Download - 12.Advanced L2TPv3 Case Studies
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Advanced L2TPv3 Case Studies
Majid Asadpoor
rayka-co.ir
rayka-co.ir Chapter 12 1
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Index
• ATM Cell Packing
• L2TPv3 Path MTU Discovery
• Quality of Service
rayka-co.ir Chapter 12 2
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ATM Cell Packing
rayka-co.ir Chapter 12 3
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ATM Cell Packing
rayka-co.ir Chapter 12 4
maximum number of cells to be packed equal to 14 cells
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ATM Cell Packing
48 bytes/cell * 14 cells = 672 bytes
16 Byte AAL5-LLC/SNAP encapsulation of CE
rayka-co.ir Chapter 12 5
16 Byte AAL5-LLC/SNAP encapsulation of CE
672 – 16 = 656
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ATM Cell Packing
rayka-co.ir Chapter 12 6
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ATM Cell Packing
rayka-co.ir Chapter 12 7
Each IP packet now needs 15 ATM cells; first L2TPv3 packet with 14 cells and a second
L2TPv3 packet with just 1 cell.
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L2TPv3 Path MTU Discovery
rayka-co.ir Chapter 12 8
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
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L2TPv3 Path MTU Discovery
sending 500 ICMP ping packets that total 1464 bytes
rayka-co.ir Chapter 12 9
DF bit in this outer IPv4 delivery header is not set. Therefore, oversized packets are being
fragmented after tunnel encapsulation
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L2TPv3 Path MTU Discovery
packets coming into
Serial5/0 interface and
sent into the tunnel are
rayka-co.ir Chapter 12 10
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
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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.
rayka-co.ir Chapter 12 11
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
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L2TPv3 Path MTU Discovery
Understanding PMTUD
rayka-co.ir Chapter 12 12
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L2TPv3 Path MTU Discovery
Understanding PMTUD
rayka-co.ir Chapter 12 13
PMTUD forces the CPU-intensive reassembly to happen in the receiving CE device
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L2TPv3 Path MTU Discovery
Implementing PMTUD
rayka-co.ir Chapter 12 14
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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
rayka-co.ir Chapter 12 15
packet with the DF bit set sent from CE device
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L2TPv3 Path MTU Discovery
Combining PMTUD with DF Bit
rayka-co.ir Chapter 12 16
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L2TPv3 Path MTU Discovery
Combining PMTUD with DF Bit
rayka-co.ir Chapter 12 17
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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
rayka-co.ir Chapter 12 18
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L2TPv3 Path MTU Discovery
Combining PMTUD with DF Bit
rayka-co.ir Chapter 12 19
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Quality of Service
Traffic Marking
rayka-co.ir Chapter 12 20
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Quality of Service
Traffic Policing
rayka-co.ir Chapter 12 21
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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)
rayka-co.ir Chapter 12 22
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Quality of Service
Queuing and Shaping
rayka-co.ir Chapter 12 23