add more srv6 encap mode consts

[kaey]

Summary by CodeRabbit

• New Features
  • Added support for three additional SEG6 encapsulation modes.
  • Added functionality to encode Segment Routing Headers for network segments.

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[coderabbitai]

Walkthrough

Added three new SEG6 encapsulation mode constants (L2ENCAP, ENCAP_RED, L2ENCAP_RED) and introduced the EncodeSEG6Srh function to encode Segment Routing Headers for IP segments. Extended SEG6EncapModeString to handle the new modes.

Changes

Cohort / File(s)	Summary
SEG6 Encapsulation Modes and Encoding nl/seg6_linux.go	Added three new enumeration constants for seg6 encap modes (L2ENCAP, ENCAP_RED, L2ENCAP_RED); introduced EncodeSEG6Srh function to encode Segment Routing Headers with segment validation; extended SEG6EncapModeString to support string representations of the new modes

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

• EncodeSEG6Srh function: Verify validation logic for non-empty input and SRH payload construction correctness
• Constant definitions: Confirm values align with kernel definitions and existing SEG6_IPTUN_MODE_\* pattern
• SEG6EncapModeString extension: Ensure string mappings ("l2encap", "encap.red", "l2encap.red") are accurate

Poem

🐰 Three modes hopping in with care,
L2ENCAP, RED in the air,
SRH headers now encode,
Along the segment routing road,
Rabbit's gift to routes so fair! 🥕

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'add more srv6 encap mode consts' accurately describes the main change: adding new SRV6/SEG6 encapsulation mode constants and related functionality to the codebase.

✨ Finishing touches

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

nl/seg6_linux.go (1)

130-149: Consider refactoring to eliminate code duplication.

The function logic is correct and consistent with EncodeSEG6Encap. However, the SRH encoding logic (lines 137-148) is duplicated from EncodeSEG6Encap (lines 69-79). Consider refactoring EncodeSEG6Encap to call EncodeSEG6Srh and prepend the mode bytes, reducing maintenance burden.

Example refactor for EncodeSEG6Encap:

 func EncodeSEG6Encap(mode int, segments []net.IP) ([]byte, error) {
-	nsegs := len(segments) // nsegs: number of segments
-	if nsegs == 0 {
+	srh, err := EncodeSEG6Srh(segments)
+	if err != nil {
-		return nil, errors.New("EncodeSEG6Encap: No Segment in srh")
+		return nil, err
 	}
-	b := make([]byte, 12, 12+len(segments)*16)
+	b := make([]byte, 4, 4+len(srh))
 	native := NativeEndian()
 	native.PutUint32(b, uint32(mode))
-	b[4] = 0                      // srh.nextHdr (0 when calling netlink)
-	b[5] = uint8(16 * nsegs >> 3) // srh.hdrLen (in 8-octets unit)
-	b[6] = IPV6_SRCRT_TYPE_4      // srh.routingType (assigned by IANA)
-	b[7] = uint8(nsegs - 1)       // srh.segmentsLeft
-	b[8] = uint8(nsegs - 1)       // srh.firstSegment
-	b[9] = 0                      // srh.flags (SR6_FLAG1_HMAC for srh_hmac)
-	// srh.reserved: Defined as "Tag" in draft-ietf-6man-segment-routing-header-07
-	native.PutUint16(b[10:], 0) // srh.reserved
-	for _, netIP := range segments {
-		b = append(b, netIP...) // srh.Segments
-	}
+	b = append(b, srh...)
 	return b, nil
 }

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 6e61cd4 and 2f6addb.

📒 Files selected for processing (1)

• nl/seg6_linux.go (4 hunks)

🔇 Additional comments (2)

nl/seg6_linux.go (2)

158-163: Implementation verified and correct.

The string representations ("l2encap", "encap.red", "l2encap.red") match the iproute2 format for SEG6 encapsulation modes, confirming the implementation is correct.

---

40-47: Based on my verification through the Linux kernel source code, the kernel's net/ipv6/seg6_iptunnel.c uses all five modes in switch statements: SEG6_IPTUN_MODE_INLINE, SEG6_IPTUN_MODE_ENCAP, SEG6_IPTUN_MODE_ENCAP_RED, SEG6_IPTUN_MODE_L2ENCAP, and SEG6_IPTUN_MODE_L2ENCAP_RED. This confirms that all the constants you've defined are valid and exist in the kernel.

Your Go code correctly defines these constants using iota continuation, which assigns sequential values 0, 1, 2, 3, and 4 respectively. This matches the expected enumeration pattern for these tunnel modes.

No changes are needed to the constant definitions.

[akerouanton]

LGTM

[akerouanton]

These matches https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/tree/ip/iproute_lwtunnel.c?id=2a82227f984b3f97354e4a490d3f172eedf07f63#n133:

static const char *seg6_mode_types[] = {
	[SEG6_IPTUN_MODE_INLINE]	= "inline",
	[SEG6_IPTUN_MODE_ENCAP]		= "encap",
	[SEG6_IPTUN_MODE_L2ENCAP]	= "l2encap",
	[SEG6_IPTUN_MODE_ENCAP_RED]	= "encap.red",
	[SEG6_IPTUN_MODE_L2ENCAP_RED]	= "l2encap.red",
};