Data frame
A data frame is a protocol data unit on the data link layer of the OSI model . The term frame is in the OSI model clear from the term package to distinguish because of this for the network layer is used.
A data frame consists of:
- Destination and source addresses: class-specific, even hardware or MAC addresses called
- Control information for data flow control
- Payload of the packet of the network layer
- Checksums to ensure data integrity
The data frame for Ethernet (IEEE 802.3)
| Layer 4: TCP segment | TCP headers | Payload (1460 bytes) | |||||||||
| Layer 3: IP packet | IP header | Payload (1480 bytes) | |||||||||
| Layer 2: Ethernet frame | MAC receiver | MAC sender | 802.1Q tag (opt.) | EtherType | Payload (1500 bytes) | Frame check sequence | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Layer 1: Ethernet packet + IPG | preamble | Start of frame | Payload (1518/1522 bytes) | Interpacket gap | |||||||
| Octets | 7th | 1 | 6th | 6th | (4) | 2 | 20th | 20th | (6-) 1460 | 4th | 12 |
By default, the Ethernet data frame has a maximum size of 1518 bytes, of which 18 bytes are reserved for the header and trailer . 1500 bytes are then available for the data field, of which at least 46 bytes must be used so that the minimum frame size of 64 bytes is maintained to ensure collision detection . When using IEEE 802.1Q tags (for VLAN or prioritization), the header is extended by four bytes and the frame is therefore a maximum of 1522 bytes.
Since an IP packet can be 64 kB in size, it does not always fit into a data frame. For this reason, larger IPv4 packets are broken down before they are transmitted to Layer 2 so that the fragments each fit into a data frame. This decomposition is called fragmentation . IPv6 basically uses Path MTU Discovery , so that no fragmentation is necessary in the transport. The Maximum Transmission Unit (MTU) determines the maximum IP packet size that can be transmitted over a link in the network without the data packet having to be fragmented.