Advance Computer Networks Lecture#08 Instructor: Engr. Muhammad Mateen Yaqoob

Network Information Source and Update Timing routing decisions usually based on knowledge of network (not always) ◦distributed routing ◦using local knowledge, info from adjacent nodes, info from all nodes on a potential route ◦central routing ◦collect info from all nodes issue of update timing ◦when is network info held by nodes updated ◦fixed - never updated ◦adaptive - regular updates MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Fixed Routing use a single permanent route for each source to destination pair determined using a least cost algorithm route is fixed ◦at least until a change in network topology ◦hence cannot respond to traffic changes advantage is simplicity disadvantage is lack of flexibility MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Flooding packet sent by node to every neighbor eventually multiple copies arrive at destination no network info required each packet is uniquely numbered so duplicates can be discarded need some way to limit incessant retransmission ◦nodes can remember packets already forwarded to keep network load in bounds ◦or include a hop count in packets MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Properties of Flooding all possible routes are tried ◦very robust at least one packet will have taken minimum hop count route ◦can be used to set up virtual circuit all nodes are visited ◦useful to distribute information (eg. routing) disadvantage is high traffic load generated MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Random Routing simplicity of flooding with much less load node selects one outgoing path for retransmission of incoming packet selection can be random or round robin a refinement is to select outgoing path based on probability calculation no network info needed but a random route is typically neither least cost nor minimum hop MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Adaptive Routing used by almost all packet switching networks routing decisions change as conditions on the network change due to failure or congestion requires info about network disadvantages: ◦decisions more complex ◦tradeoff between quality of network info and overhead ◦reacting too slowly means info may be irrelevant MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Adaptive Routing - Advantages improved performance aid congestion control but since is a complex system, may not realize theoretical benefits ◦cf. outages on many packet-switched nets MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Classification of Adaptive Routing Startegies based on information sources ◦local (isolated) ◦route to outgoing link with shortest queue ◦can include bias for each destination ◦Rarely used - does not make use of available info ◦adjacent nodes ◦takes advantage on delay / outage info ◦distributed or centralized ◦all nodes ◦like adjacent MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm finds shortest paths from given source node s to all other nodes by developing paths in order of increasing path length algorithm runs in stages each time adding node with next shortest path algorithm terminates when all nodes processed by algorithm (in set T) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm finds the shortest path from the start vertex to every other vertex in the network. We will find the shortest path from A to G A C D B F E G MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm 1.Label the start vertex with permanent label 0 and order label 1 2Assign temporary labels to all the vertices that can be reached directly from the start 3Select the vertex with the smallest temporary label and make its label permanent. Add the correct order label. 4Put temporary labels on each vertex that can be reached directly from the vertex you have just made permanent. The temporary label must be equal to the sum of the permanent label and the direct distance from it. If there is an existing temporary label at a vertex, it should be replaced only if the new sum is smaller. 5Select the vertex with the smallest temporary label and make its label permanent. Add the correct order label. 6Repeat until the finishing vertex has a permanent label. 7To find the shortest paths(s), trace back from the end vertex to the start vertex. Write the route forwards and state the length. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm Order in which vertices are labelled. Distance from A to vertex Working A C D B F E G Label vertex A 1 as it is the first vertex labelled MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to A with a ‘working value’ for its distance from A. 1 0 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G Vertex C is closest to A so we give it a permanent label 3. C is the 2 nd vertex to be permanently labelled. 1 0 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm We update each vertex adjacent to C with a ‘working value’ for its total distance from A, by adding its distance from C to C’s permanent label of A C D B F E G < 7 so replace the t-label here MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertex with the smallest temporary label is B, so make this label permanent. B is the 3 rd vertex to be permanently labelled. 3 4 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to B with a ‘working value’ for its total distance from A, by adding its distance from B to B’s permanent label of < 6 so replace the t-label here MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertex with the smallest temporary label is D, so make this label permanent. D is the 4 th vertex to be permanently labelled. 4 5 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to D with a ‘working value’ for its total distance from A, by adding its distance from D to D’s permanent label of 5. 7 < 8 so replace the t-label here < 8 so replace the t-label here 7 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertices with the smallest temporary labels are E and F, so choose one and make the label permanent. E is chosen - the 5 th vertex to be permanently labelled. 5 7 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to E with a ‘working value’ for its total distance from A, by adding its distance from E to E’s permanent label of 7. 9 < 12 so replace the t-label here 9 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertex with the smallest temporary label is F, so make this label permanent.F is the 6 th vertex to be permanently labelled MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to F with a ‘working value’ for its total distance from A, by adding its distance from F to F’s permanent label of > 9 so do not replace the t-label here MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G G is the final vertex to be permanently labelled. 7 9 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G To find the shortest path from A to G, start from G and work backwards, choosing arcs for which the difference between the permanent labels is equal to the arc length. The shortest path is ABDEG, with length 9. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra's ◦each node needs complete topology ◦must know link costs of all links in network ◦must exchange information with all other nodes MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Evaluation dependent on ◦processing time of algorithms ◦amount of information required from other nodes implementation specific under static topology and costs resulting to same solution as other algorithms if link costs change, algs attempt to catch up if link costs depend on traffic, which depends on routes chosen, may have feedback instability MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Hierarchical routing scale: with 600 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy  internet = network of networks  each network admin may want to control routing in its own network our routing study thus far - idealization  all routers identical  network “flat” … not true in practice

