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Versions: 00 01 02

Network Working Group                                         R. Whittle
Internet-Draft                                          First Principles
Intended status: Experimental                            August 22, 2008
Expires: February 23, 2009


                      Ivip4 ETR Address Forwarding
                draft-whittle-ivip4-etr-addr-forw-01.txt

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   This Internet-Draft will expire on February 23, 2009.


















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Abstract

   ETR Address Forwarding (EAF) is a novel method by which an IPv4 Core-
   Edge Separation solution to the Internet's routing scaling problem
   can tunnel packets from an ITR to an ETR.  EAF involves using 31 bits
   of the IPv4 header for new purposes: bit 48, the More Fragments flag,
   the Fragment Offset field and the Header Checksum field.
   Consequently, packets in this format need to be handled by routers
   with modified functionality.  EAF is an alternative to encapsulation
   (map-encap) or address translation (Six/One Router) and has
   advantages including: simpler ITRs and ETRs, direct support for
   conventional RFC 1191 PMTUD, no encapsulation overhead and full
   compatibility with IPsec AH and Traceroute.






































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Table of Contents

   1.  Quick note on other uses of bit 48, Flag 0, Evil bit . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Advantages . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Disadvantages  . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Ivip4 with or without encapsulation  . . . . . . . . . . . . .  9
   4.  Fragmented and Fragmentable Packets  . . . . . . . . . . . . . 10
   5.  Choice of MinCoreMTU value . . . . . . . . . . . . . . . . . . 11
   6.  Changed bits in the IPv4 header  . . . . . . . . . . . . . . . 13
     6.1.  The evil bit 48  . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  More Fragments flag and Fragment Offset  . . . . . . . . . 15
     6.3.  Header Checksum  . . . . . . . . . . . . . . . . . . . . . 15
   7.  Reconstructing the normal IPv4 header  . . . . . . . . . . . . 16
   8.  ITR Functionality  . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Rejecting fragments or fragmentable packets which are
           too long . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.2.  Setting bit 48 . . . . . . . . . . . . . . . . . . . . . . 17
     8.3.  Writing the ETR address  . . . . . . . . . . . . . . . . . 17
     8.4.  Forwarding according to the ETR address  . . . . . . . . . 17
   9.  Upgraded Router Functionality  . . . . . . . . . . . . . . . . 19
     9.1.  Recognizing the EAF format . . . . . . . . . . . . . . . . 19
     9.2.  No checksum calculations . . . . . . . . . . . . . . . . . 19
     9.3.  Forward according to 30 bit ETR address  . . . . . . . . . 19
     9.4.  ICMP generation  . . . . . . . . . . . . . . . . . . . . . 20
       9.4.1.  RFC 1191 PMTU with much less complexity than with
               map-encap  . . . . . . . . . . . . . . . . . . . . . . 20
   10. ETR Functionality  . . . . . . . . . . . . . . . . . . . . . . 22
   11. ITR and ETR placement: MTU and upgraded routers  . . . . . . . 23
     11.1. PMTU and MinCoreMTU  . . . . . . . . . . . . . . . . . . . 23
     11.2. Core and internal router upgrades  . . . . . . . . . . . . 23
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
   14. Informative References . . . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Scenarios where bit errors alter the EAF
                formatted header  . . . . . . . . . . . . . . . . . . 31
     A.1.  Error in bit 48  . . . . . . . . . . . . . . . . . . . . . 31
     A.2.  Errors in bits 50 to 63 and 80 to 96 . . . . . . . . . . . 32
     A.3.  Errors in bits other than 50 to 63 and 80 to 96  . . . . . 32
   Appendix B.  Applicability to core-edge separation schemes
                other than Ivip . . . . . . . . . . . . . . . . . . . 35
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
   Intellectual Property and Copyright Statements . . . . . . . . . . 37








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1.  Quick note on other uses of bit 48, Flag 0, Evil bit

   Within hours of version 00 of this I-D appearing, Fred Templin
   alerted me to other proposals for using bit 48 of the IPv4 header.

   Bob Briscoe and colleagues have an extensive I-D regarding "re-ECN" -
   a new approach to Explicit Content Notification.  This I-D has been
   in active development since 2005: "Re-ECN: Adding Accountability for
   Causing Congestion to TCP/IP draft-briscoe-tsvwg-re-ecn-tcp-06":
   [I-D.briscoe-tsvwg-re-ecn-tcp].

   Two other proposals regarding bit 48 are cited in briscoe-tsvwg-re-
   ecn-tcp.  Firstly, a 2005 BT Journal paper by J. Adams and colleagues
   and secondly RFC 3514 which I discuss below.  The aims of the J.
   Adams et al paper would apparently be met by the proposal of Bob
   Briscoe and colleagues.

   Fred also mentioned that some ca. 2002 proposals from Jim Fleming
   proposed new uses of IPv4 header bits, and setting bit 48 to 1.
   Searching for "Jim Fleming" with "IPv8" and "IPv16" turns up a number
   of mailing list discussions, but I can find no sign of ongoing
   development of these proposals from ca. 1998 and 2002.

   The re-ECN proposal is substantial and active.  Please let me know of
   any other proposals concerning bit 48.


























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2.  Introduction

   The Internet's routing and addressing scaling problem, as discussed
   in RAWS [RFC4984] has given rise to a number of Core-Edge Separation
   (CES) proposals which create a new form of address space for end-user
   networks.  The term SPI (Scalable Provider Independent) space is used
   here to describe this form of address space.

   In the absence of a successful CES system, most of demand driving the
   unsustainable growth in the number of advertised prefixes is likely
   to arise from hundreds of thousands or millions of end-user networks
   wanting their own conventional BGP advertised PI prefix - since this
   would remain the only method by which each such end-user network
   could be multihomed and by which its address space could be portable
   between ISPs.

   SPI space is portable between different ISPs (providers) and can be
   used for multihoming - including the use of inbound Traffic
   Engineering (TE) to control the balance of ingress traffic over the
   links from multiple providers.

   The purpose of the CES system is to make SPI space highly attractive
   to user networks of all sizes, and to ensure that their use of this
   space is highly scalable.  Only if the new type of space is
   attractive to the great majority of end-user networks will it be
   possible to ensure that most such networks choose to use this space,
   rather than the conventional BGP approach to PI space, which is the
   main cause of the routing scaling problem.

   To be scalable, each SPI end-user network's address space must not be
   a separately advertised BGP prefix.  More generally, the interdomain
   routing system with the CES extensions should be able to scale
   gracefully to millions and perhaps billions of SPI-based end-user
   networks.

