lans/ethernet-faq by james messer

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Path: senator-bedfellow.mit.edu!dreaderd!not-for-mailMessage-ID: Supersedes: Expires: 22 Sep 2003 10:09:14 GMTX-Last-Updated: 1999/07/09Organization: noneFrom: [email protected] (James Messer)Newsgroups: comp.dcom.lans.ethernet,comp.answers,news.answers

Subject: comp.dcom.lans.ethernet FAQApproved: [email protected]: comp.dcom.lans.ethernetOriginator: [email protected]: 09 Aug 2003 10:12:58 GMTLines: 786NNTP-Posting-Host: penguin-lust.mit.eduX-Trace: 1060423978 senator-bedfellow.mit.edu 3930 18.181.0.29Xref: senator-bedfellow.mit.edu comp.dcom.lans.ethernet:76537 comp.answers:54463news.answers:255966

Archive-name: LANs/ethernet-faqPosting-Frequency: monthly

Last-modified: 1999/07/09Version: 990709URL: http://www.NetworkUptime.com/faqs/ethernetCopyright: (c) 1999 James MesserMaintainer: James Messer

comp.dcom.lans.ethernet Frequently Asked Questions--------------------------------------------------

This document is provided as is without any express or impliedwarranties. While every effort has been taken to ensure the accuracy of

the information contained in this article, the authors assume noresponsibility for errors or omissions, or for damages resulting from the

use of the information contained herein. The contents of this articlereflect my opinions only and not necessarily those of my employer.

FAQ Table of Contents---------------------

1.0 FAQ Administration

[1.1] What is this FAQ?[1.2] Who maintains this FAQ?[1.3] Where can this FAQ be found?[1.4] Who provides information to the FAQ?[1.5] Can I use this FAQ on my web page?

[1.6] Copyright Information

2.0 Introduction to Ethernet

[2.1] What is Ethernet?[2.2] What is the history of Ethernet?[2.3] What is CSMA/CD?[2.4] What is the Open Systems Interconnect (OSI) model?[2.5] Where are the IEEE specifications?

3.0 Ethernet Physical Layer


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[3.1] What are the different physical Ethernet network types?[3.2] What does baseband and broadband mean?[3.3] What is the difference between a bus topology and a star topology?[3.4] What physical Ethernet topologies are no longer popular?[3.5] What are the most common physical Ethernet networks used today?[3.6] What digital signal encoding is used in an Ethernet network?[3.7] What types of cabling are used for Ethernet?

[3.8] What pin assignments are used in twisted-pair Ethernet cabling?[3.9] Can two Ethernet stations be directly attached with 10BASE-T?[3.10] How many stations are supported by a single Ethernet network?[3.11] What is propagation delay?[3.12] What is an interframe gap?

4.0 Ethernet Data Link Layer

[4.1] What are the different Ethernet frame formats?[4.2] What is transparent bridging?[4.3] What is the spanning tree protocol?[4.4] What is Ethernet switching?

5.0 Ethernet Errors and Troubleshooting

[5.1] What is a collision, and how many collisions are bad?[5.2] What is Signal Quality Error (SQE)?[5.3] What is jam?[5.4] What is a late collision, and why is it bad?[5.5] What is a runt?[5.6] What is jabber?[5.7] What is a CRC/Alignment error?[5.8] What non-commercial software is available to monitor an Ethernet

network?

6.0 Other Information

[6.1] What Ethernet-related books are available?[6.2] What certifications are available regarding Ethernet networks?

1.0 FAQ Administration

[1.1] What is this FAQ?

This FAQ will attempt to explain and decipher the intricacies ofEthernet networking and answer some of the most common questionsrelating to Ethernet networks. Although it contains technicalinformation, this FAQ is best used as an introduction to Ethernet

networking. See section [6.1] for Ethernet book and publicationinformation.

[1.2] Who maintains this FAQ?

This FAQ is maintained by James Messer .Questions, comments, corrections, and contributions are encouraged!

[1.3] Where can this FAQ be found?

