why llc and snap headers are used

On other network types, the LLC and SNAP headers are required in order to multiplex different protocols on the link layer, as the MAC layer doesn't itself have an EtherType field, so there's no alternative framing that would have a larger available payload.

Full Answer

What is the SNAP header used for?

A Sub Network Access Protocol (SNAP) header The SNAP header is used when the LLC protocol carries IP packets and contains the information which would otherwise have been carried in the 2-byte MAC frame type field.

What is the purpose of the LLC header in Ethernet frames?

The IEEE 802.3 standard moved LLC header to payload field and replaced the Type field by a Length field in the Ethernet frame as shown in Fig. 3.3 to indicate total length of the payload. The MAC layer portion of the layer 2 (data link layer) constructs frames for transmission and analyzes the received frames.

What are the additional headers of the LLC protocol?

When the LLC protocol is used, the MAC layer SDU (the payload data) is further encapsulated , adding two additional headers. The format of the complete frame with the additional headers is shown below:

What is the LLC sublayer in a packet?

The LLC is the Data Link layer closest to the Network layer. The LLC header will contain the protocol stack of the packet. For example, if you are on a TCP/IP (Transmission Control Protocol/Internet Protocol) network and request access to a resource, the LLC sublayer will identify that the Network layer protocol being used is IP.

What is a snap header used for?

SNAP is enclosed in a Logic Link Control (LLC IEEE 802.2) header extension. It is intended for the encapsulation of ARP requests and replies, and IP datagrams on IEEE 802 networks.

What is LLC snap?

The Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point (SAP) fields.

What is a 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.

What is LLC packet?

The logical link control (LLC) is the upper sublayer of the data link layer of the open system interconnections (OSI) reference model for data transmission. It acts act an interface between the network layer and the medium access control (MAC) sublayer of the data link layer.

What layer of OSI is Ethernet?

layer 2 data link protocolEthernet is a layer 2 data link protocol that is widely used with the TCP/IP protocol, which resides at layers 3 and 4.

What is an Ethernet Snap frame?

In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload.

What is LLC and MAC in networking?

The data link layer is an interface between the network and physical layer. It is further subdivided into two modules: Medium Access Control (MAC) and Logical Link Control (LLC). The MAC module plays a critical role in conserving network life by efficiently allocating medium access to the contending nodes.

What is LLC protocol Wireshark?

Logical Link Control (LLC, IEEE 802.2) protocols as its transport protocol: Ethernet: LLC can be used for packets on Ethernet networks, although many protocols don't use it when running atop Ethernet. IEEE_802. 11: LLC is used for data packets on IEEE_802.

What is the purpose of Layer 2 LLC sublayer?

What is the purpose of the layer 2 LLC sublayer? The LLC sub-layer acts as an interface between the Media Access Control (MAC) sublayer and the network layer.

What are the functions of LLC in computer network?

The Logical Link Control (LLC) sublayer provides the logic for the data link; thus it controls the synchronization, flow control, and error-checking functions of the data link layer.

What layer is LLC in the OSI model?

data link layerIn the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface between the media access control (MAC) sublayer and the network layer.

What is LLC network?

Introduction. The standards for local area networks (LANs) are generally comprised of the physical layer, the medium access control (MAC) sublayer, and the logical link control logical link control (LLC) sublayer. Two address spaces are used for addressing within the LLC sublayer: 1. The Ethernet type; and 2.

What is the highest position in an LLC?

The PresidentThe President is essentially the highest ranking manager in the LLC. The Operating Agreement typically gives the President general management powers of the business of the LLC, as well as full power to open bank accounts. Other titles of LLC officers and managers are Secretary and Treasurer for example.

What is an example of a LLC business?

Many well-known companies are structured as LLCs. For example, Anheuser-Busch, Blockbuster and Westinghouse are all organized as limited liability companies.

What does LLC stand for when someone dies?

Death is almost always a complicated event for the survivors, who have not only emotional but also logistical considerations to manage. However, when an owner (typically called a member) of a limited liability company (LLC) dies, it exacerbates the difficulties for the surviving members.

Is the manager of an LLC the owner?