   CES systems achieve this goal be retaining provider prefixes in the
   existing core routing system, whilst excluding individual SPI (edge)
   prefixes.  This gives rise to the term Core-Edge Separation scheme.
   An early use of this term was in a taxonomy by Jari Arkko in July
   2008 [Arkko-2008a] [Arkko-2008b] [Arkko-2008c].

   The first CES proposals were known as "Locator / ID separation"
   and/or "map-and-encaps" (map-encap) systems, and include LISP
   [I-D.farinacci-lisp], APT (A Practical Transit Mapping Service)
   [I-D.jen-apt], Ivip4e and Ivip6e [I-D.whittle-ivip-arch] and TRRP
   (Tunneling Route Reduction Protocol) [TRRP].  These involve ITRs
   (Ingress Tunnel Routers) tunneling a packet to an ETR (Egress Tunnel
   Router), typically in another provider network, where the ETR



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   forwards the packet to the destination end-user network.  Map-encap
   systems suffer from problems of encapsulation overhead, this overhead
   causing more PMTU problems, and with problems of complexity and extra
   management traffic in order that ITRs and ETRs can overcome the
   barriers encapsulation presents to RFC 1191 PMTUD [Ivip-PMTUD-Frag].

   A second class of CES system is "Translation", in which the
   equivalents of ITRs send packets to the equivalents of ETRs (both
   known as Translation Routers) by rewriting their source and
   destination addresses.  There is no encapsulation, and the packets
   with translated addresses are forwarded across the core by ordinary
   BGP routers.  To date, the only instance of the Translation class is
   Six/One Router [SixOneRouter] - which is not practical for IPv4.
   Translation schemes have the great advantage that the packets are not
   made any longer.  A translation scheme involves less problems with
   PMTU limits than an encapsulation scheme, and in principle may be
   able to support full RFC 1191 [RFC1191] PMTUD without excessive
   complexity in the translation routers.

   ETR Address Forwarding (EAF) is a separate class of CES proposal from
   map-encap and translation schemes.  It is somewhat related to Prefix
   Label Forwarding (PLF) [Ivip6f] - an IPv6-only system which also
   involves using bits in the existing IP header in a novel fashion.  As
   with Translation, there is no encapsulating header so there is no
   overhead or worsening of PMTUD difficulties.  Unlike Translation, the
   source and destination addresses are unaltered.

   A brief description of EAF is that the ITR sets bit 48 of the IPv4
   header to 1 and 30 other bits of the header (currently used for the
   More Fragments flag, the Fragment Offset field and the Header
   Checksum) are then used to carry the 30 most significant bits of the
   ETR's address.  Routers (core BGP routers and potentially internal
   routers) with modified functionality recognise the EAF formatted
   header, and forward the packet to the ETR according to this address,
   rather than according to the destination address.  The ETR converts
   the header back to its normal state and delivers the packet to the
   destination network.  Routers sending ICMP messages, such as Packet
   Too Big or other messages in response to traceroute, perform a
   similar conversion on the modified header to reconstruct the IPv4
   version of the packet - so PMTUD works directly for all routers
   between the ITR and the ETR.

2.1.  Advantages

   EAF has the following advantages over map-encap:

   1.  There is no transmission overhead - the packet is not made any
       longer.



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   2.  Conventional RFC 1191 PMTUD is supported over the entire path,
       including between the ITR and ETR, without any involvement of the
       ITR.

   3.  Traceroute is expected to work over the entire path.

   Avoiding encapsulation overhead is a major long-term benefit,
   especially for VoIP packets.

   The retention of conventional RFC 1191 PMTUD through the ITR to ETR
   path removes the need for a great deal of complexity which would
   otherwise be required in map-encap ITRs and ETRs.  While that
   complexity could be implemented in ITRs and ETRs, it will be highly
   desirable to avoid this, and the consequent need for the protocols,
   occasional probe packets, reply packets etc.  A discussion of the
   arrangements necessary to handle PMTUD problems [Ivip-PMTUD-Frag]
   gives some idea of the extra complexity, state etc. required in ITRs
   and ETRs for any practical map-encap system.

   A survey of higher level protocols' references to bits within the
   IPv4 and IPv6 headers [Checksums] shows that none of these protocols
   depend on the bits which are changed in the EAF formatted IPv4
   header.

2.2.  Disadvantages

   The major disadvantage of EAF is the need to upgrade the capabilities
   of most or all core routers, and likewise any internal routers
   between ITRs or ETRs and the border routers.  However, since the
   modified functionality is relatively slight compared to the current
   functionality of these routers, it seems likely that many existing
   routers could be upgraded with a firmware update.

   As discussed below, EAF ITRs do not accept fragmented packets, or
   fragmentable packets longer than some globally agreed constant, which
   would be somewhat below 1500 bytes.  However, identical restrictions
   would apply to a map-encap scheme which properly handled PMTUD
   problems [Ivip-PMTUD-Frag].

   RFC 1191 PMTUD will have been available for 20 years by the time any
   practical scalable routing system is deployed.  The design of such a
   system should not be constrained by hosts which avoid RFC 1191 and
   instead burden the network with the task of fragmenting packets which
   exceed the PMTU to the destination host - and which likewise burden
   the destination host with the reassembly task.

   Care must be exercised to ensure that packets with the new EAF format
   IPv4 headers are only forwarded to upgraded routers, or to an ETR.



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   Any other device will regard the header as malformed, and could be
   expected to drop the packet without generating any ICMP message.

   EAF provides only the 30 most significant bits of an ETR address.
   This means that ETRs can be located no more densely than one every 4
   IP addresses.  This is not expected to be a problem, since it is
   common for routers to each use a /30 prefix.












































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3.  Ivip4 with or without encapsulation

   There are three notional approaches to Ivip for IPv4: Ivip4e
   (encapsulation only), Ivip4ef (encapsulation at first, transitioning
   to EAF in the long term) and Ivip4f (EAF only).  Ideally, it would be
   possible to introduce Ivip4f - so avoiding the need for many complex
   functions in ITRs and ETRs.  If this was not possible, then Ivip4ef
   would be introduced using encapsulation, but with all ITRs and ETRs
   ready to switch to EAF as soon as the routers have been upgraded.
   The long term benefits of no encapsulation overhead and
   straightforward PMTUD are very significant.

   Since the EAF system is quite simple, it would make no sense to
   introduce Ivip4e - with encapsulation, but without all ITRs and ETRs
   having a full EAF functionality.  In the long term, the cost of
   upgrading all routers approaches nil, and the cost of including EAF
   functionality in all ITRs and ETRs will be very low - so a properly
   designed system should be able to transition to using EAF exclusively
   whenever the core and internal routers are able to support it.
































