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a~icl, pcrhaps Inorc important, ofdemanding that nct-\vork addrcss reassignment be supported throughoutthe arcl-~itccturc. Tlic two neu. addrcssing services areStatclcss Address Autoconfi g~rration and the stateful,transaction-based Dynamic Host Configuration Protocolversion 6 (1>H(:l'\~6).7Y~n the st.ltcless model,address prctiscs arc Icarncci by listcni~ig for routerad\fertiscrncnt packets. Addrcsscs ~ rc formed by combiningtlic prcf s \\.it11 linlc-specific token such as the48-bit Ethcrnct liard\\,are addrcss. In tlie statcfi~l proccclure,hosts Ilia!, rcqucst addrcsscs, configurationi~iformntion, and scr\.iccs from drdicatcd contigurationser\,crs, \\,it11 routers potcnti,~ll!r scr\'ing as relaystations cil~ring the initid pIi3sc. In botli cases, tlieresulting ;iddrcsscs have associated lifetimes, and systeltlsr~lust be prepared to both learn addrcsscsand rcleasc cspjred addrcsscs. Combined \vitli theability to rcgistcr updated addrcss information withDNS serlrers, these mcclianisms provide a path toward~ictc\lork rcnunibcring, a goal that has provcd difficultto achieve in the I l'v4 world.Finally, the Internet Control Mcssagc Protocol version6 (ICMPv6) was dcvclopcd.~This specificationaimed to merge the fi~nctions of two distinct Ih14 protocolsfor reporting errors and status, ICMP for unicastpackct trans~nission and tlic Internet Group~Mcssagc Protocol (IGMP) for multicast traffic.Tlic messages defined in this protocol arc catcgorizcdas either crror or inhr~national, ~4th a family ofIiiessages in the second gl-0~1~ LISC~ to provide theNeighbor Disco\rcry Protocol."' Nciglibor ciisco\,erysen8es mi~ltiplc purposes \\,it11 tlic o\,crall theme ofpro\,iding a systc~n \\,it11 ropologicnl and cn\,ironmcntalhints. For csnmplc, link-la\rcr addrcss resolution,router discovcr\l, destination addrcss redirection, 2ndaddrcss autoconfigurntio11 mechanisms arc a11 specifiedusing neighbor disco\,cry packct ~ pcs.Although thc ncn\,ork Inycr did experience the largestamount of change, Fig~~rc 2 sho\\,s tliat the effects ofthis \\fork touch nearly all aspects of the Digital UNISsystem. IiVc point out csamplcs ofdecisions made due toour fi~nda~~~c~lt;~l design pllilos~phy, \\,liicli is bascdupon integration \\lit11 tlic UNIX system framc\\,ork,modular and cstcnsiblc sohvarc, support for m~~ltipleoperational policies, and a dcsirc to take advantage ofthe Alpha platti)nn \vitliout compromising portabiliq.In the follo\ving sections, bvc study these topics indepth, beginning with tlic nct\vork laycr, then coveringthe transport laycr modifications and the newneighbor discovery algorithms. Aticr that, wc discussaddress autoconfiguratio~i mechanisms and theireffects upon tlic system. We concludc with servicestliat will be affcctcd by the transition ti-om IPv4 toIPv6 SLICII as thc socltct application progra~nrninginterface (API) nncl DNS.COMMANDSUSER....................KERNELFigure 2Base Platform ChangesNetwork LayerI LINK-LAYERMODULESIAND NEIGHBORIn this section, \ve review the processing rcquircmcntsof the IPv6 modules, including lli-st\.lc nctnlaslu,from Classless Inter-Do~iiain Routing), \\,liich ,11-cappropriate to botli Ih.4 and IP\,G.'? Wc ha\r alsotried to take rnasi~ni~m aditantage of the 64-bit Alphaarchitecture \\,hen i~nplementing 11'\.6, \\,liilc ~naltingcertain that this implementation \\,auld run on 32-bitCPUs as \veil. For csaniplc, the cliccksi~m routinesoperate on 32-bit quantities (~llo\ving the carry too\,erflo\\~ into the upper 32 bits of a 64-bit rcgistcr).The checlzsum routine is also designed to allo\\l it to bcissued to multiple Alpha csccution units, \\,hichremains a topic for fi~rther in\rcstigation.Adaptations to Existing IP and ICMP RoutinesThe IPv6 and ICMPv6 routines arc completelyindependent of the corresponding IPv4 and ICMPv4routines, and tlie processing styles have distinct differences.In IPv6, the incoming packet is treated as bci~lgread-only, \vhile the BSD Il'v4 code ~iianipulatcs f cldswithin the Ih4 heaclcr. We also avoici unnecessary useof the 111-pullup routine (\\~liicli co~isolidatcs chainedmemory b~rff2rs into a single large buffer) bccnl~sc thiscould cailsc the packet to bc nccdlcssl!. lost. Fin~ill!?,instead of passing nulmerous arguments \\,hen callingfro111 hnction to fi~nction, a comliion data structure isDigital Technical Journnl 1'01. 8 So 3 I996 7