Talk:Maximum transmission unit

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MTU and Modems[edit]

For example, a 1500-byte packet, the largest allowed by Ethernet at the network layer (and hence over most of the Internet), ties up a 14.4k modem for about one second.

Shouldn't that be one tenth of a second? (talk) 15:50, 3 March 2015 (UTC)

Nope. 1500 byte = 12,000 bit ≈ 14,250 serial bits (8N1). -- Zac67 (talk) 18:16, 3 March 2015 (UTC)


Moved because MTU is a TLA, and half the references were to MTU Aero Engines.

I like the style[edit]

It is true that this may not be the suitable Wikipedia style, but I like it and, as a student, I find it clear and useful. Doru001 (talk) 13:11, 20 January 2009 (UTC)

Does anyone have any comments about what is or was formerly wrong with the style/tone? I may be one of the offenders/improvers? Should the marker now be removed?CecilWard (talk) 13:49, 30 January 2009 (UTC)

Does anyone think that the descrition should refer to the fact that the MTU is the size of the PDU of the higher layer that is transported? This would make it clearer that the PDU of the transporting layer will be bigger than the MTU i.e. Ethernet II has an MTU of 1500 Bytes (of say IP) but the size of the Ethernet II frame will be larger than this at 1518 or more? Amore proprio (talk) 12:35, 6 May 2009 (UTC)

rename for better understanding[edit]

I would like to vote for remaning this article from MTU (networking) to Maximum transmission unit and to change the redirection the other way round.

I was just about to say the same thing. It seems pretty obvious that this page should be Maximum transmission unit rather than MTU. Richard W.M. Jones 21:08, 17 February 2006 (UTC)
Agreed.CecilWard (talk) 13:50, 30 January 2009 (UTC)

Requested move[edit]

MTU (networking) -> maximum transmission unit -- expanding acronym will avoid need for ugly bracketed disambiguation Plugwash 16:56, 23 April 2006 (UTC)


Add *Support or *Oppose followed by an optional one-sentence explanation, then sign your opinion with ~~~~
  • Support per nom; seems sensible idea. Regards, David Kernow 01:03, 24 April 2006 (UTC)
  • Support, pretty straight forward. Shouldn't need a vote. —Pengo 07:01, 24 April 2006 (UTC)
    • Unforuntately the redirect has a (trivial) edit history so afaict it has to go through requested moves Plugwash 17:21, 24 April 2006 (UTC)
Done. —Nightstallion (?) Seen this already? 07:25, 28 April 2006 (UTC)

ATM section[edit]

I think the section about ATM optimisation sounds a bit wrong. For instance, the ethernet header (14 bytes) (I thought) would be stripped off before being sent via ATM. It also seems a bit irrelevant to be in Wikipedia - may be a personal website or something. -- 22:59, 26 November 2006 (UTC)

Ethernet header stripped off? Think again... how would you implement a layer 2 network over ATM then? —Preceding unsigned comment added by (talkcontribs) 2007-07-29 21:05:40

My understanding is that with any WAN technology, the WAN would be the "layer 2 network". Ethernet headers are only used within the LAN (if Ethernet technology is used); layer 3 packets remain basically unchanged when going from one network to another, but layer 2 headers have to be stripped off every time the packet passes a router, and new headers - depending on the technology - added again for the next hop. Layer 2 does NOT offer connectivity between different networks (in the above example, Ethernet and ATM); that's what layer 3 (e.g., IP) is for.--Hilmarz (talk) 21:15, 31 December 2007 (UTC)

If the ATM network is using LANE, there will be an ethernet header. Otherwise there will not be, just raw L3 packets. I've seen LANE used, but only rarely. ( (talk) 15:45, 11 September 2010 (UTC))

I agree with the earlier criticisms. I have expanded the section on ATM and added a concrete example of PPPoA optimal MTU choice which I hope is correct. I have marked _my own_ contribution as requires review by an expert.

