multimedia communication & networks

13PIT101

Multimedia Communication & Networks

UNIT – II

Dr.A.Kathirvel

Professor & Head/IT - VCEW

Unit - II

Intra AS routing – Inter AS routing – Router

Architecture – Switch Fabric – Active Queue

Management – Head of Line blocking –

Transition from IPv4 to IPv6 – Multicasting –

Abstraction of Multicast groups – Group

Management – IGMP – Group Shared

Multicast Tree – Source based Multicast Tree –

Multicast routing in Internet – DVMRP and

MOSPF – PIM – Sparse mode and Dense

mode

INTRA AS ROUTING

#4

The Internet Network layer

routing

table

Host, router network layer functions:

Routing protocols

•path selection

•RIP, OSPF, BGP

IP protocol

•addressing conventions

•datagram format

•packet handling conventions

ICMP protocol

•error reporting

•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Network

layer

#5

Hierarchical Routing

scale: with 50 million

destinations:

• can’t store all dest’s in routing tables!

• routing table exchange would

swamp links!

administrative autonomy

• internet = network of networks

• each network admin may want to

control routing in its own network

Our routing study thus far - idealization

all routers identical

network “flat”

… not true in practice

#6

Hierarchical Routing

• aggregate routers into regions, “autonomous systems” (AS)

• routers in same AS run same routing protocol

– “intra-AS” routing protocol

– routers in different AS can run different intra-AS routing protocol

• special routers in AS

• run intra-AS routing

protocol with all other

routers in AS

• also responsible for routing

to destinations outside AS

– run inter-AS routing

protocol with other

gateway routers

gateway routers

#7

Intra-AS and Inter-AS routing

Gateways: •perform inter-AS

routing amongst

themselves

•perform intra-AS

routers with other

routers in their AS

inter-AS, intra-AS routing

in

gateway A.c

network layer

link layer

physical layer

a

b

b

a

a C

A

B

d

A.a

A.c

C.b B.a

c

b

c

#8

Intra-AS and Inter-AS routing

Host

h2 a

b

b

a

a C

A

B

d c

A.a

A.c

C.b B.a

c

b

Host

h1

Intra-AS routing

within AS A

Inter-AS

routing

between

A and B

Intra-AS routing

within AS B

We’ll examine specific inter-AS and intra-AS Internet

routing protocols shortly

#9

Routing: Example

AS A

(OSPF)

AS B

(OSPF intra routing)

AS D

AS C

i

b

a1

a2

d

E

F

AS I

i2

No Export

to F

#10

Routing: Example

AS A

(OSPF)

AS B

(OSPF intra routing)

AS D

AS C

i

b

How to specify?

a1

a2

d

E

F

AS I

d1

d2

#11

IP Addressing Scheme

• We need an address to uniquely identify each destination

• Routing scalability needs flexibility in aggregation of destination addresses – we should be able to aggregate a set of

destinations as a single routing unit

• Preview: the unit of routing in the Internet is a network---the destinations in the routing protocols are networks

#12

IP Addressing: introduction

• IP address: 32-bit identifier for host, router interface

• interface: connection between host, router and physical link

– router’s typically have multiple interfaces

– host may have multiple interfaces

– IP addresses associated with interface, not host, or router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 1 1

#13

IP Addressing • IP address:

– network part

• high order bits

– host part

• low order bits

• What’s a network ? (from

IP address perspective)

– device interfaces with

same network part of IP

address

– can physically reach each

other without intervening

router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

network consisting of 3 IP networks

(for IP addresses starting with 223,

first 24 bits are network address)

LAN

#14

IP Addressing

How to find the networks?

