announcements list [email protected]@csd.uoc.gr lab is still under construction next...
Post on 22-Dec-2015
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Announcements
• List [email protected]
• 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