Network Layer4-1 IP: Internet Protocol r Datagram format r IPv4 addressing r DHCP: Dynamic Host Configuration Protocol r NAT: Network Address Translation r ICMP r IPv6

Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router “signaling” Transport layer: TCP, UDP Link layer physical layer Network layer

Network Layer4-3 IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. how much overhead with TCP? r 20 bytes of TCP r 20 bytes of IP r = 40 bytes + app layer overhead

Network Layer4-4 IP Addresses 0 network host 10 network host 110 networkhost 1110 multicast address A B C D class to to to to bits given notion of “network”, let’s re-examine IP addresses: “class-full” addressing:

Network Layer4-5 IP Addressing: introduction r IP address: 32-bit identifier for host, router interface r interface: connection between host/router and physical link m router’s typically have multiple interfaces m host may have multiple interfaces m IP addresses associated with each interface =

Network Layer4-6 IP Fragmentation & Reassembly r network links have MTU (max.transfer size) - largest possible link-level frame. m different link types, different MTUs r large IP datagram divided (“fragmented”) within net m one datagram becomes several datagrams m “reassembled” only at final destination m IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

Network Layer4-7 IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =1480 fragflag =1 length =1500 ID =x offset =2960 fragflag =0 length =1040 One large datagram becomes several smaller datagrams Example r 4000 byte datagram r MTU = 1500 bytes

Network Layer4-8 IP addressing: CIDR CIDR: Classless InterDomain Routing m subnet portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part /23

Network Layer4-9 IP addresses: how to get one? Q: How does host get IP address? r hard-coded by system admin in a file m Wintel: control-panel->network->configuration- >tcp/ip->properties m UNIX: /etc/rc.config r DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server m “plug-and-play”

Network Layer4-10 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an “on” Support for mobile users who want to join network (more shortly) DHCP overview: m host broadcasts “DHCP discover” msg m DHCP server responds with “DHCP offer” msg m host requests IP address: “DHCP request” msg m DHCP server sends address: “DHCP ack” msg

Network Layer4-11 DHCP client-server scenario A B E DHCP server arriving DHCP client needs address in this network

Network Layer4-12 DHCP client-server scenario DHCP server: arriving client time DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: , 67 dest: , 68 yiaddrr: transaction ID: 654 Lifetime: 3600 secs DHCP request src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 Lifetime: 3600 secs

Network Layer4-13 IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

Network Layer4-14 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers m allocates addresses m manages DNS m assigns domain names, resolves disputes

Network Layer4-15 NAT: Network Address Translation local network (e.g., home network) /24 rest of Internet Datagrams with source or destination in this network have /24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: , different source port numbers

Network Layer4-16 NAT: Network Address Translation r Motivation: local network uses just one IP address as far as outside word is concerned: m no need to be allocated range of addresses from ISP: - just one IP address is used for all devices m can change addresses of devices in local network without notifying outside world m can change ISP without changing addresses of devices in local network m devices inside local net not explicitly addressable, visible by outside world (a security plus).

Network Layer4-17 NAT: Network Address Translation Implementation: NAT router must: m outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr. m remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair m incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer4-18 NAT: Network Address Translation S: , 3345 D: , : host sends datagram to , 80 NAT translation table WAN side addr LAN side addr , , 3345 …… S: , 80 D: , S: , 5001 D: , : NAT router changes datagram source addr from , 3345 to , 5001, updates table S: , 80 D: , : Reply arrives dest. address: , : NAT router changes datagram dest addr from , 5001 to , 3345

Network Layer4-19 NAT: Network Address Translation r 16-bit port-number field: m 60,000 simultaneous connections with a single LAN-side address! r NAT is controversial: m routers should only process up to layer 3 m violates end-to-end argument NAT possibility must be taken into account by app designers, eg, P2P applications m address shortage should instead be solved by IPv6

Network Layer4-20 ICMP: Internet Control Message Protocol r used by hosts & routers to communicate network-level information m error reporting: unreachable host, network, port, protocol m echo request/reply (used by ping) r network-layer “above” IP: m ICMP msgs carried in IP datagrams r ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

Network Layer4-21 Traceroute and ICMP r Source sends series of UDP segments to dest m First has TTL =1 m Second has TTL=2, etc. m Unlikely port number r When nth datagram arrives to nth router: m Router discards datagram m And sends to source an ICMP message (type 11, code 0) m Message includes name of router& IP address r When ICMP message arrives, source calculates RTT r Traceroute does this 3 times Stopping criterion r UDP segment eventually arrives at destination host r Destination returns ICMP “host unreachable” packet (type 3, code 3) r When source gets this ICMP, stops.

