Microsoft PowerPoint Chapter 03 Network layer and IP ST 1 Network Layer IP Address Chapter 3 Network Layer and IP Address 4 2 Contents 3 1 Introduction 3 2 Internet Protocol Datagram format.
Contents Chapter 3: Network Layer and IP Address Network Layer IP Address 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 3.3 Routing algorithms Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP 4-2 How TCP/IP works … Network layer segment datagram transport segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving side, delivers segments to transport layer application transport network data link physical network data link physical network layer protocols in every host, router network data link physical network data link physical network data link physical network data link physical network network data link data link physical physical network data link physical router examines header fields in all IP datagrams passing through it network data link physical network data link physical network data link physical application transport network data link physical 4-4 Interplay between routing and forwarding Two Key Network-Layer Functions routing algorithm forwarding: move packets from router’s input to appropriate router output local forwarding table header value output link 0100 0101 0111 1001 routing: determine route taken by packets from source to dest routing 2 value in arriving packet’s header algorithms 0111 Datagram networks Forwarding table no call setup at network layer routers: no state about end-to-end connections packets forwarded using destination host address Destination Address Range no network-level concept of “connection” packets between same source-dest pair may take different paths application transport network data link Send data physical application transport Receive data network data link physical billion possible entries Link Interface 11001000 00010111 00010000 00000000 through 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 11001000 00010111 00011111 11111111 otherwise 4-7 Longest prefix matching Prefix Match 11001000 00010111 00010 11001000 00010111 00011000 11001000 00010111 00011 otherwise Contents Link Interface 3.1 Introduction 3.2 Internet Protocol Examples DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 Which interface? Which interface? Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 3.3 Routing algorithms Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP 4-10 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 IP – Internet Protocol IP is the main protocol of the TCP/IP protocol suite Data packet is transmitted as a datagram IP provides an unreliable, connectionless datagram delivery service ICMP protocol •error reporting •router “signaling” Link layer physical layer IP - Datagram delivery service Unreliable: Review: Connectionless vs Connection-oriented Protocols guarantees that an IP datagram successfully gets to its destination Provides a best effort service Reliability must be provided by the upper layers (e.g., TCP) Send data across the network to its destination without guaranteeing receipt Fast; require little overhead No Connectionless: not maintain any state information about successive datagrams IP datagrams can get delivered out of order Connectionless protocols Connection-oriented protocols Establish a formal connection between two computers, guaranteeing the data will reach its destination Slower; more reliable IP datagram format IP Datagram IP protocol version number header length (bytes) “type” of data max number remaining hops (decremented at each router) Header 10101011101010101010010101010100101010100 11010010101010010101111111010000011101111 10100001011101010100110101011110100000101 00100000000010101000011010000111111010101 1011011001010100011001001010110 Data 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 type of ver head len service 16-bit identifier upper time to layer live flgs total datagram length (bytes) length fragment offset header checksum 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 IP Address IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link 223.1.1.