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Lecture 9:Internetworking
CSE 123: Computer NetworksAlex C. Snoeren
FDDIEthernet
Router
TCP HTTPIPEth
data packet data packet
TCP HTTPIPFDDI
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IP Networking
CSE 123 – Lecture 8: Internetworking
CSE 123 – Lecture 9: Internetworking
What does the destination address in the Ethernet frame header correspond to?
A. Host A B. Host BC. The routerD. The closest bridge/switch to host A
Host AHost B
Routers● A router is a store-and-forward device
u Routers are connected to multiple networksu On each network, looks just like another hostu 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 bridge or switch, which are transparentu Removes link-layer header, parses IP header (L3)
● Looks up next hop, forwards on appropriate networku Each router need only get one step closer to destination
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CSE 123 – Lecture 8: Internetworking
CSE 123 – Lecture 9: Internetworking
● Impose few demands on networku Make few assumptions about what network can dou No QoS, no reliability, no ordering, no large packetsu No persistent state about communications; no connections
● Manage heterogeneity at hosts (not in network)u Adapt to underlying network heterogeneityu 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
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IP Philosophy
CSE 123 – Lecture 8: Internetworking
CSE 123 – Lecture 9: Internetworking
So what does IP do?● Addressing – compicated topic so we’ll save it for later● 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
● We’ll start by deconstructing the header…
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
length
0 15 16
options (if any)
data (if any)
31
20 bytes
ver HL TOS
identificationRES
MFDF offset
TTL protocol header checksum
source address
destination address
IP Packet Header
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CSE 123 – Lecture 8: Internetworking
CSE 123 – Lecture 9: Internetworking
Version field● Which version of IP is this?
u Plan for changeu Very important!
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CSE 123 – Lecture 8: Internetworking
What versions of IP are in use today?
A. IPv4B. IPv6C. BothD. More than two
CSE 123 – Lecture 9: Internetworking
Header length● How big is IP header?
u Counted in 32-bit wordsu Variable length
» Optionsu Engineering consequences of variable length…
● Most IP packet headers are 20 bytes long
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
What is the content of the HL field for most IP packets?
A. 20B. 5C. Something else
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)
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
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
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
Protocol● Which transport protocol is the data using?
u i.e. how should a host interpret the data
● TCP = 6● UDP = 17
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
IP Checksum● Header contains simple checksum
u Validates content of header only
● Recalculated at each hopu Routers need to update TTLu Hence straightforward to modify
● Ensures correct destination receives packet
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
Length● How long is whole packet in bytes?
u Includes headeru Limits total packet to 64Ku Redundant?
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
Fragmentation● Different networks may have different
maximum frame sizesu Maximum Transmission Unit (MTUs)u Ethernet 1.5K, FDDI 4.5K
● Router breaks up single IP packet into two or more smaller IP packetsu Each fragment is labeled so it can be correctly
reassembledu End host reassembles them into original packet
R1
H4
H5
H3H2H1
Network 2 (Ethernet)
H6
Network 3 (FDDI)
Fragment?
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CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
IP ID and Bitflags● Source inserts unique value in identification field
u Also known as the IPIDu If packet is fragmented, the router copies this value into any
fragments● Offset field indicates position of current fragment (in bytes/8)
u Zero for non-fragmented packet
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● Bitflags provide additional informationu More Fragments bit helps identify last fragmentu Don’t Fragment bit prohibits (further) fragmentationu Note recursive fragmentation easily supported—just requires care with More Fragments bit
CSE 123 – Lecture 11: Fragmentation & Addressing
CSE 123 – Lecture 9: Internetworking
For Next Time
● Keep reading 3.2 in P&D
● Keep plugging away on Project 1
16CSE 123 – Lecture 9: Internetworking