Customer-Provider Routing Relationships

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Customer-Provider Routing Relationships
Advertises to its neighbors that it has no paths
to any other destinations except itself

• The Global Internet consists of Autonomous
  Systems (AS) interconnected with each other
• Customer Stub AS small corporation
• Customer Multihomed AS large corporation (no
  transit)
• Provider Transit AS backbone provider networks

e.g. w, y
e.g. x
e.g. A, B, C
Group of routers
B
x
A
w
C
All traffic entering must be destined for w, all
traffic leaving must have originated from w
y
Stub AS must be prevented from forwarding traffic
between Transit ASs using Selective Route
Advertisement Policy
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Routing in the Internet

• Two-level routing
• Intra-AS administrator is responsible for choice
• Inter-AS unique standard

Border Gateway Protocol (BGP4)

• de facto standard inter-AS routing protocol
• in todays Internet
• provides each AS a means to
• obtain subnet reachability information
• (i.e. via one of its neighboring AS)
• propagate the reachability information to all
• routers internal to the AS
• determine good routes to subnets based on the
• reachability information and on AS policy.

Allows each subnet to advertise its existence to
the rest of the Internet
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Internet AS Hierarchy
AS border (exterior gateway) routers
AS interior (gateway) routers
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Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)
• Most common IGPs
• RIP Routing Information Protocol (lower-tier
  ISPs and Enterprise networks)
• OSPF Open Shortest Path First (upper-tier ISPs)
• IGRP Interior Gateway Routing Protocol (Cisco
  proprietary)

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RIP ( Routing Information Protocol)

• Distance vector algorithm
• Included in (Berkeley Software Distribution)
  BSD-UNIX Distribution in 1982
• Distance metric
• of hops (max 15 hops) (AS lt 15 hops in
  diameter)
• Can you guess why?
• Distance vectors exchange routing updates via
  Response Message (also called advertisement)
  every 30 sec
• Each advertisement route to up to 25 destination
  subnets within the AS, including the senders
  distance from each of them

Hop no. of subnets traversed along the shortest
path from Source Router to Destination Subnet,
including the Destination Subnet.
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RIP (Routing Information Protocol)
Example
z

w
x
y
A
D
B
subnet
C
Routing table in Router D
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RIP (Routing Information Protocol)
Example
z

w
x
y
A
D
B
C
Routing table in Router D
Router A has a shorter path to Z!
(30 secs. later.. D receives an advertisement
from Router A )
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RIP (Routing Information Protocol)
Example
z

w
x
y
A
D
B
C
Routing table in Router D
Router D updates its entry for destination Z
Advertisement from Router A
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RIP Link Failure and Recovery
Example

• If no advertisement is heard after 180 sec --gt
  the neighbour/link is declared dead
• Modifies routing table - routes via neighbor
  invalidated
• new advertisements sent to neighbors
• neighbours in turn send out new advertisements
  (if tables changed)
• link failure info quickly propagates to entire
  net
• poisoned reverse used to prevent ping-pong loops
  (infinite distance 16 hops)

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Routing Info Protocol (RIP) Table processing

• RIP routing tables managed by application-level
  process called route-d (daemon)
• advertisements sent in UDP packets, periodically
  repeated

Able to manipulate routing tables within the UNIX
kernel
via UDP, port 520
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OSPF (Open Shortest Path First)

• Open means publicly available
• Uses Link-State algorithm
• LS packet dissemination
• Topology map at each node
• Route computation using Dijkstra's algorithm
• OSPF advertisement carries one entry per neighbor
  router
• Advertisements disseminated to entire AS (via
  flooding)
• Carried in OSPF messages directly over IP (rather
  than TCP or UDP with upper-layer protocol of 89

Broadcasts information to all not just
neighboring routers
OSPF Protocol Functionalities reliable data
transfer, link-state broadcast, check for links
operability, extraction of neighboring routers
database of network-wide link state
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OSPF advanced features (not in RIP)
Allow only trusted routers

• Security all OSPF messages authenticated (to
  prevent malicious intrusion)
• Multiple same-cost paths allowed (only one path
  in RIP)
• Integrated uni- and multicast routing support
• Multicast OSPF (MOSPF) uses same topology data
  base as OSPF
• Hierarchical OSPF in large domains.

