The Link Layer of the router accepts the datagram from the link, and hands If the Network Layers drops some datagrams, TCP will retransmit. A TCP receiver continuously signals to the TCP sender dropped or when there aren't enough packets to run an ACK- clock unknown to the link operator. the ExtraHop "retransmissions out" counter may be less than "dropped segments out" counter, because multiple consecutive dropped segments may be retransmitted together in one retransmission episode. The appliance counts retransmission episodes, not packets (see explanation below).
First, the application creates data to send the "User data" box in Figure 1 and then calls the write system call to send the data. Assume that the socket fd in Figure 1 has been already created. When the system call is called, the area is switched to the kernel area. POSIX-series operating systems including Linux and Unix expose the socket to the application by using a file descriptor.
The file layer executes a simple examination and calls the socket function by using the socket structure connected to the file structure. The kernel socket has two buffers. One is the send socket buffer for sending; And the other is the receive socket buffer for receiving. When the write system call is called, the data in the user area is copied to the kernel memory and then added to the end of the send socket buffer.
This is to send data in order. In the Figure 1, the light-gray box refers to the data in the socket buffer. Then, TCP is called. If data transmission is impossible due to flow control or such a reason, the system call is ended here and then the mode is returned to the user mode in other words, the control is passed to the application.
There are two TCP segments as shown in Figure 2: TCP header; And payload. TCP Frame Structure source. The payload includes the data saved in the unacknowledged send socket buffer. The maximum length of the payload is the maximum value among the receive window, congestion window, and maximum segment size MSS. Then, TCP checksum is computed. In this checksum computation, pseudo header information IP addresses, segment length, and protocol number is included.
One or more packets can be transmitted according to the TCP state. In fact, since the current network stack uses the checksum offload, the TCP checksum is computed by NIC, not by the kernel. However, we assume that the kernel computes the TCP checksum for convenience. After the IP layer has computed and added the IP header checksum, it sends the data to the Ethernet layer.
It then adds the Ethernet header to the packet. The host packet is completed by adding the Ethernet header. The interface is used for transmitting a packet to the next hop IP and the IP. Therefore, the transmit NIC driver is called. At this time, if a packet capture program such as tcpdump or Wireshark is running, the kernel copies the packet data onto the memory buffer that the program uses.
In that way, the receiving packet is directly captured on the driver.How TCP Works - MTU vs MSS
Generally, the traffic shaper function is implemented to run on this layer. The driver requests packet transmission according to the driver-NIC communication protocol defined by the NIC manufacturer. After receiving the packet transmission request, the NIC copies the packets from the main memory to its memory and then sends it to the network line.
Packet transmission is started based on the physical speed of the Ethernet and the condition of Ethernet flow control. It is like getting the floor and speaking in a conference room. Every interrupt has its own interrupt number and the OS searches an adequate driver to handle the interrupt by using the number. The driver registers a function to handle the interrupt an interrupt handler when the driver is started.
Understanding TCP/IP Network Stack & Writing Network Apps | CUBRID blog
The OS calls the interrupt handler and then the interrupt handler returns the transmitted packet to the OS. So far we have discussed the procedure of data transmission through the kernel and the device when an application performs write.
However, without a direct write request from the application, the kernel can transmit a packet by directly calling TCP. For example, when an ACK is received and the receive window is expanded, the kernel creates a TCP segment including the data left in the socket buffer and sends the TCP segment to the receiver. Data Receiving Now, let's take a look at how data is received.
Data receiving is a procedure for how the network stack handles a packet coming in. Figure 3 shows how the network stack handles a packet received. First, the NIC writes the packet onto its memory. It checks whether the packet is valid by performing the CRC check and then sends the packet to the memory buffer of the host. This buffer is a memory that has already been requested by the driver to the kernel and allocated for receiving packets.
After the buffer has been allocated, the driver tells the memory address and size to the NIC. When there is no host memory buffer allocated by the driver even though the NIC receives a packet, the NIC may drop the packet. Then, the driver checks whether it can handle the new packet or not. So far, the driver-NIC communication protocol defined by the manufacturer is used. When the driver should send a packet to the upper layer, the packet must be wrapped in a packet structure that the OS uses for the OS to understand the packet.
The driver sends the wrapped packets to the upper layer. The Ethernet layer checks whether the packet is valid and then de-multiplexes the upper protocol network protocol. At this time, it uses the ethertype value of the Ethernet header. The IPv4 ethertype value is 0x It removes the Ethernet header and then sends the packet to the IP layer.
