GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1029

Network Working Group G. Parr Request For Comments: 1029 University of Ulster

                                                             May 1988
      A MORE FAULT TOLERANT APPROACH TO ADDRESS RESOLUTION FOR
                  A MULTI-LAN SYSTEM OF ETHERNETS

STATUS OF THIS MEMO

 This memo discusses an extension to a Bridge Protocol to detect and
 disclose changes in neighbouring host address parameters in a Multi-
 LAN system of Ethernets.  The problem is one which is appearing more
 and more regularly as the interconnected systems grow larger on
 Campuses and in Commercial Institutions.  This RFC suggests a
 protocol enhancement for the Internet community, and requests
 discussion and suggestions for improvements.  Distribution of this
 memo is unlimited.

ABSTRACT

 Executing a protocol P, a sending host S decides, through P's routing
 mechanism, that it wants to transmit to a target host T located
 somewhere on a connected piece of 10Mbit Ethernet cable which
 conforms to IEEE 802.3.  To actually transmit the Ethernet packet, a
 48 bit Ethernet/hardware address must be generated.  The addresses
 assigned to hosts within protocol P are not always compatible with
 the corresponding Ethernet address (being different address space
 byte orderings or values).  A protocol is presented which allows
 dynamic distribution of the information required to build tables that
 translate a host's address in protocol P's address space into a 48
 bit Ethernet address.  An extension is incorporated to allow such a
 protocol to be flexible enough to exist in a Transparent Bridge, or
 generic Host.  The capability of the Bridge to detect host reboot
 conditions in a multi-LAN environment is also discussed, emphasising
 particularly the effect on channel bandwidth.  To illustrate the
 operation of the protocol mechanisms, the Internet Protocol (IP) is
 used as a benchmark [6], [8].  Part 1 presents an introduction to
 Address Resolution, whilst Part 2 discusses a reboot detection
 process.

DEFINITIONS:

    CATENET: a group of IP networks linked together
    IP     : Internet Protocol

Parr [Page 1] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

                               PART 1

INTRODUCTION

 In the Ethernet, while all packets are broadcast, the hardware
 interface selects only those with either the explicit hardware
 broadcast address or the individual hardware address of this
 interface.  Packets which do not have one of these two addresses are
 rejected by the interface and do not get passed to the host software.
 This saves a great deal of otherwise wasted effort by the host
 software having to examine packets and reject them.  If the interface
 hardware selected packets to pass to the host software by means of
 the protocol address, there would be no need for any translation from
 protocol to Ethernet address.  Although it is very important to
 minimize the number of packets which each host must examine, so
 reducing especially needless inspections, use of the hardware
 broadcast address should be confined to those situations where it is
 uniquely beneficial.  Perhaps if one were designing a new local
 network one could eliminate the need for an address translation, but
 in the real world of existing networks it fills a very important
 purpose.  A rare use of the broadcast hardware address, which avoids
 putting any processing load on the other hosts of the Ethernet, is
 where hosts obtain the information they need to use the specific and
 individual hardware addresses to exchange most of their packets.

REASONING BEHIND ADDRESS RESOLUTION

 The process of converting from the logical host address to the
 physical Ethernet address has been termed ADDRESS RESOLUTION, and has
 prompted research into a method which can be easily interfaced,
 whilst at the same time remaining portable.
 The Ethernet requires 48 bit addresses on the physical cable [11] due
 to the fact that the manufacturers of the LAN interface controllers
 assign a unique 48 bit address during production.  Of course, Network
 Managers do not want to be bothered using this address to identify
 the destination at the higher-level.  Rather, they would prefer to
 assign their logical names to the hosts within their supervision, and
 allow some lower level protocol to perform a resolving operation.
 Most of these logical protocol addresses are not 48 bits long, nor do
 they necessarily have any relationship to the 48 bit address space.
 For example, IP addresses have a 32 bit address space [6], thus
 giving rise to the need to distribute dynamically the correspondences
 between a <PROTOCOLTYPE,PROTOCOL-ADDRESS> pair, and a 48 bit Ethernet
 address.

