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Thursday, May 17, 2012

UCEN 3133 Advanced Computer Networks - Tutorial 4 - Year 2012

Tutorial 4 – QOS and RSVP

1. List and briefly describe the activities that a router or switch is likely to engage in to provide a certain QOS guarantee in a network.

     (i)            Policing: verifying that incoming traffic conforms to its agreed specification
   (ii)            Admission control: checking to see if there are enough resources to meet a request for QoS
 (iii)            Classification: recognizing those packets that need particular levels of QoS 
 (iv)            Queuing and scheduling: making decisions about when packets are transmitted and which packets are dropped that are consistent with the QoS guarantees


2. What is the primary difference between IntServ and DiffServ approaches to allocating resources for QOS purposes ? Which approach scales better for a large IP network and why ?

IntServ enables end-to-end QoS guarantees for a single flow (microflow) from one sender to one or more receivers through a signalling protocol like RSVP
DiffServ a coarse-grained model groups traffic of many different flows into several small classes and provides QOS guarantees on a per-class basis

DiffServ scales better. In a large IP network, the number of microflows is very large and to keep QOS and state information about each one provides a heavy burden on the routers in the network, making it difficult to scale. Grouping flows into classes allows easier management of QOS information.

3. In what way does the deficit round robin (DRR) approach provide for a more fairer QOS allocation of transmission resources on a router as compared to the weighted round robin approach ? Briefly explain how it works.

It computes weights allocated to different flows in terms of total bytes rather than number of packets. This is fairer QOS allocation because IP packets vary in size
DRR allocates a number of bytes to each flow. When a flow is selected, DRR transmits as many packets as possible without exceeding the allotted number of bytes.
The remainder (i.e. the difference between the number of bytes that was allocated and the size of the packets actually sent) becomes a deficit which is added to the amount that will be sent in the next round


4. Explain briefly how RSVP works with reference to the PATH and RESV messages.

     (i)            PATH messages travel from a sender to one or more receivers (explicitly designed to support multicast)  and include TSpecs and classification information provided by the sender.
   (ii)            When a receiver gets a PATH message, it can send a RESV message back toward the sender. The RESV message identifies the session for which the reservation is to be made and indicates the level of QoS required by this receiver.
 (iii)            Messages are intercepted by every router along the path, so that resource allocation can take place at all the necessary hops.
 (iv)            Each router must agree to reserve the resources the request specifies. Reservation is unidirectional for a single flow direction


5. Three clients are receiving shared multicast video traffic flow across a MPLS network from a media server. The routes from these different client converge on a router that the server is directly connected to. All 3 clients make a QOS reservation requests for a delay latency of 30ms, 15 ms and 10 ms respectively to this router.

     (i)            How would RSVP handle these requests in the router ?
   (ii)            How does RSVP ensure that these reservation requests are propagated correctly across the MPLS network to this router at the transmitting end ?


i) These requests are merged into a single reservation request for the common flow, to which the router will select the smallest latency of 10ms

ii) When a path message in RSVP is sent to a client, each router at each hop inserts its own IP address as the message’s last hop. Each router can look at the last hop field to learn where the flow came from. Should it later receive a reservation request for this flow, this last hop information tells it where to send the reservation request next.


6. Explain briefly what the term soft state means within the context of RSVP. Consider an ongoing video conference involving 2 clients receiving multicast traffic  from a central server. A new client wishes to join this multicast group. Outline the series of steps that it will undergo to achieve this, and show how soft state helps in this registration process.

Soft state means that PATH and RESV messages in RSVP must be sent periodically to refresh a reservation. If they are not sent for some interval (the time-out period), then the reservation is automatically torn down

  1. The client sends an IGMP group membership report message to join this multicast group.
  2. The local router receives this message and to update the rest of the network, the router builds an OSPF link state advertisement and floods it through the network.   Once all routers are updated, multicast traffic begins to flow to the new client.
  3. Path messages for the flow can reach it as well, and soft state assures that a periodic path message from the sender will eventually reach the new client.
  4. With this path message in hand, the client can identify the flow and place its own reservations.

