Router Architecture Overview

1
Router Architecture Overview

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

2
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 if datagrams arrive faster than
  forwarding rate into switch fabric

Data link layer e.g., Ethernet see chapter 5
3
Input Port Queuing

• Fabric slower that 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!

4
Three types of switching fabrics
5
Switching Via Memory

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

• Modern routers
• input port processor performs lookup, copy into
  memory
• Cisco Catalyst 8500

6
Switching Via a 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)

7
Switching Via An Interconnection Network

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

8
Output Ports

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

9
Output port queueing

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

10
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

11
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
12
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

13
Transition From IPv4 To IPv6

• Not all routers can be upgraded simultaneous
• 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

14
Dual Stack Approach
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
B-to-C IPv4
B-to-C IPv6
B-to-C IPv4
15
Tunneling
tunnel
Logical view
IPv6
IPv6
IPv6
IPv6
Physical view
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
E-to-F IPv6
B-to-C IPv6 inside IPv4
B-to-C IPv6 inside IPv4
16
What is mobility?

• spectrum of mobility, from the network
  perspective

mobile user, using same access point
mobile user, passing through multiple access
point while maintaining ongoing connections (like
cell phone)
mobile user, connecting/ disconnecting from
network using DHCP.
17
Mobility Vocabulary
home network permanent home of mobile (e.g.,
128.119.40/24)
home agent entity that will perform mobility
functions on behalf of mobile, when mobile is
remote
wide area network
Permanent address address in home network, can
always be used to reach mobile e.g.,
128.119.40.186
correspondent
18
Mobility more vocabulary
visited network network in which mobile
currently resides (e.g., 79.129.13/24)
Permanent address remains constant (e.g.,
128.119.40.186)
Care-of-address address in visited
network. (e.g., 79,129.13.2)
wide area network
home agent entity in visited network that
performs mobility functions on behalf of mobile.
correspondent wants to communicate with mobile
19
How do you contact a mobile friend
I wonder where Alice moved to?
Consider friend frequently changing addresses,
how do you find her?

• search all phone books?
• call her parents?
• expect her to let you know where he/she is?

20
Mobility approaches

• Let routing handle it routers advertise
  permanent address of mobile-nodes-in-residence
  via usual routing table exchange.
• routing tables indicate where each mobile located
• no changes to end-systems
• Let end-systems handle it
• indirect routing communication from
  correspondent to mobile goes through home agent,
  then forwarded to remote
• direct routing correspondent gets foreign
  address of mobile, sends directly to mobile

21
Mobility approaches

• Let routing handle it routers advertise
  permanent address of mobile-nodes-in-residence
  via usual routing table exchange.
• routing tables indicate where each mobile located
• no changes to end-systems
• let end-systems handle it
• indirect routing communication from
  correspondent to mobile goes through home agent,
  then forwarded to remote
• direct routing correspondent gets foreign
  address of mobile, sends directly to mobile

not scalable to millions of mobiles
22
Mobility registration
visited network
home network
wide area network

• End result
• Foreign agent knows about mobile
• Home agent knows location of mobile

23
Mobility via Indirect Routing
visited network
home network
wide area network
24
Indirect Routing comments

• Mobile uses two addresses
• permanent address used by correspondent (hence
  mobile location is transparent to correspondent)
• care-of-address used by home agent to forward
  datagrams to mobile
• foreign agent functions may be done by mobile
  itself
• triangle routing correspondent-home-network-mobil
  e
• inefficient when
• correspondent, mobile
• are in same network

25
Forwarding datagrams to remote mobile
Permanent address 128.119.40.186
Care-of address 79.129.13.2
26
Indirect Routing moving between networks

• suppose mobile user moves to another network
• registers with new foreign agent
• new foreign agent registers with home agent
• home agent update care-of-address for mobile
• packets continue to be forwarded to mobile (but
  with new care-of-address)
• Mobility, changing foreign networks transparent
  on going connections can be maintained!

27
Mobility via Direct Routing
correspondent forwards to foreign agent
visited network
home network
wide area network
correspondent requests, receives foreign address
of mobile
28
Mobility via Direct Routing comments

• overcome triangle routing problem
• non-transparent to correspondent correspondent
  must get care-of-address from home agent
• What happens if mobile changes networks?

29
Mobile IP

• RFC 3220
• has many features weve seen
• home agents, foreign agents, foreign-agent
  registration, care-of-addresses, encapsulation
  (packet-within-a-packet)
• three components to standard
• agent discovery
• registration with home agent
• indirect routing of datagrams

30
Mobile IP agent discovery

• agent advertisement foreign/home agents
  advertise service by broadcasting ICMP messages
  (typefield 9)

H,F bits home and/or foreign agent
R bit registration required
31
Mobile IP registration example
32
Network Layer summary

• What weve covered
• network layer services
• routing principles link state and distance
  vector
• hierarchical routing
• addressing
• IP
• Internet routing protocols RIP, OSPF, BGP
• whats inside a router?
• IPv6
• mobility

• Next stop
• the Data
• link layer!