Parr [Page 2] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

EXAMPLE ARP OPERATION

 Here is a review of the operation of ARP as defined in RFC-826 [5].
 Let hosts X and Y exist on the same Ethernet cable.  They have
 physical Ethernet addresses EA(X), and EA(Y), and DoD Internet
 addresses IPA(X), and IPA(Y).  Let the Ethernet type of Internet be
 ET(IP).  Host X begins an application, and sooner or later wishes to
 communicate an Internet packet to host Y.  Host X has knowledge of
 the Internet address of Y, i.e., (IPA(Y)), and informs the lower
 level that it wishes to talk to IPA(Y).  The lower-level subsequently
 consults the ARP Module (ARM) to convert <ET(IP),IPA(Y)> into a 48
 bit Ethernet address but because X has not talked to Y previously, it
 does not have this information in its Translation Cache (TC).  It
 discards (or queues) the Internet packet, and creates a new Address
 Resolution packet with:
     PACKET FIELD             VALUE ASSIGNED
      HRDTYP                   ETHERNET
      PROTYP                   ET(IP)
      HRDLEN                   length (EA(X))
      PROTLEN                  length (IPA(X))
      ARPOPC                   REQUEST
      SOURCE HWR               EA(X)
      SOURCE PROT              IPA(X)
      TARGET HWR               don't know
      TARGET PROT              IPA(Y)
 It then broadcasts this packet to all hosts on the connecting cable.
 Host Y picks up this packet and determines that it understands the
 hardware type (Ethernet), that it speaks the indicated protocol
 (Internet), and that the packet is for it, that is, TARGET PROTOCOL
 ADDRESS = IPA(Y).  Replacing any previous entry, it enters the
 information that <ET(IP),IPA(X) translates to EA(X).  It then learns
 that this is an ARREQ packet, so it swaps fields, placing EA(Y) in
 the new sender Ethernet address field SOURCE HARDWARE ADDRESS, EA(X)
 as TARGET HARDWARE ADDRESS, IPA(X) as TARGET PROTOCOL ADDRESS, IPA(Y)
 as SOURCE PROTOCOL ADDRESS, and sets the opcode to REPLY.  The packet
 is then sent with direct routing address information to EA(X).  Thus,
 Y now knows how to send to X, but X still doesn't know EA(Y).

Parr [Page 3] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 When X receives the ARREP packet from Y, it gets the address
 information into its translation cache ET(IP),IPA(Y)>-->EA(Y),
 notices that it is a REPLY, and discards the packet (i.e., disposes
 of the dynamic packet buffer).  However, if the original Internet
 Module packet had been queued, it could have been accessed and given
 the full addressing information from the translation cache.
 Alternatively, had it been discarded, the higher level would have
 succeeded on a subsequent attempt, and the Internet packet would be
 transmitted immediately.

OBTAINING GREATER NETWORKING RANGE

 There are many benefits to be gained in dividing a large multiuser
 network into smaller, more manageable networks.  These include : Data
 Security; Overall Network Reliability; Performance Enhancement; not
 to mention the most obvious: Greater Networking Range.  In some
 network technologies, cable length may be stipulated not to exceed a
 certain range due to electrical limitations.  By installing a Bridge,
 this restriction is effectively eliminated.  An important
 consideration is the effect the induced Bridge delays will have on
 the protocol timeouts in operation on each LAN/Subnet.  Careful
 analysis of upper bounds on timeouts would have to be made in order
 to gain full benefit from the increased range.  In the case of
 Ethernet the following system parameters exist [11], [12]:
  1. the bus bandwidth is 10Mbit/s
  1. the maximum node-to-node cable length is 1500 m
  1. the maximum point-to-point link cable length is 1000 m
  1. the maximum number of repeaters between two nodes is two
  1. the worst case end-to-end bus propagation delay is 22.5 us
  1. the jam time after collision is 32bit
  1. the minimum interframe time is 9.6 us
  1. the slot size is 512 bit = 51.2 us
 Once a decision has being taken to subnet, the resulting subLANs may
 be connected by including a Bridge to link them together and
 providing a protocol which makes the collection of subnets appear as
 a single network.  The basic idea of the Bridge providing 'repeater'
 facilities would not suffice in this application.  Moreover, the
 Bridge would have to have further 'intelligence' to enable it to
 select those packets which are destined for remote networks based on