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.1 223.1.2.9 223.1.2.2 223.1.3.27 router’s typically have multiple interfaces host typically has one interface IP addresses associated 223.1.1.1 = 11011111 00000001 00000001 00000001 with each interface IPv4: 32-bits IPv6: 128-bits 223.1.3.2 223.1.3.1 223 Version: Addressing and Delivering 1 IPv4 Size: bytes (32 bits) Format: — Each byte is represented by a decimal number, called a octet — two octets separated by a dot “.” Example: 10101100.00011101.00000001.00001010 172.29.1.10 Divided into parts: Network ID (NetID) Host ID IPv4 IPv4 Subnet mask: Use to identify the NetID and HostID parts of the IPv4 address bytes in size NetID bits have a value of bits have a value of HostID IPv4 IPv4 NetAddr = SubnetMask AND HostIP Network address (NetAddr): Example: 172.29.5.128/255.255.192.0 (or 172.29.5.128/18) HostIP 1010 1100 0001 1101 0000 0101 Subne 1111 1111 1111 1111 1100 0000 tMask Net 1010 1100 0001 1101 0000 0000 Addr 0000 0000 0000 0000 Broadcast address: 1000 0000 NetID bits: keep the same Host ID bits: up Example: 192.168.1.2/24 NetID bits: keep the same Host ID bits: clear to NetAddr: 192.168.1.0 Broadcast: 192.168.1.255 Two nodes with the same network address belong to same network: Example: 192.168.1.2 and 192.168.1.200: same network 192.168.1.2 and 192.168.2.1: different network IPv4 IPv4: class Number of valid host addresses in a network: 2^m-2: m is the number of bits in the HostID Host addresses range: (Network address + 1) -> (Broadcast address – 1) Example: 172.29.1.1/16 =>m = 32-SM=32-16 =>Number of hosts in this network = 2^16-2 IPv4: class IPv4: Default subnet mask: Class A: 255.0.0.0 (/8) B: 255.255.0.0 (/16) Class C: 255.255.255.0 (/24) Class Example: 15.19.18.29 Class: A Default Subnet mask: 255.0.0.0 IPv4: IPv4: Exercise For IP address: 172.29.7.10 Class: B SubnetMask: 255.255.0.0 (/16) NetAddr = IP address AND SubnetMask 172.29.7.10 AND For IP address: 191.24.197.12/20 Class: ? SubnetMask: ? NetAddr :? of hosts in the network: ? Host addresses range: ? Broadcast address: ? Number 255.255.0.0 172.29.0.0 Number of hosts in the network: 2^m-2=2^16-2 Host addresses range: 172.29.0.1->172.29.255.254 Broadcast address: 172.29.1111 1111 1111 1111 IPv4: Public address Vs Private IPv4: Classify Public address: Used to exchange on the Internet Real address Private address: address Used to address LANs within an organization Virtual Address Clas s Address (range) Network s Total Private Hosts A 10.0.0.0 16,777,214 B 172.16.0.0-172.31.0.0 16 1,048,544 C 192.168.0.0-192.168.255.0 256 65,024 Loopback address: 127.0.0.0 – 127.255.255.255 IPv4: Subnetting Reasons for subnetting: the number of nodes => Increase network throughput Increased security Ease of administration Ease of maintenance Avoid wasting IP addresses IPv4: Subnetting Reduce Rule: Borrow the first bits in HostID => NetID of subnets = 2^n (n: number of bits borrowed from HostID) New Subnet Mask = SM + n Number Planning: (1) Number of subnets to divide n=? Number of nodes in each subnet n=? (2) IPv4: Subnetting IPv4: Subnetting A company is granted the site address 192.168.1.0 The company needs subnets Design the subnets: + The number of the appropriate subnets? + The number of Hosts in each subnet? + New Subnet Mask? + Host addresses range of each subnet? + Broadcast address of each subnet? 192.168.1.0/SM Class: C (N.N.N.H), =>SM=24 + The number of the appropriate subnets: subnets n=3 (n: number of bits borrowed from HostID) The number of the appropriate subnets: 2^3 = + The number of Hosts in each subnet: m is the number of bits in the HostID: m= (32-24) – = The number of Hosts in each subnet: 2^m – = 2^5 – = 30 + New Subnet Mask? NewSM=SM+n=24+3=27 255.255.255.1110 0000 255.255.255.224 IPv4: Subnetting IPv4: Subnetting + Host addresses range of each subnet? 192.168.1.xxxh hhhh + Host addresses range of each subnet? 192.168.1.0110 0000 : 192.168.1.96 (Subnet No.4) 192.168.1.0000 0000 : 192.168.1.0 (Subnet No.1) Host range: 192.168.1.1 - > 192.168.1.30 Host range: 192.168.1.97 - > 192.168.1.126 Broadcast: 192.168.1 0111 1111 : 192.168.1.127 Broadcast: 192.168.1.0001 1111 : 192.168.1.31 192.168.1.0010 0000 : 192.168.1.32 (Subnet No.2) Host range: 192.168.1.33 - > 192.168.1.62 192.168.1.1000 0000 : 192.168.1.128 (Subnet No.5) Host range: 192.168.1.129 - > 192.168.1.158 Broadcast: 192.168.1.0011 1111 : 192.168.1.63 Broadcast: 192.168.1 1001 1111 : 192.168.1.159 192.168.1.0100 0000 : 192.168.1.64 (Subnet No.3) 192.168.1.1010 0000 Host range: 192.168.1.65 - > 192.168.1.94 Broadcast: 192.168.1 0101 1111 : 192.168.1.95 192.168.1.1100 0000 192.168.1.1110 0000 IPv4: Subnetting IPv4: Homework Exercise: A company is granted the site address 172.29.0.0/18 The company needs subnets Design the subnets: + The number of the appropriate subnets? + The number of Hosts in each subnet? + New Subnet Mask? + Host addresses range of each subnet? + Broadcast address of each subnet? Given 172.100.112.4/19 Please indicate: Which network does the above address belong to? The number of IP addresses that can be used in the network And please tell me which addresses include? Broadcast address of that network With the above network address, divide it into subnets + The number of the appropriate subnets? + The number of Hosts in each subnet? + New Subnet Mask? + Host addresses range of each subnet? + Broadcast address of each subnet? 