I would also like someone to check and clarify the example of PPPoE (which PPPoE variant?) which I'm not sure about.CecilWard (talk) 13:06, 30 January 2009 (UTC)

ATM section is at very confusing or even wrong. I've attempted to rewrite it, but could not make it much better. While the "cell tax" is correct (although explanation is very bad - what is 'INT((payload_length+47)/48)' ??), the rest does not explain why changing MTU (which is MAXIMUM size) is a good thing at all. One can deduct that changing MTU for PPPoA connections is good because packet of MTU size are the most common (not true, large packets are only 20-40% of traffic) so it makes sense to optimize their transport (again not true, because for a 1500 byte packet 1 cell overhead is just 3%, while for a small 64-byte packet that make 40% of traffic there is 100% overhead). For PPPoE (_Ethernet_) 1492B MTU size (for PPP payload) is a result of maximum Ethernet MTU (1500B) minus 8 bytes space for PPP headers. It was not designed as optimized to fit ATM cells. What's more, MTU-sized packet will NOT fit exactly into 31 cells, because it is not 1492 or 1488 (detail: 1492 - 6 = 1486, not 1488) bytes! Headers are also transported (+6 bytes) and there is another 8 bytes for AAL5 PDU headers. That invalidates whole assumption because ATM payload has a much different size. —Preceding unsigned comment added by (talk) 08:08, 4 November 2009 (UTC)

It think getting too far into detail in this article is a mistake. There is a huge variety of possible configuration scenarios for ATM which depend on what exactly is being carried on it (PPPoE versus raw IP packets versus whatever) and what encapsulation you are using (not everyone uses AAL5, AAL3/4 has also seen usage). Specific numbers are best left to network engineering literature. In addition this applies to cell/frame based networks in general and also to packet based networks where low MTU values on intermediate hops are present for other reasons (e.g. packet interleaving for QoS). ATM is, however, a convenient example to use. It would suffice to remove the section and just say "When large packets are fragmented, the majority of those fragments will necessarily be of MTU size. Cell based networks (or any underlying encapsulation layer which may cause additional re-fragmentation) incur less overhead if those fragments fit efficiently. As such, MTUs are sometimes "tuned" by network engineers to reduce overhead." ( (talk) 15:45, 11 September 2010 (UTC))

Ethernet MTU[edit]


Isn't it 1522 (2000 soon?) (talk) 20:25, 24 June 2008 (UTC)

It depends how you define MTU. Internet RFCs tend to talk in terms of the MTU seen by the TCP/IP stack, that is the figure includes overhead from IP and higher layers but excludes overhead from lower layers. Plugwash (talk) 23:06, 24 July 2008 (UTC)
The length of the frame depends on what you count -- see the table/diagram at Ethernet#Physical_layer. 1500 is the max payload size for Type II Ethernet (1492 for 802.3 with SNAP.) Dunno about any proposed extensions.. -- (talk) 23:11, 24 July 2008 (UTC)
What you count is defined in the RFC
Please have a look at the german MTU article which has good references to the rfc's defining the MTU. I'll fix it but right now i don't want to.
In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet or frame that a given layer of a communications protocol can pass onwards.
is wrong since or frame is not right, see german wikipedia article for references. —Preceding unsigned comment added by (talk) 16:54, 12 February 2009 (UTC)
Agree with plugwash, indeed it is all about how your define MTU, which protocol layer and whether you are talking about the boundary immediately above or below that layer, (SDUs versus PDUs), and often confusion results from discussion including or excluding a header. Can we / do we need to improve the article in certain places in this respect?CecilWard (talk) 13:10, 30 January 2009 (UTC)

What about Ethernet "jumbo frames"? Any particular reason for not mentioning them? That would be 9000 bytes. — Preceding unsigned comment added by Dwandelt (talkcontribs) 18:36, 7 February 2014 (UTC)

The table tells that the MTU size is in bytes, while the reference is rather in octets, at least for the ethernet V2 line. Which one is correct? — Preceding unsigned comment added by (talk) 19:44, 8 July 2015 (UTC)

These days, octets and bytes are the same thing. ~Kvng (talk) 14:12, 18 November 2015 (UTC)

MTU versus TCP Window Confusion[edit]

The final paragraph of "Path MTU Discovery" has made a fundamental mistake regarding the implementation of Receive Window Tuning in Vista. The Receive window has nothing to do with the ethernet frame size or IP packet size (or MTU) but is in fact to do with the TCP window size - which is the number of TCP packets which may be transmitted before an acknowledgement is received. The paragraph which stands is correct - its just in the wrong place. —Preceding unsigned comment added by (talk) 00:04, 28 January 2009 (UTC)

This is the paragraph that has been removed:

This problem has surfaced more frequently since the introduction of Windows Vista which introduces the 'Next Generation TCP/IP Stack'. This implements "Receive Window Auto-Tuning that continually determines the optimal receive window size by measuring the bandwidth-delay product and the application retrieve rate, and adjusts the maximum receive window size based on changing network conditions".[1] This has been seen to fail in conjunction with older routers and firewalls that appeared to work with other operating systems. It is most often seen in ADSL routers and can often be rectified by a firmware update.


