Lecture 9: Internetworking CSE 123: Computer Networks Aaron Schulman HW 1 due TODAY Combing Networks ● Main challenge is heterogeneity of link layers: u Addressing » Each network media has a different addressing scheme u Bandwidth » Modems to teraBits u Latency » Seconds to nanoseconds u Frame size » Dozens to thousands of Bytes u Loss rates » Differ By many orders of magnitude u Service guarantees » “Send and pray” vs reserved Bandwidth CSE 123 – Lecture 8: From networks to Internetworks 2 Internetworking ● Cerf & Kahn74, “A Protocol for Packet Network Intercommunication” u Foundation for the modern Internet ● Routers forward packets from source to destination u May cross many separate networks along the way ● All packets use a common Internet Protocol u Any underlying data link protocol u Any higher layer transport protocol CSE 123 – Lecture 8: From networks to Internetworks 3 TCP/IP Protocol Stack host host HTTP Application Layer HTTP TCP Transport Layer TCP router router I I Network Layer I I P P P P Ethernet Ethernet SONET SONET Ethernet Ethernet interface interface interfaceLink Layerinterface interface interface CSE 123 – Lecture 9: Internetworking 4 IP Networking Router WiFi Ethernet data packet data packet Eth IP TCP HTTP FDDI IP TCP HTTP CSE 123 – Lecture 9: Internetworking 5 Routers ● A router is a store-and-forward device u Routers are connected to multiple networks u On each network, looks just like another host u A lot like a switch, But supports multiple datalink layers and makes decisions at the network layer ● Must Be explicitly addressed By incoming frames (L2) u Not at all like a switch, which is transparent u Removes link-layer header, parses IP header (L3) ● Looks up next hop, forwards on appropriate network u Each router need only get one step closer to destination CSE 123 – Lecture 9: Internetworking 6 IP Philosophy ● Impose few demands on network u Make few assumptions aBout what network can do u No QoS, no reliaBility, no ordering, no large packets u No persistent state aBout communications; no connections ● Manage heterogeneity at hosts (not in network) u Adapt to underlying network heterogeneity u Re-order packets, detect errors, retransmit lost messages… u Persistent network state only kept in hosts (fate-sharing) ● Service model: Best effort, a.k.a. send and pray CSE 123 – Lecture 9: Internetworking 7 IP Packet Header 0 15 16 31 ver HL TOS length R M D identification E F F offset S TTL protocol header checksum 20 bytes source address destination address options (if any) data (if any) CSE 123 – Lecture 9: Internetworking 8 Version field ● Which version of IP is this? u Plan for change u Very important! ● Current versions u 4: most of Internet today u 6: new protocol with larger addresses u What happened to 5? Standards Body politics. CSE 123 – Lecture 9: Internetworking 9 Header length ● How Big is IP header? u Counted in 32-bit words u VariaBle length » Options u Engineering consequences of variaBle length… ● Most IP packet headers are 20 Bytes long CSE 123 – Lecture 9: Internetworking 10 Type-of-Service ● How should this packet be treated? u Care/don’t care for delay, throughput, reliaBility, cost u How to interpret, how to apply on underlying net? u Largely unused until 2000 (hijacked for new purposes, ECN & Diffserv) CSE 123 – Lecture 9: Internetworking 11 Length ● How long is whole packet in Bytes? u Includes header u Limits total packet to 64K u Redundant? CSE 123 – Lecture 9: Internetworking 12 TTL (Time-to-Live) ● How many more routers can this packet pass through? u Designed to limit packet from looping forever ● Each router decrements TTL field ● If TTL is 0 then router discards packet CSE 123 – Lecture 9: Internetworking 13 Protocol ● Which transport protocol is the data using? u i.e. how should a host interpret the data ● TCP = 6 ● UDP = 17 CSE 123 – Lecture 9: Internetworking 14 IP Checksum ● Header contains simple checksum u Validates content of header only ● Recalculated at each hop u Routers need to update TTL u Hence straightforward to modify ● Ensures correct destination receives packet CSE 123 – Lecture 9: Internetworking 15 So what does IP do? ● Addressing ● Fragmentation u E.g. FDDI’s maximum packet is 4500 Bytes while Ethernet is 1500 Bytes, how to manage this? ● Some error detection ● Routers only forward packets to next hop u They do not: » Detect packet loss, packet duplication » ReassemBle or retransmit packets ● Today we’ll talk aBout fragmentation CSE 123 – Lecture 9: Internetworking 16 For Next Time ● Read 3.2.5 in P&D ● Homework 1 due TODAY ● Homework 2 out on Monday ● Project 1 due in 1.5 weeks! CSE 123 – Lecture 9: Internetworking 17.