1. W. Mark Townsley townsley@cisco.com
Pseudowires and L2TPv3
W. Mark Townsley

2. Goals Define the term“Pseudowire” and its relation to an “L2VPN”
Discuss motivations for a converged network
Overview of the IETF PWE3 framework
Overview of L2TP as a tunneling protocol for PWE3 over IP

3. Pseudowire
Defined by the IETF PWE3 (Pseudowire Edge to Edge Emulation) WG
Emulates the essential attributes of a (typically layer 2) service, such as Frame Relay, PPP, T1, Ethernet, ATM, etc. over a packet switched network.
The packet switched network could be an IP network or an MPLS network (this talk will focus more on IP)

4. L2VPN
A collection of pseudowires carrying emulated data links over a converged network.
For operation over an IP network, an important building block is an interoperable tunneling protocol for carrying each link to the participating edge routers.

5. Sample L2VPN Using Pseudowires
IP Network
Tunnelled LAN
Tunnelled
serial interface
Pseudowire
(Layer 2 Tunnel) R3 R4 R1 R2
LAN1
LAN2
tu1
tu2 e1 e2
pos1
pos2
pos3
pos4

6. Network Convergence Before: Parallel Networks
Duplication of international links
Separate equipment in the PoPs for each distinct service
Paris
Miami
IP PoP
IP PoP
Global IP
Backbone
FR PoP
FR PoP
Global FR
Backbone
Milan
IP PoP
FR PoP

7. Network Convergence After: Unified Network
Single set of backbone links for the IP network
FR and IP services from the same set of platforms in the IP + FR PoP
L2TPv3 tunnels across the IP backbone for FR services
Paris
Miami
IP + FR PoP
Global IP
Backbone
IP + FR PoP
Milan
IP + FR PoP

8. Other Examples of Convergence
Frame Relay over ATM networks [FRF.5]
T1, E1 and T3 circuits over ATM networks [ATMCES].
Voice over ATM (AAL2), Frame Relay [FRF.11], IP (VoIP) and MPLS networks.
PPP is carried over IP, ATM and Frame Relay networks [L2TP].

9. Tunneling Protocol Requirements for PWE3 over IP
An efficient layer 2 tunneling and multiplexing encapsulation
Explicit configuration or signaled negotiation of service specific parameters between edge routers
Method for signaling, timing, order or other aspects of the service between edge routers
Light and heavy duty security options

10. PWE3 Encapsulation Layering
Focus of PWE3 - above a PSN specific multiplexing layer
Payload (circuit/cell/packet)
Bit type
specific
Cell type
specific
Packet type
specific
PWE3 Payload
encapsulation
definition
Optional RTP/Sequencing
PWE3 IP
convergence
definition
L2TPv3
Fragmentation
Length
PWE3 MPLS
convergence
Definition based
on draft-martini
Inner Label
IPv4
IPv6
MPLS
MAC/Data-Link
Physical
[PWE3LYR]

11. L2TPv3 Encapsulation [L2TPv3] Payload IP 20 Bytes L2TPv3 Header
int3
int2
L2TPv3 Tunneled LAN
IP Network
L2TPv3 L2 Tunnel
Payload IP
L2TP IP
20 Bytes
L2TPv3 Header
Bytes
Payload
Session Identifier
4 Bytes
Cookie
0,4,8 Bytes
[L2TPv3]

12. Tunneling Protocol Requirements for PWE3 over IP
An efficient layer 2 tunneling and multiplexing encapsulation
Easy tunnel setup, and negotiation of service specific parameters between edge routers
Method for signaling, timing, order or other aspects of the service between edge routers
Light and heavy duty security options

13. L2TP Control Plane
L2TP has an in-band, reliable, control plane used for tunnel setup and maintenance
Control plane operates its own reliable datagram protocol, documented as a part of RFC2661
By design, it is not TCP, though it borrows from TCP with adapted windowing, congestion control, and slow start methods.

14. L2TP Control Connection
Three message handshake establishes the reliable Control Connection and advertises capabilities between peers.
SCCRQ
SCCRP
SCCCN

15. L2TP Control Connection
Once a Control Connection is established, multiple tunnels (sessions) may be setup “automagically” as needed
Includes optional Challenge/Handshake mutual peer authentication method (a “light-duty” security choice)
3-way handshake is used to establish identity, advertise, and negotiate capabilities between peers.

