Telecomunicazioni. Docente: Andrea Baiocchi. DIET - Stanza 107, 1 piano palazzina P. Piga Via Eudossiana 18
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1 University of Roma La Sapiena Telecomunicaioni Docente: Andrea Baiocchi DIET - Stana 07, piano palaina P. Piga Via Eudossiana 8 andrea.baiocchi@uniroma.it Corso di Laurea in Ingegneria Gestionale A.A. 03/04 Computers Computers make it easier to do a lot of things, but most of the things they make it easier to do don't need to be done. [Andy Rooney]
2 Programma. SERVIZI E RETI DI TELECOMUNICAZIONE. ARCHITETTURE DI COMUNICAZIONE 3. MODI DI TRASFERIMENTO 4. FONDAMENTI DI COMUNICAZIONI 5. LO STRATO DI COLLEGAMENTO 6. ACCESSO MULTIPLO 7. LO STRATO DI RETE IN INTERNET 8. LO STRATO DI TRASPORTO IN INTERNET Part of the slides are adapted from companion material of Chapter 4 (Network layer) of the book: Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July 007.
3 Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP ICMP: Internet Control Message Protocol! Used by hosts & routers to communicate networklevel control information! error reporting: unreachable host, network, port, protocol! echo request/reply (used by ping)! ICMP msgs are 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 dest host unreachable 3 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 0 0 router discovery 0 TTL expired 0 bad IP header
4 ICMP approach! ICMP aim is to notify mulfunctioning to the host originating the packet that triggered malfunctioning detection! It does not specify ensuing actions! It does not locate the source of the problem! e.g. intermediate system between packet origin and system detecting the error and issuing the ICMP msg! Each notification ICMP message is related to a specific IP packet
5 Traceroute! Source sends series of UDP segments to dest! First has TTL =, second has TTL=, etc.! Unlikely port number! When nth datagram arrives to nth router:! Router discards datagram and sends to source an ICMP message (type, code 0)! Message includes name of router & IP address! 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. From my laptop, via IPsec, to US
6 From my laptop, via LAN to the server next building traceroute to ( ), 64 hops max, 40 byte packets ms.58 ms.88 ms ms 4.58 ms 4.03 ms ms 4.66 ms ms ms 4.96 ms ms ms 4.3 ms 4.7 ms ms 4.6 ms.356 ms ms 9.44 ms 7.99 ms ms 8.5 ms 8.39 ms ms.5 ms.65 ms PDU e bit! Trama di livello (Ethernet) che contiene un pacchetto IP, catturata da Wireshark! E una sequena di bit!!!! Caratteri hex per comodità ( hex = 4 bit) 00d9d8d734009df7bb c eaec0a8cd59c0a8cd008004d a6b6c6d6e6f Header Ethernet Payload Ethernet Header IP Payload IP 00d9d8d df7bb 0800=IP c0a8cd59 c0a8cd008 0=ICMP Header Payload ping request ICMP ICMP MAC MAC Protocol typeprotocol Indirio Indirio typeicmp IP IPType Destinaione Sorgente Sorgente Destinaione
7 Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP Interplay between routing, forwarding routing algorithm local forwarding table header value output link value in arriving packet s header 0 3
8 ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B s IP address? F7-B LAN A-F-BB AD D7-FA-0-B0! Each IP node (host, router) on LAN has ARP table! ARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL>! TTL (Time To Live): time after which address mapping will be forgotten (typically 0 min) C-C4--6F-E3-98 ARP cache (ARP table)! Updated each time an ARP request or ARP reply is read from the broadcast medium! Gratuitous ARP
9 ARP protocol: Same LAN (network)! A wants to send datagram to B, and B s MAC address not in A s ARP table.! A broadcasts ARP query packet, containing B's IP address! dest MAC address = FF-FF- FF-FF-FF-FF! all machines on LAN receive ARP query! B receives ARP packet, replies to A with its (B's) MAC address! frame sent to A s MAC address (unicast)! A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)! soft state: information that times out (goes away) unless refreshed! ARP is plug-and-play :! nodes create their ARP tables without intervention from net administrator Addressing: routing to another LAN walkthrough: send datagram from A to B via R assume A knows B s IP address C-E8-FF-55 A... E6-E BB-4B A-3-F9-CD-06-9B 88-B-F-54-A-0F CC-49-DE-D0-AB-7D R... B 49-BD-D-C7-56-A! two ARP tables in router R, one for each subnet (LAN)
10 ! A creates IP datagram with source A, destination B! A uses ARP to get R s MAC address for...0! A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram This is a really important! A s NIC sends frame example make sure you! R s NIC receives frame understand!! R removes IP datagram from Ethernet frame, sees its destined to B! R uses ARP to get B s MAC address! R creates frame containing A-to-B IP datagram sends to B C-E8-FF-55 A... E6-E BB-4B A-3-F9-CD-06-9B 88-B-F-54-A-0F CC-49-DE-D0-AB-7D R... B 49-BD-D-C7-56-A Graph abstraction 5 u v x 3 3 w y 5 Graph: G = (N,E) = (NODES, EDGES) N = set of routers = { u, v, w, x, y, } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,), (y,) } Remark: Graph abstraction is useful in other network contexts Example: PP, where N is set of peers and E is set of TCP connections; social networks where nodes are users and edges exist iff two users have a direct contact
