Lab 4 Dynamic Routing CS 144 Review Session

Lab 4 Dynamic Routing CS 144 Review Session 5 October 30, 2009 Samir Selman

Announcements • Lab 4 : Due in 2 weeks (Thurs , Nov 12) • Lab 3 : Due Yesterday • For those of you submitting late, contact us before your deadline if you need an additional extension. Tell us: – Where you are – How much more time you need

Routing • Static Routing (Implemented Lab 3) • Dynamic Routing: – Unicast Routing: • Interior Gateway Protocols (RIP, OSPF, IS-IS) • Exterior Gateway Protocols (BGP) – Multicast Routing • Examples: Multicast OSPF (MOSPF) , Multicast BGP(MBGP) • This Lab will cover RIP

Routing

Routing Information Protocol(RIP) • RIP: – Routing Protocol based on Bellman-Ford, Distance Vector algorithm. – Intra-domain routing algorithm or interior gateway protocol (IGP) – Cost is usually hop count – Limitations: • The protocol is limited to networks whose longest path is 15 hops • The protocol depends upon "counting to infinity" to resolve certain unusual situations • The protocol uses fixed "metrics" to compare alternative routes

RIP-Lab 4: Implementation • Lab 4 is just a subset of RIP • Want you to focus on the RIP part of the router => Supplied you with ‘dr’ binary • Binary relies on API implemented in a shared library to handle dynamic routing. You should implement this library. • Write code in dr_api. h and dr_api. c

RIP-Lab 4 • Each Router has a forwarding table containing an entry for every destination network. • Forwarding table Entry: (Dest Network, Cost, Next-hop, Time Stamp). – Dest Network: Represented by a network address and a network mask (X. X / Y) – Cost: Sum of link costs to reach destination. – Next-hop: IPv 4 address of next router along the path to destination – Time Stamp: Used to timeout routing entries

RIP-Lab 4: RIP Packet Format • RIP packet: • Command: Indicates whether packet is request or response – Request: Asks that a router sends all or part of its routing table. (For this Lab you don’t need to worry about RIP Requests) – Response: Either unsolicited routing update or a response to request. • Version: RIP version used. Lab 4 uses RIP V 2 • AFI: Address family used. (AF_INET) • Route Tag: Not used in this lab (replaced by pad)

RIP-Lab 4: RIP Packet Format • RIP packet: • Network address: IP address for the entry • Subnet Mask: Contains subnet mask for the entry • Next Hop: Indicates IP address of the next hop to which packets for the entry is forwarded. • Metric: Indicates how many hops(routers) have been traversed in the trip to the destination. (16=unreachable)

RIP-Lab 4: Periodic Updates • Send routing updates: – At regular intervals (periodic) – When a router changes the metric to a route (Triggered Updates) • Every 10 sec send RIP reply message to all neighbors containing complete routing table • Triggered update when an interface goes down or cost of a local interface changes.

RIP-Lab 4: Route Timeouts • Dynamic Route: – Route learned from a neighboring router – Have a Timestamp field per entry – When receive an update, a router sets the entry Timestamp value to the current time. – Periodically check if entries expired (current time – updatetime > 20 sec. – If an entry expires => set the Cost = INFINITY • Local Route: – Directly Connected Networks – Invalid TTL= -1 – Next_hop_ip = 0

RIP-Lab 4: Routing Loops • Split Horizon with Poisoned Reverse – Prevent a loop from occurring between two nodes – Advertise information back to source with cost INFINITY. – What about loops with more than 2 nodes. eq: A -> B, B -> C, C -> A ? • Cannot stop such a loop using split horizon. • This loop will only be resolved when the metric reaches infinity and the network involved is then declared unreachable • Speed up process using triggered update.

RIP-Lab 4: Routing Loops

RIP-Lab 4: Code • You should only touch the safe_ methods (threading handled already for you) • Routing table entry /** a single entry in the routing table */ typedef struct route_t { uint 32_t subnet; /* destination subnet which this route is for */ uint 32_t mask; /* mask associated with this route */ uint 32_t next_hop_ip; /* next hop on on this route */ uint 32_t outgoing_intf; /* interface to use to send packets on this route */ uint 32_t cost; struct timeval last_updated; int is_garbage; /* boolean which notes whether this entry is garbage */ route_t* next; /* pointer to the next route in a linked-list */ } route_t;

RIP-Lab 4: Code • safe_dr_get_next_hop(ip): Called when router needs to know how to route a packet. Returns which interface packet should be sent out and IP address of next hop. • safe_dr_handle_packet: Method called when router receives a dynamic routing packet. • safe_dr_handle_periodic(): Method called periodically by the router. • safe_dr_interface_changed: When interface goes up or down or upon a change in direct link cost • dr_init: Used to initialize any internal data structures.

RIP-Lab 4: Running the Lab • First start the lvns server –. /lvns -t simple. topo • If the server is running on the same machine as the dr instance, then you can start a router which takes control of a node named "dr 1" like this: –. /dr -v dr 1 • If you are running dr from a separate location you'll want to specify the server‘s address and port explicitly: –. /dr -v dr 1 -s myth 5. stanford. edu -p 9999

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