The Netwok Layer IPv4 and IPv6 Part 1

Transcription

1 ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE The Netwok Layer IPv4 and IPv6 Part 1 Jean Yves Le Boudec Contents 1. The Two Principles of IP Unicast 2. IPv4 addresses 3. IPv6 addresses 4. Subnets and Masks 5. NATs 6. ARP Textbook Chapter 5: The Network Layer 2

2 IP Principle #1 = Structured addresses + Longest prefix match Recall goal of Internet Protocol (IP) = interconnect all systems in the world Principle #1: every interface has an IP address IP address is structured to reflect where the system is in the world every packet contains IP address of destination every system has a forwarding table ( = routing table) and performs longest prefix match on address destination 3 Lisa A.H1 Forwarding table to output B.* 2 A.* 0 1 Forwarding table to output A.* 1 B.D.* 2 B.* router R1 to: B.D.H2 router R2 router R3 2 Forwarding table to output A.* 1 B.D.* 1 B.C.* 0 Homer B.C.H2 to output A.* 1 B.* 2 1 router R4 2 Bart B.D.H2 Forwarding table 4

3 IP Principle #2 = Don t use routers inside a LAN B WiFi base station P W E Ethernet concentrator B E and W P should not go through router W E goes through router Terminology: LAN = subnet IP principle 2 says: between subnets use routers, inside subnet don t 5 We observe a packet from W to P at 1. Which IP destination address do we see? 1. The IP address of P 1 25% 25% 25% 25% 2. The IP address of an Ethernet interface of the Ethernet concentrator 3. There is no destination IP address in the packet since communication is inside the subnet and does not go through a router 4. I don t know The IP address of P dress of an Ethernet... no destination IP a... I don t know 6

4 The Internet Protocol (IP) Communication between IP hosts requires knowledge of IP addresses An IP address is unique across the whole network (= the world in general) An IP address is the address of an interface There are two versions: IPv4 (current version) and IPv6 (next version) There are two network layers: Internet4 and Internet6 Terminology: packet = IP data unit intermediate system = system that forwards data units to another system; an IP intermediate system is called a router an IP system that does not forward is called a host 7 2. IPv4 addresses IPv4 address Uniquely identifies one interface in the world (in principle) An IPv4 address is 32 bits, usually noted in dotted decimal notation dotted decimal: 4 integers (one integer = 8 bits) example 1: example 2: hexadecimal: 8 hexa digits (one hexa digit = 4 bits) example 1: x80 bf example 2: x81 c binary: 32 bits example 1: b example 2: b

5 Binary, Decimal and Hexadecimal Given an integer B the basis : any integer can be represented in base B by means of an alphabet of B symbols Usual cases are decimal: 234 binary: hexadecimal: ea Mapping binary < > hexa is simple: one hexa digit is 4 binary digits e = 1110 a = 1010 ea = b Mapping binary < > decimal is best done by a calculator = = 234 Special Cases to remember f = 1111 = 15 ff = = lrcsuns :00:20:71:0D:D4 lrcpc :00:C0:B8:C2:8D Example Modem + PPP sic500cs in-inr :00:0C:02:78:36 LRC ed2-in DI ETHZ-Backbone ezci7-ethz-switch sw-zu-03 Switch x.x sw-la ed0-ext EPFL-Backbone ed0-swi stisun in-inj :00:0C:17:32:96 disun lrcmac4 08:00:20:20:46:2E Ring SIDI SUN 08:00:07:01:a2:a5 ezci7-ethz-switch ed2-el Komsys LEMA lrcmac :00:07:01:a2:a5 10

6 Network Prefix Network prefixes are used in routing tables /24 is the prefix length in bits Extract from routing table at sw-la Destination Next hop / / Extract from routing table at ed0-swi Destination Next hop / / / Special Addresses absence of address /24 this host (loopback address) for example /8, /12, /16 private networks (e.g in IEW) cannot be used on the public Internet /16 link local address (can be used only between systems on same LAN) 224/4 multicast 240/5 reserved /32 link local broadcast 12

