TCP with Explicit Link Failure Notification (TCP-ELFN)


Transmission Control Protocol (TCP) is a standard protocol for transmitting data over the internet. It is one of the main protocols in the TCP/IP suite, along with Internet Protocol (IP).

TCP provides reliable, ordered and error−checked delivery of data between applications running on hosts communicating via an IP network. It manages the transmission control part of the communication by dividing data into smaller packets, numbering them in sequence to ensure proper reassembly at the receiving end, and acknowledging receipt of each packet.

TCP is widely used in client−server architecture where a server provides services to multiple clients. Examples include web servers that serve web pages to clients using HTTP, email servers that receive and send emails using SMTP or POP/IMAP protocols, and file transfer servers that allow file transfer over FTP or SFTP protocols.

TCP−ELFN: What is it?

TCP with explicit link failure notification (TCP−ELFN) is an enhanced version of the traditional TCP protocol that improves the performance and reliability of network communication. The original TCP protocol was designed to provide reliable data transfer over a wide range of network conditions, but it does not handle link failures very well.

When a link fails in traditional TCP, it can take several seconds before the cause of the failure is detected and communication resumes. This delay can be especially problematic in high−speed networks or in mobile networks where handovers are frequent.

Definition of TCP−ELFN and how it differs from traditional TCP

TCP−ELFN adds an explicit link failure notification mechanism to the standard TCP protocol. With this mechanism, when a link fails, the failure is immediately signaled to both ends of the connection using a new type of control message called an Explicit Link Failure Notification (ELFN).

Upon receiving this message, both sides know that they need to retransmit lost or unacknowledged data over another available path. In contrast to traditional TCP, which relies on timeouts and packet loss as indicators of congestion or network issues, ELFN allows for quick detection and resolution of connectivity problems.

ELFN also allows for faster recovery from error conditions by providing information about alternates paths before retries. TCP−ELFN operates at the transport layer; hence it runs transparently over almost any IP network technology such as Ethernet, WiFi, cellular data networks − 2G/3G/4G/LTE − WiMAX etc.

Benefits of using TCP−ELFN over traditional TCP

The addition of an explicit link failure notification mechanism provides several benefits over traditional TCP:

  • Faster detection and recovery from failed links

    • By quickly signaling failures via ELFN messages instead of waiting for timeouts or packet losses.

  • Improved reliability and stability of network connections

    • TCP−ELFN can recover from link failures much faster than traditional TCP, making it more resilient against network disruptions.

  • Better performance over high−speed or mobile networks

    • By enabling quicker detection and reaction to link failures, TCP−ELFN can maintain higher throughput and lower latency in fast−moving scenarios. TCP−ELFN reduces the time to detect and recover from link failures, thereby reducing data loss and improving communication quality in a wide range of networking scenarios.

How does TCP−ELFN work?

Overview of the protocol and its key features

TCP−ELFN is an extension to the traditional TCP protocol that provides explicit notification of link failures. It works by adding an additional signaling message, called the Explicit Link Failure Notification (ELFN) message, to the existing TCP packet structure.

This message is sent from one endpoint to another when it detects a link failure or congestion on the network path. The recipient endpoint then uses this information to adjust its congestion control and retransmission behavior accordingly.

One of the key features of TCP−ELFN is its ability to quickly detect and respond to link failures, which can significantly reduce packet loss and improve overall network performance. Additionally, TCP−ELFN provides better fairness between flows by allowing each flow to independently react to link failures rather than relying on global congestion control algorithms.

Explanation of how explicit link failure notification is implemented in TCP−ELFN

The implementation of explicit link failure notification in TCP−ELFN involves several steps. First, each endpoint must support the protocol extension by including it in their initial handshake messages.

Next, when a sender detects a link failure or congestion on the network path, it sends an ELFN message along with the regular TCP packets. Upon receipt of an ELFN message, a receiver updates its estimates of current network conditions and adjusts its congestion control algorithm accordingly.

This may involve reducing its sending rate or adjusting its retransmission behavior based on whether it believes the cause of packet loss was due to a congested or failed link. Overall, by explicitly notifying endpoints about network conditions that affect their communications, TCP−ELFN enables faster response times and better utilization of available bandwidth while avoiding unnecessary congestion and packet loss.

Use Cases for TCP−ELFN

TCP−ELFN provides essential benefits in situations where traditional TCP would fail to provide reliable communication over a network. This section presents examples of such scenarios, highlighting how TCP−ELFN can address the limitations of traditional TCP.

High−speed networks with frequent link failures

High−speed networks are susceptible to frequent link failures due to interference and congestion, causing packets to be dropped or delayed. Deploying TCP−ELFN in such a network provides better fault tolerance compared to traditional TCP, which only discovers link failures after timeouts on unacknowledged packets. The explicit notification allows the sender to adopt an alternative route or switch to a different transport protocol, ensuring that data transmission proceeds uninterrupted and with minimal delay.

Networks with limited bandwidth or high latency

TCP assumes that packet loss is an indication of network congestion; it reduces its sending rate accordingly. However, in networks with limited bandwidth or high latency, packet loss can occur due to other reasons such as intermittent connectivity or hardware faults.

In this case, response time can become intolerable as it takes longer for the sender's retransmissions after timeout messages have been sent. Explicit notification provided by TCP−ELFN allows the sender not only to detect packet losses but also distinguish between congestion and non−congestion−related losses.

Mobile networks with frequent handovers

Mobile networks are characterized by frequent handovers between base stations causing abrupt link disruptions when switching from one station's coverage area to another station's area far away. In these cases, traditional tcp would take a long time before resuming normal transmission owing since it relies on timeouts on unacknowledged packets after detecting a link failure .

In contrast, using TCP ELFN in mobile communication systems enables fast detection of broken links and faster resumption times than for conventional tcp protocols through its explicit link failure notification capability. The notification can trigger the sender to quickly switch to alternative networks, thereby maintaining a continuous data flow without prolonged delay or disruption.

TCP−ELFN provides reliable communication in challenging network environments such as high−speed networks with frequent link failures, networks with limited bandwidth or high latency, and mobile networks with frequent handovers. It enables faster detection of broken links and faster resumption times improving overall performance of the network.


TCP−ELFN is an important protocol that offers significant benefits over traditional TCP, particularly in networks where link failures are common. By providing explicit notification of link failures to both endpoints of a connection, TCP−ELFN enables faster recovery and more efficient use of network resources. Moreover, TCP−ELFN can be combined with other advanced networking technologies such as Multipath TCP to further enhance its performance and reliability.

Updated on: 12-Jul-2023


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