Parr [Page 4] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 the protocol address of the target host.  Thereby preventing it from
 forwarding packets needlessly that will not be accepted.  If this
 procedure was not adhered to, the channel bandwidth on the remote
 networks would be inundated with packets, causing local valid traffic
 to backoff and the efficiency of the respective networks to rapidly
 decrease.
 One problem fundamental to the operation of the Bridge is how it
 discovers on which LAN a particular host is interfaced.  If there are
 only two LANs in the system, each will have a dedicated cache at the
 Bridge, and when a packet is received at the particular interface,
 the source host's address parameters are entered in the respective
 LAN cache.  However, when we consider a Multi-LAN environment, the
 procedure becomes more complicated.
 ___
  |
  |-----h3
  |                                            E4
  |-----hq                            |-----------------------|
  |                _                             |        |
  |-----hx        | | B1                         |        |
  |---------------| |                            |        |
  |-----h1        |_|                            |        |
  |                |     h19                     |        |      ______
  |                |    |                       | |        -----|______|  B4
  |                |    |                       | | B3              |
  |-----he       |-------------------| E2       |_|                 |
  |                    |                         |                  |
  |-----h5             |                         |                  |
  |                    |                         |                  |
  |                   ---                ---     |                  |
 ---                  | |                 |-------                  |
 E1                   | | B2              |                         |
                      | |-----------------|                         |
                      ---                 |                         |
                                          |          |---------------------
                                         ---                              |
                                          E3                              |
                                                                          |
                    FIGURE 1.  A MULTI-LAN TOPOLOGY
 In the normal set-up, whenever B3 or B4 would receive a packet on E4,
 they would both update the caches on their E4 interface.  In
 addition, a method must be provided to permit B4 to distinguish
 between packets arriving on E4 from E1, E2, E3, and those which
 actually originated on E4.

Parr [Page 5] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 This is so that packets can be categorized as being of remote or
 local source and processed accordingly.  The most obvious solution is
 for each Bridge to act as an AGENT and plug in its address as the
 source of any packets it cascades to a remote network, instead of the
 packet being cascaded with its original source address.  At Bridge
 boot, it may issue a broadcast request for all locally connected
 hosts/devices to return their local network protocol addresses.  On
 subsequent receipt of this information, the Bridge could then update
 the cache for each of its interfaces so that it would now have a base
 from which to perform future operations.
 The alternative to this automatic procedure is to permit manual
 intervention in the Bridge software which could be activated by the
 network manager in order to key in the addresses of the hosts
 connected to each LAN interface.
 Thus, having provided a means for the Bridge to obtain the original
 state of the LAN addresses when it boots, how then does the Bridge
 distinguish the arrival of a new host on the locally connected system
 from transmissions which were sent from a remote source and cascaded
 by an adjacent Bridge?  Two approaches are currently under
 consideration to solve this problem, namely Explicit Subnets, and
 Transparent Subnets [4], [7], [9], [14].
 In the Explicit Subnet approach, the location of the host in the
 system is important.  The address of the host in the protocol suite
 will reflect which subnet the host is interfaced to.  Consequently
 the protocol address space is divided into a three level hierarchy of
 <network,subnet,host>.  Within the Internet there are five addressing
 divisions in operation [10], classes A, B, C, D, and E.  Classes D
 and E relate to an addressing technique that will be used for
 management of multi-casting groups and will not be discussed here.
 With such a structure, it is possible to provide an address mask at
 each interface so that received packets may have their source address
 fields examined and compared with the address mask of this LAN.  In
 so doing, the component which is being verified is actually the
 subnet address.  If the masking operation is successful the source
 must exist on this LAN, otherwise it must be remote.
 With the Transparent scheme, the first time a newly booted host
 'speaks' it will be looking for addressing information (probably
 using BOOTSTRAP [1], RARP [2] or ARP [5]).  Accordingly, the Bridge
 will detect these respective requests and be in a position to perform
 operations on the address parameters.  The current approach in
 Transparent Subnetting is that before any such requests can be
 cascaded by the Bridge to an adjacent LAN, that Bridge will place its
 interface address parameters into the source address fields, thus
 acting as the AGENT.  Therefore, this Bridge will 'see' either

Parr [Page 6] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 packets arriving from the remote Bridge address, or local packets.
 By virtue of the RARP/ARP operation, which hosts perform when they
 first come up, any hi-level packets received on to the network not
 having the bridge address, and not having a mapping in the cache for
 that LAN, can be considered as being remote.
 Currently, there is a move toward the Transparent subnet proposal
 originally described by Postel [7].  This has been due mainly to
 practical problems of incompatible implementations from different
 vendors, and the restrictions that the Explicit address space place
 on the adaptability of the system to change (class C addresses are
 not flexible enough for the Explicit scheme).  It is also the opinion
 of the Author of this paper that the Agent technique adopted by the
 Bridges could have shortcomings in a dynamic environment which would
 be detrimental to its operation; for example, where the bridges
 themselves relocate or crash, or in the management of the "Agent For
 Who" cache at the bridge.  Insofar as Loop Resolution and
 SelfStabilization after failure are Bridge problems that need to be
 addressed, it is strongly felt their satisfactory solution will be
 supported by elimination of the Agent technique [13].