There is no explanation of why Jabber redirects here. Please add this. —Preceding unsigned comment added by (talk) 08:49, 29 March 2011 (UTC)

Network Engineers Talking Among Themselves[edit]

I'll be the first to say it: Lots of people come here because there's an MTU Size adjustment for their routers, and they don't know the first thing about what do with it, except that if adjusting it might improve their throughput, they'd like to maximize it.

But, before the first paragraph in the article is finished, the direction of the content diverts from Basic Definition of MTU to assorted technical details, such as how MTU varies with different systems, etc.

I would like to see more dwelling on the Basic Definition for those of us coming here with our router-concerns. For instance, MTU Size is on the Basic Settings page of my router's web-interface, so I come here expecting to be able to find a description that will round out my Basic Understanding. That would require mixing in some Customer-Service perspective into all of this Engineering talk, which obviously requires talents that go beyond engineering.

I believe my comment correlates with why this article has a "WikiProject Article Quality Grade of "C."

Thanks, Nei1 (talk) 15:47, 29 June 2011 (UTC)

802.11 MTU[edit]

I'm finding conflicting and confusing information for 802.11 MTU. Anyone have have a definitive reference and explanation? --Kvng (talk) 15:59, 23 December 2011 (UTC)

See the 802.11 2012 Specification on page 413, section - Data Frame Format — Preceding unsigned comment added by Roy muzz (talkcontribs) 01:42, 25 May 2012 (UTC)

At the time I'm posting this, the article claims that the 802.11 MTU 7981 bytes. However, all the 802.11 specifications show the frame size to be 2346 bytes (34 byte header and 2312 byte body). Furthermore, the reference points to page 413, which is a flowchart, and has no MTU information at all. — Preceding unsigned comment added by (talk) 20:57, 29 January 2016 (UTC)

Same user as above; I take back some of what I said. The 802.11 reference is correct, for the 2012 publication of the same. The older publications with the same name have different content. — Preceding unsigned comment added by (talk) 21:00, 29 January 2016 (UTC)

PDU vs SDU?[edit]

Article's first line says: "the size (in bytes) of the largest protocol data unit" -- but shouldn't it read "the size (in bytes) of the largest service data unit"? — Preceding unsigned comment added by (talk) 23:43, 12 January 2012 (UTC)

It is confusing, very confusing and so badly defined. But I explained this to myself this way: MTU of Ethernet is 1500, and 1500 is the max. payload which fits in Ethernet frame, then MTU is the largest PDU that comes from higher layer (IP) to the observed layer (Ethernet) that becomes its SDU and that it (Ethernet) can pass onward to lower layer. Hope I'm correct? Why didn't they just say "it's the maximum size of payload for the observed layer"?! -- (talk) 17:54, 10 July 2014 (UTC)

PDU and SDU are only useful terms while you're talking about a single layer, otherwise they need to be layer specific (ie. "MAC PDU". MTU is a layer 3 term, so "PDU" is the correct term here – until you mention "Ethernet frame" which is L2 and the MTU becomes the SDU in L2... "Payload" isn't correct because the MTU is the maximum payload (SDU) of the N-1 layer. Zac67 (talk) 18:43, 10 July 2014 (UTC)
I don't agree with you Zac67. Something is fishy here. If MTU is maximal PDU of a protocol, than PDU of Ethernet is 1522 (or something). Maximal SDU of Ethernet is 1500. — Preceding unsigned comment added by (talk) 00:11, 15 November 2015 (UTC)
The MTU is L3's maximum sized PDU. The L3 PDU becomes L2's (e.g. Ethernet MAC) SDU when it's transported. --Zac67 (talk) 11:15, 15 November 2015 (UTC)

PPoE rarely used?[edit]

I found the MTU of 1492 for PPoE and the note rarely used interesting. Is it refereing to PPoE being rarely used or the 1492? I thought PPoE is still fairly common for DSL. When I set up my DSL connection something that was recommended to me was to set the MTU to 1492 on the router to maximize the connection / to ensure full compatibility. I don't know enough to make a proper edit but perhaps the note needs to be updated or PPoE given it's own section? — Preceding unsigned comment added by (talk) 00:15, 25 April 2012 (UTC)

External links modified[edit]

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Question? Archived sources still need to be checked

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  1. ^ "The Cable Guy" (2005). "Performance Enhancements in the Next Generation TCP/IP Stack". Microsoft Corporation.  Unknown parameter |month= ignored (help)