Rummaging through the SIP Closet

Holding on to anger is like grasping a hot coal with the intent of throwing it at someone else; you are the one who gets burned.
-- Buddha

I have a difficult time throwing things out. Perhaps it's due to my optimistic nature. I always feel that no matter how long something has sat around unused, there is a good chance that it will prove useful sometime in the near future. Just take a look at my dresser drawers and closets, for example. They are filled with shirts, pants, coats, suits, and ties that I haven't worn in years. Some are practically threadbare and some show little signs of wear. The condition is never the deciding factor, though. I keep things because, well ... just because I keep things.

SIP has a little of that, too. While it hasn't been around as long as some of the t-shirts I own (I am not kidding), there are a few SIP relics that deserve a second look. For the next few minutes let's take a journey through the SIP closet and see what, if anything, can be tossed or at least donated to the Goodwill.

As you should know, SIP headers are used to identify the characteristics of a session. For instance, To indicates the destination of a session and From identifies the creator. There are many more headers that you see on a regular basis in both requests and responses. However, there are several headers that in all my years of SIP programming and Wireshark traces, I've never come across. So, in no particular order, let's take a look at a few of them.

The first on my list is Subject. Like the subject of an email, this header can be used to provide a brief reason as to why the session exists. For example, I might tag a phone call as "lunch" if I were calling to invite a coworker out for a quick bite.

Several months ago, I wrote about providing the context of a call in my article Contextual Communications: Ranking Interruptions. Subject would be perfect for that. You might want to answer the call tagged with the subject "Quarterly sales numbers" over the one labeled "My vacation to Akron" -- or vice versa. A little extra context to phone calls would go a long way in terms of prioritizing your day.

Subject: I am calling about today's meeting

However, as cool as having a subject associated with a phone call is, I have never actually seen it implemented. There is nothing on any of my hard, soft, or mobile SIP phones that allows me to enter a subject. Even if such a parameter existed, none of my devices have a designated spot to display it. A perfectly good header that provides a very cool feature sits unused.

I can say the same thing about the Priority header. Similar to an email priority flag, you can set different levels of session priority. So, instead of just a subject that says "Quarterly sales number," you could also tag it as "High Priority." This adds even more context to another otherwise uncategorized call.

Sadly, I have never seen the Priority header used, either. Would I love to click a priority flag on an outgoing call? You bet I would. I would love the recipient to know when he or she can safely ignore my call or when it absolutely needs to be answered.

There are a few more headers that fall into the "defined, but I have never seen" camp. I can find uses for the Alert-Info and Organization headers, but sadly, I've never seen a SIP device that supports either one.

As with a few SIP headers, there are response codes that I've never come across in all my dealings with SIP. It's not that I think they shouldn't exist. I just can't seem to find anyone who has actually implemented them.

We should all be familiar with a 200 Ok response. You send or receive one of those every time a call is answered. If you read my SIP Adventures article, REFER Revisited, you should also understand 202 Accepted.

Who out there is familiar with the 204 No Notification response? The SIP specification says that this response indicates that "the request was successful, but the corresponding response will not be received." Huh? I've poked around the Internet and found a document that says it will only be sent after a SUBSCRIBE, but it was completely vague as to when and why. It might have a perfectly wonderful use, but until I find a better explanation, it goes into my "Why is this here?" pile.

I am sure that some of you have your own list of unused, little used, or "huh?" aspects of SIP. However, like my drawers filled with old race t-shirts, I expect that that they will be around for quite some time. After all, you never know when you just might need something. If you are like me, it's probably a couple days after it has been tossed out.

Andrew Prokop writes about all things unified communications on his popular blog, SIP Adventures.

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