Links and Buffer Management - UBC ECEteerawat/publications/NS2/08-Links.pdfLinks and Buffer Management ... Introduction to Network Simulator NS2, Springer 2008. 1. OliOutline ... b

Links and Buffer ManagementLinks and Buffer ManagementLinks and Buffer ManagementLinks and Buffer Management

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008. 1

O liOutline• OverviewOverview• Packet Departure• Buffer Management• Buffer Management• Summary


Li k d Si l Li kLinks and Simple Links• Connect two nodesConnect two nodes

n0 n1Link

• Consider “Simple Links” Onlyp y• Simple Links models

– Packet transmission time = pkt_size/BWk l– Link propagation delay

– Buffer management Unlike real implementation (buffering is a functionality in

Textbook: T. Issariyakul and E. Hossain, Introduction to Network Simulator NS2, Springer 2008.

mp m ntat n ( uff r ng a funct na ty n routers)

3

Si l Li k A hiSimple Links: Architecture• A stand-alone composite OTcl objectA stand alone composite OTcl object

– head_ = the entry– queue_ = buffering

li k d l kt t s issi ti– link_ = model pkt transmission timeand link propagation delay

– ttl = time to live


_ t m t – drophead_ = dropping point

4

Si l Li k A hiSimple Links: Architecture• A stand-alone composite OTcl objectA stand alone composite OTcl object

– enqT_ (+) = Trace pkt entering queue_– deqT_ (-) = Trace pkt leaving queue_

drpT (d) = Trace pkt dropped from queue– drpT_ (d) = Trace pkt dropped from queue_(Buffer overflow)

– rcvT_ (r) = Trace pkt received by


the terminating object

5

Si l Li k A hiSimple Links: Architecture• An example of trace fileAn example of trace file

– enqT_ = Trace enqueue, “+”– deqT_ = Trace dequeue, “-”– rcvT = Trace receive “r”


rcvT_ Trace receive, r– drpT_ = Trace packet drop, “d”, (Buffer overflow)

6

Si l Li k C iSimple Links: Creation• From the OTcl (e.g., Tcl simulation script)( g p )

– simplex-link: Create a one-directional simple link object– duplex-link: Create two simplex-links in two directions

What are simple-link and duplex-link??

E l s

What are simple-link and duplex-link??( instprocs of class Simulator )

• Examples


Si l Li kSimplex Links• Instproc simplex-link of class simulatorp p f

– Create a uni-directional SimpleLink object


SimpleLink: OT l COTcl Constructor

//~ns/tcl/lib/ns-link.tcl SimpleLink instproc init { src dst bw delay q {

lltype "DelayLink"} } {set ns [Simulator instance] set drophead [ne Connector]set drophead_ [new Connector] $drophead_ target [$ns set nullAgent_] set head_ [new Connector] $head target $q

What is the class of drophead_?( C t )$ _ g $q

set queue_ $q... }

( Connector )What is “target” ?( OTcl command )


q nullAgent

SimpleLink: OT l COTcl Constructor

//~ns/tcl/lib/ns-link.tcl SimpleLink instproc init { src dst bw delay q {

lltype "DelayLink"} } {...set link [ne $llt pe]set link_ [new $lltype] $link_ set bandwidth_ $bw $link_ set delay_ $delay $queue target $link

What is the class of lltype_?( DelayLink)

$q _ g $ _$link_ target [$dst entry] $queue_ drop-target $drophead_ set ttl_ [new TTLChecker] $ttl_ target [$link_ target] $self ttl-drop-trace $link_ target $ttl_ }


}

10



R P k F diRecap: Packet Forwarding• Sending packet *p to node *targetSending packet *p to node *target• Two types of packet forwarding

1 I di t k t f di1. Immediate packet forwarding target->recv(p,h)

l k 2. Delayed packet forwarding schedule(target, p, delay)


P k D PPacket Departure Process• The amount of time needed to deliver a packet to p

the next node– Actual implementation

• From: A packet enters the router Ap• To: The packet reaches the router B

packetA B

– NS2 implementation• From: A packet enters the Link

A B

• To: A packet leaves the Link• Note: A packet is assumed to be at Node B as soon as it

leaves the Link


LinkpacketB

P k D PPacket Departure Process• The time consists of The time consists of

1. Packet transmission time– Time a packet spend inside router A

Pkt siz /link sp d– Pkt_size/link_speed

2. Link propagation delay– Start after the entire packet leaves router A– Time the packet spend in the link

A B


Packet transmission time Link propagation delay

P k D PPacket Departure ProcessLINK

head_

queue_ link_ ttl_dst_

drophead_

• Which components implement the time? ( link ) ( link_ )

• C++ Class = LinkDelay• OTcl Class = DelayLink


y

15

C Cl Li kD lC++ Class LinkDelay//~ns/link/delay.hclass LinkDelay : public Connector { public:

