Communication and Network Security

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In this article by M. L. Srinivasan, the author of the book CISSP in 21 Days, Second Edition, the communication and network security domain deals with the security of voice and data communications through Local area, Wide area, and Remote access networking. Candidates are expected to have knowledge in the areas of secure communications; securing networks; threats, vulnerabilities, attacks, and countermeasures to communication networks; and protocols that are used in remote access.

(For more resources related to this topic, see here.)

Observe the following diagram. This represents seven layers of the OSI model. This article covers protocols and security in the fourth layer, which is the Transport layer:

Transport layer protocols and security

The Transport layer does two things. One is to pack the data given out by applications to a format that is suitable for transport over the network, and the other is to unpackthe data received from the network to a format suitable for applications.

In this layer, some of the important protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), Datagram Congestion Control Protocol (DCCP), and Fiber Channel Protocol (FCP).

The process of packaging the data packets received from the applications is called encapsulation, and the output of such a process is called a datagram.

Similarly, the process of unpacking the datagram received from the network is called decapstulation.

When moving from the seventh layer down to the fourth one, when the fourth layer’s header is placed on data, it comes as a datagram. When the datagram is encapsulated with the third layer’s header, it becomes a packet, the encapsulated packet becomes a frame, and puts on the wire as bits.

The following section describes some of the important protocols in this layer along with security concerns and countermeasures.

Transmission Control Protocol (TCP)

It is a core Internet protocol that provides reliable delivery mechanisms over the Internet. TCP is a connection-oriented protocol. A protocol that guarantees the delivery of datagram (packets) to the destination application by way of a suitable mechanism (for example, a three-way handshake SYN, SYN-ACK, and ACK in TCP) is called a connection-oriented protocol. The reliability of the datagram delivery of such protocol is high due to the acknowledgment part by the receiver.

This protocol has two primary functions. The primary function of TCP is the transmission of datagram between applications, and the secondary one is in terms of controls that are necessary for ensuring reliable transmissions.

Applications where the delivery needs to be assured such as e-mail, the World Wide Web (WWW), file transfer,and so on use TCP for transmission.

Threats, vulnerabilities, attacks, and countermeasures

One of the common threats to TCP is a service disruption. A common vulnerability is half-open connections exhausting the server resources.

The Denial of Service attacks such as TCP SYN attacks as well as connection hijacking such as IP Spoofing attacks are possible.

A half-open connection is a vulnerability in the TCP implementation.TCP uses a three-way handshake to establish or terminate connections. Refer to the following diagram:

In a three-way handshake, the client first (workstation) sends a request to the server (for example This is called a SYN request. The server acknowledges the request by sending a SYN-ACK, and in the process, it creates a buffer for this connection. The client does a final acknowledgement by ACK. TCP requires this setup, since the protocol needs to ensure the reliability of the packet delivery.

If the client does not send the final ACK, then the connection is called half open. Since the server has created a buffer for that connection,a certain amount of memory or server resource is consumed. If thousands of such half-open connections are created maliciously, then the server resources maybe completely consumed resulting in the Denial-of-Service to legitimate requests.

TCP SYN attacks are technically establishing thousands of half-open connections to consume the server resources. There are two actions that an attacker might do. One is that the attacker or malicious software will send thousands of SYN to the server and withheld ACK. This is called SYN flooding. Depending on the capacity of the network bandwidth and the server resources, in a span of time,all the resources will be consumed resulting in the Denial-of-Service. If the source IP was blocked by some means, then the attacker or the malicious software would try to spoof the source IP addresses to continue the attack. This is called SYN spoofing.

SYN attacks such as SYN flooding and SYN spoofing can be controlled using SYN cookies with cryptographic hash functions. In this method, the server does not create the connection at the SYN-ACK stage. The server creates a cookie with the computed hash of the source IP address, source port, destination IP, destination port, and some random values based on the algorithm and sends it as SYN-ACK. When the server receives an ACK, it checks the details and creates the connection.

A cookie is a piece of information usually in the form of text file sent by the server to a client. Cookies are generally stored in browser disk or client computers, and they are used for purposes such as authentication, session tracking, and management.

User Datagram Protocol (UDP)

UDP is a connectionless protocol and is similar to TCP. However, UDP does not provide the delivery guarantee of data packets. A protocol that does not guarantee the delivery of datagram (packets) to the destination is called connectionless protocol. In other words, the final acknowledgment is not mandatory in UDP.

UDP uses one-way communication. The speed delivery of the datagram by UDP is high. UDP is predominantly used where a loss of intermittent packets is acceptable such as video or audio streaming.

Threats, vulnerabilities, attacks, and countermeasures

Service disruptions are common threats, and validation weaknesses facilitate such threats.

UDP flood attacks cause service disruptions, and controlling UDP packet size acts as a countermeasure to such attacks.

Internet Control Message Protocol (ICMP)

ICMP is used to discover service availability in network devices, servers ,and so on. ICMP expects response messages from devices or systems to confirm the service availability.

Threats, vulnerabilities, attacks, and countermeasures

Service disruptions are common threats. Validation weaknesses facilitate such threats.

ICMP flood attacks, such as the ping of death, causes service disruptions; and controlling ICMP packet size acts as a countermeasure to such attacks.

Pinging is a process of sending the Internet Control Message Protocol (ICMP) ECHO_REQUEST message to servers or hosts to check whether they are up and running. In this process,the server or host on the network responds to a ping request, and such a response is called echo.

A ping of death refers to sending large numbers of ICMP packets to the server to crash the system.

Other protocols in transport layer

  • Stream Control Transmission Protocol (SCTP): This is a connection-oriented protocol similar to TCP, but it provides facilities such as multi-streaming and multi-homing for better performance and redundancy. It is used in UNIX-like operating systems.
  • Datagram Congestion Control Protocol (DCCP): As the name implies, this is a Transport layer protocol that is used for congestion control. Applications her include the Internet telephony and video/audio streaming over the network.
  • Fiber Channel Protocol (FCP): This protocol is used in high-speed networking. One of the prominent applications here is Storage Area Network (SAN).

Storage Area Network (SAN) is a network architecture used to attach remote storage devices, such as tape drives anddisk arrays, to the local server. This facilitates using storage devices as if they are local devices.


This article covers protocols and security in thetransport layer, which is the fourth layer.

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