Question

The IP datagram for a TCP ACK message is 40 bytes long: It contains 20 bytes of TCP header and 20 bytes of IP header. Assume that this ACK is traversing an ATM network that uses AAL 5 to encapsulate IP packets. How many ATM packets will it take to carry the ACK? What if AAL3/4 is used instead?

Short answer

1 ATM cell for both AAL 5 and AAL 3/4.

Step-by-step solution

- Understand the Problem

The exercise involves determining how many ATM cells are required to transmit a 40-byte IP datagram using two different ATM Adaptation Layers: AAL 5 and AAL 3/4.

- Calculate the Number of ATM Cells for AAL 5

AAL 5 typically adds an 8-byte trailer to the IP packet. This makes the total data size 48 bytes (40 bytes of IP datagram + 8 bytes of AAL 5 trailer). Each ATM cell can carry 48 bytes of payload, so only 1 ATM cell is needed to carry the data.

- Calculate the Number of ATM Cells for AAL 3/4

AAL 3/4 fragments the IP datagram into multiple ATM cells. Each cell has a payload capacity of 44 bytes due to additional 4-byte protocol overhead. Therefore, the 40-byte IP datagram fits into 1 ATM cell for AAL 3/4 since 40 bytes is less than the 44 bytes payload limit.

- Provide the Solutions

For AAL 5, a total of 1 ATM cell is required. For AAL 3/4, also 1 ATM cell is required since the data fits within the maximum payload of a single cell.

Key concepts

IP datagram

An IP datagram is a basic unit of data in the Internet Protocol (IP). It contains crucial information for routing packets across networks. An IP datagram consists of two main parts:

• Header: This typically has 20 bytes of data that include source and destination IP addresses, version information, and other routing information.
• Payload: This is the actual data being transported, which in this case is a packet from the Transmission Control Protocol (TCP).

An IP datagram allows flexible data transfer across different network types and structures.

TCP ACK

A TCP ACK is an acknowledgment message in the Transmission Control Protocol (TCP). When a device receives a TCP packet, it sends back a TCP ACK to confirm receipt. This process is essential for reliable data transmission. In our exercise, the TCP ACK message's IP datagram is 40 bytes long: 20 bytes for the TCP header and 20 bytes for the IP header. The ACK ensures that both parties in a data exchange acknowledge the successful receipt of packets, thus maintaining the integrity of the communication.

ATM Adaptation Layers

Asynchronous Transfer Mode (ATM) Adaptation Layers (AALs) are crucial for segmenting and reassembling higher-level packets into ATM cells. These layers ensure seamless data transfer between networks that use different protocols. In this exercise, we focus on two specific types of ATM Adaptation Layers: AAL 5 and AAL 3/4. Each AAL involves different methods to encapsulate data frames into ATM cells, ensuring that IP datagrams can be efficiently transported over the ATM network.

AAL 5

AAL 5 is designed to minimize overhead and is mainly used for handling IP traffic efficiently. Here is a deeper look into AAL 5:

• Structure: AAL 5 encapsulates the IP packet with an 8-byte trailer, making the total size 48 bytes in our situation.
• Efficiency: Since each ATM cell can carry a 48-byte payload, a 40-byte IP datagram along with an 8-byte AAL 5 trailer fits perfectly into a single ATM cell.

This makes AAL 5 very efficient as there is no wastage of cell space when transporting the given IP datagram.

AAL 3/4

AAL 3/4 is another method of encapsulating higher-level packets into ATM cells but works differently compared to AAL 5:

• Structure: AAL 3/4 includes a 4-byte protocol overhead for each cell, reducing the usable payload of each ATM cell to 44 bytes.
• Fragmentation: The 40-byte IP datagram fits within the 44-byte limit, meaning only one ATM cell is needed for transport.
• Overhead: Although it handles the IP datagram efficiently, AAL 3/4's structure includes slightly more overhead due to the protocol-specific data included in each cell.

Both AAL 5 and AAL 3/4 ensure the IP datagram is correctly segmented and reassembled, making ATM networks versatile for various data types.