This FAQ will be posted to the comp.dcom.lans.ethernet newsgroup


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on the first of each month. An official archive of the FAQ can befound at:

ftp://rtfm.mit.edu/pub/faqs/LANs/ethernet-faq

The HTTP version of this FAQ can be found at:

http://www.NetworkUptime.com/faqs/ethernet

[1.4] Who provides information to the FAQ?

In many cases, the FAQ questions and answers are summarized from thecomp.dcom.lans.ethernet newsgroup. Much of this information is alsoobtained from the IEEE standards (http://www.ieee.com) and relatedtechnical documents.

Send any corrections or FAQ additions to [email protected] thanks to all who have provided information to this FAQ! Yourcontributions are crucial for keeping this FAQ updated andtechnically correct.

Since the comp.dcom.lans.ethernet newsgroup has not had an'official' FAQ for a few years, this FAQ is in the early stages ofbeing rebuilt. There are certainly other questions that need to beadded to the FAQ, questions that need to be answered, and there areprobably inaccuracies with the answers that already exist in theFAQ. If you have any suggestions or corrections for the FAQ, pleasedo not hesitate to e-mail [email protected] with additions orcorrections.

[1.5] Can I use this FAQ on my web page?

Since this FAQ changes constantly, a copy of the FAQ on your webpage would be out of date in a very short time. Please don't do

this! A more appropriate method would be to set a hyperlink to theURL found in the secondary header of this FAQ. Please send an e-mailto [email protected] if you plan on adding a link to this FAQto your web page. I reserve the right to restrict the use of thisFAQ.

[1.6] Copyright Information

Copyright (c) 1999 by James Messer, all rights reserved.

This FAQ may be posted to any USENET newsgroup, on-line service, orBBS as long as it is posted in its entirety, includes this copyrightstatement, and includes written permission from

[email protected].

2.0 Introduction to Ethernet

[2.1] What is Ethernet?

Ethernet is the IEEE 802.3 series standard, based on theCSMA/CD access method that provides two or more stations to share acommon cabling system. This access method, Carrier Sense MultipleAccess with Collision Detection, is the basis for Ethernet systems


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3.0 Ethernet Physical Layer

[3.1] What are the different physical Ethernet network types?

Some of the physical Ethernet types as defined in the 802.3specification are:

10BASE5 - 10BASE5 is the original design of the traditionalEthernet backbone, designed to be left in placepermanently or for extended periods.

10BASE2 - 10BASE2 is the original design for a departmental orworkgroup sized Ethernet environment. It is designed to besimple, inexpensive, and flexible as people and stationsmove.

10BROAD36 - 10BROAD36 is a seldom used Ethernet specification whichuses a physical medium similar to cable television, withCATV-type cables, taps, connectors, and amplifiers.

1BASE5 - 1BASE5 is a specification of Ethernet that runs at 1 Mb/sover twisted pair wiring. This physical topology usescentralized hubs to connect the network devices.

10BASE-T - 10BASET provides Ethernet services over twisted paircopper wire.

FOIRL - Fiber Optic Inter-Repeater Link - This specification of the802.3 standard defines a standard means of connectingEthernet repeaters via optical fiber.

10BASE-F - 10BASE-F is a set of optical fiber medium specifications

which define connectivity between devices.

100BASE-T - 100BASE-T is a series of specifications that provides100 megabit speeds over copper or fiber. Thesetopologies are often referred to as Fast Ethernet.

Gigabit Ethernet - Gigabit Ethernet provides speeds of 1000 Mb/sover copper and fiber.

[3.2] What does baseband and broadband mean?

A baseband network has a single channel that is used forcommunication between stations. Ethernet specifications which use

BASE in the name refer to baseband networks.

A broadband network is much like cable television, where differentservices communicate across different frequencies on the same cable.Broadband communications would allow a Ethernet network to share thesame physical cable as voice or video services. 10BROAD36 is anexample of broadband networking.

[3.3] What is the difference between a bus topology and a star topology?

A bus topology is a networking architecture that is linear, usually


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by using one or more pieces of cable to form a single line, or bus.The signals sent by one station extend the length of this cable tobe heard by other stations.

