN mark IP packets as usual, but needs an additional protocol above UDP to return congestion feedback Use with RTP RFC 6679 Use of UDP as a substrate making ECN support increasingly desirable Goal: determine if ECN is usable with UDP on the public Internet

3 Methodology Measurement study to determine if ECN is usable with UDP traffic Use servers from the NTP pool as measurement targets Global pool of publicly available time servers Round-robin DNS on pool.ntp.org repeated queries discovered 2500 servers NTP runs over UDP implemented an NTP probe that makes an ECN-marked query Many pool hosts also run a web server that redirects to Probe each pool host in turn with: NTP request in unmarked UDP packet NTP request in ECT(0)-marked UDP packet HTTP request using TCP without ECN HTTP request using TCP with ECN negotiation Figure 1: Geographic locations of NTP pool servers from the authors homes, University of Glasgow, Regionand from VMs NTPin Server each Count of the nine regions (N. Virginia, Oregon, N. California, Africa Ireland, Frankfurt, Tokyo, Sydney, Sao Paulo) 22 Asia 190 Australia 68

4 Reachability using ECN with UDP (1) Does use of ECT(0) marks affect reachability of UDP servers? 98% 96% 94% 92% 90% Perkins home McQuistinUGla home wired UGla w'less Cal Fra (a) Percentage of servers reachable by not-ect marked UDP that are also reachable by ECT(0) marked UDP Ire Ore Sao Sin Syd Tok Vir Not significantly on average, 98.97% of servers that are reached by regular UDP are also reachable using ECT(0) marked requests

5 Reachability using ECN with UDP (2) Are servers consistently unreachable using ECT(0) marked requests? Server ID: 0 California Servers always unreachable using ECT(0)-marked requests Servers mostly unreachable using ECT(0)-marked requests Frankfurt (comparable results from other locations) Ireland 0% 2500 Unreachable using ECT(0) Evidence ECT(0) marked traffic is deterministically dropped near, but not necessarily adjacent to, some servers

6 Servers only reachable with ECN A few servers (0.55%) are often, but not always, unreachable by UDP packets sent without ECN: California Server ID: 0 Servers often unreachable without ECN Frankfurt (comparable results from other locations) Ireland 0% 2500 Unreachable without ECN Further investigation required to determine cause

7 Are ECN marks stripped from UDP? 4.3 Modified traceroute to send ECT(0) We marked UDP packets Re also with the NT Encapsulated UDP packet returned with ECN. in ICMP shows whether the ECT(0) the pool th mark was stripped in the network this to reac Results a number of w using ECN requested ( Traceroute from each measurement location to all 2500 NTP hosts:syn with IP level hops, ~1400 ASes able to reac mark, 99.2% of IP hops always pass ECT(0)significantl others (sometimes) set it back to not-ect using UDP Majority (59%) of locations where ECT mark removed appear to be at AS boundaries run a web s is little var that run we

8 Reachability using ECN with TCP Proportion of NTP pool hosts running web servers, and those that will negotiate ECN with TCP: Reachable using TCP, but unwilling to negotiate ECN Reachable using TCP and will negotiate ECN Perkins home McQuistinUGla home wired UGla w'less Cal Fra Ire Ore Sao Sin Syd Tok Vir 53% of the servers tested are reachable with TCP 82% of those are willing to negotiate and use ECN

9 Trends in TCP ECN Capability 100 Negotiated ECN (%) measured (Trammell) 20 (Kuhlewind) (Kuhlewind) (Bauer) 0 (Medina) (Medina) (Langley) Studies used differing methodology, so perhaps not directly comparable

10 Summary For the servers tested, enabling ECN with UDP had a small (~1%) negative impact of reachability UDP packets with ECN marks successfully traverse network core in >99% cases In remaining cases, ECN mark generally removed at AS boundary over zealous attempt to zero ToS bits? Fraction of hosts willing to negotiate ECN with TCP increasing, in line with previous studies