7 IPv4 Packet Format Higher layer protocol 1= ICMP, 6 = TCP, 17 = UDP) Header 20 bytes (+ options, if any) payload We will see the functions of the fields other than the addresses in a following module IPv6 Addresses The current IP is IPv4. IPv6 is the next version of IP Why a new version? IPv4 address space is too small (32 bits 4 10 addresses ) What does IPv6 do? Redefine packet format with a larger address: 128 bits ( 3 10 addresses) Otherwise essentially the same as IPv4 IPv6 is incompatible with IPv4; routers and hosts must handle both separately A can talk to W, B can talk to W, A and B cannot communicate Web browser W Application IPv6 TCP MAC IPv4 Dual Stack Local router IPv6 MAC IPv4 B HTTP TCP IPv6 A HTTP TCP IPv4 14

8 Routing Tables at ed0 swi IP address of next hop lrcsuns :00:20:71:0D:D4 Destination Next hop 2001:620:618:1a4/64 fe80::1%1 2001:620:618/48 fe80::4%2 ::/0 fe80::1% sw-la :620:618:10a::1 fe80::1 %1 stisun1 ed0-ext ed0-swi % :620:618:10b::1 2001:620:618:1a6:1:80b2:f66:1 fe80:: ed2-in :620:618:1ac::3 2001:620:618:1ac:: in-inr in-inj 2001:620:618:1ac:: DI :620:618:1ad::1 2001:620:618:1ab::1 00:00:0C:02:78:36 00:00:0C:17:32:96 LRC interface number :620:618:10b::4 fe80::4 ed2-el :620: 618:1a4::1 lrcpc :00:C0:B8:C2:8D lrcmac4 2001:620:618:1ad:0a00:20ff:fe78:30f9 08:00:20:78:30:F9 15 IPv6 addresses are 128 bit long and are written using hexadecimal digits an EPFL public address: 2001:620:618:1a6:0a00:20ff:fe78:30f9 an EPFL private address: fd24:ec43:12ca:1a6:0a00:20ff:fe78:30f9 This is a private address EPFL private 16

9 Compression Rules for IPv6 Addresses 1 piece = 16 bits = [0 4 ]hexa digits; leading zeroes in one piece are omitted ; prefer lower case pieces separated by : (colon) one IPv6 address uncompressed = 8 pieces :: replaces any number of 0s in more than one piece; appears only once in address uncompressed 2002:0000:0000:0000:0000:ffff:80b2:0c :0620:0618:01a6:0000:20ff:fe78:30f9 compressed 2002::ffff:80b2:c :620:618:1a6:0:20ff:fe78:30f9 17 A Few IPv6 Global Unicast Addresses The block 2000/3 (i.e. 2xxx and 3xxx) is allocated for global unicast addresses 2001:620::/32 Switch 2001:620:618::/48 EPFL 2001:620:8::/48 ETHZ 2a02:1200::/27 Swisscom 2001:678::/29 provider independent address 2001::/32 Teredo 2002::/16 6to4 18

10 Examples of Special Addresses EPFL Private fc00::/7 (i.e. fcxx: and fdxx:) For example fd24:ec43:12ca:1a6:a00: 20ff:fe78:30f9 fe80::/10 ::/128 absence of address ::1/128 this host (loopback address) Unique local addresses = private networks (e.g in IEW) cannot be used on the public Internet link local address (can be used only between systems on same LAN) ff00::/8 multicast ff02::1:ff00:0/104 Solicited node multicast ff02::1/128 ff02::2/128 link local broadcast all link local routers 19 IPv6 Packet Format e.g. Higher layer protocol 1= ICMP, 6 = TCP, 17 = UDP) 16 bytes Header 40 bytes (+ options, if any) We will see the functions of the fields other than the addresses in a following module payload 20

11 The dotted decimal notation for 80 1: is I don t know 33% 33% 33% I don t know 21 The hexadecimal notation «2001::bad:babe» denotes a string of bits bits bits bits bits 6. None of the above 7. I don t know 14% 14% 14% 14% 14% 14% 14% 32 bits 44 bits 48 bits 64 bits 128 bits None of the above I don t know 22