• Detach each interface

from router, host

• create “islands of isolated networks

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2 223.1.2.1

223.1.2.6

223.1.3.2 223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1

223.1.8.0 223.1.8.1

223.1.9.1

223.1.9.2

Interconnected

system consisting

of six networks

#15

IP Addresses

0 network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class

1.0.0.0 to

127.255.255.255

128.0.0.0 to

191.255.255.255

192.0.0.0 to

223.255.255.255

224.0.0.0 to

239.255.255.255

32 bits

given notion of “network”, let’s re-examine IP addresses:

“class-full” addressing:

#16

IP addressing: CIDR

• classful addressing: – inefficient use of address space, address space exhaustion

– e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network

• CIDR: Classless InterDomain Routing – network portion of address of arbitrary length

– address format: a.b.c.d/x, where x is # bits in network portion of address

11001000 00010111 00010000 00000000

network

part

host

part

200.23.16.0/23

#17

CIDR Address Aggregation

AS A

(OSPF)

AS D

i

a1

a2

d

i->a1: I can reach

130.132/16; my path:

I

AS I

d1

130.132.1/24

130.132.2/24

130.132.3/24

intradomain routing

uses /24

#18

CIDR Address Aggregation

x00/24: B

x01/24: C

x10/24: E

x11/24: F

A

B

C

E

F

G

#19

IP addresses: how to get one?

Hosts (host portion):

• hard-coded by system admin in a file

• DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play”

– host broadcasts “DHCP discover” msg

– DHCP server responds with “DHCP offer” msg

– host requests IP address: “DHCP request” msg

– DHCP server sends address: “DHCP ack” msg – The common practice in LAN and home access (why?)

#20

IP addresses: how to get one?

Network (network portion):

• get allocated portion of ISP’s address space: ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23

... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

#21

Hierarchical addressing: route aggregation

“Send me anything

with addresses

beginning

200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7 Internet

Organization 1

ISPs-R-Us “Send me anything

with addresses

beginning

199.31.0.0/16”

200.23.20.0/23

Organization 2

.

.

.

.

.

.

Hierarchical addressing allows efficient advertisement of routing

information:

#22

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anything

with addresses

beginning

200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7 Internet

Organization 1

ISPs-R-Us “Send me anything

with addresses

beginning 199.31.0.0/16

or 200.23.18.0/23”

200.23.20.0/23

Organization 2

.

.

.

.

.

.

#23

Network Address Translation: Motivation

192.168.1.2

192.168.1.3

192.168.1.4

192.168.1.1

138.76.29.7

local network

(e.g., home network)

192.168.1.0/24

rest of

Internet

Datagrams with source or

destination in this network

have 192.168.1/24 address for

source, destination (as usual)

All datagrams leaving local

network have same single source NAT IP

address: 138.76.29.7,

different source port numbers

A local network uses just one public IP address as far as outside world is

concerned

Each device on the local network is assigned a private IP address

#24

NAT: Network Address Translation

Implementation: NAT router must:

– outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

– remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

– incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

#25


192.168.1.2

S: 192.168.1.2, 3345

D: 128.119.40.186, 80

1

192.168.1.1

138.76.29.7

1: host 192.168.1.2

sends datagram to

128.119.40.186, 80

NAT translation table

WAN side addr LAN side addr

138.76.29.7, 5001 192.168.1.2, 3345

…… ……

S: 128.119.40.186, 80

D: 192.168.1.2, 3345

4

S: 138.76.29.7, 5001

D: 128.119.40.186, 80 2

2: NAT router

changes datagram

source addr from

192.168.1.2, 3345 to

138.76.29.7, 5001,

updates table

S: 128.119.40.186, 80

D: 138.76.29.7, 5001

3

3: Reply arrives

dest. address:

138.76.29.7, 5001

4: NAT router

changes datagram

dest addr from

138.76.29.7, 5001 to 192.168.1.2, 3345

192.168.1.3

192.168.1.4

#26

Network Address Translation: Advantages

• No need to be allocated range of addresses from ISP: - just one public IP address is used for all devices

– 16-bit port-number field allows 60,000 simultaneous connections with a single LAN-side address !

– can change ISP without changing addresses of devices in local network

– can change addresses of devices in local network without notifying outside world

• Devices inside local net not explicitly addressable, visible by outside world (a security plus)

#27


• If both hosts are behind different NAT, they will have difficulty establishing connection

• NAT is controversial:

– routers should process up to only layer 3

– violates end-to-end argument

• NAT possibility must be taken into account by app designers, e.g., P2P applications

– address shortage should instead be solved by having more addresses --- IPv6

#28

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers

– allocates addresses

– manages DNS

– assigns domain names, resolves disputes

#29

Getting a datagram from source to dest.

