draft-ietf-mpls-ttl-02.txt   draft-ietf-mpls-ttl-03.txt 
Internet Draft Puneet Agarwal Internet Draft Puneet Agarwal
Pluris Pluris
Bora A. Akyol Bora A. Akyol
Document: draft-ietf-mpls-ttl-02.txt Cisco Systems Document: draft-ietf-mpls-ttl-03.txt Cisco Systems
Category: Informational Category: Standards Track
Expires: November 2002 May 2002 Expires: December 2002 June 2002
Time to Live (TTL) Processing in MPLS Networks Time to Live (TTL) Processing in MPLS Networks (Updates RFC 3032)
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026. with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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reference material or to cite them other than as "work in progress." reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
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Abstract Abstract
This document describes TTL processing in hierarchical MPLS This document describes TTL processing in hierarchical MPLS
networks. TTL processing in both pipe and uniform model hierarchical networks. It updates rfc-3032 "MPLS Label Stack Encoding". TTL
tunnels are specified with examples for both "push" and "pop" cases. processing in both pipe and uniform model hierarchical tunnels are
The document also complements rfc-3270 "MPLS Support of specified with examples for both "push" and "pop" cases. The
Differentiated Services" and ties together the terminology document also complements rfc-3270 "MPLS Support of Differentiated
introduced in that document with TTL processing in hierarchical MPLS Services" and ties together the terminology introduced in that
networks. document with TTL processing in hierarchical MPLS networks.
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC-2119]. this document are to be interpreted as described in [RFC-2119].
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TTL Processing in MPLS Networks May 2002 Time to Live (TTL) Processing in MPLS Networks June 2002
1. Introduction and Motivation 1. Introduction and Motivation
This document describes Time to Live (TTL) processing in This document describes Time to Live (TTL) processing in
hierarchical MPLS networks. We believe that this document adds hierarchical MPLS networks. We believe that this document adds
details that have not been addressed in [MPLS-ARCH, MPLS-ENCAPS], details that have not been addressed in [MPLS-ARCH, MPLS-ENCAPS],
and that the methods presented in this document complement [MPLS- and that the methods presented in this document complement [MPLS-
DS]. DS].
2. TTL Processing in MPLS Networks 2. TTL Processing in MPLS Networks
2.1. Changes to RFC 3032 [MPLS-ENCAPS] 2.1. Changes to RFC 3032 [MPLS-ENCAPS]
a) [MPLS-ENCAPS] only covers the Uniform Model and does NOT a) [MPLS-ENCAPS] only covers the Uniform Model and does NOT
address the Pipe Model or the Short Pipe Model. This draft address the Pipe Model or the Short Pipe Model. This draft
will address these 2 models and for completeness will also addresses these 2 models and for completeness will also
address the Uniform Model. address the Uniform Model.
b) [MPLS-ENCAPS] does not cover hierarchical LSPs. This draft b) [MPLS-ENCAPS] does not cover hierarchical LSPs. This draft
will address this issue. addresses this issue.
c) [MPLS-ENCAPS] does not define TTL processing in the presence c) [MPLS-ENCAPS] does not define TTL processing in the presence
of Penultimate Hop Popping (PHP). This draft will address of Penultimate Hop Popping (PHP). This draft addresses this
this issue. issue.
2.2. Terminology and Background 2.2. Terminology and Background
As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header
that indicates the following information about a packet: that indicates the following information about a packet:
a. MPLS Label (20 bits) a. MPLS Label (20 bits)
b. TTL (8 bits) b. TTL (8 bits)
c. Bottom of stack (1 bit) c. Bottom of stack (1 bit)
d. Experimental bits (3 bits) d. Experimental bits (3 bits)
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QoS treatments. On the other hand, the Uniform model makes all the QoS treatments. On the other hand, the Uniform model makes all the
nodes that a LSP traverses visible to nodes outside the tunnel. We nodes that a LSP traverses visible to nodes outside the tunnel. We
will extend the Pipe and Uniform models to include TTL processing in will extend the Pipe and Uniform models to include TTL processing in
the following sections. Furthermore, TTL processing when performing the following sections. Furthermore, TTL processing when performing
PHP is also described in this document. For a detailed description PHP is also described in this document. For a detailed description
of Pipe and Uniform models, please see [MPLS-DS]. of Pipe and Uniform models, please see [MPLS-DS].
