Firewall - Networking - Lecture Slides, Slides of Computer Networks

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What Is Firewall?

1

Blocks malicious traffic

Blocks unauthorized traffic

ext firewall int VM Linux TCP/IP network stack

**1. Decode the packet

2. Check the firewall rules
3. Pass or drop the packet**

Firewall rules

Type 1: a combination of

 Protocol (TCP/UDP/ICMP)  IP address or country (e.g., Canada)  Port number

Type 2: domain names

 E.g., block DNS queries for *.facebook.com 4

5

Last Time

 Three approaches to improving content delivery

 Compensate for TCP’s weaknesses  Caching and replication  Exploit economies of scale

HTTP Performance

 Most Web pages have multiple objects

 e.g., HTML file and a bunch of embedded images

 How do you retrieve those objects (naively)?

 One item at a time

 New TCP connection per (small) object  Slow!

 Minimum of 2RTTs per object

Improving HTTP Performance: Persistent Connections  Maintain TCP connection across multiple requests (and even user “sessions”)  Amortize overhead of connection set-up and tear-down  Allow TCP to learn more accurate RTT estimate  Allow TCP congestion window to increase  Default in HTTP/1.

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Improving HTTP Performance:

Pipelined Requests & Responses

 Batch requests and responses to reduce the number of packets  Multiple requests can be contained in one TCP segment R T R T

Scorecard: Getting n Large Objects

Time dominated by bandwidth

(F is object size, B is bandwidth)

 One-at-a-time: ~ nF/B

 M concurrent: ~ [n/m] F/B

 assuming shared with large population of users  and each TCP connection gets the same bandwidth

 Pipelined and/or persistent: ~ nF/B

 The only thing that helps is getting more bandwidth..

Improving HTTP Performance:

Caching

 Why does caching work?

 Exploits locality of reference

 How well does caching work?

 Very well, up to a limit  Large overlap in content  But many unique requests

Improving HTTP Performance:

Caching: How

 Modifier to GET requests:

 If-modified-since – returns “not modified” if resource not modified since specified time

 Response header:

 Expires – how long it’s safe to cache the resource  No-cache – ignore all caches; always get resource directly from server

Improving HTTP Performance:

Caching: Where?

 Options

 Client  Forward proxies  Reverse proxies  Content Distribution Network