Layered Communication Layers work together Encapsulation and Deencapsulation

Layered Communication Layers work together Encapsulation and Deencapsulation

Indirect Communication • Application programs on different machines cannot communicate directly – They are on different machines! Browser HTTP Request Web App Trans Int DL DL Phy User PC Webserver

Layer Cooperation on the Source Host • Application layer process passes HTTPrequest to transport layer process Application Transport HTTP Request Internet Data Link User PC Physical

Layer Cooperation on the Source Host • Transport layer makes TCP segments – HTTP message is the data field – Adds TCP header fields shown earlier – Transport process “encapsulates” HTTP request within a TCP segment TCP Segment HTTP Request TCP-H Data Field TCP Header

Encapsulation • Encapsulation is delivering a message in the data field of another message – TCP encapsulates HTTP request messages – Can also encapsulate other types of messages TCP Segment HTTP Request TCP-H Data Field TCP Header

Layer Cooperation on the Source Host • Transport layer process passes the TCP segment down to the internet layer process Application Transport TCP segment Internet Data Link User PC Physical

Layer Cooperation on the Source Host • Internet Layer Process Encapsulates TCP Segment within an IP packet – An IP packet to deliver a TCP segment has a TCP segment in its data field Data IP Packet TCP segment IP-H Data Field IP Header

Layer Cooperation on the Source Host • The internet layer process passes the IP packet to the data link layer process – Internet layer messages are called packets Application Transport Internet IP packet Data Link User PC Physical

Layer Cooperation on the Source Host • Data Link Layer Encapsulates IP Packet Within a PPP Frame – Data link layer messages are called frames – Data PPP frame has IP packet in data field PPP Frame Encapsulating an IP Packet PPP-T IP packet PPP-H

Layer Cooperation on the Source Host • The data link layer process passes the PPP frame to the physical layer process, which delivers it to the physical layer process on the first router, one bit at a time (no message at the physical layer) Application Transport Internet Data Link User PC PPP frame Physical (10110 …) To first router

Layer Cooperation on the Source Host • Recap: Adding Headers and Trailers: Application HTTP msg Transport HTTP msg TCP-H Internet HTTP msg TCP-H IP-H PPP-H Data Link User PC PPP-T Physical

Layer Cooperation on the Source Host • Encapsulation in Layering – Whenever a process at Layer N (the application, transport, internet, or data link layer) creates a message, – That Layer N process passes the message down to the next-lower-layer process, the process at layer N 1 – The N-1 process encapsulates the Layer N message by placing it in the data field of a Layer N-1 message and adding headers and perhaps trailers to create the full Layer N-1 Message

Layer Cooperation on the Source Host • Small but important detail on naming • Layer 3 (internet) messages are called packets – IP message is a packet • Layer 2 (data link) messages are called frames – PPP message is called a frame

Layer Cooperation: Destination Host • Destination host reverses processes on the sending host – Delivers HTTP message to the webserver application program Application Transport Internet Data Link User PC Physical Webserver

Layer Cooperation: Destination Host • Successively pass up layer messages Data link layer program processes the data link frame’s header and trailer, deencapsulates the IP packet, and passes the IP packet to the next higher layer, the internet layer IP-Packet DL-Frame (protocol unknown) containing IP packet in data field Final Router Physical Application Transport Internet Data Link Webserver

Layer Cooperation: Destination Host • Successively pass up layer messages – Other layers pass successive data fields (containing next-layer messages) up to the next higher layer HTTP msg TCP segment IP-Packet DL-Frame (protocol unknown) Final Router Physical Application Transport Internet Data Link Webserver

Layer Cooperation: Destination Host • Successively pass up layer messages – Other layers process headers & trailers, pass up message in data field Application HTTP msg TCP segment HTTP seg TCP-H IP Packet HTTP msg TCP-H IP-H PPP-T HTTP msg TCP-H IP-H PPP-H Data Link Physical Webserver Final Router Transport Internet

Indirect Communication • Two processes on different machines cannot communicate directly with one another because they are physically separated • So they communicate indirectly, using cooperation with lower layers

The First Router • First router receives an IP packet (encapsulated in a frame) in one port (interface) • Must make a router forwarding decision: select the port to use to send it back out B B? D? Router A D Packet C? C

Layer Cooperation on the First Router • So far, we have only looked at hosts – But deencapsulation and encapsulation also occur on EACH router • Frame arrives at a port on the first router – Port’s data link layer process receives the PPP frame containing an IP packet Internet PPP Frame Data Link First Router

Layer Cooperation on the First Router • Incoming Data Link Process on the Router – Deencapsulates the IP packet from the PPP frame – Passes the IP packet to the router’ internet layer process First Router IP Packet Internet Data Link Incoming Port on First Router

Layer Cooperation on the First Router • Routers only have physical, data link, and internet layer processes – So internet layer process is the highest-layer process on a router forwarding – Internet layer process decides where to send the packet next: another router or the destination host Internet Data Link First Router

Layer Cooperation on the First Router • Internet layer process passes IP packet to data link layer process on the selected output port that will carry the IP packet to the next router or the destination host First Router Internet Data Link IP Packet Data Link Selected Output Port on First Router

Layer Cooperation on the First Router • The data link and physical layer process on the selected port sends the frame encapsulating the IP packet onto the next router (or destination host) Internet Data Link Frame Selected Output Port On First Router Physical Layer Input Port On Next Router (Or Destination Host)

Layer Cooperation on the First Router • Notes • For router forwarding, routers only use physical, data link, and internet processes • Routers First Receive Frames – Receiving interface deencapsulates the IP packet, passes the packet to the internet layer process • Routers Then Send Frames Out – On a different output interface (port) – This requires encapsulating of the IP packet in a data link layer frame