UNIT III THE NETWORK LAYER

Transcription

1 UNIT III THE NETWORK LAYER Introduction-Virtual Circuit and Datagram Networks- Inside a Router- The Internet Protocol (IP): Forwarding and Addressing in the Internet-Routing Algorithms Routing in the Internet-Broadcast and Multicast Routing- Mobile IP Objective: To understand the responsibilities network layer Understand the concepts of routers Learning routing algorithms Page 1

2 UNIT III THE NETWORK LAYER 1.INTRODUCTION: KEY NETWORK-LAYER FUNCTIONS forwarding: move packets from router s input to appropriate router output routing: determine the path taken by packets as they flow from a sender to a receiver Routing algorithms run at routers to determine paths ; Routers have a forwarding table Destination address-based in Datagram networks Virtual circuit number-based in VC Networks 2.VIRTUAL CIRCUIT 2.1 VC NETWORKS: CONNECTION SETUP Important function in some network architectures: ATM, frame relay, X.25 Before datagrams flow, two hosts and intervening routers establish virtual connection Routers get involved Network and transport layer cnctn service: Network: between two hosts Transport: between two processes 2.2 NETWORK SERVICE MODEL Example services for individual datagrams guaranteed delivery Guaranteed delivery with less than 40 msec delay Page 2

3 Example services for a flow of datagrams In-order datagram delivery Guaranteed minimum bandwidth to flow Restrictions on changes in inter-packet spacing 2.3 NETWORK LAYER CONNECTION AND CONNECTION-LESS SERVICE Datagram network provides network-layer connectionless service VC network provides network-layer connection service Analogous to the transport-layer services, but: Service: host-to-host No choice: network provides one or the other Implementation: in the core 2.4 Virtual circuits source-to-dest path behaves much like telephone circuit performance-wise network actions along source-to-dest path call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-dest path maintains state for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC 2.5 VC IMPLEMENTATION A VC consists of: Path from source to destination VC numbers, one number for each link along path Entries in forwarding tables in routers along path Packet belonging to VC carries a VC number. Page 3

4 VC number must be changed on each link. New VC number comes from forwarding table Forwarding table Forwarding table in Northwest router: Figure 2.5 Routers maintain connection state information! 2.6 VIRTUAL CIRCUITS: SIGNALING PROTOCOLS used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today s Internet Page 4

5 Figure DATAGRAM NETWORKS no call setup at network layer routers: no state about end-to-end connections no network-level concept of connection packets forwarded using destination host address packets between same source-dest pair may take different paths Figure 3 Page 5

6 3.1 DATAGRAM OR VC NETWORK Internet data exchange among computers elastic service, no strict timing req. smart end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at edge many link types different characteristics uniform service difficult ATM evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside network 4. INSIDE A ROUTER 4.1 ROUTER ARCHITECTURE OVERVIEW Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link Page 6

7 Figure INPUT PORT FUNCTIONS Figure 4.2 Physical layer: bit-level reception Data link layer: e.g., Ethernet Page 7

8 4.3 DECENTRALIZED SWITCHING: given datagram dest., lookup output port using forwarding table in input port memory. goal: complete input port processing at line speed queuing: if datagrams arrive faster than forwarding rate into switch fabric SWITCHING VIA MEMORY First generation routers traditional computers with switching under direct control of CPU packet copied to system s memory speed limited by memory bandwidth (2 bus crossings per datagram) Figure SWITCHING VIA A BUS Figure 4.3 Page 8

9 datagram from input port memory to output port memory via a shared bus bus contention: switching speed limited by bus bandwidth 1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone) SWITCHING VIA AN INTERCONNECTION NETWORK overcome bus bandwidth limitations Banyan networks, other interconnection nets initially developed to connect processors in multiprocessor Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches Gbps through the interconnection network 4.4 OUTPUT PORTS Figure 4.4 Buffering required when datagrams arrive from fabric faster than the transmission rate Need Queue Management Policy (Drop-Tail, AQM) Also need Packet Scheduling Policy (FCFS, WFQ) Page 9