10 IP addresses: how to get one? Goal: allow host to dynamically obtain its IP address from network server when it joins network Q: How does a host get IP address? hard-coded by system admin in a file Windows: 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” 223.1.1.2 223.1.1.4 223.1.3.1 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 local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 223.1.2.9 B 223.1.1.3 DHCP overview: rest of Internet 223.1.2.1 DHCP server 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) NAT: Network Address Translation DHCP client-server scenario A 223.1.1.1 DHCP: Dynamic Host Configuration Protocol 223.1.3.27 223.1.2.2 223.1.3.2 10.0.0.2 138.76.29.7 E arriving DHCP client needs address in this network 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) 11 NAT: Network Address Translation NAT: Network Address Translation Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP address 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) ARP: Address Resolution Protocol 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table RARP 48-bit Ethernet Address S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3: Reply arrives dest address: 138.76.29.7, 5001 10.0.0.4 S: 128.119.40.186, 80 D: 10.0.0.1, 3345 10.0.0.1 10.0.0.2 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 RARP: Reverse Address Resolution Protocol ARP 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 138.76.29.7, 5001 10.0.0.1, 3345 …… …… 138.76.29.7 32-bit Internet Address NAT translation table WAN side addr LAN side addr RARP = Reverse ARP RARP is the opposite of ARP ARP is used when the IP address is known but the physical address is not known RARP is used when the physical address is known but the IP address is not known RARP is often used in conjunction with the BOOTP protocol (boot PROM) to boot diskless workstations 12 ICMP - Internet Control Message Protocol Data sent to a remote computer often travels through one or more routers These routers can encounter a number of problems in sending the message to its ultimate destination Routers use Internet Control Message Protocol (ICMP) messages to notify the source IP of these problems ICMP is also used for other diagnosis and troubleshooting functions ICMP messages encapsulated within an IP datagram 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 IPv6 Initial motivation: 32-bit address space soon to be completely allocated Additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed Contents 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 3.3 Routing algorithms Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP 4-52 13 Interplay between routing, forwarding Graph abstraction routing algorithm local forwarding table header value output link 0100 0101 0111 1001 v u 2 Graph: G = (N,E) x w z y N = set of routers = { u, v, w, x, y, z } value in arriving packet’s header E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } 0111 Remark: Graph abstraction is useful in other network contexts Example: P2P, where N is set of peers and E is set of TCP connections Graph abstraction: costs u v x • c(x,x’) = cost of link (x,x’) w z y - e.g., c(w,z) = • cost could always be 1, or inversely related to bandwidth, or inversely related to congestion Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp) Question: What’s the least-cost path between u and z ? Routing algorithm: algorithm that finds least-cost path Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology, link cost info “link state” algorithms Decentralized: router knows physicallyconnected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors “distance vector” algorithms Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly periodic update response to link cost changes in 14 Contents 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 A Link-State Routing Algorithm Dijkstra’s algorithm 3.3 Routing algorithms Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP Notation: c(x,y): link cost from node x net topology, link costs known to all nodes accomplished via “link state broadcast” all nodes have same info computes least cost paths from one node (‘source”) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k dest.’s 