Routing. Internet Layer. Routing. Routing Tables

Transcription

1 Internet Layer Raw division into three tasks: ata transfer over a global network Route decision at the sub-nodes ontrol of the network or transmission status Routing Most important functionality on layer beside addressing is routing. ach computer has been assigned an IP address very router manages a table which indicates, which outgoing connection has to be selected for a certain destination. L To destination by connection Routing Protocols Routing Tables Transfer Protocols: IPv, IPv ontrol Protocols: IMP, RP, RRP, IGMP L L L L L L The routing tables can be determined ally; it is better however to adapt them continuously to the current network status. With connectionless communication, the routing must be done for each packet separately. The routing decision can vary from packet to packet. Using virtual connections routing is done only once (connection establishment, see TM), but the routing tables are more extensive Page Page Routing Tables estination ddress Next Hop Routing How to construct a routing table? /8 7.../.7../ 9.../.../ Task: determine most favorable route from the sender to the receiver: Short response times High throughput voidance of local overload situations Security requirements Shortest path onsider: ormation can change with time routing tables have to be adapted! Page Page

2 Optimal Routing Optimal route decision is in principle not possible, since no complete ormation about the network is present within the individual nodes a routing decision effects the network for a certain period conflicts between fairness and optimum can arise: Routing in the Internet The Internet consists of a large number of autonomous systems. ach autonomous system is operated by its own operator and can use its own routing protocols. y standardization of usable protocols, gateways can forward packets at the borders of the autonomous systems. X X High traffic amount between and, and, and In order to optimize the total data flow, no traffic may occur between X and X X and X would see this totally different! fairness criteria! Internal Protocol xternal Protocol Internal Protocol Internal Protocol utonomous Systems Page Page Routing Protocols Routing in a Sub-Network Interior Gateway Protocols (Routing in autonomous systems, for efficient transmission) Routing Information Protocol (RIP) Internet Gateway Routing Protocol (IGRP) nhanced IGRP Open Shortest Path irst (OSP) Intermediate System to Intermediate System (IS-IS) xterior Gateway Protocols (Routing between domains, adherence of policies for the domains) order Gateway Protocol (GP) xterior Gateway Protocol (GP) Router iscovery Protocols IMP Router iscovery Protocol (IRP) Hot Standby Router Protocol (HSRP) K G L H M I O N J Sub-network consists of many routers Routers are connected by several links onnections are partially redundant onnections have different characteristics There possibly exist redundant connections Therefore optimization of the routes by elimination of long paths Page 7 Page 8

3 Sink or Sink Tree Routing lgorithms K G L H M I O N J s result of the optimization principle a Sink or Sink Tree is constructed (here for router ) Sink Tree contains no loops an be used as a good indicator for the quality of a routing algorithm Sink Tree can change immediately (e.g. by crash of a router or by loss of a link) deterministic Routing stochastic centralized isolated distributed deterministic stochastic deterministic stochastic. local global deterministic stochastic deterministic stochastic Page 9 Page Static Routing looding Source Routing: Route is determined by the sender Header,,7,8 ata Internal Routing: - Routing decision by intermediate nodes - Static tables are used within the routers, no reaction to changes in the network looding of the network with copies of the data packet Router propagates packet over all links, except over the incoming one Incoming packet - Stable -Simple - No reaction to changing network conditions - reakdowns in links or routers can have catastrophic results by node via line L L L L L L L High reliability (in case of failure of single routers) Meaningful for military applications (robustness) ut: large number of copied packets Possible loops are problematic Hop counter (TTL), list of all packets already sent Usable as reference for the quality of routing algorithms (delay) Used to support other routing algorithms, e.g. OSP Page Page

