Chapter 4 Network Layer slides are modified from J. Kurose & K. Ross CPE 400 / 600 Computer Communication Networks Lecture 17

Network Layer 2 Chapter 4: Network Layer r 4. 1 Introduction r 4.2 Virtual circuit and datagram networks r 4.3 What’s inside a router r 4.4 IP: Internet Protocol  Datagram format, IPv4 addressing, ICMP, IPv6 r 4.5 Routing algorithms  Link state, Distance Vector, Hierarchical routing r 4.6 Routing in the Internet  RIP, OSPF, BGP r 4.7 Broadcast and multicast routing

Network Layer 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.

Network Layer 4 IP Fragmentation & Reassembly r network links have MTU (max.transfer size) - largest possible link-level frame.  different link types, different MTUs r 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

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

Network Layer 6 Subnets r IP address:  subnet part (high order bits)  host part (low order bits) r What’s a subnet ?  device interfaces with same subnet part of IP address  can physically reach each other without intervening router network consisting of 3 subnets subnet

Network Layer 7 IP addressing: CIDR CIDR: Classless InterDomain Routing  subnet portion of address of arbitrary length  address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part /23

Network Layer 8 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 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

Network Layer 9 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 Layer 10 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 Layer 11 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 Layer 12 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 Layer 13 NAT: Network Address Translation r 16-bit port-number field:  60,000 simultaneous connections with a single LAN- side address! r 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  address shortage should instead be solved by IPv6

Network Layer 14 ICMP: Internet Control Message Protocol r used by hosts & routers to communicate network-level information  error reporting: unreachable host, network, port, protocol  echo request/reply (used by ping) r network-layer “above” IP:  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 Layer 15 Traceroute and ICMP r Source sends series of UDP segments to dest  First has TTL =1, Second has TTL=2, etc.  Unlikely port number r When nth datagram arrives to nth router:  Router discards datagram  And sends to source an ICMP message (type 11, code 0)  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 Layer 16 IPv6 r Initial motivation: 32-bit address space soon to be completely allocated. r Additional motivation:  header format helps speed processing/forwarding  header changes to facilitate QoS r IPv6 datagram format:  fixed-length 40 byte header  no fragmentation allowed

Network Layer 17 IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).

Network Layer 18 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  additional message types, e.g. “Packet Too Big”  multicast group management functions

Network Layer 19 Transition From IPv4 To IPv6 r Not all routers can be upgraded simultaneous  no “flag days”  How will the network operate with mixed IPv4 and IPv6 routers? r Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 IPv4

Network Layer 20 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 Layer 21 Lecture 17: Outline r 4.4 Internet Protocol  Datagram format  IPv4 addressing  NAT  ICMP  IPv6 r 4.5 Routing algorithms  Link state  Distance Vector  Hierarchical routing

Network Layer value in arriving packet’s header routing algorithm local forwarding table header value output link Interplay between routing and forwarding

Network Layer 23 u y x wv z Graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Graph abstraction Remark: Graph abstraction is useful in other network contexts Example: P2P, where N is set of peers and E is set of TCP connections

Network Layer 24 Graph abstraction: costs u y x wv z c(x,x’) = cost of link (x,x’) - e.g., c(w,z) = 5 cost could always be 1, or inversely related to bandwidth, or inversely related to congestion Cost of path (x 1, x 2, x 3,…, x p ) = c(x 1,x 2 ) + c(x 2,x 3 ) + … + c(x p-1,x p ) Question: What’s the least-cost path between u and z ? Routing algorithm: algorithm that finds least-cost path

Network Layer 25 Routing Algorithm classification Global: r all routers have complete topology, link cost info r “link state” algorithms Decentralized: r router knows physically-connected neighbors, link costs to neighbors r iterative process of computation, exchange of info with neighbors r “distance vector” algorithms Static: r routes change slowly over time Dynamic: r routes change more quickly  periodic update  in response to link cost changes

Network Layer 26 A Link-State Routing Algorithm Dijkstra’s algorithm r net topology, link costs known to all nodes  accomplished via “link state broadcast”  all nodes have same info r computes least cost paths from one node (‘source”) to all other nodes  gives forwarding table for that node r iterative: after k iterations, know least cost path to k destinations Notation:  c(x,y): link cost from node x to y; = ∞ if not direct neighbors  D(v): current value of cost of path from source to dest. v  p(v): predecessor node along path from source to v  N': set of nodes whose least cost path definitively known

Network Layer 27 Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'

Network Layer 28 Dijkstra’s algorithm: example Step N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v) 2,u D(w),p(w) 5,u 4,x 3,y D(x),p(x) 1,u D(y),p(y) ∞ 2,x D(z),p(z) ∞ 4,y u y x wv z

Network Layer 29 Dijkstra’s algorithm: example (2) u y x wv z Resulting shortest-path tree from u: v x y w z (u,v) (u,x) destination link Resulting forwarding table in u:

Network Layer 30 Dijkstra’s algorithm, discussion Algorithm complexity: n nodes r each iteration: need to check all nodes, w, not in N r n(n+1)/2 comparisons: O(n 2 ) r more efficient implementations possible: O(nlogn) Oscillations possible: r e.g., link cost = amount of carried traffic A D C B 1 1+e e 0 e A D C B 2+e e 1 A D C B 0 2+e 1+e A D C B 2+e 0 e 0 1+e 1 initially … recompute routing … recompute

Network Layer 31 Lecture 17: Outline r 4.4 Internet Protocol  Datagram format  IPv4 addressing  NAT  ICMP  IPv6 r 4.5 Routing algorithms  Link state  Distance Vector  Hierarchical routing

Network Layer 32 Distance Vector Algorithm Bellman-Ford Equation (dynamic programming) Define d x (y) := cost of least-cost path from x to y Then d x (y) = min {c(x,v) + d v (y) } where min is taken over all neighbors v of x v

Network Layer 33 Bellman-Ford example u y x wv z Clearly, d v (z) = 5, d x (z) = 3, d w (z) = 3 Node that achieves minimum is next hop in shortest path ➜ forwarding table d u (z) = min { c(u,v) + d v (z), c(u,x) + d x (z), c(u,w) + d w (z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 B-F equation says:

Network Layer 34 Lecture 17: Summary r Internet Protocol  Datagram format  IPv4 addressing  NAT  ICMP  IPv6 r Routing algorithms  Link state  Distance Vector