Comment by MichaelGG | Hacker News Reader

Comment by MichaelGG | Hacker News Reader

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SIP signalling over UDP was a bad idea, especially with the mandatory TCP failover. SIP's actually a spectacular clusterfuck of a design overall anyways. It's actually impossible to implement SIP in an unambiguous manner on today's networks because even the parsing rules are so bad, that multiple, popular, stacks have incompatible ways of dealing with things.

UDP for RTP makes sense because transmitting packets is pointless. In case of packetloss, the end user has to interpolate the result either via software or in their head.

This private MPLS network... it wouldn't happen to run over the exact same equipment that'd be handling your IP traffic, would it?

Just a small correction for anyone else following along. This:

> UDP for RTP makes sense because transmitting packets is pointless

Should be this:

> UDP for RTP makes sense because retransmitting packets is pointless

TCP will retransmit if no ACK is received; UDP won't (there being no ACK and all).

At the same time, beware of "helpful" lower layer protocols.

Mobile wireless stuff (EDGE/3G) is going to great lengths to avoid dropping packets, so if you get conditions right (moving train in the areas with poor coverage is one place where you can easily see this), you can get the packets "reliably delivered" in 20 seconds and more.

This really bugs me. It seems like they should be leaving the retransmission to TCP.

Well, yes and no.

TCP has been designed in such a way that it interprets packet drops as a sign of "congestion" (which was typically true in ye olden days of purely wired networking), and it will start sending less data in response.

Whereas in wireless networking, occasional packet drops are just a fact of life and are not indicative of competing flows trying to share the channel. So it actually makes [some] sense that wireless protocols try to compensate for the behaviour of the transport protocol used by 90% of all data: TCP.

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