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May 2026

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Data compression on Snowflake rendezvous messages
by David Fifield 26 May '26

26 May '26

I did some experiments to evaluate the potential of compressing Snowflake rendezvous messages. This is looking forward to a DNS-based rendezvous, where the size of messages is important. I patched snowflake-client and the standalone proxy to record offer/answer rendezvous messages, then measured the sizes of the messages before and after compression. Compressing rendezvous messages straightforwardly with zlib reduces the median size of offers by 50% and of answers by 60%. Adding a preset dictionary containing frequenly occurring strings saves another 20% in offers and 15% in answers. Overall it's possible to reduce the median size of messages by about 75%. Compression also makes the distribution of sizes narrower. type=offer treatment=uncompressed Min. 1st Qu. Median Mean 3rd Qu. Max. 630 882 1181 1251 1458 4533 type=offer treatment=zlib Min. 1st Qu. Median Mean 3rd Qu. Max. 433 527 560 563 599 937 type=offer treatment=zlib+dict Min. 1st Qu. Median Mean 3rd Qu. Max. 232.0 276.0 311.0 316.3 340.0 652.0 type=answer treatment=uncompressed Min. 1st Qu. Median Mean 3rd Qu. Max. 758 1519 1646 1522 1800 1880 type=answer treatment=zlib Min. 1st Qu. Median Mean 3rd Qu. Max. 450.0 594.2 608.5 583.3 617.0 665.0 type=answer treatment=zlib+dict Min. 1st Qu. Median Mean 3rd Qu. Max. 265.0 370.0 381.5 363.4 394.5 416.0 A zlib preset dictionary lets you start compression with a non-empty context window. The compressor and decompressor need to agree on the contents of the dictionary. I made a dictionary of about 700 bytes by inspection of commonly appearing strings in my sample of messages. The byte after "1.0" is a newline character. I don't claim this dictionary is optimal, but its effect is pretty good. 1.0.{"offer":"{\"type\":\"offer\"{"answer":"{\"type\":\"answer\",\"sdp\" :\"v=0\\r\\no=- IN IP4 127.0.0.1\\r\\ns=-\\r\\nt=0 0\\r\\nc=IN IP4 0.0.0 .0m=application 9 UDP/DTLS/SCTP webrtc-datachannela=sendrecva=setup:acti vepassivea=mid:0a=group:BUNDLE 0a=fingerprint:sha-256 a=extmap-allow-mix eda=candidate:3263612507 1694498815 udp 2130706431 typ host tcptype acti ve generation 0 network-id 1 network-cost 10 ufrag typ srflx raddr 0.0.0 .0 :: rport a=ice-ufrag:a=ice-pwd:a=ice-options:tricklea=end-of-candidat esa=sctp-port:5000a=max-message-size:1073741823a=msid-semantic:WMS *\\r\ \n\"}"},"nat":"unknownrestricted","fingerprint":"2B280B23E1107BB62ABFC40 DDCC8824814F80A72"}8838024498816A039FCBBAB14E6F40A0843051FA"} This is the source code, with the patch and analysis programs: https://gitlab.torproject.org/dcf/snowflake-rendezvous-zlib Currently at this commit: https://gitlab.torproject.org/dcf/snowflake-rendezvous-zlib/-/tree/6da5f2b2…

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