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4.  Fragmented and Fragmentable Packets

   EAF involves some important restrictions on the packets which will be
   accepted by ITR for tunneling to an ETR.

   Fragments will not be accepted.  Neither will be fragmentable packets
   longer than some globally agreed limit - MinCoreMTU - likely to be
   set at about 1470 bytes.  The same restrictions would be necessary
   for encapsulation in order to properly manage PMTUD problems and to
   avoid inefficient traffic patterns such as from fragmenting 9k
   packets into smaller than 1500 byte fragments [Ivip-PMTUD-Frag].
   There are two reasons for these restrictions.

   Firstly, it is undesirable for the network in general and the ITR-ETR
   system in particular to have to carry fragmented packets.  All host
   operating systems support RFC 1191 (1990) PMTUD and 18 years later,
   applications can reasonably be expected to use it - rather than
   sending out packets as long as the next hop MTU and expecting routers
   and the receiving host to do extra work if the PMTU to the host is
   less than the packet's length.  In the mid-1990s, fragmentation in
   the network was judged unworthy of inclusion in IPv6.  Since a core-
   edge separation system for IPv4 may be in operation indefinitely,
   since it would be much harder to engineer such a system to cope with
   fragmented and fragmentable packets, and since few hosts currently
   send fragmentable packets longer than the proposed ~1470 byte limit,
   it is best to simplify the system design by setting these limits on
   the types of packets it handles.

   Secondly, it is technically impossible to implement EAF with ITRs
   accepting fragments, or with the packets sent by ITRs being
   fragmented en-route to the ETR.

   The EAF system will carry fragmentable packets which are no longer
   than MinCoreMTU.  After these packets emerge in their original form
   from the ETR, they may be fragmented en-route to the ETR.

   Currently it is common for fragmentable packets to be sent across the
   Internet's core.  For instance, Google servers typically send TCP
   packets as long as the mutually negotiated MSS value allows, with the
   Google server offering an MSS of 1430.  This can result in the
   servers sending DF=0 packets of up to 1470 bytes long.










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5.  Choice of MinCoreMTU value

   Ivip ITRs will drop incoming fragmentable packets which exceed a
   particular length - MinCoreMTU.  The value of this global constant
   must be chosen with some care.

   The primary or sole reason for making it a high value is the
   desirability of rejecting fewer fragmentable packets.

   The primary reason for making it lower is because all ITRs and ETRs
   must be located such that every ITR has a PMTU to every ETR of at
   least MinCoreMTU bytes.

   At present, it is common for there to be 1500 byte MTU limits at many
   locations in the core of the Internet.  It is also common for servers
   to be connected with 100Mbps Ethernet switches, even when they have
   Gigabit Ethernet interfaces.  One reason for this is the simple
   expedient of limiting each server's ability to create peak outgoing
   packet rates which would overload upstream links. 100Mbps Ethernet
   switches typically impose a 1500 byte MTU, while an Ethernet switch
   operating with all ports running at 1Gbps typically enables an MTU on
   all links of ~9k bytes.

   It is highly desirable to maximise the flexibility with which ITRs
   and ETRs can be located.  The deeper they can be located in networks
   - the further from the border routers - the more numerous they can be
   and so the better the total load can be shared amongst them.
   Furthermore, it will be possible to implement the ITR function in the
   sending host.  This is likely to involve zero incremental cost,
   assuming the host has sufficient CPU and memory capacity to spare.
   Such ITFH (ITR Function in sending Host) hosts cannot be behind NAT.
   They may be on conventional addresses or on the new kind of SPI
   address.

   Even in the long-term future, it is safe to assume that there will be
   1500 byte MTU limits in some part of the Internet or edge networks
   through which ITR to ETR packets will travel.  For the foreseeable
   future, MinCoreMTU must be at or more likely somewhat below 1500
   bytes.  It is unlikely that a higher value could be contemplated in
   the future, and by then, hopefully, there would be no impetus to
   alter it, since (hopefully) there would be no remaining applications
   still sending fragmentable packets across the core.

   Choosing a lower value, such as 1400 bytes, would make for few
   restrictions in the location of ITRS and ETRs, but would cause more
   trouble with hosts which currently send DF=0 packets longer than
   this.  The task is to find a value which is as high as possible
   without unreasonably restricting the potential locations of ITRs and



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   ETRs.  Ideally, the value would not constrain existing widespread use
   of fragmentable packets.

   The final decision about MinCoreMTU's value will be made some years
   in the future.  For now, it will be assumed to be 1470 bytes.














































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6.  Changed bits in the IPv4 header

   The IPv4 header is defined in RFC 791 [RFC0791].


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Version|  IHL  |Type of Service|          Total Length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Identification        |Flags|      Fragment Offset    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Time to Live |    Protocol   |         Header Checksum       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Source Address                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Destination Address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Options                    |    Padding    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Figure 1: IPv4 header format.

   When the header is modified to the EAF format, only bit 48, bits 50
   to 63 and bits 80 to 95 are altered.


      4       5                                       6
      8   9   0   1   2   3   4   5   6   7   8   9   0   1   2   3
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |   | D | M |   |   |   |   |   |   |   |   |   |   |   |   |   |
      | 0 | F | F | f | f | f | f | f | f | f | f | f | f | f | f | f |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 0   1   2 |                                                   |
      |   Flags   |              Fragment Offset                      |


        8                                       9
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | c | c | c | c | c | c | c | c | c | c | c | c | c | c | c | c |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                        Header Checksum                        |



   Figure 2: IPv4 header bits altered in EAF format.



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   Bit 48 is set to 1.  Bits 50 to 63 and 80 to 95 are set to the 30
   most significant bits of the ETR address.


        4       5                                       6
        8   9   0   1   2   3   4   5   6   7   8   9   0   1   2   3
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |   | D |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
      | 1 | F | F | F | F | F | F | F | F | F | F | F | F | F | F | F |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 0   1 |                                                       |
      | Flags | 14 Bits of address to which packet will be Forwarded  |


        8                                       9
        0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | F | F | F | F | F | F | F | F | F | F | F | F | F | F | F | F |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |    16 Bits of address to which packet will be Forwarded       |



   Figure 3: IPv4 header bits as used in EAF format.

   ITRs will write bits 0 to 29 of the ETR address into the F bits shown
   in Figure 3 in an order TBD.

6.1.  The evil bit 48

   RFC 791 defines bit 48 as Flag Bit 0: "reserved, must be zero".

