What Is The Minimum Ethernet Frame Size That Will Not Be Discarded By The Receiver As A Runt Frame

Ethernet networks have seen a phenomenal growth due to their ability to satisfy 5 critical requirements: operation (response time, bandwidth and scalability), resilience, ruggedness, economy, and interoperability. Due to this rapid growth, selecting optimal nonetheless cost effective infrastructure components for a network from the litany of network equipment manufacturers and models available has go a daunting task even for seasoned systems engineers.

And then what should be kept in mind while selecting and testing network components?

We are going to shed light on that in our upcoming editions but get-go let us take a look at the nuts.

To give a cursory introduction about Ethernet to our readers, it is an asynchronous, frame-based protocol originally intended to provide a means of communication between more than two data devices, using shared media. Ethernet, fully divers by the IEEE802.iii standard, has changed and evolved over fourth dimension, increasing in speeds with total duplex manual and extended link distances doable using optical cobweb as the transmission medium. The most common Ethernet physical interfaces in current apply are:

x/100Base-T – 10Mbit/south or 100Mbit/southward systems over twisted pair cablevision
100Base-Fx – 100Mbit/s over single mode or multimode optical fiber.
1000Base-T – 1000Mbit/s over twisted pair cable
1000Base-Sx – 1000Mbit/s over fiber @ 850nm over multimode cobweb
1000Base-Lx – 1000Mbit/due south over cobweb @ 1310nm over singlemode or multimode fiber

Fiber has taken over as the preferred medium where required distances exceed 100m.

We volition take a structured approach by looking commencement at an Ethernet frame and its constituent fields, so nosotros will discuss Ethernet's place in the OSI model, switching methodologies and finally the characteristics that should be tested to define the limits of a network. Hither is a typical Ethernet frame:

Figure 1: Structure of an Ethernet Frame

Preamble/Commencement of Frame Delimiter, eight Bytes: These bytes allow for receiver Synchronization and marking the start of a frame.

Destination Accost, 6 Bytes: Written usually in Hex, the MAC destination address of the frame is used to route it between devices.

Source Accost, 6 Bytes: Mac address of the sending station. MAC address is frequently called a burnt-in accost as these addresses are hard coded within a network component at the time of manufacture. The first three bytes are likewise called Organizational Unique Identifiers (OUI) and as the name suggests, these identify the manufacturer. The remaining 3 Bytes are unique to the equipment.

VLAN Tag, iv Bytes: This is optional. If a VLAN tag is present in the frame, it provides a ways of separating data in to virtual LANs, irrespective of the MAC accost. It likewise provides a priority tag which tin be used to implement quality of service (QoS) functions.

Length/Type, 2 Bytes: This field determines either the length of the frame or blazon of data beingness carried in the data field (indication of the protocol carried past the data).

Data, 46-1500 Bytes: This is the data to be transported. Information technology unremarkably consists of higher layer protocols such as IP.

Frame Bank check Sequence, 4 Bytes: Using the information provided in this field, switches can discover if the full frame has been received. A frame gets dropped if it has incorrect or missing FCS.

The minimum legal frame size, including the FCS but excluding the preamble & VLAN tag, is 64 bytes. Frames below the minimum size are known as "runts" and would be discarded by most Ethernet equipment.

The maximum standard frame size is 1522 bytes if VLAN tagging is being used and 1518 bytes if VLAN is not being used. It is possible to use frames larger than the maximum size. Such frames are called "Jumbo frames" which accept a better ratio of overhead bytes to data bytes only these are non-standard and manufacturer specific, therefore interoperability cannot be guaranteed.

Frames are transmitted from left to right (come across Figure 1), to the lowest degree pregnant bit first. Frames are separated from each other past an Inter-packet gap. This is very useful for half-duplex operation where the medium has to go quiet before next frames starts transmission. Although not required for full duplex operation, information technology is still used for consistency. Minimum length of an inter-parcel gap is 12 Bytes.

