ECE 526 – Network ECE 526 – Network Processing Systems Processing Systems

DesignDesignPacket Processing II:  algorithms and

data structures Chapter 5: D. E. Comer

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Goal and OutlineGoal and Outline• Goal:

─ Understand basic network processing operations─ Learn how to perform operations – data structures &

algorithms

• Outline:─ IP fragmentation and reassembly─ IP forwarding and routing ─ TCP connection recognition and splicing ─ Summary─ For next class

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IP FragmentationIP Fragmentation• Needed when datagram larger than network MTU

─ Ethernet 1500 Byte

• FLAGs bits in datagram header

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Fragmentation ExampleFragmentation Example• How to identify a fragment

─ Flags─ Offset: multiple of 64 bit─ Ident: unique to send machine

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IP Fragmentation AlgorithmIP Fragmentation Algorithm

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IP ReassemblyIP Reassembly• Process to join fragments and produce original

datagram• Only ultimate destination perform IP reassembly

(NAT: exception)• Four factors influencing reassembly

─ Out of order delivery─ Duplication─ Loss─ Concurrent reception

• Key fields help reassembly─ Source IP address─ ID field─ Flags and Offset

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Reassembly AlgorithmReassembly Algorithm

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Reassembly Data StructureReassembly Data Structure• Two parts

─ Buffer larger enough to hold original datagram─ Linked list of pieces that have arrived

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IP Datagram ForwardingIP Datagram Forwarding• Conceptual mapping

─ (next hop, interface) f(datagram, routing table)

• Routing table─ one entry per destination─ entry contents: IP address, address mask, next-hop

address and N-bit interface number

• Example IP routing table

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IP Forwarding AlgorithmIP Forwarding Algorithm

Assuming: routing table sorted from most specific to less specific

Can I use hashing?

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High-Speed ForwardingHigh-Speed Forwarding• Example routing tree

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Routing Exercises Routing Exercises • Draw tire of the following prefixes:

─ A: 0010*─ B: 010*─ C: 0101*─ D: 0*─ E: 10*─ F: 1011*─ G: 100*

• Which prefixes match the following lookups?─ 01─ 101─ 0001─ 1

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TCP Connection RecognitionTCP Connection Recognition• Key function of traffic monitors, firewalls and NAT• State of TCP connection

─ Being established ─ Completely established─ Being terminated─ Completely terminated (remove from record)

• Code bits in TCP header:─ Reset:

• error occurred when one end has no record connection • regarded as a completely terminated here

─ Syn: • to start new connection • completely established need “see” syn from both sides

─ Fin: • to terminate connection• completely terminated need “see” fin from both sides

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TCP Connection Recognition AlgorithmTCP Connection Recognition Algorithm

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TCP Splicing TCP Splicing • Join two TCP connections

─ Allow data to pass between them─ To avoid termination overhead ─ By translating segment header fields

• Acknowledgment number; sequence number

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TCP Splicing AlgorithmTCP Splicing Algorithm

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SummarySummary• Packet processing operations and algorithms

─ Ethernet bridging (layer 2)─ IP fragmentation, reassembly and forwarding (layer 3)─ TCP splicing, connection recognition (layer 4)─ Flow classification (mixed layer)

• Important data structure─ Linked list─ Hashing table─ Routing table─ Tire

• Table lookup─ Hashing─ Full match for layer 2─ Longest prefix match (LPM) for layer 3

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For Next ClassFor Next Class

• “Networking Algorithmics” ─ Chapter 17: Network Security (handout)

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