Transport Layer Services Mul9plexing/Demul9plexing. Transport Layer Services

Transcription

1 Computer Networks Mul9plexing/Demul9plexing Transport services and protocols provide logical communica+on between app processes running on different hosts protocols run in end systems send side: breaks app messages into segments, passes to layer rcv side: reassembles segments into messages, passes to app layer more than one protocol available to apps Internet: TCP and UDP 1

2 Transport vs. layer layer: logical communica9on between hosts layer: logical communica9on between processes relies on, enhances, layer services Household analogy: 12 kids sending le8ers to 12 kids processes = kids app messages = lejers in envelopes hosts = houses protocol = Ann and Bill - layer protocol = postal service Internet - layer protocols reliable, in- order delivery (TCP) conges9on control flow control connec9on setup unreliable, unordered delivery: UDP no- frills extension of best- effort IP services not available: delay guarantees bandwidth guarantees 2

3 Mul9plexing/demul9plexing Demul9plexing at rcv host: delivering received segments to correct socket = socket = process Mul9plexing at send host: gathering data from mul9ple sockets, enveloping data with header (later used for demul9plexing) P3 P1 P1 P2 P4 link link link host 1 host 2 host 3 How demul9plexing works host receives IP datagrams each datagram has source IP address, des9na9on IP address each datagram carries 1 - layer segment each segment has source, des9na9on port number host uses IP addresses & port numbers to direct segment to appropriate socket 32 bits source port # dest port # other header fields data (message) TCP/UDP segment format 3

4 Connec9onless demul9plexing Create sockets with port numbers: DatagramSocket mysocket1 = new DatagramSocket(12534); DatagramSocket mysocket2 = new DatagramSocket(12535); UDP socket iden9fied by two- tuple: (dest IP address, dest port number) When host receives UDP segment: checks des9na9on port number in segment directs UDP segment to socket with that port number IP datagrams with different source IP addresses and/or source port numbers directed to same socket Connec9onless demux (cont) DatagramSocket serversocket = new DatagramSocket(6428); P2 P3 P1 P1 SP: 6428 DP: 9157 SP: 6428 DP: 5775 SP: 5775 IP: A DP: 6428 server IP: C DP: 6428 Client IP:B SP provides return address 4

5 Connec9on- oriented demux TCP socket iden9fied by 4- tuple: source IP address source port number dest IP address dest port number receiving host uses all four values to direct segment to appropriate socket Server host may support many simultaneous TCP sockets: each socket iden9fied by its own 4- tuple Web servers have different sockets for each connec9ng non- persistent HTTP will have different socket for each request Connec9on- oriented demux (cont) P1 P4 P5 P6 P2 P1 P3 SP: 5775 IP: A S-IP: A server IP: C Client IP:B 5

6 Connec9on- oriented demux: Threaded Web Server P1 P4 P2 P1 P3 SP: 5775 IP: A S-IP: A server IP: C Client IP:B 6