BRIDGE OPERATIONS

 Referring to figure 1, assume that at some stage during its
 processing [E1H3] wishes to communicate with [E2H19].  [E1H3] obtains
 knowledge of the Internet address of [E2H19] from its translation
 cache, but will not require the knowledge that [E2H19] exists on a
 completely different subnet.  [E1H3] calls its Internet Module to
 transmit the packet.  As detailed, the usual procedure of passing
 control to its ARM is performed in an attempt to obtain a
 translation.  If we assume that [E1H3], and [E2H19] have not talked
 before, the ARM in [E1H3] will not be able to resolve the addresses
 on the first attempt.
 In such a case, an ARREQ packet is assembled and broadcast to all
 hosts on the network [E1].  The packet traverses the cable and is
 eventually picked up by the (B1) Bridge Address Resolution Module
 (BARM), whereupon it determines whether or not it should intervene in
 the request.  If the target is determined as remote (i.e., having no
 match in the local cache), the BARM examines its Global Translation
 Cache (GTC) to determine if it has an entry for <protocol,[E2H19]>.
 Should a mapping be obtained at the Bridge, there is no need for the
 broadcast REQUEST packet to be cascaded on to the remote network
 [E2].  It is therefore assumed that the entries in the GTC reflect
 the most current addressing information.  A match thus obtained, the
 original ARREQ packet buffer is adapted as required and returned
 directly to [E1H3] via the Bridges hardware interface IFE1.

Parr [Page 7] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 On the other hand, should the Bridges' GTC have no information on
 [E2H19], the BARM would have to perform the following steps:
    1.  drop the current ARREQ from [E1H3],
    2.  create its own ARREQ using the Bridge source addresses
        and copy the target_internet_addr from the original
        [E1H3] ARREQ packet,
    3.  broadcast the ARREQ on network E2 via network interface
        IFE2, and go into a timeout awaiting a REPLY.
 Should this timeout period expire, a number of retries will be
 permitted under control of the BARM.  Alternatively, if a REPLY is
 received within the timeout interval, then the BARM will update its
 GTC.  The ARM of [E1H3] next will attempt to transmit another ARREQ,
 but this time a mapping will be obtained at the BARM'S GTC, and the
 appropriate REPLY will be returned.
 Part 1 has described the state of the art of the behaviour of Address
 Resolution.  Part 2 now extends the study to the more serious problem
 of rebooting hosts in a multi-LAN system of Ethernets, and the
 effects such changes have on the integrity of state information held
 in ARP caches and routing tables.
                               PART 2

THE CAPTURE OF REBOOTS

 Because Address Resolution packets are broadcast, all hosts on the
 connecting cable including the Transparent Bridge will pick them up
 and determine what they are.  Referring to figure 1, it may well be
 the case that a host on E1 wishes to communicate with a fellow host
 on the same physical ether.  Hence, if Hx wishes to talk to Hw on the
 same ether, but has not done so previously, it will broadcast an
 Address Resolution packet in the normal fashion.  The Bridge will
 also 'see' the packet as it passes by, and will act as described
 above, unless that is, there is some method of preventing it doing
 so; there is no point in the Bridge invoking its ARM, and wasting
 processing time if the problem is going to be resolved locally.
 It may occur however, that H1 wants to communicate with H5.  If
 however, H5 has not talked with anyone before (i.e., it has been
 "dormant"), H1 will issue an ARREQ.  The Bridge will not know that H5
 is local because it won't have been entered in the local address
 cache from previous conversations.  To avoid broadcasting an ARREQ to
 all networks/subnets, one way around this problem is to set up the
 contents of the local cache at Bridge startup time.  Therefore, the