   RFC 3514 [RFC3514] redefines this bit as the "evil" bit "E": "Benign
   packets have this bit set to 0; those that are used for an attack
   will have the bit set to 1."

   However - while the Internet is awash with packets which upon close
   inspection are found to be of questionable moral integrity - RFC 3514
   has not been widely implemented.

   We therefore retain RFC 3514's nomenclature of "E" and define new
   semantics for bit 48 (Flag 0) as the EAF flag: setting it to 0 to
   denote the conventional IPv4 header format and to 1 to denote the EAF
   usage of the abovementioned bits.


       0
      +-+



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      |E|
      +-+


   Currently-assigned values are defined as follows:

   0  The packet's header bits conform to RFC 791 semantics.

   1  The packet's header bit 50 to 63 and 80 to 95 contain the 30 most
      significant bits of an address which modified routers will use to
      determine forwarding, rather than the destination address.  In
      addition, the header will be subject to special processing as
      described below to restore its state to that of a conventional
      IPv4 header, in the event that it is either received by an ETR or
      to be included in an ICMP message.

6.2.  More Fragments flag and Fragment Offset

   As mentioned above, ITRs will drop all packets which are fragments.
   Consequently, the states of the More Fragments flag (bit 50) and the
   Fragment Offset bits (bits 51 to 63) are all zero when the packet is
   accepted to be tunneled to an ETR.  Whenever the packet is converted
   back to the conventional IPv4 header format, all these bits will be
   set back to zero.

6.3.  Header Checksum

   Since the EAF format is used for the packet's travel from the ITR to
   the ETR, and since the IPv4 Header Checksum bits are used for another
   purpose, the routers in that path - all of which must have the
   modified functionality which recognises the EAF formatted header -
   will not check the header's integrity via any checksum facility.

   Significant problems due to transmission processing errors are not
   anticipated, primarily due to the low error rate in most L2 layer
   implementations.  These low error rates were presumably considered
   when the IPv6 header was designed without any header checksum.  The
   possible bit error scenarios are discussed in Appendix 1.

   The checksum will be recalculated by the ETR when it converts the
   packet to normal IPv4 format, or by a router en-route to the ETR
   which needs to convert the packet to this format in order to send an
   ICMP message to the sending host.








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7.  Reconstructing the normal IPv4 header

   ITRs are the only devices which are required to convert an ordinary
   IPv4 header into an EAF formatted IPv4 header.  We discuss that
   functionality in a section below.  Here we discuss a function which
   needs to be performed by ETRs, and in some circumstances by any of
   the modified routers between the ITR and ETR.  Since most ITRs also
   have the additional capabilities of a modified router, most ITRs need
   the following functionality as well.

   ETRs perform this operation when they accept a packet forwarded from
   an ITR - to convert the packet into an ordinary IPv4 packet to be
   forwarded to the destination network.  Modified routers (including
   ITRs) may need to convert the packet back to its IPv4 format in order
   to include the packet, or part of it, in an ICMP message.

   The first step in converting the header is to set bit 48 and bits 50
   to 63 to 0.  As noted previously and in the following section on
   converting the packet to EAF format, all packets which are accepted
   by an ITR have the MF flag and all Fragment Offset bits set to 0.

   The second step is to calculate a new Header Checksum from the
   current state of the header, after bits 80 to 95 have been set to 0.
   Once this is value is written into bits 80 to 95, the packet is
   identical to that accepted by the ITR, except for the lower value of
   TTL which results from the EAF format packet having passed through
   routers, and for the consequently different Header Checksum.
























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8.  ITR Functionality

   This section formally defines the functions the ITR performs when EAF
   forwarding a packet, which are additional to its basic functionality
   documented elsewhere regarding deciding which packets need to be
   tunneled to an ETR, which ETR to tunnel them to etc.

   In a map-encap system, the ITR encapsulates the packet in an IP
   header with the outer header's destination address being that of the
   ETR.  (The details vary - for instance Ivip only uses an IP header,
   while LISP uses IP, UDP and then a LISP header.)

8.1.  Rejecting fragments or fragmentable packets which are too long

   The first step of EAF processing involves rejecting packets which are
   fragments, or which are fragmentable but longer than MinCoreMTU.  In
   both cases, the packets are dropped and an ICMP message sent to the
   sending host, including some number of bytes of the original packet.

   There is an unresolved question about what type of ICMP message to
   send in these circumstances.  The sending host will not be expecting
   an ICMP PTB (Packet Too Big: Type 4 Fragmentation required, and DF
   flag set) message.  There is no point defining a new ICMP message,
   since the problem only arises from hosts which are not using RFC 1191
   PMTUD or which are ignoring future BCP guidance not to send
   fragmentable packets which are longer than MinCoreMTU to any host
   which is reachable only via the ITR-ETR network.  If the host OS and
   application could be upgraded to accept a new kind of ICMP message,
   it would be better to upgrade them never to send fragments or
   fragmentable packets.

8.2.  Setting bit 48

   The router sets bit 48 to 1.

8.3.  Writing the ETR address

   The router writes the 30 most significant bits of the ETR address
   into bits 50 to 63 and 80 to 96.  The packet's header is now valid
   according to the EAF format.

8.4.  Forwarding according to the ETR address

   Generally, ITRs are conceived of as additional functions built into a
   conventional internal or border router.  However, there are two
   instances where this is not the case.  Firstly, the ITR functions can
   be built into a sending host, which has no routing functions, due to
   it having a single link to its network.  Secondly an ITR could be



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   implemented as a device which advertises prefixes in the local (or
   interdomain, in the case of an OITRD) routing system, and which
   converts the packet to the EAF format (or encapsulates it, with map-
   encap) - however the ITR has a single link to its network and
   therefore makes no forwarding decisions about the resulting EAF (or
   encapsulated) packet.

   Where the ITR function is in a device with no routing functions, the
   next step is to forward the packet out of the one interface which
   leads to the upstream router.  This router must have modified
   functionality so it recognises the EAF formatted packet.

   Where the ITR functions are performed within a router (necessarily
   with EAF modified functionality), the next step is to present the EAF
   formatted packet to the FIB - which will forward the packet to or
   towards the ETR address specified in bits 50 to 63 and 80 to 95.
   This modified forwarding functionality is described in the next
   section.

   Also, when the ITR function takes place in a router, the router's FIB
   must be able to generate an ICMP message as described in the next
   section if the EAF formatted packet cannot be forwarded.

   Converting the packet from conventional IPv4 format to EAF format
   does not involve decrementing the TTL value.  The subsequent
   forwarding step in the ITR itself, or in whichever upstream router
   the ITR sends the packet to, will decrement the packet's TTL.
























