Routing with BGP Dr. Nils Kammenhuber Chair for Network - PowerPoint PPT Presentation

Chair for Network Architectures and Services – Prof. Carle Department for Computer Science TU München Routing with BGP Dr. Nils Kammenhuber Chair for Network Architectures and Services Institut für Informatik Technische Universität München http://www.net.in.tum.de

Overview  Routing algorithms  Link state  Distance Vector  Path Vector  Hierarchical routing  Internet routing protocols  OSPF  BGP  Business considerations  Policy routing  Traffic engineering Internet­Praktikum: Routing mit BGP, WS2011/2012 2 2 Network Security, WS 2008/09, Chapter 9

Short note on pronunciation of the word “routing” ɪ ] r­oo­ting = British English  [‘ru:t ŋ ʊ ɪ ] r­ow­ding = American English  [‘ra d ŋ  Both are correct! Internet­Praktikum: Routing mit BGP, WS2011/2012 3 3 Network Security, WS 2008/09, Chapter 9

Recall: Interplay between routing and forwarding routing algorithm Routing = signalling plane = local forwarding table offline header output link value 0100 3 0101 2 0111 2 1001 1 value in arriving packet’s header 1 0111 2 3 Forwarding = data plane = online Internet­Praktikum: Routing mit BGP, WS2011/2012 4 4 Network Security, WS 2008/09, Chapter 9

Recall: Prefix notation  Separate IP address into  Network address part (the prefix)  Host address part  Prefix notation: 10.11.12.0/24  10.11.12 (i.e., the first 24 bits) is the network part  All following bits identify the host within that network  /24 is the prefix length. • Smaller number = more hosts = „larger“ (!) prefix  Prefixes may be aggregated into larger prefixes  Example: 10.11.12.0/25 and 10.11.12.128/25 into 10.11.12.0/24 Internet­Praktikum: Routing mit BGP, WS2011/2012 5 5 Network Security, WS 2008/09, Chapter 9

Graph abstraction: costs • c(x,x’) =: cost of link (x,x’) e.g.: c(w,z) = 5 5 3 • cost could always be 1, v w 5 2 • or inversely related to u z 2 1 bandwidth, 3 1 2 • or inversely related to x y 1 congestion Cost of path (x 1 , x 2 , x 3 ,…, x p ) = c(x 1 ,x 2 ) + c(x 2 ,x 3 ) + … + c(x p­1 ,x p ) Question: What’s the least­cost path between u and z ? Routing algorithm: algorithm that finds least­cost path Internet­Praktikum: Routing mit BGP, WS2011/2012 6 6 Network Security, WS 2008/09, Chapter 9

Routing Algorithm classification Global or decentralized Static or dynamic? information? Global: Static:  All routers have complete  Routes change slowly topology and link cost info over time  link state algorithms (L­S) Decentralized: Dynamic:  Router only knows physically­  Routes change more connected neighbors and quickly link costs to neighbors  periodic update  Iterative process of computation  in response to link = exchange of info with cost changes neighbors  distance vector algorithms (D­V)  Variant: path vector algorithms Internet­Praktikum: Routing mit BGP, WS2011/2012 7 7 Network Security, WS 2008/09, Chapter 9

A Link-State Routing Algorithm  Net topology and link costs made known to each node  Accomplished via link state broadcasts  All nodes have same info  Each node independently computes least­cost paths from one node (“source”) to all other nodes  Usually done using Dijkstra’s shortest­path algorithm • refer to any algorithms & data structures lecture/textbook ⇒ O( n ²) or O( n log n ) • n nodes in network  Gives forwarding table for that node  Result:  All nodes have the same information,  … thus calculate the same shortest paths,  … hence obtain consistent forwarding tables Internet­Praktikum: Routing mit BGP, WS2011/2012 8 8 Network Security, WS 2008/09, Chapter 9

Distance Vector Algorithm (1)  No node knows entire topology  Nodes only communicate with neighbours (i.e., no broadcasts)  Nodes jointly calculate shortest paths  Iterative process  Algorithm == protocol  Distributed application of Bellman­Ford algorithm  Refer to any algorithms&data structures lecture/textbook Internet­Praktikum: Routing mit BGP, WS2011/2012 9 9 Network Security, WS 2008/09, Chapter 9

Distance Vector Algorithm (2) Bellman­Ford Equation (dynamic programming) Let  c( x , y ) := cost of edge from x to y  d x ( y ) := cost of least­cost path from x to y Then d x ( y ) = min {c( x , v ) + d v ( y ) } where min is taken over all neighbours v of x Internet­Praktikum: Routing mit BGP, WS2011/2012 10 10 Network Security, WS 2008/09, Chapter 9

Bellman-Ford example 5 Clearly, d v (z) = 5, d x (z) = 3, d w (z) = 3 3 v w 5 2 B­F equation says: u z 2 1 3 1 d u (z) = min { c(u,v) + d v (z), 2 x y 1 c(u,x) + d x (z), c(u,w) + d w (z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path → forwarding table Internet­Praktikum: Routing mit BGP, WS2011/2012 11 11 Network Security, WS 2008/09, Chapter 9