LinkDelay(): dynamic_(0), latest_time_(0), itq_(0){ bind b ("band idth " &band idth )bind_bw("bandwidth_", &bandwidth_); bind_time("delay_", &delay_);

} void recv(Packet* p, Handler*); ( p, );inline double txtime(Packet* p) { /* Packet TXT Time */

return (8. * hdr_cmn::access(p)->size() / bandwidth_);}

protected: double bandwidth_; double delay_; PacketQueue* itq ;


PacketQueue* itq_; Event intr_; /* In transit */

};

16

Dummy Event

C Cl Li kD lC++ Class LinkDelay

Variable Meaningb d idth Li k s d (i b s)bandwidth_ Link speed (in bps)delay_ Link propagation delay (in seconds)itq_ Contains all packet in the linksintr_ Dummy event

A B


C Cl Li kD lC++ Class LinkDelay• Function txtime(p)Function txtime(p) Compute Packet transmission time

• Function recv(p,h)– Main packet reception function– Delay the packet reception at the downstream

NsObjectNsObject• Packet transmission time• Link propagation delay

How to delay it? Use ( Scheduler )


– How to delay it? Use ( Scheduler )

18

F i Li kD l ( h)Function LinkDelay::recv(p,h)

(1)

(2)

(3)

1. Compute packet transmission time (txt)



(1)

(2)

(3)

2. Schedule an event where a packet leave the router invoke h->handle(&intr ) after txt sec.


_

20

Q: What is the input argument here?


(1)

(2)

(3)

3. Schedule an event where the packet arrive at the next node after txt + propagation delay seconds.




B ff M O liBuffer Management: Outline• OverviewOverview• Main C++ Classes

Q d Q H dl • Queue and Queue Handler • Callback Mechanism


B ff MBuffer Management• Buffer: Put incoming packets in the buffer g p

before emitting them out

• Why do need a buffer ? Packet In Packet Out

1

Q: Suppose no buffer what will happen if packet A B

12

Q: Suppose no buffer, what will happen if packet 2 arrives when the router is serving packet 1?

(The packet “2” would be lost.)


– Handle burstiness of input traffic

24

B ff MBuffer Management• NS2 implementationNS2 implementation• SimpleLink architecture

• Instvar queueq _– Receive packets,– Hold them until the link is clear, and


– Transmit the next packet in queue. 25

C++ Main Classes• C++ Class PacketQueue: a Q

– Model the buffer– Storage, enque, and deque packets

C Cl Q • C++ Class Queue: – Contain PacketQueue – Other functionalities, e.g., user interface, g ,

• C++ Class QueueHandler:– Derived from class Handler

P s D fi d f lt ti f Q bj t– Purpose: Define default action for a Queue object


Cl P k V i blClass Packet: Variables

…

//~/ns/common/packet.hclass Packet : public Event {

… public: idti h dltpublic:

Packet* next_; static int hdrlen_; …

uid_time_ handler_next_

• The packet supports link list by itself.B d

} uid_time_ handler_next_

• But it needs a manager:• Where the first packet is.• The number of packet in the link list.


PacketQueue

27

C++ Class PacketQueue


C++ Class Queue

kReceiving packets

CallbackMechanism





C Cl Q H dlC++ Class QueueHandlerDefine default actionsDefine default actions

Function handle(e) function resume() of the associated Q object


the associated Queue object

Q d Q H dl• Highly associated

Queue and QueueHandlerHighly associated

• Queue accesses a QueueHandler object using a variable qh_

• QueueHandler accesses a Queue object using a reference to queue_

QueueQueue

queue

QueueHandler

• Now Queue and QueueHandler can invoke

qh_queue_


functions of each other.

32

Q d Q H dlQueue and QueueHandler• To associated a Queue object to a To associated a Queue object to a QueueHandler object

• The constructor of Queue: configuring qhhe constructor of Queue conf gur ng q _

//~ns/queue/queue.h class Queue : public Connector { QueueQ p {

...protected:

QueueHandler qh_; ... qh

QueueHandler

};qh_


Q d Q H dlQueue and QueueHandler• The constructor of class QueueHandler: The constructor of class QueueHandler:

configuring queue_

Queue

QueueHandler

• The connection between Queue

qh_queue_

The connection between Queueand QueueHandler is established


QueueHandlerQ

• Key functionality = ? ( default actions for a Queue object )

• QueueHandler::handle(Event*)

• Queue::resume() Q () Callback mechanism





Callback Mechanism: Overview• Recap: Recap

– NsObject • does not send

T ll th d nstr m bj ct t r c iv• Tell the downstream object to receive– Packets are passed to downstream objects

only. • A signal which passed to an upstream

object is called a callback signalTh i ll d “ llb k h i ”• The process is called “callback mechanism”

• Discussion context: Queue-Link connection


Queue-Link Connection: Implementation• Main componentsMain components

– The SchedulerPacket: base class = ( Event ) – Packet: base class = ( Event )

– QueueHandler: base class = ( Handler ) Li kD l : P t th p k t in th – LinkDelay: Put the packet in the simulation timeline


A Queue-Link Connection: ConceptBuffering D lBuffering Delay

target->recv(p &qh )

1. queue_ sends a packet to link_2 i h k

target->recv(p,&qh_)

2. link_ receives the packet 3. queue_ waits until link_ indicate that it receives the

packet4. link_ sends a “callback” signal to queue_ after a certain

delay time5. queue_ realizes that the packet is received


p6. Go to 1.

