Announcements. CS 5565 Network Architecture and Protocols. Queuing. Demultiplexing. Demultiplexing Issues (1) Demultiplexing Issues (2)
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1 Announcements CS 5565 Network Architecture and Protocols Problem Set 1 due Feb 18 Project 1A due Feb 19 Lecture 5 Godmar Back 2 Queuing Demultiplexing send queues Layer k+1 Layer k recv queues End systems must decide which layer instances should process an incoming packet Layer k has type information in header to say which instance of layer k+1 to pick Issues: speed and flexibility Ethernet Frame as received Layer k-1 Ethernet Type >IP IP Protocol 06 ->.. Port 50 ->http.. Http Request GET /~gback.. Ethernet Header IP Header Header HTTP Request 3 4 Demultiplexing Issues (1) How does a layer k determine which layer k+1 should get a packet? Generally done using dispatch/function pointer table indexed by known/registered protocols Works well layer to layer But can t see if a packet will be accepted by (or is useful to) higher layers 5 Demultiplexing Issues (2) Receiver Livelock: Spending entire CPU cycles doing low-level processing on packets that eventually need to be discarded. Goal: Early Packet Discard (On interrupt): don t waste time on packets that you need to discard anyway Because no one s listening Or: because listener doesn t keep up ( slow, buffers full, etc.) Combined with Lazy Receiver Processing (LRP) After initial acceptance, postpone further processing to accounted context See for details 6 1
2 Demultiplexing Issues (3) How do you achieve early packet discard? Often: optimize common case, support a few wellknown protocols Better/more general solution: Allow higher layers to register demultiplexing predicates, e.g. (eth.type == IP && ip.prot == && tcp.port == http && http.req matches /~gback/ ) Ideally composable Must be safe BPF: Berkeley Packet Filter interpreted language to decide if a packet matches or not Fast Demux: DPF [Engler 96] Uses dynamic code generation to install demux filters 7 8 Scout OS Arguments against Layering Developed at U Arizona by Larry Peterson Idea: center entire OS around network paths communication OS data-driven scheduling Path 1 Path2 Design Some functions don t fit in a layer network management Possibility of making (and having to live with) bad design decisions Possibility of duplication Performance Possible performance penalties from crossing layers: memory-to-memory copies Mistuning: (Old) layer implementation at k+1 may be suboptimal when (new) layer k is improved 9 10 Layering not always strict Example: FDDI Station Management 11 End-To-End Argument If you were to design a network from scratch, how would you decide which functionality goes in which layer? E2E Argument [Saltzer/Reed/Clark 84] says: Functionality whose complete & correct implementation requires knowledge that only end points have should be implemented at the end points. Corollary: only put in layer k that which benefits all instances of layer k+1 Example: file transfer Where should you put error recovery? NB: be careful to define end points Human phone conversation vs. speech message system Guiding principle, not hard rule 12 2
3 Architecture vs. Engineering Clark & Tennenhouse [SIGCOMM 1990] Architectural Considerations for a New Generation of Protocols Layering but one design/engineering principle: not always best Argue for flexible decomposition Suggest two alternate ideas -level Framing (ALF) Integrated Layer Processing (ILP) The Layer 13 Layer Creating a network app Let s look at some s (in keeping with top-down) architectures: Client/Server, P2P, Hybrid Client/Server Architecture vs Client Process/Server Process Transport Layer Requirements Write programs that run on different end systems and communicate over a network. No software typically written for devices in network core Network core devices do not function at app layer This design allows for rapid app development Exception: extensible routers Research: active networks transport network data link physical transport network data link physical transport network data link physical Client-Server Architecture server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other Pure P2P Architecture no always on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses example: Gnutella Highly scalable But difficult to manage
4 Hybrid of client-server and P2P App-layer Protocols (Original) BitTorrent File transfer P2P; Tracker centralized Instant messaging Presence detection/location centralized: User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies Chatting between two users can be P2P File transfer between users typically P2P Defines: Types of messages exchanged, e.g., request & response messages Syntax of message types: what fields in messages & how fields are delineated Semantics of the fields, ie, meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP Proprietary protocols: e.g., Skype Transport Service Requirements Internet apps:, transport protocols Data loss Bandwidth Time Sensitive layer protocol Underlying transport protocol file transfer web documents real-time audio/video stored audio/video interactive games instant messaging loss-tolerant loss-tolerant loss-tolerant audio: 5kbps-1Mbps video:10kbps-5mbps same as above few kbps up no no no (?) yes, 100 s msec yes, few secs yes, 100 s msec yes and no remote terminal access Web file transfer streaming multimedia Internet telephony SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Skype) or UDP typically UDP Communicating Processes Process: program running within a host. Access services through sockets Client process: process that initiates communication Server process: process that waits to be contacted Note 1: client/server process is really a property of socket state Note 2: process can be both server & client process for different communications when multiple sockets are used 23 Addressing Processes For a process to receive messages, it must have an identifier A host has a unique 32- bit IP address Not sufficient because multiple processes can be on same host Add 16-bit port number Identifier includes both the IP address and port number associated with the process on the host. Example port numbers: HTTP server: 80 Mail server: 25 Well-known port numbers /etc/services 24 4