12 4. Subnets and Masks Recall the IP principles: longest prefix match + routers between subnets only An IP system needs to know (in its forwarding table) which addresses are in same LAN as self ( = on link ) ed2 in has an IPv4 packet to destination address : packet is sent directly to : packet is sent to in inr This is done using the subnet mask dest next-hop interface /24 On-link eth /24 On-link eth /24 On-link eth / eth / eth1 23 One IP subnet must correspond to one network part An IP address is usually interpreted a network part a host part :620:618:1a6:0a00:20ff:fe78:30f9 Network part identifies subnet One subnet = one LAN All hosts in same LAN must have same network part The size of the network part may vary EPFL IPv4 network part is 24 bits ETHZ IPv4 network part 26 bits IPv6 network part is very often 64 bits 24

13 A system computes its network part from its IP address using the subnet mask, configured with the address = string of bits equal to 1 in network part, to 0 in host part network == IP address && mask At EPFL, IPv4 mask = and are on same subnet because && = && = IPv6 mask is very often 64 bits i.e. = ffff:ffff:ffff:ffff:: The notation /<length of network part> is also used Subnet Mask We could use the notation /24 instead of Same as saying Mask = ffff:ffff:ffff:ffff:: 26

14 Reserved Addresses with IPv4 0 and «all 1» are often avoided as host part Example: and are avoided (to prevent confusions with broadcast) 27 IPv4 address classes Long ago, IPv4 addresses had a class subnet mask was not necessary This is now obsolete but many people continue to use it class A 0 Net Id Subnet Id Host Id class B 10 Net Id Subnet Id Host Id class C 110 Net Id Host Id class D 1110 Multicast address class E Reserved Class A B C D E Range to to to to to

15 bridge? ?? ? X? bridge host A Can Host A have this address? Masks are all Yes 2. No 3. I don t know 33% 33% 33% Yes No know 29 The IPv4 Subnet Mask at ETHZ is ffff:ffff:ffff:ffff:: 8. ffff:ffff:ffff:ffff:c000:: 9. I don t know 11% 11% 11% 11% 11% 11% 11% 11% 11% ffff:ffff:ffff:ffff:: ffff:ffff:ffff:ffff:c000:: I don t know 30

16 The IPv6 Subnet Mask at ETHZ is /48 7. ffff:ffff:ffff:ffff:: 8. ffff:ffff:ffff:ffff:c000:: 9. I don t know 11% 11% 11% 11% 11% 11% 11% /48 31 ffff:ffff:: ff:c000:: What is the subnet broadcast address for subnet /26? I don t know 17% 17% 17% 17% 17% 17% I don t know 33

17 5. NATs: Why invented? (Network Address Translation boxes) Goal: re use same IP address for several devices / use private addresses This is a special type of «middle box», that is violating the TCP/IP architecture Used in residential networks («ADSL Modem») Used in companies to save IP addresses 35 How does Network Address Translation Work? NAT box modifies IP address and port numbers (port numbers are in TCP and UDP headers see transport protocol module) Maps (IP address, protocol type, port number) Exact matching from NAT Table To: UDP : 1029 To: UDP : 3441 To: UDP : 1029 To: UDP : 3442 LAN IPv 4 NAT box LAN Internet udp udp udp udp 3442 NAT table 36

18 Creating a NAT table entry: on the fly From: TCP : 1765 To: Port 80 From: TCP : 2343 To: Port 80 LAN IPv 4 NAT box LAN Internet udp udp udp udp tcp tcp 2343 NAT table Why some applications don t work with NATs S NAT Assume A behind a NAT and S in the internet Communication between A and S must be initiated by A «punch a hole through the NAT» If A and S are both behind a NAT (e.g. with voice over IP), we have a bootstrap problem A does not know its IP address as seen by S Solving this requires a third party this is what made Skype s fortune Cone or restricted NATs: third party used only to discover translated address Symmetric NAT: third party relays all traffic 38