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

A

B

E

IP datagram:

misc

fields

source

IP addr dest

IP addr data

datagram remains unchanged,

as it travels source to

destination

addr fields of interest here

mainly dest. IP addr

Dest. Net. next router Nhops

223.1.1 1 223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2

routing table in A

#30


223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

A

B

E

Starting at A, given IP datagram

addressed to B:

look up net. address of B

find B is on same net. as A

link layer will send datagram directly

to B inside link-layer frame

B and A are directly connected


223.1.1 1 223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2

misc

fields 223.1.1.1 223.1.1.3 data

#31


223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

A

B

E


223.1.1 1 223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2 Starting at A, dest. E:

look up network address of E

E on different network

A, E not directly attached

routing table: next hop router to E

is 223.1.1.4

link layer sends datagram to router

223.1.1.4 inside link-layer frame

datagram arrives at 223.1.1.4

continued…..

misc

fields 223.1.1.1 223.1.2.2 data

#32


223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2 223.1.3.1

223.1.3.27

A

B

E

Arriving at 223.1.4, destined for

223.1.2.2

look up network address of E

E on same network as router’s interface 223.1.2.9

router, E directly attached

link layer sends datagram to

223.1.2.2 inside link-layer frame

via interface 223.1.2.9

datagram arrives at 223.1.2.2!!!

(hooray!)

misc

fields 223.1.1.1 223.1.2.2 data network router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

Dest. next

#33

IP datagram format

ver T

32 bits

data

(variable length,

typically a TCP

or UDP segment)

16-bit identifier

Internet

checksum

time to

live

32 bit source IP address

IP protocol version

number

header length

(bytes)

max number

remaining hops

(decremented at

each router)

for

fragmentation/

reassembly

total datagram

length (bytes)

upper layer protocol

to deliver payload to

head.

len

type of

service “type” of data

flgs fragment

offset upper

layer

32 bit destination IP address

Options (if any) E.g. timestamp,

record route

taken, specify

list of routers

to visit.

4-34

IP Fragmentation & Reassembly

• network links have MTU

(max.transfer size) - largest

possible link-level frame.

– different link types, different

MTUs

• large IP datagram divided

(“fragmented”) within net – one datagram becomes several

datagrams

– “reassembled” only at final destination

– IP header bits used to identify,

order related fragments

fragmentation:

in: one large datagram

out: 3 smaller datagrams

reassembly

Network Layer 4-35

IP Fragmentation and Reassembly

ID

=x offset

=0

fragflag

=0

length

=4000

ID

=x offset

=0

fragflag

=1

length

=1500

ID

=x offset

=185

fragflag

=1

length

=1500

ID

=x offset

=370

fragflag

=0

length

=1060

One large datagram becomes

several smaller datagrams

Example

4000 byte datagram

MTU = 1500 bytes

1480 bytes in

data field

offset =

1480/8

Lecture 6: Network Layer #36

Routing in the Internet

• The Global Internet consists of Autonomous Systems (AS)

interconnected with each other:

– Stub AS: small corporation

– Multihomed AS: large corporation (no transit)

– Transit AS: provider

• Two-level routing:

– Intra-AS: administrator is responsible for choice

– Inter-AS: unique standard


Internet AS Hierarchy

Inter-AS border (exterior gateway) routers

Intra-AS interior (gateway) routers


Intra-AS Routing

• Also known as Interior Gateway Protocols (IGP)

• Most common IGPs:

– RIP: Routing Information Protocol

– OSPF: Open Shortest Path First

– IGRP: Interior Gateway Routing Protocol (Cisco

propr.)


RIP ( Routing Information Protocol)

• Distance vector algorithm

• Included in BSD-UNIX Distribution in 1982

• Distance metric: # of hops (max = 15 hops)

– why?

• Distance vectors: exchanged every 30 sec via Response

Message (also called advertisement)

• Each advertisement: route to up to 25 destination nets


RIP (Routing Information Protocol)

Destination Network Next Router Num. of hops to dest.

w A 2

y B 2

z B 7

x -- 1 …. …. ....

w x y

z

A

C

D B

Routing table in D


RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor/link declared

dead

– routes via neighbor invalidated

– new advertisements sent to neighbors

– neighbors in turn send out new advertisements (if

tables changed)

– link failure info quickly propagates to entire net

– poison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)


OSPF (Open Shortest Path First)

• “open”: 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)


OSPF “advanced” features (not in RIP)

• Security: all OSPF messages authenticated (to prevent

malicious intrusion); TCP connections used

• Multiple same-cost paths allowed

– only one path in RIP

• For each link, multiple cost metrics for different ToS (eg,

satellite link cost set “low” for best effort; high for real time) • Integrated uni- and multicast support:

– Multicast OSPF (MOSPF) uses same topology data base as OSPF

• Hierarchical OSPF in large domains.


