Datagram Networks: Internet Protocol (IPv4)

The Internet Network layer: IP Internet Network Layer Components: IP protocol (addressing, datagram format and handling), Routing Protocols and ICMP protocol forwarding table 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

IP Addressing: Introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router’s typically have multiple interfaces host may have multiple interfaces IP addresses associated with each interface 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 223.1.3.1 223.1.3.2 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1

network consisting of 3 IP networks: IP Addressing IP address: network part (high order bits) host part (low order bits) NetworkID.HostID What’s an IP network ? device interfaces with same network part of IP address Hosts within the same IP network can reach each other without intervening router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.2.9 223.1.2.2 223.1.1.3 223.1.3.27 223.1.3.1 223.1.3.2 network consisting of 3 IP networks: 223.1.1.0, 223.1.2.0, 223.1.3.0

IP Addressing IP networks are not restricted to Ethernet segments 223.1.1.2 IP networks are not restricted to Ethernet segments Here we have 3 point-to-point links and each have a different IP network defined over them. 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 Interconnected system consisting of 6 IP networks 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2

IP Addressing How to find IP networks? 223.1.1.2 How to find IP networks? Detach each interface from router, host Create “islands of isolated IP networks Each island defines an IP network Internet consists of millions of such IP networks 223.1.1.1 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Interconnected system consisting of 6 IP networks

IP Addresses given notion of “IP network”, let’s re-examine IP addresses: We have “class-full” addressing Original Internet design class 1.0.0.0 to 127.255.255.255 A network host B 128.0.0.0 to 191.255.255.255 10 network host 192.0.0.0 to 223.255.255.255 C 110 network host 224.0.0.0 to 239.255.255.255 D 1110 multicast address 32 bits

IP addressing: CIDR Classful addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network No longer used in the current Internet Solution? Classless Inter Domain Routing (CIDR) CIDR: Classless InterDomain Routing Standardized in 1993 Network portion of address of arbitrary length Address format: a.b.c.d/x, where x is # bits in network portion of address 11001000 00010111 00010000 00000000 network part host 200.23.16.0/23

Netmask With CIDR a new way is needed to determine the IP network given an IP address: Solution: Define a netmask Given an IP address of the form networkID.hostID, the netmask of the IP address is obtained by putting all “1”s in the networkID portion and all “0”s in the hostID portion 11001000 00010111 00010000 00000000 network part host 200.23.16.0/23 11111111 11111111 11111110 00000000 = 255.255.254.0 Given an IPAddr and a netmask, we bit-wise AND IPAddr and netmask to obtain the IP network. The rest is the hostID. NetworkID = IPAddr & Netmask HostID = IPAddr & ~Netmask

IP Layer Broadcast Recall that a host can send a LL broadcast message by putting FF-FF-FF-FF-FF-FF in destination MAC address How can a host send IP-layer broadcast packet? 2 ways: Put 255.255.255.255 in destination IP Means that all IP hosts within the same LL broadcast domain will receive this IP datagram Make <hostID> all 1’s Means that all hosts within the same IP subnet will receive this datagram Example: If IP subnet is 192.169.34.0, then a packet with a destination IP: 192.169.34.255 will be received by all hosts whose IP subnet is 192.169.34.0

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

IP addresses: how to get one? Q: How does network get network part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Hierarchical addressing: route aggregation CIDRized addresses facilitate hierarchical routing Fly-By-Night-ISP advertises that any IP datagram whose addresses begin with 200.23.16.0/20 should be sent to it. The rest of the world need not that there are 8 other organizations each with its own IP network. This is called route aggregation Dividing an IP network into smaller IP networks as done in here is called subnetting. Each organization can further divide their IP address range into smaller IP subnets Organization 0 200.23.16.0/23 Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” 200.23.18.0/23 Organization 2 200.23.20.0/23 . Fly-By-Night-ISP . Internet Organization 7 200.23.30.0/23 “Send me anything with addresses beginning 199.31.0.0/16” ISPs-R-Us

Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1. By the longest prefix matching rule, packets with destination addresses beginning with 200.23.18.0/23 are sent to ISPs-R-Us Organization 0 200.23.16.0/23 “Send me anything with addresses beginning 200.23.16.0/20” Organization 2 200.23.20.0/23 . Fly-By-Night-ISP . Internet Organization 7 200.23.30.0/23 “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” ISPs-R-Us Organization 1 200.23.18.0/23

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

32 bit destination IP address IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Header 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? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead

Forwarding: Getting a datagram from source to dest. forwarding table in A Dest. Net. Next Hop Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 IP datagram: misc fields source IP addr dest data 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E datagram remains unchanged, as it travels from source to destination

Forwarding: Getting a datagram from source to dest. forwarding table in A misc fields Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.1.3 data Starting at A, send IP datagram addressed to B: look up network address of B in forwarding table find B is on same net. as A B and A are directly connected link layer will send datagram directly to B inside link-layer frame. How? 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E

Delivering the packet from A to B 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E Starting at A, given IP datagram addressed to B: look up net. address of B, find B on same net. as A link layer send datagram to B inside link-layer frame How does A know the MAC address of B?  ARP protocol frame source, dest address datagram source, dest address B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload datagram frame

ARP: Address Resolution Protocol Question: how to determine MAC address of B given B’s IP address? Each IP node (Host, Router) on LAN has ARP module, table ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> < ………………………….. > TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

ARP protocol A knows B's IP address, wants to learn MAC address of B A broadcasts ARP query pkt, containing B's IP address all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address A caches (saves) IP-to-MAC address pairs until information becomes old (times out) soft state: information that times out (goes away) unless refreshed

Forwarding: Getting a datagram from source to dest. forwarding table in A misc fields Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.2.3 data Starting at A, dest. E: look up network address of E in forwarding table E on different network A, E not directly attached routing table: next hop router to E is 223.1.1.4 link layer sends datagram to router 223.1.1.4 inside link-layer frame datagram arrives at 223.1.1.4 continued….. 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E

Forwarding: Getting a datagram from source to dest. forwarding table in router Dest. Net router Nhops interface 223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9 223.1.3 - 1 223.1.3.27 misc fields 223.1.1.1 223.1.2.3 data Arriving at 223.1.4, destined for 223.1.2.2 look up network address of E in router’s forwarding table E on same network as router’s interface 223.1.2.9 router, E directly attached link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9 datagram arrives at 223.1.2.2 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E

Another IP Packet Forwarding Example A creates IP packet with source A, destination E A uses ARP to get R’s MAC address for 111.111.111.110 A creates Ethernet frame with R's MAC as dest, Ethernet frame contains A-to-E IP datagram A’s data link layer sends Ethernet frame to R R’s data link layer receives Ethernet frame R removes IP datagram from Ethernet frame, sees its destined to E R uses ARP to get E’s MAC address R creates frame containing A-to-E IP datagram sends to E A R E

IP packet forwarding algorithm D = destination IP address Bool found = false; For each forwarding table entry (SubnetNumber, SubnetMask, NextHop) do { D1 = SubnetMask & D; if (D1 == SubnetNumber) { if (NextHop is an Interface) { Deliver the datagram directly to the destination within a LL frame } else { Deliver the datagram to NextHop (a router) } //end-else found = true; break; } //end-if } //end-for If (!found) { if (there is a <default> router) Deliver the datagram to the <default> router else “Report: Destination unreachable”

IP packet forwarding: Example Subnet Number SubnetMask NextHop 128.96.34.0 255.255.255.128 Interface0 128.96.34.128 255.255.255.128 Interface1 128.96.33.0 255.255.255.0 R2 <default> R3 Assume Destination IP = 128.96.34.68 D1 = 128.96.34.68 & 255.255.255.128 = 128.96.34.0  Deliver the datagram to the destination over Interface0 Assume Destination IP = 128.96.34.150 D1 = 128.96.34.150 & 255.255.255.128 = 128.96.34.128  Deliver datagram to the destination over Interface1 Assume Destination IP = 128.96.35.44 IP subnet will not match any of the known IP subnetsDeliver the packet to the <default> router, R3

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

IP Fragmentation & Reassembly ID =x offset =0 fragflag length =4000 =1 =1500 =1480 =2960 =1040 One large datagram becomes several smaller datagrams Example 4000 byte datagram 3980 byte payload MTU = 1500 bytes 1st packet payload: 1480 bytes 2nd packet payload: 1480 bytes 3rd packet payload: 1020 bytes

ICMP: Internet Control Message Protocol Used by hosts, routers, gateways to communication network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) ICMP runs over IP: ICMP msgs carried in IP datagrams 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

Host IP Address Configuration Q: How does a host get an IP address? hard-coded by system admin in a file Wintel: control-panel->network->configuration->tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play” (more shortly)

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: host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg

DHCP client-server scenario 223.1.2.1 223.1.1.1 server 223.1.1.2 223.1.1.4 223.1.2.9 B 223.1.2.2 arriving DHCP client needs address in this network E 223.1.1.3 223.1.3.27 223.1.3.1 223.1.3.2

DHCP client-server scenario DHCP server: 223.1.2.5 arriving client DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs time DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs

NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual)

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

NAT: Network Address Translation NAT translation table WAN side addr LAN side addr 1: host 10.0.0.1 sends datagram to 128.119.40, 80 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 10.0.0.1 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 2 10.0.0.4 10.0.0.2 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 3: Reply arrives dest. address: 138.76.29.7, 5001

NAT: Network Address Translation Implementation: NAT router must: 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. remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair 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

NAT: Network Address Translation 16-bit port-number field: ~60,000 simultaneous connections with a single LAN-side address! NAT is controversial: routers should only process up to layer 3 violates end-to-end argument NAT possibility must be taken into account by app designers, eg, P2P applications What about embedded IP addresses? FTP, SMTP, SIP etc. address shortage should instead be solved by IPv6