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Transcript
Computer Networks Network layer (Part 2) 1 Network layer (part II) • Last class – Network layer functions • Addressing, security, fragmentation, delivery semantics, quality of service • In the middle of talking about routing…. • This class and beyond – Finish network layer functions (routing) – Specific implementations • IP (addressing, security, fragmentation, delivery semantics, quality of service, routing) • Network layer devices and implementations 2 NL: DBF (count-to-infinity example) Link cost changes: • good news travels fast • bad news travels slow - “count to infinity” problem! • see book for explanation of this example 60 X 4 Y 50 1 Z algorithm continues on! 3 NL: DBF: (count-to-infinity example) dest B C cost 1 2 dest cost 1 X A B A C 1 1 1 25 C dest cost A B 2 1 4 NL: DBF: (count-to-infinity example) C Sends Routes to B dest B C cost 1 2 dest cost A B A C ~ 1 1 25 C dest cost A B 2 1 5 NL: DBF: (count-to-infinity example) B Updates Distance to A dest B C cost 1 2 dest cost A B A C 3 1 1 25 C dest cost A B 2 1 6 NL: DBF: (count-to-infinity example) B Sends Routes to C dest B C dest cost cost 1 2 A B A C 3 1 1 25 C dest cost A B 4 1 7 NL: DBF: (count-to-infinity example) C Sends Routes to B dest B C cost 1 2 dest cost A B A C 5 1 1 25 C dest cost A B 4 1 8 NL: How are loops caused? • Observation 1: – B’s metric increases • Observation 2: – C picks B as next hop to A – But, the implicit path from C to A includes itself! 9 NL: Solutions to looping • Split horizon – Do not advertise route to X to an adjacent neighbor if your route to X goes through that neighbor – If C routes through B to get to A, C does not advertise (C=>A) route to B. • Poisoned reverse – Advertise an infinite distance route to X to an adjacent neighbor if your route to X goes through that neighbor – If C routes through B to get to A, C advertises to B that its distance to A is infinity • Works for two node loops – Does not work for loops with more nodes 10 NL: Split-horizon with poisoned 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? 60 X 4 Y 50 1 Z algorithm terminates 11 NL: Solutions to looping • Example Where Split Horizon Fails 1 A B 1 1 C 1 D • When link breaks, C marks D as unreachable and reports that to A and B • Suppose A learns it first – A now thinks best path to D is through B – A reports D unreachable to B and a route of cost=3 to C • C thinks D is reachable through A at cost 4 and reports that to B • B reports a cost 5 to A who reports new cost to C • etc... 12 NL: Solutions to looping • Path Holddown – If metric increases, delay propagating information – In our example, A and B delay changing and advertising new routes – A and B both set route to D to infinity after single step – Delay too large • Adversely affects convergence – Delay too small • Count-to-infinity more probable • Route poisoning – Advertise infinite cost when cost from next hop starts to increase 13 NL: Solutions to looping • Path vector – – – – – Select loop-free paths Each route advertisement carries entire path If a router sees itself in path, it rejects the route BGP does it this way Space proportional to diameter 14 NL: Solutions to looping • Do solutions completely eliminate loops? – No! Transient loops are still possible – Why? Because implicit path information may be stale – See this in BGP convergence • Only way to fix this – Ensure that you have up-to-date information by explicitly querying 15 NL: Link State vs. Distance Vector • Network bandwidth – DV • send everything you know to your neighbors • large messages, transfers only to neighbors – LS • send info about your neighbors to everyone • small messages, transfers O(nE) 16 NL: Link State vs. Distance Vector • Convergence speed: – LS • faster – don’t need to process LSPs before forwarding • single SPT calculation – DV • fast with triggered updates • count-to-infinity problem • Space requirements: – LS • maintains entire topology – DV • maintains only neighbor state • path vector maintains routes proportional to network diameter 17 NL: Link State vs. Distance Vector • Robustness: – LS can broadcast incorrect/corrupted LSP • Can be made robust since sources are aware of alternate paths – DV can advertise incorrect paths to all destinations • Incorrect calculation can spread to entire network 18 NL: DUAL • Distributed Update Algorithm – Garcia-Luna-Aceves 1989 – Goal: Avoid transient