1. 4: Network Layer4a-1 Network Layer Goals: r understand principles behind network layer services: m routing (path selection) m dealing with scale m how a router works m advanced topics: IPv6, multicast r instantiation and implementation in the Internet Overview: r network layer services r routing principle: path selection r hierarchical routing r IP r Internet routing protocols reliable transfer m intra-domain m inter-domain r what’s inside a router? r IPv6 r multicast routing

2. 4: Network Layer4a-2 Network layer functions r transport packet from sending to receiving hosts r network layer protocols in every host, router (Recall transport layer is end-to-end) three important functions: r path determination: route taken by packets from source to dest. Routing algorithms r switching: move packets from router’s input to appropriate router output r call setup: some network architectures (e.g. telephone, ATM) require router call setup along path before data flow network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

3. 4: Network Layer4a-3 Protocol stack: packet forwarding HTTP TCP IP ethernet Host A IP ethernet Router R link HTTP TCP IP ethernet Router W Host B IP ethernetlink

4. 4: Network Layer4a-4 Network service model Q: What service model for “channel” transporting packets from sender to receiver? r guaranteed bandwidth? r preservation of inter-packet timing (no jitter)? r loss-free delivery? r in-order delivery? r congestion feedback to sender? ? ? ? virtual circuit or datagram? The most important abstraction provided by network layer: service abstraction Which things can be “faked” at the transport layer?

5. 4: Network Layer4a-5 Virtual circuits r call setup, teardown for each call before data can flow; associates VC identifier with the path r each packet carries VC identifier (not destination host OD) r every router on source-dest path s maintain “state” for each passing connection m transport-layer connection only involved two end systems r link, router resources (bandwidth, buffers) may be allocated to VC m to get circuit-like performance “source-to-dest path behaves much like telephone circuit” m performance-wise m network actions along source-to-dest path

6. 4: Network Layer4a-6 Virtual circuits: signaling protocols r used to setup, maintain teardown VC r setup gives opportunity to reserve resources r used in ATM, frame-relay, X.25 r not used in today’s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data

7. 4: Network Layer4a-7 Datagram networks: the Internet model r no call setup at network layer r routers: no state about end-to-end connections m no network-level concept of “connection” r packets typically routed using destination host ID m packets between same source-dest pair may take different paths r Best effort application transport network data link physical application transport network data link physical 1. Send data 2. Receive data

8. 4: Network Layer4a-8 Best Effort What can happen to datagrams? r Corrupted at the physical level r Datagrams dropped because of full buffers r Destination unreachable r Routing loops

9. 4: Network Layer4a-9 Datagram or VC network: why? Datagram (Internet) r data exchange among computers m “elastic” service, no strict timing req. r “smart” end systems (computers) m can adapt, perform control, error recovery m simple inside network core, complexity at “edge” r many link types m different characteristics m uniform service difficult Virtual Circuit (ATM) r evolved from telephony r human conversation: m strict timing, reliability requirements m need for guaranteed service r “dumb” end systems m telephones m complexity inside network

10. 4: Network Layer4a-10 The Internet Network layer routing 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

11. 4: Network Layer4a-11 Internet Protocol r The Internet is a network of heterogeneous networks: m using different technologies (ex. different maximum packet sizes) m belonging to different administrative authorities (ex. Willing to accept packets from different addresses) r Goal of IP: interconnect all these networks so can send end to end without any knowledge of the intermediate networks r Routers, switches, bridges: machines to forward packets between heterogeneous networks

12. 4: Network Layer4a-12 IP Addressing: introduction r IP address: 32-bit identifier for host, router interface r interface: connection between host and physical link m router’s must have multiple interfaces m host may have multiple interfaces m IP addresses (unicast addresses) associated with interface, not host, router 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 223.1.1.1 = 11011111 00000001 00000001 00000001 223 111

13. 4: Network Layer4a-13 IP Addressing r IP address: m 32 bits m network part (high order bits) m host part (low order bits) m Defined by class of IP address? m Defined by subnet mask r What’s a network ? ( from IP address perspective) m device interfaces with same network part of IP address m can physically reach each other without intervening router 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 network consisting of 3 IP networks (223.1.1, 223.1.2, 223.1.3) LAN

14. 4: Network Layer4a-14 IP Addressing How to find the networks? r Detach each interface from router, host r create “islands of isolated networks 223.1.1.1 223.1.1.3 223.1.1.4 223.1.2.2 223.1.2.1 223.1.2.6 223.1.3.2 223.1.3.1 223.1.3.27 223.1.1.2 223.1.7.0 223.1.7.1 223.1.8.0223.1.8.1 223.1.9.1 223.1.9.2 Interconnected system consisting of six networks

15. 4: Network Layer4a-15 IP Addresses (Classes) 0 network host 10 network host 110 networkhost 1110 multicast address A B C D class 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 32 bits given notion of “network”, let’s re-examine IP addresses: “class-full” addressing Unicast Multicast 1111 reserved E 240.0.0.0 to 255.255.255.255 Reserved

16. 4: Network Layer4a-16 IP Address Space Allocation CAIDA 1998

17. 4: Network Layer4a-17 Unicast vs Broadcast vs Multicat r Unicast Addresses m IP Datagram destined for single host m Type of IP address you normally thing of m Class A-C + some special IP addresses r Broadcast m IP Datagram sent to all hosts on a given network m Some unicast network id + special host id m Some part of reserved E class r Multicast m IP Datagram sent to a set of hosts belonging to a “multicast” group m Class D m We will return to IP multicast later

