IEEE 802.2
IEEE 802.2 is the original name of the ISO/IEC 8802-2
LLC is a software component that provides a uniform interface to the user of the data link service, usually the network layer. LLC may offer three types of services:
- Unacknowledged connectionless mode services (mandatory)
- Connection mode services (optional)
- Acknowledged connectionless mode services (optional)
Conversely, the LLC uses the services of the
The IEEE 802.2 sublayer adds some control information to the message created by the upper layer and passed to the LLC for transmission to another node on the same data link. The resulting packet is generally referred to as LLC protocol data unit (PDU) and the additional information added by the LLC sublayer is the LLC HEADER. The LLC Header consist of DSAP (Destination Service Access Point), SSAP (Source Service Access Point) and the Control field.
The two 8-bit fields DSAP and SSAP allow multiplexing of various upper layer protocols above LLC. However, many protocols use the Subnetwork Access Protocol (SNAP) extension which allows using EtherType values to specify the protocol being transported atop IEEE 802.2. It also allows vendors to define their own protocol value spaces.
The 8 or 16 bit
Operational modes
IEEE 802.2 provides two
- Type 1 is an unacknowledged connectionless mode for a datagram service. It allows for sending frames
The use of multicasts and broadcasts reduces network traffic when the same information needs to be propagated to all stations of the network. However the Type 1 service provides no guarantees regarding the order of the received frames compared to the order in which they have been sent; the sender does not even get an acknowledgment that the frames have been received.
- Type 2 is a connection-orientedoperational mode. Sequence numbering ensures that the frames received are guaranteed to be in the order they have been sent, and no frames are lost.
- Type 3 is an acknowledged connectionlessservice. It supports point-to-point communication only.
Each device conforming to the IEEE 802.2 standard must support service type 1. Each network node is assigned an LLC Class according to which service types it supports:
LLC Class |
Supported Service Types | ||
---|---|---|---|
1 | 2 | 3 | |
I | X | ||
II | X | X | |
III | X | X | |
IV | X | X | X |
LLC header
Any 802.2 LLC PDU has the following format:
802.2 LLC Header | Information | ||
---|---|---|---|
DSAP address | SSAP address | Control | |
8 bits | 8 bits | 8 or 16 bits | multiple of 8 bits |
When Subnetwork Access Protocol (SNAP) extension is used, it is located at the start of the Information field:
802.2 LLC Header | SNAP extension | Upper layer data | |||
---|---|---|---|---|---|
DSAP | SSAP | Control | OUI | Protocol ID | |
8 bits | 8 bits | 8 or 16 bits | 24 bits | 16 bits | multiple of 8 bits |
The 802.2 header includes two eight-bit address fields, called
- SSAP (Source SAP) is an 8-bit long field that represents the logical address of the network layer entity that has created the message.
- DSAP (Destination SAP) is an 8-bit long field that represents the logical addresses of the network layer entity intended to receive the message.
LSAP values
Although the LSAP fields are 8 bits long, the low-order bit is reserved for special purposes, leaving only 128 values available for most purposes.
The low-order bit of the DSAP indicates whether it contains an individual or a group address:
- if the low-order bit is 0, the remaining 7 bits of the DSAP specify an individual address, which refers to a single local service access point (LSAP) to which the packet should be delivered. and
- if the low-order bit is 1, the remaining 7 bits of the DSAP specify a group address, which refers to a group of LSAPs to which the packet should be delivered.
The low-order bit of the SSAP indicates whether the packet is a command or response packet:
- if it's 0, the packet is a command packet, and
- if it's 1, the packet is a response packet.
The remaining 7 bits of the SSAP specify the LSAP (always an individual address) from which the packet was transmitted.
LSAP numbers are globally assigned by the IEEE to uniquely identify well established international standards.