4 Network ontrol enter (N) Local stimation Procedure N Network ontrol enter daptive, centralized technique The central control center collects ormation about the network state The N provides routing strategies and network ormation in regular time intervals to all routers to via router a b c L L L 8 7 min router a b c via L L L Routing decisions are made locally by routers Of use only for very small networks collecting network state ormation needs time, also the transmission of these ormation to the routers thus the ormation is outdated till it comes to the routers. in L L L Routing takes place on the basis of transmission delay estimations very router estimates for itself Problem: Routers cannot see the whole network centralized isolated Page Page Hot Potato Probabilistic Routing hoose the outgoing link with the lowest load for the next packet L % L % L 9% L L L Router decides regarding performance measures of sent packets (e.g. delay, jitter) about best routes for a packet Routing can be made proportionally by assigning a proportionality factor p i to each link i Problem: circling of messages Solution: carry a list of the routers visited last variants: Hot Potato = Shortest Queue without ias Hot Potato with carried router list to detect cycles = Shortest Queue with ias L L p(l) % % % % L p(l) 7% % - - L L L L p p p n isolated / isolated Page Page

5 Routing: Shortest Path Shortest Path: ijkstra () Static variant Router administrates a table with distances to each destination and lines to be used ased on a constant metric (e.g. distance, line costs, transmission capacity etc.) omputation of the shortest path (regarding the metric) according to ijkstra daptive variant ynamic routing algorithms ynamic metrics (e.g. current delay, actual transmission capacity) (Regular) update of the routing tables lgorithm of ijkstra (99) for Routing. Mark source node (the Work node ) as permanent (i.e. distance and line do not change any more). onsider neighbored nodes of the node currently marked as permanent (i.e. the Work node ) and compute the distance to them based on own knowledge and link costs. hoose the node with the smallest distance to the source from the nodes not marked yet and mark it as permanent, it becomes the new Work node. Goto. Page 7 Page 8 Shortest Path: ijkstra () Shortest Path: ijkstra () xample: G 7 H s metric, exemplarily the distance in kilometers is chosen. Searched is the shortest path from to. Step: Marking of node as permanent. Step: Marking of the neighbor nodes of (, ) 7 (, -) G (, ) (, -) (, -) (, -) H (, -) In order to be able to trace back the path later on, the predecessor node is stored. Step: becomes permanent, because distance is (< ).. Step: Tentative labeling of the neighbor nodes of Page 9 Page

6 Shortest Path: ijkstra () Shortest Path: ijkstra () (, ) 7 (, ) G (, ) (9, ) (, -) (, -) H (, -). Step: becomes permanent, since the distance to is (< < 9).. Step: Tentative labeling of the neighbor nodes of (, ) 7 (, ) G (, ) (9, ) (, ) (, -) H (, -). Step: Preliminary label of G is overwritten. Step: G becomes permanent, since the distance of is (< < 9). 7. Step: Tentative labeling of the neighbor nodes of G (, ): istance of to sums up to, using the path Page Page Shortest Path: ijkstra () Shortest Path: ijkstra (7) (, ) 7 (, ) G (, ) (9, ) (, ) (, -) H (9, G) 8. Step: Tentative label of G is overwritten 9. Step: becomes permanent, since the of is (< 9).. Step: Tentative labeling of the neighbor nodes of (, ) 7 (, ) G (, ) (9, ) (, ) (, H) H (8, ). Step: H becomes permanent, since the distance of is 8 (< 9).. Step: Tentative labeling of the neighbor nodes of H. Step: becomes permanent, since the distance of is 9 (< ). Page Page

7 Shortest Path: ijkstra (8) Shortest Path: ijkstra (9) (, ) 7 (, ) G (, ) (9, ) (, ) (, H) H (8, ) The distance to using is larger than the tentative label of. No more paths exist, no states are changed - the algorithm terminates. (, ) 7 (, ) G (, ) (9, ) (, ) (, H) H (8, ). Step: is reached on the shortest path and finally becomes permanent. Page Page Implementation of ijkstra () Implementation of ijkstra () Page 7 Page 8