Hierarchical OSPF


Hierarchical OSPF

• Two-level hierarchy: local area, backbone.

– Link-state advertisements only in area

– each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.

• Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

• Backbone routers: run OSPF routing limited to backbone.

• Boundary routers: connect to other ASs.


IGRP (Interior Gateway Routing Protocol)

• CISCO proprietary; successor of RIP (mid 80s)

• Distance Vector, 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


Inter-AS routing


Internet inter-AS routing: BGP

• BGP (Border Gateway Protocol): the de facto standard

• Path Vector protocol:

– similar to Distance Vector protocol

– each Border Gateway broadcast to neighbors

(peers) entire path (I.e, sequence of ASs) to

destination

– E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z



Suppose: gateway X send its path to peer gateway W • W may or may not select path offered by X

– cost, policy (don’t route via competitors AS), loop prevention reasons.

• If W selects path advertised by X, then: Path (W,Z) = W, Path (X,Z)

• Note: X can control incoming traffic by controlling its route advertisements to peers: – e.g., don’t want to route traffic to Z -> don’t advertise any routes

to Z



• BGP messages exchanged using TCP.

• BGP messages:

– OPEN: opens TCP connection to peer and

authenticates sender

– UPDATE: advertises new path (or withdraws old)

– KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN request

– NOTIFICATION: reports errors in previous msg;

also used to close connection


Why different Intra- and Inter-AS routing ?

Policy:

• Inter-AS: admin wants control over how its traffic routed, who

routes through its net.

• Intra-AS: single admin, so no policy decisions needed

Scale:

• hierarchical routing saves table size, reduced update traffic

Performance:

• Intra-AS: can focus on performance

• Inter-AS: policy may dominate over performance

Extra


Network Layer 4-53

ICMP: Internet Control Message Protocol

• used by hosts & routers to

communicate network-level

information

– error reporting: unreachable host,

network, port, protocol

– echo request/reply (used by ping)

• network-layer “above” IP: – ICMP msgs carried in IP

datagrams

• ICMP message: type, code plus first 8

bytes of IP datagram causing error

Type Code description

0 0 echo reply (ping)

3 0 dest. network unreachable

3 1 dest host unreachable

3 2 dest protocol unreachable

3 3 dest port unreachable

3 6 dest network unknown

3 7 dest host unknown

4 0 source quench (congestion

control - not used)

8 0 echo request (ping)

9 0 route advertisement

10 0 router discovery

11 0 TTL expired

12 0 bad IP header

Network Layer 4-54

Traceroute and ICMP

• Source sends series of UDP

segments to dest

– First has TTL =1

– Second has TTL=2, etc.

– Unlikely port number

• When nth datagram arrives to nth

router:

– Router discards datagram

– And sends to source an ICMP

message (type 11, code 0)

– Message includes name of

router& IP address

• When ICMP message arrives, source calculates RTT

• Traceroute does this 3 times

Stopping criterion

• UDP segment eventually arrives at destination host

• Destination returns ICMP “dest port unreachable” packet (type 3, code 3)

• When source gets this ICMP, stops.

Example: tracert www.yahoo.com

Tracing route to www-real.wa1.b.yahoo.com [69.147.76.15]

over a maximum of 30 hops:

1 <1 ms <1 ms <1 ms 132.67.250.1

2 <1 ms 1 ms <1 ms dmz-cc-gw.math.tau.ac.il [132.67.252.2]

3 <1 ms <1 ms <1 ms tel-aviv.tau.ac.il [132.66.4.1]

4 1 ms <1 ms <1 ms gp1-tau-ge.ilan.net.il [128.139.191.70]

5 1 ms * 1 ms gp0-gp1-te.ilan.net.il [128.139.188.2]