7.  How does label distribution in MPLS assist in the creation of a RSVP reserved flows ? Describe briefly the steps involved in this process which lead to the creation of a label switched path (LSP).

Labels are bound between flows that have RSVP reservations (through the use of a FEC) and then distributed through the MPLS network.

When an LSR wants to send a RESV message for a new RSVP flow:
     (i)            It allocates a label from its pool of free labels
   (ii)            Creates an entry in its LFIB with the incoming label set to the allocated label
 (iii)            Sends out the RESV message containing this label to the upstream LSR.
 (iv)            Upon receipt of a RESV that contains this label, an LSR populates its LFIB with this label as the outgoing label
   (v)            It then allocates a new label to use as the incoming label and inserts that in the RESV message before sending it upstream.
 (vi)            As RESV messages propagate upstream, an LSP is established along the path.


8. What is meant by Per Hop Behaviour (PHB) in the context of the DiffServ approach to QOS allocation and how is it indicated ? Briefly describe the 3 main types of PHBs

DiffServ allows each node along the path to define the service that a given class will receive and this can vary from node to node along a given path, therefore DiffServ does not provide QOS end-to-end guarantees. Each of the standard PHBs is indicated by a recommended DSCP value, and each router has a table that maps the DSCP found in a packet to the PHB that will determine how that packet is treated

Default - No special treatment, equivalent to best effort.
Expedited forwarding (EF) - Packets marked EF should be forwarded with minimal delay and experience low loss.
Assured forwarding (AF) - Packets go into different priority queues where they have different drop preferences if congestion occurs

9. Briefly describe the standard approach towards congestion avoidance in TCP and state two disadvantages with this approach.   Explain how Explicit Congestion Notification (ECN) overcomes these disadvantages.

Congestion avoidance in TCP assumes that packet losses in the network are an indication of congestion, and reduce their sending rates when they experience packet loss.
Disadvantages:
     (i)            For real time applications, the lost packet will need to be transmitted and its late arrival will cause degradation
   (ii)            A lost packet consumes resources up to the point it is lost, it would be preferable not to send the packet at all if it is just going to be thrown away

Router sets “congestion experienced” (CE) bit in packet header when it detects congestion, and then forward the packets rather than dropping it. The router must have some form of queue management to actively monitor congestion. When a packet with the CE bit arrives at its destination, the receiver must send a signal back to the sender that will cause the sender to reduce its sending rate.


10. A multimedia network that provides QOS guarantees uses a leaky bucket policer in one of its routers to ensure that the incoming packet traffic does not exceed the TSpec specification agreed upon during an initial session of Integrated Services (IntServ). The following are the features of this policer:

·         The token buffer can hold at most three (3) tokens, and is initially filled with two (2) tokens at time slot t = 0.

·         New tokens arrive into the bucket at a rate of two (2) tokens per time slot. Packets arrive at the beginning of a time slot and enter the packet queue, where they are processed and transferred to the output link in a First In First Out (FIFO) manner.

·         The size of the packet queue is four  (i.e. it can queue a maximum of 4 packets at any given time slot); any extra arriving packets are dropped.

·         Packets that obtain available tokens in a given time slot go together on the same time slot in the output link.             





Time slot
Incoming Packets
0
A B C D
1
E F
2
G
3
-
4
-
5
H I J K
6
L M N O
7
P Q
8
-
9
R S T


The table shows incoming packets from the network into the router with the policer, from time slot t = 0 to time slot t = 9. Based on this information, construct a new table with columns showing the packets in queue, tokens in bucket and packets on output link from time slot t = 0 to t = 9.                                            



Time slot
Packets in queue
Tokens in bucket
Packets at output
0
A B C D
2
A B
1
C D E F
2
C D
2
E F G
2
E F
3
G
2
G
4
-
3
-
5
H I J K
3
H I J
6
K L M N
2
K L
7
M N P Q
2
M N
8
P Q
2
P Q
9
R S T
2
R S

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