TTL processing in MPLS networks can be broken down into two logical TTL processing in MPLS networks can be broken down into two logical
blocks: (i) the incoming TTL determination to take into account any blocks: (i) the incoming TTL determination to take into account any
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tunnel egress due to MPLS Pop operations; (ii) packet processing of tunnel egress due to MPLS Pop operations; (ii) packet processing of
(possibly) exposed packet & outgoing TTL. (possibly) exposed packet & outgoing TTL.
We also note here that signaling the LSP type (pipe, short pipe or We also note here that signaling the LSP type (pipe, short pipe or
uniform model) is out of the scope of this document, and that is uniform model) is out of the scope of this document, and that is
also not addressed in the current versions of the label distribution also not addressed in the current versions of the label distribution
protocols, e.g. LDP [MPLS-LDP] and RSVP-TE [MPLS-RSVP]. protocols, e.g. LDP [MPLS-LDP] and RSVP-TE [MPLS-RSVP].
2.3. New Terminology 2.3. New Terminology
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(n) represents the TTL value in the corresponding header (n) represents the TTL value in the corresponding header
(x) represents non-meaningful TTL information (x) represents non-meaningful TTL information
(I) represents the LSP ingress node (I) represents the LSP ingress node
(P) represents the LSP penultimate node (P) represents the LSP penultimate node
(E) represents the LSP Egress node (E) represents the LSP Egress node
This picture shows TTL processing for a uniform model MPLS LSP. Note This picture shows TTL processing for a uniform model MPLS LSP. Note
that the inner and outer TTLs of the packets are synchronized at that the inner and outer TTLs of the packets are synchronized at
tunnel ingress and egress. tunnel ingress and egress.
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3.2. TTL Processing for Short Pipe Model LSPs 3.2. TTL Processing for Short Pipe Model LSPs
3.2.1. TTL Processing for Short Pipe Model LSPs without PHP 3.2.1. TTL Processing for Short Pipe Model LSPs without PHP
========== LSP =============================> ========== LSP =============================>
+--Swap--(N-1)-...-swap--(N-i)-----+ +--Swap--(N-1)-...-swap--(N-i)-----+
/ (outer header) \ / (outer header) \
(N) (N-i) (N) (N-i)
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/ \ / \
>--(n)--Push.............(n-1)............Pop-(n-1)-Decr.-(n-2)-> >--(n)--Push.............(n-1)............Pop-(n-1)-Decr.-(n-2)->
(I) (inner header) (P) (E) (I) (inner header) (P) (E)
(N) represents the TTL value (may have no relationship to n) (N) represents the TTL value (may have no relationship to n)
(n) represents the tunneled TTL value in the encapsulated header (n) represents the tunneled TTL value in the encapsulated header
(I) represents the LSP ingress node (I) represents the LSP ingress node
(P) represents the LSP penultimate node (P) represents the LSP penultimate node
(E) represents the LSP egress node. (E) represents the LSP egress node.
Since the label has already been popped by the LSPs penultimate Since the label has already been popped by the LSP's penultimate
node, the LSP egress node just decrements the header TTL. node, the LSP egress node just decrements the header TTL.
Also note that at the end of short pipe model LSP, the TTL of the Also note that at the end of short pipe model LSP, the TTL of the
tunneled packet has been decremented by two either with or without tunneled packet has been decremented by two either with or without
PHP. PHP.
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3.3. TTL Processing for Pipe Model LSPs (without PHP only): 3.3. TTL Processing for Pipe Model LSPs (without PHP only):
========== LSP =============================> ========== LSP =============================>
+--Swap--(N-1)-...-swap--(N-i)-----+ +--Swap--(N-1)-...-swap--(N-i)-----+
/ (outer header) \ / (outer header) \
(N) (N-i) (N) (N-i)
/ \ / \
>--(n)--Push...............(n-1)....................Pop--(n-2)-> >--(n)--Push...............(n-1)....................Pop--(n-2)->
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i.e. the TTL contained in the subsequent label is essentially i.e. the TTL contained in the subsequent label is essentially
ignored and replaced with the iTTL computed during the previous pop. ignored and replaced with the iTTL computed during the previous pop.
3.5. Outgoing TTL Determination and Packet Processing 3.5. Outgoing TTL Determination and Packet Processing
After the iTTL computation is performed, the oTTL check is performed. After the iTTL computation is performed, the oTTL check is performed.
If the oTTL check succeeds, then the outgoing TTL of the If the oTTL check succeeds, then the outgoing TTL of the
(labeled/unlabeled) packet is calculated and packet headers are (labeled/unlabeled) packet is calculated and packet headers are
updated as defined below. updated as defined below.