6 87 ms 86 ms 87 ms iucc.rt1.fra.de.geant2.net [62.40.125.121]

7 87 ms 87 ms 87 ms TenGigabitEthernet7-3.ar1.FRA4.gblx.net [207.138.144.45]

8 177 ms 177 ms 177 ms 204.245.39.226

9 180 ms 177 ms 265 ms ae1-p151.msr2.re1.yahoo.com [216.115.108.23]

10 177 ms 177 ms 177 ms te-9-4.bas-a2.re1.yahoo.com [66.196.112.203]

11 177 ms 177 ms 177 ms f1.www.vip.re1.yahoo.com [69.147.76.15]

Trace complete.

Network Layer 4-56

IPv6

• Initial motivation: 32-bit address space soon to

be completely allocated.

• Additional motivation:

– header format helps speed processing/forwarding

– header changes to facilitate QoS

IPv6 datagram format:

– fixed-length 40 byte header

– no fragmentation allowed

Network Layer 4-57

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

Network Layer 4-58

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

Network Layer 4-59

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?

• Tunneling: IPv6 carried as payload in IPv4

datagram among IPv4 routers

Network Layer 4-60

Tunneling A B E F

IPv6 IPv6 IPv6 IPv6

tunnel Logical view:

Physical view: A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: X

Src: A

Dest: F

data

Flow: X

Src: A

Dest: F

data

Flow: X

Src: A

Dest: F

data

Src:B

Dest: E

Flow: X

Src: A

Dest: F

data

Src:B

Dest: E

A-to-B:

IPv6

E-to-F:

IPv6 B-to-C:

IPv6 inside

IPv4

B-to-C:

IPv6 inside

IPv4

IPv6 status report • Operating systems –

– wide support – early 2000

– Windows (2000, XP, Vista), BSD, Linux, Apple

• Networking infrastructure – Cisco

• Deployment – Slow

• Penetration – Host - minor (less than 1%)

– Used in 2008 in China Olympic games

• Motivation: CIDR & NAT

Lecture 7: Network Layer II #61

Active Queue Management

Queuing Disciplines

• Each router must implement some queuing

discipline

• Queuing allocates both bandwidth and buffer

space:

– Bandwidth: which packet to serve (transmit) next

– Buffer space: which packet to drop next (when

required)

• Queuing also affects latency

Typical Internet Queuing

• FIFO + drop-tail – Simplest choice

– Used widely in the Internet

• FIFO (first-in-first-out) – Implies single class of traffic

• Drop-tail – Arriving packets get dropped when queue is full regardless

of flow or importance

• Important distinction: – FIFO: scheduling discipline

– Drop-tail: drop policy

FIFO + Drop-tail Problems

• Leaves responsibility of congestion control to

edges (e.g., TCP)

• Does not separate between different flows

• No policing: send more packets get more

service

• Synchronization: end hosts react to same

events

Active Queue Management

• Design active router queue management to aid

congestion control

• Why?

– Routers can distinguish between propagation and

persistent queuing delays

– Routers can decide on transient congestion, based

on workload

Active Queue Designs

• Modify both router and hosts

– DECbit – congestion bit in packet header

• Modify router, hosts use TCP

– Fair queuing

• Per-connection buffer allocation

– RED (Random Early Detection)

• Drop packet or set bit in packet header as soon as

congestion is starting

Internet Problems

• Full queues

– Routers are forced to have have large queues to maintain high utilizations

– TCP detects congestion from loss

• Forces network to have long standing queues in steady-state

• Lock-out problem

– Drop-tail routers treat bursty traffic poorly

– Traffic gets synchronized easily allows a few flows to monopolize the queue space

Design Objectives

• Keep throughput high and delay low

• Accommodate bursts

• Queue size should reflect ability to accept

bursts rather than steady-state queuing

• Improve TCP performance with minimal

hardware changes

Lock-out Problem

• Random drop

– Packet arriving when queue is full causes some

random packet to be dropped

• Drop front

– On full queue, drop packet at head of queue

• Random drop and drop front solve the lock-out

problem but not the full-queues problem

Full Queues Problem

• Drop packets before queue becomes full (early

drop)

• Intuition: notify senders of incipient

congestion

– Example: early random drop (ERD):