Ethernet & the OSI Model

Figure 2: Seven layers of OSI Model

Figure 2: 7 layers of OSI Model

If you call up the OSI model and its vii layers, Ethernet covers the bottom two layers that are Layer i (Concrete layer) and Layer 2 (Data link layer). Layer 1 simply consists of the concrete medium (UTP or fiber) over which the information is transferred in the form of 1s and 0s and layer two provides the control mechanism for transmitting data on to the medium and receiving information from a medium. Network switches operate at these layers.

The part of Ethernet is to ensure that data is transferred over a single link in a communication network whereas layer 3 protocols ensure that the information is transferred from the original source to the destination using several Ethernet links (Figure ii). Information technology is interesting to note that the pop "Ping" control is a layer 3 command which measures round trip time of a network package and records whatsoever packet loss.

Higher layers have tasks of ensuring the integrity of transmitted data and its presentation to the user or application and are of fiddling interest in a transmission environment.

Figure 3 - A typical Network

Figure 3 – A typical Network

The Ethernet Switch

Before we go in to the details of testing methodologies, let united states have a expect at a typical layer two switch. A layer 2 switch maintains a table of MAC addresses and looks this table up when it receives a frame. A switch chooses to exercise the following to the frame:

Alluvion – If the destination MAC address is not in the MAC accost tabular array, the switch floods the frame which means information technology gets sent to all ports except for the port through which information technology arrived. Such a frame is called an unknown unicast frame. The destination MAC responds. The destination MAC accost and the associated port are stored in the tabular array.

Filter – A frame volition be discarded if the destination and source MAC addresses stored on the MAC address table are located on the same port.

Forward – If the destination MAC address is in the MAC address table, the frame will be forwarded to the port to which the destination MAC accost is continued.

Once the switch has decided to forrad the frame, information technology uses one of the post-obit three processes to do information technology.

Store and Frontwards – As the name suggests, the switch stores the entire frame and checks the FCS field for errors before forwarding the frame. By checking the FCS, this method gives a means for error detection. When a frame is found to have errors, it will be discarded.

Cut-through – The switch reads the destination MAC addresses and frontwards the frame without storing it and without checking the FCS field. No error detection here but the fastest forwarding method. This method results in low switch latency. Both expert and bad information frames become sent to destination ports.

Fragment Gratuitous – The switch stores start 64bytes of the incoming frame and if there is no error/corruption found in these bytes, it forrard the frame. It is a middle ground between store and forward and cut-through forwarding methods.

Switches also support auto-negotiation betwixt each other to advertise their capabilities and configure themselves at the highest mutual setting. These capabilities include speed (10Mbits/s, 100Mbits/south & 1000Mbits/south), full or half duplex and the use of flow control.

OSD switches like the 4 port ten/100BaseT OSD2044 fiber optic switch utilizes the store and forward switching mechanism for greater error detection with ½ Megabit frame buffer memory. All OSD Ethernet modems come with a feature chosen auto MDI/MDIX which allows the user to connect the ten/100/1000BaseT ports on the OSD modems to any kind of Ethernet device without worrying about the type of Ethernet cable (straight-through or cross-over) being used for the connectedness.

Testing Ethernet Services

Ethernet connections must be tested to ensure right operation at the required levels of traffic. The procedures for performing these tests & specifications for frame sizes, examination durations and number of exam iterations have been detailed by the IETF and documented in RFCs (Requests for Comments, which are technical proposals and/or documentation). The most relevant RFCs for testing network equipment are RFC2544 (Benchmarking Methodology for Network Interconnect Devices) and RFC2889 (Benchmarking Methodology for LAN Switching Devices).

While RFC2544 is written as a general methodology for networking devices of all types, RFC2889 is written specifically to benchmark the functioning of a layer 2 LAN switching device.

In this newsletter, nosotros volition discuss RFC2544 and some of the tests it outlines and building on this information, we will look at RFC2889 in our next newsletter.

