nanog mailing list archives
Re: Anycast provider for SMTP?
From: "Joe Abley" <jabley () hopcount ca>
Date: Thu, 18 Jun 2015 09:08:13 -0400
On 18 Jun 2015, at 7:51, Ray Soucy wrote:
You can certainly do anycast with TCP, and for small stateless services it can be effective. You can't do anycast for a stateful application withouttaking the split-brain problem into account.
It's really difficult to apply broad "can" or "can't", "works" or "doesn't work" advice here since there really are no absolutes. What works and what doesn't depends on the intersection between theory and practice (including other peoples' networks), and is broader than the architectural decision to use or not use anycast.
The text I pasted much earlier from RFC 4786 was a result of a lot of discussion (and more than a handful of objections to our attempts to answer this question, and to the document as a whole existing at all).
In the general, mathematical sense, it's never safe to use anycast with TCP; "safe" here means "entirely safe in all circumstances". Since we live on the Internet, we know nowhere is safe, so this answer is unsatisfying and doesn't help us make real-world decisions.
In the pragmatic, throw it at the wall and see what sticks sense, it's usually fine to use anycast with TCP; "usually" means things like "pretty sure I remember this working just fine at my last job" and "in our very particular situation the helpdesk phone didn't seem to ring". There's usually very little science attached to this answer, either in terms of comprehensive data about failures or in terms of characterising the precise environment and considering the ways in which it is similar or dissimilar to others.
If anycast is being considered as part of a solution to a particular problem, we might consider an answer of the form "anycast, when it works, is expected to solve that problem; anycast might introduce new problems, though, so we also need to think about a fall-back to a situation where the old problems are reintroduced but the new ones are gone". This kind of fudges around the difficulty in confidently enumerating all the new problems with an anticipation that anycast will work enough of the time to make it worth using at all.
So, in the example at hand, using an MX RRSet that tries first to deliver to an SMTP service that is distributed using anycast but will fall back to SMTP service that is not might be a reasonable approach, e.g.
$ORIGIN QUIRKAFLEEG.ORG. @ MX 10 ANY.MX ; service provided at DEFRA, NLAMS, USIAD, HKHKG MX 20 DEFRA.MX ; service provided just at DEFRA MX 20 NLAMS.MX ; service provided just at NLAMS MX 20 USIAD.MX ; service provided just at USIAD MX 20 HKHKG.MX ; service provided just at HKHKG.so a client will first attempt to deliver to ANY.MX.QUIRKAFLEEG.ORG, and if that fails we'll try one of the others.
For this particular question I still think that geoip/dns is a more straightforward approach, since it avoids the possible timeout and retry behaviour of the client that might delay delivery of mail in the event that the anycast MX is unavailable.
Joe
Current thread:
- Re: Anycast provider for SMTP?, (continued)
- Re: Anycast provider for SMTP? Robert Blayzor via NANOG (Jun 16)
- Re: Anycast provider for SMTP? Rafael Possamai (Jun 16)
- Re: Anycast provider for SMTP? Joe Hamelin (Jun 16)
- Re: Anycast provider for SMTP? Ray Soucy (Jun 17)
- RE: Anycast provider for SMTP? Chuck Church (Jun 17)
- Re: Anycast provider for SMTP? Joe Abley (Jun 17)
- Re: Anycast provider for SMTP? Ray Soucy (Jun 17)
- Re: Anycast provider for SMTP? Kurt Kraut via NANOG (Jun 18)
- Re: Anycast provider for SMTP? Christopher Morrow (Jun 18)
- Re: Anycast provider for SMTP? Ray Soucy (Jun 18)
- Re: Anycast provider for SMTP? Joe Abley (Jun 18)
- Re: Anycast provider for SMTP? Ben (Jun 18)
- Re: Anycast provider for SMTP? Rob Seastrom (Jun 18)
- Re: Anycast provider for SMTP? Jonas Björk (Jun 18)
- Re: Anycast provider for SMTP? Joe Abley (Jun 18)
- Re: Anycast provider for SMTP? Nick Hilliard (Jun 18)
- Re: Anycast provider for SMTP? Baldur Norddahl (Jun 18)
- Re: Anycast provider for SMTP? Jonas Björk (Jun 18)
- Message not available
- Re: Anycast provider for SMTP? Larry Sheldon (Jun 18)
- Re: Anycast provider for SMTP? Jonas Björk (Jun 18)
- Message not available
- Re: Anycast provider for SMTP? Larry Sheldon (Jun 18)
- RE: Anycast provider for SMTP? Chuck Church (Jun 17)