1. 1 Improving Web Servers performance Objectives:  Scalable Web server System  Locally distributed architectures  Cluster-based Web systems  Distributed Web systems  Cluster-based solutions  Distributed Web-based solutions  Dispatching algorithms for cluster-based Web systems

2. 2 Reference “The State of the Art in Locally Distributed Web-server Systems” Valeria Cardellini, Emiliano Casalicchio, Michele Colajanni and Philip S. Yu

3. 3 Concepts  Web server System is a system that Provides web services  The trend is  Increasing number of clients  Growing complexity of web applications  Scalable Web server systems  The ability to support large numbers of accesses and resources while still providing adequate performance

4. 4 Architecture solutions for scalable Web-server systems

5. 5 Model architecture for a locally distributed Web system

6. 6 Locally Distributed Web System  Cluster Based Web System  The server nodes mask their IP addresses to clients, using a Virtual IP address corresponding to one device (web switch) in front of the set of the servers  Web switch receives all packets and then sends them to server nodes  Distributed Web System  The IP addresses of the web server nodes are visible to clients  No web switch, just a layer 3 router may be employed to route the requests

7. 7 Cluster based Architecture

8. 8 Distributed Architecture

9. 9 Request routing mechanisms  After classifying the two Web systems  Cluster Based Web System  Distributed Web System  The question now becomes “how are packets routed to each of the web servers?

10. 10 Request routing mechanisms for cluster-based Web systems  layer-4 switch  Content-blind routing  layer-7 switch  Content-aware switches  Also called Layer 5 switches in TCP/IP protocol What are the trade-offs between layer-4 and layer- 7 switches?

11. 11 Two Approaches

12. 12 Taxonomy of cluster-based architecture

13. 13 Layer-4 two-way architecture

14. 14 Layer-4 one-way architecture

15. 15 Layer-4 one-way mechanisms  Packet single-rewriting  Same as two-way architecture. The only difference is in the modification of the source address of outbound packets  Packet tunneling  This is also known as IP encapsulation  IP datagrams with IP datagrams  Requires that all servers support IP tunneling  Packet frowarding  Assumes that the Web switch and the server nodes are on the same LAN  All nodes share the VIP address  Server nodes need to disable ARP  Web switch forwards the inbound packet to the target server without modifying the TCP/IP header

16. 16 LAN Addresses Each adapter on LAN has unique LAN address

17. 17 LAN Address (more)  MAC address allocation administered by IEEE  manufacturer buys portion of MAC address space (to assure uniqueness)  Analogy:  MAC address: like Social Security Number  IP address: like postal address  MAC flat address => portability  IP hierarchical address NOT portable

18. 18 Routing discussion 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E Starting at A, given IP datagram addressed to B:  look up net. address of B, find B on same net. as A  link layer send datagram to B inside link-layer frame B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload datagram frame frame source, dest address datagram source, dest address

19. 19 ARP: Address Resolution Protocol  Each IP node (Host or Router) on LAN has ARP table  ARP Table: IP/MAC address mappings for some LAN nodes  TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B knowing B’s IP address?

20. 20 ARP protocol  A wants to send datagram to B, and A knows B’s IP address.  Suppose B’s MAC address is not in A’s ARP table.  A broadcasts ARP query packet, containing B's IP address  all machines on LAN receive ARP query  B receives ARP packet, replies to A with its (B's) MAC address  frame sent to A’s MAC address (unicast)  A caches (saves) IP-to- MAC address pair in its ARP table until information becomes old (times out)  soft state: information that times out (goes away) unless refreshed  ARP is “plug-and-play”:  nodes create their ARP tables without intervention from net administrator

21. 21 Layer-7 two-way architecture

22. 22 Layer-7 two-way mechanisms  TCP gateway An application level proxy running on the web switch mediates the communication between the client and the server  Makes separate TCP connections to client and server  TCP splicing reduce the overhead in TCP gateway. For outbound packets, packet forwarding occurs at network level by rewriting the client IP address

23. 23 Layer-7 two-way Mechanisms  TCP gateway An application level proxy running on the web switch mediates the communication between the client and the server  TCP splicing reduce the overhead in TCP gateway. Packet forwarding occurs at network level between the network interface driver and the TCP/IP stack, is carried out directly by OS kernel user kernel user

24. 24 Content-aware Switch Switch Image Server Application Server HTML Server www.yahoo.com Internet GET /cgi-bin/form HTTP/1.1 Host: www.yahoo.com… APP. DATATCPIP Front-end of a web servers Route packets based on layer 5/7 (content) information

25. 25 Why use Context-aware Switching  Servers can be specialized for certain types of request  Content segregation  Exploit locality  Affinity-based routing  Increase the performance because of the improved hit rate  Partial replication of server file set  Partition the server’s file set over different nodes

26. 26 URL Parsing is expensive!!  Performing content-aware routing implies that some kind of string searching and matching algorithm is required  Such a time-consuming function is expensive in a heavy traffic web site  Experience showed that the system performance would be severely degraded if we implement some URL parsing functions in the distributor

27. 27 TCP splicing  Once the two TCP connections are established, they are spliced  IP packets are forwarded at the network layer  TCP splicing requires  Connection binding  Packet analyzer to rewrite packets Appropriate address translation Sequence number modifications to be performed on the packets  Basically, we are deploying connection re-use

28. 28 Operation of Content-aware Distributor

29. 29 Layer-7 one-way architecture

30. 30 Layer-7 one-way mechanisms  TCP handoff  The switch hands off the TCP connection endpoint to the server  Needs changes to the OS on both components  TCP connection hop  Software-based proprietary solution  encapsulating the IP packet and sending it to the server

31. 31 Layer-7 one-way mechanisms  Migrate the created TCP connection from the switch to the back-end sever  Create a TCP connection at the back-end without going through the TCP three-way handshake  Retrieve the state of an established connection and destroy the connection without going through the normal message handshake required to close a TCP connection  Once the connection is handed off to the back- end server, the switch must forward packets from the client to the appropriate back-end server

32. 32 Summary  So far, we have discussed: