The Internet protocol

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Gwen Lawrence
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1 The Internet prtcl Party time! IPv4 datagram frmat IPv4 2 bytes n ptins delue r ecnmy? header + data mre later CS242 Cmputer Netwrks Department f Cmputer Science Wellesley Cllege decremented by each ruter; TTL = marks end f the line TCP, UDP? (Plays a rle similar t prt numbers.) treats each tw bytes f header as a number, adds and takes nes cmplement *Datagrams requiring lw delay, high thrughput, r reliability. Cisc interprets first 3 TOS bits as defining different levels f service that ruters can prvide. IP 14-2 Optin field* cmplicates things Since datagram headers can be f variable, ne cannt determine a priri where the data field will start. *Intended t be used nly rare and drpped altgether frm IPv6. Als, sme datagrams may require ptin prcessing, thers n; erg prcessing time varies at the ruter. IP 14-3 IP fragmentatin and reassembly Sme link-layer prtcls carry big packets; sme d nt. The maimum amunt f data a link-layer packet can hld is called its maimum transfer unit (MTU). What t d when a packet arriving at in link is t big t fit int the ut link? Reassembly at destinatin Fragmentatin 1 large datagram in 3 smaller datagrams IP

back tgether. If ne f the pieces is missing, the entire datagram is tssed.

2 Time t wrry Hey, that's nt my jb Suppse a 4 byte datagram arriving at a ruter s incming link is t be shipped ut an utging link whse MTU equals 15 bytes. 4 ffset One large datagram becmes several smaller datagrams ffset ffset 148 While the ruters in IPv4 fragment the datagram, it is the respnsibility f the destinatin hst t put the pieces back tgether. If ne f the pieces is missing, the entire datagram is tssed. 4 End hst reassembles the pieces ffset 1 1 ffset ffset ffset ffset 296 IP 14-5 IP 14-6 Fragmentatin is a pain Fragmentatin and reassembly puts an additinal burden n Internet ruters and hsts. Fr this reasn is it desirable t keep fragmentatin t a minimum. This is ften dne by limiting the TCP and UDP segments t a relatively small size, s that fragmentatin f the crrespnding datagrams is unlikely. IP 14-7 Fragmentatin is als ripe fr eplits Have t keep track f all fragments until packet is reassembled? Resurce allcatin is necessary befre all validatin is pssible Lts f fragments frm different packets can ehaust available memry; perfect grunds fr resurce ehaustin attacks. Implementatin is tricky. Incrrect implementatins can be caed int crashing machine (a simple denial f service attack). IP

Implementatin Attack: The Ping f Death Attack: Send ICMP ech with fragmented packets : ping -L 6551 <victim IP address> Result: Maimum legal size f an ICMP ech packet: 65535-2 - 8 = 6557.

3 Implementatin Attack: The Ping f Death Attack: Send ICMP ech with fragmented packets : ping -L 6551 <victim IP address> Result: Maimum legal size f an ICMP ech packet: = 6557.* Fragmentatin allws bypassing the maimum size: (ffset + size) > Reassembled packet wuld be larger than bytes Implementatin Attack: Teardrp Fragmented (i.e. brken) packet is reassembled using ffset fields Attack: Send fragments that verlap Gal: Crash, rebt and hang machine Nrmal fragment cncatenatin: Overlapping fragments: *65535 is maimum IP datagram, 2 is IP header and 8 is ICMP ech request. IP 14-9 IP 14-1 Teardrp: Mechanism Deep in the prtcl implementatin if (prev!= NULL && ffset < prev->end) // if there are verlapping fragments { i = prev->end ffset; ffset += i; /* ptr int datagram */ fp->len = end - ffset; /* number f bytes t cpy */ } First Secnd ffset (befre) Cpy this ffset (after) prev->end end Teardrp Attack Create secnd fragment that fits entirely within first, s ffset nw pints utside f the secnd datagram's buffer! Prgram calculates the number f bytes t cpy fp->len = end - ffset; Very large unsigned number! Can write huge number f bytes in places they re nt suppsed t be, causing machine t crash. Attacker takes advantage f knwledge f implementatin details/bugs. First Secnd end ffset prev->end IP IP