A star topology is an architecture that includes a central device orhub to connect all stations together. Signals sent by a station mustpass through (and are usually regenerated) by these central hubs.Since the hub sits in the center and all other stations are linked

through the hub, the architecture resembles a star.

[3.4] What physical Ethernet topologies are no longer popular?

There are a number of physical networking components specified inthe IEEE 802.3 specification, but many of those early physicalnetworking components are not used in most modern Ethernet networks.However, there may be instances where an existing legacy networkstill exists which uses these older components. Since these olderpieces of equipment are still part of the 802.3 specification, thereare no technical reasons why an Ethernet network would not operateproperly with these components. The two most popular older Ethernettechnologies are 10BASE5 and 10BASE2.

10BASE5-------

10BASE5 is the original Ethernet backbone, and is occasionallyreferred to as thicknet or thick Ethernet because of the thick 50ohm coax that was used as the physical medium. 10BASE5 is a bustopology that uses transceiver cables to attach stations to thecentral 10BASE5 cable.

Maximum segment length: 500 metersMaximum number of segments connected with repeaters: 5 (2500 meters)

Maximum attachments per segment: 100Minimum separation between attachments: 2.5 meters

10BASE2-------10BASE2 is designed as a smaller and less expensive alternative to10BASE5, and is sometimes referred to as Thinnet or Thin Ethernetbecause of the much smaller cables. 10BASE2 is also a bus topology,but each of the workstations use a 'T' BNC connector to connectworkstations to the central bus.

Maximum segment length: 200 meters

Maximum number of segments connected with repeaters: 5 (1000 meters)Maximum attachments per segment: 30Minimum separation between attachments: .5 meters

[3.5] What are the most common physical Ethernet networks used today?

Most modern Ethernet networks use twisted pair copper cabling orfiber to attach devices to the network. The 10BASE-T, 100BASE-T, andGigabit Ethernet topologies are well suited for the modern cablingand fiber infrastructures.


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[3.6] What digital signal encoding is used in an Ethernet network?

** Please contribute! See Section [1.4] **

[3.7] What types of cabling are used for Ethernet?


[3.8] What pin assignments are used in twisted-pair Ethernet cabling?

Twisted-pair Ethernet (10BASE-T, 100BASE-T, or 1000BASE-T) uses anRJ-45 connector, which is an eight-pin modular connector.

Contact 1 Transmit +Contact 2 Transmit -Contact 3 Receive +Contact 4 Not UsedContact 5 Not UsedContact 6 Receive -Contact 7 Not UsedContact 8 Not Used

When looking at an RJ-45 wall jack (female), contact 1 is on theleft and contact 8 is to the right. When looking at the RJ-45connector on the end of a cable (male) with the tab on the bottomand the contacts on the top, contact 8 is on the left and contact 1is to the right.

[3.9] Can two Ethernet stations be directly attached with 10BASE-T?

Two Ethernet stations can be directly attached to each other, butthe cabling will be wired differently than a normal 10BASE-TEthernet network connection. The 802.3 specification refers to thisdirect connection between two stations as a crossover function.

The crossover function is accomplished by simply wiring the receivepins to the transmit pins:

Contact 1 - Contact 3Contact 2 - Contact 6Contact 3 - Contact 1Contact 6 - Contact 2

[3.10] How many stations are supported by a single Ethernet network?


[3.11] What is propagation delay?

The propagation speed of a medium refers to the speed that the datatravels through that medium. Propagation delays differ betweenmediums, which affect the maximum possible length of the Ethernettopology running on that medium.

In the following table, c refers to the speed of light in a vacuum,or 300,000 kilometers per second.

Medium Propagation Speed


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------ -----------------Thick Coax .77c (231,000 km/sec)Thin Coax .65c (195,000 km/sec)Twisted Pair .59c (177,000 km/sec)Fiber .66c (198,000 km/sec)AUI Cable .65c (195,000 km/sec)

From these values, the size of a bit on 10BaseT can be calculated.

10BaseT is twisted pair, which has a propagation delay of 177,000km/sec. 177,000 km/sec divided by 10 million bits per second is17.7 meters, or the size of a single bit on a 10BaseT network.