10 OUTPUT PORT QUEUEING Figure buffering when arrival rate via switch exceeds output line speed queueing (delay) and loss due to output port buffer overflow 4.5 INPUT PORT QUEUING Fabric slower than input ports combined -> queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward (even though o/p port is free for the other datagram) Figure 4.5 Page 10

11 5.THE INTERNET PROTOCOL (IP) The Network layer consist of Host, router network layer functions: Figure 5 Page 11

12 5.1 IP DATAGRAM FORMAT Figure IP FRAGMENTATION & REASSEMBLY Figure 5.2 Page 12

13 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 5.3 IPV4 ADDRESSING 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 Page 13

14 Figure SUBNETS IP address: subnet part (high order bits) host part (low order bits) Subnets device interfaces with same subnet part of IP address can physically reach each other without intervening router Page 14

15 Figure ADDRESSING IN THE INTERNET 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 Before CIDR, Internet used a class-based addressing scheme where x could be 8, 16, or 24 bits. These corrsp to classes A, B, and C resp Page 15

16 5.5.1 IP ADDRESSES 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 a server this is becoming very popular NAT: Network Address Translation Figure 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). Page 16

17 5.6 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 IPV6 HEADER 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 Figure Page 17

18 Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by Next Header field ICMPv6: new version of ICMP additional message types, e.g. Packet Too Big multicast group management functions 5.7 TRANSITION FROM IPV4 TO IPV6 Not all routers can be upgraded simultaneous no flag days How will the network operate with mixed IPv4 and IPv6 routers Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers Tunneling Figure 5.7 Page 18

19 6.ROUTING ALGORITHMS 6.1 ROUTING ALGORITHM CLASSIFICATION 1. Global, decentralized Global: all routers have complete topology, link cost info link state algorithms Decentralized: router knows about physically-connected neighbors Iterative, distributed computations distance vector algorithms 2. Static, dynamic Static: routes change slowly over time Dynamic: routes change more quickly 3. Load sensitivity periodic update in response to link cost changes Many Internet routing algos are load insensitive 6.2 A LINK-STATE ROUTING ALGORITHM Dijkstra s algorithm net topology, link costs known to all nodes accomplished via link state broadcast all nodes have same info Page 19

20 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 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 Dijsktra s Algorithm Page 20

21 Dijkstra s algorithm: example Figure 6.2(a) Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n 2 ) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic Figure 6.2(b) Page 21

22 6.3 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 of x D x (y) = estimate of least cost from x to y Distance vector: D x = [D x (y): y є N ] Node x knows cost to each neighbor v: c(x,v) Node x maintains D x Node x also maintains its neighbors distance vectors For each neighbor v, x maintains D v = [D v (y): y є N ] Basic idea: Each node periodically sends its own distance vector estimate to neighbors When node a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: ` D x (y) min v {c(x,v) + D v (y)} for each node y N Under some conditions, the estimate D x (y) converge the actual least cost d x (y) Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor Distributed: each node notifies neighbors when its DV changes neighbors then notify their neighbors if necessary Page 22

23 Figure 6.3(a) Figure 6.3(b) Page 23

24 6.3.1 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 At time t 0, y detects the link-cost change, updates its DV, and informs its neighbors. At time t 1, 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 t 2, 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. Figure Link cost changes: good news travels fast bad news travels slow - count to infinity problem! 44 iterations before algorithm stabilizes: see text Page 24

25 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. Figure COMPARISON OF LS AND DV ALGORITHMS Message complexity LS: with n nodes, E links, O(nE) msgs sent DV: exchange between neighbors only Speed of Convergence LS: O(n 2 ) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may have routing loops count-to-infinity problem Page 25

