TCP / IP应用程序Keepalive大小和带宽开销

Question

I'm writing a Java server ( java.net.Socket , java.net.ServerSocket , java.io.ObjectOutputStream , java.io.ObjectInputStream ) and I know I'm going to have limited bandwidth allocated for it.

I've written a decorator object for my output and input streams so I can count how many bytes go through it for profiling purposes. But this won't give me any indication of the amount of overhead I'm using for the connection.

I don't anticipate it will be much, but I'd like to prepare for it. I'm not going try to optimize it, I just want to know how much it will be for logistical reasons (how much bandwidth must I request, etc.)

I can't be the first person to try to get this information, but I can't seem to find good resources on the overhead of Java Sockets and TCP/IP in general. (Perhaps that's because there's nothing noteworthy to find... If we're on the order of kb per minute, it's really not much of a concern, but I'd still like to know!)

Thanks!

Answer 1

This question is challenging to answer with the information we have right now... for instance, what are you calling 'overhead'? Is it only TCP ACK packets, or all packet overhead (for instance ethernet, IP and tcp headers) for anything other than your data payload?

How many connections per minute? What is the average data transfer, per connection? If there are many very short-lived connections, your overhead requirements go up (due to 3-way handshake, and connection close requirements)... you could also have high overhead if the clients don't read much data, but many clients keep the connections open for days at a time.

Honestly, you're 50x better off modeling this in a lab and making some assumptions about hit rate per minute and concurrent clients... that will give you some ballpark numbers. Play around with limiting the bandwidth afforded to the application to the maximum your budget would allow... then start backing off... you can throttle bandwidth by using wanem on a dual-port linux machine.

Getting lab results like this is far better than theoretical calculations.

HTH, \\mike (who spends all day testing network gear)

Answer 2

TCP overhead varies based on a number of factors, but is typically around 5% at full capacity.

Basically each "packet" has 20 bytes of IP header (and 20 more if IPv6) plus 20-32 bytes of TCP header. Packet sizes vary based on the network devices and conditions, but are often in the neighborhood of 1500 bytes.

This page has some detail: http://sd.wareonearth.com/~phil/net/overhead/

In my opinion you can completely ignore keep-alives, as they are only used when the connection is idle anyway.