8 istance Vector Routing istance Vector Routing Problem: Static procedures are lexible, they do not react to problems and must be computed again after each change of the topology etc. Solution: Routers mutually exchange (regularly) ormation about the current state of outgoing lines daptive variant of Shortest Path Routing: istance Vector Routing (ellman et al. 97) very router manages a table with (known/estimated) distances to each destination and the assigned lines to neighbor nodes. ach router is assumed to know the distances to its neighbors. Regularly, the distance ormation of the routing tables is communicated to the neighbors; due to the ormation from the neighbors and the known distances to the neighbors every router computes its routing table again (without use of the own old routing ormation). lso: istributed ellmann-ord Routing, ord-ulkerson Routing, RIP (RPNT, Internet); improved in isco routers xample of a istance Vector: V j = ((=), (=), (=), ) ( is reachable with costs, with costs, and with costs ) Page 9 Page Information xchange xample () L () L () L L s transmission costs for each line, is assumed. () L () L Table of router after system initialization or cold start rom to Link osts locally ssential here: Table of router after system initialization or cold start rom to Link osts locally Global ormation is exchanged only between neighbors! Page Page

9 xample xample transfers (=) to its neighbors and () L () L () L L now transfers its vector (=, =) using link, and to its neighbors, and () L () L () L L and know, from where the vector comes and so the costs to can be computed Routing tables of routers and, after the vector of router is processed () L () L rom to Link osts locally L rom to Link osts receives this vector over link and updates its table as follows: = is larger than = discard = for link Similar to it the processing of the vector of takes place () L () L Routing table of after the actualization coming from and rom to Link osts locally L locally L L Page Page xample xample Router receives (=, =) rom to Link osts locally L L Router receives the vector (=, =) which it uses to update the routing table with = and = using link The entry for over link is already registered with costs of, therefore no new entry for is necessary () L () L () L L () L () L Router receives (=, =) () L () L () L L () L () L rom to Link osts locally L L Routing table of after the update, and now have new routing tables generate distance vectors rom to Link osts locally L L L Page Page

10 xample xample rom : (=, =, =) over link and rom : (=, =, =) over link and rom : (=, =, =, =) over link, and () L () L () L L () L () L rom to Link osts rom : (=, =, =) over link and rom : (=, =, =) over link and rom : (=, =, =, =) over link, and rom to Link osts locally L L L rom to Link osts Routing table of : locally L L L L () L () L () L L () L () L locally L L L L Page 7 Page 8 xample xample rom : (=, =, =, =, =) over linj, and rom : (=, =, =, =) over link and rom : (=, =, =, =, =) over link, and () L () L () L L () L () L rom to Link osts locally L L L L rom to Link osts locally L L L L rom : (=, =, =, =, =) over link, and rom : (=, =, =, =) over link and rom : (=, =, =, =, =) over link, and () L () L () L L () L () L rom to Link osts locally L L L L lgorithm terminates since, and create and send new vectors, but, and do not have to apply updates any longer. Page 9 Page

11 Successive Information Propagation onnection Loss Problem with this procedure: Information must be reliable Otherwise: hristmas eadlock router j announced U j = (,, ) onsequence: Nearly the entire traffic was led over j ollapse () L () L () L L () L () L rom to Link osts locally L L L L isadvantages of istance Vector Unreliable ormation is dangerous ycling load conditions dditional overhead nd: ormation propagation lasts a certain time! Router and notice the interruption e.g. by control packets Update of their own routing tables rom to Link osts locally L L L L = inite Page Page onnection Loss onnection Loss rom : (=, =, =, =, =) over link rom : (=, =, =, =, =) over link and () L () L () L L () L () L rom : (=, =, =, =, =) over link rom : (=, =, =, =, =) over link und rom to link costs locally L L L L receives the vector from and updates with =, =, =, =, = rom to Link osts locally L L L L () L () L () L L () L () L rom to link costs locally L L L L Page Page

12 onnection Loss onnection Loss rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link, and () L () L () L L () L () L rom to Link osts locally L L L L rom to Link osts locally L L L L rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link, and () L () L () L L () L () L rom to Link osts locally L L L L rom to Link osts locally L L L L Page Page onnection Loss onnection Loss rom : (=, =, =, =, =) over link rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link, and rom to Link osts locally L L L L rom to Link osts rom : (=, =, =, =, =) over link rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link and rom : (=, =, =, =, =) over link, and rom to Link osts locally L L L L () L () L () L L () L () L locally L L L L () L () L () L L () L () L lgorithm terminates, because, and create and send new vectors, but don t cause further updates in the routing tables. Page 7 Page 8