A limited liability company (LLC) managing member is both an LLC owner and someone who keeps the business running on a day-to-day basis. The managerial aspect generally includes having the authority to make decisions and enter into contracts on behalf of the business.

What is LLC sublayer?

The LLC sublayer is primarily concerned with multiplexing protocols transmitted over the MAC layer (when transmitting) and demultiplexing them (when receiving). It can also provide node-to-node flow control and error management.

What is the logical link control layer?

In the IEEE 802 reference model of computer networking, the logical link control ( LLC) data communication protocol layer is the upper sublayer of the data link layer ( layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface between the media access control (MAC) sublayer and the network layer.

Does 802.2 use LLC?

However, most protocol stacks running atop 802.2 do not use LLC sublayer flow control and error management. In these cases flow control and error management are taken care of by a transport layer protocol such as TCP or by some application layer protocol.

What is an LLC frame?

An LLC frame is called an LLC Protocol Data Unit (LPDU), and is formatted as shown here:

Why is there an excessive retransmit in LLC2?

Excessive retransmits can occur due to drops, multiple paths, congestion and excessive latency. Some errors in LLC2 are unrecoverable and always result in a session outage. These errors are exclusively protocol violations. They can occur when stations send undefined control fields or other erroneous information.

What is LLC2 in NetBIOS?

LLC2 is implemented whenever Systems Network Architecture (SNA) runs over a LAN or virtual LAN. LLC2 is also directly encapsulated into Frame Relay. Sometimes the router simply passes LLC2 frames and sometimes the router implements an LLC2 linkstation. NetBIOS also uses LLC. NetBIOS uses LLC1 to locate a resource. LLC2 connection-oriented sessions are then established.

How many sessions are there in LLC2?

There are two separate LLC2 sessions. Therefore, configure LLC2 parameters as shown here:

How many modes of LLC2?

There are two modes of LLC2 operation:

Does connectionless service require data links?

Connectionless service does not require you to establish data links or link stations. After a Service Access Point (SAP) has been enabled, the SAP can send and receive information to and from a remote SAP that also uses connectionless service.

Is LLC1 a good knowledge?

A basic knowledge of LLC is enough to isolate and resolve most problems. Because there are no link states or sessions to maintain, problems are rare in LLC1.

When is the Snap header used?

The SNAP header is used when the LLC protocol carries IP packets and contains the information which would otherwise have been carried in the 2-byte MAC frame type field. Note that since the maximum size of Ethernet frame is fixed, the maximum size of SDU (payload data) is reduced to 1492 bytes (the MTU in IP) when LLC/SNAP encapsulation is used.

What is LLC protocol?

The LLC protocol is based on the HDLC link protocol and uses an extended 2-byte addres. The first address byte indicates a Destination Service Access Point (DSAP) and the second address a Source Service Access Point (SSAP). These identify the network protocol entities which use the link layer service.

What is the function of LLC?

The primary function of LLC is to multiplex protocols over the MAC layer while transmitting and likewise to de-multiplex the protocols while receiving.

What is a logical link control?

The logical link control (LLC) is the upper sublayer of the data link layer of the open system interconnections (OSI) reference model for data transmission. It acts act an interface between the network layer and the medium access control (MAC) sublayer of the data link layer.

What does 0x42 mean in SAP?

Absolutely correctly. The 0x42 SAP designates the Spanning Tree Protocol. I am not sure about the 0xE0 value - in an older document, I have found that it could designate the ancient Novell IPX protocol encapsulated in 802.2 LLC frames.

Is there an upper layer protocol for DSAP?

I appreciate that you join this discussion and provide this case that there is no upper layer protocols DSAP and SSAP.

Is 802.2 a strict SSAP?

Hmm, I think the 802.2 is not that strict as to limit the DSAP to only these two values. I have went over it briefly and I see no such requirement. What I see, and that is absolutely natural, is that both the DSAP and SSAP are 1B fields and therefore they can hold a value in the range of 0 to 255 inclusive, i.e 0x00 till 0xFF. The 802.2 standard itself defines these two values; other standards or recommendations add their own DSAP/SSAP codepoints.