19 Types of NATs Cone: translated (address, port) = f (internal (address, port)) ; remains valid as long as refreshed Symmetric: translated (address, port) = f(internal (address, port), correspondent (address, port)) Restricted: translated (address, port) = f (internal (address, port)) ; but valid only for specific correspondent address traffic from an unknown correspondent address is silently discarded This is only a rough classification; many things may happen in practice; NATs are a hack! 39 NAT44 and NAT66 NATs are motivated primarily by shortage of IPv4 addresses NAT44 maps IPv4 IPv4 addresses Widespread Many believe that there is no need for NAT66 since there are as many IPv6 addresses as one may ever need NAT66 are not widespread But NAT66 may be needed for other reasons use private addresses eg IEWv6 to EPFL v6 RFC 6296 specifies NAT66 40

20 When a NAT has a packet to forward and an association exists in the NAT table A. The NAT looks for a longest prefix match B. The NAT looks for an exact match C. None of the above D. I don t know The NAT looks for a longest. The NAT looks for an exact. 0% 0% 0% 0% None of the above I don t know 41 From WAN to LAN, the NAT may modify A. The source port B. The destination port C. None of the above D. I don t know 0% 0% 0% 0% The source port The destination port None of the above I don t know 42

21 ed2 in has a packet to destination address: ; packet is sent directly to ; packet is sent to MAC Address Resolution Q: What does «send packet directly» mean? A: send in an Ethernet frame, with destination MAC address = MAC address of Pb: what is the MAC address of ? Solution: ed2 in learns MAC address of using an address resolution procedure 44 Address Resolution with IPv4 : ARP Protocol 1: ed2-in has a packet to send to (stisun1) 1 No dest IP address Dest MAC addr = ff:ff:ff:ff:ff:ff ed2-in stisun1 lrcpc2 ed0-ext :00:20:71:0d:d :00:c0:b3:d2:8d :00:0c:02:78:36 this address is on the same subnet lrcsuns sends an ARP request to all systems on the subnet (Ethernet broadcast Ethernet type = ARP) target IP address = ARP request is received by all IP hosts on the local network is not forwarded by routers 45

22 Address Resolution with IPv4 : ARP Protocol 1 2 No dest IP address Dest MAC addr = 08:00:20:71:0d:d4 ed2-in stisun1 lrcpc2 ed0-ext :00:20:71:0d:d :00:c0:b3:d2:8d :00:0c:02:78:36 2: stisun1 has recognized its IPv4 address sends an ARP reply packet to the requesting host with its IPv4 and MAC addresses 46 Address Resolution with IPv4 : ARP Protocol Dest IP addr = Dest MAC Addr = 00:00:c0:b3:d2:8d ed2-in stisun1 lrcpc2 ed0-ext :00:20:71:0d:d :00:c0:b3:d2:8d :00:0c:02:78:36 3: ed2-in reads ARP reply, stores in a cache and sends IPv4 packet to stisun1 1 and 2 are ARP packets; Ethertype = ARP (0806) 3 is an IPv4 packet; Ethertype = IPv4 (0800) ed2 in keeps mapping in cache; expires if there is no traffic from stisun2 for some period of time > cache timeout 47

23 Address Resolution with IPv6 is part of the Neighbor Discovery Protocol (NDP) 1: ed2-in has a packet to send to 2001:620:618:1a6:1:80b2:f66:1(stisun1) 1 Dest IP address = ff02::1:ff66:1 Dest MAC address = 33:33:ff:66:00:01 ed2-in stisun1 lrcpc2 ed0-ext :620:618:1a6:1: 2001:620:618:1a6:1: 80b2:f01:1 08:00:20:71:0d:d4 80b2:f66:1 00:00:d0:b3:d2:8d this address is on the same subnet lrcsuns sends a Neighbor Solicitation (NS) packet to the solicited node multicast address ff02::1:ff66:1 NS is received by all IPv6 hosts on the local network that have the same solicited node multicast address (here: only stisun1) 48 The Solicited Node Multicast Address Add last 24 bits of target IP address to ff02::1:ff00:0/104 A packet with such a destination address is forwarded by layer 2 to all nodes that listen to this multicast address Using MAC multicast address 33:33:<last 32 bits of IP address> This is better than broadcast Target address Solicited Node multicast address Compressed Uncompressed Uncompressed Compressed 2001:620:618:1a6:001:80b2:f66:1 2001:0620:0618:01a6:0001:80b2:0f66:0001 ff02:0000:0000:0000:0000:0001:ff66:0001 ff02::1:ff66:1 49

