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