Is SSAP a TCP socket?

In that cases as there is no upper layer protocols DSAP and SSAP may play a role similar to a TCP socket as suggested by Peter.

What are the two types of LLC frames?

There are two types of LLC frames. These are SAP (Service Access Point) and SNAP (Subnetwork Access Protocol).

What switches support Ethernet and Fast Ethernet?

The switches that support both Ethernet and Fast Ethernet are mentioned as 10/100 Swicthes.

What is the LLC sublayer?

The LLC sublayer sits above the MAC sublayer and acts as a liaison between the upper layers and the protocols that operate at the MAC sublayer such as Ethernet, Token Ring, and so on (IEEE (Institute of Electrical and Electronics Engineers, Inc.) standards). The LLC sublayer itself is defined by IEEE 802.2. Link addressing, sequencing, and definition of service access points (SAPs) also take place at this layer.

What is LLC layer protocol?

LLC layer protocols include specifications for the frames of many types of physical networks, including Ethernet, Token Ring (rarely used today because it has become nearly impossible to find parts to maintain the hardware), and FDDI (Fiber Distributed Data Interface).

How does the physical layer of an Ethernet frame work?

The physical layer, before sending the actual bits of the frame, sends the encoded first 62 bits of the preamble to allow the receivers and repeaters along the channel to synchronize their clocks and other circuitry. The preamble is then followed by 2-bit–long sync character to indicate that the Ethernet frame would follow next. It then begins translating the bits of the frame into encoded form and passes them to the transceiver for actual transmission over the medium. When transmission has completed without contention, the data link layer informs the transmitting end station and awaits the next request for frame transmission.

What is the logic link control layer?

The Logical Link Control (LLC) sublayer provides the logic for the data link; thus it controls the synchronization, flow control, and error-checking functions of the data link layer. This layer can handle connection-oriented transmissions (unlike the MAC sublayer below it), although connectionless service can also be provided by this layer. Connectionless operations are known as Class I LLC, whereas Class II can handle either connectionless or connection-oriented operations. With connection-oriented communication, each LLC frame that is sent is acknowledged. The LLC sublayer at the receiving end keeps up with the LLC frames it receives (these are also called protocol data units (PDUs)), and if it detects that a frame has been lost during the transmission, it can send back a request to the sending computer to start the transmission over again, beginning with the PDU that never arrived.

What is LLC in a protocol stack?

The organization of a frame, however, depends on the type of MAC protocol that will be used. This means that the LLC is hardware-dependent, unlike the upper layers in the protocol stack.

How does a LAN address work?

In a LAN, each node is assigned a physical address , also known as a MAC/Ethernet address. This address is unique to each of the nodes on the LAN and is 6 bytes (48 bits) long, which is burnt on the Ethernet card (also known as the network interface card, NIC). Ethernet is a byte-count protocol. A node on a LAN broadcasts a frame that is heard by all other nodes, and only the node whose Ethernet address matches with the DA in the Ethernet frame copies the frame into its buffer. If the DA address is a broadcast address (all 1’s), then all the nodes on that LAN would copy the frame. Hackers can disguise the MAC address of their data packets to protect their identity by spoofing their MAC address. The utility that allows spoofing the MAC address is known as SMAC, which is available from Internet sites.

Introduction

Prerequisites

The SNAP is an extension of the 802.2 LLC specified in the IEEE 802 Overview and Architecture document. The 5-octet SNAP header follows the 802.2 LLC header if the destination SAP (DSAP) and the source SAP (SSAP) contain hexadecimal values of AA or AB:
The SNAP header consists of a 3-octet IEEE organizationally unique identifier (OUI) followed by a 2-octet protocol ID. If the OUI is zero, the protocol ID is the registered EtherType value for the proto…

Background Information

Implementations of LLC

In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface between the media access control (MAC) sublayer and the network layer.
The LLC sublayer provides multiplexing mechanisms that make it possible for several network pr…

Basic Information You Must Know in Order to Troubleshoot

LLC Frame Formats

LLC2 Modes and Session Establishment

LLC2 Asynchronous Balanced Mode Operation

LLC2 Error Conditions