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9.  Upgraded Router Functionality

   The additional functionalities described in this section must be
   present in all internal and core routers through which the EAF
   formatted packet travels between the ITR and the ETR.  These
   additional functionalities must also be present in ETRs and in any
   ITR which is itself a router.

9.1.  Recognizing the EAF format

   The router must recognise bit 48 being set as indicating that the
   packet should be handled somewhat differently, as described below,
   rather than discarded.

   The packet can be treated as an ordinary IPv4 packet in all respects
   apart from those mentioned in this section.

9.2.  No checksum calculations

   The router still decrements TTL and sends an ICMP message if it
   reaches zero.  Therefore, Traceroute will work over the ITR to ETR
   part of the path.  However, there is no checking of the header's
   checksum, or altering the former checksum bits to account for the
   change to the TTL bits.

9.3.  Forward according to 30 bit ETR address

   Instead of forwarding the packet according to its destination
   address, the 30 bit ETR address in bits 50 to 63 and 80 to 95 is
   used, and extended to 16 bits with two zeroes in the least
   significant bits.

   Any filtering, statistics gathering etc. regarding source and
   destination address can proceed as usual.

   The EAF formatted packet should not be forwarded to any router which
   is not upgraded to handle it - therefore it must be forwarded to a
   modified router or to an ETR.  (An EAF-formatted packet may also be
   forwarded to a modified router with ITR capabilities, but the ITR
   function in that router will ignore it, since it is already in EAF
   format.)  However it is not a task of the router to ensure that the
   next hop router is upgraded.  This requirement must be achieved by
   ensuring that only upgraded routers advertise prefixes by which ETRs
   can reached.







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9.4.  ICMP generation

   There are a variety of circumstances in which an ordinary router may
   drop a packet and send an ICMP message to the sending host, including
   TTL reaching zero, the packet being DF=1 and too long for the next-
   hop MTU, and there being no path to forward the packet on.

   Modified routers use the same criteria for these actions as with
   normal packets, except that the address for determining next hop is
   the ETR address bits, not the destination address.  (An ICMP
   destination network unreachable packet which results from this
   contains the reconstructed Internet header, which contains the
   destination address and no mention of the ETR address.)

   In order to create an ICMP message which includes the start of the
   original packet, the router first reconstructs the header into
   ordinary IPv4 format - as described in a section above.

   Packets will also be dropped if their length exceeds the next hop
   MTU, with an ICMP PTB message being sent to the sending host.  Again,
   in the resulting ICMP message, the attached start of the packet will
   contain no mention of the ETR address or of the EAF format of the
   packet at the router where this MTU limit was exceeded.

9.4.1.  RFC 1191 PMTU with much less complexity than with map-encap

   In a map-encap system where the outer source address is that of the
   ITR, a router generating a PTB would send it to the ITR, requiring
   arguably impossible amounts of state and processing in the ITR to
   securely transform it into an appropriate PTB message addressed to
   the sending host [ITR-complexity].  (Authenticating the original PTB
   involves storing huge quantities of state of recently tunneled
   packets, and altering the MTU value in the PTB to a lower value, to
   account for the encapsulation overhead, so the sending host will send
   packets short enough not to exceed the MTU limit, once encapsulated.)

   In Ivip's encapsulation approach, the outer source address is that of
   the sending host.  The PTB would go back to the sending host, but be
   ignored, since it resulted from an encapsulated packet.  There are
   methods of ensuring that PMTUD works with Ivip's encapsulation
   approach [Ivip-PMTUD-Frag], but they involve undesirable complexity
   in the ITR and ETR.

   One of the most important benefits of EAF over encapsulation is that
   the router which generates a PTB message as just described generates
   a PTB message which will always be recognised by the sending host.
   The MTU value it contains is the one the sending host will comply
   with - which ensures that further packets will pass through this



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   limiting next-hop without any problems.  This is because there is no
   encapsulation overhead, and because the router is able to reconstruct
   the packet into a form identical to that which would be expected by
   the sending host after it has passed through any routers before
   reaching the router where the MTU limit was exceeded.

   The above-mentioned method of reconstructing the ordinary IPv4
   version of the packet does not alter the DF flag.  It sets the More
   Fragments flag and the Fragment Offset bits to zero - which is in
   accordance with the fact that the ITR only accepts packets with those
   bits set to zero.

   It is a requirement of the whole system - in terms of the placement
   of ITRs and ETRs, the placement of upgraded routers, and the MTUs of
   the paths between them all - that an EAF packet can be sent from any
   ITR to any ETR without encountering a non-upgraded router.  An
   additional condition is that the MTUs of all these paths must be at
   least MinCoreMTU bytes.

   Consequently, no modified router will handle a fragmentable packet
   (DF=0) which exceeds the next hop MTU.  (Once the EAF packet has been
   converted to an ordinary IPv4 packet at the ETR, it may hit an MTU
   limit - at the ETR's next-hop or at the next-hop of subsequent
   routers - and so be fragmented by that router according to
   established principles.)

   Therefore, PTB messages will only be generated for DF=1 packets.
   Conventional RFC 1191 PMTU will function correctly and the sending
   host will send subsequent packets with lengths which do not exceed
   the current router's next-hop MTU.





















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10.  ETR Functionality

   We assume that the ETR has sufficient functionality to recognise a
   packet whose destination address matches an end-user network it
   connects to, and that the ETR is able to transport an ordinary IPv4
   packet with such a destination address to that destination network.
   This may be achieved by forwarding in the local routing system, by
   tunneling or by forwarding the packets out of a specific interface
   which leads directly to that destination network.

   We assume that the ETR also has the extra functionality of modified
   routers, but this is minimal and would only in fact be needed if the
   ETR was functioning as a router handling packets with EAF addresses
   of other ETRs.  This would be the case if the ETR was a CE or PE
   router, and there were any ITRs inside the end-user network it serves
   - those ITRs or ITR functions in hosts would be sending EAF packets
   upstream, to be forwarded to other ETRs.

   With this assumption, the functionality required of an ETR is very
   minimal.

   When an ETR receives a packet in EAF format, it must recognise if the
   ETR address in the header matches its address (or one of its
   addresses).  If not, then if the ETR device functions as a router
   (that is, it has two outgoing interfaces to the rest of the network)
   then it must have the upgraded router functions.  In that case, it
   will forward the packet to whichever interface has a path for that
   ETR's prefix - or drop the packet and generate an ICMP destination
   network unreachable message as described above.

   Assuming the ETR recognises the ETR address in the EAF formatted
   header as corresponding to itself, then the ETR converts the header
   to ordinary IPv4 format as described above and then processes it
   normally - which includes transporting the packet to the destination
   network.
















