On 12-Sep-19 10:59, Bob Hinden wrote: > Fred, > >> On Sep 11, 2019, at 7:48 AM, Templin (US), Fred L >> <fred.l.temp...@boeing.com> wrote: >> >> Geoff, the 1280 MTU came from Steve Deering's November 13, 1997 proposal to >> the ipngwg. The exact message from the ipng archives is reproduced below. >> >> 1280 isn't just a recommendation - it's *the law*. Any link that cannot do >> 1280 >> (tunnels included) is not an IPv6 link. > > Yes from IPv6’s view, but you can make a link that can’t do 1280 work if it > has its own local L2 fragmentation / reassembly as noted in Steve’s email. > ATM with is 53 byte cells comes to mind.
IPv4 with a small PMTU also comes to mind, as discussed in Section 3.2.2 of RFC 4213: In this case, the IPv6 layer has to "see" a link layer with an MTU of 1280 bytes and the encapsulator has to use IPv4 fragmentation in order to forward the 1280 byte IPv6 packets. Brian > > Bob > > >> >> Fred >> >> --- >> From owner-i...@sunroof.eng.sun.com Thu Nov 13 16:41:01 1997 >> Received: (from majordomo@localhost) >> by sunroof.eng.sun.com (8.8.8+Sun.Beta.0/8.8.8) id QAA14339 >> for ipng-dist; Thu, 13 Nov 1997 16:38:00 -0800 (PST) >> Received: from Eng.Sun.COM (engmail1 [129.146.1.13]) >> by sunroof.eng.sun.com (8.8.8+Sun.Beta.0/8.8.8) with SMTP id QAA14332 >> for <ipng@sunroof>; Thu, 13 Nov 1997 16:37:51 -0800 (PST) >> Received: from saturn.sun.com (saturn.EBay.Sun.COM [129.150.69.2]) >> by Eng.Sun.COM (SMI-8.6/SMI-5.3) with SMTP id QAA28654 >> for <i...@sunroof.eng.sun.com>; Thu, 13 Nov 1997 16:37:48 -0800 >> Received: from postoffice.cisco.com (postoffice.cisco.com [171.69.200.88]) >> by saturn.sun.com (8.8.8/8.8.8) with ESMTP id QAA28706 >> for <i...@sunroof.eng.sun.com>; Thu, 13 Nov 1997 16:37:49 -0800 (PST) >> Received: from [171.69.199.124] (deering-mac.cisco.com [171.69.199.124]) by >> postoffice.cisco.com (8.8.5-Cisco.1/8.6.5) with ESMTP id QAA20862; Thu, 13 >> Nov 1997 16:37:48 -0800 (PST) >> X-Sender: deer...@postoffice.cisco.com >> Message-Id: <v03110702b0598e80008d@[171.69.199.124]> >> Mime-Version: 1.0 >> Content-Type: text/plain; charset="us-ascii" >> Date: Thu, 13 Nov 1997 16:37:00 -0800 >> To: IPng Working Group <i...@sunroof.eng.sun.com> >> From: Steve Deering <deer...@cisco.com> >> Subject: (IPng 4802) increasing the IPv6 minimum MTU >> Cc: hin...@ipsilon.com >> Sender: owner-i...@eng.sun.com >> Precedence: bulk >> >> In the ipngwg meeting in Munich, I proposed increasing the IPv6 minimum MTU >> from 576 bytes to something closer to the Ethernet MTU of 1500 bytes, (i.e., >> 1500 minus room for a couple layers of encapsulating headers, so that min- >> MTU-size packets that are tunneled across 1500-byte-MTU paths won't be >> subject to fragmentation/reassembly on ingress/egress from the tunnels, >> in most cases). >> >> After the short discussion in the Munich meeting, I called for a show of >> hands, and of those who raised their hands (about half the attendees, if >> I recall correctly), the vast majority were in favor of this change -- >> there were only two or three people opposed. However, we recognized that >> a fundamental change of this nature requires thoughtful discussion and >> analysis on the mailing list, to allow those who were not at the meeting >> and those who were there but who have since had second thoughts, to express >> their opinions. A couple of people have already, in private conversation, >> raised some concerns that were not identified in the discussion at the >> meeting, which I report below. We would like to get this issue settled as >> soon as possible, since this is the only thing holding up the publication >> of the updated Proposed Standard IPv6 spec (the version we expect to advance >> to Draft Standard), so let's see if we can come to a decision before the ID >> deadline at the end of next week (hoping there isn't any conflict between >> "thoughtful analysis" and "let's decide quickly" :-). >> >> The reason I would like to increase the minimum MTU is that there are some >> applications for which Path MTU Discovery just won't work very well, and >> which will therefore limit themselves to sending packets no larger than >> the minimum MTU. Increasing the minimum MTU would improve