Advanced L2TPv3 Case Studies

Majid Asadpoor

rayka-co.ir

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Index

• ATM Cell Packing

• L2TPv3 Path MTU Discovery

• Quality of Service

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ATM Cell Packing

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ATM Cell Packing

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maximum number of cells to be packed equal to 14 cells

ATM Cell Packing

48 bytes/cell * 14 cells = 672 bytes

16 Byte AAL5-LLC/SNAP encapsulation of CE

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16 Byte AAL5-LLC/SNAP encapsulation of CE

672 – 16 = 656

ATM Cell Packing

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ATM Cell Packing

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Each IP packet now needs 15 ATM cells; first L2TPv3 packet with 14 cells and a second

L2TPv3 packet with just 1 cell.

L2TPv3 Path MTU Discovery

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Don’t send IP packets larger than core

MTU minus 36 bytes

• 20 bytes of IPv4 Delivery header

• 4 bytes of L2TPv3 Session ID

• 4 bytes of L2TPv3 cookie

• 4 bytes Layer 2-Specific Sublayer used

for sequencing

• 4 bytes HDLC

1500- 36 = 1464

L2TPv3 Path MTU Discovery

sending 500 ICMP ping packets that total 1464 bytes

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DF bit in this outer IPv4 delivery header is not set. Therefore, oversized packets are being

fragmented after tunnel encapsulation

L2TPv3 Path MTU Discovery

packets coming into

Serial5/0 interface and

sent into the tunnel are

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sent into the tunnel are

fast switched , but

packets that are sent out

of interface Serial5/0

coming from the L2TPv3

session are process

switched

L2TPv3 Path MTU Discovery

Understanding PMTUD• Copies DF bit from inner IP header into outer IPv4 header

• Find out and record path MTU for the session.

• If received IPv4 packet from CE has DF bit cleared and resulting L2TPv3 packet exceeds discovered MTU– It fragments CE IPv4 packet, copies original Layer 2 header and

appends it into each of the generated fragments.

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appends it into each of the generated fragments.

– Pushes computational expensive IPv4 reassembly into the receiving CE device and relieves the PE from being a centralized reassembly point.

• If received IPv4 packet from CE has DF bit set and resulting L2TPv3 packet exceeds discovered MTU– generates ICMP unreachable messages to the CE device

L2TPv3 Path MTU Discovery

Understanding PMTUD

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L2TPv3 Path MTU Discovery

Understanding PMTUD

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PMTUD forces the CPU-intensive reassembly to happen in the receiving CE device

L2TPv3 Path MTU Discovery

Implementing PMTUD

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L2TPv3 Path MTU Discovery

Combining PMTUD with DF Bit

• If PMTUD is configured but path MTU is not

discovered, reassembly occurs in PE device

– to discover path MTU, you need to have a large

packet with the DF bit set sent from CE device

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packet with the DF bit set sent from CE device

L2TPv3 Path MTU Discovery

Combining PMTUD with DF Bit

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L2TPv3 Path MTU Discovery

Combining PMTUD with DF Bit

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L2TPv3 Path MTU Discovery

Combining PMTUD with DF Bit

• PE device can take active role in PMTUD

process by setting DF bit in all packets in outer

IPv4 header

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L2TPv3 Path MTU Discovery

Combining PMTUD with DF Bit

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Quality of Service

Traffic Marking

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Quality of Service

Traffic Policing

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Quality of Service

Queuing and Shaping

• Low-latency queuing (LLQ)

• Class-based weighted fair queuing (CBWFQ)

• Weighted Random Early Detection (WRED)

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Quality of Service

Queuing and Shaping

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