[e2e] Is a non-TCP solution dead?

[Hari Balakrishnan]

David,

I didn't say anything about how to run wireless (particularly multi-hop) 
networks at close to available capacity.  I do believe that integrated 
approaches are needed for that to work well.

My comments were restricted to dealing with wireless bit errors induced by 
corruption.  Period.  I don't think having TCP deal with such link errors is a 
useful optimization.

I do agree that TCP doesn't do a great job of estimating available capacity in 
many kinds of wireless networks, for some of the reasons you mention, and also 
because wireless capacity in many networks is tied to the details of the MAC 
protocol and we don't deal well with that kind of thing in TCP.  Also, I do 
think one can integrate wireless routing better with link characteristics.

My comments were not about general partitioning of functionality in wireless 
systems; they were about who should deal with bit corruption.  (I have been 
quite guilty of exploring end-to-end approaches to traditional lower-layer 
problems, including routing.)

Hari

> At 11:47 AM 3/31/2003 -0800, Mark Handley wrote:
> >This raises a higher level issue: to what extent is a wireless link
> >error a sign of congestion?
> 
> Relating this to my other mail:  in the RF medium, any number of 
> communications can happen simultaneously.   Congestion of a sort (too many 
> signals impinging on a single receiver antenna) results in a link error, 
> but this depends significantly on the systems design of the receiver and 
> its assumptions about the kinds of signals that may concurrently be 
> transmitted.
> 
> There are many more dimensions of adaptation in the RF medium to adapt to 
> variations in end-to-end bitrate demand.  These are extraordinarily 
> interesting dimensions, far more so than mere rate control.  Simple 
> examples include power control, rate adaptation, frequency adaptation, 
> antenna steering, reconfiguration of network repeaters into new topologies, 
> etc.   Many of these are not mere "link level" adaptations, but can involve 
> the whole wireless network topology.
> 
> Given that the entire capacity of a wireless system varies tremendously 
> depending on demand, one cannot layer wireless networks in the traditional 
> way by assuming a set of independent fixed capacities at the physical level 
> and then managing capacity above that layer as if the networks consist of 
> fixed capacity links.
> 
> Thus Hari Balakrishnan's comment implying that partitioning wireless 
> networks into a link layer and an end-to-end layer are misleading.   I 
> personally suspect that end-to-end approaches are far more important for 
> managing latency, throughput, etc. than he'd suggest.   But those 
> end-to-end approaches will need to be worked out in a context that includes 
> a better abstraction of the tradeoffs than has been traditionally used in 
> modeling wired networks. 
>