"reply ERR" tcpdump output means what (TCP mounts over WAN)

I'm mounting a v3 export from a 2.4.22-rc2 server to a 2.4.22-pre4 client, over a WAN connection (it's over an IPSEC tunnel, 1.5Mbits server -> client and 128Kbits client -> server, 90ms round trip). I simply mounted -o rw,tcp. I used a reliable transport because I thought it would be more approporate over a link like this.

It works fine, but it's REALLY slow.

Here's some output from loading a Maildir-format spool (the below is from the server)

17:22:31.910279 10.20.8.30.2049 > 10.0.0.8.1698577506: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:32.029639 10.20.8.30.2049 > 10.0.0.8.544434554: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:32.159093 10.20.8.30.2049 > 10.0.0.8.2848670243: reply ok 236 (DF) [tos 0x2,ECT] 17:22:32.260640 10.20.8.30.2049 > 10.0.0.8.2865447459: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:32.260679 10.20.8.30.2049 > 10.0.0.8.1026770003: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:32.370613 10.20.8.30.2049 > 10.0.0.8.1634623304: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:32.370638 10.20.8.30.2049 > 10.0.0.8.2002875168: reply ERR 68 (DF) [tos 0x2,ECT] 17:22:32.589138 10.20.8.30.2049 > 10.0.0.8.2899001891: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:32.809437 10.20.8.30.2049 > 10.0.0.8.2932556323: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:32.809484 10.20.8.30.2049 > 10.0.0.8.1733515061: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:32.919132 10.20.8.30.2049 > 10.0.0.8.1229416566: reply ERR 100 (DF) [tos 0x2,ECT] 17:22:33.020512 10.20.8.30.2049 > 10.0.0.8.2949333539: reply ok 736 (DF) [tos 0x2,ECT] 17:22:33.121527 10.20.8.30.2049 > 10.0.0.8.2966110755: reply ok 236 (DF) [tos 0x2,ECT] 17:22:33.218590 10.20.8.30.2049 > 10.0.0.8.2982887971: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:33.218627 10.20.8.30.2049 > 10.0.0.8.1932422757: reply ERR 816 (DF) [tos 0x2,ECT] 17:22:33.342968 10.20.8.30.2049 > 10.0.0.8.2999665187: reply ok 236 (DF) [tos 0x2,ECT] 17:22:33.437201 10.20.8.30.2049 > 10.0.0.8.3016442403: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:33.437239 10.20.8.30.2049 > 10.0.0.8.812402511: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.547541 10.20.8.30.2049 > 10.0.0.8.896350316: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.547566 10.20.8.30.2049 > 10.0.0.8.1112298345: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.659641 10.20.8.30.2049 > 10.0.0.8.927545443: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.698638 10.20.8.30.2049 > 10.0.0.8.943937290: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.707383 10.20.8.30.nfs > 10.0.0.8.798: . ack 1967536866 win 22512 <nop,nop,timestamp 244760041 7925237> (DF) 17:22:33.718735 10.20.8.30.nfs > 10.0.0.8.798: . ack 129 win 22512 <nop,nop,timestamp 244760042 7925237> (DF) 17:22:33.769462 10.20.8.30.2049 > 10.0.0.8.1111057268: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.769492 10.20.8.30.2049 > 10.0.0.8.894593401: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.818440 10.20.8.30.2049 > 10.0.0.8.1295467855: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.881110 10.20.8.30.2049 > 10.0.0.8.1647340856: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.919503 10.20.8.30.2049 > 10.0.0.8.946622820: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.965985 10.20.8.30.2049 > 10.0.0.8.1314288944: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:33.990356 10.20.8.30.2049 > 10.0.0.8.1448767600: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:34.029968 10.20.8.30.2049 > 10.0.0.8.794908245: reply ERR 1376 (DF) [tos 0x2,ECT] 17:22:34.102846 10.20.8.30.2049 > 10.0.0.8.1181575987: reply ERR 608 (DF) [tos 0x2,ECT] 17:22:34.217498 10.20.8.30.nfs > 10.0.0.8.798: . ack 545 win 22512 <nop,nop,timestamp 244760092 7925288> (DF) 17:22:34.224557 10.20.8.30.2049 > 10.0.0.8.3117105699: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:34.442064 10.20.8.30.2049 > 10.0.0.8.3150660131: reply ok 1376 (DF) [tos 0x2,ECT] 17:22:34.442103 10.20.8.30.2049 > 10.0.0.8.1935897193: reply ERR 1376 (DF) [tos 0x2,ECT]

what are these ERRs and how to avoid them? I assume they're not normal?

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