Parr [Page 8] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 Bridge will already know not to intervene.  Thus, if the Bridge (with
 2 nets) finds that a particular IP destination address is not in the
 local cache of interface 1, it would have to examine its GTC and scan
 it for a mapping.  Should no mapping be obtained at interface 2, one
 of two possibilities exist:
      1. the target host doesn't exist locally
      2. the caches are corrupt (the eventuality of this should
         be negligible!)
 If it is assumed that each of the translation caches contains have
 the most recent addressing information regarding its own domain of
 the network then, in this example, if the Bridge does not get a
 mapping at the GTC it would appear that the host must exist remotely
 from E1, and E2.
 Such a conclusion would ignore cases in which a host unplugs from a
 particular hardware interface and plugs into another hardware
 interface, or where logical names are reassigned to different
 interfaces due to host user change.  Either of these events could
 happen had the host being accessed on E2, which would mean that a
 REBOOT has taken place.
 Anticipating these possiblities local caches are essential.  In
 normal operation, the Bridge will process and forward IP packets
 received from one network, and destined for another.  If the Bridge
 picks up an ARREQ, it will first look for a mapping in its GTC before
 discarding the original ARREQ, and transmitting its own to the remote
 network.  In any case, the Bridge will always examine the local cache
 entries at the receiving interface, so that it may determine if the
 target address is local or remote.  When the Bridge first scans the
 local cache, it does so with the source IP address as the key.  If no
 mapping is retrieved, it then scans the GTC with the same key.
 Should a mapping now be obtained, it remains for the Bridge to insert
 the source IP into the local cache, where it has either been
 previously deleted or corrupted.
 However, if the source IP exists in the respective local cache, the
 validity of the source Ethernet address should also be verified by
 examining the respective entry in the GTC.  A scan of the GTC is then
 performed with <protocol,source_prot_addr> as the key.  If a mapping
 is retrieved, the respective <et_addr> should be checked against the
 source Ethernet address in the packet header.  If the addresses do
 not match, then we have uncovered a Hardware Reboot condition (i.e.,
 a change in Ethernet ID).  On the other hand, should the scan of the
 GTC with <protocol,source_prot_addr> fail to obtain a mapping, then
 the Bridge would scan the GTC with the current Ethernet address in

Parr [Page 9] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 the packet header.  If this obtains a mapping, then a Protocol Reboot
 condition (i.e., change in logical ID) has been detected.
 In the next section, the implications of these forms of 'Reboot' are
 discussed.

REBOOT SCENARIO

 In normal operation, packets will uneventfully traverse each subnet
 either as complete Internet packets, broadcast ARREQ's, or direct
 ARREP's.  The Bridge attached to each subnet will 'hear', and 'see'
 all packets as they travel past its connected interfaces.  Because of
 the existence of the local caches at each interface, the Bridge can
 decide whether or not to intervene.  In general circumstances, each
 host on the Catenet will have a translation cache containing
 <protocol,source_prot_addr,source_et_addr> entries for all packets it
 has observed.  Most of these entries will have been due to processing
 ARREQ packets, which were broadcast, and by receiving REPLY packets.
 In accordance with the foregoing , the Bridge will have a cache
 attached to each subnet interface containing entries for protocol
 addresses.
 Within the Bridge's Global Translation Cache (GTC) will be entries of
 all <protocol,source_prot_addr,source_hrd_addr> triplets relating to
 valid hosts which have been recognised.  If we assume that we have
 just connected up a Catenet such as that illustrated in figure 1,
 then at power-up no stations will have knowledge about their
 neighbours.  If the Bridges are to remain transparent, the
 translation caches at each host will be totally empty.  The only
 addressing details that will be in existence will be the protocol
 addresses stored in the local caches of the Bridges.
 The hosts subsequently begin to run applications and will want to
 communicate with one another.  The first ARREQ is broadcast on the
 respective subnet and all hosts, including the Bridge's interface to
 the subnet, will pick it up and store the details.  If, for example,
 Hx issues an ARREQ for Hq, the Bridge will not intervene since there
 is no need (providing no reboot has occurred at Hq).  However, if Hx
 wishes to talk with Hz, B1 will determine that the target IP in the
 respective ARREQ does not exist in the local cache of IFE1, so it
 will examine the GTC, with the <protocol,target_prot_addr> of Hw as
 the key.
 It is assumed that there will be a timeout mechanism in operation at
 the source of any packet.  In addition, the Bridge may also place the
 target address in a 'search list' of currently sought hosts, so as to
 prevent ARREQs from different sources being cascaded for the same
 target.  Under these conditions, Hx may re-issue its original ARREQ,

Parr [Page 10] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 but will be ignored until the host Hw has replied to the ARREQ
 transmitted by the Bridge.

