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CSE524: Lecture 6 Network Layer (Part 1) 1 Administrative • Reading assignment – Chapter 4 • Homework #2 due Monday • Homework #1 – Solutions will be handed out 2 Last classes • Data-link layer – Functions – Specific implementations, devices 3 Next classes • Network layer – Functions • • • • • • • • Addressing Security Fragmentation Delivery semantics Quality of service Routing Demux to upper layer Error detection – Specific implementations • IP • Router devices, implementations 4 Network layer functions • Transport packet from sending to receiving hosts • Network layer protocols in every host, router application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical • Important functions: – Addressing: address assignment – Security: provide privacy, authentication, etc. at the network layer – Fragmentation: break-up packets based on data-link layer properties – Delivery semantics: unicast, multicast, anycast, broadcast, ordering – Quality-of-service: provide predictable performance – Routing: path selection and packet forwarding 5 NL: Addressing • Hierarchical vs. flat – Routing table size • Global vs. local – Applications (NAT) – Processing speed • Variable-length vs. fixed-length – Flexibility – Processing costs – Header size 6 NL: Security • Secrecy – No eavesdropping • Integrity – No man-in-the-middle attacks • Authenticity – Ensure identity of source • If time permits, we will look at network security at the end of course….. 7 NL: Fragmentation • Different link-layers have different MTUs • Split packets into multiple fragments • Where to do reassembly? – End nodes – avoids unnecessary work – Dangerous to do at intermediate nodes • Buffer space • Must assume single path through network • May be re-fragmented later on in the route again • Path MTU Discovery – Network layer does no fragmentation – Host does Path MTU discovery 8 NL: Fragmentation is Harmful • Uses resources poorly – Forwarding costs per packet – Best if we can send large chunks of data – Worst case: packet just bigger than MTU • Poor end-to-end performance – Loss of a fragment • Reassembly is hard – Buffering constraints 9 NL: Fragmentation • References – Characteristics of Fragmented IP Traffic on Internet Links. Colleen Shannon, David Moore, and k claffy -- CAIDA, UC San Diego. ACM SIGCOMM Internet Measurement Workshop 2001. http://www.aciri.org/vern/sigcomm-imeas2001.program.html – C. A. Kent and J. C. Mogul, "Fragmentation considered harmful," in Proceedings of the ACM Workshop on Frontiers in Computer Communications Technology, pp. 390--401, Aug. 1988. http://www.research.compaq.com/wrl/techreports/abstracts/87 .3.html 10 NL: Delivery semantics • Communication modes – – – – Unicast (One source to one destination) Anycast (One source to any of a set of destinations) Multicast (One or more sources to a set of destinations) Broadcast (One source to all destinations) • Ordering – In-order vs. out-of-order delivery • If time permits, we will look at multicast at the end of the course. 11 NL: Quality-of-Service Q: What service model for “channel” transporting packets from sender to receiver? • guaranteed bandwidth? • preservation of inter-packet timing (no jitter)? • loss-free delivery? • in-order delivery? • congestion feedback to sender? The most important abstraction provided by network layer: ? ? ? virtual circuit or datagram? 