Re: [16NG] regarding my comments on MTUs

["Samita Chakrabarti" <[e-mail]>]

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: [e-mail] [mailto:[e-mail]] On Behalf Of
>Wesley George
>Sent: Thursday, March 20, 2008 8:24 AM
>To: gabriel montenegro
>Cc: Samita Chakrabarti; [e-mail]
>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 <[e-mail]>
>> To: [e-mail]
>> 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
>>
>>
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