Distance Vector Algorithm (3)  Define D x ( y ) := estimate of least cost from x to y  Node x knows cost to each neighbour v : c( x , v )  Node x maintains distance vector D x = [ D x ( y ): y ∈ N ] ( N := set of nodes)  Node x also maintains its neighbours’ distance vectors:  For each neighbour v , x maintains D v = [ D v ( y ): y ∈ N ] Internet­Praktikum: Routing mit BGP, WS2011/2012 12 12 Network Security, WS 2008/09, Chapter 9

Distance vector algorithm (4) Basic idea:  From time­to­time, each node sends its own distance vector estimate D to neighbors  Asynchronously  When a node x receives new DV estimate from neighbour, it updates its own DV using B­F equation: ← v {c( x , v ) + D v ( y )} for each node y ∈ N D x ( y ) min  Under minor, natural conditions, these estimates D x ( y ) converge to the actual least cost d x (y) Internet­Praktikum: Routing mit BGP, WS2011/2012 13 13 Network Security, WS 2008/09, Chapter 9

Distance Vector Algorithm (5) Iterative, asynchronous: Each node: Each local iteration caused by: Forever:  local link cost change  DV update message from wait for (change in local link neighbour cost or message arriving from Distributed: neighbour}  Each node notifies neighbors recompute estimates only when its DV changes  neighbours then notify their neighbours if this caused their DV to change if (DV to any destination has  etc. changed) { notify neighbours } Internet­Praktikum: Routing mit BGP, WS2011/2012 14 14 Network Security, WS 2008/09, Chapter 9

Distance Vector Algorithm (6) node x table D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} cost to cost to = min{2+0 , 7+1} = 2 x y z x y z x 0 2 7 x 0 2 3 from from D x (z) = min{ c(x,y) + y y 2 0 1 ∞ ∞ ∞ D y (z), c(x,z) + D z (z) } z z 7 1 0 ∞ ∞ ∞ = min{2+1 , 7+0} = 3 node y table cost to y x y z 2 1 x ∞ ∞ ∞ z x from y 7 2 0 1 z ∞ ∞ ∞ node z table cost to x y z x ∞ ∞ ∞ from y ∞ ∞ ∞ z 7 1 0 time Internet­Praktikum: Routing mit BGP, WS2011/2012 15 15 Network Security, WS 2008/09, Chapter 9

D x (z) = min{ c(x,y) + D x (y) = min{c(x,y) + D y (y), c(x,z) + D z (y)} D y (z), c(x,z) + D z (z) } = min{2+0 , 7+1} = 2 = min{2+1 , 7+0} = 3 node x table cost to cost to cost to x y z x y z x y z x 0 2 7 x 0 2 3 x 0 2 3 from from from y y 2 0 1 ∞ ∞ ∞ y 2 0 1 z z 7 1 0 ∞ ∞ ∞ z 3 1 0 node y table cost to cost to cost to y x y z x y z x y z 2 1 x ∞ ∞ x 0 2 7 ∞ x 0 2 3 z x from from from y y 7 2 0 1 2 0 1 y 2 0 1 z z ∞ ∞ ∞ 7 1 0 z 3 1 0 node z table cost to cost to cost to x y z x y z x y z x 0 2 7 x 0 2 3 x ∞ ∞ ∞ from from from y y 2 0 1 y 2 0 1 ∞ ∞ ∞ z z z 3 1 0 3 1 0 7 1 0 time Internet­Praktikum: Routing mit BGP, WS2011/2012 16 16 Network Security, WS 2008/09, Chapter 9

Distance Vector: link cost changes (1) Link cost changes: 1  node detects local link cost change y 4 1  updates routing info, recalculates x z 50 distance vector  if DV changes, notify neighbors At time t 0 , y detects the link­cost change, updates its “good DV, and informs its neighbors. news At time t 1 , z receives the update from y and updates its travels table. It computes a new least cost to x and sends its fast” neighbors its DV. At time t 2 , y receives z ’s update and updates its distance table. y ’s least costs do not change and hence y does not send any message to z . Internet­Praktikum: Routing mit BGP, WS2011/2012 17 17 Network Security, WS 2008/09, Chapter 9

Distance Vector: link cost changes (2)  But: bad news travels slow — “count to infinity” problem!  In example: Many iterations before algorithm stabilizes! → : 1. Cost increase for y r r ∞  y consults DV, (i.e., link down) 1  y selects “cheaper” route via z (cost 2+1 = 3), y  4 1 Sends update to z and x x (cost to r now 3 instead of 1) z 50 2. z detects cost increase for path to r :  was 1+1, is now 3+1  Sends update to y and x (cost to r now 4 instead of 2) 3. y detects cost increase, sends update to z 4. z detects cost increase, sends update to y 5. …. Internet­Praktikum: Routing mit BGP, WS2011/2012 18 18 Network Security, WS 2008/09, Chapter 9