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Versions: (draft-agarwal-mpls-ttl) 00 01 02
03 RFC 3443
Internet Draft Puneet Agarwal
Pluris
Bora A. Akyol
Document: draft-ietf-mpls-ttl-03.txt Cisco Systems
Category: Standards Track
Expires: December 2002 June 2002
Time to Live (TTL) Processing in MPLS Networks (Updates RFC 3032)
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
This document describes TTL processing in hierarchical MPLS
networks. It updates rfc-3032 "MPLS Label Stack Encoding". TTL
processing in both pipe and uniform model hierarchical tunnels are
specified with examples for both "push" and "pop" cases. The
document also complements rfc-3270 "MPLS Support of Differentiated
Services" and ties together the terminology introduced in that
document with TTL processing in hierarchical MPLS networks.
Conventions used in this document
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 [RFC-2119].
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 1
Time to Live (TTL) Processing in MPLS Networks June 2002
1. Introduction and Motivation
This document describes Time to Live (TTL) processing in
hierarchical MPLS networks. We believe that this document adds
details that have not been addressed in [MPLS-ARCH, MPLS-ENCAPS],
and that the methods presented in this document complement [MPLS-
DS].
2. TTL Processing in MPLS Networks
2.1. Changes to RFC 3032 [MPLS-ENCAPS]
a) [MPLS-ENCAPS] only covers the Uniform Model and does NOT
address the Pipe Model or the Short Pipe Model. This draft
addresses these 2 models and for completeness will also
address the Uniform Model.
b) [MPLS-ENCAPS] does not cover hierarchical LSPs. This draft
addresses this issue.
c) [MPLS-ENCAPS] does not define TTL processing in the presence
of Penultimate Hop Popping (PHP). This draft addresses this
issue.
2.2. Terminology and Background
As defined in [MPLS-ENCAPS], MPLS packets use a MPLS shim header
that indicates the following information about a packet:
a. MPLS Label (20 bits)
b. TTL (8 bits)
c. Bottom of stack (1 bit)
d. Experimental bits (3 bits)
The experimental bits were later redefined in [MPLS-DS] to indicate
the scheduling and shaping behavior that could be associated with a
MPLS packet.
[MPLS-DS] also defined two models for MPLS tunnel operation: Pipe
and Uniform models. In the Pipe model, a MPLS network acts like a
circuit when MPLS packets traverse the network such that only the
LSP ingress and egress points are visible to nodes that are outside
the tunnel. A Short variation of the Pipe Model is also defined in
[MPLS-DS] to differentiate between different egress forwarding and
QoS treatments. On the other hand, the Uniform model makes all the
nodes that a LSP traverses visible to nodes outside the tunnel. We
will extend the Pipe and Uniform models to include TTL processing in
the following sections. Furthermore, TTL processing when performing
PHP is also described in this document. For a detailed description
of Pipe and Uniform models, please see [MPLS-DS].
TTL processing in MPLS networks can be broken down into two logical
blocks: (i) the incoming TTL determination to take into account any
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 2
Time to Live (TTL) Processing in MPLS Networks June 2002
tunnel egress due to MPLS Pop operations; (ii) packet processing of
(possibly) exposed packet & outgoing TTL.
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
also not addressed in the current versions of the label distribution
protocols, e.g. LDP [MPLS-LDP] and RSVP-TE [MPLS-RSVP].
2.3. New Terminology
iTTL: The TTL value to use as the incoming TTL. No checks are
performed on the iTTL.
oTTL: This is the TTL value used as the outgoing TTL value (see
section 3.5 for exception). It is always (iTTL - 1) unless otherwise
stated.
oTTL Check: Check if oTTL is greater than 0. If the oTTL Check is
false, then the packet is not forwarded. Note that the oTTL check is
performed only if any outgoing TTL (either IP or MPLS) is set to
oTTL (see section 3.5 for exception).
3. TTL Processing in different Models
This section describes the TTL processing for LSPs conforming to
each of the 3 models (Uniform, Short Pipe and Pipe) in the
presence/absence of PHP (where applicable).
