1. IP v6 fragmentation is limited from 1280 bytes up to 1500 bytes. Any
higher than that is optional to implement, and may not work.
2. What kind of UDP applications use such big packets, over 1280 bytes ?
On Feb 7, 2016 5:07 PM, "Yoav Nir" <ynir(_dot_)ietf(_at_)gmail(_dot_)com> wrote:
On 7 Feb 2016, at 2:47 PM, Alexey Eromenko <al4321(_at_)gmail(_dot_)com>
wrote:
Hi All,
I'm re-evaluating TCP/IP stack again with my ongoing IP-FF research.
My question: Is packet fragmentation at IP layer even needed ?
Short answer: yes.
Basically here are few possibilities:
1. Fragmentation-and-reassembly at every hop. (I don't know if anybody
implements it)
This is the Internet. Nobody controls every hop. There can be devices who
do this, but it does not force other devices on the path to do this. This
would be slower than passing the fragments as they are, so routers
typically don’t do this unless they’re modifying the packet. Our VPN
gateway may or may not re-assemble and re-fragment when doing IPsec for
IPv4, depending on configuration.
2. IPv4 style-fragmentation -- fragmentation per every hop, reassembly
at destination end.
Definitely implemented widely.
3. IPv6-style-fragmentation -- fragmentation only at source end,
reassembly at destination end.
Definitely implemented by everyone who wants to be compliant with IPv6.
4. No fragmentation at all (the advantage here: faster Router processing
vs #1 or #2 and less implementation bugs); Assuming standard packet size is
defined at 1280 bytes, like in IPv6
5. MTU path discovery via ICMP -- RFC-1981
6. MTU path discovery via TCP (or other Transport) -- RFC-4821 (or other
way)
And what if you’re using anything other than TCP/SCTP? If your
application is UDP and it is sending a 3000-byte payload you have to do the
fragmentation at some layer.
I'm leaning towards 4 + 6 solution in my own protocol, IP-FF.
What do you think ?
Should IP layer provide fragmentation ?
Some layer must, and assuming backwards compatibility with existing
transport and higher layers is required, I don’t see how an IP layer
without fragmentation would work.
Yoav