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Network Working Group                                          P. SavolaRequest for Comments: 3627                                     CSC/FUNETCategory: Informational                                   September 2003Use of /127 Prefix Length Between Routers Considered HarmfulStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2003).  All Rights Reserved.Abstract   In some cases, the operational decision may be to use IPv6 /127   prefix lengths, especially on point-to-point links between routers.   Under certain situations, this may lead to one router claiming both   addresses due to subnet-router anycast being implemented.  This   document discusses the issue and offers a couple of solutions to the   problem; nevertheless, /127 should be avoided between two routers.1.  Introduction   [ADDRARCH] defines Subnet-router anycast address: in a subnet prefix   of n bits, the last 128-n bits are all zero.  It is meant to be in   use of any one router in the subnet.   Even though having prefix length longer than /64 is forbidden by   [ADDRARCH]section 2.4 for non-000/3 unicast prefixes, using /127   prefix length has gained a lot of operational popularity; it seems   like that these prefix lengths are being used heavily in point-to-   point links.  The operational practise has often been to use the   least amount of address space especially in the presence of a large   number of point-to-point links; it may be unlikely that all of these   links would start to use /64's.  Using /127 has also other   operational benefits: you always know which address the other end   uses, and there is no "ping-pong" [PINGPONG] problem with older ICMP   implementations (fixed now in [ICMPv3]).Savola                       Informational                      [Page 1]

RFC 3627         /127 Prefix Length Considered Harmful    September 20032.  Scope of this Memo   This memo does not advocate the use of long prefixes, but brings up   problems for those that do want to use them, for one reason or   another.   Detailed discussion on what is the "right" solution is out of the   scope; it is not the goal of this memo to try to find the "best"   addressing solution for everyone.3.  Problem with /127 and Two Routers   Note that this problem does not exist between a router and a host,   assuming the PREFIX::0/127 address is assigned to the router.   Using /127 can be especially harmful on a point-to-point link when   Subnet-router anycast address is implemented.  Consider the following   sequence of events:   1. Router A and Router B are connected by a point-to-point link.   2. Neither has anything configured or set up on this link.   3. 3ffe:ffff::1/127 address is added to Router A; now it performs      Duplicate Address Detection (DAD) [NDISC] for 3ffe:ffff::1.      Router A also adds the Subnet-router anycast address      3ffe:ffff::0/127.  (DAD is not performed for anycast addresses.)   4. Now Router B has been planned and configured to use      3ffe:ffff::0/127 as its unicast IPv6 address, but adding it will      fail DAD, and Router B does not have any address.   Similar scenarios also happen during router reboots, crashes and   such.   The usability of subnet-router anycast address between two routers on   a point-to-point link is very questionable, but it is still a   mandated feature of [ADDRARCH].  Workarounds for this are presented   in the next section.   As of yet, this kind of unexpected behavior hasn't been seen at large   perhaps because the Subnet-router anycast address hasn't been   implemented or too widely used.Savola                       Informational                      [Page 2]

RFC 3627         /127 Prefix Length Considered Harmful    September 20034.  Solutions   1. One could use /64 for subnets, including point-to-point links.   2. One could use only link-local addresses, but that may make network      maintenance and debugging impractical at least in bigger networks;      for example, "traceroute" can only return a list of nodes on the      path, not the links which would have been used.   3. Failing that, /126 does not have this problem, and it can be used      safely on a point-to-point link (e.g., using the 2nd and the 3rd      address for unicast).  This is analogous to using /30 for IPv4.      Using two /128 addresses is also one, though often cumbersome,      approach.  Naturally, not much would be lost if even a shorter      prefix was used, e.g., /112 or /120.      The author feels that if /64 cannot be used, /112, reserving the      last 16 bits for node identifiers, has probably the least amount      of drawbacks (also seesection 3).   4. [ADDRARCH] could be revised to state that Subnet-router anycast      address should not be used if the prefix length of the link is not      /64 (or even longer than /120).  This does not seem like a good      approach, as we should avoid making assumptions about prefix      lengths in the specifications, to maintain future flexibility.      Also, in some cases, it might be usable to have a Subnet-router      anycast address in some networks with a longer prefix length.      A more conservative (implementation) approach would be not using      Subnet-router anycast addresses in subnets with a prefix length of      /127 if there are only two routers on the link: this can be      noticed with [NDISC] 'Router' bit in Neighbor Advertisement      messages.  However, this seems to overload the functionality of      'R' bit, so it does not look like a good approach in the long run.   5. It's also possible to improve implementations: if /127 is used on      a point-to-point link, never claim two addresses.  This has the      drawback that even if the router using the combined unicast and      anycast address is down, the packets to subnet-router anycast      address will be lost as the other cannot claim the address.  This      approach might lead to unpredictability which would be hard to      trace when debugging problems.  However, this would normally be an      issue only when the Subnet-router anycast address is used from      outside of the link; usually, this cannot be done reliably as the      prefix length or EUI64 u/g bits cannot be known for certain.      There are other problems with an address being anycast and unicastSavola                       Informational                      [Page 3]

RFC 3627         /127 Prefix Length Considered Harmful    September 2003      too: use of it as a source address, whether to use unicast or      anycast semantics in [NDISC], and others: allowing this behavior      would seem to only add a lot of complexity to the implementations.   1) is definitely the best solution, wherever it is possible.  2) may   be usable in some scenarios, but in larger networks (where the most   often the desire would be to use longer prefix length) it may be   deemed very impractical.  There are some situations where one of   these may not be an option; then an operational work-around for this   operational problem, that is 3), appears to be the best course of   action.  This is because it may be very difficult to know whether all   implementations implement some checks, like ones described in 4) or   5).5.  Other Problems with Long Prefixes   These issues are not specific to /127.   One should note that [ADDRARCH] specifies universal/local bits (u/g),   which are the 70th and 71st bits in any address from non-000/3 range.   When assigning prefixes longer than 64 bits, these should be taken   into consideration; in almost every case, u should be 0, as the last   64 bits of a long prefix is very rarely unique.  'G' is still   unspecified, but defaults to zero.  Thus, all prefixes with u or g=1   should be avoided.   [MIPV6] specifies "Mobile IPv6 Home-Agents" anycast address which is   used for Home Agent Discovery.  In consequence, 7 last bits of have   been reserved in [ANYCAST] of every non-000/3 non-multicast address,   similar to [ADDRARCH].  Thus, at least /120 would seem to make sense.   However, as the sender must know the destination's prefix length,   this "reserved anycast addresses" mechanism is only applicable when   the sender knows about the link and expects that there is a service   it needs there.  In the case of e.g., /126 between routers, the only   to node to be found on this link would be the other router, so the   mechanism does not seem useful.  At least, Mobile IPv6 Home Agent   Discovery should not be performed if the prefix length is longer than   /120.6.  References6.1.  Normative References   [ADDRARCH]  Hinden, R. and S. Deering, "IP Version 6 (IPv6)               Addressing Architecture",RFC 3513, April 2003.   [ANYCAST]   Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast               Addresses",RFC 2526, March 1999.Savola                       Informational                      [Page 4]

RFC 3627         /127 Prefix Length Considered Harmful    September 20036.2.  Informative References   [NDISC]     Narten, T., Nordmark, E. and W. Simpson, "Neighbor               Discovery for IP Version 6 (IPv6)",RFC 2461, December               1998.   [MIPV6]     Johnson, D., Perkins, C., Arkko, J., "Mobility Support in               IPv6", Work in Progress.   [ICMPv3]    Conta, A., Deering, S., "Internet Control Message               Protocol (ICMPv6)", Work in Progress.   [PINGPONG]  Hagino, J., Jinmei, T., Zill, B., "Avoiding ping-pong               packets on point-to-point links", Work in Progress.7.  Security Considerations   Beyond those already existing in other specifications, solution 4)   might lead to denial of service in the case that one router is down:   the packet to subnet-router anycast address would be lost.8.  Acknowledgements   Thanks to Robert Elz and many others on the IPv6 Working Group for   discussion, and Alain Durand for pointing out [ADDRARCH] requirements   for prefix lengths.  Charles Perkins pointed out MIPv6 HA   requirements.  Randy Bush and Ole Troan commented on the document   extensively, and Erik Nordmark pointed out issues with u-bit.9.  Author's Address   Pekka Savola   CSC/FUNET   Espoo, Finland   EMail: psavola@funet.fiSavola                       Informational                      [Page 5]

RFC 3627         /127 Prefix Length Considered Harmful    September 200310.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assignees.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Savola                       Informational                      [Page 6]