Re: [16NG] WG last call on draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: regarding my comments on MTUs
"Riegel, Maximilian (NSN - DE/Muenich)" <maximilian.riegel@nsn.com> Fri, 11 April 2008 07:22 UTC
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Thread-Topic: [16NG] WG last call on draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: regarding my comments on MTUs
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From: "Riegel, Maximilian (NSN - DE/Muenich)" <maximilian.riegel@nsn.com>
To: Samita Chakrabarti <samitac@ipinfusion.com>
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Cc: 16ng@ietf.org, Samita Chakrabarti <samitac2@gmail.com>, Burcak Beser <Burcak.Beser@telsima.com>, gabriel montenegro <g_e_montenegro@yahoo.com>
Subject: Re: [16NG] WG last call on draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: regarding my comments on MTUs
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If 'L2 tunneling' is replaced by 'L3 tunneling' I am fine with option 3. There are too many protocols and options in 'L2 tunneling', to make a recommendation without providing further details. Burcak may have one particular solution for fixed and nomadic deployments in mind, which works well, and I see issues with L2 tunneling in a mobile environment. Bye Max -----Original Message----- From: ext Samita Chakrabarti [mailto:samitac@ipinfusion.com] Sent: Thursday, April 10, 2008 11:01 PM To: Riegel, Maximilian (NSN - DE/Muenich); 'Burcak Beser'; 'Wesley George' Cc: 'Samita Chakrabarti'; 'gabriel montenegro'; 16ng@ietf.org Subject: RE: [16NG] WG last call on draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: regarding my comments on MTUs Hi Max, Not clear to me whether you are also supporting option 3 as suggested below by Burcak. Please confirm. Thanks, -Samita >-----Original Message----- >From: Riegel, Maximilian (NSN - DE/Muenich) >[mailto:maximilian.riegel@nsn.com] >Sent: Thursday, March 27, 2008 10:17 AM >To: Burcak Beser; Samita Chakrabarti; Wesley George >Cc: Samita Chakrabarti; gabriel montenegro; 16ng@ietf.org >Subject: RE: [16NG] WG last call on draft-ietf-16ng-ipv4-over-802-dot-16- >ipcs-02 + RE: regarding my comments on MTUs > >I would also support to prevent MTU limitations in the I-D, which would >be easy based on L3 tunneling, e.g. GRE over IP (IP can fragment and >reassemble the tunneled payload inside the access network). L2 tunneling >is much more critical, because L2 technologies often miss the >functionality to fragment and reassembly packets over a link. If L2 >breaks, there is no way to recover. > >Bye >Max > >-----Original Message----- >From: 16ng-bounces@ietf.org [mailto:16ng-bounces@ietf.org] On Behalf Of >ext Burcak Beser >Sent: Tuesday, March 25, 2008 8:33 PM >To: Samita Chakrabarti; 'Wesley George' >Cc: 'Samita Chakrabarti'; 'gabriel montenegro'; 16ng@ietf.org >Subject: Re: [16NG] WG last call on >draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: regarding my comments >on MTUs > >As far as I can see we tree options: > >1. Get WiMAX Forum to handle 1500 byte packets. I am not sure how this >may be achieved other than traveling back in time to impact NWG v1. > >2. Change this draft to 'personal'-ipv4-over-wimax-ipcs and use 1400 >byte limitation from WiMAX Forum NWG v1. > >3. Write the draft for IPCS assuming L2 tunneling and no MTU >limitations; and put the 1400 byte case on the appendix stating that >WiMAX is expected to be the widest 802.16 deployment and NWG v1 has the >1400 byte limitation as a special case, and offer the remedies and state >the problems. > >Personally I like the option 3 which is cleaner and straight forward. > >-burcak >________________________________________ >From: Samita Chakrabarti [samitac@ipinfusion.com] >Sent: Friday, March 21, 2008 4:07 PM >To: 'Wesley George'; Burcak Beser >Cc: 'gabriel montenegro'; 'Jari Arkko'; 'Samita Chakrabarti'; >16ng@ietf.org >Subject: RE: [16NG] WG last call on >draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: [16NG] regarding my >comments on MTUs > >Hi Wes, > >Please