Heirarchial Routing

8/12/2019 Heirarchial Routing

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Hierarchical Routing

Problem:as size of network grows, routing table, complexity grows millions of nodes (hosts, routers) in Internet

Solution:hierarchically aggregate nodes into "regions" (domain)

node have full knowledge of routes, topological structure within region

one (or more) nodes in region responsible for routing to the outside

Teminology:

intradomain routing: within domain

interdomain routing: between domains

autonomous system (AS): domain, region, administrative domain

gateway: routes to/from domain, a.k.a. border router


2/23

Hierarchical Routing (cont)

Three domains: A, B, C

A.a, A.b A.c run interdomain routing

protocol

A.c, B.a, B.b, C.a run intradomain

routing protocol among themselves


3/23

Hierarchical Routing (cont)

Different routing protocols can be

used for interdomain and

intradomain routing

A.a routing table:

A look inside A.c:

destination next hoph6 A.b

. A.b

h9 A.b

all other

(default route)A.c


4/23

Hosts and routers

Hosts (end systems) typically perform no routing

start packets on their way

send packets to nearest router

Q:how do hosts learn identity of nearby router:

A1:IP address of router hard-coded into file (see /etc/networks

on many UNIX systems)

A2:router discovery: RFC 1256 router periodically broadcasts its existence to attached hosts

host (on startup) broadcasts query (who is my router) on attached

links/LANs


5/23

Network Layer Case Study: the Internet

Fields in IP packet:

vers ion n umber :(of IP protocol),current version is 4, new version is 6

header length :because of options,

length of header is variable

TOS:not used, idea was to allow

different levels of reliability, real-time,

etc packet length:header plus data

identi f ier:used with IP fragmentation

to identify fragments belonging to

same original IP packet

f lags:2 bits: do not fragment, more

fragments

f ragmentat ion o f fset:if this a fragment, where it belongs in original packet

t ime-to-l ive:decremented by each router, so a packet will not loop forever in

the net

protoco l :which upper layer protocol to demultiplex to. See RFC 1700

header checksum:recomputed at each hop, as TTL changes

sourc e, dest IP address :of original sender, and eventual recipient


6/23

IP fragmentation and Reassembly

transport layer packet may be too big to send in single IP packet

underlying data link protocol will constraint maximum IP length

fragmentation: IP packet divided into fragments by IP

each fragment becomes its own IP packet

each address has same identifier, source, destination address

fragment offset gives offset of data from start of original packet

more fragment bit: 0 means last bit in this fragment

fragments not reassembled until final destination


7/23

Internet Intradomain Routing: RIP

RIP:Routing Information Protocol, uses distance vector algorithm, with

link costs of 1 shortest path

routing table sent to neighbors every 30 seconds, or when route costs change

Implemented as a daemon (user-level process)

communicates with other attached router using UDP packets note:UDP packets can be lost!

if route via neighbor not updated in 3 minutes, timeout route (set cost to infinity)

called routed on UNIX systems


8/23

A RIP routing table

Example table taken from freya.cs.umass.edu:

~ netstat -rn (note: on freya.cs.umass.edu)

DestinationGateway l RefcntUseInterf

127.0.0.1127.0.0.1 UH25 2260Io0

Default 128.119.40.254UG5 15223In0

128.119 128.119.40.195U2818 In0


9/23

Internet Intradomain Routing: OSPF

OSPF:open shortest path first open: a published standard (RFC 1247)

interior gateway protocol: for intradomain outing within an autonomous

system (AS)

uses link state algorithm to determine routes

each outgoing link (interface) assigned dimensionless cost different cost can be used for different TOS

load balancing :with several equal-cost-paths to destination, will distribute

load across both paths

Support for hierarchy:

autonomous system divided into "areas"

one area designated "backbone"

area border routers in backbone route between areas

other routers in backbone also

AS boundary router talks to outside world


10/23

Internet Intradomain Routing: OSPF

(cont)

Intra-area routing:

never cross backbone

To get from one area to another:

source area -> backbone -> destination area

area router:red

boundary router:blue


11/23

Interdomain Internet Routing: BGP

BGP:Border Gateway Protocol

routing between nodes in different autonomous systems (i.e.,

routing between networks)

RFC 1267, 1268

uses a distance verctor approach

Policy-Based Routing

rather than costs to destinations, BGP routers exchange full

path information (networks crossed) to destination router can

decide on policy basis which route to take

e.g. "traffic from my AS should not cross AS's a,b,c,d"