3.1. TTL Processing for Uniform Model LSPs (with or without PHP)
(consistent with [MPLS-ENCAPS]):
========== LSP =============================>
+--Swap--(n-2)-...-swap--(n-i)---+
/ (outer header) \
(n-1) (n-i)
/ \
>--(n)--Push...............(x).....................Pop--(n-i-1)->
(I) (inner header) (E or P)
(n) represents the TTL value in the corresponding header
(x) represents non-meaningful TTL information
(I) represents the LSP ingress node
(P) represents the LSP penultimate node
(E) represents the LSP Egress node
This picture shows TTL processing for a uniform model MPLS LSP. Note
that the inner and outer TTLs of the packets are synchronized at
tunnel ingress and egress.
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 3
Time to Live (TTL) Processing in MPLS Networks June 2002
3.2. TTL Processing for Short Pipe Model LSPs
3.2.1. TTL Processing for Short Pipe Model LSPs without PHP
========== LSP =============================>
+--Swap--(N-1)-...-swap--(N-i)-----+
/ (outer header) \
(N) (N-i)
/ \
>--(n)--Push...............(n-1).....................Pop--(n-2)->
(I) (inner header) (E)
(N) represents the TTL value (may have no relationship to n)
(n) represents the tunneled TTL value in the encapsulated header
(I) represents the LSP ingress node
(E) represents the LSP Egress node
Short Pipe Model was introduced in [MPLS-DS]. In the short pipe
model, the forwarding treatment at the egress LSR is based on the
tunneled packet as opposed to the encapsulating packet.
3.2.2. TTL Processing for Short Pipe Model with PHP:
========== LSP =====================================>
+-Swap-(N-1)-...-swap-(N-i)-+
/ (outer header) \
(N) (N-i)
/ \
>--(n)--Push.............(n-1)............Pop-(n-1)-Decr.-(n-2)->
(I) (inner header) (P) (E)
(N) represents the TTL value (may have no relationship to n)
(n) represents the tunneled TTL value in the encapsulated header
(I) represents the LSP ingress node
(P) represents the LSP penultimate node
(E) represents the LSP egress node.
Since the label has already been popped by the LSP's penultimate
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
tunneled packet has been decremented by two either with or without
PHP.
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 4
Time to Live (TTL) Processing in MPLS Networks June 2002
3.3. TTL Processing for Pipe Model LSPs (without PHP only):
========== LSP =============================>
+--Swap--(N-1)-...-swap--(N-i)-----+
/ (outer header) \
(N) (N-i)
/ \
>--(n)--Push...............(n-1)....................Pop--(n-2)->
(I) (inner header) (E)
(N) represents the TTL value (may have no relationship to n)
(n) represents the tunneled TTL value in the encapsulated header
(I) represents the LSP ingress node
(E) represents the LSP Egress node
From the TTL perspective, the treatment for a Pipe Model LSP is
identical to the Short Pipe Model without PHP.
3.4. Incoming TTL (iTTL) determination
If the incoming packet is an IP packet, then the iTTL is the TTL
value of the incoming IP packet.
If the incoming packet is a MPLS packet and we are performing a
Push/Swap/PHP, then the iTTL is the TTL of the topmost incoming
label.
If the incoming packet is a MPLS packet and we are performing a Pop
(tunnel termination), the iTTL is based on the tunnel type (Pipe or
Uniform) of the LSP that was popped. If the popped label belonged to
a Pipe model LSP, then the iTTL is the value of the TTL field of the
header exposed after the label was popped (note that for the purpose
of this draft, the exposed header may be either an IP header or an
MPLS label). If the popped label belonged to a Uniform model LSP,
then the iTTL is equal to the TTL of the popped label. If multiple
Pop operations are performed sequentially, then the procedure given
above is repeated with one exception: the iTTL computed during the
previous Pop is used as the TTL of subsequent label being popped;
i.e. the TTL contained in the subsequent label is essentially
ignored and replaced with the iTTL computed during the previous pop.