see in-line. > >> >>Samita, your proposed update merely adds more words to say the >>same thing that the current draft says - host MTU should be 1400. It's >>helpful in that it adds more justification to your argument, but I >still >>disagree >>with the underlying conclusion. >> >>My recommendation was to treat MTU as something that must be taken into >>consideration at the design phase of a WiMax network and the >>initial configuration of the devices, not a hard and fast limit based >on >>the limitations and assumptions of ONE case out of many, especially >since >>the >>806.16 over Ethernet draft completely contradicts your recommendation. >It >>views MTU as non-negotiable (must be at least 1500), and puts the >>responsibility on the network operator to ensure that they've build the >>network to support the right underlying MTU or deal with the >>fragmentation that will be inevitable. >>I don't see why we wouldn't be doing the same thing here. >[SC>] > As far as I know for IPCS network WiMAX forum has decided that >their >default link MTU would be 1400 bytes. Checkout RFC 5121 (appendix D) >which >chose the same value 1400 bytes for that reason. But one difference from >IPv6 case is that IPv6 specification requires to send RA with MTU option >if >the MTU supported by the link is not the default value. Thus by >following >IETF specification WiMAX AR can send MTU option with 1400 value to the >clients and the clients can adjust the MTU accordingly; while RFC 5121 >can >specify 1500 bytes as the default MTU. > >Now IPV4CS would be happy to set the same default 1500 bytes following >RFC5121 example - the only major issue is that there is no IPv4 >mandatory >requirement from the server to communicate (through RA or DHCP option >etc.) >the non-default link MTU unless the client itself asks for it. > >The problem that Jari and others are pointing to is that if we specify >1500 >bytes and the clients do not know that they are in WiMAX link and start >sending packets over 1400 bytes, the packets will not be delivered at >all. >Those clients will not operate in the WiMAX network. > >Please note that WiMAX forum can change their default MTU value in the >next >revision, but the current specifications (Rls 1.1) cannot change. > >This is the data behind the decision on 1400 bytes as initial value; it >matches the criteria that was given by Jari on the list. This IPv4CS >recommends the new clients to request MTU option from DHCP requests for >efficiency. > >If you or Burcak can work with WiMAX forum and produce a note from the >chair >that 1500byte IPv4CS payload is OK for WiMAX network for the current >NWG >releases in the next one or two weeks, we can really appreciate that and >change to 1500 bytes. Currently, I do not know of any other deployment >of >IPv4CS other than WMF. > > > >> >>Also, I do not understand how the statement >>"The WiMAX forum [WMF] has specified the Max SDU size as 1522 octets." >>aligns with the current statement in your draft >>"The Length parameter of IEEE 802.16 MAC frame has a size of 11 bits. >>Hence the total PDU size is 2048 bytes. The IPv4 payload can be a >>maximum value of 2038 bytes ( Total PDU size (2048) - (MAC Header (6) + >>CRC (4)), which is the maximum possible MTU." >> >>Where are those other ~500 octets going? Forgive my ignorance, but is >>there some layer between SDU and PDU that has that sort of overhead, or >is >>this another example of arbitrary limits that conflict with each other? >[SC>] > I do not know why WMF has specified 1522 bytes as MAX SDU and not >beyond that. You need to ask that question to the WiMAX forum. For >reference, please check RFC 5121 Appendix D. > > >Thanks, >-Samita > > > > > >> >>On Thu, 20 Mar 2008, Burcak Beser wrote: >> >>> I have to admit that I fail to understand the reason for the MTU >>limitation in the first place. The draft talks about Layer 2 tunnels >>between BS and AR, and than GRE is being given as an example which >causes >>the problems in the first place. Instead of GRE, if 802.1Q VLANs are to >be >>used this artificial limitation will not be taken into considereation. >>> >>> Is it possible to clarify the sentence "Hence if a IPv4CS MS is >>configured for 1500 bytes it will have to be communicated by the access >>router(AR) about the default link MTU (1400 bytes) in WiMAX network."