the bandwidth >> efficiency, i.e., reduce the header overhead (ratio of header bytes to >> payload bytes), for those applications. Some examples of such applications >> are: >> >> (1) Large-fanout, high-volume multicast apps, such as multicast video >> ("Internet TV"), multicast netnews, and multicast software >> distribution. I believe these applications will end up limiting >> themselves to packets no large than the min MTU in order to avoid >> the danger of incurring an "implosion" of ICMP Packet-Too-Big >> messages in response. Even though we have specified that router >> implementations must carefully rate-limit the emission of ICMP >> error messages, I am nervous about how well this will work in >> practice, especially once there is a lot of high-speed, bulk >> multicasting happening. An appropriate choice of rate or >> probability of emission of Packet-Too-Big responses to multicasts >> really depends on the fan-out of the multicast trees and the MTUs of >> all the branches in that tree, which is unknown and unknowable to >> the routers. Being sensibly conservative by choosing a very low >> rate could, in many cases, significantly increase the delay before >> the multicast source learns the right MTU for the tree and, hence, >> before receivers on smaller-MTU branches can start receiving the >> data. >> >> (2) DNS servers, or other similar apps that have the requirement of >> sending a small amount of data (a few packets at most) to a very >> large and transient set of clients. Such servers often reside on >> links, such as Ethernet, that have an MTU bigger than the links on >> which many of their clients may reside, such as dial-up links. If >> those servers were to send many reply messages of the size of their >> own links (as required by PMTU Discovery), they could incur very >> many ICMP packet-too-big messages and consequent retransmissions of >> the replies -- in the worse case, multiplying the total bandwidth >> consumption (and delivery delay) by 2 or 3 times that of the >> alternative approach of just using the min MTU always. Furthermore, >> the use of PMTU Discovery could result in such servers filling up >> lots of memory withed cached PMTU information that will never be >> used again (at least, not before it gets garbage-collected). >> >> The number I propose for the new minimum MTU is 1280 bytes (1024 + 256, >> as compared to the classic 576 value which is 512 + 64). That would >> leave generous room for encapsulating/tunnel headers within the Ethernet >> MTU of 1500, e.g., enough for two layers of secure tunneling including >> both ESP and AUTH headers. >> >> For medium-to-high speed links, this change would reduce the IPv6 header >> overhead for min MTU packets from 7% to 3% (a little less than the IPv4 >> header overhead for 576-byte IPv4 packets). For low-speed links such as >> analog dial-up or low-speed wireless, I assume that header compression will >> be employed, which compresses out the IPv6 header completely, so the IPv6 >> header overhead on such links is effectively zero in any case. >> >> Here is a list of *disadvantages* to increasing the IPv6 minimum MTU that >> have been raised, either publically or privately: >> >> (1) This change would require the specification of link-specific >> fragmentation and reassembly protocols for those link-layers >> that can support 576-byte packets but not 1280-byte packets, >> e.g., AppleTalk. I think such a protocol could be very simple, >> and I briefly sketch such a protocol in Appendix I of this >> message, as an example. >> >> Often, those links that have a small native MTU are also the ones >> that have low bandwidth. On low-bandwidth links, it is often >> desirable to locally fragment and reassemble IPv6 packets anyway >> (even 576-byte ones) in order to avoid having small, interactive >> packets (e.g., keystrokes, character echoes, or voice samples) >> be delayed excessively behind bigger packets (e.g., file transfers); >> the small packets can be interleaved with the fragments of the >> big packets. Someone mentioned in the meeting in Munich that the >> ISSLL WG was working on a PPP-specific fragmentation and >> reassembly protocol for precisely this