13 The ouncing ffect The ouncing ffect So far: osts of for each link Reality: ifferent costs per link xample: Link has costs of () L () L () L L () L () L ssumption : onnection breaks down. detects the failure and sets its costs to. () L () L () L L () L () L In the following, only the paths to router are examined. rom Link osts Temporarily, the following table results: rom Link osts In the case of stable conditions the tables of the other routers have these entries for : to L to L to locally to L to L ssumption : has sent its vector before. (in case of regular transmission) to L to L to locally to L to L Page 9 Page The ouncing ffect The ouncing ffect reports over link with costs of Router adds costs of to link for Value is lower than Table entry for is replaced by link and costs of Passing on the table to and Message comes over link resp., which and are using for Update of and () L () L () L L () L () L rom Link osts to L to L to locally to L to L Routing tables contain a loop! Packets from are bouncing between and sends its vector over link adds costs of to its own costs of ignores the message, because the costs are higher as before and send vectors Update of and () L () L () L L () L () L rom Link osts to L to L to locally to L to L Page Page

14 The ouncing ffect ount to Infinity fter several iterations, the following tables result: ntries depend however on random processes (e.g. on the order of the update messages, the arrival times of the vectors, losses of vectors etc.) rom Link osts to L to L to locally to L to L ount to Infinity problem: The istance Vector Routing achieves a correct solution, but possibly many (up to inite) update steps are necessary. xample: routers,,,,, are connected linear, the distance between neighbor routers in each case is Page Page ount to Infinity ount to Infinity xample : at first is switched off, then it is switched on xample : is switched off switched off to. Update switched on. Update. Update. Update. Update. Update. Update. Update 7. : : 8 : :. 7 : :. 8. : : Page Page

15 Split Horizon lgorithm Routing Information Protocol (RIP) irst solution approach: Split-Horizon lgorithm o not send the distance to X over that link used to transfer packets for X (resp. path is reported as ) However, it does not work always: Link is switched off announces and that is unreachable () announces however in the meantime to : has distance updates with (=, =) and calculates with distance sends out its update updates for with distance... ount to Infinity Router arly internal gateway routing protocol used in the Internet ases on the istance Vector Protocol RIP messages are sent every seconds as UP datagrams s metric for the evaluation of the paths the number of hops is used (The maximal possible number of hops is limited to!) In a message (only) up to entries of the routing table can be sent its good for small systems Problems: slow convergence (duration of minutes), ount to Infinity, no considering of subnets RIPv: subnets, authentication, multicast, however: the maximal possible number of hops is still limited to. s reaction to the restrictions of RIP, isco introduced the Internet Gateway Routing Protocol (IGRP): xtension of the metric, load sharing, more efficient packet format. The protocols did not become generally accepted, because they were isco-specific Replacement by a Link State Protocol (OSP) Page 7 Page 8 Information xchange Link State Routing.) xchange of global ormation locally (only with neighbors) istance Vector Routing.) Global exchange of local ormation Link State Routing Routers exchange Link State dvertisements (LS) very router determines its neighbors and their addresses (with so-called HLLO packets) measures the distances to the neighbors (with so-called HO packets) sends these ormation in a packet to all other routers (LS Link State dvertisement) computes on the basis of all the received LSs from other routers the shortest paths to the other routers (e.g. with the ijkstra algorithm) This is repeated regularly. Page 9 Page

16 Link State Routing Link State Routing H irst step: etermination of all neighbored routers Sending of a HLLO message on all links Routers at the other end answer with their identification If several routers are connected in a (broadcast) network, a new artificial node is introduced for simplification LN G N I G H I Second step: iscovery of link costs Transmission of HO messages Routers at the other end answer immediately (measurement of the delay) Inclusion of load leads to the choice of the lowest loaded link Side-effect in having two possible links: the less loaded link is loaded immediately heavily and the other link becomes free, with the next measurement the same happens for the other link, (cycling load) I G J H Page Page Link State Routing Packet uffer for Router Third step: reate link state messages ontain list of neighbors with appropriate link costs (elay, queue length, jitter etc.) Messages additionally contain sender identification, sequence number, age Seq.No. ge Seq.No. ge Seq.No. ge Seq.No. ge 7 8 Seq.No. ge 8 7 Seq.No. ge 7 8 ourth step: Sending of link state messages looding (problem: loops, duplicates, packet losses etc.) Sequence numbers are counted up, packets with outdated numbers (duplicates) are discarded very router reduces the age by one, with zero a packet is discarded ach router confirms the arrival of a link state packet to the sending router ata structure of router Transmission flags onfirmation flags Source Seq.No. ge 9 9 ata Page Page