18. 4: Network Layer4a-18 Special IP Addresses: Unicast and Broadcast net ID Subnet ID Host ID Can be source? Can be dest? Description 00YNThis host on this net 0HostidYNSpecified host on this net 127AnyYYLoopback NY255.255.255.255 Limited broadcast (do not forward!) NetidNYnetid.255.255.255 Net directed broadcast to netid NetidSubnetidNYSubnet directed broadcast to netid, subnetid Netid NYAll subnets directed broadcast to netid

19. 4: Network Layer4a-19 Broadcast r Limited Broadcast m 255.255.255.255 m Not forwarded! r Net-directed Broadcast m netid.255.255.255 r Subnet-directed Broadcast m All bits in host portion 1’s m Requires knowledge of subnet mask m 128.1.2.255 is a subnet-directed broadcast with subnet mask 255.255.255.0 but not with 255.255.254.0 r All-subnets-directed Broadcast m All bits in host and subnet portions are 1’s m Need to know subnet mask to distinguish from net- directed

20. 4: Network Layer4a-20 Note r Broadcast and multicast make sense for UDP and not for TCP

21. 4: Network Layer4a-21 IP addressing: CIDR r classful addressing: m inefficient use of address space, address space exhaustion m e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network r CIDR: Classless InterDomain Routing m network portion of address of arbitrary length m address format: a.b.c.d/x, where x is # bits in network portion of address 11001000 00010111 00010000 00000000 network part host part 200.23.16.0/23

22. 4: Network Layer4a-22 Recall: How to get an IP Address? r Answer 1: Normally, answer is get an IP address from your upstream provider m This is essential to maintain efficient routing! r Answer 2: If you need lots of IP addresses then you can acquire your own block of them. m IP address space is a scarce resource - must prove you have fully utilized a small block before can ask for a larger one and pay $$ (Jan 2002 - $2250/year for /20 and $18000/year for a /14)

23. 4: Network Layer4a-23 How to get lots of IP Addresses? Internet Registries RIPE NCC (Riseaux IP Europiens Network Coordination Centre) for Europe, Middle-East, Africa APNIC (Asia Pacific Network Information Centre ) for Asia and Pacific ARIN (American Registry for Internet Numbers) for the Americas, the Caribbean, sub-saharan Africa Note: Once again regional distribution is important for efficient routing! Can also get Autonomous System Numbers (ASNs) from these registries

24. 4: Network Layer4a-24 Classful vs Classless r Class A = /8 r Class B = /16 r Class C = /24

25. 4: Network Layer4a-25 IP addresses: how to get one? revisted Network (network portion): r get allocated portion of 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

26. 4: Network Layer4a-26 Hierarchical addressing: route aggregation “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23200.23.18.0/23200.23.30.0/23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16” 200.23.20.0/23 Organization 2...... Hierarchical addressing allows efficient advertisement of routing information:

27. 4: Network Layer4a-27 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23200.23.18.0/23200.23.30.0/23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” 200.23.20.0/23 Organization 2......

28. 4: Network Layer4a-28 IP Address Allocation r CIDR is great but must work around existing allocations of IP address space m Company 1 has a /20 allocation and has given out sub portions of it to other companies m University has a full class B address m Company 2 has a /23 allocation from some other class B m ALL use the same upstream ISP – that ISP must advertise routes to all these blocks that cannot be described with a simple CIDR network ID and mask! r Estimated reduction in routing table size with CIDR m If IP addresses reallocated, CIDR applied to all, IP addresses reallocated based on geographic and service provider divisions that current routing tables with 10000+ entries could be reduced to 200 entries [Ford, Rekhter and Brown 1993] m How stable would that be though? Leases for all?

29. 4: Network Layer4a-29 Current Allocation r Interesting to exam current IP address space allocation (who has class A’s ? Etc) m Who has A’s? m Computer companies around during initial allocation (IBM, Apple) m Universities (Stanford, MIT) m CAIDA has info on complete allocation

30. 4: Network Layer4a-30 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 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, pecify list of routers to visit.

31. 4: Network Layer4a-31 IP Header: Version and Header Length r Version number (4-bit ) m 4 for IPv4, 6 for IPv6 m Fields that follow can vary based on this number r Header length (4-bit ) m Number of 32 bit words (2 4 -1 32 bits = 60 bytes) m Includes length of options (40 bytes max)

32. 4: Network Layer4a-32 IP Header: TOS r Type-of-service (TOS) field m 3 Bit precedence field m 4 TOS bits (only one may be turned on) Minimize delay Maximize throughput Maximize reliability Minimize monetary cost m 1 unused bit r Many implementations ignore; most implementations don’t allow application to set this to indicate preference anyway

33. 4: Network Layer4a-33 IP Header r Total length field (16 bits) m Length in bytes m Max Total length 2 16 -1 = 65535 bytes m Max Data = 65535 –Header Length r Can you really send that much? m Link layer might not be enough to handle that much; Various link layer technologies have different limits m As pass over various link layers, IP datagram will be fragmented if necessary m Total length field will change when fragmented

34. 4: Network Layer4a-34 Next time r Continue with details of IP Fragmentation

35. 4: Network Layer4a-35 Outtakes

36. 4: Network Layer4a-36 Network layer service models: Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ? r Internet model being extended: Intserv, Diffserv m KR Chapter 6