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 4-57 Dijsktra’s Algorithm Initialization: N' = {u} for all nodes v if v adjacent to u then D(v) = c(u,v) else D(v) = ∞ Loop 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' Dijkstra’s algorithm: example Step 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 u v x w z y 15 Dijkstra’s algorithm: example (2) Dijkstra’s algorithm, discussion Resulting shortest-path tree from u: v Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n ) more efficient implementations possible: O(nlogn) w u z x Oscillations possible: e.g., link cost = amount of carried traffic y Resulting forwarding table in u: destination link v x (u,v) (u,x) y (u,x) w (u,x) z (u,x) D 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 0 C A 1+e 2+e e D 1+e B 0 C B e … recompute routing initially Contents A D A 2+e 2+e 0 B 1+e C … recompute A D 1+e B e C … recompute Distance Vector Algorithm Bellman-Ford Equation (dynamic programming) Define dx(y) := cost of least-cost path from x to y 3.3 Routing algorithms Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP Then dx(y) = {c(x,v) + dv(y) } v where is taken over all neighbors v of x 4-63 16 Bellman-Ford example u v x w Clearly, dv(z) = 5, dx(z) = 3, dw(z) = z y Distance Vector Algorithm B-F equation says: du(z) = { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = {2 + 5, + 3, + 3} = Dx(y) = estimate of least cost from x to y Node x knows cost to each neighbor v: c(x,v) Node x maintains distance vector Dx = [Dx(y): yєN] Node x also maintains its neighbors’ distance vectors For each neighbor v, x maintains Dv = [Dv(y): y є N ] Node that achieves minimum is next hop in shortest path ➜ forwarding table Distance vector algorithm (4) Basic idea: From time-to-time, each node sends its own distance vector estimate to neighbors Asynchronous 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 Distance Vector Algorithm (5) Iterative, asynchronous: each local iteration caused by: Under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y) local link cost change DV update message from neighbor Distributed: Each node: wait for (change in local link cost or msg from neighbor) recompute estimates each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary if DV to any dest has changed, notify neighbors 17 from z time 4-69 Link cost changes: node detects local link cost change updates routing info, recalculates distance vector if DV changes, notify neighbors from from x y z cost to x y z cost to x y z x y z 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} = x y z cost to x y z x y z x y z time Distance Vector: link cost changes Example: x x ∞∞ ∞ y ∞∞ ∞ z x y z 4-70 Distance Vector: link cost changes “good news travels fast” x ∞ ∞ ∞ y z ∞∞ ∞ node z table cost to x y z cost to x y z from from x cost to x y z from x ∞ ∞ ∞ y z ∞∞ ∞ node z table cost to x y z from y x ∞∞ ∞ y ∞∞ ∞ z x y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z from x y z node x table cost to x y z from from from x y ∞∞ ∞ z ∞∞ ∞ node y table cost to x y z cost to x y z from node x table cost to x y z Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = from Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = y 50 Consider the three-node topology shown in Figure z At time t0, y detects the link-cost change, updates its DV, and informs its neighbors 4.30 Rather than having the link costs shown in Figure 4.30, the link costs are c(x,y) = 3, c(y,z) = 6, c(z,x) = Compute the distance tables after the initialization step and after each iteration of a synchronous version of the distance-vector algorithm 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 not change and hence y does not send any message to z x y z 18 Comparison of LS and DV algorithms Message complexity Robustness: what happens if router malfunctions? LS: LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only convergence time varies Speed of Convergence Contents node can advertise incorrect link cost each node computes only its own table DV: LS: O(n2) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP DV node can advertise incorrect path cost each node’s table used by others 3.3 Routing algorithms error propagate thru network 4-74 Hierarchical Routing Hierarchical Routing Our routing study thus far - idealization all routers identical network “flat” … not true in practice scale: with 200 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol “intra-AS” routing protocol routers in different AS can run different intra-AS routing protocol Gateway router Direct link to router in another AS 4-76 