Value | Meaning | |
---|---|---|
Dec | Hex | |
0 | 00 | Null LSAP[2][3] |
2 | 02 | Individual LLC Sublayer Mgt[2][3] |
4 | 04 | SNA Path Control (individual)[2][3] |
6 | 06 | Reserved for DoD IP[2] |
14 | 0E | ProWay-LAN[2][3] |
24 | 18 | Texas Instruments[3] |
66 | 42 | IEEE 802.1 Bridge Spanning Tree Protocol[3] |
78 | 4E | EIA-RS 511[2][3] |
94 | 5E | ISI IP[2] |
126 | 7E | ISO 8208 (X.25 over IEEE 802.2 Type LLC)[3] |
128 | 80 | Xerox Network Systems (XNS)[3] |
130 | 82 | BACnet/Ethernet[4] |
134 | 86 | Nestar[3] |
142 | 8E | ProWay-LAN (IEC 955)[2][3] |
152 | 98 | ARPANET Address Resolution Protocol (ARP)[3] |
166 | A6 | RDE (route determination entity) |
170 | AA | SNAP Extension Used[2][3] |
188 | BC | Banyan VINES[3] |
224 | E0 | Novell NetWare[3]
|
240 | F0 | IBM NetBIOS[3] |
244 | F4 | IBM LAN Management (individual)[3] |
248 | F8 | IBM Remote Program Load (RPL)[3] |
250 | FA | Ungermann-Bass[3] |
254 | FE | OSI Connectionless-mode Network Service:[3] CLNP,[5] ISIS,[6] ESIS[7] |
Value | Meaning | |
---|---|---|
Dec | Hex | |
3 | 03 | Group LLC Sublayer Mgt[2][3] |
5 | 05 | SNA Path Control (group)[3] |
245 | F5 | IBM LAN Management (group)[3] |
255 | FF | Global DSAP (broadcast to all)[2][3] |
The protocols or families of protocols which have assigned one or more SAPs may operate directly on top of 802.2 LLC. Other protocols may use the Subnetwork Access Protocol (SNAP) with IEEE 802.2 which is indicated by the hexadecimal value 0xAA (or 0xAB, if the source of a response) in SSAP and DSAP. The SNAP extension allows using EtherType values or private protocol ID spaces in all IEEE 802 networks. It can be used both in datagram and in connection-oriented network services.
IPv4, IPX, and 802.2 LLC
Although
The IPX protocol used by Novell NetWare networks supports an additional Ethernet frame type, 802.3 raw, ultimately supporting four frame types on Ethernet (802.3 raw, 802.2 LLC, 802.2 SNAP, and Ethernet II) and two frame types on FDDI and other (non-Ethernet) IEEE 802 networks (802.2 LLC and 802.2 SNAP).
It is possible to use diverse framings on a single network. It is possible to do it even for the same upper layer protocol, but in such a case the nodes using unlike framings cannot directly communicate with each other.
Control Field
Following the destination and source SAP fields is a
- Unnumbered format PDUs, or U-format PDUs, with an 8-bit control field, which are intended for connectionless applications;
- Information transfer format PDUs, or I-format PDUs, with a 16-bit control and sequence numbering field, which are intended to be used in connection-oriented applications;
- Supervisory format PDUs, or S-format PDUs, with a 16-bit control field, which are intended to be used for supervisory functions at the LLC (Logical Link Control) layer.
To carry data in the most-often used unacknowledged connectionless mode the U-format is used. It is identified by the value '11' in lower two bits of the single-byte control field.
References
- ISBN 1-55937-959-6.
- ^ .
- ^ ISBN 1-55558-234-6.
- ^ The BACnet Standard—Standard 135-2012, Ashrae.
- ^ Final Text of DIS 8473, Protocol for Providing the Connectionless-mode Network Service, RFC, IETF.
- ^ "ISO/IEC 10589:2002". ISO. Retrieved 2020-12-18.
- ^ "ISO 9542:1988". ISO. Retrieved 2020-12-18.
- ^ LKML, 2011-07-27.
External links
- 802.2 (online ed.), IEEE.