11 Graph abstraction: costs 5 c(x,x ) = cost of link (x,x ) u v x 3 3 w y 5 - e.g., c(w,) = 5 cost could always be, or proportionl to link fee or inversely related to bandwidth, or inversely related to congestion Cost of path (x, x, x 3,, x p ) = c(x,x )+c(x,x 3 )+ +c(x p-,x p ) Question: What s the least-cost path between u and? Routing algorithm: algorithm that finds least-cost path Routing Algorithm classification Global or decentralied information? Global: ( link state algorithms)! all routers have complete topology, link cost info Decentralied:( distance vector algorithms)! router knows physically-connected neighbors, link costs to neighbors! iterative process of computation, exchange of info with neighbors Static or dynamic? Static: management plane! routes change only on network (re)-configuration Dynamic: control plane! routes change more quickly! periodic update! in response to link cost changes
12 Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP 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! 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 destinations 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
13 Dijsktra s Algorithm Initialiation: N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) and p(v)=u 6 else D(v) =! 8 Loop 9 find w not in N' such that D(w) is a minimum 0 add w to N' update D(v) for all v adjacent to w and not in N' : D(v) = min{ D(v), D(w) + c(w,v) ) 3 if D(w)+c(w,v) < D(v) then p(v)=w 4 /* new cost to v is either old cost to v or known 4 shortest path cost to w plus cost from w to v */ 5 until all nodes in N' Dijkstra s algorithm: example Step N' u ux uxy uxyv uxyvw uxyvw D(v),p(v),u,u,u D(w),p(w) 5,u 4,x 3,y 3,y D(x),p(x),u D(y),p(y)!,x D(),p()!! 4,y 4,y 4,y 5 u v x 3 3 w y 5
14 Dijkstra s algorithm: example () Resulting shortest-path tree from u: v w u x y Resulting forwarding table in u: destination v x y w link (u,v) (u,x) (u,x) (u,x) (u,x) Dijkstra s algorithm, discussion Algorithm complexity: n nodes! each iteration: need to check all nodes, w, not in N! n(n+)/ comparisons: O(n )! more efficient implementations possible: O(n logn) Oscillations possible:! e.g., link cost = amount of carried traffic A +e D 0 0 B 0 e C e initially +e A 0 D B 0 +e 0 C recompute routing 0 A +e D 0 0 B C +e recompute +e A 0 D B 0 +e 0 C recompute
15 Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP 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) } v where min is taken over all neighbors v of x
16 Bellman-Ford example u 5 Assume we know that d v () = 5, d x () = 3, d w () = 3 v x 3 3 w y 5 B-F equation says: d u () = min { c(u,v) + d v (), c(u,x) + d x (), c(u,w) + d w () } = min { + 5, + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path!"forwarding table Distance Vector Algorithm! D x (y) = estimate of least cost from x to y! Node x knows cost to each neighbor v: c(x,v)! Node x maintains distance vector DV D x = [D x (y): y " N ]! Node x also maintains its neighbors distance vectors: for each neighbor v, x maintains D v = [D v (y): y " N ]
17 Distance vector algorithm (4) Basic idea:! From time-to-time, each node sends its own distance vector estimate to neighbors! Asynchronous update! When 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! Neighbor v* attaining minimum for destination y is next hop to y! Under minor, natural conditions, the estimate D x (y) converge to the actual least cost d x (y) Distance Vector Algorithm (5) Iterative, asynchronous:! Each local iteration caused by:! local link cost change! DV update message from neighbor Distributed:! each node notifies neighbors only when its DV changes! neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors
18 D x (y) = min{c(x,y) + D y (y), c(x,) + D (y)} = min{+0, 7+} = node x table cost to cost to x y x y x 0 7 y!!!!!! node y table cost to x y from from from x y!!! 0!!! node table cost to x y x!!! y!!! 7 0 from x y time D x () = min{c(x,y) + D y (), c(x,) + D ()} = min{+, 7+0} = 3 x y 7 D x (y) = min{c(x,y) + D y (y), c(x,) + D (y)} = min{+0, 7+} = node x table cost to cost to cost to x y x y x y x 0 7 x 0 3 x 0 3 y!!! y 0 y 0!!! node y table cost to cost to cost to x y x y x y x!!! x 0 7 x 0 3 y 0 y 0 y 0!!! from from node table cost to x y from x!!! y!!! 7 0 from from from cost to x y x y from from from x y cost to x y time D x () = min{c(x,y) + D y (), c(x,) + D ()} = min{+, 7+0} = 3 x y 7
19 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 x 4 y 50 good news travels fast At time t 0, y detects the link-cost change, updates its DV, and informs its neighbors. At time t, 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, y receives s update and updates its distance table. y s least costs do not change and hence y does not send any message to. Distance Vector: link cost changes Link cost changes:! good news travels fast! bad news travels slow - count to infinity problem!! 44 iterations before algorithm stabilies: see text 60 x 4 y 50 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?
