Revision Lecture: 6th December 2019

Question 1

What is RSVP?

Answer 1

RSVP = Resource Reservation Protocol = a signalling protocol within INTSERV that tries to reserve and describe resources across a network. It is a state management protocol that uses signalling to make and preserve states that are used to make network reservations.

Question 2

How does RSVP work?

Answer 2

The sender sends out path messages to find a path to the receiver, who then replies with Recv packet if the reservation can be made (if not RecvErr). Recv packet reaches the sender, at which point network reservation is made. Takes 2 passes.

Question 3

Where are the parameters for RSVP state gained from?

Answer 3

TSpec messages from the token bucket parameters from the sender, in Resv messages.

Question 4

What is TSpec?

Answer 4

= Token Bucket TSpec = packets containing information about the data being sent, transmitted in RSVP Resv messages. This involves the parameters of the token bucket mechanism used by the sender.

Question 5

What is a token bucket?

Answer 5

The token bucket is an algorithm used in packet switched computer networks and telecommunications networks which can be used to ensure that data transmissions, in the form of packets, conform to defined limits on bandwidth and burstiness (a measure of the unevenness or variations in the traffic flow).

Question 6

How does a token bucket work?

Answer 6

The bucket fills at rate r to a maximum size of b. It starts full.

Packets given tokens by the bucket <= the peak trasnsmit rate, p.

The amount of data sent in a burst is <= (r * t) + b

t = is the time taken for the burst to transmit (but actually general to any period of time)

m = minimum policed unit = min siz
• All IP datagram of size less than m will be counted when policed
and tested for conformance as m
> M is the maximum datagram size
• Flows requesting M greater than the link MTU must be rejected.

Question 7

Who makes the reservation in RSVP?

Answer 7

The receiver.

Question 8

What is flow control?

Answer 8

The end-to-end management of the sending rate of a flow by its receiver.

Question 9

Who executes flow control?

Answer 9

The receiver.

Question 10

Does RSVP involve flow control?

Answer 10

Not during its initialisation as the flow is not live: there is not a flow to control. Arguably if state is updated after then, after transmission begins.

Question 11

What are PathTear and ResvTear packets?

Answer 11

A type of packet sent in RSVP that destroy the soft-state reservations between a sender and receiver. PathTear sent by the sender, ResvTear from the receiver: same as who sent the Path and Resv packets when making the reservations.

Question 12

What are PathErr packets?

Answer 12

Packets returned during the attempted construction of a network reservation along a path between two hosts with RSVP that signals that the reservation could not be made.

Question 13

What is the difference between PathTear and ResvTear packets?

Answer 13

PathTear sent by the sender, ResvTear from the receiver: same as who sent the Path and Resv packets when making the reservations.

Question 14

Should PathTear and ResvTear packets be executed simultaneously? Why?

Answer 14

Yes, because in “well-behaved” applications using RSVP, the sender and receiver should simultaneously decide to terminate the connection, and therefore their reservations.

Question 15

Is path information stored within RSVP state?

Answer 15

No. Just reservation information.

Question 16

Is RSVP state soft or hard state? What does that mean?

Answer 16

Soft state; it will time out and be destroyed without needing an explicit instruction to do so (as is needed with hard-state).

Question 17

Why is RSVP state soft-state? What is the drawback of this?

Answer 17

To allow for reservations to time out if hosts go down (either endpoints or routers). Otherwise, reservations would stay up forever and eventually no traffic could get through.

Increased traffic in (Resv) messages to keep the soft-state info alive.

Question 18

Why should PathTear and ResvTear packets both be sent when ending a reservation?

Answer 18

The paths between the sender and receiver and vice versa may not be symmetrical, and you need to end reservations along both.

Question 19

Is information relating to routing state in routers soft-state? Why?

Answer 19

Yes. So that if parts of the network fail the routes to them don’t stay valid, instead timing out. This would otherwise cause data to be lost as routers try to send it along paths that no longer exist.

Question 20

How may reservations cause deadlocks?

Answer 20

If they go in circles through network nodes (devices). This would mean they would fill the capacity on the devices they loop through until they fill them. Until this happens they still occupy their requested reserved capacity as pending reservations are held until they are confirmed or dropped after an error. This would mean that no other reservations could be made. This situation is more likely to occur with large reservations because few routers are likely to be able to handle them, which will result in them looping through the few which can until the reservations fill their capacity.

Question 21

Do you need to continually send resv packets to keep reservations open? Why?

Answer 21

Yes. Because RSVP is soft-state.

Question 22

What is RSpec?

Answer 22

RSVP data describing the requirements (reservations to be made) for the receiver to work properly, contained in Resv messages passed to the sender to make reservations along the path.