D u k e S y s t e m s

Network Servers: URIs, HTTP, RPC

Jeff ChaseDuke University

Heap manager

• Hours spent for 90+ points:

– 4, 4, 5, 6, 6, 8, 10, 10, 10, 10, 12,…

– 20, 20+, 24,

– 40, 65

• If it were a contest, winners are:

– 13 students: 92-93% success rate on canned test

– Tyler Nisonoff: consumes half the CPU as runner up, 92%

– Ben Berg

– Tamara Silbergleit

– Ang Li

– Kuang Han

– Matthew Tse

MacOS X

“tiny” heap

MacOS X

“small” heap

Heap manager: lessons

• “Real” heap managers are more complex:– They maintain multiple free lists for different size

blocks.

– And possibly different data structures for different size blocks.

– Be sure that you understand why.

• Debugging takes a lot of time and doesn’t teach you much and forces you to sit in front of a computer which is unhealthy and painful and frustrating when you could be outside in sunlight and fresh air.– Thought question: what do you wish we had told you?

End-to-end application delivery

Cloud and Software-as-a-Service (SaaS)Rapid evolution, no user upgrade, no user data management.Agile/elastic deployment on virtual infrastructure.

Where is your application?Where is your data?Where is your OS?

Services

RPC

GET (HTTP)

etc.

service

content provider

Clientsinitiate connection and send requests.

Serverlistens for and

accepts clients, handles requests,

sends replies

Networking

channelbinding

connection

endpointport

Some IPC mechanisms allow communication across a network.E.g.: sockets using Internet communication protocols (TCP/IP).Each endpoint on a node (host) has a port number.

Each node has one or more interfaces, each on at most one network.Each interface may be reachable on its network by one or more names.

E.g. an IP address and an (optional) DNS name.

node A node B

operationsadvertise (bind)listenconnect (bind)close

write/sendread/receive

A simple, familiar example

“GET /images/fish.gif HTTP/1.1”

URL

URIs and URLs

[image: msdn.microsoft.com]

Android content providers: URIs

[images from http://www.tutos-android.com/contentprovider-android]

Define the provider's authority string, its content URIs, and column names….To avoid conflicts with other providers, you should use Internet domain ownership (in reverse) as the basis …for Android package names…define your provider authority as an extension of the name of the package containing [it]…

Developers usually create content URIs from the authority by appending paths that point to individual tables…

By convention, providers offer access to a single row in a table by accepting a content URI with an ID value for the row at the end of the URI. …

Taking it to the net

The network stack

NFS(files)NFS

(files)HTTP(web)HTTP(web)

SMTP(email)SMTP

(email)SSH

(login)SSH

(login)

RPCRPC

TCPTCPUDPUDP

IPIP

EthernetEthernet ATMATM PPPPPP

Applications

Abstraction

Transport(L4)

Network packet (L3)

Interfaces

RPC

call

Server stub

Client stub

send

recvcall

return

return

send

recv

RPC: Language Integration

Stubs link with the client/server code to “hide” the boundary crossing.

– Marshal arguments/results

– Propagate exceptions

– Binding: need some way to name the server

– Stubs are auto-generated from an Interface Description Language (IDL) file.

RPC Execution

• How is this different from a local procedure call?

• How is it different from a system call?

The network stack, simplified

TCP/IP

Client

Networkadapter

Global IP Internet

TCP/IP

Server

Networkadapter

Internet client host Internet server host

Sockets interface(system calls)

Hardware interface(interrupts)

User code

Kernel code

Hardwareand firmware

Web services• HTTP is the standard for web systems.

– GET, PUT, POST, DELETE

• Various standards and styles layer above it.

• The Android content provider URI form is in the style of REST, as used in popular SaaS frameworks.

• What’s important is that the URI/URL authority always has the info to bind a channel to the server.– Translate domain name to an IP address and port using DNS service

(later).

