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Long Polling vs Forever Frame

25 Sep

One of the oft-cited advantages of forever frame over long polling is that it does not suffer from the 3x max latency issue. This is when an event occurs the instant after a long poll response is sent to a client, so the event must wait for that response and the subsequent long poll request before sending a response containing that event. Thus while the average latency of long polling is very good, the theoretical max latency is 3x the average latency, which is the time taken to transit the network one-way.

Forever frame is said not to suffer from this issue, as it can send a response back at any time, even the instant after a previous event has been sent. Strictly speaking, that is not always the case, as forever frame implementations also need to terminate responses and issue new requests, at the very least to prevent memory leaks on the client. But for the purposes of this musing, let’s assume that it is true.

Does this theoretical lowering of the maximum latency actually enable any applications to be developed that would be impossible with the 3x max latency? For example, could forever frame be used to implement a first person shooter game that would be unplayable with long polling injecting a 3x latency on occasions (normally just as you charge into the room full of enemy guns lagging…).

Unfortunately, I think not. The problem is that comet will never be suitable for any application that cannot accept a bit of jitter in the application latency. Comet can achieve great average latency, often <100ms over the internet, but it is always going to suffer from the possibility of an occasional long delay.

The reason is that TCP/IP is by definition the transport that will be used for comet (that adheres to open standards) and TCP/IP is simply not a protocol that can guarantee constant low latency. Like long polling, TCP/IP gives very good average latency, but all it takes is 1 dropped packet and you will incur a TCP/IP timeout and resend, which by definition will be at least 3 x the network traversal time (sender must wait at least 2x network time before deciding that the ack will never come, then it must resend). Sure TCP/IP has lots of tricks and optimizations that are designed to help with latency for missed packets (e.g. fast resend, piggyback ack), but they rely on other traffic being sent in order to quickly detect the dropped packet. If a lone event is sent in a single packet, then at least 3x latency will result. One could even argue that the client’s need to send a new poll request with long polling will provide a convenient data packet on which an ack can piggyback, and could improve latency in some situations.

So any application that cannot tolerate 3x max latency is an application that should not be considered for comet. Comet is ideal for applications that thrive on good average latency, but that can tolerate the odd delay. For such applications, long polling is a good match and the theoretical latency gains of forever frame are probably just that—theoretical.

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Posted by on September 25, 2008 in COMET

 

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