Routing

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Routing
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Connectionless Network Layers

• Destination, source, hop count
• Maybe other stuff
• fragmentation
• options (e.g., source routing)
• error reports
• special service requests (priority, custom
  routes)
• congestion indication
• Real diff size of addresses

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Comparative Addresses

• IPv4 4 bytes, boundary depends on mask
• IPX 10 bytes 4link, 6node
• AppleTalk 2link, 1node
• CLNP variable length, 14area, 6node
• IPv6 16 bytes 8link, 8node (?)

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IPv4 data packet
version
hdr lnth
TOS
total length
2
pkt id
2
df mf
offset
offset (contd)
Dont Fragment More Fragments
TTL (time to live)
protocol
hdr checksum
2
source
4
TCP, UDP
destination
4
options
variable
variable
padding
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IPv6
vers
TOS
flow label (20 bits)
(4 bits)
(8 bits)
payload length
next
hops remain
source
destination
hop by hop hdr, or rtg hdr, or authentication
hdr, or end-to-end, or TCP, or ...
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Distributed Routing Protocols

• Rtrs exchange control info
• Use it to calculate forwarding table
• Two basic types
• distance vector (DECnet, old ARPANET, RIP)
• link state (new ARPANET 1980, DECnet Phase V
  1985, IS-IS 1988, OSPF version 2 1998).

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Distance Vector Routing

• Rtr knows
• own ID
• how many cables hanging off box
• cost, for each cable, of getting to nbr

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Distance Vector (DV) Routing

• Initialize distances to all rtrs in the network
  to be 0, except to its nbrs.
• Rtr learns from nbrs their distances to all nodes
  in the network, calculate own distances, and
  forward the distance vector to nbrs. This repeats
  until the distance vector converges.
• Rtr updates the distance vector whenever it
  receives different distance vector from some nbr,
  or whenever some link breaks.
• Distance vector is either sent periodically or
  when the network configuration changes.

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Example of DV Routing
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Problems with Distance Vector Routing

• B does not conclude that C is unreachable but
  that d(B,C)d(B,A)d(A,C) 3
• When A receives DV from B it concludes that
  d(A,C)4
• DV increases in this until infinity, or maximum
  value which is set by administrator. For this
  reason, the cost field has the small size.

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Split Horizon

• This technique sometime prevents counting toward
  infinity.
• If R1 forwards packets to D through R2, then R2
  informs R1 that its distance to D is infinity.
• So, when the link toward node D fails, R2
  concludes that its distance to D is infinity
  immediately, i.e. that D is unreachable.

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Split Horizon

• Unfortunatelly, split horizon does not always
  work.
• When link to D fails, R1 concludes that D is
  unreachable.
• R2 gets the information from R1 that D is
  unreachable, and sets the path to D through R2,
  calculating DV based on DC of R2, and vice versa.
  

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Link State Routing

• Construct Link State Packet (LSP)
• who you are
• list of (nbr, cost) pairs
• Broadcast LSPs to all rtrs
• Store latest LSP from each rtr received from nbrs
• Compute Routes
• Forward LSPs from each nbr to other nbrs

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Building Link State Packets

• (a) A subnet. (b) The link state packets for
  this subnet.

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Broadcasting LSP

• LSPs are distributed through flooding
• send to every nbr except from which LSP rcvd
• LSP is updated only if it has a higher sequence
  number than the existing one, or if its age
  exceeded the maximum age.
• Rtr forwards only updated LSPs, and it generates
  new LSPs periodically or when there is a
  configuration change (link cost has changed, nbr
  is down).

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Fixing the Algorithm

• Require LSPs to age at every hop
• Make sequence number large and linear
• Careful synchronization between nbrs
• At most one LSP from one source
• Each LSP has flags for acknowledgements and
  transmissions to nbrs.
• When LSP is received from some nbr its
  corresponding ack flag is set, as well as its
  send flags to other nbrs.
• Acknowledgments for LSP reception from one nbr
  are sent to it in a round-robin fashion. LSPs
  with the send flags for some nbr set, are sent to
  it also in a round-robin fashion.

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Arithmetic in Circular Space

• Sequence number a is smaller than sequence number
  b when it holds

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Distributing the Link State Packets

• The packet buffer for router B in the previous
  slide (Fig. 5-13).

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Computing Routes

• Edsgar Dijkstras algorithm
• calculate tree of shortest paths from self to
  each
• also calculate cost from self to each
• Algorithm
• step 0 put (SELF, 0) on tree
• step 1 look at LSP of node (N,c) just put on
  tree. If for any nbr K, this is best path so far
  to K, put (K, cdist(N,K)) on tree, child of N,
  with dotted line
• step 2 make dotted line with smallest cost
  solid, go to step 1

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Example of Dijkstra Algorithm
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Example of Dijkstra Algorithm
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Example of Dijkstra Algorithm
Forwarding table A/B B/B C/self D/B E/B F/F G/F
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Distance Vector vs Link State

• Memory distance vector wins (but memory is
  cheap)
• Computation debatable
• Simplicity of coding simple distance vector
  wins.
• Convergence speed link state better
• Functionality link state can have custom routes,
  mapping the net, easier troubleshooting,

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Specific Routing Protocols

• Interdomain vs Intradomain
• Intradomain link state (OSPF, IS-IS) vs distance
  vector (RIP)
• Interdomain
• static routing
• EGP
• BGP
• ?

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Routing Information Protocol (RIP)

• Packets are requests and responses.
• Report through response every destination every
  30 seconds, or as a reply to request.
• Throw away info if too old (90? for IP)
• Request when a rtr comes up or when info is too
  old
• Maximum cost is 16
• Most implementations of IP RIP do
• split horizon
• triggered updates
• poison reverse (rtr that learns about link fail
  announce the distance through it as infinity).