Most significant advancement! Has the ability to
structure an autonomous system hierarchically
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Hierarchical Open Shortest Path First (OSPF)
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Hierarchical OSPF

• Two-level hierarchy local area, backbone.
• Link-state advertisements are sent only within an
  area
• each node has detailed area topology only know
  direction (shortest path) to nets in other areas.
• Each area runs its own OSPF link-state routing
  algorithm
• Area border routers responsible for routing
  packets outside the area.
• Backbone routers run OSPF routing limited to
  backbone.
• Boundary routers connect to other ASs.

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IGRP (Interior Gateway Routing Protocol)

• CISCO proprietary successor of RIP (mid 80s)
• Uses the Distance Vector algorithm, like RIP
• several cost metrics (delay, bandwidth,
  reliability, load, etc.)
• uses TCP to exchange routing updates
• Loop-free routing via Distributed Updating Alg.
  (DUAL) based on diffused computation

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Router Architecture Overview

• Two key router functions
• run routing algorithms/protocol (RIP, OSPF, BGP)
• switching datagrams from incoming to outgoing link

Physical layer functions
Data link layer functions
computes routing tables, performs Network
management functions
Lookup forwarding functions
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Input Port Functions
Physical layer bit-level reception

• Decentralized switching
• given datagram dest., lookup output port using
  routing table in input port memory
• goal complete input port processing at 'line
  speed'
• queuing happens if datagrams arrive faster than
  forwarding rate into switch fabric

Data link layer e.g., Ethernet see chapter 5
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Input Port Queuing
Slot for Green packet is free, but there is HOL
blocking, so Green packet will have to wait

• Fabric slower than input ports combined -gt
  queueing may occur at input queues
• Head-of-the-Line (HOL) blocking queued datagram
  at front of queue prevents others in queue from
  moving forward
• queueing delay and loss due to input buffer
  overflow!

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Three types of switching fabrics
No routing processor 1 packet at a time
Like shared memory multiprocessors
2n buses that connect n input ports to n output
ports
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Switching Via Memory

• First generation routers
• packet copied by system's (single) CPU
• speed limited by memory bandwidth (2 bus
  crossings per datagram)

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Switching Via Bus

• datagram from input port memory
• to output port memory via a shared bus
• bus contention switching speed limited by bus
  bandwidth
• 1 Gbps bus, Cisco 1900 sufficient speed for
  access and enterprise routers (not regional or
  backbone)

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Switching Via An Interconnection Network

• overcome bus bandwidth limitations
• Banyan networks, other interconnection nets
  initially developed to connect processors in
  multiprocessor
• Other Advanced design fragmenting datagram into
  fixed length cells, switch cells through the
  fabric.
• Cisco 12000 switches 60 Gbps through the
  interconnection network

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Output Ports

• Buffering required when datagrams arrive from the
  fabric faster than the transmission rate
• Scheduling discipline chooses among queued
  datagrams for transmission

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Output port queueing
It is more advantageous to mark a packet before
the buffer is full in order to provide a
congestion signal to the sender

• buffering when arrival rate via switch exceeeds
  ouput line speed
• queueing (delay) and loss due to output port
  buffer overflow!

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END OF SESSION
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IPv6

• Initial motivation 32-bit address space
  completely allocated by 2008.
• Additional motivation
• header format helps speed processing/forwarding
• header changes to facilitate QoS
• new anycast address route to best of several
  replicated servers
• IPv6 datagram format
• fixed-length 40 byte header
• no fragmentation allowed

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IPv6 Header (Cont)
Priority identify priority among datagrams in
flow Flow Label identify datagrams in same flow.
(concept of flow not well
defined). Next header identify upper layer
protocol for data
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Other Changes from IPv4

• Checksum removed entirely to reduce processing
  time at each hop
• Options allowed, but outside of header,
  indicated by Next Header field
• ICMPv6 new version of ICMP
• additional message types, e.g. ''Packet Too Big''
• multicast group management functions

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Transition From IPv4 To IPv6

• Not all routers can be upgraded simultaneously
• no flag days
• How will the network operate with mixed IPv4 and
  IPv6 routers?
• Two proposed approaches
• Dual Stack some routers with dual stack (v6, v4)
  can translate between formats
• Tunneling IPv6 carried as payload in IPv4
  datagram among IPv4 routers

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Dual Stack Approach
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Tunneling
IPv6 inside IPv4 where needed