Monday, November 17, 2008

SBCs are Killing SIP

Wow... Over a month since my last post! My how time flies.

No time to reminisce or catch up. I've got a rant that needs to get out - NOW.

SBCs (Session Border Controllers) are killing SIP. Breaking SIP. Smothering SIP. Especially when used by "carriers". Carriers and their SBCs I tells ya.

SBCs, technically, are pretty cool devices. While I certainly understand their purpose they tend to be overused, misconfigured, and misunderstood. Many entities deploy SBCs without any idea of the other components (I'm looking at you, proxies) that make up a well designed SIP network.

Why do I hate SBCs so much?

1) SIP is cool because it is end to end and designed with intelligent endpoints in mind (endpoints that can think for themselves).
2) SIP is very flexible, especially with regards to handling media.
3) Ubiquity.

SBCs (especially when misconfigured) break many of these features:

1) SBCs (by design) hide endpoints from one another. Both endpoints support G.722? The SBC doesn't and it's going to rewrite the SDP with it's capabilities. Too bad.
2) SBCs (by design) handle media. While this can be good often times it isn't and there are other, less drastic ways to ensure quality of media.
3) When the only tool you have is a hammer, every problem starts to look like a nail.

My biggest concerns with SBCs relate to the last point. I swear, there are many providers, enterprises, etc that have deployed SIP in some capacity using ONLY SBCs and simple UACs and UASs. They've never heard of a proxy. Or a registrar. Heck, I'd even go for a signalling-only B2BUA and call it a compromise. Chances are they've never heard of that either.

I have dealt with several devices that break down, utterly fall apart when used with a proxy. I've covered it on this blog before. I'm just too mad to look up the link now. Again, $MANUFACTURER designs and markets a SIP device. They only test it against SBCs and they've (apparently) never heard of a proxy. Guess what happens...

Some poor soul like myself tries to deploy said device in what I consider to be a well designed SIP network. Unfortunately for me, this call path might not involve an SBC. Guess what happens? The device doesn't understand traversing proxies (Record-Route, Via, etc) and does something silly like parse the Contact header when trying to send a response. Call failure and all kinds of brokenness ensue.

So... I talk to $MANUFACTURER and get the standard "We've deployed this device thousands of times and never seen this problem before". Let's assume that's true. I don't know what's more depressing: the fact that they skipped over multiple sections of a basic SIP RFC like 3261 or the fact that no one noticed it for this long because (apparantly) no one uses proxies anymore. Ugh. Gross.

It's not just device manufacturers. Carriers do this too. Often times the actual issue lies with their SBC. Many carriers (especially those using ACME SBCs, it seems) parse To: instead of the Request-URI. Probably because their customers are using SBCs too and Request-URI and To: match. Not so with a proxy. I don't blame the carrier's use of an SBC. This makes sense. That's what they were designed for. However, please test your device and configuration against something other than another SBC.

What happens if your Request-URI and To: don't match? They send a 404! Yet another RFC3261 violation. Section allows for a UAS to route based off To (although it doesn't sound preffered). However, for the love of God, if you are going to deny a request because of the content of a To header, please send a 403 as specified in the RFC. Your 404s are confusing and ignorant. Was it really not found, or are you just routing based off To instead of the Request-URI? Once again I blame SBCs and a world where it's becoming common for SBCs to talk to each other (and nothing else).

This is yet another situation where assumptions are made based on the behavior of SBCs. It's bad. Please stop.