12.Advanced L2TPv3 Case Studies

Advanced L2TPv3 Case Studies

Majid Asadpoor

rayka-co.ir

rayka-co.ir Chapter 12 1

Index

• ATM Cell Packing

• L2TPv3 Path MTU Discovery

• Quality of Service


ATM Cell Packing


ATM Cell Packing


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


16 Byte AAL5-LLC/SNAP encapsulation of CE

672 – 16 = 656

ATM Cell Packing


ATM Cell Packing


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


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


sending 500 ICMP ping packets that total 1464 bytes


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

fragmented after tunnel encapsulation


packets coming into

Serial5/0 interface and

sent into the tunnel are


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


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.


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


Understanding PMTUD



Understanding PMTUD


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


Implementing PMTUD



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


packet with the DF bit set sent from CE device



• PE device can take active role in PMTUD

process by setting DF bit in all packets in outer

IPv4 header


Quality of Service

Traffic Marking


Quality of Service

Traffic Policing


Quality of Service

Queuing and Shaping

• Low-latency queuing (LLQ)

• Class-based weighted fair queuing (CBWFQ)

• Weighted Random Early Detection (WRED)


Quality of Service

Queuing and Shaping


12.Advanced L2TPv3 Case Studies

Documents

Transcript of 12.Advanced L2TPv3 Case Studies