5 UDP & Socket Programming Service provided Demultiplexing Payload checksum Passes segments straight to IP UDP Length, in bytes of UDP No congestion control So simple it fits on one slide segment, including header source port # length 32 bits data (message) dest port # checksum UDP segment format 26 Socket Programming Goal: learn how to build client/server that communicate using sockets socket Socket API introduced in BSD 4.1 UNIX, 1981 explicitly created, used, released by apps used for both local and remote communication a host-local, -created, OS-controlled interface (a door ) into which process can both send and receive messages to/from another process 27 Network Socket Programming Socket: (narrow definition:) a door between process and end-end-transport protocol (UDP or ) developer operating system process socket Stack w/ buffers, variables host or server internet process socket Stack w/ buffers, variables host or server developer operating system 28 Addressing For UDP/IP or /IP socket communication, generally need 4 parameters: Source Identifier (32-bit IP Address) Source Port (16-bit) Destination Identifier (32-bit IP Address) Destination Port (16-bit) Notice that the relationship of local and remote (also called peer ) to source/destination depends on direction of communication Note: UDP uses only Destination (IP+Port) for demultiplexing uses Source + Destination (quadruple: Src IP, Src Port, Dst IP, Dest Port) 29 BSD Socket API API Programming Interface Provides access to services Specified in C language Implemented on many platforms in one way or the other (Windows: WinSock2, CSocket MFC classes for BSD-like look) Sockets (in Unix) are file descriptors General idea: writing to the socket is sending data to network, reading from socket is receiving data Good because read(2), write(2), close(2) and others (select(2), poll(2), ioctl(2), SIGIO, fcntl(2)) can be reused Bad because? Study C API first and observe how other languages embedded sockets 30 5
6 UDP Sockets: Overview socket(2) int socket(int domain, int type, int protocol) connect() (optional) send() recv() acts like acts like Client sendto() recvfrom() bind() recvfrom() sendto() Server domain: PF_INET, PF_UNIX, PF_INET6,. type: SOCK_DGRAM, SOCK_STREAM, protocol: 0 for Unspecified (or IPPROTO_UDP) returns integer file descriptor entirely between process and OS no network actions involved whatsoever man pages: ip(7), udp(7), tcp(7), socket(2), socket (7), unix(7) type man 2 socket, man 7 socket bind(2) How are addresses represented? int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen) sockfd: return by my_addr: socket address addrlen length of address (address is variable-sized data structure) binds socket to (local) address specified This affects network protocol namespace, but no information is transmitted over network Typically: one socket per port, exception: multicast 33 struct sockaddr { /* GENERIC TYPE, should be abstract */ sa_family_t sa_family; /* address family */ char sa_data[14]; /* address data */ }; /* This is the concrete subtype you ll use */ struct sockaddr_in { sa_family_t sin_family; /* address family: AF_INET */ u_int16_t sin_port; /* port in network byte order */ struct in_addr sin_addr; /* internet address */ }; /* Internet address. */ struct in_addr { u_int32_t s_addr; /* address in network byte order */ }; 34 More on bind(2) Address specified is the local address Which is destination for receive and source for sends. sin_addr.s_addr useful for multi-homed hosts, otherwise use INADDR_ANY sin_port may be zero if any port would do Use getsockname() to retrieve assigned port s_addr and sin_port are specified in network byte order This convention holds throughout socket API 35 Common Pitfalls (1) Network vs. Host Byte order Network order is big endian, most-significant byte first Host order may be either big or little: little endian on x86 Use ntohs(), ntohl(), htons(), htonl() to convert 16-bit ( short ) and 32-bit ( long ) values portably Never convert addresses/ports in sockaddr_in structures in place Not as much an issue in languages that hide data representation (e.g., Java) 36 6
7 Common Pitfalls (2) Pay attention to length parameters Example int getsockname(int s, struct sockaddr *name, socklen_t *namelen); namelen here is not OUT, its INOUT aka value-result You know what socket it is, the OS doesn t! Always check return codes! 37 sendto(2), recvfrom(2) ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen); ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from, socklen_t *fromlen); s, buf, len as in read/write flags: MSG_OOB, MSG_PEEK mostly 0 to/from are of type struct sockaddr_in These are remote/peer addresses: where did the packet come from, where should it be sent to NB: fromlen is value-result! 38 UDP & connect(2) Then what does connect() do??? UDP Demultiplexing No notion of connections in UDP connect(2) can be used to tell OS default destination for future sends Then can use send(2) and write(2) instead of sendto(2), or can omit the destination address in sendto(2) What does ECONNREFUSED mean for UDP? Courtesy extended by other nodes which send ICMP packet saying no one s listening for UDP packets at the port number you sent it to OS relayed this information to UDP recvfrom(&from) from: :3045 recvfrom(&from) from: :512 recvfrom(&from) from: :512 S: :3045 D: :80 Payload bind(*, 80) S: :80 S: :53 bind(*, 53) S: :512 D: : S: :512 Payload sendto( :80) S: S: : :??? S: :512 D: :80 Payload bind( ,512)
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