• The URI path is interpreted by the server: it may encode the name of a file on the server, or a program entry point and arguments, or…

“Web-oriented architecture”

“CRUD”

TCP/IP connection

TCP byte-stream connection(128.2.194.242, 208.216.181.15)

ServerClient

Client host address128.2.194.242

Server host address208.216.181.15

[adapted from CMU 15-213]

socket socket

TCP/IP connection

Connection socket pair(128.2.194.242:51213, 208.216.181.15:80)

Server(port 80)

Client

Client socket address128.2.194.242:51213

Server socket address208.216.181.15:80

Client host address128.2.194.242

Server host address208.216.181.15

Note: 51213 is anephemeral port allocated

by the kernel

Note: 80 is a well-known portassociated with Web servers

[adapted from CMU 15-213]

TCP/IP Ports

• What port number to connect to?– We have to agree on well-known ports for common services

– Look at /etc/services

• Ports 1023 and below are ‘reserved’ This port abstraction is an Internet Protocol (L4) concept.– Source/dest port is named in every packet.

– Kernel looks at port to demultiplex incoming traffic.

• Clients need a return port, but it can be an ephemeral port assigned dynamically by the kernel.

Packet demultiplexing

WebServer Flow

TCP socket space

state: listeningaddress: {*.6789, *.*}completed connection queue: sendbuf:recvbuf:

128.36.232.5128.36.230.2

state: listeningaddress: {*.25, *.*}completed connection queue:sendbuf:recvbuf:

state: establishedaddress: {128.36.232.5:6789, 198.69.10.10.1500}sendbuf:recvbuf:

connSocket = accept()

Create ServerSocket

read request from connSocket

read local file

write file to connSocket

close connSocketDiscussion: what does step do and how longdoes it take?

Server listens on a socket

struct sockaddr_in socket_addr;sock = socket(PF_INET, SOCK_STREAM, 0);

int on = 1;setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &on, sizeof on);

memset(&socket_addr, 0, sizeof socket_addr);socket_addr.sin_family = PF_INET;socket_addr.sin_port = htons(port);socket_addr.sin_addr.s_addr = htonl(INADDR_ANY);

if (bind(sock, (struct sockaddr *)&socket_addr, sizeof socket_addr) < 0) {perror("couldn't bind");exit(1);

}listen(sock, 10);

Accept loop

while (1) {int acceptsock = accept(sock, NULL, NULL);char *input = (char *)malloc(1024*sizeof (char));recv(acceptsock, input, 1024, 0);int is_html = 0;char *contents = handle(input,&is_html);free(input);

…send response…

close(acceptsock);}

Send HTTP/HTML response

const char *resp_ok = "HTTP/1.1 200 OK\nServer: BuggyServer/1.0\n";const char *content_html = "Content-type: text/html\n\n";

send(acceptsock, resp_ok, strlen(resp_ok), 0);send(acceptsock, content_html, strlen(content_html), 0);send(acceptsock, contents, strlen(contents), 0);send(acceptsock, "\n", 1, 0);

free(contents);

unix> telnet www.aol.com 80 Client: open connection to serverTrying 205.188.146.23... Telnet prints 3 lines to the terminalConnected to aol.com.Escape character is '^]'.GET / HTTP/1.1 Client: request linehost: www.aol.com Client: required HTTP/1.1 HOST header Client: empty line terminates headers.HTTP/1.0 200 OK Server: response lineMIME-Version: 1.0 Server: followed by five response headersDate: Mon, 08 Jan 2001 04:59:42 GMTServer: NaviServer/2.0 AOLserver/2.3.3Content-Type: text/html Server: expect HTML in the response bodyContent-Length: 42092 Server: expect 42,092 bytes in the resp body Server: empty line (“\r\n”) terminates hdrs<html> Server: first HTML line in response body... Server: 766 lines of HTML not shown.</html> Server: last HTML line in response bodyConnection closed by foreign host. Server: closes connectionunix> Client: closes connection and terminates

[CMU 15-213]

Anatomy of an HTTP Transaction

A Short Quiz: HTTPS/SSL

1. What is the most important advantage of symmetric crypto (DES) relative to asymmetric crypto (RSA)?

2. What is the most important advantage of asymmetric crypto relative to symmetric crypto?

3. What is the most important limitation/challenge for asymmetric crypto with respect to security?

4. Why does SSL “change ciphers” during the handshake?

5. How does SSL solve the key distribution problem for symmetric crypto?

6. Is key exchange vulnerable to man-in-the-middle attacks?