Hierarchical routing aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol ◦“intra-AS” routing protocol ◦routers in different AS can run different intra-AS routing protocol gateway router: at “edge” of its own AS has link to router in another AS MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table 3c Interconnected ASs  forwarding table configured by both intra- and inter-AS routing algorithm  intra-AS sets entries for internal dests  inter-AS & intra-AS sets entries for external dests

Inter-AS tasks  suppose router in AS1 receives datagram destined outside of AS1:  router should forward packet to gateway router, but which one? AS1 must: 1.learn which dests are reachable through AS2, which through AS3 2.propagate this reachability info to all routers in AS1 job of inter-AS routing! MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks

Distance vector algorithm Bellman-Ford equation (dynamic programming) let d x (y) := cost of least-cost path from x to y then d x (y) = min {c(x,v) + d v (y) } MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE v cost to neighbor v min taken over all neighbors v of x cost from neighbor v to destination y

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE Bellman-Ford example u y x wv z clearly, d v (z) = 8, d x (z) = 3, d w (z) = 5 d u (z) = min { c(u,v) + d v (z), c(u,x) + d x (z), c(u,w) + d w (z) } = min {2 + 8, 1 + 3, 5 + 5} = 4 node achieving minimum is next hop in shortest path, used in forwarding table B-F equation says:

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE RIP (Routing Information Protocol) included in BSD-UNIX distribution in 1982 distance vector algorithm ◦distance metric: # hops (max = 15 hops), each link has cost 1 ◦DVs exchanged with neighbors every 30 sec in response message (aka advertisement) ◦each advertisement: list of up to 25 destination subnets (in IP addressing sense) D C BA u v w x y z subnet hops u 1 v 2 w 2 x 3 y 3 z 2 from router A to destination subnets:

RIP: link failure, recovery if no advertisement heard after 180 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 network MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

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 advertisements flooded to entire AS ◦carried in OSPF messages directly over IP (rather than TCP or UDP IS-IS routing protocol: nearly identical to OSPF MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

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 ToS; high for real time ToS) integrated uni- and multicast support: ◦Multicast OSPF (MOSPF) uses same topology data base as OSPF hierarchical OSPF in large domains. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto inter-domain routing protocol ◦BGP uses two primary modes of information exchange, internal BGP (IBGP) and external BGP (EBGP), to communicate with internal and external peers, respectively BGP provides each AS a means to: ◦eBGP: obtain subnet reachability information from neighboring ASs. ◦iBGP: propagate reachability information to all AS-internal routers. ◦determine “good” routes to other networks based on reachability information and policy. allows subnet to advertise its existence to rest of Internet: “I am here” MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

BGP basics when AS3 advertises a prefix to AS1: ◦AS3 promises it will forward datagrams towards that prefix ◦AS3 can aggregate prefixes in its advertisement AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks  BGP session: two BGP routers (“peers”) exchange BGP messages:  advertising paths to different destination network prefixes (“path vector” protocol)  exchanged over semi-permanent TCP connections BGP message

BGP basics: distributing path information  using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1.  1c can then use iBGP do distribute new prefix info to all routers in AS1  1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session  when router learns of new prefix, it creates entry for prefix in its forwarding table. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE AS3 AS2 3b 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks eBGP session iBGP session

Path attributes and 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 17 ◦NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be multiple links from current AS to next-hop-AS) gateway router receiving route advertisement uses import policy to accept/decline ◦e.g., never route through AS x ◦policy-based routing MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

BGP route selection  router may learn about more than 1 route to destination AS, selects route based on: 1.local preference value attribute: policy decision 2.shortest AS-PATH 3.closest NEXT-HOP router: hot potato routing 4.additional criteria MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Why different Intra-, 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 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE