programing practices : how to choose a packet size for UDP datagrams?

4
votes

Disclaimer : this is not a "how to" question. I would more like to know, as a background information what are the different actual practices actually used.

We know that UDP does not have a PMTU discovery like TCP. So I see several approaches to avoid IP fragmentation with UDP :

  • Sending 512 bytes packets max (the UDP approach)
  • re-implementing some PMTU  (using ICMP "needs fragmentation" message).
  • Relying on local MTU (but how far is it reliable, as UDP isn't a connected protocol, how can it knows which interface its packets are going to go through ?)
  • others... ?

So what I would like is to have a "background" idea of which approaches are being used by current UDP programs/protocols, especially regarding streaming/VoIP common applications ?

Thanks by advance,

Jocelyn

1
network-programmingudpnetwork-protocols

1 Answers

4
votes

Limiting to 576 bytes is very common. Most of the Internet protocols such as DNS do this. Most real-time streaming protocols also use smaller packets since it has the added benefit of providing lower serialization delay and less impact if a single packet is lost.

Some protocols have ways of negotiating a larger packet size, though often not in a way that's as robust as PMTU discovery (DHCP, for example allows a maximum message size negotiation).

There's also stuff that defaults to 1500 or so and lets the user lower it if necessary. Most implementations of SNMP seem to do something like this.

In any event, the DF bit isn't generally set, so the consequence of being overly optimistic is fragmentation, not brokenness.