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Customer-Provider Routing Relationships Advertises to its neighbors that it has no paths to any other destinations except itself The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Customer: Stub AS: small corporation e.g. w, y Customer: Multihomed AS: large corporation (no transit) e.g. x Provider: Transit AS: backbone provider networks e.g. A, B, C Group of routers B w All traffic entering must be destined for w, all traffic leaving must have originated from w x A C y Stub AS must be prevented from forwarding traffic between Transit ASs 1 using Selective Route Advertisement Policy Network Layer – part 3 Routing in the Internet Two-level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard Border Gateway Protocol (BGP4) Allows each subnet to advertise its existence to the rest of the Internet de facto standard inter-AS routing protocol in today’s Internet provides each AS a means to: • obtain subnet reachability information (i.e. via one of its neighboring AS) • propagate the reachability information to all routers internal to the AS • determine “good” routes to subnets based on the reachability information and on AS policy. Network Layer – part 3 2 Internet AS Hierarchy AS border (exterior gateway) routers AS interior (gateway) routers Network Layer – part 3 3 Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common IGPs: RIP: Routing Information Protocol (lower-tier ISPs and Enterprise networks) OSPF: Open Shortest Path First (upper-tier ISPs) IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Network Layer – part 3 4 RIP ( Routing Information Protocol) Distance vector algorithm Included in (Berkeley Software Distribution) BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) = (AS < 15 hops in diameter) Can you guess why? Hop – no. of subnets traversed along the shortest path from Source Router to Destination Subnet, including the Destination Subnet. Distance vectors: exchange routing updates via Response Message (also called advertisement) every 30 sec Each advertisement: route to up to 25 destination subnets within the AS, including the sender’s distance from each of them Network Layer – part 3 5 Example RIP (Routing Information Protocol) z w A x D y B subnet C Routing table in Router D Destination Subnet Next Router ... w y z x … A B B -... Num. of hops to dest. 2 2 7 1 .... Network Layer – part 3 6 Example RIP (Routing Information Protocol) z w A x D C … y B Routing table in Router D Destination Subnet Next Router Num. of hops to dest. w y z x A B B -- 2 2 7 1 ... ... .... Router A has a shorter path to Z! (30 secs. later.. D receives an advertisement from Router A ) Destination Subnet Next Router Num. of hops to dest. z w x C --- 4 1 1 ... ... .... Network Layer – part 3 7 Example RIP (Routing Information Protocol) z w A x D C … y B Routing table in Router D Destination Subnet Next Router Num. of hops to dest. w y z x A B A -- 2 2 4 1 ... ... .... Advertisement from Router A Destination Subnet Next Router Num. of hops to dest. z w x C --- 4 1 1 ... ... .... Router D updates its entry for destination Z Network Layer – part 3 8 Example RIP: Link Failure and Recovery If no advertisement is heard after 180 sec --> the neighbour/link is declared dead Modifies routing table - routes via neighbor invalidated new advertisements sent to neighbors neighbours in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net poisoned reverse used to prevent ping-pong loops (infinite distance = 16 hops) Network Layer – part 3 9 Routing Info Protocol (RIP) Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated Able to manipulate routing tables within the UNIX kernel via UDP, port 520 Network Layer – part 3 10 OSPF (Open Shortest Path First) “Open” means publicly available Uses Link-State algorithm Broadcasts information to LS packet dissemination all not just neighboring routers 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 with upper-layer protocol of 89 OSPF Protocol Functionalities: reliable data transfer, link-state broadcast, check for Network Layer – part 3 11 links operability, extraction of neighboring router’s database of network-wide link state OSPF advanced features (not in RIP) Allow only trusted routers Security: all OSPF messages authenticated (to prevent malicious intrusion) Multiple same-cost paths allowed (only one path in RIP) Integrated uni- and multicast routing support: Multicast OSPF (MOSPF) uses same topology data base as OSPF Hierarchical OSPF in large domains. Most significant advancement! Has the ability to structure an autonomous system hierarchically Network Layer – part 3 12 Hierarchical Open Shortest Path First (OSPF) Network Layer – part 3 13 Hierarchical OSPF Two-level hierarchy: local area, backbone. Link-state advertisements are sent only within an area each node has detailed area topology; only know direction (shortest path) to nets in other areas. Each area runs its own OSPF link-state routing algorithm Area border routers: responsible for routing packets outside the area. Backbone routers: run OSPF routing limited to backbone. Boundary routers: connect to other ASs. Network Layer – part 3 14 IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Uses the Distance Vector algorithm, like RIP several cost metrics (delay, bandwidth, reliability, load, etc.) uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation Network Layer – part 3 15 Router Architecture Overview Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) switching datagrams from incoming to outgoing link Physical layer functions Data link layer functions Lookup & forwarding functions computes routing tables, performs Network management functions Network Layer – part 3 16 Input Port Functions Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5 Decentralized switching: given datagram dest., lookup output port using routing table in input port memory goal: complete input port processing at 'line speed' queuing: happens if datagrams arrive faster than forwarding rate into switch fabric Network Layer – part 3 17 Input Port Queuing Slot for Green packet is free, but there is HOL blocking, so Green packet will have to wait 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 queueing delay and loss due to input buffer overflow! Network Layer – part 3 18 Three types of switching fabrics Like shared memory multiprocessors No routing processor; 1 packet at a time 2n buses that connect n input ports to n output ports Network Layer – part 3 19 Switching Via Memory First generation routers: packet copied by system's (single) CPU speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Workstation’s Memory Output Port System Bus Modern routers: input port processor performs lookup, copy into memory Cisco Catalyst 8500 Network Layer – part 3 20 Switching Via Bus 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) Network Layer – part 3 21 Switching Via An Interconnection Network overcome bus bandwidth limitations Banyan networks, other interconnection nets initially developed to connect processors in multiprocessor Other Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the interconnection network Network Layer – part 3 22 Output Ports Buffering required when datagrams arrive from the fabric faster than the transmission rate Scheduling discipline chooses among queued datagrams for transmission Network Layer – part 3 23 It is more advantageous to mark a packet before the buffer is full in order to provide a congestion signal to the sender Output port queueing buffering when arrival rate via switch exceeeds ouput line speed queueing (delay) and loss due to output port buffer overflow! Network Layer – part 3 24 END OF SESSION Network Layer – part 3 25 IPv6 Initial motivation: 32-bit address space completely allocated by 2008. Additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS new anycast address: route to best of several replicated servers IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed Network Layer – part 3 26 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 Network Layer – part 3 27 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 Network Layer – part 3 28 Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneously no flag days How will the network operate with mixed IPv4 and IPv6 routers? Two proposed approaches: Dual Stack: some routers with dual stack (v6, v4) can translate between formats Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers Network Layer – part 3 29 Dual Stack Approach Network Layer – part 3 30 Tunneling IPv6 inside IPv4 where needed Network Layer – part 3 31