NORMAL RUNNING STATE

 Assuming that a few ARP's have been issued, IP packets will start
 traversing the Catenet with full addressing information.  Again, the
 Bridges will 'see' all the packets.  If we extend the situation one
 step further, and assume that several conversations have taken place
 across the Catenet, there will be entries in the translation caches
 of the hosts concerned, regarding the
 <protocol,target_prot_addr,target_hrd_addr> triplets of those hosts
 with which the conversations took place.  The Bridges also, will have
 details in their GTC's for packets which they cascaded.
 If a host is relocated, any connections initiated by that host will
 still work, provided that its own translation cache is cleared when
 it does physically move.  However, any connections subsequently
 initiated to it by other hosts on the Catenet will have no particular
 reason to know to discard their old translation for that host.
 Ideally, 48 bit Ethernet addresses will be unique and fixed for all
 time.

RECOGNITION OF THESE REBOOT CONDITIONS

 With reference to figure 1, assume that for some reason a fault
 occurs on the hardware interface of <E1He>.  The result of this is
 that a new interface is installed with a newly acquired hardware
 address.  When <E1He> is powered up, the previous contents of its
 translation cache are cleared and it has no recollection of local, or
 remote host addresses.  Accordingly, <E1He> begins to issue ARREQ's
 to hosts it requires.  Whenever <E1He> transmits its first ARREQ, it
 could be termed a 'HELLO PACKET', since everyone on the subnet can
 pick up the packet, and store the relevant information in their
 translation caches.  Within hosts, a mapping will be found on the old
 <protocol,source_prot_addr> pair, and the current <et_addr> of the
 packet header will replace whatever is entered in the translation
 cache.
 At this point it would be easy for each host with an entry to
 recognise the Hardware Reboot situation and inform the subnet with a
 respective broadcast reboot packet.  But allowing such a procedure
 would be extremly inefficient on the broadcast medium, and would
 drastically outweigh any improvements in performance which might be
 obtained in the long term.  In any case, given the fact that the
 ARREQ is broadcast, all stations on the subnet will recognise the
 reboot.  The important point to consider is the effect such a reboot
 will have on subsequent conversations which are initiated remotely.

Parr [Page 11] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 Can redundant transmissions be thwarted before they tie up processing
 time on hosts en-route to the rebooted target?  How these
 difficulties are resolved is critical to the level of performance
 obtained in a Catenet configuration.  Since it is not optimal for
 hosts to inform the system of a reboot, it is left to the Bridge.
 Whenever the Bridge receives a packet, be it IP, or ARP, it examines
 the source address parameters in the packet header, in the hope of
 detecting any incompatibilities between them and the entries in its
 caches.  There are three distinct possibilities, namely, a difference
 in the 48 bit hardware address only, a difference in the protocol
 address, and two completely new addresses.  If an incompatibility is
 discovered, a "REBOOT" packet is constructed and issued on all remote
 interfaces containing the appropiate information, allowing Bridges to
 update their GTC's and generic hosts their ARP caches.
 The structure of the Reboot packet is as depicted in figure 2.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | P A C K E T     O P C O D E   |REB OPC|      S O U R C E      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        H A R D W A R E            A D D R E S S               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       S O U R C E   P R O T O C O L     A D D R E S S         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     M U L T I C A S T   T A R G E T    H A R D W A R E        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    A D D R E S S      |   M U L T I C A S T     T A R G E T   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   P R O T O C O L     |
 +-+-+-+-+-+-+-+-+-+-+-+-+
  1. ——–> NEXT FOLLOWS A VARIANT FIELD ON REBOOT OPCODE
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  O L D         S O U R C E        H A R D W A R E             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  A D D R E S S        |
 +-+-+-+-+-+-+-+-+-+-+-+-+
  OR
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  O L D     S O U R C E    P R O T O C O L      A D D R E S S  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        FIGURE 2. REBOOT PACKET