3.5. Outgoing TTL Determination and Packet Processing
After the iTTL computation is performed, the oTTL check is performed.
If the oTTL check succeeds, then the outgoing TTL of the
(labeled/unlabeled) packet is calculated and packet headers are
updated as defined below.
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 5
Time to Live (TTL) Processing in MPLS Networks June 2002
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
label(s) are determined as described in section 3.6.
For packets that are routed as MPLS, we have four cases:
1) Swap-only: The routed label is swapped with another label
and the TTL field of the outgoing label is set to oTTL.
2) Swap followed by a Push: The swapped operation is performed
as described in (1). The TTL value(s) of any pushed label(s)
are determined as described in section 3.6.
3) Penultimate Hop Pop (PHP): The routed label is popped. The
oTTL check should be performed irrespective of whether the
oTTL is used to update the TTL field of the outgoing header.
If the PHPed label belonged to a short Pipe model LSP, then
the TTL field of the PHP exposed header is neither checked
nor updated. If the PHPed label was a Uniform model LSP,
then the TTL field of the PHP exposed header is set to the
oTTL. The TTL value(s) of additional labels are determined
as described in section 3.6
4) Pop: The pop operation happens before routing and hence it
is not considered here.
3.6. Tunnel Ingress Processing (Push)
For each pushed Uniform model label, the TTL is copied from the
label/IP-packet immediately underneath it.
For each pushed Pipe model or Short Pipe model label, the TTL field
is set to a value configured by the network operator. In most
implementations, this value is set to 255 by default.
3.7. Implementation Remarks
1) Although iTTL can be decremented by a value larger than 1
while it is being updated or oTTL is being determined, this
feature should be only used for compensating for network
nodes that are not capable of decrementing TTL values.
2) Whenever iTTL is decremented, the implementer must make sure
that the value does not go negative.
3) In the short pipe model with PHP enabled, the TTL of the
tunneled packet is unchanged after the PHP operation.
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 6
Time to Live (TTL) Processing in MPLS Networks June 2002
4. Conclusion
This Internet Draft describes how TTL field can be processed in a
MPLS network. We clarified the various methods that are applied in
the presence of hierarchical tunnels and completed the integration
of Pipe and Uniform models with TTL processing.
5. Security Considerations
This document does not add any new security issues other than the
ones defined in [MPLS-ENCAPS, MPLS-DS]. In particular, the document
does not define a new protocol or expand an existing one and does
not introduce security problems into the existing protocols. The
authors believe that clarification of TTL handling in MPLS networks
benefits service providers and their customers since troubleshooting
is simplified.
6. References
6.1. Normative References
[MPLS-ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031.
[MPLS-ENCAPS] E. Rosen, D. Tappan, G. Fedorkow, Y. Rekhter, D.
Farinacci, T. Li, A. Conta, "MPLS Label Stack Encoding", RFC3032.
[MPLS-DS] F. Le Faucheur, L. Wu, B. Davie, S. Davari, P. Vaananen,
R. Krishnan, P. Cheval, J. Heinanen, "MPLS Support of Differentiated
Services", RFC3270.
6.2. Informative References
[MPLS-LDP] L. Andersson, P. Doolan, N. Feldman, A. Fredette, B.
Thomas, "LDP Specification", RFC 3036.
[MPLS-RSVP] D. Awduche, L. Berger, D. Gan, T. Li, V. Srinivasan, G.
Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209.
7. Acknowledgements
The authors would like to thank the members of the MPLS working
group for their feedback. We would especially like to thank Shahram
Davari and Loa Andersson for their careful review of the document
and their comments.
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 7
Time to Live (TTL) Processing in MPLS Networks June 2002
8. Author's Addresses
Puneet Agarwal
Pluris
10455 Bandley Drive
Cupertino, CA 95014
Email: puneet@pluris.com
Bora Akyol
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
Email: bora@cisco.com
Agarwal & Akyol draft-ietf-mpls-ttl-03.txt 8
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