Chapter 1: The TCP/IP Protocol Suite

Example

At Total Seminars, one of the servers handles internal Web pages for essential company policies. But this is not its only job; it also acts as a file server and a print server. Clients request each of these different services using different port numbers.

UDP Header

-8 bytes (64 bits) long, divided into four sections/fields: 1) source port 2) destination port 3) message length 4) checksum

List of port numbers

0-1023: Well-known port numbers 1024-49151: Registered ports 49152-65535: Dynamic or private ports

Port numbers used by TCP/UDP

A TCP/IP client computer initiates contact with a server computer and uses specific values, called port numbers, to request a specific service. Servers listen on ports that correspond to the services they offer.

netstat

A TCP/IP utility that shows the status of each active connection.

Session Initiation Protocol (SIP)

A VoIP signaling protocol used to set up, maintain, and tear down VoIP phone calls.

67, 68 UDP

Dynamic Host Configuration Protocol is used to assign IP addresses automatically to clients on the network. Servers use port 67, and clients use port 68.

20/21 TCP

FTP

1720 TCP

H.323 ( ITU Telecomm. H.32x Prot. series ) Microsoft - Voice over IP signaling

Application Protocols

HTTP, DNS, DHCP, FTP Application-level protocols are responsible for initiating some sort of request (on the client) or answering that request (on the server).

Internet Layer Protocols

Internet-layer protocols handle addressing in a TCP/IP network. They help devices determine whether an intended recipient is local or remote.

137 & 138 & 139 TCP

NetBIOS

Ephemeral ports

Port numbers that devices allocate on the fly for a source port number are also called ephemeral ports.

110 TCP

Post Office Protocol (POP3)

Transmission Control Protocol (TCP)

Provides connection-oriented data management and reliable data transfer

5004/5005 UDP

Real-time Transport Protocol delivers audio and video, and compensates for jitter and UDP datagrams that arrive out of sequence

3389 TCP

Remote Desktop Protocol (RDP)

Time to Live

Routers on the Internet are not perfect and sometimes create loops. The Time to Live (TTL) field prevents an IP packet from indefinitely spinning through the Internet by using a counter that decrements by one every time a packet goes through a router. This number cannot start higher than 255.

Real-time Transport Protocol (RTP)

SIP uses either TCP or UDP ports 5060 and 5061. RTP has no fixed port number, though many implementations use UDP ports 5004 and 5005.

Socket or Endpoint

Session Information (IP address and Port Number) stored on a single computer

5060 (clear text), 5061(encrypted)

Session Initiation Protocol handles the initiation, setup, and delivery of VoIP sessions. SIP is an alternative to H.323.

25 TCP

Simple Mail Transfer Protocol (SMTP)

Transport Protocols

TCP, UDP

TCP/IP Protocols

TCP/IP consists of protocols that work together as a powerful networking language. Each protocol in the TCP/IP protocol suite performs a specific role; this section starts with an explanation of ports, a defining feature of TCP/IP protocol communication, then identifies some of the popular protocols Application-layer protocols. (There're a lot of them! Other sections cover Transport-layer and Internet-layer protocols.)

Exam Tip

TCP/IP port numbers between 0 and 1023 are called well-known port numbers. They are reserved for specific TCP/IP applications. You'll find them at every party.

443 TCP

TLS/SSL ( Transport Layer Secure. and Secure Sockets Layer ) Secure Protocols for web browsing

socket pairs or endpoints

The connection data stored on two computers about the same connection

version

The version field defines the IP address type: 4 for IPv4, 6 for IPv6.

69 UDP

Trivial File Transfer Protocol (TFTP)

445 TCP

When NetBIOS support is disabled, Microsoft File and Print Sharing uses port 445.

User Datagram Protocol (UDP)

An alternative to TCP that achieves higher transmission speeds at the cost of reliability

port number

Clearly defined port numbers exist for every well-known TCP/IP application. A port number is a 16-bit value between 0 and 65535.

53 udp/tcp

DNS ( Domain Name System ) Convert domain names to IP addresses

Exam Tips

Data gets chopped up into chunks at the Transport layer. The chunks are called segments with TCP and datagrams with UDP.

List of Application Protocols

Here are some examples of popular Application-level protocols and the port number or numbers they use by default. And yes, you need to memorize the port numbers for the CompTIA Network+ exam.

80 TCP

Hypertext Transfer Protocol (HTTP)

143 TCP

IMAP (Internet Message Access Protocol)

2427/2727 TCP

Media Gateway Control Protocol is designed from the ground up to be a complete VoIP or video presentation connection and session controller; in essence, it takes over all the work from VoIP the SIP protocol used to do and all the work from video presentation done by H.323.

22 TCP

SSH ( Secure Shell ) Encrypted console login

TCP three-way handshake

SYN, SYN/ACK, ACK

161 UDP

Simple Network Management Protocol (SNMP)

Compound Application Protocols

Some common Application-layer protocols, such as HTTPS and VoIP, use multiple protocols to accomplish their tasks.

TCP Header

Source Port, Destination Port, Sequence Number, Acknowledgment Number, Header Number, Reserved, Code Bits, Window, Checksum, Urgent, Options, Data

23 TCP

Telnet ( Telecomm. Network ) Remote console login to network devices

Connection or Session

Terms for the whole interconnection

Differentiated Services (DS

The DS field contains data used for prioritization by bandwidth-sensitive applications, like VoIP. (Network techs with long memories will note that this field used to be called the Type of Service field.)

destination and source port numbers

The client uses a destination port number to request a service. To communicate back and establish a session with the client, the server uses a port number specified by the client. The Web client, in essence, says:

Header Length

The number of 32-bit words in the TCP header. This indicates where the data begins.

Registered and Dynamic Port Numbers

The port numbers from 1024 to 49151 are called registered ports. Less-common TCP/IP applications can register their ports with the Internet Assigned Numbers Authority (IANA). Unlike well-known ports, anyone can use these port numbers for their servers or for ephemeral numbers on clients.

Protocol

This field specifies what's encapsulated in the IP packet.In the vast majority of cases, the protocol field is TCP, UDP, or ICMP (covered next). The image below shows a highly simplified IP header.

HTTP over Secure Sockets Layer (SSL)

an encrypted connection between a Web server and a Web client. Most Web browsers display a closed padlock to indicate a secure connection. HTTPS provides the basic fabric for e-commerce on the Internet. Although the protocol name still refers to SSL, current HTTPS implementations use Transport Layer Security (TLS), an improved cryptographic protocol. HTTPS uses TCP port 443.

Internet Protocol (IP)

responsible for the addressing and routing of data to the remote system. Addressing means that IP is responsible for some sort of addressing scheme used to identify each system on the network (or Internet). It's also responsible for determining how to use that address to route the data to the destination. The addressing scheme that IP uses is known as an IP address. The IP address is a unique number assigned to your system that looks something like this: 192.168.1.10.

Voice over IP (VoIP) (303)

uses TCP/IP and packet switching to send voice transmissions over a private or public IP network