Re: [16NG] WG last call on draft-ietf-16ng-ipv4-over-802-dot-16-ipcs-02 + RE: regarding my comments on MTUs

[Burcak Beser <[e-mail]>]

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?

Also within the draft "IPv4 over IEEE 802.16's IP CS" section 6 it is metioned that "each MS shall be on different IP subnet", does this mean that the DHCP subnet returned has to be 255.255.255.255? If so why? I do not see the reason for ech MS being at a different subnet. I understand that each MS is within the smallest broadcast domain possible. I am sure that each MS belonging a different subnet will have unintended consequences within the network. By the statement of Appendix A it looks like that every MS will need a default gateway address. I do not understand the benefit of such a hidden requirement.

I assume the sentence "DHCP [4] SHOULD be used for assigning IPv4 address for the MSs." will be removed.

Is there a hidden normative requirement on AR having a DHCP server in the sentence "In case DHCP server does not reside in AR, the AR SHOULD implement DHCP relay Agent [5]." ?

I would like a stronger wording for the sentence "In a point-to-point link model, address resolution may not be needed." This is the basis of IPv4CS.

For the sentence "IPv4 multicast packets are carried over the point-to-point link between the AR and the MS (via the BS)."; it is my understanding that multicast CIDs can carry IP packets to multiple MS'. The sentence must be removed. The following sentence is one of the methods for supporting multicast, and at least multicast CIDs method must be explained as well. Even for the multi-unicast multicast case, a standards based AR will not work.

As nit-picking point the reference 12 is a zip file that contains 12 documents. It will be better if these are stated individually.

Second nit-pick; sentence "all the packets originated at.." must start with capital A.

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: [e-mail] [mailto:[e-mail]] On Behalf Of Samita Chakrabarti
Sent: Thursday, March 20, 2008 7:18 PM
To: 'Wesley George'; 'gabriel montenegro'; 'Jari Arkko'
Cc: 'Samita Chakrabarti'; [e-mail]
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: [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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>>
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