Anycast 101

Fri Dec 17 14:27:25 UTC 2004

Iljitsch van Beijnum wrote:

>
> On 17-dec-04, at 3:06, Steve Gibbard wrote:
>
>>> under some very
>>> specific circumstances it can also happen with per packet load
>>> balancing.
>>
>
>> You're misunderstanding how per-packet load balancing is generally used.
>
>
> I wasn't saying anything about how per packet load balancing is 
> generaly used, the point is that it's possible that subsequent packets 
> end up at different anycast instances when a number of specific 
> prerequesites exists. In short: a customer must pplb across two 
> routers at the same ISP, and each of those routers must have different 
> preferred paths to different anycast instances. This isn't going to 
> happen often, but it's not impossible, and it's not bad engineering on 
> the customer's or ISP's part if it does, IMO.
>
You're wrong.  That's VERY bad engineering!

PPLB requires 2 routers, one at each end of the link bundle. 

More than 1 router at any end will lead to a lot more problems than
anycast, including multicast and any stateful protocol (like TCP).

For one thing, the load balancing will be only in 1 direction, and will
lead to congestion in the reverse path....  Self defeating.

-- 
William Allen Simpson
    Key fingerprint =  17 40 5E 67 15 6F 31 26  DD 0D B9 9B 6A 15 2C 32