26 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 6.5 HIERARCHICAL ROUTING routing study thus far - idealization o all routers identical, network flat scale: with 200 million destinations: o can t store all dest s in routing tables! o routing table exchange would swamp links! administrative autonomy o internet = network of networks o 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 o intra-as routing protocol o routers in different AS can run different intra-as routing protocol Gateway router o Direct link to router in another AS o Establishes a peering relationship o Peers run an inter-as routing protocol Page 26

27 Interconnected ASes Figure 6.5 Inter-AS tasks Obtain reachability information from neighboring AS(s) Propagate this info to all routers within the AS All Internet gateway routers run a protocol called BGPv4 (we will talk about this soon) Figure 6.5 Page 27

28 6.5.1 HIERARCHICAL ROUTING: ROUTING IN THE INTERNET Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols: o RIP: Routing Information Protocol (DV protocol) o OSPF: Open Shortest Path First (Link-State) o IGRP: Interior Gateway Routing Protocol (Cisco proprietary) 6.6 RIP ( ROUTING INFORMATION PROTOCOL) Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) Figure OSPF (OPEN SHORTEST PATH FIRST) open : publicly available Uses Link State algorithm LS packet dissemination Topology map at each node Page 28

29 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 OSPF advanced features (not in RIP) 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: o Multicast OSPF (MOSPF) uses same topology data base as OSPF Hierarchical OSPF in large domain 6.8 HIERARCHICAL OSPF Page 29 Figure 6.8

30 6.9 INTERNET INTER-AS ROUTING: BGP BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate the reachability information to all routers internal to the AS. 3. Determine good routes to subnets based on reachability information and policy. Allows a subnet to advertise its existence to rest of the Internet: BGP basics Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP conctns: BGP sessions Note that BGP sessions do not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisemen Figure 6.9 Page 30

31 BGP route selection Router may learn about more than 1 route to some prefix. Router must select route. Elimination rules: 1. Local preference value attribute: policy decision 2. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato routing 4. Additional criteria 7.MULTICAST/BROADCAST Figure 7 Page 31

32 8.MOBILE IP Mobile IP was developed as a means for transparently dealing with problems of mobile users Enables hosts to stay connected to the Internet regardless of their location Enables hosts to be tracked without needing to change their IP address Requires no changes to software of non-mobile hosts/routers Requires addition of some infrastructure Has no geographical limitations Requires no modifications to IP addresses or IP address format Supports security Could be even more important than physically connected routing IETF standardization process is still underway. 8.1 MOBILE IP ENTITIES Mobile Node (MN) The entity that may change its point of attachment from network to network in the Internet Detects it has moved and registers with best FA Assigned a permanent IP called its home address to which other hosts send packets regardless of MN s location Home Agent (HA) Since this IP doesn t change it can be used by long-lived applications as MN s location changes This is router with additional functionality Located on home network of MN Page 32

33 Does mobility binding of MN s IP with its COA Forwards packets to appropriate network when MN is away Does this through encapsulation Foreign Agent (FA) Another router with enhanced functionality If MN is away from HA the it uses an FA to send/receive data to/from HA Advertises itself periodically Forward s MN s registration request Decapsulates messages for delivery to MN Care-of-address (COA) Address which identifies MN s current location Sent by FA to HA when MN attaches Usually the IP address of the FA Correspondent Node (CN) End host to which MN is corresponding (eg. a web server) 8.2 MOBILE IP SUPPORT SERVICES Agent Discovery HA s and FA s broadcast their presence on each network to which they are attached Registration Beacon messages via ICMP Router Discovery Protocol (IRDP) When MN is away, it registers its COA with its HA Typically through the FA with strongest signal Registration control messages are sent via UDP to well known port Page 33

34 Encapsulation just like standard IP only with COA Decapsulation again, just like standard IP. 8.3 MOBILE IP OPERATION A MN listens for agent advertisement and then initiates registration If responding agent is the HA, then mobile IP is not necessary After receiving the registration request from a MN, the HA acknowledges and registration is complete Registration happens as often as MN changes networks HA intercepts all packets destined for MN This is simple unless sending application is on or near the same network as the MN There is a specific lifetime for service before a MN must re-register There is also a de-registration process with HA if an MN returns home Registration Process Figure 8.3 Page 34

