Outline Computer Networking. IPv4 Header Fields. IP Service Model. The IP protocol Lecture 6 The Internet Protocol

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1 Outlie Computer Networkig Lecture 6 The Iteret Protocol Peter Steekiste Fall The protocol v4 v6 i practice Network address traslatio Address resolutio protocol Tuels 2 Service Model v4 Fields Low-level commuicatio model provided by Iteret gram Each packet self-cotaied All iformatio eeded to get to destiatio No advace setup or coectio maiteace Aalogous to letter or telegram v4 Format versio HLe TOS Legth Idetifier Flag Offset TTL Protocol Checksum Source Address Destiatio Address Optios (if ay) 3 Versio: Versio versio TOS Legth 1 HLe Fl 4 for v4 Idetifier ag Offset s TTL Protocol Checksum Source Address HLe: Legth Destiatio Address Optios (if ay) 32-bit words (typically 5) TOS: Type of Service Priority iformatio Legth: Legth Bytes (icludig header) format ca chage with versios First byte idetifies versio Legth field limits packets to 65,535 bytes I practice, break ito much smaller packets for etwork performace cosideratios 3 4 1

2 v4 Fields Delivery Model Idetifier, flags, fragmet offset used for fragmetatio Time to live Must be decremeted at each s with TTL=0 are throw away Esure packets exit the etwork Protocol versio TOS HLe Demultiplexig to higher layer protocols Idetifier TCP = 6, ICMP = 1, UDP = 17 checksum Esures some degree of header itegrity Relatively weak 16 bit Source ad destiatio addresses Optios E.g. Source routig, record route, etc. Performace issues Poorly supported Legth Fl ag Offset s TTL Protocol Checksum Source Address Destiatio Address Optios (if ay) 3 1 Best effort service Network will do its best to get packet to destiatio Does NOT guaratee: Ay maximum latecy or eve ultimate success Iformig the seder if packet does ot make it Delivery of packets i same order as they were set Just oe copy of packet will arrive Implicatios Scales very well (really, it does) Higher level protocols must make up for shortcomigs Reliably deliverig ordered sequece of bytes TCP Some services ot feasible (or hard) Latecy or badwidth guaratees 5 6 Fragmetatio Fragmetatio Related Fields Every etwork has ow Maximum Trasmissio Uit (MTU) Largest datagram it ca carry withi its ow packet frame E.g., Etheret is 1500 bytes Do t kow MTUs of all itermediate etworks i advace Solutio Whe hit etwork with small MTU, fragmets packet Destiatio reassembles the paper why? Legth Legth of fragmet Idetificatio To match up with other fragmets Flags Do t fragmet flag More fragmets flag Fragmet offset Where this fragmet lies i etire datagram Measured i 8 octet uits (13 bit field) 7 8 2

3 Fragmetatio Example #1 Fragmetatio Example #2 Legth = 3820, M=0 Legth = 3820, M=0 Legth =, M=1, Offset = bytes 1980 bytes Legth = 1840, M=0, Offset = bytes 9 10 Fragmetatio is Harmful Uses resources poorly Forwardig costs per packet Best if we ca sed large chuks of data Worst case: packet just bigger tha MTU Poor ed-to-ed performace Loss of a fragmet Path MTU discovery protocol determies miimum MTU alog route Uses ICMP error messages Commo theme i system desig Assure correctess by implemetig complete protocol Optimize commo cases to avoid full complexity Iteret Cotrol Message Protocol (ICMP) Short messages used to sed error & other cotrol iformatio Some fuctios supported by ICMP: Pig request /respose: check whether remote reachable Destiatio ureachable: Idicates how packet got & why could t go further Flow cotrol: Slow dow packet trasmit rate Redirect: Suggest alterate routig path for future messages Router solicitatio / advertisemet: Helps ewly coected discover local Timeout: exceeded maximum hop limit How useful are they fuctios today?