Parr [Page 12] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 The following definitions apply:
      PACKET FIELD              VALUE
      OPCODE                    REBOOT
      REBOOT OPCODE             HARDWARE
      REBOOT OPCODE             PROTOCOL
 The format is then as follows:
      48 bit broadcast Ethernet address for the destination,
      48 bit Ethernet address of source Bridge,
      16 bit Protocol type = PACKET OPCODE - REBOOT.
 For completeness and error checking it may be an advantage to have a
 field which specifies the length of addresses in the Ethernet and
 protocol address spaces.  Thus, the Reboot packet structure contains
 the following:
 FIELD          FIELD SIZE                    DESCRIPTION
 HRDLEN          4 bit             byte length of Ethernet address
 PROTLEN         4 bit             byte length of Protocol address
 SOURCE
 PROTOCOL
 ADDRESS        32 bit            current protocol address of host
 TARGET
 PROTOCOL
 ADDRESS        32 bit           broadcast target protocol address
 REBOOT
 OPCODE          4 bit            will be either PROTOCOL or HARDWARE
 if   PROTOCOL       32 bit         old protocol address
 else HARDWARE       48 bit         old hardware  address

Parr [Page 13] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 As shown, depending on the REBOOT-OPCODE, the structure will continue
 with either the 48 bit old hardware address or the 32 bit old
 protocol address.  The choice of a variant packet structure is for
 reasons of curtailing the size of the packet to the fields that are
 truely necessary in each situation.  From this Reboot packet
 structure, the process of generating such a packet can be considered.
 When the Bridge algorithm detects a reboot, it should create a reboot
 packet structure containing the relevant addressing information and
 subsequently multicast it on the interface(s) which access(es) the
 remote subnet(s).  The decision as to which interface(s) is/are
 local, and which is/are remote, can be resolved automatically
 whenever a packet is received.  With respect to this packet transfer
 the receive interface at the Bridge becomes local, and all others are
 tagged as remote.
 Thus, hosts on the subnet remote from the reboot are informed of the
 situation immediately as it is detected by the Bridge.  In the
 Catenet configuration illustrated in fig 1, this will have the effect
 of updating the Translation Cache within each host, whenever it
 receives the packet.  If for example, <E4Hw> reboots under hardware,
 B3 will detect this occurance.  There is no reason for the subnets
 E1, E2, E3 to be aware of this episode.  In normal operation, B3 will
 recognise the reboot from the first ARREQ issued from <E4Hw>.  With
 this reboot detection facility, B3 will be in a position to inform
 the hosts on E1, E2, and E3.  B3 can then create and issue the Reboot
 packet via its interface with E3.  When B3 picks it up, it will
 update its own caches and subsequently cascade the packet onto E2,
 where it will be passed on to E1 via B1.

ARGUMENTS FOR REBOOT PACKETS

 It is envisaged that introducing Reboot packets, will serve to
 enhance the bandwidth achievable within a Catenet system.  Problems
 of addressing 'dead' hosts will no longer exist in a correctly
 functioning configuration.  Translation Caches will have on hand the
 most recent addressing information available, which should also serve
 to enhance the performance of the routing strategy in operation.
 Multiple, redundant processing of packets destined for 'dead' hosts
 will be avoided.  Weighing this against the processing involved with
 a single multicast of Reboot packets, it is expected that the latter
 will be is the most economically viable in relation to the long-term
 traffic presented to the system.

CONCLUSION

 It appears that reboots are becoming increasingly common on internet
 networks.  Many sites use Personal Computers (PC) as terminals and
 the typical way to finish a session is to switch them off!  With the