20 Comparison of LS and DV algorithms Message complexity! LS: with n nodes, E links, O(nE) msgs sent! DV: exchange between neighbors only! convergence time varies Speed of Convergence! LS: O(n ) algorithm requires O(nE) msgs! may have oscillations! DV: convergence time varies! may be routing loops! count-to-infinity problem Robustness: what happens if router malfunctions? LS: DV:! node can advertise incorrect link cost! each node computes only its own table! DV node can advertise incorrect path cost! each node s table used by others! error propagate thru network Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP
21 Intra-AS Routing! also known as Interior Gateway Protocols (IGP)! most common Intra-AS routing protocols:! RIP: Routing Information Protocol! OSPF: Open Shortest Path First! IGRP: Interior Gateway Routing Protocol (Cisco proprietary) RIP (Routing Information Protocol)! distance vector algorithm! included in BSD-UNIX Distribution in 98! distance metric: # of hops (max = 5 hops) From router A to subnets: u A C B D v y w x destination hops u v w x 3 y 3
22 RIP advertisements! distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)! each advertisement: list of up to 5 destination subnets within AS RIP: Example w x y A D B C Destination Network Next Router Num. of hops to dest. w A y B B 7 x Routing/Forwarding table in D
23 RIP: Example Dest Next hops w - x - C Advertisement from A to D w x y A D B C Destination Network Next Router Num. of hops to dest. w A y B B A 7 5 x Routing/Forwarding table in D RIP: Link Failure and Recovery If no advertisement heard after 80 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 = 6 hops)
24 RIP Table processing! RIP routing tables managed by application-level process called route-d (daemon)! advertisements sent in UDP packets, periodically repeated route-d route-d Transport (UDP) Transport (UDP) network (IP) forwarding table forwarding table network (IP) link link physical physical Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP
25 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 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:! Multicast OSPF (MOSPF) uses same topology data base as OSPF! hierarchical OSPF in large domains.
26 Hierarchical OSPF 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: summarie 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.
27 Outline! 4. Internet architecture! 4. What s inside a router! 4.3 Internet Protocol (IP)! Datagram format! IPv4 addressing! ICMP! 4.4 Routing algorithms! Link state! Distance Vector! 4.5 Routing in the Internet! RIP! OSPF! BGP Internet inter-as routing: BGP! BGP (Border Gateway Protocol): the de facto standard! BGP provides each AS a means to:. Obtain subnet reachability information from neighboring ASs.. Propagate reachability information to all ASinternal 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
28 BGP basics! pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions! BGP sessions need not correspond to physical links.! when AS advertises a prefix to AS:! AS promises it will forward datagrams towards that prefix.! AS can aggregate prefixes in its advertisement 3c 3a 3b AS3 a AS c d ebgp session ibgp session b a c b AS Distributing reachability info! using ebgp session between 3a and c, AS3 sends prefix reachability info to AS.! c can then use ibgp to distribute new prefix info to all routers in AS! b can then re-advertise new reachability info to AS over b-to-a ebgp session! when router learns of new prefix, it creates entry for prefix in its forwarding table. 3c 3a 3b AS3 a AS c b d ebgp session ibgp session a c b AS
29 Path attributes & BGP routes! advertised prefix includes BGP attributes.! prefix + attributes = route! two important attributes:! AS-PATH: contains ASs through which prefix advertisement has passed: e.g, AS 67, AS 7! NEXT-HOP: indicates specific internal-as router to next-hop AS. (may be multiple links from current AS to next-hop-as)! when gateway router receives route advertisement, uses import policy to accept/decline. BGP route selection! router may learn about more than route to some prefix. Router must select route.! elimination rules:. local preference value attribute: policy decision. 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 BGP routing policy (/) W A B C X legend: provider network customer network: Y! A,B,C are provider networks! X,W,Y are customer (of provider networks)! X is dual-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
31 BGP routing policy (/) W A B C X legend: provider network customer network:! A advertises path AW to B! B advertises path BAW to X! Should B advertise path BAW to C?! 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! Y 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 sie, reduced update traffic Performance:! Intra-AS: can focus on performance! Inter-AS: policy may dominate over performance
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