Announcements

• List hy535-list@csd.uoc.gr

• 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 0.0.0.0/0

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