Parr [Page 14] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 increasing popularity of multitasking Operating Systems on these
 types of machines, problems are more likely to occur, particularly
 when the PCs are diskless, or participating in a distributed file
 system of some kind.  Given the importance of correct addressing in
 communications networks running Ethernet, it is anticipated the
 reboot mechanism described will serve to improve the correctness and
 validity of the protocol/network address mappings which may be stored
 in the translation caches.  To this degree, simulation is expected to
 show that the volume of invalid traffic will decrease, to the benefit
 of hosts, Bridges and servers alike.  Likewise, ratification of the
 routing policy is anticipated and since redundant/obsolete packets
 will be thwarted, the efficient utilization of available channel
 bandwidth across the catenet is also expected to improve.  Thus,
 effectively increasing Catenet throughput for 'valid' packets, and
 therefore enhancing the level of service provided to the end users.
 It is obvious that the proposed scheme implies the alteration of the
 packet processing code in Bridges/Gateways.  The point to remember is
 the increased favour with which larger, more complex Multi-LAN
 systems of Ethernets are being received.  The recent adaption of
 extra telephone cables to serve as the transmission media for the
 Ethernet can only result in installation costs being reduced, therein
 making the Ethernet more attractive within large corporate buildings,
 etc.  It is sensible to suggest that the probability of host address
 re-assignment shall increase in proportion to the number of physical
 systems attached, component failure rate (for whatever reason),
 relocation of resources, and the size and turnover of the workforce
 (i.e., people moving from one room to another).  Simulation
 experiments are currently being developed to analyse the resultant
 traffic patterns under this scheme, and it is hoped to highlight
 thresholds where adoption of the scheme becomes a necessity.
 In addition, the Author is currently extending the boundaries of this
 problem to encompass the reboot, or relocation of Bridges themselves.
 Involved with this are the phenomena of loop resolution, load sharing
 and duplicate packet suppression.  It is envisaged that a Self-
 Stabilizationg Bridge Protocol will result that will be more "light-
 weight" than those adhering to the Spanning Tree Algorithm.
 The Author would appreciate feedback/comments on this RFC.  My
 network address is: CBAD13%UCVAX.ULSTER.AC.UK@CUNYVM.CUNY.EDU.

ACKNOWLEDGEMENTS

 The Author acknowledges with gratitute the help and comments
 contributed by Mr. Piotr Bielkowitz (Supervisor) of the Computing
 Science Department, and the time devoted my Mr. Raymond Robinson for
 painstakingly preparing the first draft of this paper on 'Pagemaker'.

Parr [Page 15] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

 Thanks are due also to Dr. M. W. A. Smith of Information Systems for
 his assistance.  Finally, this work was supported under a grant from
 the Department of Education for Northern Ireland of which the Author
 is extremely grateful.

REFERENCES

 [1]  Croft, Bill, and John Gilmore, "Bootstrap Protocol", RFC-951,
      Stanford University, September 1985.
 [2]  Finlayson, Mann, Mogul, and Theimer, "A Reverse Address
      Resolution Protocol", RFC-903, Computer Science Dept, Stanford
      University, June 1984.
 [3]  Lorimer, Alan, and Jim Reid, "ARP Information Communique",
      Computer Science Dept, Strathclyde University, 1987.
 [4]  Mogul, Jeffrey, "Internet Subnets", RFC-917, Computer Science
      Dept, Stanford University, October 1984.
 [5]  Plummer, David, "An Ethernet Address Resolution Protocol", RFC-
      826, MIT, November 1982.
 [6]  Postel, Jon, "DARPA Internet Program Protocol Specification",
      RFC-791, USC/Information Sciences Institute, September 1981.
 [7]  Postel, Jon, "Multi-LAN Address Resolution", RFC-925,
      USC/Information Sciences Institute, October 1984.
 [8]  Postel, Jon, Carl Sunshine, and Danny Cohen, "The ARPA Internet
      Protocol", Computer Networks, no. 5, pp. 261-271, 1981.
 [9]  Postel, Jon, and Jeff Mogul, "Internet Standard Subnetting
      Procedure", RFC-950, USC/Information Sciences Institute and
      Stanford University, August 1985.
 [10] Reynolds, Joyce, and Jon Postel, "Assigned Numbers", RFC-1010,
      USC/Information Sciences Institute, May 1987.
 [11] "The Ethernet: a local area network, data link layer and
      physical layer specification", Version 1.0 DEC, Intel and Xerox
      Corporations, USA 30 September 1980).
 [12] Hughes, H.D., and L. Li, "Simulation model of an Ethernet",
      Computer Performance, Vol 3, no. 4, December 1982.
 [13] Parr, Gerald P., "Address Resolution For An Intelligent
      Filtering Bridge Running On A Subnetted Ethernet System", ACM

Parr [Page 16] RFC 1029 Fault Tolerant ARP for Multi-LANs May 1988

      SIGCOMM Computer Communication Review, (July/August 1987), vol.
      17, no. 3.
 [14] Smoot, Carl-Mitchell, and John S. Quarterman, "Using ARP to
      Implement Transparent Subnet Gateways", RFC-1027, Texas Internet
      Consulting, October 1987.

Parr [Page 17]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1029.txt · Last modified: 1988/05/09 19:46 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki