CS 1652 The slides are adapted from the publisher’s material All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Jack Lange University of Pittsburgh 1

Network Layer4-2 The Internet Network layer forwarding table Host, router network layer functions: Routing protocols path selection RIP, OSPF, BGP IP addressing conventions datagram format packet handling conventions ICMP 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 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? r 20 bytes of TCP r 20 bytes of IP r = 40 bytes + app layer overhead

Network Layer4-4 IP Fragmentation & Reassembly r Network links have MTU (max.transfer size) - largest possible link-level frame. m Dial-up – 576 m Ethernet – 1500 m FDDI 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 Layer4-5 IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =185 fragflag =1 length =1500 ID =x offset =370 fragflag =0 length =1040 One large datagram becomes several smaller datagrams (=fragments) Example r 4000 byte datagram r MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 same ID -> same packet

IPv4 Addressing 6

Network Layer4-7 IP Addressing: introduction r IP address: 32-bit identifier for host, router interface r Interface: connects between host/router and physical link m router’s typically have multiple interfaces m host typically has one interface m IP addresses associated with each interface =

Network Layer4-8 Subnets r IP address: m subnet part (high order bits) m host part (low order bits) r What’s a subnet ? m device interfaces with same subnet part of IP address m can physically reach each other without intervening router network consisting of 3 subnets subnet

Network Layer4-9 Subnets How many?

Network Layer4-10 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 Layer4-11 Classful IP addressing? Before CIDR (1995), we used classful IP m Class A : a.b.c.d/8 : addr starts with 0…. m Class B : a.b.c.d/16 : addr starts with 10…. m Class C : a.b.c.d/24 : addr starts with 110…. Why CIDR? m IP allocation efficiency 300 hosts => one class B vs. two class C ? m Classful IP increased # of C allocation More entries in the forwarding table! m CIDR allows supernetting Aggregating N continuous address blocks into 1

Network Layer4-12 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 gets it from ICANN) ISP's block /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

Network Layer4-13 Hierarchical addressing: route aggregation “Send me anything with addresses beginning /20” / / /23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning /16” /23 Organization Hierarchical addressing allows efficient advertisement of routing information:

Network Layer4-14 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning /20” / / /23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning /16 or /23” /23 Organization

Network Layer4-15 IP addresses: how to get one? Q: How does a host get IP address? 1. hard-coded by system admin m Provided by network admin 2. DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server m “plug-and-play”

Network Layer4-16 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network m Can renew its lease on address in use m Allows reuse of addresses (only hold address while connected and “on”) m 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 Layer4-17 DHCP client-server scenario DHCP server arriving DHCP client needs address in this network

Network Layer4-18 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 Layer4-19 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 Layer4-20 NAT: Network Address Translation r Motivation: local network uses just one IP address as far as outside world is concerned: m range of addresses not needed from ISP: just one IP address 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 Layer4-21 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 Layer4-22 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 Layer4-23 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 m Incoming connection not possible NAT traversal (static binding, UPnP, relaying - Skype)

ICMP 24

Network Layer4-25 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 Layer4-26 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.

IPv6 27

Network Layer4-28 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 Layer4-29 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 Layer4-30 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 Layer4-31 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 Dual stack: run IPv4 & IPv6 at the same time r Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

Network Layer4-32 Tunneling A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 IPv4

Network Layer4-33 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