Re: [RRG] The use of UDP in LISP

[Dino Farinacci <dino@cisco.com>]

> <recap>
>
> Basically, you need a UDP header so that boxes that sit between the  
> ITR and ETR that don't know about LISP will be able to do load  
> balancing of data streams towards an ETR. However, you're really not  
> looking to calculate a checksum over the resulting UDP packet. In  
> IPv4 UDP checksums are optional, but not in IPv6.

To be general, any tunneling mechanism looks to core routers as a  
single flow. And if they can't look deeper into the packet, they  
cannot load-split efficiently on LAGs.

The checksum is orthogonal to the discussion, but I guess if you are  
stating that UDP is an advantage to solve LAGs but is a disadvantage  
because of requiring checksum computation for IPv6, then I agree.

> When a router (switch?) has multiple links that are equal cost paths  
> toward a certain destination, you can generally let the router load  
> balance across these links, but to avoid packet reordering, the  
> router creates a hash of at least the destination IP address  
> (possibly also the source IP address) and the source/destination  
> port numbers if the packet is TCP or UDP and sends packets with the  
> same hash over the same output port. If the packet isn't TCP or UDP  
> the router can only look at the destination address and possibly the  
> source address so load balancing becomes a lot coarser to the point  
> of possibly not working to a usable degree anymore.

Right.

> I think this can be solved by giving each ETR a small range of  
> addresses rather than a single one. For instance, if the mapping  
> system tells the ITR that an ETR has 240.0.0.5/29 then the ITR could  
> send one flow towards 240.0.0.0, another flow to 240.0.0.1 and so on.

In popular routers, that would need to be 16. That is too much  
administration and a wastage of address space. There is nothing wrong  
with UDP encapsulation.

> This way, the checksum issue goes away and we also get to save 8  
> bytes per packet.

Sounds like a really bad tradeoff to me.

Dino


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