The maximum propagation delay through the network can be calculatedby dividing the maximum length by the speed. For 10Base2 thin coaxnetwork, this is 185 meters divided by 195,000 km/sec, or 950nanoseconds. If the actual propagation delay from one end of thenetwork to the other is greater than 950 nanoseconds, latecollisions may occur. See section [5.4] for more information onlate collisions.

[3.12] What is an interframe gap?

The inteframe gap is the amount of time that is specified betweenframes transmitted from a workstation. The designers of theEthernet specification arbitrarily chose 96 bit times to occurbetween frames from a transmitting station.

This delay is designed to provide the workstations on the Ethernetnetwork with some 'breathing time' between frames to perform normalEthernet housekeeping functions on the network interface card.

4.0 Ethernet Data Link Layer

[4.1] What are the different Ethernet frame formats?

Ethernet Version 2 and IEEE 802.3 Frame Formats-----------------------------------------------

The Ethernet Version 2 frame format was designed before the IEEEspecifications, but is almost identical to the 802.3 frame type.With the Ethernet Version 2 frame type, a two-byte Type fieldfollows the source station's six-byte MAC address. In the 802.3frame type, this two-byte field after the source address is a lengthfield specifying the number of bytes in the LLC and data fields. Ifthese two bytes are greater than 05DC hex (1500 decimal), the frameis a Version 2 frame. Since all type fields are greater than 1500

decimal (the maximum Ethernet frame size), both frame types caneasily coexist on the same network. Some network protocol analyzerscall a Version 2 frame an Ethertype frame because of this two-byteType field.

This is an Ethernet Version 2 frame:

+--------------+| | The preamble consists of 62 bits of alternating| Preamble | ones and zeros that allows the Ethernet card to| 7 bytes | synchronize with the beginning of a frame.


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| |+--------------+ The Start Frame Delimiter is the sequence| SFD - 1 byte | 10101011, and indicates the start of a frame.+--------------+| | The destination address is a six byte Media Access| Destination | Control (MAC) address, usually burned into the| 6 bytes | ROM of the Ethernet card.+--------------+

| | The source address is a six byte MAC address, and| Source | can signify a physical station or a broadcast.| 6 bytes |+--------------+| Type | The Type field (see explanation above).| 2 bytes |+--------------+| | Any higher layer information is placed in the| Data | data field, which could contain protocol| | information or user data.~ ~~ ~| 46 to 1500 |

| bytes || |+--------------+| FCS | The Frame Check Sequence is a cyclic redundancy| 4 bytes | check used by the sending and receiving stations+--------------+ to verify a successful transmission. The FCS is

based on the contents of the destination address,source address, type, and data.

The 802.2 Logical Link Control (LLC) Information------------------------------------------------

The IEEE 802.3 Ethernet specification was intended to be used with

the 802.2 Logical Link Control (LLC) specification. The LLCinformation envelops the data of the frame, and the 802.3 headersenvelop this 802.2 LLC protocol data unit (PDU).

This is the frame structure for an 802.3 Ethernet frame thatcontains the 802.2 LLC information:

+----------------+| || Preamble || 7 bytes || |+----------------+

| SFD - 1 byte |+----------------+| || Destination || 6 bytes |+----------------+| || Source || 6 bytes |+----------------+| Frame Length |


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| 2 bytes |+----------------+| DSAP - 1 byte | The Destination and Source Service Access Point+----------------+ fields determine the protocol used for the upper| SSAP - 1 byte | protocol type of the frame.+----------------+|Control - 1 byte| The Control field is used for administration by+----------------+ certain protocols.

| Data || |~ ~~ ~| 46 to 1500 || bytes || |+----------------+| FCS || 4 bytes |+----------------+

The 802.2 Sub-Network Access Protocol (SNAP)--------------------------------------------After the 802.2 frame type was defined, many people felt that asingle byte for DSAP and SSAP would not be sufficient to handle thegrowth of protocols into the future. A single byte DSAP or SSAP canonly specify 256 separate protocols, and many of those werepredefined from the beginning of the 802.2 specification.