loops in DV and LS algorithms – 2 ideas • A path shorter than current path cannot contain a loop • Based on diffusing computation (Dijkstra-Scholten 1980) – Wait until computation completes before changing routes in response to a new update – Similar to path-holddown – 3 kinds of messages • Update, query, reply – 2 states for routers • Active (queries outstanding), passive 19 NL: DUAL On update if (lower cost) adopt else if (higher cost) { if (from next hop) { if (any path exists < old length from next hop) switch path else freeze route send query to all neighbors except next hop go into active wait for reply from all neighbors update route return to passive } send reply to all querying neighbors } 20 NL: Routing Hierarchies • Flat routing doesn’t scale – Each node/router cannot be expected to store routes to every destination (or destination network) – 50 million destinations – Route table exchange would swamp network • Key observation – Need less information with increasing distance to destination • Two radically different approaches for routing – The area hierarchy – The landmark hierarchy (discuss in routing alternatives) 21 NL: Areas • Divide network into areas – – – – Areas can have nested sub-areas No path between two sub-areas of an area can exit that area Within area, each node has routes to every other node Outside area • Each node has routes for other top-level areas only • Inter-area packets are routed to nearest appropriate border router • Can result in sub-optimal paths • Hierarchically address nodes in a network – Sequentially number top-level areas – Sub-areas of area are labeled relative to that area – Nodes are numbered relative to the smallest containing area 22 NL: Landmark Hierarchy • Details about things nearby and less information about things far away • Not defined by arbitrary boundaries – Thus, not well suited to the real world that does have administrative boundaries • Example: My apartment • MtHood.Portland.USBancorpTower.Oba.KearneyPlaza • From Beaverton – – – – Go towards Mt. Hood See USBancorpTower before running into Mt.Hood See Oba before running into USBancorpTower Reach Oba and route to Kearney Plaza 2 blocks away • From The Dalles – – – – Go towards Mt. Hood, reach it Go towards Portland, see USBancorpTower Go towards and reach USBancorpTower Go towards and reach Oba, route to Kearney Plaza 2 blocks away 23 NL: A Landmark 9 6 5 11 8 7 3 10 4 1 Router 1 is a landmark of radius 2 2 24 NL: Landmark Overview • Landmark routers have “height” which determines how far away they can be seen (visibility) – Routers within radius n can see a landmark router LMn • See means that those routers have LMn’s address and know next hop to reach it. – Router x as an entry for router y if x is within radius of y • Distance vector style routing with simple metric • Routing table: Landmark (LM2(d)), Level(2), Next hop 25 NL: LM Hierarchy Definition • Each LM i associated with level (i) and radius (ri) • Every node is an LM0 landmark • Recursion: some LMi are also LMi+1 – Every LMi sees at least one LMi+1 • Terminating state when all level j LMs are seen by entire network 26 NL: LM Self-configuration • Bottom-up hierarchy construction algorithm – Goal to bound number of children – Every router is L0 landmark – All Li landmarks run election to self-promote one or more Li+1 landmarks • LM level maps to radius (part of configuration), e.g.: – LM level 0: radius 2 – LM level 1: radius 4 – LM level 2: radius 8 • Dynamic algorithm to adapt to topology changes – Efficient hierarchy 27 NL: LM Addresses • LM(2).LM(1).LM(0) (C.B.A) • If destination is more than two hops away, will not have complete routing information, refer to LM(1) portion of address, if not known then refer to LM(2) LM0A aka C.B.A R0 R1 LM1B LM2C R2 28 NL: LM Routing • LM does not imply hierarchical forwarding • It is NOT a source route • En route to LM1,packet may encounter router that is within LM0 radius of destination address (like longest match) • Paths may be asymmetric 29 NL: Landmark Routing: Basic Idea • Source wants to reach LM0[a], whose address is c.b.a: •Source can see LM2[c], so sends packet towards c •Entering LM1[b] area, first router diverts packet to b •Entering LM0[a] area, packet delivered to a • Not shortest path • Packet may not reach landmarks LM0[a] r1[b] r0[a] LM1[b] LM2[c] r2[c] Network Node Path Landmark Radius 30 NL: Landmark Routing: Example d.d.f d.i.k d.i.g d.d.e d.i.i d.d.d