4 IPv4 addressing IPv4 addresses are 32 bits lng A hst typically has a single link int the netwrk thrugh which IP talks. The bundary between a hst and the link is called an interface Ruters are cnnected t multiple links thrugh multiple interfaces. IP addresses are assciated with interfaces. These addresses are usually written using a dtteddecimal ntatin s IP address is usually written Each interface n every hst and ruter in the glbal Internet must have an IP address that is glbally unique. IP IP A methd t their madness Netwrk definitin recipe Each subnet itself has an address. E.g., subnet cnsists f 3 hsts & 1 ruter interface has address: /24 IP netwrk intercnnected by a single physical link sharing the same netwrk prefi. First detach each interface frm its hst r ruter. This creates islands f islated netwrks, with interfaces terminating the endpints f the islated netwrks. Each islated netwrk is called a subnet. The current Internet cnsists f millins f such netwrks. IP *Hw many subnets are shwn and what are they? IP

IPv4 address frmats Classless Interdmain Ruting (CR) Wellesley Cllege was a Class B netwrk. There were up t 2 14 such netwrks each with up t 2 16 interfaces.

The prblem with classful addressing is that a class C netwrk des nt have nearly enugh addresses fr mst rganizatins, while a class B has t many.

Only the leading prefi bits are cnsidered by ruters utside the rganizatin s netwrk.

5 IPv4 address frmats Classless Interdmain Ruting (CR) Wellesley Cllege was a Class B netwrk. There were up t 2 14 such netwrks each with up t 2 16 interfaces. In 1993, the IETF [RFC 1519] did away with classful addressing. Class A Class B Class C Class D Class E 1111 was reserved fr future use. The prblem with classful addressing is that a class C netwrk des nt have nearly enugh addresses fr mst rganizatins, while a class B has t many. The netwrk part f an IP address can nw be any number f bits lngs. The mst significant bits f an address f the frm a.b.c.d/ cnstitutes the netwrk prtin f the IP address. Only the leading prefi bits are cnsidered by ruters utside the rganizatin s netwrk. IP IP Classless Interdmain Ruting (CR) Obtaining a netwrk address A ruter utside the rganizatin frwards a datagram t a destinatin address inside the rganizatin using nly the leading bits f the address. This cnsiderably reduces the size f the frwarding table in these ruters, a single entry f the frm a.b.c.d/ suffices. An ISP btains a blck f IP addresses (a.b.c.d/) frm an Internet registry, then further divides the blck t create its wn internal netwrks. ISP blck / Organizatin / Organizatin / Organizatin / *The / is knwn as the subnet mask. IP IP

IP address management authrity In practice this lks like IP addresses are managed under the authrity f the Internet Crpratin fr

The ICANN nt nly allcates IP addresses, but als t manage the DNS rt servers and assign dmain names.

within their regins. *The ability t use a single prefi t advertise multiple netwrks is referred t as address aggregatin.

it can assign individual IP addresses t hsts by either 18 Bug! Organizatin in Fly-by-night 2.23.16.

6 IP address management authrity In practice this lks like IP addresses are managed under the authrity f the Internet Crpratin fr Assigned Names and Numbers (ICANN). The ICANN nt nly allcates IP addresses, but als t manage the DNS rt servers and assign dmain names. The ICANN allcates addresses t reginal internet registries (ARIN, RIPE, and APNIC), which handle the allcatin/ management f addresses within their regins. *The ability t use a single prefi t advertise multiple netwrks is referred t as address aggregatin. IP But suppse Organizatin 1 mves IP Obtaining a hst address Once an rganizatin has btained a blck f addresses frm an ISP, it can assign individual IP addresses t hsts by either 18 Bug! Organizatin in Fly-by-night / Organizatin 1 in ISPs-R-Us / IP Manually cnfiguring each hst (easy t understand; nightmare t administer), r Using dynamic hst cnfiguratin prtcl (DHCP).* *Allws hst t als get subnet mask, the address f its first-hp ruter, and the address f its lcal DNS server. IP

Plug-and-play prtcl Dynamic hst cnfiguratin prtcl* Wendy Wellesley walking acrss campus with her lap is likely accessing different subnets, requiring different IP addresses. Enter DHCP.

Client bradcasts a UDP datagram n prt 67 *A client-server prtcl. If n server is present n the subnet, a DHCP relay agent (ruter) that knws the server's address is needed.

7 Plug-and-play prtcl Dynamic hst cnfiguratin prtcl* Wendy Wellesley walking acrss campus with her lap is likely accessing different subnets, requiring different IP addresses. Enter DHCP. Furthermre, nt all students are likely t be nline at nce allwing ISPs t get away with a smaller blcks f IP addresses than the size f the custmer base. Client bradcasts a UDP datagram n prt 67 *A client-server prtcl. If n server is present n the subnet, a DHCP relay agent (ruter) that knws the server's address is needed. IP IP Newbie arrives and bradcasts a DHCP discver message using IP DHCP clientserver interactin DHCP clientserver interactin DHCP server respnses with a bradcast message cntaining newbie's transactin & ffering IP address, netwrk mask, & an IP address lease time. IP IP

.. the interactin is cmplete and the client can use the DHCP-allcated IP address fr the duratin f the lease.

8 DHCP clientserver interactin DHCP clientserver interactin Newbie accepts with a DCHP request message eching back the parameters. DHCP cnfirms requested parameters IP IP 14-3 Once the client receives DHCP ACK... This week s puzzler... the interactin is cmplete and the client can use the DHCP-allcated IP address fr the duratin f the lease. LTS ften set up devices like printers s that they have a static IP, but they're still cnfigured thrugh DHCP. There is a mechanism fr renewing the lease if necessary. A client sends a bradcast message asking fr an IP address. If it had ne befre, it usually tries t ask fr that ne again. But n guarantees. This is bad news fr anyne trying find a particular printer. IP IP

The rest f the stry Bming ecnmies If the requested address is nt available, the dhcp server picks a dynamic address frm a

This is dne by hand-entering a line int the dhcp cnfig file which cntains the MAC address f the client and its fied IP

If the subnet grew bigger, a larger blck wuld have t be allcated.

IP 14-33 IP 14-34 Netwrk Address Translatin (NAT) NAT-enabled ruters hide LAN details An ffice with ne IP address sets up a

../8 is ne f three prtins f IP space reserved fr a private netwrk.

9 The rest f the stry Bming ecnmies If the requested address is nt available, the dhcp server picks a dynamic address frm a pl f addresses UNLESS the client is specifically defined in the dhcp server as having a particular IP address. This is dne by hand-entering a line int the dhcp cnfig file which cntains the MAC address f the client and its fied IP address. T install an ffice subnet a range f addresses wuld be needed. If the subnet grew bigger, a larger blck wuld have t be allcated. What if the ISP had already allcated the cntiguus prtins f the ffice's current address range? IP IP Netwrk Address Translatin (NAT) NAT-enabled ruters hide LAN details An ffice with ne IP address sets up a small netwrk. All datagrams arriving at the NAT ruter have the same destinatin IP. Address space 1.../8 is ne f three prtins f IP space reserved fr a private netwrk. A NAT-enabled ruter behaves t the utside wrld like a single device with a single IP. It gets its address and the ISP's DHCP server, The NAT translatin table at the NAT ruter uses prt numbers as well as IP address t rute within the netwrk. IP and runs a DHCP server t prvide addresses t cmputers within the NAT-DHCP-rutercntrlled netwrk. IP

Nt everyne is happy with NAT 1. Prt numbers are meant fr addressing prcesses, nt fr addressing hsts. 2. Ruters are suppse t prcess packets nly up t layer 3.

and prt numbers. 4. We shuld use IPv6 t slve the shrtage f IP addresses. IPv6 datagram frmat What abut fragmentatin?

(ptins sneak in) IP 14-37 IP 14-38 Transitining frm IPv4 t IPv6 January 1, 1983 Estimates vary, but 32-bit IP addresses are prjected t run ut smetime in the

10 Nt everyne is happy with NAT 1. Prt numbers are meant fr addressing prcesses, nt fr addressing hsts. 2. Ruters are suppse t prcess packets nly up t layer NAT prtcl vilates the scalled end-t-end argument ; that is, hsts shuld be talking directly with each ther, withut interfering ndes mdifying IP addresses and prt numbers. 4. We shuld use IPv6 t slve the shrtage f IP addresses. IPv6 datagram frmat What abut fragmentatin? Assign pririties and/r special handling instructins N checksums? Identifies prtcl t which cntents will be delivered. (ptins sneak in) IP IP Transitining frm IPv4 t IPv6 January 1, 1983 Estimates vary, but 32-bit IP addresses are prjected t run ut smetime in the "net few years". Flag day? The Internet transitined frm ARPANET Netwrk Cntrl Prgram (NCP) t TCP. Even back then, when the Internet was tiny and still administered by a small number f wizards, flag day was bad news. IP IP

11 Tunneling IP

Chapter 5. The Network Layer IP

Chapter 5. The Network Layer IP Chapter 5 The Netwrk Layer IP These slides are taken frm the bk Cmputer etwrking, A Tp Dwn Apprach Featuring the Internet by Kurse & Rss and frm the bk Cmputer etwrks by Andrew Tanenbaum. The Netwrk Layer