Datos interesantes sobre MTU

Publicado: abril 19, 2011 en Uncategorized

extraido de http://whirlpool.net.au/wiki/adsl_theory_ppp

Point to Point Protocol (PPP) is, as its name suggests, a protocol for establishing a link between two points. In the case of ADSL it’s between your modem or PC to our BRAS (Broadband Remote Access Server – the PPP server). PPP isn’t technically required for an ADSL connection, the internet could run straight over the ADSL’s ATM network. PPP is used on most of ADSL connection because of the legacy of dialup; an authorisation/accounting system is needed and PPP is the defacto standard. PPP can be run in two different ways and the preferred style is dependant on your ISPs set up. PPPoE is best if they run an ethernet link between the DSLAM and BRAS, PPPoA is better if they are running an ATM network between the DSLAM and BRAS. Why the difference? Maximum Transmissable Unit (MTU) is the largest individual data packet that can be sent over a network, in the case of Ethernet it’s 1500, in the case of ATM it’s not limited. What this means is that as the packet passes from ADSL to Ethernet is can exceed Ethernet’s MTU and be dropped. By forcing PPPoE your modem forces the packets to stay within Ethernet’s 1500 MTU limit. That said PPPoA is marginally more efficient with overheads and processing by both ends. Many people are asking: – should I use PPPoE or PPPoA – what MTU is better Some people are reporting troubles with PPPoA and some with PPPoE. So use whatever works for you, however if you are really in a position to choose, then you will be slightly better off by using PPPoA. As for MTU, set it to 1462 in case you have settled on PPPoA. So the short answer is: use PPPoA with MTU 1462 bytes. The long (and much more involved) answer. Part one: PPPoE vs PPPoA. PPPoE uses one extra eight bytes long header which eats into the payload. PPPoA does not have this header so it has less overhead and each packet can carry more useful data (8 bytes more) which results in slight (around one percent) speed improvement. The long (and much more involved) answer. Part two: MTU issues. The default MTU for PPPoA is 1500 bytes. The same default for PPPoE is 1492 bytes (8 bytes less due to increased overhead because of one extra 8 bytes header mentioned above). Your ADSL modem always talks to DSLAM using ATM with either PPPoE or PPPoA (whatever you have chosen) on top of ATM. DSLAM is in turn connected to a server called BRAS/LNS using either ATM (in case of Telstra Wholesale DSLAMs) of Gigabit Ethernet – GE (in case of iiNET DSLAMs). If backhaul is ATM based, then DSLAM can process both PPPoE and PPPoA and it can digest both MTU of 1500 bytes for PPPoA and MTU of 1492 bytes for PPPoE. These are the default MTU values so no probs here. If backhaul is GE based, then DSLAM can still process both PPPoE and PPPoA, however it can digest only MTU of no more than 1492 bytes. If you have chosen PPPoE then 1492 is the PPPoE’s default MTU and everything is fine. If you have chosen PPPoA and kept its default MTU equal to 1500 bytes, then you are in trouble unless you lowered MTU to at least 1492 bytes. This is the reason why iiNet recommends PPPoE – less potential issues with MTU exceeding 1492 bytes because iiNet DSLAMs are set to max MTU equal to 1492 bytes for both PPPoE and PPPoA. The long (and much more involved) answer. Part three: What MTU to choose. When choosing MTU you should aim at increased speed. So don’t think in terms of choosing between 1500 and 1492 bytes MTUs as these values are only relevant when you consider how to avoid the potential trouble when a DSLAM (with GE backhaul) drops your packets with 1500 bytes MTU. But we are after the increased speed aren’t we? It will be achieved by getting 53 bytes long ATM cells filled better. Which happens when the packet has 1454 bytes MTU for PPPoE and 1462 bytes MTU for PPPoA. PPPoA is itself more efficient for the reason described above. Hence the answer: PPPoA with 1462 bytes MTU.

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