? >Due >>to fragmentation support in the 802.16 transport aribitrary PDU sizes >are >>supported even when the maximum SDU size is limited. In the light of >>fragmentation the "The WiMAX forum [WMF] has specified the Max SDU size >as >>1522 octets." only means that a 1600 byte PDU will be transmitted in at >>least two chunks. The 802.16 transport will carry almost anything that >the >>underlying native protocol can. >>> >>> The normative 'should' in the sentence "Therefore, IEEE 802.16 frame >>SHOULD support for 1500 byte IP payload..." is unnecessary. >>> >>> The use of normative 'should' in ".. IPv4 and IPV4CS clients SHOULD >>implement DHCP interface MTU option..." sentence looks like >conditionally >>mandating the use of DHCP as well. A clear requirement will be better. >The >>inclusion of IPv4 clients is first not correct since ARP would work, >and >>second is out of scope for this draft by definition. >>> >>> I understand that IPv4CS cannot transmit ARP messages but "An IPv4CS >>client is not capable of doing ARP probing either to find out the link >>MTU." looks like stating that "IPv4CS client MUST NOT use ARP >messages". At >>the same time IPv4CS client has to be defined. >>> >>> "Consequently, the clients are encouraged to run PMTU[RFC 1191] >algorithm >>or PPMTUD[RFC 4821] for appropriating the bandwidth usage over the >route- >>path or end-to-end transport layers. However, PMTU mechanism has >inherent >>problems of packet loss due to ICMP messages not reaching the sender >and >>IPv4 routers not fragmenting the packets due to DF bit being set in the >IP >>packet." Can we state this sentence in normative manner? >>> >>> I have to admit that the draft struggles around the artificial MTU >>limitation which is not justified, and omits other important subjects. >>> >>> -burcak >>> >>> -----Original Message----- >>> From: 16ng-bounces@ietf.org [mailto:16ng-bounces@ietf.org] On Behalf >Of >>Samita Chakrabarti >>> Sent: Thursday, March 20, 2008 7:18 PM >>> To: 'Wesley George'; 'gabriel montenegro'; 'Jari Arkko' >>> Cc: 'Samita Chakrabarti'; 16ng@ietf.org >>> Subject: Re: [16NG] regarding my comments on MTUs >>> >>> Hi Jari and Gabriel, >>> Thanks for the guidelines on re-wording the text on MTU determination >for >>IP4CS link. Thanks to Wes for your proposed text as well; I have taken >some >>ideas from your text. >>> >>> >>> Here is the proposed text on MTU based on the wg discussion and the >AD >>> guideline: >>> ------------------------------------ >>> >>> 5. Maximum Transmission Unit considerations for IPv4CS link >>> >>> [RFC894] specifies 1500 bytes as a maximum length of IPv4 over >Ethernet >>and encourages to support full-length packets. Therefore, IEEE 802.16 >frame >>SHOULD support for 1500 byte IP payload in the IPv4CS link by >supporting >>larger than 1500 bytes link MTU between the MS and the Access router >(AR). >>> The extra overhead that IEEE 802.16 frame should support in this >case, >>depends on the tunneling protocols or any other virtual or >pseudo-wiring >>protocols packet overhead. >>> >>> The current architecture of IPv4CS in IEEE 802.16 networks is >defined >>in >>> the WiMAX (Worldwide Interoperability for Microwave Access) forum >[WMF]. >>> The addressing and operation of IPv4CS described in this document >are >>> applicable to the WiMAX networks as well. The WiMAX forum [WMF] has >>> specified the Max SDU size as 1522 octets. However, it is >recommended >>> that IP-payload in WiMAX architecture [WMF] specified network is >1400 >>> bytes due to overhead of GRE tunnel and possible >>> additional over-head of different types of tunneling (such as >IPSec/UDP >>> tunneling) between the base station (BS) and Access Router. >>> >>> Hence if a IPv4CS MS is configured for 1500 bytes it will have