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11.  ITR and ETR placement: MTU and upgraded routers

   EAF provides a much simpler method of transporting packets from ITR
   to ETR than map-encap.  This can best be appreciated by considering
   what has to be done in a map-encap system to support RFC 1191 PMTUD.
   It is also 100% efficient, with no overhead from encapsulating
   headers.

   However, EAF requires that all routers between ITRs and ETRs - in the
   core and in internal networks - be upgraded to handle the new header
   format.  Hopefully these changes can be implemented with existing
   routers via a firmware upgrade by the time Ivip4 or any comparable
   core-edge separation system is deployed.

   There are actually two requirements on routers between any ITR and
   any ETR - which in turn sets limits on where ITRs and ETRs can be
   located.  Firstly, the intervening routers must be upgraded to handle
   EAF format.  Secondly, the PMTU between all these routers, and
   between the ITRs and ETRs and their neighbouring routers, must be at
   least MinCoreMTU bytes.

11.1.  PMTU and MinCoreMTU

   In practice, with a wisely chosen value of MinCoreMTU, the PMTU
   requirement should not prove too constraining.  For instance,
   MinCoreMTU = 1470 enables fragmentable packets to be carried without
   fragmentation over a DSL service, with 8 bytes of overhead due to
   PPPoE, and for instance 28 bytes of IPv4 + UDP or GRE tunnel
   overhead.

   The sole purpose of MinCoreMTU is to support hosts currently sending
   DF=0 packets.  It is not a constraint to how long DF=1 packets can
   be.  As the core of the Net changes so that PMTUs of 9k or so become
   commonplace, hosts with 9k next-hop MTUs will naturally send 9k
   packets to destination hosts all over the world, where possible.
   EAF's uncomplicated support of RFC 1191 enables all sending hosts to
   adapt quickly to any lower PMTU it must comply with on the path to
   any particular destination host.

11.2.  Core and internal router upgrades

   The biggest hurdle an EAF-only system would need to overcome is the
   task of upgrading essentially all core routers to handle EAF-
   formatted packets.  "Core" routers in this context includes provider
   border routers.  In this discussion we refer to four kinds of
   network: ordinary provider networks, upgraded provider networks,
   ordinary end-user networks and SPI end-user networks.




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   Ordinary provider networks and ordinary end-user networks do not
   contain any ITRs or ETRs.  Therefore, these ordinary provider
   networks cannot be used by an SPI network.  By definition, an
   ordinary end-user network is not used by any SPI or any other end-
   user networks for connectivity to the Net, because then it would be
   functioning as a provider network.

   SPI end-user networks do not have any routers connected to the BGP
   interdomain core.  All their border routers (CE - Customer Edge)
   either link to ETRs in one or more upgraded provider networks - or
   the upgraded provider network provides a link (from a PE - Provider
   Edge - router) to the CE router, which itself functions as an ETR.

   With map-encap, it is possible to locate ITRs in a provider network
   which has no ETRs.  Likewise, ITRs could be located in conventional
   end-user networks which connect to provider networks which lack ITRs
   or ETRs.  In these cases, the purpose of the ITRs is to encapsulate
   packets from local sending hosts, rather than send those packets to
   the core and rely on OITRDs to do the encapsulation.

   However, with EAF, this flexibility of locating ITRs deep within
   provider and end-user networks is restricted by the need to ensure
   all internal routers, (provider) border routers and connected core
   routers are upgraded to handle EAF packets - and that the PMTUs of
   all links equal or exceed MinCoreMTU bytes.

   Within all networks, and the core, there is no need to upgrade a
   router if it can be assured that it never advertises a prefix which
   ETRs are located within.  For networks with internal ITRs, including
   ITR functions in the sending hosts, this means that most or all
   internal routers need to be upgraded.  Likewise, for any upgraded
   provider network which houses ETRs.

   With the exception of their CE routers (which have direct links to
   provider networks), SPI end-user networks never contain ETRs.  ETRs
   must be located on conventional address space, and so can only be in
   upgraded provider networks.  (Again, a conventional end-user network
   with an ETR is not an end-user network for the purposes of this
   discussion, since it is providing connectivity to some other network,
   including perhaps parts of its own network which use SPI space.)

   Any SPI end-user network, upgraded provider network or upgraded
   conventional end-user network may contain ITRs - in which case these
   networks need upgraded internal routers to handle all EAF packets
   emitted by these ETRs.

   There may be routers in the core which do not need to be upgraded:
   any border router of any network which contains no ITRs or ETRs, and



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   (the next condition may be impossible to assure) any transit router
   which never handles prefixes which contain ETRs.  This means that
   networks which do not want to use ITRs or ETRs need take no action
   while other networks upgrade their routers to support EAF.  Hosts
   within these non-upgraded networks will still have full connectivity
   to hosts in SPI end-user networks, via OITRDs.

   There may be some techniques within the BGP control plane to restrict
   the advertisement of ETR-containing prefixes only to routers which
   have been upgraded.  This may be a fruitful technique in the event
   that some core routers remain without the EAF upgrades.  However,
   this does not alter the requirement that there be enough core routers
   with EAF capabilities to ensure robust connectivity between all ITRs
   and ETRs.

   In a transitional arrangement, where map-encap Ivip is progressively
   converted to EAF, any such BGP techniques could prove valuable in
   ensuring that for each prefix in which all ETRs are reachable via EAF
   upgraded internal routers, such prefixes are only advertised to
   upgraded core routers.  However, the most attractive option remains
   to upgrade the core routers to EAF capabilities from the outset of
   operations, and therefore avoid the need to implement encapsulation
   with all its attendant complexities to handle PMTUD.

   Why should ISPs want to upgrade their core routers?  Perhaps because
   they could do so for minimal cost with a firmware upgrade, and
   because by doing so they are making it more possible to implement a
   core-edge separation system without the inefficiencies and
   complexities of encapsulation.

   Why would router vendors want to develop such upgrades for their core
   routers - and make available, at minimal cost?  Perhaps because they
   want to contribute to the establishment of the most efficient
   scalable routing solution, and/or because they would not want their
   installed routers to be viewed as being incapable of supporting the
   modest enhancements this entails.

   Upgrading sufficient (most, or ideally all) core routers remains the
   biggest challenge to deploying a scalable routing system based on EAF
   from the outset.  Quite a few years development needs to be done on
   these proposals before a sufficiently strong global consensus could
   be reached to choose and implement one scheme.  By then, it may be
   relatively easy to upgrade the core routers within the desired
   timeframe.