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Link State Routing Protocols

• Intermediate system-intermediate system (IS-IS)
  is ISO standard Netware link state protocol
  (NLSP) is modification of IS-IS Private
  network-to-network interface (PNNI) for ATM Open
  shortest path first (OSPF)
• Similarities and differences hierarchy, area
  addresses, LANs, parameter synchronization,
  number of destinations per LSP, LSP database
  overload, authentication.

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IS-IS Pkt Types

• Hello
• pt-to-pt
• LAN (extra stuff like LAN name, 2-way
  connectivity check)
• Sequence number packet (SNP)
• CSNP (complete), for LAN sync, and startup
• PSNP (partial), for acking one or more LSPs
• LSPs.

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OSPF Pkt Types

• Hellos
• Database description
• Startup
• Link state request
• Link state update
• Multiple LSAs
• Link state ack
• Links state advertisement (LSA)
• type 1 LSA (like IS-IS ordinary LSP)
• type 2 LSA (like IS-IS LSP on a LAN)
• types 3, 4, 5, external info

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OSPF types 3, 4, and 5 LSAs
area border router
IP prefix
AS border rtr
3
AS
3
3
area
3
5
5
4
4
5
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OSPF

• Runs on the top of IP with protocol field 89.
• Comprises two levels of hierarchy areas and
  backbone.
• Boarder routers of some domain calculate their
  costs to the destinations outside the domain and
  flood the information into the area, so that area
  routers can calculate optimal path.

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OSPF

• Hierarchy OSPF has two levels of hierarchy.
  Boarder routers of any area calculate their costs
  to the boarder routers of the autonomous system
  (AS) and inject those to the area. The AS boarder
  routers report their cost to the destinations
  outside of the area.
• Area addresses area has ID (4 bajta), where
  0.0.0.0 denotes level 2 in hierarchy. No
  possibility for dynamic merging or splitting the
  areas.

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OSPF

• Routing in LAN DR expects an acknowledgment from
  LAN routers for each link stage advertisement
  (LSA). A backup DR (BDR) keeps the replicated LSA
  database. Whenever some LAN router sends LSA it
  multicasts it to DR and BDR. Acks are also
  multicast to DR and BDR. If there is no ack, LSA
  is sent to the individual router.
• Parameter sync. HelloInterval and
  RouterDeadInterval are specified in Hello
  messages, and have to be the same in the nbrs.
  This is limitation when the parameters is to be
  changed.
• Startup master/slave database description
  protocol where LSAs are explicitly sent and acked
  and only after that is complete does link come
  up.

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OSPF

• One destination can be advertised in one LSA.
• An overload protection is option in RFC 1765. All
  routers receive the same max external link state
  information. Rtrs can purge the info that they
  transmit if their databases are overloaded.
• Authentication is set in the link state update
  message comprising multiple LSAs. It is same for
  the two directions of a link. Each rtr changes
  authentication.

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Hierarchical Routing

• Hierarchical routing.

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Exterior Gateway Protocol (EGP)

• Like RIP, but no metrics. Just if reachable. Rtr
  inside a domain collects reachability information
  and informs the rtr on the boarder of the domain.
  Boarder rtr informs the internal rtr about
  reachability outside the domain.
• Rtrs establish com with pkts nbr acquisition
  request, nbr acquisition reply or refusal, nbr
  cease request, nbr cease ack.
• Theoretically only legal topology (but tree would
  work)

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EGP Does not Support Loops
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Border Gateway Protocol (BGP)

• Replacement of EGP, with policies
• Path vector Instead of distances, rtrs exchange
  info about path, sequence of AS. Given reported
  paths to D from each nbr, and configured
  preferences, choose your path to D
• dont ever route through domain X, or not to D,
  or only as last resort
• Other policies dont tell nbr about D, or lie to
  nbr about D making path look worse

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BGP Atributes and Pkts

• Origin (well-known, mandatory) can be IGP, EGP or
  incomplete AS path (well-known, mandatory) 2
  octets for each AS along the path Next hop
  (well-known, mandatory), Unreachable (well-known,
  discretionary) Intra AS metric (optional,
  non-transitive) to help to rtrs of nbr AS to
  calculate optimal path Community (optional,
  non-transitive) to establish a unique policy
• Packets are Open establish com between rtrs of
  different AS Update carries routing info
  Notification last message before a connection is
  closed Keepalive to inform about presence of
  nbr.

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BGP Configuration

• Path preference rules
• Which nbr to tell about which destinations
• How to edit the path when telling nbr N about
  prefix P (add fake hops to discourage N from
  using you to get to P)
• Possible policies that dont converge
• Lots of theoretical problems, and in practice

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E-BGP vs I-BGP

• Talking to peer within domain I-BGP
• Talking to peer in another domain E-BGP
• Original I-BGP had to be fully connected
• To improve things
• use confederations to break domain into smaller
  domains (each fully connected I-BGP)
• use route reflecter, start topology with BGP
  router in domain in center, passing routing info

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BGP Confederations

• Originally so could group lots of domains into
  super-domain
• only one policy
• path looks shorter
• does constrain path (since cant have domain
  twice)

S
d1
d2
d3
FOO
d4
d7
d5
d6
D
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Multicast Routing
(a) A network. (b) A spanning tree for the
leftmost router. (c) A multicast tree for
group 1. (d) A multicast tree for group 2.
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Routing for Mobile Hosts

• A WAN to which LANs, MANs, and wireless cells are
  attached.

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References

• Radia Perlman, Interconnections Bridges,
  Routers, Switches and Internetworking Protocols,
  Addison-Wesley January 2000.