to >be >>> communicated by the access router(AR) about the default link MTU >(1400 >>> bytes) in WiMAX network. >>> However, currently in IPv4 client architecture a node is not >required >>> to ask for MTU option in its DHCP messages nor the >>> IPv4 router-advertisements >>> are required to advertise link MTU option when the link does not >>support >>> 1500 byte de-facto MTU size. An IPv4CS client is not capable of >doing >>ARP >>> >>> probing either to find out the link MTU. >>> Thus current specifications of WiMAX network access routers cannot >>> communicate its link MTU to the IPV4CS MS. On the other hand, it is >>> imperative for an MS to know the link MTU size if it is not the >default >>> MTU value for de-facto standard in order to >>> successfully send packets in the network towards the first hop. >>> This document can not also assume that the legacy IPv4 client >>> implementation with IEEE 802.16 layer 2 support, >>> would be able to dynamically sense IPv4CS WiMAX network and adjust >>> their MTU size accordingly. >>> >>> Thus for IPv4CS over IEEE 802.16 the default MTU size is 1400 >bytes. >>> >>> This document recommends that all future implementations of IPv4 >and >>> IPV4CS clients SHOULD implement DHCP interface MTU option [RFC2132] >in >>> order to configure its interface MTU according to the access >network in >>> order to maximize the capacity of the bandwidth of >>> the network. Consequently, the clients are encouraged to run >>> PMTU[RFC 1191] algorithm or PPMTUD[RFC 4821] for >>> appropriating the bandwidth usage over the route-path or end-to-end >>> transport layers. However, PMTU mechanism has inherent problems of >>packet >>> >>> loss due to ICMP messages not reaching the >>> sender and IPv4 routers not fragmenting the packets due to DF bit >being >>> set in the IP packet. The above mentioned path MTU mechanisms will >take >>> care of the MTU size between the MS and its correspondent node >across >>> different flavors of convergence layers in the WiMAX networks, >>> IEEE 802.16 networks and other types of networks such as Wi-Fi, >>> Ethernet or 3G networks. >>> >>> -------------------------- >>> >>> Review comments are welcome. >>> >>> Regards, >>> -Samita >>> >>>> -----Original Message----- >>>> From: 16ng-bounces@ietf.org [mailto:16ng-bounces@ietf.org] On Behalf >Of >>>> Wesley George >>>> Sent: Thursday, March 20, 2008 8:24 AM >>>> To: gabriel montenegro >>>> Cc: Samita Chakrabarti; 16ng@ietf.org >>>> Subject: Re: [16NG] regarding my comments on MTUs >>>> >>>> I have been thinking about the discussions regarding MTU, and would >>>> like to propose some alternate language for section 5 of the ipcs >>>> draft. I will caveat this by saying that it is my first attempt at >an >>>> ID (or a part of >>>> one) but I thought that it might be helpful to propose an >alternative >>>> rather than simply saying that the language that is in the -02 draft >>>> currently in Last Call that IMO is not adequate, due to its conflict >>>> with the .16 over ethernet draft. >>>> >>>> ** proposed draft language to replace the first paragraph of section >5 >>>> ** >>>> >>>> When considering end host MTU, there are two points at which MTU >must >>>> be considered. First, the end to end path MTU between one IP host >and >>>> another. In 802.16, this is expected to follow existing convention >[rfc >>>> citation needed?] for fragmentation and path MTU discovery between >end >>>> hosts, and will not be treated here. >>>> >>>> The point at which MTU must be considered and will be treated by >this >>>> document is the path between the Base Station (BS) and the ASN >Gateway >>>> or HA, where traffic is usually encapsulated using one of several >>>> methods before being allowed to transit the remaining path as native >IP >>>> traffic. >>>> On this portion of the path, there are two cases which must be >>considered: >>>> >>>> 1) Where the underlying MTU of the transport mechanism between the >BS >>>> and the ASN GW or HA is configurable to a value > 1600 bytes or is >>>> known to support the same >>>> 2) Where the underlying MTU of the transport mechanism between the >BS >>>> and the ASN GW or HA is not a known value, or is not able to be >>>> configured to MTU > 1600 bytes >>>> >>>> In case 1, the end host MTU SHOULD be set to at least 1500 bytes, >but >>>> can be set to any higher value supported by the underlying network >up >>>> to the maximum supported size on 802.16 of 2038 bytes. >>>> >>>> With an end host MTU of 1500 bytes or more, the underlying network >MUST >>>> support an MTU of at least 1500 bytes plus the overhead of the >chosen >>>> encapsulation method between the BS and the ASN GW or HA. For ease >of >>>> standardization, the underlying network SHOULD support an MTU of at >>>> least 100 bytes larger than the chosen host MTU value, as this is >large >>>> enough to ensure that there is no dropping or fragmentation of >oversize >>>> packets regardless of the encapsulation method. >>>> >>>> In case 2, the end host MTU SHOULD be set to 1400 bytes to ensure >that >>>> there is no dropping or fragmentation of oversize packets regardless >of >>>> the encapsulation method. If the end host MTU is set higher, or the >MTU >>>> of the underlying transport network is lower than 1500 bytes, the >>>> network MUST be able to accept any fragmentation of packets larger >than >>>> the supported transport MTU, or MUST employ a method of MTU >discovery >>>> to prevent oversize packets from being generated by the end host. >>>> >>>> In order to preserve the defacto standard of MTU 1500 bytes for end >>>> host use, case 1 SHOULD be followed if at all possible. If adhering >to >>>> case 1 is not possible, case 2 is presented as an alternative. >>>> >>>> ** end proposed draft language ** >>>> >>>> The second paragraph of section 5 can remain as-is. >>>> >>>> Some additional comments: >>>> It may be a good idea to add in language from the background slides >>>> that were presented in Philadelphia which identify why 100 bytes is >the >>>> recommended padding between host MTU andNetwork MTU for further >>>> clarification. >>>> >>>> I concede that it's not really feasible to rely on an MTU discovery >>>> method to compensate for MTU on the transport network that is not >>>> capable of supporting the full 1500 bytes of host MTU. However, I >think >>>> that the proposed language provides ways to work this based on >whether >>>> that is a known value or not (via the two different cases) and >>>> addresses that concern. I think that this is much closer in idea >with >>>> the way that the EthernetCS draft is written, treating 1500 as the >>>> defacto standard that we should prefer, but covering what the >reasoning >>>> is for MTU settings and identifying ways to compensate for >limitations >>>> in the underlying transport network, rather than simply assuming >that >>>> those limitations exist and catering to the lowest common >denominator. >>>> >>>> Thanks, >>>> Wes >>>> >>>> _________________________________________ >>>> Wesley George >>>> Sprint IP Engineering >>>> 703-689-7505 (O) 703-864-4902 (PCS) >>>> http://www.sprint.net >>>> _________________________________________ >>>> >>>> >>>> On Tue, 18 Mar 2008, gabriel montenegro wrote: >>>> >>>>> For the IPv4 case, I have no problem with the MTU staying at 1400 >>>>> bytes >>>> per the current draft. >>>>> There is no sure-fire way to indicate to the MS what the MTU is. >The >>>> draft currently says: >>>>> >>>>> The MTU value of the MS can be configured via Path MTU Discovery >[6], >>>>> Packetization layer path MTU discovery [7], DHCP MTU option [8] >or >>>>> static configuration of each MS.As pointed out by Jari, what >we're >>>> talking about is the *interface* MTU, not the *path* MTU >>>>> (minimum hop MTU of all the hops in a path). Accordingly, when >>>>> applying >>>> [6] or [7] >>>>> to discover the interface MTU, they must be used between the MS and >>>>> its >>>> immediate IP hop, >>>>> the access router. Of course, in order to use [6] or [7], some >>>>> interface >>>> MTU must be assumed >>>>> to begin the discovery process. >>>>> >>>>> 1400 is a very reasonable value for this initial default MTU. Given >>>>> the >>>> well-known issues with PMTU, >>>>> and the scant support for the DHCP Interface MTU option, 1400 will >>>>> work >>>> well in the absence of >>>>> any further MTU discovery or configuration. It will also avoid >>>> fragmentation in what is expected >>>>> to be the most common deployment for 802.16: WiMAX networks. 