To provide future growth, the Sub-Network Access Protocol (SNAP) wascreated as an extension to the 802.2 specification. To differentiatethis protocol from the original 802.2 specification, 802.2 SNAP usesthe DSAP and SSAP of 0xAA.

This is an 802.2 SNAP frame encapsulated in an 802.3 frame:

+----------------+| || Preamble || 7 bytes || |+----------------+| SFD - 1 byte |+----------------+| || Destination || 6 bytes |

+----------------+| || Source || 6 bytes |+----------------+| Frame Length || 2 bytes |+----------------+| DSAP - 1 byte |+----------------+| SSAP - 1 byte |


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+----------------+|Control - 1 byte|+----------------+ The Organizationally Unique ID (OUI) is assigned| OUI - 3 bytes | to unique vendors to help differentiate protocols| | from different manufacturers.+----------------+| Type - 2 bytes | The two-byte protocol type defines a specific+----------------+ protocol in the SNAP. This also maintains a

| | compatibility with Ethernet v2.| Data || |~ ~~ ~| 46 to 1500 || bytes || |+----------------+| FCS || 4 bytes |+----------------+

Novell 802.3 'Raw' Frame Format-------------------------------

Before the final 802.2 LLC specifications were finalized, Novellimplemented IPX/SPX over Ethernet. For this reason, Novelloriginally utilized 802.3 Ethernet without using 802.2 LLC. Becauseof this lack of LLC header, this frame type was nicknamed 802.3'raw'. Since Novell created this proprietary frame type for theirown use, no other manufacturer uses this frame type.

To implement their 'raw' frame type, Novell used the first two bytesof the 802.3 data field as 0xFFFF. Since the DSAP and SSAP values of

0xFF do not exist, it becomes easy to differentiate between the802.3 and 802.3 'raw' frame types.

[4.2] What is transparent bridging?

Transparent bridging is a method to connect two similar networksegments to each other at the datalink layer. It is done in a waythat is transparent to end stations, hence end-stations do notparticipate in the bridging algorithm.

Transparent bridges are sometimes called learning bridges.When they are turned on and receive data packets from a networksegment they:

1) learn the relation between MAC address and segment/port, and2) forward the packet to all (!) other segments/ports.

The first step in this process is essential to the "learning" aspectof the bridge. After some time the bridge has learned that aparticular MAC address, say MACa, is on a particular segment/port,say PORT1. When it receives a packet destined for the MAC addressMACa (from any port not being PORT1) it will no longer forward thepacket to all ports (step 2). It knows that MACa is associated withPORT1 and will only forward the packet to PORT1.


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[4.3] What is the spanning tree protocol?

Spanning tree is a protocol defined in IEEE 802.1q to preventbridges from creating network loops. Using the spanning treeprotocol, bridges communicate to each other and disable certainports/segments to prevent looping of packets.

Many implementations of the spanning tree protocol are configured soan alternate path is available to network traffic, should theoriginal path become disabled.

[4.4] What is Ethernet switching?

From a functional point of view, switching is exactly the same asbridging. However switches use specially designed hardware calledApplication Specific Integrated Circuits (ASICs) to perform thebridging and packet-forwarding functionality (as supposed toimplementations using a central CPU and special software).Consequently, switches are much faster than bridges.

Ethernet switches also offer additional capabilities such as virtualLANs (VLANs) and full duplex connectivity.

5.0 Ethernet Errors and Troubleshooting

[5.1] What is a collision, and how many collisions are bad?

Ethernet networking uses collisions as one of the contention accessmethods. When the network carrier is not active, any station cansend information. If two stations attempt to send information at thesame time, the signals overlap with each other, creating a

collision.

Collisions are not errors! Many people misinterpret a flashingcollision light or a collision counter as a network problem!Although the term 'collision' may bring to mind a terrible crash, beassured that a collision is a normal part of Ethernet networking.

The total number of collisions that occur on a network may berelated to traffic patterns or utilization. Because of thisvariability of collisions, it is not applicable to define a 'good'or 'bad' level of collisions. In most cases, detailed analysis ofcollisions alone yields very little qualitative network healthinformation.