12 NL: Virtual circuits “source-to-dest path behaves much like telephone circuit” – performance-wise – network actions along source-to-dest path • call setup, teardown for each call before data can flow • each packet carries VC identifier (not destination host OD) • every router on source-dest path s maintain “state” for each passing connection – transport-layer connection only involved two end systems • link, router resources (bandwidth, buffers) may be allocated to VC – to get circuit-like perf. 13 NL: Virtual circuits: signaling protocols • used to setup, maintain teardown VC • used in ATM, frame-relay, X.25 • not used in today’s Internet on an end-to-end basis application transport 5. Data flow begins network 4. Call connected data link 1. Initiate call physical 6. Receive data application 3. Accept call transport 2. incoming call network data link physical 14 NL: Datagram networks: the Internet model • no call setup at network layer • routers: no state about end-to-end connections – no network-level concept of “connection” • packets typically routed using destination host ID – packets between same source-dest pair may take different paths application transport network data link 1. Send data physical application transport 2. Receive data network data link physical 15 NL: Network layer service models: Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no • Internet model being extended: Intserv, Diffserv – Chapter 6 16 NL: Datagram or VC network: why? Internet ATM • data exchange among computers • evolved from telephony – “elastic” service, no strict • human conversation: timing req. – strict timing, reliability • “smart” end systems (computers) requirements – can adapt, perform control, – need for guaranteed service error recovery • “dumb” end systems – simple inside network, – telephones complexity at “edge” – complexity inside network • many link types – different characteristics – uniform service difficult 17 NL: Routing • Routing algorithms and architectures – Link state algorithms – Distance vector algorithms • Routing hierarchies – Area routing – Landmark routing 18 NL: Routing algorithms Routing protocol 5 Goal: determine “good” path (sequence of routers) thru network from source to dest. 2 A Graph abstraction for routing algorithms: • graph nodes are routers • graph edges are physical links – link cost: delay, $ cost, or congestion level B 2 1 D 3 C 3 1 5 F 1 E 2 • “good” path: – typically means minimum cost path – other def’s possible 19 NL: Routing algorithms Global or decentralized information? Global: • all routers have complete topology, link cost info • “link state” algorithms Decentralized: • router knows physicallyconnected neighbors, link costs to neighbors • iterative process of computation, exchange of info with neighbors • “distance vector” algorithms Static or dynamic? Static: • routes change slowly over time Dynamic: • routes change more quickly – periodic update – in response to link cost changes 20 NL: What to look for in routing algorithms • • • • Communication costs Processing costs Optimality Stability – Convergence time – Loop freedom – Oscillation damping 21 NL: Link state routing algorithms • Used in OSPF (intra-domain routing protocol) • Basic steps • Start condition – Each node assumed to know state of links to its neighbors • Step 1 – Each node broadcasts its state to all other nodes – Reliable flooding mechanism • Step 2 – Each node locally computes shortest paths to all other nodes from global state – Dijkstra’s shortest path tree (SPT) algorithm 22 NL: Step 1 • Link State Packets (LSPs) to broadcast state to all nodes • Periodically, each node creates a link state packet containing: – – – – – Node ID List of neighbors and link cost Sequence number Time to live (TTL) Node outputs LSP on all its links 23 NL: Step 1 • Reliable Flooding – When node J receives LSP from node K • If LSP is the most recent LSP from K that J has seen so far, J saves it in database and forwards a copy on all links except link LSP was received on • Otherwise, discard LSP – How to tell more recent • • • • • Use sequence numbers Same method as sliding window protocols Needed to avoid stale information from flood Sequence number wrap-around Lollipop sequence space 24 NL: Step 1 and wrapped sequence numbers • Wrapped sequence numbers – 0-N where N is large – If difference between numbers is large, assume a wrap – A is older than B if…. • A < B and |A-B| < N/2 or… • A > B and |A-B| > N/2 • What