reason, so maybe the job >> of specifying such a protocol is already being taken care of. >> >> (2) Someone raised the concern that, if we make the minimum MTU close >> to Ethernet size, implementors might never bother to implement PMTU >> Discovery. That would be regrettable, especially if the Internet >> evolves to much more widespread use of links with MTUs bigger >> than Ethernet's, since IPv6 would then fail to take advantage of >> the bandwidth efficiencies possible on larger MTU paths. >> >> (3) Peter Curran pointed out to me that using a larger minimum MTU for >> IPv6 may result in much greater reliance on *IPv4* fragmentation and >> reassembly during the transition phase while much of the IPv6 >> traffic is being tunneled over IPv4. This could incur unfortunate >> performance penalties for tunneled IPv6 traffic (disasterous >> penalties if there is non-negligible loss of IPv4 fragments). >> I have included Peter's message, describing his concern in more >> detail, in Appendix II of this message. >> >> (4) Someone expressed the opinion that the requirement for link-layer >> fragmentation and reassembly of IPv6 over low-cost, low-MTU links >> like Firewire, would doom the potential use of IPv6 in cheap >> consumer devices in which minimizing code size is important -- >> implementors of cheap Firewire devices would choose IPv4 instead, >> since it would not need a fragmenting "shim" layer. This may well >> be true, though I suspect the code required for local frag/reasm >> would be negligible compared to the code required for Neighbor >> Discovery. >> >> Personally, I am not convinced by the above concerns that increasing the >> minimum MTU would be a mistake, but I'd like to hear what the rest of the >> WG thinks. Are there other problems that anyone can think of? As I >> mentioned earlier, the clear consensus of the Munich attendees was to >> increase the minimum MTU, so we need to find out if these newly-identified >> problems are enough to swing the consensus in the other direction. Your >> feedback is heartily requested. >> >> Steve >> >> ---------- >> >> Appendix I >> >> Here is a sketch of a fragmentation and reassembly protocol (call it FRP) >> to be employed between the IP layer and the link layer of a link with native >> (or configured) MTU less than 1280 bytes. >> >> Identify a Block Size, B, which is the lesser of (a) the native MTU of the >> link or (b) a value related to the bandwidth of the link, chosen to bound >> the latency that one block can impose on a subsequent block. For example, >> to stay within a latency of 200 ms on a 9600 bps link, choose a block size >> of .2 * 9600 = 2400 bits = 240 bytes. >> >> IPv6 packets of length <= B are transmitted directly on the link. >> IPv6 packets of length > B are fragmented into blocks of size B >> (the last block possibly being shorter than B), and those fragments >> are transmitted on the link with an FRP header containing the following >> fields: >> >> [packet ID, block number, end flag] >> >> where: >> >> packet ID is the same for all fragments of the same packet, >> and is incremented for each new fragmented packet. The size of >> the packet ID field limits how many packets can be in flight or >> interleaved on the link at any one time. >> >> block number identifies the blocks within a packet, starting at >> block zero. The block number field must be large enough to >> identify 1280/B blocks. >> >> end flag is a one-bit flag which is used to mark the last block >> of a packet. >> >> For example, on a 9600 bps serial link, one might use a block size of >> 240 bytes and an 8-bit FRP header of the following format: >> >> 4-bit packet ID, which allows interleaving of up to 16 packets. >> 3-bit block number, to identify blocks numbered 0 through 5. >> 1-bit end flag. >> >> On a 256 kpbs AppleTalk link, one might use the AppleTalk-imposed block >> size of ~580 bytes and an 8-bit FRP header of the following format: >> >> 5-bit packet ID, which allows for up to 32 fragmented packets in >> flight from each source across the AppleTalk internet. >> 2-bit block number, to identify blocks numbered 0 through 2. >> 1-bit end flag. >> >> On a multi-access link, like AppleTalk, the