4 MTU Discovery with ICMP MTU Discovery with ICMP Typically sed series of packets from oe to aother Typically, all will follow same route Routes remai stable for miutes at a time Makes sese to determie path MTU before sedig real packets Operatio: Sed max-sized packet with do ot fragmet flag set If ecouters problem, ICMP message will be retured Destiatio ureachable: Fragmetatio eeded Usually idicates MTU problem ecoutered ICMP abuse? Other solutios? ICMP Frag. Needed Legth = 4000, Do t Fragmet MTU Discovery with ICMP MTU Discovery with ICMP ICMP Frag. Needed Legth =, Do t Fragmet Legth = 1500, Do t Fragmet Whe successful, o reply at level No ews is good ews Higher level protocol might have some form of ackowledgemet

5 Importat Cocepts Base-level protocol () provides miimal service level Allows highly decetralized implemetatio Each step ivolves determiig ext hop Most of the work at the edpoits ICMP provides low-level error reportig forwardig global addressig, alteratives, lookup tables addressig hierarchical, CIDR service best effort, simplicity of s packets header fields, fragmetatio, ICMP Outlie The protocol v4 v6 i practice Network address traslatio Address resolutio protocol Tuels v6 v6 Address Size Discussio Next geeratio. Most urget issue: icreasig address space. 128 bit addresses Simplified header for faster processig: No checksum (why ot?) No fragmetatio (really?) Support for guarateed services: priority ad flow id Optios hadled as ext header reduces overhead of hadlig optios V/Pr Legth Flow label Next Source address Destiatio address Hop L Do we eed more addresses? Probably, log term Big paic i 90s: We re ruig out of addresses! Big worry: Devices. Small devices. Cell phoes, toasters, everythig. 128 bit addresses provide space for structure (good!) Hierarchical addressig is much easier Assig a etire 48-bit sized chuk per LAN use Etheret addresses Differet chuks for geographical addressig, the v4 address space, Perhaps help clea up the routig tables - just use oe huge chuk per ISP ad oe huge chuk per customer. Sub 010 Registry Provider Subscriber Host Net

6 Router Implemetatio: Fast Path versus Slow Path Commo case: Switched i silico ( fast path ) Almost everythig Weird cases: Haded to CPU ( slow path, or process switched ) Fragmetatio TTL expiratio (traceroute) optio hadlig Slow path is evil i today s eviromet Christmas Tree attack sets weird optios, bits, ad overloads Developers caot (really) use thigs o the slow path Slows dow their traffic ot good for busiess If it became popular, they are i trouble! v6 Cleaup: Optios 32 v4 optios variable legth header Rarely used No developmet / may s/s do ot support Worse tha useless: s w/optios ofte eve get dropped! Processed i slow path. v6 optios: Next header poiter Combies protocol ad optios hadlig Next header: TCP, UDP, etc. Extesios header: Chaied together Makes it easy to implemet -based optios Oe value hop-by-hop examied by itermediate s E.g., source route implemeted oly at itermediate hops v6 Cleaup: o No checksum Motivatio was efficiecy: If packet corrupted at hop 1, do t waste b/w trasmittig o hops 2..N. Useful whe corruptio frequet, b/w expesive Today: corruptio is rare, badwidth is cheap No fragmetatio Router discard packets, sed ICMP Too Big does MTU discovery ad fragmets Reduced packet processig ad etwork complexity. Icreased MTU a boo to applicatio writers Hosts ca still fragmet - usig fragmetatio header. Routers do t deal with it ay more. Migratio from v4 to v6 Iteroperability with v4 is ecessary for icremetal deploymet. No flag day Fudametally hard because a (sigle) protocol is critical to achievig global coectivity across the iteret Process uses a combiatio of mechaisms: Dual stack operatio: v6 odes support both address types Tuel v6 packets through v4 clouds v4-v6 traslatio at edge of etwork NAT must ot oly traslate addresses but also traslate betwee v4 ad v6 protocols v6 addresses based o v4 o beefit! 20 years later, this is still a major challege!