   Similar principles apply to internal routers.  These are likely to
   include a larger variety of models, including more older, non-
   upgradable, models than in the core.  However, to the extent that it



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   is harder or impossible to upgrade these older, smaller, routers, it
   is also true that they are more due for replacement - so the
   incremental cost of replacing them in order to support an EAF-only
   scalable routing solution is not necessarily prohibitive.















































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12.  Security Considerations

   Other than the extremely unlikely possibility of bit errors causing a
   packet to be presented to an upper layer protocol at some host other
   than the desired destination (Appendix 1) the EAF techniques do not
   appear to create any security problems beyond those inherent in the
   routing system or in map-encap.

   IPsec Authentication Header is unaffected by EAF.

   Where a border router - or any other router - handles the EAF format
   packet (this is necessarily a router with upgraded functionality
   which recognises the EAF format) the router can still perform
   filtering on the packet according to its source and destination
   addresses, and any other aspects of the packet - since only bit 48,
   the MF flag, fragment offset bits and the header checksum have been
   altered.  Consequently EAF should not reduce the effectiveness of
   existing filtering arrangements.

































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13.  IANA Considerations

   [To do as more detail is developed about data formats and
   communication protocols.]















































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14.  Informative References

   [Arkko-2008a]
              Arkko, J., "[RRG] thoughts on the design space 1: the
              space http://psg.com/lists/rrg/2008/msg01942.html",
              July 2008.

   [Arkko-2008b]
              Arkko, J., "Design Space (Dataplane)
              http://www.arkko.com/ietf/rrg/designspace_dataplane.jpg",
              July 2008.

   [Arkko-2008c]
              Arkko, J., "[RRG] thoughts on the design space 2: upper
              layer implications
              http://psg.com/lists/rrg/2008/msg01943.html", July 2008.

   [Checksums]
              Whittle, R., "IPTM - Protocols which involve bits from the
              IPv4 or IPv6 headers in their checksums
              http://www.firstpr.com.au/ip/ivip/checksums/",
              August 2008.

   [I-D.briscoe-tsvwg-re-ecn-tcp]
              Briscoe, B., Jacquet, A., Moncaster, T., and A. Smith,
              "Re-ECN: Adding Accountability for Causing Congestion to
              TCP/IP", draft-briscoe-tsvwg-re-ecn-tcp-06 (work in
              progress), August 2008.

   [I-D.farinacci-lisp]
              Farinacci, D., Fuller, V., Oran, D., Meyer, D., and S.
              Brim, "Locator/ID Separation Protocol (LISP)",
              draft-farinacci-lisp-08 (work in progress), July 2008.

   [I-D.jen-apt]
              Jen, D., Meisel, M., Massey, D., Wang, L., Zhang, B., and
              L. Zhang, "APT: A Practical Transit Mapping Service",
              draft-jen-apt-01 (work in progress), November 2007.

   [I-D.whittle-ivip-arch]
              Whittle, R., "Ivip (Internet Vastly Improved Plumbing)
              Architecture", draft-whittle-ivip-arch-02 (work in
              progress), August 2008.

   [ITR-complexity]
              Whittle, R., "ITR complexity & security (ICMP) in LISP-
              NERD/CONS & eFIT-APT http://www.ietf.org/mail-archive/web/
              ram/current/msg01766.html", July 2007.



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   [Ivip-PMTUD-Frag]
              Whittle, R., "IPTM - Ivip's approach to solving the
              problems with encapsulation overhead, MTU, fragmentation
              and Path MTU Discovery
              http://www.firstpr.com.au/ip/ivip/pmtud-frag/",
              April 2008.

   [Ivip6f]   Whittle, R., "Prefix Label Forwarding (PLF) for Ivip6
              http://www.firstpr.com.au/ip/ivip/ivip6/", August 2008.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC3514]  Bellovin, S., "The Security Flag in the IPv4 Header",
              RFC 3514, April 1 2003.

   [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
              Workshop on Routing and Addressing", RFC 4984,
              September 2007.

   [SixOneRouter]
              Vogt, C., "Six/One Router A Scalable and Backwards-
              Compatible Solution for Provider-Independent Addressing
              and IPv4/IPv6 Interworking http://users.piuha.net/chvogt/
              pub/2008/vogt-2008-six-one-router.pdf", July 2008.

   [TRRP]     Herrin, W., "TRRP
              http://bill.herrin.us/network/trrp.html", February 2008.




















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Appendix A.  Scenarios where bit errors alter the EAF formatted header

   Bit errors are unlikely to go undetected by L2 protection mechanisms.
   In the unlikely event that such errors are undetected, it is highly
   unlikely that the packet would be forwarded to a node which accepts
   it.  Silent packet loss is an acceptable outcome in the event of an
   bit errors in the header.  The only outcome of concern would be the
   packet arriving at the wrong node and being recognised, so that it is
   accepted by an upper level protocol.  This would disrupt that node,
   and potentially lead to information being divulged to inappropriate
   recipients.  However, the chances of this occurring are remote, as
   the following exhaustive discussion indicates.

A.1.  Error in bit 48

   An error in bit 48 (setting it to zero) will cause the packet to be
   recognised by the recipient router as an IPv4 packet.  That router
   will test the header against the presumed checksum in bits 80 to 95 -
   which are in fact part of the ETR's address.  There is a 2^-16 chance
   that the header would pass this test.  Of those which do, the great
   majority can be expected to have at least one of bits 51 to 63 set,
   indicating to the receiving host that this packet is a fragment.  The
   resulting packet would be forwarded towards whichever router
   advertises a prefix which matches the destination address.

   Assuming the destination address is unaltered, since the destination
   address is an SPI address and is part of an Ivip MAB (Mapped Address
   Block) the packet would be forwarded to the nearest OITRD (Open ITR
   in the DFZ) which advertises this MAB.  There it would be dropped,
   unless either: (1) all bits 50 to 63 were zero; or (2) bits 51 to 63
   were zero, bit 50 was 1 and the packet was shorter than MinCoreMTU
   bytes.  In the unlikely event the bit was not dropped by the OITRD,
   it would be turned back into a valid EAF format packet and forwarded
   to the ETR specified in the mapping for the packet's destination
   address - and so arrive correctly at the destination host in spite of
   the bit errors!

   If there were also errors in the destination address bits, then the
   packet would be forwarded to some host which is not its intended
   destination.  (If the altered destination address was an SPI address,
   then it would be forwarded to an OITRD and most likely be dropped, as
   just described.)  Assuming the damaged packet was delivered to some
   host other than its intended recipient, the IP layer of the recipient
   host would fail to pass the packet upwards unless it was received
   with all bits 50 to 63 set to zero.  If bit 50 was set, this would
   indicate that more fragments were to follow - yet no more fragments
   would be received.  If any bits 51 to 63 were set, this would
   indicate this packet was a second or subsequent fragment - yet no



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   other fragments would be received.