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If the packet was routed as an IP packet, the TTL value of the IP If the packet was routed as an IP packet, the TTL value of the IP
packet is set to oTTL (iTTL - 1). The TTL value(s) for any pushed packet is set to oTTL (iTTL - 1). The TTL value(s) for any pushed
label(s) are determined as described in section 3.6. label(s) are determined as described in section 3.6.
For packets that are routed as MPLS, we have four cases: For packets that are routed as MPLS, we have four cases:
1) Swap-only: The routed label is swapped with another label 1) Swap-only: The routed label is swapped with another label
and the TTL field of the outgoing label is set to oTTL. and the TTL field of the outgoing label is set to oTTL.
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while it is being updated or oTTL is being determined, this while it is being updated or oTTL is being determined, this
feature should be only used for compensating for network feature should be only used for compensating for network
nodes that are not capable of decrementing TTL values. nodes that are not capable of decrementing TTL values.
2) Whenever iTTL is decremented, the implementer must make sure 2) Whenever iTTL is decremented, the implementer must make sure
that the value does not go negative. that the value does not go negative.
3) In the short pipe model with PHP enabled, the TTL of the 3) In the short pipe model with PHP enabled, the TTL of the
tunneled packet is unchanged after the PHP operation. tunneled packet is unchanged after the PHP operation.
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4. Conclusion 4. Conclusion
This Internet Draft describes how TTL field can be processed in a This Internet Draft describes how TTL field can be processed in a
MPLS network. We clarified the various methods that are applied in MPLS network. We clarified the various methods that are applied in
the presence of hierarchical tunnels and completed the integration the presence of hierarchical tunnels and completed the integration
of Pipe and Uniform models with TTL processing. of Pipe and Uniform models with TTL processing.
5. Security Considerations 5. Security Considerations
This document does not add any new security issues other than the This document does not add any new security issues other than the
ones defined in [MPLS-ENCAPS, MPLS-DS]. In particular, the document ones defined in [MPLS-ENCAPS, MPLS-DS]. In particular, the document
does not define a new protocol or expand an existing one and does does not define a new protocol or expand an existing one and does
not introduce security problems into the existing protocols. The not introduce security problems into the existing protocols. The
authors believe that clarification of TTL handling in MPLS networks authors believe that clarification of TTL handling in MPLS networks
benefits service providers and their customers since troubleshooting benefits service providers and their customers since troubleshooting
is simplified. is simplified.
6. References 6. References
6.1. Normative References
[MPLS-ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol [MPLS-ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031. Label Switching Architecture", RFC 3031.
[MPLS-ENCAPS] E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D. [MPLS-ENCAPS] E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D.
Farinacci, T. Li, A. Conta, "MPLS Label Stack Encoding", RFC3032. Farinacci, T. Li, A. Conta, "MPLS Label Stack Encoding", RFC3032.
[MPLS-DS] F. Le Faucheur, L. Wu, B. Davie, S. Davari, P. Vaananen, [MPLS-DS] F. Le Faucheur, L. Wu, B. Davie, S. Davari, P. Vaananen,
R. Krishnan, P. Cheval, J. Heinanen, "MPLS Support of Differentiated R. Krishnan, P. Cheval, J. Heinanen, "MPLS Support of Differentiated
Services", RFC3270. Services", RFC3270.
6.2. Informative References
[MPLS-LDP] L. Andersson, P. Doolan, N. Feldman, A. Fredette, B. [MPLS-LDP] L. Andersson, P. Doolan, N. Feldman, A. Fredette, B.
Thomas, "LDP Specification", RFC 3036. Thomas, "LDP Specification", RFC 3036.
[MPLS-RSVP] D. Awduche, L. Berger, D. Gan, T. Li, V. Srinivasan, G. [MPLS-RSVP] D. Awduche, L. Berger, D. Gan, T. Li, V. Srinivasan, G.
Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209.
7. Acknowledgements 7. Acknowledgements
The authors would like to thank the members of the MPLS working The authors would like to thank the members of the MPLS working
group for their feedback. We would especially like to thank Shahram group for their feedback. We would especially like to thank Shahram
Davari and Loa Andersson for their careful review of the document Davari and Loa Andersson for their careful review of the document
and their comments. and their comments.
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8. Author's Addresses 8. Author's Addresses
Puneet Agarwal Puneet Agarwal
Pluris Pluris
10455 Bandley Drive 10455 Bandley Drive
Cupertino, CA 95014 Cupertino, CA 95014
Email: puneet@pluris.com Email: puneet@pluris.com
Bora Akyol Bora Akyol
Cisco Systems Cisco Systems
170 W. Tasman Drive 170 W. Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
Email: bora@cisco.com Email: bora@cisco.com
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