Tcp Reliability versus Udp Burdens for serious, high-performance server

Question

Speed, optimization, and scalability are the typical comparisons between the Udp and Tcp protocols. Tcp touts reliability with the disadvantage of a little extra overhead, but speed is good to excellent. Once a Tcp socket is instanced, keeping the socket open requires some overhead. But compared to the oft described burdens of Udp, which protocol actually has more overhead?. I've also heard that there are scalability issues with Tcp...yet the Internet (Web pages/servers) runs on Tcp - so what is it about Tcp that inhibits scalability?

Okay...so Udp doesn't require that overhead of keeping a connection open. But, it requires that you write extra methods to ensure all of the packet gets there, hopefully in the order that you want it received. If a packet isn't received in full, then you have to tell the client or server to resend. And you also have to keep some sort of message collection for partial packets, rebuild the partial messages, and check for a complete message before the message can finally be processed. Not to mention if the second part of a message never makes it, you have to either say resend the entire thing, or resend the part we are missing, or whatever.

Basically, my questions are:

1. Why would I choose Udp over Tcp for a serious, high-performance server with the added "overhead" of message checking and manual ACK versus the "overhead" of a continuous stream?
2. If Tcp is good enough for the likes of World of Warcraft, why isn't Tcp more widely accepted as the protocol to use for a game server?

Note: I am not opposed to implementing Udp options for a server. We are using C# on .Net 3.5 framework. So I would also be interested in the best practices for dealing with Udp burdens. I am also using the asynchronous methods at the socket level rather than using TcpListener, TcpClient, etc. etc.

Answer 1

Well, I would recommend reading up some more. There are plenty places to look at the pro's and con's of TCP vs. UDP and vice versa, here are a few:

• What Are The Advantages Of Using TCP Over UDP?
• When should I use UDP instead of TCP?
• TCP and UDP
• What are the advantages of UDP over TCP?

However, this link may interest you the most, as it is directly about networked game programming:

• Gaffer on Games - UDP vs. TCP

If I were to quote something small:

The decision seems pretty clear then, TCP does everything we want and its super easy to use, while UDP is a huge pain in the ass and we have to code everything ourselves from scratch. So obviously we just use TCP right?

Wrong.

Using TCP is the worst possible mistake you can make when developing a networked game! To understand why, you need to see what TCP is actually doing above IP to make everything look so simple!

I still recommend doing your own research on the matter though, and make sure which of the protocols suits your needs at the end of the day. This being said, it does seem to be the case that majority of games use UDP for their data. Anything that updates the entire state continuously does not need the overhead of guaranteed packet delivery.

Answer 2

I think the biggest part of TCP/IP that inhibits scalability is that it maintains a buffer on all incoming / outgoing connections up to basically the size of the window. So if I have a high latency but high throughput client i'm talking to, I have to keep all sent packets in buffer until I receive an ack. So for a few connections this is fine, but for handling 100K connections, it can start to be problematic overhead. On the receiving end, if a packet is dropped, again it will buffer all new packets received until the one required is retransmitted.

If you're going to implement retransmission, you need to do the same thing, and hence will have the same overhead. However, UDP does give you an advantage, if you know the end-to-end link speeds, or if certain message can be delivered out of order, or certain messages don't need retransmission. Keeping the gaming scenario:

packet 1 = move to 1,1 packet 2 = shoot packet 3 = move to 2,2

Most game designers, if packet 1 is lost, but packet 3 is received, packet 1 is no longer important because it contains out of date information anyways. However, you could opt to say packet 2 is important, so if it's not acked, send a retransmission.

If you need high throughput, and connect two servers directly with 1000Mbps ethernet, TCP/IP will take awhile to scale and have additional overhead, and will likely never achieve a true gigabit connection due to the congestion avoidance mechanisms. However, you know it's 1 Gbps, so you can set up you're UDP to transmit at up to a 1 Gbps (minus overhead) yourself.

To answer you're questions more directly: If you are going to ack every packet anyways, there isn't a massive benefit to having UDP, other than you can process some messages while waiting for retransmission (unless you want in-order delivery as well).

Udp isn't considered for game servers as much, mainly out of the scenario above, and real time combat systems such as First person shooters, where a message can be dropped, and the new message to come will invalidate the dropped message anyways. World of warcraft can get away with using TCP, since they don't have to be as precise with timing, and likely have some good logic that makes it more difficult for you to tell the difference anyways. The combat system simply doesn't require the speed.

I'd also contend that some of the justification is holdover from years ago, when everyone had less-reliable, and slower Internet connections. TCP is also more lenient for sharing the network, so if there's a lot going on, it will slow down so everyone gets a share of the connection (congestion avoidance).