1500 >>>>> bytes, >>>> the other MTU value >>>>> folks have proposed for IPv4 CS does not have this virtue and will >>>>> incur >>>> fragmentation. >>>>> >>>>> Notice that for DHCP we should really say "Interface MTU option" >>>>> to clarify that we're talking about option 26 (Section 5.1, >RFC2132). >>>>> >>>>> Going forward we may have a more reliable and less time consuming >>>> mechanism to >>>>> derive the interface IP MTU by learning the layer 2 maximum SDU via >a >>>>> new >>>> TLV in the SBC exchange >>>>> in 802.16. This is being proposed this week at the Orlando IEEE >>meeting. >>>>> >>>>> As for the ethernet CS MTU, as Max said during his presentation >last >>> week, >>>> there is no choice. We must >>>>> preserve the ethernet MTU as expected from "Ethernet" CS, >regardless >>>>> of >>>> the fact that 1500 byte IP packets will incur >>>>> fragmentation in common WiMAX networks. >>>>> >>>>> >>>>> -gabriel >>>>> >>>>> >>>>> ----- Original Message ---- >>>>> From: Jari Arkko <jari.arkko@piuha.net> >>>>> To: 16ng@ietf.org >>>>> Sent: Thursday, March 13, 2008 1:18:55 PM >>>>> Subject: [16NG] regarding my comments on MTUs >>>>> >>>>> >>>>> As a clarification of my comments during the WG meeting I would >like >>>>> to state the following: >>>>> >>>>>> From AD perspective the WG is free to decide whatever MTU value >that >>>>>> you >>>>> wish to have, as long as you are both aware of and document the >>>>> implications of this choice. Some of the implications include: >>>>> >>>>> - difficulty of manual configuration >>>>> - ability to raise/lower the values automatically >>>>> - capability of 802.16 backhaul networks to handle larger MTUs >(ipcs, >>>>> ethcs, wimax and non-wimax) >>>>> - ability or inability to determine what network you are in >>>>> - inability of L2 devices to send ICMP messages >>>>> >>>>> Hope this clarifies, >>>>> >>>>> Jari >>>>> >>>>> >>>>> _______________________________________________ >>>>> 16NG mailing list >>>>> 16NG@ietf.org >>>>> https://www.ietf.org/mailman/listinfo/16ng >>>>> >>>>> >>>>> >>>>> >>>> _______________________________________________ >>>> 16NG mailing list >>>> 16NG@ietf.org >>>> https://www.ietf.org/mailman/listinfo/16ng >>> >>> >>> >>> _______________________________________________ >>> 16NG mailing list >>> 16NG@ietf.org >>> https://www.ietf.org/mailman/listinfo/16ng >>> >>> >>> > > > > >_______________________________________________ >16NG mailing list >16NG@ietf.org >https://www.ietf.org/mailman/listinfo/16ng _______________________________________________ 16NG mailing list 16NG@ietf.org https://www.ietf.org/mailman/listinfo/16ng
- [16NG] regarding my comments on MTUs Jari Arkko
- Re: [16NG] regarding my comments on MTUs gabriel montenegro
- Re: [16NG] regarding my comments on MTUs Wesley George
- Re: [16NG] regarding my comments on MTUs Samita Chakrabarti
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Burcak Beser
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Wesley George
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Samita Chakrabarti
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Samita Chakrabarti
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Burcak Beser
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Wesley George
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Riegel, Maximilian (NSN - DE/Muenich)
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Samita Chakrabarti
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Riegel, Maximilian (NSN - DE/Muenich)
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Wesley George
- Re: [16NG] WG last call on draft-ietf-16ng-ipv4-o… Samita Chakrabarti