39

Queue-Link Connection: Delay a callback message

//~ns/link/delay.ccvoid LinkDelay::recv(Packet* p Handler* h)void LinkDelay::recv(Packet* p, Handler* h) {

double txt = txtime(p); Scheduler& s = Scheduler::instance(); ();if (dynamic_) { /* See ~ns/link/delay.cc */ } else if (avoidReordering_) { /* See ~ns/link/delay.cc */

}l {else {

s.schedule(target_, p, txt + delay_); (a)}

s sched le(h &intr t t) (b)s.schedule(h, &intr_, txt); (b)} Q: What do (a) and (b) do? When do these actions occur?

a.Send a packet to a downstream objectb Send a callback message to the upstream Q


b.Send a callback message to the upstream Queue- Indicate that the packet has left the Queue- Queue can send another packet

40

Queue-Link Connection: Queue blocking mechanism

• How do a queue “wait” for a callback message?H w a q _ wa t f r a ca ac m ag ?• I.E., How do we stop a Queue object from sending

out packets after target->recv(p,&qh_) is t dexecuted.

• Queue blocking mechanism• Queue blocking mechanism– Not caused by buffer being filled– Use the variable blocked_ of class Queue//~ns/queue/queue.h class Queue : public Connector {

... protected:


int blocked_; int unblock_on_resume_;

};

Queue-Link Connection: Queue blocking mechanism

• Blocked (blocked =1)( _ )– Not being able to send packets– After sending– Waiting for the callback signal from link– Waiting for the callback signal from link_

• Not blocked (blocked_=0)– Able to send packetsp– After receiving the callback signal from link_


A Queue-Link Connection: ConceptBuffering D lBuffering Delay

Queue::resume()

1. queue_ sends a packet to link_2 i h k

Queue::resume()

2. link_ receives the packet 3. queue_ waits until link_ indicate that it receives the

packet4. link_ sends a “callback” signal to queue_ after a certain

delay time5. queue_ realizes that the packet is received


p6. Go to 1.

43

Queue-Link Connection: Main Functions

Function What it does Invoked byFunction What it does Invoked byresume() Reception of a

callback messageAn associated QueueHandler object

recv(p,h) Reception of a packet An upstream object


Queue-Link Connection: Delay a callback message

//~ns/link/delay.ccvoid LinkDelay::recv(Packet* p, Handler* h) {

double txt = txtime(p); Sched ler& s Sched ler instance()Scheduler& s = Scheduler::instance(); if (dynamic_) { /* See ~ns/link/delay.cc */ } else if (avoidReordering_) { /* See ~ns/link/delay.cc */

}else {

s.schedule(target_, p, txt + delay_); }

s.schedule(h, &intr_, txt);

}


Callback Mechanism in Queue-Link Interaction

(2) (3) (6)

(5)

target_ -> recv(p,h)

(1) (4)(4)

_


O liOutline• OverviewOverview• Packet Departure• Buffer Management• Buffer Management

– Queue and QueueHandler– Callback Mechanism– Callback Mechanism

• Summary


SSummary• Link is a composite OTcl object used to Link is a composite OTcl object used to

connect two nodes

• Class SimpleLink– models buffer management and Link delaymodels buffer management and L nk delay– is created by instrocs simplex-link or duplex-link of class Simulator

– consists of : head_, queue_, link_, ttl_, and drophead_


SSummary• Packet delay = transmission delay + propagation Packet delay transmission delay propagation

delay

• NS2 Link– C++ Class = LinkDelay– OTclClass = DelayLinkOTclClass DelayLink

• C++ Class LinkDelay– Two main variables: bandwidth_ and delay_. – Function recv(p,h) schedule two events:

• Packet departure from the upstream object


Packet departure from the upstream object• Packet arrival at the downstream object

49

SSummary• Queue handle burstiness: Queue handle burstiness Put a packet in queue and sent it out when the transmitter is ready.Cl d • Class Queue and QueueHandler:– Queue: A variable qh_– QueueHandler: A reference variable queue .q _

• Callback Mechanism– Send a message upstream

N ll k t ti i f d i d t – Normally packet reception is performed in a downstream direction.

– E.g., a LinkDelay object sends a message to a Queue to i di t d th t th k t h l ft th


indicated that the packet has left the Queue.

50

Documents

Links and Buffer Management - UBC ECEteerawat/publications/NS2/08-Links.pdfLinks and Buffer Management ... Introduction to Network Simulator NS2, Springer 2008. 1. OliOutline ... b