d.d.a d.i.w d.d.j d.d.b d.i.v d.d.c d.d.l d.d.k d.i.u d.n.h d.n.x d.n.t d.n.n d.n.q d.n.s d.n.o d.n.p d.n.r 31 NL: Routing Table for Router g Landmark Level Next hop LM2[d] LM1[i] 2 1 f LM0[e] 0 f LM0[k] 0 k LM0[f] 0 f k d.d.f Router g d.i.k d.i.g d.d.e d.i.i d.d.d d.d.a d.i.w d.d.j r0 = 2, r1 = 4, r2 = 8 hops • How to go from d.i.g to d.n.t? g-f-e-d-u-t • How does path length compare to shortest path? g-k-I-u-t d.d.b d.d.c d.d.l d.d.k d.i.u d.n.h d.n.x Router t d.n.t d.n.n d.n.q d.n.s d.n.o d.n.p d.n.r 32 NL: Demux to upper layer • Sends payload to the next layer in protocol stack (usually transport layer) 33 NL: Error detection • Protection of data and/or header at the network layer – Provide extra protection on top of data-link layer and below transport layer – End-to-end principle • Is this necessary? 34 NL: Network layer functions summary • Each networking layer provides varying degrees of functionality including…. – security, delivery semantics, quality of service, demux to upper layer, error detection, fragmentation, addressing, routing • Next: IP network layer – – – – – – – – IP error detection IP demux to upper layer IP security IP fragmentation IP delivery semantics IP quality-of-service IP addressing IP routing 35 NL: The Internet Network layer (IP) 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 routing table ICMP protocol •error reporting •router “signaling” Link layer physical layer 36 NL: How is IP Design Standardized? • IETF – Voluntary organization – Meeting every 4 months – Working groups and email discussions • “We reject kings, presidents, and voting; we believe in rough consensus and running code” (Dave Clark 1992) – Need 2 independent, interoperable implementations for standard • IRTF – End2End – Reliable Multicast, etc.. 37 NL: IP datagram format (RFC 791) IP protocol version number (currently 4) header length (bytes) “type” of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to 32 bits type of ver head. len service length fragment 16-bit identifier flgs offset time to upper header layer live checksum total datagram length (bytes) for fragmentation/ reassembly 32 bit source IP address 32 bit destination IP address Options (if any) + padding data (variable length, typically a TCP or UDP segment) E.g. timestamp, record route taken, pecify list of routers to visit. 38 NL: IP header • Version – Currently at 4, next version 6 • Header length – Length of header (20 bytes plus options) • Type of Service – Typically ignored – Values • • • • 3 bits of precedence 1 bit of delay requirements 1 bit of throughput requirements 1 bit of reliability requirements – Replaced by DiffServ and ECN • Length – Length of IP fragment (payload) 39 NL: IP header (cont) • Identification – To match up with other fragments • Flags – Don’t fragment flag – More fragments flag • Fragment offset – Where this fragment lies in entire IP datagram – Measured in 8 octet units (11 bit field) 40 NL: IP header (cont) • Time to live – Ensure packets exit the network • Protocol – Demultiplexing to higher layer protocols • Header checksum – Ensures some degree of header integrity – Relatively weak – 16 bit • Source IP, Destination IP (32 bit addresses) • Options – E.g. Source routing, record route, etc. – Performance issues • Poorly supported 41 NL: IP error detection • IP checksum – IP has a header checksum, leaves data integrity to TCP/UDP – Catch errors within router or bridge that are not detected by link layer – Incrementally updated as routers change fields – http://www.rfc-editor.org/rfc/rfc1141.txt 42 NL: IP demux to upper layer • http://www.rfc-editor.org/rfc/rfc1700.txt – Protocol type field • • • • • • • • • • • • • 1 = ICMP 2 = IGMP 3 = GGP 4 = IP in IP 6 = TCP 8 = EGP 9 = IGP 17 = UDP 29 = ISO-TP4 80 = ISO-IP 88 = IGRP 89 = OSPFIGP 94 = IPIP http://www.rfc-editor.org/rfc/rfc2003.txt 43 NL: IP demux to upper layer • ICMP: Internet Control Message Protocol – Essentially a network-layer protocol for passing control messages – used by hosts, routers, gateways 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 • http://www.rfc-editor.org/rfc/rfc792.txt Type 0 3 3 3 3 3 3 4 Code 0 0 1 2 3 6 7 0 8 9 10 11 12 0 0 0 0 0 description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header 44 NL: IP security • IP originally had no provisions for security • IPsec – Retrofit IP network layer with encryption and authentication – http://www.rfc-editor.org/rfc/rfc2411.txt – If time