Network Layer4-22 IPv6 r Initial motivation: 32-bit address space soon to be completely allocated. r Additional motivation: m header format helps speed processing/forwarding m header changes to facilitate QoS IPv6 datagram format: m fixed-length 40 byte header m no fragmentation allowed

Network Layer4-23

Network Layer4-24 IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data

Network Layer4-25 IPv4 Header 4 for IPv4

Network Layer4-26 Other Changes from IPv4 r Checksum: removed entirely to reduce processing time at each hop r Options: allowed, but outside of header, indicated by “Next Header” field r ICMPv6: new version of ICMP m additional message types, e.g. “Packet Too Big” m multicast group management functions

Network Layer4-27 Features of IPv6 r Larger Address r Extended Address Hierarchy r Flexible Header Format r Improved Options r Provision For Protocol Extension r Support for Auto-configuration and Re-numbering r Support For Resource Allocation.

Network Layer4-28 IPv6 availability r Generally available with (new) versions of most operating systems. m BSD, Linux 2.2 Solaris 8 r An option with Windows 2000/NT r Most routers can support IPV6

Network Layer4-29 IPv6 Design Issues r Overcome IPv4 scaling problem m lack of address space. r Flexible transition mechanism. r New routing capabilities. r Quality of service. r Security. r Ability to add features in the future.

Network Layer4-30 IPv6 Header Fields r VERS: 6 (IP version number) r Priority: will be used in congestion control r Flow Label: experimental - sender can label a sequence of packets as being in the same flow. r Payload Length: number of bytes in everything following the 40 byte header, or 0 for a Jumbogram.

Network Layer4-31 IPv6 Headers r Simpler header - faster processing by routers. m No optional fields - fixed size (40 bytes) m No fragmentation fields. m No checksum r Support for multiple headers m more flexible than simple “protocol” field.

Network Layer4-32 IPv6 Header Fields r Next Header is similar to the IPv4 “protocol” field - indicates what type of header follows the IPv6 header. r Hop Limit is similar to the IPv4 TTL field (but now it really means hops, not time).

Network Layer4-33 Extension Headers r Routing Header - source routing r Fragmentation Header - supports fragmentation of IPv6 datagrams. r Authentication Header r Encapsulating Security Payload Header

Network Layer4-34 IPv6 Addresses r 128 bits - written as eight 16-bit hex numbers. 5f1b:df00:ce3e:e200:0020:0800:2078:e3e3 r High order bits determine the type of address. The book shows the breakdown of address types.

Network Layer4-35

Network Layer4-36

Network Layer4-37 Transition From IPv4 To IPv6 r Not all routers can be upgraded simultaneous m no “flag days” m How will the network operate with mixed IPv4 and IPv6 routers? r Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

Network Layer4-38 Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest: F data Src:B Dest: E A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4

Network Layer4-39 IPv4-Mapped IPv6 Address r IPv4-Mapped addresses allow a host that support both IPv4 and IPv6 to communicate with a host that supports only IPv4. r The IPv6 address is based completely on the IPv4 address.

Network Layer4-40 IPv4-Mapped IPv6 Address r 80 bits of 0s followed by 16 bits of ones, followed by a 32 bit IPv4 Address: IPv4 AddressFFFF 80 bits32 bits16 bits

Network Layer4-41 Works with DNS r An IPv6 application asks DNS for the address of a host, but the host only has an IPv4 address. r DNS creates the IPv4-Mapped IPv6 address automatically. r Kernel understands this is a special address and really uses IPv4 communication.

Network Layer4-42 IPv4-Compatible IPv6 Address r An IPv4 compatible address allows a host supporting IPv6 to talk IPv6 even if the local router(s) don’t talk IPv6. r IPv4 compatible addresses tell endpoint software to create a tunnel by encapsulating the IPv6 packet in an IPv4 packet.

Network Layer4-43 IPv4-Compatible IPv6 Address IPv4 Address bits32 bits16 bits 80 bits of 0s followed by 16 bits of 0s, followed by a 32 bit IPv4 Address:

Network Layer4-44 Tunneling (done automatically by kernel when IPv4- Compatible IPv6 addresses used) IPv6 Host IPv6 Host IPv4 Routers IPv6 Datagram IPv4 Datagram

Network Layer4-45 Dual Server r In the future it will be important to create servers that handle both IPv4 and IPv6. r The work is handled by the O.S. (which contains protocol stacks for both v4 and v6): m automatic creation of IPv6 address from an IPv4 client (IPv4-mapped IPv6 address).

Network Layer4-46 IPv4 client IPv4 client TCP IPv4 Datalink IPv6 client IPv6 client TCP IPv6 Datalink IPv6 server IPv6 server TCP Datalink IPv4 IPv6 IPv4-mapped IPv6 address IPv4-mapped IPv6 address