CSC 4900 Computer Networks: Routing Protocols Professor Henry Carter Fall 2017
Last Time Link State (LS) versus Distance Vector (DV) algorithms: What are some of the differences? What is an AS? Why do they exist? 2
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 3
Hierarchical Routing Our routing study thus far - idealization all routers identical network flat not true in practice scale: with 600 million destinations: can t store all dest s in routing tables! administrative autonomy Internet = network of networks each network admin may want to control routing in its own network routing table exchange would swamp links! 4
Hierarchical Routing aggregate routers into regions, autonomous systems (AS) routers in same AS run same routing protocol Gateway router Direct link to router in another AS intra-as routing protocol routers in different AS can run different intra- AS routing protocol 5
Interconnected ASes Forwarding table is configured by both intra- and inter-as routing algorithm Intra-AS sets entries for internal dests Inter-AS & Intra-As sets entries for external dests 6
Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols: OSPF: Open Shortest Path First RIP: Routing Information Protocol IGRP/EIGRP: Interior Gateway Routing Protocol/ Enhanced IGRP (Cisco proprietary) 7
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 8
OSPF (Open Shortest Path First) open : publicly available Uses Link State algorithm 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) IS-IS routing protocol: nearly identical to OSPF 9
OSPF advanced features 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. 10
Hierarchical OSPF 11
Hierarchical OSPF Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. 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. 12
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 13
RIP (Routing Information Protocol) Distance vector algorithm distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec in response message (aka advertisement) each advertisement: list of up to 25 destination subnets (in IP addressing sense) 14
RIP: Example 15
RIP: Example 16
RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new 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) 17
RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated 18
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 19
Interconnected ASes Forwarding table is configured by both intra- and inter-as routing algorithm Intra-AS sets entries for internal dests Inter-AS & Intra-As sets entries for external dests 20
Inter-AS tasks Suppose router in AS1 receives datagram for which the dest is outside of AS1 Router should forward packet towards one of the gateway routers, but which one? AS1 needs: 1. to learn which dests are reachable through AS2 and which through AS3 2. to propagate this reachability info to all routers in AS1 Job of inter-as routing! 21
Example: Setting forwarding table in router 1d Suppose AS1 learns (via inter-as protocol) that subnet x is reachable via AS3 (gateway 1c) but not via AS2. Inter-AS protocol propagates reachability info to all internal routers. Router 1d determines from intra-as routing info that its interface I is on the least cost path to 1c. Puts in forwarding table entry (x,i). 22
Example: Choosing among multiple ASes Now suppose AS1 learns from the inter-as protocol that subnet x is reachable from AS3 and from AS2. To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. This is also the job on inter-as routing protocol! 23
Example: Choosing among multiple ASes Now suppose AS1 learns from the inter-as protocol that subnet x is reachable from AS3 and from AS2. To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. This is also the job on inter-as routing protocol! Hot potato routing: send packet towards closest of two routers. Learn from inter-as protocol that subnet x is reachable via multiple gateways Use routing info from intra-as protocol to determine costs of least-cost paths to each of the gateways Hot potato routing: Choose the gateway that has the smallest least cost Determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,i) in forwarding table 24
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 25
Internet inter-as routing: BGP BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to: 1. ebgp: Obtain subnet reachability information from neighboring ASs. 2. ibgp: Propagate reachability information to all AS-internal routers. 3. Determine good routes to subnets based on reachability information and policy. allows subnet to advertise its existence to rest of Internet: I am here 26
BGP basics BGP session: two BGP routers ( peers ) exchange BGP messages: advertising paths to different destination network prefixes ( path vector protocol) exchanged over semi-permanent TCP connections When AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix AS3 can aggregate prefixes in its advertisement 27
Distributing reachability info Using an ebgp session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use ibgp to distribute this new prefix reach info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-2a ebgp session When router learns of new prefix, creates entry for prefix in its forwarding table. 28
Path attributes & BGP routes When advertising a prefix, advert includes BGP attributes. prefix + attributes = route Two important attributes: AS-PATH: contains ASs through which prefix advertisement has passed: AS 67 AS 17 NEXT-HOP: Indicates specific internal-as router to next-hop AS. (There may be multiple links from current AS to next-hop-as.) When gateway router receives route advertisement, uses import policy to accept/decline. e.g., never route through AS x policy-based routing 29
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 30