The IP layer also checks whether the packet is valid. In other words, it checks the IP header checksum. It logically determines whether it should perform IP routing and make the local system handle the packet, or send the packet to the other system. If the packet must be handled by the local system, the IP layer de-multiplexes the upper protocol transport protocol by referring to the proto value of the IP header. The TCP proto value is 6.
Like the lower layer, the TCP layer checks whether the packet is valid. It also checks the TCP checksum. As mentioned before, since the current network stack uses the checksum offload, the TCP checksum is computed by NIC, not by the kernel. Then it searches the TCP control block where the packet is connected. After searching the connection, it performs the protocol to handle the packet. If it has received new data, it adds the data to the receive socket buffer.
The size of the receive socket buffer is the TCP receive window. To a certain point, the TCP throughput increases when the receive window is large. In the past, the socket buffer size had been adjusted on the application or the OS configuration.
TCP Series #3: Network Packet Loss, Retransmissions, and Duplicate Acknowledgements
The latest network stack has a function to adjust the receive socket buffer size, i. When the application calls the read system call, the area is changed to the kernel area and the data in the socket buffer is copied to the memory in the user area. The copied data is removed from the socket buffer.
And then the TCP is called. The TCP increases the receive window because there is new space in the socket buffer. And it sends a packet according to the protocol status. If no packet is transferred, the system call is terminated.
Network Stack Development Direction The functions of network stack layers described so far are the most basic functions. The network stack in the early s had few more functions than the functions described above.
TCP Series #3: Network Packet Loss, Retransmissions, and Dup Acks
However, the latest network stack has many more functions and complexity as the network stack implementation structure gets higher. The latest network stack is classified by purpose as follows.
By inserting the user-controllable code to the basic processing flow, the function can work differently according to the user configuration. Protocol Performance It aims to improve the throughput, latency, and stability that the TCP protocol can achieve within the given network environment. The protocol improvement will not be discussed here since it is out of the scope. Packet Processing Efficiency The packet processing efficiency aims to improve the maximum number of packets that can be processed per second by reducing the CPU cycle, memory usage, and memory accesses that one system consumes to process packets.
There have been several attempts to reduce the latency in the system. Control Flow in the Stack Now, we will take a more detailed look at the internal flow of the Linux network stack.
Like a subsystem which is not a network stack, a network stack basically runs as the event-driven way that reacts when the event occurs. Therefore, there is no separated thread to execute the stack.
Figure 1 and Figure 3 showed the simplified diagrams of control flow. Control Flow in the Stack. If one of these packets in the stream goes missing, the receiving socket can indicate which packet was lost using selective acknowledgments. These allow the receiver to continue to acknowledge incoming data while informing the sender of the missing packet s in the stream.
As shown above, selective acknowledgements will use the ACK number in the TCP header to indicate which packet was lost.
Most network analyzers will flag these packets as duplicate acknowledgements because the ACK number will stay the same until the missing packet is retransmitted, filling the gap in the sequence. Typically, duplicate acknowledgements mean that one or more packets have been lost in the stream and the connection is attempting to recover. They are a common symptom of packet loss. In most cases, once the sender receives three duplicate acknowledgments, it will immediately retransmit the missing packet instead of waiting for a timer to expire.
- Understanding TCP/IP Network Stack & Writing Network Apps
These are called fast retransmissions. Connections with more latency between client and server will typically have more duplicate acknowledgement packets when a segment is lost. In high latency connections, it is possible to observe several hundred duplicate acknowledgements for a single lost packet.
For example, if a service provider is connecting end users to applications in a data center, or if the application is hosted in a cloud environment, there are several connections that are beyond the control and visibility of the network team.
End users may perceive performance as normal, but a small number of retransmissions may exist. However, when troubleshooting an application performance problem with incrementing retransmissions for the very users who are complaining, the underlying culprit is likely packet loss. Lost packets require retransmissions, which take time, which will slow applications down. Depending on how many occur and how fast the endpoints can recover the missing packets, they can significantly impact application performance.
In these cases, walk the link between client and server, analyzing link-level errors for all infrastructure devices you control. It may be that you discover the faulty cable, Frame Check Sequence counter FCSor discard indicator that is contributing to the packet loss. It can help us to hone in on which connections are suffering packet loss and identify if this is significantly impacting the application or if these are occurring during normal performance.
One of the keys in diagnosing packet loss is understanding where which systems are suffering from packet losswhen continuously or momentarilyand in which conditions only for certain services or all of them.