• If qlen > drop level, drop each new packet with fixed

probability p

• Does not control misbehaving users

Random Early Detection (RED)

• Detect incipient congestion, allow bursts

• Keep power (throughput/delay) high

– Keep average queue size low

– Assume hosts respond to lost packets

• Avoid window synchronization

– Randomly mark packets

• Avoid bias against bursty traffic

• Some protection against ill-behaved users

RED Algorithm

• Maintain running average of queue length

• If avgq < minth do nothing

– Low queuing, send packets through

• If avgq > maxth, drop packet

– Protection from misbehaving sources

• Else mark packet in a manner proportional to

queue length

– Notify sources of incipient congestion

RED Operation

Min thresh Max thresh

Average Queue Length

minth maxth

maxP

1.0

Avg queue length

P(drop)

RED Algorithm

• Maintain running average of queue length

– Byte mode vs. packet mode – why?

• For each packet arrival

– Calculate average queue size (avg)

– If minth ≤ avgq < maxth

• Calculate probability Pa

• With probability Pa

– Mark the arriving packet

• Else if maxth ≤ avg

– Mark the arriving packet

Queue Estimation

• Standard EWMA: avgq - (1-wq) avgq + wqqlen

– Special fix for idle periods – why?

• Upper bound on wq depends on minth

– Want to ignore transient congestion

– Can calculate the queue average if a burst arrives

• Set wq such that certain burst size does not exceed minth

• Lower bound on wq to detect congestion relatively quickly

• Typical wq = 0.002

Extending RED for Flow Isolation

• Problem: what to do with non-cooperative flows?

• Fair queuing achieves isolation using per-flow state – expensive at backbone routers – How can we isolate unresponsive flows without

per-flow state?

• RED penalty box – Monitor history for packet drops, identify flows

that use disproportionate bandwidth

– Isolate and punish those flows

FRED

• Fair Random Early Drop (Sigcomm, 1997)

• Maintain per flow state only for active flows

(ones having packets in the buffer)

• minq and maxq min and max number of

buffers a flow is allowed occupy

• avgcq = average buffers per flow

• Strike count of number of times flow has

exceeded maxq

FRED – Fragile Flows

• Flows that send little data and want to avoid

loss

• minq is meant to protect these

• What should minq be?

– When large number of flows 2-4 packets

• Needed for TCP behavior

– When small number of flows increase to avgcq

FRED

• Non-adaptive flows

– Flows with high strike count are not allowed more

than avgcq buffers

– Allows adaptive flows to occasionally burst to

maxq but repeated attempts incur penalty

Stochastic Fair Blue

• Same objective as RED Penalty Box

– Identify and penalize misbehaving flows

• Create L hashes with N bins each

– Each bin keeps track of separate marking rate (pm)

– Rate is updated using standard technique and a bin size

– Flow uses minimum pm of all L bins it belongs to

– Non-misbehaving flows hopefully belong to at least one

bin without a bad flow

• Large numbers of bad flows may cause false positives

Stochastic Fair Blue

• False positives can continuously penalize same

flow

• Solution: moving hash function over time

– Bad flow no longer shares bin with same flows

– Is history reset does bad flow get to make

trouble until detected again?

• No, can perform hash warmup in background

# 83

Head of Line

blocking

# 84

Buffers

• Input ports

• Output ports

• Inside fabric

• Shared Memory

• Combination of all

Buffer locations

Fabric

# 85

Input Queuing

fabric

Inp

uts

Outp

uts

# 86

• Input speed of queue – no more than input line

• Need arbiter (running N times faster than input)

• FIFO queue

• Head of Line (HoL) blocking .

• Utilization:

• Random destination

• 1- 1/e = 59% utilization

• due to HoL blocking

Input Buffer : properties

# 87

Head of Line Blocking

# 90


Stadium

Beer/Soda/Chips

Kwiky Mart

# 91

Stadium

Output Queuing

Beer/Soda/Chips

Kwiky Mart

# 92


B C A C B

A

B

C

# 93


B C A C B C A B

A

B

C

# 94


C B C B C A B C B A

A

B

C

# 95

A

B

C

VOQ—Virtual Output Queues

B C A C B

ARB

# 96


B

C

A A

A

B

C

ARB

C B C A B

# 97


B

C

A

B A C C B

A A A

A

B

C

ARB

# 98

Performance Issue with Cross-Bars

Source: M. J. Karol, M.G. Hluchyj, S. P. Morgan, “Input Versus Output Queueing [sic] on a Space-Division Packet Switch”, IEEE Transactions on Communications, Vol COM-35, No 12,

December 1987, page 1353

58.6%

# 99

The fabric looks ahead into the input buffer for packets that may be transferred if they were not blocked by the head of line.