17 Packet uffer for Router Link state message from arrives directly, therefore ge=, Seq.No.= Message is sent to and and confirmed for Message of must be transferred to and and be confirmed for Message of came twice (over and ) therefore only forwarding to, confirmation for and Transmission flags Source Seq.No. ge 9 9 onfirmation flags ata 8 7 Route nquiry ifth step: ecision on best routes Router collects link state ormation from all other routers path graph for the entire sub-network is determined Local execution of e.g. the ijkstra algorithm for the determination of the optimal route Results are written into the routing table Problems: With n routers and m neighbors, nm table entries are necessary In the case of router failures, the graphs of all routers are outdated xtremely susceptibly to attacks Page Page Routing Tables Hierarchical Routing Problem: extensive routing tables hierarchical routing Region Region Large tables are needing too much memory, PU time, transmission capacity for link state messages etc. virtual division of the network (Regions) Region Region Region Page 7 Page 8

18 Hierarchical Routing Internal Gateway Routing Protocol - OSP ull routing table for Hierarchical routing table for Open Shortest Path irst Region Region Region Region Region isadvantage: Possibly increasing path length estination Link to Hops - - estination Link to Hops standardized by IT (R 7) Open protocol (not manufacturer specific) Supports a multiplicity of metrics (distance, delay etc.) ynamic algorithm for fast adjustment to changing conditions in the network Load sharing between redundant links Supports hierarchical systems ontains security mechanisms to protect routers from wrong routing ormation or attacks Three types of connections are supported: Point-to-point links between routers roadcast networks (mostly LNs) Multi-access networks without broadcasting (e.g. packet switching WNs) Page 9 Page 7 OSP OSP The Internet is divided into autonomous systems (S) Very large autonomous systems are divided in areas ach autonomous system has a backbone, which connects all parts of the S very router, which belongs to two or more areas, is part of the backbone Within these areas every router has the same link state database and implements the same algorithm for determination of the shortest path router, which connect two areas, needs the link state databases from both areas OSP distinguishes four router classes (for reducing the extent of routing tables): Internal routers, which only belong to one area rea routers at the border of areas, which connect two or more areas ackbone routers, which are placed at the backbone S border routers, which mediate between several autonomous systems S S Internal router S S ackbone ackbone router rea xternal protocols connect the autonomous systems order router of the autonomous system rea router Relationship between autonomous systems, backbones and areas in OSP Page 7 Page 7

19 xternal Gateway Protocol - GP xternal Gateway Protocol - GP Internal gateway protocols are designed for efficiency: find the best way to the destination host. xternal gateway protocols must have to consider policies (political, economical, ) ssumption: uses G to reach Information sent to receives the following ormation about from its neighbors : GP - order Gateway Protocol n external routing protocol Variant of the istance Vector Protocol: not the costs of a transmission path are being monitored and exchanged, but the complete description of paths onsiders security and other rules (Routing Policies) ommunicates the neighbor routers the whole path which is to be used (deterministically) Needs reliable data exchange I G J H from : I use from G: I use G from I: I use IG from : I use G searches for the optimal route Paths of I and are directly discarded, because they cross and G are possible options pplication of Policies Routes, which violate policies, are being set to Page 7 Page 7 hoke Messages Routers can also exchange control ormation. y means of a so-called hoke Message a router can instruct other routers to send less data. xample: Router is overloaded and instructs to send fewer data. The message is sent back to the source. reduces its data amount. hoke Variant: each crossed router directly reduces its data amount. hoke hoke Page 7