Announcements List Lab is still under construction Next session we will have paper discussion, assign papers, 3x15 min presentation first homework will be given next class

Review IP protocol –Address formats IPv4, IPv6 –Address per interface –Subnets, network mask, prefix len IP is connectionless, packet forwarding –Packets follow independent paths to destination Based on destination address –DNS to allow me to use symbolic names

Network layer vs. Link Layer Each link has also a link layer address that is technology specific –Ethernet MAC –ATM etc … Network layer address (IP) is independent of the networking technology

Sending to the subnet Nodes are “close” can reach them directly in layer 2 - same “broadcast domain” ! Need to know the MAC address Need to map IP to MAC address ARP, cache response, broadcast No need for any other mechanism, this is not routing But I can not have everything in a single subnet

What is routing/router Device with multiple ports of different networking technologies Forward a packet between subnets Forwarding table –Contains prefixes (and not addresses) –Device –Gateway Longest prefix match (LPM) Default route /0

Concept of ASes A set of systems under the same administration –Forth-net, EDET k.o.k –Same rules, policies Different protocols inside and among the domains –Inside is relatively small Intra-domain –Among is massively huge… Inter-domain

The Big picture Multiple Ases talking to each other PoPs We will revisit later… Some numbers –ASes: 23,400 –Prefixes: 214,000 –Average AS Path lengths: 3,6 –Average Prefix length: 22,3

Hierarchy, what makes it all work Can aggregate multiple routing table entries in large ones (less specific, larger prefix) Is convenient to allocate addresses hierarchically –Global provider, local ISPs, customers There are some problems though: multi- homing

What is important Scale: can have tons of prefixes Speed: need to forward fast Resilience to faults: some link somewhere is bound to fail Management and misconfigurations: there are 23,400 entities that collaborate to make all this work

Some generalities about routing Attempt to find a “good” path for the packet In reality, I just find the best “next-hop” –Routing is packet-packet EXAMPLE

Cost in Routing Good can have multiple definitions –Small delay Do not send the packet to athens through the US –Less loaded (related to delay) –Less expensive (real money) –Less cost (administrative cost) –Less hops In practice it is least cost routing today See example SPF: cost is set according to some recipies/rules of thumb

ECMP May have multiple next-hops with the same cost –Why not use them all –Router will load balance –But have to be done carefully to avoid out-of- order packets ECMP, 8 or 16 in today;s routers EXAMPLE

Standards The role of standards –Necessary if different boxes are to work together –Standards bodies, IETF, ISO The role of IETF –Democratic, collaborative –Working groups Rough consensus and working code Requests for Comments –Standards –Proposed standards –Informational –Historical

The local view – Intra-domain routing Link state routing –The most commonly used today Basic concept: –Each router has a complete view of the topology of the network –Pros: simple and fast convergence –Cons: expensive to maintain reliably Flooding –Compute SPF routes Link state routing allows me to do much-much more

What is important The view of each router about the network has to agree –Else routing loops TTL will catch it EXAMPLE

Basic Structures Each router has –A list of neighbors –The topology database that describes the network –And the routing table

Basic Operations Join the network –Discover neighbors –Forming adjacency –Database exchange Monitor for faults and handle changes –Monitor neighbor’s up status –Reliable Flooding Route Computation Scaling –Multiple areas

OSPF Open SPF, standard protocol today –Not the only one though IS-IS is also strong Has all the elements: –HELLO protocol for neighbor discovery and health monitoring –Database exchange for database syn on start –Reliable flooding for propagating changes

Details Packets sent as an IP protocol (OSPF protocol 89) –Does not use TCP/UDP etc… 5 packet types: hello, LS-req, LS-upd, LS- ack and DD-desc LS-* packets carry link states

What is a LS Describes an object in the network –Router, network, external prefix Is originated by a specific router, has an id and a sequence number –Each OSPF router has a unique router-id Routers exchange LS through flooding, build their LS database and then compute routes EXAMPLE EXAMPLE for link failure

Flooding When receiving an update send it to all your adjacencies except the one it came from It is reliable, each LS sent must be acknowledged (with an LS-ack packet) Can receive duplicates –Discard It is a bit expensive

Joining an OSPF network EXAMPLE

Route computation Build the shortest path tree rooted at the computing node and derive the next hop information for each destination EXAMPLE