Improvement depends on the depth of the look ahead.

This corresponds to virtual output queues where each input port has buffer for each output port.

Overcoming HoL blocking:

look-ahead

# 100

Input Queuing Virtual output queues

# 101

Each output port is expanded to L output

ports

The fabric can transfer up to L packets to

the same output instead of one cell.

Overcoming HoL blocking:

output expansion

Karol and Morgan,

IEEE transaction on communication, 1987: 1347-1356

# 102

fabric

L

Input Queuing

Output Expansion

# 103

Output Queuing The “ideal”

1

1

1

1

1

1

1

1

1

1 1

1

2

2

2

2

2

2

# 104

Output Buffer : properties

• No HoL problem

• Output queue needs to run faster than input lines

• Need to provide for N packets arriving to same queue

• solution: limit the number of input lines that can be destined to the output.

# 105

Shared Memory

a common pool of buffers divided into

linked lists indexed by output port number

FA

BR

IC

FA

BR

IC

MEMORY

# 106

Shared Memory: properties

• Packets stored in memory as they arrive

• Resource sharing

• Easy to implement priorities

• Memory is accessed at speed equal to sum of the

input or output speeds

• How to divide the space between the sessions

Multicast: one sender to many receivers

• Multicast: one sender to many receivers

– analogy: one teacher to many students

• Question: how to achieve multicast

Internet Multicast Service Model

multicast group concept:

– hosts send IP datagram pkts to multicast group

– hosts that have “joined” that multicast group will receive pkts sent to that group

Multicast groups

• host group semantics:

– anyone can “join” (receive) multicast group

– anyone can send to multicast gorup

– no network layer identification to hosts of members

• session/application-level mechanisms needed for membership identification, privacy

• needed: infrastructure to deliver mcast-addressed packets to all hosts that have joined that multicast group

Internet Multicast Addressing

• indirection: mcast address does not name a

destination, but host group to receive packet

• class D Internet addresses reserved for multicast:

packet addr: 226.17.30.197

Joining a mcast group: a two-step process

• local: host informs local mcast router of desire to join group: IGMP

• wide area: local router interacts with other routers to receive mcast packet flow

– many protocols (e.g., DVMRP, MOSPF, PIM)

IGMP: Internet Group Management Protocol

• host: sends IGMP report when application

joins mcast group

– IP_ADD_MEMBERSHIP socket option

– host need not explicitly “unjoin” group when leaving

• router: sends IGMP query at regular intervals

– host belonging to a mcast group must reply to

query

IGMP

IGMP version 1

• router: Host Membership Query msg broadcast on LAN to all hosts

• host: Host Membership Report msg to indicate group membership

– randomized delay before responding

– implicit leave via no reply to Query

• RFC 1112

IGMP v2: additions include

• group-specific Query

• Leave Group msg

– last host replying to Query can

send explicit Leave Group msg

– router performs group-specific

query to see if any hosts left in

group

– RFC 2236

IGMP v3: under development as

Internet draft

Multicast Issues

• Naming

• Membership Management

• Routing

IP Multicast Naming

• Class D address represents multicast group

– E.g. 226.17.30.197

• Datagram with destination address set to group delivered to all hosts in the group

– Indirection

– 226.17.30.197 => 65.30.1.2, 66.8.3.53, 128.32.75.60, …

– Sender may or may not be in the group

• No address hierarchy or subnets

– How is routing done?

Membership Management

• Some other questions:

– Who is part of the group?

– How does one join?

– How does one leave?

– Who decides if it’s OK?

