* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Network layer (IP)
Survey
Document related concepts
Multiprotocol Label Switching wikipedia , lookup
Backpressure routing wikipedia , lookup
Network tap wikipedia , lookup
Airborne Networking wikipedia , lookup
Computer network wikipedia , lookup
Piggybacking (Internet access) wikipedia , lookup
Wake-on-LAN wikipedia , lookup
Internet protocol suite wikipedia , lookup
List of wireless community networks by region wikipedia , lookup
IEEE 802.1aq wikipedia , lookup
Routing in delay-tolerant networking wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
Transcript
Announcement CTEC code for TA CS 340 Lin Code: 322 1 Last class Network layer - introduction Virtual circuit and datagram networks Routing algorithms Link state 2 Link-state algorithm: example Step 0 1 2 3 4 5 N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v) D(w),p(w) 2,u 5,u 2,u 4,x 2,u 3,y 3,y D(x),p(x) 1,u D(y),p(y) ∞ 2,x D(z),p(z) ∞ ∞ 4,y 4,y 4,y 5 2 u v 2 1 x 3 w 3 1 5 1 y z 2 3 Overview Distance vector IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4 Distance Vector Algorithm (1) Bellman-Ford Equation (dynamic programming) Define dx(y) := cost of least-cost path from x to y Then dx(y) = min {c(x,v) + dv(y) } where min is taken over all neighbors of x 5 Bellman-Ford example (2) 5 2 u v 2 3 w 3 Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3 5 1 z B-F equation says: du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), 1 c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path ➜ 1 x y 2 6 Distance Vector Algorithm (3) Estimates: Dx(y) = estimate of least cost from x to y Distance vector: Dx = [Dx(y): y є N ] Each node x: Node x knows cost to each neighbor v: c(x,v) Node x maintains Dx = [Dx(y): y є N ] Node x also maintains its neighbors’ distance vectors • For each neighbor v, x maintains Dv = [Dv(y): y є N ] 7 Distance vector algorithm (4) Basic idea: Each node periodically sends its own distance vector estimate to neighbors When a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N Under minor, natural conditions, the estimate Dx(y) converge the actual least cost dx(y) 8 Distance Vector Algorithm (5) Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor Distributed: Each node: wait for (change in local link cost of msg from neighbor) each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary The algorithm doesn’t know the entire path – only knows the next hop recompute estimates if DV to any dest has changed, notify neighbors 9 Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2 node x table cost to x y z x ∞∞ ∞ y ∞∞ ∞ z 71 0 from from from from x 0 2 7 y 2 0 1 z 7 1 0 cost to x y z x 0 2 7 y 2 0 1 z 3 1 0 x 0 2 3 y 2 0 1 z 3 1 0 cost to x y z x 0 2 3 y 2 0 1 z 3 1 0 x 2 y 7 1 z cost to x y z from from from x ∞ ∞ ∞ y 2 0 1 z ∞∞ ∞ node z table cost to x y z x 0 2 3 y 2 0 1 z 7 1 0 cost to x y z cost to x y z from from x 0 2 7 y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z cost to x y z Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3 x 0 2 3 y 2 0 1 z 3 1 0 time 10 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates routing info, recalculates distance vector if DV changes, notify neighbors “good news travels fast” 1 x 4 y 50 1 z At time t0, y detects the link-cost change, updates its DV, and informs its neighbors. At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV. At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z. 11 Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - “count to infinity” problem! 44 iterations before algorithm stabilizes: see text 60 x 4 y 50 1 z Poissoned reverse: If Z routes through Y to get to X : Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) will this completely solve count to infinity problem? 12 Comparison of LS and DV algorithms Message complexity LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only convergence time varies Speed of Convergence LS: O(n2) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node’s table used by others • error propagate thru network 13 Overview Distance vector IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 14 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer IP protocol •addressing conventions •datagram format •packet handling conventions Routing protocols •path selection •RIP, OSPF, BGP forwarding table ICMP protocol •error reporting •router “signaling” Link layer physical layer 15 IP datagram format IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead 32 bits head. type of length ver len service fragment 16-bit identifier flgs offset upper time to Internet layer live checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, specify list of routers to visit. 16 IP Fragmentation & Reassembly network links have MTU (max.transfer size) - 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 17 IP Fragmentation and Reassembly Example 4000 byte datagram MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =185 length ID fragflag offset =1040 =x =0 =370 18 Overview Distance vector IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 19 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 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 223.1.3.2 223.1.3.1 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 20 Subnets IP address: subnet part (high order bits) host part (low order bits) What’s a subnet ? device interfaces with same subnet part of IP address can physically reach each other without intervening router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 LAN 223.1.3.1 223.1.3.2 network consisting of 3 subnets 21 Subnets Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Subnet mask: /24 22 Subnets 223.1.1.2 Q. How many? 223.1.1.1 A. The IP definition of a subnet is not restricted to Ethernet segments 223.1.1.4 223.1.1.3 223.1.9.2 223.1.7.0 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.2.1 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 23 IP addressing: CIDR Before CIDR: only 8-, 16-, and 24- bit masks were available (A, B, and C class networks) 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 11001000 00010111 00010000 00000000 200.23.16.0/23 24 IP addresses: how to get one? Q: How does host get 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 in a couple of weeks) 25 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 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 Organization 1 Organization 2 ... 11001000 00010111 00010000 00000000 11001000 00010111 00010010 00000000 11001000 00010111 00010100 00000000 ….. …. 200.23.16.0/23 200.23.18.0/23 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 26 Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 200.23.16.0/23 Organization 1 200.23.18.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16” 27 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 200.23.16.0/23 Organization 2 200.23.20.0/23 Organization 7 . . . . . . Fly-By-Night-ISP “Send me anything with addresses beginning 200.23.16.0/20” Internet 200.23.30.0/23 ISPs-R-Us Organization 1 200.23.18.0/23 “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” 28 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 assigns domain names, resolves disputes 29 NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.4 10.0.0.1 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) 30 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). 31 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 32 NAT: Network Address Translation 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table 2 NAT translation table WAN side addr LAN side addr 1: host 10.0.0.1 sends datagram to 128.119.40, 80 138.76.29.7, 5001 10.0.0.1, 3345 …… …… S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 138.76.29.7 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3: Reply arrives dest. address: 138.76.29.7, 5001 3 1 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 10.0.0.1 10.0.0.2 4 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 33 NAT: Network Address Translation 16-bit port-number field: 60,000 simultaneous connections with a single LAN-side address! NAT is controversial: Port numbers are to address processes – not hosts 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 34