receiver uses the link-level >> source address as well as the packet ID to identify blocks belonging to >> the same packet. >> >> If a receiver fails to receive all of the blocks of a packet by the time >> the packet number wraps around, it discards the incompletely-reassembled >> packet. Taking this approach, no timers should be needed at the receiver >> to detect fragment loss. We expect the transport layer (e.g., TCP) checksum >> at the final IPv6 destination to detect mis-assembly that might be caused by >> extreme misordering/delay during transit across the link. >> >> On links on which IPv6 header compression is being used, compression is >> performed before fragmentation, and reassembly is done before decompression. >> >> ---------- >> >> Appendix II >> >> From: Peter Curran <pe...@gate.ticl.co.uk> >> Subject: Re: IPv6 MTU issue >> To: deer...@cisco.com (Steve Deering) >> Date: Mon, 22 Sep 1997 11:50:34 +0100 (BST) >> >> Steve >> >> My problem was that moving the MTU close to 1500 would have an adverse >> effect on the transition strategy. The current strategy assumes that the >> typical Internet MTU is >576, and that sending an IPv6 packet close to the >> minimum MTU will not require any IPv4 fragmentation to support the tunnel >> transparently. The PMTU discovery mechanism will 'tune' IPv6 to use a >> suitable MTU. >> >> If the IPv4 MTU is <= 576 then IPv4 fragmentation will be required to >> provide a tunnel with a minimum MTU of 576 for IPv6. This clearly places >> a significant strain on the tunnelling nodes - as these will normally be >> routers then there will be a demand for memory (for reassembly buffers) >> as well as CPU (for the frag/reassembly process) that will have an overall >> impact on performance. >> >> This is an acceptable risk, as Internet MTU's of <= 576 are not too common. >> >> However, if the minimum MTU of IPv6 is increased to something of the order >> of 1200-1500 octets then the likelihood of finding an IPv4 path with an >> MTU lower than this value increases (I think significantly) and this will >> have a performance impact on these devices. >> >> During the brief discussion of this matter in the IPNG session at Munich >> you stated that MTU's less than 1500 where rare. I don't agree with this >> completely - it seems to be pretty common practise for smaller 2nd and 3rd >> tier ISP's in the UK to use an MTU of 576 for connection to their transit >> provider. Their objective, I believe, is to 'normalize' the packet sizes >> on relatively low bandwidth circuits (typically <1Mbps) to provide better >> performance for interactive sessions compared to bulk-file transfer users. >> >> I think that before we go ahead and make a decision on an increased minimum >> MTU for IPv6 then we should discuss the issues a little more. >> >> Incidentally, I am not convinced of the benefits of doing this anyway >> (ignoring the issue raised above). With a properly setup stack the PMTU >> discovery mechanism seems to be able to select a good MTU for use on the >> path - at least that is my experience on our test network and the 6Bone. >> >> I appreciate that you are trying to address the issues of PMTU for multi- >> casting but I don't see how raising the minumum MTU is going to help much. >> PMTU discovery will still be required irrespective of the minimum MTU >> adopted, unless we adopt a value that can be used on all link-layer technolo- >> gies. >> >> I would welcome wider discussion of these issues before pressing ahead >> with a change. >> >> Best regards >> >> Peter Curran >> TICL >> >> >> -------------------------------------------------------------------- >> IETF IPng Working Group Mailing List >> IPng Home Page: http://playground.sun.com/ipng >> FTP archive: ftp://playground.sun.com/pub/ipng >> Direct all administrative requests to majord...@sunroof.eng.sun.com >> -------------------------------------------------------------------- > > > _______________________________________________ > Int-area mailing list > Int-area@ietf.org > https://www.ietf.org/mailman/listinfo/int-area > _______________________________________________ Int-area mailing list Int-area@ietf.org https://www.ietf.org/mailman/listinfo/int-area