TCP/IP is a protocol designed by people far smarter than I over years of research. Tuning in the last several years has allowed it to perform better with the faster and faster average network speeds we are seeing, and doesn't require a great understanding to use.

However, replacing this with UDP, does require a significant understanding of networking. I've seen badly written UDP programs saturate 1Gbps links and kill all traffic on the link, because they implemented a rather naive retransmission algorithm.

Here's a list of things TCP/IP can now do that you'd loose by going UDP: - In order arrival to you're program - retransmission (Now with Fast retransmit, selective acknowledgement, and other features) - Maximum segment size - Path MTU Discovery - Black Hole Detection (extension of Path MTU) - Congestion avoidance

Because of this, I'd highly recommend sticking with TCP/IP if it suits you're needs.

Also not to nit pick, but you're comment about the Internet running on TCP/IP is wrong, there are in fact dozens of Internet routeable protocols check them out here. I think you were referring to web pages and web servers are all running on top of TCP/IP. Which again for the web is great where us humans won't notice a delay as long as the page shows up correctly. Even for TCP/IP, their is some challenge that TCP/IP isn't aggressive enough for the web: Google thinks tcp/ip should be more aggressive by default

Answer 3

Define "serious high performance" - how many concurrent connections are you talking about and how much data is flowing?

Take a look at the answers to this question http://stackoverflow.com/questions/107668/what-do-you-use-when-you-need-reliable-udp which list some of the reliable protocols that have already been built on UDP. You might find one that works for your situation, or you may at least find some useful ideas.

The key to using UDP effectively here is to have some level of reliability and some level of unreliability and you get more of an advantage the more each datagram is able to be handled independently of others. The advantage over TCP is that you get to act on each datagram and decide if you can use it as it arrives. This is why it works for action games.

So, IMHO, if you need 100% reliability AND in order delivery then go with TCP; don't try and reimplement TCP in UDP.

Answer 4

First, I'll just paraphrase Stevens from Unix Network Programming Section 22.4 "When to Use UDP instead of TCP":

He basically says the following:

1. UDP is the only option for broadcast / multicast - so you have to use it there.
2. UDP can be used for simple request / reply apps. But you have to add your own error detection meaning at least acks, timesouts and retransmission.
3. UDP should not be used for bulk data transfer ( file transfers ) since you would have to build in all the functionality arleady in TCP to make it work right.
4. UDP should be used for real time data where speed of delivery is most important and some data loss is not an issue such as real time sensor data, live multimedia streams, real time stock quotes, etc.

The answer to your first question is very dependent on your definition of "high-performance". If you're primary concern is low latency, i.e. the individual data packets / requests arriving as quickly as possible than UDP would be the way to go. There are two primary reasons for this. Assuming packets / requests are fairly independent of each other than using TCP would introduce a problem known as head-of-line blocking.

Let's say you send two independent packets / requests. First A then B. Since TCP is stream based, if A get's lost in the network and needs to be retransmitted then even if B has already successfully arrived it can't be delivered to the application by the stack until A arrives, introducing unnecessary latency. Not only that, but until A arrives, B can't be acknowledged by the stack which might cause B to also be retransmitted causing needless network congestion.

One way around this problem is to use separate connections for each request, however this also introduces latency and hogs system resources. UDP bypasses all these problems.

Another issue in high performance ( low latency ) servers is the Nagle Algorithm which can add significant latency in TCP communications.

The answer to your second question is that WoW probably sends streams of data, not independent request / reply pairs. Also, some of the latency of TCP can be removed by disabling the Nagle algorithm. If they do use some request / reply communications they may have simply made a design decision that reliability is more important to them than latency.

Answer 5

It's Reliability vs Performance.

FPS games don't require -all- the packets to reach the destination, to reach it in order, to be exceptionally big, or to assure big throughput. They only require the packets to reach the server AS SOON AS POSSIBLE. This is the ultimate priority and overhead of TCP is simply an unnecessary burden.

WoW, in its "not quite realtime" communication and often tons of data to transmit (in crowded areas), may have to deal with packets exceeding MTU (requiring fragmentation) and requires reliability (fewer bigger packets = packet lost hurts more). So its choice of TCP is logical. Same would go for most turn-based strategy games and the like. In games where the player with ping of 30ms beats the player with ping 50ms UDP is the king.

Answer 6

There are online game engines and frameworks out there that help to take away the "pain in the ass" ...

Open-Source:
C/C++: ENet (enet.bespin.org)
C#: Lidgren v3 (code.google.com/p/lidgren-network-gen3/)

Commercial:
C/C++ & hosted CLR: Photon (http://exitgames.com/Photon)
(Photon actually uses eNet as the reliable UDP protocol.)