Sunday, 3 May 2015

TCP/IP Model & TCP Sequence and Acknowledgment Numbers

The Open Systems Interconnection Basic Reference Model (OSI Model) is an abstract description for network protocol design, developed as an effort to standardize networking.

The TCP/IP model is basically a shorter version of the OSI model. It consists of four instead of seven layers.

             TCP/IP Model

Application Layer: The Application layer deals with representation, encoding and dialog control issues. All these issues are combined together and form a single layer in the TCP/IP model whereas three distinctive layers are defined in the OSI model. 
Application Layer deals with DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.

Transport: Transport protocol in the TCP/IP model provides more or less the same services with its equivalent Transport protocol in the OSI model. Its responsibilities include application data segmentation, transmission reliability, flow and error control.

Internet: Again Internet layer in TCP/IP model provides the same services as the OSIs Network layer. Their purpose is to route packets to their destination independent of the path taken.

Network Access: Network Access Layer defines details of how data is physically sent through the network. The network access layer deals with all the physical issues concerning data termination on network media. It includes all the concepts of the data link and physical layers of the OSI model for both LAN and WAN media.

The diagram below shows clearly the way TCP/IP protocol suite relates to the TCP/IP model.


The TCP/IP suite is illustrated here in relation to the OSI model:

Consider the data flow when you open a website.


TCP utilizes a number of flags, or 1-bit boolean fields, in its header to control the state of a connection. The three we're most interested in here are:
  • SYN - (Synchronize) Initiates a connection
  • FIN - (Final) Cleanly terminates a connection
  • ACK - Acknowledges received data
As we'll see, a packet can have multiple flags set.

The client on either side of a TCP session maintains a 32-bit sequence number it uses to keep track of how much data it has sent. This sequence number is included on each transmitted packet, and acknowledged by the opposite host as an acknowledgement number to inform the sending host that the transmitted data was received successfully.

 When a host initiates a TCP session, its initial sequence number is effectively random; it may be any value between 0 and 4,294,967,295, inclusive. However, protocol analyzers like Wireshark will typically display relative sequence and acknowledgement numbers in place of the actual values. These numbers are relative to the initial sequence number of that stream. This is handy, as it is much easier to keep track of relatively small, predictable numbers rather than the actual numbers sent on the wire.

For example, the initial relative sequence number shown in packet #1 is 0 (naturally), while the ASCII decode in the third pane shows that the actual sequence number is 0xf61c6cbe, or 4129057982 decimal.

The display of relative sequence numbers can optionally be disabled by navigating to Edit > Preferences... and un-checking Relative sequence numbers and window scaling under TCP protocol preferences. However, be aware that the remainder of this article will reference relative sequence and acknowledgement numbers only.

To better understand how sequence and acknowledgement numbers are used throughout the duration of a TCP session, we can utilize Wireshark's built-in flow graphing ability. Navigate to Statistics > Flow Graph..., select TCP flow and click OK. Wireshark automatically builds a graphical summary of the TCP flow.


 Each row represents a single TCP packet. The left column indicates the direction of the packet, TCP ports, segment length, and the flag(s) set. The column at right lists the relative sequence and acknowledgement numbers in decimal. Selecting a row in this column also highlights the corresponding packet in the main window.

We can use this flow graph to better understand how sequence and acknowledgement numbers work.

Packet #1

Each side of a TCP session starts out with a (relative) sequence number of zero. Likewise, the acknowledgement number is also zero, as there is not yet a complementary side of the conversation to acknowledge.

(Note: The version of Wireshark used for this demonstration, 1.2.7, shows the acknowledgement number as an apparently random number. This believed to be a software bug; the initial acknowledgement number of a session should always be zero, as you can see from inspecting the hex dump of the packet.)

Packet #2

The server responds to the client with a sequence number of zero, as this is its first packet in this TCP session, and a relative acknowledgement number of 1. The acknowledgement number is set to 1 to indicate the receipt of the client's SYN flag in packet #1.

Notice that the acknowledgement number has been increased by 1 although no payload data has yet been sent by the client. This is because the presence of the SYN or FIN flag in a received packet triggers an increase of 1 in the sequence. (This does not interfere with the accounting of payload data, because packets with the SYN or FIN flag set do not carry a payload.)

Packet #3

Like in packet #2, the client responds to the server's sequence number of zero with an acknowledgement number of 1. The client includes its own sequence number of 1 (incremented from zero because of the SYN).

At this point, the sequence number for both hosts is 1. This initial increment of 1 on both hosts' sequence numbers occurs during the establishment of all TCP sessions.

Packet #4

This is the first packet in the stream which carries an actual payload (specifically, the client's HTTP request). The sequence number is left at 1, since no data has been transmitted since the last packet in this stream. The acknowledgement number is also left at 1, since no data has been received from the server, either.

Note that this packet's payload is 725 bytes in length.

Packet #5

This packet is sent by the server solely to acknowledge the data sent by the client in packet #4 while upper layers process the HTTP request. Notice that the acknowledgement number has increased by 725 (the length of the payload in packet #4) to 726; e.g., "I have received 726 bytes so far." The server's sequence number remains at 1.

Packet #6

This packet marks the beginning of the server's HTTP response. Its sequence number is still 1, since none of its packets prior to this one have carried a payload. This packet carries a payload of 1448 bytes.

Packet #7

The sequence number of the client has been increased to 726 because of the last packet it sent. Having received 1448 bytes of data from the server, the client increases its acknowledgement number from 1 to 1449.

For the majority of the capture, we will see this cycle repeat. The client's sequence number will remain steady at 726, because it has no data to transmit beyond the initial 725 byte request. The server's sequence number, in contrast, continues to grow as it sends more segments of the HTTP response.

Packet #38

After acknowledging the last segment of data from the server, the client processes the HTTP response as a whole and decides no further communication is needed. Packet #38 is sent by the client with the FIN flag set. Its acknowledgement number remains the same as in the prior packet (#37).

Packet #39

The server acknowledges the client's desire to terminate the connection by increasing the acknowledgement number by one (similar to what was done in packet #2 to acknowledge the SYN flag) and setting the FIN flag as well.

Packet #40

The client sends its final sequence number of 727, and acknowledges the server's FIN packet by incrementing the acknowledgement number by 1 to 22952.

At this point, both hosts have terminated the session and can release the software resources dedicated to its maintenance.

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