Re: Is Fragmentation at IP layer even needed ?

[Yoav Nir <[e-mail]>]

> On 7 Feb 2016, at 2:47 PM, Alexey Eromenko <[e-mail]> 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