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L23_network_layer_01

L23_network_layer_01 - The Network Layer Forwarding Routing...

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1 The Network Layer Forwarding, Routing, and Addressing (Part I) Lecture 23 6.02 Spring 2008 May 2, 2008 Network layer functions Difference between forwarding and routing Link-state routing with Dijkstra’s shortest-paths alg. Data Link Physical Ethernet, WiFi, … Framing, coding, [limited] rxmits, channel access Modulation/demodulation Application HTTP, FTP, SMTP, … Transport TCP, UDP End-to-End Layer Layering in the Internet Network IP Forwarding & routing (and addressing) Everything else! Reliability, integrity, ordering, jitter ctrl, congestion response, … Today
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2 Network Layer Functions Main goal : best-effort connectivity Addressing : How to name nodes? Forwarding : How does switch process packet? Routing : How to build routing tables to ensure that forwarding is correct? A B C D E F G ? Addressing A good addressing scheme helps scalability Influences mobility (Can nodes move a little? A lot?) For now, assume each node has a unique arbitrary address (“just a name”) • We’ll revisit when we discuss network scalability • Hierarchical and structured addresses
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3 Forwarding: It’s All in the Header Packet header is crucial: switches look at this info to decide what to do • E.g., to send a letter via USPS, the “To” (and “From”) fields are crucial Header depends on network layer protocol • Generally has dst address , src address, demultiplexing info (which higher layer should get packet) • Also packet length, hop limit (or “time to live”), type-of-service (e.g., priority level), checksum Example: IPv4 Header Payload TCP, UDP, ICMP, etc. Hop limit
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4 Forwarding Core function is conceptually simple
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