   Even if the packet was passed upwards, the changed destination
   address bits would have a 65,535 / 65,536 chance of being detected by
   the checksum of the higher level protocol header: UDP (if a checksum
   was used), TCP, DCCP and UDP-Lite.  IPsec AH would also detect the
   altered destination address with one of its hash functions.  ICMP,
   IGMP, RSVP, GRE, ESP, OSPF and SCTP would not detect the altered
   destination address [Checksums].  However this would only occur after
   a series of highly improbable - and unfortunate - events.

A.2.  Errors in bits 50 to 63 and 80 to 96

   Assuming bit 48 unaltered was (still set to 1), an error in any of
   the other modified bits 50 to 63 and 80 to 95 would cause the packet
   to be forwarded to a different 30 bit ETR address than the one set by
   the ITR.

   There is a remote chance, this may result in the packet arriving at
   another ETR which accepts the packet, due to this ETR recognising the
   destination address as one belonging to an end-user network it
   connects to.  More likely, the packet would be forwarded to the wrong
   address.  If there was an ETR at that address - which is unlikely -
   the packet would most likely be dropped because that ETR does not
   recognise the destination address as matching one of its end-user
   networks.  Alternatively, the packet may be recognised by an upgraded
   router as having a forwarding address which did not match any prefix
   for which it has a path - in which case the router would drop the
   packet and generate an ICMP destination network unreachable message.
   Alternatively, the packet would arrive at a host or at a non-upgraded
   router - in which case a host would be expected to view the header as
   invalid, due to bit 48 being set.

   In the scenarios mentioned in this subsection and the one before -
   all involving errors in the bits which have new functions in the EAF
   format - there is an extremely low probability that the packet would
   arrive at a node other than its intended destination in a form which
   caused it to be presented to any upper layer protocol.

A.3.  Errors in bits other than 50 to 63 and 80 to 96

   Initially, we discuss changes to bits in the header other than bit
   48, or 50 to 63 and 80 to 96 - assuming them to be unchanged.  It is
   assumed the packet arrives at the correct ETR, which converts the
   header into an ordinary IPv4 format, calculating a fresh checksum.
   In this scenario, the resulting packet will contain any altered bits
   we consider below, with a checksum which matches the altered state of
   those bits.  Consequently, we may expect the altered packet to be



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   sent to the correct recipient host, where the altered bits may have a
   variety of impacts.

   Changes to bits 0 to 3 would result in the packet being dropped by
   any router, including the ETR or any router between the ITR and the
   ETR, due to it failing to have the proper IP version bits.

   Changes to bits 4 to 7 (header length) would probably result in the
   packet being dropped by a router - due to the value being too low, or
   indicating that there were options in the header which the router
   does not accept or recognise.  If such a packet was forwarded to the
   recipient host, it is unlikely that the altered header length would
   allow the packet to be accepted as valid by the IP layer of any
   receiving host.

   Changes to the DiffServ and ECN bits would go undetected, but since
   the error is a random event affecting a single packet, it is unlikely
   that any serious negative consequences would arise.

   Changes to the Identification field would go undetected in most
   instances.  The only exception which comes to mind are ping packets.
   Identification is used for reassembling fragmented packets, but the
   ITR does not accept fragmented packets.  If a fragmentable packet
   (less than MinCoreMTU in length) was emitted by an ETR with an
   altered Identification field, and then fragmented en-route to the
   destination host, there would almost certainly be no ill effect,
   since the value of the Identification field in all the fragments
   would be the same.  Only in some extreme circumstances might a
   changed Identification field prove problematic: when it is changed to
   the value of another packet which is also fragmented, and where there
   is some out of order delivery, so disrupting the host's ability to
   correctly reassemble the two packets.  This would result in data loss
   of one packet, and potential data loss in the packet which was
   reassembled.

   Changes to the TTL bits 64 to 71 would generally go unnoticed, but
   the consequences are at most the loss of the packet and the
   generation of a spurious ICMP Time Exceeded message.

   Changes to the Next Protocol bits 72 to 79 would probably result in
   the packet not being accepted due to lack of an appropriate protocol
   handler in the destination host.  In the event that the new value was
   accepted, it would not match the first four bits of the next header
   and so the packet would be dropped.

   Changes to the source or destination address would be detected by
   many protocols, as noted above at the end of the subsection "Error in
   bit 48".



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   Bit errors in both the EAF bits and in others would result in more
   complex scenarios, but for the present discussion, it suffices to
   conclude that this is extremely unlikely to result in anything worse
   than data loss, and that comparable problems are considered
   acceptable in IPv6.














































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Appendix B.  Applicability to core-edge separation schemes other than
             Ivip

   There is nothing in EAF which depends on any other aspect of Ivip,
   such as the fast hybrid fast-push mapping distribution system.
   However, there would be little point in applying EAF to any system
   which required extra bits to be sent with each traffic packet.

   LISP requires such data to be included with each traffic packet: The
   encapsulation involves an IP header, a UDP header and then a LISP
   header.  EAF could not be applied to LISP without significant
   modification.  Also, the vision of simplicity of Ivip4f - EAF-only
   Ivip - involves no communication between ITRs and ETRs.  LISP
   involves messages going back and forth between ITRs and ETRs for
   purposes including testing reachability.

   APT also uses a three layer encapsulation technique involving IP, UDP
   and an APT header.  As with LISP's header, the APT header carries
   information which is vital to the functioning of the system.  (Ivip
   does not need these complex functions, because it does no multihoming
   reachability testing or service restoration decisions - end-users are
   required to do this in their own chosen manner, and send mapping
   changes to change the behaviour of ITRs in real-time.)

   APT relies on its encapsulation technique for an ITR to signal to a
   Default Mapper that the ITR needs some mapping information for
   whatever EID prefix the current packet's destination address is
   within.  Converting this to EAF would not be possible, since there is
   no record of the ITR address in the packet.

   TRRP uses GRE encapsulation - an IP header followed by a GRE header.
   Perhaps this could be replaced with EAF, as long as there was no need
   to convey data from the ITR to the ETR, or a need for the ETR to know
   the address of each ITR which was tunneling packets to it.

















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Author's Address

   Robin Whittle
   First Principles

   Email: rw@firstpr.com.au
   URI:   http://www.firstpr.com.au/ip/ivip/












































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Full Copyright Statement

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