1. Why 750ms connect timeout is a bad idea

One of the benefits of TCP is that it is a reliable communication protocol. No need to worry about data being lost, rearranged or duplicated: receiving application reads exactly the same sequence of bytes that was written by sending application. Reliability doesn’t come for free, though. To provide reliability, TCP uses acknowledges (ACK) under the hood. Sending host keeps copy of sent data in the buffer up to the moment when it receives ACK from receiving host that acknowledges the delivery of the data. If ACK was not received in some amount of time after the data was sent, then sending host assumes that data was lost in transit and retransmits it from the buffer.

The question is: how long does sending host is going to wait for ACK before retransmission? It is obvious that network latency (commonly defined as round-trip time, RTT) is the key to this timeout. If retransmit timeout is shorter than RTT, then sender will unnecessary send the same data multiple times. If it is larger than RTT, then detection of lost data will be delayed. Both situations are not healthy. Thus, learning RTT for a TCP connection is crucial for defining optimal retransmit timeout.

RTT significantly depends on the geographical distance between sending and receiving hosts, on the number of router-to-router hops and on the network technology. It takes less than 1 millisecond to ping/pong a host connected to the same switch over Ethernet. At the same time, RTT between two hosts located on the opposite sides of the globe can be as large as one second. This is a three orders of magnitude difference. TCP learns and updates RTT dynamically by remembering send and receive timestamps. When the connection is being established, one of the hosts sends SYN and another one responds with ACK/SYN during 3-way handshake. This first round trip is enough to derive approximate RTT value. Further deliveries are used to compute more precise RTT values and to handle possible sudden changes in RTT. Hence, retransmit timeout value is kept sane for the whole duration of the TCP connection.

There is still one problem left. What happens when the very first SYN segment is lost? When remote port is closed or destination IP address is unreachable, correctly configured network sends ICMP notification back, signaling that connection couldn’t be established, which is immediately propagated to the user program as an error. But situation is different if no explicit notification is sent back. There is a variety of reasons why this can happen. Remote host may be down, or it may appear as a black hole (no ICMP notifications) because of firewall rules, or network may be overloaded. Because sender of the very first SYN didn’t receive anything in response, it can’t figure out RTT of the network. You, as the developer, may know that transmitting packet from to takes only couple of milliseconds because these two addresses are both on the local network, but TCP engine has no knowledge of that. When connection is being established, RTT is not known yet, and TCP treats all destinations identically: it defines retransmit timeout to a fixed, very high for a local network, value. On Linux this timeout is initially 1 second and is progressively increased by a factor of two after each unsuccessful retransmit attempt. Thus, if the remote host doesn’t respond, SYN segment is retransmitted at timepoints of 0 seconds, 1, 3, 7, 15, and so on. Below is the example:


What it means is that setting connect timeout to values like 750 milliseconds for hosts on local network is nonsense. If connection wasn’t established in, let’s say, 50 milliseconds, it won’t happen in the next 950 milliseconds either. There is no need in waiting. You’d better close the socket and retry connection from the very beginning. Below piece of pseudocode demonstrates the logic:

long deadline = getCurrentTime() + kConnectTimeout;
while (getCurrentTime() < deadline) {
  int fd = socket();
  int status = connect(socket, address, 50 /*ms*/); // couple of times larger than true RTT
  if (status == OK) {
    return fd;
  if (status != TIMEOUT) {
    return status;
return TIMEOUT;
2. 64K ports should be enough for everybody

Developers often forget than spare port is required not only for listening servers, but also for outgoing connections. When you create a server, you have to explicitly bind a socket to a given pair of IP address and port, simply because you want a server to be reachable on some well-known port. But when you establish outgoing connections, you do not care very much about what local port will be used: any would be fine. Operating system automatically selects free ports for outgoing connections during connect(). If there are 100 simultaneous client connections, then they will occupy 100 ports in total, no matter whether they connect to the same remote service or to the different services. One port is for one connection.

Operating system chooses ports for outgoing connections from the range of ephemeral ("dynamical") ports, which in Linux occupies about half of all possible ports by default: 32768-60999. This doesn’t cause any harm if you are running only well-known services, because traditionally they listen to the ports below 32000. However, in modern days of multicore CPUs and microservices it is typical to run dozens of daemons on the same host. Remembering all port numbers is hard, so developers often create some easy-to-remember schemes of assigning ports to daemons. For example, all instances of daemon type A should listen to the ports starting from 40000, while all instances of daemon B should listen to the ports starting with 50000. Rationale for using large numbers is to avoid possible collisions with well-known services. However, such large numbers do create collisions with the range of ephemeral ports.


Collisions with ephemeral ports can go unnoticed for long time. A socket can be still bound to the ephemeral port if it is not occupied. Operating system doesn’t warn about that. A daemon listening to ephemeral port can work for years without troubles. But at some unlucky day, after manual or automatic shutdown, you may find out that it failed to start again because of EADDRINUSE error, "Address already in use". This means that some outgoing connection acquired port that was meant to be used solely by the daemon.