BGP messages BGP messages exchanged using TCP. BGP messages: OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection 31
BGP routing policy A,B,C are provider networks X,W,Y are customer (of provider networks) X is multi-homed: attached to two networks X does not want to route from B via X to C... so X will not advertise to B a route to C 32
BGP routing policy (2) A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No way! B gets no revenue for routing CBAW since neither W nor C are B s customers B wants to force C to route to w via A B wants to route only to/from its customers! 33
Why different Intra- and Inter-AS routing? Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance 34
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 35
Software Defined Network Why a logically centralized control plane? easier network management: avoid router misconfigurations, greater flexibility of traffic flows table-based forwarding (recall OpenFlow API) allows programming routers centralized programming easier: compute tables centrally and distribute distributed programming : more difficult: compute tables as result of distributed algorithm (protocol) implemented in each and every router open (non-proprietary) implementation of control plane 36
SDN Perspective Control applications Implement network functionality SDN Controller Maintain state in a distributed database routing network-control applications access control load balance northbound API SDN Controller (network operating system) southbound API control plane Data plane switches Fast, simple, communicate with controller SDN-controlled switches data plane 37
Components of the controller Interface layer to network control apps: abstractions API Network-wide state management layer: state of networks links, switches, services: a distributed database communication layer: communicate between SDN controller and controlled switches routing network graph Network-wide distributed, robust state management Link-state info statistics OpenFlow access control RESTful API host info load balance Interface, abstractions for network control apps flow tables SNMP intent switch info Communication to/from controlled devices SDN controller 38
Example network graph statistics Link-state info Dijkstra s link-state Routing 3 4 5 2 OpenFlow RESTful API host info flow tables SNMP intent switch info 1 2 3 S1, experiencing link failure using OpenFlow port status message to notify controller SDN controller receives OpenFlow message, updates link status info Dijkstra s routing algorithm application has previously registered to be called when ever link status changes. It is called. s1 1 s3 s2 s4 4 Dijkstra s routing algorithm access network graph info, link state info in controller, computes new routes 39
Example network graph statistics Link-state info Dijkstra s link-state Routing 3 4 5 2 OpenFlow RESTful API host info flow tables SNMP intent switch info 5 6 link state routing app interacts with flow-tablecomputation component in SDN controller, which computes new flow tables needed Controller uses OpenFlow to install new tables in switches that need updating 1 s1 s3 s2 s4 40
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 41
ICMP: Internet Control Message Protocol used by hosts & routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer above IP: ICMP msgs carried in IP datagrams 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 42
Traceroute and ICMP Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0) When ICMP message arrives, source calculates RTT Traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host Destination returns ICMP host unreachable packet (type 3, code 3) When source gets this ICMP, stops. Message includes name of router& IP address 43
Smurf Attack ICMP Messages can be used in a classic amplification attack. An ICMP ping is sent to the broadcast address in a subnet (255.255.255.255) or network (192.168.1.255). All hosts receiving this message would automatically respond, thereby clogging the network. Only took one message to initiate. 44
Chapter 5: Network Layer 5.1 Introduction 5.2 Routing algorithms Link State Distance Vector 5.3 Intra-AS routing OSPF 5.4 BGP Hierarchical routing BGP protocol 5.5 SDN 5.6 ICMP 5.7 SNMP RIP 45
Network Management autonomous systems (aka network ): 1000s of interacting hardware/software components other complex systems requiring monitoring, control: jet airplane, nuclear power plant, others? "Network management includes the deployment, integration and coordination of the hardware, software, and human elements to monitor, test, poll, configure, analyze, evaluate, and control the network and element resources to meet the real-time, operational performance, and Quality of Service requirements at a reasonable cost." 46
Infrastructure definitions: managing entity managing entity network management protocol data agent data managed device agent data managed device agent data managed device managed devices contain managed objects whose data is gathered into a Management Information Base (MIB) agent data managed device agent data managed device 47
SNMP managing entity managing entity request response trap msg agent data agent data managed device managed device request/response mode trap mode 48
Message Types Message type GetRequest GetNextRequest GetBulkRequest InformRequest SetRequest Response Trap Function manager-to-agent: get me data (data instance, next data in list, block of data) manager-to-manager: here s MIB value manager-to-agent: set MIB value Agent-to-manager: value, response to Request Agent-to-manager: inform manager of exceptional event 49
Next Time... Textbook Chapter 6.1-6.3 Remember, you need to read it BEFORE you come to class! Homework HW #2 and Project #2 due next week! 50