Network Working Group Jin Lizhong (ED)
Internet Draft Feng Jun
Expiration Date: Septembet 2007 ZTE Corporation
March 2007
Inter-AS Pseudo Wire Extention
draft-jin-pwe3-pw-inter-as-ext-00.txt
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Abstract
This document describes another way to set up a PW between two ASs
in the scenario of directly connected ASBR between the two ASs.
The Single-Segment PW or PW Segment of a MS-PW setup between two
directly connected ASBRs dose not need PSN tunnel, so as to provide
a more simple way to set up inter-as PW. In the scenario described
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in this draft, MP-EBGP can also be used to signal the mapping of PW,
then no other protocols are existed between the two ASBRs.
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
In many network topology, the ASBRs between two different ASs are
directly connected. The new way to set up inter-as PW described in
this draft bases on this scenario. Figure 1 shows the scenario.
In the MS-PW draft [I-D.draft-ietf-pwe3-segmented-pw-04] and [I-D.
draft-ietf-pwe3-dynamic-ms-pw-02], if we want to set up a MS-PW
across two different AS, then a PSN tunnel is needed between the two
ASs or ASBRs according [RFC4447], whatever the ASBRs are directly
connected.
\ /
AS1 | | AS2
ST-PE----S-PE1(ASBR1)---------S-PE2(ASBR2)----TT-PE
| |
/ \
Figure 1: Inter-AS scenario
The two S-PEs are at the same time ASBRs in Figure 1. ST-PE needs
to set up a MS-PW to TT-PE, then a segment PW between S-PE1 and
S-PE2 needs an PSN tunnel according the PW setup mechanism. An
inter-as MPLS tunnel is needed according [RFC4447], maybe it is
RSVP-TE tunnel or LDP tunnel. As we know, it is a little difficult
to provide such a tunnel between two ASs, for the PE devices in the
two ASs may not support the same type of inter-as tunnel. If the two
S-PEs are not ASBRs or not directly connected, then an inter-as
tunnel is a MUST; but if they are also ASBRs and directly connected,
then the inter-as tunnel is not needed. It is more simple of not
providing a PSN tunnel between the two ASBRs and no additional
requirements are needed for the ASBRs.
3. PW without PSN tunnel
If a Single-Segment PW or PW Segment of a MS-PW is setup between
two directly connected ASBRs, then the PSN tunnel is not needed.
In the MPLS label based network, the ASBR received PW packets from
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intra-as MPLS tunnel, pops the external label if PHP is disabled or
explicit label is used, switches the PW label with the one sent by
peer ASBR, then sends the packets to peer ASBR. The peer ASBR
switches the PW label with the one sent by peer PE, pushes the
external label if necessary, then sends the packets to the peer PE
through a new intra-as MPLS tunnel. PW packets switched between the
two ASBRs will not have PSN tunnel label, only PW label is switched.
But the packets switched in the same AS still need two-label stack
switching. The LDP Target session is still set up between the two
ASBRs using the same address with BGP.
The method described above can be a complement for both [I-D.
draft-ietf-pwe3-segmented-pw-04] and [I-D.draft-ietf-pwe3-dynamic-ms-
pw-02].
4. MS-PW without LDP signaling between two ASBRs
For the dynamic MS-PW described in [I-D.draft-ietf-pwe3-dynamic-ms-
pw-02], MP-BGP is already used to exchanged the MS-PW path
information. In the above scenario, LDP Target session is still
needed and exists between two ASBRs. This results LDP protocol
enabled between the two ASBRs. We can use MP-EBGP to signal the
mapping of PW instead of LDP session between the two ASBRs. A new
type of NLRI with AFI and SAFI should be assigned by IANA. MP-EBGP
UPDATA packet needs to contain the PWid FEC or Generalized PWid FEC
element and Generic Label TLV, see figure 2. The PWid FEC/Generalized
PWid FEC and Generic Label are encoded in NLRI. The encoding method
is the same with that defined in [RFC3107]. The processing of
reachable NLRI is the same with that of LDP mapping message that
described in [RFC 4447], while processing of unreachable NLRI is the
same with that of LDP withdraw message described in [RFC 4447].
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Label (3 octets) |
+---------------------------+
| PWid FEC (variable) |
+---------------------------+
Figure 2: Encoding of NRLI for PW
In this case, there is only BGP Protocol existed between the two
ASBRs, LDP and other protocols are not needed. Then a very clear
protocol border exists between the two ASs. In this draft, using of
MP-EBGP to signal the mapping of dynamic Multi-Segment PW is
suggested, since MP-BGP has now been used to exchange MS-PW path
information.
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The method described above can be a complement for [I-D.
draft-ietf-pwe3-dynamic-ms-pw-02].
6. Applicability
In the scenario of setting up a PW between two different ASs, and
the two ASs support different kind of PSN tunnel, or Service
Provider does not want to enable any PSN tunnel between two ASs or
ASBRs, then a MS-PW can be set up with one PW Segment between two
ASBRs that are directly connected. PSN tunnel between the two ASBRs
is not needed.
In the scenario that the Service Provider neither wants to
distribute any loopback address to other AS, nor enable any other
protocols except BGP between the two ASBRs, so as to ensure a clear
border between two ASs, then MP-EBGP can be used to signal the
mapping of PW and exchange MS-PW path information.
7. QoS considerations
When the label is switched between the two ASBRs, the processing
method of EXP is the same with before.
8. Intellectual Property Statement
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ipr@ietf.org.
9. Full Copyright Statement
Copyright (C) The IETF Trust (2007).
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10. Acknowledgments
The authors wish to acknowledge the contributions of the authors
[RFC 4447], [I-D.draft-ietf-pwe3-segmented-pw-04] and [I-D.draft-ietf
-pwe3-dynamic-ms-pw-02].
11. Normative References
[RFC3107] "Carrying Label Information in BGP-4", Y. Rekhter., et al.,
rfc 3107 May 2001
[RFC4447] "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", Martini, L., et al., rfc4447
April 2006.
12. Informative References
[I-D.draft-ietf-pwe3-segmented-pw-04] Martini et.al. "Segmented
Pseudo Wire", draft-ietf-pwe3-segmented-pw-04.txt, IETF Work
in Progress, February 2007
[I-D.draft-ietf-pwe3-dynamic-ms-pw-02] Martini et.al. "Dynamic
Placement of Multi Segment Pseudo Wires", draft-ietf-pwe3-
dynamic-ms-pw-02.txt, IETF Work in Progress, October 2006
13. Author Information
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Jin Lizhong
ZTE Corporation
No.68 Zijinghua Rd,Yuhuatai District,
Nanjing. P.R.China.
e-mail: jin.lizhong@zte.com.cn
Feng Jun
ZTE Corporation
No.68 Zijinghua Rd,Yuhuatai District,
Nanjing. P.R.China.
e-mail: feng.jun99@zte.com.cn
Liu Kebo
ZTE Corporation
No.68 Zijinghua Rd,Yuhuatai District,
Nanjing. P.R.China.
e-mail: liu.kebo@zte.com.cn
Hu Kan
ZTE Corporation
No.68 Zijinghua Rd,Yuhuatai District,
Nanjing. P.R.China.
e-mail: hu.kan@zte.com.cn
Wang Liangjun
ZTE Corporation
No.68 Zijinghua Rd,Yuhuatai District,
Nanjing. P.R.China.
e-mail: wang.liangjun@zte.com.cn
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