• Membership management answers these

IGMP

• Internet Group Management Protocol

• Runs only between host and router

– Multicast routing takes care of communication

between routers

IGMP

hosts

routers

host-to-router protocol

(IGMP)

multicast routing protocols

(various)

IGMP query

• IGMP membership_query

– Router sends query

– Find out all groups a host belongs to

– Can query a specific group instead

– Sent to the “all systems group” (224.0.0.1) with

TTL=1

IGMP report

• IGMP membership_report

– Response from host to a query

– Can send report unsolicited

• Join group this way!

• IGMP leave_group

– Optional

– Router will clean up membership info on next

membership_query

IGMP properties

• Minimalist semantics

– Host controlled membership

• No decision about:

– Who controls membership

– Invitations

– How to find groups and join them

• Move these decisions to application layer

Soft state

• Host is authoritative on group membership

• Router maintains “soft state”

• A crashed router soon recovers

– Sends a new membership_query

– Misdelivers packets for a little while

• OK by IP service model!

CS 640 123

Protocol types

• Dense mode protocols

– assumes dense group membership

– Source distribution tree and NACK type

– DVMRP (Distance Vector Multicast Routing Protocol)

– PIM-DM (Protocol Independent Multicast, Dense Mode)

– Example: Company-wide announcement

• Sparse mode protocol

– assumes sparse group membership

– Shared distribution tree and ACK type

– PIM-SM (Protocol Independent Multicast, Sparse Mode)

– Examples: a Shuttle Launch

Multicast Routing

• A number of routers have hosts that belong to

a multicast group

• How to connect them (and others) in a tree?

– Shared tree: single tree for all

– Source-based tree: many trees

Core-Based Tree

• Tree rooted at a core

• To join a group, send unicast message towards

core

– Add all links traversed until hit existing tree

Diagram

Core

Choice of Core

• If core close to source, efficiency is good

• If core far from source, efficiency falls

– Delay up to twice optimal

• Optimal core placement is NP-hard

– Use heuristics

Source-based Trees

• Different tree for each possible source

– Why?

• Reverse path forwarding to figure out tree

• Pruning to leave out routers

Pruning

• Prune when no attached members or

downstream routers

• Propagate prune messages upstream

R1

R2

R3

R4

R5

R6 R7

router with attached

group member

router with no attached

group member

prune message

S: source

links with multicast

forwarding

P

P

P

DVMRP

• Distance Vector Multicast Routing Protocol

• DV + RPF + Pruning

• DV vector carries distance to multicast sources

• Pruning carries a timeout

– Afterwards, traffic delivery is resumed

• Explicit graft message to reverse pruning

– Done upon join

MOSPF

• Multicast Extensions to OSPF

• Link-state advertisements include multicast group

membership

– Only report directly connected hosts

• Compute shortest-path spanning tree rooted at

source

– On demand, when receiving packet from source for the

first time

– Forward multicast traffic along tree

MOSPF performance

• Global state allows source-based trees to be

used

– Faster delivery of messages

• Overhead

– Joins and leaves flooded to all routers

– Any change may cause whole tree to be

recomputed

PIM

• Protocol Independent Multicast

– Uses routing tables, but agnostic of how they are built

• Two settings:

– Dense: most routers members of a group

• Use RPF flooding with pruning

– Sparse: most routers not members of a group

• Use shared tree or source-based tree based on data characteristics

• Uses soft-state

Sparse vs. Dense

Dense Mode

• Dense participants

• B/W plentiful

• Membership assumed

until pruned

• Data driven

Sparse Mode

• Sparse participants

• B/W overhead

significant

• Membership explicitly

requested

• Receiver driven

Shared v. Source-based Trees

• Shared trees used initially

– Tree rooted at rendezvouz-point (RP)

• Can switch to source-based trees when data

rate is high

– RP sends a Join message to source

– Each router independently decides to switch to

source-based tree, sends Join to source

Shared Tree Example

RP

S

G

G G

PIM Receiver Join

RP

S

G

G G

G

Join *,G

Report G

What if

join is here?

PIM Shared Tree After Join

RP

S

G

G G

G

G

PIM Source Based Tree

RP

S

G

G G

G

G

Join s,g

PIM Source Based Tree

RP

S

G

G G

G

G

PIM routing tables

• Routing entries of the form (s,g)

– s - source

– g - group

• Wildcard entries (*,g) for shared-group trees

• Packets are routed using best match

Queries

multimedia communication & networks

Education