> -----Original Message----- > From: Int-area [mailto:[email protected]] On Behalf Of Templin (US), > Fred L > Sent: Thursday, September 12, 2019 6:31 AM > To: Brian E Carpenter <[email protected]>; Bob Hinden > <[email protected]> > Cc: [email protected]; Suresh Krishnan <[email protected]> > Subject: Re: [Int-area] Discussion about Section 6.1 in > draft-ietf-intarea-frag-fragile > > Brian, > > > -----Original Message----- > > From: Brian E Carpenter [mailto:[email protected]] > > Sent: Wednesday, September 11, 2019 4:14 PM > > To: Bob Hinden <[email protected]>; Templin (US), Fred L > > <[email protected]> > > Cc: [email protected]; Suresh Krishnan <[email protected]> > > Subject: Re: [Int-area] Discussion about Section 6.1 in > > draft-ietf-intarea-frag-fragile > > > > On 12-Sep-19 10:59, Bob Hinden wrote: > > > Fred, > > > > > >> On Sep 11, 2019, at 7:48 AM, Templin (US), Fred L > > >> <[email protected]> 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. > > Yes, IP fragmentation - exactly.
See also Section 7 of RFC2473. > Fred > > > Brian > > > > > > > > Bob > > > > > > > > >> > > >> Fred > > >> > > >> --- > > >> From [email protected] 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 <[email protected]>; 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 <[email protected]>; 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: [email protected] > > >> 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 <[email protected]> > > >> From: Steve Deering <[email protected]> > > >> Subject: (IPng 4802) increasing the IPv6 minimum MTU > > >> Cc: [email protected] > > >> Sender: [email protected] > > >> 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 <[email protected]> > > >> Subject: Re: IPv6 MTU issue > > >> To: [email protected] (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 [email protected] > > >> -------------------------------------------------------------------- > > > > > > > > > _______________________________________________ > > > Int-area mailing list > > > [email protected] > > > https://www.ietf.org/mailman/listinfo/int-area > > > > > _______________________________________________ > Int-area mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/int-area _______________________________________________ Int-area mailing list [email protected] https://www.ietf.org/mailman/listinfo/int-area