16. L2TP Session Establishment
In L2TP, each tunnel between the same endpoints is referred to as a “session”. A similar 3 message exchange is used to establish each session.
ICRQ
ICRP
ICCN

17. L2TP Control Messages
Control plane is designed to easily accept new messages for reliable delivery
Standard methods for “vendor specific” message type number space, as well as IETF number space
Attribute-value pair (AVP) message construction

18. Tunneling Protocol Requirements for PWE3 over IP
An efficient layer 2 tunneling and multiplexing encapsulation
Explicit configuration or signaled negotiation of service specific parameters between edge routers
Method for signaling, timing, order or other aspects of the service between edge routers
Light and heavy duty security options

19. L2TP Maintenance
Messages are sent over the in-band reliable control plane to signal all line events, advertise a state changes, establish and teardown new sessions, etc.
Single keepalive operates for all sessions between two endpoints
Sequencing and TDM emulation operates above the tunnel

20. Tunneling Protocol Requirements for PWE3 over IP
An efficient layer 2 tunneling and multiplexing encapsulation
Explicit configuration or signaled negotiation of service specific parameters between edge routers
Method for signaling, timing, order or other aspects of the service between edge routers
Light and heavy duty security options

21. L2TP Security “Heavy Duty” choice: RFC 3193 “Securing L2TP with IPsec”
IPSec operates in Transport Mode, L2TP is responsible for tunneling
Gives operator the option of turning security on or off at will, decoupling the tunneling system from the security method

22. L2TP Security Light duty options: Control Connection Authentication
L2TPv3 “Cookie field” random 64 bit value in each data packet associated with session to protect against a malicious blind attack, or inadvertent insertion of data into the tunnel stream.

23. Blind Insertion Attack
Blind – The attacker as no access to any data flowing on the provider’s network, only the ability to insert spoofed data at will.
In order for the packet to not be dropped, the attacker will have to guess a 64-bit random value.

24. Brute Force Insertion
Goal, to get one 40 byte spoofed packet inserted onto a VPN at OC48
20 bits – 130 ms
32 bits – Under 10 min
64 bits – 75K years

25. Brute Force Insertion
Goal, to get one 40 byte spoofed packet inserted onto a VPN at OC192
20 bits – 34 ms
32 bits – Under 3 min
64 bits – 18K years

26. What’s new in L2TPv3? Majority of functionality unchanged
Tunnel setup, control channel, maintenance…
New encapsulation for IP, resurrection of Cookie field
Separation of the base tunneling protocol from PPP
draft-ietf-l2tpext-l2tp-base-01.txt
draft-ietf-l2tpext-l2tp-ppp-01.txt

27. L2TP Timeline
August First version of L2TP Internet Draft published
May First multivendor interoperability workshop (“bakeoff”) at Pacific Bell
Nov First version of L2TP over IPsec Internet Draft submitted
Aug 1999 – RFC2661 published

28. L2TP Timeline Jun 2000 – Ethernet over L2TP Internet draft submitted
July 2001 – First version of “l2tp-base” a.k.a. L2TPv3 submitted to WG
Aug 2001 – First PWE3 WG Meets at 51st IETF in London

29. Summary
Pseudowires provide network convergence by emulating a variety of data links over a common packet switched network
Pseudowires may be operated over IP without modification of IP core routers
L2TPv3 is a tunneling protocol that has a large base of operational experience and standardization in the IETF that is being used for pseudowire tunneling

30. References [PWE3] draft-ietf-pwe3-framework-00.txt
[PWE3LYR] draft-bryant-pwe3-protocol-layer-00.txt
[L2TPv3]draft-ietf-l2tpext-l2tp-base-01.txt
[L2TP] RFC2661
[ATMCES] ATM Forum, "Circuit Emulation Service Interoperability Specification Version 2.0" (af-vtoa ), January 1997.
[FRF.5] O'Leary et al, "Frame Relay/ATM PVC Network Interworking Implementation Agreement", Frame Relay Forum FRF.5, December 20, ITU Recommendation Q.933, Annex A, Geneva, 1995.
[FRF.11] R. Kocen and T. Hatala, "Voice over frame relay implementation agreement", Implementation Agreement FRF.11, Frame Relay Forum, Foster City, California, Jan

31. End