Johan J. Lukkien, [e-mail]

1
Computer Networks2002/2003

• Routing
• Johan Lukkien

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Layer tasks

• Basic service
• addressing, data format, fragmentation,
  congestion, error messages
• Neighbor greeting
• configuration routers find out about their
  environment, (other routers, end-stations)
• Routing
• forwarding, route computation selection
• methods, protocols
• dynamic, based on metrics (e.g., delay,
  bandwidth, link error rate) or static
• Quality of the service
• Requirements
• robustness, stability
• preferable automatic configuration (e.g.
  assigning addresses)
• Slides following assume connection-less service

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Neighbor greeting

• ....(Auto-)configuration
• End nodes
• find adjacent routers
• see difference between locally accessible nodes
  and those accessible through a router
• find link addresses of adjacent nodes
• find out own network address
• Routers, in addition
• find network addresses of end nodes
• Note
• dependent on type of connection (point-to-point,
  LAN)

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Soft state

• Typically, state information that is only
  temporary valid
• values of variables of other processes, machines
• state of environment, connections
• in principle, invalidation is interference
• Note making copies violates the principle
  never copy volatile information
• however, for scalability a replication strategy
  of shared objects is needed
• Methods to maintain consistency
• send updates keep-alive messages
• depends on modification frequency timers
• only upon interaction, request Im alive
  message
• e.g. refresh a router table entry only upon
  message sent

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Finding things service advertisement discovery

• Use a local database
• e.g. look it up in a phone book
• Use a special location to find that database
• e.g. dial the telephone directory service
• giving either the nearest or all service
  providers
• Broadcast what youre looking for
• dial a number that rings all phones and tell what
  you ned
• Broadcast the service that you provide
• dial the same number and tell what youve got

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Issues on a LAN

1. E? need their layer 3 addresses
2. R? need to know and announce enough info (e.g.,
   layer 3 addresses) such that packets for E1,E2
   will be forwarded to the LAN
3. R? need to know link addresses of adjacent nodes
4. E? need to know the data link address of an
   adjacent router
5. E1, E2 should be able to communicate directly
6. E? should be able to find the best router per
   partner
7. Nodes on the same LAN do not depend on routers to
   communicate

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IP solutions

• Routers know network number and mask of each of
  their outgoing links (2)
• manually configured
• On that link you find the link address of the
  next hop (endnode or router) through the Address
  Resolution Protocol (3,5,7)
• A router can send an ICMP redirect message
  for D, use this router IP, not me (6)
• why IP and not data link address?

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ARP (RFC 826)

• Message as payload of LAN message
• next protocol is (R)ARP (e.g. 16806 on
  ethernet)
• cannot leave the LAN as such
• protocol where resolution is requested for
• Broadcast request, unicast reply
• test use own IP address
• Upon receipt of request or reply
• add association to cache
• answer request, if requested target

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ARP Example

• Router F1, F3 can serve as ARP proxy
• advantage/disadvantage
• ...or source knows if destination is off the
  network

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Reverse ARP (RFC 903), BOOTP (RFC 951)

• Find IP address corresponding to given data link
  address (1)
• basic auto-configuration
• although it needs a server that is configured
  with this association
• Extended into BOOTP (1,4)
• message in IP packet
• router (relay agents) can forward the message
• how to deal with addressing?
• can specify a host/bootfile combination
• vendor specific options
• rather than (type, length, value) encoded fields
• repaired by the special vendor-specific field
  63.82.53.63 (4 bytes, hex.) .... magic cookie

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DHCP (1,4)

• Extends BOOTP
• packet includes magic cookie and option 1653 ?
• Extras
• dynamic address assignment
• using leases
• re-use of addresses
• no database needed
• no addresses needed for powered-off nodes

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DHCP

• Fields
• Seconds either to obtain priority or specifying
  relay delay
• flagsltgt0 reply to broadcast address
• client IP0 server fills in your id
• relay IPltgt0 server responds
• to that address
• options routers, subnet mask,
• DNS servers, time servers,
• lease time
• reuse host-name and
• boot-file name space
• option overload to store
• options there as well ?
• at most 256 options, half of which are free to use

