© 2004 The MITRE Corporation. All rights reserved SCPS-TP Updates Cislunar WG Meeting CCSDS...

© 2004 The MITRE Corporation. All rights reserved

SCPS-TP Updates

Cislunar WG Meeting

CCSDS Toulouse

November 2004


Agenda

Current Status

Clarifications of existing SCPS-TP capabilities– Compressed header– ECN support

Extending the SCPS Capabilities Option– Rationale– Xiphos’ Proposal– TLV Proposal– Discussion


Current Status

SCPS-Capabilities Option (TCP Option 20)– Allows endpoints to negotiate which SCPS options they

support Compressed SCPS Transport Protocol (IPPROTO 105) A number of vendors using SCPS-TP

– Interoperability issues SCPS-RI Mapping of compressed bit vector not matching spec. What connection IDs to use Protocol# to use in pseudo-header checksum for compressed

connections Compressed (short-form) SNACK option ECN Reserved bits use

– Extended capabilities for vendor-specific


Clarification issues wrt compressed header1. Which "Connection ID" do you use in constructing compressed

SCPS-TP headers?

2. When computing the checksum for the compressed SCPS-TP headers, what protocol number do you use for populating the pseudo-header?

3. Encoding of SNACK options: Do you treat any/all SNACK options on a TCP header as incompressible options or do you make use of the SNACK bit in the Compressed Header Bit-Vector for encoding the first SNACK option on a segment?

4. Encoding of ECN bits in a TCP Header: RFC 3168 defines two of the previously Reserved 6 bits in a TCP header:

– Bit 8 CWR (Congestion Window Reduced)

– Bit 9 ECE (ECN-Echo)

– Do any other implementors handle these with respect to compressed SCPS-TP Headers, and if so how?

– How could we support ECN?

5. Usage of reserved bits in the Compressed Header Bit-Vector


Which "Connection ID" do you use in constructing compressed SCPS-TP headers? Connection ID you provided the peer?Connection ID the peer provided?

SCPS-RI The one provided _TO_ the peer.

Global Protocols The one provided _TO_ the peer.

Xiphos The one provided _TO_ the peer.


When computing the header checksum for the compressed SCPS-TP

headers, what protocol number do you use for populating the pseudo-

header?

Protocol 105 – Compressed SCPS-TPProtocol 6 – TCP

SCPS-RI SCPS-RI now uses 105, and tries to _decode_ using 6 if 105 doesn’t work.

Global Protocols 105

Xiphos 105


Do you treat any/all SNACK options on a TCP header as incompressible options or do you make use of the SNACK bit in the Compressed Header Bit-Vector for encoding the first SNACK option on a segment?

SCPS-RI Does not implement compressed SNACK (all SNACK options uncompressed).

Global Protocols Compresses first SNACK option. 2nd, 3rd, etc. SNACK options are carried uncompressed.

Xiphos SNACK bit included in compressed header, all SNACK options uncompressed.


Encoding of ECN bits in a TCP Header

Supports ECNDoesn’t support ECN

SCPS-RI No ECN support.

Global Protocols No ECN support for compressed connections

Xiphos No ECN support.


Usage of reserved bits in the Compressed Header Bit-Vector

SCPS-RI None.

Global Protocols No, but they want to

Xiphos No, but they want to


Recommendations

The Connection ID used is always the one provided _to_ the peer during the syn/syn-ack exchange

Protocol #105 shall be used in the pseudo-header when computing the checksum for compressed headers.

Deal with compressed SNACKs in a minute ECN (RFC3168) should be supported. Deal with reserved bits in a minute.


Recommendation: Clarify the SNACK bit in the compressed header. Table 3-2: Compressed Header Bit Vector Contents

– Bit Name: SNACK– Meaning when set to 1: Short form SNACK option present– Notes: <None>

Insert between 3.6.2.8 and 3.6.2.9 in current spec:– 3.6.2.9 Compressed Short-form SNACK Option– The compressed short-form SNACK bit shall be set whenever a

short-form (i.e. NOT including a SNACK bit-vector) SNACK option is present, and shall occupy 4 octets immediately following the Sequence Number field.

– The first two bytes of the compressed short-form SNACK option shall contain the hole1 offset (see yyy). The next two bytes shall contain the hole1 size (see yyy2).

– Other SNACK options, including _all_ long-form options (those with uncompressed option lengths greater than 6 bytes) must be carried in the UNCOMPRESSED TCP Options portion of the compressed header.

Spec – C2.5.2


Recommendation: Define the SNACK bit in the compressed header (cont’d) Update fig. 3-5 to reflect encoding of compressed short-

form SNACK option.


Extending the SCPS Capabilities Option

Rationale Xiphos’ Proposal TLV Proposal Discussion


Rationale for Extending the SCPS Capabilities Option There are now a number of vendors implementing SCPS

PEPs– Desire to differentiate themselves by providing ‘special’

services Associate ‘session’ kinds of information with connections, signal

payload compression, …

– Need for additional capability signaling/negotiation. This can be done:

As part of the SCPS Capabilities Option exchange As a (set of) other TCP Options (would need experimental RFCs) Other

SCPS-Capabilities Option goes from fixed-length (4 bytes) to variable length– Need to check with IANA

Changing SCPS Capabilities option to variable-length Managing assignment of ‘extension types’


Xiphos’ Proposed Extensions

Xiphos’ extension proposal– Define a number of vendor types– Each vendor can define vendor-specific stuff– One vendor type per implementation (all SCPS-RI derived PEPs

would share one vendor space?)


General TLV Extension Mechanism

General TLV extension of SCPS Capabilities– # of bits for type, length?

SCPS Option Type (20)

SCPS Option Length

BETS SN1 SN2 Com NL TS ext

Connection ID

Type Len

Stuff…

SCPS Option Type (20)

SCPS Option Length

BETS SN1 SN2 Com NL TS ext

Connection ID

Type

Stuff…

Length

6 bits of type, 2 bits of length

8 bits of type, 8 bits of length


Next Steps for SCPS-TP

Post ‘interoperability resolutions’ to the list– Discussion– Accept– Mid-Dec.

More discussion of extension options on the list– Make sure all (known) vendors are aware– Mid-end of Dec.

Final resolutions to the list– WG consensus– Dec. – Revised blue book.

Generate ‘Pink Sheets’ for agency review– Jan – (getting the pink sheets pushed out to agencies)


Next Steps for Cislunar Documents

Other SCPS Protocols (FP, SP, NP)– Updated web site ($$$)

SCPS-RI Vegas modifications

Green Book– architecture,– Use cases

Document describing candidate protocols (solutions)– X– X– X– X– Scps

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Documents

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