19 Inter-AS tasks Interconnected ASes 3c 3a AS3 3b 2c 2a 1c 1a AS2 1b AS1 1d Intra-AS Routing algorithm Inter-AS Routing algorithm 2b 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 should forward packet to gateway router, but which one? 3c intra-AS sets entries for internal dests inter-AS & intra-As sets entries for external dests Contents AS1 must: learn which dests are reachable through AS2, which through AS3 propagate this reachability info to all routers in AS1 Job of inter-AS routing! router forwarding table configured by both intraand inter-AS routing algorithm Forwarding table suppose router in AS1 receives datagram destined outside of AS1: 3b 3a AS3 2a 1c 1a 1d 2c AS2 2b 1b AS1 Intra-AS Routing 3.3 Routing algorithms Link state Distance Vector Hierarchical routing RIP: 3.4 Routing in the Internet RIP OSPF BGP also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary) 4-79 20 RIP advertisements RIP ( Routing Information Protocol) distance vector algorithm included in BSD-UNIX Distribution in 1982 distance metric: # of hops (max = 15 hops) From router A to subnets: u z destination hops u v w x y z v A B C D w x distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) each advertisement: list of up to 25 destination subnets within AS y RIP: Example RIP: Example z w A x y D Dest w x z … Next hops - - C … w B A Advertisement from A to D z x C Destination Network w y z x … Next Router Num of hops to dest A B B 2 … Routing/Forwarding table in D y D B C Destination Network w y z x … Next Router Num of hops to dest A B BA 2 75 … Routing/Forwarding table in D 21 RIP: Link Failure and Recovery If no advertisement heard after 180 sec > neighbor/link declared dead routes via neighbor invalidated advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickly (?) propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) new RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated routed Transprt (UDP) network (IP) link physical OSPF (Open Shortest Path First) Transprt (UDP) forwarding table forwarding table network (IP) link physical Link-State Advertisement (LSA) “open”: publicly available uses Link State algorithm routed LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm OSPF advertisement carries one entry per neighbor router advertisements disseminated to entire AS (via flooding) carried in OSPF messages directly over IP (rather than TCP or UDP 22 OSPF “advanced” features (not in RIP) Hierarchical OSPF security: all OSPF messages authenticated (to prevent malicious intrusion) multiple same-cost paths allowed (only one path in RIP) For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains Hierarchical OSPF two-level hierarchy: local area, backbone advertisements only in area each nodes has detailed area topology; only know direction (shortest path) to nets in other areas Internet inter-AS routing: BGP Link-state BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to: area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers backbone routers: run OSPF routing limited to backbone boundary routers: connect to other AS’s Obtain subnet reachability information from neighboring ASs Propagate reachability information to all ASinternal routers Determine “good” routes to subnets based on reachability information and policy allows subnet to advertise its existence to rest of Internet: “I am here” 23 BGP basics Why different Intra- and Inter-AS routing ? pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to physical links when AS2 advertises a prefix to AS1: AS2 promises it will forward datagrams towards that prefix AS2 can aggregate prefixes in its advertisement 3a AS3 iBGP session Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance 2c 2a 1c 1a AS1 1d Inter-AS: admin wants control over how its traffic routed, who routes through its net Intra-AS: single admin, so no policy decisions needed eBGP session 3c 3b Policy: 2b AS2 1b Summary 3.1 Introduction 3.2 Internet Protocol Datagram format IPv4 addressing DHCP, NAT, ARP ICMP IPv6 3.3 Routing algorithms Link state Distance Vector Hierarchical routing 3.4 Routing in the Internet RIP OSPF BGP 4-95 24 ... 4-10 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... Two nodes with the same network address belong to same network: Example: 192.168.1.2 and 192.168.1.200: same network 192.168.1.2 and 192.168.2.1: different network IPv4 IPv4: class Number of... Subnet mask: 255.0.0.0 IPv4: IPv4: Exercise For IP address: 172.29.7.10 Class: B SubnetMask: 255.255.0.0 (/16) NetAddr = IP address AND SubnetMask 172.29.7.10 AND For IP address: 191.24.197.12/20