BGP implementation:

implemented as a daemon (user-level process)

communicates with other BGP routers using TCP


12/23

ICMP: Internet Message Control Protocol

used to communicate network-

level error conditions and info

to IP/TCP/UDP protocols or

user processes

often considered part of IP, but

ICMP message sent within IP

datagram

IP demultiplexes up to ICMP

using IP protocol field ICMP message contains IP

header and first 8 bytes of IP

contents that causes ICMP

mesage to be generated

I C M P t y p e c o d e d e s c r i p t io n

0 0 e c h o re p ly (to p in g )3 0 d e s tin a tio n n e tw o rk

u n r e a c h a b l e3 1 d e s tin a tio n h o s t

u n r e a c h a b l e

3 2 d e s tin a tio n p ro to c o lu n r e a c h a b l e

3 3 d e s tin a tio n p o rtu n r e a c h a b l e

3 6 d e s tin a tio n n e tw o rk u n k n o w n

3 7 d e s tin a tio n h o s tu n k n o w n

4 0 so u rc e q u e n c h(c o n g e s t io n c o n t ro l )

8 0 e c h o re q u e s t9 0 ro u te r a d v e r tise m e n t1 0 0 ro u te r d is c o v e ry1 1 0 T T L e x p ire d1 2 0 IP h e a d e r b a d


13/23

IPv6: next generation IP

Changes to Ipv4:

128 bit addresses (so we don't run

out of IP addresses)

header simplification (faster

processing)

more support for type of service

priorities flow identifier: identifiy packets in a

connection

security

Notes:

no fragmentation in network

packet too big generates ICMP error to source source fragmentation via extension header

no checksum (already done at transport and data link layer)


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Transitioning from IPv4 to IPv6

Internet too big for "flag day":

can't turn off all IP routers, install IPv6 and reboot

IPv4 nodes will be legacy

IPv6 nodes can route IPv4 packets

IPv4 nodes can not route IPv6 packets

Tunneling:

source and destination speak network protocol X

physically intermediate nodes speak network protocol Y

source takes protocol X packet, sticks it inside (encapsulates) protocol Y

packet

intermediate nodes route using protocol Y

destination receives packet using protocol Y, removes protocol X packet

network between source and destination looks like a single link to

protocol X


15/23

Tunneling: a pictorial view


16/23

Case Study: ATM Network Layer

ATM: packet (cell) format:

UNI: user-network interface

(host-to-switch)

NNI: network-network interface

(switch-to-switch) GFC: generic flow control

(unused)

VPI: virtual path identifier VCI: virtual circuit identifer

VPI and VCI together a call/connection identifier

PTI: payload type: 3 bits

111: RM cell (recall RM congestion control)

000: user cell 010: user cell, congestion experienced (recall EFCI)

CLP: cell loss priority (1 bit)

priority bit for discarding

HEC: header error correction

DATA: 48 bytes of data


17/23

Observations about ATM Cell

very small

reflecting telephony origins

48 bytes a compromise, halfway 64 and 32

no explicit source/destination address VCI/VPI used instead

faster switching (VPI/VCI can index into table)

28 bit VPI/VCI for switching instead of 128 bit IP address in IPv6

(savings)

fixed length for faster switching

minimal priority


18/23

ATM networks: Virtual-circuit Oriented

VCI/VPI together identify call

multiple calls (VCI) bundled into same VP

network can switch on VP basis only

less state (network only sees VP's)

all VC's in VP follow same path


19/23

Connection Setup in ATM

messages ("signaling") used to setup up call through network

state info (VP switching info - which output line to switch

incoming VC) set up in switches

meaning of call setup messages:

message sent byhost tonet receivedfromnetSETUP establish

connectionincomingcall

CALLPROCEEDING

host seescall Networkattemptingcall

CONNECT I accept incomingcall

Your call accepted

CONNECTACK

ACKreceivedCONNECT

ACKreceivedCONNECT


20/23

ATM Call Setup (cont)Observations:

unlike Internet, switches involved in call setup

state creation

ACKing between switches

wait one RTT before sending data

unlike UDP

same as TCP

what if connection breaks?

other switches must remove state

ATM standard does not specify a routing protocol


21/23

Switches and Routers: What's Inside

Input interface cards:

physical layer processing

memory buffers to hold incoming packet

Switch fabric:

to move packets from input to output

Output interface cards:

memory buffers to hold outgoing packets

physical layer processing

Control processor:routing table updates, supervisory (management)functions

will typically not touch the packets being switched


22/23

Switching Fabrics

Two popular ways to switch:

switching via memory:input line ports write to memory, output

ports read from memory

switching via a bus:bus (backplane) connects input and

output ports

e.g.: Cisco AGS+ has 533 Mbps backblane bus


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Network Layer: Summary

Network service:datagram versus VC

Theory of routing protocols

link state and distance vector multicast

broadcasting

Case studies:

Internet

IPv4, IPv6

protocols for exchanging routing information: RIP, OSPF, BGP

ATM

Heirarchial Routing

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