1. Network Core and QoS

2. The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
IP protocol
addressing conventions
datagram format
packet handling conventions
Routing protocols
path selection
RIP, OSPF, BGP
Network
layer
Routing/
Forwarding
Table
ICMP protocol
error reporting
router “signaling”
Link layer
physical layer

3. Forwarding Engine –Fast path of the code
Stage 1
Basic error checking to see if header is from a IP datagram
Confirm packet/header lengths are reasonable
Confirm that IP header has no options
Compute hash offset into route cache and load the route
Start loading of next header

4. Forwarding Engine – Stage 2
Checks to see if the cached route matches the destination of the datagram
If not, the code jumps to an extended lookup which examines the routing table in the Bcache
The code then checks the IP time-to-live (TTL) field and computes the updated TTL and IP checksum
The TTL and checksum are the only header fields that normally change and they must not be changed if the datagram is destined for the router

5. Forwarding Engine – software Fast path of the code
Stage 3
The updated TTL and checksum are put in the IP header
The necessary routing information is extracted from the forwarding table entry and the updated IP header is written out along with link-layer information from the forwarding table
The routing information includes the flow classifier (currently classifiers are associated with destination prefixes)

6. Questions
Given link speed and packet size how do we calculate the no. of packets that can be sent on the link. Vary the packet size from 32byte to 16K bytes and calculate no. of packets on the link. What is the impact of packet size on a network element
Given no. of lines of C code, average machine instructions for each line, and average no. of clock cycles per each instruction, calculate the time required for forwarding a packet.
If you now forwarding time for each packet, calculate the MIPS requirement

7. Different Types of Switching
Circuit Switching (telephone network)
dedicated circuit, sending and receiving bit streams
Message Switching
Packet Switching
store and forward, sending and receiving packets
Virtual Circuit Switching
Cell Switching (ATM)
What are Packets?
Data to be transmitted is divided into discrete blocks

8. Network Core: Circuit Switching
End-end resources reserved for “call”
link bandwidth, switch capacity
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup required

9. Cost-Effective Resource Sharing
Must share (multiplex) network resources among multiple users.
Common Multiplexing Strategies
Time-Division Multiplexing (TDM)
Synchronous TDM (STDM)
Frequency-Division Multiplexing (FDM)
Multiplexing multiple logical flows over a single physical link.

10. Network Core: Circuit Switching
network resources (e.g., bandwidth) divided into “pieces”
pieces allocated to calls
resource piece idle if not used by owning call (no sharing)
dividing link bandwidth into “pieces”
frequency division
time division

11. Network Core: Packet Switching
10 Mbs
Ethernet C A
statistical multiplexing
1.5 Mbs B
queue of packets
waiting for output
link
45 Mbs D E
Packet-switching versus circuit switching: human restaurant analogy
other human analogies?

12. Packet switching versus circuit switching
Packet switching allows more users to use network!
1 Mbit link
each user:
100Kbps when “active”
active 10% of time
circuit-switching:
10 users
packet switching:
with 35 users, probability > 10 active less that .004
N users
1 Mbps link

13. Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
Great for bursty data
resource sharing
no call setup
Excessive congestion: packet delay and loss
protocols needed for reliable data transfer, congestion control
Q: How to provide circuit-like behavior?
bandwidth guarantees needed for audio/video apps