35 Tables maintained on routers Mobility Binding Table Maintained on HA of MN Maps MN s home address with its current COA Visitor List Maintained on FA serving an MN Maps MN s home address to its MAC address and HA address HA then encapsulates all packets addressed to MN and forwards them to FA IP tunneling FA decapsulates all packets addressed to MN and forwards them via hardware address (learned as part of registration process) NOTE that the MN can perform FA functions if it acquires an IP address eg. via DHCP Bidirectional communications require tunneling in each direction 8.4 SECURITY IN MOBILE IP Authentication can be performed by all parties Only authentication between MN and HA is required Keyed MD5 is the default Replay protection Timestamps are mandatory Random numbers on request reply packets are optional HA and FA do not have to share any security information. Page 35

36 8.5 MOBILE IP TUNNELING Figure PROBLEMS WITH MOBILE IP Suboptimal triangle routing Single HA model is fragile Frequent reports to HA if MN is moving Security. Page 36

37 Review Questions: 1. Virtual circuit and datagram networks. Identify three important differences between a virtual circuit network (for example, ATM) and a datagram network (for example, Internet). 2. Virtual circuits. Consider Figure 4.3 on page 308 of the textbook and the virtual circuit (VC) table for router R1 shown above the figure. Write the set of VC table entries in router R2 in Figure 4.3 that are needed to ensure that the VC tables in R1 and R2 are consistent, that is, that the VCs entering/leaving interface 2 in router R1 are consistent with the VCs leaving/entering interface 1 in router R2. 3. IP addressing. a. Write the IP address in its binary form. b. Consider an IP subnet with prefix /27. Provide the range of IP addresses (of form xxx.xxx.xxx.xxx to yyy.yyy.yyy.yyy) that can be assigned to this subnet. c. Suppose an organization owns the block of addresses of the form /27. Suppose it wants to create four IP subnets from this block, with each block having the same number of IP addresses. What are the prefixes (of form xxx.xxx.xxx/y) for the four IP subnets? 4. IP datagram. Suppose a host has a file consisting of 2 million bytes. The host is going to send this file over a link with an MTU of 1,500 bytes. How many datagrams are required to send this file? 5. IP fragmentation. Consider sending a 2,000-byte datagram into a link with a MTU of 980 bytes. Suppose the original datagram has the identification number 227. How many fragments are generated? For each fragment, what is its size, what is the value of its identification, fragment offset, and fragment flag? 6. Longest prefix matching. Consider a datagram network using 32-bit host addresses. Suppose that a router has three interfaces, numbered 0 through 2, and that packets are to be forwarded to these link interfaces as follows. Any address not within the ranges in the table below should not be forwarded to an outgoing link interface. Create a forwarding table using longest prefix matching. Destination address range Outgoing link interface Through through through Longest prefix matching. Consider the same datagram network using 32-bit host addresses, and a router that has three interfaces, numbered 0 through 2 Page 37

38 (see Question 6). Packets are to be forwarded to these link interfaces as follows. The address ranges for the first, third, and fourth entries in the table below are the same as in Question 6; the second entry below is new. Any address not within the ranges in the table below should not be forwarded to an outgoing link interface. Destination address range Outgoing link interface through through Through Destination address range Outgoing link interface Through Through through Through Longest prefix matching. Consider the same datagram network using 32-bit host addresses, and a router that has three interfaces, numbered 0 through 2 (see Question 7). Packets are to be forwarded to these link interfaces as follows. The address ranges for the second and third entries in the table below are the same as in the earlier problem; the first entry below has an upper-end of the address range that is smaller than before. Any address not within the ranges in the table below should not be forwarded to an outgoing link interface. Destination address range Outgoing link interface Through through Through Page 38