permits, we may cover this at the end of the course….or someone should do a research paper on this. 45 NL: IP fragmentation (and reassembly) • network links have MTU (max.transfer size) - largest possible link-level frame. – different link types, different MTUs fragmentation: in: one large datagram out: 3 smaller datagrams • IP packets can be 64KB – – – – – – large IP datagram divided (“fragmented”) within net IP header on each fragment Intermediate router may fragment further as needed one datagram becomes several datagrams “reassembled” only at final destination IP header bits used to identify, order related fragments reassembly 46 NL: IP fragmentation length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =1480 length ID fragflag offset =1040 =x =0 =2960 47 NL: IP fragmentation • Path MTU Discovery in IP – http://www.rfc-editor.org/rfc/rfc1191.txt – Hosts dynamically discover minimum MTU of path – Algorithm: • Initialize MTU to MTU for first hop • Send datagrams with Don’t Fragment bit set • If ICMP “pkt too big” msg, decrease MTU – What happens if path changes? • Periodically (>5mins, or >1min after previous increase), increase MTU – Some routers will return proper MTU – MTU values cached in routing table – C. Shannon, D. Moore, k claffy, “Characteristics of Fragmented IP Traffic on Internet Links” ACM SIGCOMM Internet Measurement Workshop 2001. 48 NL: IP delivery semantics • The waist of the hourglass – Unreliable datagram service – Out-of-order delivery possible – Compare to ATM and phone network… • Unicast mostly – IP broadcast not forwarded – IP multicast supported, but not widely used • If there is time, we will talk about IP multicast…. 49 NL: IP quality of service • IP originally had “type-of-service” (TOS) field to eventually support quality – Not used, ignored by most routers • Then came int-serv (integrated services) and RSVP signalling – Per-flow quality of service through end-to-end support • Setup and match flows on connection ID • Per-flow signaling • Per-flow network resource allocation (*FQ, *RR scheduling algorithms) 50 NL: IP quality of service • RSVP – – – – http://www.rfc-editor.org/rfc/rfc2205.txt Provides end-to-end signaling to network elements General purpose protocol for signaling information Not used now on a per-flow basis to support int-serv, but being reused for diff-serv. • int-serv – Defines service model (guaranteed, controlled-load) • http://www.rfc-editor.org/rfc/rfc2210.txt • http://www.rfc-editor.org/rfc/rfc2211.txt • http://www.rfc-editor.org/rfc/rfc2212.txt – Dozens of scheduling algorithms to support these services • WFQ, W2FQ, STFQ, Virtual Clock, DRR, etc. • If this class was being given 5 years ago…. 51 NL: IP quality of service • Why did RSVP, int-serv fail? – Complexity • Scheduling • Routing • Per-flow signaling overhead – Lack of scalability • Per-flow state • Route pinning – Economics • Providers with no incentive to deploy • SLA, end-to-end billing issues – QoS a weak-link property • Requires every device on an end-to-end basis to support flow 52 NL: IP quality of service • Now it’s diff-serv… – – – – – – – Use the “type-of-service” bits as a priority marking http://www.rfc-editor.org/rfc/rfc2474.txt http://www.rfc-editor.org/rfc/rfc2475.txt http://www.rfc-editor.org/rfc/rfc2597.txt http://www.rfc-editor.org/rfc/rfc2598.txt Core network relatively stateless AF • Assured forwarding (drop precedence) – EF • Expedited forwarding (strict priority handling) – If there is time, we may cover IP quality of service more completely at the end of the class…. 53 NL: IP Addressing • IP address: fixed-length, 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 interface, not host, router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 223.1.3.2 223.1.3.1 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 54 NL: IP Addressing • IP address: – network part (high order bits) – host part (low order bits) • What’s a network ? (from IP address perspective) – device interfaces with same network part of IP address – can physically reach each other without intervening router 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.2.9 223.1.3.27 223.1.2.2 LAN 223.1.3.1 223.1.3.2 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) 55 NL: IP Addressing 223.1.1.1 223.1.1.2 223.1.1.4 How to find the networks? 