Frame Sizes – In guild to ensure that an Ethernet network is capable of supporting a variety of services (such as VoIP, video, etc.), both RFC 2544 & RFC 2889 support 7 pre-defined frame sizes (64, 128, 256, 512, 1024, 1280 and 1518 bytes & 1522 bytes including VLAN) to simulate various traffic conditions. Small-scale frame sizes increase the number of frames transmitted, thereby stressing the network device as it must switch a big number of frames.

RFC 2544 Tests

Throughput – Data throughput is simply the maximum amount of information that tin exist transported from source to destination. A throughput test defines the maximum number of frames per 2d that can be transmitted without whatever error. This examination is done to measure the rate-limiting capability of an Ethernet switch.

The maximum throughput doable for various frame sizes is given in tables 1, 2 and 3 or 10, 100 and 1000Mbit/s respectively. IGP stands for the inter-packet gap and a minimum size of 12Bytes has been used.

Back-to-Back – The back-to-back test (also known as burstability or burst examination) assesses the buffering capability of a switch. Dorsum-to-back frame testing involves sending a burst of frames with minimum inter-frame gaps to the device under examination (DUT) and count the number of frames forwarded by the DUT. If the count of transmitted frames is equal to the number of frames forwarded the length of the burst is increased and the exam is rerun. If a frame is lost, burst length is shortened.

Frame Loss Test – The frame loss test measures the network's response in overload conditions—a critical indicator of the network's ability to support existent-time applications in which a big corporeality of frame loss will rapidly degrade service quality. As there is no re-manual in real-time applications, these services might rapidly become unusable if frame loss is non controlled.

For case, 1000 frames are transmitted just merely 950 were received the frame loss rate would exist: (1000 – 950) / chiliad x 100% = 5%. Frames can be lost, or dropped, for a number of reasons including errors caused by wrong FCS, excessive filibuster in reception of frames etc.

Latency Exam – The latency test measures the time required for a frame to travel from the originating device through the network to the destination device (too known as stop-to-stop testing). This test can also be configured to measure the round-trip time; i.e., the time required for a frame to travel from the originating device to the destination device so back to the originating device.

Organisation Reset Test – The objective of the test is to narrate the speed at which a DUT recovers from a device or software reset. Both hardware & software resets should be tested and results should be recorded in a simple gear up of statements for each type of reset.

System Recovery Test – This test checks the speed at which a DUT recovers from an overload condition. Information technology is performed by sending a stream of frames at a rate 110% of the recorded throughput rate for at least 60 seconds, stressing the DUT with high traffic. At time stamp A, frame rate is reduced to l % of the above rate and fourth dimension of the last frame that was lost due to high traffic is recorded (time stamp B). The organisation recovery time is adamant by subtracting fourth dimension stamp B from fourth dimension stamp A.

RFC 2544 primarily describes not-meshed traffic which means that the test frames are offered to a single port and addressed to a unmarried output port on a switch under test for both uni-directional and bi-directional traffic. RFC2889 on the other manus provides methodology for testing switches with partially meshed and fully meshed traffic patterns. In a fully meshed pattern all ports offering traffic destined to all other ports on a switch nether exam whereas in partially meshed traffic scenario, one test port send traffic to many ports and vice versa.

More comprehensive tests are outlined in RFC2889 which provides methodology for testing switches with partially meshed and fully meshed traffic patterns.

Partially meshed traffic pattern ways that frames are offered to one or more than input interfaces of a switch and addressed to one or more than output interfaces where input and output interfaces are mutually exclusive and mapped one-to-many, many- to-one or many-to-many.

Fully Meshed Traffic pattern ways that frames are offered to a designated number of interfaces of a switch such that each one of the interfaces nether test receives frames addressed to all of the other interfaces nether exam.

This is to be continued in our adjacent newsletter….

Don't hesitate to contact OSD Systems Engineers for any further information on this subject field.