One possible solution is just not to use ephemeral range of ports (32768-60999) for daemons. But since non-ephemeral ports may collide with well-known system services (something that we wanted to avoid in the first place), a better solution would be to adjust the list of ephemeral ports itself, i.e. to exclude port numbers of interest from it. In Linux, it is done by adjusting the options ip_local_reserved_ports and ip_local_port_range. For example, assuming that there would be no more than 100 instances of daemon of each type, we could permanently set net.ipv4.ip_local_reserved_ports=40000-40099,50000-50099 in /etc/sysctl.conf. Ports listed in this option are not considered ephemeral.

Very rarely the opposite issue can happen: when the number of outgoing connections is so big that default number of approximately 32K ephemeral ports is not enough. For example, this may happen if you operate large-scale web robot. When a new connection is initiated and there is no free ephemeral port available, application gets EADDRNOTAVAIL error ("Cannot assign requested address"). As a quick fix, ip_local_port_range can be used to increase the range of ephemeral ports. But since number of ephemeral ports can be increased only twice, up to 64K, a better solution would be to allocate more IP addresses and use them in round-robin fashion.

3. Improving throughput of high-RTT connections

Default buffer sizes are meant to be used for connections with typical RTTs and network bandwidths. If actual connection characteristics fall under the definition of "typical", no adjustment is required. But when RTT is too high, which happens for example during transmission between two data centers located on the opposite sides of the Earth, default buffer sizes become a bottleneck that limits effective throughput. It is critically important to adjust both send and receive buffer sizes with respect to RTT and network bandwidth in such case.


When application calls write() or similar syscall, data is not immediately sent but is copied into in-kernel buffer instead. Operating system transmits data from this buffer in background. Parts of the buffer are cleared only when ACK is received informing OS that corresponding piece of data was successfully delivered. Till then, data is kept in buffer for the purpose of retransmission if delivery fails. Such mode of operation can severely limit throughput in high-RTT networks if send buffer size is not adjusted for RTT. When RTT is large (hundreds of milliseconds), acknowledges are received with significant delay. Consequently, if buffer size is of default value, then at some point during a call to write(), OS won’t be able to copy data immediately from application buffer to kernel buffer because there will be no free space in in-kernel buffer. OS will have to wait until it gets ACK that will allow it to free some space in buffer and continue IO. This will limit throughput. The solution is to increase send buffer size to a value large enough so that it won’t be a bottleneck. For example, if true network throughput is 100Mb/s and RTT is 200ms, then acknowledge for the very first sent portion of data will be delivered no faster than in 200ms. This period of time would be enough to transmit 100Mb/s / 8 * 0.2 = 2.5MB of data, provided that send buffer size is at least this large.

Situation with receive buffer is no better. Operating system on receiving side doesn’t know how fast application will extract data from the buffer. And obviously it would be a bad idea to send more data than there is free space in receive buffer. In order not to overwhelm receive buffer, receive side notifies sending side how much data it can accept by sending "window" value. Under optimal circumstances, if application reads the data as soon as it gets into the buffer, receiving host will advertise its window equal to the receive buffer size. Hence it is not enough to increase the sending buffer size: receive buffer size must be adjusted as well.

In Linux, two things must be done to override buffer sizes. First of all, system-wide options net.core.wmem_max and net.core.rmem_max must be increased to reasonably high values. They control maximum allowed buffer size per socket. After that, applications can increase actual buffer sizes by calling setsockopt(fd, SOL_SOCKET, …​) with options SO_SNDBUF and SO_RCVBUF respectively. Of course, you have to keep in mind that if you have a lot of sockets, then you can’t set too large buffer sizes since they will occupy too much RAM. Also it doesn’t make any sense to override buffer sizes if you can control only one side of the connection. It won’t provide any benefits but will consume extra RAM.

4. Client requests are delayed

By default, data is not sent immediately after you write() it. OS postpones sending data if amount of buffered data is small. This is called Nagle’s algorithm. The rationale is that sending small portions of data is inefficient, so let’s wait for more data from application. Delay can rich hundreds of milliseconds. The downside of Nagle’s algorithm is that in client/server applications requests may be unexpectedly delayed. For example, this happens to HTTP requests because they are relatively small even with large number of HTTP fields.

Nagle’s algorithm can be easily disabled by setting SO_NODELAY socket option. Once you do this, of course, you should avoid issuing a lot of small writes. Instead, you should combine data yourself into buffer no smaller than MSS before writing.

5. Daemon fails to restart

When daemon is restarted and listening TCP socket is closed, port is not immediately available for reuse. There may be some orphan packets still traveling in the network which have relation to this port. Immediately allowing reuse of the port could theoretically lead to unexpected consequences. While such behavior may sound reasonable, it makes fast daemon restart impossible. The attempt to bind socket to recently closed port results in an error. Think about this functionality as about anachronism implemented for backward compatibility. Listening sockets should always be configured with SO_REUSEADDR option.