223.1.1.3 • Detach each interface 223.1.7.0 223.1.9.2 from router, host • create “islands of isolated networks 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 Interconnected system consisting of six networks 223.1.2.1 223.1.3.27 223.1.2.2 223.1.3.1 223.1.3.2 56 NL: Initial IP Addressing (1981) • Classful structure (1981) • Total IP address size: 4 billion – Class A: 128 networks, 16M hosts – Class B: 16K networks, 64K hosts – Class C: 2M networks, 256 hosts High Order Bits 0 10 110 Format 7 bits of net, 24 bits of host 14 bits of net, 16 bits of host 21 bits of net, 8 bits of host Class A B C 57 NL: IP address classes 8 16 Class A 0 Network ID 24 32 Host ID 1.0.0.0 to 127.255.255.255 Class B 10 Network ID Host ID 128.0.0.0 to 191.255.255.255 Class C 110 Network ID Host ID 192.0.0.0 to 223.255.255.255 Class D 1110 Multicast Addresses 224.0.0.0 to 239.255.255.255 Class E 1111 Reserved for experiments 58 NL: Special IP Addresses • 127.0.0.1: local host (a.k.a. the loopback address) • Private addresses – – – – http://www.rfc-editor.org/rfc/rfc1918.txt Class A: 10.0.0.0 - 10.255.255.255 (10/8 prefix) Class B: 172.16.0.0 - 172.31.255.255 (172.16/12 prefix) Class C: 192.168.0.0 - 192.168.255.255 (192.168/16 prefix) • 255.255.255.255 – IP broadcast to local hardware that must not be forwarded – http://www.rfc-editor.org/rfc/rfc919.txt – Same as network broadcast if no subnetting • IP of network broadcast=NetworkID+(all 1’s for HostID) • 0.0.0.0 – IP address of unassigned host (BOOTP, ARP, DHCP) 59 NL: IP Addressing Problem #1 (1984) • Subnet addressing – http://www.rfc-editor.org/rfc/rfc917.txt – Address problem of inefficient use of address space – For class B & C networks • Class A (rarely given out, not many of them given out by IANA) • Class B = 64k hosts – Very few LANs have close to 64K hosts – Electrical/LAN limitations, performance or administrative reasons – e.g., class B net allocated enough addresses for 64K hosts, even if only 2K hosts in that network • Class C = 256 hosts – Need simple way to get multiple “networks” • Use bridging, multiple IP networks or split up single network address ranges (subnet) • Reduce the total number of addresses that are assigned, but not used 60 NL: Subnetting • Variable length subnet masks – Subnet a class B address space into several chunks Network Host Network Subnet 1111.. ..1111 Host 00000000 Mask 61 NL: Subnetting Example • Assume an organization was assigned address 150.100 • Assume < 100 hosts per subnet • How many host bits do we need? – Seven • What is the network mask? – 11111111 11111111 11111111 10000000 – 255.255.255.128 62 NL: IP Address Problem #2 (1991) • Address space depletion – In danger of running out of classes A and B – Class A • Very few in number • IANA frugal in giving them out – Class B • Subnetting only applied to new allocations • sparsely populated • people refuse to give it back – Class C too small for most domains 63 NL: Some Problems • Solution – Assign multiple consecutive class C address blocks – Supernetting • http://www.rfc-editor.org/rfc/rfc1338.txt – Later known as Classless Inter-Domain Routing (CIDR) • http://www.rfc-editor.org/rfc/rfc1518.txt • http://www.rfc-editor.org/rfc/rfc1519.txt 64 NL: IP addressing: CIDR • Original classful addressing – Use class structure (A, B, C) to determine network ID for route lookup • CIDR: Classless InterDomain Routing – Do not use classes to determine network ID – network portion of address of arbitrary length – address format: a.b.c.d/x, where x is # bits in network portion of address network part host part 11001000 00010111 00010000 00000000 200.23.16.0/23 65 NL: CIDR • Assign any range of addresses to network – Use common part of address as network number – e.g., addresses 192.4.16.* to 192.4.31.* have the first 20 bits in common. Thus, we use this as the network number – netmask is /20, /xx is valid for almost any xx – 192.4.16.0/20 • Enables more efficient usage of address space (and router tables) • More on how this impacts routing later…. 66 NL: IP addressing: How are they allocated? • Hosts (host portion): – From organization via static configuration or DHCP • Network (network portion) – Organization gets of ISP’s assigned address space – ISP gets assigned address space from ICANN ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 ... 11001000 00010111 00010100 00000000 ….. …. 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/2367 NL: IP addressing Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers – allocates addresses – manages DNS – assigns domain names, resolves disputes 68