Telecomunicazioni. Docente: Andrea Baiocchi DIET - Stanza 107, 1 piano palazzina P. Piga Via Eudossiana 18

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1 University of Roma La Sapienza Telecomunicazioni Docente: Andrea Baiocchi DIET - Stanza 07, piano palazzina P. Piga Via Eudossiana 8 andrea.baiocchi@uniroma.it Corso di Laurea in Ingegneria Gestionale a.a. 206/207 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 2. ARCHITETTURE DI COMUNICAZIONE 3. MODI DI TRASFERIMENTO 4. FONDAMENTI DI COMUNICAZIONI 5. MULTIPLAZIONE E ACCESSO MULTIPLO 6. LO STRATO DI COLLEGAMENTO 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 2007.

3 Outline 4. Internet architecture 4.2 What s inside a router 4.3 Internet Protocol (IP) Datagram format IPv4 addressing ICMP 4.4 Routing algorithms Link state 4.5 Routing in the Internet OSPF BGP ICMP: Internet Control Message Protocol Used by hosts & routers to communicate network-level 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 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 0 0 router discovery 0 TTL expired 2 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=2, etc. Unlikely port number When n-th datagram arrives to n-th 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 Roma, Italy, via IPsec, to US

6 From my laptop, via LAN to the server next building traceroute to ( ), 64 hops max, 40 byte packets ms.528 ms.882 ms ms 4.58 ms ms ms 4.66 ms ms ms 4.96 ms ms ms 4.23 ms ms ms ms.356 ms ms 9.44 ms ms ms 8.25 ms ms ms.5 ms 2.65 ms PDU e bit Trama di livello 2 (Ethernet) che contiene un pacchetto IP, catturata da Wireshark E una sequenza di bit!!! Caratteri hex per comodità ( hex = 4 bit) 002d9d8d df27bb c eae2c0a8cd59c0a8cd008004d a6b6c6d6e6f Header Ethernet Payload Ethernet Header Payload IP IP c0a8cd59 c0a8cd0 Header d9d8d df27bb 0800=IP 0=ICMP Payload ICMP ping request ICMP MAC MAC Protocol typeprotocol Indirizzo Indirizzo typeicmp IP IP Type Destinazione Sorgente Sorgente Destinazione

7 Outline 4. Internet architecture 4.2 What s inside a router 4.3 Internet Protocol (IP) Datagram format IPv4 addressing ICMP 4.4 Routing algorithms Link state 4.5 Routing in the Internet OSPF BGP Interplay between routing, forwarding routing algorithm local forwarding table header value output link value in arriving packet s header 0 3 2

8 ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B s IP address? F7-2B LAN A-2F-BB AD D7-FA-20-B0 0C-C4--6F-E3-98 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 20 min) 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-23-F9-CD-06-9B 88-B2-2F-54-A-0F CC-49-DE-D0-AB-7D R B 49-BD-D2-C7-56-2A 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 understand! R s NIC receives frame 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-23-F9-CD-06-9B 88-B2-2F-54-A-0F CC-49-DE-D0-AB-7D R B 49-BD-D2-C7-56-2A Graph abstraction 5 2 u v 2 x 3 3 w y 5 2 z Graph: G = (N,E) = (NODES, EDGES) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Remark: Graph abstraction is useful in other network contexts Example: P2P, 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 u 2 5 v x w y 5 2 z c(x,x ) = cost of link (x,x ) - e.g., c(w,z) = 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 2, x 3,, x p ) = c(x,x 2 )+c(x 2,x 3 )+ +c(x p-,x p ) Question: What s the least-cost path between u and z? Routing algorithm: algorithm that finds least-cost path Routing Algorithm classification Global: link state algorithm (Dijkstra) all routers have complete topology, link cost info distributed implementation: all-to-all router messages, no central server. centralized implementation: all router send messages to a central server (Software Defined Network, SDN) Local: distance vector algorithm (Bellman-Ford) router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of messages only with neighbors

12 Outline 4. Internet architecture 4.2 What s inside a router 4.3 Internet Protocol (IP) Datagram format IPv4 addressing ICMP 4.4 Routing algorithms Link state 4.5 Routing in the Internet 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 for node a Initialization: 2 N' = {a} 3 for all nodes x 4 if x adjacent to a 5 then D(x) = c(a,x) and p(x)=a 6 else D(x) = 8 Loop 9 find y not in N' such that D(y) is a minimum 0 add y to N' update D(z) for all z adjacent to y and not in N' : 2 D(z) = min{ D(z), D(y) + c(y,z) } if D(y)+c(y,z) < D(z) then p(z)=y /* new cost to z is either old cost to z or known 4 shortest path cost to y plus cost from y to z */ 5 until all nodes in N' Dijkstra s algorithm: example Step N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v) 2,u 2,u 2,u D(w),p(w) 5,u 4,x 3,y 3,y D(x),p(x),u D(y),p(y) 2,x D(z),p(z) 4,y 4,y 4,y 5 u 2 v x w y 5 2 z

14 Dijkstra s algorithm: example (2) Resulting shortest-path tree from u: v w u z x y Resulting forwarding table in u: destination v x y w z 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+)/2 comparisons: O(n 2 ) 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 2+e A 0 D B 0 +e 0 C recompute routing 0 A 2+e D 0 0 B +e C recompute 2+e A 0 D +e B 0 0 C recompute

15 Distance-vectors!29 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 its own distance vector (DV) D x = [d x (y): y є N ] Node x also maintains its neighbors DVs: for each neighbor v, x maintains D v = [d v (y): y є N ] DV are computed by means of the iterative Bellman-Ford algorithm Distance-vector algorithm!30 Basic idea: From time-to-time, each node sends its own DV estimate to neighbors (can be asynchronous) When a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: d x (y) = min {c(x,v) + d v (y) } for each node y in the network. v Under minor, natural conditions, the estimate D x (y) converge to the actual least cost d x (y)

16 Outline 4. Internet architecture 4.2 What s inside a router 4.3 Internet Protocol (IP) Datagram format IPv4 addressing ICMP 4.4 Routing algorithms Link state 4.5 Routing in the Internet OSPF BGP 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

17 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. Hierarchical OSPF

18 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. Outline 4. Internet architecture 4.2 What s inside a router 4.3 Internet Protocol (IP) Datagram format IPv4 addressing ICMP 4.4 Routing algorithms Link state 4.5 Routing in the Internet OSPF BGP

19 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. 2. 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 BGP basics pairs of routers (BGP peers) exchange routing info over semipermanent TCP connections: BGP sessions BGP sessions need not correspond to physical links. when AS2 advertises a prefix to AS: AS2 promises it will forward datagrams towards that prefix. AS2 can aggregate prefixes in its advertisement 3c 3a 3b AS3 a AS c d ebgp session ibgp session b 2a AS2 2c 2b

20 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 AS2 over b-to-2a ebgp session when router learns of new prefix, it creates entry for prefix in its forwarding table. 3c 3a 3b AS3 a AS ebgp session ibgp session c b d 2a AS2 2c 2b 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.

21 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 2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing 4. additional network provider specific criteria 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

22 BGP routing policy (/2) 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 BGP routing policy (2/2) 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

23 Another BGP policy example!45 AS X AS X AS Y AS Y Perth Sydney Perth Sydney Hot potato routing: each ISP aims at minimizing the path of the outbound traffic within his own AS 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