about new nodes out of sync with sequence number space? • Lollipop sequence (Perlman 1983) 25 NL: Step 1 and lollipop sequence numbers • Divide sequence number space • Special negative sequence for recovering from reboot • When receiving an old number, nodes inform new node of current sequence number • A older than B if – A < 0 and A < B – A > 0, A < B and (B – A) < N/4 – A > 0, A > B and (A – B) > N/4 -N/2 0 N/2 - 1 26 NL: Step 2 A Link-state routing algorithm Dijkstra’s algorithm • all link costs on the network are known • all nodes have same info • computes least cost paths from one node (‘source”) to all other nodes – gives routing table for that node • iterative: after k iterations, know least cost path to k destinations Notation: • c(i,j): link cost from node i to j. cost infinite 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, that is next v • N: set of nodes whose least cost path definitively known 27 NL: Step 2 (Dijkstra’s algorithm example) 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infinity 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N 28 NL: Step 2 (Dijkstra’s algorithm example) 5 B 2 A 2 1 SPT A C C F 2 E 1 5 1 3 D B step 0 3 D E F D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f) 2, A 5, A 1, A ~ ~ 29 NL: Step 2 (Dijkstra’s algorithm example) 5 B 2 A 2 1 SPT A AD C C F 2 E 1 5 1 3 D B step 0 1 3 D E F D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f) 2, A 5, A 1, A ~ ~ 2, A 4, D 2, D ~ 30 NL: Step 2 (Dijkstra’s algorithm example) 5 B 2 A 2 1 SPT A AD ADE C C F 2 E 1 5 1 3 D B step 0 1 2 3 D E F D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f) 2, A 5, A 1, A ~ ~ 2, A 4, D 2, D ~ 2, A 3, E 4, E 31 NL: Step 2 (Dijkstra’s algorithm example) 5 B 2 A 2 1 SPT A AD ADE ADEB C C F 2 E 1 5 1 3 D B step 0 1 2 3 3 D E F D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f) 2, A 5, A 1, A ~ ~ 2, A 4, D 2, D ~ 2, A 3, E 4, E 3, E 4, E 32 NL: Step 2 (Dijkstra’s algorithm example) 5 B 2 A 2 1 SPT A AD ADE ADEB ADEBC C C F 2 E 1 5 1 3 D B step 0 1 2 3 4 3 D E F D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f) 2, A 5, A 1, A ~ ~ 2, A 4, D 2, D ~ 2, A 3, E 4, E 3, E 4, E 4, E 33 NL: Step 2 (Dijkstra’s algorithm example) 5 B 2 A 2 1 SPT A AD ADE ADEB ADEBC ADEBCF C C F 2 E 1 5 1 3 D B step 0 1 2 3 4 5 3 D E F D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f) 2, A 5, A 1, A ~ ~ 2, A 4, D 2, D ~ 2, A 3, E 4, E 3, E 4, E 4, E 34 NL: Link State Characteristics • With consistent LSDBs, all nodes compute consistent loop-free paths • Limited by Dijkstra computation overhead, space requirements • Can still have transient loops B 1 1 3 A 5 C 2 D Packet from CA may loop around BDC if B knows about failure and C & D do not 35 NL: Dijkstra’s algorithm, discussion Algorithm complexity: n nodes • each iteration: need to check all nodes, w, not in N • n*(n+1)/2 comparisons: O(n**2) • more efficient implementations possible: O(nlogn) Oscillations possible: • e.g., link cost = amount of carried traffic D 1 1 0 A 0 0 C e 1+e e initially B 1 2+e A 0 D 1+e 1 B 0 0 C … recompute routing 0 D 1 A 0 0 C 2+e B 1+e … recompute 2+e A 0 D 1+e 1 B e 0 C … recompute 36 NL: Distance vector routing algorithms • Variants used in – Early ARPAnet – RIP (intra-domain routing protocol) – BGP (inter-domain routing protocol) • Distributed next hop computation • Unit of information exchange – Vector of distances to destinations 37 NL: Distance vector routing algorithms • Exchange known distance information iteratively • Example (Bellman 1957) – Start with link table (as with Dijkstra), calculate distance table iteratively through table exchanges with adjacent nodes – Distance table data structure • • • • X D (Y,Z) table of known distances and next hops kept per node row for each possible destination column for each directly-attached neighbor to node example: in node X, for dest. Y via neighbor Z: distance from X to = Y, via Z as next hop Z = c(X,Z) + minw{D (Y,w)} X Minimum known D (Y,*) = distance from X to Y X Next hop node H (Y) = from X to Y 38 NL: Distance Table: example 7 A B 1 E cost to destination via D () A B D A 1 14 5 B 7 8 5 C 6 9 4 D 4 11 2 2 8 1 C E 2 D E D D (C,D) = c(E,D) + minw {D (C,w)} = 2+2 = 4 E D c(E,D) + min {D (A,w)} D (A,D) = w = 2+3 = 5 loop! E B D (A,B) = c(E,B) + minw{D (A,w)} = 8+6 = 14 loop! X H (Y) = 39 NL: Distance table gives routing table X H (Y) E cost to destination via Outgoing link to use, cost D () A B D A 1 14 5 A A,1 B 7 8 5 B D,5 C 6 9 4 C D,4 D 4 11 2 D D,4 Distance table Routing table 40 NL: Bellman algorithm while there is a change in D { for all k not neighbor of i { for each j neighbor of i { Di(k,j) = c(i,j) + Dj(k,*) if Di(k,j) < Di(k,*) { Di(k,*) = Di(k,j) Hi(k) = j } } } } Dj(k,*) Di(k,*) c(i,j) i k j c(i,j’) j’ Dj’(k,*) k’ 41 NL: Distributed Bellman-Ford • Make Bellman algorithm distributed (Ford-Fulkerson 1962) – Each node i knows part of link table – Iterative • Each node sends around and recalculates D[i,*] • continues until no nodes exchange info. • self-terminating: no “signal” to stop – Asynchronous • nodes need not exchange info/iterate in lock step! • “triggered updates” – Distributed • each node communicates only with directly-attached neighbors 42 NL: Distributed Bellman-Ford overview Iterative, asynchronous: each local iteration caused by: • local link cost change • message from neighbor: its least cost path change from neighbor Distributed: • each node notifies neighbors only when its least cost path to any destination changes Each node: wait for (change in local link cost of msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors – neighbors then notify their neighbors if necessary 43 NL: Distributed Bellman-Ford algorithm At all nodes, X: 1 Initialization: 2 for all adjacent nodes v: 3 D X(*,v) = infinity /* the * operator means "for all rows" */ X 4 D (v,v) = c(X,v) 5 for all destinations, y X 6 send min D (y,w) to each neighbor /* w over all X's neighbors */ w 44 NL: Distributed Bellman-Ford algorithm (cont.): 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) 11 12 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D X(y,V) = D X(y,V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min w DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D X(Y,V) = c(X,V) + newval 22 23 if we have a new minw DX(Y,w)for any destination Y 24 send new value of min w D X(Y,w) to all neighbors 25 26 forever 45 NL: DBF example Initial Distance Vectors 1 B C 7 8 A 1 2 2 E D Distance to Node Info at Node A B C D E A 0 7 ~ ~ 1 B 7 0 1 ~ 8 C ~ 1 0 2 ~ D ~ ~ 2 0 2 E 1 8 ~ 2 0 46 NL: DBF example E Receives D’s Routes; Updates Cost 1 B C 7 8 A 1 2 2 E D Distance to Node Info at Node A B C D E A 0 7 ~ ~ 1 B 7 0 1 ~ 8 C ~ 1 0 2 ~ D ~ ~ 2 0 2 E 1 8 4 2 0 47 NL: DBF example A receives B’s; Updates Cost 1 B C 7 8 A 1 2 2 E D Distance to Node Info at Node A B C D E A 0 7 8 ~ 1 B 7 0 1 ~ 8 C ~ 1 0 2 ~ D ~ ~ 2 0 2 E 1 8 4 2 0 48 NL: DBF example A receives E’s routes; Updates Costs 1 B C 7 8 A 1 2 2 E D Distance to Node Info at Node A B C D E A 0 7 5 3 1 B 7 0 1 ~ 8 C ~ 1 0 2 ~ D ~ ~ 2 0 2 E 1 8 4 2 0 49 NL: DBF example Final Distances 1 B C 7 8 A 1 2 2 E D Distance to Node Info at Node A B C D E A 0 6 5 3 1 B 6 0 1 3 5 C 5 1 0 2 4 D 3 3 2 0 2 E 1 5 4 2 0 50 NL: DBF example E’s routing table 1 B C E’s routing table Next hop 7 8 A 1 2 2 E D dest A B D A 1 14 5 B 7 8 5 C 6 9 4 D 4 11 2 51 NL: DBF (another example) • See book for explanation of this example X 2 Y 7 1 Z 52 NL: DBF (another example) X 2 Y 7 1 Z Z X D (Y,Z) = c(X,Z) + minw{D (Y,w)} = 7+1 = 8 Y X D (Z,Y) = c(X,Y) + minw {D (Z,w)} = 2+1 = 3 53 NL: DBF (good news example) Link cost changes: • node detects local link cost change • updates distance table (line 15) • if cost change in least cost path, notify neighbors (lines 23,24) • See book for explanation of this example “good news travels fast” 1 X 4 Y 50 1 Z algorithm terminates 54