[5.2] What is the Signal Quality Error (SQE) Test?

The SQE Test is used to test for the collision present circuitbetween a transceiver and a network interface card (NIC). After datais successfully transmitted, the Ethernet transceiver asserts theSQE signal on the collision presence circuit of the NIC. The NICsees this test signal as a verification that the transceiver willinform the NIC when a collision occurs.

In most modern Ethernet networks, the SQE test is not used or


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applicable. Most NICs now have an integrated transceiver andtherefore have a hard-wired AUI, so a test for the collisionpresence circuit is unnecessary.

[5.3] What is jam?

When a collision is recognized by a transmitting station, a bitsequence called jam is transmitted. This jam is 32 bits long, which

is long enough to traverse the entire collision domain so that alltransmitting stations can detect the collision.

Interestingly enough, the actual format of jam is unspecified inthe 802.3 specifications. Most manufacturers have used alternating1s and 0s as jam, which is displayed as 0x5 (0101) or 0xA (1010)depending on when the jam is captured in the data stream.

In many Fast Ethernet implementations, the jam has been seen asother arbitrary values, such as 1101000 (0xD0) or 10000110 (0x43).The reasoning for this particular jam pattern isn't very obvious. Ifanyone has more information on this jam sequence, please [email protected].

[5.4] What is a late collision, and why is it bad?

A collision is considered late if the jam occurs after 512bit-times, or 64 bytes. Collisions that occur after the first 64bytes of a frame may be indicative of a network design problem (thenetwork is so large the jam cannot traverse the entire length in 32bit-times), or a hardware or Ethernet firmware issue.

When collisions do not propagate the network quickly enough, acollision could occur between two stations without the stationsaware that the packets collided. In this situation, the frames aresimply lost, and the upper-layer protocols must begin a

retransmission process to retransmit the information. Theseretransmissions can cause large delays, especially at theapplication layer.

[5.5] What is a runt?

In Ethernet networks, any frame shorter than the minimum 64 bytesbut with a valid CRC is considered a runt. Other frame-length errorsin Ethernet are long frames, which are longer than 1518 bytes yethave a valid CRC.

[5.6] What is jabber?

Jabber is described most often as a frame greater than the maximumof 1518 bytes with a bad CRC. A jabbering NIC is often indicative ofa hardware problem with a NIC or transceiver.

[5.7] What is a CRC/Alignment error?

When a station sends a frame, it appends a Cyclical RedundancyCheck to the end of the frame. This CRC has been generated from analgorithm and is based on the data in the frame. If the frame isaltered between the source and destination, the receiving stationwill recognize that the CRC does not match the actual contents of


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the packet.

All frames should end on an 8-bit boundary, but problems on thenetwork could cause the number of bits to deviate from the multipleof 8.

Both CRC errors and alignment errors are grouped together as thesingle CRC/Alignment error counter.

[5.8] What non-commercial software is available to monitor an Ethernetnetwork?

A list of commercial, shareware, and freeware software is availableat:

http://www.NetworkUptime.com/tools

6.0 Other Information

[6.1] What Ethernet-related books are available?

The Certified Network Expert (CNX) consortium described in section[7.2] has an excellent list of Ethernet books. This list isdesigned for the network professional who is studying for the CNXcertification, and is a very comprehensive list of technicalpublications. This CNX reading list can be found at:

http://www.cnx.org/reading.htm

An updated CNX study library can also be found on NetworkUptime.com:

http://www.NetworkUptime.com/cnx/

[6.2] What certifications are available regarding Ethernet networks?

An Ethernet-specific certification is available through theCertified Network eXpert (CNX) program. This certification isEthernet topology specific, and does not emphasize any networkoperating system. Visit http://www.cnx.org for more information onthe CNX exam. Sylvan Prometric administers the CNX exam, and theirweb page is http://www.sylvanprometric.com. NetworkUptime.com alsokeeps a CNX resources page at http://www.NetworkUptime.com/cnx.

--- End of comp.dcom.lans.ethernet FAQ ---

lans/ethernet-faq by james messer

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