Monday, March 28, 2011

OSI Model : Layer 2 (DLC)

Further to last post on OSI model, let me continue with some information on it. There’re some key parameters that embedded in every layer in one traffic.


Legend :
DA=Destination Address
SA=Source Address
FCS=Frame Check Sequence based on Cyclic Redundancy Check (CRC)
SP=Source Port
DP=Destination Port

Layer 2 frame header Overview


Offset 0-5 and Offset 6-11


At layer 2 (Data Link Control Layer)  frame header, source & destination MAC address will be the key parameters ( as shown in the ASCII packet below ). It takes up 6 Bytes for each layer-2 DA & SA and start from offset 0000 in your sniffer’s pcap file.

*( Note : Sniffer will start the capture from Layer-2 DA & SA range, and normally Preamble & 32-bit FCS/CRC will be left )


Offset 12-13

The following 2 Bytes are allocated for the Ethertype, in the example above shows it’s IP type ( 0x0800 ). More info in IEEE Ethertype. This 2 Bytes will indicate the following layer-3 IP header type.

Beside the example here which is Ethernet Version 2 Frame, there’re also some other 802.3 type e.g. IEEE 802.3 LLC Frame, IEEE 802.3 SNAP Frame & Novell Netware 802.3 RAW Frame.


Offset 15-17

If the 2 Bytes following the SA is greater than 05DC HEX ( 1500 Decimal ), the station will identified by Ethernet Version 2. Else, they’re assumed to be a length field. When the following 2 Bytes showing 802.3 length as 38 Decimal ( 26 HEX ) means the following Bytes will be LLC ( Logical Link Control ) header. This sub-layer is acts as an interface between MAC sublayer and the network layer.

Both DSAP ( Destination Source Service Access Point ) & SSAP ( Source Service Access Point ) will be taking up 1 Bytes respectively. This is simply to be the pointer to a memory buffer in the receiving station & of the sending process. Some example will DSAP/SSAP=43, means the following header will be BPDU ( Bridge Protocol Data Unit ).

HEX AA (DEC 170) values are usually employed for the DSAP and SSAP values. Netware uses a HEX 03 in the SNAP Control field and will use the 0x81-37 Ethernet Type value. Usually, the Organizational Code is coded as all 0s (0x00-00-00) and NetWare is no exception.

Offset 17 is simply the Control Byte.


Offset 15-17 

The first 3 Bytes i.e. Offset 15-20 is the vendor code. Refers IANA assigned Ethernet-numbers.

Offset 21-22 is the local code, that typically indicate the IEEE Ethertype. This is where the backwards compatibility with Version II Ethernet is implemented.

VLAN (  Optional frame header )

802.1q VLAN tagged frame, is an IEEE standard, uses an additional header in the frames that identifies the VLAN. The 4 Bytes of 802.1q VLAN tag after Layer-2 DA & SA, the Offset 12-15, but it’s optional.


VLAN tag takes up 32 bits in the frame header. The first 16 bits, which located at Offset 12-15 which same with the EtherType/Size field in the untagged frame, is the Tag Protocol Identifier (TPID).

The following 2 Bytes ( 16 bits ) is the Tag Control Identifies (TCI). Priority Code Point( (PCP) the 3-bit field which refers to the IEEE802.1p priority. This indicates the frame priority level, from value DEC 0 (Best Effort) to value DEC 7 (Highest). CFI, or known as Canonical Format Identifier, 1 bit field. CFI normally used to identify between Ethernet ( value 0 ) or Token Ring ( value 1 ).

Another 12-bit field specifying the VLAN to which frame belongs. A value of 0 means that the frame does not belong to any VLAN.

In the case of Q-in-Q ( IEEE802.1ad ), another additional 4 Bytes of VLAN tagging is needed in the frame header. The first 4 Bytes which next to SA will representing the S-TAG ( Service Tag ) and the following 4 Bytes will be representing the C-TAG ( Customer Tag ). IN this case, TPID of the outer S-TAG will be using 0x88A8 to identify it as Q-in-Q packet.


To be continued….

Related Post : OSI Model Standard

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