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DHCP example

• Can have server on different net

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ICMP Router Discovery (RFC 1256) (4)

• Routers advertise every 7-10 minutes
• valid for 30 min. default
• to broadcast address or unicast to configured
  address
• Clients send sollicitations
• destination see above

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IPX solutions

• Node number is link local address
• Broadcast request for network number (1,3,5,7)
• no reply 0
• Broadcast request for best router to given
  destination network (4,6)
• replying router is indeed alive

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Routing protocols

• Generally robust, stable, simple, ....
• Provide routers with information issues
• convergence, settling time after change
• count-to-infinity problem
• lazy, reactive
• existence of (temporary) loops
• long- or short-lived
• hierarchy
• Calculate routes (routing algorithm) issues
• optimality
• delay per packet, network use
• adaptive
• deal with special conditions
• congestion, mobility
• support multiple paths

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The Optimality Principle

• Optimal paths to a certain point form a tree (or
  acyclic graph)
• Each router B needs to be able to determine
  NextB(x) the next step on the path to from B to x

1. A subnet.
2. A sink tree for router B.

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Determine the tables

• Using a distributed algorithm
• distributed shortest path
• similar to distributed spanning tree but now for
  each node as root
• basis for Distance Vector Routing
• Using a local, sequential algorithm
• based on information obtained directly from the
  point of change
• e.g. locally, the shortest path algorithm of
  Dijkstra
• basis for Link State Routing

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Dijkstras algorithm sketch

• Note the dot denotes infinity

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Distance Vector Routing

1. A subnet
2. Input from A, I, H, K, and the new routing table
   for J

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Counting to infinity

• Good news spreads rapidly
• Bad news spreads slowly
• since the distance to a destination D through a
  neighbor cannot differ very much from your own

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Link State Routing

• Each router does the following (repeatedly)
• discover neighbors, particularly, learn their
  network addresses
• using one of the neighbor discovery methods
  discussed before
• measure cost to each neighbor
• e.g. by exchanging a series of packets
• construct a packet containing this
• send this packet to all other routers
• using what route information? chicken / egg
• what if re-ordered? or delayed?
• compute locally the shortest path to every other
  router when this information is received

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Distributing the Link State Packets

• Typically, flooding
• routers recognize packets passed earlier
• sequence number
• thus avoiding the exponential packet explosion
• first receivers start changes already while
  changes are being reported
• sequence numbers wrap around or might be
  corrupted
• add an additional age field that is decremented
  once a second packets wont be too long in the
  system
• but, need additional robustness in order to deal
  with effect of packet forgeing
• acknowledgements

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Building Link State Packets

1. A subnet
2. The link state packets for this subnet

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Hierarchical Routing
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Routing for Mobile Hosts
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Routing for Mobile Hosts
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The IP way

• Internet is made up of autonomous systems (AS)
• routing within an AS interior gateway protocol
• originally, RIP routing information protocol
• distance vector, derived from Bellman-Ford
• first link state protocol was not robust enough
• Open Shortest Path First (OSPF) since 90
• routing between ASs exterior gateway protocol
• Border Routing Protocol

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OSPF

• Resources assumed to be freely usable since
  within AS
• Requirements
• Open, known to everyone
• Support several metrics
• Dynamic adaptation
• note stability is important here
• Use multiple paths load balancing
• Hierarchy
• Safeguard against forged routing info
• Tunneling support

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Represent network as a graph

• Essentially three types of connection
• point-to-point
• LAN (broadcast link)
• multi-access without broadcast

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ASs, backbones, and areas

• AS divided into numbered areas
• One area(0) is called the backbone area
• All areas connected to the backbone
• inter-area routes go through backbone
• Each router runs the link-state algorithm for
  each area it is part of
• flooding between adjacent routers
• on a LAN designated router

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Message

• IP packets

5-66
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BGP

• Between managerial (political?) domains
• not my problem....
• rule-based
• Distance vector
• but keeps track of paths
• which are matched against the rules
• and solve the counting to infinity