24 Address Resolution with IPv6 : NDP Protocol 1 2 Dest IP address = 2001:620:618:1a6:1:80b2:f01:1 Dest MAC address = 08:00:20:71:0d:d4 ed2-in stisun1 lrcpc2 ed0-ext :620:618:1a6:1: 2001:620:618:1a6:1: 80b2:f01:1 08:00:20:71:0d:d4 80b2:f66:1 00:00:c0:b3:d2:8d 2: stisun1 has received the NS packet and recognized its IPv6 address as the target sends a Neighbor Advertisement in reply with its IPv6 and MAC addresses 50 Address Resolution with IPv6 : NDP Protocol Dest IP address = 2001:620:618:1a6:1:80b2:f66:1 Dest MAC address = 00:00:c0:b3:d2:8d ed2-in stisun1 lrcpc2 ed0-ext :620:618:1a6:1: 2001:620:618:1a6:1: 80b2:f01:1 08:00:20:71:0d:d4 80b2:f66:1 00:00:c0:b3:d2:8d 3: ed2-in reads NA, stores in a cache and sends IPv6 packet to stisun1 1,2 and 3 are IPv6 packets; Ethertype = ARP (86DD) ed2 in keeps mapping in cache; expires if there is no traffic from stisun2 for some period of time > cache timeout 51

25 Look inside an ARP packet Ethernet II Destination: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff) Source: 00:03:93:a3:83:3a (Apple_a3:83:3a) Type: ARP (0x0806) Trailer: Address Resolution Protocol (request) Hardware type: Ethernet (0x0001) Protocol type: IP (0x0800) Hardware size: 6 Protocol size: 4 Opcode: request (0x0001) Sender MAC address: 00:03:93:a3:83:3a (Apple_a3:83:3a) Sender IP address: ( ) Target MAC address: 00:00:00:00:00:00 (00:00:00_00:00:00) Target IP address: ( ) 52 Look Inside an ICMPv6 NDP Neighbour Solicitation Packet Solicited Node Multicast Address corresponding to this IPv6 target address Neighbor Solicitation (=ARP Request) 53

26 ed2 in has a packet to destination address ; packet is sent to ; the packet is sent by «ed2 in» inside an Ethernet frame with destination MAC address = 08:00:20:71:0d:d inside an Ethernet frame with destination MAC address = 00:00.0d:0d:9a:75 3. None of the above 4. I don t know 00:00.0d:0d:9a:75 0% 0% 0% 0% Security Issues with ARP/ NDP ARP requests / replies may be falsified (ARP spoofing) 1 2 No dest IP address Dest MAC addr = 08:00:20:71:0d:d4 ed2-in stisun1 lrcpc2 ed0-ext :00:20:71:0d:d :00:c0:b3:d2:8d :00:0c:02:78:36 Can we prevent ARP spoofing? 55

27 DHCP Snooping and Dynamic ARP Inspection can prevent ARP spoofing in LANs DHCP snooping = switch/ethernet concentrator/wifi base station observes all DHCP traffic and remembers mappings IP addr MAC addresses (DHCP is used to automatically configure the IP address at system boot) Dynamic ARP inspection: switch filters all ARP (or NDP) traffic and allows only valid answers This solution is deployed in enterprise networks, rarely in homes or WiFi access points 56 Conclusion The network layer (= IP) is the center piece of communication networks IP is built on two principles: one IP address per interface and longest prefix match; this allows to compress routing tables by aggregation inside subnet, don t use routers There are (unfortunately) two versions of IP, IPv4 and IPv6; they are not compatible interworking requires some tricks (see later). NATs came as an after thought and use a different principle than IP unicast (exact match versus longest prefix match) are widely deployed NATs hide IP addresses and complicate the operation of networks The goal of ARP/NDP is to find the MAC address corresponding to an IP address 57